President-elect Trump says he wants to bring back the coal industry by building "clean coal" power plants. But he hasn't clarified what exactly he means.

Clean coal comprises a broad category of technologies that means different things to different people.

If the focus is on climate change, then the clean coal conversation quickly turns to carbon capture and storage, or CCS. If you are really in the know, the technology derivative is carbon capture, utilization and storage, with an even more long-winded acronym, CCUS.

Both technologies focus on removing carbon dioxide gas from a power plant's exhaust, or even as the coal is being burned, so as to not release it into the atmosphere and contribute to global warming. The carbon can be cooled and injected as a liquid deep underground where the power plant company hopes it will remain forever.

More advanced processes, or CCUS, recycle the carbon dioxide to use as a chemical precursor, power hydraulics, make renewable fuels and even produce oil.

The big problem with carbon capture technologies is that they are currently not cost effective, and therefore are not available for commercial use on power plants. Although large utility companies such as Southern Co. are investing millions of dollars in attempting to make the technology viable, they aren't there yet.

Trump has been silent on whether his ideas about clean coal mean continued investment in carbon capture, or something else.

Some media reports have envisioned, although no one has confirmed it, that Trump could resurrect former President George W. Bush's FutureGen program, which established the goal of building the world's first zero-emission coal-fired power plant. Bush killed off the program due to cost overruns.

President Obama tried to resurrect it as FutureGen 2.0, but then pulled the plug in 2015 for financing troubles. It is not certain if FutureGen 3.0 would be a priority for Trump, but it is an option.

The National Mining Association, representing the coal industry, has been in contact with the Trump transition team, but the group still doesn't know exactly what the president-elect means by clean coal, according to long-time spokesman Luke Popovich.

"We've been in contact with his transition team, as have just about everyone else in the fossil energy end," Popovich said in an email. "If I had to guess, I doubt he focuses on any one technology but maybe refers to the whole suite of high efficiency, low emissions technologies you've listed here."

Other more near-term technologies include what are known as critical and supercritical coal power plants, which burn coal at higher temperatures than their more conventional and less efficient brethren.

The plants are much cleaner than conventional coal plants and can easily be retrofitted to include carbon capture once it is commercially available.

One such plant is running in Arkansas. It is called the Turk Plant, built by a subsidiary of utility American Electric Power. Former Texas environmental regulator Kathleen Hartnett-White, who has been advising the Trump transition team, told the Washington Examiner last month that she recently visited the plant, but wouldn't say for sure if plants like it would be the focus of Trump's initiative.

The Turk plant is the last new coal-fired power plant to be built in the country. It is the last because the Environmental Protection Agency issued climate change rules for new power plants that essentially ban any new coal plants being built if they do not use carbon capture, which is not currently feasible.

Plants such as Turk are cleaner and produce fewer emissions, but because of the EPA's technology restrictions cannot be built. But many argue that even without the EPA standards, coal is on the wane. The low cost of natural gas has put coal in the backseat as the dominant source of electricity generation in the country. For now, natural gas is king.

But coal is expected to come back this winter as natural gas prices rise, making coal more competitive in some regions, according to the government's Energy Information Administration.

Much of the increased electricity from coal expected will come from more conventional coal plants that the industry says fits into the clean coal category, the "scrubbed" coal power plants.

Scrubbed means the plants have been fitted with devices called scrubbers that pull pollutants out of the flue stack, sort of like a gigantic cigarette filter or your car's catalytic converter. They are cost effective and most of the coal fleet have them now, thanks to the EPA.

The scrubbers and bag houses make the plants clean because of separate EPA regulations for conventional pollutants, but they do not reduce the plants' carbon dioxide emissions. Still, the plants could be another option in Trump's clean coal initiative. It just depends how far he wants to go to address climate change.