President Trump's pledge to keep coal alive in a cleaner form hinges on his administration helping deploy a big rollout of long-elusive and expensive technology that captures carbon emissions from power plants and stores it underground, according to energy experts.

The Trump administration used the recent United Nations' international climate change conference in Bonn, Germany, to promote the “clean” use of coal, irking other nations that planned to build off the Paris international climate change agreement intended to facilitate a switch to other forms of lower-emissions energy sources.

But the White House may be on to something, energy experts say, if it means what it says.

That’s because they expect fossil fuels will continue to play a role in the energy mix.

Technologies such as carbon capture and storage, known as CCS, will be key to fulfilling the goal of the Paris Agreement to limit global warming to “well below” 2 degrees Celsius, or 3.6 degrees Fahrenheit, the temperature at which many scientists say the world would see irreversible effects of climate change.

“We are going to need this technology to effectively address domestic and global emissions,” said Paul Bledsoe, a former Clinton White House adviser who has studied carbon capture and storage for 20 years. “The problem is no administration has been truly serious about this. Trump’s emphasis on coal production and employment is going nowhere unless CCS commercializes. That's his only hope of actually increasing coal deployment in the U.S.”

CCS technology removes carbon dioxide from a power plant's exhaust, so as to not release it into the atmosphere. The carbon can be cooled and injected as a liquid underground. Some technologies can use the captured carbon for other energy uses.

For example, the Petra Nova plant outside Houston, America’s only successfully running carbon capture facility, sends the carbon dioxide via pipeline to nearby oil fields, where it is used to assist in the extraction of crude oil. The sale of carbon dioxide as a commodity helps to offset the added cost of the CCS technology.

The UN's scientific body determined in 2014 that if CCS isn't widely deployed, keeping global temperatures below the 2-degree Celsius threshold over the next century would be 138 percent more expensive. A U.N. Environment Program report released this month found that to stay below 2 degrees Celsius, the world would have to close nearly every coal plant or outfit them to include carbon capture technology.

Yet, the concept has not been widely deployed, mostly because of the high cost to retrofit a coal plant for the technology.

Seventeen carbon capture projects exist around the world, according to a report released this month by the Global CCS Institute. Just two are capturing carbon from coal, which is responsible for more than 40 percent of global carbon dioxide emissions. Carbon capture also can be used on facilities from refineries to iron and steel plants.

“We are doing pretty well on renewables and fuel switching. We are not doing well enough on the other things,” said Julio Friedmann, the CEO of Carbon Wrangler and a former Obama administration official who was the principal deputy assistant of the Energy Department’s Office of Fossil Energy. “That's where we need to have inventive policies that take different approaches to tackling carbon where it is.”

American Electric Power, one of the nation’s largest investor-owned electric utilities, has seen firsthand the promises, and challenges, of carbon capture and storage.

In 2009, the Energy Department chose AEP to receive funding of up to $334 million to pay for part of the costs for installing a commercial-scale CCS system at the utility’s Mountaineer coal plant in New Haven, W.Va.

AEP, working with Battelle Co. and Alstom, operated a pilot version of the project that captured more than 50,000 metric tons of carbon dioxide, and stored just less than 38,000 metric tons of the carbon underground, becoming the first fully integrated CCS facility in the world.

But in 2011, AEP announced it would not complete the project as planned, because the cost of implementing the technology overshot expectations, approaching $1 billion.

“We didn't have an appreciation for the cost required for such a system to be scaled up,” said Matt Usher, AEP’s director of new technology development and policy support who was the lead engineer on the Mountaineer project. “It's a very different animal than when you treat other pollutants. When you scale these systems, the cost just gets enormous."

AEP has invested about $9 billion in efficiency improvements to its coal plant fleet and has reduced its carbon emissions by 44 percent since 2000.

“We are very proud of the fact we have taken a energy source that has been sort of given a black eye and are making these units much much cleaner and more efficient,” Usher said. “We still value coal units and think there is significant life in our coal units.”

A number of countries are looking to build new coal plants equipped with more efficient designs.

George David Banks, Trump’s special assistant on international energy and environment, said during the Bonn conference that the administration is considering forming a "clean coal alliance.”

Banks said the alliance, which could include Japan, Australia, India, and some African countries, would focus on technology known as "high-efficiency, low-emission," which could provide 30 percent fewer carbon emissions than older plants, according to the World Coal Association.

Some experts say technologies that focus only on reducing emissions inside a coal plant, without CCS, do not help combat climate change.

"High-efficient, low-emission coal plants in my book do not qualify as clean coal, period,” Friedmann said. “Unless you have extremely low carbon emissions as well as extremely low criteria pollutant emissions, it's immaterial. The world should not be thinking about building new coal plants."

Angelos Kokkinos, the Energy Department’s director of advanced fossil technology, said the Trump administration is focusing its research and development both on improving efficiency in coal plants and carbon capture technologies.

He contends that investments in new high-efficiency, low-emission coal plants are worth making, even if they can’t match the impact of more expensive CCS solutions that promise to capture up to 90 percent of carbon emitted from a plant.

"Based on the materials and technology we are looking at, the coal plant of the future will have CO2 emissions 20 to 25 percent lower than what the average emissions of the fleet is right now,” Kokkinos said. "Making the technology available to do that is a win-win situation. From a cost and emissions point of view, it's awfully good.”

Yet, he also said the Energy Department is researching ways to reduce the cost of carbon capture.

In October, the department announced up to $26 million in grants for “novel and enabling” carbon capture technologies. By 2025, the department hopes to help develop technology that would cost half of what it takes to operate Petra Nova.

"That is what we are working on — lowering the cost [for carbon capture],” Kokkinos said. “Not just the cost of building it, not just the cost of the chemicals you use to capture, but also the energy cost to capture the emissions."

Bledsoe and Friedmann say the Trump administration could do more to support bipartisan legislative proposals introduced in Congress to help fund carbon capture deployment.

For example, the Future Act, introduced in the Senate Finance Committee with 24 co-sponsors, would expand a limited tax credit for carbon capture. Sponsors include Sens. Shelley Moore Capito, R-W.Va., Heidi Heitkamp, D-N.D., and Rob Portman, R-Ohio.

Another bill by Sens. Chris Coons, D-Del., and Jerry Moran, R-Kan., with companion legislation in the House, would provide low-cost capital for CCS projects.

But critics say the Trump administration is undermining those goals by its policy decisions and rhetoric.

The administration has proposed slashing the Energy Department's research and development budget, including for the fossil energy office, although Congress is moving toward restoring much of those cuts.

Trump has downplayed the risk of climate change, and agencies such as the Environmental Protection Agency and Interior Department have acted to delay or eliminate a number of regulations intended to reduce emissions of carbon and other pollutants.

“There is an opportunity for the Trump administration to play a big role in the climate debate, but I have trouble believing that an administration that doesn’t take climate science seriously will be a leader in carbon technology,” said Erin Burns, a senior adviser for the Clean Energy Program at Third Way, a centrist Washington think tank.

Trump's plan to leave the Paris Agreement, meanwhile, would limit the administration’s leverage to influence other countries to adopt more efficient coal technology, Bledsoe says.

“China is still financing coal plants globally that aren't super efficient,” Bledsoe said. “The Trump administration should criticize them for that, but it will only have grounds to do so if they push for CCS and play a more active role internationally. We should be the innovators who would be exporting it to the world.”