Research On Migration And Transformation Mechanism Of Heavy Metals Lead,Selenium And Arsenic From Coal-fired Units Flue Gas

After the implementation of ultra-low emission of flue gas pollutants in coal-fired power stations,the emission control of heavy metals in flue gas has become an urgent problem.In this paper,lead,selenium and arsenic,three typical heavy metals produced in the process of coal burning,are taken as the research objects.The physical and chemical conversion experiments of gaseous heavy metals are carried out by tubular furnace to explore the conversion mechanism of gaseous heavy metals in the process of flue gas cooling.The main research work and conclusions are as follows:(1)The homogenous nucleation of Arsenic,selenium,lead under single atmosphere was studied.The results show that PbCl2,SeO2 and As2O3 homogenous condensed particles are all submicron particles.With the decrease of temperature,the particle size gradually increases,and the variation range is PbCl2,As2O3 and SeO2,respectively.The roundness of particles is affected by impurities in the filter cartridge,and the roundness is SeO2,PbCl2 and As2O3,respectively.According to the amount of condensation,the main homogenous condensation temperature ranges of PbCl2,SeO2 and As2O3 are below 500℃,100℃and 200℃,respectively.(2)Arsenic,selenium,lead three kinds of heavy metal was studied under the condition of single atmosphere in three kinds of different specific surface area and particle size of SiO2 particles on heterogeneous nucleation,the results showed that the heavy metals in heterogeneous condensation as the main way of condensation,with lower temperature,the proportion of homogeneous condensation increases gradually,the smaller the specific surface area of the particles,the phenomenon is more obvious;There is a direct relationship between the amount of heavy metal condensation on the particles and the specific surface area of the particles.The larger the specific surface area of the particles,the more heavy metals are enriched.SeO2,PbCl2 and As2O3 are the most abundant heavy metals in particulate matter.In the presence of particulate matter,the main condensation temperature points of PbCl2,SeO2 and As2O3 are 700-500℃,200℃ and 200℃,respectively,which indicates that PbCl2 has completed condensation at a higher temperature,while SeO2 and As2O3 with a lower melting boiling point are completely condensed at a lower temperature.(3)The chemical reactions of arsenic,selenium and lead with six different components of fly ash in complex flue gas were studied.The results show that some of the lead in fly ash exists in the form of PbCl2 and PbSO4.The PbSO4 comes from two aspects.Second,PbCl2 reacts with sulfur minerals in fly ash,and some lead may exist in the form of PbO.x Al2O3,because the content of lead in fly ash is positively correlated with the content of aluminum in fly ash.Most of the selenium in fly ash exists in the form of Se elemental substance and selenite,and the content of CaSO4 in fly ash after adsorption of selenium is higher.The reason is that SO2 reacts with SeO2 to form Se elemental substance and H2SO4,and H2SO4 reacts with CaO to form CaSO4 more easily.Arsenic in fly ash exists in the form of trivalent arsenic and pentavalent arsenic,and there is a positive correlation between the content of calcium in fly ash and the adsorption capacity of arsenic in fly ash.Arsenic reacts with CaO to form Ca3(AsO3)2 and Ca3(AsO4)2.