- Email: [email protected]
Liquid Organic Hydrogen Carrier (LOHC) – Assessment based on chemical and economic properties
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Review Article
Liquid Organic Hydrogen Carrier (LOHC) e Assessment based on chemical and economic properties Matthias Niermann a,*, Alexander Beckendorff a, Martin Kaltschmitt a, Klaus Bonhoff b a
Hamburg University of Technology, Institute of Environmental Technology and Energy Economics, Eißendorfer Str. 40, 21073, Hamburg, Germany b National Organisation Hydrogen and Fuel Cell Technology, Fasanenstr. 5, 10623, Berlin, Germany
article info
abstract
Article history:
Hydrogen can be transported via long distances based on Liquid Organic Hydrogen Carriers
Received 23 July 2018
(LOHC). Such a transport is realized based on a two-step cycle: (1) loading/storage of
Accepted 21 January 2019
hydrogen (hydrogenation) into the LOHC molecule and (2) unloading/release of hydrogen
Available online 16 February 2019
(de-hydrogenation). During the storage period, hydrogen is covalently bound to the respective LOHC. Since the (optimal) LOHC is liquid at ambient conditions and shows
Keywords:
similar properties as crude oil based liquids (e.g. diesel, gasoline), it can easily be handled,
Liquid Organic Hydrogen Carrier
transported and stored; thus a stepwise implementation using the existing crude oil based
LOHC
infrastructure would be possible. Against this background this paper reviews the current
Hydrogen economy
knowledge in hydrogenation and de-hydrogenation of various LOHC. Therefore, a variety
Alternative fuels
of LOHC is evaluated based on their properties and compared to each other. By applying
Hydrogen storage
different evaluation criteria representing the requirements of the three different applica-
Assessment
tion areas (energy-storage, energy-transport, mobility application), the LOHCs can be assigned to a field they suit best. The analysis shows that the most promising LOHC candidates to date are dibenzyltoluene for energy-transport and energy-storage as well as Nethylcarbazole for mobility applications. In addition, a use of toluene in the transport sector is also conceivable. Methanol can potentially be applied in all three application fields due to its properties if a compromise between de-hydrogenation temperature and gas flow can be achieved based on further R&D-activities. For future implementation phenazine and formic acid show great potential, but also additional R&D especially regarding catalysis and solvents is necessary. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (M. Niermann). https://doi.org/10.1016/j.ijhydene.2019.01.199 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
6632
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6632 Storage molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6634 N-ethylcarbazole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6634 Dibenzyltoluene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6636 1,2-dihydro-1,2-azaborine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6637 Formic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6638 Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6639 Naphthalene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6640 Toluene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6641 Phenazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6642 Assessment and comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6642 Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6643 Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6644 Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 De-hydrogenation temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Energy demand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Material handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Process design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Gas flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Technical readiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6647 Energy-transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6648 Energy-storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6648 Comparison to other storage options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6649 Final considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6649 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6650 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6650
Introduction Our modern energy and transportation system relies mainly on fuels based on fossil energy resources. Because of the a priori limited resources of these energy carrier, their strong price fluctuations, their contribution to global warming, as well as their negative impact on human health and the natural environmental interest in renewable energy sources has grown significantly in recent years; 2016 already 19.5% of the world's energy demand has been covered by renewable sources of energy [1]. However, this is just a first step in an on-going development towards an increasingly more sustainable energy system. Germany, as one example, has set the objective to reach a share of 80% renewable energies related to the overall gross electricity generation by 2050 [2]. Such a transition of the overall energy system in general and the electricity supply system in particular faces substantial challenges that need to be addressed in an adequate way. Especially the fluctuating character of electricity from wind mills and photovoltaic systems might result in regional
over- and underproduction of electricity. Thus, balancing as well as storage of e.g. electrical energy is needed to ensure a secure energy supply. Additionally, regional differences in solar radiation and wind speed influencing the energy respectively the electricity output contribute to this imbalance. To counteract and balance this uneven distribution of energy/electricity, efficient systems for (long-term) energystorage and (long-distance) energy-transportation are urgently required. Beside this, to contribute to the decarbonization of the mobility sector the electricity supply system could be much better coupled with the transportation sector [3,4]; this is one of the driver to develop battery-electric vehicles as well as hydrogen driven fuel cell cars. To realize such approaches successfully, especially the weight and size of on-board storage systems for renewable fuels need to be greatly reduced. Hydrogen can be used in all three application areas (mobility, energy-storage, energy-transport). It is due to its very specific properties an attractive energy carrier. However, the low energy density and the difficult handling of molecular hydrogen as a low density gas complicates its usage and results in a high economic and energetic effort to store it.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
6633
Fig. 1 e Concept of the LOHC storage.
So far, different storage options exist characterized by specific advantages and disadvantages. Most widely used are “Compressed Hydrogen (CGH2)” and “Liquefied Hydrogen (LH2)” storage devices. Both storage options are characterized by high security requirements due to a high pressure and a low temperature, respectively. Additionally, the necessary conditioning of the hydrogen gas to be stored into the storage device results in a high electricity demand. Additionally, 0.3e3% of the hydrogen is lost in a liquid H2 storage system due to boil-off [5e7]. Low and high temperature metal hydrides (MH) are another storage option showing in theory high storage capacities. But here thermal management and packing limitations greatly decreases these values [8]. Also, the limited reversibility of the hydrogen storage, decomposition of the storage material and slow reaction kinetics are challenges to be tackled by further research [9]. Another method to store hydrogen at ambient conditions are Liquid Organic Hydrogen Carriers (LOHC). LOHCs are potentially cheap, safe, and easily manageable. Moreover, they allow for a long-term energy storage without boil-off or other hydrogen losses as well as an uncomplicated transportation. The storage concept of hydrogen within a LOHC (Fig. 1) is typically based on reversible hydrogenation and dehydrogenation of carbon double bounds. During the hydrogenation process the double bounds are saturated with hydrogen. This process is exothermic and typically takes place at elevated temperatures and pressures. Vice versa the hydrogen can be released again in its pure form based on a catalytic endothermic de-hydrogenation reaction taking place mostly close to atmospheric pressure, although at elevated temperatures. Catalysis plays a crucial role within the underlying reaction processes. Hydrogenation of the unloaded LOHC (H0LOHC) as well as de-hydrogenation of the loaded LOHC (HnLOHC1) is 1
n: number of hydrogen atoms.
typically catalyzed. A heterogenic catalysis is beneficial for such LOHC-storage systems, because then there is no need to separate the catalyst from the reaction mixture. Thus, it is possible to operate the reactor and the storage tank separately; this might have advantages especially for large systems [10]. Due to such a storage process (i.e. hydrogen bound into the LOHC molecule), the volumetric energy density can be greatly increased and the handling can be simplified as the properties of LOHCs are typically similar to crude oil derivate (e.g. diesel, gasoline). Therefore, LOHC systems might use the existing crude oil infrastructure. This allows a stepwise implementation of this technology and offers an opportunity for the crude oil industry to reduce increasingly its carbon footprint. Another advantage resulting from the similarity to known fossil energy based fuels (e.g. diesel, gasoline) is the good public acceptance as the people are used to deal with these substances since decades. Hydrogenation and de-hydrogenation of different substances has been assessed in depth [11e21]. For example, Crabtree [22] suggested organic heterocycles to be used as LOHC outlining the safe storage and easy heat management. Mu¨ller et al. [14,23] evaluated LOHCs based on their thermodynamic properties emphasizing that nitrogencontaining aromatics are well suited to be used as a LOHC due to their enthalpy change. Additionally, a systematic toxicity evaluation has been carried out outlining that this aspect need to be treated seriously [24]. Also potential LOHC candidates have been analyzed for various potential application fields [10,25e34]; such fields of applications were energy-storage in buildings, energy-transport and fuel applications in vehicles. Each of these investigation mainly focus on one LOHC, including N-ethylcarbazole, dibenzyltoluene, toluene and 1,2-dihydro-1,2-azaborine. Also the coupling or linking of the endothermic de-dehydrogenation with industrial waste heat have been examined exemplarily for a cement plant [35]. Wang et al. [36] carried out a comparison between a LOHC-based storage system and a Compressed Hydrogen Gas (CHG) storage system. However, a broad evaluation and comparison of different LOHC with
6634
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
each other related to the potential implementation field based on their chemical/physical properties does not exist for the time being. Against this background this paper examines different substances based on a broad literature review for their potential use as a LOHC. To find potential weak spots for each substance the physical properties, hydrogenation and dehydrogenation conditions as well as selected economic factors are considered and analyzed. Based on these data, for each LOHC assessed here possible application areas are identified and allocated. Thus, for the first time properties and fields of application are combined for a holistic overview of potential LOHC.
Storage molecules Possible storage molecules, their hydrogenation and dehydrogenation reaction as well as their properties are evaluated below. The following frame conditions are taken as a basis for further evaluation. Storage. The ultimate objective for on-board storage capacity and volumetric energy density are according to the US-DoE 6.5 wt.-% and 1.7 kWh/L, respectively [37]. These values are recognized internationally so far as the given benchmark. The storage capacity and energy density is stated for the pure substance (i.e., without possible solvents and without the tank system) and, if applicable, the diluted substance with the de-hydrogenation limitation. Availability. From an economic point of view, the LOHC should be cheap and unlimitedly available. The current prices for these substances are taken, if not otherwise available, from an internet wholesale platform for large distributors [38]. Toxicity. The LOHC substances should of course be nontoxic. Typically the toxicity is described by the Toxicity Potential Indicator (TPI) [39]. This key figure is calculated based on the R-phrases, the water hazard class, the maximum admissible concentration and the EU carcinogenicity. The toxicity potential indicator (TPI) is given for a range between “0” (substance with no known hazard) and “100” (extremely toxic substance). De-hydrogenation temperature. A low de-hydrogenation temperature simplifies the integration of the storage into a heating network and ideally makes it possible to use lowtemperature waste heat to cover the de-hydrogenation demand (e.g. the waste heat of a Polymer Electrolyte Fuel Cell (PEFC)). Energy demand. During hydrogenation energy is released and typically not used. During de-hydrogenation more energy is needed to release the hydrogen again from the LOHC molecule. Thus the energy demand of this storage system is determined by the energy demand/heat demand of de-hydrogenation. In this respect low enthalpies of reaction require a higher heat demand, which would make the storage energetically less favorable. If a cleaning step for the hydrogen is needed after the release the respective energy demand adds up to the overall energy demand for such a storage system.
Material handling. Ideally, no precaution based on the guidelines for working with chemicals (GHS-symbols) have to be taken to store and transport the LOHC. Handling is further characterized by the physical properties of the LOHC, which in an ideal case includes a low melting and high boiling point (i.e., the loaded and unloaded LOHC is liquid during the overall storage process). This also includes a low vapor pressure for low requirements for storage safety and a low dynamic viscosity for easy pumping. Process design. If the LOHC is solid in the needed temperature range (high melting point), a solvent has to be added. To ensure hydrogen purity after de-hydrogenation, a downstream purification has to be added to separate the solvent securely from the hydrogen. A low boiling point of the LOHC is also disadvantageous since hydrogen separation from the unloaded LOHC will also require an extra purification step. And each additional step complicates the process design and needs more energy. A simple design further includes a high ignition temperature as well as a high flashpoint for explosion prevention. Stability. The optimal substance is completely stable in the relevant temperature range and no by-products occur during the hydrogenation resp. the de-hydrogenation reaction. In reality the LOHC needs to be replaced at regular intervals as a certain amount will inevitably be lost/ degraded during the storage process. This happens through unwanted side reactions like isomerization and disproportionation. The more stable the LOHC, the more recirculation intervals are possible. The process stability is further characterized by a high catalyst stability. The information on stability is not uniform in literature; a high stability is either marked by a high turnover number or by a long operation time, respectively. Gas flow. This parameter shows how fast hydrogen can be released from the loaded LOHC. It describes the mass of hydrogen released from the reaction mixture (LOHC plus solvents) per hour. If a certain amount of hydrogen remains bounded to the LOHC (i.e. cannot be released again), this is synonymous with a reduced gas flow. Technical readiness. For comparison of the technical development status, the Technology Readiness Level (TRL) is applied [40]. It rates the progress on a scale from 1 to 9 (i.e., from the observation of the basic principle to successfully operating systems). In addition to the substances examined here (Table 1) and described in detail below, there are several other chemical compounds potentially to be used as a LOHC; these LOHC are not further investigated here (Table 1).
N-ethylcarbazole N-ethylcarbazole (NEC) is a nitrogen-substituted heterocycle and was first considered as a LOHC in the mid-2000's [57]. The hydrogenated form is called perhydro-N-ethylcarbazole. The main material parameters of this LOHC system are listed in Table 2. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 2) are as follows.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
6635
Table 1 e Possible LOHCs. LOHC
Reason for further consideration
N-ethylcarbazole Dibenzyltoluene 1,2-dihydro-1,2-azaborine Formic acid Methanol Naphthalene Toluene Phenazine
Well-studied nitrogenous LOHC Already existing application as LOHC; safe and convenient handling Unique characteristics through integration of boron and nitrogen Safe and convenient handling Very high storage density Well-studied cycloalkane; high storage density Well-studied cycloalkane; planned application as LOHC Promising stability and sustainable raw material production
LOHC Benzyltoluene 3-Methyl-1,2-BN-cyclopentan 2-Aminoethanol Benzene Indoline Chinoline Fluorene 4-Aminopyridine Bicyclohexyl 1,2,4-Triazolidin Lithiated Primary Amine 2-Methyl-1,2,3,4-tetra-hydroquinoline Perhydro-dibenzofuran 2,6-Dimehtyldecahdro-1,5-naphthyridin N-ethylindole N-propylcarbazole
Ref. [18] [41] [18] [18] [18] [42] [43] [44]
Reason for no further consideration
Ref.
Similar to dibenzyltoluene, however, more toxic Energy intensive regeneration and trimerization Low selectivity, long storage cycles High process temperatures (400e450 C) and very toxic Low gravimetric and volumetric energy density De-hydrogenation of only one ring possible, thus low storage capacity Slow de-hydrogenation Low selectivity Low selectivity Unstable at room temperature To date no successful hydrogenation Low selectivity Low selectivity Low selectivity Similar to N-ethylcarbazole, but less studied Similar to N-ethylcarbazole, but less studied
[45] [46] [47] [48] [18] [49] [48] [50] [51] [20] [52] [18] [53] [54] [55] [56]
Hydrogenation. The hydrogenation reaction is normally catalyzed by heterogenic catalysts consisting of a precious metal like palladium (Pd) and ruthenium (Ru) supported by aluminum oxide (Al2O3). Bru¨ckner et al. [45] have demonstrated a full hydrogenation of N-ethylcarbazole after 180 min at 150 C and 50 bar with a Ru/Al2O3 -catalyst. De-hydrogenation. For de-hydrogenation similar catalyst compositions as for hydrogenation are found. A full dehydrogenation can for example be achieved using a Pd/ Al2O3-catalyst at 270 C in 25 min or at 180 C in 250 min [45,61]. For the N-ethylcarbazole/perhydro-N-ethylcarbazole system, the following results can be summarized for the assessment criteria defined above.
hydrogenation is restricted to 90% to ensure a liquid state throughout the process. This decreases its storage capacity to 5.2 wt.-%. The energy density is 2.5 kWh/L and 2.25 kWh/ L, respectively. Availability. The raw material price of N-ethylcarbazole is ca. 40 V/kg [62]. The substance is produced by distillation; the production capacity currently installed globally is less than 10,000 t/a [18]. Toxicity. N-ethylcarbazole has a toxicity potential indicator (TPI) of 5.1 TPI/mg. For perhydro-N-ethylcarbazole no data is available to calculate the TPI. However, a first analysis shows, that the toxicity level is more or less equal to N-ethylcarbazole [24]. De-hydrogenation temperature. At 180e270 C a full dehydrogenation can be achieved. The higher the temperatures, the shorter the reaction time [45,61].
Storage. The storage capacity is 5.8 wt.-%. N-ethylcarbazole is not liquid at ambient conditions. Therefore, the de-
Table 2 e N-ethylcarbazole/Perhydro-N-ethylcarbazole system [58e60].
Density [kg/L] Liquid temperature range [ C] Flashpoint [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes
N-ethylcarbazole
Perhydro-Nethylcarbazole
1.10 68e270
0.94 <20e280
186 121
146 5.9
0.05
4.4
08
No data
Fig. 2 e Storage process of the N-ethylcarbazole/perhydroN-ethylcarbazole system.
6636
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Energy demand. De-hydrogenation is characterized by a heat demand of 53.2 kJ/molH2 [45]. Material handling. N-ethylcarbazole raises only health hazard concerns; additional precautions are not necessary. Unfortunately, data for perhydro-N-ethylcarbazole are missing. The low vapor pressure of both forms are advantageous for storage and handling, while the relatively high viscosity of the N-ethylcarbazole increases the pumping resistance. Process design. N-ethylcarbazole is solid at ambient conditions. To avoid adding a solvent, de-hydrogenation is restricted. Thus, a hydrogen purity of 99.99% after dehydrogenation can be achieved [61]. A downstream hydrogen purification step is not necessary. Stability. At temperatures of 270 C and a reaction time longer than 72 h in the presence of a catalyst a small number of by-products (<2%) is formed due to dealkylation [45]. Process temperatures above 270 C to enhance the reaction kinetic lead to a reduced stability. The catalyst lifetime is estimated to allow for 500,000 kgmat/kgcat [30]. Gas flow. The de-hydrogenation temperature influences the reaction time and therefore the gas flow. At 180 C with the de-hydrogenation restriction a gas flow of 68.0 gH2/(L h) can be achieved; this can be increased at 270 C to 163.1 gH2/ (L h) with the de-hydrogenation restriction of 90%. Technical readiness. N-ethylcarbazole has been tested on a lab-scale. The implementation in an operating environment still needs to be demonstrated. Thus the technology readiness level is 3.
Dibenzyltoluene Dibenzyltoluene (DBT) is a cycloalkane used in industry as a heat transfer oil. The hydrogenated form is called perhydrodibenzyltoluene. Important parameters of this LOHC system are listed in Table 3. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 3) are as follows. Hydrogenation. The hydrogenation reaction is normally catalyzed by heterogenic catalysts consisting of a precious
Table 3 e Dibenzytoluene/perhydro-dibenzyltoluene system [58,60].
Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a
Dibenzyltoluene
Perhydrodibenzyltoluene
1.04 39e390
0.91 45e354
450 44.1
No data 258
0.07a
0.04a
09
No data
The values were extrapolated based on the experimental data of Mu¨ller et al. [14].
Fig. 3 e Storage process of the dibenzyltoluene/perhydrodibenzyltoluene system.
metal like platinum (Pt) and ruthenium (Ru) supported by aluminum oxide (Al2O3). For example, Bru¨ckner et al. [45] have demonstrated a full hydrogenation of dibenzyltoluene after 240 min at 150 C and 50 bar with a Ru/Al2O3catalyst. De-hydrogenation. For the de-hydrogenation palladium (Pd) and ruthenium (Ru) catalysts supported by carbon (C) are used. An almost full de-hydrogenation (97%) can for example be achieved using a Pd/C-catalyst at 310 C in 120 min [45]. For the dibenzyltoluene/perhydro-dibenzyltoluene system, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity is 6.2 wt.-% and the energy density is 1.9 kWh/L. Considering the de-hydrogenation limitation the storage capacity is 6.0 wt.-% and the energy density 1.8 kWh/L. Availability. The price for dibenzyltoluene is around 4 V/kg. This relative low price level supports large scale applications [62]. Toxicity. Dibenzyltoluene has a toxicity potential indicator (TPI) of 13,8 TPI/mg. For the perhydro-dibenzyltoluene no data about the toxicity is available. De-hydrogenation temperature. Temperatures above 310 C are needed to achieve an almost full dehydrogenation. Lower temperatures are possible for this reaction; but a lower temperature level significantly reduce the hydrogen yield. At 270 C only 40% of the hydrogen is released within 2 h [45]. Energy demand. De-hydrogenation is characterized by a heat demand of 65.4 kJ/molH2 [45]. Material handling. The handling of dibenzyltoluene is only characterized by a possible environmental hazard. Other precautions are not necessary. Unfortunately, data for perhydro-dibenzyltoluene are missing. The low vapor pressure is advantageous for storage and handling. The relatively high viscosity of dibenzyltoluene increases pumping resistance. Process design. Dibenzyltoluene is liquid during the overall hydrogenation resp. de-hydrogenation process. Thus a
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
hydrogen purification step after de-hydrogenation is not necessary. Stability. At temperatures of 270 C and a reaction time longer than 72 h in the presence of a catalyst no significant number of by-products (<0.01%) has been formed [45]. The catalytic stability for hydrogenation is assumed with 14,000 h and the de-hydrogenation with 8,000 h [34]. Gas flow. De-hydrogenation temperature is crucial for the hydrogen yield after a specific reaction and therefore influences the gas flow. At 270 C a gas flow of 11.3 gH2/(L h) can be achieved to be increased at 310 C to 27.5 gH2/(L h). Technical readiness. Dibenzyltoluene is well analyzed. A demonstration project for overall storage system including transportation by trucks has been realized in Germany. Hydrogenation (<400 Nm3/h) and de-hydrogenation (<110 Nm3/h) plants are available on the small scale as well as in an industrial scale. This system will be demonstrated within the US in an industrial demonstration project [41]. Therefore, the technology readiness level is 9.
1,2-dihydro-1,2-azaborine 1,2-dihydro-1,2-azaborine (AB) is a material synthesized for the first time in 2011 [63]. It is a boron (B)- and nitrogen (N) substituted heterocycle and the hydrogenated form is called 1,2-BN-cyclohexane. The main material parameters of this LOHC system are shown in Table 4. The conditions for hydrogenation and de-hydrogenation of the storage process (Fig. 4) are as follows. Hydrogenation. The carbon (C) atoms of the storage molecule can be hydrogenated with a palladium catalyst supported on carbon. However, a catalytic hydrogenation of the B- and N-atoms induced by elevated pressure levels and higher temperatures has not been demonstrated so far. Up to now, the addition of hydrogen to these atoms is only possible via a two-step addition by hydride (KH) and proton (HCl) equivalents [65]. The calculated equilibrium results in a theoretical hydrogenation rate of 95% at 80 C and 10 bar [28]. De-hydrogenation. De-hydrogenation can be catalyzed by an iron chloride (FeCl2) or cobalt chloride (CoCl2) catalysts
Table 4 e 1,2-dihydro-1,2-azaborine/1,2-BN-cyclohexane system [23,63,64]. 1,2-dihydro1,2-azaborine Density [kg/L] Liquid temperature range [ C] Flash point [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a b
1,2-BNcylcohexane
No data 45e87a
1.01 63e87a
22.7 H 22.6b 0.55a
No data 0.57a
18,300a
18,300a
No data
No data
Calculated with Aspen Properties. Predictions by ACD/labs.
6637
Fig. 4 e Storage process of the 1,2-dihydro-1,2-azaborine/ 1,2-BN-cyclohexane system.
at 80 C in 20 and 10 min, respectively. To date in this catalytic reaction only two hydrogen molecule equivalents can be released through isomerization from the storage molecule [66]. The theoretical equilibrium was also calculated for the de-hydrogenation with a rate of 99% at 80 C and atmospheric pressure [28]. For the 1,2-dihydro-1,2-azaborine/1,2-BN-cyclohexane system, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity is 7.1 wt.-% and the energy density is 2.4 kWh/L. By adding tetrahydrofuran as a solvent the storage capacity is reduced to 2,3 wt.-% and the storage capacity to 0.8 kWh/L and by trimerization to 4.7 wt.-% and 1.6 kWh/L [63]. Availability. Data about market prices are not available. However, it is expected that the price might be at least as high as boron (B) due to the limited availability of boron. Toxicity. To date no reliable data about the toxicity of 1,2dihydro-1,2-azaborine or 1,2-BN-cyclohexane is available. Nevertheless, tests of B-N-heterocycles suggest a low toxicity [67]. De-hydrogenation temperature. De-hydrogenation takes place at temperatures of 80 C. Existing calculations [28] promise a nearly full yield at this temperature level. Energy demand. De-hydrogenation is characterized by a heat demand of 35.9 kJ/molH2 [23]. This low enthalpy is achieved through the coupling of the exothermic dehydrogenation of the B- and N-atoms of the molecule with an endothermic de-hydrogenation of the C-atoms. Material handling. No reliable statement about the handling of the material can be given. The predicted high vapor pressure suggests a difficult storage, while the relatively low viscosity reduces the pumping resistance. Process design. 1,2-BN-cyclohexane is not liquid at ambient temperatures. Thus a solvent like tetrahydrofuran has to be added. Therefore, hydrogen purification after dehydrogenation is necessary. The predicted low flashpoint makes the de-hydrogenation operation more difficult as the substance might ignite on the hot surface of the de-
6638
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
hydrogenation reactor. The multi-step hydrogenation process demands a more complicated process design. Stability. 1,2-dihydro-1,2-azaborine is air and moisture stable. Additionally, it is thermally stable up to 150 C and de-hydrogenation proceeds cleanly without side reactions [46,68]. The stability in several storage cycles has yet to be demonstrated. Gas flow. With tetrahydrofuran as a solvent a gas flow of 132.5 gH2/(L h) catalyzed by CoCl2 and 66.2 gH2/(L h) catalyzed by FeCl2 can be achieved, if trimerization can be avoided. Technical readiness. The storage of hydrogen in the 1,2dihydro-1,2-azaborine/1,2-BN-cyclohexane system still faces major obstacles to be addressed in the years to come. If a regeneration of the loaded state is not possible with pressure alone, in the future energetically and economically justifiable hydrogenation can take place in large-scale plants. Further research regarding long-term stability, liquefaction, regeneration, trimerization and toxicity is indispensable. Its technology readiness level is assessed to be 2.
Formic acid Formic acid (FA) is a colorless, corrosive and water-soluble liquid. In this LOHC system carbon dioxide (CO2) is used as a starting material for formic acid generation. The main material parameters of the carbon dioxide/formic acid system are listed in Table 5. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 5) are as described below. Hydrogenation. Hydrogenation is exothermic but also endergonic because the equilibrium can be shifted by adding solvents (e.g. water) and/or bases (e.g. ammonia). The solubility of hydrogen and carbon dioxide is the limiting factor for final formic acid-concentration. Thus, it greatly decreases storage capacity. Homogenous catalysts are normally used to catalyze the reaction in a pressure range from 1 to 100 bar and a temperature range from 50 to 100 C [69e72]. De-hydrogenation. De-hydrogenation can take two different reaction pathways, producing (1) hydrogen and carbon dioxide or (2) water and carbon monoxide. The second pathway needs to be avoided for hydrogen storage. The de-hydrogenation is also catalyzed by homogenous catalysts. To avoid downstream separation,
Table 5 e Carbon dioxide /formic acid system [58].
Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes
Carbon dioxide
Formic acid
0.47 e e e
1.22 8e101 520 1.8 11,400 02, 05, 06
04
Fig. 5 e Storage process for carbon dioxide/formic acid system.
catalysts, which are able to catalyze both reactions, are used [69e72]. This eliminates the need to separate the catalyst from the LOHC stream. However, these catalysts are less productive than those specialized for one reaction. Each reaction can be induced for example by changing the pH and the temperature or pressure [18]. The unfavorable equilibrium of the hydrogenation is advantageous for the de-hydrogenation, liberating the hydrogen at temperatures below 100 C. An alternative way to the carbon dioxide/formic acid system for hydrogen storage is the bicarbonate/formate system. This system (Equation (1)) is thermodynamic more favorable and the carbon dioxide remains dissolved throughout the overall process. Water is here the source for the hydrogen [73]. Six release/storage cycles with a heterogeneous palladium catalyst supported by reduced graphite oxide (Pd/r-GO) were performed with potassium formate (KHCOO) [74]. The hydrogenation was realized at 100 C and 40 bar in a 4.8 M aqueous potassium bicarbonate (KHCO3) solution with formate yield of 96.8% and a de-hydrogenation at 80 C and atmospheric pressure in 4.8 M aqueous KHCOO solution with a conversion efficiency of 96.6%. The reaction time were 10 and 2 h, respectively. MHCOO þ H2 O#MHCO3 þ H2
with M ¼ Na; K
(1)
For the carbon dioxide/formic acid system, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity of pure formic acid is 4.4 wt.% and the energy density is 1.8 kWh/L. The needed solvents for shifting the equilibrium reduce the capacity to 0.3 wt.-% and the energetic density to 0.1 kWh/L (final formic acid concentration 1.53 M [70]). For the potassium bicarbonate/ potassium formate system in solution it is 0.65 wt.-% and 0.25 kWh/L, respectively. Availability. Carbon dioxide (CO2) is the starting material for a hydrogen storage in formic acid. Although CO2 is abundantly available in the earth atmosphere, it remains energetically challenging to capture it. Exhaust air are
6639
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
potential sources. Here CO2 is available in higher proportion; thus it can be captured more easily. The CO2-price through a pipeline distribution is assumed to be about 30 V/t2 [76]. Toxicity. Formic acid has a toxicity potential indicator (TPI) of 9,0 TPI/mg. De-hydrogenation temperature. The de-hydrogenation can be achieved at temperatures below 100 C. Energy demand. De-hydrogenation is characterized by a heat demand of 31.2 kJ/molH2 [77]. Material handling. Formic acid is flammable, corrosive and toxic. Therefore, handling is challenging. The high vapor pressure causes high requirements during storage, while the low viscosity is advantageous. Process design. Although formic acid is liquid at ambient temperatures, solvents still need to be added. Thus a downstream hydrogen purification step is necessary to clean the released hydrogen and recapture the CO2 for renewed hydrogenation. To realize this within a closed cycle the storage of CO2 is challenging. Thus CO2 can also be released into the atmosphere and removed from the ambient air where it is needed again. An alternative design, which can be applied to avoid the use of gaseous CO2, is the use of the bicarbonate/formate system. The CO2 remains solved during the overall process; a hydrogen purification is therefore not necessary [73]. Stability. Formic acid dissolved in water shows a high thermal stability [78]. Challenging is the stability of the catalysts; depending on the catalyst the turn over numbers vary between 2,000 and 200,000 for the hydrogenation and between 120 and 20,000 for the de-hydrogenation [69e72]. Gas flow. The added solvent reduces the gas flow. A flow up to 0.7 gH2/(L h) can be achieved. Technical readiness. The potential use of formic acid as a LOHC has been experimentally demonstrated on a labscale level. Further research to reduce the dilution is necessary to increase gas flow and storage capacity in technical applications. The deactivation of catalysts is also an issue. The technology readiness level can be assessed to be 3.
Table 6 e Carbon dioxide /methanol system.
Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes
Carbon dioxide
Methanol
0.46 e e e e 04
0.79 98e65 440 0.6 35,400 02, 06, 08
Hydrogenation is catalyzed by heterogeneous multicomponent copper (Cu) based catalysts at temperatures between 220 and 270 C and pressures between 20 and 80 bar [79e82]. The conversion rates are comparatively low. De-hydrogenation. De-hydrogenation production stream consists of carbon dioxide, carbon monoxide and hydrogen. Due to the toxicity of carbon monoxide this reaction product needs to be avoided. With increasing temperature, the carbon monoxide content within the production stream increases. Additionally, the water/ methanol molar ratio is crucial for the formation of carbon monoxide [83]. De-hydrogenation can be achieved by hightemperature steam reforming of methanol (SRM) and lowtemperature de-hydrogenation. For steam reforming of methanol (SRM) at 420 C with a heterogeneous catalyst containing iridium (Ir) and platinum (Pt) supported by cerium-zirconium-oxide (Ce0,5Zr0,5O2) the total reforming in an aqueous medium (H2O/CH3OH: 1,5 mol/mol) was achieved in 2 h. However, 6 mol-% carbon monoxide was detected in the production stream [84]. Low-temperature de-hydrogenation (<100 C) facilitates the heat management and makes the hydrogen delivery on-board more feasible. The reaction takes place in an aqueous medium and achieves depending on the used catalyst a hydrogen yield between 15 and 84%. The reaction lasts between 600 and 1,440 min; a higher yield does not necessarily correspond with the longer reaction time. Water or other solvents like tetrahydrofuran or triglyme have to be added also. Only very small amounts of carbon monoxide were detected [85e88].
Methanol Methanol (MET) is an alcohol. In the respective LOHC system carbon dioxide (CO2) is used as a starting material for methanol generation. The main material parameters of the CO2/ methanol system are listed in Table 6. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 6) are as outlined below. Hydrogenation. Methanol can be synthesized by hydrogenation of carbon dioxide. The reaction follows two pathways, which yield either (1) methanol and water or (2) carbon monoxide and water. For hydrogen storage pathway (2) must be avoided to optimize this storage process. Also this reaction can be steered by catalysts.
2
Exchange rate 0.86 V ¼ 1.0 US$ [75].
Fig. 6 e Storage process of the carbon dioxide/methanol system.
6640
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
For the LOHC system carbon dioxide/methanol, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity of methanol is 12.1 wt.-% and the energy density is 3.3 kWh/L. The solvents necessary for de-hydrogenation reduce the capacity and the energetic density. For high-temperature steam reforming a capacity of 10 wt.-% and an energy density of 2.7 kWh/L can be realized. For low-temperature de-hydrogenation they are reduced to 4 wt.-% and 1.1 kWh/L. Availability. Carbon dioxide (CO2) is the starting material for hydrogen storage in formic acid. Although CO2 is abundantly available in the earth atmosphere, it remains energetically challenging to capture it from ambient air. Flue gases from the combustion of fossil fuel energy are potential sources; here CO2 is available in higher proportion and thus can be captured more easily. The CO2 price through a pipeline distribution is assumed to be about 30 V/t3 [76]. De-hydrogenation temperature. De-hydrogenation can be realized at temperatures below 100 C. For a faster release of hydrogen temperatures around 400 C are needed. Energy demand. De-hydrogenation is characterized by a heat demand of 16.5 kJ/molH2 [18]. Material handling. Methanol is flammable and toxic. It is also characterized by a health hazard. The handling is therefore relatively challenging. Additionally, the high vapor pressure causes higher storage requirements. Process design. Although methanol is liquid at ambient temperatures, solvents still need to be added. Therefore, a downstream hydrogen purification step is necessary to purify the released hydrogen and recapture the CO2 for renewed hydrogenation. The low viscosity of methanol is further advantageous. Stability. Methanol is stable up to a temperature of 320 C [89]. The catalyst for hydrogenation show a good stability after 3,000 h operation time [79]. For high-temperature dehydrogenation only a minor decrease in the methanol conversion have been detected after 200 h [84]. The catalysts for low-temperature de-hydrogenation show turnover numbers of 10,000 and 350,000 [85,88]. Gas flow. At 420 C a gas flow of 44.8 gH2/(L h) can be realized. Below 100 C the gas flow is reduced to 1 to 3 gH2/(L h). Technical readiness. The hydrogenation process of CO2 to methanol is currently being demonstrated in a commercial-scale facility (George Olah Plant, Island). It produces more than 5 Mio. L/a [90]. Also high-temperature steam reforming of methanol is a well-known process. But, the low-temperature de-hydrogenation characterized by the benefit of a simpler heat management must be investigated more closely and has so far only been demonstrated. The system in combination with high-temperature reforming has a technology readiness level of 9, while the system in combination with low-temperature de-hydrogenation shows a technology readiness level of 3.
3
Exchange rate 0.86 V ¼ 1.0 US$ [75].
Naphthalene Naphthalene (NAP) is a polycyclic aromatic hydrocarbon. This molecule has also been proposed as a LOHC. The hydrogenated form is called decalin. The main material parameters of this LOHC system are listed in Table 7. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 7) are as follows. Hydrogenation. Full hydrogenation can be catalyzed by an aluminum mobile composition of matter (Al-MCM) catalyst at 300 C or 200 C and 69 bar in 150 min or 480 min, respectively. During hydrogenation decalin is formed. This molecule is an isomer consisting of trans-decalin and cisdecalin. Trans-decalin is thermodynamically favorable; nonetheless, these two isomers are usually equimolar distributed. The exact ratio depends on the catalyst used and the reaction temperature [91]. De-hydrogenation. De-hydrogenation is usually catalyzed by a platinum (Pt)-catalyst on carbon (C) support. It can almost be fully achieved at 280 C in 150 min. By adding rhenium (Re) to the catalyst (Pt-Re/C) the reaction time can be decreased to 120 min by otherwise identical conditions [42]. For the naphthalene/decalin system, the following results are found for the characteristic criteria defined above. Storage. The storage capacity of the naphthalene is 7.4 wt.% and the energy density is 2.2 kWh/L. Adding toluene as a
Table 7 e Naphthalene/decalin system [58]. Naphthalene Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a
Decalin
1.14 80e218 540 2.0
0.90 43e185 235 3.0
114a 02, 07, 08, 09
360a 02, 05, 06, 08, 09
Calculated with Aspen Properties.
Fig. 7 e Storage process of the naphthalene/decalin system.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
solvent, the capacity and the energetic density is reduced to 3.8 wt.-% and 1.1 kWh/L. Availability. The current market price of approximately 0.6 V/kg is favorable for large scale application [92]. Toxicity. Naphthalene has a high toxicity potential indicator (TPI) of 45,8 TPI/mg and is probably carcinogenic. Compared to that the toxicity potential indicator (TPI) for decalin is only 1,2 TPI/mg. De-hydrogenation temperature. De-hydrogenation can be realized at temperatures below 280 C. Energy demand. De-hydrogenation is characterized by a heat demand of 66.3 kJ/molH2 [93]. Material handling. Naphthalene is flammable, possess a health hazard, is irritating to the eyes and skin as well as dangerous to the natural environment. Decalin is not irritating to the eyes and skin; nevertheless, it is toxic and corrosive. The handling is therefore challenging. The low vapor pressure causes only little storage requirements, while the low dynamic viscosity is advantageous for pumping. Process design. Naphthalene is solid at ambient temperatures. Thus a solvent like toluene needs to be added. The consequence is that a downstream hydrogen purification step is necessary to purify the released hydrogen. The low ignition temperature of decalin, however, makes the operation more difficult as the substance might ignite on the hot surface of the de-hydrogenation reactor. Stability. Decalin is stable up to temperatures of 450 C [94]. High temperatures cause catalyst deactivation and carbonaceous deposits [95]. But within the process temperature range, this problem is manageable. Gas flow. A gas flow of 16.1 gH2/(L h) is achievable. Technical readiness. Even though naphthalene has been well-studied and different reactor concepts have been tested, it still faces serious technical problems (e.g., catalyst deactivation). A pilot scale demonstration has not been realized yet. The technology readiness level is assessed to be 4.
Toluene Toluene (TOL) is an aromatic hydrocarbon; it is a colorless and water-insoluble liquid. The hydrogenated form is called methylcyclohexane. The main material parameters of this LOHC system are listed in Table 8. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 8) are as follows.
Table 8 e Toluene/methylcyclohexane system [58]. Toluene Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a
Methylcyclohexane
0.88 95e111 535 0.6
0.77 127e101 260 0.7
7,880a 02, 07, 08
10,900a 02, 07, 08, 09
Calculated with Aspen Properties.
6641
Fig. 8 e Storage cycle for toluene/methylcyclohexane.
Hydrogenation. Full hydrogenation can be catalyzed by platinum (Pt) catalyst supported on zeolite at 120 C and 30 bar [96]. Alternatively, hydrogenation is also possible at 200 C and 20 bar in 2 h with a nickel (Ni) cobalt (Co) molybdenum (Mo) catalyst supported zeolite [97]. De-hydrogenation. De-hydrogenation is usually catalyzed by platinum (Pt) or nickel (Ni) catalysts supported on aluminum oxide (Al2O3). The temperature varies from 350 to 450 C with yields between 50 and 92%; no information about the reaction time is available [98,99]. A Raney-Nicatalyst allowing for a 65% yield after 30 min at 250 C can be used as well; but then isomerization and disproportionation reaction take place [100] greatly reducing the recyclability. A potassium platinum catalyst supported on aluminum oxide (K-Pt/Al2O3) achieved the highest hydrogen yield of 95% at 320 C with selectivity higher than 99,9% [101]. For the toluene/methylcyclohexane system, the following results are found for the characteristic criteria defined above. Storage. The storage capacity of toluene is 6.2 wt.-% and the energy density is 1.6 kWh/L. With the dehydrogenation limit of 95% it is 5.9 wt.-% and 1.5 kWh/L. Availability. The current market price of ca. 0.3 V/kg is favorable for a large scale application [102]. Toxicity. Toluene has a toxicity potential indicator (TPI) of 19,3 TPI/mg and is presumably toxic to reproduction. The toxicity potential indicator (TPI) of methylcyclohexane is 7.3 TPI/mg. De-hydrogenation temperature. De-hydrogenation can be achieved at temperatures between 250 and 450 C. Energy demand. De-hydrogenation is characterized by a heat demand of 68.3 kJ/molH2 [101]. Material handling. Toluene is flammable, possess a health hazard and is irritating to the eyes as well as the skin. Beside this, methylcyclohexane is dangerous to the environment. Handling is therefore relative challenging. The low vapor pressures cause higher storage requirements, while the low dynamic viscosity is advantageous for pumping. Process design. Toluene is liquid at ambient temperatures; i.e., there is no need for a solvent. Nevertheless, a
6642
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
downstream hydrogen purification step is necessary to purify the released hydrogen because toluene is gaseous during de-hydrogenation; thus the gaseous state allows for the use of simple fixed bed reactors. The low ignition temperature of methylcyclohexane, however, makes the process more difficult as the substance might ignite on the hot surface of the de-hydrogenation reactor. Stability. Toluene used as a LOHC results in side reactions (for example disproportionation and de-alkylation). Additionally, the catalyst will be deactivated. But adding an additional metal like rhenium (Re) can increase the stability of the catalyst [103]. A stable process for 6,000 h with the impregnated K-Pt/Al2O3 catalyst have been demonstrated [101]. Gas flow. A gas flow of 61.5 gH2/(L h) is achievable. Technical readiness. Toluene has been well studied so far and was also tested as truck fuel combined with a combustion engine [104]. The process also has been recently applied in demonstration plants [105,106]. Additionally, a demonstration project to import hydrogen to Japan by ship with the toluene/methylcyclohexane system is currently under planning [43]. Its technology readiness level is 8.
Phenazine Phenazine (PHE) is a polycyclic aromatic molecule being a yellow solid at ambient conditions. The hydrogenated form is called tetradecahydrophenazine. The main material parameters of this LOHC system are listed in Table 9. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 9) are as follows. Hydrogenation. Hydrogenation of phenazine controlled by the homogeneous catalyst Pd2Ru@SiCN at 115 C and 50 bar after 1,440 min have been demonstrated [44]. For this reaction dioxane and water must be added. De-hydrogenation. De-hydrogenation is catalyzed by the same catalyst at 190 C. After 1,440 min a full conversion to phenazine (i.e. 100%) has been achieved. The solvents of the hydrogenation must be replaced by diglyme [44].
For the phenazine/tetradecahydrophenazine system, the following results can be summarized for the assessment criteria defined above. Storage. Storage capacity of toluene is 7.2 wt.-% and the energy density is 2.4 kWh/L. Adding diglyme for dehydrogenation reduces the capacity to 2.4 wt.-% and the energy density to 0.8 kWh/L. Availability. The average market price of up to 26 V/kg4 is unfavorable for large scale applications [107]. But a way to synthesize phenazine from lignin has been demonstrated [44]. This could drastically reduce production costs, as lignin is abundantly available and hardly used. Toxicity. Phenazine has a high toxicity potential indicator (TPI) of 38.0 TPI/mg. For tetradecahydrophenazine no data is available for the time being. De-hydrogenation temperature. De-hydrogenation can be achieved at temperatures at around 190 C. Energy demand. De-hydrogenation is characterized by a heat demand of 61.3 kJ/molH2. Material handling. Phenazine only cause irritation to the skin and eyes; no other precautions have to be taken. The predicted low vapor pressure suggests a simple storage, while the low dynamic viscosities are advantageous for pumping. Process design. Phenazine is solid at ambient temperatures; i.e., a solvent need to be added. Therefore, a downstream hydrogen purification step is necessary to purify the released hydrogen as well as an intermediate step to separate the solvent from the hydrogen after dehydrogenation. The low flash point of phenazine, however, makes the operation more difficult as the substance might ignite on the hot surface of the de-hydrogenation reactor. Stability. Seven cycles with hydrogenation conversion efficiencies varying between 85 and 96% and dehydrogenation conversion efficiencies between 77 and 100% have been demonstrated [44]. Despite these variations, a final capacity above 7 wt.-% after the seven cycles has been achieved [44]. Gas flow. A gas flow of 0.9 gH2/(L h) is achievable. Technical readiness. Phenazine can be synthesized from lignin; therefore, it is basically available. However, the different solvents for hydrogenation and de-hydrogenation complicate the technical use considerably. Its technology readiness level is assessed to be 3.
Table 9 e Phenazine/tetradecahydrophenazine system [64]. Phenazine Density [kg/L] Liquid temperature range [ C] Flash point [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a b
Tetradecahydrophenazine
1.30 H 0.1a 172e360
1.00 H 0.1a No data
160.3 H 11.7a 3.1a
No data 2.2a
0.01b
0.18b
07
No data
Prediction by ACD/Labs. Calculated with Aspen Properties.
Assessment and comparison In the following section, the LOHCs examined are evaluated according to a rating scale and compared with regard to their applicability in the fields of mobility, energy-transport and energy-storage. A general overview of the important properities of the LOHC systems examined are given in Table 10.
4
Exchange rate 0.86 V ¼ 1.0 US$ [75].
6643
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Table 10 e Properties of the evaluated LOHCs and diesel and gasoline as reference (NEC: N-ethylcarbazole, AB: 1,2-dihydro1,2-azaborine, FA: formic acid, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: Phenazine, MO: mineral oils, D: diesel, G: gasoline, n.d.: no data).
Storage capacity [wt.%] Enthalpy of reaction [kJ/molH2] De-hydrogenation temperature [ C] Gas flow [gH2/L/h] Energy density [kWh/L] Viscosity at 20 C [mPa s] Vapor pressure at 40 C [Pa] Toxicity [TPI/mg] Price [V/kg] GHS
NEC
DBT
AB
FA
MET
NAP
TOL
PHE
MO
H0/H12
H0/H18
H0/H6
H0/H2
H0 H6
H0/H10
H0/H6
H0/H14
D/G
5.8 53.2
6.2 65.4
7.1 35.9
4.4 31.2
12.1 16.5
7.3 66.3
6.2 68.3
7.2 61.3
180e270
270e310
80
50e100
90e420
210e300
250e450
190
68.0e163.1 2.5 0.5/5.9 0.1/4.4
11.0e27.5 1.9 44/258 0.07/0.04
66.2e132.5 2.4 0.5/0.6 18,300
0.7 1.8 -/1.8 -/1,300
0.8e44.8 3.3 -/0.6 -/35,400
16.1 2.2 2.0/3.0 114/481
61.5 1.6 0.6/0.7 7,880/10,900
0.9 2.4 9.8/8.6 3.1/2.2 1.9/5.2 0.01/0.18 40/62,300
5.1/n.d. 40 09
13.8/n.d. 4 08
n.d. n.d. n.d.
-/9.0 -/29.7 45.8/1.2 19.3/7.3 0.03 0.03 0.6 0.3 02,04,05,06 02,04,06,08 02,05,06,07,08,09 02,07,08.09
38.0/n.d. 19.1/96.4 <25 1.3/1.7 07 02,07.08,09
Within the relative valuation, the rating depends on the bandwidth defined by the various LOHC. The evaluated parameters can be summarized as follows.
Fig. 9 e Phenazine/tetradecahydrophenazine system.
Assessment The assessment of the different LOHCs discussed above is based on the parameters examined for each LOHC. Each of these parameter is now rated on a scale of 1 (worst) to 10 (optimal). A distinction is made between (a) a relative rating of the parameters among the various LOHCs assessed here and (b) an absolute rating on a fixed scale (Table 11).
Storage. The storage targets of the US-DoE do not represent physical limits; this development goal is more defined by aspects related to practical implementation. Therefore, values above the target are possible. A rating is therefore realized on a relative scale. The highest values among the examined LOHCs are 12.1 wt.-% and 3.3 kWh/L (both rating 10) and the lowest 0.3 wt.-% and 0.1 kWh/L (both rating 1). Availability. Low prices and unlimited occurrence of the raw material is essential for large scale application. CO2, which is abundantly available and potentially cheap (0.03 V/kg), is the target for a rating of 10, while N-ethylcarbazole is rated with 1 because of the limited availability and the high price (40 V/kg). De-hydrogenation temperature. The lower the temperature of de-hydrogenation, the easier the heat management and the less energy is needed. The lowest temperatures found is 50 C (rating 10) and the highest is 450 C (rating 1). Energy demand. To release the stored hydrogen from loaded LOHC during de-hydrogenation, heat is necessary. This effectively reduces the storage efficiency. The lower
Table 11 e Values of the rating scale.
Relative rating Storage
Storage capacity [wt.-%] Energy density [kWh/L]
Availability De-hydrogenation temperature [ C] Energy demand [kJ/molH2] Material handling Process design Stability Gas flow [gH2/(L h)] Absolute rating Toxicity Technical readiness
Value 1
Value 10
0.3 0.1 40 V/t 450 16.5 Six precautions Intermediate/downstream purification; explosion prevention Frequently exchanged system 0.7
12.1 3.3 0.03 V/t 50 68.3 One precaution Simple 2-step system without additional explosion prevention Robust system 163.1
TPI 100 TRL 1
TPI 0 TRL 9
6644
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
this heat demand, the higher the storage efficiency. A spectrum from 16.5 kJ/molH2 (rating 10) to 68.1 kJ/molH2 (rating 1) has been identified. Material handling. Ideally no precaution must be taken dealing with the LOHC. For the examined LOHCs, a variety between only one (rating 10) to six (rating 1) different precautions per LOHC system have been detected. Process design. Additional process steps (e.g., hydrogen purification) or special reactor designs for explosion prevention lead to complex overall process designs. On the one hand there are two-step LOHC systems, consisting of hydrogenation and de-hydrogenation (rating 10), and on the other hand multi-step LOHC systems including hydrogenation, de-hydrogenation and purification with additional precautions regarding explosion prevention (rating 1). Stability. Ideally the stability of the LOHC system can be quantified with possible operating times. Unfortunately, such information is missing in most cases. Robust LOHC systems (rating 10) show only marginal losses of the LOHC over time (<0.02%) and a long-term catalytic activity (>8,000 h), while frequently exchanged systems (rating 1) are characterized by high losses and the catalyst can only convert limited amounts of starting material (low turnover numbers). Gas flow. The gas flow as an indicator for fast or slow hydrogen release varies for the examined LOHC between 163.1 gH2/(L h) (rating 10) and 0.7 gH2/(L h) (rating 1).
an overview of each rating and the total grade from 1 to 10 are presented in Table 12. A distinction is made between the storage density and the energy density of the pure substance and the diluted substance. For methanol, results are given for the high-temperature and low-temperature reforming.
