Rawhide Energy Station lowers mercury emissions

By Kenneth Jessen
North Forty News

This summer, the Rawhide Energy Station north of Wellington put into effect a new mercury removal system using powdered activated carbon.

With the new system, station owner Platte River Power Authority will reach the Colo-rado mandated standard of 0.0174 pounds per gigawatt hour, which will take effect in 2014. There will be even lower standards for 2018. In addition, the federal government is expected to put into effect similar nationwide standards for mercury that will be bundled into multi-pollutant regulations next year.

To reduce emissions at Rawhide, carbon is injected as a dry sorbent into the flue gas, trapping the mercury. It is then separated for disposal. PRPA officials say mercury emissions were already low at 0.0289 pounds per gigawatt hour in 2009. However, large concentrations of mercury can be a health problem, and coal-fired plants disperse the byproducts of combustion over a wide area.

Rawhide Energy Station's installation investment was about $3 million, with expected yearly operational and maintenance costs of about the same amount.

PRPA officials say the community owned utility's investment in advanced technologies at the Rawhide Energy Station has produced one of the cleanest coal-fired plants in the United States. Low-sulfur coal arrives by rail from a mine located about 250 miles away in Wyoming's Powder River Basin. The coal cars are owned by Platte River and are delivered by the BNSF Railway. The coal is pulverized in four coal mills as fine as talcum powder and blown into the boiler. Injected at multiple points, the burning coal creates a suspended ball of fire.

The main byproduct of the coal combustion process at Rawhide is fly ash. This powdery nonhazardous material is composed mainly of silicon dioxide, calcium oxide (lime) and aluminum oxide. It is trapped in large bag filters as part of the pollution control process. A portion of the fly ash is mixed with water to form slurry. The calcium oxide becomes calcium hydroxide (slaked lime). This mixture is sprayed into the flue gas coming from the boiler to trap sulfur dioxide, a pollutant that becomes sulfuric acid in the presence of atmospheric moisture. The chemical reaction produces gypsum, a stable, nontoxic mineral found naturally in Larimer County, and the gypsum is placed in a mono-fill. The fill is covered with topsoil and planted with native grasses.