In his rallies, President Donald Trump repeatedly talks about “clean coal,” using the phrase more than a dozen times over the past three months. He also has said in other appearances that clean coal can be exported or “loaded up” on railway cars.

But “clean coal” refers to technologies deployed at power plants that make coal cleaner to burn, not to the fuel itself. Modern definitions require cuts in carbon dioxide emissions, and the only way to do that in a substantial way is through carbon capture. Just two coal power plants in the world use the technique, and it makes up less than 0.1 percent of American coal-fired capacity.

“Clean coal” has become a popular talking point for the president, especially when he addresses crowds in coal-producing states, such as West Virginia, Pennsylvania and Montana.

On Nov. 3 in Belgrade, Montana, Trump said: “And we then did the war on clean, beautiful coal, and we are putting — and you see it better than almost anybody — our coal miners. They’re all back to work, and they’re going back to work. Clean coal, clean coal. Nobody thought that was going to happen so fast, either.”

But what does “clean coal” mean? And how does the technology work? We’ll dive into the various ways coal combustion can be made cleaner, and take a look at the future of coal in a world grappling with the effects of climate change.

What Is ‘Clean Coal’?

The term “clean coal,” which is used by industry, the government and academics, is annoyingly nebulous.

“Clean coal turns out to mean largely whatever one wants it to mean,” said Edward S. Rubin, a professor of mechanical engineering and public policy at Carnegie Mellon University, in a phone interview. He has worked on energy and environmental issues for almost half a century, and on coal technologies nearly as long.

Julio Friedmann, a senior research scholar at Columbia University’s Center on Global Energy Policy and CEO of the company Carbon Wrangler, agrees.

“There is no formal definition of clean coal,” he said in a phone interview. “And it is often in the eye of the beholder.”

Rubin recalls the term first being widely used in the mid-1980s, when the Department of Energy launched its Clean Coal program. At the time, acid rain was a top priority, and coal plants were a key culprit. Coal contains impurities, that, when burned, escape into the air as harmful air pollutants, including acid-rain-causing chemicals such as sulfur dioxide and nitrogen oxides. Fortunately, scrubbers and catalytic converters could be added to power plants to chemically remove the majority of these pollutants.

Over time, the definition expanded as society recognized that burning coal resulted in other pollution problems. In the ‘90s, for example, as scientists realized the importance of mercury emissions from coal, that too became a clean coal signifier. Mercury released into the air eventually makes its way back down to Earth, where it can become a health hazard for humans and damage ecosystems and wildlife. The Environmental Protection Agency estimates coal-burning power plants are responsible for about 42 percent of all man-made mercury emissions in the United States.

Finally, the “clean” label evolved to include reductions in carbon dioxide, the greenhouse gas scientists say is driving climate change. The EPA formally designated carbon dioxide as an air pollutant in 2009, in recognition of the harm the gas poses “to public health and welfare” in its role in global warming.

Coal plants pump out inordinate amounts of the gas. According to the Energy Information Administration, coal power plants released 1,207 million metric tons of CO 2 in 2017 — far more than any other source of electricity. And that’s despite a decline in coal use. As the EPA notes, “although coal accounted for about 67 percent of CO 2 emissions from the sector, it represented only about 32 percent of the electricity generated in the United States in 2016.”

“These days, my own sense is that talking about clean coal without extending it to mean significant reductions in carbon emissions is basically ignoring the climate-related impacts of coal combustion,” Rubin said.

It’s a sentiment Friedmann shares. “I personally believe coal cannot be clean unless it controls CO 2 emissions as well,” he said.

But Rubin notes that coal industry definitions are often vague, referring to environmental performance improvements that may make only incremental advancements. Even if industry groups accept that carbon dioxide control is part of clean coal, as some do, it’s often a question of how much. Someone could argue, Rubin said, that a 5 percent reduction in emissions is enough to make coal “clean.”

Clearly, the “clean” part of “clean coal” is relative, and is better described as “cleaner.” For this reason, many environmentalists think that “clean coal” doesn’t exist and consider the phrase an oxymoron. Coal can never be clean, they argue, because even if sophisticated power plants capture pollutants, there is environmental damage during the mining and transport of coal. Mountaintop removal in particular is notorious for polluting streams and harming ecosystems, especially aquatic ones. This environmental damage is well documented by the EPA.

