NOx reduction for oxygen enriched combustion with two-stage tubular flame

Oxygen-enriched combustion is frequently applied to industrial furnaces for requiring high temperature heating. The burned gas temperature in the oxygen-enriched combustion becomes high because of a decrease of inert gas (nitrogen). However, it is difficult to avoid the increase of NOx formation. This research tries to reduce the NOx emission from the oxygen-enriched combustion by the two-stage combustion using the tubular flame in the first combustion zone. The oxygen-enriched combustion experiments were implemented using enriched air (the oxygen concentration; up to 27%) with a prototype tubular flame burner of 1 kW output. The NOx emission from the first combustion zone became approximately zero even when using oxygen-enriched air by applying the rich premixed combustion at the nearly upper flammability limit. The NOx emission of the two-stage combustion using tubular flame was lower than the single-stage combustion. The reduction rate was 58% at 21% O2 air, 73% at 24% O2 and 79% at 27% O2. As the oxygen concentration rose, the NOx reduction rate became higher. Since the heat release rate in the second combustion zone can be mild when introducing the tubular combustion as a flame of the first stage, the NOx emission became decreased. This effect may become clear under the higher temperature condition of higher oxygen concentration.