Simulation Study Of Turbulent Combustion Of Pulverized Coal And Soot Formation

As conventional fossil fuel with abundant reserves in China,more than 50%of the total consumption of coal is burned to generate electricity.Soot is produced by pyrolysis or incomplete combustion of hydrocarbon fuels,for instance,coal and oil,and defined as carbonaceous particles,which has tremendous effects on human health,ecological environment and industrial production.Ultrafine soot particles can deep into the lungs and lead to respiratory system diseases.In addition,soot contributes to global warming only second to carbon dioxide,and the presence of soot in combustion equipment has an important impact on its thermal efficiency.Therefore,in order to cut down soot emission and promote efficiency of combustion equipment,it is important to study the formation process of soot and the factors which affect soot formation.In this paper,the mathematical model of pulverized coal combustion was firstly constructed,and described the soot formation model.The steady numerical simulation was executed for the pulverized coal burner by ANSYS Fluent software.The Moss-Brookes soot model was combined with the H abstraction-C₂H₂-addition（HACA）mechanism to study the soot formation characteristics.The prediction results are in accordance with the experimental data,which validates the validity of the mathematical model of pulverized coal combustion.The results indicate that the ignition pattern of pulverized coal has a big impact on the ignition stability,and there is a long and narrow high temperature zone in the center of the burner.The soot surface growth reached the maximum at the downstream of the burner center.The soot oxidation was accompanied by the soot growth,and the change of acetylene,hydrogen and hydrogen concentration verified the applicability of HACA mechanism.Then,the burner model was intensified and the LES（Large Eddy Simulation）of cold flow distribution was implemented.The Moss-Brookes soot model was combined with the LES-PDF（Probability Density Function）model for the hot state simulation.The contrast between the mean cold velocity of the LES and the measured values indicated that the LES could predict the velocity distribution of the fluid in the flow field commendably,and the large vortices could be straight reported.The thermal simulation showed that the high temperature was concentrated in the narrow and long area near the axis,and the pulsation performance of the temperature field was better demonstrated.Moreover,the spread of OH free radicals was consistent with the experimental results.Then,the moment soot model and Moss-Brookes model were used to simulate the steady-state flame of a swirling pulverized coal combustion furnace,and the effects of swirling and soot on the radiation temperature field in the furnace were studied.The results show that the swirling secondary air at the nozzle of the swirling combustion furnace has obvious effects on the temperature field,component field and soot particle distribution.The mass density of soot is mainly distributed from the entrance of the combustion furnace to the center space of 1/4 of the main body of the combustion furnace.The soot superficial area tend to increasing first and then decreasing by degrees in a confined area with the secondary swirling air.The nucleation,surface growth and oxidation of soot were continuous in time and space,and soot volume fraction was positively correlated with the radiation emitted in the furnace.Finally,the LES of combustion furnace flame characteristics and the interaction between soot formation and radiation characteristics and temperature field under different fuel equivalent ratio were analyzed.The results indicate that the maximum temperature of pulverized coal flame declines evidently when considering turbulence pulsation in the LES,and the maximum temperature decreases again by 127 K when considering soot formation.Soot nucleation,surface growth and oxidation mainly exist in the high temperature region of the main space of the combustion furnace,and have a consistent consistency.Different fuel equivalent ratio has great impact on flame temperature field,but the distribute tendency of soot volume fraction is similar.