But while Friedmann considers these fair complaints, he also thinks they should be put in perspective, as just 5 percent of U.S. coal production is done by mountaintop removal. “It doesn’t represent a big fraction of how coal is produced,” he said.

The best technology cannot make coal completely pollution-free or carbon-neutral, but it can mitigate a lot of the negative environmental impacts. This technology, known as carbon capture and sequestration (or storage), or CCS, is a process in which CO 2 is trapped and kept out of the atmosphere, usually by forcing the gas into below-ground formations. As we’ll explain, there are several ways to do CCS. But the technology is expensive and is not yet widespread.

Other “clean coal” methods also address carbon dioxide emissions, including novel ways of burning coal more efficiently. In a standard coal-fired power plant, coal is burned to heat a boiler, which creates steam that turns a turbine, which powers a generator to create electricity. Increasing the temperature and pressure of the steam, as is done in so-called supercritical and ultrasupercritical plants, makes this process more efficient. Because less coal needs to be burned to create the same amount of energy, the plants have smaller carbon footprints. These improvements, however, usually yield only modest reductions in carbon dioxide, such that they still do not match the lower emissions of modern natural gas plants. Rubin said these are also not widely used in the United States.

What Trump Could Mean

The White House did not reply to requests for clarification on what Trump means when he says “clean coal.” Trump could simply be thinking of the basic scrubbers that prevent acid rain — but do nothing to reduce CO 2 emissions.

Some of Trump’s more specific comments, however, incorrectly describe coal itself as being “clean,” not the burning of it.

Take, for instance, the idea that coal sitting in railway cars is clean, and that the U.S. exports clean coal, both ideas he floated in September.

Trump, United Nations speech, Sept. 25: We have become the largest energy producer anywhere on the face of the Earth. The United States stands ready to export our abundant, affordable supply of oil, clean coal, and natural gas.

Trump, aboard Air Force One, Sept. 7: No administration has done what I’ve done. I just left Montana, and I looked at those trains and they’re loaded up with clean coal — beautiful clean coal. And those trains were empty two years ago. They were empty; they were dying. Nobody’s done what I’ve done.

Setting aside Trump’s inflated claims about reviving the coal industry, which we’ve addressed before, Rubin said Trump’s words don’t make a lot of sense, given the normal definitions of clean coal.

“It isn’t the coal that’s clean,” said Rubin. “It’s using coal, and using it in a way that generates useful energy, without significant environmental emissions.” All of the meaningful ways to do this, he said, happen at a power plant.

There is only one plausible way Trump could argue his case, and it’s a stretch.

He could be thinking of coal cleaning or coal washing, an older technology that removes impurities, such as non-combustible mineral matter and sulfur, from coal. This can cut down on coal ash pollution, and slightly reduce sulfur dioxide emissions. But this processing hardly makes coal clean.

“Today, no one really thinks of coal cleaning as a significant clean coal technology,” said Rubin.

According to the Energy Information Administration, less than 20 percent of U.S. coal is washed, usually higher-sulfur coal in the East, and cleaning does not change coal’s carbon dioxide emissions.

Coal washing, notably, appears on a list of technologies posted on the American Coal Council’s website, but the industry group is careful to label the entire list “cleaner” rather than “clean.”

As for the export claim, Friedmann is equally baffled.

“In my mind, I don’t see how you can export coal and call it clean unless you export clean coal technology with it,” said Friedmann, adding that this is something the Trump administration is not doing.

And no exported coal heads to carbon capture facilities. Of the two coal-fired CCS plants, only one — the Boundary Dam Power Station in Saskatchewan, Canada — is outside the United States, and it gets its coal domestically.

How Coal CCS Works

Understanding the basics of how coal-based carbon capture and sequestration works will help explain why it isn’t more widespread.

There are three main ways to do carbon capture, which differ in when and how the CO 2 is removed:

Post-combustion : CO 2 is captured after the coal has been burned. This is the only capture method in commercial operation. Essentially, before the flue gases are sent up the chimney, the CO 2 is pulled out with chemicals, usually nitrogen-rich amines. This technique can be applied to existing coal plants as a retrofit — a huge advantage over the other methods. But it still takes quite a bit of energy to run the system, Rubin explained, mainly because the CO 2 -absorbing chemicals need to be stripped of CO 2 so they can be regenerated and used again.

