Analytical correlation to model diluent concentration repercussions on the burning velocity of biogas lean flames: Effect of CO2 and N2

The application of alternative and more sustainable fuels such as biogas or diluted natural gas can dramatically decrease greenhouse gas emissions and their impact on global climate. To do so, a knowledge on its typical combustion characteristics is crucial so that it can be burned in an efficient way in safe conditions. Simulations using CANTERA with the latest USC-Mech chemical-kinetic mechanism were conducted to assess the influence of diluents (CO2 and N-2) on the laminar burning velocity of lean CH4/air flames, as biogas behaves fundamentally as a CH4/diluents blend. Equivalence ratios tested ranged from 0.7 to 1.0 and diluent mass fractions (y(dil)) from 0 to 0.38. A new analytical correlation relating S-L, and y(dil), S-L/S-L,S-ref = (P/P-ref)(sigma)(T-u/T-u,T-ref)(gamma)(1 + alpha y(dil) + beta y(dil)(2)) was obtained from the simulations and validated with (e)xperimental results available in the literature. Good agreement was found between the correlation predictions and the experimental results indicating that it can be used to predict S-L in the ranges of this work: up to y(CO2,dil) = 0.38 and y(N2,dil) = 0.28, 298 K< T-u < 500 K and 1 atm < p < 10 atm. It was verified that N-2 and CO2 produce noticeably different effects when reducing S-L, with the latter exhibiting a characteristic contribution for small diluent concentrations, mainly due to a chemical impact.