Energy Forum / Alternative Energy

On a steamy, torpid summer morning in Florida, the Polk power plant is performing a small feat of modern alchemy. Every hour it converts 100 tons of the dirtiest fuel on the planet—coal—into 250 million watts of power for about 56,000 homes and businesses around Tampa. The alchemy part? Vernon Shorter, a tall, bluff consultant for the Tampa Electric Company (TECO), points to a looming smokestack. "Look at the top of that stack," he shouts over the cacophony of generators and coal-grinding machines. "That is the main emissions source. You can't see anything. You don't even see a heat plume."

He's right. No smoke mars the lazy blue Florida sky. The Polk plant captures all its fly ash, 98 percent of its sulfur—which causes acid rain—and nearly all its nitrogen oxides, the main component of the brown haze that hangs over many cities. Built to demonstrate the feasibility of a new way to wring economical power from coal without belching assorted toxins into the air, the $600 million plant has been running steadily since 1996. "It makes the lowest-cost electricity on TECO's grid," Shorter says. "It also has very, very low emissions. Particulate matter is almost undetectable."

What is both distressing and remarkable about the Polk plant is that it could do much more. "There's no requirement for mercury capture, but 95 percent of it could be captured very easily," Shorter adds. More important, the plant could also capture nearly all of coal's most elusive and potentially disastrous emissions: carbon dioxide, the main gas that drives global warming.

That capability could prove vital. With oil and natural gas prices rising rapidly and nuclear power stuck in political limbo, the world's appetite for coal is soaring. In the United States, the Department of Energy estimates that 153 new coal-fired power plants will be built by 2025. Meanwhile, China and India, the world's second and third largest coal producers, are embarking on a coal power plant building spree. China alone is expected to construct 562 new coal-fired plants over the next eight years. Since the life span of a typical coal-fired plant is 50 years, coal's share of the world's energy production will rival oil's for most of the century.

Industry advocates brag that the United States, which has 27 percent of all known coal reserves, is "the Saudi Arabia of coal," with enough to burn for the next 180 years at the current rate of use. Unfortunately, coal is as filthy as it is cheap and abundant. When burned it releases three pounds of sulfur dioxide and four pounds of nitrogen oxide for every megawatt-hour of operation. The nation's plants produce a total of about 48 tons of mercury annually. "If all the coal-burning power plants that are scheduled to be built over the next 25 years are built, the lifetime carbon dioxide emissions from those power plants will equal all the emissions from coal burning in all of human history to date," says John Holdren, a professor of environmental policy at Harvard University's Kennedy School of Government.

Holdren and many others are especially concerned about the carbon dioxide, which unlike coal's other emissions is completely unregulated in the United States. By 2012, the new coal plants in the United States, China, and India will send 2.7 billion tons of carbon dioxide into the atmosphere each year. According to leading climate models, all the added CO2 could trigger an average global temperature rise of up to 10 degrees Fahrenheit by 2100. That much warming could raise sea levels several feet, flooding the world's coastlines and shifting global weather patterns in ways that could cause massive recurring crop failures.

The smoke-free skies above the Polk plant hint at a way out. We now have the technology to capture and store most of the carbon dioxide generated by burning coal. "It's very important what we do with the next 25 years of coal plants," says Holdren. "If all those coal plants are built without carbon control, the amount of carbon dioxide added to the atmosphere would make it virtually impossible to stabilize atmospheric carbon dioxide concentrations at a moderate level." Right now the Polk power plant is one of just four of its kind in the world. If we are going to survive our coal-fueled future, we will probably need a whole lot more like it.

The technology behind the Polk plant is called an integrated gasification combined cycle—a mouthful usually shortened to IGCC. Unlike conventional coal-fired generators, IGCC plants don't actually burn the coal itself; they convert it into gas and burn the gas. This highly efficient process makes it possible to selectively pull out the resulting emissions, including carbon dioxide, which could then be collected and buried rather than released into the air.

Vernon Shorter walks through the maze of pipes and towers that is the Polk power plant, giving me a tour of how IGCC works. He points out a conveyor belt that carries a steady stream of coal from a 5,000-ton storage silo to a grinding mill, where the coal is mixed with water. The resulting mudlike slurry is then pumped under 400 pounds per square inch of pressure to the plant's most novel feature, the 300-foot-tall gasification tower.

The tower looks like an unfinished skyscraper, a boxy skeleton of steel. At its top sits a 30-foot-tall vessel filled with 96 percent pure oxygen heated to 2500ºF. When the slurry is injected into the chamber, it doesn't ignite. Instead, the coal reacts with the oxygen and immediately starts to break down into its component gases, mostly hydrogen and carbon monoxide. Those gases are cooled and pumped through a series of filters that remove sulfur, particulate matter, and other pollutants; only then is remaining synthetic gas, or syngas, burned for power.

Shorter then points out the progress of the syngas through a set of pipes descending from the gasifier to a building that houses a combustion turbine—essentially a jet engine mounted on the floor. The syngas ignites inside the turbine, spinning the turbine blades that generate about half the plant's electricity. Torrid exhaust gases from the turbine are captured and used to heat water, which is fed to a separate steam turbine to yield another 125 megawatts. This two-­turbine scheme makes an IGCC plant about 15 percent more energy efficient than a conventional coal plant.