Rawhide utilizes Powder River Basin subbituminous 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 billion) percentage of
the flue gas. The oxidized mercury compounds are generally soluble and
are captured with the existing SO2
control system. Particulate
mercury is readily captured with the existing particulate
control system.
Removing the remaining oxidized 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 become effective for Rawhide
in 2012 and are then lowered in 2018. In 2010, Platte River installed
a state-of-the-art mercury control system that utilizes powdered
activated carbon (PAC) injection (ACI) technology. A continuous mercury
monitoring
system was previously installed to meet the state's monitoring
requirements and will also be used to show compliance with the emission
limits.
The ACI system injects
the PAC into the flue gas upstream from the SO2 control system. The
PAC absorbs the mercury as the flue
gas is pulled through the SO2 system and is collected in
the particulate control system baghouses. Additional mercury is removed
while the PAC is present on the surface of the Teflon®-coated fiberglass
bag filters prior to the periodic baghouse compartment cleaning.
