Research On Characteristics And Mechanism Of NO_x Emission From Biomass/Sewage Sludge Co-Combustion

Co-combustion of biomass and sewage sludge is an advantage technology of waste energy utilization.However,the characteristics and mechanism of NO_x emission during co-combustion of the two fuels are not clear.China’s NO_x emission standard is increasingly stringent（the emission standard for key areas is 30 mg/m³）.In order to promote the practical application of the co-combustion technology in the reality of increasingly stringent NO_x emission standard and realize the harmless and resource utilization of biomass/sewage sludge,it is urgent to investigate the NO_x emission characteristics and mechanism of biomass/sewage sludge co-combustion.In the present study,systematic research on combustion characteristics of biomass/sewage sludge blends,NO_x emission characteristics and synergistic effect from whole and each stage of co-combustion process,condition optimization and simulation prediction of co-combustion were carried out thoroughly.Follows are the main conclusions and contents in this current dissertation:（1）Combustion characteristics and NO_x emission characteristics of biomass/sewage sludge co-combustion was investigated by TG-FTIR experimental platform.The results showed that the comprehensive combustion index of co-combustion was 32.67%higher than that of biomass mono-combustion.There was synergetic effect towards NO_x reduction between biomass and sewage sludge through the changing of fuel-N transfer pathway and the delaying of the reaction rates.（2）The optimal conditions of co-combustion were explored using fixed bed system.The results show that 60 wt.%moisture content was optimal for NO_x reduction.NO_x emission was more dependent on heating rate than sewage sludge water content.A CFD model was established for NO_x and precursor release from biomass/sewage sludge blended fuel.The results show that the prediction error of NO_x for biomass and low sludge ratio mixed fuel is less than 8%.But when the sludge ratio was higher than40 wt.%,the deviation between simulation and experimental results can reach 20%.（3）The migration of fuel-N and the interaction between two fuels as far as NO_xemission in each combustion stage were investigated in detail under the optimal mixing ratio which was determined based on the combustion and NO_x emission characteristics.As follows:（1）The fuel-N conversion in volatilization stage of co-combustion was systematically explored by TG-FTIR platform and XPS characterization,etc.The results showed that there was interaction between biomass and municipal sludge,and the selectivity of fuel-N to HCN was significantly improved,and CO formation promoted the conversion of NH₃ to HCN.The selectivity of HCN from mixed fuel-N is 11.61%higher than that of biomass and sludge mono-fuel.At the same time,NO_xemission reduction could reach 22.5 wt.%.（2）Chemkin simulation software was employed to study the fuel-N migration path in volatile combustion stage of co-combustion.The results showed that co-combustion could promote the oxidation of volatile-N to NO_x.In the early stage of volatilization,HCN mainly generated NH₃ through hydrogenation,while in the later stage,HCN mainly transformed to NO_x.At the same time,the conversion intensity of NH₃ to NO_xwas higher than that of NO_x.（3）The NO_x formation and reduction during char combustion stage of co-combustion were studied in a vertical tubular furnace.The results showed that the thermal stability of biomass and sewage sludge blended char was higher than that of single fuel char,so the NO_x release intensity of char was lower at the initial stage of combustion.But the NO_x release was more intense than that of single fuel char at the later stage.Lignin could not only enhance the formation of NO_x in municipal sludge char combustion,but also enhanced the heterogeneous reduction of NO_x in sewage sludge char.In this paper,the feasibility of biomass/sewage sludge co-combustion and the characteristics of fuel-N migration and release were studied to provide theoretical support for the development of clean and efficient co-combustion technology.