University of Wales, Newport



BIOHYTHANE AS AN ENERGY FEEDSTOCK FOR SOLID OXIDE FUEL CELLSG.K. Veluswamya,c , C.J. Laycockb , K.Shahc, A. S. Balla , A. J. Guwyb, R. M. DinsdalebaCentre for Environmental Sustainability and Remediation (EnSuRe), School of Science, RMIT University, Victoria, Melbourne, Australia.bUniversity of South Wales, Sustainable Environment Research Centre (SERC), Llantwit?Road, Treforest, Pontypridd, Rhondda Cynon Taff, Wales, United?Kingdom. CF37 1DL.cAdvance Energy Technologies and Waste Utilisation Research Group, School of Engineering, RMIT University, Victoria, Melbourne, Australia. ABSTRACT:Biogas (60%-CH4, 40%- CO2) is a potential source of renewable energy when used as energy feedstock for solid oxide fuel cells (SOFC), but releases biogenic CO2 emissions. Hybrid SOFC performance can be affected by fuel composition and reformer performance. Biohythane (58%-CH4, 35%-CO2 and 7% H2 ) can be a better alternative providing balance between energy and biogenic emissions. Biohythane performance is studied for a 120 kW SOFC stack using ASPEN process model and compared with other feed stocks. This work is the first to study and report on the application of biohythane in SOFC systems. Biohythane was found to produce less biogenic CO2 emissions and 6% less CO at the reformer than biogas. Comparisons show that biohythane provides better efficiencies in hybrid SOFC systems. Sensitivity studies recommends operation of stack with biohythane at Steam to Carbon Ratio (STCR) = 2.0, i = 200 mA cm-2 and UF = 0.85 respectively. Highlights: Biohythane can be a better alternative to biogas as a renewable fuel for SOFCBiohythane provides better performance than biohydrogen and syngasBiohythane produces less biogenic CO2 emissions than biogasAt the reformer unit, biohythane produces 6% less CO compared with biogasHydrogen in biohythane have very little effect on water-gas shift reactionsKeywords: Biohythane, Biogas, Anaerobic Digestion, Solid Oxide Fuel Cell, Hydrogen and Biogenic CO2.INTRODUCTIONThe continuous increase in energy demand due to rapidly expanding global rural population and industrialization has created a demand for alternatives to fossil fuels [1]. Along with the energy demand, the rise in carbon emissions has advanced major efforts on improving alternative renewable technologies with minimized carbon footprint (CFP) [2]. There have been many studies to improve the carbon neutrality of renewable energy, as even renewable energy resources make some contribution to CFP [3,4]. The CFP of renewable energy resources can be minimised by cogeneration of energy, which is the simultaneous production of thermal energy with either mechanical or electrical energy [5]. Fuel cell technology has gained more interest among researchers recently as an alternative to conventional cogeneration systems, owing to its size and better conversion efficiency [6].Solid oxide fuel cell (SOFC) technology is a very promising technology for stationary standalone power generation, combined heat and power, and hybrid systems where the fuel cell is coupled with turbines to improve overall system efficiency. Fuel cells can beneficially produce more electrical energy per unit of input energy than conventional electrical generation systems [7,8]. Fuel cells have a modular design that can be configured in a number of different applications depending on the required capacity, allowing for more flexibility in the scaling of these systems. Due to the high operating temperature of SOFCs, they can utilise a wide range of fuels including syngas, natural gas and liquid fuels such as methanol and kerosene [9]. The working principle of SOFC technology is discussed elsewhere [10-14]. Combining SOFC technology with gas turbines (GT) has many advantages including increased efficiencies and low carbon emissions, and can alleviate current and future energy and environmental demands [15]. The fuel composition can potentially have a considerable effect on SOFC-GT performance. CO in the fuel significantly reduces both SOFC and hybrid module efficiency [16, 17]. This is attributed to the lower heating value and change in Gibbs free energy of CO, resulting in slightly lower cell temperatures and therefore requiring more external energy to maintain the desired anode temperature. Recently, Lv et al., 2019 [18], studied the effect