Effect of various coal contaminants on the performance of solid oxide fuel cells: Part I. Accelerated testing

The contaminants that are potentially present in the coal-derived gas stream and their thermochemical nature are discussed. Accelerated testing was carried out on Ni-YSZ/YSZ/LSM solid oxide fuel cells (YSZ: yttria stabilized zirconia and LSM: lanthanum strontium manganese oxide) for eight main kind of contaminants: CH₃Cl, HCl, As, P, Zn, Hg, Cd and Sb at the temperature range of 750-850 °C. The As and P species, at 10 and 35 ppm, respectively, resulted in severe power density degradation at temperatures 800 °C and below. SEM and EDX analysis indicated that As attacked the Ni region of the anode surface and the Ni current collector, caused the break of the current collector and the eventual cell failure at 800 °C. The phosphorous containing species were found in the bulk of the anode, they were segregated and formed “grain boundary” like phases separating large Ni patches. These species are presumably nickel phosphide/phosphate and zirconia phosphate, which could break the Ni network for electron transport and inhibit the YSZ network for oxygen ion transport. The presence of 40 ppm CH₃Cl and 5 ppm Cd only affected the cell power density at above 800 °C and Cd caused significant performance loss. Whereas the presence of 9 ppm Zn, 7 ppm Hg and 8 ppm Sb only degraded the cell power density by less than 1% during the 100 h test in the temperature range of 750-850 °C.     

Effect of various coal contaminants on the performance of solid oxide fuel cells: Part II. ppm and sub-ppm level testing

The poisoning effects of various trace contaminants in the coal-derived syngas stream at ppm and sub-ppm level on the performance of Ni-YSZ/YSZ/LSM solid oxide fuel cells were studied at extended duration. The thermochemical nature of impurities such as PH₃(g) and CH₃Cl(g) in presence and absence of water steam was analyzed by a high temperature mass spectrometer. Only less than half of PH₃(g) is hydrolyzed, and CH₃Cl(g) also co-exist with HCl(g). After a certain duration of exposure, 1 ppm AsH₃(g), 0.5 ppm PH₃(g), and 2.5 ppm CH₃Cl(g) all caused some degree of degradation to the power density at 750 °C. Whereas 1 ppm of H₂S(g) resulted in immediate performance loss. The mechanisms of degradation are mainly divided into two categories: surface adsorption effect (for S and Cl) and bulk reaction effect (for As and P). The controversies regarding the poisoning effect and mechanism of S are also discussed with the aid of thermodynamic equilibrium composition calculation.     

Effects of coal syngas impurities on anodes of solid oxide fuel cells

A literature review is conducted to summarize the studies on the identification of impurities in coal syngas and their effects on the performance of Ni-yttria stabilized zirconia (Ni-YSZ) anode of solid oxide fuel cells (SOFCs). Coal syngas typically contains major species, CO, H₂, CO₂, H₂O, CH₄, N₂, and H₂S as well as trace impurities. Thermodynamic equilibrium calculations have indicated that trace impurities species such as Be, Cr, K, Na, and V in the coal syngas form condensed phases under warm gas cleanup conditions and can be effectively removed by the cleanup processes. For meaningful data comparison, a practical parameter is formulated to quantify the level of degradation normalized with respect to the relevant experimental parameters. Experimental results show that the existence of Hg, Si, Zn and NH₃ in the coal syngas does not significantly affect the performance of the Ni-YSZ anode. The presence of Cd and Se in the syngas impacts the SOFC anode performance to some extent. Impurity species such as Cl, Sb, As, and P cause severe cell voltage degradation due to attack on the Ni-YSZ anode. Sb, As and P have the potential to react with Ni to form secondary phases in the Ni-YSZ anode, which deteriorate the catalytic activity of the anode.     

