Mechanism for Inhibition of Atmospheric-Pressure Syngas/Air Flames by Trimethylphosphate

The effect of the addition of trimethylphosphate on the propagation of atmospheric-pressure syngas/air flames of different stoichiometry was investigated experimentally and numerically. The inhibition effectiveness determined from experimental observations was shown to have a minimum at the equivalence ratio of 1.5 and to increase substantially with an increasing equivalence ratio. The modeling did not predict the minimum of the inhibition effectiveness but satisfactorily predicted its increase with an increase of the equivalence ratio. Sensitivity analysis showed that the effectiveness rise correlated with the relative increase in the net rate of the reaction CO + OH = CO2 + H, which is known to be the main pathway for CO oxidation. The decrease in the net reaction rate was explained by a reduction in the OH mole fraction as the inhibitor was added to the flames. The reduction in the OH concentration is not due to OH recombination in reactions involving phosphorus but is due to a decrease in its production rate in syngas oxidation reactions because of the removal of H atoms from the flames via their recombination catalyzed by phosphorus-containing species.