OCR Text |
Show 2 2. A Model of NOx Formation and Reduction during Coal Combustion Figure 1 shows a schematic of NOx reaction mechanism near a coal combustion burner. The primary air is used to transport pulverized coal into a boiler furnace. Ignition of coal is enhanced by the hot recirculation zone formed downstream of the flame holder at the exit of the primary air nozzle. In the immediate downstream of the burner exit (zone CD in Figure 1), oxygen in the primary air is consumed by the combustion of volatile matter evolved from the coal. At the same time, the volatile-N (which is nitrogen in the volatile) matter reacts with oxygen and forms NOx in this zone. In the further downstream of a point at which all oxygen in the primary air is depleted, the formed NOx is reduced in the reduction zone (zone @ in Figure 1). When this, point comes closer to the burner, the reduction zone is wider. in the downstream of the burner exit. In the reduction zone, the NOx formed in the oxidation zone reacts with reducing agent being N 2 gas. Flame Holder __ _ •..........~. .4 ~R~~k ~~bUO:·~D~·~~~""7~.~J~_- -:.:~:.::: ZOne··~·::::~'J Coal .~~ + E~:;;'~~ I Volatile Evolution1 l'ry Air ._. =IN=+" O==2-+=N=O=~~I_--I~.L.(mJ.ua~fLLl.tuL.~ Oxidation Zone Reduction Zone (ZoneQ» (Zone~ Fig.l Schematic of NOx Reaction Mechanism near Coal Combustion Burner A simple reaction model for such volat~le evolution, NOx formation and reduction is shown in Figure 2. KVM' KN and KR are the rates of volatile evolution, NOx formation and reduction, respectively. These reaction rates are assumed to be expressed by the Arrhenius type equation and defined as follows ; KYM = AYM exp [-EVM/(RTs)] (1) KN = (d[NOx]/dt)/([N][O~) = AN exp [-EN/(RTg)] (2) KR = (-d[NOx]/dt)l([NOx][VMD = AR exp [-ER/(RTg)] (3) |