AT the Central Landfill in Johnston, R.I., trash is going in and 12.3 megawatts of electricity is going out.
Unlike "waste-to-energy" plants that burn solid waste, the energy source in this case is the methane gas that forms as the trash decomposes.
The landfill, New England's largest, is dotted with 75 wells that extract gas for burning in eight huge internal-combustion engines.
The engines can produce enough electricity to supply 20,000 homes for 30 to 50 years, according to the Rhode Island Solid Waste Management Corporation, which runs the landfill.
This is one of 114 landfill-gas energy projects that have sprung up across the nation in the last decade, supplying 344 MW of power and reducing emission of "greenhouse" gases that contribute to global warming.
The future growth of the industry depends on several factors, including the price of competing sources of power and whether Congress decides to renew tax credits for such projects, an incentive that is due to expire at the end of the year. But another incentive is due to kick in: A new clean-air rule, to be issued by the Environmental Protection Agency (EPA) this fall, will require about 620 of the nation's 6,000 landfills to collect and control gas emissions.
This can be done by simply flaring the gas. But experts say many of those landfills could make viable power projects.
"When part of the expenditure has to be taken" to meet air-quality rules, then electricity generation becomes more attractive, says Don Walgren, a vice president of Waste Management Inc. of Oak Brook, Ill. The company currently has 25 methane-to-energy projects.
Mr. Walgren says the sites affected by the new clean-air rule will be "good candidates" for new energy projects. "We expect to develop new projects as they become viable," he says.
Viability depends to a large extent on prevailing energy prices. Under the Public Utility Regulatory Policy Act of 1978, electric utilities must buy power from alternative sources at a price comparable to what the utility would have to pay to produce it.
This "avoided cost" varies across the country, Walgren says. In the Midwest, the presence of nuclear power sets the avoided cost at 2 or 3 cents per kilowatt-hour; in the Northeast, which relies more on coal, oil, and gas, the cost is more like 6 cents per kwh.
"We have several successful projects here in the Midwest," Walgren says. Two of them, in Cincinnati and Detroit, sell power directly to industrial users rather than to a utility.
ONE company, Vermont Energy Recovery, is trying to show that methane projects are viable even if they produce only half a megawatt of power.
"Presently ... the philosophy is that people don't develop anything smaller than one megawatt," says Susan Thorneloe, an EPA specialist on landfill gas in Raliegh, N.C. The largest producer is the 50 MW Puente Hills landfill in Whittier, Calif.
Vermont Energy Recovery targets projects that can produce 0.5 to 2 MW. To keep costs low, the company sets up its engines on concrete pads without any building around it. The engines can then be moved to another landfill when the gas runs out.
For example, Vermont Energy just signed contracts for gas rights at landfills in Hartford, Vt., and nearby Lebanon, N.H. As the Hartford landfill, which recently closed, begins to provide less gas, the company plans to move one of its modular units across the river to Lebanon. That should occur around the year 2000, says Dan Ingold, the company's chief financial officer.
Vermont Energy has run a 0.7 MW project at its Brattleboro, Vt., headquarters since 1981.
Mr. Ingold says the company's use of standardized "building blocks" is like the difference between "making race cars and making Chevy Novas."
Many landfill operators lack the capital and technology to develop power projects on their own, but contract with independent companies. That was what Rhode Island's Central Landfill did when faced with mounting complaints about the odor emanating from the site. The Northeast Landfill Power Joint Venture, financed by Palmer Capital Corporation, acquired a 30-year lease to mine the gas and contracted to sell electricity to the New England Power Company. The state-run landfill gets 15 percent of the revenu es.
Landfill gas typically contains 50 to 55 percent methane, 45 to 50 percent carbon dioxide, and less than 1 percent nonmethane organic compounds (NMOCs). Although the EPA rule will target the NMOCs, which contribute to smog formation and contain some toxic elements, the rule will effectively require the capture of the whole gas mixture. Methane is a potent "greenhouse" gas; each molecule contributes 30 times more to global warming than a carbon dioxide molecule.