Elsevier

Combustion and Flame

Volume 157, Issue 10, October 2010, Pages 1879-1898
Combustion and Flame

A consistent chemical mechanism for oxidation of substituted aromatic species

https://doi.org/10.1016/j.combustflame.2010.07.009Get rights and content

Abstract

Computational studies of combustion in engines are typically performed by modeling the real fuel as a surrogate mixture of various hydrocarbons. Aromatic species are crucial components in these surrogate mixtures. In this work, a consistent chemical mechanism to predict the high temperature combustion characteristics of toluene, styrene, ethylbenzene, 1,3-dimethylbenzene (m-xylene), and 1-methylnaphthalene is presented. The present work builds on a detailed chemical mechanism for high temperature oxidation of smaller hydrocarbons developed by Blanquart et al. [Combust. Flame 156 (2009) 588–607]. The base mechanism has been validated extensively in the previous work and is now extended to include reactions of various substituted aromatic compounds. The reactions representing oxidation of the aromatic species are taken from the literature or are derived from those of the lower aromatics or the corresponding alkane species. The chemical mechanism is validated against plug flow reactor data, ignition delay times, species profiles measured in shock tube experiments, and laminar burning velocities. The combustion characteristics predicted by the chemical model compare well with those available from experiments for the different aromatic species under consideration.

Introduction

The study of real engine fuels is essential for two main reasons: to monitor their performance and to understand the effects of their chemical structure and composition on the combustion behavior and the resulting consequences of their emissions on the environment. However, this is complicated by the fact that real fuels like gasoline, diesel, kerosene, and jet fuels are mixtures of several hydrocarbon species of variable composition. Therefore, in numerical simulations of combustion processes, more complex real fuels are represented by surrogates [1], [2]. This surrogate fuel mixture consists of a blend of several hydrocarbon species; the constituents are chosen in such a way that the physical and chemical properties of the surrogate fuel are similar to those of the real fuel.

Aromatic compounds are major constituents of real engine fuels: 25% by volume in gasoline, 33% in diesel, and 16% in jet fuels (JP-8, Jet A/A-1) [3]. They are used as anti-knock additives to enhance the octane number of the fuels, for they have high resistance to auto-ignition [4]. Aromatic species also play a crucial role in the formation of soot as they enhance the formation of soot precursors such as Polycyclic Aromatic Hydrocarbons (PAHs). Therefore, understanding and accurately modelling the chemistry of aromatic compounds is essential to include them as components of surrogate fuels.

Much work in the past has been devoted to modeling combustion of toluene. Chemical mechanisms for high temperature (875–1500 K) gas-phase oxidation of benzene, toluene, ethylbenzene, and propylbenzene are described and discussed in detail by Brezinsky et al. [4], [5]. A detailed chemical kinetic mechanism for combustion of toluene at intermediate and high temperatures has been assembled and evaluated by Lindstedt and Maurice [6] for a wide range of oxidation regimes.

Mechanisms for dimethylbenzenes (xylenes) have also been established in the literature. Battin-Leclerc et al. [7] measured ignition delay times of xylenes in a shock tube and proposed a reaction scheme to reproduce the experimental results. Gail and Dagaut [8] studied oxidation of p-xylene (1,4-dimethylbenzene) and m-xylene (1,3-dimethylbenzene) in a Jet Stirred Reactor (JSR) at atmospheric pressure. Similarly, a detailed kinetic model was developed to describe the oxidation of p-xylene that represents the experimental JSR results. Finally, a detailed chemical mechanism was proposed for the oxidation of o-xylene (1,2-dimethylbenzene) and m-xylene by the same group [9], [10] to represent their JSR and ignition delay time results.

