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Biohydrogen purification using metal hydride technologies
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Biohydrogen purification using metal hydride technologies D. Dunikov a,b,*, V. Borzenko a, D. Blinov a, A. Kazakov a, C.-Y. Lin c, S.-Y. Wu c, C.-Y. Chu c a
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia National Research University “Moscow Power Engineering Institute”, Moscow, Russia c Green Energy Development Center, Feng Chia University, Taichung 40724, Taiwan b
article info
abstract
Article history:
Metal hydrides are known for their ability of selective hydrogen absorption and might be
Received 31 March 2016
used for hydrogen purification. We demonstrate separation of hydrogen/carbon dioxide
Received in revised form
mixtures with the use of two AB5-type alloys. A metal hydride reactor was filled with 1 kg of
24 July 2016
“high-pressure” alloy La0.9Ce0.1Ni5 (Peq ¼ 1.96 bar at 293 K) and 1 kg of “low-pressure” alloy
Accepted 1 August 2016
LaNi4.8Mn0.3Fe0.1 (Peq ¼ 0.38 bar at 293 K), maximum H2 capacity is 140 st.L, nominal
Available online xxx
operating H2 capacity is 110 st.L. Hydrogen concentration was in the range 40e60 vol.%, feed pressure 5.6 bar. Separation efficiency and hydrogen recovery depend on equilibrium
Keywords:
pressure of absorption, which has to be as low as possible to increase hydrogen recovery.
Metal hydride
The purification rate of 81 st.L/h from a mixture containing 59 vol.% of hydrogen with
Hydrogen purification
recovery 94% was achieved for the “low-pressure” alloy. The results show that metal hy-
Hydrogen storage
dride H2/CO2 separation unit can be a second stage of a biohydrogen upgrade system after a
Biohydrogen
membrane module. Polymer membranes can defend metal hydrides from poisonous im-
Dark fermentation
purities and high selectivity of metal hydrides can improve the overall performance of the purification system. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A problem of decentralized energy supply and efficient processing of organic wastes is actual for highly populated regions with scarce resources as well as for vast poor populated or remote territories, which can be found both in Russia, where up to 20 million people live outside the Unified Power System and another 20 million live in the Moscow agglomeration, and Taiwan, where highly populated regions have difficulties with efficient processing of organic wastes.
Significant increase of fossil fuel consumption and waste production rises concerns about protection of environment and leads to research and development of new green technologies. Hydrogen is a universal clean and efficient secondary energy carrier with the highest energy value, it can be used for energy storage and transportation and converted into electricity via fuel cells with high efficiency without pollution. Russia recognizes “Novel and renewable energy sources, including hydrogen energy” as a Critical Technology in “Energy efficiency and energy saving”, which is one of the 8 top
* Corresponding author. Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia. E-mail address: [email protected] (D. Dunikov). http://dx.doi.org/10.1016/j.ijhydene.2016.08.190 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Dunikov D, et al., Biohydrogen purification using metal hydride technologies, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.190
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priority science and technology development directions in Russia. Distributed generation becomes a significant part of Russian energy sector [1]. Biologically produced hydrogen generally referred as biohydrogen is considered as a perspective renewable energy source, and sustainable and competitive development can be reached by valorization of wastes and effluents using an emerging biorefinery approach [2]. Fermentative hydrogen production from biomass can provide clean and sustainable energy supply with the additional advantage of processing biomass waste and wastewater [3,4] of different origin. Practical application and commercialization of biohydrogen technologies requires a gradual transition from a basic research of biohydrogen production in laboratory conditions with clean substrates (glucose, sucrose) to a development of large-scale pilot bioreactors for hydrogen production with the use of various organic wastes [5]. Different substrate pretreatment techniques [6e8], new types of bioreactors [9,10] were used to improve biohydrogen production. Chu et al. [6,10] reported high hydrogen production rates up to 16.32 st.L H2/L/day in dark fermentation from rice straw hydrolysate. Chen et al. [11] announced hydrogen production rate up to 26 st.L H2/L/day from starch hydrolysate by Clostridium species. Sivagurunathan et al. reported peak hydrogen production rates of 37.5 st.L H2/L/day [12] and 55 st.L H2/L/day [13] from beverage industry wastewater. There are some reports of pilot scale biohydrogen production systems principally operating with different type of wastes with fairly high hydrogen production rates [14e18]. Several technical obstacles prevent widespread usage of biohydrogen [19], and the main are: uncertain hydrogen yield from various feedstocks, especially wastewater; heat management of bioreactors; low purity (high content of CO2, even more than 50 vol.%); low partial pressure of hydrogen (less than 1 bar). As pointed in Ref. [20] classical dark fermentation produces a complex gaseous mixture of 35e65 vol.%. H2, CO2 and minor impurities (e.g. H2S, N2, water vapor, etc.) and has to be upgraded. Moreover, since reduced hydrogen partial pressure conditions intensify H2 production, one of the strategies to improve hydrogen production is sparging with N2 or CO2 [21], thus increasing the necessity of biohydrogen purification. It is estimated that separation and purification equipment contributes at least 50% and sometimes up to 80% of the capital investment costs [22] and consumes significant part of the total in-plant energy expenditures. Industrial solutions, especially cryogenic purification and pressure swing adsorption (PSA), are designed primarily for large-scale hydrogen production and cannot be used without modification for upgrade of relatively small quantities of biohydrogen to levels of purity required for a PEM fuel cell operation (>99.95 vol.%). Indeed, cryogenic and membrane processes normally produce hydrogen at 90e98 vol.%, whereas the PSA process normally produces hydrogen at 99þ vol.%, on the other hand for most economically viable PSA process feed streams have to be already compressed at 15e30 bar and contain 75e90 vol.% hydrogen [23]. Several
studies for downscaling of PSA hydrogen purification were reported recently [24,25], also some emerging membrane technologies have been reported [26], but polymer membranes suffer from low H2/CO2 selectivity. Use of highly selective metallic membranes for biohydrogen purification is limited by high operating temperatures, expensive components (platinum, palladium) and degradation upon contact with the impurity gases [27]. Separation of H2/CO2 gas mixtures obtained by bioconversion of organic wastes into hydrogen with the aid of active membrane systems (membrane contactors) with moving liquid carriers was proposed [28], and H2 recovery can be successfully realized as combination of standard membrane method (H2 preconcentrating) and PSA (H2 conditioning), though improving of the whole process requires the development of high selective membranes [29]. Metal hydrides are known for their ability of selective hydrogen absorption and might be used for hydrogen purification [30]. It is possible to successfully separate hydrogen e carbon dioxide mixtures with the use of metal hydrides [19,31]. The scheme of metal hydride hydrogen purification method is presented in Fig. 1. Mixture is fed to a metal hydride reactor, and hydrogen is selectively absorbed by alloy bed with formation of metal hydride, reaction heat is transferred to a cooling liquid. Absorption is stopped after hydrogen breakthrough. Purified hydrogen can be obtained by desorption, outflow hydrogen pressure can be controlled by heating liquid temperature. A number of fundamental investigations of impurity gas interactions with metal hydrides were undertaken since 1970s [32e37]. Some of the developments were implemented on industrial scale for hydrogen extraction from ammonia production [38,39]. Feasibility of metal hydride applications for hydrogen extraction from gas mixtures containing CO2 were shown in Refs. [40e42]. Impurities has different influence on the performance of metal hydride devices [43], depending on the alloyeimpurity combination, hydrogen storage properties can deteriorate as a result of various types of damages [44] such as poisoning (H2S), retardation and corrosion (water vapor, CO2) and inert gas blanketing (N2).
