What Trump has said about 'clean coal' and what it is It is widely seen as a propaganda for the coal industry.

 -- President Donald Trump on Tuesday night told supporters at a rally in Phoenix that "clean coal" was back. Only his description of "clean coal" was scientifically wrong.

"We've ended the war on beautiful, clean coal. and it's just been announced that a second, brand new coal mine where they're going to take out clean coal — meaning they're taking out coal, they're going to clean it — is opening in the state of Pennsylvania," Trump said.

His comment puzzled scientists and people with an understanding of how coal mining works.

"He clearly doesn't know what he's talking about," said Pieter Tans, the lead scientist of the Global Greenhouse Gas Reference Network of the National Oceanic and Atmospheric Administration in Boulder, Colorado.

"It sounds like he thinks that they're going to wash the coal," Tans told ABC News. "It doesn't make any sense."

He added, "The concept of 'clean coal' is industry propaganda."

It was not the first time Trump referred to "clean coal" in a speech.

At a February 2016 rally in Virginia, he said, "Clean coal is coming back" and took a dig at China's coal production practices.

"We sell coal. The coal mines are dying, but the only coal we give is coal to China. Do you think they clean the coal? Believe me, they don't," he said.

During the second presidential debate, Trump mentioned it again but didn't talk about what it was or how it would help the coal industry.

"Hillary Clinton wants to put all the miners out of business. There is a thing called clean coal. Coal will last for 1,000 years in this country," he said last October.

Steve Clemmer, the energy research director at the Union of Concerned Scientists, said Trump's clean coal references show that he does not have an understanding of the technology.

"Every time he refers to the word 'coal,' he puts the word 'clean' in front of it. Or 'beautiful,'" Clemmer told ABC News. "That signals to me that he doesn't understand what most people refer to 'clean coal' as."

He continued, "The way that most people refer to clean coal is the process of removing the carbon emissions from the coal and injecting them into geologic formations to address the impact that coal has on climate change."

According to Paul Bailey, the CEO of the American Coalition for Clean Coal Energy, "clean coal technology" was first used by then-Sen. Robert Byrd, D-W.V., in the 1980s. However, that term has become shortened by many to just "clean coal."

"The terms 'clean coal' and 'clean coal technology' have gotten conflated," Bailey explained.

"So when people refer to 'clean coal,' whether they know it or not, they're really referring to 'clean coal technology,'" he told ABC.

Clean coal technology refers to the advancements that have been made in the past several decades to limit the amount of air pollution created as a result of coal mining and burning.

Bailey denied that his organization uses the term "clean coal" as propaganda and said he never considered that people may interpret Trump's frequent references to "clean coal" as a type of coal that is cleaner than others.

"There's no such thing as clean coal. If you want to clean up coal, you've got to burn it and capture the particles, and the mercury will come out anyway as a gas," Tans said.

U.S. production of coal has increased more than 14 percent year to date compared with the same period in 2016, the U.S. Energy Information Administration said. West Virginia, Pennsylvania and Wyoming have experienced strong bumps in coal production since the beginning of the Trump presidency.

"As with many of Trump's asinine proposals, his insistence on promoting coal is a big step backwards," said Pushker Kharecha, the deputy director of the Climate Science, Awareness and Solutions program at Columbia University.

Kharecha noted that "coal is by far the dirtiest energy source in terms of both greenhouse gas emissions and fatal particulate emissions. In the U.S., coal plants are responsible for the large majority of both of these emission types."