Is steam addition necessary for the landfill gas fueled solid oxide fuel cells?

Landfill gas in Hong Kong – a mixture of about 50% (by volume) CH₄ and 50% CO₂ – can be utilized for power generation in a solid oxide fuel cell (SOFC). Conventional way of utilizing CH₄ in a SOFC is by adding H₂O to CH₄ to initiate methane steam reforming (MSR) and water gas shift reaction (WGSR). As the methane carbon dioxide reforming (MCDR: CH₄ + CO₂ ↔ 2CO + 2H₂) is feasible in the SOFC anode, it is unknown whether H₂O is needed or not for landfill gas fueled SOFC. In this study, a numerical model is developed to investigate the characteristics of SOFC running on landfill gas. Parametric simulations show that H₂O addition may decrease the performance of short SOFC at typical operating conditions as H₂O dilute the fuel concentration. However, it is interesting to find that H₂O addition is needed at reduced operating temperature, lower operating potential, or in SOFC with longer gas channel, mainly due to less temperature reduction in the downstream and easier oxidation of H₂ than CO. This preliminary study could help identify strategies for converting landfill gas into electrical power in Hong Kong.     

Modeling of methane fed solid oxide fuel cells: Comparison between proton conducting electrolyte and oxygen ion conducting electrolyte

An electrochemical model was developed to study the methane (CH₄) fed solid oxide fuel cell (SOFC) using proton conducting electrolyte (SOFC-H) and oxygen ion conducting electrolyte (SOFC-O). Both the internal methane steam reforming (MSR) and water gas shift (WGS) reactions are considered in the model. Previous study has shown that the CH₄ fed SOFC-H had significantly better performance than the SOFC-O. However, the present study reveals that the actual performance of the CH₄ fed SOFC-H is considerably lower than the SOFC-O, partly due to higher ohmic overpotential of SOFC-H. It is also found that the CH₄ fed SOFC-H has considerably higher cathode concentration overpotential and lower anode concentration overpotential than the SOFC-O. The anode concentration overpotentials of the CH₄ fed SOFC-H and SOFC-O are found to decrease with increasing temperature, which is different from previous analyses on the H₂ fed SOFC. Therefore, high temperature is desirable for increasing the potential of the CH₄ fed SOFC. It is also found that there exist optimal electrode porosities that minimize the electrode total overpotentials. The analyses provided in this paper signify the difference between the CH₄ fed SOFC-H and SOFC-O. The model developed in this paper can be extended to 2D or 3D models to study the performance of practical SOFC systems.     

Electrochemical properties and thermal neutral state of solid oxide fuel cells with direct internal reforming of methane

Solid oxide fuel cells (SOFCs) with direct internal reforming (DIR) provide a promising method to realize clean and efficient utilization of hydrocarbon fuels. Thse endothermic reforming reactions occur simultaneously with exothermic electrochemical reactions at the anode, making thermal neutral state achievable inside a fuel cell, providing reference to the thermal management. In this study, a calculation model combining experimental data and thermodynamic results was established, validating the possibility of achieving thermal neutral state in DIR-SOFCs. In the process of modeling, the electrochemical and thermodynamic characteristics in direct internal steam and dry reforming were elaborately compared, contributing to a more scientific understanding of anode reaction mechanism. Detailed experimental investigation was carried out to determine the influence of H₂O/CO₂ on the electrochemical properties of DIR-SOFCs, based on which the optimum steam-carbon ratio (S/C) and CO₂ to CH₄ ratios were obtained. Besides, analysis of distribution of relaxation times (DRT) combined with elementary reactions in CH₄H₂O and CH₄CO₂ atmospheres were proposed to distinguish different physical and chemical processes within anodes. The results of this study can be conducive to a more precise understanding of reaction mechanism on SOFC anodes and meaningful for practical application of DIR-SOFCs.     

