Study On The Release,Migration,Transformation And Enrichment Mechanism Of Typical Heavy Metals During Coal Combustion

Coal-fired power generation is the most important way of coal consumption.However,the toxic heavy metals released during coal combustion have been threatening the ecological environment and human health greatly due to their persistent hazards,non-biodegradability and the accumulation in organisms,soil and atmosphere.Therefore,this work investigate the releasing behavior,migration and transformation characteristics of typical heavy metals(Pb and Cd)and the related enrichment mechanism during coal combustion systematically and comprehensively.In this work,the samples collected from five different coal-fired power plants were studied and the indoor coal combustion experiments were simultaneously conducted to investigate the releasing behaviors of the typical heavy metals.By performing an arrays of techniques of Thennogravimetric analysis,Scanning Electron Microscopy-Energy Spectroscopy,Transmission Electron Microscopy-Energy Spectroscopy,Laser Particle Size Analyzer,X-Ray Diffraction Spectroscopy,X-Ray Fluorescence Spectroscopy and inductively coupled plasma mass spectrometry,etc,and applying the theoretical knowledge involving the geochemistry of trace elements,coal petrology,mineralogy,combustion,environmental chemistry and so on,the concentration distribution,migration characteristics,form transformation and enrichment mechanism of Pb and Cd during coal combustion in power plants have been systematically studied.The main results obtained are as follows:(1)The indoor coal combustion experiment was conducted with different types of coals and the blend of bituminous coal and calcium carbonate minerals,using atmosphere-controllable and temperature-programmable tube furnace.The results indicated that Pb and Cd in lignite reached the equilibrium point of volatilization first with the increasing temperature,and the volatilization rate of Pb and Cd in the three types of coals were in the sequence of:anthracite>lignite>bituminous coal.Besides,it was observed that Pb and Cd bound to the carbonate,sulfide and organic matters were more likely to be released with decomposition of minerals during the coal combustion,while the Pb and Cd bound to silicates could ultimately remain in the combustion residues.Furthermore,the study demonstrated that the capture mechanism of Pb by calcium carbonate minerals was chemical adsorption at 800?900?,while that of Cd was the fixed adsorption.(2)The mineral composition,chemical composition,micro morphology,the contents and occurrence forms of Pb and Cd in the raw coals and the combustion residues(bottom ash and raw ash,fine ash and coarse ash)from various coal-fired power plants was investigated.The result showed that the fly ash particles from the circulating fluidized bed boilers mostly existed in irregular molten states and contained a large amount of calcite and anhydrite,while the fly ash in the pulverized coal boiler mainly consisted of spherical or elliptical solid particles,and its minerals were primarily derived from quartz and mullite.In addition,it was found that Pb and Cd tended to accumulated in the fly ash,and the smaller the fly ash particles,the higher the degree of enrichment Overall,the typical heavy metals bound to organic matters,sulfide and carbonate could be converted into stable silicate bound form at high temperatures during the coal combustion.(3)The size-fractionated particulate matters(<10 ?m)in the flue gas,collected from an industrial circulating fluidized bed coal-fired power plant,was analyzed with morphology,mineral composition,chemical composition(including Pb)and particle size distribution.It was observed that the submicron particles(<0.7 ?m)mainly consisted of Si-rich and Si-Al phases.And the fragmentation of limestone additives and newly-formed CaSO4 made remarkable contribution to the formation of fine particulate matters in the size range of 5.8-4.7 ?m.Moreover,the mass particle size distribution of size-fractionated particulate matters followed a unimodal mode,while that for the inherent Pb fowled a bimodal Mode.Consequently,the mechanism of Pb enrichment in the supermicron and submicron particles was proved to be controlled by the chemical adsorption and heterogeneous condensation,respectively.(4)The morphology,mineral composition,chemical composition,contents of heavy metals(Cd and Ti)and the occurrence forms of a series of samples(feed coal,bottom ash,fly ash and PM10,PM2.5,PM1.0)collected from the pulverized coal-fired power plant were analyzed.The results revealed that the kaolinite and pyrite in the feed coal could be converted into minerals like mullite,hematite,hercynite and magnetite in the combustion residues at high temperatures.Furthermore,a large amount of Ti-rich particles were recognized in PM2.5 and the formation mechanism was also discussed.Besides,abundant Cd on Ti-rich particles was identified,which was further interpreted that the retention of gaseous Cd(g)by Ti in fine particles was dominated by chemisorption at high temperatures and physical adsorption or condensation at lower temperatures.