PAC Silo – Installed in 2010 as a part of Rawhide's Activated Carbon Injection System for minimizing mercury emissions
Rawhide utilizes 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 are 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 utilizes 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.