The effects of the initial NO volume fractions on the NOx generation and reduction routes under natural gas MILD combustion conditions

Abstract

• The thermal and N 2 O-intermediate routes are dominant in NOx generation under MILD combustion. • Contributions of each NO generation and reduction route to NO emission are quantitatively determined. • The EINO in exhaust gas increases with the increase of initial NO volume fraction. Nitrogen oxides (NO x ) generated from fossil fuel combustion are the main sources of air pollutants and photochemical smog. Moderate and Intense Low-oxygen Dilution (MILD) combustion has been regarded as a novel combustion technology which can significantly improve combustion efficiency and reduce pollutant emissions. The natural gas MILD combustion in the International Flame Research Foundation (IFRF) semi-industrial test furnace were simulated by using standard k-ε model and Eddy Dissipation Concept (EDC) model with GRI-Mech 2.11 mechanism. The predicted results of flue gas velocity, temperature and species volume fractions were compared with the experimental data to validate better performance of the present model. The relative contributions of various NO generation and reduction routes to total NO x emissions were quantitatively determined to obtain the dominant NO formation pathway under MILD combustion conditions. The NO in the preheated oxidizer and the in-furnace recycled flue gas may have an important effect on the NO x generation and reduction during the combustion process. The effects of the initial NO volume fraction on the key reactions of individual NO x generation and reduction route were studied by adjusting the NO volume fraction in the oxidizer. The results showed that the net NO x emission and CO emission in the IFRF furnace were as low as 13.6 mg/m 3 and 3.7 mg/m 3 referring to 15% O 2 volume fraction, respectively, which were much less than the NO x and CO emission thresholds (40 mg/m 3 for NO and 67 mg/m 3 for CO) for natural gas MILD combustion. Moreover, the thermal route and N 2 O-intermediate route played the dominant roles in NO x generation and their contributions to the total NO x emission in the exhaust gas were 48% and 34% for IFRF experiment case, respectively. Although the kinetic rates of NO-reburning reactions in the furnace continued to increase, the net emission index of NO ( EINO ) in the exhaust gas increased by 27% due to the enhanced thermal route when the initial NO volume fraction in the oxidizer increased from 0 ppm to 500 ppm.