Model of the low-temperature heat release and ignition of n-butanol

Abstract

The low-temperature ignition of n-butanol and air mixtures is studied in this work. A short chemistry model of 14 reactions, for this low-temperature phenomenon, was obtained from a reduced chemical mechanism of 279 reactions with 62 chemical species that reproduces a wide range of combustion experiments: ignition delay times, jet-stirred reactor measurements and laminar flame propagation. Both chemical mechanisms include low-temperature chemistry to more accurately model ignition times for temperature ranges below 1000 K. Since there is no conclusive experimental evidence about the existence of NTC (negative temperature coefficient) behaviour for n-butanol, the present work addresses a theoretical–numerical analysis showing that this alcohol, does not present an NTC behaviour. It was found that the low-temperature kinetics is able to increase only slightly the temperature in a relatively short time. Additionally, the reaction involving CHO and OH represents the chain termination reaction in the low-temperature regime (T<1000 K) that prevents an NTC behaviour of n-butanol. The final ignition event then requires the chain branching mechanism via HO-HO-2OH to be activated, which is very slow at low temperatures. Analytical results show the parametric influence of the resulting two-stage ignition process of n-butanol at low temperatures.