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Process analysis of refuse derived fuel hydrogasification for producing SNG
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 ) 2 1 4 7 0 e2 1 4 8 0
Available online at www.sciencedirect.com
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Process analysis of refuse derived fuel hydrogasification for producing SNG Silvano Tosti a,*, Manuel A. Sousa b, Giuliano Buceti a, Luis M. Madeira b, Alfonso Pozio c a
ENEA, Dipartimento Fusione e Tecnologie per La Sicurezza Nucleare, Via E. Fermi 45, 00044 Frascati, Italy LEPABE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias S/ n, 4200-465 Porto, Portugal c ENEA, Dipartimento Tecnologie Energetiche, Via Anguillarese 301, 00123 Roma, Italy b
highlights Gasification of refuse derived fuel with electrolytic hydrogen is modelled via Aspen. Synthetic natural gas is produced with a yield of 65 wt%. The process exhibits an energy efficiency of 61%. Best operating conditions (300 C and 30 bar) inhibit the formation of toxic emissions (dioxins).
article info
abstract
Article history:
Toxic emissions from waste thermal treatments are a major issue that in several countries
Received 8 February 2019
hinders a full waste cycle. This paper addresses the specific topic of a sustainable exploi-
Received in revised form
tation of the refuse derived fuel (RDF). Because concerns still arise when using RDF as fuel,
10 June 2019
a new approach for its exploitation, the RDF hydrogasification for producing synthetic
Accepted 20 June 2019
natural gas (SNG), is discussed herein.
Available online 18 July 2019
The process has been simulated with Aspen code. The best operating conditions to maximize methane yield, under thermodynamic equilibrium conditions, have been stud-
Keywords:
ied while the composition of the obtained SNG has been verified in order to comply with
Hydrogasification
the requirements of the methane grid. Furthermore, the analysis looked at the effect of the
Refuse derived fuel
hydrogen excess in promoting the methane yield and the reduction of the dioxins
Electrolytic hydrogen
formation.
Process simulation Natural gas substitute
The energy analysis of the process, that has considered the use of commercial alkaline electrolysers for producing hydrogen, has been found to be mainly affected by the electric power consumption and has exhibited values of energy efficiency around 61%. Finally, a preliminary assessment of the economic competitiveness of this process has been done in order to clarify if the benefits from costs avoided for waste disposal could be the breakthrough in making RDF a sustainable competitive fuel. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (S. Tosti). https://doi.org/10.1016/j.ijhydene.2019.06.117 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Future energy scenarios will clearly foresee a growing penetration of the renewables. In fact, under the “New Policies Scenario”, renewables provide nearly 60% of power capacity additions and should reach 5170 GW in 2040, becoming the largest source of electricity supply before 2030 and achieving a share nearly 60% of all supply in 2040 [1]. Most of renewable energy sources (e.g. solar, wind) are characterized by intrinsic variability and, therefore, need efficient energy storage systems integrated with the grid. Large adoption of renewables produces an excess of electricity: a study focused on energy scenarios in Denmark has estimated an excess of about 20% of electricity production in case the wind power installed attains a share of 50% [2]. Biomass hydrogasification is a power-to-gas process thought for the renewable energy storage via the exploitation of hydrogen as energy vector [3e5]. Hydrogen as energy vector exhibits a large potential of fully satisfying the criteria of safe and clean energy use. The hydrogen technological platform consists of all the systems related to the production, purification, storage and distribution of hydrogen. Despite of noteworthy technological developments in the last years, the requirements for a reliable and cost-effective exploitation of this energy vector have still not been fully satisfied. Presently most of hydrogen is produced via steam reforming of fossil fuels (namely methane, from natural gas), a process that is not CO2-neutral. An alternative is to produce hydrogen from renewable electricity via water electrolysis: alkaline electrolysers make use of effective and reliable technologies, but they are quite expensive because of their large electricity consumption. In this vein, the use of low-cost electricity coming from the excess of renewable energy could make available cheaper hydrogen that, in turn, could be used to produce fuels and chemicals via different processes like hydrogasification. This way, hydrogen reacts with a carbon source such as coal to produce methane according to the power-to-gas philosophy [6,7]. A previous work has assessed the impact on the EU energy scenarios of coal hydrogasification [8]: hydrogen produced via electrolysis with wind and solar energy has been used to produce synthetic natural gas (SNG), demonstrating the need of a further acceleration of wind power investments in order to make achievable the process under the economic point of view. It is remarkable that, in order to keep “clean” this power-togas strategy, the hydrogasification process has to be fed by a biomass source. A process study on the hydrogasification of wood with electrolytic hydrogen has verified the achievement of high methane yield (about 0.64 kgCH4/kgbiomass) and energy efficiency (over 77%) [9]. Furthermore, the economic analysis of this study showed that the price of methane produced could be comparable to the market one in case of using excess (e.g. no cost) electricity. Starting from these premises, subsequent work has been focused on the use of cheaper biomass with the aim of reducing the cost of the methane produced. The municipal solid waste (MSW) represents an environmental concern of growing importance: its production has achieved 244 Mt in the EU in 2015 [10]. Most of the MSW is landfilled, a practice that requires about 0.3 m2 of land space per ton of waste [11]. In order to reduce the mass of MSW, and
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also aiming to valorize it, several processes have been operating especially on the unsorted residual material, e.g. the waste left downstream of separate collection (i.e. glass, paper, metals, etc.). These treatments consist mainly of thermochemical, biochemical and physicochemical processes [11]. In particular, the waste-to-energy (WTE) process is related to a thermal treatment aimed at recovering heat and electricity from waste and at minimizing the landfilling [12e15]. WTE processes use fuels derived from waste; however, these fuels have to fulfil specific standards in terms of net calorific value and content of biomass, moisture, chlorine and heavy metals. The EU defined a specific regulation for refuse derived fuels (RDF) that usually are produced in form of pellets via mechanicalebiological treatment (MBT) plants utilizing MSW as inlet material, as well as in material-recovery facilities utilizing packaging waste [12,16]. RDF results a homogeneous material which can be used as substitute for fossil fuels in a wide range of specialized waste-to-energy facilities to produce electricity and thermal energy (heat/steam) for district heating systems or industrial uses. This uniform grain size fuel consists largely of combustible components as non-recyclable plastics, paper cardboard and other materials from packaging. In several thermochemical processes proposed for the treatment of MSW, control of toxic emissions, in particular dioxins and furans, is one of the most important aspects affecting public acceptance [17]. In fact, dioxins (polychlorinated dibenzo-para-dioxins or PCDDs) and furans (polychlorinated dibenzo-para-furans or PCDFs) are among the most toxic chemicals in the products of combustion systems (burning of various fuels, such as coal, wood, and petroleum products) [17]. Due to their high toxicity, the dioxins emissions limit, in Europe and in Japan, is set by law and is equal to 0.1 ng I-TEQ m3, where I-TEQ (international toxic equivalent) is a single figure resulting from the product of the concentration and individual TEF (toxic equivalency factor) values of each congener [18]. In incineration of MSW, the formation of dioxins is due to the presence of chloride (e.g. in PVC-plastics) and metals which may act as catalysts [19]. On the other hand, dioxins can be minimized by operating at high temperature (over 1000 C) with high turbulence and longer residence time [17]. A different prospective is MSW gasification, which takes place at 600e1000 C in presence of a gasifying agent (typically air, steam, nitrogen, carbon dioxide, oxygen or a combination of these). This process produces a gaseous stream, the synthesis gas (namely CO, H2, CH4, etc.), plus ash and residual carbon (char and tar) that result from the incomplete conversion of the biomass [20]. Gasification presents some advantages compared to incineration: i) waste is converted not only into heat and electricity but also into fuels and chemicals, ii) formation of dioxins is reduced because of the low levels of oxygen present in the reaction environment and correct management of the hydrogen chloride before the combustion of syngas produced [11,18]. In principle, in gasification processes it is expected that the formation of dioxins should be more reduced when increasing the excess of hydrogen fed and, therefore, RDF hydrogasification could be proposed as a convenient way to produce methane with very low toxic emissions [21,22]. This paper discusses results from ASPEN code modelling of RDF
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hydrogasification, in particular by assessing methane yield, dioxins emissions and energy efficiency. A preliminary economic analysis is also presented: it has been carried out for Italian regions where the thermal treatments of RDF have difficult applicability and where the MSW often remains untreated and landfilled, buried or shipped away with high costs. That brings to consider that the economy of RDF hydrogasification could greatly takes advantage of the cost avoided for its disposal.
Table 2 e RDF ultimate analysis on dry basis [23]. Components
Mass fraction, %wt.