Production of H2 via sorption enhanced auto-thermal reforming for small scale Applications-A process modeling and machine learning study

Small scale production of H₂ via sorption enhanced auto-thermal reforming (SEATR) of methane is simulated using Ni based catalyst and CaO sorbent for the capturing of CO₂. One dimensional heterogeneous reactor model was developed using gPROMS model builder to study the performance of SEATR reactor. At low pressure mode, the process was evaluated for varying temperature, pressure, gas flux and steam to carbon ratio. Chemical equilibrium with application (CEA), an equilibrium based software was employed so as to compare both equilibrium and simulation results. Under a range of temperature (500–1000 K), pressure (1–10 bar), S/C ratio (1–6), and O/C ratio (0.2–0.6) close to equilibrium conditions, model outputs satisfactory results with regard to CH₄ conversion, CO₂ capturing, H₂ yield and purity. At 750 K, 2.9 bar, G_s of 0.4 kg/m² s, S/C of 3 and O/C of 0.45, H₂ purity and CH₄ conversion achieved was 97% and 94% respectively in comparison with 66% and 77% from conventional auto-thermal reforming. In Bayesian Regularization (BR), Mean square error(MSE) and R value is minimum for neural network algorithm comparison. It accounts for 1.2e⁻¹⁰ and 0.999 respectively. BR produces minimum error with increase in Epochs and gradients values highlighting maximum performance with optimize computation time for process modeling data-integration studies and generalization.     

Steam reforming of CH4 at low temperature on Ni/ZrO2 catalyst: Effect of H2O/CH4 ratio on carbon deposition

In present work, the H₂O/CH₄ on carbon deposition in SRM reaction over Ni/ZrO₂ was studied. Prepared by impregnation, the catalysts were characterized by TEM, XPS, H₂-TPR, TG-DSC-MS, Raman, and XRD. The results showed that when H₂O/CH₄ = 2 with GHSV of 14460 h⁻¹, the highest CH₄ conversion (46.8%) was achieved on the Ni/ZrO₂ catalyst at 550 °C. This was due to the relatively high surface content of Ni⁰ (27%) species and the formation of easy removal polymer carbon (C₁). When the H₂O/CH₄ was 1, a large amount of difficult to remove fiber carbon (C₂) formed and polymer carbon would wrap around the catalyst to reduce its activity. However, excess water might promote surface reconstruction by converting Ni⁰ to Ni(OH)₂, which reduced the surface Ni⁰ content and then inhibited the catalytic activity, while the amount of carbon deposited, especially C₂, reduced significantly.     

Dry and steam reforming of biomass pyrolysis gas for rich hydrogen gas

Biomass pyrolysis gas (including H₂, CO, CH₄, CO₂, C₂H₄, C₂H₆ and etc.) reforming for hydrogen production over Ni/Fe/Ce/Al₂O₃ catalysts was presented in this study. This study investigated how the operating conditions, such as the calcinations temperature of catalysts, the reaction temperature, the gas hourly space velocity (GHSV) and the ratio of H₂O/C, affect the conversion of CH₄ and CO₂ and the selectivity of hydrogen from dry and steam reforming of pyrolysis gas. The experimental results indicated that, under the conditions: the reaction temperature of 600 °C, the GHSV of 900 h⁻¹ and H₂O/C of 0.92, the reaction efficiency is the optimal. Especially, the concentration of H₂, CO, CH₄, CO₂, and C₂H_n (C₂H₄ and C₂H₆) were 36.80%, 10.48%, 9.61%, 42.62%, 0.49% respectively. The conversion of CH₄ and CO₂ reached 45.9% and 51.09%, respectively. There were all kinds of reactions during the processing of reforming of pyrolysis gas. And the main reactions changed with the operation condition. It was due to the promoting or inhibiting interaction among different constituents in the pyrolysis gas and the different activity of catalysts in the different operation condition.     