Fig. 1 e Principle of metal hydride flow-through purification technique.
Please cite this article in press as: Dunikov D, et al., Biohydrogen purification using metal hydride technologies, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.190
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 x x x ( 2 0 1 6 ) 1 e8
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Fig. 2 e Experimental test bench for hydrogen purification and storage.
AB5-type metal hydrides could be perspective solution for hydrogen purification and compression for its utilization in fuel cells. The near-ambient PCT-properties (temperature and pressure ranges 0e100 C and 1e10 bar respectively), ease of activation without any heating, absence of protective oxide layers, possibility to regenerate H2 capacity after interaction with H2O or CO2 by mild heating in hydrogen atmosphere are distinctive features of AB5 metal hydrides [45] that would be advantages for biohydrogen purification and initial compression. Current state of research and development indicates a possibility of practical use and commercialization of a biohydrogen fuel cell power supply systems. On the other hand, conventional technical solutions do not allow to effectively upgrade and use biohydrogen produced by microorganisms in practice. The objective of this study is to experimentally investigate a performance of a metal hydride reactor during separation of H2/CO2 mixture in different operational conditions. Two samples of AB5-intermetallic compounds with different equilibrium absorption pressures have been used for purification of mixtures with hydrogen content varying from 40 to 59 vol.% and different feed rates. Impact of the separation performance on hydrogen recovery ratio has been investigated, and further research needs have been discussed.
Experimental procedure Experimental investigations on hydrogen separation from different H2/CO2 gas mixtures were conducted on the experimental test bench 12-04 JIHT RAS. The experiment scheme is presented in Fig. 2 and consists of gas ramp for preparation of mixtures (1), flow-through metal hydride reactor RSP-8 (2), vacuum pump (3), gas analyzer (4), pressure reducer (PR), pressure sensors (PS), gas flow meters (FM), valves (V). The gas flow at the inlet/outlet valve of the reactor was controlled and measured by a Bronkhorst mass flow meter/ controller, the pressure inside the reactor and the gas supply were measured by Aplisens pressure transmitters model PC28, concentration of hydrogen was measured by gas analyzer AG 0012, the water temperature was measured by thin film platinum sensors Heraeus M422, 1 kU. The experiments was controlled using LabView software with 1 Hz discretization. The RSP-8 (Reactor for Storage and Purification) metal hydride reactor is presented in Fig. 3. The reactor has tubular design, inner and outer tubes form two liquid heat exchangers with an annular reaction chamber between them. The reaction chamber is filled with a metal hydride bed and has inlet and outlet tubes at the ends so gas can flow through the bed. Several samples of AB5 type alloys are used in experiments
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Fig. 5 e PCT-isotherms for LaNi4.8Mn0.3Fe0.1.
Fig. 3 e RPS-8 metal hydride reactor.
with bed weight 1 kg, maximum H2 capacity is Vmax ¼ 140 st.L, nominal operating H2 capacity is Vn ¼ 110 st.L, purity grade at desorption is up to 99.999%. A base type of RSP-8 contains two water heat exchangers (inner and outer), in an experimental type RSP-8e the outer heat exchanger is removed to install additional temperature sensors. Two types of alloys were used: alloy with relatively high equilibrium pressure La0.9Ce0.1Ni5 (HP-alloy) and with relatively low equilibrium pressure LaNi4.8Mn0.3Fe0.1 (LP-alloy). The samples were prepared by arc melting, mechanically crushed into 1e5 mm particles and loaded into reactors. Before experiments, metal hydride beds were activated by heating in hydrogen atmosphere and cycled for 10 cycles or more. PCT-isotherms were measured by modified Sievert's method as described in Ref. [46] and presented in Figs. 4 and 5.
