Research On Dilute Combustion Characteristics Of High-temperature Air And Coal Gas/Recirculated Flue Gas In Alumina Calciner

With the continuous improvement of environmental protection requirements,air pollution in the aluminum industry has gradually attracted more and more attention.Aluminum hydroxide roasting is the last process in the alumina production process.Due to the high-temperature roasting process,the roaster is the key part of NO_x emissions in the aluminum industry.The mainstream alumina roaster uses high-temperature air and fuel gas to diffuse the combustion flame.The fuel can be natural gas,coal-made gas,or a mixture thereof.Burning under the preheated air temperature of 600?750?,the NO_x emission of the roaster is between150?300mg/m3.According to the requirements of the current "Aluminum Industry Pollutant Emission Standards",NO_x should be reduced to below 100mg/m3.The operation of the roaster is facing extremely severe environmental pressure,and low-nitrogen combustion transformation is imperative.In this paper,based on the combustion characteristics of existing roasters,the idea of using flue gas recirculation to reduce NO_x generation is proposed,and the mixing characteristics,combustion characteristics and NO_x generation characteristics of coal-made gas and high-temperature air in flue gas recirculation combustion are mainly studied.Design a new burner structure,optimized the key operating parameters of the burner,and obtained a new low-nitrogen combustion transformation plan for the alumina roaster.First of all,in order to fully understand the operating environment and operating parameters of the roasting furnace system,a pilot test was carried out before the transformation to grasp the generation of NO_x in the existing combustion system,temperature distribution in the furnace and dust concentration,and to obtain the incentive for the high NO_x emission concentration.Secondly,through three-dimensional modeling,the gas combustion process of the roaster is numerically simulated,and the following results are obtained: the uniformity of the temperature distribution in the furnace is not good,and it is easy to produce local high-temperature regions,and this part of the region is the region with the highest NO_x production.This part of NO_x is mainly thermal NO_x.Based on the above results,the main design idea is to add a new flue gas recirculation system and improve the burner structure.Then,in this paper,the effect of flue gas recirculation rate on the characteristics of the velocity field,temperature field and component field in the furnace was studied.The results show that with the increase of flue gas recirculation rate,the mixing uniformity of fuel and air is obvious Better,but in the more critical thermal temperature distribution,which also determines the generation of NO_x,it can befound that the greater the proportion of flue gas that is not blended,the more uniform the temperature distribution,compared to all working conditions,at 5% The fuel mixing uniformity and temperature distribution uniformity at 5%?10% flue gas recirculation ratio are better.However,it is not enough to renovate the existing roaster system by relying on flue gas recirculation alone,and it is necessary to improve the blending characteristics of the fuel and change the structure of the existing burner.Finally,this paper optimizes and improves the structure of the prototype burner,increases the number of openings in the central area of the burner head,and makes the fuel penetrate deeper into the central area of the furnace.The numerical simulation results show that after changing the structure of the burner,the uniformity of the fuel and air mixing is significantly improved,and the uniformity of the thermal temperature distribution is also significantly increased,which also represents the reduction of the area of the local high temperature area and the generation of thermal NO_x Restricted,NO_x emissions are significantly reduced.