MEMPHIS — The Tennessee Valley Authority’s reliance on coal has dropped and the use of natural gas as fuel to generate electricity has increased.

According to The Commercial Appeal, TVA’s use of gas climbed 70 percent in the fiscal year just ended. At the same time, coal-fired generation declined 30 percent from fiscal 2011. The fiscal years end Sept. 30.

“What we’re trying to achieve, from a corporate standpoint, is a good, diverse mix of energy sources that we can adjust as the market forces change,” said Bob Dalrymple, TVA vice president of gas operations.

Dalrymple said current costs for coal and gas are reasonably competitive with one another.

The greater use of gas by TVA mirrors a national trend as hydraulic fracturing in drilling releases abundant sources of the fuel.

TVA has been building and buying natural gas-fired plants, and the resulting change has been remarkable. Gas fueled the generation of about one-fifth of TVA’s electricity generation in fiscal 2012. That was up from less than 1 percent in 2007, agency officials said.

The use of gas is likely to accelerate further. TVA has announced it will retire 18 of its 59 coal-fired units by year’s end.

The utility last week dedicated the new John Sevier Combined Cycle plant near Rogersville, Tenn., and is considering construction of gas-fired generation at the Allen Fossil Plant in Memphis.

The Rogersville plant can produce 880-megawatts — enough power for about 500,000 homes. It went on line in April.

Among other gas plants built, bought or leased by TVA are units in Southaven, Miss., and near Brownsville, Tenn.

Nuclear generation also is a major player, accounting for one-third of the power produced by TVA. Hydroelectric generation and that from solar, landfill gases and other renewable sources fill in the menu of generation sources behind nuclear, coal and gas.

There are environmental advantages, too.

“We’re very happy to see them (TVA) retire coal plants and reduce their greenhouse emissions,” said John D. Wilson, research director for the Knoxville-based Southern Alliance for Clean Energy.

Wilson said gas-fired plants better lend themselves to conservation because they are easier to shut down than coal units when their output isn’t needed. He said gas, however, is subject to greater price volatility than coal is.

Coal industry leaders also are quick to point to price swings in gas and underscore that nearly 30 percent of the world’s known coal reserves are in the U.S.