628 / 2019-01-20 20:38:27

Pressurized MILD and oxy-fuel combustion in counter-flow configuration: Emissions of NO and CO

MILD combustion, oxy-fuel combustion, pressurized combustion, NO and CO emissions

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This paper systematically investigates the effect of pressure on the NO and CO emissions from counter-flow combustion under MILD and oxy-fuel conditions. After comprehensive validation of the modeling, the effects of pressure and strain rate on the formation of nitrogen oxides (NOx) and CO are investigated in detail. Note that the terms “T-N2”, “T-CO2”, “MILD-N2” and “MILD-CO2” combustion adopted hereafter represent the traditional combustions diluted by N2 and CO2, and MILD combustions diluted by N2 and CO2, respectively.
As the pressure increases from 1 atm to 10 atm under T-N2 or T-CO2, the temperature raises by approximately 300 K and the NO emission drastically increased by 10 times. However, for MILD-N2 or MILD-CO2, the temperature raises by only 150 K. More interestingly, although both the pressure and reaction temperature increase, the NO emission from MILD combustion first increases but then decreases with the peak NO emission obtained at 3 atm, regardless of N2 or CO2 dilution. This un-expected NO reduction phenomenon at high pressure is caused by the competition between the chain reaction (H+O2 ⇌ O+OH) and recombination reaction (H+O2(+M) ⇌ HO2(+M)). The CO formation in MILD combustion is also decreased with increasing pressure. A higher temperature and lower emissions of NO and CO implies that the higher energy efficiency and lower pollution emission can be simultaneously achieved in pressurized MILD combustion (> 3 atm). Further analysis found that the conventional thermal NO route is always suppressed under pressurized MILD-N2 and MILD-CO2 conditions. The prompt NO route controls the NO formation in pressurized MILD-N2 combustion, while the N2O-intermediate route dominates the NO production in pressurized MILD-CO2 combustion. The importance of N2O-intermediate route increases significantly when the strain rate is lower than 10 s-1.