Mechanism Study On The Removal Of Elemental Mercury Based On Cobalt-Ceria Bimetallic Adsorbent From Syngas

Coal gasification technology is the core technology for clean and efficient use of coal.Recently,it has received extensive attention because of the characteristics of high efficiency,low operation and maintenance cost,and wide adaptability of raw materials.However,compared with coal-fired flue gas,mercury in the syngas is mainly released in the form of elemental mercury,and the syngas is a reducing atmosphere which makes it difficult to remove mercury.Moreover,the current mercury removal efficiency of traditional adsorbents in the syngas is low,and there are still some limitations in adsorption temperature and regeneration performance.The mechanism of mercury removal is not clear,so the mechanism of mercury removal from syngas is carried out which is of great significance for the development of high-efficiency adsorbents.On the basis of the help of oxygen in the reducing atmosphere,the mercury removal efficiency of the adsorbent can be greatly improved.It is proposed to remove the mercury by using the adsorbent with strong oxidation effect and strong oxygen storage capacity.A series of cobalt-bismuth bimetallic adsorbents with different ratios were prepared by coprecipitation method.The characteristics of Hg⁰ removal efficiency in simulated syngas were carried out on a fixed bed test bed under low temperature（80-240°C）,combined with BET,XRD,XPS,SEM and other characterization methods were used to characterize and analyze the adsorbents before and after the reaction,and combined with the first-principles software to further reveal the adsorption mechanism of mercury on the cobalt-ceria bimetallic adsorbent.Firstly,different ratios of adsorbents were prepared by co-precipitation method,and then placed in simulated syngas at different temperatures for mercury removal experiments.The experimental results showed that the mercury removal efficiency of Ce_xCo_yTi adsorbents was higher than that of TiO₂ in the research temperature scope.When x=0.2,y=0.1,the mercury removal efficiency could reach 92%at 120°C.BET characterization showed that pore structure was not the main factor affecting mercury adsorption.XRD and XPS characterization showed that there was synergy between cobalt and ceria.The presence of ceria could make cobalt exist in higher valence state,while high valence state of Co is beneficial to mercury removal.Then the effects of main components of gas（H₂,CO,H₂S,HCl,NH₃）on the removal of mercury from cobalt-bismuth bimetallics were studied.The results showed that H₂ hindered the adsorption of mercury by reducing some of Ce⁴⁺and Co³⁺and consumed part of the active sulfur.The disproportionation of CO would block the pore size of the adsorbent and hinder the removal of mercury.HCl promoted the adsorption of mercury,but it inhibited the adsorption of mercury under the condition of coexistence with H₂S.NH₃ consumed part of the active oxygen and could react with H₂S,which reduced the surface active sulfur content of the adsorbent and inhibited the adsorption of mercury.The adsorption mechanism of mercury in the presence of H₂S followed the Langmuir-Hinshelwood mechanism.At the same time,it was found that the adsorbent exhibited the best adsorption performance when the calcination temperature was 400°C.The adsorption efficiency of the adsorbent remained above 60%after repeated regeneration.Cobalt-ceria bimetallic adsorbent has good regeneration performance.Finally,the Co-CeO₂（111）surface of the adsorbent model was constructed by using the first-principles software VASP.The adsorption of Hg and H₂S on the surface was simulated,and the reaction energy barrier was searched.Results showed that Hg was optimally adsorbed at the Ce Top site,and the optimal adsorption energy was-0.9eV.H₂S had molecular adsorption,partial mechanism adsorption and complete dissociation adsorption.When H₂S was completely dissociated,the adsorption energy was the smallest;at the same time,the HgS surface also had dissociative adsorption and molecular adsorption.When the HgS molecule was parallel to the surface of the adsorbent,the adsorbent energy was minimal.By modeling the reaction product and the product model,the energy barrier of the transition state was only 0.214 eV.