Donald Trump has tossed around the term “clean coal” in his murky plan to boost jobs in the fossil fuel industry for months now. Most recently in a video in which the president-elect vowed to “cancel job killing restrictions on the production of shale energy and clean coal, creating many millions of high-paying jobs” during his first 100 days in the office.

What Trump meant by “clean coal” is unclear. Coal is a dirty fuel with a declining fortune. With production falling, at least half a dozen coal mining companies have filed for bankruptcy within the past two years. Technical advances in fracking have increased the production of natural gas and driven down prices, making it cheaper than coal and a more attractive source of energy for power generation. How Trump plans to beat that economic reality to promote the production of both coal and natural gas is puzzling.

While the term feels like an oxymoron, it’s used more often within the energy industry to refer to an expensive technology called carbon capture and storage (CCS) that once promised to keep coal power a dominant source of electricity for decades to come. Efforts to make clean coal technology affordable in the US have so far failed despite hundreds of millions of dollars in government and private funding. But public and private funding continue to pour in as proponents believe the technology will play an important role in many countries that have pledged to cut emissions and abide by the Paris climate agreement that went into effect last month.

Putting a price on carbon to force businesses to cut emissions will help to create a market for CCS, energy experts say. But how will Trump make it work if he is bent on relaxing regulations?

CCS refers to a family of technologies that catches as much as 90% of the carbon dioxide emissions from burning fossil fuels, preventing them from escaping into the atmosphere and contributing to global warming. The captured emissions are then either stored underground or used to boost oil production by pumping it underground to pry loose the sticky oil from the rocks. On a much smaller scale, carbon dioxide has been used to make a variety of products, including paint, plastic and fuels such as diesel.

Energy experts at organizations such as the International Energy Agency and the United Nations’ Intergovernmental Panel on Climate Change believe CCS is an essential tool in the battle against global warming because fossil fuels remain major sources of emissions. The technology can capture carbon dioxide released while making products such as steel, cement and chemicals.

A pricey technology

The cost of testing and building CCS projects remains high after more than a decade of significant investments. One of the most notable flops, called FutureGen, was a $1.7bn public-private project that began in 2003. FutureGen was killed twice, by both the Bush and Obama administrations, after it ballooned into an expensive undertaking that faced lawsuits and couldn’t line up the money necessary to complete it in Illinois. By its second demise, the government had spent about $116.5m of the $1.1bn it committed in 2010.

Yet, public funding for its research hasn’t stopped. The US Department of Energy has invested more than $4.8bn in CCS development and testing since 2009. Last week, it announced a $44m grants to 16 CCS projects from California to Georgia.

Currently, there are 38 large-scale CCS projects underway around the world, including three in the US, though pilot and demonstration efforts and other CCS initiatives number in the hundreds globally, the Global CCS Institute noted in its 2016 report.

The three ongoing industrial-size CCS projects in the US are the Illinois Industrial Carbon Capture and Storage Project; the Kemper project in Mississippi, which is something of a poster-child for delays and cost overruns; and the $1bn Petra Nova project, a 50-50 joint venture of NRG Energy and JX Nippon Oil & Gas Exploration Corp, Japan’s largest oil producer.

Petra Nova is scheduled to come online by the end of the year. It will capture carbon dioxide from a retrofitted, not newly built, coal-fired power plant southwest of Houston. NRG plans to pipe the captured carbon dioxide to an oil field 80 miles away to help its boost production. Petra Nova’s funding includes a grant of $190m from the energy department, $300m from JX Nippon, $300m from NRG and $250m in loans from a pair or Japanese banks.

While NRG is able to complete the project, it doesn’t plan to build another unless one of two things happen: oil prices rise to create demand for carbon dioxide, or carbon capture prices fall.

“The model that we put together was extremely financially viable at $100 oil,” said David Knox, a NRG spokesman. Currently, oil prices hover around $50 per barrel.

In the meantime, power plant owners and other businesses that rely on fossil fuels will likely face a continual pressure to cut their emissions and figure out how to dispose of them. As a result, NRG and Canada’s Oil Sands Innovations Alliance (COSIA) are sponsoring a $20m competition to find new use for carbon dioxide. They’ve selected 27 semi-finalist teams from six countries, including 18 teams from the US. The teams have proposed to convert carbon dioxide into products as varied as concrete, biofuels, toothpaste, fish food and fertilizer. The money will be divvied up among several winners.

The benefit of political pressure

NRG’s partnership with other firms in Petra Nova and the competition reflects the growing interest in CCS by the oil and gas sector. Ten of the world’s biggest oil companies, mostly European companies such as Total, BP and Shell, earlier this month pledged to invest $1bn to develop climate-friendly technologies, including a large chunk for CCS.

Critics called the pledge minuscule. Some environmental groups oppose CCS altogether because they believe backing the technology signals support for fossil fuels.

Private investments alone may not be enough to make CCS a good tool to fight climate change. One of the best ways to create a market for CCS, according to more than 80% of economists with climate expertise, is to enact a broad reaching carbon policy that forces businesses to pay for their emissions. But the Trump administration and Republican-led Congress won’t likely favor enacting this type of regulation.

“Carbon capture can’t compete in today’s environment where there’s no penalty for releasing CO2 emissions into the atmosphere, because it’s always cheaper to release emissions than to try and clean up them up,” says Howard J. Herzog, senior research engineer at the MIT Energy Initiative at the Massachusetts Institute of Technology.

Legislation is pending in Congress to increase CCS tax subsidies. Tax credits likely won’t sufficiently lower the cost of building a new coal plant, but it could encourage the use of CCS for existing power plants and industrial processes, says David Hawkins, director of the Climate Program at the Natural Resources Defense Council.

CCS could gain greater support outside of the US as countries move to meet the emission reduction targets of the Paris agreement, proponents of the technology believe.

China could be a big market for CCS because it depends heavily on coal for energy and has expressed an interest in using carbon capture to meet its Paris targets, says Matt Gray, senior utilities & power analyst for the nonprofit financial think tank Carbon Tracker, in a new report. The country’s political system also makes it easier for CCS to take off.

“And the Chinese government is suited to implementing a CCS strategy because it’s very top down,” he says.