Study On Preparation Of Seaweed-based Biomass Porous Carbon And Removal Of Elemental Mercury From Coal-fired Flue Gas

Due to its high toxicity,persistence,and bioaccumulation,mercury has been recognized as one of the toxic pollutants that endanger human health and the ecological environment.The utilization and combustion of coal is considered to be the main source of anthropogenic mercury emissions.Mercury in coal-fired flue gas includes particulate mercury（Hg^p）,oxidized mercury(Hg²⁺)and elemental mercury（Hg⁰）.Due to its low water solubility and high volatility,elemental mercury（Hg⁰）is difficult to be captured by existing flue gas purification devices.Hence,how to effectively remove Hg⁰ in flue gas is currently a hot spot in the field of mercury control.Activated carbon injection technology is a widely used technology to control mercury emission,but its development is hindered by disadvantages such as low efficiency and high cost.Due to its characteristics of easy cultivation,strong adaptability,regeneration,rapid growth and reproduction,no occupation of arable land,and wide distribution in coastal areas,seaweed biomass（Sargassum and Enteromorpha）is considered to be one of the cheap raw materials for biomass fuel production.Seaweed based biomass char is the main solid waste produced in the production of biomass fuel.If these solid wastes cannot be disposed of timely and effectively,it will lead to environmental pollution and waste of resources.Thus,in order to make full use of seaweed biomass resources,this study uses seaweed based biomass chars as raw materials to prepare modified seaweed-based porous carbon adsorbents for removing mercury from flue gas by ammonium halide impregnation,microwave hydrogen peroxide combined modification,and UV/hydrogen peroxide advanced oxidation process（UV/H₂O₂）modification,respectively.The main contents and results are as follows:Seaweed based biomass char was selected as raw material,modified seaweed-based porous carbon adsorbents were prepared by ammonium halide impregnation method.The Hg⁰removal performance and influencing parameters of seaweed-based porous carbon modified by ammonium halide were studied in a fixed bed experimental system.The physicochemical properties of seaweed-based porous carbon modified by ammonium halide were analyzed by various characterization methods,and the related adsorption mechanism and kinetics were discussed.The results show that the pyrolysis temperature has an important influence on Hg⁰removal performance and pore structure of seaweed-based porous carbon,and the optimum pyrolysis temperature is 800℃;The modification of ammonium halide significantly improves Hg⁰ removal performance of seaweed-based porous carbon,and the optimum loading amount of ammonium halide is 5 wt.%;Increasing the reaction temperature is conducive to the removal of Hg⁰;Increasing the concentration of O₂ and NO can significantly promote the removal of Hg⁰;H₂O and SO₂ have dual effects on Hg⁰ removal;C-Br and C-Cl functional groups are considered to be the main active sites for Hg⁰ removal;Adsorbed mercury can combine with C-Br or C-Cl functional groups to form mercury containing compounds.The combination of adsorbed mercury and one halogen atom is an exothermic process,while the combination of adsorbed mercury and two halogen atoms is an endothermic process.The pseudo-second-order model can well describe the Hg⁰ removal process of seaweed-based porous carbon modified by ammonium halide.Seaweed based biomass char was selected as raw material,modified seaweed-based porous carbon adsorbents were prepared by microwave hydrogen peroxide combined modification method.The Hg⁰ removal performance and influencing parameters of modified seaweed-based porous carbon were studied in a fixed bed experimental system.The physicochemical properties of modified seaweed-based porous carbon were analyzed by characterization methods,and the related Hg⁰ adsorption mechanism and kinetics were studied.The results show that microwave activation can greatly improve the pore structure of seaweed-based porous carbon;The modification of H₂O₂ not only further increases the specific surface area and pore volume of seaweed-based porous carbon,but also increases the content of oxygen functional groups on the modified seaweed-based porous carbon;The optimum H₂O₂modification concentration and adsorption temperature are 30 wt.%and 120℃,respectively;The existence of NO and O₂ promotes the removal of Hg⁰,while H₂O inhibites the removal of Hg⁰;Low concentration of SO₂ is conducive to the capture of Hg⁰,while high concentration of SO₂ inhibites the removal of Hg⁰;The C=O bonds of oxygen functional groups play an important role in the Hg⁰ removal process of modified seaweed based porous carbon.According to the results of quantum chemistry calculation,in the armchair carbonaceous model,the influence order of oxygen functional groups on Hg⁰ removal is as follows:lactone>carboxyl>semiquinone>carbonyl;In the zigzag carbonaceous model,the influence order of oxygen functional groups on Hg⁰ removal is as follows:semiquinone>carbonyl>lactone>carboxyl.Seaweed based biomass char was selected as raw material,UV/H₂O₂ modified seaweed-based porous carbon adsorbents were further prepared by UV/H₂O₂ advanced oxidation process.The Hg⁰ removal performance and influencing parameters of modified seaweed-based porous carbon were investigated in a fixed bed experimental system.The UV/H₂O₂ modified seaweed-based porous carbon was characterized by characterization techniques,and the related Hg⁰ removal mechanism and kinetics were studied.The results show that UV/H₂O₂ modification can significantly improve the Hg⁰ removal performance of modified seaweed-based porous carbon;The optimum H₂O₂ concentration and adsorption temperature are 4 wt.%and 120℃,respectively;The addition of NO is beneficial to the removal of Hg⁰,while the addition of H₂O inhibites the removal of Hg⁰;Low concentration of O₂ and SO₂ can greatly promote the Hg⁰ capture,but excessive O₂ and SO₂ inhibit the removal of Hg⁰;UV/H₂O₂ modification has a negative effect on the pore structure of seaweed-based porous carbon,but the modification is beneficial to increase the content of oxygen functional groups;The existence of oxygen functional groups（such as C=O）promotes the removal of Hg⁰ by modified seaweed based porous carbon;The Hg⁰ removal process of UV/H₂O₂ modified seaweed-based porous carbon meets the pseudo-second-order model.Based on the adsorbent surface adsorption model,mass transfer process and mass balance equation,a mathematical model that was suitable for the removal of Hg⁰ by UV/H₂O₂ modified seaweed-based porous carbon was established.The results show that the fitting results of the model are basically consistent with the experimental results,indicating that the adsorption model can be used to simulate the removal of Hg⁰;Increasing the filling height of the adsorbent increases the Hg⁰ removal efficiency of the modified seaweed-based porous carbon;Increasing flue gas flow has a negative impact on the Hg⁰ removal efficiency;Increasing the void ratio is not conducive to the capture of Hg⁰;Increasing the mass transfer coefficient promotes the removal of Hg⁰.