Article pubs.acs.org/JPCA
Effects of New Ab Initio Rate Coefficients on Predictions of Species Formed during n‑Butanol Ignition and Pyrolysis Darshan M. A. Karwat,*,† Margaret S. Wooldridge,†,‡ Stephen J. Klippenstein,§ and Michael J. Davis§ †
Department of Mechanical Engineering and ‡Department of Aerospace Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States § Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Experimental, time-resolved species profiles provide critical tests in developing accurate combustion models for biofuels such as n-butanol. A number of such species profiles measured by Karwat et al. [Karwat, D. M. A.; et al. J. Phys. Chem. A 2011, 115, 4909] were discordant with predictions from a well-tested chemical kinetic mechanism developed by Black et al. [Black, G.; et al. Combust. Flame 2010, 157, 363]. Since then, significant theoretical and experimental efforts have focused on determining the rate coefficients of primary n-butanol consumption pathways in combustion environments, including H atom abstraction reactions from n-butanol by key radicals such as HO2 and OH, as well as the decomposition of the radicals formed by these H atom abstractions. These reactions not only determine the overall reactivity of n-butanol, but also significantly affect the concentrations of intermediate species formed during n-butanol ignition. In this paper we explore the effect of incorporating new ab initio predictions into the Black et al. mechanism on predictions of ignition delay time and species time histories for the experimental conditions studied by Karwat et al. The revised predictions for the intermediate species time histories are in much improved agreement with the measurements, but some discrepancies persist. A rate of production analysis comparing the effects of various modifications to the Black et al. mechanism shows significant changes in the predicted consumption pathways of n-butanol, and of the hydroxybutyl and butoxy radicals formed by H atom abstraction from n-butanol. The predictions from the newly revised mechanism are in very good agreement with the low-pressure n-butanol pyrolysis product species measurements of Stranic et al. [Stranic, I.; et al. Combust. Flame 2012, 159, 3242] for all but one species. Importantly, the changes to the Black et al. mechanism show that concentrations of small products from n-butanol pyrolysis are sensitive to different reactions than those presented by Stranic et al.
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INTRODUCTION A large portion of recent combustion chemistry research has focused on the combustion chemistry of alcohols given that they can be produced from waste feedstocks and organic material, possibly reducing the demand for fossil-based fuels and/or reducing the overall emissions of carbon dioxide to the atmosphere over the life cycle of the fuel. n-Butanol in particular has been widely studied given its thermophysical properties (like vapor pressure and energy density) that make it suitable for a wide range of energy applications, including aviation. As detailed by Sarathy et al.,4 significant effort has been expended on ab initio kinetics calculations,5−14 and on experimental measurements of ignition delay times,1,2,15−17 rate constants,18 and intermediate species formation at enginerelevant temperatures and pressures.1,19−23 We direct the interested reader to Sarathy et al.4 for a comprehensive review of the chemistry of alcohol combustion. As described by Sarathy et al.,4 for alcohols, many reaction rates, such as some of the key H atom abstractions, have not yet been measured (and many reaction rates are difficult to measure). Researchers building chemical kinetic mechanisms have thus relied on analogies to hydrocarbon counterparts to estimate the rate coefficients for these reactions. Take, for © 2015 American Chemical Society
example, the detailed n-butanol chemical kinetic mechanism developed by Black et al.2 The authors chose key H atom abstraction reaction rates, such as those for n-butanol + OH from the 3- and 4-carbon sites of n-butanol, to be identical to the corresponding rates for n-butane. (For reference, the radicals formed from n-butanol by H atom abstraction are denoted as C4H8OH-1, C4H8OH-2, C4H8OH-3, and C4H8OH4, where the radical sites correspond to the carbon sites in nbutanol numbered 1−4, with 1 being the carbon atom attached to the OH group and 4 being the carbon furthest from the OH group. The radical formed when an H atom is abstracted from the OH group of n-butanol is denoted as PC4H9O. This is the same notation used in the mechanism file of Black et al.2) Furthermore, abstraction from the 2 site was estimated to be the average of the abstraction rates from the 3 and 4 sites; abstraction from the alcohol group was assumed to be negligible; and abstraction from the 1 site was set to the rate used by Dagaut et al.22 Received: September 13, 2014 Revised: January 3, 2015 Published: January 5, 2015 543
DOI: 10.1021/jp509279d J. Phys. Chem. A 2015, 119, 543−551
The Journal of Physical Chemistry A
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simulation. All ignition simulations presented in this paper use a mole-basis mixture composition of χ(n-butanol) = 0.0245, χ(O2) = 0.147, χ(N2) = 0.541, and χ(Ar) = 0.2875, and initial conditions of P = 3.25 atm and T = 975 K, which are the average experimental conditions for the speciation experiments presented in Karwat et al.1 While the Black et al.2 mechanism predicted the ignition delay time measured by Karwat et al.1 well (measured = 15.7 ms, predicted = 18.6 ms), several key intermediate species were significantly overpredicted, as illustrated in Figure 1. For example, small alkenes were overpredicted by factors of 2−3 for propene and 4−8 for ethene. These discrepancies pointed to the need for revisions to some of the key elementary reaction rates that dictate n-butanol consumption and intermediate species production and consumption. Karwat et al.1 explored the effects of incorporating the rates for n-butanol + OH recommended by Zhou et al.,5 which were based on G3-level ab initio calculations. The revision increased the predicted ignition delay (from 18.6 to 21.8 ms) and had contrasting effects on species predictions (as shown by the dashed black lines in Figure 1). While the predictions for methane and propene improved significantly, the discrepancy between the model predictions and experimental measurements for ethene increased dramatically due to increased branching to C4H8OH-4, the β-scission of which yields ethene. Motivated by the importance of the H atom abstractions from n-butanol at moderate combustion temperatures (∼1000 K), several research groups have recently published new kinetics results for H atom abstractions from n-butanol by OH and HO2 radicals.5−8,12−14 A theoretical kinetic analysis for the decomposition of n-butanol fuel radicals was also presented recently.9 These state-of-the-art results provide the impetus for the present study, which weaves together the threads of new theoretical ab initio kinetics calculations with those of the welltested Black et al.2 chemical kinetic model to explore the effects on predicted species profiles and on reaction pathways at the very beginning stages of n-butanol combustion. There are many recent mechanisms15,22−27 for n-butanol that could have been used as the basis for this effort, but to maintain consistency with the previous study by Karwat et al.,1 we used the Black et al.2 mechanism. While experimental measurements like those of Karwat et al.1 guide the development of detailed chemical kinetic models, this work is not a traditional modeling study in that we are not trying to update a model by fitting model results to experimental data. Instead, we investigate the effects of advances in the physical understanding of key reactions that control n-butanol combustion and small radical chemistry. These physical investigations can only happen in the context of detailed chemical kinetic models, which are built using an understanding of reaction classes, insights gained from explorations of individual reaction kinetics, and scientific judgment. Thus, while such studies can yield mechanisms that agree better with experimental data, the primary goal of this study is to understand the changes that arise in mechanism predictions and pathways from physically motivated modifications to a well-tested mechanism.