Equilibrium pressures and thermodynamic parameters are summarized in Table 1. Model hydrogen e carbon dioxide mixture or biohydrogen is fed into a reactor from a top of the reactor, which is cooled to remove heat of a hydride formation. If cooling power is sufficient a hydriding front moves slowly through the metal hydride bed, hydrogen is absorbed and only CO2 leaves the reactor from the bottom. As reactor is filled with hydrogen, cooling power becomes insufficient, so hydrogen breaks through the bed and gets lost. For any cooling power there is a critical mixture flow rate corresponding to hydrogen breakthrough. Preparation of gas mixtures is carried out by partial pressures technique based on Dalton's law. A gas cylinder is vacuumed to P ¼ 105 MPa and consequently filled with gases up to needed partial pressures. The mixture is maintained at least 72 h until equilibration of concentrations inside the cylinder, and concentration variation does not exceed 0.5% during the experiments. Chromatographic analysis of the resulting gas mixture is carried out before the experiments. During a charging step, pure hydrogen or simulated gas mixtures H2 þ CO2 (H2 content between 40 and 60 vol.%) are fed into inlet tube (V1 of Fig. 2 is open) and inlet gas flow rate is controlled by the flow meter (FMin). Simultaneously, outlet valve (V2) is opened and output gas flow rate is also controlled by the flow meter (FMout). Operating inlet pressure is 0.56 MPa, outlet is connected to atmosphere. At a discharge, hydrogen desorption is stimulated by hot water, gas flow rate is controlled by the flow meter (FMout) and hydrogen is fed to a Hoppecke H2 Power 200 PEM fuel cell.
Results and discussion
Fig. 4 e PCT-isotherms for La0.9Ce0.1Ni5.
Test run with pure hydrogen was conducted for La0.9Ce0.1Ni5. Inlet flow rate was 3 st.L/min and outlet flow rate was 1.5 st.L/ min. Metal hydride reactor was charged to nominal capacity Vn in 50 min, inlet and outlet flow rates are presented in Fig. 6. The reactor can be charged to nominal capacity with constant flow rate, part of the hydrogen stream breaks through the metal hydride bed and leaves the reactor. While the reactor is
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Table 1 e MH alloys hydrogen sorption properties. MH alloy La0.9Ce0.1Ni5 LaNi4.8Mn0.3Fe0.1
Temperature, K
С(Н2)max, wt.%
Peq, bar
DHdesо, kJ/mole
DSdesо, J/mole$K
293 293
1.3 1.3
1.96 0.38
30.7 ± 0.2 34.0 ± 0.3
110 ± 2 108 ± 2
charged over the nominal capacity, equilibrium pressure rises and gets close to inlet hydrogen pressure, thus absorption reaction slows down causing inlet hydrogen flow rate to fall down. To investigate influence of alloy equilibrium pressure on performance of the reactor, the two experiments were carried out for the HP-alloy La0.9Ce0.1Ni5 and the LP-alloy LaNi4.8Mn0.3Fe0.1. Initial conditions for the HP-alloy: feed H2 (46%) þ CO2 (54%) at 0.56 MPa, cooling water at 10.5 C, for the LP-alloy feed H2 (45%) þ CO2 (55%) at 0.56 MPa, cooling water at 21.5 C. Hydrogen recovery during the purification process is determined by the following equation: hðV=Vn Þ ¼ 1
Vout Vin
(1)
where, V is the volume of absorbed hydrogen, Vn is the nominal capacity of the reactor, Vout is the volume of hydrogen in purged stream, Vin is the volume of hydrogen in feed stream. To estimate purification efficiency, the hydrogen recovery is plotted as a function of state of charge of the reactor V/Vn. Flow rates at inlet and outlet and hydrogen recovery are presented in Fig. 7 for HP-alloy and in Fig. 8 for LP-alloy. For the HP-alloy equilibrium pressure during the experiment is close to the partial pressure of hydrogen in the feed stream, thus the reactor cannot maintain a constant inlet flow rate due to a slow absorption rate and a lot of hydrogen is lost. On the other hand, the equilibrium pressure for LP-alloy is lower than the partial pressure of hydrogen in the feed stream and the absorption rate is high enough to maintain the constant flow rate at inlet, and mixture at outlet contains a little amount of hydrogen, and hydrogen recovery is higher than 90% almost up to 60% of nominal charge of the reactor.
Fig. 6 e Inlet and outlet hydrogen flow rates at test run.
Fig. 7 e Inlet and outlet mixture flow rates and hydrogen recovery ratio for experiment with HP-alloy La0.9Ce0.1Ni5, feed H2 (46%) þ CO2 (54%) at 0.56 MPa, cooling water temperature of 10.5 C.
These results show that equilibrium pressures of alloys for biohydrogen purification have to be as low as possible to avoid preliminary compression of feed stream. Influence of gas mixture composition and feed flow rate on the recovery ratio was investigated in series of experiments with LaNi4.8Mn0.3Fe0.1 alloy. The experimental results are collected in Table 2. Our experiments show that the reactor
Fig. 8 e Inlet and outlet mixture flow rates and hydrogen recovery ratio for experiment with LP-alloy LaNi4.8Mn0.3Fe0.1, feed H2 (45%) þ CO2 (55%) at 0.56 MPa, cooling water temperature of 21.5 C.
