Mechanism Study On Mercury Removal By Co-based Sorbents From Simulated Syngas

Due to its toxicity,persistence,high volatility and bioaccumulation,mercury is seriously harmful to the ecological environment and human health.Therefore,the prevention and control of atmospheric mercury pollution has become crucial.The coal industry is one of the major sources of atmospheric mercury emissions in China.In order to achieve clean and efficient coal utilization,coal gasification technology has drawn wide attention.In the coal gasification process,the vast majority of mercury will be released as Hg0.The catalytic oxidation of mercury in syngas is more difficult because of the CO and H₂.At present,the removal of Hg0 in coal gas is still in the initial stage of research.The adsorbents still have some problems such as low mercury removal efficiency and unclear mechanism of mercury removal.Therefore,to study the adsorption mechanism of mercury under reducing atmosphere is of great scientific significance for the development of new and efficient adsorbents and coal gasification technology.In order to change the current situation that the traditional adsorbent has a weak capability of oxidizing and removing mercury under a reducing atmosphere,Co₃O₄ with strong oxidizing power was used as an active material,and titanium dioxide and activated coke were used as carriers to achieve high-efficiency mercury removal.The adsorption experiments of mercury on cobalt-based adsorbents in a simulated gas atmosphere were carried out on a fixed bed reactor.The analytical method of sorbents,such as BET,XRD and XPS,were carried out to study the mercury removal efficiency at different temperatures,different gases and different loadings.Besides,combined with the simulation calculation to further explore the cobalt-based sorbent removal mechanism of elemental mercury.Firstly,different loadings of Co-TiO₂ adsorbents were prepared by the same volume impregnation method.The average mercury removal efficiency of 15%Co-TiO₂ was 80.8%in two hours at 120?,which was 52.6%higher than pure TiO₂.H₂S can greatly promote the ability of adsorbent to remove mercury.According to XPS results,a large amount of S2-and S0 exists on the surface of sorbent,which proves that active sulfur?Sad?and HgS are formed.CO and H₂ are not conducive to the removal of mercury,both in addition to restore part of Co₃O₄,consumption of lattice oxygen.Also CO will react with S to generate COS,and competitive adsorption between H₂ and H₂S.NH3 reacts with H₂S and consumes active oxygen which leads to the low mercury removal efficience.HCl promotes the mercury removal by forming active Cl,and the ability of HCl to compete for active sites was weaker than that of H₂S.The specific surface area of 5%Co-AC was 390.80cm3/g,the mercury removal efficiency was 51,4%under N2 atmosphere at 120?,and the efficiency increased to 97.8%after adding H₂S.Due to the reaction of H₂S with reactive oxygen on the surface of adsorbent,more Hg0 transfer to HgS.High temperature will greatly inhibit the Co-AC adsorbent to remove mercury,mainly due to the low melting point temperature of S0;reaction equilibrium constant of S and Hg will decrease significantly with the temperature rising;generated HgS easily decomposed at high temperatures.From the thermal desorption experiments,it is found that mercury exists mainly in the form of HgO and HgS on the surface of sorbent and sorbents performed well after the thermal regeneration.After secondary regeneration,the mercury removal efficiency still maintained more than 90%at 120?.Finally,the adsorption of Hgo on the surface of Co₃O₄?110?was simulated by VASP based on density functional theory.The results showed that Co₃O₄ has the ability to adsorb Hg0 with the adsorption energy of-0.4498eV.According to the adsorption results of H₂S at different sites on Co₃O₄?110?surface,one of the HS bonds of H₂S molecules broken down to form HS + H.Then HS bond was broken again.After that,there were stable S-Co₃O₄?110?with-3.422eV of adsorption energy.In the end,it showd that Hg reacts with S0 on the surface to form HgS,following the Eley-Rideal mechanism.