POWER April 2013 - 58

MERCURY CONTROL
conversion routes, is depicted schematically
in Figure 4 with a list of accompanying
advantages and risks.
As noted, wet limestone and lime flue gas
desulfurization (wet FGD) based systems
are particularly suited for capturing oxidized
mercury. For example, a facility using
higher-chorine/lower-sulfur fuels, with an
installed selective catalytic reduction (SCR)
unit to enhance oxidation and a downstream
wet FGD system would achieve excellent
mercury control (likely in excess of 95% capture),
but it would need to run the SCR year
round and closely monitor scrubber chemistry
to prevent re-release of mercury from the
scrubbing liquor. If coal chlorine levels happen
to be lower, halogen injection could be
used to restore oxidation potential.
Dry FGD systems, particularly those
equipped with baghouses, circulating dry
scrubbers, spray dryers-and, to a lesser extent,
electrostatic precipitators (ESPs)-can
be effective at capturing particulate-bound
mercury, in addition to having reasonable
oxidized mercury capture efficiencies. An
example of an effective dry system would be
an Appalachian bituminous coal-fired boiler
coupled with an activated carbon injection
(ACI) system upstream of a baghouse. If only
higher-sulfur bituminous coals are available,
the detrimental impact could be partially offset
via dry sorbent injection, which can be
easily installed in tandem with ACI equipment
(see sidebar " Three Site-Specific Sorbent
Injection Options " ).
Systems having both wet and dry gascleaning
equipment can achieve the combined
benefits of both sets of options and
generally attain higher degrees of mercury
capture. This approach may be necessary
for facilities using particularly difficult coal
types, including certain grades of lignite.
The relationships in Figure 4 illustrate
the synergy that can be achieved by coupling
two or more mercury reduction unit
operations. With this general set of relationships
established, it is now possible to
construct a road map outlining a range of
equipment combinations that satisfy mercury
reduction requirements.
Matrix of Mercury Control
Upgrade Options
The development of a comprehensive assetbased
methodology is a very large undertaking,
requiring the evaluation of a vast number
of equipment combinations, fuel grades, and
other complex process factors. A summary
matrix of the results of the asset-based approach
is shown in the table. The purpose of
this table is to highlight the range of equipment
configurations capable of achieving
mercury reduction targets and to allow users
to identify the most attractive upgrade alternatives
for their site on a qualitative basis.
Using the summary table is very straightforward:
1.
Define the coal type: The coal is defined in
Step 1 (upper three rows of the table). The
coal type is specified as bituminous, subbituminous,
or lignite, and the levels of sulfur
and chlorine in the coal are defined as
high, medium, or low (coal grades/blends
in between these broad definitions will require
some interpolation of the results).
2. Determine whether existing equipment
configurations are suitable: Some facilities
4. Many paths to compliance. Summary of the challenges and opportunities for improving
Hg collection from plant stacks. Courtesy: Hatch Consultants Inc.
may be able to achieve MATS compliance
with common gas-cleaning equipment already
installed to control other pollutants
such as particulate matter, SO2
, and NOx.
Suitable existing equipment configurations
are represented by the numbered columns
in Step 2 (middle rows of the table)
for a given coal composition. Note that
for some coal compositions, no existing
configurations are suitable and mercuryspecific
equipment upgrades are required,
as specified in Step 3.
3. Identify upgrade options, if required:
Upgrade equipment configurations are
represented by lettered columns in Step
3 (bottom rows of the table). Each configuration
can be expected to achieve, on
average, the required reduction in mercury
emissions for MATS compliance.
For example, a facility firing high-sulfur,
high-chlorine, interior province bituminous
coal would have five existing combinations
of equipment that would preclude the need
for any specific mercury control upgrades:
Three Site-Specific
Sorbent Injection Options
Three primary sorbent injection processes
are used for the reduction of mercury
in the stack gas. Selection of the
right process is a function of the coal,
existing air quality management system
(principally, the presence of a baghouse
or scrubber), and impact on ash benefaction
programs.
Activated Carbon Injection (ACI).
Powdered porous activated carbon is injected
into the flue gas in a uniformly distributed
and well-mixed manner to adsorb
mercury. The spent carbon is captured in a
particulate collection device. Treated and/
or alternative agents can also be used.
Halogen Injection (HI). Halogen gas
(for example, chlorine, bromine, and/or
alternative agents) is injected into the
flue gas to promote oxidation and the
formation of Hg2+
. The oxidized mercury
is collected in a wet scrubber or other
particulate collection device.
Dry Sorbent Injection (DSI). Sorbent
(such as hydrated lime, trona, or sodium
bicarbonate) is injected into the flue
gas in a uniformly distributed manner to
absorb scavenging agents (such as SO2
)
that compete for adsorption sites on
activated carbon and would otherwise
reduce ACI performance.
58
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POWER April 2013

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