Experimental and kinetic study on the pyrolysis and oxidation of isopentane in a jet-stirred reactor

Abstract

An experimental and kinetic study on the pyrolysis and oxidation of isopentane (2-methylbutane) were conducted in this work. The experiments were performed in a jet-stirred reactor (JSR) at the equivalence ratios of 0.5, 1.0, 2.0 and ∞, across the temperature range from 700 to 1100 K, and at atmospheric pressure. Mole fractions of oxygen, hydrogen, CO, CO₂, C₁C₆ hydrocarbons and methanol were measured using a gas chromatograph (GC), at the initial fuel mole fraction of 0.5% and residence time at 2 s. Three literature kinetic models, named as the Bugler model, the NUIGMech1.1 model, and the LLNL model, were employed to predict the speciation profiles measured in this work, and the ignition delay times in the literature. Based on the model performances and kinetic analysis, some modifications were made to the LLNL model, by supplementing the beta-scission reaction aC₅H₁₁ = C₄H₈–1 +CH₃, and updating the rate constants for the reactions iC₅H₁₂ + OH = cC₅H₁₁ + H₂O, iC₅H₁₂ + OH = bC₅H₁₁ + H₂O, C₃H₄-a = C₃H₄-p, C₃H₄-a + H = C₃H₄-p + H, and C₂H₆ + CH₃ = C₂H₅ +CH₄. After the modifications, the model predictions on mole fractions of 1-butene, ethane, allene and propyne in JSR pyrolysis and oxidation were improved, and the overestimations on the ignition delay times at low temperatures are significantly reduced. Reaction pathway and sensitivity analyses were carried out using the modified model. The results indicated that fuel consumption in pyrolysis is sensitive to the unimolecular decomposition reaction iC₅H₁₂ = iC₃H₇ + C₂H₅ across the temperature range of 900–1100 K. In addition, fuel low-temperature oxidation reactivity is sensitive to the mutual conversion between HO₂ and H₂O₂, while the competition between OH and HO₂ formation has a more pronounced effect at increased temperatures.

Introduction

Isopentane (2-methylbutane, iC₅H₁₂) is the smallest isoalkane present in gasoline, with a low boiling point (27.8 °C) and a high octane number (RON∼93.2, MON∼90.8 [1]). Recently, it was found that the inclusion of isopentane into surrogate can more satisfactorily capture the physical properties of real fuels. For instance, Gail et al. [2,3] proposed a three-component gasoline surrogate with isopentane, n-heptane and toluene (THIP). Compared to traditional toluene primary reference fuels (TPRF), the THIP was found to be easier to replicate the octane number, density, distillation and Dry Vapour Pressure Equivalent (DVPE) of gasoline fuels, and also to precisely capture the ignition delay times of real fuels in rapid compression machine (RCM) experiments. Other multi-component gasoline surrogates involving isopentane were also formulated in the literature studies [4], [5], [6] to describe various combustion behaviors of real fuels experimentally and kinetically.

The development of combustion kinetic models for transportation fuels is of interests, because it can improve our understanding in the chemistry governing fuel oxidation and pyrolysis. To this end, several experimental and modeling studies on pyrolysis and oxidation of isopentane have been reported in the literature. Ribaucour et al. [7] studied autoignition characteristics of three pentane isomers in an RCM at temperatures from 640 to 900 K and at initial pressures of 0.39 and 0.53 atm. Two-stage autoignition behaviors were observed for the three isomers, and the ignition delay times of isopentane were found to be longer than those of the other two isomers. Autoignition behaviors of pentane isomers were also investigated by Bugler et al. [8] at equivalence ratios of 0.3–2.0 and at pressures of 1–20 atm in an RCM and two shock tubes (STs). The results showed that the ignition delay times of isopentane are the longest at temperatures up to 900 K, but become shorter than neopentane at higher temperatures. In addition, measured ignition delay times in a ST were also reported by Oehlschlaeger et al. [9], and the experimental conditions were extended to lower fuel concentrations (up to 0.025%) and higher temperatures (up to 1726 K). Except for the experimental studies, Bugler et al. [10] developed a detailed kinetic model for pentane isomers, based on a comprehensive evaluation of thermodynamic parameters and rate constants for C₅ species and low-temperature oxidation reactions. Good agreements were achieved between the measured and simulated ignition delay times for the three pentane isomers. Sajid et al. [11] measured the mole fractions of methane, acetylene and ethylene during the pyrolysis of n-pentane and isopentane in a ST at temperatures of 1400–2100 K. The measured speciation profiles were compared with the predictions of a model by Zhang et al. [12], which involves the sub-models of pentane isomers from Bugler et al. [10]. They stated that the model overestimated the concentration of ethylene, and underestimated the concentrations of methane and acetylene.

Table 1 summarizes the existing experimental studies on the combustion behaviors of pentane isomers in the literature. It was found that a few experimental studies on n-pentane and neopentane in flow reactors (FR) or jet-stirred reactors (JSR) were available, however, experimental data for isopentane oxidation or pyrolysis in reactors have not been reported. Therefore, this work aims to provide the experimental measurements for isopentane oxidation and pyrolysis. The experiments were performed in a JSR at equivalence ratios of 0.5, 1.0, 2.0 and ∞, and over a temperature range of 700–1100 K. Besides, the comparison between the measured species mole fractions and the prediction by the literature chemical kinetic models were provided. Reaction pathway and sensitivity analyses were conducted to improve the understanding of chemistry controlling isopentane oxidation and pyrolysis behaviors.