Editor's note: The story has been updated at 4 p.m. Oct. 27 to reflect a higher proportion of coal-fired plants being closed.

Coal's decline in Texas accelerated this week as Luminant announced it would close three coal-fired power plants in early 2018. Combined, they account for about 22 percent of the state's coal power plant capacity.

The state's largest power generator revealed Friday that the Big Brown Plant southeast of Corsicana and Sandow Plant northeast of Austin would close next year. A week ago, Luminant announced it was shutting down its Monticello Plant near Mount Pleasant.

The three plants can produce 4,167 megawatts of electricity — enough to power nearly 2.1 million Texas homes.

Luminant officials said cheap natural gas and Texas' competitive energy market has gradually made these decades-old plants unprofitable. The state has also seen a boom in wind energy (it leads the nation) and increases in solar.

The cost to generate electricity from coal plants varies from $60 to $143 per megawatt hour, compared to $48 to $78 for natural gas, according to a report last year from financial advisory firm Lazard. The unsubsidized cost for wind was $32 to $62.

"Though the long-term economic viability of these plants has been in question for some time, our yearlong analysis indicates this announcement is now necessary," according to a statement from Curt Morgan, president and CEO of Luminant parent company, Vistra Energy Corp.

These closures slash the company's coal generation capacity by 52 percent. Another coal plant, CPS Energy's J.T. Deely, is scheduled to close at the end of 2018. That closure was announced in 2013 and would remove another 840 megawatts of coal capacity, or more than 4 percent of the state's total.

A Sierra Club-commissioned study of another CPS Energy coal plant, J.K. Spruce Power Plant, found that it lost $135 million in 2015 and 2016.

Although Luminant's decision was financial, it was met with applause from environmentalists.

"This is more good news for clean air and the health of Texas families," said Luke Metzger, director of Environment Texas, in a written statement. "For decades, these power plants have pumped out dangerous pollution that have cost thousands of lives. Fortunately, clean energy is booming and we don't need these old, dirty plants anymore."

In a 2013, Environment Texas concluded that Big Brown was the 61st-most polluting power plant in the U.S. based on carbon dioxide emissions. That same report labeled Monticello as 17th-most polluting plant.

Officials with the Environmental Integrity Project said the Big Brown and Sandow closures would cut 26 million ton of carbon dioxide emissions annually or 10.8 percent of the total generated by Texas power plants.

One of President Donald Trump's major campaign promises was to revive the U.S. coal industry and bring back jobs. And he and his appointees — including Energy Secretary Rick Perry — have described coal as an important foundation of the nation's electricity supply.

Closing Sandow would cost about 450 jobs, which includes the nearby Three Oak Mine supplying the plant. Shuttering Big Brown and Monticello will each eliminate about 200 jobs.

Nationally, utility-scale coal plants accounted for 31 percent of the nation's electricity this year, up one percentage point from 2016, according to the U.S. Energy Information Administration. The federal forecast is that coal will stay at that same level in 2018.

Even with that current plateau, the trend has been moving away from coal as a power source. The Sierra Club said that with the Texas closures, more than half of U.S. coal plants have shut down or announced their retirements since 2010.

Luminant has invested in more competitive natural gas and solar plants since its parent company emerged from bankruptcy.

ERCOT, which operates most of Texas' electrical grid, will need to sign off on the closure after determining its effect on the grid.

After these closures, Luminant will have only two coal-fired plants in its fleet. Martin Lake is the company's largest coal plant with a capacity to power more than 1.2 million homes and Oak Grove is the newest at seven years old.

At 20 percent, coal has the second-largest share of Texas' power generation capacity, trailing only natural gas at 50 percent. The Luminant plant closings are expected to push wind into second place in capacity.