Experimental study of dry reforming of biogas in a tubular anode-supported solid oxide fuel cell

The performance of a tubular Ni/YSZ anode supported SOFC directly fed by an anaerobic digestion simulated biogas, with an extra addition of carbon dioxide to operate in conservative operating conditions to avoid coking on the anode support, was investigated. The fuel cell has been tested at a fixed oven temperature of 800 °C and under biogas/CO₂ mixtures with different volumetric ratios, fuel utilization (FU) and current densities. Polarization curves and performance maps were obtained to better understand the influence of the investigated operational parameters on the cell behavior. Furthermore, since the tubular geometry enables an easy separation of the anode and cathode exhaust gases, the anode off-gas has been collected and monitored through a gas-chromatograph under open circuit voltage to investigate on the catalytic behavior of a Ni-based state-of-the-art anode. For corresponding operative conditions, performances of the cell for biogas/CO₂ 1/1.5 (i.e. CH₄/CO₂ 30/70) and 1/2 (i.e. CH₄/CO₂ 24/76) were at least 2% and 4% lower than the case 1/1 (i.e. CH₄/CO₂ 20/80), respectively. The highest efficiency of 43.4% was reached at 17.5 A and FU = 70%.     

Stability and coking of direct-methane solid oxide fuel cells: Effect of CO2 and air additions

This paper concerns the stability of anode-supported solid oxide fuel cells (SOFCs), operated with fuel mixtures of methane–CO₂ and methane–air. Stability, which was evaluated in terms of voltage decrease at constant current density, was affected by coke deposits. Chemically inert anode barrier layers were shown to enhance stability and to slow catalytic endothermic reforming reactions within the Ni–YSZ anode that otherwise caused deleterious temperature variations and cell cracking. Increasing the amount of CO₂ added to CH₄ fuel led to a wider stable operating range, yet had relatively little effect on SOFC performance. Button cells operated at 800 ° C with fuel streams of 75% CH₄ and 25% CO₂ achieved maximum power densities above 1 W/cm². Adding air to methane also increased stability. In the case of air addition, SOFC temperature increased as a consequence of exothermic partial-oxidation reforming chemistry. Models were developed to predict temperature and gas-composition profiles within the button cells. The simulation results were used to assist interpretation of the experimental observations.     

Three dimensional CFD modeling and experimental validation of an electrolyte supported solid oxide fuel cell fed with methane-free biogas

In the present study a comprehensive numerical model of a planar cross-flow electrolyte-supported solid oxide fuel cell (SOFC) is reported. This model is solved in a 3D environment using COMSOL Multiphysics software. To verify the simulation results, an experimental set-up of a six-cell stack was built. Cell temperature and current–voltage measurements are used for validation of the simulation results. Good agreement between the simulation results and the experimental measurements is achieved. Temperature validation in addition to the popular current/voltage validation ensures that the model performs well in predicting local processes like chemical reactions. In this study methane-free biogas (CO₂ + H₂) is fed to the SOFC, and the performance of the system is investigated and explained. It is concluded that the methane-free biogas reduces the cooling air flow due to endothermic reverse water gas shift reaction and gives better current density distribution over the cell compared to hydrogen.     

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.     

Internal reforming SOFC running on biogas

Direct feeding of biogas to SOFC, which is derived from municipal organic wastes, has been investigated as a carbon-neutral renewable energy system. CH₄/CO₂ ratio in the actual biogas fluctuated between 1.4 and 1.9 indicating biogas composition is strongly affected by the kinds of organic wastes and the operational conditions of methane fermentation. Using anode-supported button cells, stable operation of biogas-fueled SOFC was achieved with the internal reforming mode at 800 °C. Cell voltage above 0.8 V was recorded over 800 h at 200 mA cm⁻². It has been revealed that air addition to actual biogas reduced the risk of carbon formation and led to more stable operation without compromising cell voltage due to the lowering of anodic overvoltage.     

