MERCURY SPECIATION IN COAL COMBUSTION AND GASIFICATION FLUE-GASES

Coal combustion and gasification are an anthropogenic source of mercur y emission to the atmosphere. Effectively minimizing the emission and understanding the atmospheric fate and transport of mercury require kn owledge of its speciation in flue gases. Hg-0(g) is the thermodynamica lly stable form in the highest temperature regions of combustors and g asifiers. Hg-0(g) remains as the dominant form in the relatively reduc ing conditions of a gasification flue gas, but with decreasing tempera ture in a combustion flue gas Hg-0(g) will react to form Hg2+ compound s. Current mercury speciation analysis results suggest that generally > 50% of the Hg-0(g) reacts with oxidants in coal combustion flue gase s; results for gasification conditions are lacking. Oxidation is benef icial because Hg2+ compounds are generally water-soluble and are there fore more effectively captured by wet scrubber pollution control syste ms and are more apt to deposit locally or regionally. Conversely, Hg-0 (g) is difficult to control and is likely to enter the global atmosphe ric cycle because of its high vapor pressure and low water solubility. The physical and chemical processes governing the interactions of mer cury species with flue gas components are poorly known.