Metal foam-supported Pd–Rh catalyst for steam methane reforming and its application to SOFC fuel processing

Pd–Rh/metal foam catalyst was studied for steam methane reforming and application to SOFC fuel processing. Performance of 0.068 wt% Pd–Rh/metal foam catalyst was compared with 13 wt% Ni/Al₂O₃ and 8 wt% Ru/Al₂O₃ catalysts in a tubular reactor. At 1023 K with GHSV 2000 h⁻¹ and S/C ratio 2.5, CH₄ conversion and H₂ yield were 96.7% and 3.16 mol per mole of CH₄ input for Pd–Rh/metal foam, better than the alumina-supported catalysts. In 200 h stability test, Pd–Rh/metal foam catalyst exhibited steady activity. Pd–Rh/metal foam catalyst performed efficiently in a heat exchanger platform reactor to be used as prototype SOFC fuel processor: at 983 K with GHSV 1200 h⁻¹ and S/C ratio 2.5, CH₄ conversion was nearly the same as that in the tubular reactor, except for more H₂ and CO₂ yields. Used Pd–Rh/metal foam catalyst was characterized by SEM, TEM, BET and CO chemisorption measurements, which provided evidence for thermal stability of the catalyst.     

Experimental evaluation of methanol steam reforming reactor heated by catalyst combustion for kW-class SOFC

The distributed power generation of methanol steam reforming reactor combined with solid oxide fuel cell (SOFC) has the characteristics of outstanding economic advantages. In this paper, a methanol steam reforming reactor was designed which integrates catalyst combustion, vaporization and reforming. By catalyst combustion, it can achieve stable operation to supply fuel for kW-class SOFC in real time without additional heating equipment. The optimal operating condition of the reforming reactor is 523–553 K, and the steam to carbon ratio (S/C) is 1.2. To study the reforming performance, methanol steam reforming (MSR), methanol decomposition (MD), water-gas shift (WGS) were considered. Operating temperature is the greatest factor affecting reforming performance. The higher the reaction temperature, the lower the H₂ and CO₂, the higher the CO and the methanol conversion rate. The methanol conversion rate is up to 95.03%. The higher the liquid space velocity (LHSV), the lower the methanol conversion rate, the lowest is 90.7%. The temperature changes of the reforming reactor caused by the load change of stack takes about 30 min to reach new balance. Local hotspots within the reforming reactor lead to an excessive local temperature to test a small amount of CH₄ in the reforming gas. The methanation reaction cannot be ignored at the operating temperature. The reforming gas contains 70–75% H₂, 3–8% CO, 18–22% CO₂ and 0.0004–0.3% CH₄. Trace amounts of C₂H₆ and C₂H₄ are also found in some experiments. The reforming reactor can stably supply the fuel for up to 1125 W SOFC.     

Modelling of H2 production via sorption enhanced steam methane reforming at reduced pressures for small scale applications

The production of H₂ via sorption enhanced steam reforming (SE-SMR) of CH₄ using 18 wt % Ni/Al₂O₃ catalyst and CaO as a CO₂-sorbent was simulated for an adiabatic packed bed reactor at the reduced pressures typical of small and medium scale gas producers and H₂ end users. To investigate the behaviour of reactor model along the axial direction, the mass, energy and momentum balance equations were incorporated in the gPROMS modelbuilder^®. The effect of operating conditions such as temperature, pressure, steam to carbon ration (S/C) and gas mass flow velocity (G_s) was studied under the low-pressure conditions (2–7 bar). Independent equilibrium based software, chemical equilibrium with application (CEA), was used to compare the simulation results with the equilibrium data. A good agreement was obtained in terms of CH₄ conversion, H₂ yield (wt. % of CH₄ feed), purity of H₂ and CO₂ capture for the lowest (G_s) representing conditions close to equilibrium under a range of operating temperatures pressures, feed steam to carbon ratio. At G_s of 3.5 kg m⁻²s⁻¹, 3 bar, 923 K and S/C of 3, CH₄ conversion and H₂ purity were up to 89% and 86% respectively compared to 44% and 63% in the conventional reforming process.     

