Can Coal Be Clean?

By Robin Sussingham

With a 250-year supply of coal lying beneath U.S. soils, this cheap source of energy is an obvious choice of fuel for generating electricity. But only if ongoing research can solve major pollution issues.

On the industrial west side of Salt Lake City, behind a chain-link fence, sits a nondescript building the size of an aircraft hangar. Inside, chemical engineer Jost Wendt, of the Clean Coal Institute at the University of Utah, is climbing up a set of metal steps to a platform about 10 feet high. On top of the platform, graduate student Jingwei Zhang tinkers with a metal cylinder, about the size of a refrigerator, that looks something like a miniature 1960s-era space capsule. This experimental oxycoal combustion unit, Wendt says, will demonstrate a way of producing power from coal with the ability to trap greenhouse gases. Zhang stops working for a moment and says, “Carbon dioxide is a big issue.” Then he adds, “We should use the coal, but we should reduce the pollutant emissions and enhance the efficiency.”

With that one sentence, Zhang has summed up the focus of a worldwide research effort and billions of dollars of funding for clean-coal technology. It’s perhaps even more relevant in Zhang’s own China, where a new coal-fired power plant is being built every week. And it’s vital for those of us who greatly value our ability to turn on the lights, but hope we can do so without eating fish full of mercury, seeing acid raining down on our forests, or trying to survive a global climate change.

Coal plants may seem like a Dickensian throwback in this modern era: a dark, smoky pub in a Starbucks age. But like it or not, coal is not going away. It’s the granddaddy of energy, the workhorse of electricity—choose your own metaphor. Today, half of this country’s electricity comes from coal, and the United States is often called the Saudi Arabia of coal. We may speculate about a few decades of oil reserves remaining, but we talk about having centuries of coal buried within our borders. Experts estimate that the United States holds 500 billion tons (t) of coal, with about 55%—275 billion t—recoverable. If we continue consuming coal at today’s rate of 1.1 billion t per year, our coal deposits will last us until the year 2250 or so.

Not only is coal abundant, it is cheap. Coal costs around $1 or $2 per million Btu’s of electricity produced, compared with about $7 per million Btu’s for natural gas (as of this writing). So energy officials and utility managers know they can rely on this dependable, secure source of power, and are making plans to build dozens of new coal-fired power plants—more than 150 are proposed.

But while they provide cheap energy, U.S. coal plants are putting nearly 2 billion t of carbon dioxide (CO2 ) into the air every year. The federal government so far has not put any limits on this greenhouse gas, but environmental groups are lobbying hard, and states are acting on their own to restrict emissions. In a case now before the Supreme Court, 12 states, various cities, and several environmental groups are suing the U.S. Environmental Protection Agency to take action in curbing output of CO2. Environmentalists in Utah are urging municipalities not to buy energy from old-technology, carbon-emitting plants. The Sierra Club and Environmental Defense are trying to block the governor of Texas from expediting the construction of eight new coal plants in that state. The state of California recently passed a law blocking its utility companies from buying electricity from CO2-spewing power plants. And so on.

The utility companies themselves are watching this tug-of-war closely, with a finger in the political winds. Their fleet of coal-powered plants is aging; many are reaching the end of their lifespans and need to be replaced. But with what? What will be the most practical way to build—using untested technology that will meet theoretical future pollution regulations, or tried-and-true operations that they know can get the job done? The decisions made will affect our air and water, our food, our power supply, our economy—and possibly even our climate—in the years to come. When natural gas was cheap in the 1990s, it seemed like the clean-fuel answer, and there was a massive build out of natural-gas plants. But now, natural gas is expensive, and King Coal is back on his throne.

Can King Coal Become Mr. Clean?

Coal-fired power is now generated primarily from three commercial platforms, of which pulverized coal (PC) combustion is the oldest by far. Coal finely ground to the consistency of baby powder burns faster and hotter than chunks of coal and combusts in a large boiler, where it heats water to make steam and drive a steam turbine. This technology, developed in the 1920s, powers about 1100 units in the United States and about 5000 worldwide. This country’s fleet of PC power plants was built over a span of four decades, from the 1950s through the 1980s. By the 1970s, the industry was building huge, 1000-megawatt (MW) plants that could supply power to about 1 million households.

With PC technology, says the U.S. Department of Energy’s Tom Sarkus (ACS ’80), “you’re looking at a mature technology that the industry has grown up around and become very used to.” Sarkus has been working with new coal technology for a long time at the Energy Department’s National Energy Technology Laboratory, and he is now the director of the department’s showcase project for new, clean-coal technology, FutureGen. But he’s still protective of the stalwart PC units. “I would argue that you shouldn’t consider [PC] technology stagnant; it’s not,” he says. “A brand-new PC power plant is much different than the ones built in the 1950s,” much like the cars built now are different and more efficient than those from the 1950s.

