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, the flue
gas passes
through large reaction chambers where an atomized lime-water mixture
(calcium hydroxide) is sprayed. The calcium hydroxide reacts
with the SO2 in the flue gas to produce a dry particulate material
composed of calcium sulfate,
calcium sulfite and unreacted calcium oxide (lime). This material
is then captured
in the particulate emissions control system.
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.
