King Coal is meeting its match: natural gas. Power plants fired by natural gas have set a new record by producing as much electricity in the United States as those run by coal, long the nation's dominant power producer.

Preliminary data for April show that natural gas plants generated the same share of U.S. electricity as coal ones -- 32% -- for the first time since the U.S. Energy Information Administration began keeping records decades ago. EIA is the statistical arm of the Department of Energy.

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"Natural gas prices as delivered to power plants were at a ten-year low" in April, the EIA said this week in disclosing the data, which it said may change. Another factor, as the agency reported last month, is coal's continuing decline in output -- 19% between the first quarter of 2011 and the first quarter of 2012.

The coal industry has been hit by reduced electric demand earlier this year, due to a mild winter and spring, and by new regulations from the Environmental Protection Agency that will require older coal plants to have pollution-control equipment. With the summer's heat wave in much of the U.S., the EIA said the demand for electricity from both coal and gas plants will increase, at least temporarily.

While coal faces its own challenges, natural gas has boomed because of a drilling process known as hydraulic fracturing, or "fracking," in which millions of gallons of water, sand and chemicals are pumped underground to fracture shale layers and release the gas.

Fracking has brought economic growth to many U.S. communities but has also raised environmental and other concerns.

"This is a mixed blessing from an emissions perspective," writes Stephen Lacey of the Center for American Progress, a think tank that advocates for clean energy. He notes that less coal generation will reduce the emissions of carbon dioxide but may increase those of methane, a more potent, long-term greenhouse gas. He also cite's fracking's impact on air and water quality.