* GenOn to deactivate 3,140 MW of generating capacity * Gas fired plants displacing coal in record numbers * More than 25,000 MW of coal to retire over decade By Scott DiSavino Feb 29 (Reuters) - U.S. power generator GenOn Energy Inc on Wednesday said it would deactivate 3,140 megawatts of mostly coal-fired generating capacity in Pennsylvania, Ohio and New Jersey by 2015 due to more stringent federal environmental regulations. Over the past few years, energy companies have announced the shutdown or planned retirement of more than 25,000 megawatts (MW) of coal-fired generation due to proposed more stringent federal environmental regulations, weak power market conditions and record switching from coal to natural gas-fired generators as gas prices hold near 10-year lows. In its earnings release, GenOn said it would shut the units, which are all located in the PJM power grid, between June 2012 and May 2015 because forecasted returns on investments necessary to comply with environmental regulations were insufficient. PJM operates the nation's biggest power grid covering all or parts of 13 U.S. Mid-Atlantic and Midwest states. GenOn's stock rose more than 7 percent, or 17 cents, to $2.47 on the New York Stock Exchange. Also on Wednesday, Midwest Generation, a unit of California power company Edison International, agreed to shut two coal-fired power plants in Chicago. The affected power plants are the following: Plant State Fuel Size (MW) Date Elrama PA Coal 460 Jun 2012 Niles OH Coal 217 Jun 2012 Portland PA Coal 401 Jan 2015 Avon Lake OH Coal 732 Apr 2015 New Castle PA Coal 330 Apr 2015 Shawville PA Coal 597 Apr 2015 Titus PA Coal 243 Apr 2015 Glen Gardner NJ Natgas 160 May 2015 -------------------- Total 3,140 One megawatt powers about 1,000 homes in PJM. GenOn said it expected to deactivate the units pending further review based on market conditions, noting it was still evaluating the potential return on investment if it installed environmental controls at Avon Lake in Ohio. The company also said Shawville, which is leased, would be placed in long-term protective layup and the required lease payments would continue to be made and the assets maintained in accordance with the lease. In addition to the plants announced Wednesday, GenOn also noted previously announced fleet reductions, including: Plant State Fuel Size (MW) Date Vandolah FL Natgas 630 May 2012 Potomac River VA Coal 482 Oct 2012 Contra Costa CA Natgas 674 May 2013 GenOn said its tolling agreement for the Vandolah facility in Florida would expire in May 2012. The company also said the expected retirement of Contra Costa was subject to regulatory approvals. The total fleet reductions, including the sale of the mothballed 586-MW Indian River plant in Florida and the deactivations announced Wednesday totaled 5,512 MW, GenOn said. After taking out the reductions announced through Wednesday and adding in the 719-MW Marsh Landing natural gas plant in California, which the company expects to enter service in mid-2013, GenOn said it will have 19,490 MW of generating capacity. GenOn said it had invested about $2.4 billion since 2000 on environmental controls for the existing plants the company expected to keep in its fleet. In addition, GenOn said it expects to invest another $586 to $726 million over the next 10 years to install more controls at some plants to meet air and water environmental regulations.