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The electrochemical reactions that occur inside fuel cells to generate electricity could provide a cheap way to selectively remove carbon dioxide from the exhaust gases of fossil-fuel power plants. The same reactions could concentrate the carbon dioxide, allowing it to be stored underground. The fuel cell could also be used to generate electricity, providing revenue to offset its cost.

Existing approaches to capturing carbon dioxide would nearly double the cost of electricity from a coal-fired power plant. And although using fuel cells instead would still increase the cost of electricity, that increase—based on early tests and calculations—might be one-third or less, says Shailesh Vora, a program manager at the U.S. Department of Energy’s National Energy Technology Laboratory, which is helping to fund development of the technology with a $2.4 million grant. Researchers have considered using fuel cells for capturing carbon dioxide since at least the early 1990s, but the cells are cheaper now and they last longer, which could make them more practical.

Carbon capture technology, aimed at reducing emissions from power plants, might be key to addressing climate change, especially since fossil-fuel power is growing faster than power from low-carbon sources such as wind, solar, and nuclear (see “The Carbon Capture Conundrum,” “Will Carbon Capture Be Ready on Time?” and “The Enduring Technology of Coal”). Technology already exists that could capture carbon dioxide from exhaust gases, but it’s not being used at a large scale because it’s expensive, and because it uses steam that would otherwise be used to generate electricity, cutting a plant’s power production and revenue by about a third. 

Vora says one advantage of the fuel cell approach is that while other carbon-capture technologies decrease the amount of electricity—and revenue—that a power plant generates, the use of fuel cells actually increases power production.

Not all fuel cells can be used to capture carbon dioxide. The process only works with molten carbonate fuel cells, a type made by the Danbury, Connecticut-based company FuelCell Energy, which sells them now to businesses and utilities for certain niche applications (see “Utility Dominion Buys Big into Fuel Cells”).

Molten carbonate fuel cells actually rely on carbon dioxide to operate. They take it in at one electrode. That carbon dioxide is then used to form ions that conduct current to the opposite electrode, where the carbon dioxide is emitted. Finally, it is pumped back to the first electrode to be reused, forming a complete loop.

To capture carbon dioxide, this loop would be interrupted. Instead of recycling carbon dioxide, the fuel cell would get the carbon dioxide it needs from the exhaust in a power plant. These exhaust gases contain about 5 to 15 percent carbon dioxide, diluted by other gases, mostly nitrogen. The fuel cell would selectively take up the carbon dioxide, use it to form ions, and then emit it in a much more concentrated stream at the opposite electrode. The gases emitted there would be about 70 percent carbon dioxide. Most of the rest is water vapor, which is easy to condense out, leaving an almost pure stream of carbon dioxide that can be pressurized and pumped underground for storage.

So far, the technology has been demonstrated at a small scale in a lab. The DOE funding will be used to build larger systems for capturing carbon dioxide. Several questions remain about how well it will work and how much it will cost. For one thing, exhaust gases from power plants are contaminated with sulfur and other pollutants that could interfere with the operation of a fuel cell. The costs will depend on how much the gases need to be cleaned up before they’re introduced into a fuel cell. The technology might work better at natural gas plants, which have cleaner exhaust than coal plants.

FuelCell Energy makes fuel cells that each generate only a few megawatts. It may need to make much bigger ones—capable of hundreds of megawatts—to capture large amounts of carbon dioxide at a power plant.

FuelCell Energy calculates that the process could capture carbon dioxide at between $20 and $30 per ton. At those rates, FuelCell might be able to sell it for use in greenhouses or for enhancing production at oil wells—it can be used to force out oil that otherwise would cling to pores inside an oil reservoir.

Fuel cells could, in fact, help address one key challenge to using carbon dioxide to enhance oil recovery. Usually, doing this requires pipelines to deliver the carbon dioxide to remote wells. But fuel cells could be used at the site of an oil well to generate electricity from gases that are produced along with the oil. That would produce carbon dioxide that could then be captured and piped underground to help free oil. Once the reservoir is depleted, the well could be capped, trapping carbon dioxide underground.

However, the net carbon dioxide emissions involved in using carbon dioxide to increase oil production aren’t clear—some of the advantage of trapping carbon dioxide underground would be offset by increased carbon dioxide emissions from the oil. 

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Tagged: Energy, carbon dioxide, DOE, carbon capture, fuel cell, fuel cells, CCS

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