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

Abstract

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.