On the Combustion Chemistry of n-Heptane and n-Butanol Blends

Abstract

High-speed gas sampling experiments to measure the intermediate products formed during fuel decomposition remain challenging yet important experimental objectives. This article presents new speciation data on two important fuel reference compounds, n-heptane and n-butanol, at practical thermodynamic conditions of 700 K and 9 atm, for stoichiometric fuel-to-oxygen ratios and a dilution of 5.64 (molar ratio of inert gases to O(2)), and at two blend ratios, 80%-20% and 50%-50% by mole of n-heptane and n-butanol, respectively. When compared against 100% n-heptane ignition results, the experimental data show that n-butanol slows the reactivity of n-heptane. In addition, speciation results of n-butanol concentrations show that n-heptane causes n-butanol to react at temperatures where n-butanol in isolation would not be considered reactive. The chemical kinetic mechanism developed for this work accurately predicts the trends observed for species such as carbon monoxide, methane, propane, 1-butene, and others. However, the mechanism predicts a higher amount of n-heptane consumed at the first stage of ignition compared to the experimental data. Consequently, many of the species concentration predictions show a sharp rise at the first stage of ignition, a trend that is not observed experimentally. An important discovery is that the presence of n-butanol reduces the measured concentrations of the large linear alkenes, including heptenes, hexenes, and pentenes, showing that the addition of n-butanol affects the fundamental chemical pathways of n-heptane during ignition.

A review of the combustion and emissions properties of advanced transportation biofuels and their impact on existing and future engines

“…The reduction in low-temperature ignition propensity (lower cetane numbers) when alcohols are blended with petroleum fuels is caused by the alcohol acting as a sink of reactive species that reduces the overall chain-branching reaction rates and, thus, slows ignition [171,176], as discussed above in the context of the octane ratings. The high-octane tert-butanol has a highly-branched structure that results in it having the lowest reactivity, and hence smallest cetane number, of the butanol isomers, as well as a lower cetane number than ethanol [176].…”

“…When blended with hydrocarbon fuels, ethanol acts as a sink of reactive species (OH radicals) that disrupts the chain branching of the hydrocarbon fuel under lowtemperature chemistry conditions and slows ignition of the blend [140,169]. Butanol, similarly, is seen to retard the low-temperature chemistry of n-heptane, a gasoline primary reference fuel and common surrogate fuel-blend component, by fundamentally changing the reaction pathways of the hydrocarbon fuel when it is added to a blend, thereby increasing the octane rating of the fuel blend [171,172]. As the hydrocarbon chain increases from n-butanol to n-hexanol, the reactivity at low temperatures increases, due to more low-temperature chemistry for the longchain alcohols [173].…”

Using rapid compression machines for chemical kinetics studies

“…Recent work has focused on primary reference fuels (PRFs), biofuels, and biofuel/PRF blends, including work with n-heptane [92], iso-octane [93], n-butanol [94], n-heptane/n-butanol blends [95], and methyl-butanoate [96]. These studies found that, although most chemical kinetic models are able to predict the ignition delay time fairly well (e.g., within 20% typically), the predictions of some species profiles deviate from those measured experimentally, and this may influence the model's ability to predict combustion features besides ignition delay time, e.g., soot formation and/or emissions.…”

Effect of hydrogen addition on the counterflow ignition of n-butanol at atmospheric and elevated pressures

“…Similar experiments at atmospheric pressure were conducted by Zhang et al [38], coming to similar conclusions. Using a rapid compression facility, Karwat et al [39] explored the effects of blending n-heptane/n-butanol on ignition delays, finding that butanol slowed ignition relative to pure n-heptane. In addition, Karwat et al [39] found that the presence of n-heptane caused n-butanol to react at temperatures it would otherwise be non-reactive.…”