Behaviors of Mercury in Solid Fuels and Wastes during Combustion/Gasification Processes

摘要

Mercury emissions from combustion/gasification processes of solid fuels and wastes havernbeen concerned worldwide. Many types of Hg control technology are now under intensiverndeveloped. Generally, mercury exists as elemental mercury and oxidized mercury in therncoal and/or solid wastes. The elemental mercury easily passes through particulate controlrndevices since it becomes gaseous phase even at ambient temperature. The oxidized mercury,rnhowever, is water-soluble, hence some can be removed by wet desulfurization processes orrnwet scrubbers. Speciation of the mercury compounds in combustion/gasification processesrnis, therefore, a particularly important issue to develop the appropriate mercury controlrndevices.rnIn this study, first, thermo-gravimetric (TG) experiments were carried out to understandrnfundamentals of mercury evolution behaviors under the combustion and gasificationrnconditions, using three types of coal and dried sewage sludge as the samples. Then, onernsample was burnt in an electrically heated drop tube furnace to study the evolution behaviorrnof mercury in semi-practical combustion processes. Moreover, chemical equilibriumrncalculation predicted the tendency of mercury evolution under the same experimentalrnconditions. Meanwhile, the kinetic calculations of mercury oxidation phenomena were alsornconducted to analyze the time-depended behaviors of all mercury species concentrations,rnusing CHEMKIN. The results obtained show that sulfur content in the fuels inhibits thernevolution behaviors of mercury during sludge combustion. For coal, while, the fixed carbonrncontent and Cl concentration in the coal play an important role for the mercury evolution.rnUnder the gasification condition, however, mercury evolves quickly because the elementalrnmercury is easily formed. The kinetic prediction also suggests that the mercury oxidationrnprocesses depend on some radicals, especially for Cl radical produced.