Study On Preparation And Mechanism Of Biomass Based Activated Carbon On The Elemental Mercury Removal In Coal Gas

Activated carbon from biomass has well-developed pore structures and rich surface chemical groups,resulting in a strong specific adsorption ability that can remove mercury.However,due to the limited mercury adsorption capacity,an additional modification process is often required.The additional load and modification processes to obtain a high mercury removal efficiency are often complex and time-consuming.Based on the characteristics of biomass itself,the biomass-based activated carbon with significant mercury removal effect was developed.Meanwhile,the key factors and mechanism analysis of the removal of elemental mercury from coal gas by activated carbon were analyzed,which could provide guidance for the production of activated carbon for mercury removal from coal gas.In this paper,three typical biomass-corn straw,sawdust and bamboo powder-were activated and loaded with ZnCl₂ in one step to realize the efficient removal of mercury.Combined with the characterization of TPD,XPS,XRD,Boehm titration,etc.,the effect of preparation method and raw material characteristics on activated carbon for mercury removal was studied.As H₂S is an important component of coal gas,the effect of H₂S on the removal of Hg⁰ from activated carbon was analyzed through TPD methods and thermodynamics at last.Firstly,the key factors and mechanism of removing elemental mercury from coal gas by activated carbon were analyzed.During the activation,a part of ZnCl₂ is converted into ZnO and laden on activated carbon,which promotes the mercury removal performance with the presence of H₂S.ZnO and carbonyl groups play an important role in mercury removal in coal gas,while physisorption is not significant at 130?.The differences in mercury adsorption capacity are also reflected in the ash and trace elements,although they do not promote mercury removal as much as ZnO and carbonyl groups.XPS results showed that ZnO serves as a catalytic active site and catalyses the Hg⁰ into HgS with the presence of H₂S.While the C?O group of carbonyl acts as adsorption sites and turns into C-O after the adsorption.Then the influence of preparation methods and raw material characteristics on activated carbon for mercury removal was analyzed.In terms of preparation,500?could be the ideal temperature of ZnCl₂ activation method.The drying step can be beneficial to the preparation of activated carbon,however,the advantages and disadvantages should be further weighed from the perspective of technical economy.In terms of raw material characteristics,the ZnO content varies greatly with the content of SiO₂ in the biomass due to its inactivation of ZnO and formation of Zn₂SiO₄.Biomass with high lignin content is easier to obtain high specific surface area and pore volume,but physisorption based on specific surface area is not the key factor to remove Hg⁰.Ash and trace elements have small effect on mercury removal and can be used as auxiliary reference for material selection.At last,the effect of H₂S on the removal of Hg⁰ from activated carbon was analyzed and the feasibility of using H₂S to sulfurize activated carbon at high temperature and then removing Hg⁰at low temperature with it was discussed.H₂S significantly weakens the adsorption of activated carbon on Hg⁰.Specifically,the consumption of oxygen adsorbed by H₂S on the surface of activated carbon leads to the fact that sulfide on the surface of activated carbon cannot be oxidized to form elemental sulfur to remove mercury.The substitution reaction of H₂S to oxygen functional groups results in the decrease of the catalytic effect of oxygen functional groups on mercury removal.Physisorption is enhanced by chemisorption.High temperature sulfuration cannot effectively sulfurize the activated carbon.Therefore,mercury removal in the form of HgS with H₂S as a sulfur source is not a feasible method by activated carbon.