The Release Rule Of Mercury In The Desulfurized Gypsum From Coal-fired Power Plants

Wet flue gas desulfurization(WFGD)gypsum is one of the most important by-products from the coal-fired power plants.Besides desulfurization,WFGD can also synergistically remove mercury in flue gas,which exists potential pollution to natural environment during gypsum utilization.It is significant to investigate the distribution of mercury in liquid-solid phase in gypsum slurry,and to develop suitable techniques to reduce the mercury content in the by-product of gypsum.In this thesis,five coal-fired units equipped with WFGD were selected to get gypsum slurry samples.After the separation of the liquid-solid phase in the laboratory,Atomic Absorption Spectroscopy(AAS)are used to analysis the mercury content in liquid and solid phases.Mercury content and its distribution in liquid and solid phases are analyzed.The results show that mercury content in both liquid phase and solid phase gypsum slurry sampled from the#1,#2 and#3 pulverized coal boiler units are much higher than that from the#4 and#5 circulating fluidized bed units.The distribution of mercury in liquid-solid phase is directly related to the kinds of boilers,i.e.,pulverized coal boiler and circulating fluidized bed boiler.Generally,the mercury content in the solid phase in the gypsum slurry from the pulverized coal boilers is approximately three times higher than that in the liquid phase.However,the circulating fluidized bed unit has higher total mercury content in the liquid phase than the pulverized coal boiler.Acid pickling method was used to reduce the mercury content in slurry gypsum.The effect of aqua regia,HC1 and HNO3 was compared in this study.It was found that the aqua regia has the strongest effect on removing mercury from slurry gypsum.Then the effect of different kinds of acids was compared.It is worth noting that the removal effect of HC1 is significantly stronger than that of HNO3.The effect of acid concentrations on mercury removal was also investigated.The results show that higher concentration of strong acid increases the removal rate of mercury.However,the change of weak acid concentration did not make significant effect on the removal of mercury in the range of this work.Sodium hydrosulfide(NaHS)and sodium diethyl dithiocarbamate(DDTC)were employed as additives to the gypsum slurry.The stabilizing effect of two additives on mercury in gypsum was investigated.The results show that both additives can increase the solid-phase mercury content in the gypsum slurry.By simulating plaster as raw material to produce processing environment for building materials,and investigate the thermal stability of mercury in gypsum solids at 200 ? and 300?,It was found that the introduction of additives could significantly reduce the release rate of mercury in gypsum solids at 200 ?,however the enhancement of thermal stability at 300? was not obvious.