Mercury Emission,Control and Measurement from Coal Combustion

摘要

Coal-fired electric power generation accounts for sixty-five percent of U.S.emissions of sulfur dioxide (SO2), twenty-two percent of nitrogen oxides (NOx),and thirty-seven percent of mercury (Hg). The proposed Clear Air Interstate Rule (CAIR) and Clean Air Mercury Rule (CAMR) will attempt to regulate these emissions using a cap-and-trade program to replace a number of existing regulatory requirements that will impact this industry over the next decade.Mercury emissions remain the largest source that has not yet been efficiently controlled,in part because this is one of the most expensive to control.Mercury is a toxic,persistent pollutant that accumulates in the food chain. During the coal combustion process,when both sampling and accurate measurements are challenging,we know that mercury is present in three species: elemental,oxidized and paniculate. There are three basic types of mercury measurement methods: Ontario Hydro Method,mercury continuous emission monitoring systems (CEMS) and sorbent-based monitoring.Particulate mercury is best captured by electrostatic precipitators (ESP).Oxidized mercury is best captured in wet scrubbers.Elemental mercury is the most difficult to capture,but selective catalytic reduction units (SCRs) are able to convert elemental mercury to oxidized mercury allowing it to be captured by wet flue gas desulfurization (FGD). This works well for eastern coals with high chlorine contents,but this does not work well on the Wyoming Powder River Basin (PRB) coals.However,no good explanation for its mechanism,correlations of chlorine content in coal with SCR performance,and impacts of higher chlorine content in coal on FGD reemission are available. The combination of SCR and FGD affords more than an eighty percent reduction in mercury emissions in the case of high chlorine content coals. The mercury emission results from different coal ranks,boilers,and the air pollution control device (APCD) in power plant will be discussed. Based on this UAEPA new regulation,most power plants that are only equipped with an Electrostatic Precipitator (ESP) have to look for a control method to reduce mercury emission.So far, the most economical method has been active carbon or sorbent injection before the ESP.Active carbon or sorbent injected into the flue gas ducts to oxidize the elemental mercury and then the oxidized mercury will be captured from the flue gas, then the ESP captures the active carbon or sorbent and fly ash simultaneously. Therefore, the long distance transportation of gaseous mercury is eliminated.However, the capture efficiency of mercury is extremely important in order to reduce the increase in ESP load and control the cost. The oxidation and adsorption rate of HBr and fly ash will be discussed in this presentation.