The World's Largest Digester Gas Fuel Cell Demonstration Project
Construction: April 2003; Operation: June 2004
King County, United States of America
In June 2004, King County launched a two-year project to test whether it could generate 1 MW of electricity from a fuel cell power plant powered by digester gas—a byproduct of anaerobic digestion of wastewater solids. The project, located at the 80-acre South Wastewater Treatment Plant southeast of Seattle, was the first commercial megawatt-scale project of its kind. In addition to generating electricity without combustion and the resultant air pollution, the high-temperature (molten carbonate) direct fuel cell unit also delivered a useful heat byproduct.
The project was a success. After adapting the wastewater treatment plant to accept digester gas and making other adjustments, the power plant reached generation capacity of 1 MW during the first year of operation using either digester gas or natural gas (backup to digester gas). During the second year, a heat recovery unit allowed the plant to recycle the waste heat into the plant heat loop for digester heating. The plant operated 90 percent of the time with an average of 43 to 47 percent electrical efficiency and 60 to 65 percent overall efficiency (with heat recovery) while generating almost zero emissions.
In the two-year period, the plant operated over 13,000 hours and generated over 10,000 MWh of electricity.
What is it?
Similar to a battery, the fuel cell houses hundreds of individual cells. Cells are grouped to form stacks—four stacks in this particular application, each producing 400 V. The fuel cell electrochemically combines hydrogen with oxygen to produce electricity. Each stack contains an anode, cathode and electrolyte.
The power plant occupies half an acre of land, about 1,000 feet from the digesters. The plant’s main components are: the fuel cell module; digester gas, natural gas, and water treatment systems; gas pre-converter and reforming systems; a heat recovery system; and a power inverter.
The King County fuel cell is a molten carbonate system, one of the most energy efficient fuel cell technologies. It can directly convert hydrogen from a variety of fuels and is not prone to carbon monoxide “poisoning” that can occur at lower temperature fuel cells. Most exhausts and wastes from the process are recycled with negligible emission. As a high temperature technology, the molten carbonate fuel cell is suitable for industrial and commercial applications where heat recovery is a valuable characteristic.
How does it work?
- In cooperation with the U.S. Environmental Protection Agency (US EPA) and FuelCell Energy Inc., King County has trialed the world’s largest demonstration project of a molten carbonate fuel cell (1 mega watt (MW)) using digester gas. CH2M Hill (an engineering, consulting and construction company) and Brown and Caldwell (environmental engineers and consultants) are assisting King County in the coordination and management of the overall project. CH2M HILL and Brown and Caldwell have direct responsibility for monitoring and reporting of project status, design and utility interface requirements, assistance during construction, start-up, testing, and operation, and analysis and reporting of the results of the demonstration project.
- As wastewater is treated through the sewage treatment process, it releasesbiogas (also known as “digester gas,” a gas that is produced in a container in which substances are treated by heat, enzymes, or a solvent.) The South Plant produces 4 MW of biogas in five digester tanks. The composition of the gas is about 60% methane and 40% CO2.
- In addition to using raw digester gas, the fuel cell power plant can also use natural gas that is either produced by scrubbing the digester gas or supplied by the local utility, Puget Sound Energy. The power plant runs on only one fuel at a time.
- Pretreatment (carbon scrubbing) systems remove impurities from both the digester gas and natural gas that could damage system components. Before entering the anode, high temperatures and steam convert methane gas to hydrogen. At the cathode, oxygen and carbon dioxide recycled from the anode react with the electrons in the electrolyte to form carbonate ions that replenish the electrolyte and transfer current through the fuel cell.
- Cathode exhaust gas travels to the heat recovery unit where it is distributed as a heat source for the fuel cell plant and the anaerobic digesters at the treatment plant.
- The power plant is sized to generate up to 1.5 MW to take advantage of future technology advances. A power conditioning system converts DC power from the fuel cell stack modules to AC power for export to the treatment plant. The power plant is also capable of exporting electricity to the regional power grid.
- A turbine cogeneration facility at South Plant may be used in the future to consume the remaining 3 MW of scrubbed digester gas and up to 5 MW of natural gas from the utility to produce standby power, with the potential to generate enough electricity to power the whole plant.
- The power plant is currently not operating. King County plans to replace the demonstration power plant with new equipment and to resume using fuel cell technology in 2008.
- Experience on this project points to a greater need for education and training among industry professionals until the technology becomes more common. Even though the project was safely isolated from the power grid, the plant was required to comply with inter-connection rules such as shutting down when it sensed deviations in voltage and frequency. In addition, a third-party electrical inspector was hired because local inspectors were not familiar with the unique fuel cell technology in use. Early and frequent communication with local inspectors and utilities can help smooth the process.
- Many of the processes required more tweaking and it took longer to make the transition from FuelCell Energy staff to South Plant staff to operate and maintain the plant than expected. By the end of the demonstration, however, the plant was fully automatic. Also, some of the components are expensive to maintain, such as the stack that needs to be replaced every three to five years.
- If incentives are available, the capital cost is competitive with other technologies. Much of the project costs can be attributed to research and development. The cost of a similar system today would be about $5 million ($0.04 to $0.06/kWh). The cost in the future is expected to more like $0.01/kWh. As costs are decreasing, efficiency of fuel cell technology continues to improve.