One thing different about them is how much cleaner the coal-fired plants already are—if you ignore CO2. Today’s PC power plants are not belching out a blanket of black smoke because electrostatic precipitators are removing ash particles. Acid rain is much less of a problem than it used to be, in large part because emissions of the principal acid-rain-causing chemicals, sulfur dioxide (SO2 ) and nitrogen oxide (NOx ), are down significantly. Since the Clean Air Act of 1990 went into effect, levels of SO2 have decreased 68%, and NOx is down by almost half. The effects of these reductions are starting to be seen in streams, lakes, and forests in the eastern United States.

How did these reductions come about? As a starting point, power-plant operators switched to low-sulfur coal. They also installed SO2 scrubbers, which react the SO2 emissions with calcium, precipitating out the resulting calcium sulfate. The industry deals with NOx (a mixture of NO and NO2 ) in two ways: by using a catalyst to react the NOx with ammonia, forming nitrogen and water (called selective catalytic reduction), and by using sophisticated, low-NOx burners that prevent the compounds from forming in the first place.

SO2 scrubbers and low-NOx burners are now standard practice in the coal-power industry, and as a result, says Sarkus, emissions are down significantly despite the fact that coal use has nearly tripled since the 1970s. “These are old issues,” says Wendt. “These are problems that have been solved.” But serious obstacles to clean coal still remain. Emissions of CO2 have increased 27% since 1990, and mercury remains a major environmental hazard of coal-fired power plants.

A newer commercial process, developed in the 1970s, is known as fluidized bed combustion (FBC). This type of power plant burns finely ground coal particles held aloft by jets of air. “Think of a popcorn machine or a lottery machine with ping-pong balls,” advises Sarkus. This popcorn-popper effect improves the mixing process between the fuel and the combustion air, which in turn allows the reactor temperature to be much lower—about 870 ˚C, compared with 1300–1500 ˚C in the PC boiler. The lower temperature means that NOx doesn’t have a chance to form, making it a cleaner burning process. Another advantage is that these plants can burn any type of coal, from the highest-grade to waste-grade fuel. Their drawbacks are smaller size—the largest is about 300 MW—and lower efficiency. About 200 of these FBC plants are operating worldwide.

The New Heirs?

The newest technology, the one that’s causing all the buzz and turning plenty of heads, is called coal gasification, or most specifically, integrated gasification combined cycle (IGCC) technology. In gasification, the coal itself is not combusted, but instead is exposed to steam and controlled amounts of air or oxygen at high pressure and temperature. A synthesis gas, also known as syngas, is produced that consists mainly of carbon monoxide and water. After emerging from the gasifier, the syngas is cleaned of SO2 , NOx , mercury, and other pollutants, and then burned in a gas turbine to generate one source of electricity. For even greater efficiency, the hot exhaust from the gas turbine is recycled through a conventional steam turbine, forming a “combined cycle.” This technology, developed in the 1990s, will theoretically yield an efficiency somewhere north of 50%, much higher than that obtained with the older platforms.

Much of the excitement arising from IGCC technology comes from the fact that after the syngas is produced, it can be reacted with steam to produce hydrogen and CO2 in something called a shift reactor. The hydrogen can be burned to power a gas turbine, or in the future, captured for use in fuel cells. IGCC also provides a much easier and cheaper means of capturing NOx , should it ever become a regulated pollutant. There are six IGCC plants worldwide, including two in the United States, but no commercial plants have so far put all the pieces—combustion and CO2 capture—into an integrated system.

“We’re hoping to do this on FutureGen,” says Sarkus of a $1 billion, 275-MW prototype plant being built with Energy Department funding. “The next step will be to integrate IGCC with a shift reactor, a hydrogen turbine, and carbon sequestration. There are places where CO2 is being sequestered, but we don’t have one place where all of this is being done in a power-plant setting. That’s going to be quite interesting and challenging at the same time.”

Coal gasification is “absolutely” the wave of the future, agrees Raghubir Gupta, a senior research director at RTI International. “With natural-gas prices going so high, a lot of people are taking a serious look at coal gasification,” he says. “Utilities are very interested in these systems because they think that CO2 regulations will be here eventually, probably in the next five years, and they want to be prepared.”

CO2, says Gupta, is a very big deal indeed. “The amount of CO2 you produce [with a coal-fired plant] is huge,” he says. “One kilowatt-hour of electricity produces roughly one kilogram of CO2. So a house that typically uses about 100 kilowatt-hours per day produces about 100 kilograms of CO2, which is pretty significant.”

It’s such an important issue, says Wendt, that “it doesn’t make sense to do research in coal utilization without addressing it.” In fact, the big idea behind FutureGen is that it will be a zero-emission facility—no SO2 , no NOx , no mercury, and no CO2 just electricity and water vapor. The CO2 will be “sequestered”—an idea that in the not-so-distant past would have been laughable, says Wendt, because CO2 is the main product of combustion.

Now, huge strides are being made to do just that, thanks to research efforts such as FutureGen and another technology known as oxycoal combustion, which is the approach that Wendt is taking. With oxycoal combustion, coal is burned with oxygen rather than air, producing CO2 and water vapor, with fewer products of incomplete combustion. The water is condensed out, resulting in a stream of nearly pure CO2 that can be compressed to a semiliquid state and pumped underground into an aquifer or some other geologic formation.

