POWER April 2013 - 57

MERCURY CONTROL
interference similar to SO2
), would also affect
mercury speciation, though typically to
a lesser degree.
The chlorine and sulfur contents of bituminous,
subbituminous, and lignite coals
found in the continental U.S. are generally
well documented. Representative distributions
are illustrated within four approximate
composition " quadrants " corresponding to
combinations of high or low concentrations
of both sulfur and chlorine in Figures 1, 2,
and 3. As noted, the high-chlorine/low-sulfur
quadrant is typically the most beneficial
for mercury speciation and adsorption with
activated carbon. The low-chlorine/highsulfur
quadrant is the most problematic,
and the remaining quadrants have the corresponding
mixture of speciation and adsorption
characteristics.
Although extreme variations in coal composition
exist, most bituminous coals produce
exhaust gas with fairly well-oxidized mercury
compositions and high levels of UBC. Lowersulfur
Appalachian bituminous coals generally
have higher mercury oxidizing potential,
and interior province bituminous coals have a
2. Distribution of common U.S. subbituminous coals, with low to medium
UBC. Source: Hatch Consultants Inc.
10,000
High chlorine/low sulfur
" Good " mercury speciation
High chlorine/high sulfur
Reduced ACI performance
somewhat reduced potential. Subbituminous
coals, including Powder River Basin (PRB)
coal, have lower mercury oxidizing potential
owing to lower chlorine content and less
UBC in the fly ash (though sulfur levels are
also typically much lower, which allows for
downstream gas-conditioning options). Lignite
coals normally have the lowest oxidizing
potential of all coals, with very low levels
of chlorine and production of low-carbon fly
ash. This family of coals normally requires
more extensive mercury upgrades to achieve
reasonable collection efficiencies.
MATS compliance will require the average
plant to attain an outlet mercury concentration
of no more than 1.2 lb/TBtu (4
lb/TBtu for lignite), which corresponds to a
collection efficiency of approximately 90%
(80% for lignite). However, it is possible for
certain plants with access to very low mercury
content coals to achieve compliance
with existing equipment at much lower collection
efficiencies.
For example, facilities that do not have a
1,000
100
PRB region
subbituminous
Low chlorine/low sulfur
" Poor " mercury speciation
1
Low chlorine/high sulfur
" Poor " mercury speciation
Reduced ACI performance
2
Sulfur content of coal (% wt)
3. Distribution of common U.S. lignite coals, with low UBC. Source: Hatch
Consultants Inc.
10,000
High chlorine/low sulfur
" Good " mercury speciation
High chlorine/high sulfur
Reduced ACI performance
3
4
scrubber may be able to meet standards simply
on a mass balance basis by switching to
low-mercury coal (though the coal will also
likely need to be low chlorine and, to comply
with likely future CSAPR regulations, low
sulfur). Another short-term strategy could involve
facilities with existing high-efficiency
SO2
gas-scrubbing switching to higher-chlorine
coals to enhance mercury collection,
without exceeding acid gas HAP regulations.
Consequently, the demand for these unique
coal types is likely to increase for a period
of time before more stringent " second-generation "
mercury regulations or rule changes
mandate capital improvements.
Understanding the impact of coal type
on mercury removal is an important consideration
in selecting mercury control equipment,
but a facility's fate is not entirely
bound to its fuel.
1,000
Western/Gulf
lignite
100
Low chlorine/low sulfur
" Poor " mercury speciation
April 2013 | POWER
1
2
Sulfur content of coal (% wt)
www.powermag.com
Low chlorine/high sulfur
" Poor " mercury speciation
Reduced ACI performance
3
4
Options for Enhancing Hg2+ and
Hgp Speciation
Post-combustion mercury speciation can be
significantly altered by several means. The
required extent of conversion to oxidized
or particulate-bound mercury is dictated by
the desired degree of mercury control, the
installed gas treatment equipment, and the
system upgrades (if any) being considered.
The intent is to allow the downstream gas
treatment equipment to best capitalize on the
mercury speciation profile and maximize the
degree of capture.
Options for enhancing mercury collection
efficiency have been documented
extensively in the literature. A summary
of the more common options, categorized
for particulate-bound and oxidation-based
57
Chlorine content of coal (ppm)
Chlorine content of coal (ppm)
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POWER April 2013

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