Effect Of Additives On NO_x Emission From Co-combustion Of Biomass And Coal

Co-combustion of agricultural and forestry wastes with coal can not only promote the development of non fossil energy,but also reduce the air pollution caused by direct incineration of agricultural and forestry wastes.In this paper,the influence law and mechanism of additives（desulfurizer and catalyst）on NOx emission during co-combustion of biomass and coal were studied.The research results are helpful to guide the development and application of co-combustion technology of biomass and coal.Firstly,the co-combustion experiments of biomass and coal were carried out in horizontal tubular furnace.It was found that the NO_X concentration of biomass with low cellulose,hemicellulose and lignin content mixed with coal was lower than that of biomass with high cellulose,hemicellulose and lignin content mixed with coal.The best desulfurizer was calcium acetate,calcium sulfur mol ratio was 2.3.The optimal catalyst was Pr-TiO₂（2%,W）doped with 2%Pr prepared by microwave-assisted sol-gel method,and the addition amount was 8%of the fuel mass.This method is suitable for low-volatile coal with volatile content ranging from 10%to 20%,medium-sulfur coal with sulfur content ranging from 1.51%to2%and medium-high sulfur coal with sulfur content ranging from 2.01%to 3%.This method has good adaptability to single coal and mixed coal.After No.5 coal was mixed with rice straw and added with calcium acetate and Pr-TiO₂（2%,W）,the NO_X emission decreased from 48.34μg/g（combustion alone）to 11.41μg/g.After blending coal 2 with rice straw and adding calcium acetate and Pr-TiO₂（2%,W）,NO_X emission decreased from 64.83μg/g（combustion alone）to 5.30μg/g.Secondly,the catalysts and biomass were characterized and analyzed differently.The results showed that the catalyst prepared by microwave-assisted sol-gel method had smaller particle size and increased anatase content,which increased the specific surface area of the catalyst and inhibited the grain growth.Pr was incorporated into the catalyst at three and four-valence.There were oxygen vacancies and lattice oxygen in the catalyst,and oxygen vacancies enhanced the adsorption capacity of the catalyst.Pr-TiO₂（2%,W）had good reducibility at low and high temperatures,so it had good catalytic performance.Biomass contained a large number of hydroxyl,carboxyl and other oxygen-containing functional groups.Finally,the experiment of NO adsorption by coal char was carried out,and different characterizations of coal,coal char and coal ash were analyzed,and the possible mechanism of NO_X emission reduction was speculated.NO_X emission reduction includes homogeneous reduction reaction and coal char heterogeneous reduction reaction.Homogeneous reduction reaction:Rice straw and calcium acetate precipitate a large amount of hydrocarbon reducing gases（C_nH_m）during combustion,compete with NH₃ and HCN for O₂ to reduce NO_X formation;C_nH_m reduced the generated NO to N₂.Coal char heterogeneous reduction reaction:The adsorption experiment and BET analysis of coal char showed that there was chemical adsorption in the process of NO adsorption.Additives can promote the conversion of N-5 and N-6 to N-Q in coal char,thereby reducing NO_X emissions.Additives increase the amorphous structure and defect structure in the carbon structure of coal and coal char,which enhance the reactivity of coal char.Rice straw,calcium acetate and Pr-TiO₂（2%,W）can increase the pore volume and specific surface area of coal char,and promote the heterogeneous reduction of NO.Alkaline oxides in biomass and Ca O decomposed by calcium acetate at high temperature significantly promoted the heterogeneous reduction of NO by coal char.In addition,Pr-TiO₂（2%,W）has a combustion-supporting effect,and additives reduce the ignition point of coal.Additives make different forms of sulfur in coal transform into each other,and the sulfur fixation of calcium acetate makes sulfur exist in the form of sulfate sulfur such as calcium sulfate and calcium sulfite.Rice straw and calcium acetate increased the slagging degree of coal ash,while Pr-TiO₂（2%,W）could reduce the slagging degree of coal ash;The sulfur-fixing effect of calcium acetate increases the contents of calcium sulfate and calcareous feldspar in ash;Pr-TiO₂（2%,W）increased the content of TiO₂ and quartz in ash.