Fuel cells powered by natural gas, biogas, and hydrogen are a rapidly expanding option for distributed generation, with fuel cell–based power plants being deployed in capacities into tens of megawatts. But as the technology improves and costs begin to scale, opportunities for other applications are being explored.
One such application may even go beyond power generation. Danbury, Conn.–based FuelCell Energy (FCE), one of the largest suppliers of fuel cells worldwide, and ExxonMobil announced a partnership on May 5 to explore the possibilities for using fuel cells for carbon capture.
Fuel Cells Using Flue Gas
According to Kurt Goddard, FCE’s vice president of investor relations, the application will depart from the traditional method of powering the fuel cells with ambient air and methane.
“What we found is there is a unique side reaction and benefit of our standard typical fuel cell, and instead of ambient air, we can bring in the flue gas from a coal plant or gas plant, and we can concentrate the CO2 that is coming in.”
Normally in FCE’s carbonate fuel cells, methane is reformed in the cell anode to create CO2 and H2, which is then used to generate electricity by combining with oxygen from ambient air to create an electric current and exhaust streams of water vapor and CO2. The carbon-capture fuel cell will still be powered by methane, but by using flue gas instead of ambient air, the chemical reactions in the cell can concentrate up to 90% of the incoming CO2, which flows with the CO2 generated in the reforming process into the normal exhaust stream, where it can easily be separated from the water vapor.
A critical difference—and bonus—in this process is that rather than being a parasitic drain on the plant, it actually generates additional power (Figure 1). According to FCE, the power output is larger than the parasitic drain from conventional carbon-capture technologies.
In addition, Goddard said, “We can destroy roughly two-thirds of the NOx in the flue gas.”
How much CO2 can be captured depends on how many fuel cells are employed, as each can handle only a certain amount of flue gas. A typical 500-MW combined cycle plant would need around 120 MW of fuel cells to achieve 90% capture, Goddard said, while an equivalent coal plant might need around 400 MW of fuel cells because of its lower efficiency.
“You can add a couple over time and do it in increments. You do not have to do it all at once.”
Commercial deployment is still years away, but representatives from both companies are optimistic about the potential, which Vijay Swarup, vice president for research and development at ExxonMobil Research and Engineering called a “game changer” for carbon-capture technology.
Profitable Carbon Capture?
Efforts to deploy carbon-capture at grid scale have run into serious problems with CO2 capture performance and costs. The only operational utility-scale facility, SaskPower’s Boundary Dam Project in Canada, suffered from substantial operational problems its first year of operation, though it has recently seen more success. Southern Co.’s Kemper County project in Mississippi has been plagued with delays and huge cost overruns that most recently have drawn attention from the Securities and Exchange Commission.
Other projects, such as NRG’s retrofit project at the W.A. Parish plant near Houston and the proposed Texas Clean Energy Project near Midland, have faced criticism that their reliance on enhanced oil recovery negates climate benefits of carbon capture. No matter the approach, all current carbon-capture technologies suffer from substantially increased costs of generation and reduced net power output.
The possibilities of employing fuel cells to reduce power plant emissions for both coal plants and natural-gas combined cycle plants, and potentially capture CO2 have drawn research attention in recent years, though applications have typically involved placing the fuel cells in front of the turbines or replacing typical combustion processes altogether. The FCE-ExxonMobil initiative is among the first to look at placing the fuel cells after combustion.
ExxonMobil began working with FCE on this project several years ago, Goddard said, beginning with informal discussions that led to more formal testing. With the potential now clear, the two companies have gone public with the initiative. The partnership will focus initially on how to further increase efficiency in separation and concentration of the CO2 from gas turbine exhaust. That is expected to take one to two years.
Goddard said FCE and ExxonMobil scientists will be working to better understand the chemical processes that are taking place and how they respond to different compositions of flue gas. Assuming success, the second phase will move to a small-scale pilot project for additional testing, then integration into a larger-scale pilot facility.
—Thomas W. Overton, JD is a POWER associate editor (@thomas_overton, @POWERmagazine).