Experimental Study On The Characteristics Of NO_x Formation And Reduction On Grate Combustion

The grate combustion has strong adaptability to fuels and is widely used in the combustion of coal,biomass and domestic waste,but its high NO_x emission is one technical bottleneck.General researches focus on the formation and reduction of NO_x in the upper surface of the fuel bed,while limited attention is paid to that within the fuel bed.Starting from the study on fuels’ignition characteristics and the formation of the ignition front,this thesis studies and analyzes the grate combustion process and the formation and reduction characteristics of NO_x within the bed.In grate combustion,ignition is a prerequisite for the formation and spread of the ignition front,which involves series of homogeneous and heterogeneous reactions.For fuels with different volatile content,biomass pellets,bituminous coal,lean coal and anthracite coal,we first investigate ignition characteristics and the formation of the ignition front by combined utilization of TA and a unit furnace.The results show that as the volatile content increases,the ignition temperature decreases.The ignition temperatures of biomass pellet are about 280℃,and the ignition mode is heterogeneous ignition.Increasing primary air volume will diffuse more oxygen to the surface of char to promote ignition,while also intensify the convective cooling effect.Under this competition mechanism,the ignition temperature of biomass will first decrease and then increase.The ignition temperatures of coals are around 500℃and the reaction of volatiles and oxygen plays an important role.With the increase of the primary air volume,the concentration of volatiles involved in oxidation reaction on the bed surface will decreases,and the convective cooling effect strengthens,so the ignition temperatures of coals increase.This thesis utilizes the unit furnace to further explore the effects of representative fuels and the primary air volume on the formation and reduction characteristics of NO within the bed during the grate combustion process.Based on the comparision of the NO concentration on the ignition front and on the bed surface,and the detection of char reactivity,we find that NO shows up heavily on the ignition front,and the reduction effect of char layer is obvious.Fuel N is mainly oxidized to NO in forms of volatile N and char N during the ignition and burnout stages,forming two concentration peaks respectively,and the first is much higher than the second.Under the same excess air ratio and bed height,the NO produced by bituminous coal is lower than that of biomass.For biomass pellet,when increasing the air volume in oxygen limit area,the NO concentration on the ignition front will be much higher and char reactivity will also increase,which lead to less NO emission.