Storage The theoretical volumetric and gravimetric storage density of the various LOHC investigated in chapter 2 differs significantly from the real densities reduced by solvents and the physical or self-specified de-hydrogenation limitation (Fig. 10). The methanol/water mixture after hydrogenation of carbon dioxide has the highest volumetric energy density of 3.3 kWh/L among the examined LOHC (rating 10). For the dehydrogenation/reforming, water has to be added. This reduces the density; for high-temperature reforming a rating of 8 and for low-temperature de-hydrogenation of 3 has been awarded. N-ethylcarbazole, dibenzyltoluene and toluene do not need a solvent. Still their volumetric density is reduced due to de-hydrogenation limitations, but to a much lesser extent. Completely de-hydrogenated N-ethylcarbazole has been granted by a rating of 8, with the de-hydrogenation restrictions the rating is only 7. For dibenzyltoluene the rating with and without limitation is 6 and for toluene 5. The use 1,2-dihydro-1,2-azaborine, formic acid, naphthalene and phenazine requires the addition of solvents. Thus, the ratings regarding energy density are reduced significantly compared to the pure substances. De-hydrogenation limitations further affect the rating. Pure and completely dehydrogenated 1,2-dihydro-1,2-azaborine has been given a rating of 7 and in dilution of 3. For formic acid the rating decreases from 6 to 1, for naphthalene from 7 to 4 and for phenazine from 7 to 3. In addition to reducing the volumetric energy density, dilutions and de-hydrogenation limitations also decrease the gravimetric storage capacity. The highest value again was found for the methanol/water mixture with 12.1 wt.-% (rating 10). With the addition of water, the rating for hightemperature reformed methanol is 8 and for lowtemperature de-hydrogenated methanol it is 4. As N-ethylcarbazole, dibenzyltoluene and toluene only show dehydrogenation limitations their capacities are slightly reduced. The rating for N-ethylcarbazole remains at 5, for dibenzytoluene and toluene it is reduced from 6 to 5. Because
Beside this relative rating an absolute valuation has been realized when it is possible. Parameters evaluated absolutely are summarized below. Toxicity. The toxicity is rated based on the toxicity potential indicator (TPI) scale. Substances with a TPI of 0 are potentially non-hazardous (rating 10), while substances with a TPI of 100 are extremely toxic (rating 1). Technical readiness. The readiness of each LOHC system is rated according to the technical readiness level (TRL). TRL 9 (rating 10) requires a successful operating system in the predetermined environment, while TRL 1 (rating 1) describes the observation of the basic principle. Below the various LOHC presented in chapter 2 are assessed related to the criteria discussed above. Additionally,
Table 12 e Rating of each LOHC (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
Storage capacity Energy density Availability Toxicity De-hydrogenation temperature Energy demand Material handling Process design Stability Gas flow Technical readiness
NEC
DBT
AB
FA
MET
NAP
TOL
PHE
5 7/8 1 9 5 4 10 9 8 10 3
5/6 6 9 8 4 2 10 10 10 2 10
3/6 3/7 1 e 9 7 e 1 5 8 2
1/4 1/6 10 9 10 7 5 5 6 1 3
4/8 4/8 10 7 9/2 10 5 5 6/8 1/3 3/10
4/6 4/7 10 5 6 1 1 3 7 3 3
5/6 5 10 8 3 1 5 3 8 1 9
2/6 3/7 4 6 7 2 10 1 7 1 3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Fig. 10 e Influence of solvents and de-hydrogenation limitations on volumetric and gravimetric density (NEC: N-ethylcarbazole, AB: 1,2-dihydro-1,2-azaborine, FA: formic acid, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: Phenazine).
of the need to use solvents, 1,2-dihydro-1,2-azaborine, formic acid, naphthalene and phenazine show significantly reduced gravimetric storage capacities. Without dilution and under the assumption of a complete de-hydrogenation 1,2-dihydro-1,2azaborine has a rating of 6 and in dilution 3. With the necessarily to be added solvents and the respective dehydrogenation limitation the rating is reduced for formic acid from 4 to 1, for naphthalene from 6 to 4 and for phenazine from 6 to 3.
Availability In the category availability methanol, formic acid, naphthalene and toluene accomplish the best results (rating 10). The raw material price of dibenzyltoluene (rating 9) is approximately 10 times higher than that of naphthalene and toluene, but still significantly cheaper compared to phenazine and Nethylcarbazole. Phenazine is more expensive (rating 4); the possibility to be synthesized in the future from lignin counts as a plus. N-ethylcarbazole shows high prices and is only produced to a limited extend so far (rating 1). Prices for 1,2dihydro-1,2-azaborine are not available; however, the low disposability of boron results in a poor rating (1).
Toxicity None of the examined LOHCs has a toxicity potential indicator (TPI) of “0” (substance with no known hazard). However, N-ethylcarbazole and formic acid show values close to it (rating 9). Naphthalene is the most toxic substance among the investigated LOHCs (rating 5); nevertheless, it has a lower toxicity potential indicator compared to gasoline fuel. Next in line are phenazine (rating 6) and methanol (rating 7), which are less toxic than naphthalene, but still more toxic than diesel fuel. Dibenzytoluene and toluene (both rating 8) show a similar toxicity to diesel fuel. 1,2-dihydro-1,2azaborine cannot be evaluated due to a lack of information (no rating).
De-hydrogenation temperature Formic acid shows the lowest de-hydrogenation temperature of 50 C (rating 10). The needed temperature to de-
6645
hydrogenate the 1,2-dihydro-1,2-azaborine/1,2-BN-cyclohexane system is only slightly above (rating 9). Methanol can also be de-hydrogenated at low temperatures (rating 9), but at the cost of an increased dilution and extended reaction times. To achieve better kinetics, the de-hydrogenation temperature has to increased significantly (rating 2). Phenazine is rated 7 with regard to the de-hydrogenation temperature. For fast de-hydrogenation of the N-ethylcarbazole/ perhydro-N-ethylcarbazole system the temperature is increased to 270 C (rating 5). Temperatures as low as 180 C are possible for this system (rating 7), resulting in a slower reaction kinetic. The other LOHCs have comparatively high temperatures: dibenzytoluene (rating 4), naphthalene (rating 5) and toluene (rating 3).
Energy demand The release of hydrogen from the loaded LOHC needs thermal energy. Methanol has the lowest heat demand (rating 10), followed by formic acid and 1,2-dihydro-1,2-azaborine (both rating 7). Especially the de-hydrogenation of the toluene/ methylcyclohexane and the naphthalene/decalin system (both rating 1) is very energy demanding, closely followed by dibenzyltoluene and phenazine (both rating 2). N-ethylcarbazole is somewhere in between (rating 4).
Material handling The handling of the LOHC material is significantly influenced by the precaution to be taken. Dibenzytoluene, N-ethylcarbazole and phenazine have the fewest requirements (all rating 10), keeping in mind that specific data of their hydrogenated forms is still not available. Naphthalene is the most difficult LOHC to be handled; it requires even more measures of safety than gasoline and diesel fuel (rating 1). Toluene shows the same safety measures that must be taken when dealing with diesel and gasoline fuel (rating 5). For formic acid and methanol three precautions must be taken; additionally, gaseous CO2 needs to be handled (both rating 5). In case of 1,2-dihydro-1,2-azaborine no reliable evaluation is possible (no rating).
Process design The simplest process design consists of a hydrogenation and de-hydrogenation reactor with a condensation at ambient conditions to separate hydrogen from the de-hydrogenated LOHC. For dibenzyltoluene such a process can be realized easily (rating 10). De-hydrogenation restrictions, which can be achieved by adjusting the reaction temperature and time, slightly complicates the process for N-ethylcarbazole (rating 9); still an additional hydrogen purification is not needed. Although no solvent is added together with toluene, a downstream hydrogen purification is necessary since toluene and methylcyclohexane are low-boiling. Either low condensation temperatures or further purification methods like pressure swing adsorption or membrane separation must be applied. The low ignition temperature of methylcyclohexane further decreases the rating (rating 3). When using naphthalene (rating 3), 1,2-diyhdro-1,2-azaborine (rating 1) and phenazine (rating 1), solvents are added, which contaminate the released hydrogen stream. Again, hydrogen purification methods are inevitable. All three substances also show complications
6646
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
regarding the ignition temperature and the flash point. 1,2diyhdro-1,2-azaborine further demands a multi-step hydrogenation process and for phenazine an intermediate solvent change is necessary. Both additional demands complicate the process design. For methanol and formic acid (both rating 5), the separation of hydrogen from CO2, water and eventually CO-impurities is necessary. As only gaseous products emerge from de-hydrogenation, no by-products are accumulated, if CO2 is not reused. For a closed storage cycle, an additional purification step has to be integrated to purify the CO2 for renewed hydrogenation.
Stability Dibenzyltoluene shows a high thermal stability and a longterm catalytic activity (rating 10). Although challenges regarding the stability of toluene remain, still an operation time of the de-hydrogenation of 6,000 h has been realized (rating 8). N-ethylcarbazole lacks thermal stability for high temperatures limiting faster reactions kinetics, but its catalyst stability is high (rating 8). The stability of the storage process regarding the phenazine/tetradecahydrophenazine system is generally good; nevertheless, still minor variations in the uptake and release of hydrogen occur (rating 7). Low-temperature de-hydrogenated methanol and formic acid show both good thermal stability in the respective temperature range. The catalytic stability remains challenging in their implementation (both rating 6). Compared to that, high temperature steam reforming of methanol does not show this drawback (rating 8). The naphthalene/decalin cycle is thermally stable; but catalyst deactivation can be problematic at higher temperatures (rating 7). 1,2-dihydro-1,2-azaborine is thermal stable in the considered temperature spectrum and the de-hydrogenation proceeds clean without side-reactions, but the stability in a repeated storage cycle still has to be verified (rating 5).
Gas flow The addition of solvents and de-hydrogenation limitations reduce the gas flow. As the de-hydrogenation cannot be achieved with the pure LOHC, the parameter is only evaluated under the respective circumstances. N-ethylcarbazole (rating 10) and 1,2-dihydro-1,2-azaborine (rating 8) show the highest gas flows under ideal reaction conditions. The short reaction times below 30 min are hereby of great advantage. Toluene has a lower gas flow (rating 4). The reaction proceeds fast, but the comparatively lower density of its hydrogenated form leads to a less promising result. Dibenzyltoluene shows a longer reaction time; its gas flow is therefore reduced (rating 2). The de-hydrogenation in the naphthalene/decalin cycle take a similar amount of time. However, since a solvent must be added, gas flow is reduced (rating 2). With hightemperature steam reforming (rating 3) of methanol the gas flow is significantly higher than with low-temperature reforming (rating 1), because the dilution and the reaction time can be significantly reduced. Formic acid and phenazine exhibit very low gas flows (rating 1), which is mainly due to a high dilution rate and a long reaction time, respectively.
Technical readiness N-ethylcarbazole is well researched; nevertheless, further testing especially on toxicity and the application in the
operating environment are still needed. The technology readiness level (TRL) is 3. For dibenzyltoluene the TRL is 9 as demonstration projects have been already implemented and systems at different sizes are available. The TRL of 1,2-dihydro1,2-azaborine is 2 because challenges about a pressure and temperature induced hydrogenation as well as dehydrogenation without trimerization remain. High dilution rates is a big challenge for the implementation of the CO2/ formic acid system based on homogenous catalyst catalyzing both reactions; additionally, the process has been demonstrated on lab-scale only (TRL 3). CO2 hydrogenation to methanol is used on an industrial scale. High-temperature steam reforming is also a well-known industrial process. This combination is therefore technically ready (TRL 9). The lowtemperature reforming, however, is still on lab-scale (TRL 3). Naphthalene has a TRL of 4, since demonstration projects have not yet been realized. For toluene demonstration projects have been realized and others are on the way (TRL 8). The hydrogen storage process using phenazine as the LOHC has been demonstrated on the lab-scale, while a further evaluation is still pending (TRL 3).
Comparison The various assessment parameters (Table 11) are of different importance for the different application areas (Table 13). For mobile applications storage capacity and energy density are of great significance as they directly affect the size as well as the weight of an on-board storage system (i.e., heavy and large scale tanks decrease the possible driving range). Additionally, a low cold start time and a good dynamic behavior are also important for mobile systems. Thus, typically a Polymer Electrolyte Membrane (PEM) fuel cell is used for such applications. Ideally, the thermal energy demand of the de-hydrogenation is covered by waste heat from this PEM fuel cell. Therefore, de-hydrogenation temperature is another important parameter for mobile applications. The gas flow is an indicator for the time span between start-up and hydrogen delivery and thus crucial for the dynamic behavior of mobile systems. But also the process design is critical, as every additional requirement (e.g. hydrogen purification) add to the limited space and weight demand (Table 13). For an energy transport, the availability is a critical parameter as it strongly influences the economic feasibility; and most likely even more than the storage capacity or the energy density. Low safety and other legal requirements for handling and storage of the LOHC material as well as a low toxicity are additional important criteria for a safe and simple transport (Table 13). For (stationary) energy storage applications, the availability of the LOHC is critical. Additionally, the costs directly affect the economic feasibility of the storage system. A large scale application also entails a high mass demand of the LOHC. In addition to that, the stability of the respective LOHC material is of great importance. A high cycle stability reduces the need for LOHC replacement, which lowers the operating costs. For a high storage efficiency, a low energy demand is essential (Table 13).
6647
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Table 13 e Weighting of the individual parameters with regard to the fields of application.
Storage Availability Toxicity De-hydrogenation temperature Energy demand Material handling Process design Cycle stability Gas flow
Mobility
Energy-transport
(Stationary) energy-storage
Very important Second-rate Second-rate Very important Important Important Very important Second-rate Very important
Important Very important Very important Second-rate Second-rate Very important Second-rate Important Second-rate
Second-rate Very important Important Second-rate Very important Second-rate Important Very important Second-rate
Based on the results of the evaluation (Table 12), the various LOHCs can now be assigned to the areas of application.
Mobility Fig. 11 shows the ratings of the most relevant parameters for the application area “mobility”. The requirements of mobile systems are challenging for LOHCs. A fast de-hydrogenation being the basis for a high gas flow typically demands high temperatures. Such a temperature level, in turn, is difficult to realize based on the waste heat of a PEM fuel cell (max. 180 C). N-ethylcarbazole shows a high rating in terms of process design and gas flow. But related to the storage characteristics, especially for the gravimetric storage capacity, N-ethylcarbazole does not perform so well. However, it almost reaches the onboard DoE storage targets of 6.4 wt.-% and 1.7 kWh/L. The high de-hydrogenation temperature is the real bottleneck here; this can be reduced, but with the consequence of a lower gas flow. Toluene shows comparable results to N-ethylcarbazole, with slightly less promising results in terms of energy density and gas flow. Again, it is the de-hydrogenation temperature making a heat integration with a PEM fuel cell impossible. In addition, the necessary hydrogen purification step complicates the overall process design.
Due to the integrated nitrogen and boron atoms within the storage molecule, 1,2-dihydro-1,2-azaborine shows good characteristics for mobile applications. The low dehydrogenation temperature, which allows for a coupling of the de-hydrogenation with that waste heat of a PEM fuel cell, combined with a high gas flow, is advantageous. However, the required solvent leads to a comparatively bad rating for storage; the reduced storage densities are also well below the DoEtargets. Further, the solvent also complicates the process design for the de-hydrogenation unit due as hydrogen purification step needs to be added. Formic acid shows even in pure form comparatively low storage densities, which make a use in the field of mobility difficult, even though it has the lowest de-hydrogenation temperature. Like formic acid, methanol also meets the requirements in terms of de-hydrogenation temperatures to be coupled with the waste heat of a PEM fuel cell. Prolonged reaction times and high dilution rates prevent the use as they lead to poor rating in terms of storage design and gas flow. Higher activities of the catalyst are the key to overcome this challenges. The rating regarding storage and gas flow can significantly be increased, if the de-hydrogenation temperature is raised. In this case methanol shows the highest rating among the examined LOHC in terms of storage; only the additionally added water leads to the fact that the optimal
Fig. 11 e Rating of the different LOHC in the most relevant parameters of mobility (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET-HT: high-temperature reformed methanol, MET-LT: low-temperature reformed methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
6648
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
value is not reached. Due to the increased temperature, the de-hydrogenation can then no longer be combined with the waste heat of the PEM fuel cell. Besides that, methanol and formic acid can potentially also be used with a Direct Liquid (DL) fuel cell, which would greatly enhance the integration in the mobility sector [108]. With a such a fuel cell the dehydrogenation temperature would decrease as protons are liberated from the LOHC instead of hydrogen. Also the process design, which is more complex due to a downstream hydrogen purifications step, could be simplified if the dehydrogenation unit and the fuel cell are combined in one device [109]. Dibenzyltoluene and naphthalene are not applicable in the area of mobility as their reaction kinetics are limited.
Energy-transport Fig. 12 shows the results of the assessment step discussed above for the application field energy transport. As the availability is a crucial parameter for the use of the LOHC systems in the field of energy transport, N-ethylcarbazole is inapplicable to date. Mass production of N-ethylcarbazole could push the price down in the future, whereby the availability for this system is not an exclusionary criterion. Additionally, N-ethylcarbazole is non-toxic and easily manageable. Dibenzytoluene has a good availability, is non-toxic and can be handled easily. The technical and commercial use of dibenzytoluene in such systems is already state of technology and has been demonstrated in pilot projects. One the one hand formic acid shows a very cheap raw material price and low toxicity; on the other hand, the handling is comparatively difficult, although less precautions
need to be taken to deal with it compared to diesel fuel and gasoline. The very low storage density of formic acid has to be taken into account here, since at this extent it greatly influences the economy of the transport. Compared to formic acid, toluene is slightly worse with regard to toxicity and a bit more expensive; methanol shows the same availability but an increased toxicity potential. Both substances show significantly higher storage densities in comparison to formic acid. Phenazine and naphthalene are out due to their high toxicity potential. 1,2-dihydro-1,2-azaborine cannot be evaluated for the time being due to a lack of data.
Energy-storage Fig. 13 presents the assessment results of the most relevant parameters for the application field (stationary) energy storage. Dibenzyltoluene and toluene are qualified for this use; only their high energy demand is challenging. Coupling dehydrogenation with a high-temperature fuel cell or other available excess heat sources means that the energy requirement does not diminish the storage efficiency. Both LOHC would then be an attractive storage option. Methanol presents a valid option, if the energy demand must be covered internally through hydrogen burning, since it has the lowest demand. Formic acid and 1,2-dihydro-1,2-azaborine also have comparatively low energy demands, but the stability needs to be increased. Phenazine can potentially be produced from lignin, which can significantly reduce the manufacturing costs and by that reduce the hurdles for its entry. Due to its high energy
Fig. 12 e Rating of the different LOHC in the most relevant parameters of energy transport (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET-HT: high-temperature reformed methanol, MET-LT: low-temperature reformed methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
Fig. 13 e Rating of the different LOHC in the most relevant parameters of energy storage (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET-HT: high-temperature reformed methanol, MET-LT: low-temperature reformed methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
demand, Phenazine need to be coupled with external excessive heat sources to increase the efficiency.
Comparison to other storage options To evaluate LOHCs in the broader context of H2-storages, they are compared to compressed hydrogen gas (CGH2), liquefied hydrogen (LH2) and metal hydrides regarding their storage capacity, energy density and energy demand. Compressed hydrogen gas (CGH2) needs 4.4 kWh/kgH2 if compressed to 875 bar. The liquefaction process of liquefied hydrogen (LH2) requires 15.2 kWh/kgH2 (i.e., 46% of the LHVH2 [5]). The storage capacity and energy density of compressed hydrogen gas (CGH2) (at 700 bar) and liquefied hydrogen (LH2) including the tank is 6 wt.-% and 0.8 kWh/L and 14.4 wt.-% and 1.1 kWh/L, respectively [8]. Low-temperature metal hydrides have a practical storage capacity of around 0.9 wt.-% and an energy density of 3,8 kWh/L, for high-temperatures metal hydrides it is 2.9 wt.-% and 3.6 kWh/L, respectively. The storage and releasing process at low temperatures has an energy demand of 2 kWh/kgH2, while the process at high temperatures demands 4.9 kWh/kgH2 [8]. Compared to other energy storage systems, commercially available Lithium-Ion-Batteries currently have a volumetric energy density of 0.65 kWh/L and a gravimetric storage density of 0.25 kWh/kg [110]. The volumetric energy density and the gravimetric storage capacity of the different storage options in comparison with the pure LOHCs is shown in Fig. 14. For the LOHCs the tank system is not included. The fossil fuels diesel and gasoline are presented additionally. LOHCs can provide a good compromise between gravimetric and volumetric energy density compared to the other storage options; methanol in particular shows very high values here. The values of mineral oils are still higher than those of the possible storage systems. LOHCs are characterized by a good gravimetric storage capacity while the volumetric storage capacity is sufficient. They are in between metal hydrides and liquefied hydrogen showing a low gravimetric storage capacity and a low volumetric storage density, respectively. Methanol exceeds the other LOHC in this context. Compressed hydrogen gas (CGH2)
6649
has both a low volumetric storage density and a low gravimetric storage capacity. The energy demand of LOHC is compared to metal hydrides and compressed hydrogen gas (CGH2) for most substances higher (Fig. 15). For the LOHC only the energy demand for the de-hydrogenation is taken into account; further consumptions for hydrogen purification are estimated to 2 to 4 kWh/kgH2 and marked additionally in Fig. 15. Additionally, the amount of heat consumed for de-hydrogenation is released during hydrogenation, yet at a lower temperature level. But, the heat produced during der hydrogenation process can hardly be used; therefore, it is not included here. Methanol, formic acid and 1,2-dihydro-1,2-azaborine have small enthalpies of reactions. Thus their energy demands are in the same range as metal hydrides and compressed hydrogen gas (CGH2). Liquefied hydrogen (LH2) has by far the highest energy demand due to the energy-intensive liquefaction process.
Final considerations The overall goal of this paper is to present an extensive assessment of different LOHC in terms of important characteristic aspects. Therefore, eight different LOHC have been identified and evaluated for a potential implementation in the areas of mobility, energy-transport and energy-storage. Dibenzytoluene has yielded promising results for large scale implementation in energy-transport. Energyestorage is another conceivable area of application, if the dehydrogenation heat demand can be covered by external (excessive) heat sources. N-ethylcarbazole shows a considerable potential in both fields as well, even though its high raw material price hinders an economically feasible implementation; but mass production might significantly reduce this price in the future. Even though other LOHC have better storage values, the onboard DoE storage targets are almost met and N-ethylcarbazole has the highest gas flow among the
Fig. 14 e Comparison of volumetric and gravimetric storage density of different hydrogen storage options (CGH2: compressed hydrogen gas, LH2: liquefied hydrogen, LMH: low-temperature metal hydride, HMH: high-temperature metal hydride, (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
6650
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
activities. The high raw material price is disadvantageous, but the possible production path with lignin as the feedstock can take up this challenge. The toxicity might also be problematic as the toxicity potential indicator is higher than for diesel fuel.
Fig. 15 e Energy demand for hydrogen storage and its percentage of the lower heating value (LHV) of hydrogen (CGH2: compressed hydrogen gas, LH2: liquefied hydrogen, LMH: low-temperature metal hydride, HMH: hightemperature metal hydride, (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
substances examined here. In terms of mobile applications, it is the high de-hydrogenation temperature, which makes an implementation difficult. Before 1,2-dihydro-1,2-azaborine could be applied in any field a lot of research is still needed. Nevertheless, this substance shows potential for mobile applications. However, the need of a solvent is unattractive. To maintain the advantage of the low energy demand and low dehydrogenation temperature of BN-heterocycles, 3-Methyl1,2-BN-cyclopentan has been developed, which is liquid in the intended temperature range. Therefore, it is a promising candidate. Nevertheless, its de-hydrogenation and hydrogenation remains challenging; that is why it was not included in this study. Reducing the dilution rates for formic acid, and lowtemperature reformed methanol by researching possible highly active and stable catalysts, would drastically improve their potential in all three fields of application. If the dilution can be reduced, the high storage densities combined with the low de-hydrogenation temperature and energy demand makes methanol a particularly attractive LOHC. The release of gaseous CO2 is both an advantage and a disadvantage. If CO2 is released into the atmosphere, there is no accumulation of the unloaded LOHC. However, new starting material for hydrogenation must then be provided e.g. by a separation from ambient air. Naphthalene shows bottlenecks in numerous criteria. Furthermore, the substance is probably carcinogenic. It is doubtful if a fuel, which toxicity is more questionable than diesel will be permitted in the future by legislation. Toluene is applicable in energy-transport, although its material handling is compared to dibenzyltoluene and Nethylcarbazole more challenging. It is also an alternative for energy-storage, if the de-hydrogenation heat is covered by external (excess) heat sources. Phenazine shows good qualities for the use in energystorage, yet the requirement of different solvents for hydrogenation and de-hydrogenation are impractical for everyday application and need to be addressed by R&D
Still challenges like decreasing dilution rates, improving the stability and reducing raw material prices remain. Overcoming these obstacles can make LOHC an important player in a more sustainable and highly integrated energy system based on renewable sources of energy. Admittedly, there is still a long way to go in the field of mobility to realize lowtemperature de-hydrogenation combined with high storage densities and fast reaction kinetics. This is a big challenge, as fast kinetics usually requires high de-hydrogenation temperatures, which do not allow a coupling of the de-hydrogenation with the waste heat of a PEM fuel cell. Energy-storage and especially energy-transport are fields, where business models based on LOHC are more likely to be implemented in the near future. The use of dibenzyltoluene, high-temperature reformed methanol and toluene in these areas is already in preparation and shows the greatest potential for the time being. Although methanol, with its high storage density, has potential uses in all three application areas, it is more likely to use specific LOHCs for each application for which their properties fit best. The greatest advantage of LOHCs, which applies to all examined substances, compared to other storage options is that they have similar properties compared to crude oil based substances. The public acceptance of such fuels is far more likely than, for example, gaseous hydrogen, which is regarded as very dangerous in public opinion. Furthermore, LOHC can be used with the existing infrastructure for crude oil derivate with only little modifications, which makes their use in the field of energy-transportation appealing. In conclusion, it can be stated that LOHCs have the potential to replace fossil fuels as energy carriers and to be a critical piece of the puzzle for global energy exchange. Thus a more detailed assessment of important energetic and economic parameters of LOHC process chains could provide more deep insights into the consequences of a possible implementation of such energy transport and storage system.