Effects of coal syngas major compositions on Ni/YSZ anode-supported solid oxide fuel cells

This paper presents a systematical evaluation of the effects of CO₂, H₂O, CO, N₂ and CH₄ in the coal syngas on the properties of typical Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The results show that CO₂, H₂O, CO, N₂ and CH₄ have complicated effects on the cell performance and the electrochemical impedance spectra (EIS) analysis reveals the addition of these gases influences electrode processes such as the oxygen ion exchange from YSZ to anode TPBs, the charge transfer at the anode TPBs, gas diffusion and conversion at the anode. Two kinds of mixture gases with different compositions are thus constituted and used as fuel for aging test on two cells at 750 °C. No degradation or carbon deposition is observed for the cell fueled with 40% H₂-20% CO-20% H₂O-20% CO₂ for 360 h while the cell fueled with 50% H₂-30% CO-10% H₂O-10% CO₂ exhibits an abrupt degradation after 50 h due to the severe carbon deposition.     

The effect of coal syngas containing HCl on the performance of solid oxide fuel cells: Investigations into the effect of operational temperature and HCl concentration

The performance of solid oxide fuel cells (SOFCs) using simulated coal-derived syngas, with and without hydrogen chloride (HCl), was studied. Electrolyte-supported SOFCs were tested potentiostatically at 0.7 V at 800 and 900 °C with simulated coal syngas containing 0, 20, and 160 ppm HCl. The results from the tests without HCl show good performance with little degradation over 100 h of operation. Both 20 and 160 ppm HCl were shown to cause performance losses in the SOFCs after injection into the system. Although the tests presented in this paper show that HCl does cause degradation to SOFC performance, the cell performance was recoverable upon the removal of HCl from the fuel. Also recent results from anticipated Integrated Gasification Combined Cycle IGCC warm/hot-gas-cleanup technologies suggest that HCl will be removed to levels that will not cause any significant performance losses in SOFCs.     

The use of hydrogen-depleted coal syngas in solid oxide fuel cells

Electrolyte-supported solid oxide fuel cells were operated using hydrogen, simulated coal syngas, and a hydrogen-depleted syngas as fuel. Open circuit voltages closely matched theoretical predictions, and carbon deposition was completely avoided. When 50% of the hydrogen flow rate was removed from the syngas mixture while increasing the overall flow rate to maintain the same fuel utilization, the drop in power density of the cell was less than 8%. Switching from syngas to hydrogen-depleted syngas caused an average increase in the area-specific resistance of 0.027 Ω cm², or 4%. The results of this study suggest that solid oxide fuel cells could operate successfully using syngas that has been partially stripped of hydrogen for other purposes.     

High performance solid oxide fuel cell operating on dry gasified coal

A fluidized coal bed-solid oxide fuel cell (FB-SOFC) arrangement is employed for efficient conversion of dry gasified coal into electricity at 850 °C. It consists of an anode-supported tubular solid oxide fuel cell of 24 cm² active area coupled to a Boudouard gasifier. A minimally fluidized bed of low sulfur (0.15 wt%) Alaska coal is gasified at 930 °C by flowing CO₂ to generate CO. The resulting CO fuel is oxidized at the Ni/YSZ cermet anode. The highest cell power density achieved is 0.45 W cm⁻² at 0.64 V with 35.7% electrical conversion efficiency based on CO utilization. This power density is the highest reported in the literature for such systems and corresponds to a total power generation of 10.8 W by this cell. Similarly, 48.4% is the highest conversion efficiency measured at a power density of 0.30 W cm⁻² and 0.7 V. The open circuit voltages are in good agreement with values expected based on thermodynamic data.     

Effects of coal syngas and H2S on the performance of solid oxide fuel cells: Part 2. Stack tests

The performance of two-cell planar solid oxide fuel cell stacks using coal syngas, with and without hydrogen sulfide (H₂S), was studied. All cells were tested at 850 °C with a constant current load of 15.2 A (current density of 0.22 A cm⁻² per cell) and 30% fuel utilization. The H₂S injection immediately and significantly affected the power degradation of the stack system regardless of the carrier fuel. Results for the test with only H₂ and N₂ in the presence of H₂S (119–120 ppm) indicated that the power decay and area-specific resistance (ASR) degradation values were lower than those for the tests where simulated syngas containing CO and increased water content was used. The results indicate that contact points in the stack contributed to the power degradation of the system. Other factors, including contamination from the upstream fuel gas tubing, may have contributed to the higher degradation under simulated syngas conditions. In general the data confirm previous results for single cell testing, and showed that for this specific short stacks (two-cells) arrangement both a fast and a slow response to H₂S injection that eventually stabilized.     

