Study On Mercury Removal From Coal Combustion Flue Gas Over Sorbents Derived From Cl/Br-containing Wastes

In China,coal-fired power plants are deemed as one of the largest anthropogenic source of mercury emission.The difficulty in controlling mercury emission from coal combustion flue gas is how to control elemental mercury?Hg⁰?emission.To date,activated carbon injection?ACI?upstream of dust removal device is regarded as the most promising technology for mercury removal.However,virgin activated carbon shows poor Hg⁰removal performance and a high C/Hg ratio is required in the ACI technology.The universal method for improving the Hg⁰ removal performance is modifying activated carbon with Cl/Br.But the modification procedure is complex and requires expensive chemical reagents.This would further hinder the development and commercialization of ACI technology.To solve this problem,a novel method using one-step pyrolysis of municipal solid wastes?wood,PVC,and brominated flame retarded plastic?was proposed to prepare modified biochar for Hg⁰ removal from coal combustion flue gas.In addition,biochar is a low-cost carbon material with similar properties as activated carbon.Thus,this method can reduce the cost of mercury sorbent from three aspects:active components,carbon-based material,and manufacturing process.The feasibility of using the method—co-pyrolysis of wood and PVC plastic to prepare chlorinated biochar?Cl-Biochar?for efficient removal of mercury—was investigated.During co-pyrolysis process,the addition of PVC blocked the development of pore structure of Cl-Biochar and thus the specific surface area and pore volume were decreased.The Cl emitted from PVC was fixed into the biochar in the form of KCl and C-Cl group,which is conducive to reducing the release of Cl during PVC pyrolysis.Cl-Biochar exhibited far better Hg⁰ removal performance compared to virgin biochar.The optimum pyrolysis temperature and PVC/wood mass ratio were respectively 600°C and 1:3.The Hg⁰removal over Cl-Biochar was mainly controlled by chemisorption,where C-Cl group was the active component for oxidizing Hg⁰ into HgCl₂.The mechanism of Hg⁰ adsorption over Cl-Biochar could be described as follows.Hg⁰ was first adsorbed on the surface of Cl-Biochar via physisorption,and then was oxidized by C-Cl group.The feasibility of using copyrolysis of wood and brominated flame retarded?BFR?plastic to prepare sorbents for Hg⁰ removal from coal combustion flue gas was investigated.An interaction effect between BFR plastic and wood occurred during co-pyrolysis process.The addition of BFR plastic would deteriorate the pore structure of Br-Biochar because the melted plastic residues were aggregated on the surface.But Br emitted from BFR plastic was fixed into the Br-Biochar in the form of Br^–and C-Br group.In addition,the specific surface area of Br-Biochar decreased with the increase of PVC ratio.Br-Biochar exhibited far better Hg⁰ removal performance compared to virgin biochar.The optimum pyrolysis temperature and plastic/biomass mass ratio were respectively 600°C and 1:1.The Hg⁰removal efficiency decreased with the increase of gas hourly space velocity.O₂,NO,HCl and SO₂ had positive effects on Hg⁰ removal whereas H₂O slightly inhibited Hg⁰ removal.For virgin biochar,C=O served as electron acceptor and facilitated Hg⁰ oxidization during Hg⁰ removal process.But for Br-Biochar,C-Br group provided active sites for Hg⁰adsorption due to its higher activity than C=O group.To solve the following problems:?i?the adverse effect of plastic on the pore structure of biochar;?ii?the adverse effect of carbon-based sorbent on fly ash quality,magnetic biochar with high specific surface area were developed.Both excellent magnetism and improved specific surface area were acquired under Fe catalysis,and thus the negative effect caused by PVC melting was minimized.Magnetic biochar exhibited a typical superparamagnetic feature because magnetic Fe₃O₄ was introduced into the biochar.The magnetic biochar showed far better Hg⁰ removal performance compared to non-magnetic biochar over a broad reaction temperature range.The optimum pyrolysis temperature and Fe?NO₃?₃-solution concentration were 700°C and 0.05 wt.%,respectively.O₂,HCl and NO promoted Hg⁰ removal whereas SO₂ had little effect on Hg⁰ removal.H₂O slightly suppressed Hg⁰ removal.Compared to the commercial activated carbon manufactured specifically for Hg⁰ removal,magnetic biochar was superior in both Hg⁰ adsorption capacity and adsorption rate.In addition,the Hg⁰ removal performance of brominated biochar was higher than that of chlorinated biochar.The mechanism of Hg⁰ removal over magnetic chlorinated biochar was chemisorption reaction,where Fe₃O₄,C-Cl and C=O provided active sites for Hg⁰ removal.The stability of Hg/Cl/Br in waste-derived sorbents was investigated using leaching test procedures,and the results were compared with that of activated carbon impregnated with Cl/Br and commercial activated carbon.The stability of Hg in Br-containing sorbents was higher than that of Cl-containing sorbents.The stability of Hg in waste-derived sorbents was higher than that of chemically impregnated sorbents,and the addition of iron during co-pyrolysis could further improve the stability of Hg in sorbents.The leaching rate of Hg increased with the increase of leaching time and liquid-solid ratio,but first decreased and then increased with the increase of leachate pH.The amount of Hg leaching in column leaching experiment was much higher than that in batch leaching experiment.Similarly,the stability of Cl/Br in waste-derived sorbents was higher than that of chemically impregnated sorbents,and the addition of iron during co-pyrolysis could further improve the stability of Cl/Br in sorbents.The leaching rate of Cl/Br increased with the increase of leaching time and liquid-solid ratio.The most extreme acidic leaching condition could promote the leaching of Cl/Br in sorbents.To some extent,water washing pretreatment could reduce Cl/Br pollution during the landfill process of sorbents,especially for magnetic brominated sorbents.The crystal structure and surface functional groups of waste-derived sorbents did not obviously change after leaching experiment.More X^–was removed than C-X bond in the leaching tests?X=Cl/Br?.