Acknowledgments We would like to thank the National Organisation Hydrogen and Fuel Cell Technology for supporting this work.
references
[1] REN21. Renewables 2017. Global status report. REN21. QC, al; 2017. CA: Montre [2] BMWi. Gesetz fu¨r den ausbau erneuerbarer energien. EEG; 2017. [3] Ahmed A, Al-Amin AQ, Ambrose AF, Saidur R. Hydrogen fuel and transport system. A sustainable and environmental
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
future. Int J Hydrog Energy 2016;41(3):1369e80. https:// doi.org/10.1016/j.ijhydene.2015.11.084. Albrecht J. Tradable CO2 permits for cars and trucks. J Clean Prod 2001;9(2):179e89. https://doi.org/10.1016/S09596526(00)00069-X. Adametz P, Mu¨ller K, Arlt W. Energetic evaluation of hydrogen storage in metal hydrides. Energetic evaluation of hydrogen storage in metal hydrides. Int J Energy Res 2016;40(13):1820e31. https://doi.org/10.1002/er.3563. Jepsen J. Technical and economic evaluation of hydrogen storage systems based on light metal hydrides. 2014. p. 1e150. https://www.hzg.de/imperia/md/content/hzg/ zentrale_einrichtungen/bibliothek/berichte/hzg_reports_ 2014/hzg_report_2014_2.pdf. Zu¨ttel A. Materials for hydrogen storage. Mater Today 2003;6(9):24e33. https://doi.org/10.1016/S1369-7021(03) 00922-2. Di Profio P, Arca S, Rossi F, Filipponi M. Comparison of hydrogen hydrates with existing hydrogen storage technologies. Energetic and economic evaluations. Int J Hydrog Energy 2009;34(22):9173e80. https://doi.org/10.1016/ j.ijhydene.2009.09.056. Motyka T. Metal hydrides. DOE Materials-Based Hydrogen Storage Summit. 2015. https://www.energy.gov/sites/prod/ files/2015/02/f19/fcto_h2_storage_summit_motyka.pdf. Preuster P, Papp C, Wasserscheid P. Liquid Organic Hydrogen Carriers (LOHCs). Toward a hydrogen-free hydrogen economy. ACC Chem Res 2017;50(1):74e85. https://doi.org/10.1021/acs.accounts.6b00474. Sotoodeh F, Smith KJ. Structure sensitivity of dodecahydro-N-ethylcarbazole dehydrogenation over Pd catalysts. J Catal 2011;279(1):36e47. https://doi.org/ 10.1016/j.jcat.2010.12.022. Sotoodeh F, Smith KJ. Kinetics of hydrogen uptake and release from heteroaromatic compounds for hydrogen storage. Ind Eng Chem Res 2009;49(3):1018e26. https:// doi.org/10.1021/ie9007002. Stark K, Emel'Yanenko VN, Zhabina AA, Varfolomeev MA, Verevkin SP, Mu¨ller K, Arlt W. Liquid Organic Hydrogen Carriers. Thermophysical and thermochemical studies of carbazole partly and fully hydrogenated derivatives. Ind Eng Chem Res 2015;54(32):7953e66. https://doi.org/10.1021/ acs.iecr.5b01841. Mu¨ller K, Stark K, Emel'Yanenko VN, Varfolomeev MA, Zaitsau DH, Shoifet E, Schick C, Verevkin SP, Arlt W. Liquid Organic Hydrogen Carriers. Thermophysical and thermochemical studies of benzyl- and dibenzyl-toluene derivatives. Ind Eng Chem Res 2015;54(32):7967e76. https:// doi.org/10.1021/acs.iecr.5b01840. Biniwale R, Rayalu S, Devotta S, Ichikawa M. Chemical hydrides. A solution to high capacity hydrogen storage and supply. Int J Hydrog Energy 2008;33(1):360e5. https:// doi.org/10.1016/j.ijhydene.2007.07.028. Sotoodeh F, Huber BJM, Smith KJ. Dehydrogenation kinetics and catalysis of organic heteroaromatics for hydrogen storage. Int J Hydrog Energy 2012;37(3):2715e22. https:// doi.org/10.1016/j.ijhydene.2011.03.055. Mu¨ller K, Aslam R, Fischer A, Stark K, Wasserscheid P, Arlt W. Experimental assessment of the degree of hydrogen loading for the dibenzyl toluene based LOHC system. Int J Hydrog Energy 2016;41(47):22097e103. https://doi.org/ 10.1016/j.ijhydene.2016.09.196. Zhu Q-L, Xu Q. Liquid organic and inorganic chemical hydrides for high-capacity hydrogen storage. Energy Environ Sci 2015;8(2):478e512. https://doi.org/10.1039/ C4EE03690E. Mellmann D, Sponholz P, Junge H, Beller M. Formic acid as a hydrogen storage material e development of homogeneous
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
6651
catalysts for selective hydrogen release. Chem Soc Rev 2016;45(14):3954e88. https://doi.org/10.1039/C5CS00618J. He T, Pei Q, Chen P. Liquid Organic Hydrogen Carriers. J Energy Chem 2015;24(5):587e94. https://doi.org/10.1016/ j.jechem.2015.08.007. He T, Pachfule P, Wu H, Xu Q, Chen P. Hydrogen carriers. Nat Rev Mater 2016;1(12). https://doi.org/10.1038/ natrevmats.2016.59. Crabtree RH. Hydrogen storage in liquid organic heterocycles. Energy Environ Sci 2008;1(1):134. https:// doi.org/10.1039/b805644g. € lkl J, Arlt W. Thermodynamic evaluation of Mu¨ller K, Vo potential organic hydrogen carriers. Energy Technol 2013;1(1):20e4. https://doi.org/10.1002/ente.201200045. € ssmann A, Bru¨ckner N, Markiewicz M, Zhang YQ, Bo Thoming J, Wasserscheid P, Stolte S. Environmental and health impact assessment of Liquid Organic Hydrogen Carrier (LOHC) systems. Challenges and preliminary results. Energy Environ Sci 2015;8(3):1035e45. https://doi.org/ 10.1039/C4EE03528C. Teichmann D, Arlt W, Wasserscheid P. Liquid Organic Hydrogen Carriers as an efficient vector for the transport and storage of renewable energy. Int J Hydrog Energy 2012;37(23):18118e32. https://doi.org/10.1016/ j.ijhydene.2012.08.066. Teichmann D, Arlt W, Wasserscheid P, Freymann R. A future energy supply based on Liquid Organic Hydrogen Carriers (LOHC). Energy Environ Sci 2011;4(8):2767. https:// doi.org/10.1039/c1ee01454d. Teichmann D, Stark K, Mu¨ller K, Zittl G, Wasserscheid P, Arlt W. Energy storage in residential and commercial buildings via Liquid Organic Hydrogen Carriers (LOHC). Energy Environ Sci 2012;5(10):9044. https://doi.org/10.1039/ c2ee22070a. Mu¨ller K, Stark K, Mu¨ller B, Arlt W. Amine borane based hydrogen carriers. An evaluation. Energy Fuels 2012;26(6):3691e6. https://doi.org/10.1021/ef300516m. Eypasch M, Schimpe M, Kanwar A, Hartmann T, Herzog S, Frank T, Hamacher T. Model-based techno-economic evaluation of an electricity storage system based on Liquid Organic Hydrogen Carriers. Appl Energy 2017;185(Part 1):320e30. https://doi.org/10.1016/ j.apenergy.2016.10.068. Ahluwalia RK, Hua TQ, Peng J-K, Kromer M, Lasher S, McKenney K, Law K, Sinha J. Technical assessment of organic liquid carrier hydrogen storage systems for automotive applications. Executive summary. 2011. https://www1.eere.energy.gov/hydrogenandfuelcells/ pdfs/liquid_carrier_h2_storage.pdf. [Accessed 25 June 2018]. € tzinger C, Mu¨ller S, Mu¨ller K, Preißinger M, Adametz P, Po Lechner R, Bru¨ggemann D, Brautsch M, Arlt W. Thermodynamic evaluation and carbon footprint analysis of the application of hydrogen-based energy-storage systems in residential buildings. Energy Technol 2017;5(3):495e509. https://doi.org/10.1002/ente.201600388. Haupt A, Mu¨ller K. Integration of a LOHC storage into a heat-controlled CHP system. Energy Environ Sci 2017;118: 1123e30. https://doi.org/10.1016/j.energy.2016.10.129. Scherer GWH, Newson E. Analysis of the seasonal energy storage of hydrogen in liquid organic hydrides. Int J Hydrog Energy 1998;23(1):19e25. € smann A, Preuster P, Wasserscheid P, Arlt W, Ru¨de T, Bo Mu¨ller K. Resilience of Liquid Organic Hydrogen Carrier based energy-storage systems. Energy Technol 2018;6(3):529e39. https://doi.org/10.1002/ente.201700446. Krieger C, Mu¨ller K, Arlt W. Coupling of a Liquid Organic Hydrogen Carrier system with industrial heat. Chem Eng
6652
[36]
[37]
[38] [39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Technol 2016;39(8):1570e4. https://doi.org/10.1002/ ceat.201600180. Wang H, Zhou X, Ouyang M. Efficiency analysis of novel Liquid Organic Hydrogen Carrier technology and comparison with high pressure storage pathway. Int J Hydrog Energy 2016;41(40):18062e71. https://doi.org/ 10.1016/j.ijhydene.2016.08.003. Department of Energy. Technical system targets: onboard hydrogen storage for light -duty fuel cell vehicles. 2017. https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_ myrdd_table_onboard_h2_storage_systems_doe_targets_ ldv_1.pdf. [Accessed 8 June 2018]. Alibaba: alibaba.com. https://www.alibaba.com/. [Accessed 19 June 2018]. Fraunhofer IZM. Environmental evaluation methods. Toxic potential indicator (TPI). 2018. https://www.izm.fraunhofer. de/en/abteilungen/environmental_reliabilityengineering/ key_research_areas/environmental_assessmentandecodesign/toxic-potential-indicator–tpi-.html. [Accessed 19 June 2018]. NASA. Technology readiness level. 2012. https://www.nasa. gov/directorates/heo/scan/engineering/technology/txt_ accordion1.html. Hydrogenious Technologies. Hydrogen - stored as an oil. 2018. http://www.energiewende-erlangen.de/wp-content/ uploads/2018/02/0_HydrogeniousTechnologies.pdf. [Accessed 8 June 2018]. Hodoshima S, Takaiwa S, Shono A, Satoh K, Saito Y. Hydrogen storage by decalin/naphthalene pair and hydrogen supply to fuel cells by use of superheated liquidfilm-type catalysis. Appl Catal Gen 2005;283(1e2):235e42. https://doi.org/10.1016/j.apcata.2005.01.010. Mitsui & Co. The world's first global hydrogen supply chain demonstration project. 2017. https://www.mitsui.com/jp/ en/release/2017/1224164_10832.html. [Accessed 8 June 2018]. Forberg D, Schwob T, Zaheer M, Friedrich M, Miyajima N, Kempe R. Single-catalyst high-weight% hydrogen storage in an N-heterocycle synthesized from lignin hydrogenolysis products and ammonia. Nat Commun 2016;7:13201. https:// doi.org/10.1038/ncomms13201. € smann A, Teichmann D, Arlt W, Bru¨ckner N, Obesser K, Bo Dungs J, Wasserscheid P. Evaluation of industrially applied heat-transfer fluids as Liquid Organic Hydrogen Carrier systems. Chem Sustain 2013;7(1):229e35. https://doi.org/ 10.1002/cssc.201300426. Luo W, Campbell PG, Zahkarov LN, Liu S-Y. A singlecomponent liquid-phase hydrogen storage. Material J Am Chem Soc 2011;133(48). https://doi.org/10.1021/ja208834v. 29326-19329. Hu P, Fogler E, Diskin-Posner Y, Iron MA, Milstein D. A novel Liquid Organic Hydrogen Carrier system based on catalytic peptide formation and hydrogenation. Nat Commun 2015;6:6859. https://doi.org/10.1038/ncomms7859. Sasaki K, Li H-W, Hayashi A, Yamabe J, Ogura T, Lyth SM, editors. Hydrogen energy engineering. A japenese perspective. Green energy and technology. Springer Japan; 2016. Yamaguchi R, Ikeda C, Takahashi Y, Fujita K-i. Homogeneous catalytic system for reversible DehydrogenationHydrogenation reactions of nitrogen heterocycles with reversible interconversion of catalytic species. J Am Chem Soc 2009;131(24):8410e2. https:// doi.org/10.1021/ja9022623. Cui Y, Kwok S, Bucholtz A, Davis B, Whitney RA, Jessop PG. The effect of substitution on the utility of piperidines and octahydroindoles for reversible hydrogen storage. New J Chem 2008;32(6):1027. https://doi.org/10.1039/b718209k.
[51] Hiyoshi N, Rode CV, Sato O, Shirai M. Biphenyl hydrogenation over supported transition metal catalysts under supercritical carbon dioxide solvent. Appl Catal Gen 2005;288(1e2):43e7. https://doi.org/10.1016/j.apcata. 2005.04.021. [52] Chen J, Wu H, Wu G, Xiong Z, Wang R, Fan H, Zhou W, Liu B, Chua Y, Ju X, Chen P. Lithiated primary amine-A new material for hydrogen storage. Chem Eur J 2014;20(22): 6632e5. https://doi.org/10.1002/chem.201402543. [53] Wang Z, Belli J, Jensen CM. Homogeneous dehydrogenation of Liquid Organic Hydrogen Carriers catalyzed by an iridium PCP complex. Faraday Discuss 2011;151:297. https://doi.org/ 10.1039/c1fd00002k. [54] Fujita K-i, Tanaka Y, Kobayashi M, Yamaguchi R. Homogeneous perdehydrogenation and perhydrogenation of fused bicyclic N-heterocycles catalyzed by iridium complexes bearing a functional bipyridonate ligand. J Am Chem Soc 2014;136:4829e32. [55] Dong Y, Yang M, Yang Z, Ke H, Cheng H. Catalytic hydrogenation and dehydrogenation of N-ethylindole as a new heteroaromatic Liquid Organic Hydrogen Carrier. Int J Hydrog Energy 2015;40(34):10918e22. https://doi.org/ 10.1016/j.ijhydene.2015.05.196. [56] Yang M, Dong Y, Fei S, Pan Q, Ni G, Han C, Ke H, Fang Q, Cheng H. Hydrogenation of N-propylcarbazole over supported ruthenium as a new prototype of Liquid Organic Hydrogen Carriers (LOHC). RSC Adv 2013;3(47):24877. https://doi.org/10.1039/c3ra44760j. [57] Pez, G., Scott, A., Cooper, A., Cheng, H. Hydrogen storage by reversible hydrogenation of pi-conjugated substrates. USA Patent US20050002857A1; 2006. [58] IFA. GESTIS-stoffdatenbank. Gefahrstoffinformationssystem der Deutschen gesetzlichen unfallversicherung. 2018. https://www.dguv.de/ifa%3B/gestis/gestis-stoffdatenbank/ index.jsp. [Accessed 19 June 2018]. [59] Teichmann D. Konzeption und Bewertung einer nachhaltigen Energieversorgung auf Basis flu¨ssiger € ger (LOHC). Dissertation. FriedrichWasserstofftra € t; 2014. Alexander-Universita [60] Eypasch M. Wasserstoffspeicherung in LOHC-Systemen als Basis fu¨r industrielle Energiespeicheranwendungen. € t; 2016. http:// Dissertation, Friedrich-Alexander-Universita www.shaker.de/Online-Gesamtkatalog-Download/2018.05. 02-13.44.00-134.28.31.128-radC6F02.tmp/3-8440-4946-0_ INH.PDF. [61] Yang M, Dong Y, Fei S, Ke H, Cheng H. A comparative study of catalytic dehydrogenation of perhydro-N-ethylcarbazole over noble metal catalysts. Int J Hydrog Energy 2014;39(33):18976e83. https://doi.org/10.1016/j.ijhydene. 2014.09.123. [62] Arlt W. Speicherung elektrischer energie. Chemische € ge; speicherung elektrischer energie. Mu¨nchen: VDI-Vortra 2014. http://www.verein-der-ingenieure.de/fileadmin/ mediapool_vdi/Landesverband/doc/Arlt-5.5.2014.pdf. [Accessed 8 June 2018]. [63] Luo W, Zakharov LN, Liu S-Y. 1,2-BN cyclohexane. Synthesis, structure, dynamics, and reactivity. J Am Chem Soc 2011;133(33):13006e9. https://doi.org/10.1021/ ja206497x. [64] RSC. ChemSpider. Search and share chemistry. 2018. https://www.chemspider.com. [65] Campbell PG, Zakharov LN, Grant DJ, Dixon DA, Liu S-Y. Hydrogen storage by BoronNitrogen heterocycles. A simple route for spent fuel regeneration. J Am Chem Soc 2010;132(10):3289e91. https://doi.org/10.1021/ja9106622. [66] Liu S-Y. Hydrogen storage by novel CBN heterocycle materials. Final Report. 2012. https://www.osti.gov/servlets/ purl/1221989.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
[67] Payne Jr DA, Eads EA. Boron-nitrogen heterocycles. J Chem Educ 1964;41(6):334. [68] Mikulas TC. Electronic structure investigations of titanium oxide nanoclusters, boron-nitrogen heterocycles, and reaction products of lanthanides with oxygen difluoride and lanthanides with water. Dissertation. University of Alabama; 2015. http://search.proquest.com/openview/ 8aba150f27e07b8c9d2894a2a518c034/1?pq-origsite¼ gscholar&cbl¼18750&diss¼y. [Accessed 15 September 2017]. [69] Maenaka Y, Suenobu T, Fukuzumi S. Catalytic interconversion between hydrogen and formic acid at ambient temperature and pressure. Energy Environ Sci 2012;5(6):7360. https://doi.org/10.1039/c2ee03315a. [70] Hull JF, Himeda Y, Wang W-H, Hashiguchi B, Periana R, Szalda DJ, Muckerman JT, Fujita E. Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures. Nat Chem 2012;4(5):383e8. https://doi.org/10.1038/ nchem.1295. [71] Hsu S-F, Rommel S, Eversfield P, Muller K, Klemm E, Thiel WR, Plietker B. A rechargeable hydrogen battery based on Ru catalysis. Angew Chem 2014;53(27):7074e8. https:// doi.org/10.1002/anie.201310972. [72] Filonenko GA, van Putten R, Schulpen EN, Hensen EJM, Pidko EA. Highly efficient reversible hydrogenation of carbon dioxide to formates using a ruthenium PNP-pincer catalyst. ChemCatChem 2014;6(6):1526e30. https://doi.org/ 10.1002/cctc.201402119. [73] Mu¨ller K, Brooks K, Autrey T. Hydrogen storage in formic acid: a comparison of process options. Energy Fuels 2017;31(11):12603e11. https://doi.org/10.1021/acs. energyfuels.7b02997. [74] Bi Q-Y, Lin J-D, Liu Y-M, Du X-L, Wang J-Q, He H-Y, Cao Y. An aqueous rechargeable formate-based hydrogen battery driven by heterogeneous Pd catalysis. Angew Chem 2014;53(49):13583e7. https://doi.org/10.1002/anie.201409500. € hrungsrechner: dollar - euro (USD [75] finanzen.net GmbH. Wa in EUR). 2018. https://www.finanzen.net/ waehrungsrechner/us-dollar_euro. [Accessed 5 July 2018]. [76] Markussen P, Austell J, Hustad C. A CO2-infrastructure for eor in the north sea (cens). Macroeconomic implications for host countries. In: Greenhouse gas control technologies 6th internation conference(2); 2003. p. 1077e82. [77] Eppinger J, Huang K-W. Formic acid as a hydrogen energy carrier. ACS Energy Lett 2016;2(1):188e95. https://doi.org/ 10.1021/acsenergylett.6b00574. [78] Formic acid, vol. 5; 2016. Ullmann's Encyclopedia of Industrial Chemistry, 1. [79] Toyir J, Miloua R, Elkadri NE, Nawdali M, Toufik H, Miloua F, Saito M. Sustainable process for the production of methanol from CO2 and H2 using Cu/ZnO-based multicomponent catalyst. Phys Proc 2009;2(3):1075e9. https://doi.org/10.1016/ j.phpro.2009.11.065. [80] Jadhav SG, Vaidya PD, Bhanage BM, Joshi JB. Catalytic carbon dioxide hydrogenation to methanol. A review of recent studies. Chem Eng Res Des 2014;92(11):2557e67. https://doi.org/10.1016/j.cherd.2014.03.005. [81] Saito M, Murata K. Development of high performance Cu/ ZnO-based catalysts for methanol synthesis and the watergas shift reaction. Catal Surv Asia 2004;8(4):285e94. [82] Yang C, Ma Z, Zhao N, Wei W, Hu T, Sun Y. Methanol synthesis from CO2-rich syngas over a ZrO2 doped CuZnO catalyst. Catal Today 2006;115(1e4):222e7. https://doi.org/ 10.1016/j.cattod.2006.02.077. [83] Kundu A, Shul YG, Kim DH. Methanol reforming processes. In: Zhao TS, Kreuer K-D, van Nguyen T, editors. Advances in fuel cells. Advances in fuel cells, vol. 1. Amsterdam, San Diego, CA, Oxford: Elsevier; 2007. p. 419e72.
6653
[84] Zhang C, Yuan Z, Liu N, Wang S. Study of catalysts for hydrogen production by the high temperature steam reforming of methanol. Fuel Cells 2006;6(6):466e71. https:// doi.org/10.1002/fuce.200600010. [85] Nielsen M, Alberico E, Baumann W, Drexler H-J, Junge H, Gladiali S, Beller M. Low-temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide. Nature 2013;495(7439):85e9. https://doi.org/ 10.1038/nature11891. [86] Monney A, Barsch E, Sponholz P, Junge H, Ludwig R, Beller M. Base-free hydrogen generation from methanol using a Bi-catalytic system. Chem Commun 2014;50(6):707e9. https://doi.org/10.1039/C3CC47306F. [87] Rodrı´guez-Lugo RE, Trincado M, Vogt M, Tewes F, SantisoQuinones G, Gru¨tzmacher H. A homogeneous transition metal complex for clean hydrogen production from methanolewater mixtures. Nat Chem 2013;5(4):342e7. https://doi.org/10.1038/nchem.1595. [88] Fujita K-i, Kawahara R, Aikawa T, Yamaguchi R. Hydrogen production from a methanol-water solution catalyzed by an anionic iridium complex bearing a functional bipyridonate ligand under weakly basic conditions. Angew Chem 2015;127(31):9185e8. https://doi.org/10.1002/ ange.201502194. [89] Stability of methanol, propanol, and SF6 as hightemperature tracers. In: Adams MC, Yamada Y, Yagi M, Kondo T, Wada T, editors. Proceeding World Geothermal Congress; 2000. [90] Carbon recycling international: world's largest CO2 methanol plant. 2015. http://carbonrecycling.is/georgeolah/. [Accessed 8 June 2018]. [91] Park K-C, Yim D-J, Ihm S-K. Characteristics of Al-MCM-41 supported Pt catalysts. Effect of Al distribution in Al-MCM41 on its catalytic activity in naphthalene hydrogenation. Catal Today 2002;74(3):281e90. [92] Alibaba. Preis fu¨r Naphthalene. 2018. https://www.alibaba. com/product-detail/Refined-naphthalene-CAS-NO-91-20_ 60722181200.html?spm¼a2700.7724838.2017115.29. 3b49557eoq81ev&s¼p. [Accessed 2 May 2018]. [93] Shinohara C. Local structure around platinum in Pt/C catalysts employed for liquid-phase dehydrogenation of decalin in the liquid-film state under reactive distillation conditions. Appl Catal Gen 2004;266(2):251e5. [94] Recent advances in basic and applied aspects of industrial catalysis. In: Prasada Rao TSR, Dhar GM, India CSo, editors. Proceedings of 13th national symposium and silver jubilee symposium of catalysis of India, Dehradun, India, April 2e4, 1997. Studies in surface science and catalysis. Elsevier; 1998. [95] Hodoshima S, Arai H, Saito Y. Liquid- lm-type catalytic decalin dehydrogeno-aromatization for long-term storage and long-distance transportation of hydrogen. Int J Hydrog Energy 2003;28(2):197e204. [96] Thomas K, Binet C, Chevreau T, Cornet D, Gilson J-P. Hydrogenation of toluene over supported Pt and Pd catalysts. Influence of structural factors on the sulfur tolerance. J Catal 2002;212(1):63e75. https://doi.org/10.1006/ jcat.2002.3780. [97] Shuwa SM, Jibril BY, Al-Hajri RS. Hydrogenation of toluene on Ni-Co-Mo supported zeolite catalysts. Nig J Tech 2018;36(4):1114. https://doi.org/10.4314/njt.v36i4.17. [98] Hatim MI, Fazara MU, Syarhabil AM, Riduwan F. Catalytic dehydrogenation of methylcyclohexane (MCH) to toluene in a palladium/alumina hollow fibre membrane reactor. Proc Eng 2013;53:71e80. https://doi.org/10.1016/ j.proeng.2013.02.012. € Ni/Al2O3 catalysts and their activity in [99] Yolcular S, Olgun O. dehydrogenation of methylcyclohexane for hydrogen
6654
[100]
[101]
[102]
[103]
[104]
[105]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
production. Catal Today 2008;138(3e4):198e202. https:// doi.org/10.1016/j.cattod.2008.07.020. Zhang L, Xu G, An Y, Chen C, Wang Q. Dehydrogenation of methyl-cyclohexane under multiphase reaction conditions. Int J Hydrog Energy 2006;31(15):2250e5. https://doi.org/ 10.1016/j.ijhydene.2006.02.001. Okada Y, Sasaki E, Watanabe E, Hyodo S, Nishijima H. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Int J Hydrog Energy 2006;31(10):1348e56. https://doi.org/10.1016/ j.ijhydene.2005.11.014. Intratec. Average toluene price. 2007-2010. 2017. https:// www.intratec.us/chemical-markets/toluene-price. [Accessed 8 June 2018]. Alhumaidan F, Cresswell D, Garforth A. Hydrogen storage in liquid organic hydride. Producing hydrogen catalytically from methylcyclohexane. Energy Fuels 2011;25(10):4217e34. https://doi.org/10.1021/ef200829x. Gru¨nenfelder NF, Schucan TH. Seasonal storage of hydrogen in liquid organic hydrides. Description of the second prototype vehicle. Int J Hydrog Energy 1989;14(8):579e86. Emori H, Kawamura T, Ishikawa T, Takamura T. Carbonhydride energy storage system. Hitachi Rev 2013:192e8.
