Can concept of clean coal be salvaged?

Bryn Nelson Columnist

Coal may be a four-letter word for former Vice President Al Gore and many environmentalists. But some researchers believe technology may yet salvage the concept of “clean coal” — or at least cleaner coal — as an alternative to foreign oil while the drive for longer-term alternative energy picks up steam.

Among the latest scientists to weigh in, researchers at the University of Padova in Italy have led detailed simulations concluding that a new process for transforming coal into gas could improve on the efficiency of conventional coal-to-gas, or gasification, techniques by 28 percent and release 22 percent less carbon dioxide — the main gas linked to global warming. Coupling the scheme to a coal-to-liquid process, the authors conclude, could yield 40 percent more synthetic fuel, release 32 percent less carbon dioxide and contribute to a 70 percent rise in overall energy efficiency.

The study, being published in the Nov. 19 issue of the journal Energy & Fuels, carries several big caveats, including an estimated price tag that may be hard for power companies to swallow. And many environmental organizations have dismissed the notion that coal could ever be extracted and burned in an earth-friendly way as little more than “greenwashing.”

But as part of his campaign platform, President-elect Barack Obama has pledged to enter into public-private partnerships to develop five “first-of-a-kind” commercial scale coal-fired plants with clean carbon capture and sequestration technology.

Coal-producing states such as Pennsylvania and Montana also have proposed building coal-to-liquid plants, in part to relieve dependence on foreign oil.

And with some studies suggesting the U.S. boasts an abundant (though non-renewable) supply, a host of new modifications are being proposed for cleaning up the pollution-spewing source of nearly half of all electricity produced in the country.

Economically feasible?
With its assertion that coal is “the only reasonable large-scale conventional source left in the medium term,” the new Italian study argues that “the urgent need of exploiting coal to produce both electric power and liquid fuels is a strong motivation to study how to improve the related production processes.”

Alberto Bertucco, a professor of chemical engineering at the University of Padova and a study co-author, said the big question is whether the concept will be economically feasible for a full-size plant producing syngas, a mixture of hydrogen and carbon monoxide gas.

“If the idea is good technically, probably it’s worthwhile to see if it could be applied at this scale. You get rid of the need for Saudi Arabia,” he said. “But this process must be sustained to get plants which are viable in the sense that they don’t cost too much.”

Traditional coal combustion produces steam capable of turning electricity-generating turbines. As coal’s carbon content oxidizes, however, the process also yields the potent pollutants sulfur dioxide and nitrogen oxide, as well as the greenhouse gas carbon dioxide.

A somewhat less polluting alternative, a process known as integrated gasification combined cycle (IGCC), produces syngas to power a turbine generator. Another possibility is to use that syngas mix to produce heavier hydrocarbons in the form of liquid synthetic fuels.

For the gasification reactions, burning coal typically requires pure oxygen, Bertucco said. “So there is a part of the plant that is to get pure oxygen to be fed to the gasifier. This part is expensive and energy-demanding.”

In collaboration with the global engineering and construction contractor Foster Wheeler and an independent consultant, Bertucco and post-doctoral fellow Maria Sudiro proposed burning the needed coal with cheaper air instead of oxygen.

Last month, Foster Wheeler won the right to perform a feasibility study for a coal-to-liquid (CTL) plant in China spearheaded by South Africa’s Sasol Synfuels International, owner of the only currently operational CTL plant.

The new study, akin to a how-to guide, explains how the necessary separation of the combustion and gasification processes could be safely achieved. “In the combustor, we burn roughly one-fourth of the coal, and we do this with air,” Bertucco said.

But the process needs something to transport the generated heat to the gasifier. The solution, he said, is heat-conducting sand.

“Coal plus steam makes carbon monoxide plus hydrogen, but a lot of energy is needed and this is relieved by the sand,” he said. “This is circulated continuously from one reaction to another, back and forth.”

The process requires abundant caution to avoid the risk of an explosion, though Bertucco said the danger can be overcome by properly controlling the pressure in different parts of the plant to prevent air from coming into contact with the gasifier.

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