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