Taking the Carbon Out of Coal

Taking the Carbon Out of Coal

By Robert VerBruggen - October 27, 2014

The first in a series of articles this week on energy innovation and the American economy

When coal burns, it emits a "flue gas" teeming with hard-to-separate pollutants -- but it doesn't have to. Instead, it can create a simple flow of carbon dioxide that's easy to capture.

That's the bold idea behind coal-direct chemical looping, or CDCL. The technology has been proven only in the lab, and it has many hurdles to clear before it can be used to generate electricity and capture carbon on a commercial scale. But it holds the promise of electricity from coal with very little pollution, in a nation where coal provides 18 percent of all energy and 24 percent of all carbon emissions.

"It creates the same amount of power as a traditional combustion process, but instead of a nitrogen-laden flue gas, we get a stream of pure carbon dioxide," said Andrew Tong, a postdoctoral research associate in chemical and bio-molecular engineering at Ohio State University. Tong is part of a team that created a small CDCL unit in its lab. The generator has run for more than 600 hours to date, including a 200-hour continuous run.

CDCL works by looping special metal particles back and forth between two reactors. In the first step, the particles carry oxygen into a "reducer" reactor, where the oxygen reacts with coal to produce CO2. In the second step, the particles are sent to a "combustion" reactor, where air is added, re-oxidizing the particles. Both reactors create heat, which is used to generate electricity.

According to an economic analysis Ohio State helped to conduct, if brought to a commercial scale, CDCL could capture 96.5 percent of the carbon released during the process while increasing the cost of electricity by 28.8 percent. This falls within the Department of Energy's guidelines for carbon-capture development projects, which require at least 90 percent capture for at most a 35 percent cost increase.

A major of advantage of CDCL is that it doesn't consume a lot of energy itself, said Jeff Phillips, a senior program manager at the Electric Power Research Institute, a nonprofit group funded mainly by electric utilities. For comparison, Phillips outlined three broad types of carbon capture – pre-combustion, where the coal is stripped of its carbon before being burned; post-combustion, where the coal is burned as usual and the carbon is removed from the flue gas; and oxy-combustion, where the coal is burned in pure oxygen to make the carbon easier to remove. CDCL is a type of oxy-combustion, but other types require a machine that separates oxygen from the rest of the air instead of using metal particles.

"When we look at those three generic types, ignoring chemical looping, all take about 30 percent of the power plant's energy to capture and compress carbon dioxide. That's a big hit," Phillips said. The economic analysis mentioned above compared the "energy penalty" of CDCL with that of a post-combustion-style plant: 8.8 percent vs. 27.6 percent.

The technology also surpasses what's currently available in terms of removing carbon. For example, a pre-combustion facility being constructed in Kemper, Miss., will capture just two-thirds of the carbon emitted; this gives coal roughly the carbon footprint of natural gas, which, thanks to the hydraulic fracturing boom, is increasingly used for electricity generation.

But there's an enormous difference between a lab facility and a commercial plant. Through a multi-phase contract with the Department of Energy, Babcock & Wilcox is working with the Ohio State team to bridge the gap. The contract could culminate in a small pilot plant; assuming the pilot succeeds, after that must come a demonstration-scale facility before a full-size commercial plant will be feasible. 

In a best-case scenario, the demonstration could be built later this decade, followed by a commercial plant in the 2020s, said Kip Alexander, the company's vice president of technology. "A demonstration is a heavy lift -- not tens of millions of dollars but sometimes a few hundreds of millions," he said. "A lot goes into getting something like that turned loose."

CDCL is just one part of the bigger picture of carbon capture. The Department of Energy is working to develop a variety of approaches – pre-combustion, post-combustion, and oxy-combustion -- said Briggs White, a project manager in the Advanced Energy Systems Division of the department's National Energy Technology Laboratory. These range from another chemical-looping process from the company Alstom, to post-combustion options that have the advantage of being easily added to existing coal plants.

The DOE is also working on the question of what should be done with the CO2 that's captured. Oil companies will buy the chemical -- they use it to release more oil from wells and then leave it underground -- but this market can't absorb all the carbon that's emitted nationwide.

"The majority of the projects we have today sell the CO2 to be used in advanced oil recovery, which really helps with providing an additional revenue stream," White said. "There are some other projects looking at injecting quantities of it into the ground -- injecting it into different geological formations and trying to learn from that. Those are the main approaches, but there are other efforts as well looking at how to make things out of CO2 instead of storing it in the ground."

The future of electricity is full of open questions: To what extent, and for how long, will natural gas replace coal at power plants? To what extent will government regulations and subsidies promote the development of advanced clean-coal technologies like CDCL, as opposed to alternatives to coal or incremental improvements to existing plants? Currently, for example, the Environmental Protection Agency is developing rules for power plants -- any new coal plants will be required to capture about 40 percent of their emissions, and existing plants will need to gradually reduce their emissions as well.

“There’s a big enough carrot in terms of energy savings that I think the technology will continue to receive funding. It certainly deserves to,” said the Electric Power Research Institute’s Phillips. “But scaling it up is going to take quite some time.”

Robert VerBruggen

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