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

Abstract

SUMMARYNumerical simulation with detailed chemistry has been carried out to clearly discriminate the thermal and chemical contributions of added diluents (H 2 O and CO 2 ) to major flame structures and NO emission characteristics in H 2 /N 2 counterflow diffusion flame. The pertinence of GRI, Miller-Bowman, and their recent modified mechanisms are estimated for the combined fuel of H 2 , CO 2 , and N 2 . A virtual species X ; which displaces the individual CO 2 and H 2 O in the fuel sides, is introduced to separate chemical effects from thermal effects. In the case of H 2 O addition the chain branching reaction, H+O 2 ! O+OH is considerably augmented in comparison with that in the case of CO 2 addition. It is also seen that there exists a chemically super-adiabatic effect in flame temperature due to the breakdown of H 2 O. The reaction path of CH 2 O ! CH 2 OH ! CH 3 and the C1-branch reactions become predominant due to the breakdown of CO 2 . In NO emission behaviour super-equilibrium effects caused by the surplus chain carrier radicals due to the breakdown of added H 2 O are more superior to the enhanced effects of prompt NO with the breakdown of added CO 2 . Especially, it is noted that thermal NO emission is directly influenced by the chemical super-equilibrium effects of chain carrier radicals in the case of H 2 O addition. As a result the overall NO emission in the case of the addition of H 2 O is higher than that in the case of CO 2 addition.

Evaluation of chemical effects of added CO2 according flame location

“…The governing equations for mass, momentum, chemical species and energy, and the solution technique can be found in Lutz et al (1997). In this study, we retain the sink term due to the thermal radiation and details of the thermal radiation model, based on an optically thin approximation, can be found in the previous researches (Ju et al, 1997;Kim et al, 2002;Park et al, 2002aPark et al, -c, 2003a. The thermal radiation term is particularly involved due to its importance in the flame dynamics at low strain rates and the high gas radiation absorption coefficient such as CO 2 , CO, H 2 O, and CH 4 .…”

“…The production of C 1 -and C 2 -chemical species, which mainly originated from the reaction CO 2 +H ! CO+OH, was a direct evidence of chemical effects, and thus as a result these chemical effects reduced the flame temperature and affected prompt NO in a complicated manner (Liu et al, 2001;Kim et al, 2002;Park et al, 2002aPark et al, -c, 2003a.…”

“…One reason may be the reduction of flame temperature, which is absolutely associated with thermal NO, due to high heat capacities of recirculated major combustion products such as carbon dioxide and water vapour, while the high temperature of recirculated gases can cause an increase in NO x emission. It should be additionally, kept in mind that there exist chemical effects of recirculated major gases, thereby affecting flame structure and NO x emission behaviour considerably (Park et al, , 2002aLee et al, 2001;Liu et al, 2001;Kim et al, 2002). Because of these conflicting effects, it is difficult to optimize combustors for achieving best performance with minimum emissions.…”

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

“…The temperatures at both fuel-and oxidizer-sides are 300 K, respectively. It was well recognized in the previous researches (Ju et al, 1997;Lee et al, 2001;Kim et al, 2002;Park et al, 2001) that especially at low strain rates the reduction of flame temperature was due to radiative heat loss. This was so because radiative heat loss is proportional to the volume of reaction zone and reaction zone thickness is physically inversely proportional to square root of strain rate.…”

“…Especially for the description of CO 2 -added flames Li and Williams (1999) took a different approach, which each with rate parameters were evaluated independently but might provide reasonable agreement with in references cited for other mechanisms. Park et al (2003b) took the basic skeleton with MillerBowman mechanism, and most of the rate parameters with the specific reaction rate constants were taken with those in the Li-Williams mechanism since the Miller-Bowman mechanism was less sensitive (Kim et al, 2002), and showed good agreement with the experimental data of Rrtveit et al (2002). The present study also adopted the reaction mechanism of Park et al (2003b), in which consists of 49 chemical species and 239 elementary reaction steps.…”