Carbon Hydrogen Nitrogen Sulfur Oxygen Chlorine Ash
48.93 6.38 1.17 0.22 31.47 0.38 11.45
Process modelling As first hypothesis, the hydrogen needed for the hydrogasification is supposed to come from water electrolysis: DH298 K ¼ 241.86 kJ mol1 (1)
H2O ⇔ H2 þ 0.5 O2
Still, methane is assumed to be produced by the combination of hydrogen and carbon: C þ 2H2 ⇔ CH4
DH298
K
¼ 74.87 kJ mol1 (2)
Actually, RDF has a composition which depends on the type of waste (e.g., MSW, industrial waste) and pre-treatment adopted (MBT, drying, etc.). Besides carbon, other elements appear into RDF, namely hydrogen, nitrogen, sulfur, oxygen and chloride. In this work, RDF composition reported by Vounatsos et al. has been considered [23]; Table 1 reports the moisture content and the heat values of the RDF while Table 2 lists its ultimate analysis on dry basis. Taking into account the actual RDF composition, many other reactions may occur in the gasification reactor, including the following: C þ H2 O⇔ CO þ H2
DH298K ¼ 131:30 kJ mol
(3) 1
C þ O2 ⇔ CO2 2C þ O2 ⇔ 2CO2
1
DH298K ¼ 353:90 kJ mol 1
DH298K ¼ 110:54 kJ mol
CO þ 3H2 ⇔ CH4 þ H2 O
(4) (5)
1
DH298K ¼ 206:00 kJ mol
(6)
1
CO2 þ 4H2 ⇔ CH4 þ 2H2 O
DH298K ¼ 64:90 kJ mol
N2 þ 3H2 ⇔2NH3
DH298K ¼ 92:22 kJ mol
(7)
1
(8)
1
H2 þ S⇔H2 S DH298K ¼ 20:60 kJ mol
(9) 1
Cl2 þ H2 ⇔2HCl DH298K ¼ 184:60 kJ mol
(10)
Table 1 e Moisture content and heat values of the RDF [23]. Moisture, %wt.
26.72
LHV, MJ kg1 HHV, MJ kg1
13.43 20.70
Presence of dioxins among the emissions is a critical point for the public acceptance of RDF gasification processes. During thermal and combustion treatments of waste, formation of dioxins and furans arises through several mechanisms described extensively in the literature [24]. Generally, formation of these compounds takes place either in vapor phase (homogeneous reaction) or on solid surfaces (heterogeneous reaction), being that chlorophenols and polycyclic aromatic hydrocarbons act as precursors [25,26]. Formation of these compounds is also promoted by the presence of chloride (in form of both Cl2 and HCl), oxygen, sulfur, water, and nitrogen compounds. For instance, dioxin formation is expected to occur from HCl when combined with oxygen [27]. The first step is the formation of chlorine: 2 HCl þ1/2 O2 ⇔ H2O þ Cl2 DH298
K
¼ 56.50 kJ mol1
(11)
Then, chlorine reacts with phenolic compounds to form dioxin precursors, and eventually the dioxin is formed as a product from the breakdown and molecular rearrangement of the precursor over metals acting as catalyst: phenol þ Cl2 ⇔ chlorophenol (dioxin precursor)
(12)
2-chlorophenol þ 1/2 O2 ⇔ dioxin þ Cl2
(13)
The simplified scheme of the whole hydrogasification process (Fig. 1) consists of an electrolyser producing hydrogen and oxygen, a gasification reactor where RDF reacts with hydrogen producing SNG and an upgrading unit to remove undesired components present in the SNG (e.g. ash, ammonia, water). Under steady state conditions, the hydrogasification process has been studied by using Aspen Plus software while the energy optimization has been carried out by Aspen Energy Analyzer [28]. In the model, RDF has been defined as a non-conventional solid since it is not present in the Aspen Plus components database [29,30]. In particular, RDF elementary composition described in Table 2 has been adopted in this simulation. Based on previous works [30], the Peng-Robinson state equation with Boston-Mathias modifications has been used for calculating the physical properties. The list of dioxins and furans introduced in the Aspen database are reported in Table 3. The plant layout studied by Aspen Plus is shown in Fig. 2. The first unit of the simulated process is a cracking reactor where RDF is separated into conventional components. In a
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Fig. 1 e Process schematics of RDF hydrogasification.
- Wobbe index, WI, MJ (Sm3)1.
Table 3 e List of dioxins and furans. Component
Molecular Formula
1,2,4,6,8,9-Hexachlorooxanthrene 2,3,7,8-Tetrachlorooxanthrene 1,2,3,4-Tetrachlorooxanthrene 1,2,6-Trichlorooxanthrene 1,2,7-Trichlorooxanthrene 2,3-Dichlorooxanthrene 1-Chlorooxanthrene 2-Chlorooxanthrene Dibenzo-1,4-dioxin 2,3-Dihydro-1,4-dioxine 1,4-Dioxane 1,3-Dioxane 6-Methyl-4H-1,3-dioxin 2H-1,3-Benzodioxole 2,6-Dimethyl-1,3-dioxan-4-yl acetate 2,3-Dihydro-1,4-benzodioxine
C12H2Cl6O2 C12H4Cl4O2 C12H4Cl4O2 C12H5Cl3O2 C12H5Cl3O2 C12H6Cl2O2 C12H7ClO2 C12H7ClO2 C12H8O2 C4H6O2 C4H8O2 C4H8O2 C5H8O2 C7H6O2 C8H14O4 C8H8O2
In particular, gas gravity is defined as: GG ¼
rGas rAir
(14)
where rGas and rAir are the density of SNG and of air at P ¼ 1 bar and T ¼ 15 C. The Wobbe index is given by: HHV WI ¼ pffiffiffiffiffiffiffi GG
(15)
Details of the upgrading section show a cyclone for separating solids content (i.e., the ashes) with an efficiency of 85%, and a separator removing most of ammonia and other compounds. Water separation (via condensation) is done in a cooler. Downstream, grid requirements state that methane
Fig. 2 e Scheme of the Aspen model.
following step, hydrogasification reactions occur in a Gibbs reactor where the equilibrium composition of the species in the system have been determined via a nonstoichiometric approach which involves the minimization of the Gibbs free energy [31]. Afterward, an upgrading section is adopted to treat the SNG produced in order to comply with the grid requirements. In this work, standard grid requirements established in Italy have been considered [32]. These requirements fix the values of the following parameters: - higher heating value, HHV, MJ (Sm3)1, - gas gravity, GG,
has to be provided at 30 C and 60 bar: so, a compressor and another cooler are needed before releasing the outlet stream. Through the model, a parametric analysis has been carried out for assessing methane yield and formation of dioxins by varying the main operation parameters (pressure, temperature, hydrogen excess). Methane yield (wt. %) is defined by: MYð%Þ ¼
FCH4 xC 100 FC
(16)
where FCH4 is the mass flow rate of methane produced (kg h1), FC is the mass flow rate of carbon fed (contained in RDF feed stream) (kg h1) and xC the carbon content in RDF.
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Results and discussion In this process analysis, the hydrogasification reactor is fed with 1000 kg h1 of dry RDF. In a simplified approach based on the stoichiometry of reaction (2) and the elementary composition of RDF (cf. Table 2), the theoretical amount of hydrogen needed to react with carbon is 163.1 kg h1 (81.6 kmol h1) of which 63.8 kg h1 (31.9 kmol h1) are already present in the RDF fed. This means that, to keep the hydrogasification reaction (2) under the stoichiometric ratio, at least 99.4 kg h1 (49.7 kmol h1) have to be fed together with RDF. Any additional hydrogen over 99.4 kg h1 in the feed stream will result as hydrogen excess. In practice: 1
FH2 exc ¼ FH2 fed 99:4 kg h
(17)
where FH2 exc is the hydrogen mass flow rate excess (kg h1) and FH2 fed is the hydrogen mass flow rate fed (kg h1) into the hydrogasification reactor.
Effect of temperature and pressure With an hydrogen feed flow rate of 140 kg h1 (corresponding to an excess of about 40 kg h1) and pressure of 30 bar, the hydrogasification temperature has been varied to calculate methane yield (MY), composition of the outlet stream and dioxins formation. Figs. 3 and 4 report the methane yield and gas composition in the stream leaving the hydrogasification reactor. According to the thermodynamics of reaction (2), which is exothermic, the methane formation reduces with the temperature that, indeed, favors the competitive endothermal reactions (like formation of carbon dioxide) and lowers the content of methane in SNG, while the unreacted hydrogen increases. CO formation takes place over 600 C at the expense of CO2 according to the Boudouard reaction: C þ CO2 ⇔ 2CO
DH298 K ¼ 172 kJ mol1
(18)
Fig. 3 e Methane yield vs. temperature with hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar.
