Mercury emissions from utility power stations are a complex function of coal chemistry and properties such as rank (coal quality), combustion conditions, and type and positioning of air pollution control devices (APCDs) deployed. The average mercury concentration of coals delivered to U.S. utility power stations in 1999 is 0.10 ppm (U.S. EPA, 2002). Mercury in bituminous coals occurs primarily in iron sulfides, whereas lower rank (lignite and sub-bituminous) coals have a greater proportion of organically-bound mercury. In high temperature portions of the boiler, mercury exists as gaseous elemental mercury (Hg⁰), regardless of its mode of occurrence in coal. However, as the flue gas cools, coal constituents (especially chlorine and sulfur), coal rank, and combustion characteristics can influence the formation of gaseous oxidized mercury and particulate mercury, species that are preferentially captured by ACPDs. For bituminous coals, mercury capture efficiency increases proportionally with chlorine content, primarily attributable to HgCl₂ conversion, but the fraction of mercury captured varies widely with the type of APCD employed. Sulfur in coal has an opposite effect, inhibiting oxidation of elemental mercury by unburned carbon in fly ash and reducing adsorption of mercury on ash particles. Oxidation and adsorption of mercury by unburned carbon in fly ash are important processes that influence mercury capture by particulate control devices.

The EPA Clean Air Interstate Rule (CAIR) is projected to result in a 40% reduction in mercury emissions from U.S. coal-fired utility power stations by 2010, largely as a co-benefit to reductions in NO_x and SO₂ emissions specified for 28 Eastern States and the District of Columbia. Knowledge of coal quality parameters and their effect on the performance of APCDs allows optimization of mercury capture co-benefit, helping utilities achieve mercury reduction targets under CAIR.

Reference: U.S. EPA, 2002, Results of 1999 Information Collection Request http://www.epa.gov/ttn/atw/combust/utiltox/utoxpg.html#DA2