Study On The Enrichment Characteristics Of Heavy Metal Elements In Fine Particles Of Coal-fired Fly Ash

In China,energy consumption structure dominated by coal would not be changed in the short term.The air pollution caused by coal burning brings huge challenges to the ecological environment,in which the heavy metal elements and fine fly ash from coal-fired power plant have caused great concern.Lots of the field emission test data showed that heavy metal elements such as Pb,Ni,Cr,Cd are readily to be enriched in fly ash particles,especially in fine particles.It is difficult for the existing dust removal equipment to capture the micron-scale fly ash particles effectively(such as PM₁₀,PM_2.5,etc).These ultra-fine particles are usually present as aerosols in the atmosphere,which do great potential harm to biological and natural environment.Therefore,the ultra-fine particles are either carrier of heavy metal elements or main resources of atmosphere haze.The enrichment and morphology of heavy metal elements in ultrafine particles are not understood deeply owing to the limitation of sampling and detection.Exploring the enrichment degree and occurrence forms of heavy metal elements in different size particles,as well as the effect of physical and chemical properties on the enrichment behavior of heavy metal elements can provide a fundamental base for studying the adsorption behavior of heavy metal elements on fine particles and promoting the synergistic removal of both of them.Based on this,the enrichment mechanism and occurrence forms of heavy metal elements in fly ash particulates with different particle sizes were studied experimentally and theoretically in this paper.Firstly,the concentration of heavy metal elements in raw coal and solid-phase coal combustion products（fly ash,bottom ash,desulfurized gypsum）from two different coal-fired power plants was measured.The enrichment coefficient relative to raw coal was obtained to study enrichment degree of seven heavy metal elements（Cr,Mn,Ni,As,Mo,Cd,Pb）in each solid-phase coal combustion product.In addition,the enrichment coefficient relative to fly ash was introduced to measure the heavy metal element enrichment degree in different particle size ranges.The results show that the heavy metal elements are mainly distributed in bottom ash and fly ash during coal combustion.For the 1#boiler,except Mn,the other six heavy metal elements are more easily enriched in the fly ash.For the 2#boiler,Mn is mainly distributed in fly ash and bottom ash,As and Mo are mainly enriched in fly ash,while Cr,Ni,Cd,Pb are relatively evenly distributed between bottom ash and fly ash.On the whole,the seven heavy metal elements Cr,Mn,Ni,As,Mo,Cd,and Pb in the fly ash from two different power plants display an enrichment trend in fine particles.The above seven heavy metal elements are highly enriched in PM_2.5-10and PM_10-50,namely,they have significant enrichment behavior in the particle size range of 2.5～50μm.Secondly,the Characterizations of BET,XRD,XRF and TG-DCS were conducted on the classified fly ash particle.The SPSS data processing software was used to explore the relative relationship among the concentration of heavy metal elements,surface microstructure and internal chemical composition of fly ash with different particle sizes.The result shows that the enrichment degree of Cr,Mn,Ni,Cd and Pb in 1#fly ash is higher than that in 2#fly ash,and the enrichment degree of Mo and As is lower than that of 2#fly ash.There is a negative correlation between the particle size and the concentration of heavy metal elements,and the correlation coefficient satisfies Mo>As>Ni>Cr>Cd>Pb>Mn.The contents of Cr,Mn,Ni,Cd,and Pb in fine particles are highly correlated,and the gasification characteristics and source pathways of these five heavy metal elements are relatively similar.The change of particle size has no obvious effect on proportion of inorganic components,but has a certain effect on UBC.The UBC of 1#fly ash is positively correlated with particle size,while the UBC of 2#fly ash is negatively correlated with particle size.The maximum absolute value of Si O₂content indicates the greatest contribution to the chemical composition index.The typical variable reflecting chemical composition index is dominated by Si O₂content.While the typical variable of physical characteristics index is mainly determined by average pore size.The UBC content is positively correlated with typical variable of physical characteristics index,which indicates that the UBC content dominates the development degree of surface pore structure.The contribution of chemical components in fly ash to the well-developed surface pore structure is in descending order:UBC>Fe₂O₃>Al₂O₃>Ca O>Si O₂.Then,the sequential extraction experiments were carried out on the classified fly ash to study the speciation difference of heavy meatal elements,and to investigate the correlation between each occurrence form and particle size.The experimental results show that the speciation contents of Cd,Ni,Pb and Mn in1#fly ash are higher than those in 2#fly ash in terms of the correlation between combustion mode and occurrence form.From the perspective of the correlation between particle size and occurrence form,except for Mn,the speciation contents of the other six elements are negatively correlated with particle size.With the decrease of particle size,the environmental stability of Cd,Cr,Ni,Pb and As deteriorates.Finally,the adsorption performance of main chemical components（Si O₂,Al₂O₃,Fe₂O₃,Ca O and AC）in fly ash on gaseous Pb Cl₂and Cd Cl₂was investigated at different temperatures and various atmospheres through fixed-bed adsorption experiments.The experimental results show that the unit adsorption capacity of the five substances for gaseous Pb Cl₂in N₂atmosphere declines as follows:Al₂O₃>Fe₂O₃>Ca O>AC>Si O₂.Al₂O₃shows the best adsorption performance at 800℃,and the increase of temperature is beneficial to the adsorption performance.The adsorption capacity of Fe₂O₃to Pb Cl₂decreases with the increase of temperature,and the optimum reaction temperature is 700℃.The unit adsorption capacity of the five substances for gaseous Cd Cl₂descends on order of Ca O>Al₂O₃>Fe₂O₃>AC>Si O₂.The excellent adsorption performance of Ca O occurs at 700℃,and high temperature is not conducive to its adsorption performance.The optimum adsorption temperature range of Al₂O₃is between 700℃and800℃.Compared with pure N₂atmosphere,the addition of O₂,SO₂and HCl greatly reduces the unit adsorption capacity of Pb Cl₂and Cd Cl₂by Fe₂O₃,Al₂O₃and Ca O.