of fuel composition on SOFC-GT systems and their results indicated that H2 has a slightly positive effect on performance, while CH4 and CO have a negative influence . Hence, it is important to find a good energy feedstock with less environmental impact and also provide improved SOFC-GT performance. Anaerobic Digestion (AD) in a waste water treatment plant (WWTP) can provide a renewable energy-rich gas with flexibility in fuel composition. Conventional AD processes can produce biogas (CH4-60%, CO2-40%) and biohydrogen (H2-50%, CO2-50%), depending on the bacterial conversion pathways. Detailed research has been conducted which reports on the feasibility, economics and life cycle analysis of AD-derived biogas as a fuel for SOFCs [19-24]. However, use of conventional AD-derived biogas has been shown to be problematic towards SOFC anode operation when directly fed due to carbon deposition [25]. Studies have been previously conducted into the use of methane-free hydrogen [50% H2 /50% CO2] from dark fermentation processes as a means to reduce carbon emissions from SOFCs and also to reduce carbon deposition in SOFCs [26-29]. However, use of higher H2 contents in the SOFC fuel leads to thermal shock issues because more heat is released due to the reduced cooling effect of the water-gas shift reaction (WGS) [30], with a 50 °C increase in cell temperature reported at 60% H2 content in the fuel cell. This indicates the conventional AD derived biogas and biohydrogen can be a better alternative to renewable energy feedstocks for SOFCs. In the last decade, researchers have successfully used food waste and bio waste to produce biohythane in two-stage AD pilot scale systems with a composition of CH4 (52-59%), CO2 (35-40%) and H2 (6-7%) [31-35]. This involves co-generation of biogas and biohydrogen in parallel before blending in the required proportions according to the energy and carbon emission demands for SOFCs. To the authors knowledge there is no experimental or modelling research studies reported for using biohythane as an energy feedstock for SOFCs. This current research work will be the first of its kind to study and report on the technical feasibility of using biohythane as fuel for SOFCs. An Aspen plus V10 process model is used to study the performance of biohythane in a 120 kW SOFC stack. A sensitivity analysis has been conducted to determine the ideal operating conditions of the system. Its performance against other energy feedstocks such as syngas [36-38], natural gas, biohydrogen and biogas was carried out for benchmarking. 1.1 ASPEN PROCESS MODEL DESCRIPTIONThe ASPEN process model simulator is a widely applied research tool for studying the thermo-physical behaviour of SOFCs under different conditions. The comprehensive built-in process models of the ASPEN process simulation software with extensive thermodynamic and physical property data bases, make it a very convenient and efficient way to study different chemical process systems. Recently, researches have used ASPEN to study SOFCs with planar designs in reversible modes and as hybrid systems coupled with gas turbines [39-42]. In this work, ASPEN is used to understand the technical feasibility of biohythane utilisation as an energy feedstock. Accordingly, a zero-dimensional SOFC model was built in to ASPEN plusV10 using available blocks for steady-state simulation. Design blocks and calculator blocks were used for determining the required fuel, air and steam recycle ratios. The incoming air and fuels are compressed to the desired pressure ratios in the compressor blocks. The high operating temperature requirements of the SOFC are met by combustion of the anode flue gases in a separate burner section and the same heat is utilized for heating the inlet air to avoid thermal shocks. To avoid complexity, this work assumes that a constant heat source is applied to the air in the heater block to match the anode temperature. An ambient inlet air temperature of 25 ℃ is assumed in this work. Heated air is split into nitrogen and oxygen at the cathode section which was modelled using a separator block. Compressed fuel is heated and reformed by the steam present in the recycled anode flue gas coming from the anode outlet in the reformer block. In the reformer block, all the methane reforming reactions (Equation 1, 2 and 3) are expected to occur. Equation 1 