One of Texas' largest coal plants is shutting down, a victim of plentiful and cheap natural gas, plant owner Luminant announced today.

The Monticello Power Plant near Mount Pleasant, about 115 miles east of Dallas, is expected to close in January after ERCOT, which manages Texas' electric grid, signs off on the decision.

The plant, once seen as a key source of smog-causing pollution, has an operating capacity of 1,880 MW — enough to power about 940,000 homes in normal conditions and 376,000 homes in periods of peak demand. It employs about 200 workers.

The shutdown decision follows a year-long review of the plant's economic viability, according to Allan Koenig, a spokesman for Vistra, Luminant's parent company. Ultimately, Koenig said, it was Texas' highly competitive electricity market, rather than environmental regulations, that led to the closure.

"It's purely economic," he said. "The plant guys tried everything they could to keep it open, but it was a money loser. In a competitive market, you've got to take these steps. This is a coal plant operating in a market that's flooded with cheap natural gas."

Koenig said there are efforts to find the 200 affected workers other jobs in the company when possible.

Even as President Donald Trump has promoted coal as a critical power source, the cheap natural gas and plummeting prices for wind and solar energy have made coal a less financially viable fuel.

Environmental groups say Monticello was once one of the nation's most polluting power plants. In 2013, an report from Environment Texas found that Monticello emitted the 17th most carbon of any U.S. power plant. Martin Lake, which is also owned by Luminant and is still open, was the third largest emitter.

"It seems like they've been bluffing for a few years now that it was the regulations that were dragging them under when by their admission today, it's the economics," said Luke Metzger, director of Environment Texas. "Coal is just not competitive like it was."

In 2011, Luminant announced that it would shut down two of the three Monticello units because of new federal air pollution regulations. The company blamed the Environmental Protection Agency's Cross-State Air Pollution Rule, which targeted harmful emissions that cross state lines.

That rule was initially blocked by litigation. And later, Texas was removed from the list of states that must comply with that rule.

In recent years, Luminant started operating Monticello only at times when demand and electricity prices were higher. That's mostly been during summer and winter peaks.

Since emerging from its parent company's bankruptcy, Luminant has worked to diversify its energy mix. This year, it bought a 1,054 megawatt natural gas plant and purchased what would be the state's second largest solar plant (182 megawatts).

The Monticello units opened in 1974, 1975 and 1978. Koenig said there was a boom in coal plant construction in the mid-1970 and some of those are starting to close.

The Sierra Club said Monticello is the 259th coal plant to close since 2010. That's nearly half the coal plants in operation seven years ago, according to the environmental group.

"Our communities in Dallas have borne the brunt of dangerous smog pollution for decades and coal plants like Monticello have been a huge source of that pollution," said Misti O'Quinn of the Sierra Club's Beyond Coal Campaign in Texas.

Emissions from Monticello and other East Texas coal plants have contributed to local air pollution as well as haze in other states.

Luminant had already closed a pair of nearby lignite coal mines that supplied Monticello. Instead, higher-quality coal was shipped by rail from the Powder River Basin.

Even as coal plants continue to close, it hasn't slowed Washington's efforts to roll back Obama-era environmental regulations. Bloomberg reported that the Trump administration was planning to argue that President Barack Obama's Clean Power Plan — meant to cut greenhouse gases that contribute to climate change — violates U.S. law.

Also, Energy Secretary Rick Perry, the former Texas governor, wants to enact rules to protect coal and nuclear power plants as a means to increase the reliability of the nation's electric grid by diversifying the sources of power.