In the study of bi-ringed substituted species, Shaddix et al. [11] proposed reaction pathways for the high temperature oxidation of 1-methylnaphthalene based on the observed intermediate species profiles in plug flow reactor (PFR) measurements [11]. Pitsch [12] proposed a mechanism for the oxidation of 1-methylnaphthalene and validated the mechanism with PFR and ignition delay time data. More recently, Karim et al. [13] also developed a model for 1-methylnaphthalene oxidation and validated their mechanism with JSR measurements and ignition delay times.

As is evident, in almost all of the work done thus far, the focus has been on developing detailed chemical mechanisms for the oxidation of individual fuel species. However, the development of a single consistent mechanism valid for all the individual species becomes crucial. Ultimately, such a mechanism could be directly incorporated in an engine study to describe an appropriate surrogate fuel mixture. Towards this goal, this work presents a complete and consistent reaction scheme for the oxidation of a set of substituted aromatic species from moderate to high temperatures.

The current mechanism is based on the detailed mechanism recently published by Blanquart et al. [14]. This mechanism has been developed for the oxidation of thirteen fuels spanning from C1 to C8 species including alkanes such as n-heptane and iso-octane and aromatic species such as benzene and toluene. It has been extensively validated against ignition delay times and laminar burning velocities over a wide range of temperatures and pressures. Finally, it has been applied to a series of laminar premixed and diffusion flames. In all the cases investigated, the mechanism was found to reproduce the experimental measurements well.

In the current work, this base mechanism is extended to include combustion of some other aromatic species, specifically ethylbenzene, styrene, 1-methylnaphthalene, and m-xylene. In addition, the chemical scheme describing the oxidation of toluene is revised to extend the mechanism to moderate temperatures. The resulting chemical mechanism is validated against ignition delay times, species concentration profiles in shock tube experiments, PFR data, and laminar burning velocities.

The choice of the species considered in the current work was motivated by the necessity to model the oxidation of certain substituted aromatic species which are crucial components of engine fuels. In addition, these species were chosen based on their relative importance in surrogate formulations and the availability of experimental data for their oxidation. Toluene makes up 12% by weight in gasolines. Ethylbenzene and xylenes constitute roughly 7% with m-xylene being the dominant species (3.5%). In steps of increasing complexity, toluene is the simplest alkane-substituted aromatic, ethylbenzene being the next simplest. Styrene, the simplest alkene-substituted aromatic species, is an important intermediate in ethylbenzene oxidation. Therefore, reaction pathways of styrene need to be understood in order to predict ethylbenzene oxidation characteristics accurately. Xylenes are the simplest aromatics with two alkane substituents. In addition, a bi-ringed substituted aromatic compound, specifically 1-methylnaphthalene, has been considered. PAHs are present to varying extents in modern distillation fuels and are important constituents of the surrogate fuel mixture. While their concentrations might remain low, they are characterized by large sooting tendencies which render them extremely important.

In this article, the presentation of the work has been organized as follows. First, the mechanism development is explained, providing details on the thermochemical properties and the different reactions included in the reaction scheme for each of the species considered. Following this, the validation results are presented, comparing the simulation results with the data available in the literature.

For clarity, in the rest of the article, abbreviations according to those introduced by Frenklach et al. [15] have been used. For instance, A1 is benzene, A1CH3 is a methyl-substituted species consisting of one aromatic ring (i.e. toluene), A1(CH3)2 refers to a bi-methyl substituted one-ringed aromatic (i.e. xylene), A1-refers to a phenyl radical, A1CH3 refers to a methylphenyl radical, A2-is the naphthyl radical, and A2CH3 refers to a methyl-substituted species consisting of two aromatic rings (i.e. methylnaphthalene). In the following sections, substitutions on a naphthyl ring refer to the α site if not stated otherwise. A complete list of the species nomenclature can be found in the supporting material.

Section snippets

Thermochemical properties

The thermodynamic properties used in the present chemical model are built upon those used in the base kinetic mechanism [14]. Most of the species needed for the development of the current mechanism were already included in the base mechanism. However, with the introduction of xylene as a new component of the chemical model, new thermodynamic properties of the combustion derivatives of xylene were required. During the validation of the kinetic mechanism, especially for plug flow reactors, it was

Results and discussion

Table 3 lists the validation cases considered in the present study, which are also described in more detail in the following sub-sections. The numerical calculations have been performed using FlameMaster version 3.3.9 [49].

