BOILER OPTIMIZATION FORMERCURY AND Nox EMISSIONS REDUCTION

摘要

U.S. EPA's studies have determined that mercury emissions from coal- fired power plantspose significant hazards to public health and must be reduced. Coal- fired power plants representa significant fraction of the anthropogenic emissions of mercury into the atmosphere. Mercuryemissions are impacted by factors such as coal type, boiler operation, fly ash characteristics andtype of environmental control equipment installed on the unit. Field data show a wide variationin the fraction of mercury that is emitted in the gaseous phase, as elemental and oxidizedmercury vapor, and as particulate-bound mercury. Oxidized mercury is a preferable speciesbecause is less volatile at stack temperatures, water-soluble, and tends to interact with mineralmatter and char, and cold-end air pollution control devices. There is also evidence that boileroperating conditions could be used to influence mercury beha vior in the boiler, since they affectthe thermo-chemical conditions for mercury formation and reduction. However, operation ofboiler control settings, which result in mercury removal benefits, should also consider thetradeoff between mercury reduction and other emissions (for example, Nox and CO), the level ofunburned carbon in the fly ash, unit efficiency and the performance of particulate controlequipment.This paper reports work performed to investigate the benefit of affecting mercuryemissions from coal- fired plants by manipulating boiler control settings. Full-scale testing wasperformed at three units that fire bituminous coals. Two of the units are equipped with a back-endtrain that includes a rotary air preheater followed by two electrostatic precipitators in series.The other unit is equipped with hot and cold precipitators and a tubular air preheater. A strategyfor mercury control by selectively manipulating boiler control settings, if used in combinationwith other control measures such as sorbent injection, will provide a cost-effective option formercury control.