is the methane steam reforming (MSR) reaction, which is highly endothermic. The carbon monoxide yielded can be further reacted with steam to produce more hydrogen and carbon dioxide via the water-gas shift reaction (WGS), which is exothermic (Equation 2). Apart from steam present in the anode flue gas, the carbon dioxide present in the biohythane fuel is also considered to be a good methane oxidising agent [43]. This dry reforming reaction (Equation 3) is highly endothermic and requires high operating temperatures to achieve high methane conversion [44]. The net heat duty of the reformer is set to zero; hence, the endothermic nature of the reforming equations and exothermic nature of the water-gas shift reaction are properly captured in the system. The hot gas from the reformer is mixed with the oxygen coming from the cathode and passed to the anode which is set at a desired operating temperature. High operating temperatures at the anode can lead to direct oxidation of CO and hydrocarbons in the incoming fuel, but this is less favourable compared with the water-gas shift of CO to H2 and reforming of hydrocarbons to H2 [45]. Due to the complexity of the reaction system, thermodynamic equilibrium analysis is determined using the non-stoichiometric approach. In this approach, the equilibrium composition of the system is found by the direct minimization of the Gibbs free energy for a given set of species without any specification of the possible reactions that might take place in system. The anode in this system is defined as an equilibrium R-Gibbs reactor (Equation 1, 2, 3 and 4). The assumption of the R-Gibbs reactor to define the water-gas shift reaction was validated with experimental data on the SOFC [46]. The experiments were carried out for different H2 and CO2 mixtures. The R-Gibbs assumption is able to correctly predict the formation of CO and CO2 from the WGS reaction as shown in Table 1 below. A fraction of the flue gases from the anode outlet which are rich in steam is recycled back to the reformer unit while the remainder is assumed to leave for the burner unit. The recycled steam to carbon ratio (STCR) is calculated using a design specification block according to incoming hydrocarbon percentile. The ASPEN base case model is developed and validated for T-SOFC of 120 kW stack, which has been widely used in earlier literature [47,48], where a similar anode gas recycle [AGR] has been used for reforming. Voltage and fuel requirement calculations are carried out in separate calculator blocks and are discussed in the following sections. CH4+H2O ?3H2 + CO (1)CO+H2O?CO2 + H2 (2)CH4+CO2 ?2H2 + 2CO (3) H2 + 0.5 O2 →H2O (4)MATERIALS AND METHODS2.1 Voltage and Current density calculationThe cell voltage is first calculated based on the Nernst Equation. The fuel cell achieves maximum reversible voltage when no current from an external load is applied (Equation 5) [37]. The same voltage is less than the reversible voltage when current inform an external load is applied due to irreversible ohmic, activation and concentration losses (Equation 6). In Eq. (5), g is the molar Gibbs free energy of formation (J/mol) at standard pressure (1 bar), 2 represents the number of electrons produced per mole of H2 fuel reacted, F is the Faraday constant (96 485 C/mol), Tavg is the average temperature between the SOFC inlet and outlet streams (K), Rg is the molar gas constant of 8.314 J mol-1 K and Pi is the partial pressure (in bar) of the gaseous component i. The partial pressures were taken as average values of the anode and cathode inlet and outlet streams.The ohmic losses due to resistance of electron flow through the fuel cell is calculated using Equations 7-10 [49]. The subscript terms A, C, E and int refer to anode, cathode, electrolyte and interconnection respectively. Ap (radians) and Bp (radians) are the angles related to the extent of electrical contact and interconnection respectively. ρa, ρc, ρe and ρint (? m) are the resistivity terms and are calculated based on the temperature-dependent relationships shown in Table 2 [50]. The cell diameter is given by Dm (m), while the thickness of the components is given by tA, tC, tE and tInt (m) respectively. The interconnection width is defined by Wint (m). The current density is defined as i (A/m2) (Equation (10)) and is calculated based on the imposed current