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Mercury: Much Ado About Nothing?

How the Clean Air Mercury Rule will affect coal prices.

Fortnightly Magazine - June 2005

lignite or subbituminous coal. The average capture of mercury in plants with a cold-side electrostatic precipitator (ESP), for example, is about 35 percent for bituminous coal, 3 percent for subbituminous coal and 0 percent for lignite. 

The concentration of mercury in coal often depends on its geographical origin. Using data obtained from the U.S. Geological Survey CoalQual database, a comprehensive national coal information database that contains chemical analyses of more than 7,000 coal samples taken over a 20 year period, the distribution of mercury and other constituents can be seen. Bituminous coal from Northern and Central Appalachia, for example, tends to have higher concentrations of mercury. Western subbituminous coal typically has lower concentrations of mercury. Figure 3 shows the average mercury concentrations at coal producing counties across the United States.

During combustion, the mercury in coal is volatilized into elemental mercury vapor by the high temperatures in the boilers. As the flue gas cools, a series of complex reactions take place where the elemental mercury is speciated into elemental mercury, ionic mercury compounds, and mercury compounds. This partitioning can play a major role in the selection of mercury control approaches. As a rule of thumb, the emissions from bituminous coal-fired boilers are ionic mercury compounds and the emissions from subbituminous- and lignite-fired boilers are typically elemental mercury.

If the co-benefit from existing emissions control technology doesn’t sufficiently reduce mercury emissions, other mercury specific options are available. Activated carbon injection (ACI) has been used successfully to remove mercury by up to 90 percent at municipal waste incinerators. The activated carbon is injected in the flue-gas stream prior to entry into the particulate matter control device, which typically is either an electrostatic precipitator or a fabric filter (FF) baghouse. The mercury binds with the activated carbon and is collected downstream in the particulate matter control device. Fabric filters generally remove a greater percentage of mercury because the sorbent builds up on the bags, allowing for more capture pathways for mercury.

Activated carbon injection removes mercury successfully from bituminous coals, but lignite and subbituminous coals tend to have lower mercury capture rates because the flue gas from burning these coals has a higher proportion of elemental mercury emissions. The flue gas from burning bituminous coal usually has less gaseous elemental mercury and a much higher proportion of particle-bound and oxidized mercury. Oxidized and particle-bound mercury are more readily adsorbed onto sorbents than elemental mercury. Oxidation of elemental mercury is improved with lower temperature combustion, higher halogen ( i.e., chlorine, iodine, bromine, fluorine, or astatine) content, longer residence time, and the absence of ammonia injection. While ACI successfully may reduce mercury emissions from bituminous coal, the presence of activated carbon in ash may prevent the sale of ash for use in concrete.

Subbituminous coals have higher elemental mercury concentrations in the flue gas because they have lower concentrations of halogens, such as chlorine. Figure 4 shows the average county level chlorine concentrations based on data obtained from the USGS CoalQual database. Northern, Central and Southern Appalachian coals all have relatively high concentrations of chlorine and halogens.