Design and performance analysis of a de-coupled solid oxide fuel cell gas turbine hybrid

Hybrid solid oxide fuel cells (SOFC) cycles of varying complexity are widely studied for their potential efficiency, carbon recovery and co-production of chemicals. This study introduces an alternative de-coupled fuel cell-gas turbine hybrid arrangement that retains the high efficiency thermal integration of a topping cycle without the high temperature heat exchanger of a bottoming cycle. The system utilizes a solid-state oxygen transport membrane to divert 30%–50% of the oxygen from the turbine working fluid to the intermediate temperature SOFC. Thermodynamic modeling delineates design trade-offs and identifies a flexible operating regime with peak fuel-to-electric efficiency of 75%. Co-production of electricity and high purity hydrogen result in net energy conversion efficiencies greater than 80%. The potential to retrofit existing turbine systems, particularly micro-turbines and stand-by ‘peaker’ plants, with minimal impact to compressor stability or transient response is a promising pathway to hybrid fuel cell/turbine development that does not require turbomachinery modification.     

Coal gas fuel utilization effects on electrolyte supported solide oxide fuel cell performance

Solid oxide fuel cells (SOFCs) transform the energy of the fuel instantly into electric energy with a large fuel option. Coal, which is a local energy source, is a preferred fuel despite its negative features because it is cheap and abundant. The use of coal and coal-based fuels in SOFCs has recently attracted considerable attention. In this study, performance analysis of the SOFC has been performed experimentally by using hydrogen, generator gas (contained 12% H₂), and water-gas (contained 50% H₂) in an electrolyte-supported SOFC (ES-SOFC). The numerical modelling of the fuel cell had been previously performed. In addition, the effect of inlet gas fuel flow rates on the ES- SOFC has been investigated numerically in this study. The temperature effect on the performance of ES-SOFC has been examined experimentally. It is seen that the performance of SOFCs fueled hydrogen is favorable than fueled water gas and generator gas. This is because of the higher hydrogen substance in the water gas measure against the other gas. In addition, it is seen that the increase in temperature increases the performance with positive effects on the reactions. It is also concluded that the performance of SOFC increases when inlet fuel flow rates increase.     

Investigation of the effect of ion transition type on performance in solid oxide fuel cells fueled hydrogen and coal gas

Renewable energy will be a panacea for environmental difficulties due to the extensive usage of carbon-rich fuels as a main source of energy. As a result, hydrogen-fueled solid oxide fuel cell is a revolutionary clean technology that has a great contribution in solving the current energy and environmental-related challenges. Thus, a 3D model of hydrogen and coal gases fueled solid oxide fuel cell (H₂–SOFC) using different electrolytes has been developed and simulated using COMSOL commercial software to explore the performance of electrolyte supported SOFC. The performance of the developed model has been studied and characterized using different differential equations. Accordingly, it has been found that the performance of hydrogen-fueled oxide ion conducting electrolytes (SOFC–O) is lower than that of protonic conducting one (SOFC–H) at 800 °C. Furthermore, a numerical simulation has been conducted to investigate the result of temperature changes on SOFC performance at 400 °C, 600 °C, and 800 °C for proton-conducting SOFC and 800 °C and 1000 °C for oxygen-conducting SOFC. It has been demonstrated that SOFC–O shows a better performance at high temperatures compared with SOFC–H while SOFC–H can be an agreeable selection at medium temperatures. Therefore, this study reveals that the temperature augments the performance of both electrolytes, yet at higher working temperatures SOFC–H becomes more advantageous than SOFC–O to use hydrogen and coal gas as a primary fuel. Besides, the effect of channel height was also analyzed numerically and the finding disclosed that decreasing the channel height emerges in a curtly current path. Thus, it can be reasoned out that the performance of SOFC decreases when the channel height is increased.     

