Sustainable Energy

Exxon Has a Clever Way to Capture Carbon—If It Works

Sticking a fuel cell in a smokestack could dramatically reduce emissions.

Determined to solve one of the biggest challenges in reducing the carbon dioxide emissions from the production of electricity, ExxonMobil says it will partner with fuel-cell developer FuelCell Energy to pursue a novel form of carbon capture—cleansing the carbon dioxide from the exhaust from natural gas- and coal-fired power plants and storing it rather than releasing it into the atmosphere.

Carbon capture and storage, as it’s known, has long been a goal of big oil, gas, and coal producers and the utilities that generate power. Power plants that burn fossil fuels are the largest single source of carbon emissions. Research and development to date has focused on derivatives of ammonia known as amines, which can separate carbon dioxide from smokestack exhaust.

That technology, though, is costly and energy-intensive, adding to the cost of electricity and siphoning off a large amount of the power produced by the plant. Part of the problem is that the concentration of carbon dioxide in the exhaust is so low: 12 to 15 percent for a coal plant and as little as 5 percent for a modern natural gas plant. Attempts to build power plants at large scale incorporating carbon capture, including the ill-fated FutureGen project, have largely foundered (see “Peabody Energy’s Bankruptcy Shows the Limits of ‘Clean Coal’”).

Fuel cells like this one could capture carbon from fossil-fuel plant smokestacks.

Funded by the U.S. Department of Energy, research on carbon-capturing fuel cells has been under way for several years. The FuelCell Energy technology would use what’s known as a carbonate fuel cell, which uses carbon dioxide as one of its inputs, to capture the carbon dioxide and concentrate it into a form that can be transported and stored, most likely in deep underground repositories. The system would generate additional electricity, rather than consuming it, and power from the overall plant would cost less than electricity from plants using earlier, amine-based solutions.

FuelCell Energy makes fuel cells that generate power in more than 50 locations around the world. Using those fuel cells to capture carbon dioxide while also producing electricity would be a novel application. It would dramatically improve the economics of carbon capture. For a 500-megawatt coal plant, for example, capturing 90 percent of the carbon emissions would require a 400-megawatt fuel-cell system and increase the cost of the electricity from 6 cents per kilowatt-hour to 8 cents. Capturing 90 percent of the carbon dioxide from a natural gas plant of the same size would entail a 120-megawatt fuel-cell system and would add about the same amount to the cost of the electricity.

“I have been a proponent of carbonate-based electrochemical CO2 capture for years now,” says William Mustain, an associate professor of chemical engineering at the University of Connecticut who has done extensive research on carbonate fuel cells. “I believe that these types of technologies are exactly what we should be looking at.”

While promising, this technology does not offer a complete solution. Adding 2 cents per kilowatt-hour to the cost of electricity would be less than half the increase from more conventional carbon-capture technology, but it’s still a 33 percent increase—a significant hike, particularly in an era of abundant, low-cost electricity from natural gas. And adding fuel-cell systems of hundreds of megawatts would be a costly undertaking for power generators.

“That's enormous,” says Mustain. “They would have to build something unprecedented.” Initial laboratory research will involve kilowatt-scale systems, the companies said, later scaled up to a pilot plant of around 2.5 megawatts, a far cry from the size needed to work on operating power plants.

What’s more, capturing the carbon is only a first step; it must then be transported and stored underground. The back end, building the pipelines and developing the underground storage reservoirs, would cost billions of dollars. But solving the front end would be a huge step forward.

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Fuel cells like this one could capture carbon from fossil-fuel plant smokestacks.

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