488

a Removals as percentage of mercury in coal calculated by the Electric Power Research Institute (EPRI).

Note: EPRI, Electric Power Research Institute; ICRs, Information Collection Requests; ESP, electrostatic precipitator; FGD, flue gas desulfurization; FBC, fluidized-bed combustion (FBC). Source: Adapted from Pavlish et al. [78].

a Removals as percentage of mercury in coal calculated by the Electric Power Research Institute (EPRI).

Note: EPRI, Electric Power Research Institute; ICRs, Information Collection Requests; ESP, electrostatic precipitator; FGD, flue gas desulfurization; FBC, fluidized-bed combustion (FBC). Source: Adapted from Pavlish et al. [78].

Individual Test Runs

FIGURE 6-24. Mercury capture across cold-side ESPs. (From Feeley, T. J. et al., A Review of DOE/NETL's Mercury Control Technology R&D Program for Coal-Fired Power Plants, DOE/NETL Hg R&D Review (National Energy Technology Laboratory, U.S. Department of Energy, Washington, D.C.), www.netl.doe.gov/, 2003.

Individual Test Runs

FIGURE 6-24. Mercury capture across cold-side ESPs. (From Feeley, T. J. et al., A Review of DOE/NETL's Mercury Control Technology R&D Program for Coal-Fired Power Plants, DOE/NETL Hg R&D Review (National Energy Technology Laboratory, U.S. Department of Energy, Washington, D.C.), www.netl.doe.gov/, 2003.

bituminous coal, subbituminous coal, and lignite [77]. Mercury removal across a cold-side ESP followed by a WFGD averaged 65% for bituminous coal compared to 35% for low-rank coal [78]. The ICR data indicate that, for pulverized coal-fired units, the greatest co-benefit for mercury control is obtained for bituminous coal-fired units equipped with a fabric filter for particulate matter control and either a WFGD or spray dryer absorber for sulfur dioxide control. The worst performing pulverized bituminous coal-fired units were equipped with only a hot-side ESP [77].

The rank-dependency on mercury removal is due to the speciation of the mercury in the flue gas, which can vary significantly among power plants depending on coal properties. Power plants that burn bituminous coal typically have higher levels of oxidized mercury than power plants that burn subbituminous coal or lignite, possibly due to the higher chlorine and sulfur content of the bituminous coal. The oxidized mercury, as well as the par-ticulate mercury, can be effectively captured in some conventional control devices such as an ESP, fabric filter, or FGD system, while elemental mercury is not as readily captured. The oxidized mercury can be more readily adsorbed onto fly ash particles and collected with the ash in either an ESP or fabric filter. Also, because the most likely form of oxidized mercury present in the flue gas—mercuric chloride (HgCl2)—is water soluble, it is more readily absorbed in the scrubbing slurry of plants equipped with wet FGD systems compared to elemental mercury, which is not water soluble [77]. It has been speculated that the installation of SCRs or SNCRs could significantly increase oxidation and improve removal of mercury. This suggestion is based on European reports and testing performed at the University of North Dakota Energy and Environmental Research Center [78].

Near-Term Control Technologies

Many research organizations, federal agencies, technology vendors, and utilities are actively in the process of identifying, developing, and demonstrating cost-effective mercury control technologies for the electric utility industry. Many technology options are available at various levels of testing, demonstration, and commercialization but, based on the current state of development, sorbent injection, FGD, and coal cleaning represent the best potential for reducing mercury emissions and meeting the future mercury regulations.

Sorbent Injection Injection of activated carbon upstream of either an ESP or a baghouse is the retrofit technology that offers the greatest potential for controlling mercury emissions in plants that are not equipped with FGD scrubbers, which includes 75% of all U.S. power plants [78-80]. This is a challenging technology due to the low concentrations of mercury in the flue gas, the wide range of concentrations of acid gases and chlorine species that are present, and the relatively short gas residence time upstream of the particulate control device. Contact time, though, can be increased by using fabric filters. In addition, carbon injection upstream of a COHPAC fabric filter offers one of the most efficient and cost-effective approaches for reducing mercury emissions from coal-fired boilers. This combination of activated-carbon injection and COHPAC represents EPRI's patented TOXE-CON process and has the additional benefit of minimizing the impact on fly ash and its subsequent reuse because most of the fly ash is removed upstream by the ESP [81].

Wet Flue Gas Desulfurization Wet FGD systems are currently installed on about 25% of the electric-power-generating capacity in the United States. Although the primary function of wet scrubbers is to reduce sulfur dioxide emissions, bench- to large-scale testing has indicated that oxidized mercury (80-95%) can also be effectively captured in wet scrubbers. They are not effective, though, in capturing elemental mercury, and there is evidence that a portion of the oxidized mercury can be reduced to elemental mercury within the wet FGD system and emitted from the stack [77]. Bituminous coal, which typically has high concentrations of oxidized mercury, has the potential for achieving high overall mercury reduction. Low-rank coals also exhibit high capture of the oxidized mercury but because the concentration of oxidized mercury is low, they have low overall mercury reduction. Techniques to oxidize the vapor-phase elemental mercury prior to the wet scrubber are being aggressively studied. Also, methods to prevent the reduction of oxidized mercury to elemental mercury are also being investigated.

Coal Cleaning Coal cleaning is an option for removing mercury from the coal prior to utilization. Of the over 1 billion short tons of coal mined each year in the United States, about 600 to 650 million short tons are processed to some degree [82]. Coal cleaning removes pyritic sulfur and ash. Mercury tends to have a strong inorganic association (i.e., it is associated with the pyrite), especially for Eastern bituminous coals, but mercury removal efficiencies reported for physical coal cleaning vary considerably. Physical coal cleaning is effective in reducing the concentration of many trace elements, especially if they are present in the coal in relatively high concentrations. The degree of reduction achieved is coal specific, relating in part to the degree of mineral association of the specific trace element and the degree of liberation of the trace element-bearing mineral. High levels of mercury removal (up to ~80%) have been demonstrated with advanced cleaning techniques such as column flotation and selective agglomeration [83], while conventional cleaning methods, such as heavy media cyclone, combined water-only cyclone/spiral concentrators, and froth flotation, have been shown to remove up to 62% of the mercury [84]. In both the conventional and advanced cleaning techniques, the results varied widely and were coal dependent.

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