Optimization Of Fuel Injection Ratio In Rotary Kiln For Pellet Roasting

Grate rotary kiln is an important equipment for pelletizing production,it uses anthracite as fuel to provide a high-temperature environment for pelletizing roasting.With the continuous consumption of coal resources,the price of high-quality anthracite continues to rise.In order to reduce the production cost and reduce the consumption of high-quality coal,the enterprise is prepared to add an appropriate amount of bituminous coal into the anthracite.The change of mixed coal composition and calorific value will inevitably affect the flow field and temperature field in the rotary kiln,and then affect the pellet roasting quality.Therefore,it is necessary to analyze the feasibility of burning bituminous coal mixed with anthracite.This article usesΦ6.1 m×40 m rotary kiln as the research object,which which is used to roast oxidized iron ore pellets of a sintering plant,a three-dimensional mathematical model of combustion,heat transfer and flow is established to simulate and calculate the combustion law of mixed coal in the rotary kiln.Based on the calculation results of anthracite combustion,the working conditions after blending bituminous coal are compared and analyzed,and the optimization scheme of coal blending is determined.The simulation calculation of the original working condition with only anthracite injection shows that the primary air blows from the single channel burner to the large space of the rotary kiln,forming a typical free jet.A low temperature zone,also known as a black burner,appears near the burner outlet,with a length of about 7 m,which is conducive to protecting the burner.The flame is in the shape of a mallet.The flame section gradually expands to the section that fills the rotary kiln.The flame length is about 1/3 of the length of the rotary kiln.The temperature distribution in the kiln tail area is uniform,and the flue gas temperature out of the kiln is 1250 K.NO_xdistribution and temperature distribution are similar.When the coal blending ratio is 20%～55%,the numerical simulation shows that the overall flow field structure is almost unchanged with the increase of coal blending ratio;However,the jet velocity near the nozzle attenuates slightly faster,and the velocity attenuation area is slightly larger;The black flame head becomes longer,the flame length becomes shorter,and the flame temperature decreases slightly.Bituminous coal contains more volatile matter and is easier to burn.The concentration of O₂and CO₂in the flue gas at the kiln tail increases,and the generation of NO_xdecreases.Taking the mixing ratio of 20%as an example,the influence of coal injection rate on combustion is simulated.With the increase of coal injection,the concentration of pulverized coal carried by the primary air in unit time increases,the time and travel required for combustion slightly extend,the flame length slightly increases,the average temperature in the rotary kiln increases,and the concentration of CO₂and NO_xboth increase,but it has little impact on the velocity distribution.When the primary wind speed increases from 40 m/s to 80 m/s,the simulation calculation shows that the entrainment of primary air on secondary air is strengthened,the momentum loss of primary air is greater,the velocity attenuation is faster,and the velocity gradient is increased;The pulverized coal and air are mixed more fully,the combustion is more intense,the flame is shorter and thicker,and the combustion temperature is higher.The generation area of NO_xfirst expands and then narrows;Reducing the primary air speed appropriately can reduce the generation of NO_xwithin a certain range.The combustion parameters of blended coal are optimized based on the temperature distribution of the pellet plane in the original working condition.When the coal blending ratio is lower than 30%,there are three optimization schemes:the first is to adjust the coal injection only,which is simple and easy to operate;The second is to adjust the coal injection rate and primary wind speed at the same time;The third is to adjust the primary air speed and the position of the coal gun at the same time,which is the most economical.When the coal blending ratio is in the range of30%～35%,the second or third optimization scheme can be adopted.When the coal blending ratio is between 35%and 45%,only the third optimization scheme can be adopted.The simulation calculation shows that the coal blending ratio should not exceed 45%.