Kinetic study of the effect of sub-atmospheric conditions on the laminar burning velocity of high C2H6 content natural gas mixtures

The laminar burning velocity (SL) was measured at sub-atmospheric pressure (0.84
atm) and an environmental temperature of 295 ± 2K for two high C2H6 content
fuel mixtures, 75% CH4 – 25% C2H6 (mixture M1), and 50% CH4 – 50% C2H6
(mixture M2), as well as the pure constituent fuels. The equivalence ratios for the
experiments ranged between 0.8 and 1.4. Numerical calculations predicting SL were
performed using 3 detailed reaction mechanisms, finding GRI-Mech 3.0 to achieve
the best agreement at the pressure conditions evaluated. The pre-exponential factor
of reaction H + O2 = O + OH (R38) was modified in order to improve the numerical
results at sub-atmospheric conditions. Kinetic analysis by means of the defined
reaction factor (F ±j R,i) was carried out to identify the mechanism for SL changes at
sub-atmospheric conditions. According to the experimental results, SL increased by
15.9% and 26.3% for mixtures M1 and M2, respectively, at 0.84 atm as compared to
1.0 atm. The reaction pathways elaborated employing FR indicate that the increase
in SL at sub-atmospheric conditions is caused by increased CH3 radical production
by reaction C2H5 + H = 2CH3 (R159), which increases the formation of H radical
through reactions O + CH3 = H + CH2O (R10) and O + CH3 = H + H2 + CO
(R284). The recombination reactions associated with the production of CH4 and C2H6
also contribute to SL increases at sub-atmospheric conditions.