Chemical effect of diluents on flame structure and NO emission characteristic in methane-air counterflow diffusion flame

Abstract

SUMMARYThe dilution effect of air stream according to agent type on flame structure and NO emission behaviour is numerically simulated with detailed chemistry in CH 4 /air counterflow diffusion flame. The volume percentage of diluents (H 2 O, CO 2 , and N 2 ) in air stream is systematically changed from 0 to 10. The radiative heat loss term, based on an optically thin model, is included to clearly describe the flame structure and NO emission behaviour especially at low strain rates. The effect of dilution of air stream on the decrease of maximum flame temperature varies as CO 2 >H 2 O>N 2 , even if heat capacity of H 2 O is the highest. It is also found that the addition of CO 2 shows the tendency towards the reduction of flame temperature in both the thermal and chemical sides, while the addition of H 2 O enhances the reaction chemically and restrains it thermally due to a super-equilibrium effect of the chain carrier radicals caused by the breakdown of H 2 O in high-temperature region. The comparison of the nitrogen chemical reaction pathway between the cases of the addition of CO 2 and H 2 O clearly displays that the addition of CO 2 is much more effective to reduce NO emission.

Thermal and chemical contributions of added H2O and CO2 to major flame structures and NO emission characteristics in H2/N2 laminar diffusion flame

“…However, it should be noted that the addition of diluents reduce the flame temperature and reaction rate due to the high heat capacity and the reduction of the concentrations of reactive species, and the breakdown of diluents such as CO 2 and H 2 O in high-temperature regions may change reaction pathways partly. The breakdown of CO 2 may alter the C1-and C2-pathways directly while that of H 2 O may increase the population of chain carrier radicals and so there may exist super-equilibrium effects in flame structure and NO emission behaviour (Park et al, 2001a). For the extensive application of the fuels of low calorific heating value and of their combined form to industrial combustors further cautious inspection may be, in the present status, required to distinguish the chemical and thermal effects clearly in the flame structure and NO emission characteristic since these effects have been failed to notice in most of previous researches (Payne et al, 1989;Hainsworth et al, 1996;Li and Williams, 1999).…”

Numerical study on NO formation in CH4-O2-N2 diffusion flame diluted with CO2

“…The detailed explanation can be made as follows. In hydrogen flames the principle chain branching reaction, H+O 2 =O+OH, which is competing with the principle recombination reaction, H+O 2 +M=HO 2 +M, is a direct measure of overall reaction rate and thereby maximum flame temperature (Park et al, 2002a(Park et al, -c, 2003aSohn and Chung, 2002). In methane flames the principle chain branching reaction, in general, competes with the H-removal reactions which are the reaction, CH 4 +H !…”

Chemical effects of CO2 addition to oxidizer and fuel streams on flame structure in H2-O2 counterflow diffusion flames

“…The previous researches for the understanding of combustion characteristics in flue gas recirculation system (Kim et al, 2002;Park et al, 2002c) displayed the different behaviors of added H 2 O and CO 2 on flame structures. That is, the dissociation of added H 2 O implies the production of chain carrier radicals (H, O, and OH), that is a measure of flame strength, and thus the chemical effects of added H 2 O may play a role of enhancing the burning.…”

“…It was also shown that the chemical effects of CO 2 addition on the fuel side are small but becomes significant when introduced on the oxidizer side. It should be noted that the direct participation of an additive in chemical reactions also altered the flame temperature chemically (Kim et al, 2002) and the chemical effects of CO 2 addition suppressed NO x formation (Park et al, 2002c;Liu et al, 2001).…”