Study On Mercury Removal From Coal-fired Flue Gas By Modified Wheat Straw Chars/red Mud

Mercury is one of the most hazardous pollutants due to its long persistence,neurological toxicity and strong bioaccumulation.Mercury emissions have caused worldwide attention because of its significant impact on the human health and ecological environment.The total amount of anthropogenic mercury emissions is more than 1000 tons each year.The combustion of fossil fuel has become the main anthropogenic mercury emissions source,which accounts for about 30 percent of anthropogenic mercury emissions.Activated carbon injection technology is considered as one of the most effective mercury removal technologies.However,the higher operating costs limit the application and development of this technology.Biomass straw,a kind of common renewable resource,is highly abundant and low cost.Biomass char,a by-product of biomass pyrolysis,is considered to be the best alternative to activated carbon in the field of flue gas mercury control because of its well-developed pore structure and better surface properties.Red mud is a by-product produced by aluminum making enterprises in the process of producing alumina.Disposal of red mud by the ways of landfill and stockpile has been adopted by most alumina plants.However,this disposal method not only wastes land resources,but also seriously damages the ecological environment.Thus,using red mud as a sorbent to remove mercury from flue gas can not only reduce the cost of mercury removal,but also achieve the purpose of comprehensive utilization of red mud.Therefore,biomass wheat straw char and red mud were selected as raw materials due to their lower prices.A series of modified sorbents were synthesized by an impregnation method,and their mercury removal capacity and reaction mechanism were comprehensively investigated in a fixed bed reactor system.The primary research contents and achievements are as follows:Mn-Ce mixed oxides modified wheat straw chars were prepared by an ultrasonic-assisted impregnation method,and were employed to remove elemental mercury?Hg⁰?from flue gas.A series of characterization methods were employed to analyze the physicochemical properties of the sorbents,and the mercury removal mechanism of sorbents was also studied.The effects of ultrasonic-assisted impregnation,Mn/Ce molar ratios,calcination temperatures,loading values,reaction temperatures and main flue gas components on mercury removal were studied in a fixed bed reactor.The results showed that the sorbent with a Mn/Ce molar ratio of 2/1 exhibited high mercury removal activity at 150?.The optimal Mn-Ce loading value and calcination temperature were 0.12 mol/L and 250?,respectively.The presence of O₂ and NO obviously promoted Hg⁰removal.Low concentrations of water vapor and SO₂ strengthened Hg⁰removal,but high concentrations of water vapor and SO₂ inhibited Hg⁰removal.The synergistic effect between manganese and cerium oxides greatly improved the mercury removal efficiency of modified samples.The adsorbed Hg⁰was oxidized by the active sites on the surface of samples,and the chemical adsorption of Hg⁰played a key role in the mercury removal process.The presence of CeO₂ contributed to the form of the redox cycle(Mn³⁺+Ce⁴⁺?Mn⁴⁺+Ce³⁺).The solid waste residue derived from the production of alumina,red mud,was modified by potassium halides impregnation,and was used to capture elemental mercury?Hg⁰?from flue gas.The effects of some key parameters such as loading value,reaction temperature and main flue gas components on mercury removal were investigated in a fixed bed reactor.The physicochemical properties of samples were analyzed using a series of characterization techniques,and the mercury removal mechanism of sorbents was also studied.The results showed that the modification of potassium halides significantly improved the Hg⁰removal performance of red mud,and the KI modified red mud exhibited the highest Hg⁰removal efficiency.Increasing reaction temperature,loading value,O₂ concentration and NO concentration enhances Hg⁰removal.Increasing SO₂ concentration greatly reduces Hg⁰capture.Water vapor has a dual effect on mercury removal,and the low concentration of water vapor was conducive to the Hg⁰removal,but the high concentration of water vapor inhibited the removal of Hg⁰.The adsorbed Hg⁰was oxidized by the active sites on the surface of samples.Halogen played an important role in the mercury removal process.The process of Hg⁰adsorption on modified red mud conforms to the pseudo-second order kinetic model.In addition,the mercury removal process of the modified red mud sample is a spontaneous and endothermic process.