Numerical Simulation And Optimum Design Of Oxygen Enriched Combustion In A Cement Rotary Kiln

Cement industry is the main energy consuming industry and CO₂ emission industry in China.With the proposal of the goal of"carbon peaking and carbon neutrality",the application of oxygen-enriched combustion technology in the new dry process cement production system has received widespread attention,but due to the lack of systematic theoretical and technical research,the cement production has not yet formed a mature application system of oxygen-enriched combustion.In view of this,this paper conducts a theoretical study of the oxygen-enriched combustion process in rotary kilns and optimizes the design of the oxygen-enriched method,and the research work has important theoretical and engineering implications.Aiming a real-size rotary kiln,numerical simulation of air combustion conditions was carried out using ANSYS Fluent software,and the airflow field,temperature field,component concentration field and NO_xconcentration in the kiln were analyzed in detail,and the relationship between the factors was profiled.Meanwhile,based on the same oxygen flow rate,the primary air and secondary air are enriched with oxygen respectively.By comparing and analyzing the distribution of airflow,temperature and component concentration fields in the rotary kiln under the three conditions,as well as the parameters of pulverized coal combustion rate,radiation and NO_xgeneration,the best oxygen enrichment method was found.Then,the effect of oxygen enrichment concentration on the parameters of rotary kiln was studied under the optimal oxygen enrichment mode,and the optimal oxygen enrichment concentration was found.The results of numerical simulation of air combustion conditions show that the high-temperature zone is"willow leaf-like"with a reasonable flame shape,radiant heat transfer accounts for 94%of the total heat transfer in the firing zone,and the thermal NO_xgeneration rate is the fastest and the generation amount is the largest,in which the total NO_xgeneration at the rotary kiln outlet is 1099 ppm.Based on the same oxygen flow rate,the primary air and secondary air were enriched with oxygen,and the research results showed that the flow field in the kiln was not significantly affected before and after the enrichment.the corresponding high temperature zone before and after the enrichment was in the shape of"willow leaf-like",and the black flame head length was almost equal,and the overall shape did not change significantly compared with the air combustion condition.Compared with the air combustion condition,the combustion rate of pulverized coal in the burning zone were all improved to a certain extent after the oxygen enrichment,and the maximum and average temperatures in the kiln were increased for both oxygen enrichment conditions.Compared with the air combustion condition,the content of H₂O,CO₂ and CO in the kiln increased after oxygen enrichment,which led to an increase in radiation,with the largest increase in total radiation in the primary air enrichment.Compared with the air combustion condition,the NO_x generation in the kiln increased significantly after oxygen enrichment,with the largest increase in the primary air enrichment,which is inseparable from the increase in the kiln temperature and oxygen concentration after oxygen enrichment.On balance,oxygen enrichment in the primary air brings the highest benefit to the rotary kiln.The primary air was enriched with different concentrations of oxygen,and the research results showed that the primary air enriched with oxygen concentration would not affect the flow field in the kiln.When the oxygen concentration increases,the overall shape of the flame does not change significantly and the length of the black flame head is almost equal,but the burning rate of volatiles and char accelerates,which makes the high temperature zone cover a longer position,the average temperature rises,the local high temperature increases and the temperature uniformity decreases,and when the oxygen concentration is greater than 30%,the effect on the increase of burning speed and temperature becomes smaller.With the increase of oxygen concentration,the front end of the kiln H₂O,CO₂,CO.When the concentration of oxygen is greater than 30%,the change of total radiation in the first 18 m and 31 m becomes smaller compared with the low concentration.For NO_x,the total NO_xgeneration increases with the increase of oxygen concentration,and there is a sudden increase of NO_xwhen the oxygen concentration is greater than 35%.On the whole,30%oxygen enrichment in primary air is the best.