Experimental Study On Mercury Emission Characteristics Of A 350MW Ultra-low Emission Coal-fired Power Plant

Mercury emission from coal-fired power plant,as the main anthropogenic emission resource,has become the fourth largest pollutant after particulate matter,SO₂ and NO_x.Mercury emission control is of significant importance in coal-fired power plants.With the implementation of the ultra-low emission retrofitting in coal-fired power plants,the mercury migration and emissions have changed greatly compared with conventional units.Therefore,it is of great significance to investigate the mercury emissions and synergistic mercury removal of the air polltutant control devices?APCDs?in ultra-low emission coal-fired power plants,which is also benificial to the mercury emission reduction and environmental protection.Field test of mercury emission characteristics was conducted in a 350 MW ultra-low emission coal-fired power plant equipped with selective catalytic reduction?SCR?,electrostatic-fabric integrated precipitator?EFIP?,and wet flue gas desulurazation?WFGD?.The standard method of Ontario Hydro Method?OHM?was used for simultaneous sampling of flue gas mercury at the inlet and outlet of the APCDs.Samples of coal,bottom ash,fly ash from the EFIP hopper,fresh slurry of the desulfurizer,desulfurization waste water and desulfurization gypsum were collected.The synergistic removal characteristics of mercury from APCDs,mercury emission characteristics from flue gas and mercury emission from by-products of coal-fired power plants were studied.The experimental results show that the mercury mass balance ratios of the boiler system at the loads of 100%MCR,85%MCR,and 68%MCR are 70.18%¹29.16%,75.4%¹29.16%,and 83.67%-116.63%,respectively.Mercury emission is mainly distributed in flue gas,followed by EFIP fly ash,WFGD gypsum,waste water,and bottom ash,with the Hg⁰being the major emission from the stack.The conversion efficiency of Hg⁰ to Hg²⁺and Hg^p for SCR is in the range of 46.92%-64.3%.The concentration of Hg^p at the outlet of SCR increases significantly and the concentration of Hg²⁺decreases considerably.It is different from the result of the previous studies that the Hg^p concentration varies little.It is likely that some of geseous Hg²⁺is converted to gaseous Hg^p,resulting in the decrease in the concentration of Hg⁰ and Hg²⁺and the notable increase in the concentration of Hg^p.The removal efficiency of EFIP for particulate mercury?Hg^p?and total mercury(Hg^T,Hg^T=Hg⁰+Hg²⁺+Hg^p)is 99.95%-99.97%,43.7%-64.34%,respectively,which is higher than that of the conventional results.This is due to the co-benefit of ESP and FF for particlute matter capture.The former electric field area can remove over 80%of dust,with the remaining being captured by FF.The regular and loose dust layer formed on the surface of FF with the help of the electrostatic force promotes the mercury removal by FF.The mercury enrichment factors in fly ash of EFIP area and bag area are 1.7×10^-3-3.8×10^-33 and 9.4×10^-3-1.28×10^-2,respectively.The concentration of mercury in fly ash from FF is 4 times of that from ESP.FF contributes much more to the mercury removal,compared with the ESP.The removal efficiencies of gaseous Hg²⁺,Hg⁰,and Hg^T by WFGD are 17%-25%,-8.26-5.2%,and 0.05%-5.6%,respectively,indicating that WFGD has better removal performance for Hg²⁺than Hg⁰.The poor Hg^T removal performance is probably due to the reduction of Hg²⁺to Hg⁰ in WFGD.The overal synergistic removal efficiency of Hg^T from flue gas by SCR+EFIP+WFGD is 60.13%-73.14%.The mercury emission concentration at the outlet of WFGD is in the range of 1.8?g/m³to 2.54?g/m³,which remarkably meets the mercury emission regulation of less than 30?g/m³.The mercury content in slag is 0.005-0.014 mg/kg,0.13-0.216 mg/kg in EFIP fly ash,0.019-0.063 mg/kg in gypsum,and 0.31-0.37 mg/L in desulfurization waste water.The content of mercury in slag,EFIP fly ash and gypsum is lower than the limit of soil in GB15618-1995?0.5 mg/kg?,while the concentration of mercury in desulfurization waste water is much higher than the limit of underground water in GB/T 1448-9?0.001 mg/L?.Finally,based on the above-mentioned results,the regenerable mercury removal adsorbents,reduction of Hg⁰ re-release from WFGD slurry and further mercury removal from desulfurization waste water are supposed to be highlighted for the research of zero mercury emission stratigy in the current cola-fired power plants.