Experimental Study And Quantum Chemical Simulation On The Adsorption Of Heavy Metals And Alkali Metals In Coal Combustion By Mineral Additives

Mixing mineral additives in the furnace is an economical and effective coal-burning heavy metals emission reduction technology.Although there are many researches on mineral additives in the field of coal-burning heavy metal control,there is still a lack of systematic research in the selection,modification,performance testing of additives,the influence of al Fkali metals on them and related mechanisms.To this end,this article has carried out in-depth research on the above content.Use the tube furnace to carry out the dual-factor test of additive type and temperature to explore the optimal additive and temperature of reducing heavy metals Pb,Cd,Zn and Cr during coal combustion,and to detect the performance of the optimal additive through test of changing coal type.Meanwhile according to the ash morphology and XRD results to analyze and speculate related influence mechanisms.The results show that at 900?1300?,kaolin has better performance in reducing these four heavy metals(The retention rate can reach 60?68%,which is significantly higher than that of pure coal(34?53%)),followed by diatomaceous earth and palygorskite(About60%),illite and bauxite have good effects on Cd and Cr(About 55%and 69%respectively),zeolite and pyrophyllite is only effective for Zn and Cr respectively(More than 60%),and the effects of bentonite on the four heavy metals are relatively general(42?58%).Most minerals have a better effect of reducing heavy metals at 900?1100?.Kaolin can still effectively and stably reduce these heavy metals under the condition of changing coal type(The retention rate increase is 22?31%).Kaolin can delay coal ash coking and increase active components,thus promoting the fixation of heavy metals.Kaolin was modified by intercalation peeling,intercalation peeling combined with acid/alkali and metal salt impregnation methods,respectively,and the emission reduction effects of these modified kaolins on Pb,Cd,Zn and Cr were investigated through tube furnaces and settling furnaces experiments.In addition,relevant characterization(BET,XRD and SEM)and mechanism analysis were also carried out.The results show that under the condition of tube furnace,these modified kaolins have a better emission reduction effect at 900?1000?,among them,calcium acetate modified kaolin(Ca-Kaol)and hydroxyl aluminum modified kaolin(Al-Kaol)both have better modification effects on the four heavy metals(The fixation rate is 10?14%higher than that of raw kaolin),potassium acetate combined with acid modified kaolin(KH-Kaol)and potassium acetate combined with alkali modified kaolin(KNa-Kaol)have good effects on Pb,Cd and Zn(More than 10%higher than the original),while potassium acetate modified kaolin(KAc-Kaol)has a fair effect(The increase is 3?9%).Under the conditions of the settling furnace,whether kaolin was modified or not can reduce the emission of the above heavy metals,and except that KNa-Kaol has lower emission reduction effects on Pb and Ca-Kaol on Zn and Cr than raw kaolin,other modified kaolins have significantly better emission reduction effects on the four heavy metals than raw kaolin(Modification can increase the fixation rate of kaolin to Pb,Cd and Zn by up to about15%,and up to 9%for Cr).Modification can enrich the pore structure of kaolin and increase its adsorption sites and active components,thereby strengthening the adsorption reaction between additives and heavy metals.The adsorption characteristics of kaolin before and after modification on Pb and influence of alkali metal were investigated through the adsorption experiment platform at 900?.The results show that the modification can improve the high temperature adsorption performance of kaolin for Pb.The adsorption capacity and rate of raw kaolin for Pb are about 151mg/g and 2.12?g/(s?m²).Modified kaolin can reach more than 200 mg/g,and the adsorption rate is also close to 3.6?g/(s?m²).The increase of alkali metal will cause the Pb adsorption capacity and adsorption rate of kaolin to decrease continuously.Quantum chemistry tools were used to study the mechanism of action of heavy metals Pb(Pb O,Pb Cl₂)and alkali metals(Na Cl,KCl)on the surface of kaolin.The results show that the adsorption reaction strength of alkali metals on the surface of kaolin is greater than that of heavy metal Pb.The energy of each system will decrease with the occurrence of adsorption behavior,and there are electron transfer and bonding effects in the adsorption process.Regardless of whether the surface of kaolin is loaded with alkali metals,Pb O is more likely to undergo adsorption reaction than Pb Cl₂(For example,the adsorption energy of Pb O before loading is 230 k J/mol more than that of Pb Cl₂,and the adsorption energy of Pb O after loading Na Cl/KCl is 172.06 k J/mol and 134.31 k J/mol more than Pb Cl₂,respectively;The energy required for the reaction of Pb O with the adsorption surface before(after)loading on the surface is 0.68e V lower than that of Pb Cl₂).In addition,whether it is Pb O or Pb Cl₂,the surface of clean kaolin is more likely to undergo adsorption reaction than the surface of kaolin loaded with alkali metals(For example,the adsorption energy of Pb O and Pb Cl₂before loading is about 100k J/mol and 30k J/mol more than after loading;The energy required for the reaction of heavy metal Pb with the adsorption surface before loading Na Cl/KCl is about0.22e V and 0.48e V lower than after loading,respectively).In the presence of alkali metals,the adsorption stability of heavy metal Pb on the surface of kaolin becomes poor.Therefore,promoting the transformation of coal-burning heavy metals to oxidation state and reducing the alkali metal content in coal have a positive effect on the control of heavy metal emissions.