Effect of inlet flow maldistribution in the stacking direction on the performance of a solid oxide fuel cell stack

This study examines the performance of a ten-cell solid oxide fuel cell (SOFC) stack with a non-uniform flow rate in the stacking direction. The author develops a two-dimensional numerical method to solve the electrochemical, mass and energy equations one stack at a time. The energy equations couple the heat exchange between the interconnector and both the cell and the flowing gas of adjacent cells. Moreover, this paper considers two boundary conditions, adiabatic and constant temperature, on the top and bottom faces of the SOFC. The results show that the non-uniform inlet flow rate of the fuel dominates the current density distribution; it causes the cell voltage to vary by over 13% for both boundary conditions. In addition, the constant temperature condition in this study can produce 3% more power than with the adiabatic condition. On the other hand, the air dominates the temperature field of a SOFC, and the non-uniform inlet flow rate of the air produces a variation of 3% in the average cell temperature of the cells when the boundary condition is adiabatic. This non-uniform effect on the electrical performance of each stack is apparently larger than in the transverse direction, which has been examined in our previous research.     

Effect of thermal cycling on durability of a solid oxide fuel cell stack with external manifold structure

A 5-cell stack with external manifold is thermal cycled between room temperature and 750 °C fifteen times. The electric performances after each cycle are measured and compared. The stack has an initial peak output of 328.44 W and shows excellent stability in thermal cycling. The average operating voltage degradation rate is only 0.8% corresponding each thermal cycle. A cell from the stack is randomly chosen for electrochemical evaluation. Its performance is found to be comparable to a cell which is not thermal cycled. Post-test examination shows deterioration of cathode contact materials at points of contact and cracks throughout the oxide layer between corrugated and bipolar plates to be the main causes of the degradation.     

Effect of reduced graphene oxide addition on cathode functional layer performance in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) operating at high temperatures are highly efficient electrochemical devices since they convert the chemical energy of a fuel directly into heat and electrical energy. The electrochemical performance of an SOFC is significantly influenced by the materials and microstructure of the electrodes since the electrochemical reactions in SOFCs take place at three/triple phase boundaries (TPBs) within the electrodes. In this study, graphene in the form of reduced graphene oxide (rGO) is added to cathode functional layer (CFL) to improve the cell performance by utilizing the high electrical properties of graphene. Various cells are prepared by varying the rGO content in CFL slurry (1–5 wt %), the number of screen printing (1–3) and the cathode sintering temperature (900–1100 °C). The electrochemical behavior of the cells is evaluated by electrochemical performance and impedance tests. It is observed that there is a ∼26% increase in the peak performance of the cell coated with single layer CFL having 1 wt % graphene and 1050 °C sintering temperature, compared to that of the reference cell.     

Developing fuel map to predict the effect of fuel composition on the maximum voltage of solid oxide fuel cells

A thermodynamic model is developed to determine the fuels that would yield an identical maximum cell voltage (MCV) for solid oxide fuel cells (SOFCs) at a given operating condition. These fuels make a continuous curve in the ternary coordinate system. A fuel map is established by developing the continuous fuel curves for different MCVs at the same operating condition and representing them in the carbon-hydrogen-oxygen (C-H-O) ternary diagram. Using the fuel map, the effect of the composition of a fuel containing carbon, hydrogen, oxygen, and inert gas atoms on the MCV of SOFCs can be easily studied. In addition to the effect of the fuel composition, the graphical representation of fuel maps can be applied to study the effect of the fuel processors on the MCV of SOFCs. As a general result, among fuels that can be directly utilized in SOFCs, at the same temperature and pressure, the one located at the intersection of the H-C axis and the carbon deposition boundary (CDB) curve in the C-H-O ternary diagram, provides the highest MCV for SOFCs. The results also show that for the fuels that cannot be directly utilized in SOFC, the steam reforming fuel processor always yields a higher MCV than the autothermal reforming or the partial oxidation fuel processors at the same inlet fuel temperature.     

