New ways to scrub out the carbon

Taylorville Energy Center CCS converts coal to methane with carbon capture, combined-cycle efficiency, and CO2 sequestration; flexible operation sells SNG to pipelines or generates power, alongside IGCC, oxy-fuel, and flue gas scrubbing alternatives.

Three technologies lead the pack for capturing the carbon dioxide in coal while also harnessing the energy. The Energy Department endorsed the “oxyburn” strategy, which involves filtering the nitrogen out of air and burning coal in pure oxygen, with a resulting flue gas that is almost pure carbon dioxide it will attempt that in Illinois.

The other two technologies involve scrubbing the carbon dioxide out of flue gases in coal that is conventionally burned, something that American Electric Power is trying out at a decades-old plant in West Virginia, and cooking the coal into a hydrocarbon gas (coal-to-gas project) and taking the carbon dioxide out before combustion, which Duke Energy is planning to do in Edwardsport, Ind.

But there are some others. Not far from the Illinois plant that the Energy Department said it planned to retrofit for oxyburning, a form of clean coal technology, Tenaska, a company that builds power plants around the country, has purchased land and is designing an “integrated” plant with a different concept.

The front end of the plant will take coal, a mixture of hydrogen and carbon, and turn it into methane through coal gasification processes. Then, depending on market conditions, it will either burn the methane in a combined-cycle power plant, making electricity at very high efficiency, or push the methane into a nearby natural gas pipeline and sell it as the final product. If the part of the plant that makes methane from coal runs into mechanical problems, Tenaska will buy methane from the pipeline to keep making electricity.

Because the price of electricity varies sharply over the course of a day, and the price of natural gas runs all over the map but is generally higher in winter, the plant could end up changing modes daily and certainly seasonally.

The conversion may make sense economically, because per unit of heating value, natural gas is more expensive than coal. But the idea is not inherently good for limiting releases of carbon dioxide, though proponents argue coal can be green in some contexts. Coal is a mixture of carbon and hydrogen, with the emphasis on the former; methane, the main component of natural gas, consists of one carbon atom and four hydrogen atoms. But in the conversion, the extra carbon has to go somewhere.

A plant in North Dakota that was conceived as part of a synthetic fuels project in the 1970s takes the excess carbon, in the form of carbon dioxide, and pipes it through to an oil field in Saskatchewan, where it is used to force oil to the surface. Again, the amount by which carbon dioxide emissions would be reduced is uncertain for the energy future debate because the oil that is recovered will release carbon when it is burned. The oil sells for something in the range of $80 a barrel.

The Tenaska plant, called the Taylorville Energy Center, will not release its carbon dioxide but will inject it into the earth as part of carbon capture efforts, Tenaska says. It is proceeding under special financing incentives approved by the state Legislature.

Taylorville also recently received a $419 million federal tax credit from the Energy Department and the United States Treasury during a period when stimulus money put clean coal projects on a faster track, that it describes as the largest of its kind ever given.

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