The role of radical + fuel-radical well-skipping reactions in ethanol and methylformate low-pressure flames

Although the reactions of fuel-radicals with other dominant flame radicals such as H and CH3 are important reactions in low-pressure flames, they have not been well studied. These reactions may occur through either recombination to form stabilized molecular complexes or direct abstractions and chemically activated addition-eliminations to yield bimolecular products. Here, the role of such reactions in low-pressure flames of ethanol and methylformate is studied through a combination of theoretical characterizations of key reactions and detailed kinetic modeling. In particular, H and CH3 + fuel-radical reactions have been characterized theoretically in this work and these are shown to make a pronounced impact on the formation of intermediates. Theoretical calculations for H + CH3CHOH and CH3 + CH3CHOH predict that at low pressures recombinations are minor processes with well-skipping (addition-eliminations) dominating the reaction flux. Direct abstraction was also considered in H + CH3CHOH and theory suggests that abstraction at the CH3-site forming CH2CHOH is the only important channel. Notably, this result is counter to analogy based predictions that CH3CHO should be the dominant abstraction product. Low-pressure ethanol flame simulations indicate that addition-elimination reactions from H + CH3CHOH and CH3 + CH3CHOH are a major source for C2H4 and C3H6 profiles, respectively. Similar results are observed in simulations of a low-pressure methylformate flame, where addition-elimination reactions of H + CH2OCHO and CH3 + CH2OCHO have a significant impact on CH3OH and C2H4 mole fraction profiles, respectively. The present results suggest that the well-skipping reactions of relatively stable fuel-radicals with ubiquitous flame radicals such as H, O, OH, and CH3 should be considered extensively in combustion models.