Mercury emission and removal of a 135 MW CFB utility boiler

Abstract

To evaluate characteristic of the mercury emission and removal from a circulating fluidized bed (CFB) boiler, a representative 135 MW CFB utility boiler was selected to take the onsite measurement of mercury concentrations in feeding coal, bottom ash, fly ash and flue gas using the US EPA recommended Ontario Hydro Method (OHM). The results show that particulate mercury is of majority in flue gas of the CFB boiler. Mercury removal rate of the electrostatic precipitator (ESP) reaches 98%. Mercury emission concentration in stack is only 0.062μg/Nm3, and the mass proportion of mercury in bottom ash is less than 1%. It was found that the fly ashes were highly adsorptive to flue gas mercury because of its higher unburned carbon content. Adsorption effect is related to carbon pore structural properties of fly ash and temperature of flue gas. However mercury adsorption capacity by fly ash can not be improved any more when unburned carbon content in fly ash increases further.