Mercury speciation in coal combustion and gasification flue gases

Coal combustion and gasification are an anthropogenic source of mercury emission to the atmosphere. Effectively minimizing the emission and understanding the atmospheric fate and transport of mercury require knowledge of its speciation in flue gases. Hg-0(g) is the thermodynamically stable form in the highest temperature regions of combustors and gasifiers. Hg-0(g) remains as the dominant form in the relatively reducing conditions of a gasification flue gas, but with decreasing temperature in a combustion flue gas Hg-0(g) will react to form Hg2+ compounds. Current mercury speciation analysis results suggest that generally > 50% of the Hg-0(g) reacts with oxidants in coal combustion flue gases; results for gasification conditions are lacking. Oxidation is beneficial because Hg2+ compounds are generally water-soluble and are therefore more effectively captured by wet scrubber pollution control systems and are more apt to deposit locally or regionally. Conversely, Hg-0(g) is difficult to control and is likely to enter the global atmospheric cycle because of its high vapor pressure and low water solubility. The physical and chemical processes governing the interactions of mercury species with flue gas components are poorly known.