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Table 2 e Experimental results for hydrogen purification with LaNi4.8Mn0.3Fe0.1 alloy. Experiment
A
Hydrogen in feed, vol.% Inlet/outlet flow rate set point, st.L/min Time, h 0.5
1
1.5
2
2.5
3
C
D
59
55
55
40
20/5
6/3
3/1.5
10/5
Parameter State of charge V/Vn Integral flow rate Q, st.L/h Recovery h V/Vn Q, st.L/h h V/Vn Q, st.L/h h V/Vn Q, st.L/h h V/Vn Q, st.L/h h V/Vn Q, st.L/h h
B
Results 0.37 81 94% 0.61 67 89% 0.77 56 84% 0.87 48 79% 0.92 40 73% 0.94 34 68%
0.30 66 97% 0.52 57 93% 0.69 51 89% 0.81 45 85% 0.88 39 80% 0.92 34 75%
0.30 66 97% 0.52 57 93% 0.68 50 89% 0.79 43 86% 0.87 38 81% 0.91 33 76%
0.18 40 99% 0.33 36 98% 0.47 34 96% 0.58 32 94% 0.66 29 92% 0.73 27 89%
RSP-8 can separate hydrogen from the mixture with carbon dioxide with 90% recovery at 30e40 st.L/min for 40% of hydrogen in feed, 55e65 st.L/min for 55%, and 70e80 st.L/min for 59%. After the experiments absorbed hydrogen was desorbed and fed to a Hoppecke H2 Power 200 PEM fuel cell. The RSP-8 reactor was heated by hot water with temperature 45e55 C, flow rate and quality of purified hydrogen were sufficient to operate the fuel cell at maximum capacity 200 W (including auxiliary power). Our results on hydrogen purification are better than reported data on small scale PSA hydrogen purification, for example in Ref. [24], with breakthrough experiments and a four-bed PSA simulation, the production of H2 with a purity of 96e99.5% and a recovery of 71e85% from feed stream (H2/CO2/ CH4/CO/N2; 38/50/1/1/10 vol.%) at pressure 5e8 bar was achieved, and in Ref. [25] in rapid vacuum PSA experiment with feed at 3 bar (70e80 vol.% H2) hydrogen recovery was 61.8% for purity 99.993%. We compared our experimental results with similar experiments conducted by Miura et al. [31]. For 30 min cycle they
obtained 94% hydrogen recovery at 100 st.L/min from a feed steam at 8 bar with hydrogen content 70 vol.%. In our experiment we have 94% hydrogen recovery at 81 st.L/min from the feed stream at 5.6 bar with hydrogen content 59%. Performance is quite close, taking into account difference in feed pressure and composition (see Table 3). Our study shows that metal hydride method can be applied to separation of H2/CO2 mixtures with low hydrogen content. The process is more cost efficient than current small scale PSA techniques due to much lower capital investments, which require significant amount of valves per bed. The capacity of metal hydride purification system can be easily increased by installation of additional metal hydride reactors without significant loss of kinetics. On the other hand, in case of upscaling the process a cost of absorbent will count for much. In 1999 the cost of raw materials for AB5 alloys estimated as 7e10 USD/kg [45], but recently the lanthanum prices has significantly risen higher than 160 USD/kg [47] and retail prices are even higher. This increase in possible capital investments constrains possible upscaling of the metal hydride separation technology. Another issue is the presence of minor impurities in biohydrogen, especially water and sulphur compounds, which can damage a metal hydride bed. Special pretreatment is needed to use a real biohydrogen. In our opinion metal hydride H2/CO2 separation unit can be a second stage of a biohydrogen upgrade system after a membrane module. Polymer membranes can defend metal hydrides from poisonous impurities and high selectivity of metal hydrides can improve the overall performance of the purification system. Further study proposes a comprehensive approach to implement a coherent sequence of “bioreactor for hydrogen production e purification and solid-state storage system e power unit”, where all the stages are seen as key in the framework of a joint Russia-Taiwan project on biohydrogen production and utilization. The project involves two research groups from Joint Institute for High Temperatures RAS (Russia) and Feng Chia University (Taiwan) and is aimed to solve interdisciplinary problems for decentralized production of renewable energy carrier e hydrogen e by different types of organic materials conversion using microbial technology, hydrogen purification and storage in the solid phase for use in distributed and autonomous power units. The goal is to assess the potential application of the various biohydrogen systems, to elucidate the design guideline for scale-up and full-scale application in bioenergy generation from organic wastes.
Table 3 e Comparison with results of other authors. Cycle time, min
Feed pressure, MPa
H2 in feed, vol.%
Performance, st.L/h 81 100 550 (per 1 kg of adsorbent) up to 400 Nm3/h
Metal hydride, present study Metal hydride [31] RVPSA [25]
30 30 1
0.56 0.8 0.3
59 70 70e80
Breakthrough experiment with 2- and 4-bed PSA simulation [24]
~10
0.5e0.8
38
H2 recovery, %
H2 purity, vol.%
94 94 61.8
99.9þ 99.9þ 99.9þ
71e85
96e99.5
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Conclusions [5]
The separation of hydrogen e carbon dioxide mixtures with the use of two AB5-type alloys was performed. The RSP-8 metal hydride reactor was filled with 1 kg of “high-pressure” alloy La0.9Ce0.1Ni5 (Peq ¼ 1.96 bar at 293 K) and 1 kg of “lowpressure” alloy LaNi4.8Mn0.3Fe0.1 (Peq ¼ 0.38 bar at 293 K), the maximum H2 capacity is 140 st.L, the nominal operating H2 capacity is 110 st.L. For the HP-alloy the equilibrium pressure during the experiment was close to the partial pressure of hydrogen in the feed stream and purification was inefficient. For LP-alloy the purification rate of 81 st.L/h from a mixture containing 59 vol.% of hydrogen with recovery 94% was achieved. Thus, equilibrium pressures of alloys for biohydrogen purification have to be as low as possible to avoid preliminary compression of feed stream. Purified hydrogen was discharged to a Hoppecke H2 Power 200 PEM fuel cell. The flow rate and quality of purified hydrogen were sufficient to operate the fuel cell at maximum capacity of 200 W (including auxiliary power). Our results show that metal hydride purification can be a valuable part of a coherent sequence of “bioreactor for hydrogen production e purification and solid-state storage system e power unit”. The goal of further joint Russia-Taiwan research is to assess the potential application of the various biohydrogen systems, to elucidate the design guideline for scale-up and full-scale application in bioenergy generation from organic wastes.
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Acknowledgements [12]
Development of the experimental equipment and experimental investigations were performed with the financial support of Ministry of Education and Science of Russian Federation (subsidy 14.604.21.0010 RFMEFI60414X0010). Russia-Taiwan collaboration was supported by the joint NSC-RFBR project NSC 103-2923-E-035-001-MY3 (14-08-92001).
