Mercury emissions control
Rawhide uses Powder River Basin sub-bituminous coal. During coal combustion, trace mercury compounds in the coal are either vaporized as elemental mercury, chemically converted into gaseous oxidized mercury compounds or chemically bound with particulate matter. These forms of mercury represent an extremely small (parts per trillion) percentage of the flue gas. The oxidized mercury compounds are generally soluble and are captured with the existing SO2 control system as a co-benefit. Particulate mercury is readily captured with the existing particulate control system. Removing the remaining elemental mercury is more difficult and requires a separate mercury control system.
In 2008, the State of Colorado implemented its Utility Mercury Reduction Regulation specifying mercury emission rate limits that became effective for Rawhide in 2012, and were then lowered in 2018. The EPA Mercury and Air Toxics Standard (MATS) requires additional mercury reductions that became effective in 2015. In 2010, Platte River installed a state-of-the-art mercury control system that uses brominated powdered activated carbon (B-PAC) injection (ACI) technology. A continuous mercury monitoring system is installed to meet the state’s monitoring requirements and to demonstrate compliance with the emission limits.
The ACI system injects the B-PAC into the flue gas upstream from the SO2 control system. The bromide in the B-PAC reacts with elemental mercury to produce an oxidized compound which is more easily absorbed by the activated carbon particles as the flue gas is pulled through the SO2 system. After absorbing mercury, the B-PAC is collected in the particulate control system baghouses along with ash and other particulates. Additional mercury is removed while the B-PAC is present on the surface of the Teflon®-coated fiberglass bag filters prior to the periodic baghouse compartment cleaning.
Particulate emissions control
During coal combustion and sulfur dioxide (SO2) emissions control operations, ash and other particulate materials are produced by Rawhide Energy Station Unit One that must be removed from the flue gas before being discharged at the plant stack. The Rawhide Operating Permit issued by the State of Colorado limits the particulate emission rate and requires greater than 99.9% of these particulates be removed using a baghouse control system.
The system consists of two baghouses where the particulate is filtered from the flue gas using 34-foot-long, 12-inch-diameter Teflon®-coated fiberglass bag filters. Each baghouse contains 3,276 bag filters separated into 12 compartments. The compartments are periodically cleaned by reversing the air flow through the bag filters and the collected particulate is then either reused in the SO2 emissions control process, used by other industries or disposed of in Rawhide’s on site monofill.
Sulphur dioxide (SO2) emissions control
Rawhide uses 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% 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 flue gas 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 on-site 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.