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ENHANCING MERCURY CONTROL ON 

COAL-FIRED BOILERS USING 

SCR OR OXIDATION CATALYST AND FGD 

Mercury capture in existing emissions control equipment offers a cost effective mercury 

control option for coal-fired power plants. The co-benefit of mercury capture from coal-fired 

power plants varies significantly depending on the existing emissions control configuration 

and type of coal being burned. Different coals and control configurations produce varying 

amounts of the three basic forms of mercury in flue gas including elemental, oxidized, and 

particulate mercury. Particulate mercury will typically be captured in the particulate control 

device and oxidized mercury is water soluble and can therefore be captured in certain control 

equipment such as wet flue gas desulfurization (FGD) systems. 

In order to enhance mercury capture, the elemental form of mercury can be catalytically 

oxidized from the elemental metallic form, Hg0

, to the oxidized form of mercury, Hg+2, and 

captured by a downstream FGD system. Selective catalytic reduction (SCR) catalysts or 

oxidation catalysts may be used for the catalytic oxidation step, followed by downstream wet 

or dry FGD technology to capture the oxidized mercury. High levels of oxidized mercury 

and total mercury capture are achieved commercially when firing bituminous coal on units 

equipped with SCR and FGD due to relatively high halogen levels in the coal. Enhancement 

of mercury capture efficiency for low-halogen coals (i.e. coals with low chloride content 

such as subbituminous or lignite coals) equipped with SCR and FGD can be accomplished by 

several methods: addition of halogen-containing compounds to the coal, co-firing higherhalogen content coals, usage of advanced SCR catalysts with improved mercury oxidation 

performance, or addition of a sorbent system such as activated carbon injection. Co-firing 

involves blending in 15 to 50 percent bituminous coal when firing low-halogen 

subbituminous or lignite coal types. Additives may also be used in the FGD system with all 

coal types to prevent the reemission of captured mercury to the flue gas. 

Technology Overview 

Flue gas desulfurization is a widely used post-combustion technology for controlling SO2

emissions. FGD also captures a large percentage of oxidized mercury present in coal-fired 

combustion flue gas because the oxidized mercury is soluble in the calcium-containing liquid 

solution in the FGD. However, FGD does not effectively capture elemental mercury from 

the flue gas because elemental mercury is not soluble in the liquid solution. For bituminous 

firing, there is a medium to high level of oxidized mercury at the FGD inlet resulting in a 

comparable level of mercury capture. For sub-bituminous and lignite firing, the relative 

levels of oxidized mercury and subsequent capture are both low unless means are provided to 

enhance oxidation of elemental mercury as described above. Neither “wet” FGD nor “dry” 

FGD is effective in capturing elemental mercury. In a dry FGD unit, also known as spray 

dryer absorber (SDA), a calcium-containing liquid solution is sprayed into the flue gas to 

capture SO2 but does not saturate the flue gas like wet FGD. For dry FGD, the subsequent 
ICAC  1220 N. Fillmore Street, Suite 410, Arlington, VA, 22201 703-812-4811  www.icac.com

baghouse normally provides additional contact time thus effectively allowing the combined 

SDA/baghouse configuration to capture oxidized mercury as efficiently as wet FGD units. 

Selective catalytic reduction (SCR) is a widely-employed post-combustion method for 

controlling NOx emissions. Although SCR catalyst reduces NOx, it also has the dualfunction of oxidizing elemental mercury. This provides a significant co-benefit since SCR 

installed upstream of an FGD will result in higher overall mercury capture. An increasing 

number of plants are operating with SCR/FGD control combinations due to EPA regulations 

that require significant SO2 and NOx reductions from power plants. For bituminous coal 

firing units, 80 percent to over 90 percent overall mercury capture has been observed on a 

sustained basis in full-scale plant tests where SCR followed by wet FGD is employed. A 

large bituminous-fired plant equipped with SCR and wet FGD showed sustained high Hg 

capture performance over multiple ozone operating seasons. Firing of low chlorine 

containing subbituminous and lignite coals will require process enhancements, such as those 

outlined below, in order to achieve high mercury capture. 

Oxidation catalysts containing precious metals have been used in a commercial 

demonstration for the oxidation and improved capture of mercury by downstream FGD, and 

may provide an alternative for units not employing SCR that are equipped with FGD. These 

catalysts can be installed upstream of an FGD and perform in a similar manner to SCR 

catalyst. Oxidation catalyst might also be installed between the SCR catalyst and the FGD to 

enhance the overall performance of the SCR/FGD system for mercury oxidation and capture. 

Greater than 90 percent mercury oxidation efficiency has been achieved firing PRB 

(subbituminous) and North Dakota lignite coals in slipstream tests at power plants, and 80% 

oxidation was observed in the 200 MW PRB demonstration1

. 

Process Augmentation for Mercury Capture Enhancement 

The following options to increase the efficiency of mercury capture of the SCR/FGD or 

Oxidation Catalyst/FGD process are in various stages of development, field demonstration, 

or commercial utilization at a small number of units: 

(a) One technique is the addition of bromine, chlorine, or other halogens to the coal (precombustion), directly to the boiler (combustion), or after the boiler (post-combustion). 