Pre-combustion : CO 2 is captured from a coal-sourced gas before being burned. The carbon capture part is less expensive than in post-combustion capture, but the overall cost of gasification plants is high. To Rubin’s knowledge, just two American power plants use gasification at all. One plant, in Kemper County, Mississippi, was designed to do coal gasification with carbon capture, but had to scrap those plans after cost overruns and delays. It now runs on natural gas.

Oxy-fuel : Coal is burned in oxygen rather than air. This makes it easier to collect the CO 2 , but according to Rubin, running an oxygen plant takes a lot of energy, and the economics may or may not make sense compared with post-combustion capture.

Perhaps surprisingly, the basic technology hasn’t changed much since we last wrote about it in 2009, when then-President Barack Obama was taking heat from environmental groups for supporting clean coal. What has changed is that there are now two commercial coal CCS plants in operation. One is the Petra Nova plant in Houston, Texas, which came online in 2017, and the other is Canada’s Boundary Dam, which debuted in 2014. Both grab the CO 2 after combustion using chemicals, although their setups are different.

One of the differences is in how the plants get the extra energy to run their capture systems, which Rubin said can take 20 to 25 percent of an efficient coal-fired power plant’s energy to run. Boundary Dam uses internal power, reducing its output. Petra Nova, in contrast, uses a separate natural gas plant — without carbon capture. As a result, Petra Nova can produce more electricity, but its overall carbon footprint is higher than one might think. Even though Petra Nova captures 90 percent of its CO 2 emissions, Rubin estimates that the net reduction is only about 70 percent. That’s still substantial, but it’s important to keep in mind when evaluating how clean even this type of advanced technology actually is. “An efficient natural gas plant would have roughly half the emissions of a modern coal plant, without doing anything,” explains Rubin. Along with reducing the plant’s green credentials, the extra energy required to do carbon capture drives up the cost of electricity, which is arguably the bigger hurdle keeping coal CCS from going mainstream. One workaround, which is what the Petra Nova and Boundary Dam plants are doing, is to profit off of their captured CO 2 through something called enhanced oil recovery. It turns out that CO 2 is useful to oil companies because injecting the gas into the ground can help squeeze out more oil. Of course, not everyone is on board with using captured CO 2 to find more oil, which as a fellow fossil fuel would be contributing to our CO 2 problem. But Rubin said it depends on one’s assumptions as to whether on net, there’s a decrease in CO 2 emissions or not. “It’s something that needs to be considered,” he said, “but there’s not a clear answer.” The Coal CCS Outlook With just two coal plants in the world that make significant reductions in all air pollutants, anything approaching clean coal is happening on an extremely small scale. In fact, the carbon capture systems at both plants are only running on a portion of the total coal that is burned. At Boundary Dam, the total capacity of all the units is 672 megawatts, but the single carbon capture unit is capable of about 120 megawatts. It is often down for repairs or running below capacity. At Petra Nova, carbon capture is applied to 240 megawatts of the 654 megawatt unit. That makes Petra Nova the largest coal CCS project in the world — an impressive feat. But given that the country’s total coal-fired power plant capacity tops 250,000 megawatts, it still means that less than 0.1 percent of American capacity uses the technique. Will we see more? “With the price of natural gas being as low as it currently is, there is relatively little interest in building new coal plants of any sort,” said Rubin. What’s needed, he said, are regulations and policies that make it economically worthwhile for more companies to build these sorts of power plants, such as a sufficiently high price on CO 2 emissions, or a standard that requires emission reductions. “We have neither of these today,” he said, “nor any prospects of such incentives from the current administration.” In February, Trump signed into law a set of expanded tax credits that could encourage various carbon capture projects. The measure provides tax credits for every ton of CO 2 that’s either stored or reused. But while Friedmann thinks the credits might allow for a few existing coal plants to do CCS retrofits, even those projects would likely need extra support. In the end, Friedmann said, the focus shouldn’t really be on coal or any other fuel, it should be on the power plant. “You shouldn’t care if it’s using coal, natural gas, or tires,” he said. “You should care about what is emitted. That’s the only thing that the Earth cares about.” Clarification, Nov. 9: We clarified that only two American power plants use gasification.