[106]
[107]
[108]
[109]
[110]
https://www.scopus.com/record/display.uri?eid¼2-s2.084877081726&origin¼inward&txGid¼7aba1c692b5aa 933ecd745dc5d69fe59. Okada Y, Shimura M. Development of large-scale H2 storage and transportation technology with Liquid Organic Hydrogen Carrier (LOHC). In: The 21st joint GCC-Japan environment symposium; 2013. https://www.jccp.or.jp/ international/conference/docs/15rev-chiyoda-mr-shimurachiyoda-h2-sturage-and-transpor.pdf. Alibaba. Preis fu¨r Phenazine. 2018. https://www.alibaba. com/product-detail/Phenazine-92-82-0_1185968472.html? spm¼a2700.7724857.main07.82.7c38c74dXa0use. [Accessed 2 May 2018]. Ong BC, Kamarudin SK, Basri S. Direct liquid fuel cells. A review. Int J Hydrog Energy 2017;42(15):10142e57. https:// doi.org/10.1016/j.ijhydene.2017.01.117. Joghee P, Malik JN, Pylypenko S, O'Hayre R. A review on direct methanol fuel cells e in the perspective of energy and sustainability. MRS Energy Sustain 2015;2. https://doi.org/ 10.1557/mre.2015.4. Manthiram A. An outlook on Lithium ion battery technology. ACS Cent Sci 2017;3(10):1063e9. https://doi.org/ 10.1021/acscentsci.7b00288.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Review Article
Liquid Organic Hydrogen Carrier (LOHC) e Assessment based on chemical and economic properties Matthias Niermann a,*, Alexander Beckendorff a, Martin Kaltschmitt a, Klaus Bonhoff b a
Hamburg University of Technology, Institute of Environmental Technology and Energy Economics, Eißendorfer Str. 40, 21073, Hamburg, Germany b National Organisation Hydrogen and Fuel Cell Technology, Fasanenstr. 5, 10623, Berlin, Germany
article info
abstract
Article history:
Hydrogen can be transported via long distances based on Liquid Organic Hydrogen Carriers
Received 23 July 2018
(LOHC). Such a transport is realized based on a two-step cycle: (1) loading/storage of
Accepted 21 January 2019
hydrogen (hydrogenation) into the LOHC molecule and (2) unloading/release of hydrogen
Available online 16 February 2019
(de-hydrogenation). During the storage period, hydrogen is covalently bound to the respective LOHC. Since the (optimal) LOHC is liquid at ambient conditions and shows
Keywords:
similar properties as crude oil based liquids (e.g. diesel, gasoline), it can easily be handled,
Liquid Organic Hydrogen Carrier
transported and stored; thus a stepwise implementation using the existing crude oil based
LOHC
infrastructure would be possible. Against this background this paper reviews the current
Hydrogen economy
knowledge in hydrogenation and de-hydrogenation of various LOHC. Therefore, a variety
Alternative fuels
of LOHC is evaluated based on their properties and compared to each other. By applying
Hydrogen storage
different evaluation criteria representing the requirements of the three different applica-
Assessment
tion areas (energy-storage, energy-transport, mobility application), the LOHCs can be assigned to a field they suit best. The analysis shows that the most promising LOHC candidates to date are dibenzyltoluene for energy-transport and energy-storage as well as Nethylcarbazole for mobility applications. In addition, a use of toluene in the transport sector is also conceivable. Methanol can potentially be applied in all three application fields due to its properties if a compromise between de-hydrogenation temperature and gas flow can be achieved based on further R&D-activities. For future implementation phenazine and formic acid show great potential, but also additional R&D especially regarding catalysis and solvents is necessary. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (M. Niermann). https://doi.org/10.1016/j.ijhydene.2019.01.199 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
6632
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6632 Storage molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6634 N-ethylcarbazole . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6634 Dibenzyltoluene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6636 1,2-dihydro-1,2-azaborine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6637 Formic acid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6638 Methanol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6639 Naphthalene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6640 Toluene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6641 Phenazine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6642 Assessment and comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6642 Assessment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6643 Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6644 Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Toxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 De-hydrogenation temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Energy demand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Material handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Process design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6645 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Gas flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Technical readiness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6646 Mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6647 Energy-transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6648 Energy-storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6648 Comparison to other storage options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6649 Final considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6649 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6650 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6650
Introduction Our modern energy and transportation system relies mainly on fuels based on fossil energy resources. Because of the a priori limited resources of these energy carrier, their strong price fluctuations, their contribution to global warming, as well as their negative impact on human health and the natural environmental interest in renewable energy sources has grown significantly in recent years; 2016 already 19.5% of the world's energy demand has been covered by renewable sources of energy [1]. However, this is just a first step in an on-going development towards an increasingly more sustainable energy system. Germany, as one example, has set the objective to reach a share of 80% renewable energies related to the overall gross electricity generation by 2050 [2]. Such a transition of the overall energy system in general and the electricity supply system in particular faces substantial challenges that need to be addressed in an adequate way. Especially the fluctuating character of electricity from wind mills and photovoltaic systems might result in regional
over- and underproduction of electricity. Thus, balancing as well as storage of e.g. electrical energy is needed to ensure a secure energy supply. Additionally, regional differences in solar radiation and wind speed influencing the energy respectively the electricity output contribute to this imbalance. To counteract and balance this uneven distribution of energy/electricity, efficient systems for (long-term) energystorage and (long-distance) energy-transportation are urgently required. Beside this, to contribute to the decarbonization of the mobility sector the electricity supply system could be much better coupled with the transportation sector [3,4]; this is one of the driver to develop battery-electric vehicles as well as hydrogen driven fuel cell cars. To realize such approaches successfully, especially the weight and size of on-board storage systems for renewable fuels need to be greatly reduced. Hydrogen can be used in all three application areas (mobility, energy-storage, energy-transport). It is due to its very specific properties an attractive energy carrier. However, the low energy density and the difficult handling of molecular hydrogen as a low density gas complicates its usage and results in a high economic and energetic effort to store it.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
6633
Fig. 1 e Concept of the LOHC storage.
So far, different storage options exist characterized by specific advantages and disadvantages. Most widely used are “Compressed Hydrogen (CGH2)” and “Liquefied Hydrogen (LH2)” storage devices. Both storage options are characterized by high security requirements due to a high pressure and a low temperature, respectively. Additionally, the necessary conditioning of the hydrogen gas to be stored into the storage device results in a high electricity demand. Additionally, 0.3e3% of the hydrogen is lost in a liquid H2 storage system due to boil-off [5e7]. Low and high temperature metal hydrides (MH) are another storage option showing in theory high storage capacities. But here thermal management and packing limitations greatly decreases these values [8]. Also, the limited reversibility of the hydrogen storage, decomposition of the storage material and slow reaction kinetics are challenges to be tackled by further research [9]. Another method to store hydrogen at ambient conditions are Liquid Organic Hydrogen Carriers (LOHC). LOHCs are potentially cheap, safe, and easily manageable. Moreover, they allow for a long-term energy storage without boil-off or other hydrogen losses as well as an uncomplicated transportation. The storage concept of hydrogen within a LOHC (Fig. 1) is typically based on reversible hydrogenation and dehydrogenation of carbon double bounds. During the hydrogenation process the double bounds are saturated with hydrogen. This process is exothermic and typically takes place at elevated temperatures and pressures. Vice versa the hydrogen can be released again in its pure form based on a catalytic endothermic de-hydrogenation reaction taking place mostly close to atmospheric pressure, although at elevated temperatures. Catalysis plays a crucial role within the underlying reaction processes. Hydrogenation of the unloaded LOHC (H0LOHC) as well as de-hydrogenation of the loaded LOHC (HnLOHC1) is 1
n: number of hydrogen atoms.
typically catalyzed. A heterogenic catalysis is beneficial for such LOHC-storage systems, because then there is no need to separate the catalyst from the reaction mixture. Thus, it is possible to operate the reactor and the storage tank separately; this might have advantages especially for large systems [10]. Due to such a storage process (i.e. hydrogen bound into the LOHC molecule), the volumetric energy density can be greatly increased and the handling can be simplified as the properties of LOHCs are typically similar to crude oil derivate (e.g. diesel, gasoline). Therefore, LOHC systems might use the existing crude oil infrastructure. This allows a stepwise implementation of this technology and offers an opportunity for the crude oil industry to reduce increasingly its carbon footprint. Another advantage resulting from the similarity to known fossil energy based fuels (e.g. diesel, gasoline) is the good public acceptance as the people are used to deal with these substances since decades. Hydrogenation and de-hydrogenation of different substances has been assessed in depth [11e21]. For example, Crabtree [22] suggested organic heterocycles to be used as LOHC outlining the safe storage and easy heat management. Mu¨ller et al. [14,23] evaluated LOHCs based on their thermodynamic properties emphasizing that nitrogencontaining aromatics are well suited to be used as a LOHC due to their enthalpy change. Additionally, a systematic toxicity evaluation has been carried out outlining that this aspect need to be treated seriously [24]. Also potential LOHC candidates have been analyzed for various potential application fields [10,25e34]; such fields of applications were energy-storage in buildings, energy-transport and fuel applications in vehicles. Each of these investigation mainly focus on one LOHC, including N-ethylcarbazole, dibenzyltoluene, toluene and 1,2-dihydro-1,2-azaborine. Also the coupling or linking of the endothermic de-dehydrogenation with industrial waste heat have been examined exemplarily for a cement plant [35]. Wang et al. [36] carried out a comparison between a LOHC-based storage system and a Compressed Hydrogen Gas (CHG) storage system. However, a broad evaluation and comparison of different LOHC with
6634
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
each other related to the potential implementation field based on their chemical/physical properties does not exist for the time being. Against this background this paper examines different substances based on a broad literature review for their potential use as a LOHC. To find potential weak spots for each substance the physical properties, hydrogenation and dehydrogenation conditions as well as selected economic factors are considered and analyzed. Based on these data, for each LOHC assessed here possible application areas are identified and allocated. Thus, for the first time properties and fields of application are combined for a holistic overview of potential LOHC.
Storage molecules Possible storage molecules, their hydrogenation and dehydrogenation reaction as well as their properties are evaluated below. The following frame conditions are taken as a basis for further evaluation. Storage. The ultimate objective for on-board storage capacity and volumetric energy density are according to the US-DoE 6.5 wt.-% and 1.7 kWh/L, respectively [37]. These values are recognized internationally so far as the given benchmark. The storage capacity and energy density is stated for the pure substance (i.e., without possible solvents and without the tank system) and, if applicable, the diluted substance with the de-hydrogenation limitation. Availability. From an economic point of view, the LOHC should be cheap and unlimitedly available. The current prices for these substances are taken, if not otherwise available, from an internet wholesale platform for large distributors [38]. Toxicity. The LOHC substances should of course be nontoxic. Typically the toxicity is described by the Toxicity Potential Indicator (TPI) [39]. This key figure is calculated based on the R-phrases, the water hazard class, the maximum admissible concentration and the EU carcinogenicity. The toxicity potential indicator (TPI) is given for a range between “0” (substance with no known hazard) and “100” (extremely toxic substance). De-hydrogenation temperature. A low de-hydrogenation temperature simplifies the integration of the storage into a heating network and ideally makes it possible to use lowtemperature waste heat to cover the de-hydrogenation demand (e.g. the waste heat of a Polymer Electrolyte Fuel Cell (PEFC)). Energy demand. During hydrogenation energy is released and typically not used. During de-hydrogenation more energy is needed to release the hydrogen again from the LOHC molecule. Thus the energy demand of this storage system is determined by the energy demand/heat demand of de-hydrogenation. In this respect low enthalpies of reaction require a higher heat demand, which would make the storage energetically less favorable. If a cleaning step for the hydrogen is needed after the release the respective energy demand adds up to the overall energy demand for such a storage system.
Material handling. Ideally, no precaution based on the guidelines for working with chemicals (GHS-symbols) have to be taken to store and transport the LOHC. Handling is further characterized by the physical properties of the LOHC, which in an ideal case includes a low melting and high boiling point (i.e., the loaded and unloaded LOHC is liquid during the overall storage process). This also includes a low vapor pressure for low requirements for storage safety and a low dynamic viscosity for easy pumping. Process design. If the LOHC is solid in the needed temperature range (high melting point), a solvent has to be added. To ensure hydrogen purity after de-hydrogenation, a downstream purification has to be added to separate the solvent securely from the hydrogen. A low boiling point of the LOHC is also disadvantageous since hydrogen separation from the unloaded LOHC will also require an extra purification step. And each additional step complicates the process design and needs more energy. A simple design further includes a high ignition temperature as well as a high flashpoint for explosion prevention. Stability. The optimal substance is completely stable in the relevant temperature range and no by-products occur during the hydrogenation resp. the de-hydrogenation reaction. In reality the LOHC needs to be replaced at regular intervals as a certain amount will inevitably be lost/ degraded during the storage process. This happens through unwanted side reactions like isomerization and disproportionation. The more stable the LOHC, the more recirculation intervals are possible. The process stability is further characterized by a high catalyst stability. The information on stability is not uniform in literature; a high stability is either marked by a high turnover number or by a long operation time, respectively. Gas flow. This parameter shows how fast hydrogen can be released from the loaded LOHC. It describes the mass of hydrogen released from the reaction mixture (LOHC plus solvents) per hour. If a certain amount of hydrogen remains bounded to the LOHC (i.e. cannot be released again), this is synonymous with a reduced gas flow. Technical readiness. For comparison of the technical development status, the Technology Readiness Level (TRL) is applied [40]. It rates the progress on a scale from 1 to 9 (i.e., from the observation of the basic principle to successfully operating systems). In addition to the substances examined here (Table 1) and described in detail below, there are several other chemical compounds potentially to be used as a LOHC; these LOHC are not further investigated here (Table 1).
N-ethylcarbazole N-ethylcarbazole (NEC) is a nitrogen-substituted heterocycle and was first considered as a LOHC in the mid-2000's [57]. The hydrogenated form is called perhydro-N-ethylcarbazole. The main material parameters of this LOHC system are listed in Table 2. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 2) are as follows.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
6635
Table 1 e Possible LOHCs. LOHC
Reason for further consideration
N-ethylcarbazole Dibenzyltoluene 1,2-dihydro-1,2-azaborine Formic acid Methanol Naphthalene Toluene Phenazine
Well-studied nitrogenous LOHC Already existing application as LOHC; safe and convenient handling Unique characteristics through integration of boron and nitrogen Safe and convenient handling Very high storage density Well-studied cycloalkane; high storage density Well-studied cycloalkane; planned application as LOHC Promising stability and sustainable raw material production
LOHC Benzyltoluene 3-Methyl-1,2-BN-cyclopentan 2-Aminoethanol Benzene Indoline Chinoline Fluorene 4-Aminopyridine Bicyclohexyl 1,2,4-Triazolidin Lithiated Primary Amine 2-Methyl-1,2,3,4-tetra-hydroquinoline Perhydro-dibenzofuran 2,6-Dimehtyldecahdro-1,5-naphthyridin N-ethylindole N-propylcarbazole
Ref. [18] [41] [18] [18] [18] [42] [43] [44]
Reason for no further consideration
Ref.
Similar to dibenzyltoluene, however, more toxic Energy intensive regeneration and trimerization Low selectivity, long storage cycles High process temperatures (400e450 C) and very toxic Low gravimetric and volumetric energy density De-hydrogenation of only one ring possible, thus low storage capacity Slow de-hydrogenation Low selectivity Low selectivity Unstable at room temperature To date no successful hydrogenation Low selectivity Low selectivity Low selectivity Similar to N-ethylcarbazole, but less studied Similar to N-ethylcarbazole, but less studied
[45] [46] [47] [48] [18] [49] [48] [50] [51] [20] [52] [18] [53] [54] [55] [56]
Hydrogenation. The hydrogenation reaction is normally catalyzed by heterogenic catalysts consisting of a precious metal like palladium (Pd) and ruthenium (Ru) supported by aluminum oxide (Al2O3). Bru¨ckner et al. [45] have demonstrated a full hydrogenation of N-ethylcarbazole after 180 min at 150 C and 50 bar with a Ru/Al2O3 -catalyst. De-hydrogenation. For de-hydrogenation similar catalyst compositions as for hydrogenation are found. A full dehydrogenation can for example be achieved using a Pd/ Al2O3-catalyst at 270 C in 25 min or at 180 C in 250 min [45,61]. For the N-ethylcarbazole/perhydro-N-ethylcarbazole system, the following results can be summarized for the assessment criteria defined above.
hydrogenation is restricted to 90% to ensure a liquid state throughout the process. This decreases its storage capacity to 5.2 wt.-%. The energy density is 2.5 kWh/L and 2.25 kWh/ L, respectively. Availability. The raw material price of N-ethylcarbazole is ca. 40 V/kg [62]. The substance is produced by distillation; the production capacity currently installed globally is less than 10,000 t/a [18]. Toxicity. N-ethylcarbazole has a toxicity potential indicator (TPI) of 5.1 TPI/mg. For perhydro-N-ethylcarbazole no data is available to calculate the TPI. However, a first analysis shows, that the toxicity level is more or less equal to N-ethylcarbazole [24]. De-hydrogenation temperature. At 180e270 C a full dehydrogenation can be achieved. The higher the temperatures, the shorter the reaction time [45,61].
Storage. The storage capacity is 5.8 wt.-%. N-ethylcarbazole is not liquid at ambient conditions. Therefore, the de-
Table 2 e N-ethylcarbazole/Perhydro-N-ethylcarbazole system [58e60].
Density [kg/L] Liquid temperature range [ C] Flashpoint [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes
N-ethylcarbazole
Perhydro-Nethylcarbazole
1.10 68e270
0.94 <20e280
186 121
146 5.9
0.05
4.4
08
No data
Fig. 2 e Storage process of the N-ethylcarbazole/perhydroN-ethylcarbazole system.
6636
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Energy demand. De-hydrogenation is characterized by a heat demand of 53.2 kJ/molH2 [45]. Material handling. N-ethylcarbazole raises only health hazard concerns; additional precautions are not necessary. Unfortunately, data for perhydro-N-ethylcarbazole are missing. The low vapor pressure of both forms are advantageous for storage and handling, while the relatively high viscosity of the N-ethylcarbazole increases the pumping resistance. Process design. N-ethylcarbazole is solid at ambient conditions. To avoid adding a solvent, de-hydrogenation is restricted. Thus, a hydrogen purity of 99.99% after dehydrogenation can be achieved [61]. A downstream hydrogen purification step is not necessary. Stability. At temperatures of 270 C and a reaction time longer than 72 h in the presence of a catalyst a small number of by-products (<2%) is formed due to dealkylation [45]. Process temperatures above 270 C to enhance the reaction kinetic lead to a reduced stability. The catalyst lifetime is estimated to allow for 500,000 kgmat/kgcat [30]. Gas flow. The de-hydrogenation temperature influences the reaction time and therefore the gas flow. At 180 C with the de-hydrogenation restriction a gas flow of 68.0 gH2/(L h) can be achieved; this can be increased at 270 C to 163.1 gH2/ (L h) with the de-hydrogenation restriction of 90%. Technical readiness. N-ethylcarbazole has been tested on a lab-scale. The implementation in an operating environment still needs to be demonstrated. Thus the technology readiness level is 3.
Dibenzyltoluene Dibenzyltoluene (DBT) is a cycloalkane used in industry as a heat transfer oil. The hydrogenated form is called perhydrodibenzyltoluene. Important parameters of this LOHC system are listed in Table 3. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 3) are as follows. Hydrogenation. The hydrogenation reaction is normally catalyzed by heterogenic catalysts consisting of a precious
Table 3 e Dibenzytoluene/perhydro-dibenzyltoluene system [58,60].
Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a
Dibenzyltoluene
Perhydrodibenzyltoluene
1.04 39e390
0.91 45e354
450 44.1
No data 258
0.07a
0.04a
09
No data
The values were extrapolated based on the experimental data of Mu¨ller et al. [14].
Fig. 3 e Storage process of the dibenzyltoluene/perhydrodibenzyltoluene system.
metal like platinum (Pt) and ruthenium (Ru) supported by aluminum oxide (Al2O3). For example, Bru¨ckner et al. [45] have demonstrated a full hydrogenation of dibenzyltoluene after 240 min at 150 C and 50 bar with a Ru/Al2O3catalyst. De-hydrogenation. For the de-hydrogenation palladium (Pd) and ruthenium (Ru) catalysts supported by carbon (C) are used. An almost full de-hydrogenation (97%) can for example be achieved using a Pd/C-catalyst at 310 C in 120 min [45]. For the dibenzyltoluene/perhydro-dibenzyltoluene system, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity is 6.2 wt.-% and the energy density is 1.9 kWh/L. Considering the de-hydrogenation limitation the storage capacity is 6.0 wt.-% and the energy density 1.8 kWh/L. Availability. The price for dibenzyltoluene is around 4 V/kg. This relative low price level supports large scale applications [62]. Toxicity. Dibenzyltoluene has a toxicity potential indicator (TPI) of 13,8 TPI/mg. For the perhydro-dibenzyltoluene no data about the toxicity is available. De-hydrogenation temperature. Temperatures above 310 C are needed to achieve an almost full dehydrogenation. Lower temperatures are possible for this reaction; but a lower temperature level significantly reduce the hydrogen yield. At 270 C only 40% of the hydrogen is released within 2 h [45]. Energy demand. De-hydrogenation is characterized by a heat demand of 65.4 kJ/molH2 [45]. Material handling. The handling of dibenzyltoluene is only characterized by a possible environmental hazard. Other precautions are not necessary. Unfortunately, data for perhydro-dibenzyltoluene are missing. The low vapor pressure is advantageous for storage and handling. The relatively high viscosity of dibenzyltoluene increases pumping resistance. Process design. Dibenzyltoluene is liquid during the overall hydrogenation resp. de-hydrogenation process. Thus a
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
hydrogen purification step after de-hydrogenation is not necessary. Stability. At temperatures of 270 C and a reaction time longer than 72 h in the presence of a catalyst no significant number of by-products (<0.01%) has been formed [45]. The catalytic stability for hydrogenation is assumed with 14,000 h and the de-hydrogenation with 8,000 h [34]. Gas flow. De-hydrogenation temperature is crucial for the hydrogen yield after a specific reaction and therefore influences the gas flow. At 270 C a gas flow of 11.3 gH2/(L h) can be achieved to be increased at 310 C to 27.5 gH2/(L h). Technical readiness. Dibenzyltoluene is well analyzed. A demonstration project for overall storage system including transportation by trucks has been realized in Germany. Hydrogenation (<400 Nm3/h) and de-hydrogenation (<110 Nm3/h) plants are available on the small scale as well as in an industrial scale. This system will be demonstrated within the US in an industrial demonstration project [41]. Therefore, the technology readiness level is 9.
1,2-dihydro-1,2-azaborine 1,2-dihydro-1,2-azaborine (AB) is a material synthesized for the first time in 2011 [63]. It is a boron (B)- and nitrogen (N) substituted heterocycle and the hydrogenated form is called 1,2-BN-cyclohexane. The main material parameters of this LOHC system are shown in Table 4. The conditions for hydrogenation and de-hydrogenation of the storage process (Fig. 4) are as follows. Hydrogenation. The carbon (C) atoms of the storage molecule can be hydrogenated with a palladium catalyst supported on carbon. However, a catalytic hydrogenation of the B- and N-atoms induced by elevated pressure levels and higher temperatures has not been demonstrated so far. Up to now, the addition of hydrogen to these atoms is only possible via a two-step addition by hydride (KH) and proton (HCl) equivalents [65]. The calculated equilibrium results in a theoretical hydrogenation rate of 95% at 80 C and 10 bar [28]. De-hydrogenation. De-hydrogenation can be catalyzed by an iron chloride (FeCl2) or cobalt chloride (CoCl2) catalysts
Table 4 e 1,2-dihydro-1,2-azaborine/1,2-BN-cyclohexane system [23,63,64]. 1,2-dihydro1,2-azaborine Density [kg/L] Liquid temperature range [ C] Flash point [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a b
1,2-BNcylcohexane
No data 45e87a
1.01 63e87a
22.7 H 22.6b 0.55a
No data 0.57a
18,300a
18,300a
No data
No data
Calculated with Aspen Properties. Predictions by ACD/labs.
6637
Fig. 4 e Storage process of the 1,2-dihydro-1,2-azaborine/ 1,2-BN-cyclohexane system.
at 80 C in 20 and 10 min, respectively. To date in this catalytic reaction only two hydrogen molecule equivalents can be released through isomerization from the storage molecule [66]. The theoretical equilibrium was also calculated for the de-hydrogenation with a rate of 99% at 80 C and atmospheric pressure [28]. For the 1,2-dihydro-1,2-azaborine/1,2-BN-cyclohexane system, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity is 7.1 wt.-% and the energy density is 2.4 kWh/L. By adding tetrahydrofuran as a solvent the storage capacity is reduced to 2,3 wt.-% and the storage capacity to 0.8 kWh/L and by trimerization to 4.7 wt.-% and 1.6 kWh/L [63]. Availability. Data about market prices are not available. However, it is expected that the price might be at least as high as boron (B) due to the limited availability of boron. Toxicity. To date no reliable data about the toxicity of 1,2dihydro-1,2-azaborine or 1,2-BN-cyclohexane is available. Nevertheless, tests of B-N-heterocycles suggest a low toxicity [67]. De-hydrogenation temperature. De-hydrogenation takes place at temperatures of 80 C. Existing calculations [28] promise a nearly full yield at this temperature level. Energy demand. De-hydrogenation is characterized by a heat demand of 35.9 kJ/molH2 [23]. This low enthalpy is achieved through the coupling of the exothermic dehydrogenation of the B- and N-atoms of the molecule with an endothermic de-hydrogenation of the C-atoms. Material handling. No reliable statement about the handling of the material can be given. The predicted high vapor pressure suggests a difficult storage, while the relatively low viscosity reduces the pumping resistance. Process design. 1,2-BN-cyclohexane is not liquid at ambient temperatures. Thus a solvent like tetrahydrofuran has to be added. Therefore, hydrogen purification after dehydrogenation is necessary. The predicted low flashpoint makes the de-hydrogenation operation more difficult as the substance might ignite on the hot surface of the de-
6638
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
hydrogenation reactor. The multi-step hydrogenation process demands a more complicated process design. Stability. 1,2-dihydro-1,2-azaborine is air and moisture stable. Additionally, it is thermally stable up to 150 C and de-hydrogenation proceeds cleanly without side reactions [46,68]. The stability in several storage cycles has yet to be demonstrated. Gas flow. With tetrahydrofuran as a solvent a gas flow of 132.5 gH2/(L h) catalyzed by CoCl2 and 66.2 gH2/(L h) catalyzed by FeCl2 can be achieved, if trimerization can be avoided. Technical readiness. The storage of hydrogen in the 1,2dihydro-1,2-azaborine/1,2-BN-cyclohexane system still faces major obstacles to be addressed in the years to come. If a regeneration of the loaded state is not possible with pressure alone, in the future energetically and economically justifiable hydrogenation can take place in large-scale plants. Further research regarding long-term stability, liquefaction, regeneration, trimerization and toxicity is indispensable. Its technology readiness level is assessed to be 2.
Formic acid Formic acid (FA) is a colorless, corrosive and water-soluble liquid. In this LOHC system carbon dioxide (CO2) is used as a starting material for formic acid generation. The main material parameters of the carbon dioxide/formic acid system are listed in Table 5. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 5) are as described below. Hydrogenation. Hydrogenation is exothermic but also endergonic because the equilibrium can be shifted by adding solvents (e.g. water) and/or bases (e.g. ammonia). The solubility of hydrogen and carbon dioxide is the limiting factor for final formic acid-concentration. Thus, it greatly decreases storage capacity. Homogenous catalysts are normally used to catalyze the reaction in a pressure range from 1 to 100 bar and a temperature range from 50 to 100 C [69e72]. De-hydrogenation. De-hydrogenation can take two different reaction pathways, producing (1) hydrogen and carbon dioxide or (2) water and carbon monoxide. The second pathway needs to be avoided for hydrogen storage. The de-hydrogenation is also catalyzed by homogenous catalysts. To avoid downstream separation,
Table 5 e Carbon dioxide /formic acid system [58].
Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes
Carbon dioxide
Formic acid
0.47 e e e
1.22 8e101 520 1.8 11,400 02, 05, 06
04
Fig. 5 e Storage process for carbon dioxide/formic acid system.
catalysts, which are able to catalyze both reactions, are used [69e72]. This eliminates the need to separate the catalyst from the LOHC stream. However, these catalysts are less productive than those specialized for one reaction. Each reaction can be induced for example by changing the pH and the temperature or pressure [18]. The unfavorable equilibrium of the hydrogenation is advantageous for the de-hydrogenation, liberating the hydrogen at temperatures below 100 C. An alternative way to the carbon dioxide/formic acid system for hydrogen storage is the bicarbonate/formate system. This system (Equation (1)) is thermodynamic more favorable and the carbon dioxide remains dissolved throughout the overall process. Water is here the source for the hydrogen [73]. Six release/storage cycles with a heterogeneous palladium catalyst supported by reduced graphite oxide (Pd/r-GO) were performed with potassium formate (KHCOO) [74]. The hydrogenation was realized at 100 C and 40 bar in a 4.8 M aqueous potassium bicarbonate (KHCO3) solution with formate yield of 96.8% and a de-hydrogenation at 80 C and atmospheric pressure in 4.8 M aqueous KHCOO solution with a conversion efficiency of 96.6%. The reaction time were 10 and 2 h, respectively. MHCOO þ H2 O#MHCO3 þ H2
with M ¼ Na; K
(1)
For the carbon dioxide/formic acid system, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity of pure formic acid is 4.4 wt.% and the energy density is 1.8 kWh/L. The needed solvents for shifting the equilibrium reduce the capacity to 0.3 wt.-% and the energetic density to 0.1 kWh/L (final formic acid concentration 1.53 M [70]). For the potassium bicarbonate/ potassium formate system in solution it is 0.65 wt.-% and 0.25 kWh/L, respectively. Availability. Carbon dioxide (CO2) is the starting material for a hydrogen storage in formic acid. Although CO2 is abundantly available in the earth atmosphere, it remains energetically challenging to capture it. Exhaust air are
6639
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
potential sources. Here CO2 is available in higher proportion; thus it can be captured more easily. The CO2-price through a pipeline distribution is assumed to be about 30 V/t2 [76]. Toxicity. Formic acid has a toxicity potential indicator (TPI) of 9,0 TPI/mg. De-hydrogenation temperature. The de-hydrogenation can be achieved at temperatures below 100 C. Energy demand. De-hydrogenation is characterized by a heat demand of 31.2 kJ/molH2 [77]. Material handling. Formic acid is flammable, corrosive and toxic. Therefore, handling is challenging. The high vapor pressure causes high requirements during storage, while the low viscosity is advantageous. Process design. Although formic acid is liquid at ambient temperatures, solvents still need to be added. Thus a downstream hydrogen purification step is necessary to clean the released hydrogen and recapture the CO2 for renewed hydrogenation. To realize this within a closed cycle the storage of CO2 is challenging. Thus CO2 can also be released into the atmosphere and removed from the ambient air where it is needed again. An alternative design, which can be applied to avoid the use of gaseous CO2, is the use of the bicarbonate/formate system. The CO2 remains solved during the overall process; a hydrogen purification is therefore not necessary [73]. Stability. Formic acid dissolved in water shows a high thermal stability [78]. Challenging is the stability of the catalysts; depending on the catalyst the turn over numbers vary between 2,000 and 200,000 for the hydrogenation and between 120 and 20,000 for the de-hydrogenation [69e72]. Gas flow. The added solvent reduces the gas flow. A flow up to 0.7 gH2/(L h) can be achieved. Technical readiness. The potential use of formic acid as a LOHC has been experimentally demonstrated on a labscale level. Further research to reduce the dilution is necessary to increase gas flow and storage capacity in technical applications. The deactivation of catalysts is also an issue. The technology readiness level can be assessed to be 3.
Table 6 e Carbon dioxide /methanol system.
Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes
Carbon dioxide
Methanol
0.46 e e e e 04
0.79 98e65 440 0.6 35,400 02, 06, 08
Hydrogenation is catalyzed by heterogeneous multicomponent copper (Cu) based catalysts at temperatures between 220 and 270 C and pressures between 20 and 80 bar [79e82]. The conversion rates are comparatively low. De-hydrogenation. De-hydrogenation production stream consists of carbon dioxide, carbon monoxide and hydrogen. Due to the toxicity of carbon monoxide this reaction product needs to be avoided. With increasing temperature, the carbon monoxide content within the production stream increases. Additionally, the water/ methanol molar ratio is crucial for the formation of carbon monoxide [83]. De-hydrogenation can be achieved by hightemperature steam reforming of methanol (SRM) and lowtemperature de-hydrogenation. For steam reforming of methanol (SRM) at 420 C with a heterogeneous catalyst containing iridium (Ir) and platinum (Pt) supported by cerium-zirconium-oxide (Ce0,5Zr0,5O2) the total reforming in an aqueous medium (H2O/CH3OH: 1,5 mol/mol) was achieved in 2 h. However, 6 mol-% carbon monoxide was detected in the production stream [84]. Low-temperature de-hydrogenation (<100 C) facilitates the heat management and makes the hydrogen delivery on-board more feasible. The reaction takes place in an aqueous medium and achieves depending on the used catalyst a hydrogen yield between 15 and 84%. The reaction lasts between 600 and 1,440 min; a higher yield does not necessarily correspond with the longer reaction time. Water or other solvents like tetrahydrofuran or triglyme have to be added also. Only very small amounts of carbon monoxide were detected [85e88].
Methanol Methanol (MET) is an alcohol. In the respective LOHC system carbon dioxide (CO2) is used as a starting material for methanol generation. The main material parameters of the CO2/ methanol system are listed in Table 6. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 6) are as outlined below. Hydrogenation. Methanol can be synthesized by hydrogenation of carbon dioxide. The reaction follows two pathways, which yield either (1) methanol and water or (2) carbon monoxide and water. For hydrogen storage pathway (2) must be avoided to optimize this storage process. Also this reaction can be steered by catalysts.
2
Exchange rate 0.86 V ¼ 1.0 US$ [75].
Fig. 6 e Storage process of the carbon dioxide/methanol system.
6640
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
For the LOHC system carbon dioxide/methanol, the following results can be summarized for the assessment criteria defined above. Storage. The storage capacity of methanol is 12.1 wt.-% and the energy density is 3.3 kWh/L. The solvents necessary for de-hydrogenation reduce the capacity and the energetic density. For high-temperature steam reforming a capacity of 10 wt.-% and an energy density of 2.7 kWh/L can be realized. For low-temperature de-hydrogenation they are reduced to 4 wt.-% and 1.1 kWh/L. Availability. Carbon dioxide (CO2) is the starting material for hydrogen storage in formic acid. Although CO2 is abundantly available in the earth atmosphere, it remains energetically challenging to capture it from ambient air. Flue gases from the combustion of fossil fuel energy are potential sources; here CO2 is available in higher proportion and thus can be captured more easily. The CO2 price through a pipeline distribution is assumed to be about 30 V/t3 [76]. De-hydrogenation temperature. De-hydrogenation can be realized at temperatures below 100 C. For a faster release of hydrogen temperatures around 400 C are needed. Energy demand. De-hydrogenation is characterized by a heat demand of 16.5 kJ/molH2 [18]. Material handling. Methanol is flammable and toxic. It is also characterized by a health hazard. The handling is therefore relatively challenging. Additionally, the high vapor pressure causes higher storage requirements. Process design. Although methanol is liquid at ambient temperatures, solvents still need to be added. Therefore, a downstream hydrogen purification step is necessary to purify the released hydrogen and recapture the CO2 for renewed hydrogenation. The low viscosity of methanol is further advantageous. Stability. Methanol is stable up to a temperature of 320 C [89]. The catalyst for hydrogenation show a good stability after 3,000 h operation time [79]. For high-temperature dehydrogenation only a minor decrease in the methanol conversion have been detected after 200 h [84]. The catalysts for low-temperature de-hydrogenation show turnover numbers of 10,000 and 350,000 [85,88]. Gas flow. At 420 C a gas flow of 44.8 gH2/(L h) can be realized. Below 100 C the gas flow is reduced to 1 to 3 gH2/(L h). Technical readiness. The hydrogenation process of CO2 to methanol is currently being demonstrated in a commercial-scale facility (George Olah Plant, Island). It produces more than 5 Mio. L/a [90]. Also high-temperature steam reforming of methanol is a well-known process. But, the low-temperature de-hydrogenation characterized by the benefit of a simpler heat management must be investigated more closely and has so far only been demonstrated. The system in combination with high-temperature reforming has a technology readiness level of 9, while the system in combination with low-temperature de-hydrogenation shows a technology readiness level of 3.
3
Exchange rate 0.86 V ¼ 1.0 US$ [75].
Naphthalene Naphthalene (NAP) is a polycyclic aromatic hydrocarbon. This molecule has also been proposed as a LOHC. The hydrogenated form is called decalin. The main material parameters of this LOHC system are listed in Table 7. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 7) are as follows. Hydrogenation. Full hydrogenation can be catalyzed by an aluminum mobile composition of matter (Al-MCM) catalyst at 300 C or 200 C and 69 bar in 150 min or 480 min, respectively. During hydrogenation decalin is formed. This molecule is an isomer consisting of trans-decalin and cisdecalin. Trans-decalin is thermodynamically favorable; nonetheless, these two isomers are usually equimolar distributed. The exact ratio depends on the catalyst used and the reaction temperature [91]. De-hydrogenation. De-hydrogenation is usually catalyzed by a platinum (Pt)-catalyst on carbon (C) support. It can almost be fully achieved at 280 C in 150 min. By adding rhenium (Re) to the catalyst (Pt-Re/C) the reaction time can be decreased to 120 min by otherwise identical conditions [42]. For the naphthalene/decalin system, the following results are found for the characteristic criteria defined above. Storage. The storage capacity of the naphthalene is 7.4 wt.% and the energy density is 2.2 kWh/L. Adding toluene as a
Table 7 e Naphthalene/decalin system [58]. Naphthalene Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a
Decalin
1.14 80e218 540 2.0
0.90 43e185 235 3.0
114a 02, 07, 08, 09
360a 02, 05, 06, 08, 09
Calculated with Aspen Properties.
Fig. 7 e Storage process of the naphthalene/decalin system.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
solvent, the capacity and the energetic density is reduced to 3.8 wt.-% and 1.1 kWh/L. Availability. The current market price of approximately 0.6 V/kg is favorable for large scale application [92]. Toxicity. Naphthalene has a high toxicity potential indicator (TPI) of 45,8 TPI/mg and is probably carcinogenic. Compared to that the toxicity potential indicator (TPI) for decalin is only 1,2 TPI/mg. De-hydrogenation temperature. De-hydrogenation can be realized at temperatures below 280 C. Energy demand. De-hydrogenation is characterized by a heat demand of 66.3 kJ/molH2 [93]. Material handling. Naphthalene is flammable, possess a health hazard, is irritating to the eyes and skin as well as dangerous to the natural environment. Decalin is not irritating to the eyes and skin; nevertheless, it is toxic and corrosive. The handling is therefore challenging. The low vapor pressure causes only little storage requirements, while the low dynamic viscosity is advantageous for pumping. Process design. Naphthalene is solid at ambient temperatures. Thus a solvent like toluene needs to be added. The consequence is that a downstream hydrogen purification step is necessary to purify the released hydrogen. The low ignition temperature of decalin, however, makes the operation more difficult as the substance might ignite on the hot surface of the de-hydrogenation reactor. Stability. Decalin is stable up to temperatures of 450 C [94]. High temperatures cause catalyst deactivation and carbonaceous deposits [95]. But within the process temperature range, this problem is manageable. Gas flow. A gas flow of 16.1 gH2/(L h) is achievable. Technical readiness. Even though naphthalene has been well-studied and different reactor concepts have been tested, it still faces serious technical problems (e.g., catalyst deactivation). A pilot scale demonstration has not been realized yet. The technology readiness level is assessed to be 4.
Toluene Toluene (TOL) is an aromatic hydrocarbon; it is a colorless and water-insoluble liquid. The hydrogenated form is called methylcyclohexane. The main material parameters of this LOHC system are listed in Table 8. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 8) are as follows.
Table 8 e Toluene/methylcyclohexane system [58]. Toluene Density [kg/L] Liquid temperature range [ C] Ignition temperature [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a
Methylcyclohexane
0.88 95e111 535 0.6
0.77 127e101 260 0.7
7,880a 02, 07, 08
10,900a 02, 07, 08, 09
Calculated with Aspen Properties.
6641
Fig. 8 e Storage cycle for toluene/methylcyclohexane.
Hydrogenation. Full hydrogenation can be catalyzed by platinum (Pt) catalyst supported on zeolite at 120 C and 30 bar [96]. Alternatively, hydrogenation is also possible at 200 C and 20 bar in 2 h with a nickel (Ni) cobalt (Co) molybdenum (Mo) catalyst supported zeolite [97]. De-hydrogenation. De-hydrogenation is usually catalyzed by platinum (Pt) or nickel (Ni) catalysts supported on aluminum oxide (Al2O3). The temperature varies from 350 to 450 C with yields between 50 and 92%; no information about the reaction time is available [98,99]. A Raney-Nicatalyst allowing for a 65% yield after 30 min at 250 C can be used as well; but then isomerization and disproportionation reaction take place [100] greatly reducing the recyclability. A potassium platinum catalyst supported on aluminum oxide (K-Pt/Al2O3) achieved the highest hydrogen yield of 95% at 320 C with selectivity higher than 99,9% [101]. For the toluene/methylcyclohexane system, the following results are found for the characteristic criteria defined above. Storage. The storage capacity of toluene is 6.2 wt.-% and the energy density is 1.6 kWh/L. With the dehydrogenation limit of 95% it is 5.9 wt.-% and 1.5 kWh/L. Availability. The current market price of ca. 0.3 V/kg is favorable for a large scale application [102]. Toxicity. Toluene has a toxicity potential indicator (TPI) of 19,3 TPI/mg and is presumably toxic to reproduction. The toxicity potential indicator (TPI) of methylcyclohexane is 7.3 TPI/mg. De-hydrogenation temperature. De-hydrogenation can be achieved at temperatures between 250 and 450 C. Energy demand. De-hydrogenation is characterized by a heat demand of 68.3 kJ/molH2 [101]. Material handling. Toluene is flammable, possess a health hazard and is irritating to the eyes as well as the skin. Beside this, methylcyclohexane is dangerous to the environment. Handling is therefore relative challenging. The low vapor pressures cause higher storage requirements, while the low dynamic viscosity is advantageous for pumping. Process design. Toluene is liquid at ambient temperatures; i.e., there is no need for a solvent. Nevertheless, a
6642
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
downstream hydrogen purification step is necessary to purify the released hydrogen because toluene is gaseous during de-hydrogenation; thus the gaseous state allows for the use of simple fixed bed reactors. The low ignition temperature of methylcyclohexane, however, makes the process more difficult as the substance might ignite on the hot surface of the de-hydrogenation reactor. Stability. Toluene used as a LOHC results in side reactions (for example disproportionation and de-alkylation). Additionally, the catalyst will be deactivated. But adding an additional metal like rhenium (Re) can increase the stability of the catalyst [103]. A stable process for 6,000 h with the impregnated K-Pt/Al2O3 catalyst have been demonstrated [101]. Gas flow. A gas flow of 61.5 gH2/(L h) is achievable. Technical readiness. Toluene has been well studied so far and was also tested as truck fuel combined with a combustion engine [104]. The process also has been recently applied in demonstration plants [105,106]. Additionally, a demonstration project to import hydrogen to Japan by ship with the toluene/methylcyclohexane system is currently under planning [43]. Its technology readiness level is 8.
Phenazine Phenazine (PHE) is a polycyclic aromatic molecule being a yellow solid at ambient conditions. The hydrogenated form is called tetradecahydrophenazine. The main material parameters of this LOHC system are listed in Table 9. The conditions for the hydrogenation and dehydrogenation of the storage process (Fig. 9) are as follows. Hydrogenation. Hydrogenation of phenazine controlled by the homogeneous catalyst Pd2Ru@SiCN at 115 C and 50 bar after 1,440 min have been demonstrated [44]. For this reaction dioxane and water must be added. De-hydrogenation. De-hydrogenation is catalyzed by the same catalyst at 190 C. After 1,440 min a full conversion to phenazine (i.e. 100%) has been achieved. The solvents of the hydrogenation must be replaced by diglyme [44].
For the phenazine/tetradecahydrophenazine system, the following results can be summarized for the assessment criteria defined above. Storage. Storage capacity of toluene is 7.2 wt.-% and the energy density is 2.4 kWh/L. Adding diglyme for dehydrogenation reduces the capacity to 2.4 wt.-% and the energy density to 0.8 kWh/L. Availability. The average market price of up to 26 V/kg4 is unfavorable for large scale applications [107]. But a way to synthesize phenazine from lignin has been demonstrated [44]. This could drastically reduce production costs, as lignin is abundantly available and hardly used. Toxicity. Phenazine has a high toxicity potential indicator (TPI) of 38.0 TPI/mg. For tetradecahydrophenazine no data is available for the time being. De-hydrogenation temperature. De-hydrogenation can be achieved at temperatures at around 190 C. Energy demand. De-hydrogenation is characterized by a heat demand of 61.3 kJ/molH2. Material handling. Phenazine only cause irritation to the skin and eyes; no other precautions have to be taken. The predicted low vapor pressure suggests a simple storage, while the low dynamic viscosities are advantageous for pumping. Process design. Phenazine is solid at ambient temperatures; i.e., a solvent need to be added. Therefore, a downstream hydrogen purification step is necessary to purify the released hydrogen as well as an intermediate step to separate the solvent from the hydrogen after dehydrogenation. The low flash point of phenazine, however, makes the operation more difficult as the substance might ignite on the hot surface of the de-hydrogenation reactor. Stability. Seven cycles with hydrogenation conversion efficiencies varying between 85 and 96% and dehydrogenation conversion efficiencies between 77 and 100% have been demonstrated [44]. Despite these variations, a final capacity above 7 wt.-% after the seven cycles has been achieved [44]. Gas flow. A gas flow of 0.9 gH2/(L h) is achievable. Technical readiness. Phenazine can be synthesized from lignin; therefore, it is basically available. However, the different solvents for hydrogenation and de-hydrogenation complicate the technical use considerably. Its technology readiness level is assessed to be 3.
Table 9 e Phenazine/tetradecahydrophenazine system [64]. Phenazine Density [kg/L] Liquid temperature range [ C] Flash point [ C] Dynamic viscosity at 20 C [mPas] Vapor pressure at 40 C [Pa] Hazard classes a b
Tetradecahydrophenazine
1.30 H 0.1a 172e360
1.00 H 0.1a No data
160.3 H 11.7a 3.1a
No data 2.2a
0.01b
0.18b
07
No data
Prediction by ACD/Labs. Calculated with Aspen Properties.
Assessment and comparison In the following section, the LOHCs examined are evaluated according to a rating scale and compared with regard to their applicability in the fields of mobility, energy-transport and energy-storage. A general overview of the important properities of the LOHC systems examined are given in Table 10.
4
Exchange rate 0.86 V ¼ 1.0 US$ [75].
6643
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Table 10 e Properties of the evaluated LOHCs and diesel and gasoline as reference (NEC: N-ethylcarbazole, AB: 1,2-dihydro1,2-azaborine, FA: formic acid, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: Phenazine, MO: mineral oils, D: diesel, G: gasoline, n.d.: no data).
Storage capacity [wt.%] Enthalpy of reaction [kJ/molH2] De-hydrogenation temperature [ C] Gas flow [gH2/L/h] Energy density [kWh/L] Viscosity at 20 C [mPa s] Vapor pressure at 40 C [Pa] Toxicity [TPI/mg] Price [V/kg] GHS
NEC
DBT
AB
FA
MET
NAP
TOL
PHE
MO
H0/H12
H0/H18
H0/H6
H0/H2
H0 H6
H0/H10
H0/H6
H0/H14
D/G
5.8 53.2
6.2 65.4
7.1 35.9
4.4 31.2
12.1 16.5
7.3 66.3
6.2 68.3
7.2 61.3
180e270
270e310
80
50e100
90e420
210e300
250e450
190
68.0e163.1 2.5 0.5/5.9 0.1/4.4
11.0e27.5 1.9 44/258 0.07/0.04
66.2e132.5 2.4 0.5/0.6 18,300
0.7 1.8 -/1.8 -/1,300
0.8e44.8 3.3 -/0.6 -/35,400
16.1 2.2 2.0/3.0 114/481
61.5 1.6 0.6/0.7 7,880/10,900
0.9 2.4 9.8/8.6 3.1/2.2 1.9/5.2 0.01/0.18 40/62,300
5.1/n.d. 40 09
13.8/n.d. 4 08
n.d. n.d. n.d.
-/9.0 -/29.7 45.8/1.2 19.3/7.3 0.03 0.03 0.6 0.3 02,04,05,06 02,04,06,08 02,05,06,07,08,09 02,07,08.09
38.0/n.d. 19.1/96.4 <25 1.3/1.7 07 02,07.08,09
Within the relative valuation, the rating depends on the bandwidth defined by the various LOHC. The evaluated parameters can be summarized as follows.
Fig. 9 e Phenazine/tetradecahydrophenazine system.
Assessment The assessment of the different LOHCs discussed above is based on the parameters examined for each LOHC. Each of these parameter is now rated on a scale of 1 (worst) to 10 (optimal). A distinction is made between (a) a relative rating of the parameters among the various LOHCs assessed here and (b) an absolute rating on a fixed scale (Table 11).
Storage. The storage targets of the US-DoE do not represent physical limits; this development goal is more defined by aspects related to practical implementation. Therefore, values above the target are possible. A rating is therefore realized on a relative scale. The highest values among the examined LOHCs are 12.1 wt.-% and 3.3 kWh/L (both rating 10) and the lowest 0.3 wt.-% and 0.1 kWh/L (both rating 1). Availability. Low prices and unlimited occurrence of the raw material is essential for large scale application. CO2, which is abundantly available and potentially cheap (0.03 V/kg), is the target for a rating of 10, while N-ethylcarbazole is rated with 1 because of the limited availability and the high price (40 V/kg). De-hydrogenation temperature. The lower the temperature of de-hydrogenation, the easier the heat management and the less energy is needed. The lowest temperatures found is 50 C (rating 10) and the highest is 450 C (rating 1). Energy demand. To release the stored hydrogen from loaded LOHC during de-hydrogenation, heat is necessary. This effectively reduces the storage efficiency. The lower
Table 11 e Values of the rating scale.
Relative rating Storage
Storage capacity [wt.-%] Energy density [kWh/L]
Availability De-hydrogenation temperature [ C] Energy demand [kJ/molH2] Material handling Process design Stability Gas flow [gH2/(L h)] Absolute rating Toxicity Technical readiness
Value 1
Value 10
0.3 0.1 40 V/t 450 16.5 Six precautions Intermediate/downstream purification; explosion prevention Frequently exchanged system 0.7
12.1 3.3 0.03 V/t 50 68.3 One precaution Simple 2-step system without additional explosion prevention Robust system 163.1
TPI 100 TRL 1
TPI 0 TRL 9
6644
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
this heat demand, the higher the storage efficiency. A spectrum from 16.5 kJ/molH2 (rating 10) to 68.1 kJ/molH2 (rating 1) has been identified. Material handling. Ideally no precaution must be taken dealing with the LOHC. For the examined LOHCs, a variety between only one (rating 10) to six (rating 1) different precautions per LOHC system have been detected. Process design. Additional process steps (e.g., hydrogen purification) or special reactor designs for explosion prevention lead to complex overall process designs. On the one hand there are two-step LOHC systems, consisting of hydrogenation and de-hydrogenation (rating 10), and on the other hand multi-step LOHC systems including hydrogenation, de-hydrogenation and purification with additional precautions regarding explosion prevention (rating 1). Stability. Ideally the stability of the LOHC system can be quantified with possible operating times. Unfortunately, such information is missing in most cases. Robust LOHC systems (rating 10) show only marginal losses of the LOHC over time (<0.02%) and a long-term catalytic activity (>8,000 h), while frequently exchanged systems (rating 1) are characterized by high losses and the catalyst can only convert limited amounts of starting material (low turnover numbers). Gas flow. The gas flow as an indicator for fast or slow hydrogen release varies for the examined LOHC between 163.1 gH2/(L h) (rating 10) and 0.7 gH2/(L h) (rating 1).
an overview of each rating and the total grade from 1 to 10 are presented in Table 12. A distinction is made between the storage density and the energy density of the pure substance and the diluted substance. For methanol, results are given for the high-temperature and low-temperature reforming.
