Analysis of Transient Behavior of Fuel Reformer to Fuel Supply Variation in Solid Oxide Fuel Cell Systems with Anode Off-Gas Recycle

Solid oxide fuel cells (SOFCs) are known for having high power generating efficiency among various types of fuel cells and being able to utilize various fuels such as fossil fuels. In the case of applying SOFC to vehicles, improvement of vehicle efficiency can be expected, using fossil fuels.

However, in the case of using fossil fuels for fuel cells, fuels must be reformed on board using H₂O, CO₂, and so on. Therefore, in order to utilize SOFCs to vehicles, methods of supplying these gasses on board need to be considered. To solve this problem, one of the promising methods is the anode off-gas recycle (AGR). The AGR is a technology in which SOFC exhaust gas, which contains residual fuels, is partially mixed with supplied fuels into the system prior to entering a fuel reformer, and recirculates these gasses in the system. By introducing AGR to SOFC system, i.e., by utilizing H₂O and CO₂included in the SOFC exhaust,, fuel utilization of the system can be improved and the steam reforming of fuel become possible without mounting water tanks on vehicles.

In addition, in the case of utilizing SOFC to vehicles, complicated responses to precipitous load variations must be considered, unlike stationary systems operated under moderate load variations. For example, in the case of increasing fuel supply in order to increase output power from the system, it could be possible that the output power is temporarily lowered because of decrease of temperature of the SOFC and reformer due to increase of heat requirement for fuel reforming. This behavior is due to transient variation of heat balance in the reformer. Therefore, for the analysis of transient response of SOFC systems, the response of reformer is very important and needs to be paid attention.

Especially, further complicated behavior of reformers is expected in SOFC systems with AGR (AGR SOFC systems) because of off-gas recycle. For instance, in the case of increasing fuel supply to the system in order to increase the output power from the SOFC, possible delay of fuel reforming in the reformer may cause the decrease in the ratio of increase of recycled gas to that of fuel supply. In such a case, the amount of H₂O and CO₂in the recycled gas decreases compared to the fuel supply, and the carbon deposition occurs, depending on the ratio of increase of fuel supply.

Hence, in this research, the transient behavior of the fuel reformer in AGR SOFC systems under various fuel supply rate is discussed by numerical simulation. Specifically, the effect of the change ratio of fuel supply and the delay of fuel reforming in the fuel reformer on system response is analyzed, by modeling composition of input gas to the fuel reformer. As a result of numerical simulation, the operating conditions in which the carbon deposition occurs are clarified. In addition, because ratio of increase of fuel supply needs to be limited in order to prevent carbon deposition, mitigation methods for this limitation are also discussed.