Rawhide utilizes low-sulfur Powder River Basin subbituminous coal. During coal combustion the trace sulfur compounds in the coal are converted to SO2, a gaseous air pollutant that must be removed from the flue gas before being discharged from the plant stack. The Rawhide Operating Permit issued by the State of Colorado limits the SO2 emission rate and requires that at least 80 percent of the SO2 is removed.
The SO2 emissions are controlled using a spray dryer absorber (SDA) flue gas desulfurization system. During the SDA process, hot flue gas with entrained coal flyash passes through large reaction chambers where a mixture of recycled flyash/SDA product slurry, and lime-water slurry (calcium hydroxide) is sprayed and atomized. The calcium hydroxide in the atomized slurry reacts with the SO2 in the flue gas to produce calcium sulfate and calcium sulfite. The hot fluegas evaporates the slurry and cools to produce a dry particulate material composed of calcium sulfate, calcium sulfite and unreacted calcium oxide (lime). This dry SDA product and flyash is then captured in the particulate emissions control system baghouse. A significant portion of the collected flyash and SDA product is conveyed back to the SDA and used in slurry preparation in order to enhance performance. The remaining baghouse collected flyash and SDA product is either recycled offsite for beneficial uses (i.e. block manufacturing) or disposed of in the onsite monofill. .
Rawhide is consistently among the lowest SO2-emitting coal-fired plants in the U.S. according to data available from the U.S. Environmental Protection Agency.