Study On Mercury Removal Of Bromine Modified Biomass Coke In Oxy-fuel Combustion Atmosphere

Oxy-fuel combustion technology can effectively control greenhouse gas emission,however,the mercury emission not only poses a serious threat to human health and ecological environment,but also causes the erosion and dissolution of the aluminum metal parts in the compression equipment,which leads to the safe operation seriously.The activated carbon injection technology is the most promising flue gas mercury removal technology,but more researches focus on the traditional air combustion compared to the oxy-fuel combustion that the flue gas components are complex.Therefore,in this paper,the influence and mechanism of single and complex components of flue gas on mercury removal performance of the modified walnut shell coke were studied on a fixed bed under the condition of simulated oxy-fuel combustion flue gas.Firstly,the walnut shell was pyrolyzed to coke in a tubular furnace,and then impregnated by 1%NH₄Br solution to prepare mercury removal adsorbent.The physical and chemical properties of the modified adsorbent were characterized by SEM,BET,FT-IR and XRF.The mercury removal performance of the adsorbent before and after modification was investigated in a fixed bed experimental device.The effects of reaction temperature and inlet mercury concentration on mercury removal were obtained,and the rate control step affecting mercury adsorption process was analyzed based on the kinetic analysis.The results showed that Hg⁰removal performance of the modified adsorbent was obviously improved,and the active component Br on adsorbent surface was the key factor to promote Hg⁰removal.Br on the adsorbent surface may exist in many forms.When Hg⁰ in flue gas passed through the adsorbent bed,one part reacted with Br to form Hg Br₂,the other part was oxidized to gaseous Hg²⁺.Lower reaction temperature and higher inlet mercury concentration was found to promote the removal of Hg⁰.The adsorption process was controlled by both external mass transfer and chemical adsorption.Secondly,the effects of single flue gas component of CO₂,O₂,NO,SO₂ and HCl on the Hg⁰ removal performance was studied in O₂/CO₂ atmosphere,and the results were compared with those in O₂/N₂ atmosphere.The results indicated that CO₂ improved the Hg⁰ removal efficiency and did not change the mercury species.It might be that CO₂ can increase oxygen-containing functional groups thus promote Hg⁰ adsorption and heterogeneous oxidation.O₂ was always conducive to Hg⁰ adsorption.In O₂/N₂ atmosphere,O₂ promoted the formation of active center on the sorbent surface,which made Hg⁰ more easily adsorbed and then reacted with Br-functional group to form more Hg Br₂.In O₂/CO₂ atmosphere,O₂ promoted the formation of some reactive carbon–oxygen complexes which acted as catalyst to facilitate Hg⁰ oxidation and forming Hg O.NO improved the formation of C-O functional group on the sorbent surface and catalyzed NO to NO₂ in the adsorbed state and then formed Hg₂（NO₃）₂.SO₂ consumed oxygen on the sorbent surface and inhibited Hg⁰ removal.In O₂/CO₂ atmosphere,SO₂reacted with O₂to form SO₃that competed with Hg⁰ on the adsorption site,as a result of restraining Hg⁰adsorption and oxidation.HCl obviously inhibited mercury adsorption because of generating Hg Cl₂.As an acidic specie,HCl may make Br varied in its form and inhibit mercury removal.Finally,the influence of the mixture of two flue gas components on performance of bromine modified adsorbent was studied.The results showed that the low concentration of SO₂would form C-S bond on the surface of the adsorbent,and react with Hg⁰ to form Hg S.At the same time,SO₂ would occupy the corresponding active site and compete adsorption with Hg⁰,and reduce the Hg⁰ removal efficiency.The high concentration of SO₂would make SO₂dissociate on surface of adsorbent and generate active O site,which promoted the oxidation and adsorption of Hg⁰.When SO₂ and NO coexisted,NO played a leading role in the Hg⁰ removal efficiency because Hg₂（NO₃）₂ was the main form in the adsorbed material.The binding capacity of mercury to the acid gas was N->S-.When SO₂ and HCl coexisted,mercury in the adsorbent was mainly in the form of Hg Br₂,and the proportion of adopted Hg dominated.The active component bromine and Hg⁰ was more likely to combined to form stable Hg Br₂.When HCl and NO coexisted,the homogeneous oxidation ratio of Hg⁰ in flue gas increased obviously as a result of Hg₂（NO₃）₂was the main form in the adsorbed material.The binding capacity of mercury to the acid gas was N->Cl-.