Fig. 4 e Gas composition in terms of: a) CH4, H2, CO2 and CO and b) N2, H2O and NH3 in SNG vs. temperature with hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar.
Based on the above considerations it seems a good choice to fix the temperature at 300 C since lower temperatures could reduce too much the reaction kinetics. It is noteworthy that, at this temperature, the characteristics of SNG produced are optimized also in terms of grid specifications. Table 4 shows that values of grid parameters decrease when increasing the temperature because of the reduction of the methane content that affects mainly the HHV. In particular, at 400e500 C the HHV, WI and GG of the produced SNG already go below the values required by the methane grid specifications. For that reason, 300 C was selected as the best temperature in this work (although it might be low for hydrogasification, leading in practice to big reactor size, but of course this needs to be experimentally assessed under conditions where kinetic aspects are relevant). The presence of dioxins in the product stream vs. temperature is described in Fig. 5; it is noticed that the dioxins
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Table 4 e Characteristics of the SNG produced vs. temperature (hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar). Temperature C
HHV MJ (Sm3)1
Gas gravity
Wobble Index MJ (Sm3)1
grid specifications 300 400 500 600 700 800
34.95e45.28 36.5 35.0 32.6 29.0 24.7 20.4
0.5548e0.800 0.555 0.546 0.527 0.470 0.390 0.340
47.31e52.33 49.0 47.4 44.9 42.3 39.6 35.0
formation starts over 700 C with a sharp increase from 750 C. As discussed in Section Process modelling, due to the complexity of dioxins formation schemes that involve multiple solid- and gas-phase reactions, the comparison with literature can be only qualitative. In particular, literature reports critical temperatures for dioxins formation in homogeneous gas-phase reactions in the range of 500e800 C, while lower temperatures are expected to promote heterogeneous reactions [26]. In a further step, once fixed the temperature at 300 C, the process analysis has been carried out by varying the pressure. Reaction (2) takes place with a reduction of the number of moles and, therefore, from the thermodynamics point of view, it is promoted by a pressure increase. However, such effect is quite modest: Fig. 6 and Fig. 7 show that both methane yield and methane concentration raise moderately with the pressure in the range 5e35 bar while hydrogen content reduces slightly. Content of CO and hydrocarbons (C2H4, C2H6 and C3H8) is negligible (less than 104 M fraction). The analysis of the grid parameters reported in Table 5 shows the achievement of a plateau at 25e30 bar of HHV as well as of the Wobbe index, both important parameters for qualifying the SNG under the economic point of view. Although the methane yield increases only slightly moving from 5 to 30 bar, the operation at high pressure (30 bar) is suggested for complying the grid parameters. These parameters are dramatically affected by the hydrogen content in SNG
Fig. 5 e Dioxin produced vs. temperature with hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar.
that reduces with the pressure (see Fig. 7a). In particular, Table 5 shows that gas gravity overcomes the minimum (0.5548) starting from 20 bar. The choice of a pressure like 30 bar is also relevant to promote kinetics. Finally, at 300 C no formation of dioxins has been found whatever the pressure.
Effect of hydrogen excess As discussed before, from a theoretical point of view any amount of hydrogen feeding the hydrogasification reactor over 99.4 kg h1 could be considered as an excess with respect to the stoichiometry of the reaction (2). The effect of the hydrogen excess has been studied at 300 C and 30 bar by varying the hydrogen feed flow rate between 50 and 180 kg h1 (corresponding around to under-stoichiometric conditions and a hydrogen excess of 80 kg h1, respectively). According to thermodynamics, excess of reagents promotes reversible reactions forward, and Fig. 8 shows that methane yield rises with the hydrogen gas fed into the hydrogasification unit. However, as reported by Fig. 9, when using 180 kg h1 of hydrogen fed (excess of 80.6 kg h1) the methane concentration reduces due to the growing presence of unreacted hydrogen that dilutes the methane produced; still, according to reaction (7), CO2 decreases with the hydrogen fed. Fig. 9b shows that nitrogen content reduces
Fig. 6 e Methane yield vs. pressure with hydrogen feed flow rate of 140 kg h¡1 and temperature of 300 C.
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Fig. 8 e Methane yield vs. hydrogen feed flow rate at a temperature of 300 C and pressure of 30 bar.
Fig. 7 e Gas composition in terms of: a) CH4, H2 and CO2 and b) N2, H2O and NH3 in SNG vs. pressure with hydrogen feed flow rate of 140 kg h¡1 and temperature of 300 C.
with the hydrogen fed although only a few of this nitrogen reacts to form ammonia that, indeed, is very slightly increasing. Content of CO and hydrocarbons (C2H4, C2H6 and C3H8) is negligible (less than 104 M fraction).
Table 5 e Characteristics of the SNG produced vs. pressure (hydrogen feed flow rate of 140 kg h¡1 and temperature 300 C). Pressure bar grid ref. values 5 10 15 20 25 30 35
HHV MJ (Sm3)1
Gas gravity
Wobble Index MJ (Sm3)1
34.95e45.28
0.5548e0.800
47.31e52.33
35.7 36.1 36.3 36.4 36.4 36.5 36.6
0.550 0.553 0.554 0.555 0.555 0.555 0.556
48.1 48.5 48.7 48.8 48.9 49.0 49.0
Following the behavior of methane yield and content in the reactor outlet stream, both HHV and WI of the SNG exhibit a maximum at 140 kg h1 of hydrogen fed. On the other hand, the presence of unreacted hydrogen dilutes the methane produced and, then, gas gravity reduces with the hydrogen added. The results shown by Table 6 confirm that the best option is to fix the hydrogen feed flow rate at around 140 kg h1. Being anyhow negligible at 300 C, the formation of dioxins vs. the hydrogen fed has been assessed at a higher temperature (600 C). From Fig. 10, it is clear the effect of the hydrogen fed in reducing the formation of dioxins e it not only dilutes the dioxins, but the reducing environment withdraws oxygen otherwise needed to the dioxins formation reactions (eqs. (11)e(13)). In practice, according to thermodynamics of reaction (2) the RDF hydrogasification for producing methane has to be advantageously operated at low temperature because of its exothermicity and with hydrogen excess to promote the reaction yield. Since these operating conditions prevent the dioxins formation, this process for the treatment of RDF can be thought as a gasification process with very low toxic emissions.
Energy analysis The energetic integration of the different cold and hot streams has been carried out through Aspen Energy Analyzer. The heat exchangers' network has been designed to comply with the minimum energy requirements under the hypotheses of heat exchangers efficiency of 80% and minimum DT of 10 C between the heat exchanging streams. This analysis exhibited no pinch points and low complexity of the exchangers’ network (data not shown). The results of the energy assessment are significantly affected by the efficiency of the electrolysers. In the following analysis, the use of commercial alkaline electrolysers has
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Fig. 10 e Dioxin produced vs. hydrogen feed flow rate at a temperature of 600 C and pressure of 30 bar.
Fig. 9 e Gas composition in terms of: a) CH4, H2 and CO2 and b) N2, H2O and NH3 in SNG vs. hydrogen feed flow rate at a temperature of 300 C and pressure of 30 bar.
Perspective economic analysis
Table 6 e Characteristics of the SNG produced vs. hydrogen feed flow rate at 300 C and 30 bar. Hydrogen gas fed kg h1 grid ref. values 50 100 140 180
HHV MJ (Sm3)1 34.95e45.28 34.0 35.6 36.5 29.5
Gas gravity Wobble Index MJ (Sm3)1 0.5548e0.800 0.621 0.584 0.555 0.405
47.31e52.33 43.1 46.6 49.0 46.4
been assumed with power consumption of 61.8 kW h per kg of hydrogen produced [33]. The energy efficiency of the process, h, has been defined by: h¼
FSNG LHVSNG FRDF LHVRDF þ Pel þ Pcomp þ
Pcold EER
100
where FSNG is the mass flow rate of the SNG produced (kg h1), LHVSNG and LHVRDF stand for the low heating value of the SNG and RDF (kJ kg1), respectively, Pel is the electric power required by the electrolysers (kW), Pcomp is the electric power required by the pumps and compressors (kW), Pcold is the waste heat power to be removed by external cooling systems (kW) and EER stands for the thermal performance coefficient of the cooling system, which was fixed equal to 4.42 [9]. The energy analysis has been performed for a plant treating 1000 kg h1 (on dry basis) of RDF at 300 C and 30 bar and with 140 kg h1 of hydrogen fed. The results are summarized in Table 7. Energy content of the synthetic natural gas produced (8714 kW) is about the same as the power consumption for water electrolysis (8652 kW), while the energy losses of the system are nearly balanced by the RDF power content (5417 kW) plus the power needed for removing waste heat and for the compressors (433 and 73 kW). The resulting overall energy efficiency is 61.2%.
