Numerical simulation of pulverized coal combustion in rotary kilns with different oxygen concentrations

In the present study, CFD were used to investigate the effect of a new type of rotary kiln on the velocity field, temperature field, material component distribution, and NOx concentration distribution in poor oxygen (18% O2), normal oxygen (21% O2), oxygen-enriched (24% O2, 27% O2, 30% O2) and full oxygen (100% O2) combustion conditions. The reliability of the simulations was demonstrated by grid independence and the thermal calibration data of the cement plant. The results revealed that the dual recirculation zone could strengthen the burner’s stabilizing effect. The local low-temperature zone near the burner could prolong the burner nozzle’s service life. Furthermore, the increase in oxygen concentration would change the flame length; the flame length under oxygen enrichment and full oxygen were significantly shorter than that under oxygen depletion. The highest temperature in the kiln was directly proportional to the oxygen concentration. The average temperature changes were relatively smooth, resulting in poor combustion uniformity in the kiln. The maximum temperature of poor oxygen combustion below 1730 K is not conducive to the intensified production of clinker. Full oxygen combustion can maximize the combustion temperature in the kiln and is accompanied by a significant increase in CO2 and thermal NOx concentration. As the oxygen concentration increased from 21% to 27%, the temperature in the kiln rapidly increased, and NOx slowly increased; when the O2 concentration was greater than 27%, the temperature slowly increased, and NOx rapidly increased. Therefore, oxygen-enriched conditions should be controlled by 27%, and the NOx emission reduction effect would be more obvious.