Numerical Simulations Of Gas-solid Flow Field And Coal Combustion In Precalciners Of Cement Industry For Optimization

Precalciner is not only a key equipment in cement industry on which the optimization of structure parameter are carried on frequently,but also an energy consumer.High-quality bituminous coal is commonly used in precalciners of cement industry in china for a long time.And low-volatile coal is almost not used since it is hard to be ignite and burnout.Recently,in order to reduce the energy consumption,more and more attentions have been paid on inferior coal combustion in precalciners.In this paper,combining qualitative studies and quantitative analysis, numerical simulations was carried out aiming at three actual precalciners under operational-based boundary conditions.What obtained from the analyzed result provides important theoretical information and significant referenced value for optimization of structure parameter and low-volatile or high ash inferior coal combustion in precalciners.In this paper,aiming at spouted precalciner,swirling precalciner and swirling-spouted precalciner,the gas flow field,gas-solid flow field and coal combustion are simulated respectively with the k-εmodel,the Discrete Phase Model and theβ-PDF modle used.The optimum performance parameters for gas-solid flow field and low-volatile combustion were presented,and also the differences between the three types were studied with the strongpoint or shortcoming of each type illustrated legibly.Results are as follows:(1)There is a simple spraying-liked flow field only with reversed-flow appears near the wall at the bottom in the spouted precalciner.Though the pressure drop from the tertiary inlet to the outlet is low,there is an ununiform particle dispersion condition.The dispersion condition can be improved by adding a new raw meal inlet to this type of precalciner,and the optimal horizontal projection angle between the two raw meal pipes is larger than or equal to 135°,especially at 157.5°.The research results of coal combustion show that this type of precalciner has a good adaptability for different quality coal combustion,and it is suitable for inferior coal combustion. The researches of low-volatile coal combustion for optimum condition show that it is optimum when the tertiary air velocity is 30 m/s with the incident angle of coal particles downward and with the angle between the two coal pipes is 180°.(2)There is a high-velocity stream rising spirally near the wall with a series of spirally rising vortexes exist around the center axis in the swirling precalciner. Particles disperse uniform throughout the precalciner.By changing the initial tertiary air velocity but fixing the initial flue gas velocity-and vice versa-the predicted results were compared.It is advised to set the raw meal inlet along the direction of the initial tertiary air with the horizontal projection angle between the raw meal inlet and the tertiary air inlet is in the range of 45°and 135°,especially, 135°is the best.The research results of coal combustion show that the coal burn-off rate is high coming from the coal pipe located on the same side of the tertiary air inlet,while it is quite low coming from the coal pipe located on the different side of the tertiary air inlet,resulting in a low total coal burn-off rate,indicating the importance of the location of coal pipe.It is suggusted that when one of the coal pipe is near the tertiary air inlet and have the same initial moving direction with the tertiary air,it is better to keep the other coal flow out of the initial direction of the tertiary air.(3)There is a very complex and turbulent flow field in the swirling-spouted precalciner:in the half down-body of the precalciner,there is a high velocity region in the center while two series of "c-shaped" low-velocity regions occur around the center;at the center-body,there appear two high-velocity vortexes cause by the "neck",resulting in a spraying-liked flow field with a uniform velocity in the half up-body;in the top space,a bracket-shaped vortex appears caused by both the two outlets located on the same side and the coping on the top.The simulation results of the gas solid flow filed show that there are many particles near the raw meal inlets but few in the center of high gas velocity region.With the the fixed initial tertiary air velocity,it is optimum when the flue gas velocity is between 20m/s and 30m/s.It is optimum when the angle between the raw meal pipe and the precalciner body is in the range of 24°and 34°.The better,the smaller of the angle between two raw meal inlets on the same side,and it is optimum when the angle is 0°in this research.The research results of coal combustion show that this type of precalciner has a good adaptability for different quality coal combustion,and it is suitable for low-volatile inferior coal combustion.The optimum researches results show that it is beneficial when the angle between the two coal pipes on the same side is 40°;with the angle of 90°,the larger the velocity is,the higher the coal burn-off rate is,and it is optimum when the tertiary air velocity is 24 m/s.(4)Comparing research indicates that both the spouted precalciner and the swirling precalciner in this research have a good adaptability for coal combustion with different quality,and it is suitable for low-volatile inferior coal combustion.If adjust parameters properly,the coal burn-off rate will increase greatly.There is a great potential for inferior coal combustion in the swirling-spouted precalciner if the position and direction of the coal pipes are adjusted suitably.