(19)
In general, biomass hydrogasification here described is penalized under the economic point of view by the high cost of hydrogen produced via water electrolysis. Previous studies on coal and wood hydrogasification have verified that methane produced in this way has a price comparable with the market one only when low-cost electricity is used to produce hydrogen. This is the case, for example, when electrolysis works with excess of electricity produced by renewables during off-peak hours. Such a scenario could be credible in case of substantial penetration of wind and solar energy as foreseen, for instance, beyond 2040 when renewables are expected to achieve a share over 50e60% of the energy produced [3,4]. In particular, in some regions RDF hydrogasification could exhibit the economic benefit coming from the cost avoided for waste disposal. As discussed in the Introduction section, due to the low public acceptance, thermal treatments of RDF are of
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Table 7 e Energy analysis for the treatment of 1000 kg h¡1 of dry RDF. Methane produced (kg h1) SNG produced (kg h1) SNG power content (kW) RDF power content (kW) Electric power for electrolysis (kW) Power for removing waste heat (kW) Compression power (kW) Energy efficiency (%)
649 677 8714 5417 8652 433 73 61.2
Table 8 e Main direct cost items of hydrogasification of 1 ton of RDF. 3
Methane produced (Nm ) Electricity consumption (kWh) Methane revenue (V) Cost avoided for RDF disposal (V) Electricity cost (V) Ratio revenue/cost
922 9725 552.90 80.00 583.50 1.08
difficult applicability in Italy where disposal of pre-treated MSW costs over 100 V/t [34]. Based on the encouraging results of this process analysis in terms of dioxins emission reduction, it appears appropriate to carry out a preliminary simplified economic analysis aimed at verifying the perspective economic potentialities of this approach. In this calculation, only direct costs have been taken into account. Actual prices of household utilities in Italy have been considered for electricity and methane while a conservative value has been assumed for the cost avoided with RDF disposal. In particular:
processes the costly supply of biomass (coal, wood, etc.) brings the ratio revenue/cost below 1 [9].
Hydrogen production via steam reforming Use of hydrogen from water electrolysis introduces the main cost in the studied process, which is made viable only when renewable energies penetration in the market will reduce the cost of electricity. In an alternative process, the hydrogen needed for the hydrogasification could be obtained via steam reforming of methane: CH4 þ 2H2O ⇔ CO2 þ 4H2
DH298 K ¼ 165.00 kJ mol1
(20)
In this reaction, 4 mol of hydrogen are produced per mole of methane consumed; this means that, in principle, half of the methane produced by hydrogasification has to be fed to the reformer. As represented in Fig. 11, the new process scheme of the hydrogasification plant should foresee the presence of two units for producing hydrogen: water electrolysis and methane steam reforming. In general, the increase of hydrogen produced via methane steam reforming reduces both the electricity cost and the methane revenue, while it introduces additional costs for heat to be provided to the reformer and for CO2 separation. As future work, a dedicated process and economic analyses should be carried out to establish the optimum configuration based on the combination of the two units for producing hydrogen versus the availability of low-cost electricity.
Conclusions - electricity cost of 0.06 V kWh1, - methane price of 0.6 V (Nm3)1, - cost avoided for RDF disposal of 80 V t1. The main cost items for the treatment of 1 ton of RDF are summarized in Table 8. The methane revenue (552.90 V) balances approximately the electricity cost (583.50 V), while the revenue coming from the cost avoided for RDF disposal (80.00 V) makes the ratio revenue/cost definitely attractive. It is noteworthy that, generally, in other hydrogasification
The simulation of RDF hydrogasification with hydrogen produced by electrolysis has established 300 C and 30 bar as the best thermodynamic operating conditions capable to maximize the methane yield and comply the Italian methane specification grid. A maximum methane yield of about 65 wt% has been assessed with a SNG characterized by a higher heating value (HHV) of 36.5 MJ (Sm3)1 and Wobbe index of 49.0 MJ (Sm3)1. The analysis has also studied the effect of the hydrogen excess to promote the methane formation by finding the optimum value of 140 kg of hydrogen fed per ton of
Fig. 11 e Schematic of RDF hydrogasification with hydrogen produced via electrolysis and methane steam reforming.
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 ) 2 1 4 7 0 e2 1 4 8 0
RDF treated. The energy assessment of the process based on commercial alkaline electrolysers is mainly affected by the electric power consumption and has exhibited values of energy efficiency around 61%. Formation of dioxins has been studied in order to verify the effectiveness of this gasification process to reduce toxic emissions. In particular, the process analysis has verified that, from the thermodynamic point of view, the formation of dioxins: i) takes place at temperatures over 600 C, and ii) is dramatically cut down by the hydrogen excess. Based on these model results, future test campaigns shall verify the process kinetics and address, where necessary, the development of specific proper catalysts. Once demonstrated, the absence of dioxins emissions is an important aspect making favorable the economy of the process. Due to environmental concerns, in many areas (such as some Italian regions) thermal treatments of RDF are of difficult applicability and the resulting MSW disposal represents a cost for the municipalities. On the contrary, RDF hydrogasification without dioxins emissions could avoid the costs for waste disposal and represent an income for the waste thermal treatment plant. Next work should be also focused on hydrogasification done with hydrogen obtained via steam reforming of part of the methane produced, thus reducing significantly the process direct costs.
Acknowledgements This work has been carried out at ENEA Frascati laboratories in the framework of the Erasmus internship of M.A. Sousa from the Faculty of Engineering, University of Porto. L.M. Madeira thanks financial supported by project POCI01-0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy e UID/EQU/00511/ 2013), funded by the European Regional Development Fund (ERDF) through COMPETE2020 e Programa Operacional ~ o (POCI), and by naCompetitividade e Internacionalizac¸a ~ o para a Ciencia e a tional funds through FCT e Fundac¸a Tecnologia.
Acronyms EU GG HHV I-TEQ LHV MBT MSW PCDD PCDF PVC RDF SNG TEF WI WTE
european union gas gravity higher heating value international toxic equivalent lower heating value mechanicalebiological treatment municipal solid waste polychlorinated dibenzo-para-dioxin polychlorinated dibenzo-para-furan polyvinyl chloride refuse derived fuel synthetic natural gas toxic equivalency factor Wobbe index waste-to-energy
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Nomenclature EER thermal performance coefficient of the cooling system (dimensionless) mass flow rate of species i (kg h1) Fi mass flow rate excess of species i (kg h1) Fi exc Fi fed mass flow rate fed of species i (kg h1) GG gas gravity (dimensionless) HHV higher heating value (MJ kg1 or MJ (Sm3)1) LHV lower heating value (MJ kg1 or MJ (Sm3)1) MY methane yield (wt.%) P total pressure (bar) waste heat power removed by external cooling Pcold systems (kW) electric power required by pumps and compressors Pcomp (kW) electric power required by the electrolysers (kW) Pel standard cubic meter of gas at a temperature of 20 C Sm3 and pressure of 1.012 bar T absolute temperature (ºC or K) WI Wobbe index (MJ (Sm3)1) DH heat of reaction (kJ mol1) density of species i (kg m3) ri h energy efficiency of the process (%)
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circular economy and energy recycling in Europe. Energy 2017:2013e44. Arena U. Process and technological aspects of municipal solid waste gasification. A Review, Waste Management 2012;32:625e39. A, Kropa J, Kermes V, Nemet A, Stehlı´k P. Waste-toTabasova energy technologies: impact on environment. Energy 2012;44:146e55. Blanco PH, Wu C, Williams PT. Influence of Ni/SiO2 catalyst preparation methods on hydrogen production from the pyrolysis/reforming of refuse derived fuel. Int J Hydrogen Energy 2014;39(11):5723e6573. Galeno G, Minutillo M, Perna A. From waste to electricity through integrated plasma gasification/fuel cell (IPGFC) system. Int J Hydrogen Energy 2011;36(2):1692e701. Bosmans A, Vanderreydt I, Geysen D, Helsen L. The crucial role of Waste-to-Energy technologies in enhanced landfill mining: a technology review. J Clean Prod 2013;55:10e23. Vounatsos P, Atsonios K, Itskos G, Agraniotis M, Grammelis P, Kakaras E. Classification of refuse derived fuel (RDF) and model development of a novel thermal utilization concept through air-gasification. Waste and Biomass Valorization 2016;7:1297e308. McKay G. Dioxin characterisation, formation and minimisation during municipal solid waste (MSW) incineration: review. Chem Eng J 2002;86:343e68. Lopes EJ, Okamura LA, Yamamoto CI. Formation of dioxins and furans during municipal solid waste gasification. Braz J Chem Eng 2015;32:87e97. Shibamoto T, Yasuhara A, Katami T. Dioxin formation from waste incineration. Rev Environ Contam Toxicol 2007;190:1e41. Kumar 1 A, Jones DD, Hanna MA. Thermochemical biomass gasification: a review of the current status of the technology. Energies 2009;2:556e81. S. Tosti, G. Buceti, A. Pozio, Process for the production of methane from refuse-derived fuel and plant thereof, European Patent Application EP18185183.3 (priority 27.07.2017). Minutillo M, Perna A. Renewable energy storage system via coal hydrogasification with co-production of electricity and synthetic natural gas. Int J Hydrogen Energy 2014;39:5793e803. P. Vounatsos, G. Koufodimos, M. Agraniotis, K. Roufos, C. Eleftheriadis, P. Grammelis, E. Kakaras, Characterization and classification of refuse derived fuel in the materials recovery facility of EPANA S.A., http://ec.europa.eu/environment/life/
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Process analysis of refuse derived fuel hydrogasification for producing SNG Silvano Tosti a,*, Manuel A. Sousa b, Giuliano Buceti a, Luis M. Madeira b, Alfonso Pozio c a
ENEA, Dipartimento Fusione e Tecnologie per La Sicurezza Nucleare, Via E. Fermi 45, 00044 Frascati, Italy LEPABE, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias S/ n, 4200-465 Porto, Portugal c ENEA, Dipartimento Tecnologie Energetiche, Via Anguillarese 301, 00123 Roma, Italy b
highlights Gasification of refuse derived fuel with electrolytic hydrogen is modelled via Aspen. Synthetic natural gas is produced with a yield of 65 wt%. The process exhibits an energy efficiency of 61%. Best operating conditions (300 C and 30 bar) inhibit the formation of toxic emissions (dioxins).
