Among currently used methods for generating electricity, coal is by far the dirtiest.
But--in the U.S., at least--it's also on the decline as an energy source.
The U.S. Energy Information Administration (EIA) now predicts that coal's share of U.S. electricity generation will decline from 44 percent in 2011 to 32 percent in 2040.
That trend is already becoming apparent in a significant number of retirements of coal-fired power plants.
It's a trend that will accelerate under proposed Environmental Protection Agency (EPA) rules, notes a recent Navigant Research blog post.
In March, the EIA predicted that 13 gigawatts of coal generation capacity would be retired this year, due to aging infrastructure and upcoming environmental mandates.
Coal, by Flicker user oatsy40 (Used Under CC License)
Most of these plants are located in or near the Appalachian region, with Virginia, West Virginia, Kentucky, Ohio, and Indiana accounting for 8GW of the expected retirements.
The rest of the 13-GW total will include plants in Alabama and the Midwest.
In June, the EIA released an estimate of how the stricter rules of the EPA's proposed Clean Power Plan would affect the mix of energy-generation sources between now and 2040.
ALSO SEE: EPA Power Plant Carbon Regulations Late, Hostile Congress Likely To Fight Them (Dec 2014)
The Clean Power Plan was recently struck down by the Supreme Court, and has faced aggressive opposition from lawmakers representing coal-producing states.
Yet even if the proposed rules never take effect, analysts predict 40 GW of current coal generation capacity, and 46 GW of oil and natural gas capacity, will still be retired by 2040.
If the rules are implemented, the amounts of retired capacity could increase to 90 GW for coal, and 62 GW for oil and natural gas.
Electricity grid substation (Image: FirstEnergy Corp on Flickr, used under CC license)
A possible extension of those rules could see retirement figures increase slightly more--to 100 GW of coal-generation capacity, and 74 GW of oil and natural gas capacity.
Of course, this scenario is dependent on the fate of the proposed EPA rules, which would make coal-fired power plants non-viable for more utility companies.
However, the current age of much of the coal-generation infrastructure, and increased interest in renewable energy on the state level, could lead to continued retirements of coal-fired power plants over the next couple of decades.
In the interim, cheap natural gas will likely entice many utilities to move away from coal as renewable alternatives are developed.
[EDITOR'S NOTE: An earlier version of this article incorrectly said that the Clean Power Plan was recently struck down by the Supreme Court. The case actually involved Mercury and Air Toxics Standards.]