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Biohydrogen purification using metal hydride technologies D. Dunikov a,b,*, V. Borzenko a, D. Blinov a, A. Kazakov a, C.-Y. Lin c, S.-Y. Wu c, C.-Y. Chu c a
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia National Research University “Moscow Power Engineering Institute”, Moscow, Russia c Green Energy Development Center, Feng Chia University, Taichung 40724, Taiwan b
article info
abstract
Article history:
Metal hydrides are known for their ability of selective hydrogen absorption and might be
Received 31 March 2016
used for hydrogen purification. We demonstrate separation of hydrogen/carbon dioxide
Received in revised form
mixtures with the use of two AB5-type alloys. A metal hydride reactor was filled with 1 kg of
24 July 2016
“high-pressure” alloy La0.9Ce0.1Ni5 (Peq ¼ 1.96 bar at 293 K) and 1 kg of “low-pressure” alloy
Accepted 1 August 2016
LaNi4.8Mn0.3Fe0.1 (Peq ¼ 0.38 bar at 293 K), maximum H2 capacity is 140 st.L, nominal
Available online xxx
operating H2 capacity is 110 st.L. Hydrogen concentration was in the range 40e60 vol.%, feed pressure 5.6 bar. Separation efficiency and hydrogen recovery depend on equilibrium
Keywords:
pressure of absorption, which has to be as low as possible to increase hydrogen recovery.
Metal hydride
The purification rate of 81 st.L/h from a mixture containing 59 vol.% of hydrogen with
Hydrogen purification
recovery 94% was achieved for the “low-pressure” alloy. The results show that metal hy-
Hydrogen storage
dride H2/CO2 separation unit can be a second stage of a biohydrogen upgrade system after a
Biohydrogen
membrane module. Polymer membranes can defend metal hydrides from poisonous im-
Dark fermentation
purities and high selectivity of metal hydrides can improve the overall performance of the purification system. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction A problem of decentralized energy supply and efficient processing of organic wastes is actual for highly populated regions with scarce resources as well as for vast poor populated or remote territories, which can be found both in Russia, where up to 20 million people live outside the Unified Power System and another 20 million live in the Moscow agglomeration, and Taiwan, where highly populated regions have difficulties with efficient processing of organic wastes.
Significant increase of fossil fuel consumption and waste production rises concerns about protection of environment and leads to research and development of new green technologies. Hydrogen is a universal clean and efficient secondary energy carrier with the highest energy value, it can be used for energy storage and transportation and converted into electricity via fuel cells with high efficiency without pollution. Russia recognizes “Novel and renewable energy sources, including hydrogen energy” as a Critical Technology in “Energy efficiency and energy saving”, which is one of the 8 top
* Corresponding author. Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia. E-mail address: [email protected] (D. Dunikov). http://dx.doi.org/10.1016/j.ijhydene.2016.08.190 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Dunikov D, et al., Biohydrogen purification using metal hydride technologies, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.190
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priority science and technology development directions in Russia. Distributed generation becomes a significant part of Russian energy sector [1]. Biologically produced hydrogen generally referred as biohydrogen is considered as a perspective renewable energy source, and sustainable and competitive development can be reached by valorization of wastes and effluents using an emerging biorefinery approach [2]. Fermentative hydrogen production from biomass can provide clean and sustainable energy supply with the additional advantage of processing biomass waste and wastewater [3,4] of different origin. Practical application and commercialization of biohydrogen technologies requires a gradual transition from a basic research of biohydrogen production in laboratory conditions with clean substrates (glucose, sucrose) to a development of large-scale pilot bioreactors for hydrogen production with the use of various organic wastes [5]. Different substrate pretreatment techniques [6e8], new types of bioreactors [9,10] were used to improve biohydrogen production. Chu et al. [6,10] reported high hydrogen production rates up to 16.32 st.L H2/L/day in dark fermentation from rice straw hydrolysate. Chen et al. [11] announced hydrogen production rate up to 26 st.L H2/L/day from starch hydrolysate by Clostridium species. Sivagurunathan et al. reported peak hydrogen production rates of 37.5 st.L H2/L/day [12] and 55 st.L H2/L/day [13] from beverage industry wastewater. There are some reports of pilot scale biohydrogen production systems principally operating with different type of wastes with fairly high hydrogen production rates [14e18]. Several technical obstacles prevent widespread usage of biohydrogen [19], and the main are: uncertain hydrogen yield from various feedstocks, especially wastewater; heat management of bioreactors; low purity (high content of CO2, even more than 50 vol.%); low partial pressure of hydrogen (less than 1 bar). As pointed in Ref. [20] classical dark fermentation produces a complex gaseous mixture of 35e65 vol.%. H2, CO2 and minor impurities (e.g. H2S, N2, water vapor, etc.) and has to be upgraded. Moreover, since reduced hydrogen partial pressure conditions intensify H2 production, one of the strategies to improve hydrogen production is sparging with N2 or CO2 [21], thus increasing the necessity of biohydrogen purification. It is estimated that separation and purification equipment contributes at least 50% and sometimes up to 80% of the capital investment costs [22] and consumes significant part of the total in-plant energy expenditures. Industrial solutions, especially cryogenic purification and pressure swing adsorption (PSA), are designed primarily for large-scale hydrogen production and cannot be used without modification for upgrade of relatively small quantities of biohydrogen to levels of purity required for a PEM fuel cell operation (>99.95 vol.%). Indeed, cryogenic and membrane processes normally produce hydrogen at 90e98 vol.%, whereas the PSA process normally produces hydrogen at 99þ vol.%, on the other hand for most economically viable PSA process feed streams have to be already compressed at 15e30 bar and contain 75e90 vol.% hydrogen [23]. Several
studies for downscaling of PSA hydrogen purification were reported recently [24,25], also some emerging membrane technologies have been reported [26], but polymer membranes suffer from low H2/CO2 selectivity. Use of highly selective metallic membranes for biohydrogen purification is limited by high operating temperatures, expensive components (platinum, palladium) and degradation upon contact with the impurity gases [27]. Separation of H2/CO2 gas mixtures obtained by bioconversion of organic wastes into hydrogen with the aid of active membrane systems (membrane contactors) with moving liquid carriers was proposed [28], and H2 recovery can be successfully realized as combination of standard membrane method (H2 preconcentrating) and PSA (H2 conditioning), though improving of the whole process requires the development of high selective membranes [29]. Metal hydrides are known for their ability of selective hydrogen absorption and might be used for hydrogen purification [30]. It is possible to successfully separate hydrogen e carbon dioxide mixtures with the use of metal hydrides [19,31]. The scheme of metal hydride hydrogen purification method is presented in Fig. 1. Mixture is fed to a metal hydride reactor, and hydrogen is selectively absorbed by alloy bed with formation of metal hydride, reaction heat is transferred to a cooling liquid. Absorption is stopped after hydrogen breakthrough. Purified hydrogen can be obtained by desorption, outflow hydrogen pressure can be controlled by heating liquid temperature. A number of fundamental investigations of impurity gas interactions with metal hydrides were undertaken since 1970s [32e37]. Some of the developments were implemented on industrial scale for hydrogen extraction from ammonia production [38,39]. Feasibility of metal hydride applications for hydrogen extraction from gas mixtures containing CO2 were shown in Refs. [40e42]. Impurities has different influence on the performance of metal hydride devices [43], depending on the alloyeimpurity combination, hydrogen storage properties can deteriorate as a result of various types of damages [44] such as poisoning (H2S), retardation and corrosion (water vapor, CO2) and inert gas blanketing (N2).
