Simulation of Start-up of Anode Gas Recycle SOFC System Using Catalytic Combustion Method

Solid oxide fuel cells (SOFCs) applied to stationary power generation systems have advantages such as the highest efficiency among all fuel cells and lower cost by the use of non-platinum catalysts. Furthermore, hybrid systems based on SOFCs are being actively researched to obtain even higher efficiencies by utilizing the high temperature exhaust gas from the SOFC. For an anode gas recycle (AGR) SOFCs, steam produced by the electrochemical reaction can be used in the steam reforming process to avoid carbon formation; therefore, there is no need for an external steam supply under a large recirculation ratio of anodic off-gas. In addition, the overall efficiency of an SOFC hybrid system, including bottoming engines, can be improved with such an approach. In our previous report, the experiments using a 1 kW tubular-type SOFC with an AGR system driven by a variable flow ejector were conducted, it was clarified that the AGR-SOFC with recirculation ratio of 0.62 was stable and generated about 2% more electricity compared to one-path SOFC systems under rated-load conditions. In this study, the cycle analyses were carried out for the AGR system using the catalytic combustion method, to remove methane efficiently in the anode gas at the start-up operations. In this systems, oxidation air is injected with main methane fuel, then methane can be converted by the catalytic combustion reaction in the combustion plenum located upstream the pre-reformer. Furthermore, steam produced by the reaction can be used in the steam reforming reaction at the pre-reformer and the fuel cell. To clarify the effects of operating conditions on the AGR system with catalytic combustion method, the chemical equilibrium calculations were conducted by using the experimental data on stack temperature and recirculation ratio determined using an actual SOFC stack.

(1) Heat-up mode

Cathode and anode gases were heated from ambient to high temperature by an electric heater to heat up the AGR-SOFC system. Methane fuel and oxidation air is injected into the anodic loop, thus methane is converted at the combustion plenum. It is assumed that catalytic combustion reaction fully processed in the combustion plenum heated by electric heater. Three operating thresholds were defined as a stable AGR-SOFC system operation as follows: (i) steam-to-carbon ratio S/C defined as the mole ratio of the H2O to CH4 and CO feed rate is larger than 2.0 to prevent carbon deposition; (ii) anode gas at the combustion plenum exit is not extremely high temperature; and (iii) oxygen is fully removed by the combustion process to maintain the reductive gas atmosphere. For recirculation ratio is larger than 0.7, the results shows enough steam product by the catalytic combustion reaction at air-to-methane ratio is about 7-9. In addition, excessive temperature rise of the anode gas can be prevented at high recirculation ratio greater than 0.7 for recycled gas temperature is 873 K, the required recirculation ratio strongly depends on the recycled gas temperature. An ejector drive system can easily function at the extremely high temperature, such as that of the anodic off-gas, because there are no rotating parts. Recirculation ratio 0.7 is the proven value in actual SOFC experiments under only rated-load condition, thus it is necessary to develop a high-efficiency binary fluid twin-fluid nozzle type ejector for the start-up operations.

(2) Load-up mode

CH4 fuel was increased gradually with an increase in current for the load. Finally, only CH4 is fed for a specified full load period. Fuel utilization is defined based on methane flow rate derived from feed rate by loss of consumption by catalytic combustion. The results shows that S/C decreases with increasing fuel utilization due to decrease in combustion reaction rate. In addition, the reduction of recirculation ratio cause a decrease of S/C and an increase of gas temperature at the exit of the combustion plenum. Consequently, it is important to keep up a sufficiently high recirculation ratio to maintain high S/C and low gas temperature at the exit of the combustion plenum.

(3) Rated-load power-generating mode

Only CH4 is fed in anode recycle system. AGR-SOFC showed stable output power with no external steam supply, because recycle enough steam via the electrical reaction can be used in steam reforming of methane fuel. It is important to maintain a sufficiently high recirculation ratio to avoid carbon deposition. The threshold of the recirculation ratio is dependent on fuel utilization; therefore, the AGR system requires a recirculation ratio greater than ca. 0.6.