Mechanism Study On Flue Gas Mercury Speciation Transformation By Adding Additives In Coal And Trace Elements Migration In Coal Combustion Process

Coal-fired power plants are considered to be the main anthropogenic emission sources of mercury and trace elements?TEs?to the environment.Mercury and TEs emitted from coal combustion have caused worldwide concern due to their toxicity,bioaccumulation and environmental durability.Adding additives in coal for mercury removal from coal-fired flue gas is an easy-to-operate,low-cost,high-efficiency mercury removal technology.The current researches on this technology mainly focus on the mercury removal efficiency,the mechanism of mercury transformation and removal in coal-fired flue gas with additives addition is not very clear.The study on the migration and transformation of TEs in coal-fired flue gas by additives is seldom reported.The most studies about TEs in coal combustion process are the distribution of TEs in fly ash,flue gas,bottom ash,and the enrichment characteristics of them in solid combustion products,or simply using chemical thermodynamic software for the simulation calculations of TEs,etc.Although researchers have conducted lots of the field tests on mercury emitted from coal-fired power plants,the mingration and transformation of mercury in each air pollution control device?APCD?in ultra low emission coal-fired power plants and that of other TEs in normal coal-fired power plants is not understood deeply.Therefore,clarifing the influencing mechanism of putting additives in coal on transformation and removal of mercury speciation in coal-fired flue gas,and the migration/transformation mechanism and emission characteristics of mercury and TEs in coal-fired power plants,have important guiding significance for development of relevant emissions tandards for mercury and TEs in coal-fired power plants,as well as their removal.In view of the above problems,this paper systematically and deeply carried out the experimental and mechanistic study on flue gas mercury speciation transformation by adding additives in coal and TEs migration in coal combustion process.The experimental study on the transformation and removal of mercury speciation in flue gas by putting NH₄Br in coal was conducted on a 6 kW_th circulating fluidized bed?CFB?device.The Ontario Hydro Method?OHM?was adopted to sample the mercury speciation and concentration in flue gas.The specific surface area and porosity analyzer?BET?,scanning electron microscope?SEM?,and energy dispersive X-ray fluorescence?EDXRF?analysis were used to characterize the physical and chemical properties of the coal-fired fly ash.The mercury forms in the fly ash was determined by the mercury programmed temperature desorption method?Hg-TPD?.The results show that:for the Guizhou anthracite combustion,the Hg^p concentration is much higher than that of Hg⁰ and Hg²⁺in the flue gas,amd the content of mercury in the bottom ash is negligible.The high unburned carbon content in the fly ash and strong gas-solid interactions contribute to high Hg^p share the flue gas of raw coal combustion.With NH₄Br addition amount increasing,the proportion and concentration of both Hg⁰ and Hg²⁺decrease while Hg^p correspondingly augments.Both the Hg⁰transformation rate and gas-phase mercury(Hg^g,Hg^g=Hg⁰+Hg²⁺)removal rate rise with the addition amount of NH₄Br increasing,which reach the maximal value at 0.3%addition amount.It indicates adding NH₄Br in the coal has great positive effects on the transformation of Hg⁰ to Hg²⁺/Hg^p and mercury removal during coal combustion.The added NH₄Br is decomposed to active bromine in the furnace,which promotes mercury homogeneous and heterogeneous oxidation.Under the addition of NH₄Br,some mercury compounds on fly ash(such as HgS?red?,HgO,and HgSO₃are further oxidized by active bromine to form more HgO and HgSO₄.The experimental study on the migration and transformation of Cl,As,Cr,Mn and Ba in the bituminous coal combustion process was conducted on a drop tube furnace?DTF?device.The US EPA Method 26 and the US EPA Method 29 were adopted to sample the Cl and other four TEs in flue gas,respectively.The high performance liquid chromatography?HPLC?coupled with inductively coupled plasma mass spectrometry?ICP-MS?was used to determine the concentration of As forms in coal-fired fly ash.Combining with the chemical thermodynamic calculations,the species transformation of Cl,As,Cr,Mn and Ba during coal combustion,as well as the influence of the Cl content in coal on them,were discussed.The experimental results show that:Cl is almostly distributed equally in the gas phase and particulate matter in the flue gas,and the distribution amount of gas-phase Cl is higher than that of its particulate form.The four TEs?namely,As,Cr,Mn and Ba?are mainly distributed in the fly ash,which are with little amount of them in gaseous form.The concentration of HCl and Cl₂ in the flue gas is much higher than that of the other four gaseous TEs?As_g,Cr_g,Mn_g,Ba_g?.HCl is the main form of gas-phase Cl in the flue gas.The concentration of particulate TEs?As_p,Cr_p,Mn_p,Ba_p?is much higher than that of their gaseous form.The relative enrichment factors of Cl,As,Cr,Mn and Ba in coal ash are calculated.Cl and As have certain volatility,and the volatility of Cl is much higher than that of As.Cr,Mn and Ba have certain enrichment characteristics.As⁵⁺is the main form of As in fly ash,which accounts for 93.79%of the total As amount in the fly ash.Based on the chemistry thermodynamic theory,the transformation of Cl,As,Cr,Mn,Ba in coal combustion process and influencing factors were in-depth