A chemical kinetic study of n-butanol oxidation at elevated pressure in a jet stirred reactor

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Abstract

Biofuels are attractive alternatives to petroleum derived transportation fuels. n-Butanol, or biobutanol, is one alternative biofuel that can replace gasoline and diesel in transportation applications. Similar to ethanol, n-butanol can be produced via the fermentation of sugars, starches, and lignocelluloses obtained from agricultural feedstocks. n-Butanol has several advantages over ethanol, but the detailed combustion characteristics are not well understood. This paper studies the oxidation of n-butanol in a jet stirred reactor at 10 atm and a range of equivalence ratios. The profiles for CO, CO2, H2O, H2, C1–C4 hydrocarbons, and C1–C4 oxygenated compounds are presented herein. High levels of carbon monoxide, carbon dioxide, water, hydrogen, methane, formaldehyde, ethylene, and propene are detected. The experimental data are used to validate a novel detailed chemical kinetic mechanism for n-butanol oxidation. The proposed mechanism well predicts the concentration of major product species at all temperatures and equivalence ratios studied. Insights into the prediction of other species are presented herein. The proposed mechanism indicates that n-butanol consumption is dominated by H-atom abstraction from the α, β, and γ carbon atoms. A sensitivity analysis is also presented to show the effects of reaction kinetics on the concentration of several poorly predicted species.

Introduction

A potential biofuel for use in both gasoline and diesel engines is n-butanol. Historically, industrial scale production of n-butanol from biomass feedstocks was the second largest fermentation process, exceeded only by ethanol. However, its demise was brought about in the early 1960s when petroleum derived n-butanol became more economically feasible [1]. Recent advances in n-butanol production in the laboratory have spurred interest in commercial scale production of the n-butanol [2], [3]. Recently, BP and Dupont announced that they would commercially produce n-butanol, which they call biobutanol, as a gasoline blending component for automotive fuels [4], [5]. n-Butanol is produced via a fermentation process similar to that of ethanol, and therefore its feedstocks could include sugar beet, sugar cane, corn, wheat and also cellulosic biomass. n-Butanol has several advantages over ethanol including enhanced tolerance to water contamination allowing the use of existing distribution pipelines, the ability to blend at higher concentrations without retrofitting vehicles, and better fuel economy.

Relatively few engine studies of n-butanol have been published. Yacoub et al. used gasoline blended with a range of C1–C5 alcohols (including n-butanol) to fuel a single-cylinder spark ignition (SI) engine [6]. They found that the n-butanol blends had less knock resistance than neat gasoline. The n-butanol blends also had reduced CO and hydrocarbon emissions but increased NOx emissions. This may be due to the n-butanol blends having a higher flame temperature and earlier spark timing. Of particular interest to the present study is that the primary oxygenated hydrocarbon emissions were n-butanol, formaldehyde and to a lesser extent, acetaldehyde. A study by Miller et al. successfully operated unmodified gasoline and diesel engines on blends containing 0–20% n-butanol in gasoline and 0–40% n-butanol in diesel fuel [7]. Another study successfully ran a compression ignition (CI) engine fueled with n-butanol and diesel fuel microemulsions [8].

Predictive models provide a better understanding of the combustion performance and emissions characteristics of biofuel compositions and why they differ from petroleum derived materials. The development of an n-butanol model requires understanding of its fundamental pyrolysis and oxidation kinetics. However, few studies have examined the combustion chemistry of n-butanol, while none have developed a detailed chemical kinetic mechanism of the fuel. A 1959 study by Barnard examined the pyrolysis of n-butanol [9]. The experiments were carried out in a static reactor at temperatures between 579 and 629 °C. Barnard suggested that, in the absence of oxygen, n-butanol primarily reacts by the fission of the molecule at the C3H7–CH2OH bond. This produces formaldehyde, ethylene and a methyl radical, following the decomposition of the n-propyl radical. Barnard also conducted a similar study of t-butanol [10]. A study by Roberts measured the burning velocities of n-butanol using schlieren photographs of the flames [11], and found that the maximum burning velocity of n-butanol is similar to that of isopropyl alcohol and isopentyl alcohol. A recent study by McEnally and Pfefferle [12] measured the temperature and species in an atmospheric-pressure coflowing laminar nonpremixed flames. The fuels consisted of methane doped with one of the four isomers of butanol. They claimed that unimolecular dissociation was dominant, not H-atom abstraction. For n-butanol, this consisted of C–C fission followed by β scission of the resulting radicals. In the case of n-butanol, complex fission involving four-center elimination of water was estimated to account for only 1% of n-butanol decomposition. The most important measured species included ethylene (C2H4) and propene (C3H6). More recently, Yang and co-workers [13] studied laminar premixed flames fuelled by one of four isomers of butanol (including n-butanol). Their results identify combustion intermediates in the butanol flames, but do not provide concentration profiles. The qualitative data provided lends support to the aforementioned dissociation mechanism proposed by McEnally and Pfefferle [12].

In this paper, we report new experimental data obtained in a jet stirred reactor (JSR) for the oxidation of n-butanol at a pressure of 10 atm and a range of equivalence ratios (0.5–2.0) and temperatures (800–1150 K). In addition, a chemical kinetic model of n-butanol is developed using the JSR experiments as validation data. Both experimental and kinetic insights are offered below.

Section snippets

Experimental methods

The JSR experimental setup used in this study has been described earlier [14], [15]. The JSR consists of a small sphere of 4 cm diameter (39 cm3) made of fused silica (to minimize wall catalytic reactions), equipped with four nozzles of 1 mm i.d. for the admission of the gases which achieve stirring. The reactants were diluted by high-purity nitrogen (<50 ppm O2, <1000 ppm Ar, <5 ppm H2) and mixed at the entrance of the injectors. A high degree of dilution (0.1% volume of fuel) was used, reducing

Computational methods

The kinetic modeling was performed using the PSR computer code [16] that computes species concentrations from the net rate of production of each species by chemical reactions and the difference between the input and output flow rates of the species. These rates are computed from the kinetic reaction mechanism and the rate constants of the elementary reactions calculated at the experimental temperature.

The reaction mechanism used here is based on a previously proposed oxidation mechanism [17],

Results and discussion

Molecular species concentration profiles were measured by sonic probe sampling and GC and FT-IR analyses from the oxidation of n-butanol in a JSR: hydrogen (H2), water (H2O), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), acetylene (C2H2), ethylene (C2H4), ethane (C2H6), propene (C3H6), 1-butene (C4H8), acetaldehyde (CH3HCO), formaldehyde (CH2O), butyraldehyde (C3H7CHO), and n-butanol (C4H9OH). Figure 1 presents the experimental measurements and modeling results of n-butanol

Conclusions

New experimental data for n-butanol oxidation in a JSR at 10 atm and equivalence ratios between 0.5 and 2.0 are compared to a novel chemical kinetic model for n-butanol oxidation. The most abundant measured product species were carbon monoxide, carbon dioxide, water, hydrogen, methane, formaldehyde, ethylene, and propene. Measured in lesser amounts were butyraldehyde, 1-butene, acetaldehyde, ethane, and acetylene. The model proposed herein provides good overall agreement with the experimental

Acknowledgment

This research acknowledges funding from NSERC.

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