Some think that the only way to clean up coal is to stop burning it altogether. Now a handful of researchers and companies are pursuing a technology to do just that, but one that has the potential of keeping part of the nation's economy coal-fired.
They are skipping the burning step. "Direct carbon" fuel cells efficiently produce electricity straight from the carbon source, which can be anything from coal to coconut shells.
The nascent, laboratory-scale technology has the potential of making electricity by using less than half the coal burned today and sharply reducing the costs of capturing carbon dioxide emissions from the fuel.
Provided major scale-up hurdles can be surmounted, the cells would offer an "attractive option" for electric utilities that need to reduce their carbon emissions, said Dan Rastler, energy storage program manager for Electric Power Research Institute, a utility-funded think tank. Fifty percent of the nation's electricity comes from coal-fired power plants, many of which are entering the final years of their expected lives.
But the scale-up and commercialization of the new technology would require a significant bump in government research funds. "Without the federal money, I don't really see this viable technology making real headway," said Turgut Gur, chief technology officer of Direct Carbon Technologies LLC, one of several companies working to push the idea out of the laboratory. It does receive some DOE funding.
"People do not deem this green because it involves coal. In my mind, that's a short-sighted view," he added.
Today, most coal-fired power plants convert the heat from coal combustion into steam. The steam turns a turbine which generates electricity. Energy is lost in each step of this process so that most power plants hover near 30 percent efficiencies.
Making electricity directly with carbon fuel
By contrast, direct carbon fuel cells (DCFCs) -- an umbrella term that encompasses a range of fuel cell technologies -- convert carbon sources to electricity in a single reaction step, just as current fuel cells do with hydrogen. Under various schemes that pulverize coal and feed it into the fuel cell in a slurry of molten salt or other superheated material, the carbon in the coal reacts in the cell to produce a moving stream of electrons, or electricity.
Researchers estimate that real-world efficiencies of DCFCs could approach up to 70 percent, the highest of any fuel cell class.
Even though fuel cells tend to be synonymous with hydrogen, DCFCs have a history dating back to the mid-19th century. By the 1980s, labs including Lawrence Livermore National Laboratory and SRI International had small research programs to delve into the chemistry of these cells.
But with coal so plentiful and cheap, there was never much motivation to seriously pursue the idea. Since the late 1990s, DOE's interest in fuel cells has focused mainly on solid-state technologies tailored to the use of hydrogen as a fuel. Carbon-powered alternatives were "simmering on the back burner," explained Rastler.
More recently, though, as the future of the coal industry has come under serious question, the DCFC concept has garnered a small and slowly growing research following.
The cells reduce carbon dioxide emissions in proportion to their higher efficiencies. What is emitted from the cells is a pure carbon dioxide stream that is easy to capture. A 2008 EPRI study estimates that with complete CO2 capture, the cells could produce electricity more cheaply than today's state-of-the-art power plant designs that use a lot of energy to gasify coal, then remove the CO2 and burn the other gases to make power.
Direct Carbon Technologies plans to make a 10-kilowatt pre-commercial prototype in the next two years, according to Gur, who founded the company based on his research as a consulting professor at Stanford University. Funded mainly by a DOE grant, the company has already tested the system on low-sulfur coals from the Powder River Basin and Alaska.
Alvin Duskin, the company's CEO, foresees eventually producing modular 100 kW and 1,000 kW units. The latter might be stacked within the footprint of an existing coal-fired power plant that needs to bump up its generation capacity without building a whole new facility, he said.
Starting with off-grid generation
"Coal plants have great economies of scale. It's just not economic to make a small one. But there are no economies of scale to make a fuel cell," said Duskin. That is actually a big advantage for this smaller niche. The cells, he said, could be used in a plant that wants to co-fire more biomass.
Another startup, Contained Energy LLC, which licensed the research done at DOE's Lawrence Livermore, is taking a different approach. It will first develop smaller-scale systems to power generators and other off-the-grid needs. Alex Perwich, the company's president, also highlights its cells' potential to back up a utility's baseload power, a function that will be increasingly necessary as more renewable energy enters the grid.
The Department of Defense is also funding Contained Energy to develop a power source for its front-line operations. Perwich noted that biomass, such as wood or farm wastes, will likely be the initial fuel for the cells, since it is a cleaner and simpler source of carbon compared with coal.
In fact, handling coal -- and all its impurities and contaminants -- will be a major challenge for the cells, experts noted. If not removed beforehand with chemical processing, these can degrade the cell and reduce its lifetime. For that reason, many early applications will likely rely on smaller, biomass-powered cells, Rastler said.
Improvements are also needed to ramp down the cells' so-called power density and increase their lifetime. To be viable, DCFCs will have to produce more electricity in a smaller space than they do now, Rastler said.
But the biggest hurdles, say the startup companies, are more prosaic. "Money and time," said Gur, who has had trouble attracting private investors for such a nascent and unproven venture.
Perwich estimated it will take his company $20 million to get to a demonstration prototype. "It then takes boatloads of money to scale something up," he noted, putting the price tag at hundreds of millions. He had hope that as DOE's solid oxide fuel cell program winds down, the agency might take more of an interest in DCFCs.
"Personally, I feel that it's an area that needs more attention. There's a role for good basic science at this point," Rastler said.