Evaluation of the Impact of Chlorine on Mercury Oxidation in a Pilot-Scale Coal Combustor - The Effect of Coal Blending

摘要

Coal-fired power plants are a major source of mercury (Hg) released into the environment and the utility industry is currently investigating options to reduce Hg emissions. The EPA Clean Air Mercury Rule (CAMR) depends heavily on the co-benefit of mercury removal by existing and new wet flue gas desulfurization (FGD) scrubbers. The split (speciation) between chemical forms of mercury (Hg) species has a strong influence on the control and environmental fate of Hg emissions from coal combustion. The high-temperature coal combustion process releases Hg in elemental form (Hg~0). A significant fraction of the Hg~0 can be subsequently oxidized in the lowtemperature, post-combustion environment of a coal-fired boiler. Relative to Hg~0, oxidized Hg (Hg~(2+)) is more effectively removed by air pollution control systems (APCS). For example, the water-soluble Hg~(2+) is much more easily captured than insoluble Hg~0 in FGD units. Selective catalytic reduction (SCR) technology widely applied for reducing NO_x emissions from power plants also affects the speciation of Hg in the coal combustion flue gases. Recent full-scale field tests conducted in the U.S. showed increases in Hg oxidation across the SCR catalysts for plants firing bituminous coals with sulfur (S) content ranging from 1.0 to 3.9%. However, plants firing subbituminous Powder River Basin (PRB) coals which contains significantly lower chlorine (Cl) and sulfur (S) content and higher calcium (Ca) content than those of the bituminous coals, showed very little change in mercury speciation across the SCR reactors. A field study conducted by EPRI showed blending of PRB coal with a bituminous coal (60% PRB/40% bituminous) resulted in increased Hg~(2+) from 45% at the SCR inlet to 93% at the outlet. Coal blending appears to be a potentially cost effective approach for increasing Hg oxidation for PRB coal-fired SCR systems. A study has been undertaken to investigate the effect of blending PRB coal with an Eastern bituminous coal on the speciation of Hg across an SCR catalyst. In this project, a pilot-scale (1.2 MWt) coal combustor equipped with an SCR reactor for NO_x control was used for evaluating the effect of coal blending on improving Hg oxidation across an SCR catalyst. Several parameters such as the ratio of PRB/bituminous coal blend and the concentrations of hydrogen halides (HCl, HBr, and HF) and halogens (Cl_2 and Br_2) in the flue gas were evaluated to determine their effects on the oxidation of Hg~0 under typical SCR NO_x emission control conditions. The objective of this project was to evaluate the effectiveness of firing PRB/bituminous coal blends to enhance mercury oxidation in a coal fired power plant equipped with an SCR system.