一次风速对高浓度煤粉预燃式低氮燃烧器性能影响的数值模拟

Numerical simulation of the effect of primary air velocity on the performance of high-concentration pulverized coal pre-combustion low-nitrogen burner

高浓度煤粉燃烧器能稳定燃烧和显著降低NO_(x)排放,是一种经济环保的燃烧技术,其一次风速对炉内着火延迟、煤粉燃烧稳定性以及NO_(x)排放量都有重要的影响。为了确定适合新型高浓度煤粉预燃式低氮燃烧器的一次风速,为燃烧器的现场试验和实际运行提供指导依据,采用ANSYS Fluent软件模拟计算了一次风速对煤粉燃烧稳定性和NO_(x)排放的影响。先进行网格无关性检验,并用一台25 t/h全尺寸煤粉工业锅炉进行试验,验证了模型的准确性。数值模拟计算结果表明:新型高浓度煤粉预燃式低氮燃烧器可在预燃室和炉膛内形成2个回流区,预燃室内的回流区保证煤粉稳定燃烧,炉膛内的回流区降低NO_(x)。一次风速过低时,一、二次风的后期混合减弱,煤粉燃烧不稳定,NO_(x)排放量略微升高;一次风速过高时,二次风与煤粉的混合被削弱,煤粉燃烧同样不稳定,且焦炭转化率明显下降,NO_(x)排放大幅增加;一次风速从17 m/s增加到20 m/s,出口截面NO_(x)浓度提高约10%;适当的一次风速不仅能稳定煤粉着火和燃烧,还能实现NO_(x)低排放。试验研究燃烧器的最佳一次风速在14~17 m/s。

The high-concentration pulverized coal burner can stabilize the combustion and significantly reduce NO_(x) emissions,which is a very economical and environmentally friendly combustion technology.Its primary wind speed has a very important impact on the ignition delay in the furnace,the stability of pulverized coal combustion and the NO_(x) emissions.In order to determine the primary wind speed suitable for the new high-concentration coal pre-combustion low-nitrogen burner,and provide guidance for the on-site experiment and actual operation of the burner,ANSYS Fluent software was used to simulate and calculate the influence of primary wind speed on the combustion stability of pulverized coal and NO_(x) emissions.In this paper,a grid-independent test was performed first,and a 25 t/h full-scale pulverized coal industrial boiler was tested to verify the accuracy of the model.The numerical simulation results show that the new high-concentration coal pre-combustion low-nitrogen burner can form two recirculation zones in the pre-combustion chamber and the furnace.The recirculation zone in the pre-combustion chamber ensures stable combustion of pulverized coal,and the recirculation zone in the furnace reduces NO_(x).When the primary wind speed is too low,the later mixing of primary and secondary air weakens,the combustion of pulverized coal is unstable,and NO_(x) emission increases slightly.Also,when the primary wind speed is too high,the mixing of the secondary air and the pulverized coal is weakened,and the combustion of pulverized coal is unstable,resulting in a significant decrease in the coke conversion rate and a significant increase in NO_(x) emissions.The primary wind speed increases from 17 m/s to 20 m/s,and the NO_(x) concentration in the outlet section increases by about 10%.An appropriate primary wind speed can not only stabilize the ignition and combustion of pulverized coal,but also achieve low NO_(x) emissions.The best primary wind speed of the burner studied is between 14-17 m/s.