I (A) and total active area of the fuel cell S (m2). The corresponding ohmic losses are calculated as the summation of the aforementioned individual losses (Equation 12). The activation losses due to the electrochemical reactions is defined by Equations (13-14) as outlined in [51]. Ract,a, Ract,c (? m2) represent the specific resistance at the anode and cathode respectively, while Ka, Kc are the pre-exponential factors. The activation losses are calculated as the product of the resistance terms and current flux density (i) as shown in Equation 15. The losses due to mass transfer limitations are calculated as voltage concentration losses. A simplified method based on Fick’s Law is used (Equation 16-17), [37]. The terms iLa, ILc, are the limiting current density and their values are given Table 2. The cell concentration over potential losses can be obtained using Equation 18. The calculations described above are carried out using a design specification and calculator blocks, where the input fuel flow is varied until the SOFC stack power equals a specified value as a function of imposed current (Equation 19-23). Uf and Ua are fuel and air utilization factors. VR = -?gf 2.f+-Rg. Tavg 2.flnPH2 . P0.5O2 PH2O (5)V= VR - Vohm - Vact - Vcon (6)Vohm,a = i.ρa.(A.π.Dm)28.ta(7)Vohm,c = i.ρc.A.π.Dc28.tc.A.(A+2.(1-A-B)(8)Vohm,e =i.ρa.te (9)Vohm,int =i.ρint.(π.Dm)tintwint(10)i=IS(11)VR =Vohm,a + Vohm,c + Vohm,e + Vohm,int (12)1Ract,a=Ka.2.FRg.TPH2P0M1exp?-EaRg.T(13)1Ract,c=Kc.4.FRg.TPO2P0M2exp?-EcRg.T(14)Vact =Ract,a+ Ract,c.i(15)Vcon,a=-R.Tne.FiiL,a(16)Vcon,c=-R.Tne.FiiL,c(17)Vcon= Vcon,a+ Vcon,c(18)I= 2.F.nH2,Consumned (19)nH2,in =nH2, gas +1nCOgas + 4nCH4gas + ….. (20)Uf = nH2,consumed nH2,in (21)nO2,consumed = 0.5nH2,consumed (22)Ua = nO2,consumed nO2,in (23)2.2 Model ValidationThe developed ASPEN model predictions are compared and validated against published data from earlier work in which gas compositions and power capacities were varied (Table 3, Table 4 and Table 5). The gross and net efficiencies are calculated by Equations 24 and 25. The power output is calculated based on the gas compositions at the anode for a 92% DC to AC conversion efficiency [47, 48]. The compressor loads are also taken in to consideration in the evaluation of net efficiency, where LHV is defined as the lower heating value of the fuel. The model predictions are in good agreement with earlier works. The outlet gas compositions from the anode and pre-reformer are similar to earlier reported literature results. The voltage and current density predictions are well within the error limits of 2-3%. These slight deviations are possible due to the assumptions that the reformer net heat duty is zero and that the inlet air temperature is not heated by energy from stack gas combustion. Assumptions made in the model development do not have any major impact on the results and match well with previous published data. The aim of this research is to determine the feasibility of using biohythane derived from two-stage AD in order to reduce the carbon footprint, rather than to optimize current SOFC technology. The validated ASPEN process model is used for further studies in the work. The optimal blended mixture proportion is identified, and its results are discussed in the following sections.?eff, gross = PEL,AC nFuel,in.LHVFuel (24)?eff, net = PEL,AC -PComp nFuel,in.LHVFuel (25)RESULTS AND DISCUSSIONThe validated model was run for a biohythane composition (CH4-58%, CO2-35% AND H2-7%) derived from a two-stage AD pilot facility utilising food waste [35]. As there are no previous pilot scale studies on the integration, it was assumed the incoming fuel is at 200 °C and the air is at 25 °C. To understand the effect of various operating parameters, a sensitivity analysis has been conducted for an industrial-sized SOFC stack of 120 kW, as reported in earlier publications with baseline operating parameters of Uf/Ua/STCR at 0.85/0.19/2.3.1 Effect of Current DensityThe imposed current density was varied from 50 to 550 mA/cm2 to study its effect on the fuel cell stack operating performance (see Figure 2). An increase in current density has a positive effect on the power while it has a very significant and negative impact on the voltage which drops by 80%. The current density has an undesirable effect on the system efficiencies by creating higher fuel demands. An increase in fuel flow rates affects the loading capacity of compressors, lowering the efficiencies. Peak power performance is observed at around 450 mA/cm2; after that the power reduces and fuels cells are usually operated left of this range [48]. Hence, it is desirable to operate the stack at higher power outputs but at a reduced efficiency. However, a trade-off between operating cost and performance has to be achieved. It is therefore recommended to operate the stack between 180-200 mA/cm2, where both voltages and efficiencies are higher for this biohythane mixture. 