A comprehensive combustion chemistry study of 2,5-dimethylhexane

Abstract

Iso-paraffinic molecular structures larger than seven carbon atoms in chain length are commonly found in conventional petroleum, Fischer-Tropsch (FT), and other alternative hydrocarbon fuels, but little research has been done on their combustion behavior. Recent studies have focused on either mono-methylated alkanes and/or highly branched compounds (e.g., 2,2,4-trimethylpentane). In order to better understand the combustion characteristics of real fuels, this study presents new experimental data for the oxidation of 2,5-dimethylhexane under a wide variety of temperature, pressure, and equivalence ratio conditions. This new dataset includes jet stirred reactor speciation, shock tube ignition delay, and rapid compression machine ignition delay, which builds upon recently published data for counterflow flame ignition, extinction, and speciation profiles. The low and high temperature oxidation of 2,5-dimethylhexane has been modeled using a comprehensive chemical kinetic model developed using established reaction rate rules. The agreement between the model and data is presented, along with suggestions for improving model predictions. The importance of propene chemistry is highlighted as critical for correct prediction of high temperature ignition delay. The oxidation behavior of 2,5-dimethylhexane is also compared with oxidation behavior of other linear and branched octane isomers, in order to determine the effects of the number of methyl branches on combustion properties. Both experimental data and model predictions indicate that increasing the level of branching decreases fuel reactivity at low and intermediate temperatures. The model is used to elucidate the structural features and reaction pathways responsible for inhibiting the reactivity of 2,5-dimethylhexane. FUEL or CNF, Sarathy et al. submitted July 2013Introduction Detailed chemical kinetic models for transportation fuels have reached a level of fidelity where accurate predictions can be made of combustion phenomenon relevant to the operation of practical devices. Schofield [1] states that these large scale models are adequate as engineering tools for studying the combustion of new fuel molecules. A recent review paper by Pitz and Mueller [2] describing the development of diesel surrogate fuel models concluded that major research gaps remain in modeling high molecular weight (i.e., C 8 and greater) aromatics, alkyl aromatics, cyclo-alkanes, and lightly branched iso-alkanes. The present study is concerned with the combustion of branched alkanes, specifically 2,5-dimethylhexane, which has been reported as a component of petroleum combustion exhaust, smog, and tobacco smoke [3]. Branched alkanes are important components of conventional diesel and jet fuels derived from petroleum [2,4]; synthetic Fischer-Tropsch diesel and jet fuels derived from coal, natural gas, and/or biomass [5,6]; and renewable diesel and jet fuels derived from thermochemical treatment of bio-derived fats and oils (e.g., hydrotreated renewable jet (HRJ) fuels) [7,8]. Detailed com...

Chemical Kinetic Insights into the Octane Number and Octane Sensitivity of Gasoline Surrogate Mixtures

“…It is well-known that iso-paraffins (e.g., iso-octane) display less low-temperature reactivity as compared to n-paraffins (e.g., nheptane) because the presence of multiple methyl substitutions inhibits the kinetics of low chain branching reactions in isoparaffins. 53,67,68 The simulation results for PRF91 at 750 K, shown in Figure 6a, clearly display the prominent two-stage ignition characteristic expected for paraffinic mixtures. The production of OH and HO 2 radicals shows a sharp increase up to the LTHR regime.…”

“…Griffiths et al 42 correlated the ignition delay time of PRFs from RCM experiments and their composition (corresponding to the octane number) at 900 K. They found quantitative agreement for low RON fuels (up to RON 85). Sarathy et al 43 tried to relate RON values to homogeneous gas-phase ignition delay times at 835 K and 20 bar. Badra et al 44 considered various TPRF mixtures and developed correlations of simulated ignition delay time with RON and MON using both constant volume and variable volume reactor simulations.…”

Additional chain-branching pathways in the low-temperature oxidation of branched alkanes

“…Therefore, the molecular formulas proposed for specific m/z signals must be judiciously determined based on mechanistic and kinetic analyses. Simulations for DMH oxidation under the present JSR conditions using the detailed chemical kinetic model of Sarathy et al [55] demonstrate that high molecular weight intermediates are produced via the classical low-temperature oxidation mechanism (i.e., radical plus O 2 reactions, concerted eliminations, etc.). We postulate that the measured m/z signals correspond to molecular formulas for C 8 alkenes (C 8 H 16 , 112.13 u), cyclic ethers (C 8 H 16 O, 128.12 u) and ketohydroperoxides (C 8 H 16 O 3 , 160.11 u), respectively.…”

Site-Specific Rate Constant Measurements for Primary and Secondary H- and D-Abstraction by OH Radicals: Propane and n-Butane

“…The method and setup were described previously [18][19][20] and only a brief description is given here. The shock tube is made of a 9 m driver section and a 9 m driven section, with an inner diameter of 14.2 cm.…”