Tolerance tests of H2S-laden biogas fuel on solid oxide fuel cells

Biogas is a variable mixture of methane, carbon dioxide and other gases. It is a renewable resource which comes from numerous sources of plant and animal matter. Ni-YSZ anode-supported solid oxide fuel cell (SOFC) can directly use clean synthesized biogas as fuel. However, trace impurities, such as H₂S, Cl₂ and F₂ in real biogas can cause degradation in cell performance. In this research, both uncoated and coated Ni-YSZ anode-supported cells were exposed to three different compositions of synthesized biogases (syn-biogas) with 20 ppm H₂S under a constant current load at 750-850 °C and their performance was evaluated periodically using standard electrochemical methods. Postmortem analysis of the SOFC anode was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results show that H₂S causes severe electrochemical degradation of the cell when operating on biogas, leading to both complete electrochemical and mechanical failure. The Ni-CeO₂ coated cell showed excellent stability during CH₄ reforming and some tolerance to H₂S contamination.     

Biohythane as an energy feedstock for solid oxide fuel cells

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 U_F = 0.85 respectively.     

Electrochemical impedance study of the poisoning behaviour of Ni-based anodes at low concentrations of H2S in an MCFC

The effect was investigated of low H₂S concentrations, simulating biogas impurity, on the poisoning behaviour of a Ni-based, molten carbonate fuel cell anode. A conventional Ni–Cr anode was coated with ceria using dip coating to form a rare earth metal oxide thin layer on the surface of the anode. Electrochemical studies of the Ni-based samples were performed in symmetric cells under anode atmosphere (H₂, CO₂, H₂O and N₂ as balance) with 2, 6, 12, and 24 ppm of H₂S by means of electrochemical impedance spectroscopy.     

Dealing with fuel contaminants in biogas-fed solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) plants: Degradation of catalytic and electro-catalytic active surfaces and related gas purification methods

Fuel cell and hydrogen technologies are re-gaining momentum in a number of sectors including industrial, tertiary and residential ones. Integrated biogas fuel cell plants in wastewater treatment plants and other bioenergy recovery plants are nowadays on the verge of becoming a clear opportunity for the market entry of high-temperature fuel cells in distributed generation (power production from a few kW to the MW scale).High-temperature fuel cell technologies like molten carbonate fuel cells (MCFCs) and solid oxide fuel cells (SOFCs) are especially fit to operate with carbon fuels due to their (direct or indirect) internal reforming capability. Especially, systems based on SOFC technology show the highest conversion efficiency of gaseous carbon fuels (e.g., natural gas, digester gas, and biomass-derived syngas) into electricity when compared to engines or gas turbines. Also, lower CO₂ emissions and ultra-low emissions of atmospheric contaminants (SO_X, CO, VOC, especially NO_X) are generated per unit of electricity output. Nonetheless, stringent requirements apply regarding fuel purity. The presence of contaminants within the anode fuel stream, even at trace levels (sometimes ppb levels) can reduce the lifetime of key components like the fuel cell stack and reformer. In this work, we review the complex matrix (typology and amount) of different contaminants that is found in different biogas types (anaerobic digestion gas and landfill gas). We analyze the impact of contaminants on the fuel reformer and the SOFC stack to identify the threshold limits of the fuel cell system towards specific contaminants. Finally, technological solutions and related adsorbent materials to remove contaminants in a dedicated clean-up unit upstream of the fuel cell plant are also reviewed.     

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.     