Fig. 1 e Principle of metal hydride flow-through purification technique.
Please cite this article in press as: Dunikov D, et al., Biohydrogen purification using metal hydride technologies, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.190
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Fig. 2 e Experimental test bench for hydrogen purification and storage.
AB5-type metal hydrides could be perspective solution for hydrogen purification and compression for its utilization in fuel cells. The near-ambient PCT-properties (temperature and pressure ranges 0e100 C and 1e10 bar respectively), ease of activation without any heating, absence of protective oxide layers, possibility to regenerate H2 capacity after interaction with H2O or CO2 by mild heating in hydrogen atmosphere are distinctive features of AB5 metal hydrides [45] that would be advantages for biohydrogen purification and initial compression. Current state of research and development indicates a possibility of practical use and commercialization of a biohydrogen fuel cell power supply systems. On the other hand, conventional technical solutions do not allow to effectively upgrade and use biohydrogen produced by microorganisms in practice. The objective of this study is to experimentally investigate a performance of a metal hydride reactor during separation of H2/CO2 mixture in different operational conditions. Two samples of AB5-intermetallic compounds with different equilibrium absorption pressures have been used for purification of mixtures with hydrogen content varying from 40 to 59 vol.% and different feed rates. Impact of the separation performance on hydrogen recovery ratio has been investigated, and further research needs have been discussed.
Experimental procedure Experimental investigations on hydrogen separation from different H2/CO2 gas mixtures were conducted on the experimental test bench 12-04 JIHT RAS. The experiment scheme is presented in Fig. 2 and consists of gas ramp for preparation of mixtures (1), flow-through metal hydride reactor RSP-8 (2), vacuum pump (3), gas analyzer (4), pressure reducer (PR), pressure sensors (PS), gas flow meters (FM), valves (V). The gas flow at the inlet/outlet valve of the reactor was controlled and measured by a Bronkhorst mass flow meter/ controller, the pressure inside the reactor and the gas supply were measured by Aplisens pressure transmitters model PC28, concentration of hydrogen was measured by gas analyzer AG 0012, the water temperature was measured by thin film platinum sensors Heraeus M422, 1 kU. The experiments was controlled using LabView software with 1 Hz discretization. The RSP-8 (Reactor for Storage and Purification) metal hydride reactor is presented in Fig. 3. The reactor has tubular design, inner and outer tubes form two liquid heat exchangers with an annular reaction chamber between them. The reaction chamber is filled with a metal hydride bed and has inlet and outlet tubes at the ends so gas can flow through the bed. Several samples of AB5 type alloys are used in experiments
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Fig. 5 e PCT-isotherms for LaNi4.8Mn0.3Fe0.1.
Fig. 3 e RPS-8 metal hydride reactor.
with bed weight 1 kg, maximum H2 capacity is Vmax ¼ 140 st.L, nominal operating H2 capacity is Vn ¼ 110 st.L, purity grade at desorption is up to 99.999%. A base type of RSP-8 contains two water heat exchangers (inner and outer), in an experimental type RSP-8e the outer heat exchanger is removed to install additional temperature sensors. Two types of alloys were used: alloy with relatively high equilibrium pressure La0.9Ce0.1Ni5 (HP-alloy) and with relatively low equilibrium pressure LaNi4.8Mn0.3Fe0.1 (LP-alloy). The samples were prepared by arc melting, mechanically crushed into 1e5 mm particles and loaded into reactors. Before experiments, metal hydride beds were activated by heating in hydrogen atmosphere and cycled for 10 cycles or more. PCT-isotherms were measured by modified Sievert's method as described in Ref. [46] and presented in Figs. 4 and 5.
