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How to Clean Coal
Page 2

On the Carbon Trail

I decided to take an exploratory journey down that pathway, in effect following coal's carbon trail from cradle to grave. That's what took me to the Freedom Mine near Beulah, North Dakota. The mine serves as the fuel source for a sprawling complex that includes two large coal-based energy plants: the Dakota Gasification Company's Great Plains Synfuels Plant, which gasifies coal to produce a form of natural gas, and the Antelope Valley Station, a 900-MW traditional coal-fired plant. Together, these two plants, and a third generating station nearby, consume the Freedom Mine's annual output of 16 million tons of a type of coal called lignite. "We call it dirt that burns," said Floyd Robb, my guide to the complex. "It's as soft and as low in energy density as coal gets. Any less than that and it's peat."

Photo of a coal scoopMost coal started out as peat -- plant debris that accumulated over many thousands of years in moist bogs. Deep beds of peat were eventually buried under sedimentary deposits, which gradually compressed the peat and subjected it to geothermal heat for a few hundred million years. In general, the longer coal bakes in its geologic oven, the harder and blacker it gets. Lignite, the youngest and brownest type of coal, occupies the bottom rank of the coal hierarchy. Next up, in terms of hardness, carbon content, and heating value, is sub-bituminous coal, found largely in the Powder River Basin of eastern Wyoming and Montana, which is now home to the largest coal mines in the country (all open-pit surface mines, the domain of mammoth draglines like the ones at the Freedom Mine). Bituminous coal, the rank above sub-bituminous, is typical of the eastern half of the United States, from Illinois to Appalachia. It has a higher sulfur content than most western coal but packs a bigger energy wallop, pound for pound. Hardest and hottest-burning of all is anthracite. So black it's iridescent, anthracite comes mainly from those shaft mines in western Pennsylvania that haven't already been exhausted and abandoned.

Unlike hard, dry anthracite, lignite has a high moisture content. "Our lignite here is about 35 percent water," said Robb, vice president of communications for the Basin Electric Power Cooperative, which owns the coal from the Freedom Mine as well as the two energy plants adjacent to it. "You can't economically ship it, because you'd be shipping so much water. That's why the power plant is right next to the mine. The only way to ship lignite economically is on wires, as electricity."

Basin Electric is shipping its lignite a second way as well: through a pipeline, as "synthetic natural gas" (an oxymoron of the energy business). The Great Plains Synfuels Plant is a product of the Arab oil embargo of the early 1970s. Not only was oil in short supply, but predictions of a natural-gas shortage made America's energy situation seem all the more precarious. Building an ambitiously large-scale facility in western North Dakota would take some of the region's cheap, abundant lignite and convert it, through a carefully orchestrated series of chemical reactions, into synthetic gas -- a process known as coal gasification. (The process is not new: It fueled, for example, the German Luftwaffe in World War II.) Plans, permits, and financing came together in the late 1970s; construction of the North Dakota plant began in 1980.

By the time it started operating four years later, however, the plant was already a white elephant. It was a technical success, capable of gasifying enough coal to produce 150 million cubic feet of synthetic gas per day, enough to keep 300,000 houses toasty through a North Dakota winter's night. But the price of natural gas (that is, "natural" natural gas, which consists mainly of methane and comes from underground deposits, much like oil) had come down to the point where the plant's synthetic product was no longer cost-competitive. The U.S. Department of Energy operated the plant at a loss for a few years, then Basin Electric bought it at auction for a bargain price.

When you gasify coal, you don't actually burn it. You heat it to about 2,000 degrees Fahrenheit in a sealed chamber. Along with adding some steam, you inject a bit of oxygen, but not enough to allow the coal to burst into flames. Instead, the coal breaks down into its chemical building blocks. Dozens of chemical reactions occur in the gasifier. The gas that emerges is made up mostly of carbon monoxide, hydrogen, sulfur, and nitrogen compounds, plus smaller amounts of elements such as mercury. Most of the gasification facility at Basin Electric -- a square mile covered with a Brobdingnagian rat's nest of pipes, minaret-like distillation towers, storage tanks, and mustard-yellow steel buildings -- is devoted to cleaning up the synthesis gas, removing impurities from the methane stream, which is the desired end product.

Many of the impurities are, in fact, valuable by-products, and Basin Electric has greatly improved the finances of the plant by finding markets for them. It sells anhydrous ammonia and ammonium sulfate as agricultural fertilizers. A steady procession of railroad cars and semitrucks hauls the stuff away. The plant sells phenol, mainly to a Canadian company that manufactures resins for wood products, such as plywood. Naphtha and liquid nitrogen leave the plant by the millions of gallons.

The gasification plant also produces carbon dioxide -- 200 million cubic feet of it per day, or just over four million tons per year. In that respect, the plant is no different from the 900-MW pulverized-coal power plant next door: Whether you burn coal outright in a boiler or break it down chemically in a gasifier, there's no getting around the CO2 problem. But there is a crucial difference between the two ways of producing it. Capturing the CO2 from a conventional power plant, while theoretically possible, is prohibitively expensive and impractical. With a gasification plant, by contrast, separating CO2 from the rest of the synthetic-gas stream is a straightforward chemistry project that requires little or no added expense. The North Dakota synfuels plant did not capture its CO2 stream, however. For its first two decades of operation, it had nowhere to put it except up a 300-foot-tall stack.

That changed in the late 1990s, when Basin Electric actually found a customer for its CO2. PanCanadian Petroleum, one of Canada's largest oil and natural-gas producers, operated an oil field near Weyburn, Saskatchewan, about 200 miles northwest of the Beulah gasification plant. Production from the Weyburn field was declining, and its owners were interested in extending the field's life using a technique known as enhanced oil recovery -- basically, pumping CO2 into the ground to push more oil out of the source rock, 4,600 feet below the surface. Enhanced oil recovery by means of CO2 had been a standard practice for more than 20 years in the aging oil fields of west Texas. But these operations used CO2 from naturally occurring reservoirs of the gas in southern Colorado -- a natural "recycling" that did not result in a net reduction of greenhouse gases escaping into the atmosphere. The Weyburn project would represent the first time in North America that man-made CO2 destined for atmospheric release would instead be pumped deep into the earth, where it might potentially be sequestered for thousands or even millions of years.

Photo of carbon dioxide pipesPanCanadian Petroleum (now called EnCana) agreed to pay handsomely for the CO2 (it's now paying $2.5 million every month for what was formerly a waste product). But Basin Electric first had to transport the gas, so it built a 205-mile pipeline from Beulah to Weyburn. Basin Electric also had to pressurize the CO2 so it would arrive at Weyburn compressed to just over 2,000 pounds per square inch, the force required to push it nearly a mile below the ground and make the oil flow. This would require some of the most powerful gas compressors of their kind ever built.

I saw these brutish compressors during my tour of the gasification plant. There were two of them, housed in a hangar-like yellow building, each powered by a 20,000-horsepower electric motor. They appeared to be the size of the jet engines on a Boeing 747, and were just about as noisy. "When you compress the CO2 that much, it gets very hot," said Daren Eliason, a chemical engineer who showed me around the plant. "We have to bring the temperature down with air-cooled units." We walked around behind the battleship-gray coolers. A white pipe the circumference of a watermelon emerged from the coolers, made a 90-degree bend, and disappeared into the brown gravel, beginning its underground trip to Weyburn.

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Photos: Mitch Epstein
Map: Small World Maps

Illustration: Jim Kopp

OnEarth. Fall 2005
Copyright 2005 by the Natural Resources Defense Council