The effects of fuel variability on the electrical performance and durability of a solid oxide fuel cell operating on biohythane

Biohythane is typically composed of 60/30/10 vol% CH₄/CO₂/H₂ and can be produced via two-stage anaerobic digestion of renewable and low carbon biomass with much greater efficiency compared with CH₄/CO₂ biogas. This work investigates the effects of fuel variability on the electrical performance and fuel processing of a commercially available anode supported solid oxide fuel cell (SOFC) operating on biohythane mixtures at 750 °C. Cell electrical performance was characterised using current-voltage curves and electrochemical impedance spectroscopy. Fuel processing was characterised using quadrupole mass spectroscopy. It is shown that when H₂/CO₂ is blended with CH₄ to make biohythane, the SOFC efficiency is significantly increased, high SOFC durability is achieved, and there are considerable savings in CH₄ consumption. Enhanced electrical performance was due to the additional presence of H₂ and promotion of CH₄ dry reforming, the reverse Boudouard and reverse water-gas shift reactions. These processes alleviated carbon deposition and promoted electrochemical oxidation of H₂ as the primary power production pathway. Substituting 50 vol% CH₄ with 25/75 vol% H₂/CO₂ was shown to increase cell power output by 81.6% at 0.8 V compared with pure CH₄. This corresponded to a 3.4-fold increase in the overall energy conversion efficiency and a 72% decrease in CH₄ consumption. A 260 h durability test demonstrated very high cell durability when operating on a typical 60/30/10 vol% CH₄/CO₂/H₂ biohythane mixture under high fuel utilisation due to inhibition of carbon deposition. Overall, this work suggests that decarbonising gas grids by substituting natural gas with renewably produced H₂/CO₂ mixtures (rather than pure H₂ derived from fossil fuels), and utilising in SOFC technology, gives considerable gains in energy conversion efficiency and carbon emissions savings.     

Numerical investigation of components influence on characteristics of autothermal reforming of methane in micro premix chamber

This paper focuses on investigating that the influence of O₂, CO₂ and H₂O on characteristics of autothermal reforming of methane in micro premix chamber on Ni catalysts. In addition, the effect of catalytic wall temperature on autothermal reforming reaction of methane under a certain ratio of CH₄/CO₂/H₂O/O₂ is simulated. The results indicate that appropriately increasing O₂ concentration can increase the conversion efficiency of CH₄, so does adding CO₂ or H₂O. The positive effect of O₂, CO₂ and H₂O is more pronounced at the higher temperature. The temperature range of 650–750 K is the important transitional region in the reactions of CH₄/O₂, CH₄/H₂O and CH₄/CO₂. It also gives a guide to the available range of parameters in the high efficiency reforming process of micro-reactor.     

Steam enhanced carbon dioxide reforming of methane in DBD plasma reactor

Considering the inevitable high energy input to implement the CO₂ reforming of methane under high-temperature operation using conventional catalysis method, the low temperature conversion of CO₂ and methane in the coaxial dielectric barrier discharge (DBD) plasma reactor was investigated in this work. Steam was introduced to enhance the CO₂ reforming of methane with synergetic catalysis effect by cold plasma and catalyst. The experimental results showed that a certain percent of steam could promote the conversion of both CH₄ and CO₂. Meanwhile, the carbon deposition was evidently reduced compared with the dry reforming of methane. With the increase of steam input, the steam reforming occurred predominantly. As a result, the hydrogen volume percentage in the product gases increased. In this way, the products with different H₂/CO ratio could be achieved by changing the mole ratio of CH₄/CO₂/H₂O at the reactor inlet. In particular, when the mole ratio of H₂O/CH₄ increased to almost 3 corresponding to the pure steam reforming process, the conversion of CH₄ reached almost 0.95 and the selectivity to H₂ was almost 0.99 at 773K.     

Development of paper-structured catalyst for application to direct internal reforming solid oxide fuel cell fueled by biogas

A flexible paper-structured catalyst (PSC) that can be applied to the anode of a solid oxide fuel cell (SOFC) was examined for its potential to enable direct internal reforming (DIR) operation. The catalytic activity of three types of Ni-loaded PSCs: (a) without the dispersion of support oxide particles in the fiber network (PSC-A), (b) with the dispersion of (Mg,Al)O derived from hydrotalcite (PSCB), and (c) with the dispersion of (Ce,Zr)O_2-δ (PSCC), for dry reforming of CH₄ was evaluated at operating temperatures of 650–800 °C. Among the PSCs, PSC-C exhibited the highest CH₄ conversion with the lowest degradation rate. The electrochemical performance of an electrolyte-supported cell (ESC) was evaluated under the flow of simulated biogas at 750 °C for cases without and with the PSCs on the anode. The application of the PSCs improved the cell performance. In particular, PSC-C had a remarkably positive effect on stabilizing DIRSOFC operation fueled by biogas.     

