| In recent years,circulating fluidized bed(CFB)boilers have been rapidly employed for power generation in China and many other coal-produced countries due to their excellent fuel flexibility,low NOx emission.high sulfur capture efficiency and so on.By the end of 2015.total installation capacity of CFB coal-fired boilers has been reached 70 GW in China.As results,mercury emissions from CFB boiler units should be considered as a high-priority regulatory environment concern.In this thesis,mercury emissions and its migration behavior in CFB boiler units were explored based on the field test.Then,the effect of CFB fly ash characteristics on mercury enrichment behavior,mechanism of mercury adsorption on CFB fly ashes and release characteristics of mercury in CFB fly ashes during heat treatment were investigated through laboratory analytical testing and dynamic theory.Finally,CuCl2 was selected to modify CFB fly ash.and the adsorption ability of gaseous elemental mercury was measured.The field test of mercury was conducted in a commercial coal-fired CFB unit with the electricity generation capacity of 25 MW.The decrease of the flue gas temperature through the air preheater(APH)was beneficial to the migration of Hg from flue gas to fly ash.The average Hg contents in the fly ash samples collected from the ESP,the first FF and the second FF were in a increasing order,while the mean particle diameters of these fly ash samples tend to decrease along the flow pass in the PCD system.Compare with Pulverized coal(PC)boiler units,mercury emissions from CFB boiler units were solid discharge.The Hg mass balance in the range of 104.07%to 112.87%was obtained throughout the CFB unit.The majority of Hg contained in the feed fuel was captured by the fly ash.whilst Hg contain was very low in both the bottom ash and flue gas.The physicochemical property and mercury content in fly ash showed that the CFB boiler fly ash had higher mercury content than PC boiler fly ash due to small particle size,high UBC content and big specific surface area.The mercury adsorption capacity of CFB fly ash increased with decreasing particle size and reaction temperature,but increasing unburned carbon content and specific surface area.On the other hand,the mercury adsorption capacity of PC fly ash was too complicated,which not only depended on the surface structure and unburned carbon content but also was related to the reactivity.The experimental results of Hg0(g)adsorption showed that the penetration time and adsorption capacity of the CFB fly ash were much higher than those of the PC fly ash under the same operational conditions.The adsorption capacity of fly ash samples was the highest at 150℃ in the range of this study.Mercury adsorption on fly ash samples increased with an increase of initial mercury concentration due to the enhancement of mercury diffusion force onto the fly ash surface and the reduction of the external diffusion resistance.The mercury adsorption on the fly ash was affected by external diffusion,internal diffusion and surface adsorption,among of which surface chemical adsorption played a key role.Therefore,Hg adsorption on fly ash could be predicted by the pseudo-second order kinetic model and Elovich kinetic model.The mercury release rate of the fly ash was closely related to the heating temperature and heat decompostion time.Mercury species in the fly ash mainly included HgCl2,Hg2Cl2,HgO,HgSO4,HgS(black)and HgS(red),among of which HgS(black)and HgS(red)were in dominant.Elovich thermo-analytic kinetic equation was more applicable to reflect the Hg release from the fly ash.Hg0(g)adsorption on CuCl2-modified CFB fly ash had higher Hg0(g)adsorption ability than original fly ash because Cl and Cu2+could oxidize the Hg0(g)into Hg2+(g).The mercury adsorption capacity of CuCl2-modified CFB fly ash increased with an increase of CuCl2 concentration,whilst the adsorption capacity increased firstly and then decreased with the increase of temperature with the highest value at 150℃ in the range of this study.Generally,Hg adsorption on fly ash could be described by the pseudo-second order kinetic model. |
