Experimental And Dft Study On Enrichment Of Heavy Metals By Mineral Additives During Coal Combustion At High Temperature

Coal plays a major role in the energy structure of China.At the same time,the use of coal has also brought huge environmental problems.In addition to common pollutants,heavy metals have also received increasing attention.In the field of heavy metal pollution control during coal conbustion,solid additive adsorption technology has attracted people’s attention because of its low cost,high efficiency,and easy operation.In this paper,a series of experiments and theoretical studies on solid additive adsorption technology are carried out in order to obtain mineral additives with excellent properties and clarify the adsorption mechanism of additives on heavy metals,so as to provide experimental and theoretical basis for heavy metal emission control during coal conbustion.Experiments were performed in a high-temperature tube furnace to study the effects of different additives,addition ratios,and combustion temperatures on the retention of Pb,Zn,Cr,and Cd.Studies have shown that as the temperature rises,the retention rates of Pb,Zn,and Cd decrease,while the retention rates of Cr increase first and then decrease.The best enrichment temperature range of kaolin,attapulgite,montmorillonite and limestone for heavy metals is 900～1100℃.When the temperature reaches 1200℃,the performance of all additives decreases significantly.With the increase of the addition ratio,the retention rates of Pb,Cr,Cd,and Zn all increased,but the increase range was different.Among these four mineral additives,kaolin has the best comprehensive performance and limestone has the worst.Kaolin and montmorillonite were modified by intercalation-exfoliation method,and co-combustion experiments and adsorption tests were performed.The XRD,FTIR,SEM,and BET characterization results show that the layer spacing of kaolin and montmorillonite was increased after modification and more active surface area could be exposed.Combustion experiments show that,with the exception of a few temperature points,the retention of intercalation-exfoliation modified kaolin on four heavy metals has been improved.Modified montmorillonite also improved the retention of Pb,Zn and Cr,but the effect on Cd was not ideal.Among the three modified kaolins,potassium-acetat（KAc）-kaolin has the best comprehensive performance.As for the two modified montmorillonites,the comprehensive performance of cetyltrimethylammonium bromide（CTAB）-montmorillonite is better than sodium dodecylbenzenesulfonate（SDBS）-montmorillonite.Adsorption tests showed that the adsorption of Pb by kaolin and montmorillonite after intercalation-exfoliation modification had a significant increase,and KAc-kaolin and CTAB-montmorillonite showed the largest increase.In addition,the change in the adsorption of Pb by kaolin and montmorillonite is consistent with the change in the retention rate during the combustion experiment,indicating that the intercalation-exfoliation modification can increase the active area of these two minerals,increase the active adsorption sites,and thereby increase their adsorption activity for heavy metals.The thermodynamic equilibrium calculation software was used to study the main occurrence forms of heavy metal Pb during coal combustion,and based on the thermodynamic equilibrium calculations,DFT calculation software was used to study the adsorption of PbO and PbCl₂ molecules on kaolin and montmorillonite surfaces.Thermodynamic calculations show that the main forms of Pb during combustion of pure coal are PbCl_2（g）,solid PbO and gaseous PbO_（g）.After adding 5%kaolin or montmorillonite,the main morphology of Pb has not changed.It is still PbCl_2（g）,solid PbO,and gaseous PbO_（g）,but the proportion of solid PbO and Pb Si O₃ will increase.The DFT studies have shown that the Al（001）-surface of kaolin has good adsorption for PbO and PbCl₂ molecules,and the stability of adsorption to PbO is greater than that of PbCl₂.The Si（None-001）surface of montmorillonite has strong adsorption ability for PbO molecules,but has no adsorption ability for PbCl₂ molecules,and even repulsion occurs.For the Si（Na-001）surface of montmorillonite,due to the presence of Na atoms,the Si（Na-001）surface cannot form bonds with PbO molecules,and the attraction of Na atoms to Cl atoms is not enough to promote stable adsorption of PbCl₂ molecules on the Si（Na-001）surface.