Performance of an anode-supported solid oxide fuel cell with direct-internal reforming of ethanol

A theoretical study of a solid oxide fuel cell (SOFC) fed by ethanol is presented in this study. The previous studies mostly investigated the performance of ethanol-fuelled fuel cells based on a thermodynamic analysis and neglected the presence of actual losses encountered in a real SOFC operation. Therefore, the real performance of an anode-supported SOFC with direct-internal reforming operation is investigated here using a one-dimensional isothermal model coupled with a detailed electrochemical model for computing ohmic, activation, and concentration overpotentials. Effects of design and operating parameters, i.e., anode thickness, temperature, pressure, and degree of ethanol pre-reforming, on fuel cell performance are analyzed. The simulation results show that when SOFC is operated at the standard conditions (V = 0.65 V, T = 1023 K, and P = 1 atm), the average power density of 0.51 W cm⁻² is obtained and the activation overpotentials represent a major loss in the fuel cell, followed by the ohmic and concentration losses. An increase in the thickness of anode decreases fuel cell efficiency due to increased anode concentration overpotential. The performance of the anode-supported SOFC fuelled by ethanol can be improved by either increasing temperature, pressure, degree of pre-reforming of ethanol, and steam to ethanol molar ratio or decreasing the anode thickness and fuel flow rate at inlet. It is suggested that the anode thickness and operating conditions should be carefully determined to optimize fuel cell efficiency and fuel utilization.     

Performance of SOFC coupled with n-C4H10 autothermal reformer: Carbon deposition and development of anode structure

The performance deterioration of solid oxide fuel cells (SOFCs, Nickel-Yttria stabilized zirconia (Ni-YSZ)/YSZ/lanthanum doped strontium manganite-YSZ (LSM–YSZ)) coupled with n–C₄H₁₀ steam reformers (SR), autothermal reformers (ATR), or catalytic partial oxidation reformers (CPOX) was examined using an integrated system of a micro-reactor reformer and SOFC unit. The terminal voltage rapidly degraded in CPOX-driven SOFC (oxygen to carbon ratio (OCR) = 0.5) while it was fairly stable for SR-driven SOFC (steam to carbon ratio (SCR) = 2) over 250 h. For ATR-driven SOFC at near the thermoneutral point (OCR = 0.5 and steam to carbon ration (SCR) = 1.3), significant deterioration of the terminal voltage was observed in 100 h of operation. The main precursors of carbon deposition on the SOFC were identified by reformate gas analysis during the tests. In this study, we reveal that the carbon deposition on the SOFC anode can be affected by not only lower-order hydrocarbons (C₁∼C₄), but also by the CO/H₂ gas mixture. The change in electrical conductivity of the Ni-YSZ cermet used for the SOFC anode was investigated under different gas mixtures. To investigate the propensity for carbon deposition by each carbon-containing gas mixture, we defined the ratios of steam to specific carbon (C₁∼C₄ lower-order hydrocarbons and CO) in the reformate gas (SSCR, steam to specific carbon ratio). To inhibit carbon deposition on SOFC anode, the SSCR must be sufficiently high. However, the reformer operates near its maximum efficiency at low SSCR value and the higher the SSCR value, the lower the open circuit voltage and operating power density due to Nernst potential. In this study, a metal-foam supported SOFC single cell (Ni-YSZ/YSZ/Gd-doped ceria (CGO) buffer layer/lanthanum strontium cobalt ferrite-samarium doped ceria (LSCF-SDC)), impregnated with catalyst was designed; this novel SOFC was then examined for operation at a low SSCR value of the autothermal reformer conditions (near maximum efficiency of n-C₄H₁₀ reformer and thermal neutral point, SSCR = 0.5, OCR = 0.5 and SCR = 1.3). The voltage for the metal-foam supported SOFC impregnated with 0.5 wt% Rh/CGO remained at a nearly constant value, around 0.8 V, for 200 h under a constant temperature of 750 °C and current load of 250 mA cm⁻².     

