In SOFC systems, raising the fuel utilization ratio (Uf) is an effective way to improve the electrical efficiency. There are two main approaches to raise the system’s Uf up to around 90% while keeping lower Uf for individual cell-stack. One is to include a fuel recycling system, which reuses the unreacted anode off-gas and mixes it with the fresh gas as fuel. However, the system may require expensive and unreliable fueling components. Therefore, we focused on the other approach, a multi-stage SOFC system, which uses the anode off-gas of the first SOFC stack as the fuel for the second stack. Additionally, a fuel regenerating technique was employed, that is, removing CO2 and H2O from the off-gas of the first stack, hence enables to realize a higher operating voltage as well as to prevent an irreversible damage to the second stack under high Uf conditions of the system. To date, we have developed three types of SOFC hotboxes by combining stacks, a reformer, a combustor, a vaporizer, regenerators, and heat exchangers.
The 1st generation hotbox was built to enhance the system’s Uf up to higher than 90% by removing CO2 and H2O from anode off-gas using a CO2 absorber and a vapor condenser. A stable operation was confirmed under the system condition with an Uf value of 92.0% while methane is used as the fuel. However, note that this result is achieved while the heat loss from the hotbox was compensated by electric heaters. Next, the 2nd generation hotbox was made with the aim of eliminating the assisting heaters. A vapor condenser was adopted as the fuel regenerator. The operation was successful, and typical results at the thermal self-sustainable condition were as following: DC-power output 2.27 kW, current density 0.44 A/cm2, system Uf 86.3%, and DC-electrical efficiency 69.2%LHV. The 3rd generation hotbox was modified from the 2nd model with an objective of enhancing the system’s Uf by controlling the heat loss from the hotbox. To control the heat losses, it is important to lower the gas temperature exhausted from the hotbox as well as to reduce the heat loss from the surface of the hotbox body. In the multi-stage stack system, as lower temperature for the anode off-gas of the first stack and the exhaust gas were desired, heat exchangers were further optimized and fabricated to be installed in the 3rd model hotbox. The power generation test was performed with the conditions below: CH4 4.47 NL/min, Air 64.0 NL/min, S/C ratio 2.5. The DC efficiency reached 71.2%LHV at the system’s Uf of 88.4%, with the current density of 0.40 A/cm2, and 2.03 kW of DC-power output. Assuming the efficiencies of the auxiliary machines and the power conditioner to be respectively 94% and 95%, the estimated AC-power generation efficiency of the system is 63.6%LHV. The heat loss of the anode off-gas and the exhaust gas has decreased by 273 W and 19 W respectively compared to the 2nd model hotbox. Additionally, we have succeeded to generate power continuously for more than 500 hours using the system with the 3rd generation hotbox connected with the auxiliary machines and an automatic operation controller.
As described above, our SOFC system demonstrated an electrical efficiency higher than that of state-of-the-art thermal power plants with combined cycles, even at much smaller power output scale such as 2 kW by employing both the multi-stage SOFC system flow and the fuel regenerating techniques. We are aiming to increase the power output with higher efficiency, and to achieve a longer operation term by further improving our hotboxes.