Experiments that measure intermediate species formed during ignition provide stringent tests for chemical kinetic mechanisms. For example, using the University of Michigan Rapid Compression Facility (UM RCF), Karwat et al.1 measured the intermediate species formed during n-butanol ignition, and compared their experimental results to model predictions of the Black et al.2 mechanism, the most up-to-date mechanism at the time. In Figure 1, the solid black symbols
Figure 1. Comparison of experimental data and model predictions for stable intermediate species observed1 during n-butanol ignition. The solid black symbols represent the experimental measurements made by Karwat et al.1 Predictions using the unmodified reaction mechanism of Black et al.2 are presented as black solid lines. The effects of using the reaction rates for n-butanol + OH recommended by Zhou et al.5 with the Black et al.2 mechanism were explored by Karwat et al.1 and are reproduced here as black dashed lines.
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represent the experimental measurements made by Karwat et al.1 and the solid black lines represent model predictions. All data are presented on a normalized time scale (consistent with Karwat et al.1), with 0 representing the end of the compression in the UM RCF and the beginning of the chemical kinetic simulations, and 1 representing the time of ignition, which is the point of maximum rate of pressure rise in the experiment or
STUDY METHODOLOGY We systematically consider the effects of the newly predicted elementary rate coefficients on the Black et al.2 model predictions of the intermediate species measured by Karwat et al.1 via sequential substitution of the available sets of 544
DOI: 10.1021/jp509279d J. Phys. Chem. A 2015, 119, 543−551
The Journal of Physical Chemistry A
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RESULTS AND DISCUSSION Step 1. Updating Old Rates for and Incorporating New Hydroxybutyl and Butoxy Decomposition Reactions. Zhang et al.9 used an ab initio transition-state-theorybased master equation analysis to obtain rate predictions for all hydroxybutyl (C4H8OH-1, C 4H8OH-2, C4H8OH-3, and C4H8OH-4) and butoxy (PC4H9O) radical decomposition and isomerization reactions over a wide range of temperatures (300−2500 K) and pressures (1.3 × 10−3−100 atm). Their analysis properly treated well-skipping and species-merging phenomena. In contrast, the Black et al.2 mechanism ignored all pressure effects for these rate coefficients. Zhang et al.9 showed that the dominant pathway for both C4H8OH-4 and PC4H9O radical decomposition is a well-skipping pathway that forms propyl + formaldehyde, rather than C4H8OH-4 predominantly forming ethene + CH2CH2OH as presumed by Black et al.2 We replaced the original rates in the Black et al.2 mechanism with the Zhang et al.9 rates and reactions, and several new reactions determined to be important by Zhang et al.,9 and listed below, were also incorporated into the Step 1 mechanism. (The reverse rates for the forward reactions9 were calculated using CHEMKIN algorithms and the thermochemistry of Black et al.2)
theoretical rate constants. These substitutions were carried out in the following order: Step 1 is updating old rates for and incorporating new hydroxybutyl and butoxy decomposition reactions9 into the original Black et al.2 mechanism. Step 2 consists of Step 1 + updating rates for H atom abstractions from n-butanol by OH7 and by HO2.6,8 Step 3 includes Step 2 + updating the rate for HO2 + HO2.28 For each step, we briefly review the incorporated reaction rates and compare the predicted intermediate species concentrations with those presented in Karwat et al.1 The results of these steps are shown in Figure 2. For Step 3, we also compare the final mechanism predictions for the products of nbutanol pyrolysis with the speciation measurements of Stranic et al.3 and present the results of a sensitivity analysis for the conditions studied by Stranic et al.3
C4 H8OH‐1 ↔ NC3H 7 + CH 2O C4 H8OH‐2 ↔ CH3 + IC3H5OH
C4 H8OH‐2 ↔ NC3H 7 + CH 2O C4 H8OH‐3 ↔ C4 H8OH‐4
C4 H8OH‐3 ↔ NC3H 7 + CH 2O C4 H8OH‐4 ↔ NC3H 7 + CH 2O C4 H8OH‐4 ↔ C2H5 + C2H3OH PC4 H 9O → NC3H 7 + CH 2O
PC4 H 9O → C4 H8OH‐3
Furthermore, Zhang et al.9 calculated that several hydroxybutyl decomposition reactions initially included by Black et al.2 were insignificant (