3.2 Effect of Fuel Utilization FactorThe fuel utilization factor was varied from 0.60 to 0.95 to determine the effects on overall SOFC stack performance (Figure 3). Any increase in the fuel utilization factor leads to an increase in power density as more H2 is utilised for power generation. However, this increase in power density also leads to higher voltage losses across the cell, as according to Nernst Equation. This trend is captured in the model and reflected in Figure 3. A positive effect on efficiencies was observed, resulting in a higher power output; at higher fuel utilization, less fuel is required and hence higher efficiencies are observed. The demand for higher power generation and better efficiency will drive the operation at higher Uf; however, it is important to consider that SOFCs are a better technology when used for driving cogeneration systems irrespective of the nature of the fuel. At higher Uf, much less fuel is available for burners downstream and hence less heat energy will be generated. Owing to these characteristics, it is recommended to operate the stack for this biohythane blend at a typical range between 0.80 and 0.85, where there is still depleted fuel available for downstream combustion. 3.3 Effect of Steam to Carbon Ratio (STCR)The overall performance of the SOFC is also determined by the reforming unit characteristics. Reforming ensures maximum hydrogen is available through methane reforming reactions and reduces the risk of coke poisoning of the anode. Apart from the H2O available in the anode flue gas, CO2 present in the incoming fuel also act as a reforming agent. Earlier literature studies by Tjaden et al., 2014 and Cozzolino et al., 2017 [24, 52] have shown that for a typical biogas composition and reformer operation at 675-700°C, a low STCR of 1 is sufficient to avoid coke formation. However, coke formation is not a direct correlation for system efficiency, irrespective of whether biogas or biohythane compositions are used. Hence a detailed sensitivity analysis (Figure 4 (a-d)) needs to carried out to understand the effect of STCR on system overall performance. Figure 4a, depicts the effect of the STCR on system overall performance for various anode operating temperatures. It is observed that there is an overall improvement of 5% in system efficiency when the anode temperature is increased from 800°C to 1000?C. The increase in temperature leads to an increase of reforming flue gas temperature and it assists with the endothermic methane reforming reaction, as shown in Figure 4b. This observation is in good agreement with Cozzolino et al., 2017 [52]. Also, it is observed that there is an improvement of 3% when STCR is increased from 0.5 to 3. This result matches similar work conducted using biogas (CH4-55%, CO2-45%) reformed with recycled anode flue gas [24]. Even though SOFC systems are designed to operate at higher temperatures, the high temperatures can make the material more susceptible to faster degradation [53]. Also, as discussed earlier by La Licata et al 2011 [30], the presence of additional H2 in the incoming fuel can increase the heat stress on the reformer side due to the WGS reaction, which is exothermic. Therefore, further studies were carried out at 900 ?C operating temperature in order to avoid heat stress issues.Figure 4c depicts the performance of the reformer unit at 900°C as a function of STCR. An increase in the recycled steam flow rate to the reformer increases the reformer inlet temperature leading to a positive effect on the reforming reaction and therefore the methane conversion increases as observed in Figure 4c. Methane conversion reaches almost 100% as maximum steam is recycled.; The reformer outlet temperature is lower than the inlet temperature because the