On the Chemical Kinetics of n-Butanol: Ignition and Speciation Studies

Abstract

Direct measurements of intermediates of ignition are challenging experimental objectives, yet such measurements are critical for understanding fuel decomposition and oxidation pathways. This work presents experimental results, obtained using the University of Michigan Rapid Compression Facility, of ignition delay times and intermediates formed during the ignition of n-butanol. Ignition delay times for stoichiometric n-butanol/O(2) mixtures with an inert/O(2) ratio of 5.64 were measured over a temperature range of 920-1040 K and a pressure range of 2.86-3.35 atm and were compared to those predicted by the recent reaction mechanism developed by Black et al. (Combust. Flame 2010, 157, 363-373). There is excellent agreement between the experimental results and model predictions for ignition delay time, within 20% over the entire temperature range tested. Further, high-speed gas sampling and gas chromatography techniques were used to acquire and analyze gas samples of intermediate species during the ignition delay of stoichiometric n-butanol/O(2) (χ(n-but) = 0.025, χ(O(2)) = 0.147, χ(N(2)) = 0.541, χ(Ar) = 0.288) mixtures at P = 3.25 atm and T = 975 K. Quantitative measurements of mole fraction time histories of methane, carbon monoxide, ethene, propene, acetaldehyde, n-butyraldehyde, 1-butene and n-butanol were compared with model predictions using the Black et al. mechanism. In general, the predicted trends for species concentrations are consistent with measurements. Sensitivity analyses and rate of production analyses were used to identify reactions important for predicting ignition delay time and the intermediate species time histories. Modifications to the mechanism by Black et al. were explored based on recent contributions to the literature on the rate constant for the key reaction, n-butanol+OH. The results improve the model agreement with some species; however, the comparison also indicates some reaction pathways, particularly those important to ethene formation and removal, are not well captured.

Intermediate species measurement during iso-butanol auto-ignition

“…The average ratio between the experimental carbon balance and the predicted carbon balance (C_EXP/C_CAL) is about 84% at sampling times before ignition (t/τ ign ≤ 0.95). The experimental carbon balance is reasonably good and at level similar to the other speciation studies using RCM [28,29]. In this study, the carbon loss was primarily due to the iso-butanol condensation in the sampling probe.…”

“…The concentration of CO 2 is very low (<150 ppm) for NT<0.9, and CO 2 is also excluded from the detailed comparison. Besides, previous speciation studies in RCM [28] and JSR [8] have shown that the isomers of C 2 H 4 O may elute together with acetaldehyde, and the isomers of C 3 H 6 O may elute together with propionaldehyde. Thus acetaldehyde and propionaldehyde were excluded from the comparison.…”

Experimental and Computational Study of n-Heptane Autoignition in a Direct-Injection Constant-Volume Combustion Chamber

“…4. It is noted that similar models have been used extensively to simulate combustion in a variety of systems ranging from rapid-compression machines and shock tubes to single-cylinder engines (e.g., [25][26][27][28][29]). Zero-dimensional models can be useful in evaluating ignition delay trends and general behavior, even for heterogeneous systems such as engines.…”

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.…”