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.