Reduced mechanism for the combustion of evolved gases in forest fires
Résumé
In wildland fires, gaseous fuel released from the thermal degradation of vegetation are burnt in the flame surrounding the solid. The gaseous fuel is a complex and variable mixture including mainly CO, CH4, CO2 and other light hydrocarbons (C2H2, C2H4, C2H6, C3H6). For the first time, a detailed study of the gas-phase oxidation of a CO/CH4/CO2 mixture is reported for wildland fire modelling purpose. The experiments were performed in a perfectly stirred reactor (PSR) at atmospheric pressure over the temperature range 773 - 1273 K at fuel/air equivalence ratios of 0.6, 1 and 1.4. Mole fraction profiles as a function of temperature were obtained for molecular species via sonic probe sampling and off-line chromatography analyses. These measurements were compared to the numerical predictions obtained with the PSR code from the CHEMKIN II package with a full mechanism (GRI-Mech 3.0). A skeletal mechanism was then developed; it was derived from the full reaction mechanism through sensitivity analysis and rate-of production analysis of PSR calculations covering the range of interest. The skeletal mechanism consists in 49 elemental reactions and 20 reactive species. By using a steady state assumption for 13 reactive species, we developed a reduced four-step global mechanism with CH4, CO, H2, O2, H, CO2 and H2O as reactants and products. Both skeletal and reduced mechanisms provide a good description of the oxidation process. The numerical results for the species profiles are in agreement with full mechanism predictions and experimental data.