article info
abstract
Article history:
Toxic emissions from waste thermal treatments are a major issue that in several countries
Received 8 February 2019
hinders a full waste cycle. This paper addresses the specific topic of a sustainable exploi-
Received in revised form
tation of the refuse derived fuel (RDF). Because concerns still arise when using RDF as fuel,
10 June 2019
a new approach for its exploitation, the RDF hydrogasification for producing synthetic
Accepted 20 June 2019
natural gas (SNG), is discussed herein.
Available online 18 July 2019
The process has been simulated with Aspen code. The best operating conditions to maximize methane yield, under thermodynamic equilibrium conditions, have been stud-
Keywords:
ied while the composition of the obtained SNG has been verified in order to comply with
Hydrogasification
the requirements of the methane grid. Furthermore, the analysis looked at the effect of the
Refuse derived fuel
hydrogen excess in promoting the methane yield and the reduction of the dioxins
Electrolytic hydrogen
formation.
Process simulation Natural gas substitute
The energy analysis of the process, that has considered the use of commercial alkaline electrolysers for producing hydrogen, has been found to be mainly affected by the electric power consumption and has exhibited values of energy efficiency around 61%. Finally, a preliminary assessment of the economic competitiveness of this process has been done in order to clarify if the benefits from costs avoided for waste disposal could be the breakthrough in making RDF a sustainable competitive fuel. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. E-mail address: [email protected] (S. Tosti). https://doi.org/10.1016/j.ijhydene.2019.06.117 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Introduction Future energy scenarios will clearly foresee a growing penetration of the renewables. In fact, under the “New Policies Scenario”, renewables provide nearly 60% of power capacity additions and should reach 5170 GW in 2040, becoming the largest source of electricity supply before 2030 and achieving a share nearly 60% of all supply in 2040 [1]. Most of renewable energy sources (e.g. solar, wind) are characterized by intrinsic variability and, therefore, need efficient energy storage systems integrated with the grid. Large adoption of renewables produces an excess of electricity: a study focused on energy scenarios in Denmark has estimated an excess of about 20% of electricity production in case the wind power installed attains a share of 50% [2]. Biomass hydrogasification is a power-to-gas process thought for the renewable energy storage via the exploitation of hydrogen as energy vector [3e5]. Hydrogen as energy vector exhibits a large potential of fully satisfying the criteria of safe and clean energy use. The hydrogen technological platform consists of all the systems related to the production, purification, storage and distribution of hydrogen. Despite of noteworthy technological developments in the last years, the requirements for a reliable and cost-effective exploitation of this energy vector have still not been fully satisfied. Presently most of hydrogen is produced via steam reforming of fossil fuels (namely methane, from natural gas), a process that is not CO2-neutral. An alternative is to produce hydrogen from renewable electricity via water electrolysis: alkaline electrolysers make use of effective and reliable technologies, but they are quite expensive because of their large electricity consumption. In this vein, the use of low-cost electricity coming from the excess of renewable energy could make available cheaper hydrogen that, in turn, could be used to produce fuels and chemicals via different processes like hydrogasification. This way, hydrogen reacts with a carbon source such as coal to produce methane according to the power-to-gas philosophy [6,7]. A previous work has assessed the impact on the EU energy scenarios of coal hydrogasification [8]: hydrogen produced via electrolysis with wind and solar energy has been used to produce synthetic natural gas (SNG), demonstrating the need of a further acceleration of wind power investments in order to make achievable the process under the economic point of view. It is remarkable that, in order to keep “clean” this power-togas strategy, the hydrogasification process has to be fed by a biomass source. A process study on the hydrogasification of wood with electrolytic hydrogen has verified the achievement of high methane yield (about 0.64 kgCH4/kgbiomass) and energy efficiency (over 77%) [9]. Furthermore, the economic analysis of this study showed that the price of methane produced could be comparable to the market one in case of using excess (e.g. no cost) electricity. Starting from these premises, subsequent work has been focused on the use of cheaper biomass with the aim of reducing the cost of the methane produced. The municipal solid waste (MSW) represents an environmental concern of growing importance: its production has achieved 244 Mt in the EU in 2015 [10]. Most of the MSW is landfilled, a practice that requires about 0.3 m2 of land space per ton of waste [11]. In order to reduce the mass of MSW, and
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also aiming to valorize it, several processes have been operating especially on the unsorted residual material, e.g. the waste left downstream of separate collection (i.e. glass, paper, metals, etc.). These treatments consist mainly of thermochemical, biochemical and physicochemical processes [11]. In particular, the waste-to-energy (WTE) process is related to a thermal treatment aimed at recovering heat and electricity from waste and at minimizing the landfilling [12e15]. WTE processes use fuels derived from waste; however, these fuels have to fulfil specific standards in terms of net calorific value and content of biomass, moisture, chlorine and heavy metals. The EU defined a specific regulation for refuse derived fuels (RDF) that usually are produced in form of pellets via mechanicalebiological treatment (MBT) plants utilizing MSW as inlet material, as well as in material-recovery facilities utilizing packaging waste [12,16]. RDF results a homogeneous material which can be used as substitute for fossil fuels in a wide range of specialized waste-to-energy facilities to produce electricity and thermal energy (heat/steam) for district heating systems or industrial uses. This uniform grain size fuel consists largely of combustible components as non-recyclable plastics, paper cardboard and other materials from packaging. In several thermochemical processes proposed for the treatment of MSW, control of toxic emissions, in particular dioxins and furans, is one of the most important aspects affecting public acceptance [17]. In fact, dioxins (polychlorinated dibenzo-para-dioxins or PCDDs) and furans (polychlorinated dibenzo-para-furans or PCDFs) are among the most toxic chemicals in the products of combustion systems (burning of various fuels, such as coal, wood, and petroleum products) [17]. Due to their high toxicity, the dioxins emissions limit, in Europe and in Japan, is set by law and is equal to 0.1 ng I-TEQ m3, where I-TEQ (international toxic equivalent) is a single figure resulting from the product of the concentration and individual TEF (toxic equivalency factor) values of each congener [18]. In incineration of MSW, the formation of dioxins is due to the presence of chloride (e.g. in PVC-plastics) and metals which may act as catalysts [19]. On the other hand, dioxins can be minimized by operating at high temperature (over 1000 C) with high turbulence and longer residence time [17]. A different prospective is MSW gasification, which takes place at 600e1000 C in presence of a gasifying agent (typically air, steam, nitrogen, carbon dioxide, oxygen or a combination of these). This process produces a gaseous stream, the synthesis gas (namely CO, H2, CH4, etc.), plus ash and residual carbon (char and tar) that result from the incomplete conversion of the biomass [20]. Gasification presents some advantages compared to incineration: i) waste is converted not only into heat and electricity but also into fuels and chemicals, ii) formation of dioxins is reduced because of the low levels of oxygen present in the reaction environment and correct management of the hydrogen chloride before the combustion of syngas produced [11,18]. In principle, in gasification processes it is expected that the formation of dioxins should be more reduced when increasing the excess of hydrogen fed and, therefore, RDF hydrogasification could be proposed as a convenient way to produce methane with very low toxic emissions [21,22]. This paper discusses results from ASPEN code modelling of RDF
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hydrogasification, in particular by assessing methane yield, dioxins emissions and energy efficiency. A preliminary economic analysis is also presented: it has been carried out for Italian regions where the thermal treatments of RDF have difficult applicability and where the MSW often remains untreated and landfilled, buried or shipped away with high costs. That brings to consider that the economy of RDF hydrogasification could greatly takes advantage of the cost avoided for its disposal.