A prefilter for mitigating PH3 contamination of a Ni-YSZ anode

Ni-YSZ is used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance for operation with coal-derived syngas. However, trace impurities, PH₃ H₂S AsH₃, and Sb in coal-syngas can cause SOFC degradation. Described here is a means of removing PH₃ impurity from syngas by using a Ni-based prefilter. In one test, a thin Ni-based filter was set upstream of a Ni-YSZ anode-supported SOFC. The SOFC was exposed to syngas with PH₃ under a constant current load at 800 °C. The filter decreased 20 ppm PH₃ in the feed to a level which did not degrade the SOFC for over 400 h until the filter became saturated. In another test, both H₂S and PH₃ were co-fed to the cell with Ni-based and Fe/Ni-based filters. The interaction between these two impurities did not significantly impact the filter performance with respect to PH₃ removal for both filter formulations. The cell performance was evaluated by current-voltage measurements and impedance spectroscopy. Post-mortem analyses of the cell and filter were performed by means of XRD, SEM/EDS and XPS. With proper filter design, the Ni-YSZ SOFC can operate on contaminated coal-syngas without degradation over a prescribed period of time.     

A direct-flame solid oxide fuel cell (DFFC) operated on methane,propane, and butane

This paper presents an experimental study of a direct-flame type solid oxide fuel cell (DFFC). The operation principle of this system is based on the combination of a combustion flame with a solid oxide fuel cell (SOFC) in a simple, no-chamber setup. The flame front serves as fuel reformer located a few millimeters from the anode surface while at the same time providing the heat required for SOFC operation. Experiments were performed using 13-mm-diameter planar SOFCs with Ni-based anode, samaria-doped ceria electrolyte and cobaltite cathode. At the anode, a 45-mm-diameter flat-flame burner provided radially homogeneous methane/air, propane/air, and butane/air rich premixed flames. The cell performance reaches power densities of up to 120 mW cm⁻², varying systematically with flame conditions. It shows a strong dependence on cell temperature. From thermodynamic calculations, both H₂ and CO were identified as species that are available as fuel for the SOFC. The results demonstrate the potential of this system for fuel-flexible power generation using a simple setup.     

Kinetics of (reversible) internal reforming of methane in solid oxide fuel cells under stationary and APU conditions

The internal reforming of methane in a solid oxide fuel cell (SOFC) is investigated and modeled for flow conditions relevant to operation. To this end, measurements are performed on anode-supported cells (ASC), thereby varying gas composition (y_CO = 4–15%, y_H2=5−17%, y_CO2=6−18%, y_H2O=2−30%, y_CH4=0.1−20%) and temperature (600–850 °C). In this way, operating conditions for both stationary applications (methane-rich pre-reformate) as well as for auxiliary power unit (APU) applications (diesel-POX reformate) are represented. The reforming reaction is monitored in five different positions alongside the anodic gas channel by means of gas chromatography. It is shown that methane is converted in the flow field for methane-rich gas compositions, whereas under operation with diesel reformate the direction of the reaction is reversed for temperatures below 675 °C, i.e. (exothermic) methanation occurs along the anode. Using a reaction model, a rate equation for reforming could be derived which is also valid in the case of methanation. By introducing this equation into the reaction model the methane conversion along a catalytically active Ni-YSZ cermet SOFC anode can be simulated for the operating conditions specified above.     

Hydrogen-rich syngas production and carbon dioxide formation using aqueous urea solution in biogas steam reforming by thermodynamic analysis

Biogas is a renewable biofuel that contains a lot of CH₄ and CO₂. Biogas can be used to produce heat and electric power while reducing CH₄, one of greenhouse gas emissions. As a result, it has been getting increasing academic attention. There are some application ways of biogas; biogas can produce hydrogen to feed a fuel cell by reforming process. Urea is also a hydrogen carrier and could produce hydrogen by steam reforming. This study then employes steam reforming of biogas and compares hydrogen-rich syngas production and carbon dioxide with various methane concentrations using steam and aqueous urea solution (AUS) by Thermodynamic analysis. The results show that the utilization of AUS as a replacement for steam enriches the production of H₂ and CO and has a slight CO₂ rise compared with pure biogas steam reforming at a temperature higher than 800 °C. However, CO₂ formation is less than the initial CO₂ in biogas. At the reaction temperature of 700 °C, carbon formation does not occur in the reforming process for steam/biogas ratios higher than 2. These conditions led to the highest H₂, CO production, and reforming efficiency (about 125%). The results can be used as operation data for systems that combine biogas reforming and applied to solid oxide fuel cell (SOFC), which usually operates between 700 °C to 900 °C to generate electric power in the future.     