The elemental mercury is oxidized by the halogens to form mercury halides to improve 

mercury capture. This approach is especially useful when firing low halogen content 

coals. 

(b) Another method is blending for sub-bituminous or lignite coal firing by the addition of 

bituminous coal with higher halogen levels. This technique can be used to improve 

overall mercury capture for FGD, SCR/FGD and oxidation catalyst/FGD configurations. 

Performance tests at a large utility plant firing a 60 percent sub-bituminous and 40 

percent bituminous blend at two identical boilers (one equipped with SCR and the other 

 

1

 “Full-scale Demonstration of Oxidation Catalyst for Enhanced Mercury Control by Wet FGD”, Mega Symposium 

Conference 2008. 
ICAC  1220 N. Fillmore Street, Suite 410, Arlington, VA, 22201 703-812-4811  www.icac.com

without SCR) showed an increase in downstream oxidized mercury fraction from 63 

percent without SCR to 97 percent with SCR. An unblended PRB sub-bituminous coal 

situation would have only achieved between 0-40 percent oxidized mercury based on 

full-scale data from the 1999 EPA Information Collection Request. 

(c) Addition of catalysts to specifically convert elemental mercury to oxidized mercury is 

another method. Options for SCR-equipped units include addition of an extra SCR 

catalyst layer or usage of SCR catalyst with enhanced mercury oxidation performance. 

Possibilities for mercury-specific oxidation catalyst include catalyst addition after (or in 

lieu of) the SCR catalyst layers in a hot flue gas location, or the addition of a mercuryspecific oxidation catalyst in a relatively cool flue gas location (a cool location was used 

for the commercial demonstration). For some units firing sub-bituminous or lignite coals, 

SCR catalyst or potential oxidation catalyst systems may require augmentation of flue 

gas halogen levels. 

(d) Usage of FGD additives to avoid re-emission of captured mercury is another technique. 

Commercial tests have shown 0-20 percent re-emission of captured mercury from wet 

FGD units, and additives have shown the ability to substantially prevent this re-emission 

effect. Additives include various liquids, solids or gases that affect the FGD slurry 

chemistry. 

Performance 

 

The overall mercury capture efficiency depends on the configuration of post-combustion air 

pollution control equipment and coal type at a particular plant. EPA had previously 

developed the following estimates of co-benefit mercury capture efficiencies for some of the 

major plant configurations involving FGD or combined SCR/FGD (those shown are for 

pulverized coal fired boilers). These incidental efficiencies which were identified under 

EPA’s earlier ICR data collection are not optimized for mercury capture and may be 

optimized further. 

Post-Combustion Controls Mercury Capture Efficiency 

PM NOx SO2 Bituminous Subbituminous Lignite 

 

ESP None Wet FGD 66% 16% 44% 

ESP None Dry FGD 36% 35% 0% 

ESP SCR Wet FGD 90% 66% 44% 

ESP SCR Dry FGD 36% 35% 0% 

None None Wet FGD 42% 30% 0% 

None None Dry FGD 40% 15% 0% 

None SCR Wet FGD 90% 51% 0% 

None SCR Dry FGD 40% 15% 0% 

Note: ESP is cold-side electrostatic precipitator. 

Actual performance is somewhat variable and new ICR data is being obtained by EPA. As 

an example, a 550 MW bituminous-fired unit equipped with ESP, SCR and wet FGD resulted 
ICAC  1220 N. Fillmore Street, Suite 410, Arlington, VA, 22201 703-812-4811  www.icac.com

in greater than 90 percent overall mercury capture in the downstream wet FGD unit. With 

the usage of the SCR catalyst, the oxidized fraction of mercury in the flue gas increased from 

64 percent of total gaseous mercury to over 95 percent. However, caution should be taken 

using this data since lower mercury capture occurs at some SCR/FGD equipped units. In 

such cases it may be necessary to augment mercury oxidation or FGD capture efficiency 

using the methods described above, or alternatively add a sorbent injection system (e.g. 

activated carbon injection) to serve as a trim control. 

Installation and Availability 

Around one-third of the coal-fired capacity in the U.S. has installed FGD for SO2 control and 

SCR for NOx control. The majority of the FGD installations are wet FGD systems. A 

significant number of existing coal-fired power plants are currently retrofitting or planning 

on retrofitting their boilers with FGD and/or SCR systems, which are more suited for the low 

cost mercury control options previously discussed. The enhanced control approaches 

primarily utilize additives and catalysts that are readily available in adequate quantities for 

initial operation and continued operation of these control systems. The augmentation 

technologies being commercialized are expected to become readily available and not limited 

by installation manpower, equipment, or consumables. 

Typical Costs 

Site-specific factors result in significant variation in costs. For units that are already 

equipped or will be equipped with both SCR and FGD for NOx and SO2 control, the 

incremental operating costs are relatively low for SCR catalyst replacements, SCR catalyst 

additions/replacements, or FGD additives. FGD additives also require a relatively low 

capital investment for equipment. Costs for other SCR/FGD augmentation technologies are 

variable dependent upon site-specific factors and the technology being deployed.