Analysis of Transient Response of Solid Oxide Fuel Cells with Anode Off-Gas Recycle

Abstract

An anode off-gas recycle (AGR) technology is currently focused as a way for further improvement of efficiency of solid oxide fuel cells (SOFC) which have higher performance compared to other types of fuel cells. AGR is a technology that recycles a part of the anode off-gas containing electrochemically unreacted fuel that comes out from the SOFC anode and reutilizes it for power generation. Because the unreacted fuel component in the recycled gas is added to the input fuel to the system, the fuel flow in the cell becomes more than that for a conventional SOFC without AGR, even though the fuel supply to the system is equivalent. Therefore, fuel utilization can be easily increased and improvement of electrical efficiency of the system is expected. Another advantage is that the system, in principle, does not require external water supply due to reutilization of steam in the recycled gas. However, in the SOFC system in which AGR is introduced (AGR-SOFC), the changes in the current, the flow rate and the composition of the gasses supplied to the cell lead to the changes in the flow rate and the composition of the recycled gas. The effect of changes in the recycled gas is then feedbacked to the system inlet. Because a delay in response of a blower which recycles the anode off-gas (recycle blower) is added in this situation, the transient response performance of the system may become more complex and worse than conventional systems. Thus, in this study, we constructed an AGR-SOFC model for transient response analysis under power increase situations and analyzed the transient response of the system and dependency of the response on a degree of power increase by numerical simulation. The AGR-SOFC model is composed of a reformer, an SOFC, and a recycle blower assuming the plug flow. Transient behavior of the system was analyzed where the increase of output power in accordance with the step wise increases in required current and fuel supply was assumed. Because the transport of gasses in the SOFC systems is relatively slow, increase in the current and fuel supply without any limit leads to fuel starvation due to shortage of the fuel in the cell and carbon deposition due to shortage of the steam for fuel reforming. Due to these effects, the system may be damaged. Hence the operating conditions for avoiding these causes of damage were investigated. In this model the current and the fuel supply are chosen in accordance with the investigation. As a result of the analysis, the transient response of the AGR-SOFC is classified into three types according to the way of increase in the current and the fuel supply and characteristics of each type of responses are clarified. In addition, it is revealed that AGR-SOFC can afford more fuel and steam to the reformer and cell and can increase both the current and the fuel supply instantaneously compared to conventional non AGR SOFC systems under identical fuel utilization conditions. Therefore, superior transient response over wide range of power increase is clarified to be expected in AGR-SOFC, even though there had been a concern that transient response performance might become worse due to the complex recycling system.