Study On Mercury Removal From Coal-fired Flue Gas By Biomass-based Chars/activated Carbons Prepared From Seaweed

In recent years,mercury,which is extremely neurotoxic and volatile,has caused great harm to human health and the ecological environment.Therefore,the issue of mercury emission has become a focus of increasing concern in society.The burning of fossil fuels in coal-fired power plants has proven to be the most important source of anthropogenic mercury emissions,accounting for about one-third of the total mercury released into the atmosphere each year.In July 2011,the Ministry of Environmental Protection of China promulgated the“Emission Standard for Air Pollutants in Thermal Power Plants”,which stipulated that the mercury emission limit was 0.03 mg/m³.The US Environmental Protection Agency issued“Mercury and Air Toxics Standards”in December of the same year,which also limited the mercury emissions from coal-fired power plants.Adsorbent injection technology was the main method of current coal-fired flue gas mercury control technology.Among them,activated carbon jet adsorption technology had been used in coal-fired power plants in some regions,but its high cost,high loss and non-targeted adsorption are hindering its commercialization.In the process of preparing biomass oil from seaweed biomass,a large amount of by-product biomass chars were produced.Because of its more developed pore structure and better surface characteristics,these biomass chars were considered by scientists to be a potential substitute for activated carbon adsorbent.Therefore,two common seaweed biomass?sargassum and enteromorpha?was selected as raw materials,and multi-class adsorbents were prepared by potassium halide salt impregnation method and zinc chloride activation method.The key influencing factors?preparation parameters,reaction temperature,coal-fired flue gas component,etc.?for the Hg⁰ removal were investigated in a simulated flue gas fixed bed.The main research contents and results were as follows:The sargassum and enteromorpha were selected as the raw materials for preparation of seaweed chars by pyrolysis,and the modified seaweed char adsorbent was prepared by impregnating the potassium halide salt.The physicochemical properties of the modified seaweed char were analyzed by industrial analysis,elemental analysis,BET,SEM and XPS.In the simulated flue gas fixed bed reactor system,the effects of the preparation process parameters?such as pyrolysis temperature,loading of potassium halide salt and potassium iodide loading?on the mercury removal performance of modified seaweed char were studied.The results showed that the optimum pyrolysis temperature was 800?.The potassium iodide modification had better Hg⁰ adsorption performance than potassium chloride and potassium bromide.The optimum loading of the potassium iodide modifying reagent was 3%.Subsequently,the reaction temperature and the influence of various flue gas components on the mercury removal performance were investigated.It was found that the increase of reaction temperature was beneficial to remove Hg⁰.The O₂ in flue gas contributed to mercury removal.SO₂ inhibited the mercury removal from enteromorpha char.However,SO₂ had a dual effect on the Hg⁰ removal performance of sargassum char.Low concentrations of NO and H₂O?g?were beneficial for the removal of Hg⁰,and high concentrations of NO and H₂O?g?inhibited the removal of Hg⁰.The adsorption kinetic model analysis showed that the external mass transfer was the control factor in the process of removing Hg⁰ from modified seaweed char adsorbent at 80?.At 120?and160?,chemisorption was the main controlling factor in the process of removing Hg⁰ from adsorbents.The surface covalent functional group?C-I?and chemisorbed oxygen were the main chemisorption sites on the surface of the potassium iodide modified seaweed char?S8KI3 and E8KI3?,and participated in the Hg⁰ removal process.A new type of seaweed biomass-based activated carbon was prepared by using zinc chloride activation method using sargassum and enteromorpha as raw materials.The physico-chemical properties of seaweed-based activated carbon adsorbents were analyzed using a series of characterization techniques.The results showed that the seaweed-based activated carbon had a large specific surface area(S_BET)and a developed pore structure.The S_BETET of the sargassum-based activated carbon reached 524.57 m²/g,and the S_BETET of the enteromorpha-based activated carbon reached 565.82 m²/g.In the simulated flue gas fixed bed reactor system,the effects of activation temperature,pyrolysis temperature,adsorption reaction temperature and various flue gas components on the removal of Hg⁰ using seaweed biomass-based activated carbon were investigated.Studies had shown that the seaweed-based activated carbon adsorbents?SAC-800 and EAC-800?prepared by the zinc chloride activation method had higher Hg⁰removal efficiency than the pyrolysis method.The optimal activation temperature was 800°C;It was found that the increase of reaction temperature was beneficial to remove Hg⁰;O₂ and NO in the flue gas contributed to mercury removal,SO₂ and H₂O?g?inhibited the Hg⁰ removal.Adsorption kinetics models and other studies had shown that the adsorption of Hg⁰ by seaweed-based activated carbon was endothermic and spontaneous,including physical adsorption and chemisorption.At 120?and 160?,chemisorption was the main control factor for removal of Hg⁰ from seaweed-based activated carbon.The C-Cl functional group and the oxygen-containing functional group acted as chemical adsorption sites on the surface of the seaweed-based activated carbons,and participated in the process of removing Hg⁰.Seaweed-based activated carbons had better Hg⁰ adsorption performance than other activated carbons?traditional biomass-based activated carbon and some commercial activated carbon?.