Table 2 e RDF ultimate analysis on dry basis [23]. Components
Mass fraction, %wt.
Carbon Hydrogen Nitrogen Sulfur Oxygen Chlorine Ash
48.93 6.38 1.17 0.22 31.47 0.38 11.45
Process modelling As first hypothesis, the hydrogen needed for the hydrogasification is supposed to come from water electrolysis: DH298 K ¼ 241.86 kJ mol1 (1)
H2O ⇔ H2 þ 0.5 O2
Still, methane is assumed to be produced by the combination of hydrogen and carbon: C þ 2H2 ⇔ CH4
DH298
K
¼ 74.87 kJ mol1 (2)
Actually, RDF has a composition which depends on the type of waste (e.g., MSW, industrial waste) and pre-treatment adopted (MBT, drying, etc.). Besides carbon, other elements appear into RDF, namely hydrogen, nitrogen, sulfur, oxygen and chloride. In this work, RDF composition reported by Vounatsos et al. has been considered [23]; Table 1 reports the moisture content and the heat values of the RDF while Table 2 lists its ultimate analysis on dry basis. Taking into account the actual RDF composition, many other reactions may occur in the gasification reactor, including the following: C þ H2 O⇔ CO þ H2
DH298K ¼ 131:30 kJ mol
(3) 1
C þ O2 ⇔ CO2 2C þ O2 ⇔ 2CO2
1
DH298K ¼ 353:90 kJ mol 1
DH298K ¼ 110:54 kJ mol
CO þ 3H2 ⇔ CH4 þ H2 O
(4) (5)
1
DH298K ¼ 206:00 kJ mol
(6)
1
CO2 þ 4H2 ⇔ CH4 þ 2H2 O
DH298K ¼ 64:90 kJ mol
N2 þ 3H2 ⇔2NH3
DH298K ¼ 92:22 kJ mol
(7)
1
(8)
1
H2 þ S⇔H2 S DH298K ¼ 20:60 kJ mol
(9) 1
Cl2 þ H2 ⇔2HCl DH298K ¼ 184:60 kJ mol
(10)
Table 1 e Moisture content and heat values of the RDF [23]. Moisture, %wt.
26.72
LHV, MJ kg1 HHV, MJ kg1
13.43 20.70
Presence of dioxins among the emissions is a critical point for the public acceptance of RDF gasification processes. During thermal and combustion treatments of waste, formation of dioxins and furans arises through several mechanisms described extensively in the literature [24]. Generally, formation of these compounds takes place either in vapor phase (homogeneous reaction) or on solid surfaces (heterogeneous reaction), being that chlorophenols and polycyclic aromatic hydrocarbons act as precursors [25,26]. Formation of these compounds is also promoted by the presence of chloride (in form of both Cl2 and HCl), oxygen, sulfur, water, and nitrogen compounds. For instance, dioxin formation is expected to occur from HCl when combined with oxygen [27]. The first step is the formation of chlorine: 2 HCl þ1/2 O2 ⇔ H2O þ Cl2 DH298
K
¼ 56.50 kJ mol1
(11)
Then, chlorine reacts with phenolic compounds to form dioxin precursors, and eventually the dioxin is formed as a product from the breakdown and molecular rearrangement of the precursor over metals acting as catalyst: phenol þ Cl2 ⇔ chlorophenol (dioxin precursor)
(12)
2-chlorophenol þ 1/2 O2 ⇔ dioxin þ Cl2
(13)
The simplified scheme of the whole hydrogasification process (Fig. 1) consists of an electrolyser producing hydrogen and oxygen, a gasification reactor where RDF reacts with hydrogen producing SNG and an upgrading unit to remove undesired components present in the SNG (e.g. ash, ammonia, water). Under steady state conditions, the hydrogasification process has been studied by using Aspen Plus software while the energy optimization has been carried out by Aspen Energy Analyzer [28]. In the model, RDF has been defined as a non-conventional solid since it is not present in the Aspen Plus components database [29,30]. In particular, RDF elementary composition described in Table 2 has been adopted in this simulation. Based on previous works [30], the Peng-Robinson state equation with Boston-Mathias modifications has been used for calculating the physical properties. The list of dioxins and furans introduced in the Aspen database are reported in Table 3. The plant layout studied by Aspen Plus is shown in Fig. 2. The first unit of the simulated process is a cracking reactor where RDF is separated into conventional components. In a
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Fig. 1 e Process schematics of RDF hydrogasification.
- Wobbe index, WI, MJ (Sm3)1.
Table 3 e List of dioxins and furans. Component
Molecular Formula
1,2,4,6,8,9-Hexachlorooxanthrene 2,3,7,8-Tetrachlorooxanthrene 1,2,3,4-Tetrachlorooxanthrene 1,2,6-Trichlorooxanthrene 1,2,7-Trichlorooxanthrene 2,3-Dichlorooxanthrene 1-Chlorooxanthrene 2-Chlorooxanthrene Dibenzo-1,4-dioxin 2,3-Dihydro-1,4-dioxine 1,4-Dioxane 1,3-Dioxane 6-Methyl-4H-1,3-dioxin 2H-1,3-Benzodioxole 2,6-Dimethyl-1,3-dioxan-4-yl acetate 2,3-Dihydro-1,4-benzodioxine
C12H2Cl6O2 C12H4Cl4O2 C12H4Cl4O2 C12H5Cl3O2 C12H5Cl3O2 C12H6Cl2O2 C12H7ClO2 C12H7ClO2 C12H8O2 C4H6O2 C4H8O2 C4H8O2 C5H8O2 C7H6O2 C8H14O4 C8H8O2
In particular, gas gravity is defined as: GG ¼
rGas rAir
(14)
where rGas and rAir are the density of SNG and of air at P ¼ 1 bar and T ¼ 15 C. The Wobbe index is given by: HHV WI ¼ pffiffiffiffiffiffiffi GG
(15)
Details of the upgrading section show a cyclone for separating solids content (i.e., the ashes) with an efficiency of 85%, and a separator removing most of ammonia and other compounds. Water separation (via condensation) is done in a cooler. Downstream, grid requirements state that methane
Fig. 2 e Scheme of the Aspen model.
following step, hydrogasification reactions occur in a Gibbs reactor where the equilibrium composition of the species in the system have been determined via a nonstoichiometric approach which involves the minimization of the Gibbs free energy [31]. Afterward, an upgrading section is adopted to treat the SNG produced in order to comply with the grid requirements. In this work, standard grid requirements established in Italy have been considered [32]. These requirements fix the values of the following parameters: - higher heating value, HHV, MJ (Sm3)1, - gas gravity, GG,
has to be provided at 30 C and 60 bar: so, a compressor and another cooler are needed before releasing the outlet stream. Through the model, a parametric analysis has been carried out for assessing methane yield and formation of dioxins by varying the main operation parameters (pressure, temperature, hydrogen excess). Methane yield (wt. %) is defined by: MYð%Þ ¼
FCH4 xC 100 FC
(16)
where FCH4 is the mass flow rate of methane produced (kg h1), FC is the mass flow rate of carbon fed (contained in RDF feed stream) (kg h1) and xC the carbon content in RDF.
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Results and discussion In this process analysis, the hydrogasification reactor is fed with 1000 kg h1 of dry RDF. In a simplified approach based on the stoichiometry of reaction (2) and the elementary composition of RDF (cf. Table 2), the theoretical amount of hydrogen needed to react with carbon is 163.1 kg h1 (81.6 kmol h1) of which 63.8 kg h1 (31.9 kmol h1) are already present in the RDF fed. This means that, to keep the hydrogasification reaction (2) under the stoichiometric ratio, at least 99.4 kg h1 (49.7 kmol h1) have to be fed together with RDF. Any additional hydrogen over 99.4 kg h1 in the feed stream will result as hydrogen excess. In practice: 1
FH2 exc ¼ FH2 fed 99:4 kg h
(17)
where FH2 exc is the hydrogen mass flow rate excess (kg h1) and FH2 fed is the hydrogen mass flow rate fed (kg h1) into the hydrogasification reactor.
