Emission and Migration Characteristics of Mercury in a 0.3 MWth CFB Boiler with Ammonium Bromide-Modified Rice Husk Char Injection into Flue

Abstract

The emission and migration characteristics of flue gas mercury in a pilot-scale 0.3 MWth circulating fluidized bed combustion boiler system by injecting the adsorbent of ammonium bromide (NH4Br)-modified rice husk char (NBr-RHC) into the flue duct before a fabric filter (FF) were investigated in this study. Mercury concentration in the flue gas was sampled by the Ontario Hydro Method. Meanwhile, the feeding coal, lime, process water, fly ashes both in the flue gas and FF hopper, bottom ash, gypsum, and wet flue gas desulfurization (WFGD) effluent were also collected during the flue gas mercury sampling. The temperature-programmed desorption (TPD) method was used to identify the mercury species in the sample of fly ashes. The results show that the combination system of the selective catalytic reduction (SCR) + adsorbent injection (AI) + FF + WFGD obtains a total flue gas mercury removal efficiency of 94.62%, which is 13.42% higher than that without the AI of NBr-RHC. A total of 93.83% of mercury exists in byproducts of the solid and liquid, and the amount of mercury collected by FF accounts for 97.28% due to the physisorption and chemisorption of gaseous mercury by NBr-RHC injection. The TPD analysis for mercury compound in the particulate matters from the AI device indicates that HgBr2 formation is the key reason for NBr-RHC having high mercury removal efficiency. Elemental mercury is found to oxidize across the SCR, with 70.23% converted to oxidized mercury. WFGD exhibits a good oxidized mercury absorption rate of 86.67%, but elemental mercury increases from 0.19 to 0.26 μg/m3 due to the oxidized mercury reduction, and more attention should be given on this increase.