Equilibrium pressures and thermodynamic parameters are summarized in Table 1. Model hydrogen e carbon dioxide mixture or biohydrogen is fed into a reactor from a top of the reactor, which is cooled to remove heat of a hydride formation. If cooling power is sufficient a hydriding front moves slowly through the metal hydride bed, hydrogen is absorbed and only CO2 leaves the reactor from the bottom. As reactor is filled with hydrogen, cooling power becomes insufficient, so hydrogen breaks through the bed and gets lost. For any cooling power there is a critical mixture flow rate corresponding to hydrogen breakthrough. Preparation of gas mixtures is carried out by partial pressures technique based on Dalton's law. A gas cylinder is vacuumed to P ¼ 105 MPa and consequently filled with gases up to needed partial pressures. The mixture is maintained at least 72 h until equilibration of concentrations inside the cylinder, and concentration variation does not exceed 0.5% during the experiments. Chromatographic analysis of the resulting gas mixture is carried out before the experiments. During a charging step, pure hydrogen or simulated gas mixtures H2 þ CO2 (H2 content between 40 and 60 vol.%) are fed into inlet tube (V1 of Fig. 2 is open) and inlet gas flow rate is controlled by the flow meter (FMin). Simultaneously, outlet valve (V2) is opened and output gas flow rate is also controlled by the flow meter (FMout). Operating inlet pressure is 0.56 MPa, outlet is connected to atmosphere. At a discharge, hydrogen desorption is stimulated by hot water, gas flow rate is controlled by the flow meter (FMout) and hydrogen is fed to a Hoppecke H2 Power 200 PEM fuel cell.
Results and discussion
Fig. 4 e PCT-isotherms for La0.9Ce0.1Ni5.
Test run with pure hydrogen was conducted for La0.9Ce0.1Ni5. Inlet flow rate was 3 st.L/min and outlet flow rate was 1.5 st.L/ min. Metal hydride reactor was charged to nominal capacity Vn in 50 min, inlet and outlet flow rates are presented in Fig. 6. The reactor can be charged to nominal capacity with constant flow rate, part of the hydrogen stream breaks through the metal hydride bed and leaves the reactor. While the reactor is
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Table 1 e MH alloys hydrogen sorption properties. MH alloy La0.9Ce0.1Ni5 LaNi4.8Mn0.3Fe0.1
Temperature, K
С(Н2)max, wt.%
Peq, bar
DHdesо, kJ/mole
DSdesо, J/mole$K
293 293
1.3 1.3
1.96 0.38
30.7 ± 0.2 34.0 ± 0.3
110 ± 2 108 ± 2
charged over the nominal capacity, equilibrium pressure rises and gets close to inlet hydrogen pressure, thus absorption reaction slows down causing inlet hydrogen flow rate to fall down. To investigate influence of alloy equilibrium pressure on performance of the reactor, the two experiments were carried out for the HP-alloy La0.9Ce0.1Ni5 and the LP-alloy LaNi4.8Mn0.3Fe0.1. Initial conditions for the HP-alloy: feed H2 (46%) þ CO2 (54%) at 0.56 MPa, cooling water at 10.5 C, for the LP-alloy feed H2 (45%) þ CO2 (55%) at 0.56 MPa, cooling water at 21.5 C. Hydrogen recovery during the purification process is determined by the following equation: hðV=Vn Þ ¼ 1
Vout Vin
(1)
where, V is the volume of absorbed hydrogen, Vn is the nominal capacity of the reactor, Vout is the volume of hydrogen in purged stream, Vin is the volume of hydrogen in feed stream. To estimate purification efficiency, the hydrogen recovery is plotted as a function of state of charge of the reactor V/Vn. Flow rates at inlet and outlet and hydrogen recovery are presented in Fig. 7 for HP-alloy and in Fig. 8 for LP-alloy. For the HP-alloy equilibrium pressure during the experiment is close to the partial pressure of hydrogen in the feed stream, thus the reactor cannot maintain a constant inlet flow rate due to a slow absorption rate and a lot of hydrogen is lost. On the other hand, the equilibrium pressure for LP-alloy is lower than the partial pressure of hydrogen in the feed stream and the absorption rate is high enough to maintain the constant flow rate at inlet, and mixture at outlet contains a little amount of hydrogen, and hydrogen recovery is higher than 90% almost up to 60% of nominal charge of the reactor.
Fig. 6 e Inlet and outlet hydrogen flow rates at test run.
Fig. 7 e Inlet and outlet mixture flow rates and hydrogen recovery ratio for experiment with HP-alloy La0.9Ce0.1Ni5, feed H2 (46%) þ CO2 (54%) at 0.56 MPa, cooling water temperature of 10.5 C.
These results show that equilibrium pressures of alloys for biohydrogen purification have to be as low as possible to avoid preliminary compression of feed stream. Influence of gas mixture composition and feed flow rate on the recovery ratio was investigated in series of experiments with LaNi4.8Mn0.3Fe0.1 alloy. The experimental results are collected in Table 2. Our experiments show that the reactor
Fig. 8 e Inlet and outlet mixture flow rates and hydrogen recovery ratio for experiment with LP-alloy LaNi4.8Mn0.3Fe0.1, feed H2 (45%) þ CO2 (55%) at 0.56 MPa, cooling water temperature of 21.5 C.
