Study On Mercury Removal From Coal-fired Flue Gas By Green/magnetic Maize Straw Biomass Carbon Prepared With Microwave Assistance

With the official entry into force of the Minamata Convention on Mercury aimed at controlling mercury emissions,States parties will ban the production and import/export of mercury-containing products from 2020.Mercury is a highly toxic pollutant that can be enriched in organisms,and one third of the anthropogenic mercury emissions come from the combustion of fossil fuels such as coal.At present,the most effective mercury removal technology is active carbon injection?ACI?,but the high cost and high loss of active carbon adsorbent limit the commercial application of this technology.In China,agricultural straw is a kind of renewable biomass resource with low price and wide sources.As a byproduct of biomass pyrolysis,biochar is considered as a promising alternative of activated carbon adsorbent due to its exploitable pore structure and good surface characteristics.In order to improve the Hg⁰ removal performance of biochar,researchers tried to modify biochar by sulfur modification,halide modification,metal oxide modification and other methods,but this will cause problems such as reagent leakage and wastewater generation during the modification process,and will cause secondary pollution and solid waste treatment problems when processing inactivated adsorbents.In addition,after Hg⁰capture,the used adsorbents would be mixed with fly ash in flue gas,which makes it quite difficult for the adsorbents to be recovered from fly ash,greatly increasing the industrial cost and causing secondary mercury pollution.To solve these problems,in this paper,maize straw biochars were modified by green reagent H₂O₂ and magnetic Ce-Fe metal oxides with the assistance of microwave to prepare green and magnetic maize straw biomass carbon,respectively,and the Hg⁰ removal performances of these two biomass carbon adsorbents from coal-fired flue gas were investigated in a fixed bed experimental system.The main research contents and achievements are as follows:The green maize straw biomass carbon adsorbent was prepared by H₂O₂ impregnating the microwave-pretreated maize straw char,and the effects of H₂O₂ concentration,reaction temperature and flue gas concentration on the Hg⁰ removal performance of the modified biomass carbon were studied in a fixed bed reaction system.The surface physical and chemical characteristics of the modified biomass carbon were analyzed through a series of characterization methods including industrial analysis,elemental analysis,BET,SEM,FTIR and XPS,and the kinetic model was combined to reveal the mechanism of Hg⁰ removal.The results show that microwave steam activation and H₂O₂ modification significantly improve the pore structure of maize straw char and form a porous structure that is favorable for Hg⁰ adsorption.The H₂O₂ modification increases the number of oxygen-containing functional groups?such as C-O and O-H?on the surface of the maize straw char,thereby improving the oxidizing ability of the adsorbent.The optimal H₂O₂ concentration is 10 wt.%,and the optimal reaction temperature is 120?.O₂ and NO in the flue gas promote mercury removal,while SO₂ and H₂O inhibit Hg⁰removal.Hg⁰ can be oxidized by oxygen-containing functional groups such as C-O,O-H and chemisorbed oxygen on the surface of adsorbents,and chemisorption plays a leading role in the process of Hg⁰ removal.The magnetic maize straw biomass carbon adsorbent was prepared by Ce-Fe metal oxides impregnating the microwave-pretreated maize straw char,and the effects of ultrasonic assisted impregnation,Ce-Fe loading,Ce/Fe molar ratio,calcination temperature,reaction temperature and flue gas concentration on the Hg⁰ removal performance of the modified biomass carbon were investigated in a fixed bed reaction system.The surface physical and chemical characteristics of the modified biomass carbon were analyzed through a series of characterization methods including BET?SEM-EDS?XRD?VSM and XPS,and the kinetic model was combined to reveal the mechanism of Hg⁰ removal.The results show that the optimal Ce-Fe loading and Ce/Fe molar ratio are 10 wt.%and 3/5,respectively,and the optimal calcination temperature and reaction temperature are 700?and 140?,respectively.The porous structure is formed after the modification of Ce-Fe metal oxides,and the dispersion of Ce-Fe active components on the adsorbent surface is promoted by ultrasonic assisted impregnation.The Hg⁰ removal performance of magnetic biomass carbon can be significantly improved in the presence of O₂ and NO in the flue gas,and the biomass carbon has certain sulfur and vapor resistance.Hg⁰ adsorbed on the surface of adsorbent can be oxidized by the active sites(Ce⁴⁺,Fe³⁺and chemisorption oxygen)on the surface of adsorbent,and chemisorption plays a leading role in the process of Hg⁰ removal.The interaction between cerium and iron oxides can promote the regeneration of active components and further promote the oxidation removal of Hg⁰.The modified biomass carbon with good regeneration performance exhibits good magnetic properties under the action of external magnetic field.In addition,the pseudo-second-order kinetic model can accurately describe the Hg⁰ removal process of green and magnetic maize straw biomass carbons.The equilibrium adsorption capacities of green and magnetic biomass carbons obtained from this model reach 1674.8?g/g and 7230.8?g/g,respectively,which is much higher than the Hg⁰ adsorption capacity of commercial activated carbon and common adsorbents.Therefore,the two maize straw biomass carbons developed in this paper have good application prospects from the perspectives of environmental protection and recyclability.The research results of this article will provide a certain theoretical basis for the development of environmentally friendly,cost-effective and recyclable Hg⁰ removal adsorbents,and provide necessary theoretical guidance for the utilization of straw biomass resources.