Effect of staged air with a high-speed jet on ammonia pyrolysis and NOx formation in an ammonia-coal co-firing flame
As a zero-carbon clean fuel,ammonia plays an important role in carbon emission reduction,but its tendency of NOx generation during combustion is large.To control NOx formation during ammonia-coal co-firing,an improved ammonia doped cyclone burner struc-ture with a built-in high-speed air jet array structure was proposed using a cyclone burner used in a 50 kW one-dimensional test furnace system as a prototype,in order to realize ammonia pyrolysis before combustion and air-staged combustion in the furnace.CFD combustion numerical simulation was further used to investigate the effects of air staged ratio,excess air coefficient and ammonia fuel nozzle size on the flame structure and NOx emission of ammonia-coal co-firing,and to optimize the structure and operating parameters of the new com-bustor.The results show that compared with the prototype burner,the use of spatially dispersed high-speed tertiary air jets leads to lagging in the combustion zone,deepens the air staging,reduces the local excess air coefficient in the main combustion zone,and inhibits the ex-cessive oxidation of ammonia to form NOx,and also reduces the peak flame temperature,which is conducive to the inhibition of the forma-tion of thermal NOx.At the same time,by regulating the total excess air coefficient so that the ammonia pyrolysis occurs along the axial ex-tension of the furnace,the high-temperature under-oxygenized zone in front of the flame increases,to promote the pyrolysis of ammonia into N2 and H2,and further reduce the ammonia direct conversion to form NOx.At the air staged ratio of 20∶22∶58,the volume fraction of NOx formed by the prototype burner of 3 309×10-6 decreases to 1 069x10-6 of the improved burner,a reduction of 67.69％.Increasing the excess air coefficient,or keeping the excess air coefficient constant and controlling the primary air ratio constant while increasing the tertiary air ratio will promote the above effect and further reduce NO formation.The variation range of the inner diameter of the ammonia tube is only 5-9 mm,and the synergistic effect of the ammonia injection rate and the dispersed high-speed tertiary air need to be further investigated.
ammonia-coal co-firingdeep air stagingammonia pyrolysishigh-speed air jettertiary air
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