Mitsubishi Heavy Industries (MHI) has successfully tested a combined-cycle power generation system incorporating solid oxide fuel cells (SOFC) and a micro gas turbine (MGT), also known as a microturbine (Figure 1). As a result of the verification, MHI has started production of a combined cycle power system in the 200 kW range and will continue developing systems offering larger capacities and higher efficiency in a quest to eventually develop a system suitable for utility applications.
MHI has been developing the system since 2004 for Japan’s New Energy and Industrial Technology Development Organization (NEDO). The most recent testing verified 75 kW of capacity from the hybrid system. The SOFC is a ceramic-based fuel cell that produces electricity by chemical reaction between oxygen in the air and hydrogen removed from natural gas or syngas through reforming. The SOFC-MGT combined-cycle power generation system uses unreacted hydrogen contained in the gas exhausted from the SOFC and hot air also discharged from the SOFC. The gas and air from the SOFC are used to drive an MGT to generate electricity. During testing, this two-stage power generation system achieved power generation efficiency above 50 percent.
The SOFC/Microturbine combined cycle hybrid could lead to larger versions with utility applications. Courtesy of Mitsubishi Heavy Industries
Two technologies that enable Mitsubishi’s SOFC-MGT combined-cycle system are pressure control to organically combine the two systems and stable combustion of low heat value gas from the SOFC. The pressure of the air sent to the SOFC and natural gas that is blown into the SOFC are kept at identical levels suitable for the given pressure conditions required for stable gas turbine operation. The pressure levels of both gas and air in the SOFC must be at the same level. For stable combustion of the low heat value gas, MHI has developed and installed a combustor specifically for the SOFC-MGT power generation system, enabled by the company’s industrial-use gas turbine technologies.
MHI sees the large-scale SOFC combined-cycle power generation system as an effective way of reducing CO2 emissions alongside nuclear generating capacity and high-efficiency utility scale thermal power generation systems. Mitsubishi projects the possibility of generation efficiencies through combining SOFCs and other power generation systems to be as high as 60 percent to 70 percent with gas turbine combined-cycle (GTCC) systems for liquefied natural gas (LNG), and 55 percent to 60 percent with integrated coal gasification combined cycle (IGCC) systems for coal.
MHI aims to further accelerate its effort to establish various technologies to produce a combined-cycle power generation system integrating an SOFC and both gas and steam turbines.
– Steve Blankinship