Study On The Migration And Transformation Characteristics Of As And Se In Sludge Incineration

The release of toxic trace elements during sludge incineration poses a huge threat to the environment and human health.Studying the factors affecting its release and migration is an effective way to control its emissions.In this thesis,municipal sludge from a large sewage treatment plant in Jinan was used as the raw material.The study focuses on the speciation of As and Se in sludge,and the effect of reaction temperature and atmosphere on the release during pyrolysis and incineration.The adsorption characteristics of main minerals in sludge to As and Se are calculated through thermodynamic equilibrium calculations and experimental studies of sedimentation furnaces and vertical furnaces,which provides a theoretical basis for the control of As and Se emissions during actual sludge incinerationThe modified BCR method was used to study the speciation of As and Se in the sludge.The study found that the As form in the sludge was more uniformly distributed,while Se mainly existed in the organic state.During sludge incineration,there are two main weightless peaks of As and Se.One is 200?500 ?.As the organic matter in the sludge is decomposed,As and Se are gradually released into the flue gas.The second weightless peak is 600?1000?.As the inorganic substances and sulfides in the sludge are decomposed,the releasable As and Se in the sludge are almost completely volatile About 75%of As and 90%of Se are released into the flue gas.The atmosphere of sludge pyrolysis and incineration has a significant effect on the release.The study found that the increase of CO2 content has a certain inhibitory effect on the release of As and Se,while the increase of O2 content significantly promotes its release.Water vapor can promote the decomposition of CaCO3 into CaO with a good pore structure inhibits the release of As and Se.The calculation of thermodynamic equilibrium shows that As and Se mainly exist in the form of AsO2(g),As4O6(g),and SeO2(g)during the sludge incineration.When CaO,CaCO3,MgO,and Fe2O3 are added,As and Se in the flue gas react to form arsenate and selenate.Al2O3 can react with As to form thermally stable AIAsO4,but does not react with Se.It mainly inhibits the release of Se by physical adsorption.By adding different minerals to the sludge,it was found that CaO has the best adsorption effect on As,Fe2O3 has the best adsorption effect on Se.Water vapor has a significant inhibitory effect on the adsorption capacity of alkali metal oxides and it can promote the adsorption of As and Se by CaCO3.The mechanism study found that the CaO and MgO samples gained weight after adding water vapor,indicating that the addition of water vapor can react with it,which significantly reduced the adsorption capacity of As and Se.Water vapor mainly reduces its adsorption capacity by occupying the active sites of Fe2O3.Appropriate concentration of water vapor can improve the adsorption capacity of CaCO3,while excessively high water vapor concentration will inhibit the effect.Temperature has a significant effect on As and Se adsorption capacity of different minerals.The increase of temperature can not only improve the reactivity of the adsorbent,but also cause the adsorbent to sinter to a certain extent.When the temperature is higher than 900?,the sintering effect gradually becomes obvious,which reduces its adsorption capacity.The CO2 produced by the decomposition of CaCO3 during high-temperature decomposition can cause the CaO generated outside the particles to form a pore structure and improve its adsorption capacity.After CaCO3 and Fe2O3 are mixed,they can react to form Ca2Fe2O5 at high temperature.The formation of Ca2Fe2O5 can improve the adsorption capacity of the composite adsorbent.When the ratio of CaCO3 to Fe2O3 is 3:1,the adsorption effect of the composite adsorbent reaches the best.