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King County Fuel Cell Demonstration Project
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What is a Fuel Cell?Fuel cells are electrochemical devices that efficiently convert chemical energy in a hydrogen-rich fuel to electrical power and heat. Similar to a battery, a fuel cell is composed of many individual cells. Cells are grouped together to form a stack. Each consists of an anode, cathode and electrolyte. A hydrogen-rich fuel, such as digester gas and oxygen, is supplied to the stack, where it reacts with the cells to produce electric current. However, while a battery has a fixed supply of energy, fuel is continuously added to a fuel cell enabling production of electricity for a long time. The reaction byproducts are CO2 and water. A fuel cell stack will produce power for about five years before the stack needs to be replaced. Other components -- for example, a power converter and heat recovery equipment -- join with the fuel cell module to form a complete power plant.
Efficiency of Fuel CellsFuel cells are an emerging technology for the efficient, clean generation of electrical power from natural gas and the methane found in digester gas. Molten carbonate fuel cells (MCFCs or "Direct" fuel cells) can achieve conversion efficiencies of 60 percent and up to 80 percent if the heat produced during the catalytic reaction is captured or used. By comparison, conventional cogeneration equipment, such as reciprocating engines, turbines and combined cycles, achieve lower efficiencies. Engine generators, typically found in smaller scale applications, are efficient to 25 percent; meaning they can convert 25 percent of their fuel to energy while 75 percent is wasted. At larger-scale installations of 100 megawatt capacity or greater turbines can achieve efficiencies of 45-50 percent. Fuel cells will be a practical alternative to existing stationary power plants where power production is limited to 200 kilowatts to 2.0 megawatts. Multiple fuel cells can be connected to produce greater amounts of power. Return to top.
Electrical Generating Efficiencies Comparison Fuel Cell Power Plants To TopA fuel cell power plant consists of three distinct components as seen in the diagram below. Fuels such as methane (produced during solids digestion at wastewater treatment plants) are first processed to separate H2 gas from the other constituents. The processed fuel is converted to direct electrical current in the fuel cell stack. The power conditioner then converts the produced DC current to AC at proper voltage levels for transmission. This fuel cell demonstration included a fourth major component, heat recovery equipment to capture and utilize the heat produced during the chemical reaction. The fuel cell power plant designed for this project is typical to plants for commercial applications. Its design life is estimated at 30 years with routine maintenance.
Fuel Cell Power Plant Components The four major types of fuel cells are: PAFCs are already entering commercial markets and have been installed in several wastewater treatment plants, including Yonkers, New York; Portland, Oregon; and Boston, Massachusetts. MCFCs are just entering commercial markets and are expected to achieve operational efficiencies 50 percent higher than PAFCs. SOFCs are now beginning pilot-scale testing. PEM fuel cells are generally small, low-temperature units. They have applications in transportation and household-scale power plants. The increased operating efficiency of MCFCs is due in part to the higher operating temperature within the MCFC. Reaction temperatures in the molten carbonate fuel cell can reach 1,600 degrees Fahrenheit. MCFCs therefore produce a higher-quality heat byproduct that, when recovered, is suitable for use in many treatment plant applications, such as digester heating, building heating or pre-pasteurization of wastewater solids. Municipal wastewater treatment plants that treat wastewater solids through anaerobic digestion produce a large quantity of gas that is about 60 percent methane and 40 percent CO2. Currently, most small plants simply burn the gas in a flare. This disposal method contributes to air emissions. Larger plants more frequently reuse the combined gases on site for heat or power, or in some cases, remove the CO2 in order to sell the remaining gas (This is the current practice at South Plant). Using a molten carbonate fuel cell can eliminate CO2 removal costs and make more efficient use of gas resources while generating electricity. Siting a MCFC power plant at a wastewater treatment facility offers potential for significant economic and environmental benefits. The King County fuel cell demonstration power plant is sized to produce one megawatt (MW) of power during the two-year demonstration. The balance of the plant is designed to accommodate a 1.5 MW fuel cell stack. A wastewater treatment plant the size of Renton produces sufficient gas supplies to generate about 4 MW of electricity. That is more than 50 percent of the total power demand of the 102 mgd plant. Waste heat from the fuel cell power plant will be integrated into the existing heat distribution system offering further efficiency. Three organizations have come together to jointly develop this demonstration project. They are the U.S. Environmental Protection Agency (external link), King County Wastewater Treatment Division, and FuelCell Energy Inc. (external link). For More InformationSee project library for fact sheet, brochure and quarterly reports. |
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For questions about the Fuel Cell Demonstration Project Web page,
Fuel Cell Demonstration Project Updated: July 5, 2005
King County | Natural Resources and Parks | Wastewater Treatment Division
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