Mercury in coal: a review Part 2. Coal use and environmental problems

摘要

During combustion of pulverized coals, most of the Hg volatilizes. Variable amounts of Hg are captured by fly ash and essentially none accumulates .pi the, bottom ash. Hg is emitted to the atmosphere as gaseous (dominate) and solid-phase (subordinate) pollutants. Because of the high toxicity of Hg and its compounds, it is desirable to reduce the Hg content of coals prior to utilization and in the process of combustion. There are several ways to achieve reduction: selective mining of low-sulfur and low-Hg coals; standard coal cleaning through pyrite removal; also, a rejection of coarse, often pyrite-rich, material in pulverization at power plant can be added; perhaps, combustion of some coarser-grained coals, which will lead to more Hg capture through the formation of unburned char particles in fly ash; change of combustion regime by lowering flue gas temperatures and increasing Hg capture on collected fly ash (low-NO_x burners and other devices); oxidation of elemental Hg in flue gases to more capturable HgCl_2 through the combustion of Cl-bearing coals and in ECO process; the wider use of dry and wet scrubber systems for flue gas desulfurization and Hg capture; the greater use of carbon sorbent injection into flue gas ducts for better Hg capture, and, especially, use of modified carbon sorbents with more capacity for Hg; the use of some other very promising sorbents and filters, especially of FF type (fabric filters) as more effective scavengers of Hg than ESPs (electrostatic precipitators). Because of the potentially serious environmental impacts, additional research is required on the Hg geochemistry of coals.