Numerical study of CO2 effects on laminar non-premixed biogas flames employing a global kinetic mechanism and the Flamelet-Generated Manifold technique

- CO2 diluted methane diffusion flames are investigated numerically.
- Dilution effects are evaluated based on the CO2 content in a stoichiometric mixture.
- Chemical effects are more important in the fuel-rich side of the flame.
- A progress variable was found that was able to map the flames up to 40% of CO2.
- FGM presented better results and lower computational times than a global mechanism.

Numerical simulations of flames employing detailed kinetic mechanisms are computationally demanding problems. For this reason, reduced mechanisms and techniques of chemical kinetic reduction have been developed and used in both research and industrial applications. The objective of the present work is to compare the prediction of a global kinetic mechanism and the Flamelet-Generated Manifold (FGM) reduction technique with a detailed mechanism in numerical simulations of laminar non-premixed flames. The fuel was a biogas-like mixture modelled as methane diluted with carbon dioxide. First, the effect of CO2 addition into fuel stream was studied in one-dimensional counterflow flames. Flame structure and chemical kinetics were investigated for different levels of CO2. A new approach for evaluating the dilution effect is introduced based on the CO2 content in a stoichiometric mixture. With this definition, the effect of pure dilution on major species and temperature peak values tends to be linear. The comparisons among solutions obtained with a detailed, a skeleton and a 4-step global mechanism for different strain rates and dilution levels reveals the capabilities of these chemical kinetic approaches for modelling biogas flames. Then, the 4-step mechanism and the FGM technique were compared with a skeleton kinetic model in two-dimensional coflow flames. It was observed that the 4-step mechanism presented good results for temperature and major chemical species, being a good choice when these fields are desired. On the other hand, the FGM technique presented better quantitative results with the advantage of providing detailed information about the flame composition with lower computational time. For the FGM technique, the capability of different progress variable definitions in reproducing the composition space of CO2 diluted flames is investigated and discussed in detail. The chemical and pure dilution effects of CO2 addition to the fuel stream are also discussed for the two-dimensional results.