reforming reactions are endothermic. Figure 4d shows the reformer unit outlet gas composition as a function of STCR. As discussed above, an increase in steam flow rate will result in an increase of reforming reactions and an increase in H2O in the reformer flue gas. There is a very marginal decrease in the CO2 mole fraction by 1% as the recycled steam flow rate increases. This indicates that along with methane steam reforming, methane dry reforming is also occurring. Reforming reactions are highly endothermic and the required heat is provided by the increasing steam flow rate. This is observed by the increase in CO and H2 mole fractions. A good reformer operating characteristic is defined by high H2 and high CO output as this reduces coking and demonstrates adiabatic operation [54]. Hence, it is recommended to operate the SOFC with high STCR between 2 and 2.5, where a maximum efficiency of 49.5% is observed, beyond which the efficiency is asymptotic in nature. The extra steam can potentially be used for other heating purposes in downstream.3.4 Performance ComparisonBased on the above studies, the recommended operating stack characteristics will be STCR -2.0, i-200 and UF-0.85 for biohythane mixtures (CH4-58%/ CO2-35% / H2-7%). The performance of biohythane is first compared with biogas for reformer performance to understand the influence of the presence of H2 in the biohythane. The composition of biogas mixtures varies according to the source even though all are derived from AD process in WWTP. Methane percentiles range from 60-70 % and CO2 varies between 30-40%, with the remaining components being mostly nitrogen, oxygen and sulphides [55-58]. Therefore, a standard biogas composition of 60% CH4 and 40% CO2 was used for this comparison with a recommended STCR of 2, as reported earlier in [24]. The comparison study was conducted for a fuel and an air inlet temperature of 200 ?C and 25 ?C respectively, with fuel utilization at 85% for an anode working temperature of 900 ?C. It is observed that both renewable AD gases offer similar efficiencies as shown in Table 6. In the reformer unit, it is observed that the methane conversion is increased by 2% for biogas when compared to biohythane. Most importantly, the CO content in the biohythane reformed gas is lower by 6% when compared with biogas. The similar efficiency is due to the similar LHV energy content of biogas (CH4-60%, CO2-40% - 1 Kmol = 481 MJ) and biohythane (CH4-58%, CO2-35% , H2-7%- 1 Kmol = 482 MJ). This indicates biohythane can provide similar efficiencies as biogas and can provide better performance of hybrid SOFC systems due to low CO levels entering the anode. This observation agrees with Lv et al., 2019 [18].Another important observation from this comparative study is the role of H2 in the fuel. The presence of extra H2 in the biohythane fuel composition has very little effect on the reverse water gas shift reaction as less CO is produced when compared to reformed biogas fuels. A similar observation is made by [59], when they reformed biogas with different proportions of steam. Hence, it will be safe to assume that the presence of H2 in the biohythane, reduces the stoichiometric carbon content in the incoming fuel, rather than significantly influencing the reverse water gas shift reaction. The extra H2 is available as fuel for the anode reactions, which is evident from the high H2 and H2O content in both the reformer and anode flue gases for biohythane. On comparison of the anode flue gas compositions, it is observed that the equivalent biogenic CO2 emissions for biohythane are 6% less when compared with biogas. Hence, it is expected that the use of biohythane over biogas as energy feedstock in SOFCs will definitely reduce the equivalent biogenic CO2 emissions of the overall AD process and will improve the carbon foot print of waste water treatment considerably. The performance comparison is further extended for other commonly used fuel mixtures like natural gas and biomass derived syngas used in SOFC stack. The STCR for natural gas and syngas is taken from previous published literature. All of the gases are compared for a 120 kW SOFC stack. Biohydrogen (50% H2-50% CO2) derived from AD is also compared for performance. Table 7 lists all of the gaseous fuel mixtures studied in this work.Figure 5 compares the performance of the SOFC