Experimental study and modeling of NOx generation from char nitrogen in the bubbling bed

Char-NOx is mainly generated in the dense bed of circulating fluidized bed ( CFB) boilers. A small-scale bubbling bed test rig was used to model the dense bed and single char particles were burnt in the 850 ℃ bed tempera- ture and 10% O2 . The influences of bed material size,pyrolysis coal type,char particle diameter and fluidization veloc- ity on the conversion ratio of char-nitrogen to NOx were investigated. For three different types of char,the conversion ratio of char-nitrogen to NOx increases with the increase of bed material size due to the subsequent increase of mass transfer coefficient and the decrease of heat transfer coefficient in the emulsion phase of bubbling bed. Meanwhile,the burning process of chars was accelerated. The conversion rate decreases with the increase of initial char particle diame- ter,but is not significantly influenced by fluidization velocity. The char temperature was also measured,which verified the previous experimental results. Furthermore,the model of NOx generation from single char particle was established based on the mass and heat transfer mechanism in the dense bed,and the simplified chemistry mechansim. The model was proved to be reliable to match the experimental results and would benefit for the modelling of NOx emission in a practical CFB boiler.