Skeletal Mechanism Generation for High-Temperature Combustion of H₂/CO/C₁–C₄ Hydrocarbons

The development of the combustion mechanism for hydrogen (H₂) and C₁–C₄ hydrocarbon fuels plays critical roles in many combustion systems. In the present work, a general framework to develop an efficient skeletal mechanism, which can maintain both accuracy of predicted combustion properties and chemical reality, has been established on the basis of the combination of the directed relation graph method for mechanism reduction and the element flux analysis method for reaction pathway analysis. Within the framework, a skeletal mechanism with 56 species and 428 reactions is developed from a detailed mechanism, including 111 species and 784 elementary reactions, for high-temperature combustion of H₂, and C₁–C₄ hydrocarbons. Errors in the predicted combustion properties will be introduced via removing species from detailed mechanisms. Therefore, systematical error analysis is first performed for ignition over a wide range of conditions, including temperature, pressure, and equivalence ratio, to check the robustness of the skeletal mechanism. Results show that the accuracy of the skeletal mechanism in the prediction of ignition for hydrogen, methane, ethylene, ethane, and propene is within 5% and no more than 10% for propane and <i>n</i>-butane. Time-integrated element flux analysis is subsequently used as an efficient method to check the chemical reality of the skeletal mechanism. The results indicate that the skeletal mechanism maintains the major reaction paths for targeted fuels. Finally, the skeletal mechanism is validated via the predictions of ignition, laminar flame speed, species profiles, and diffusion counter-flow flame simulations, and the use of the skeletal mechanism in the development of simplified high-temperature combustion mechanism for large alkanes is also performed

Reaction Kinetics of Hydrogen Atom Abstraction from C4–C6 Alkenes by the Hydrogen Atom and Methyl Radical

Alkenes are important ingredients of realistic fuels and are also critical intermediates during the combustion of a series of other fuels including alkanes, cycloalkanes, and biofuels. To provide insights into the combustion behavior of alkenes, detailed quantum chemical studies for crucial reactions are desired. Hydrogen abstractions of alkenes play a very important role in determining the reactivity of fuel molecules. This work is motivated by previous experimental and modeling evidence that current literature rate coefficients for the abstraction reactions of alkenes are still in need of refinement and/or redetermination. In light of this, this work reports a theoretical and kinetic study of hydrogen atom abstraction reactions from C4–C6 alkenes by the hydrogen (H) atom and methyl (CH₃) radical. A series of C4–C6 alkene molecules with enough structural diversity are taken into consideration. Geometry and vibrational properties are determined at the B3LYP/6-31G­(2df,p) level implemented in the Gaussian-4 (G4) composite method. The G4 level of theory is used to calculate the electronic single point energies for all species to determine the energy barriers. Conventional transition state theory with Eckart tunneling corrections is used to determine the high-pressure-limit rate constants for 47 elementary reaction rate coefficients. To faciliate their applications in kinetic modeling, the obtained rate constants are given in the Arrhenius expression and rate coefficients for typical reaction classes are recommended. The overall rate coefficients for the reaction of H atom and CH₃ radical with all the studied alkenes are also compared. Branching ratios of these reaction channels for certain alkenes have also been analyzed

ReaxFF Molecular Dynamics Simulations of Oxidation of Toluene at High Temperatures

Aromatic hydrocarbon fuels, such as toluene, are important components in real jet fuels. In this work, reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field have been performed to study the high-temperature oxidation mechanisms of toluene at different temperatures and densities with equivalence ratios ranging from 0.5 to 2.0. From the ReaxFF MD simulations, we have found that the initiation consumption of toluene is mainly through three ways, (1) the hydrogen abstraction reactions by oxygen molecules or other small radicals to form the benzyl radical, (2) the cleavage of the C–H bond to form benzyl and hydrogen radicals, and (3) the cleavage of the C–C bond to form phenyl and methyl radicals. These basic reaction mechanisms are in good agreement with available chemical kinetic models. The temperatures and densities have composite effects on toluene oxidation; concerning the effect of the equivalence ratio, the oxidation reaction rate is found to decrease with the increasing of equivalence ratio. The analysis of the initiation reaction of toluene shows that the hydrogen abstraction reaction dominates the initial reaction stage at low equivalence ratio (0.5–1.0), while the contribution from the pyrolysis reaction increases significantly as the equivalence ratio increases to 2.0. The apparent activation energies, <i>E</i>_a, for combustion of toluene extracted from ReaxFF MD simulations are consistent with experimental results

Effects of Fuel Additives on the Thermal Cracking of <i>n</i>-Decane from Reactive Molecular Dynamics