Conclusion

A single consistent chemical kinetic mechanism has been developed for the oxidation of a set of substituted aromatic species at moderate to high temperatures. The current reaction scheme is based on the mechanism developed by Blanquart et al. [14]. This base mechanism has been extensively validated for a wide range of hydrocarbons at various temperatures and pressures and provides a well-validated base to build upon. Additional reactions for the oxidation of toluene, ethylbenzene, styrene,

Acknowledgments

The authors greatly acknowledge funding by the Air Force Office of Scientific Research and NASA. The authors would like to thank Supreet Singh Bahga for his help in developing the mechanism and Michael Mueller for his valuable comments on the manuscript.

References (70)

  • H. Zhang et al.

    Proc. Combust. Inst.

    (2007)
  • K. Brezinsky

    Prog. Energy Combust. Sci.

    (1986)
  • S. Gail et al.

    Combust. Flame

    (2005)
  • C.B. Shaddix et al.

    Proc. Combust. Inst.

    (1992)
  • H. Pitsch

    Proc. Combust. Inst.

    (1996)
  • G. Blanquart et al.

    Combust. Flame

    (2009)
  • M.B. Colket et al.

    Proc. Combust. Inst.

    (1994)
  • R. Frochtenicht et al.

    J. Photochem. Photobiol.

    (1994)
  • T.A. Litzinger et al.

    Combust. Flame

    (1986)
  • H.-P.S. Shen et al.

    Proc. Combust. Inst.

    (2009)
  • V. Vasudevan et al.

    Proc. Combust. Inst.

    (2005)
  • R. Sivaramakrishnan et al.

    Combust. Flame

    (2004)
  • R. Johnston et al.

    Proc. Combust. Inst.

    (2005)
  • T. Hirasawa et al.

    Proc. Combust. Inst.

    (2002)
  • H.-P.S. Shen et al.

    Combust. Flame

    (2009)
  • U. Pfahl et al.

    Proc. Combust. Inst.

    (1996)
  • P. Middha et al.

    Proc. Combust. Inst.

    (2002)
  • S.G. Davis et al.

    Proc. Combust. Inst.

    (1996)
  • M. Colket, T. Edwards, S. Williams, N. Cernansky, D.Miller, F.Egolfopoulos, P.Lindstedt, K. Seshadri, F. Dryer, C. Law,...
  • N. Grumman, Northrop Grumman Petroleum Product survey reports, updated annually,...
  • J.L. Emdee et al.

    J. Phys. Chem.

    (1992)
  • R.P. Lindstedt et al.

    Combust. Sci. Technol.

    (1996)
  • F. Battin-Leclerc et al.

    Int. J. Chem. Kinet.

    (2006)
  • S. Gail et al.

    Combust. Sci. Technol.

    (2008)
  • S. Gail et al.

    Combust. Sci. Technol.

    (2007)
  • M. Karim et al.

    Combust. Sci. Technol.

    (2007)
  • M. Frenklach et al.

    Combust. Sci. Technol.

    (1986)
  • G. Blanquart et al.

    J. Phys. Chem. A

    (2007)
  • M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N....
  • J. Emdee et al.

    J. Phys. Chem.

    (1991)
  • S.J. Blanksby et al.

    Acc. Chem. Res.

    (2003)
  • G. da Silva et al.

    J. Phys. Chem.

    (2009)
  • S. Klippenstein et al.

    Proc. Combust. Inst.

    (2007)
  • T. Ichimura et al.

    J. Phys. Chem. A

    (1986)
  • D.M. Brenner et al.

    J. Am. Chem. Soc.

    (1976)
  • Cited by (330)

    View all citing articles on Scopus
    View full text