stack running on different gas mixtures. Natural gas has the highest operating efficiency and the lowest CO2 emission when compared with other hydrocarbon based fuels. A very high CH4 percentage and minimal CO2 volume makes natural gas the ideal fuel for an SOFC system. Although natural gas gives a better overall performance (51%-? and 25% GHG-CO2), it is a fossil fuel and hence other alternative gases have to be considered. The other alternative is the biomass derived syngas, but it has a low performance overall with lower efficiency (40%) and CO2 emissions (38%). The reduced efficiency of syngas is attributed to its low energy content and the increased fuel and air required to ensure power requirements are met. Also, production of syngas has its own Capex and Opex burdens due to the purification and tar removal processes involved. The other alternative, biohydrogen derived from AD process, gives higher biogenic CO2 emissions (45%) and also works with a lower efficiency (40%). This is due to low calorific value of the fuel and its dilute nature due to presence of more CO2. Hence the best fuels for a long-term and sustainable renewable energy will be AD derived biogas and biohythane due to the better efficiency and biogenic CO2 emissions. However, as discussed previously, the two-stage AD derived biohythane is expected to provide better performance when compared to single-stage AD derived biogas in terms of reforming and biogenic CO2 emissions for hybrid systems. 3.5 SummaryThe above results and discussions are summarised as follows:The ASPEN process model sensitivity studies recommend operation of an SOFC stack operating on biohythane at STCR = 2.0, i = 200 mA cm-2 and UF = 0.85. Two-stage AD derived biohythane provides similar efficiencies to natural gas and biogas. Also, biohythane provides better efficiencies than syngas and biohydrogen.H2 in the biohythane has very little effect on the water-gas shift reactions and it directly contributes to electrochemical energy generation at the anode.CO in the anode fuel can negatively impact SOFC-hybrid systems performance due to its cooling effect. Biohythane when reformed produces 6% less CO when compared with biogas. Hence, biohythane is expected to provide better performance than biogas when used as an energy feedstock for SOFC-Hybrid systems.Biogenic CO2 emissions from biohythane are also 6% less compared with biogas at the anode flue gas section. Hence biohythane can decrease the overall carbon foot print of the process.Overall, the use of biohythane as an energy feed stock for SOFCs provides many advantages to WWTPs and the energy industry with minimal environmental impact. Promotion of biohythane production from two-stage AD is more energy efficient compared with biogas produced from single-stage AD [60]. The energy industry will benefit from higher efficiencies when SOFCs are used in a hybrid setup. Most importantly, lower biogenic emissions from biohythane provides a better alternative to fossil fuels like natural gas, syn gas and biogas, which gives high biogenic CO2 emissions.CONCLUSIONSTwo-stage AD derived biohythane was studied as an energy feedstock for SOFC systems and is proposed to be a better alternative to single-stage AD derived biogas. Biohythane produces 6% less biogenic CO2 at the anode and 6% less CO in the reformer, which will benefit the anode temperature. The H2 in the fuel affects the fuel carbon stoichiometric coefficient negatively and has very little effect on the water gas shift reactions. Overall, biohythane would give good performance in SOFC hybrid systems and reduce the carbon footprint of AD processes. 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University of Applied Sciences, Novia. Laura Bailón Allegue and Jorgen Hinge., 2012. Biogas and Bio-Syngas upgrading. Danish Technological Institute.Grzegorz, B., Remigiusz, N., Janusz, S. Szmyd., 2015. An experimental and theoretical approach for the carbon deposition problem during steam reforming of model biogas. Journal of Theoritical and applied mechanics, 53, 273-284. De Gioannis, Giorgia & Muntoni, Aldo & Polettini, A & Pomi, Raffaella. (2013). A review of dark fermentative hydrogen production from biodegradable municipal waste fractions. Waste management (New York, N.Y.). 33. 