Research For Carbonation Solidification Characteristics Of Municipal Solid Waste Incineration Fly Ash

With the improvement of populace living standards,the consumption of energy and resource have increased for many years.Thus,a large amount of municipal solid waste(MSW)was produced in daily life.The amount of national municipal solid waste transportation has reached 235 million tons in 2020,although it decreased for the first time.Thus,the more scientific and environmental-friendly treatment must be developed to deal with the huge amount MSW.The main methods are sanitary landfill and incineration,which both have own advantages.Moreover,the incineration can greatly reduce the mass and volume of MSW in a short time,and the heat during incineration can be used for power generation to utilize energy.Then,incineration has gradually become the significant method for harmless treatment of MSW.However,the incineration fly ash,which was captured after the flue gas purification equipment,is identified as hazardous waste,because of the heavy metals,organic contaminants and soluble chlorides.Thus,investigation on fly ash solidification is of significance and necessity.This study was supported by the National Key R&D Program,through the combination of experiment,characterization analysis and quantum chemistry simulation,the reaction for Ca Cl OH,the carbonation process of fly ash and the characteristics of fly ash after solidification were explored.Then the research for reaction mechanism can provide the theoretical evidence for fly ash solidification.First,the thermal characteristics of fly ash were investigated by using TG under different atmospheres,which could confirm the release behaviors during the heating process.And the TG and DTG curves in the atmospheres showed differently.The mass gain was appeared in air and CO2,showing that fly ash could capture CO2 and O2.The obvious mass loss occurred after700 ~oC,which was the decomposition temperature of the inorganic matter or the release temperature of volatile matter.The main gases were CO2 and H2O during heating,causing by the decomposition of carbonate and hydrocarbon cracking.And the Ca Cl OH still existed after the thermal treatment,revealing that the thermal stability is better than expected.Further,in order to understand Ca Cl OH better,using quantum chemistry method to analyse the formation process.As known,Ca Cl OH was produced during the HCl removal stage,which were the reactions between Ca O,Ca(OH)2 and HCl.Adsorption is the significant step of reaction,the adsorption process could be predicted using quantum chemistry method.It showed that the O atom of Ca O surface had strong adsorption ability.Combined the charge transfer,structure change,partial density of states analysis,it was confirmed that the adsorption processes were chemisorption.The electron density of O and H overlapped,and the H-Cl bond still interacted,thus,O,H and Cl formed the bond together to ensure the structure stability.Moreover,the multi HCl adsorption on the surface was conducted,the more HCl molecules can reflect the real situation.And the adsorption process was complicated,although it appeared the adsorption of O to HCl,the adsorption interacted each other.Though the transition state search,the reaction process is confirmed,showing that the chemical bond broke and reformed,such as H-Cl bond broke,Ca-O bond broke,and it could produce new structure.H2O was produced first,and Ca Cl OH had precedence over Ca Cl2 during the reaction,which was the reason for the Ca Cl OH in fly ash.Based on the adsorption analysis and activation energy,it is proposed that Ca Cl OH is the final product of the reaction.Furthermore,according to the TG curve of fly ash in CO2 atmosphere,it is proposed that fly ash can be solidified through gas-solid carbonation,thus,the test were conducted in the fixed-bed experimental system.Before the test,fly ash was calcined and washed,and the carbonation were conducted under 350-550 ~oC.The results reveals that,the mass of fly ash increased after carbonation,and the maximum mass gain were:92.8 mg/g for raw fly ash,104.3mg/g for calcined fly ash,90.5 mg/g for washed fly ash,respectively.The maximum CO2sequestration of washed fly ash was 208 mg of CO2/g of fly ash.Pretreatment changed the composition of fly ash,Ca Cl OH transformed into Ca(OH)2.The mineral phase content was calculated by QXRD,the content of Ca Cl OH decreased after calcination,pretreatment changed the phase content of fly ash.The main component of carbonated fly ash was Ca CO3,the content of Ca CO3 in carbonated washed fly ash reached 88%,which was higher than others.The content of amorphous phases in carbonated raw fly ash and carbonated calcined fly ash increased,probably causing by the agglomeration at high temperature and the high content of Cl.Pretreatments increased the content of heavy metals(some exceptions),thus the leaching concentration of heavy metals(except Pb)was also increased.Carbonation had an effective solidification on Cu,Pb,and Cr.Finally,the carbonation of fly ash was investigated by using quantum chemistry simulation.The results revealed that,it was difficult for Ca Cl OH to adsorb CO2,and the bond angle and bond length of CO2 changed after adsorption.CO2 gain electrons from the surface of Ca Cl OH,and made the electron negatively.CO2 existed with the adsorbed state,and the reaction on the surface was carried out with the following steps:CO2 combined with OH to produce HCO3,then CO3 was formed after dehydrogenation,H2O was released after the free H combined with OH.The formation of Ca CO3 was prior than Ca Cl2,and the structure change needed energy to overcome the activation energy,which also explain the carbonation should be achieved under heating.