Although the Energy Department allocates much of its coal research budget to IGCC, oxycoal (or oxyfuel, as it’s also called) combustion is garnering attention as well. The advantage of oxycoal technology, says Wendt, is that a traditional coal plant can be retrofitted with an oxycoal burner for the purpose of sequestering CO2. IGCC technology requires building an entirely new plant, and that’s a big risk. “IGCC has the advantage of 50% efficiency,” he says, “It can make hydrogen, and it can sequester CO2. But it is a brand-new technology. Do you want to be the first on your block to spend billions of dollars on it?”

One exciting oxycoal project, says Wendt, is taking place in Saskatchewan, Canada. SaskPower and other organizations are now studying the feasibility of building a $1.5 billion, 300-MW oxycoal power plant that would sequester CO2. It would also employ advanced clean-coal technology to capture SO2 , NOx, mercury, and particulates, resulting in a near-zero-emissions facility. The proposed plant would capture about 8000 t of CO2 per day and would pump that into depleted oil wells as a way of recovering more oil—possibly financing the additional costs of sequestration through sales of the CO2. Another possibility would be pumping the CO2 into saline aquifers. The utility company should decide on whether to go ahead with the project by mid-2007, and it would be built by 2011.

In another example of activity on the oxycoal/sequestration front, the Swedish company Vattenfall is planning a 30-MW pilot plant in Germany that it is billing as “the first pilot plant in the world to use the oxyfuel capture method.” The power plant is scheduled to begin operating in 2008, and Vattenfall hopes to use the plant to solve the remaining problems with oxycoal combustion, such as how to separate oxygen from air more economically.

The IGCC boosters are not sitting still, either. For example, the Australians are planning ZeroGen, and the Chinese have GreenGen. A company in Colorado recently announced a proposal to build a 350-MW IGCC power plant that would capture and store CO2.

“When FutureGen was announced, there were none of these,” says Sarkus. “Now, they are coming out of the woodwork, and I really can’t keep up with all of them. I say, the more the merrier, and if anything, these people coming on board with different ideas and different ways to achieve this near-zero-emissions concept for coal actually validates our original FutureGen premise. When we came out with FutureGen, there was nothing like it, and people were questioning whether we could do it. Now that there are 10 or 20 of these projects, I think the consensus is building that, yeah, we probably will do it, and we will learn a lot from doing it.”

IGCC holds another advantage over conventional PC technology—it’s easier to control its mercury emissions. According to an Energy Department study, it would cost about one-tenth as much to remove 90% of mercury from gasifier syngas compared with removing it from flue gas. The Environmental Protection Agency has mandated a two-phase, 70% reduction in mercury emissions, with a cap of 38 t beginning in 2010, and a cap set at 15 t beginning in 2018.

“Mercury is the major current issue for coal combustion worldwide,” says Wendt. “It is currently uncontrolled, except by chance. Mercury levels in fish are increasing. Pregnant women have to be careful about what they eat, and the time may come when whole classes of food will be out of reach because of mercury pollution.” But luckily—and as Wendt says, by chance—SO2 scrubbers are also effective at removing some of the mercury. They’re best at removing the oxidized form of mercury, which is found when combusting the high-chlorine bituminous coals of Pennsylvania, because the mercury oxides are soluble and can be scrubbed out along with the SO2 .

Another accepted method of mercury removal is activated carbon injection (ACI), but this method, too, has varying degrees of success, depending on the type of coal being burned. For example, high-sulfur coals such as those found in Ohio produce SO3 during combustion, which competes with mercury for bonding sites on the activated carbon. But ACI has shown great success with other types of coal, such as lower-rank subbituminous and lignite, which contain tough-to-remove elemental mercury. Wendt’s group, on the other hand, is hard at work developing a non-carbon-based sorbent that will work at higher temperatures. Worldwide, the area of mercury capture is generating a great deal of research activity.

Though the pathway to commercial success for any of these new technologies is bound to be a bumpy one, researchers agree overwhelmingly that coal can indeed be clean. The technology can and should be commercially available within the next few years, provided that natural-gas prices remain high and attention stays focused on coal. An extra boost would come from anticipated regulations governing CO2 emissions. But there will be costs attached—and how much will people pay for clean power?

Sarkus says that’s an urgent question. “This is a sizable industry, and it underpins every other aspect of our society,” he says. “And you have to keep that cost down. If you start monkeying around to the point where we double, triple, quadruple the cost of electricity in this country, we will be tinkering with our national competitiveness on a grand scale. And I believe that quite fervently.”

Wendt says that one day we’ll be able to do without coal, but not yet. “It’s a bridge to the future,” he says. “For the next 100 years, we need it. But we have to pay to make it clean.”

Robin Sussingham is a writer, independent radio producer, and podcaster. She lives in Salt Lake City, UT.