The King County Department of Natural Resources is participating in a $17 million demonstration project to test the feasibility of using fuel cell technology to convert digester gas into electricity and heat.
Other participating groups include the U.S. Environmental Protection Agency, Washington State Energy Office, M-C Power Corporation, Bechtel Corporation and Puget Sound Energy. The demonstration will verify the merits and applicability of Molten Carbonate Fuel Cell (MCFC) technology for use in digester gas applications throughout the United States and the world.
Wastewater treatment plants using anaerobic digestion to stabilize and reduce pathogenic microorganisms produce large quantities of gas containing about 60 percent methane and 40 percent CO2. According to research by M-C Power and CH2M Hill, a 30 mgd wastewater treatment plant generates enough digester gas to fuel a 1 megawatt fuel cell plant using molten carbonate technology.
MCFC technology is one of several types of fuel cells being developed. The technologies differ in the electrolyte used. Common electrolytes are molten carbonate, phosphoric acid, solid oxide, alkaline, and polymer. The electrolyte dictates the operating temperature and the potential electrical efficiency of the fuel cell.
The M-C Powers molten carbonate fuel cell currently is one of the most efficient fuel cell technologies. Carbonate salts serve as the electrolyte in a molten carbonate cell. Heated to 1200 degrees F, the salts melt and provide ionic conductivity. At the anode, hydrogen (H2) reacts with the carbonate ions (CO3-2) from the electrolyte to produce water (H2O), carbon dioxide (CO2) and electrons. Similarly, at the cathode, oxygen (O2) from process air and CO2 (recycled from anode products) react with electrons to form CO3-2 ions which replenish the electrolyte and transfer current through the fuel cell.
The cell produces direct electric current, which is converted to 60 cycle, 3 phase alternating current by an inverter. The voltage then is changed by a transformer to meet the needs of the end user. The fuel cell also produces heat during its exothermic reactions that can be harnessed for energy conservation.
A study by CH2M Hill indicates that fuel cells can convert digester gas into a usable form of energy at a cost lower than other technologies currently on the market. Fuel cells also produce lower air emissions. The study found that there are currently 134 wastewater treatment plants in the United States that have a capacity of 30 mgd or more and incorporate anaerobic digestion. The plants all are located in urban areas with heavy populations, providing a ready potential market for electric power.
Demonstration Project
The East Division Reclamation Plant, located in Washingtons King County, was selected as the site for a 1 Mw demonstration project funded by EPA with cost sharing provided by King County, M-C Power and Puget Sound Energy. M-C Power is a major subcontractor responsible for plant engineering, equipment purchase, plant installation and startup. Plant construction begins in 2000 and operations begin in 2001 and extend for two years.
Editors note:
This article was adapted from a paper presented at the 1999 Energy Efficiency Forum by Rene Laurens of M-C Power Corp., Burr Ridge, Il.
