MCFC projects make progress In Japan`s New Sunshine Program
Collaboration should produce a 1,000-kW MCFC demonstration plant by 1997
By Toshikatsu Mori, Hiroyoshi Uematsu, Hidemichi Tanaka and Hiroo Yasue Technology Research Association for Molten Carbonate Fuel Cell Power Generation Systems
Meeting energy and environmental demands in the 21st century will require new power-generation systems with higher efficiency. With estimated net efficiencies more than 50 percent, molten carbonate fuel cells (MCFC) should provide a competitive, low-emissions option to conventional, thermal power-generation systems. These systems also offer lower CO2 and NOx emissions and fuel flexibility.
Much effort has been put into research and development (R&D) for MCFC systems, especially in the United States, Japan and Europe. In Japan, a national project, the New Sunshine Program, was initiated in 1981 and has been promoted by the Agency of Industrial Science and Technology (AIST), the New Energy and Industrial Technology Development Organization (NEDO), the Technology Research Association for Molten Carbonate Fuel Cell Power Generation Systems (MCFC-RA) and their contractors. The project, designing a 1,000-kW MCFC pilot plant, is scheduled to begin demonstration testing in 1997.
The key components of a unit fuel cell include a cathode, an anode and a molten carbonate electrolyte matrix. Each unit operates at about 650 C. The components are porous plates with a thickness of approximately 1 mm and are made of NiO for the cathode, Ni for the anode, Li2CO3/K2CO3 for the electrolyte and LiAlO2 for the electrolyte matrix. Stacking a large number of unit cells and operating at elevated pressure attains a large power output.
A basic system of liquid natural gas (LNG)-fueled MCFC power generation designed in the Japanese national project is depicted in Figure 1. The system consists mainly of reformer, stack, ac/dc inverter, high-temperature blower, turbine compressor and heat-recovery steam generator (HRSG). A mixture of LNG and steam fed to the reformer tube produces hydrogen-rich gas (reformate) at 800 C. The resulting gas is introduced to the anode, where the hydrogen reacts with carbonate ions from the cathode through the electrolyte to generate electricity. The anode exhaust gas is recycled to the reformer, where its catalytic combustion produces heat needed for the endothermic reforming reaction and to regulate NOx emissions. The high-temperature blower recycles the cathode exhaust gas to control the gas temperature at the cathode inlet. Energy in the cathode exhaust is recovered from the recycle line by the turbine compressor and HRSG. The turbine compressor feeds compressed air to the cathode and to the combustion chamber of the reformer. Steam produced in the HRSG is also fed to the reformer.
The R&D plan for New Sunshine Program`s MCFC system is shown in Table 1.
The group operated two external, reforming-type 100-kW stacks from April 1993 to June 1994 at MCFC-RA`s test site. MCFC-RA stacked and operated 102 unit cells with an area of one square meter at three to seven atmospheres for 5,118 hours using LNG reformate. The maximum power output was 129 kW, and the cell voltage standard deviation was 13 mV. The decay rate of the mean cell voltage was 1.5 percent per 1,000 hours.
Project participants have conducted intensive R&D to reduce the decay rate using 5- and 25-kW stacks fabricated with alternative component materials and using improved fabrication procedures. They attained a decay rate of 0.4 percent per 1,000 hours in the 13,000-hour operation test of a rectangular-type 5-kW stack at ambient pressure. In addition, an improved multiple-type 25-kW stack is successfully operating with a decay rate of 0.5 percent per 1,000 hours. The group also operated an internal reforming-type 30-kW stack at one atmospheric pressure for 12,000 hours with a decay rate of 0.6 percent per 1,000 hours.
Much effort is needed toward enhancing cell life for MCFC power generation systems to become commercially viable. The target for cell life is 40,000 hours with a decay rate of less than 0.25 percent per 1,000 hours. To achieve these numbers, MCFC engineers must improve corrosion durability of metal components, retard cathodic dissolution and deposition in the electrolyte matrix, and improve the stacking technology (uniform gas-stream distribution and compression and gas sealing).
Balance of plant development
The group conducted material screening tests and simulation analyses to design the balance of plant equipment in the first R&D phase. A 1,000-kW reformer, turbine compressor and HRSG, and a 500-kW high-temperature blower were tested at the Akagi test site under simulated operation conditions for the pilot plant design. In addition, the group carried out a 100-kW plant-control test using a dummy stack. They achieved the following target values in the tests:
– reformer: carbon conversion > 95 percent;
– simulated anode exhaust: heat value = 500 kcal/m3N;
– load change response = 25 percent per minute;
– NOx less than 10 ppm;
– high-temperature blower: total adiabatic efficiency = 75 percent;
– turbine compressor: adiabatic efficiency = 65 percent; and
– HRSG: heat-recovery efficiency = 80 percent.
Pilot plant design
Table 2 summarizes the targets and specifications for the pilot plant. In order to achieve targeted values, the project group is designing a 1,000-kW pilot plant (Figure 2) using the data obtained from the operation tests and system analyses. The group will install two 250-kW stacks and a high-temperature blower in one power-generation unit. The pilot plant will be constructed in Chubu Electric Power Corp.`s Kawagoe Thermal Power Station and should begin operating in 1997.
Demonstration tests for MW-class pilot plants are forthcoming in Japan and the United States by 1997. Net efficiency of LNG-fueled commercial plants is estimated to be more than 50 percent when the plant capacity is more than 100 MW. Domestic energy loss of 3 percent or less is, in part, responsible for the high efficiency. To meet fuel flexibility needs, the group has also proposed a system combined with a coal gasification unit. Designers estimate that a 500-MW combined system would have a 47-percent net efficiency with a footprint of 0.4 m2/kW or less.
New members from a user`s group joined the national project in 1994 to accelerate R&D. Aided by project efforts, gross power output by commercial MCFC systems should exceed 1,000 MW by 2010, according to Japanese government committee estimates. To commercialize MCFC by the 21st century will require innovative technologies to enhance material stability, cell life, stacking, mass production and system simplicity.
1: 250-kW stacks
3: High-temperature blower
4: Turbine compressor
6: Control room
MCFC-RA operated 102 unit cells with 129-kW maximum output at their MCFC test site.
Dr. Toshikatsu Mori worked for MCFC-RA for three years before moving to Hitachi Ltd.`s Hitachi Research Laboratory in April 1995.
Hiroyoshi Uematsu moved to MCFC Research Association`s plant department from Ishikawajima Harima Heavy Industries Company Ltd. in July 1993. Uematsu is manager for designing the 1,000-kW MCFC pilot plant.
Hidemichi Tanaka moved to MCFC-RA`s plant department from Tohoku Electric Power Company Inc. in July 1993.
Hiroo Yasue was director of Chubu Electric Power Corp.`s Chita-Daini Thermal Power Station before moving to MCFC-RA in July 1993. Yasue is managing director of the association and will also be director of the Kawagoe MCFC test station.