The oxygen-deficient combustion and its effect on the NOx emission in a localized stratified vortex-tube combustor

Highlights: • Low NO_x emission with high stability can be achieved via a simple structure. • Steadily combustion results from the optimization structure of this combustor. • Low combustion temperature is the principal reason for the low NO_x emission. • Generated NO_x can be reduced by the stratified species distribution. • The flow field can promote the transport of the NO_x to the reduction region. The oxygen-deficient combustion characteristics of methane in a localized stratified vortex-tube combustor (LSVC) are studied by diluting combustion air with nitrogen. The influences of oxygen mole fraction (0.13–0.21) on flame configuration, combustion stability, combustion efficiency, and NO_x emission characteristics are experimental investigated at the inlet temperature of 300 K. Combined with the numerical simulation method, the NO_x generation, and emission mechanisms are analyzed in this combustor. Results show that the LSVC can achieve a wide stability limit, in which the global equivalence ratio can be as low as 0.22 at the lowest oxygen mole fraction (β) of 0.13. To ensure high combustion efficiency, the β should be kept above 0.16 since the oxygen-deficient condition reduces the reaction rate and flame temperature. The combustor can achieve ultra-low NO_x emission of below 10 ppm (@ 15 vol% O₂) due to low oxygen concentration and flame temperature. Furthermore, part of NO_x entrained into the fuel-rich reduction zone by the swirl flow field is reduced by the reductive species (i.e., CO and H₂) to further lowering NO_x emissions. The results of this paper can guide the development of the LSVC in the high-efficiency and low-emission combustion fields.