Relative Reactivity of Oxygenated Fuels: Alcohols, Aldehydes, Ketones, and Methyl Esters

This work aims at comparing and highlighting the main reaction pathways characterizing the combustion behavior of oxygenated fuels. Ethanol and heavier alcohols are already viable biofuels, despite some concern about their aldehydes and ketones emissions. Recently, the potential of 2-butanone (methyl ethyl ketone, MEK) as anti-knocking fuel was investigated at engine-relevant conditions. Moving from methyl butanoate (MB), long-chain fatty acid methyl esters are largely considered and used as biodiesels, mainly in Europe. Starting from a consistent assessment of C-H and C-C bond dissociation energies (BDEs) in n-butane, n-butanol, n-butanal, MEK, and MB, their impact on the selectivity of the different H-abstraction reactions and their relative reactivity are analyzed. Low-temperature oxidation mechanisms of 1-butanol and 2-butanone are also presented and discussed. Based on the upgraded Politecnico di Milano (POLIMI) kinetic mechanism, the relative reactivity of n-butane and the different oxygenated fuels is discussed here in depth. Stoichiometric fuel/air mixtures at 10 and 30 atm and 600-1450 K are analyzed. At low temperatures (T < 675 K), n-butanol and 2-butanone show the lowest reactivity, whereas the other fuels tend to converge to a very similar behavior. n-Butanal is the fastest to ignite in the whole T range, because it has the weakest C-H BDEs. No negative temperature coefficient (NTC) behavior is observed for n-butanal and n-butanol under the investigated conditions. A weak NTC is predicted for MB, similar to that of propane. MB and 2-butanone are the slowest to ignite between 750 and 850 K. A limited number of fuel-specific reactions characterizing each fuel and deserving more accurate investigation are highlighted, together with the lack of experimental targets below 850 K for MB and 2-butanone.