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Table 2 e Experimental results for hydrogen purification with LaNi4.8Mn0.3Fe0.1 alloy. Experiment
A
Hydrogen in feed, vol.% Inlet/outlet flow rate set point, st.L/min Time, h 0.5
1
1.5
2
2.5
3
C
D
59
55
55
40
20/5
6/3
3/1.5
10/5
Parameter State of charge V/Vn Integral flow rate Q, st.L/h Recovery h V/Vn Q, st.L/h h V/Vn Q, st.L/h h V/Vn Q, st.L/h h V/Vn Q, st.L/h h V/Vn Q, st.L/h h
B
Results 0.37 81 94% 0.61 67 89% 0.77 56 84% 0.87 48 79% 0.92 40 73% 0.94 34 68%
0.30 66 97% 0.52 57 93% 0.69 51 89% 0.81 45 85% 0.88 39 80% 0.92 34 75%
0.30 66 97% 0.52 57 93% 0.68 50 89% 0.79 43 86% 0.87 38 81% 0.91 33 76%
0.18 40 99% 0.33 36 98% 0.47 34 96% 0.58 32 94% 0.66 29 92% 0.73 27 89%
RSP-8 can separate hydrogen from the mixture with carbon dioxide with 90% recovery at 30e40 st.L/min for 40% of hydrogen in feed, 55e65 st.L/min for 55%, and 70e80 st.L/min for 59%. After the experiments absorbed hydrogen was desorbed and fed to a Hoppecke H2 Power 200 PEM fuel cell. The RSP-8 reactor was heated by hot water with temperature 45e55 C, flow rate and quality of purified hydrogen were sufficient to operate the fuel cell at maximum capacity 200 W (including auxiliary power). Our results on hydrogen purification are better than reported data on small scale PSA hydrogen purification, for example in Ref. [24], with breakthrough experiments and a four-bed PSA simulation, the production of H2 with a purity of 96e99.5% and a recovery of 71e85% from feed stream (H2/CO2/ CH4/CO/N2; 38/50/1/1/10 vol.%) at pressure 5e8 bar was achieved, and in Ref. [25] in rapid vacuum PSA experiment with feed at 3 bar (70e80 vol.% H2) hydrogen recovery was 61.8% for purity 99.993%. We compared our experimental results with similar experiments conducted by Miura et al. [31]. For 30 min cycle they
obtained 94% hydrogen recovery at 100 st.L/min from a feed steam at 8 bar with hydrogen content 70 vol.%. In our experiment we have 94% hydrogen recovery at 81 st.L/min from the feed stream at 5.6 bar with hydrogen content 59%. Performance is quite close, taking into account difference in feed pressure and composition (see Table 3). Our study shows that metal hydride method can be applied to separation of H2/CO2 mixtures with low hydrogen content. The process is more cost efficient than current small scale PSA techniques due to much lower capital investments, which require significant amount of valves per bed. The capacity of metal hydride purification system can be easily increased by installation of additional metal hydride reactors without significant loss of kinetics. On the other hand, in case of upscaling the process a cost of absorbent will count for much. In 1999 the cost of raw materials for AB5 alloys estimated as 7e10 USD/kg [45], but recently the lanthanum prices has significantly risen higher than 160 USD/kg [47] and retail prices are even higher. This increase in possible capital investments constrains possible upscaling of the metal hydride separation technology. Another issue is the presence of minor impurities in biohydrogen, especially water and sulphur compounds, which can damage a metal hydride bed. Special pretreatment is needed to use a real biohydrogen. In our opinion metal hydride H2/CO2 separation unit can be a second stage of a biohydrogen upgrade system after a membrane module. Polymer membranes can defend metal hydrides from poisonous impurities and high selectivity of metal hydrides can improve the overall performance of the purification system. Further study proposes a comprehensive approach to implement a coherent sequence of “bioreactor for hydrogen production e purification and solid-state storage system e power unit”, where all the stages are seen as key in the framework of a joint Russia-Taiwan project on biohydrogen production and utilization. The project involves two research groups from Joint Institute for High Temperatures RAS (Russia) and Feng Chia University (Taiwan) and is aimed to solve interdisciplinary problems for decentralized production of renewable energy carrier e hydrogen e by different types of organic materials conversion using microbial technology, hydrogen purification and storage in the solid phase for use in distributed and autonomous power units. The goal is to assess the potential application of the various biohydrogen systems, to elucidate the design guideline for scale-up and full-scale application in bioenergy generation from organic wastes.
Table 3 e Comparison with results of other authors. Cycle time, min
Feed pressure, MPa
H2 in feed, vol.%
Performance, st.L/h 81 100 550 (per 1 kg of adsorbent) up to 400 Nm3/h
Metal hydride, present study Metal hydride [31] RVPSA [25]
30 30 1
0.56 0.8 0.3
59 70 70e80
Breakthrough experiment with 2- and 4-bed PSA simulation [24]
~10
0.5e0.8
38
H2 recovery, %
H2 purity, vol.%
94 94 61.8
99.9þ 99.9þ 99.9þ
71e85
96e99.5
Please cite this article in press as: Dunikov D, et al., Biohydrogen purification using metal hydride technologies, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.08.190
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 x x x ( 2 0 1 6 ) 1 e8
Conclusions [5]
The separation of hydrogen e carbon dioxide mixtures with the use of two AB5-type alloys was performed. The RSP-8 metal hydride reactor was filled with 1 kg of “high-pressure” alloy La0.9Ce0.1Ni5 (Peq ¼ 1.96 bar at 293 K) and 1 kg of “lowpressure” alloy LaNi4.8Mn0.3Fe0.1 (Peq ¼ 0.38 bar at 293 K), the maximum H2 capacity is 140 st.L, the nominal operating H2 capacity is 110 st.L. For the HP-alloy the equilibrium pressure during the experiment was close to the partial pressure of hydrogen in the feed stream and purification was inefficient. For LP-alloy the purification rate of 81 st.L/h from a mixture containing 59 vol.% of hydrogen with recovery 94% was achieved. Thus, equilibrium pressures of alloys for biohydrogen purification have to be as low as possible to avoid preliminary compression of feed stream. Purified hydrogen was discharged to a Hoppecke H2 Power 200 PEM fuel cell. The flow rate and quality of purified hydrogen were sufficient to operate the fuel cell at maximum capacity of 200 W (including auxiliary power). Our results show that metal hydride purification can be a valuable part of a coherent sequence of “bioreactor for hydrogen production e purification and solid-state storage system e power unit”. The goal of further joint Russia-Taiwan research is to assess the potential application of the various biohydrogen systems, to elucidate the design guideline for scale-up and full-scale application in bioenergy generation from organic wastes.
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Acknowledgements [12]
Development of the experimental equipment and experimental investigations were performed with the financial support of Ministry of Education and Science of Russian Federation (subsidy 14.604.21.0010 RFMEFI60414X0010). Russia-Taiwan collaboration was supported by the joint NSC-RFBR project NSC 103-2923-E-035-001-MY3 (14-08-92001).
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