10.1016/j.wasman.2013.02.019.Figure 1: ASPEN PROCESS MODEL FLOW SHEETFigure 2: The effect of Current Density on Fuel Cell Stack PerformanceFigure 3: The effect of Fuel Utilization Factor on Fuel Cell Performance. Figure 4a: The Effect of Temperature on SOFC Performance Figure 4b: The Effect of Temperature on Methane conversion in ReformerFigure 4c: The Effect of STCR on Reformer performance Figure 4d: The Effect of STCR on Reformer Outlet Gas CompositionFigure 5: Comparison of SOFC performance for different gas mixturesTable 1: Validation of ASPEN MODEL –GIBBS Reactor AssumptionGas Composition (50% H2, 50% CO2) [46] ASPEN MODEL (Gibbs Reactor)CO20.480.46CO0.040.04O200H200.04H200.48a0.46a, Back calculated based on mole balance.Table 2: Model input parametersCell Length/ Diameter (m)1.5/0.022Anode Thickness, ta (m)0.0001Cathode thickness, tc (m)0.0022Electrolyte thickness, Te (m)0.00004Interconnection thickness tint (m)0.000085Interconnection width Wint (m)0.009Anode resistivity ρA (? m)2.98*10^-5 exp(-1392/Top)Cathode resistivity ρc (? m)8.114*10^-5 exp (600/Top)Electrolyte resistivity ρE (? m)2.94*10^-4 exp (10350/Top)Interconnection resistivity ρint (? m)0.025A/B0.804/0.13Pre-exponential factor, Ka/Kc (A/m2)2.13*108 / 1.49 *1010 Slope, m0.25Activation Energy, Ea/Ec (J/mol)110000/160000Anode limiting current density, iLa (A/m2)29900Cathode limiting current density, iLc (A/m2)21600Table 3: Natural Gas Composition-1 [47]InputsNatural Gas Composition, (Mole fraction)CH4-81.3%, C2H6-0.29%, C3H8-0.4%, C4H10-0.2%, N2-14.3, CO2-0.9%Active area, (S), m296.0768Anode Operating Temperature (Top), K1183.15 Input Air temperature, C630 Input Fuel temperature, C200DC Power , kW120UF/UA/STCR0.85/0.19/1.8DC-AC inverter efficiency92%Results ComparisonLiteratureModelCurrent Density (mA/cm2)178179.5Voltage (mV)700685Net AC efficiencyNA50%Gross AC efficiency52%51%Anode Inlet gas composition (mole %)H2-27%, CO-5.6%,CH4-10.1%,H2O-27.9%,CO2-23.1%,N2-6.2%H2-26.9%, CO-5.7%,CH4-10.6%,H2O-27.6%,CO2-23%,N2-6.1%Anode exhaust gas composition (mole %)H2-11.6%, CO-7.4%, H2O-50.9%,CO2-24.9%,N2-5.1%H2-11.7%, CO-7.3%, H2O-50.8%,CO2-25%,N2-5%Table 4: Natural Gas Composition -2Extrapolated data [48]InputsNatural Gas Composition, (Mass fraction)CH4-93.8%, N2-3.8%, CO2-2.4%Active area, (S), m296.0768Anode Operating Temperature (Top), K1193.15 Input Air temperature, C20Input Fuel temperature, C200DC Power , kW127.4UF/UA/STCR0.85/0.2/2DC-AC inverter efficiency92%Results ComparisonLiteratureModelCurrent Density (mA/cm2)200.6194Voltage (mV)661672Net AC efficiencyNA50%Gross AC efficiency4851%Anode Inlet gas composition (mass %)H2-3.16%, CO-11.2%,CH4-5.81%,H2O-27.3%,CO2-51.2%,N2-1.24%H2-3%, CO-9%,CH4-7.2%,H2O-27%,CO2-52.8%,N2-1.26%Anode exhaust gas composition (mass %)H2-1.39%, CO-11.9%, H2O-39.88%,CO2-45.9%,N2-0.94%H2-1%, CO-9%, H2O-41.1%,CO2-47.8%,N2-0.9%Table 5: Syngas results comparison[48]InputsSyngas Composition, (Mole fraction)H2-34%, CO-16%, CH4-7.4%, CO2-15.8%, N2-1%, H2O-25.7%Active area, (S), m296.0768Anode Operating Temperature (Top), K910 Input Air temperature, C24.9Input Fuel temperature, C200DC Power , kW120UF/UA/STCR0.85/0.167/2.5DC-AC inverter efficiency92%Results ComparisonLiteratureModelCurrent Density (mA/cm2)188182Voltage (mV)662676Net AC efficiency37%39%Gross AC efficiency43%43%Anode Inlet gas composition (mass %)H2-29.4%, CO-7.2%,CH4-3.6%,H2O-33.1%,CO2-25.8%,N2-1%H2-32%,CO-9%,CH4-2%,H2O-31%,CO2-24.7%,N2-0.9%Anode exhaust gas composition (mass %)H2-6.2%, CO-4.2%, H2O-58.7%,CO2-30%,N2-0.9%H2-7%, CO-5%, H2O-57.7%, CO2-29%,N2-0.9%Table 6: Performance comparison for biohythane vs biogasBiogas (CH4-60%, CO2-40%)Biohythane (CH4-58%, CO2-35%, H2-7%)Fuel required, kmol/hr1.651.66Reformer inlet T, K10311023Reformer outlet T, K813812Recycled anode gas, Kmol/hr6.635.95Methane conversion at Reformer 90%88%Reformer outlet gas compositionCO0.080.075CO20.370.35H20.220.24H2O0.260.27Anode outlet gas compositionCH40.000.00CO0.0820.079CO20.370.35H20.080.086H2O0.460.48System Gross efficiency, % 49.949.5Table 7: Model Input ParametersNatural GasSyngasBioHythaneBiogasBiohydrogenCH40.820.070.580.60C2H40.000.0000C2H60.030.01000C3H80.0040.0000C4H100.0020.0000N20.1430.00.00.10CO0.0090.23000CO20.00.190.350.30.5H20.00.260.0700.5H2O0.00.23000Uf0.850.850.850.850.85STCR1.82.712.02.0NAAnode Temp,°C900900900900900Inlet air ,°C2020202020Fuel ,°C200200200200200 ................
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