Gasification of torrefied oil palm biomass in a fixed-bed reactor: Effects of gasifying agents on product characteristics

Gasification represents an attractive pathway to generate fuel gas (i.e., syngas (H₂ and CO) and hydrocarbons) from oil palm biomass in Malaysia. Torrefaction is introduced here to enhance the oil palm biomass properties prior to gasification. In this work, the effect of torrefaction on the gasification of three oil palm biomass, i.e., empty fruit bunches (EFB), mesocarp fibres (MF), and palm kernel shells (PKS) are evaluated. Two gasifying agents were used, i.e., CO₂ and steam. The syngas lower heating values (LHV_syngas) for CO₂ gasification and steam gasification were in the range of 0.35–1.67 MJ m⁻³ and 1.61–2.22 MJ m⁻³, respectively. Compared with EFB and MF, PKS is more effective for fuel gas production as indicated by the more dominant emission of light hydrocarbons (CH₄, C₂H₄, and C₂H₆) in PKS case. Gasification efficiency was examined using carbon conversion efficiency (CCE) and cold gas efficiency (CGE). CCE ranges between 4% and 55.1% for CO₂ gasification while CGE varies between 4.8% and 46.2% and 27.6% and 62.9% for CO₂ gasification and steam gasification, respectively. Our results showed that higher concentration of gasifying agent promotes higher carbon conversion and that steam gasification provides higher thermal efficiency (CGE) compared to CO₂ gasification.     

Thermal field under the effect of the chemical reaction of a direct internal reforming solid oxide fuel cell DIR-SOFC

The effects of direct internal reforming in a fuel cell solid oxide (SOFC) on thermal fields are studied by mathematical modeling. This study presents the thermal fields of a standard fuel cell (Ni-YSZ/YSZ/LSM) anode supported. This study is also made in the perpendicular plane at the flow of gases. The fuel cell is powered by air and fuel, CH₄, H₂, CO₂, CO and H₂O hence the birth of the phenomenon of direct internal reforming (DIR-SOFC). It is based on reforming chemical reactions, steam reforming reaction and water–gas shift reaction. The main purpose of this work is the visualization of temperature fields under the influence of global chemical reactions and the confirmation of the thermal behavior of this chemical reaction. The thermal fields are obtained by a computer program (FORTRAN).     

Modelling of solid oxide fuel cells with internal glycerol steam reforming

The direct application of glycerol in solid oxide fuel cell (SOFC) for power generation has been demonstrated experimentally but the detailed mechanisms are not well understood due to the lack of comprehensive modeling study. In this paper, a numerical model is developed to study the glycerol-fueled SOFC. After model validation, the simulated SOFC demonstrates a performance of 7827 A m^?2 at 0.6 V, with a glycerol conversion rate of 49% at 1073 K. Then, parametric analyses are conducted to understand the effects of operation conditions on cell performance. It is found that the SOFC performance increases with decreasing operating voltage or increasing inlet temperature. However, increasing either the fuel flow rate or steam to glycerol ratio could decrease the cell performance. It is also interesting to find out that the contribution of H₂ and CO to the total current density is significantly different under various operating conditions, even sometimes CO dominates while H₂ plays a negative role. This is different from our conventional understanding that usually H₂ contributes more significantly to current generation. In addition, cooling measures are needed to ensure the long-term stability of the cell when operating at a high current density.     

Self-sustained operation of a kWe-class kerosene-reforming processor for solid oxide fuel cells