Thermal cracking of <i>n</i>-decane and <i>n</i>-decane in the presence of several fuel additives are studied in order to improve the rate of thermal cracking by using reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field. From MD simulations, we find the initiation mechanisms of pyrolysis of <i>n</i>-decane are mainly through two pathways: (1) the cleavage of a C–C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding decyl radical. Another pathway is the H-abstraction reactions by small radicals including H, CH₃, and C₂H₅. The basic reaction mechanisms are in good agreement with existing chemical kinetic models of thermal decomposition of <i>n</i>-decane. Quantum mechanical calculations of reaction enthalpies demonstrate that the H-abstraction channel is easier compared with the direct C–C or C–H bond-breaking in <i>n</i>-decane. The thermal cracking of <i>n</i>-decane with several additives is further investigated. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis. The different chemical structures of the fuel additives greatly affect the apparent activation energy and pre-exponential factors. The presence of diethyl ether (DEE), methyl <i>tert</i>-butyl ether (MTBE), 1-nitropropane (NP), 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexaoxonane (TEMPO), triethylamine (TEA), and diacetonediperodixe (DADP) exhibit remarkable promoting effect on the thermal cracking rates, compared with that of pure <i>n</i>-decane, in the following order: NP > TEMPO > DADP > DEE (∼MTBE) > TEA, which coincides with experimental results. These results demonstrate that reactive MD simulations can be used to screen for fuel additives and provide useful information for more comprehensive chemical kinetic model studies at the molecular level

Effects of Fuel Additives on the Thermal Cracking of <i>n</i>-Decane from Reactive Molecular Dynamics

Thermal cracking of <i>n</i>-decane and <i>n</i>-decane in the presence of several fuel additives are studied in order to improve the rate of thermal cracking by using reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field. From MD simulations, we find the initiation mechanisms of pyrolysis of <i>n</i>-decane are mainly through two pathways: (1) the cleavage of a C–C bond to form smaller hydrocarbon radicals, and (2) the dehydrogenation reaction to form an H radical and the corresponding decyl radical. Another pathway is the H-abstraction reactions by small radicals including H, CH₃, and C₂H₅. The basic reaction mechanisms are in good agreement with existing chemical kinetic models of thermal decomposition of <i>n</i>-decane. Quantum mechanical calculations of reaction enthalpies demonstrate that the H-abstraction channel is easier compared with the direct C–C or C–H bond-breaking in <i>n</i>-decane. The thermal cracking of <i>n</i>-decane with several additives is further investigated. ReaxFF MD simulations lead to reasonable Arrhenius parameters compared with experimental results based on first-order kinetic analysis. The different chemical structures of the fuel additives greatly affect the apparent activation energy and pre-exponential factors. The presence of diethyl ether (DEE), methyl <i>tert</i>-butyl ether (MTBE), 1-nitropropane (NP), 3,6,9-triethyl-3,6,9-trimethyl-1,2,4,5,7,8-hexaoxonane (TEMPO), triethylamine (TEA), and diacetonediperodixe (DADP) exhibit remarkable promoting effect on the thermal cracking rates, compared with that of pure <i>n</i>-decane, in the following order: NP > TEMPO > DADP > DEE (∼MTBE) > TEA, which coincides with experimental results. These results demonstrate that reactive MD simulations can be used to screen for fuel additives and provide useful information for more comprehensive chemical kinetic model studies at the molecular level

Experimental and Modeling Study on the Ignition Kinetics of Nitromethane behind Reflected Shock Waves

Nitromethane (NM) is the simplest nitroalkane fuel and has demonstrated potential usage as propellant and fuel additive. Thus, understanding the combustion characteristics and chemistry of NM is critical to the development of hierarchical detailed kinetic models of nitro-containing energetic materials. Herein, to further investigate the ignition kinetics of NM and supplement the experimental database for kinetic mechanism development, an experimental and kinetic modeling analysis of the ignition delay times (IDTs) of NM behind reflected shock waves at high fuel concentrations is reported against previous studies. Specifically, the IDTs of NM are measured via a high-pressure shock tube within the temperature from 900 to 1150 K at pressures of 5 and 10 bar and equivalence ratios of 0.5, 1.0, and 2.0. Brute-force sensitivity analysis and chemical explosive mode analysis in combination with reaction path analysis are employed to reveal the fundamental ignition kinetics of NM. Finally, a skeletal mechanism for NM is derived via the combination of directed relation graph-based methods, which demonstrates good prediction accuracy of NM ignition and flame speeds. The present work should be valuable for understanding the combustion chemistry of NM and the development of the fundamental reaction mechanism of nitroalkane fuels