氨作为零碳清洁燃料,对碳减排有重要作用,但其燃烧过程中NOx生成倾向大。针对煤掺氨燃烧NOx生成控制问题,以某50kW一维试验炉系统所用旋流燃烧器为原型,提出一种内置高速空气射流阵列结构的新型煤掺氨旋流燃烧器结构,以实现炉内氨预分解后燃烧和分级燃烧。进一步用CFD燃烧数值模拟探究空气分级比、过量空气系数及氨燃料喷口尺寸对氨煤掺烧火焰结构及NOx排放影响,优化新型燃烧器结构及运行参数。结果表明,相比于原型燃烧器,采用空间分散高速三次风射流会导致燃烧区域滞后,加深空气分级,减小主燃烧区域局部过量空气系数,抑制氨过度氧化形成NOx,也降低火焰温度峰值,利于抑制热力型NOx产生。同时通过调控总过量空气系数使氨热解发生区域沿炉膛轴向逐渐延伸,火焰前部高温欠氧区增大,促进氨受热预分解为N2和H2,进一步减少氨中燃料氮直接转化形成NOx。空气分级比20∶22∶58时,NOx生成体积分数由原燃烧器的3309×10-6降至新型燃烧器的1069×10-6,降幅达67.69%。增大过量空气系数,或过量空气系数不变、控制一次风率不变而增大三次风率将促进上述效应,进一步减少NO生成。氨管内径变化范围仅考虑5~9mm,氨喷射速度和分散高速三次风带来的协同效应有待进一步研究。

As a zero-carbon clean fuel, ammonia plays an important role in carbon emission reduction, but its tendency of NOx generationduring combustion is large. To control NOx formation during ammonia-coal co-firing, an improved ammonia doped cyclone burner structure with a built-in high-speed air jet array structure was proposed using a cyclone burner used in a 50 kW one-dimensional test furnacesystem as a prototype, in order to realize ammonia pyrolysis before combustion and air-staged combustion in the furnace. CFD combustionnumerical simulation was further used to investigate the effects of air staged ratio, excess air coefficient and ammonia fuel nozzle size onthe flame structure and NOx emission of ammonia-coal co-firing, and to optimize the structure and operating parameters of the new combustor. The results show that compared with the prototype burner, the use of spatially dispersed high-speed tertiary air jets leads to laggingin the combustion zone, deepens the air staging, reduces the local excess air coefficient in the main combustion zone, and inhibits the excessive oxidation of ammonia to form NOx, and also reduces the peak flame temperature, which is conducive to the inhibition of the formation of thermal NOx. At the same time, by regulating the total excess air coefficient so that the ammonia pyrolysis occurs along the axial extension of the furnace, the high-temperature under-oxygenized zone in front of the flame increases, to promote the pyrolysis of ammoniainto N2 and H2, and further reduce the ammonia direct conversion to form NOx. At the air staged ratio of 20 ∶ 22 ∶ 58, the vclume fractionof NOx formed by the prototype burner of 3 309×10-6 decreases to 1 069×10-6 of the improved burner, a reduction of 67.69%. Increasingthe excess air coefficient, or keeping the excess air coefficient constant and controlling the primary air ratio constant while increasing thetertiary air ratio will promote the above effect and further reduce NO formation. The variation range of the inner diameter of the ammoniatube is only 5-9 mm, and the synergistic effect of the ammonia injection rate and the dispersed high-speed tertiary air need to be furtherinvestigated.

0 引言

1 燃烧器结构设计及模拟方法

   1.1 燃烧器结构设计

   1.2 模拟方法及计算工况

   1.3 网格独立性验证

2 结果与讨论

   2.1 分散三次风射流参数对火焰结构及NO排放影响

   2.2 过量空气系数对火焰结构及NO排放影响

   2.3 氨燃料射流参数对火焰结构及NO排放影响

3 结论