In this paper, fuel-processing technologies are developed for application in residential power generation (RPG) in solid oxide fuel cells (SOFCs). Kerosene is selected as the fuel because of its high hydrogen density and because of the established infrastructure that already exists in South Korea. A kerosene fuel processor with two different reaction stages, autothermal reforming (ATR) and adsorptive desulfurization reactions, is developed for SOFC operations. ATR is suited to the reforming of liquid hydrocarbon fuels because oxygen-aided reactions can break the aromatics in the fuel and steam can suppress carbon deposition during the reforming reaction. ATR can also be implemented as a self-sustaining reactor due to the exothermicity of the reaction. The kW_e self-sustained kerosene fuel processor, including the desulfurizer, operates for about 250 h in this study. This fuel processor does not require a heat exchanger between the ATR reactor and the desulfurizer or electric equipment for heat supply and fuel or water vaporization because a suitable temperature of the ATR reformate is reached for H₂S adsorption on the ZnO catalyst beds in desulfurizer. Although the CH₄ concentration in the reformate gas of the fuel processor is higher due to the lower temperature of ATR tail gas, SOFCs can directly use CH₄ as a fuel with the addition of sufficient steam feeds (H₂O/CH₄ ≥ 1.5), in contrast to low-temperature fuel cells. The reforming efficiency of the fuel processor is about 60%, and the desulfurizer removed H₂S to a sufficient level to allow for the operation of SOFCs.     

Syngas production via partial oxidation of dimethyl ether over Rh/Ce0.75Zr0.25O2 catalyst and its application for SOFC feeding

Dimethyl ether (DME) partial oxidation (PO) was studied over 1 wt% Rh/Ce_0.75Zr_0.25O₂ catalyst at temperatures 300–700 °C, O₂:C molar ratio of 0.25 and GHSV 10000 h⁻¹. The catalyst was active and stable under reaction conditions. Complete conversion of DME was reached at 500 °C, but equilibrium product distribution was observed only at T ≥ 650 °C. High concentration of CH₄ and low contents of CO and H₂ were observed at 500–625 °C 75 cm³ of composite catalyst 0.24 wt% Rh/Ce_0.75Zr_0.25O₂/Al₂O₃/FeCrAl showed excellent catalytic performance in DME PO at O₂:C molar ratio of 0.29 and inlet temperature 840 °C which corresponded to carbon-free region. 100% DME conversion was reached at GHSV of 45,000 h⁻¹. The produced syngas contained (vol. %): 33.4 H₂, 34.8 N₂, 22.7 CO, 3.6 CO₂ and 1.6 CH₄. Composite catalyst demonstrated the specific syngas productivity (based on CO and H₂) in DME PO of 42.8 m³·L_cat⁻¹·h⁻¹ (STP) and the syngas productivity of more than 3 m³·h⁻¹ (STP) that was sufficient for 3 kW_e SOFC feeding. PO of natural gas and liquified petroleum gas can be carried out over the same catalyst with similar productivity, realizing the concept of multifuel hydrogen generation. The syngas composition obtained via DME PO was shown to be sufficient for YSZ-based SOFC feeding.     

Investigation of La0.6Sr0.4Co1-xNixO3-δ (x=0, 0.2, 0.4, 0.6, 0.8) catalysts on solid oxide fuel cells anode for biogas dry reforming

The introduction of catalyst on anode of solid oxide fuel cell (SOFC) has been an effective way to alleviate the carbon deposition when utilizing biogas as the fuel. A series of La_0.6Sr_0.4Co_1-xNi_xO_3-δ (x = 0, 0.2, 0.4, 0.6, 0.8) oxides are synthesized by sol-gel method and used as catalysts precursors for biogas dry reforming. The phase structure of La_0.6Sr_0.4Co_1-xNi_xO_3-δ oxides before and after reduction are characterized by X-ray diffraction (XRD). The texture properties, carbon deposition, CH₄ and CO₂ conversion rate of La_0.6Sr_0.4Co_1-xNi_xO_3-δ catalysts are evaluated and compared. The peak power density of 739 mW cm^?2 is obtained by a commercial SOFC with La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst at 850 °C when using a mixture of CH₄: CO₂ = 2:1 as fuel. This shows a great improvement from the cell without catalyst for internal dry reforming, which is attributed to the formation of NiCo alloy active species after reduction in H₂ atmosphere. The results indicate the benefits of inhibiting the carbon deposition on Ni-based anode through introducing the La_0.6Sr_0.4Co_0.4Ni_0.6O_3-δ catalyst precursor. Additionally, the dry reforming technology will also help to convert part of the exhaust heat into chemical energy and improve the efficiency of SOFC system with biogas fuel.