2.1 MCFC Principle
The cathode of the molten carbonate fuel cell (MCFC) is fed with air and carbon dioxide. Oxygen and carbon dioxide in the air undergo reduction reaction at the cathode to obtain electrons and generate carbonate ions, which are transmitted to the anode through the molten carbonate. The anode feed of a molten carbonate fuel cell (MCFC) is hydrogen or hydrogen-rich gas. Hydrogen and carbonate ions oxidize at the anode, losing electrons and producing water and carbon dioxide. [4,7] The matrix between the electrodes provides structural support for the molten carbonate and acts as a gas trap. Electrons supply electrical appliances with electricity through external circuits. See Fig.1.
Fig.1 Principle of MCFC
2.2 MCFC System
A set of molten carbonate fuel cell power generation system mainly includes four parts: fuel treatment system, MCFC stack system, exhaust heat recovery and CO2 circulation system, and control system . The fuel treatment system can convert hydrocarbon fuels to hydrogen-rich gas by external reforming device or internal reforming technology. High temperature fuel cell stack system transforms chemical energy of the hydrogen-rich gas into electricity. Exhaust heat recovery and CO2 cycle system uses the waste heat of exhaust gas by heat exchanger and separates CO2 from the exhaust gas for recycling . Control system controls the operation of the fuel cell system parameters such as gas flow, cell stack temperature, etc. through the control cabinet. See Fig.2.
Fig.2 The Process diagram of MCFC power generation system
The power generation system of multiple MCFC stacks can be combined by means of series and parallel connection. The parallel connection can amplify the power of the fuel cell stack, and the multi-stage series connection can improve the fuel efficiency and power generation efficiency. The combination of parallel and series can increase the power of the stack and achieve higher generation efficiency at the same time. For example, for the 100kW MCFC power generation system, four 20kW MCFC stacks can be connected in parallel and then connected in series with one 20kW MCFC stack. The four parallel battery stacks receive air separately. The anode tail gas still contains a large amount of unreacted hydrogen, reconnected it to the anode of the tandem MCFC stack, further utilizing the chemical energy of the unreacted hydrogen, and the cathode of the tandem MCFC stack is separately injected. This combination can effectively improve the fuel efficiency without changing the total amount of anode air intake, thus improving the power generation efficiency of MCFC power generation system [10-12]. See Fig.3.
Fig.3 100 kW MCFC system based on Aspen
2.3 State of Art
The main research institutions of molten carbonate fuel cell in China are Dalian Institute of Chemical Physics (Chinese Academy of Sciences), Shanghai Jiao Tong University and Huaneng Clean Energy Research Institute.
Dalian Institute of Chemical Physics（Chinese Academy of Sciences）mainly carried out basic laboratory research on materials science, and successfully assembled a MCFC stack with a working area of 110cm2. The power was less than 1kW. Shanghai Jiao Tong University carried out laboratory material preparation research and successfully assembled a 1.5kW MCFC battery stack. With the special support of GreenGen in 2008, Huaneng Clean Energy Research Institute has carried out researches on the preparation of key materials and components, the assembly of MCFC stacks, the sintering of MCFC stacks, the operation of MCFC stacks, etc. Huaneng Clean Energy Research Institute masters the core and key technologies of MCFC, successfully developed 2kW, 5kW and 10kW MCFC stacks and is working on developing 20-100kw MCFC power generation system. See Fig.4.
Fig.4 10kW MCFC stack of Huaneng Clean Energy Research Institute
For more than 20 years, the molten carbonate fuel cell (MCFC) has been proved to be the most suitable distributed power generation technology in the United States, Germany, Italy, Japan, South Korea and other developed countries after a lot of research and demonstration .
In USA, Fuel Cell Energy (FCE) has successfully developed 300kW, 1.4MW, 2.8MW MCFC product modules, and built about fifty 250kW-50MW MCFC power plants. The effective surface of a single cell is 1m2, the power generation efficiency is 47%-60%, and the cogeneration efficiency can reach 80-90% [14-15]. In the 1880s, Japanese IHI company began to study the MCFC power generation technology, mainly on the MCFC and gas turbine co-generation system. Currently the power can reach 250-300kw scale. And in 2005, IHI successfully operated two 300kW molten carbonate fuel cell and gas turbine (MCFC-GT) co-generation system with Toyota [16-17]. By adopting the MCFC technology of FCE, POSCO has built a number of MCFC power stations in South Korea, among which the largest MCFC power station can reach 59MW with a power generation efficiency of 47% [18-19].