One of the most efficient ways to produce power at future coal-gasification power plants is with solid-oxide fuel cells, which use the hydrogen from the gas stream to generate electricity through chemical reactions. This is more efficient than simply combusting the gas stream from coal gasification. And unlike other types of fuel cells, the solid-oxide variety can operate at very high temperatures and efficiencies, and be scaled up to provide cities with power.
But among the various challenges to developing the technology, manufacturing cost has been a potential deal breaker. Now, researchers at GE have demonstrated a manufacturing method that assembles layers of ceramic and electrolyte materials cheaply so that the final product can be built for about $800 a kilowatt, which starts to approach the $500-to-$550-per-kilowatt cost of building a conventional gas-fired power plant.
GE’s six-kilowatt prototype achieves 49 percent efficiency in converting fuel into electricity, which compares favorably with the 35 percent efficiency of conventional coal-burning power plants. “I do believe GE has established a new state of the art,” says Wayne Surdoval, technology manager for fuel cells at the National Energy Technology Laboratory, part of the U.S. Department of Energy, which is funding this project and others aimed at producing better solid-oxide fuel cells. “The bottom line,” he adds, is that the GE prototype “is a particularly inexpensive fuel cell to make. Basically, you are using simple manufacturing techniques using fairly inexpensive materials in the cell.”
Surdoval likens the process to making pizza dough. Three sets of materials–representing the two electrodes and one electrolyte that make up each layer of a fuel cell–are mixed and put through two rollers that squeeze them. “You have three different doughs, you flatten each one, then layer them, then flatten them,” he explains. “Then basically, you bake it.”
The process paves the way for mass manufacture, according to Kelley Fletcher, the advanced-technology leader for sustainable-energy programs at GE Global Research, in Niskayuna, NY. “People have made fuel cells that make more power, and people have also made ones that have done this efficiency level,” he says. “But to do so in one package, and at the cost estimate that we have done, is the real achievement here.” Previous prototypes have cost thousands of dollars per kilowatt to manufacture, he says.
Of course, there are other roadblocks. Sulfur in the hydrocarbon fuel can contaminate the fuel cell and degrade it. GE and others are working on various pre-treatment processes to keep the contaminant out of the fuel cell. For example, researchers at Tufts University have developed a way to use cerium and lanthanum oxides to remove sulfur.
Maria Flytzani-Stephanopoulos, a chemical engineer at Tufts who led development of the scrubber, cautions that while the GE work is impressive, large challenges lie ahead. “I think that the reported numbers represent a significant development,” she says. “Of course, scale-up systems must be shown to be equally efficient in future work.”
The work is part of a Department of Energy clean-coal initiative launched in 2003 that aims to build, within ten years, a highly efficient, multi-megawatt, solid-oxide fuel-cell power plant paired with coal-gasification technology. The United States is thought to have about 250 years’ worth of coal in the ground. But burning coal looms as a major factor in increasing global warming; indeed, coal releases more carbon dioxide for each unit of energy produced than any other fossil fuel does.
In coal-gasification plants, the coal is heated and turned into a “syngas,” a mixture of mainly carbon monoxide and hydrogen. This can then be combusted in a type of power plant called Integrated Gasification Combined Cycle. The GE technology would allow hydrogen to be pulled out of the syngas and sent through a solid-oxide fuel cell.