Effect of temperature and pressure With an hydrogen feed flow rate of 140 kg h1 (corresponding to an excess of about 40 kg h1) and pressure of 30 bar, the hydrogasification temperature has been varied to calculate methane yield (MY), composition of the outlet stream and dioxins formation. Figs. 3 and 4 report the methane yield and gas composition in the stream leaving the hydrogasification reactor. According to the thermodynamics of reaction (2), which is exothermic, the methane formation reduces with the temperature that, indeed, favors the competitive endothermal reactions (like formation of carbon dioxide) and lowers the content of methane in SNG, while the unreacted hydrogen increases. CO formation takes place over 600 C at the expense of CO2 according to the Boudouard reaction: C þ CO2 ⇔ 2CO
DH298 K ¼ 172 kJ mol1
(18)
Fig. 3 e Methane yield vs. temperature with hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar.
Fig. 4 e Gas composition in terms of: a) CH4, H2, CO2 and CO and b) N2, H2O and NH3 in SNG vs. temperature with hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar.
Based on the above considerations it seems a good choice to fix the temperature at 300 C since lower temperatures could reduce too much the reaction kinetics. It is noteworthy that, at this temperature, the characteristics of SNG produced are optimized also in terms of grid specifications. Table 4 shows that values of grid parameters decrease when increasing the temperature because of the reduction of the methane content that affects mainly the HHV. In particular, at 400e500 C the HHV, WI and GG of the produced SNG already go below the values required by the methane grid specifications. For that reason, 300 C was selected as the best temperature in this work (although it might be low for hydrogasification, leading in practice to big reactor size, but of course this needs to be experimentally assessed under conditions where kinetic aspects are relevant). The presence of dioxins in the product stream vs. temperature is described in Fig. 5; it is noticed that the dioxins
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Table 4 e Characteristics of the SNG produced vs. temperature (hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar). Temperature C
HHV MJ (Sm3)1
Gas gravity
Wobble Index MJ (Sm3)1
grid specifications 300 400 500 600 700 800
34.95e45.28 36.5 35.0 32.6 29.0 24.7 20.4
0.5548e0.800 0.555 0.546 0.527 0.470 0.390 0.340
47.31e52.33 49.0 47.4 44.9 42.3 39.6 35.0
formation starts over 700 C with a sharp increase from 750 C. As discussed in Section Process modelling, due to the complexity of dioxins formation schemes that involve multiple solid- and gas-phase reactions, the comparison with literature can be only qualitative. In particular, literature reports critical temperatures for dioxins formation in homogeneous gas-phase reactions in the range of 500e800 C, while lower temperatures are expected to promote heterogeneous reactions [26]. In a further step, once fixed the temperature at 300 C, the process analysis has been carried out by varying the pressure. Reaction (2) takes place with a reduction of the number of moles and, therefore, from the thermodynamics point of view, it is promoted by a pressure increase. However, such effect is quite modest: Fig. 6 and Fig. 7 show that both methane yield and methane concentration raise moderately with the pressure in the range 5e35 bar while hydrogen content reduces slightly. Content of CO and hydrocarbons (C2H4, C2H6 and C3H8) is negligible (less than 104 M fraction). The analysis of the grid parameters reported in Table 5 shows the achievement of a plateau at 25e30 bar of HHV as well as of the Wobbe index, both important parameters for qualifying the SNG under the economic point of view. Although the methane yield increases only slightly moving from 5 to 30 bar, the operation at high pressure (30 bar) is suggested for complying the grid parameters. These parameters are dramatically affected by the hydrogen content in SNG
Fig. 5 e Dioxin produced vs. temperature with hydrogen feed flow rate of 140 kg h¡1 and pressure of 30 bar.
that reduces with the pressure (see Fig. 7a). In particular, Table 5 shows that gas gravity overcomes the minimum (0.5548) starting from 20 bar. The choice of a pressure like 30 bar is also relevant to promote kinetics. Finally, at 300 C no formation of dioxins has been found whatever the pressure.
Effect of hydrogen excess As discussed before, from a theoretical point of view any amount of hydrogen feeding the hydrogasification reactor over 99.4 kg h1 could be considered as an excess with respect to the stoichiometry of the reaction (2). The effect of the hydrogen excess has been studied at 300 C and 30 bar by varying the hydrogen feed flow rate between 50 and 180 kg h1 (corresponding around to under-stoichiometric conditions and a hydrogen excess of 80 kg h1, respectively). According to thermodynamics, excess of reagents promotes reversible reactions forward, and Fig. 8 shows that methane yield rises with the hydrogen gas fed into the hydrogasification unit. However, as reported by Fig. 9, when using 180 kg h1 of hydrogen fed (excess of 80.6 kg h1) the methane concentration reduces due to the growing presence of unreacted hydrogen that dilutes the methane produced; still, according to reaction (7), CO2 decreases with the hydrogen fed. Fig. 9b shows that nitrogen content reduces
Fig. 6 e Methane yield vs. pressure with hydrogen feed flow rate of 140 kg h¡1 and temperature of 300 C.
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Fig. 8 e Methane yield vs. hydrogen feed flow rate at a temperature of 300 C and pressure of 30 bar.
Fig. 7 e Gas composition in terms of: a) CH4, H2 and CO2 and b) N2, H2O and NH3 in SNG vs. pressure with hydrogen feed flow rate of 140 kg h¡1 and temperature of 300 C.
with the hydrogen fed although only a few of this nitrogen reacts to form ammonia that, indeed, is very slightly increasing. Content of CO and hydrocarbons (C2H4, C2H6 and C3H8) is negligible (less than 104 M fraction).
Table 5 e Characteristics of the SNG produced vs. pressure (hydrogen feed flow rate of 140 kg h¡1 and temperature 300 C). Pressure bar grid ref. values 5 10 15 20 25 30 35
HHV MJ (Sm3)1
Gas gravity
Wobble Index MJ (Sm3)1
34.95e45.28
0.5548e0.800
47.31e52.33
35.7 36.1 36.3 36.4 36.4 36.5 36.6
0.550 0.553 0.554 0.555 0.555 0.555 0.556
48.1 48.5 48.7 48.8 48.9 49.0 49.0
Following the behavior of methane yield and content in the reactor outlet stream, both HHV and WI of the SNG exhibit a maximum at 140 kg h1 of hydrogen fed. On the other hand, the presence of unreacted hydrogen dilutes the methane produced and, then, gas gravity reduces with the hydrogen added. The results shown by Table 6 confirm that the best option is to fix the hydrogen feed flow rate at around 140 kg h1. Being anyhow negligible at 300 C, the formation of dioxins vs. the hydrogen fed has been assessed at a higher temperature (600 C). From Fig. 10, it is clear the effect of the hydrogen fed in reducing the formation of dioxins e it not only dilutes the dioxins, but the reducing environment withdraws oxygen otherwise needed to the dioxins formation reactions (eqs. (11)e(13)). In practice, according to thermodynamics of reaction (2) the RDF hydrogasification for producing methane has to be advantageously operated at low temperature because of its exothermicity and with hydrogen excess to promote the reaction yield. Since these operating conditions prevent the dioxins formation, this process for the treatment of RDF can be thought as a gasification process with very low toxic emissions.
Energy analysis The energetic integration of the different cold and hot streams has been carried out through Aspen Energy Analyzer. The heat exchangers' network has been designed to comply with the minimum energy requirements under the hypotheses of heat exchangers efficiency of 80% and minimum DT of 10 C between the heat exchanging streams. This analysis exhibited no pinch points and low complexity of the exchangers’ network (data not shown). The results of the energy assessment are significantly affected by the efficiency of the electrolysers. In the following analysis, the use of commercial alkaline electrolysers has
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Fig. 10 e Dioxin produced vs. hydrogen feed flow rate at a temperature of 600 C and pressure of 30 bar.
Fig. 9 e Gas composition in terms of: a) CH4, H2 and CO2 and b) N2, H2O and NH3 in SNG vs. hydrogen feed flow rate at a temperature of 300 C and pressure of 30 bar.
