Measurement of the adiabatic flame speed and overall activation energy of a methane enriched H2/CO/CO2/N2 low heating value mixture

Abstract

The objective of this work is to investigate the effect of the methane blending in the adiabatic laminar flame speed and overall activation energy of a CO/CO2 rich, low calorific value, syngas fuel premixed combustion with air. The measurements were performed in a McKenna flat flame burner at constant equivalence ratio, unburned temperature and pressure. A flame asymptotic model curve-fitted to the measurements provided the overall activation energy and adiabatic laminar flame speed. The mixtures tested presented lower heating values from 4.5 MJ/kg to 50 MJ/kg as the CH4 fraction was changed from 0% to 100%. While the measured laminar flame speed remained approximately constant for all fuels tested, the overall activation energy evidenced the higher reactivity of the mixture blended with 20% CH4. Predictions using GRI-Mech 3.0 and San Diego mechanisms closely followed the measurements of the laminar flame speed for higher methane content, but deviations were found for the fuels with higher hydrogen content. Conversely, the mechanisms underpredicted the values of overall activation energy for all mixtures tested. A sensitivity analysis revealed the competition between the CO wet oxidation and the chain branching formation of OH and the increase in reactivity as the fraction of CH4 increased from 0 was related to a switch from a higher sensitivity of the former by the later.