Storage The theoretical volumetric and gravimetric storage density of the various LOHC investigated in chapter 2 differs significantly from the real densities reduced by solvents and the physical or self-specified de-hydrogenation limitation (Fig. 10). The methanol/water mixture after hydrogenation of carbon dioxide has the highest volumetric energy density of 3.3 kWh/L among the examined LOHC (rating 10). For the dehydrogenation/reforming, water has to be added. This reduces the density; for high-temperature reforming a rating of 8 and for low-temperature de-hydrogenation of 3 has been awarded. N-ethylcarbazole, dibenzyltoluene and toluene do not need a solvent. Still their volumetric density is reduced due to de-hydrogenation limitations, but to a much lesser extent. Completely de-hydrogenated N-ethylcarbazole has been granted by a rating of 8, with the de-hydrogenation restrictions the rating is only 7. For dibenzyltoluene the rating with and without limitation is 6 and for toluene 5. The use 1,2-dihydro-1,2-azaborine, formic acid, naphthalene and phenazine requires the addition of solvents. Thus, the ratings regarding energy density are reduced significantly compared to the pure substances. De-hydrogenation limitations further affect the rating. Pure and completely dehydrogenated 1,2-dihydro-1,2-azaborine has been given a rating of 7 and in dilution of 3. For formic acid the rating decreases from 6 to 1, for naphthalene from 7 to 4 and for phenazine from 7 to 3. In addition to reducing the volumetric energy density, dilutions and de-hydrogenation limitations also decrease the gravimetric storage capacity. The highest value again was found for the methanol/water mixture with 12.1 wt.-% (rating 10). With the addition of water, the rating for hightemperature reformed methanol is 8 and for lowtemperature de-hydrogenated methanol it is 4. As N-ethylcarbazole, dibenzyltoluene and toluene only show dehydrogenation limitations their capacities are slightly reduced. The rating for N-ethylcarbazole remains at 5, for dibenzytoluene and toluene it is reduced from 6 to 5. Because
Beside this relative rating an absolute valuation has been realized when it is possible. Parameters evaluated absolutely are summarized below. Toxicity. The toxicity is rated based on the toxicity potential indicator (TPI) scale. Substances with a TPI of 0 are potentially non-hazardous (rating 10), while substances with a TPI of 100 are extremely toxic (rating 1). Technical readiness. The readiness of each LOHC system is rated according to the technical readiness level (TRL). TRL 9 (rating 10) requires a successful operating system in the predetermined environment, while TRL 1 (rating 1) describes the observation of the basic principle. Below the various LOHC presented in chapter 2 are assessed related to the criteria discussed above. Additionally,
Table 12 e Rating of each LOHC (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
Storage capacity Energy density Availability Toxicity De-hydrogenation temperature Energy demand Material handling Process design Stability Gas flow Technical readiness
NEC
DBT
AB
FA
MET
NAP
TOL
PHE
5 7/8 1 9 5 4 10 9 8 10 3
5/6 6 9 8 4 2 10 10 10 2 10
3/6 3/7 1 e 9 7 e 1 5 8 2
1/4 1/6 10 9 10 7 5 5 6 1 3
4/8 4/8 10 7 9/2 10 5 5 6/8 1/3 3/10
4/6 4/7 10 5 6 1 1 3 7 3 3
5/6 5 10 8 3 1 5 3 8 1 9
2/6 3/7 4 6 7 2 10 1 7 1 3
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Fig. 10 e Influence of solvents and de-hydrogenation limitations on volumetric and gravimetric density (NEC: N-ethylcarbazole, AB: 1,2-dihydro-1,2-azaborine, FA: formic acid, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: Phenazine).
of the need to use solvents, 1,2-dihydro-1,2-azaborine, formic acid, naphthalene and phenazine show significantly reduced gravimetric storage capacities. Without dilution and under the assumption of a complete de-hydrogenation 1,2-dihydro-1,2azaborine has a rating of 6 and in dilution 3. With the necessarily to be added solvents and the respective dehydrogenation limitation the rating is reduced for formic acid from 4 to 1, for naphthalene from 6 to 4 and for phenazine from 6 to 3.
Availability In the category availability methanol, formic acid, naphthalene and toluene accomplish the best results (rating 10). The raw material price of dibenzyltoluene (rating 9) is approximately 10 times higher than that of naphthalene and toluene, but still significantly cheaper compared to phenazine and Nethylcarbazole. Phenazine is more expensive (rating 4); the possibility to be synthesized in the future from lignin counts as a plus. N-ethylcarbazole shows high prices and is only produced to a limited extend so far (rating 1). Prices for 1,2dihydro-1,2-azaborine are not available; however, the low disposability of boron results in a poor rating (1).
Toxicity None of the examined LOHCs has a toxicity potential indicator (TPI) of “0” (substance with no known hazard). However, N-ethylcarbazole and formic acid show values close to it (rating 9). Naphthalene is the most toxic substance among the investigated LOHCs (rating 5); nevertheless, it has a lower toxicity potential indicator compared to gasoline fuel. Next in line are phenazine (rating 6) and methanol (rating 7), which are less toxic than naphthalene, but still more toxic than diesel fuel. Dibenzytoluene and toluene (both rating 8) show a similar toxicity to diesel fuel. 1,2-dihydro-1,2azaborine cannot be evaluated due to a lack of information (no rating).
De-hydrogenation temperature Formic acid shows the lowest de-hydrogenation temperature of 50 C (rating 10). The needed temperature to de-
6645
hydrogenate the 1,2-dihydro-1,2-azaborine/1,2-BN-cyclohexane system is only slightly above (rating 9). Methanol can also be de-hydrogenated at low temperatures (rating 9), but at the cost of an increased dilution and extended reaction times. To achieve better kinetics, the de-hydrogenation temperature has to increased significantly (rating 2). Phenazine is rated 7 with regard to the de-hydrogenation temperature. For fast de-hydrogenation of the N-ethylcarbazole/ perhydro-N-ethylcarbazole system the temperature is increased to 270 C (rating 5). Temperatures as low as 180 C are possible for this system (rating 7), resulting in a slower reaction kinetic. The other LOHCs have comparatively high temperatures: dibenzytoluene (rating 4), naphthalene (rating 5) and toluene (rating 3).
Energy demand The release of hydrogen from the loaded LOHC needs thermal energy. Methanol has the lowest heat demand (rating 10), followed by formic acid and 1,2-dihydro-1,2-azaborine (both rating 7). Especially the de-hydrogenation of the toluene/ methylcyclohexane and the naphthalene/decalin system (both rating 1) is very energy demanding, closely followed by dibenzyltoluene and phenazine (both rating 2). N-ethylcarbazole is somewhere in between (rating 4).
Material handling The handling of the LOHC material is significantly influenced by the precaution to be taken. Dibenzytoluene, N-ethylcarbazole and phenazine have the fewest requirements (all rating 10), keeping in mind that specific data of their hydrogenated forms is still not available. Naphthalene is the most difficult LOHC to be handled; it requires even more measures of safety than gasoline and diesel fuel (rating 1). Toluene shows the same safety measures that must be taken when dealing with diesel and gasoline fuel (rating 5). For formic acid and methanol three precautions must be taken; additionally, gaseous CO2 needs to be handled (both rating 5). In case of 1,2-dihydro-1,2-azaborine no reliable evaluation is possible (no rating).
Process design The simplest process design consists of a hydrogenation and de-hydrogenation reactor with a condensation at ambient conditions to separate hydrogen from the de-hydrogenated LOHC. For dibenzyltoluene such a process can be realized easily (rating 10). De-hydrogenation restrictions, which can be achieved by adjusting the reaction temperature and time, slightly complicates the process for N-ethylcarbazole (rating 9); still an additional hydrogen purification is not needed. Although no solvent is added together with toluene, a downstream hydrogen purification is necessary since toluene and methylcyclohexane are low-boiling. Either low condensation temperatures or further purification methods like pressure swing adsorption or membrane separation must be applied. The low ignition temperature of methylcyclohexane further decreases the rating (rating 3). When using naphthalene (rating 3), 1,2-diyhdro-1,2-azaborine (rating 1) and phenazine (rating 1), solvents are added, which contaminate the released hydrogen stream. Again, hydrogen purification methods are inevitable. All three substances also show complications
6646
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
regarding the ignition temperature and the flash point. 1,2diyhdro-1,2-azaborine further demands a multi-step hydrogenation process and for phenazine an intermediate solvent change is necessary. Both additional demands complicate the process design. For methanol and formic acid (both rating 5), the separation of hydrogen from CO2, water and eventually CO-impurities is necessary. As only gaseous products emerge from de-hydrogenation, no by-products are accumulated, if CO2 is not reused. For a closed storage cycle, an additional purification step has to be integrated to purify the CO2 for renewed hydrogenation.
Stability Dibenzyltoluene shows a high thermal stability and a longterm catalytic activity (rating 10). Although challenges regarding the stability of toluene remain, still an operation time of the de-hydrogenation of 6,000 h has been realized (rating 8). N-ethylcarbazole lacks thermal stability for high temperatures limiting faster reactions kinetics, but its catalyst stability is high (rating 8). The stability of the storage process regarding the phenazine/tetradecahydrophenazine system is generally good; nevertheless, still minor variations in the uptake and release of hydrogen occur (rating 7). Low-temperature de-hydrogenated methanol and formic acid show both good thermal stability in the respective temperature range. The catalytic stability remains challenging in their implementation (both rating 6). Compared to that, high temperature steam reforming of methanol does not show this drawback (rating 8). The naphthalene/decalin cycle is thermally stable; but catalyst deactivation can be problematic at higher temperatures (rating 7). 1,2-dihydro-1,2-azaborine is thermal stable in the considered temperature spectrum and the de-hydrogenation proceeds clean without side-reactions, but the stability in a repeated storage cycle still has to be verified (rating 5).
Gas flow The addition of solvents and de-hydrogenation limitations reduce the gas flow. As the de-hydrogenation cannot be achieved with the pure LOHC, the parameter is only evaluated under the respective circumstances. N-ethylcarbazole (rating 10) and 1,2-dihydro-1,2-azaborine (rating 8) show the highest gas flows under ideal reaction conditions. The short reaction times below 30 min are hereby of great advantage. Toluene has a lower gas flow (rating 4). The reaction proceeds fast, but the comparatively lower density of its hydrogenated form leads to a less promising result. Dibenzyltoluene shows a longer reaction time; its gas flow is therefore reduced (rating 2). The de-hydrogenation in the naphthalene/decalin cycle take a similar amount of time. However, since a solvent must be added, gas flow is reduced (rating 2). With hightemperature steam reforming (rating 3) of methanol the gas flow is significantly higher than with low-temperature reforming (rating 1), because the dilution and the reaction time can be significantly reduced. Formic acid and phenazine exhibit very low gas flows (rating 1), which is mainly due to a high dilution rate and a long reaction time, respectively.
Technical readiness N-ethylcarbazole is well researched; nevertheless, further testing especially on toxicity and the application in the
operating environment are still needed. The technology readiness level (TRL) is 3. For dibenzyltoluene the TRL is 9 as demonstration projects have been already implemented and systems at different sizes are available. The TRL of 1,2-dihydro1,2-azaborine is 2 because challenges about a pressure and temperature induced hydrogenation as well as dehydrogenation without trimerization remain. High dilution rates is a big challenge for the implementation of the CO2/ formic acid system based on homogenous catalyst catalyzing both reactions; additionally, the process has been demonstrated on lab-scale only (TRL 3). CO2 hydrogenation to methanol is used on an industrial scale. High-temperature steam reforming is also a well-known industrial process. This combination is therefore technically ready (TRL 9). The lowtemperature reforming, however, is still on lab-scale (TRL 3). Naphthalene has a TRL of 4, since demonstration projects have not yet been realized. For toluene demonstration projects have been realized and others are on the way (TRL 8). The hydrogen storage process using phenazine as the LOHC has been demonstrated on the lab-scale, while a further evaluation is still pending (TRL 3).
Comparison The various assessment parameters (Table 11) are of different importance for the different application areas (Table 13). For mobile applications storage capacity and energy density are of great significance as they directly affect the size as well as the weight of an on-board storage system (i.e., heavy and large scale tanks decrease the possible driving range). Additionally, a low cold start time and a good dynamic behavior are also important for mobile systems. Thus, typically a Polymer Electrolyte Membrane (PEM) fuel cell is used for such applications. Ideally, the thermal energy demand of the de-hydrogenation is covered by waste heat from this PEM fuel cell. Therefore, de-hydrogenation temperature is another important parameter for mobile applications. The gas flow is an indicator for the time span between start-up and hydrogen delivery and thus crucial for the dynamic behavior of mobile systems. But also the process design is critical, as every additional requirement (e.g. hydrogen purification) add to the limited space and weight demand (Table 13). For an energy transport, the availability is a critical parameter as it strongly influences the economic feasibility; and most likely even more than the storage capacity or the energy density. Low safety and other legal requirements for handling and storage of the LOHC material as well as a low toxicity are additional important criteria for a safe and simple transport (Table 13). For (stationary) energy storage applications, the availability of the LOHC is critical. Additionally, the costs directly affect the economic feasibility of the storage system. A large scale application also entails a high mass demand of the LOHC. In addition to that, the stability of the respective LOHC material is of great importance. A high cycle stability reduces the need for LOHC replacement, which lowers the operating costs. For a high storage efficiency, a low energy demand is essential (Table 13).
6647
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Table 13 e Weighting of the individual parameters with regard to the fields of application.
Storage Availability Toxicity De-hydrogenation temperature Energy demand Material handling Process design Cycle stability Gas flow
Mobility
Energy-transport
(Stationary) energy-storage
Very important Second-rate Second-rate Very important Important Important Very important Second-rate Very important
Important Very important Very important Second-rate Second-rate Very important Second-rate Important Second-rate
Second-rate Very important Important Second-rate Very important Second-rate Important Very important Second-rate
Based on the results of the evaluation (Table 12), the various LOHCs can now be assigned to the areas of application.
Mobility Fig. 11 shows the ratings of the most relevant parameters for the application area “mobility”. The requirements of mobile systems are challenging for LOHCs. A fast de-hydrogenation being the basis for a high gas flow typically demands high temperatures. Such a temperature level, in turn, is difficult to realize based on the waste heat of a PEM fuel cell (max. 180 C). N-ethylcarbazole shows a high rating in terms of process design and gas flow. But related to the storage characteristics, especially for the gravimetric storage capacity, N-ethylcarbazole does not perform so well. However, it almost reaches the onboard DoE storage targets of 6.4 wt.-% and 1.7 kWh/L. The high de-hydrogenation temperature is the real bottleneck here; this can be reduced, but with the consequence of a lower gas flow. Toluene shows comparable results to N-ethylcarbazole, with slightly less promising results in terms of energy density and gas flow. Again, it is the de-hydrogenation temperature making a heat integration with a PEM fuel cell impossible. In addition, the necessary hydrogen purification step complicates the overall process design.
Due to the integrated nitrogen and boron atoms within the storage molecule, 1,2-dihydro-1,2-azaborine shows good characteristics for mobile applications. The low dehydrogenation temperature, which allows for a coupling of the de-hydrogenation with that waste heat of a PEM fuel cell, combined with a high gas flow, is advantageous. However, the required solvent leads to a comparatively bad rating for storage; the reduced storage densities are also well below the DoEtargets. Further, the solvent also complicates the process design for the de-hydrogenation unit due as hydrogen purification step needs to be added. Formic acid shows even in pure form comparatively low storage densities, which make a use in the field of mobility difficult, even though it has the lowest de-hydrogenation temperature. Like formic acid, methanol also meets the requirements in terms of de-hydrogenation temperatures to be coupled with the waste heat of a PEM fuel cell. Prolonged reaction times and high dilution rates prevent the use as they lead to poor rating in terms of storage design and gas flow. Higher activities of the catalyst are the key to overcome this challenges. The rating regarding storage and gas flow can significantly be increased, if the de-hydrogenation temperature is raised. In this case methanol shows the highest rating among the examined LOHC in terms of storage; only the additionally added water leads to the fact that the optimal
Fig. 11 e Rating of the different LOHC in the most relevant parameters of mobility (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET-HT: high-temperature reformed methanol, MET-LT: low-temperature reformed methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
6648
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
value is not reached. Due to the increased temperature, the de-hydrogenation can then no longer be combined with the waste heat of the PEM fuel cell. Besides that, methanol and formic acid can potentially also be used with a Direct Liquid (DL) fuel cell, which would greatly enhance the integration in the mobility sector [108]. With a such a fuel cell the dehydrogenation temperature would decrease as protons are liberated from the LOHC instead of hydrogen. Also the process design, which is more complex due to a downstream hydrogen purifications step, could be simplified if the dehydrogenation unit and the fuel cell are combined in one device [109]. Dibenzyltoluene and naphthalene are not applicable in the area of mobility as their reaction kinetics are limited.
Energy-transport Fig. 12 shows the results of the assessment step discussed above for the application field energy transport. As the availability is a crucial parameter for the use of the LOHC systems in the field of energy transport, N-ethylcarbazole is inapplicable to date. Mass production of N-ethylcarbazole could push the price down in the future, whereby the availability for this system is not an exclusionary criterion. Additionally, N-ethylcarbazole is non-toxic and easily manageable. Dibenzytoluene has a good availability, is non-toxic and can be handled easily. The technical and commercial use of dibenzytoluene in such systems is already state of technology and has been demonstrated in pilot projects. One the one hand formic acid shows a very cheap raw material price and low toxicity; on the other hand, the handling is comparatively difficult, although less precautions
need to be taken to deal with it compared to diesel fuel and gasoline. The very low storage density of formic acid has to be taken into account here, since at this extent it greatly influences the economy of the transport. Compared to formic acid, toluene is slightly worse with regard to toxicity and a bit more expensive; methanol shows the same availability but an increased toxicity potential. Both substances show significantly higher storage densities in comparison to formic acid. Phenazine and naphthalene are out due to their high toxicity potential. 1,2-dihydro-1,2-azaborine cannot be evaluated for the time being due to a lack of data.
Energy-storage Fig. 13 presents the assessment results of the most relevant parameters for the application field (stationary) energy storage. Dibenzyltoluene and toluene are qualified for this use; only their high energy demand is challenging. Coupling dehydrogenation with a high-temperature fuel cell or other available excess heat sources means that the energy requirement does not diminish the storage efficiency. Both LOHC would then be an attractive storage option. Methanol presents a valid option, if the energy demand must be covered internally through hydrogen burning, since it has the lowest demand. Formic acid and 1,2-dihydro-1,2-azaborine also have comparatively low energy demands, but the stability needs to be increased. Phenazine can potentially be produced from lignin, which can significantly reduce the manufacturing costs and by that reduce the hurdles for its entry. Due to its high energy
Fig. 12 e Rating of the different LOHC in the most relevant parameters of energy transport (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET-HT: high-temperature reformed methanol, MET-LT: low-temperature reformed methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
Fig. 13 e Rating of the different LOHC in the most relevant parameters of energy storage (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET-HT: high-temperature reformed methanol, MET-LT: low-temperature reformed methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
demand, Phenazine need to be coupled with external excessive heat sources to increase the efficiency.
Comparison to other storage options To evaluate LOHCs in the broader context of H2-storages, they are compared to compressed hydrogen gas (CGH2), liquefied hydrogen (LH2) and metal hydrides regarding their storage capacity, energy density and energy demand. Compressed hydrogen gas (CGH2) needs 4.4 kWh/kgH2 if compressed to 875 bar. The liquefaction process of liquefied hydrogen (LH2) requires 15.2 kWh/kgH2 (i.e., 46% of the LHVH2 [5]). The storage capacity and energy density of compressed hydrogen gas (CGH2) (at 700 bar) and liquefied hydrogen (LH2) including the tank is 6 wt.-% and 0.8 kWh/L and 14.4 wt.-% and 1.1 kWh/L, respectively [8]. Low-temperature metal hydrides have a practical storage capacity of around 0.9 wt.-% and an energy density of 3,8 kWh/L, for high-temperatures metal hydrides it is 2.9 wt.-% and 3.6 kWh/L, respectively. The storage and releasing process at low temperatures has an energy demand of 2 kWh/kgH2, while the process at high temperatures demands 4.9 kWh/kgH2 [8]. Compared to other energy storage systems, commercially available Lithium-Ion-Batteries currently have a volumetric energy density of 0.65 kWh/L and a gravimetric storage density of 0.25 kWh/kg [110]. The volumetric energy density and the gravimetric storage capacity of the different storage options in comparison with the pure LOHCs is shown in Fig. 14. For the LOHCs the tank system is not included. The fossil fuels diesel and gasoline are presented additionally. LOHCs can provide a good compromise between gravimetric and volumetric energy density compared to the other storage options; methanol in particular shows very high values here. The values of mineral oils are still higher than those of the possible storage systems. LOHCs are characterized by a good gravimetric storage capacity while the volumetric storage capacity is sufficient. They are in between metal hydrides and liquefied hydrogen showing a low gravimetric storage capacity and a low volumetric storage density, respectively. Methanol exceeds the other LOHC in this context. Compressed hydrogen gas (CGH2)
6649
has both a low volumetric storage density and a low gravimetric storage capacity. The energy demand of LOHC is compared to metal hydrides and compressed hydrogen gas (CGH2) for most substances higher (Fig. 15). For the LOHC only the energy demand for the de-hydrogenation is taken into account; further consumptions for hydrogen purification are estimated to 2 to 4 kWh/kgH2 and marked additionally in Fig. 15. Additionally, the amount of heat consumed for de-hydrogenation is released during hydrogenation, yet at a lower temperature level. But, the heat produced during der hydrogenation process can hardly be used; therefore, it is not included here. Methanol, formic acid and 1,2-dihydro-1,2-azaborine have small enthalpies of reactions. Thus their energy demands are in the same range as metal hydrides and compressed hydrogen gas (CGH2). Liquefied hydrogen (LH2) has by far the highest energy demand due to the energy-intensive liquefaction process.
Final considerations The overall goal of this paper is to present an extensive assessment of different LOHC in terms of important characteristic aspects. Therefore, eight different LOHC have been identified and evaluated for a potential implementation in the areas of mobility, energy-transport and energy-storage. Dibenzytoluene has yielded promising results for large scale implementation in energy-transport. Energyestorage is another conceivable area of application, if the dehydrogenation heat demand can be covered by external (excessive) heat sources. N-ethylcarbazole shows a considerable potential in both fields as well, even though its high raw material price hinders an economically feasible implementation; but mass production might significantly reduce this price in the future. Even though other LOHC have better storage values, the onboard DoE storage targets are almost met and N-ethylcarbazole has the highest gas flow among the
Fig. 14 e Comparison of volumetric and gravimetric storage density of different hydrogen storage options (CGH2: compressed hydrogen gas, LH2: liquefied hydrogen, LMH: low-temperature metal hydride, HMH: high-temperature metal hydride, (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
6650
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
activities. The high raw material price is disadvantageous, but the possible production path with lignin as the feedstock can take up this challenge. The toxicity might also be problematic as the toxicity potential indicator is higher than for diesel fuel.
Fig. 15 e Energy demand for hydrogen storage and its percentage of the lower heating value (LHV) of hydrogen (CGH2: compressed hydrogen gas, LH2: liquefied hydrogen, LMH: low-temperature metal hydride, HMH: hightemperature metal hydride, (NEC: N-ethylcarbazole, DBT: dibenzyltoluene, AB: 1,2-dihydro-1,2-azaborine, MET: methanol, NAP: naphthalene, TOL: toluene, PHE: phenazine).
substances examined here. In terms of mobile applications, it is the high de-hydrogenation temperature, which makes an implementation difficult. Before 1,2-dihydro-1,2-azaborine could be applied in any field a lot of research is still needed. Nevertheless, this substance shows potential for mobile applications. However, the need of a solvent is unattractive. To maintain the advantage of the low energy demand and low dehydrogenation temperature of BN-heterocycles, 3-Methyl1,2-BN-cyclopentan has been developed, which is liquid in the intended temperature range. Therefore, it is a promising candidate. Nevertheless, its de-hydrogenation and hydrogenation remains challenging; that is why it was not included in this study. Reducing the dilution rates for formic acid, and lowtemperature reformed methanol by researching possible highly active and stable catalysts, would drastically improve their potential in all three fields of application. If the dilution can be reduced, the high storage densities combined with the low de-hydrogenation temperature and energy demand makes methanol a particularly attractive LOHC. The release of gaseous CO2 is both an advantage and a disadvantage. If CO2 is released into the atmosphere, there is no accumulation of the unloaded LOHC. However, new starting material for hydrogenation must then be provided e.g. by a separation from ambient air. Naphthalene shows bottlenecks in numerous criteria. Furthermore, the substance is probably carcinogenic. It is doubtful if a fuel, which toxicity is more questionable than diesel will be permitted in the future by legislation. Toluene is applicable in energy-transport, although its material handling is compared to dibenzyltoluene and Nethylcarbazole more challenging. It is also an alternative for energy-storage, if the de-hydrogenation heat is covered by external (excess) heat sources. Phenazine shows good qualities for the use in energystorage, yet the requirement of different solvents for hydrogenation and de-hydrogenation are impractical for everyday application and need to be addressed by R&D
Still challenges like decreasing dilution rates, improving the stability and reducing raw material prices remain. Overcoming these obstacles can make LOHC an important player in a more sustainable and highly integrated energy system based on renewable sources of energy. Admittedly, there is still a long way to go in the field of mobility to realize lowtemperature de-hydrogenation combined with high storage densities and fast reaction kinetics. This is a big challenge, as fast kinetics usually requires high de-hydrogenation temperatures, which do not allow a coupling of the de-hydrogenation with the waste heat of a PEM fuel cell. Energy-storage and especially energy-transport are fields, where business models based on LOHC are more likely to be implemented in the near future. The use of dibenzyltoluene, high-temperature reformed methanol and toluene in these areas is already in preparation and shows the greatest potential for the time being. Although methanol, with its high storage density, has potential uses in all three application areas, it is more likely to use specific LOHCs for each application for which their properties fit best. The greatest advantage of LOHCs, which applies to all examined substances, compared to other storage options is that they have similar properties compared to crude oil based substances. The public acceptance of such fuels is far more likely than, for example, gaseous hydrogen, which is regarded as very dangerous in public opinion. Furthermore, LOHC can be used with the existing infrastructure for crude oil derivate with only little modifications, which makes their use in the field of energy-transportation appealing. In conclusion, it can be stated that LOHCs have the potential to replace fossil fuels as energy carriers and to be a critical piece of the puzzle for global energy exchange. Thus a more detailed assessment of important energetic and economic parameters of LOHC process chains could provide more deep insights into the consequences of a possible implementation of such energy transport and storage system.
