While automakers display prototype fuel cell-powered cars as cutting-edge, fuel cells already are being used at sewage treatment plants and landfills to convert anaerobic digester gas (ADG) and landfill gas (LFG) that normally would be flared into electric power and heat.
Three fuel cell power plants currently use the ADG produced during wastewater treatment to generate electricity - in Boston, Yonkers, N.Y., and Yokohama, Japan. Additionally, the Groton, Conn., Flanders Road landfill has been converting LFG into electricity for a couple of years. And, similar fuel cell facilities are planned for California, Oregon and Japan, while Europe considers the technology.
Fuel cells generate electricity through chemical reaction rather than combustion. Similar to batteries, the cells use an electrochemical process to convert fuel into electricity, heat and water. Individual cells then are connected to produce direct current voltage. While batteries run down if not recharged, fuel cells produce energy as long as they are supplied with fuel.
The cells use hydrogen, which typically is produced from natural gas at an off-site facility. To make refueling convenient, "reformers," which extract hydrogen from traditional fuels like gasoline, ethanol or methanol, recently have been developed for use on vehicles. ADG is about 60 percent methane, but with the reformers, the methane is converted into hydrogen.
Key to ADG or LFG conversion is the gas clean-up system, which removes chlorinated compounds, sulfur compounds and contaminants from methane before it is reformed. In Groton, Conn.'s LFG clean-up system, hydrogen sulfide is absorbed by carbon pellets. The gas then is chilled to liquefy most of the other impurities so that they can be drained off. Next, the clean methane goes to a reformer, which turns it into hydrogen-rich gas. Heat from the fuel cell is used in the process.
At Yonkers' power plant, a gas processing unit removes sulfur and moisture from the ADG before the gas, which consists almost entirely of methane and carbon dioxide, enters the fuel cell. Carbon dioxide does pass through the unit and is emitted into the air, but the emissions are less than one-tenth that of methane when it is flared.
The Yonkers fuel cell - which was funded in part by the U.S. Environmental Protection Agency (EPA) and the U.S. Department of Energy, and was created by ONSI Corp., South Windsor, Conn. - provides electricity to the Westchester County Wastewater Treatment Plant, Yonkers.
Preliminary EPA data shows that New York Power Authority's (NYPA) 200-kilowatt (kW) unit emits minimal pollution and has reduced annual total emissions by more than 26 tons. Consumption of fossil fuels has been reduced, and emissions have been cut nearly 25 percent - more than six tons - by using ADG in the cell instead of flaring the gas.
ONSI, a subsidiary of International Fuel Cells Corp. (IFC), which is a joint venture between United Technologies Corp., Hartford, Conn., and Toshiba America, New York, also designed the 200 kW PC25 phosphoric acid fuel cell that has been installed at Groton's 42-acre landfill.
While Groton's Flanders Road Landfill no longer accepts garbage, it still is capable of producing gas for another 20 years. Consequently, wells and pipes collect the gas, which normally is burned off, and about 20 percent of the LFG is sent to the cell. A power conditioner converts the DC electricity produced by the cell into AC electricity. A step-up transformer increases the output to 13,800 volts for use on the Connecticut Light and Power Co.'s (CL&P) utility grid.
The 165 kW output of Groton's fuel cell while operating on LFG is said to be able to power more than 100 homes, as well as to supply electricity and heat to nearby factories or processing plants. When used at facilities near a landfill, the fuel cells are able to recover more than 80 percent of the LFG's energy value.
Groton's landfill demonstration project is a partnership between the town of Groton, CL&P, IFC and the EPA.
In Japan, Toshiba is expanding its efforts to sell fuel cells which run on gas derived from sewage sludge. The company hopes to sell approximately 10 systems per year, primarily to local governments. Yokohama city government's system is the first such operation.
The first use of fuel cell energy in California is at the Las Virgenes municipal water district, Calabasas, and Triunfo sanitation district, Ventura. The districts, which currently provide water and wastewater services to more than 80,000 residents in western Los Angeles County and east Ventura County, plan to power their operations by installing two ONSI PC25C fuel cells that will convert waste methane from the district's Rancho Las Virgenes biosolids composting facility into 400 kWs of electricity and process heat.
Compared with electricity generated from combustion-based processes, these two units are expected to reduce nearly 5 million pounds of air pollution during each operation year, including eliminating the carbon dioxide emissions that would result from combustion.
As many as 1,700 U.S. landfills are ideally suited for a fuel cell based LFG-to-electricity conversion, according to the EPA. This could generate more than 1,000 megawatts of electric energy, enough to provide electricity to nearly one half million homes.
The limiting factor for widespread fuel cell use is the high cost. Systems such as Groton's cost about $1 million to $1.5 million, including the clean-up equipment. However, the higher cost of this system can be balanced against other power generation techniques that are unable to recover methane gas, a suspected contributor to global warming.