Comparative Autoignition Trends in the Butanol Isomers at Elevated Pressure

TL;DR

This study investigates the autoignition behavior of butanol isomers at elevated pressures using rapid compression machine experiments, revealing differences in reactivity and heat release patterns, and compares experimental results with kinetic simulations to understand underlying chemistry.

Contribution

It provides new experimental data on butanol isomers' autoignition at high pressures and compares these with kinetic models to elucidate reaction pathways and reactivity trends.

Findings

Reactivity order varies with pressure among butanol isomers.

T-butanol exhibits pre-ignition heat release, especially at higher pressures.

Kinetic models show good agreement with experimental ignition delays.

Abstract

Autoignition experiments of stoichiometric mixtures of s-, t-, and i-butanol in air have been performed using a heated rapid compression machine (RCM). At compressed pressures of 15 and 30 bar and for compressed temperatures in the range of 715-910 K, no evidence of a negative temperature coefficient region in terms of ignition delay response is found. The present experimental results are also compared with previously reported RCM data of n-butanol in air. The order of reactivity of the butanols is n-butanol>s-butanol$\approx$i-butanol>t-butanol at the lower pressure, but changes to n-butanol>t-butanol>s-butanol>i-butanol at higher pressure. In addition, t-butanol shows pre-ignition heat release behavior, which is especially evident at higher pressures. To help identify the controlling chemistry leading to this pre-ignition heat release, off-stoichiometric experiments are further performed at 30 bar compressed pressure, for t-butanol at $\phi$ = 0.5 and $\phi$ = 2.0 in air. For these experiments, higher fuel loading (i.e. $\phi$ = 2.0) causes greater pre-ignition heat release (as indicated by greater pressure rise) than the stoichiometric or $\phi$ = 0.5 cases. Comparison of the experimental ignition delays with the simulated results using two literature kinetic mechanisms shows generally good agreement, and one mechanism is further used to explore and compare the fuel decomposition pathways of the butanol isomers. Using this mechanism, the importance of peroxy chemistry in the autoignition of the butanol isomers is highlighted and discussed.