Acknowledgments We would like to thank the National Organisation Hydrogen and Fuel Cell Technology for supporting this work.
references
[1] REN21. Renewables 2017. Global status report. REN21. QC, al; 2017. CA: Montre [2] BMWi. Gesetz fu¨r den ausbau erneuerbarer energien. EEG; 2017. [3] Ahmed A, Al-Amin AQ, Ambrose AF, Saidur R. Hydrogen fuel and transport system. A sustainable and environmental
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
future. Int J Hydrog Energy 2016;41(3):1369e80. https:// doi.org/10.1016/j.ijhydene.2015.11.084. Albrecht J. Tradable CO2 permits for cars and trucks. J Clean Prod 2001;9(2):179e89. https://doi.org/10.1016/S09596526(00)00069-X. Adametz P, Mu¨ller K, Arlt W. Energetic evaluation of hydrogen storage in metal hydrides. Energetic evaluation of hydrogen storage in metal hydrides. Int J Energy Res 2016;40(13):1820e31. https://doi.org/10.1002/er.3563. Jepsen J. Technical and economic evaluation of hydrogen storage systems based on light metal hydrides. 2014. p. 1e150. https://www.hzg.de/imperia/md/content/hzg/ zentrale_einrichtungen/bibliothek/berichte/hzg_reports_ 2014/hzg_report_2014_2.pdf. Zu¨ttel A. Materials for hydrogen storage. Mater Today 2003;6(9):24e33. https://doi.org/10.1016/S1369-7021(03) 00922-2. Di Profio P, Arca S, Rossi F, Filipponi M. Comparison of hydrogen hydrates with existing hydrogen storage technologies. Energetic and economic evaluations. Int J Hydrog Energy 2009;34(22):9173e80. https://doi.org/10.1016/ j.ijhydene.2009.09.056. Motyka T. Metal hydrides. DOE Materials-Based Hydrogen Storage Summit. 2015. https://www.energy.gov/sites/prod/ files/2015/02/f19/fcto_h2_storage_summit_motyka.pdf. Preuster P, Papp C, Wasserscheid P. Liquid Organic Hydrogen Carriers (LOHCs). Toward a hydrogen-free hydrogen economy. ACC Chem Res 2017;50(1):74e85. https://doi.org/10.1021/acs.accounts.6b00474. Sotoodeh F, Smith KJ. Structure sensitivity of dodecahydro-N-ethylcarbazole dehydrogenation over Pd catalysts. J Catal 2011;279(1):36e47. https://doi.org/ 10.1016/j.jcat.2010.12.022. Sotoodeh F, Smith KJ. Kinetics of hydrogen uptake and release from heteroaromatic compounds for hydrogen storage. Ind Eng Chem Res 2009;49(3):1018e26. https:// doi.org/10.1021/ie9007002. Stark K, Emel'Yanenko VN, Zhabina AA, Varfolomeev MA, Verevkin SP, Mu¨ller K, Arlt W. Liquid Organic Hydrogen Carriers. Thermophysical and thermochemical studies of carbazole partly and fully hydrogenated derivatives. Ind Eng Chem Res 2015;54(32):7953e66. https://doi.org/10.1021/ acs.iecr.5b01841. Mu¨ller K, Stark K, Emel'Yanenko VN, Varfolomeev MA, Zaitsau DH, Shoifet E, Schick C, Verevkin SP, Arlt W. Liquid Organic Hydrogen Carriers. Thermophysical and thermochemical studies of benzyl- and dibenzyl-toluene derivatives. Ind Eng Chem Res 2015;54(32):7967e76. https:// doi.org/10.1021/acs.iecr.5b01840. Biniwale R, Rayalu S, Devotta S, Ichikawa M. Chemical hydrides. A solution to high capacity hydrogen storage and supply. Int J Hydrog Energy 2008;33(1):360e5. https:// doi.org/10.1016/j.ijhydene.2007.07.028. Sotoodeh F, Huber BJM, Smith KJ. Dehydrogenation kinetics and catalysis of organic heteroaromatics for hydrogen storage. Int J Hydrog Energy 2012;37(3):2715e22. https:// doi.org/10.1016/j.ijhydene.2011.03.055. Mu¨ller K, Aslam R, Fischer A, Stark K, Wasserscheid P, Arlt W. Experimental assessment of the degree of hydrogen loading for the dibenzyl toluene based LOHC system. Int J Hydrog Energy 2016;41(47):22097e103. https://doi.org/ 10.1016/j.ijhydene.2016.09.196. Zhu Q-L, Xu Q. Liquid organic and inorganic chemical hydrides for high-capacity hydrogen storage. Energy Environ Sci 2015;8(2):478e512. https://doi.org/10.1039/ C4EE03690E. Mellmann D, Sponholz P, Junge H, Beller M. Formic acid as a hydrogen storage material e development of homogeneous
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
[35]
6651
catalysts for selective hydrogen release. Chem Soc Rev 2016;45(14):3954e88. https://doi.org/10.1039/C5CS00618J. He T, Pei Q, Chen P. Liquid Organic Hydrogen Carriers. J Energy Chem 2015;24(5):587e94. https://doi.org/10.1016/ j.jechem.2015.08.007. He T, Pachfule P, Wu H, Xu Q, Chen P. Hydrogen carriers. Nat Rev Mater 2016;1(12). https://doi.org/10.1038/ natrevmats.2016.59. Crabtree RH. Hydrogen storage in liquid organic heterocycles. Energy Environ Sci 2008;1(1):134. https:// doi.org/10.1039/b805644g. € lkl J, Arlt W. Thermodynamic evaluation of Mu¨ller K, Vo potential organic hydrogen carriers. Energy Technol 2013;1(1):20e4. https://doi.org/10.1002/ente.201200045. € ssmann A, Bru¨ckner N, Markiewicz M, Zhang YQ, Bo Thoming J, Wasserscheid P, Stolte S. Environmental and health impact assessment of Liquid Organic Hydrogen Carrier (LOHC) systems. Challenges and preliminary results. Energy Environ Sci 2015;8(3):1035e45. https://doi.org/ 10.1039/C4EE03528C. Teichmann D, Arlt W, Wasserscheid P. Liquid Organic Hydrogen Carriers as an efficient vector for the transport and storage of renewable energy. Int J Hydrog Energy 2012;37(23):18118e32. https://doi.org/10.1016/ j.ijhydene.2012.08.066. Teichmann D, Arlt W, Wasserscheid P, Freymann R. A future energy supply based on Liquid Organic Hydrogen Carriers (LOHC). Energy Environ Sci 2011;4(8):2767. https:// doi.org/10.1039/c1ee01454d. Teichmann D, Stark K, Mu¨ller K, Zittl G, Wasserscheid P, Arlt W. Energy storage in residential and commercial buildings via Liquid Organic Hydrogen Carriers (LOHC). Energy Environ Sci 2012;5(10):9044. https://doi.org/10.1039/ c2ee22070a. Mu¨ller K, Stark K, Mu¨ller B, Arlt W. Amine borane based hydrogen carriers. An evaluation. Energy Fuels 2012;26(6):3691e6. https://doi.org/10.1021/ef300516m. Eypasch M, Schimpe M, Kanwar A, Hartmann T, Herzog S, Frank T, Hamacher T. Model-based techno-economic evaluation of an electricity storage system based on Liquid Organic Hydrogen Carriers. Appl Energy 2017;185(Part 1):320e30. https://doi.org/10.1016/ j.apenergy.2016.10.068. Ahluwalia RK, Hua TQ, Peng J-K, Kromer M, Lasher S, McKenney K, Law K, Sinha J. Technical assessment of organic liquid carrier hydrogen storage systems for automotive applications. Executive summary. 2011. https://www1.eere.energy.gov/hydrogenandfuelcells/ pdfs/liquid_carrier_h2_storage.pdf. [Accessed 25 June 2018]. € tzinger C, Mu¨ller S, Mu¨ller K, Preißinger M, Adametz P, Po Lechner R, Bru¨ggemann D, Brautsch M, Arlt W. Thermodynamic evaluation and carbon footprint analysis of the application of hydrogen-based energy-storage systems in residential buildings. Energy Technol 2017;5(3):495e509. https://doi.org/10.1002/ente.201600388. Haupt A, Mu¨ller K. Integration of a LOHC storage into a heat-controlled CHP system. Energy Environ Sci 2017;118: 1123e30. https://doi.org/10.1016/j.energy.2016.10.129. Scherer GWH, Newson E. Analysis of the seasonal energy storage of hydrogen in liquid organic hydrides. Int J Hydrog Energy 1998;23(1):19e25. € smann A, Preuster P, Wasserscheid P, Arlt W, Ru¨de T, Bo Mu¨ller K. Resilience of Liquid Organic Hydrogen Carrier based energy-storage systems. Energy Technol 2018;6(3):529e39. https://doi.org/10.1002/ente.201700446. Krieger C, Mu¨ller K, Arlt W. Coupling of a Liquid Organic Hydrogen Carrier system with industrial heat. Chem Eng
6652
[36]
[37]
[38] [39]
[40]
[41]
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
[50]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
Technol 2016;39(8):1570e4. https://doi.org/10.1002/ ceat.201600180. Wang H, Zhou X, Ouyang M. Efficiency analysis of novel Liquid Organic Hydrogen Carrier technology and comparison with high pressure storage pathway. Int J Hydrog Energy 2016;41(40):18062e71. https://doi.org/ 10.1016/j.ijhydene.2016.08.003. Department of Energy. Technical system targets: onboard hydrogen storage for light -duty fuel cell vehicles. 2017. https://www.energy.gov/sites/prod/files/2017/05/f34/fcto_ myrdd_table_onboard_h2_storage_systems_doe_targets_ ldv_1.pdf. [Accessed 8 June 2018]. Alibaba: alibaba.com. https://www.alibaba.com/. [Accessed 19 June 2018]. Fraunhofer IZM. Environmental evaluation methods. Toxic potential indicator (TPI). 2018. https://www.izm.fraunhofer. de/en/abteilungen/environmental_reliabilityengineering/ key_research_areas/environmental_assessmentandecodesign/toxic-potential-indicator–tpi-.html. [Accessed 19 June 2018]. NASA. Technology readiness level. 2012. https://www.nasa. gov/directorates/heo/scan/engineering/technology/txt_ accordion1.html. Hydrogenious Technologies. Hydrogen - stored as an oil. 2018. http://www.energiewende-erlangen.de/wp-content/ uploads/2018/02/0_HydrogeniousTechnologies.pdf. [Accessed 8 June 2018]. Hodoshima S, Takaiwa S, Shono A, Satoh K, Saito Y. Hydrogen storage by decalin/naphthalene pair and hydrogen supply to fuel cells by use of superheated liquidfilm-type catalysis. Appl Catal Gen 2005;283(1e2):235e42. https://doi.org/10.1016/j.apcata.2005.01.010. Mitsui & Co. The world's first global hydrogen supply chain demonstration project. 2017. https://www.mitsui.com/jp/ en/release/2017/1224164_10832.html. [Accessed 8 June 2018]. Forberg D, Schwob T, Zaheer M, Friedrich M, Miyajima N, Kempe R. Single-catalyst high-weight% hydrogen storage in an N-heterocycle synthesized from lignin hydrogenolysis products and ammonia. Nat Commun 2016;7:13201. https:// doi.org/10.1038/ncomms13201. € smann A, Teichmann D, Arlt W, Bru¨ckner N, Obesser K, Bo Dungs J, Wasserscheid P. Evaluation of industrially applied heat-transfer fluids as Liquid Organic Hydrogen Carrier systems. Chem Sustain 2013;7(1):229e35. https://doi.org/ 10.1002/cssc.201300426. Luo W, Campbell PG, Zahkarov LN, Liu S-Y. A singlecomponent liquid-phase hydrogen storage. Material J Am Chem Soc 2011;133(48). https://doi.org/10.1021/ja208834v. 29326-19329. Hu P, Fogler E, Diskin-Posner Y, Iron MA, Milstein D. A novel Liquid Organic Hydrogen Carrier system based on catalytic peptide formation and hydrogenation. Nat Commun 2015;6:6859. https://doi.org/10.1038/ncomms7859. Sasaki K, Li H-W, Hayashi A, Yamabe J, Ogura T, Lyth SM, editors. Hydrogen energy engineering. A japenese perspective. Green energy and technology. Springer Japan; 2016. Yamaguchi R, Ikeda C, Takahashi Y, Fujita K-i. Homogeneous catalytic system for reversible DehydrogenationHydrogenation reactions of nitrogen heterocycles with reversible interconversion of catalytic species. J Am Chem Soc 2009;131(24):8410e2. https:// doi.org/10.1021/ja9022623. Cui Y, Kwok S, Bucholtz A, Davis B, Whitney RA, Jessop PG. The effect of substitution on the utility of piperidines and octahydroindoles for reversible hydrogen storage. New J Chem 2008;32(6):1027. https://doi.org/10.1039/b718209k.
[51] Hiyoshi N, Rode CV, Sato O, Shirai M. Biphenyl hydrogenation over supported transition metal catalysts under supercritical carbon dioxide solvent. Appl Catal Gen 2005;288(1e2):43e7. https://doi.org/10.1016/j.apcata. 2005.04.021. [52] Chen J, Wu H, Wu G, Xiong Z, Wang R, Fan H, Zhou W, Liu B, Chua Y, Ju X, Chen P. Lithiated primary amine-A new material for hydrogen storage. Chem Eur J 2014;20(22): 6632e5. https://doi.org/10.1002/chem.201402543. [53] Wang Z, Belli J, Jensen CM. Homogeneous dehydrogenation of Liquid Organic Hydrogen Carriers catalyzed by an iridium PCP complex. Faraday Discuss 2011;151:297. https://doi.org/ 10.1039/c1fd00002k. [54] Fujita K-i, Tanaka Y, Kobayashi M, Yamaguchi R. Homogeneous perdehydrogenation and perhydrogenation of fused bicyclic N-heterocycles catalyzed by iridium complexes bearing a functional bipyridonate ligand. J Am Chem Soc 2014;136:4829e32. [55] Dong Y, Yang M, Yang Z, Ke H, Cheng H. Catalytic hydrogenation and dehydrogenation of N-ethylindole as a new heteroaromatic Liquid Organic Hydrogen Carrier. Int J Hydrog Energy 2015;40(34):10918e22. https://doi.org/ 10.1016/j.ijhydene.2015.05.196. [56] Yang M, Dong Y, Fei S, Pan Q, Ni G, Han C, Ke H, Fang Q, Cheng H. Hydrogenation of N-propylcarbazole over supported ruthenium as a new prototype of Liquid Organic Hydrogen Carriers (LOHC). RSC Adv 2013;3(47):24877. https://doi.org/10.1039/c3ra44760j. [57] Pez, G., Scott, A., Cooper, A., Cheng, H. Hydrogen storage by reversible hydrogenation of pi-conjugated substrates. USA Patent US20050002857A1; 2006. [58] IFA. GESTIS-stoffdatenbank. Gefahrstoffinformationssystem der Deutschen gesetzlichen unfallversicherung. 2018. https://www.dguv.de/ifa%3B/gestis/gestis-stoffdatenbank/ index.jsp. [Accessed 19 June 2018]. [59] Teichmann D. Konzeption und Bewertung einer nachhaltigen Energieversorgung auf Basis flu¨ssiger € ger (LOHC). Dissertation. FriedrichWasserstofftra € t; 2014. Alexander-Universita [60] Eypasch M. Wasserstoffspeicherung in LOHC-Systemen als Basis fu¨r industrielle Energiespeicheranwendungen. € t; 2016. http:// Dissertation, Friedrich-Alexander-Universita www.shaker.de/Online-Gesamtkatalog-Download/2018.05. 02-13.44.00-134.28.31.128-radC6F02.tmp/3-8440-4946-0_ INH.PDF. [61] Yang M, Dong Y, Fei S, Ke H, Cheng H. A comparative study of catalytic dehydrogenation of perhydro-N-ethylcarbazole over noble metal catalysts. Int J Hydrog Energy 2014;39(33):18976e83. https://doi.org/10.1016/j.ijhydene. 2014.09.123. [62] Arlt W. Speicherung elektrischer energie. Chemische € ge; speicherung elektrischer energie. Mu¨nchen: VDI-Vortra 2014. http://www.verein-der-ingenieure.de/fileadmin/ mediapool_vdi/Landesverband/doc/Arlt-5.5.2014.pdf. [Accessed 8 June 2018]. [63] Luo W, Zakharov LN, Liu S-Y. 1,2-BN cyclohexane. Synthesis, structure, dynamics, and reactivity. J Am Chem Soc 2011;133(33):13006e9. https://doi.org/10.1021/ ja206497x. [64] RSC. ChemSpider. Search and share chemistry. 2018. https://www.chemspider.com. [65] Campbell PG, Zakharov LN, Grant DJ, Dixon DA, Liu S-Y. Hydrogen storage by BoronNitrogen heterocycles. A simple route for spent fuel regeneration. J Am Chem Soc 2010;132(10):3289e91. https://doi.org/10.1021/ja9106622. [66] Liu S-Y. Hydrogen storage by novel CBN heterocycle materials. Final Report. 2012. https://www.osti.gov/servlets/ purl/1221989.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
[67] Payne Jr DA, Eads EA. Boron-nitrogen heterocycles. J Chem Educ 1964;41(6):334. [68] Mikulas TC. Electronic structure investigations of titanium oxide nanoclusters, boron-nitrogen heterocycles, and reaction products of lanthanides with oxygen difluoride and lanthanides with water. Dissertation. University of Alabama; 2015. http://search.proquest.com/openview/ 8aba150f27e07b8c9d2894a2a518c034/1?pq-origsite¼ gscholar&cbl¼18750&diss¼y. [Accessed 15 September 2017]. [69] Maenaka Y, Suenobu T, Fukuzumi S. Catalytic interconversion between hydrogen and formic acid at ambient temperature and pressure. Energy Environ Sci 2012;5(6):7360. https://doi.org/10.1039/c2ee03315a. [70] Hull JF, Himeda Y, Wang W-H, Hashiguchi B, Periana R, Szalda DJ, Muckerman JT, Fujita E. Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and pressures. Nat Chem 2012;4(5):383e8. https://doi.org/10.1038/ nchem.1295. [71] Hsu S-F, Rommel S, Eversfield P, Muller K, Klemm E, Thiel WR, Plietker B. A rechargeable hydrogen battery based on Ru catalysis. Angew Chem 2014;53(27):7074e8. https:// doi.org/10.1002/anie.201310972. [72] Filonenko GA, van Putten R, Schulpen EN, Hensen EJM, Pidko EA. Highly efficient reversible hydrogenation of carbon dioxide to formates using a ruthenium PNP-pincer catalyst. ChemCatChem 2014;6(6):1526e30. https://doi.org/ 10.1002/cctc.201402119. [73] Mu¨ller K, Brooks K, Autrey T. Hydrogen storage in formic acid: a comparison of process options. Energy Fuels 2017;31(11):12603e11. https://doi.org/10.1021/acs. energyfuels.7b02997. [74] Bi Q-Y, Lin J-D, Liu Y-M, Du X-L, Wang J-Q, He H-Y, Cao Y. An aqueous rechargeable formate-based hydrogen battery driven by heterogeneous Pd catalysis. Angew Chem 2014;53(49):13583e7. https://doi.org/10.1002/anie.201409500. € hrungsrechner: dollar - euro (USD [75] finanzen.net GmbH. Wa in EUR). 2018. https://www.finanzen.net/ waehrungsrechner/us-dollar_euro. [Accessed 5 July 2018]. [76] Markussen P, Austell J, Hustad C. A CO2-infrastructure for eor in the north sea (cens). Macroeconomic implications for host countries. In: Greenhouse gas control technologies 6th internation conference(2); 2003. p. 1077e82. [77] Eppinger J, Huang K-W. Formic acid as a hydrogen energy carrier. ACS Energy Lett 2016;2(1):188e95. https://doi.org/ 10.1021/acsenergylett.6b00574. [78] Formic acid, vol. 5; 2016. Ullmann's Encyclopedia of Industrial Chemistry, 1. [79] Toyir J, Miloua R, Elkadri NE, Nawdali M, Toufik H, Miloua F, Saito M. Sustainable process for the production of methanol from CO2 and H2 using Cu/ZnO-based multicomponent catalyst. Phys Proc 2009;2(3):1075e9. https://doi.org/10.1016/ j.phpro.2009.11.065. [80] Jadhav SG, Vaidya PD, Bhanage BM, Joshi JB. Catalytic carbon dioxide hydrogenation to methanol. A review of recent studies. Chem Eng Res Des 2014;92(11):2557e67. https://doi.org/10.1016/j.cherd.2014.03.005. [81] Saito M, Murata K. Development of high performance Cu/ ZnO-based catalysts for methanol synthesis and the watergas shift reaction. Catal Surv Asia 2004;8(4):285e94. [82] Yang C, Ma Z, Zhao N, Wei W, Hu T, Sun Y. Methanol synthesis from CO2-rich syngas over a ZrO2 doped CuZnO catalyst. Catal Today 2006;115(1e4):222e7. https://doi.org/ 10.1016/j.cattod.2006.02.077. [83] Kundu A, Shul YG, Kim DH. Methanol reforming processes. In: Zhao TS, Kreuer K-D, van Nguyen T, editors. Advances in fuel cells. Advances in fuel cells, vol. 1. Amsterdam, San Diego, CA, Oxford: Elsevier; 2007. p. 419e72.
6653
[84] Zhang C, Yuan Z, Liu N, Wang S. Study of catalysts for hydrogen production by the high temperature steam reforming of methanol. Fuel Cells 2006;6(6):466e71. https:// doi.org/10.1002/fuce.200600010. [85] Nielsen M, Alberico E, Baumann W, Drexler H-J, Junge H, Gladiali S, Beller M. Low-temperature aqueous-phase methanol dehydrogenation to hydrogen and carbon dioxide. Nature 2013;495(7439):85e9. https://doi.org/ 10.1038/nature11891. [86] Monney A, Barsch E, Sponholz P, Junge H, Ludwig R, Beller M. Base-free hydrogen generation from methanol using a Bi-catalytic system. Chem Commun 2014;50(6):707e9. https://doi.org/10.1039/C3CC47306F. [87] Rodrı´guez-Lugo RE, Trincado M, Vogt M, Tewes F, SantisoQuinones G, Gru¨tzmacher H. A homogeneous transition metal complex for clean hydrogen production from methanolewater mixtures. Nat Chem 2013;5(4):342e7. https://doi.org/10.1038/nchem.1595. [88] Fujita K-i, Kawahara R, Aikawa T, Yamaguchi R. Hydrogen production from a methanol-water solution catalyzed by an anionic iridium complex bearing a functional bipyridonate ligand under weakly basic conditions. Angew Chem 2015;127(31):9185e8. https://doi.org/10.1002/ ange.201502194. [89] Stability of methanol, propanol, and SF6 as hightemperature tracers. In: Adams MC, Yamada Y, Yagi M, Kondo T, Wada T, editors. Proceeding World Geothermal Congress; 2000. [90] Carbon recycling international: world's largest CO2 methanol plant. 2015. http://carbonrecycling.is/georgeolah/. [Accessed 8 June 2018]. [91] Park K-C, Yim D-J, Ihm S-K. Characteristics of Al-MCM-41 supported Pt catalysts. Effect of Al distribution in Al-MCM41 on its catalytic activity in naphthalene hydrogenation. Catal Today 2002;74(3):281e90. [92] Alibaba. Preis fu¨r Naphthalene. 2018. https://www.alibaba. com/product-detail/Refined-naphthalene-CAS-NO-91-20_ 60722181200.html?spm¼a2700.7724838.2017115.29. 3b49557eoq81ev&s¼p. [Accessed 2 May 2018]. [93] Shinohara C. Local structure around platinum in Pt/C catalysts employed for liquid-phase dehydrogenation of decalin in the liquid-film state under reactive distillation conditions. Appl Catal Gen 2004;266(2):251e5. [94] Recent advances in basic and applied aspects of industrial catalysis. In: Prasada Rao TSR, Dhar GM, India CSo, editors. Proceedings of 13th national symposium and silver jubilee symposium of catalysis of India, Dehradun, India, April 2e4, 1997. Studies in surface science and catalysis. Elsevier; 1998. [95] Hodoshima S, Arai H, Saito Y. Liquid- lm-type catalytic decalin dehydrogeno-aromatization for long-term storage and long-distance transportation of hydrogen. Int J Hydrog Energy 2003;28(2):197e204. [96] Thomas K, Binet C, Chevreau T, Cornet D, Gilson J-P. Hydrogenation of toluene over supported Pt and Pd catalysts. Influence of structural factors on the sulfur tolerance. J Catal 2002;212(1):63e75. https://doi.org/10.1006/ jcat.2002.3780. [97] Shuwa SM, Jibril BY, Al-Hajri RS. Hydrogenation of toluene on Ni-Co-Mo supported zeolite catalysts. Nig J Tech 2018;36(4):1114. https://doi.org/10.4314/njt.v36i4.17. [98] Hatim MI, Fazara MU, Syarhabil AM, Riduwan F. Catalytic dehydrogenation of methylcyclohexane (MCH) to toluene in a palladium/alumina hollow fibre membrane reactor. Proc Eng 2013;53:71e80. https://doi.org/10.1016/ j.proeng.2013.02.012. € Ni/Al2O3 catalysts and their activity in [99] Yolcular S, Olgun O. dehydrogenation of methylcyclohexane for hydrogen
6654
[100]
[101]
[102]
[103]
[104]
[105]
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 6 6 3 1 e6 6 5 4
production. Catal Today 2008;138(3e4):198e202. https:// doi.org/10.1016/j.cattod.2008.07.020. Zhang L, Xu G, An Y, Chen C, Wang Q. Dehydrogenation of methyl-cyclohexane under multiphase reaction conditions. Int J Hydrog Energy 2006;31(15):2250e5. https://doi.org/ 10.1016/j.ijhydene.2006.02.001. Okada Y, Sasaki E, Watanabe E, Hyodo S, Nishijima H. Development of dehydrogenation catalyst for hydrogen generation in organic chemical hydride method. Int J Hydrog Energy 2006;31(10):1348e56. https://doi.org/10.1016/ j.ijhydene.2005.11.014. Intratec. Average toluene price. 2007-2010. 2017. https:// www.intratec.us/chemical-markets/toluene-price. [Accessed 8 June 2018]. Alhumaidan F, Cresswell D, Garforth A. Hydrogen storage in liquid organic hydride. Producing hydrogen catalytically from methylcyclohexane. Energy Fuels 2011;25(10):4217e34. https://doi.org/10.1021/ef200829x. Gru¨nenfelder NF, Schucan TH. Seasonal storage of hydrogen in liquid organic hydrides. Description of the second prototype vehicle. Int J Hydrog Energy 1989;14(8):579e86. Emori H, Kawamura T, Ishikawa T, Takamura T. Carbonhydride energy storage system. Hitachi Rev 2013:192e8.
[106]
[107]
[108]
[109]
[110]
https://www.scopus.com/record/display.uri?eid¼2-s2.084877081726&origin¼inward&txGid¼7aba1c692b5aa 933ecd745dc5d69fe59. Okada Y, Shimura M. Development of large-scale H2 storage and transportation technology with Liquid Organic Hydrogen Carrier (LOHC). In: The 21st joint GCC-Japan environment symposium; 2013. https://www.jccp.or.jp/ international/conference/docs/15rev-chiyoda-mr-shimurachiyoda-h2-sturage-and-transpor.pdf. Alibaba. Preis fu¨r Phenazine. 2018. https://www.alibaba. com/product-detail/Phenazine-92-82-0_1185968472.html? spm¼a2700.7724857.main07.82.7c38c74dXa0use. [Accessed 2 May 2018]. Ong BC, Kamarudin SK, Basri S. Direct liquid fuel cells. A review. Int J Hydrog Energy 2017;42(15):10142e57. https:// doi.org/10.1016/j.ijhydene.2017.01.117. Joghee P, Malik JN, Pylypenko S, O'Hayre R. A review on direct methanol fuel cells e in the perspective of energy and sustainability. MRS Energy Sustain 2015;2. https://doi.org/ 10.1557/mre.2015.4. Manthiram A. An outlook on Lithium ion battery technology. ACS Cent Sci 2017;3(10):1063e9. https://doi.org/ 10.1021/acscentsci.7b00288.