Effect of burnout air injection conditions on methane air-staged MILD combustion in a laboratory-scale furnace

Moderate or intense low-oxygen dilution (MILD) combustion is a promising technology with low NOx emissions. Despite this, further NOx emission reduction for MILD combustion is ongoingly required in response to the increasingly stringent NOx emission regulation. This paper proposes a scheme of MILD combustion combined with air-staged (along the furnace height) combustion to form air-staged MILD combustion for realizing ultra-low and even near-zero NOx emissions. The effect of burnout air injection conditions: staging ratio (SRbt, air = 0-50 %), position (L-bt,L- air = 550-850 mm), and velocity (U-bt,U- air = 13-52 m/s) on methane air-staged MILD combustion in a 20-kW laboratory-scale furnace is numerically investigated. In particular, the effectiveness and mechanism of NO emission reduction and recycled-NO reduction are analyzed, besides the reaction zone structure, interaction between turbulence and chemistry, and CO production and consumption. The results show that, with air staging, MILD combustion can yield a lower Damk & ouml;hler number closer to unity in a larger main/primary combustion zone with better temperature uniformity, further resulting in the extension of the combustion state from flamelets-in-eddies to well-distributed. Interestingly, further NO emission reduction can be realized for MILD combustion by combining air-staged combustion. In air-staged MILD combustion, NO emission decreases first and then increases with SRbt, air from 0 to 50 %, where the maximum NO emission reduction efficiency reaches 80.2 % at SRbt, air = 30 %, which is caused by less NO formation via the prompt, NNH, and N2O-intermediate mechanisms as well as more NO reburning. Air-staged MILD combustion can also enhance recycled-NO reduction, with the NO reduction efficiency increased by 18-46.7 % when doping 1000 to 10000 ppm NO in the fuel, which is attributed to the enhancement of NO reduction by HCCO/CHi=0-3 in an enlarged NO reduction zone. In conclusion, air-staged MILD combustion is suggested for deep NO emission control in industrial furnaces, with a moderate increase in L-bt,L- air and/or U-bt,U- air to maximize NO emission reduction.