Preparation Of High Sulfur Coal-based Sulfur Self-doped Porous Carbon Sorbent And Its Mercury Removal Performance

Mercury is a trace pollutant,which has attracted extensive attention worldwide due to its neurotoxicity,long-range mobility,and bioaccumulation.At present,coal-fired power plants are the main anthropogenic source of mercury emissions.The relatively mature mercury pollution control technology at this stage is activated carbon injection technology（ACI）.However,due to the limited mercury removal capacity of activated carbon and the high cost,the development of efficient and cheap sorbents has great practical significance for mercury pollution control.Sulfur-loaded activated carbon is currently one of the most promising sorbents for mercury removal.The sorbent was usually obtained by sulfurizing activated carbon with elemental sulfur,H2S or SO2,or by introducing metal sulfides.High-sulfur coal is limited in its use as fuel because of its high sulfur content,especially high organic sulfur coal.Based on this,this thesis prepares the sulfur-rich porous carbon mercury removal sorbent through activation of high-sulfur coal and metal doping.The effects of activation conditions on the physical and chemical structure of the sorbent and its mercury removal performance are investigated;the relationships between the sulfur form as well as minerals in coal and the mercury removal performance of the sorbent are explored;the effect of type and amount of metal on conversion of sulfur form is discussed.At the same time,the mercury removal mechanism of the sorbents is revealed.The main conclusions are as follows:（1）Using high inorganic sulfur coal ZY coal as raw material and KOH as activator,the sorbent for removing mercury was prepared.The study showed that the amount of KOH and the activation temperature affect the mercury removal performance of the sorbent mainly by changing the pore structure and sulfur content of the sorbent.Under the preparation conditions of KOH/coal of 2:4（mass ratio）and activation temperature of 800°C,the sulfur-rich carbon-based sorbent made from high-inorganic sulfur coal has the best mercury removal performance,and this sorbent is named as ZY-2:4-800.The results of analysis indicated that Fe S2 in the high organic sulfur coal is the precursor of the active sites for mercury removal,while organic sulfur and other minerals have little contribution to the mercury removal performance.In addition,the effect of mercury removal experimentals conditions on the mercury removal performance of the sorbent was also investigated.O2 can promote the removal of Hg⁰;the sorbent has good tolerance to H2O and SO2;in the atmosphere of N2+O2+NO+SO2+H2O,the removal rate of Hg⁰of ZY-2:4-800 is above 95%within 2 h;The sulfur form(S⁰,S2^2-and S^2-)of the sorbent are the main active sites for mercury removal;the sorbents prepared from other high-inorganic sulfur coals（such as ZZ coal and DEP coal）under the process conditions also exhibit similar mercury removal properties to sorbent from ZY coal.（2）Based on the optimal preparation conditions of the high inorganic sulfur coal-based sulfur self-doping porous carbon sorbent,the high organic sulfur coal was used as the raw material to prepare the mercury removal sorbent,but iron oxide is need to be added to transform organic sulfur into Fe Sx before activation.The effect of iron oxide doping content on pyrolysis/carbonization samples and sorbents was investigated.The experimental results indicated that part of the organic sulfur of the high organic coal was converted into inorganic sulfur（Fe S or Fe Sx）by doping with iron oxide to obtain the active site precursor material for mercury sorbent,and with the increase of iron oxide,the content of inorganic sulfur in the sample gradually increased.Iron doping helps to activate the pyrolysis/carbonization sample,so the specific surface area of the sample prepared by doping with iron oxide is greater than the sample prepared by high organic sulfur coal alone.However,too much iron will increase the loss of sulfur and weaken the ability of mercury removal of the sorbent.When m（Fe）:m（S）=3:4,the sorbent has the highest sulfur content and the best mercury removal activity.Doping iron oxide bridge the conversion of organic sulfur to inorganic sulfur.（3）Furthermore,different metals were used to transform the organic sulfur to inorganic sulfur during preparation of mercury removal dorbents from high organic sulfur coal,the effects of the type of doping metal on the conversion situation of organic sulfur and the mercury removal performance of the sorbent were investigated.In the experiment,transition metals Mn,Fe,Co,Ni,Cu,Zn,and Mo,which are non-precious metals and have excellent redox ability,were selected as doping metals.The results showed that the doped metal could convert part of the organic sulfur into the corresponding metal sulfide during the pyrolysis/carbonization process,and metal doping was beneficial to the formation of pore structure in the activation process.However,the stronger the promotion of the metal was,the lower the sulfur content of the sorbent was.The doped metal not only affected the sulfur content,but also affected the sulfur form on the surface of the sorbent.The results showed that the mercury removal performance of the sorbent decreases with the increase of the number of electrons in the 4s orbit and 3d orbit of the doped metal,and copper doped sorbent has the best mercury removal activity.On this basis,the influence mechanism of reaction temperature,gas hourly space velocity and atmosphere on Hg⁰ removal performance of copper doped sorbent（LF+Cu-A）was systematically investigated.It was found that the Hg⁰ removal performance of the sorbent was negligibly affected by temperature within 60-120°C.O2 and H2O had an inhibitory effect on the Hg⁰ removal performance of the sorbent,and LF+Cu-A had strong resistance to SO2.The inhibition effect of O2 and H2O could be greatly weakened in the presence of SO2 or NO.In addition,the Raman,HG-TPDD and XPS characterization techniques were used to reveal the Hg⁰ removal mechanism of LF+Cu-A.The adsorption kinetics study showed that the external mass transfer process was the controlling step of the Hg⁰ adsorption over the sorbent.The Hg⁰adsorbed was finally converted into Hg S that was retained in the used sorbent.（4）With the high organic sulfur coal as raw material,the organic sulfur can be transformed into inorganic sulfur by metal doping.The activation method and the order of activation and metal doping may have great influence on the Hg⁰ removal performance of sorbents.The experimental results show that the sulfur content of porous carbon prepared by steam activation is much higher than that prepared by KOH activation.The oder of steam activation and metal doping have little influence on the sulfur content of the sorbents prepared,but the specific surface area of the sorbent prepared by metal doping first is much larger than that prepared by steam activation first.But,when the steam reacts with the sample surface to form pore structure,the surface of the sorbents becomes passivated due to the sulfur on the surface reacts with the reducing gas in the atmosphere and migrates into the gas phase in the form of H2S,which leads to the relatively poor Hg⁰ removal performance of the sorbents prepared by metal doping first.The sulfur and metal related sites can be formed again on the surface of the sorbents by steam activation first to enrich the active sites of Hg⁰ removal,so that the sorbents show the better Hg⁰ removal performance.