Effect Of Mineral Composition On Lead Volatilization Characteristics During Coal Combustion

Coal combustion is accompanied by the emission of a large number of harmful heavy metals.With the increase of pollutant emission standards of coal-fired power plants,it is imperative to develop heavy metal emission control means for coal-fired power plants.Coal-fired power plants are one of the main sources of atmospheric lead.For the semi-volatile heavy metal lead,the mineral composition of coal is an important factor affecting the migration and volatilization of lead.However,the effect of minerals on lead migration under high temperature environment and the preparation method of targeted lead removal adsorbents need to be further studied.In regards to the above problems,this paper based on the tubular furnace system explores the occurrence mode of lead during coal combustion,as well as the capture effect of mineral composition on lead and adsorption mechanism,and according to the enrichment characteristics of lead to prepare efficient lead removal adsorbent.Three typical coals,Gongyi anthracite,Datong bitumite,and Zhaotong lignite,were used to investigate the release of lead during the combustion process of coals of different coal rank and mineral composition.Lower rank coals have a higher proportion of organically bound lead,which is more inclined to release at the low combustion temperature.The predominant form of lead in coal is lead in the sulfide bound state,which is converted to gaseous lead oxide at800～1000℃.During the combustion of anthracite,bituminous coal,and lignite at 700～1200℃,the addition of mineral components（SiO₂,Al₂O₃,Ca O,and Fe₂O₃）reduces the volatility of lead,and the capture capacity of the four mineral components for lead is:SiO₂<Al₂O₃<Ca O<Fe₂O₃.The capture efficiency of the minerals increases with temperature and then decreases gradually.Fe₂O₃ almost completely loses its capture capacity at 1000℃.The combustion process of coal is accompanied by the decomposition and transformation of the inherent minerals in the coal,which have a certain capture effect on the lead released from the combustion of coal.When pulverized coal is mixed with lead oxide powder,it is found that the low volatility of lead in anthracite up to 1100℃is due to the adsorption of lead oxide by the intrinsic mineral mullite(Al₆Si₂O₁₃),the product of which is lead silicate（Pb（Al₂Si₂O₈））,while lignite captured lead by both chemisorption and physical adsorption,whereas the intrinsic minerals in bituminous coal are mainly hematite,which has almost no effect on lead retention at high temperatures.The inherent minerals in bituminous coal are dominated by hematite,which has almost no effect on lead retention at high temperatures.On this basis,to investigate the mechanism of the adsorption reaction of mineral components on PbO during coal combustion,the adsorption mechanisms of SiO₂,Al₂O₃,Ca O,and Fe₂O₃ on PbO were investigated by differential scanning calorimetry.SiO₂,Al₂O_3,and Ca O all increase the initial weight loss temperature of PbO,while Fe₂O₃ causes the early volatilization of PbO.SiO₂,Ca O,and Fe₂O₃ immobilize PbO by adsorption through chemical reactions,with the main reaction products being Pb Si O₃,Ca₂PbO_4,and Pb Fe₁₂O₁₉,while Al₂O₃undergoes stronger physical adsorption to immobilize PbO.SiO₂ and Al₂O₃ have the highest efficiency when acting alone to adsorb PbO,while the simultaneous involvement of both in the reaction significantly increases the capture efficiency.The adsorption of PbO on mineral surfaces was investigated using density functional theory.The adsorption of SiO₂,Al₂O_3,and Fe₂O₃ occurs mainly on metal atoms,with the adsorption energy of metal atoms for PbO being higher than that of oxygen atoms for PbO,with the order of adsorption energy being:SiO₂>Fe₂O₃>Al₂O₃,while the adsorption characteristics of Ca O differ from those of the other three minerals,with oxygen atoms for The adsorption energy of oxygen atoms on PbO is greater than that of calcium atoms on PbO.The effect of both chlorinated and sulfurous atmospheres on the adsorption of PbO was discussed using thermodynamic equilibrium calculations,with both atmospheres having a hindering effect on adsorption,with chlorinated PbO being chlorinated to Pb Cl₂ and volatilized in the gaseous state at 500℃.The chlorinated atmosphere causes the four minerals to lose their ability to adsorb PbO.The sulfurous atmosphere affects the adsorption of the four adsorbents at low temperatures,but as Pb SO₄ is not volatile,it does not change the overall pattern of PbO adsorption by the minerals,except for Ca O which is unable to capture PbO due to the competing adsorption effect of SO₂.As the simultaneous action of silica and aluminum maximizes the immobilization of PbO,SiO₂ was chosen as the carrier,and aluminum nitrate Al（NO₃）₃ and aluminum acetate C₉H₉Al O₆as precursors for the preparation of targeted PbO adsorbents.At 900℃and 1000℃,the capture efficiency of kaolinite was better than that of Al（NO₃）₃-SiO₂ and C₉H₉Al O₆-SiO₂,while at1200℃,the capture efficiency of kaolinite was not as good as that of Al（NO₃）₃-SiO₂ and C₉H₉Al O₆-SiO₂.The specific surface area,pore volume,and pore size of kaolinite all decrease and change from a lamellar to a spherical structure,with the disappearance of fine pores.After calcination the Al（NO₃）₃-SiO₂ surface decomposes into small alumina particles,significantly increasing the number of surface pores,while the C₉H₉Al O₆-SiO₂ surface develops a large number of new pores from the original dense and flat surface,with a larger average pore volume.Using quantum chemistry to model and calculate the Si-Al adsorbent,the results show that the Si top sites,O top sites,and vacant sites on the surface of the silica-aluminum adsorbent all have extremely high adsorption energies for PbO molecules,with adsorption energies significantly higher than those for PbO adsorption by SiO₂ and Al₂O₃alone.