studied.The simulation results show that when the raw coal is burned:in the range of 200-1400^oC,HCl?g?is the main Cl form.As the temperature increases,HCl?g?increases firstly and then decreases.Cl₂?g?is clearly formed at temperatures below 300°C,which gradually increases with decreasing temperature.At the temperature higher than 1100^oC,Cl?g?is clearly formed,which increases with increasing temperature.FeAsO₄ is the main As form in the range of 200-1073^oC.When the temperature is higher than 1073^oC,FeAsO₄ is obviously decomposed and converted into AsO?g?and AsO₂?g?.The higher the temperature,the more severe the decomposition.Combined with the experimental and simulation results,the main As form in coal-fired ash can be inferred to be FeAsO₄.In the range of200-1400^oC,Cr₂O₃ is the main Cr form.The conversion of Cr₂O₃ to Cr₂FeO₄ occurs at the temperature higher than 636^oC,and the degree of conversion become higher at the higher temperature.The main chemical forms of Mn are different in different temperature ranges.At200-527^oC,MnSO₄ and MnO₂ are the main Mn forms.At 527-855^oC,the conversion of MnSO₄ to MnO₂ and FeMnO₄ occurs,where the path to MnO₂ is dominant.At 855-1400^oC,the conversion of MnSO₄ and MnO₂ to FeMnO₄ and Mn₂SiO₄ occurrs.At 200-1400^oC,MnSO₄,MnO₂,FeMnO₄,Mn₂SiO₄ have obvious proportions,while the content and share of gaseous Mn compounds?MnO₂?g?,MnCl₂?g?,MnS?g??are extremely low?almost zero?.At 200-1400^oC,BaSO₄ and BaSiO₃are the main Ba forms.At 200-855^oC,BaSO₄ is the main form of Ba.When the temperature is higher than 855^oC,the conversion of BaSO₄ to BaSiO₃ occurs with the increase of temperature.When the content of Cl in coal increases to 0.08 mol,it has no obvious effect on the initial formation temperature of Cl?g??1100^oC?,but broadens the Cl₂?g?formation temperature range,which increases from 300^oC to 500^oC after Cl addition.Overall,there is no significant influence on the forms transformation of As,Cr,Mn and Ba during coal combustion,but it has a certain influence on the formation of AsCl₃?g?,CrO₂Cl₂?g?,MnCl₂?g?and BaCl₂?g?.The field-test on the mercury emission in a 660 MW ultra low emission coal-fired power plant equipped with SCR,ESP,WFGD,WESP,which burns low-chlorine,low-mercury,low-sulfur bituminous coal,was investigated.The OHM was used to sample the mercury in the flue gas at the five sampling points before or after APCDs simultaneously.During the flue gas mercury sampling,the coal,boiler bottom ash,ESP ash,desulfurizer fresh slurry,desulfurization product gypsum,desulfurization wastewater,WESP fresh water and wastewater were also collected.Mercury forms in the above solid samples were determined on the Hg-TPD experimental device.The physical and chemical composition of coal and by-products were characterized by SEM and X-ray diffractometry?XRD?.The results show that:the emitted Hg⁰ in the stack accounts for the largest proportion of the total mercury distribution amount.Mercury in bottom ash occupies the smallest ratio.SCR catalyst benefits the conversion of Hg⁰ to Hg²⁺/Hg^p.ESP and WESP can capture the gaseous mercury,especially the Hg^p.Hg²⁺can be removed in the WFGD but Hg⁰ re-emission occurs in this process.HgCl₂,HgS?black?and HgS?red?are the primary mercury compounds in the gypsum.Hg⁰ oxidation across SCR plays an important role for the Hg⁰ removal.WFGD has the best effects on the Hg²⁺capture.The removal rate of flue gas total mercury across the APCDs obeys the order of WFGD>ESP>WESP.The coal-fired power plant with WESP can further remove the mercury in the flue gas.The emission factor in this study is 1.56 g/10¹² J,which is less than the mean value of Chinese plants.The mercury content in the gypsum is higher than the mercury content limit in the relevant soil standard while the mercury content in the bottom ash and ESP ash is lower than the limit in the soil standard.The mercury concentration in WFGD wastewater is higher than the relevant water quality discharge standards,which should be given sufficient attention during its discharge or treatment.The migration and transformation process and emission characteristics of mercury in furnace,SCR,ESP,WFGD and WESP are described in detail.The field tests on other TEs?Zn,Sb,Pb,Cd,As,Cr,Mn and Ba?emission was carried out on a320 MW coal-fired power plant equipped with SCR,ESP and WFGD.The US EPA Method 29 was used for flue gas other TEs sampling at the four sampling points before or after APCDs simultaneously.The coal,ESP ash,boiler bottom ash,desulfurizer fresh slurry,desulfurization wastewater,etc.,were also collected at the same time.Scanning electron microscope and energy dispersive spectrometer?SEM-EDX?,and XRD were adopted to characterize the physical and chemical structure characteristic of the above soild samples.Results show that:most of TEs distribute in ESP ash and boiler bottom ash,while little of them emit to atmosphere.Mass amount of Mn and Ba in bottom slag is more than that of other TEs.Particulate TEs account for large portion in flue gas before ESP.Combination of ESP and WFGD has great removal effects on the TEs in flue gas?99.96%-99.99%?,in which the contribution ratio of ESP is about 99.97%.Mn and Ba will enrich in bottom slag and fly ash equally,REI for which are higher than 1.During coal combustion,As,Cd,Sb,Zn,Pb and Cr are prone to enrich in fly ash.Emission concentration of the studied TEs in flue gas to atmosphere is relatively low(3.5×10^-3?g/m³-4.30?g/m³).Emission factor of the TEs is 0.001 g/10¹²J-1.473 g/10¹²J.At last,the migration and transformation mechanism of the studied TEs in coal combustion process are discussed in detail.