CFD Simulation Study On Combustion Of Municipal Solid Waste In The Large-scale Grate Incinerator

Municipal solid waste output increases rapidly which makes our cities are facing the predicament of being surrounded by waste.Incineration,as one of municipal solid waste processing methods,can efficiently treat MSW with the principles of decreasing,innocuity and resourcing.The grate incinerator has been embraced for large scale deployment in China due to favorable adaptability to waste of high moisture content and heterogeneous calorific content.CFD simulation method has advantages in economy,efficiency and security,compared to experimental methods.To provide theoretical guidance for the structure design and operation,CFD simulation method was applied in this paper to maximize MSW combustion efficiency while minimizing pollutant(NOx)emission and improving operation safety.A 500 t/d moving-grate incinerator was modeled in this work,software FLIC was used to simulate the in-bed combustion while out-of-bed combustion was simulated by ANSYS FLUENT.The SNCR reactions were described by a detailed reaction mechanism with 23 step reactions.As the SNCR simulation result indicated,the NOx reduction efficiency is 51.2%.A field test of 500t/d incinerator was launched in Nanjing waste-to-energy plant.According to the testing values of temperature in furnace and species concentration in flue,the simulation method and SNCR mechanism were validated by the good agreement between test and simulation.Simulation and optimization of secondary air distribution in a 750 t/d grate furnace was presented.The results indicate that by applying lower secondary air below arches of front and back walls,the scour by high temperature flue gas can be prevented and the strong reducing atmosphere near arches can be weakened,suggesting the high temperature corrosion of arch area can be improved;Opposed arrangement and larger velocity(ranging from 45m/s to 65m/s)of higher secondary air both can promote the full mixing of flue gas,improving the furnace fullness and temperature uniformity;Compared to staggered arrangement,opposed arrangement is more favorable for prolonging gas residence time and lowering outlet concentration of CO.A new operational mode of grate incinerator with very low NOx emissions(VLN)was introduced.The amount of NO_x emissions in VLN mode is 39.6%less than that in normal mode which was verified by simulation.The height and downward inclination angle of the VLN nozzle and the downward inclination angle of lower secondary air were optimized.The results shows that when the height of VLN nozzle is 3 meters higher than lower secondary air nozzle,the downward inclination angle of VLN nozzle is 0°,the downward inclination angle of lower secondary air is 75°,the combustion status in furnace is optimal.