President Trump speaks at a campaign rally in Phoenix on Aug. 22. (Joshua Roberts/Reuters)

In a meandering speech Tuesday evening in Phoenix, President Trump launched into one angry rant after another, belittling the media, the North American Free Trade Agreement and Arizona’s two Republican senators.

But one of the more perplexing comments during the often off-message rally was about something Trump has championed in the past: “clean coal.”

Although Trump has positioned himself as an advocate of the fledgling technology, it’s unclear whether he really understands the basics of it.

“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 told a rapt, though eventually dwindling, crowd.

A Pennsylvania company has announced the opening of two new coal mines. But Trump’s description of “clean coal” is wanting, to say the least, according to experts.

Carbon capture and storage, branded by advocates as “clean coal,” refers to methods of capturing the carbon dioxide emissions that cause climate change. That generally involves capturing the carbon dioxide from smokestacks on coal-fired power plants, and storing it underground.

“ ‘Clean coal’ refers to capturing the carbon dioxide after the coal is burned,” Matt Lucas, associate director of carbon-capture technology at the nonprofit Center for Carbon Removal, said in an email.

Indeed, the Energy Department itself describes its “clean coal” research and development as focused on “carbon capture, utilization and storage technologies.”

Trump’s comment suggests the carbon is somehow captured before the coal is burned — which is definitely not the case.

The president may have been referring to another pollution-reduction practice called “coal washing,” where soil and rock is washed away from mined coal before being burned.

But that is not the same as carbon capture and storage.

The technology has found proponents among Democrats and Republicans. Last month, a bipartisan quartet of senators — Heidi Heitkamp (D-N.D.), Shelley Moore Capito (R-W.Va.), Sheldon Whitehouse (D-R.I.) and John Barrasso (R-Wyo.) — introduced legislation to strengthen a tax credit supportive of projects attempting to sequester carbon emissions.

Even the oil and gas industry has, at times, backed carbon capture. That’s because the captured carbon can be injected underground to push out difficult-to-reach pockets of oil — a process called “enhanced oil recovery.” Captured carbon can also be used as a raw material in the manufacture of cement, steel and some industrial chemicals.

However, some environmental groups, such as the Sierra Club, argue that no coal-burning technology can be called “clean” given the levels of soot and other traditional pollutants such plants emit relative to nuclear, wind, solar and even natural-gas-fired power plants.

And the nascent technology has not been without its growing pains. While one facility, the Petra Nova carbon capture system in Texas, was turned on this year with much fanfare from Energy Secretary Rick Perry, another high-profile carbon-capture project, Southern’s Kemper plant in Mississippi, was suspended in June after running $4 billion over budget and three years behind schedule. Both had financial backing from the Energy Department.

The Phoenix rally was not the first time Trump made misleading statements about “clean coal.” Trump claimed the Paris climate agreement — which he led the United States in exiting — “effectively blocks the development of clean coal in America.” In reality, the accord allows countries to decide for themselves how to curb carbon emissions.

To the frustration of carbon-capture advocates, the Trump administration has been more supportive of the technology in word than in action.

In May, the White House proposed slashing the budget of the Energy Department’s Office of Fossil Energy, which backs carbon-capture research, and eliminating a loan program for companies developing carbon-capture and other innovative energy technologies, arguing that “the private sector is better positioned to finance innovative technologies.”

“There’s a big gulf between the president’s words and the actions of the administration on support for carbon capture,” said Dan Reicher, executive director of the Steyer-Taylor Center for Energy Policy and Finance at Stanford.