Study of a SOFC with a bimetallic Cu–Co–ceria anode directly fuelled with simulated biogas mixtures

The present work is focused in the study of the bimetallic Cu–Co formulation combined with CeO₂ as SOFC anode, at 750 °C, direct feed of methane and two different fuel mixtures that simulate biogas. Additionally, the sulphur tolerance of new anode material has been evaluated. Its single cell evaluation, based on a samaria doped ceria (SDC) solid electrolyte and a LSM perovskite cathode, together with the electrochemical characterisation and catalytic activity tests, have allowed to demonstrate the ability of this material to operate directly with simulated biogas mixtures without loss of single cell performance due to the formation of carbon deposits or sulphur anode poisoning. The activity of this material for the exothermic oxidation of methane reduces the energy requirement of the endothermic internal methane reforming process. The cobalt doping of basic copper–ceria formulation enhanced sulphur and carbon coking tolerance of the SOFC anode material.     

Modeling electrochemical partial oxidation of methane for cogeneration of electricity and syngas in solid-oxide fuel cells

This paper uses computational models to evaluate strategies for scaling electrochemical partial oxidation (EPOX) processes from the laboratory scale to practical application. In addition to producing electrical energy alone, solid-oxide fuel cells (SOFC) can be operated with hydrocarbon fuel streams to produce synthesis gas (H₂ and CO) as well. SOFC systems are usually operated to consume most of the fuel and produce electricity. However, by operating with a hydrocarbon fuel at relatively high flow rates, the exhaust-gas composition can be predominantly syngas. In this case the steam (and CO₂), produced from electrochemical and thermal reactions, reacts to reform the hydrocarbon fuel within the catalytic anode support structure. A practical limitation of electrochemical partial oxidation operation is the fact that carbon tends to deposit on Ni-based anode catalysts. The present paper explores the use of barrier layers to prevent carbon deposits. The results show that a tubular cell can be designed to deliver syngas and electricity using methane as the primary fuel.     

Improvement of anode performance by surface modification for solid oxide fuel cell running on hydrocarbon fuel

A Ni/ yttria-stabilized zirconia (YSZ) cermet anode was modified by coating with samaria-doped ceria (SDC, Sm_0.2Ce_0.8O₂) sol within the pores of the anode for a solid oxide fuel cell (SOFC) running on hydrocarbon fuel. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the triple phase boundary (TPB), which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode made it possible to have good stability for long-term operation due to low carbon deposition and nickel sintering.The maximum power density of an anode-supported cell (electrolyte; 8 mol% YSZ and thickness of 30 μm, and cathode; La_0.85Sr_0.15MnO₃) with the modified anode was about 0.3 W/cm² at 700 °C in the mixture of methane (25%) and air (75%) as the fuel and air as the oxidant. The cell was operated for 500 h without significant degradation of cell performance.     

Experimental characterization and mechanistic modeling of carbon monoxide fueled solid oxide fuel cell

The paper presents an elementary reaction based solid oxide fuel cell (SOFC) model coupled with anodic elementary heterogeneous reactions and electrochemical charge transfer reactions for CO/CO₂ fuel based on an anode supported button cell. The model is calibrated and validated using experimental data obtained for various CO/CO₂ fuel compositions at 750, 800 and 850 °C. The comparison shows that the modeling results agree well with the experimental data. The effects of operating conditions on the cell performance and the detailed species concentration distribution are predicted. Then, the carbon deposition on the SOFC anode with CO/CO₂ fuel is experimentally measured and simulated using the elementary reaction model. The results indicate that lower temperature and lower operation voltage are helpful to reduce the possibilities of carbon deposition on Ni particle surfaces.