Lessons Learned From the World's Largest Digester Gas Fuel Cell

Abstract

Increasing energy costs, more stringent air emission regulations, and an interest in exploring emerging energy technologies prompted King County in Washington state to search for new and innovative ways to provide electricity for its wastewater treatment plants. In cooperation with the U.S. Environmental Protection Agency (EPA), FuelCell Energy, Inc. (FCE), CH2M HILL, and Brown and Caldwell, King County (County) is operating the world's largest molten carbonate fuel cell (MCFC) at its South Treatment Plant in Renton, Washington. The 1-megawatt (MW) fuel cell can operate on either natural gas or digester gas. Operation started in April of 2004 as part of a 2-year demonstration project. If successful, the fuel cell will continue to operate and produce energy for the South Treatment Plant after the 2-year demonstration project is completed. The data from this demonstration will provide other municipal wastewater treatment plants with information on fuel cell power systems applicable to their interests and needs.Fuel cells produce electrical power directly through an electrochemical reaction using hydrogen and oxygen. The hydrogen can be produced from fuels such as natural gas, landfill gas, or anaerobic digester gas, while air provides the oxygen. The fuel cell has an anode and a cathode. When the fuel gas is combined with water, the fuel gas is eventually reformed to hydrogen, which is then fed to the fuel cell anode. Air is fed to the cathode. The reaction of the hydrogen moving from the anode to the cathode through an electrolyte creates electricity. By avoiding the two-step process of conventional combustion technology in which fuel is first burned and then the subsequent heat used to produce power, fuel cells are energy efficient, better for the environment, quieter, and ultimately more cost effective. Unlike combustion technologies, fuel cells have very low emissions of carbon monoxide, sulfur oxides, nitrogen oxides, and nonmethane organic carbon. The main product of the fuel cell reaction is water, which discharges in the exhaust gas. Molten carbonate fuel cells are one of the most efficient fuel cell technologies and operate between 40 and 50 percent electrical efficiency. With additional heat recovery, the overall efficiency is typically between 60 and 70 percent. 6460The King County project has two objectives:(1) Demonstrate that molten carbonate fuel cell technology can be adapted to use anaerobic digester gas as a fuel source.(2) Generate a nominal plant power output of 1 MW (net AC) using both digester gas and natural gas.To date, these two objectives have been successfully met. Provided that gas supply is consistent, the fuel cell can operate reliably. In the summer of 2005, halfway through the 2-year demonstration period, the efficiency of the power plant while operating on natural gas measured between 40 to 45 percent, and 40 to 47 percent while operating on digester gas. These measurements include the plant parasitic loads (e.g., anode gas oxidizer blower, inversion from DC to AC, water cleanup, et...