A review of numerical modeling of solid oxide fuel cells

The solid oxide fuel cell (SOFC) is one of the most promising fuel cells for direct conversion of chemical energy to electrical energy with the possibility of its use in co-generation systems because of the high temperature waste heat. Various mathematical models have been developed for three geometric configurations (tubular, planar, and monolithic) to solve transport equations coupled with electrochemical processes to describe the reaction kinetics including internal reforming chemistry in SOFCs. In recent years, considerable progress has been made in modeling to improve the design and performance of this type of fuel cells. The numbers of the contributions on this important type of fuels have been increasing rapidly. The objective of this paper is to summarize the present status of the SOFC modeling efforts so that unresolved problems can be identified by the researchers.     

Developing fuel map to predict the effect of fuel composition on the maximum efficiency of solid oxide fuel cells

At any given cell operating condition, a fuel map can be developed to predict the effect of a fuel containing carbon, hydrogen, oxygen and inert gas atoms on the maximum cell efficiency (MCE) of solid oxide fuel cells (SOFCs). To create a fuel map, a thermodynamic model is developed to obtain the fuels that would yield identical MCE for SOFCs. These fuels make a continuous curve in the ternary coordinate system. A fuel map is established by developing continuous fuel curves for different MCEs at the same operating condition of a cell and representing them in the carbon-hydrogen-oxygen (C-H-O) ternary diagram. The graphical representation of fuel maps can be applied to predict the effect of the fuel composition and fuel processor on the MCE of SOFCs. As a general result, among the fuels that can be directly utilized in SOFCs, at the same temperature and pressure, the one located at the intersection of the H-C axis and the carbon deposition boundary (CDB) curve in the C-H-O ternary diagram provides the highest MCE. For any fuel that can be indirectly utilized in SOFCs, the steam reforming fuel processor always yields a higher MCE than auto-thermal reforming or partial oxidation fuel processors at the same anode inlet fuel temperature.     

On modeling multi-component diffusion inside the porous anode of solid oxide fuel cells using Fick's model

Stefan-Maxwell model (SMM) and simple Fick's model (FM) type of relations both including Knudsen diffusion for the calculation of species mole fraction distribution inside the porous anode of a solid oxide fuel cell (SOFC) were compared and it was found that at low current densities the models agree well but as current increases the differences also increase. Based on the findings an empirical correction is proposed for the effective diffusivity used in Fick's model. The corrected diffusivity coefficient gave better agreement with the Stefan-Maxwell model and even at higher current densities the error is less than 5%. This correction was implemented via a three-dimensional, in-house SOFC simulation code (DREAMSOFC) which uses Fick's model type relations for diffusion flux calculations. The code also takes into account methane steam reforming (MSR) and water gas shift (WGS) reactions and the electrochemical oxidation of both H₂ and CO. As an application, a SOFC button cell which is being tested at West Virginia University was simulated. The results with and without the proposed correction for effective diffusivity are compared.     

Hydrogen sulfide poisoning in solid oxide fuel cells under accelerated testing conditions

This study investigates the 0.2% hydrogen sulfide poisoning of Ni/YSZ anode-supported solid oxide fuel cells (SOFCs). The deterioration degrees and recovery extents of the cell current density, cell voltage and operation temperature are monitored. The results of impedance spectroscopy analysis show that hydrogen sulfide poisoning behavior may affect oxygen ion migration and gas diffusion and conversion on the anode side. Microstructural inspection reveals sulfur or sulfide formed on the anode-active area, which accounts for the immediate and severe cell power drop upon the injection of H₂S. The nickel sulfide in the anodic functional layer cannot be completely removed after long-term regeneration and thus may be a key factor in the permanent degradation of the cell.