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

## ENHANCING MERCURY CONTROL ON

COAL-FIRED BOILERS USING

SCR OR OXIDATION CATALYST AND FGD

Mercury capture in existing em issions control equipment offers a cost effective mercury control option for coal-fired power plants. The co-benefit of mercury cap ture from coal-fired power plants varies significantly depending on the existing emissions control configuration and type of coal being burned. Different co als and control configur ations produce varying amounts of the three basic forms of mercury in flue gas including elemental, oxidized, and particulate mercury. Particulate mercury will typically be captu red in the particulate control device and oxidized mercury is water soluble an d can therefore be captu red in certain control equipment such as wet flue gas desulfurization (FGD) systems.

In order to enhance mercury cap ture, the elemental form of mercury can be catalytically oxidized from the elemental metallic form, Hg 0 , to the oxidized form of mercury, Hg +2 , and captured by a downstream FGD system. Select ive catalytic reduction (SCR) catalysts or oxidation catalysts may be used for the catal ytic 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 commer cially when firing bituminous coal on units equipped with SCR and FGD due to relatively hi gh 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) e quipped with SCR and FGD can be accomplished by several methods: addition of halogen-contai ning compounds to the co al, co-firing higherhalogen content coals, usage of advanced S CR catalysts with impr oved mercury oxidation performance, or addition of a sorbent system such as activat ed carbon injecti on. Co-firing involves blending in 15 to 50 percent b ituminous 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 technol ogy for controlling SO 2 emissions. FGD also captures a large percenta ge of oxidized mercury present in coal-fired combustion flue gas because the oxidized mercur y is soluble in the ca lcium-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 oxidi zed 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 captu re are both low unless means are provided to enhance oxidation of elemental mercury as de scribed above. Neither “wet” FGD nor “dry” FGD is effective in capturing elemental mercur y. 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 SO 2 but does not saturate the flue gas like wet FGD. For dry FGD, the subsequentICAC  1220 N. Fillmore Street, Suite 410, Arlington, VA, 22201  703-812-4811  www.icac.com baghouse normally provides additional contact ti me thus effectively allowing the combined SDA/baghouse configuration to capture oxidized mercury as efficiently as wet FGD units.

Selective catalytic reducti on (SCR) is a widely-employed post-combustion method for controlling NO

x emissions. Although SCR catalyst reduces NO

x , 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 combinat ions due to EPA regulations that require significant SO 2 and NO

x reductions from power plan ts. For bituminous coal firing units, 80 percent to over 90 percent overall mercury ca pture 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 S CR and wet FGD showed sustained high Hg capture performance over multiple ozone operating seasons. Firing of low chlorine containing subbituminous and lignite coals will re quire 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 cap ture of mercury by downstream FGD, and may provide an alternative for units not empl oying SCR that are equipped with FGD. These catalysts can be installed ups tream of an FGD and perform in a similar manner to SCR catalyst. Oxidation catalyst might also be in stalled between the SCR catalyst and the FGD to enhance the overall performance of the SCR/FG D 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 th e 200 MW PRB demonstration 1

.

Process Augmentation for Mercury Capture Enhancement

The following options to increase the efficien cy 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, chlo rine, or other halogens to the coal (precombustion), directly to the boiler (combusti on), or after the boile r (post-combustion). The elemental mercury is oxidized by the halo gens to form mercury halides to improve mercury capture. This approach is especia lly useful when firing low halogen content

coals.

(b) Another method is blending for sub-bituminous or lignite coal firi ng 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/F GD configurations. Performance tests at a large utility plan t firing a 60 percent sub-bituminous and 40 percent bituminous blend at two identical boile rs (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 dow nstream oxidized mercury fraction from 63 percent without SCR to 97 pe rcent 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-specifi c oxidation catalyst include ca talyst addition after (or in lieu of) the SCR catalyst layers in a hot flue gas locati on, or the addition of a mercuryspecific oxidation catalyst in a relatively cool flue gas loca tion (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 abilit y to substantially pr event this re-emission effect. Additives include various liquids, so lids 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 NO

x SO 2 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 e quipped with ESP, SCR and wet FGD resultedICAC  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 frac tion of mercury in the flue gas increased from 64 percent of total gaseous mercury to over 95 percent. However, ca ution 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 alterna tively 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 SO 2 control and

SCR for NO

x 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 S CR systems, which are more suited for the low cost mercury control options previously di scussed. The enhanced control approaches primarily utilize additives and cat alysts that are readily availa ble in adequate quantities for initial operation and continued operation of these control systems. The augmentation technologies being commercialized are expected to become read ily 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 S CR 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. F GD additives also require a relatively low capital investment for equipment. Costs fo r other SCR/FGD augmentation technologies are variable dependent upon site-specific f actors and the technology being deployed.