Research On Low No_x Release Characteristics Of Staged Combustion In Precalciner

With the development of China's economy,the pollution level is also increasing.After the thermal power generation and automobile exhaust,the cement industry has become the third largest emitter of nitrogen oxides in our country.Precalciner occupies an important position in the cement production process and is the main part of the cement industry to reduce NOx.Therefore,optimizing the structure and process parameters of the precalciner to achieve energy saving and emission reduction has become an inevitable trend in the design and operation of precalciner.The study object of this paper is the precalciner and the 3-d geometric model was established.The pulverized coal combustion and raw meal decomposition in the basic conditions of the precalciner were numerically studied with CFD software.The simulation data and the measured data were compared to verify the reliability and correctness of the numerical simulation method and the selected mathematical model.In addition,the numerical simulation optimization study of the precalciner was carried out under different working conditions.The numerical simulation study of the basic conditions of the precalciner mainly studied the aerodynamic field,temperature field,component field,Ca CO3 and Ca O distribution,and NOx concentration field in the furnace under this operating condition,and compared with the measured data at the same time,to verify the reliability and correctness of the results.The results show that the aerodynamic characteristics of the precalciner is well-distributed,the velocity distribution is regular,and two symmetrical recirculation zones are formed at the bottom of the precalciner,which is beneficial to the material shuttle and increase the residence time of the pulverized coal particles.Furnace temperature distribution is uniform.The temperature of the main combustion zone can reach 1500~2200 K,which meets the temperature requirements for the decomposition of raw meal.In the high-temperature region of the precalciner,the mass fraction of O2 is relatively low,and the mass fraction of CO and CO2 is relatively high.The concentration of Ca CO3 decreases with the height of the furnace,and the concentration of Ca O increases with the height of the furnace.Along the height direction of the precalciner,the high concentration region of NOx is mainly distributed in the combustion zone of the lower cylinder of the precalciner and the third air duct on the central cylinder of the precalciner.The concentration of NO x in the upper part of the precalciner gradually stabilizes,and the concentration value reached 617.7 mg/m3.The numerical simulation results are compared with the measured data,the error is within the acceptable range,and the numerical calculation method and the simulation result are correct.The precalciner was optimized by numerical simulation under different conditions.Effects of different flue gas velocities,different tertiary air velocities,different necking ratios and different raw material ratios on combustion characteristics and NOx emission of pulverized coal were studied respectively.The results show that the larger the flue gas velocity,the higher the overall temperature of the furnace,but when the flue gas velocity increases to a certain 30 m/s,the temperature does not increase but decreases.With the increase of flue gas velocity,the average NOx concentration increases from 371.2 mg/m3 to 846.8 mg/m3.The temperature of different tertiary air velocity conditions can meet the raw material decomposition,when the third tertiary air velocity is v=36 m/s,the lowest NOx concentration reaches 413.8 mg/m3.However,considering the uniform temperature distribution,local high concentration NOx at the outlet of the precalciner,and the raw material decomposition efficiency,it is suitable to select v=20.1 m/s as the lower third tertiary air velocity.At this time,the average NOx concentration at the exit of precalciner is 499.9 mg/m3,which is 117.8 mg/m3 lower than the basic working condition.When the necking radius is 2.4 m,the temperature can meet the raw material decomposition,the components are distributed reasonably,and the average NO x concentration at the precalciner outlet is the lowest,reaching 346.6 mg/m3.When the raw material ratio of A and B is 20%,and the ratio of raw materials of C and D is 30%,the temperature distribution is uniform and the average NOx concentration at the exit is the smallest,reaching 329.9 mg/m3.In this paper,the numerical simulation results of the precalciner provide the theoretical basis for the actual operation data of the precalciner,and provide a reference for the combustion of the precalciner and the reduction of NOx emissions.