Perspective economic analysis
Table 6 e Characteristics of the SNG produced vs. hydrogen feed flow rate at 300 C and 30 bar. Hydrogen gas fed kg h1 grid ref. values 50 100 140 180
HHV MJ (Sm3)1 34.95e45.28 34.0 35.6 36.5 29.5
Gas gravity Wobble Index MJ (Sm3)1 0.5548e0.800 0.621 0.584 0.555 0.405
47.31e52.33 43.1 46.6 49.0 46.4
been assumed with power consumption of 61.8 kW h per kg of hydrogen produced [33]. The energy efficiency of the process, h, has been defined by: h¼
FSNG LHVSNG FRDF LHVRDF þ Pel þ Pcomp þ
Pcold EER
100
where FSNG is the mass flow rate of the SNG produced (kg h1), LHVSNG and LHVRDF stand for the low heating value of the SNG and RDF (kJ kg1), respectively, Pel is the electric power required by the electrolysers (kW), Pcomp is the electric power required by the pumps and compressors (kW), Pcold is the waste heat power to be removed by external cooling systems (kW) and EER stands for the thermal performance coefficient of the cooling system, which was fixed equal to 4.42 [9]. The energy analysis has been performed for a plant treating 1000 kg h1 (on dry basis) of RDF at 300 C and 30 bar and with 140 kg h1 of hydrogen fed. The results are summarized in Table 7. Energy content of the synthetic natural gas produced (8714 kW) is about the same as the power consumption for water electrolysis (8652 kW), while the energy losses of the system are nearly balanced by the RDF power content (5417 kW) plus the power needed for removing waste heat and for the compressors (433 and 73 kW). The resulting overall energy efficiency is 61.2%.
(19)
In general, biomass hydrogasification here described is penalized under the economic point of view by the high cost of hydrogen produced via water electrolysis. Previous studies on coal and wood hydrogasification have verified that methane produced in this way has a price comparable with the market one only when low-cost electricity is used to produce hydrogen. This is the case, for example, when electrolysis works with excess of electricity produced by renewables during off-peak hours. Such a scenario could be credible in case of substantial penetration of wind and solar energy as foreseen, for instance, beyond 2040 when renewables are expected to achieve a share over 50e60% of the energy produced [3,4]. In particular, in some regions RDF hydrogasification could exhibit the economic benefit coming from the cost avoided for waste disposal. As discussed in the Introduction section, due to the low public acceptance, thermal treatments of RDF are of
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Table 7 e Energy analysis for the treatment of 1000 kg h¡1 of dry RDF. Methane produced (kg h1) SNG produced (kg h1) SNG power content (kW) RDF power content (kW) Electric power for electrolysis (kW) Power for removing waste heat (kW) Compression power (kW) Energy efficiency (%)
649 677 8714 5417 8652 433 73 61.2
Table 8 e Main direct cost items of hydrogasification of 1 ton of RDF. 3
Methane produced (Nm ) Electricity consumption (kWh) Methane revenue (V) Cost avoided for RDF disposal (V) Electricity cost (V) Ratio revenue/cost
922 9725 552.90 80.00 583.50 1.08
difficult applicability in Italy where disposal of pre-treated MSW costs over 100 V/t [34]. Based on the encouraging results of this process analysis in terms of dioxins emission reduction, it appears appropriate to carry out a preliminary simplified economic analysis aimed at verifying the perspective economic potentialities of this approach. In this calculation, only direct costs have been taken into account. Actual prices of household utilities in Italy have been considered for electricity and methane while a conservative value has been assumed for the cost avoided with RDF disposal. In particular:
processes the costly supply of biomass (coal, wood, etc.) brings the ratio revenue/cost below 1 [9].
Hydrogen production via steam reforming Use of hydrogen from water electrolysis introduces the main cost in the studied process, which is made viable only when renewable energies penetration in the market will reduce the cost of electricity. In an alternative process, the hydrogen needed for the hydrogasification could be obtained via steam reforming of methane: CH4 þ 2H2O ⇔ CO2 þ 4H2
DH298 K ¼ 165.00 kJ mol1
(20)
In this reaction, 4 mol of hydrogen are produced per mole of methane consumed; this means that, in principle, half of the methane produced by hydrogasification has to be fed to the reformer. As represented in Fig. 11, the new process scheme of the hydrogasification plant should foresee the presence of two units for producing hydrogen: water electrolysis and methane steam reforming. In general, the increase of hydrogen produced via methane steam reforming reduces both the electricity cost and the methane revenue, while it introduces additional costs for heat to be provided to the reformer and for CO2 separation. As future work, a dedicated process and economic analyses should be carried out to establish the optimum configuration based on the combination of the two units for producing hydrogen versus the availability of low-cost electricity.
Conclusions - electricity cost of 0.06 V kWh1, - methane price of 0.6 V (Nm3)1, - cost avoided for RDF disposal of 80 V t1. The main cost items for the treatment of 1 ton of RDF are summarized in Table 8. The methane revenue (552.90 V) balances approximately the electricity cost (583.50 V), while the revenue coming from the cost avoided for RDF disposal (80.00 V) makes the ratio revenue/cost definitely attractive. It is noteworthy that, generally, in other hydrogasification
The simulation of RDF hydrogasification with hydrogen produced by electrolysis has established 300 C and 30 bar as the best thermodynamic operating conditions capable to maximize the methane yield and comply the Italian methane specification grid. A maximum methane yield of about 65 wt% has been assessed with a SNG characterized by a higher heating value (HHV) of 36.5 MJ (Sm3)1 and Wobbe index of 49.0 MJ (Sm3)1. The analysis has also studied the effect of the hydrogen excess to promote the methane formation by finding the optimum value of 140 kg of hydrogen fed per ton of
Fig. 11 e Schematic of RDF hydrogasification with hydrogen produced via electrolysis and methane steam reforming.
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 ) 2 1 4 7 0 e2 1 4 8 0
RDF treated. The energy assessment of the process based on commercial alkaline electrolysers is mainly affected by the electric power consumption and has exhibited values of energy efficiency around 61%. Formation of dioxins has been studied in order to verify the effectiveness of this gasification process to reduce toxic emissions. In particular, the process analysis has verified that, from the thermodynamic point of view, the formation of dioxins: i) takes place at temperatures over 600 C, and ii) is dramatically cut down by the hydrogen excess. Based on these model results, future test campaigns shall verify the process kinetics and address, where necessary, the development of specific proper catalysts. Once demonstrated, the absence of dioxins emissions is an important aspect making favorable the economy of the process. Due to environmental concerns, in many areas (such as some Italian regions) thermal treatments of RDF are of difficult applicability and the resulting MSW disposal represents a cost for the municipalities. On the contrary, RDF hydrogasification without dioxins emissions could avoid the costs for waste disposal and represent an income for the waste thermal treatment plant. Next work should be also focused on hydrogasification done with hydrogen obtained via steam reforming of part of the methane produced, thus reducing significantly the process direct costs.
Acknowledgements This work has been carried out at ENEA Frascati laboratories in the framework of the Erasmus internship of M.A. Sousa from the Faculty of Engineering, University of Porto. L.M. Madeira thanks financial supported by project POCI01-0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy e UID/EQU/00511/ 2013), funded by the European Regional Development Fund (ERDF) through COMPETE2020 e Programa Operacional ~ o (POCI), and by naCompetitividade e Internacionalizac¸a ~ o para a Ciencia e a tional funds through FCT e Fundac¸a Tecnologia.
Acronyms EU GG HHV I-TEQ LHV MBT MSW PCDD PCDF PVC RDF SNG TEF WI WTE
european union gas gravity higher heating value international toxic equivalent lower heating value mechanicalebiological treatment municipal solid waste polychlorinated dibenzo-para-dioxin polychlorinated dibenzo-para-furan polyvinyl chloride refuse derived fuel synthetic natural gas toxic equivalency factor Wobbe index waste-to-energy
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Nomenclature EER thermal performance coefficient of the cooling system (dimensionless) mass flow rate of species i (kg h1) Fi mass flow rate excess of species i (kg h1) Fi exc Fi fed mass flow rate fed of species i (kg h1) GG gas gravity (dimensionless) HHV higher heating value (MJ kg1 or MJ (Sm3)1) LHV lower heating value (MJ kg1 or MJ (Sm3)1) MY methane yield (wt.%) P total pressure (bar) waste heat power removed by external cooling Pcold systems (kW) electric power required by pumps and compressors Pcomp (kW) electric power required by the electrolysers (kW) Pel standard cubic meter of gas at a temperature of 20 C Sm3 and pressure of 1.012 bar T absolute temperature (ºC or K) WI Wobbe index (MJ (Sm3)1) DH heat of reaction (kJ mol1) density of species i (kg m3) ri h energy efficiency of the process (%)
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