John Bugler

Revisiting the Kinetics and Thermodynamics of the Low-Temperature Oxidation Pathways of Alkanes: A Case Study of the Three Pentane Isomers

This paper describes our developing understanding of low-temperature oxidation kinetics. We have investigated the ignition of the three pentane isomers in a rapid compression machine over a wide range of temperatures and pressures, including conditions of negative temperature coefficient behavior. The pentane isomers are small alkanes, yet have structures that are complex enough to allow for the application of their kinetic and thermochemical rules to larger molecules. Updates to the thermochemistry of the species important in the low-temperature oxidation of hydrocarbons have been made based on a thorough literature review. An evaluation of recent quantum-chemically derived rate coefficients from the literature pertinent to important low-temperature oxidation reaction classes has been performed, and new rate rules are recommended for these classes. Several reaction classes have also been included to determine their importance with regard to simulation results, and we have found that they should be included when developing future chemical kinetic mechanisms. A comparison of the model simulations with pressure-time histories from experiments in a rapid compression machine shows very good agreement for both ignition delay time and pressure rise for both the first- and second-stage ignition events. We show that revisions to both the thermochemistry and the kinetics are required in order to replicate experiments well. A broader validation of the models with ignition delay times from shock tubes and a rapid compression machine is presented in an accompanying paper. The results of this study enhance our understanding of the combustion of straight- and branched-chained alkanes.

Modeling Nitrogen Species as Pollutants: Thermochemical Influences

To simulate emissions of nitrogen-containing compounds in practical combustion environments, it is necessary to have accurate values for their thermochemical parameters, as well as accurate kinetic values to describe the rates of their formation and decomposition. Significant disparity is observed in the literature for the former, and we therefore present herein high-accuracy ab initio gas-phase thermochemistry for 60 nitrogenous compounds, many of which are important in the formation and consumption chemistry of NOx species. Several quantum-chemical composite methods (CBS-APNO, G3, and G4) were utilized to derive enthalpies of formation via the atomization method. Entropies and heat capacities were calculated from traditional statistical thermodynamics, with oscillators treated as anharmonic based on ro-vibrational property analyses carried out at the B3LYP/cc-pVTZ level of theory. The use of quantum chemical methods, along with the treatments of anharmonicities and hindered rotors, ensures accurate enthalpy of formation, entropy, and heat capacity values across the temperature range 298.15-3000 K. The implications of these results for atmospheric and combustion modeling are discussed.

The Effect of Impurities on Ignition Delay Times and Laminar Flame Speeds of Syngas Mixtures at Gas Turbine Conditions

In addition to mostly H2 and CO, syngas also contains reasonable amounts of light hydrocarbons, CO2, H2O, N2, and Ar. Impurities such as NH3, HCN, COS, H2S, and NOx (NO, NO2, N2O) are also commonly found in syngas. The presence of these impurities, even in very low concentrations, can induce some large changes in combustion properties. Although they introduce potential design and operational issues for gas turbines, these changes in combustion properties due to the presence of impurities are still not well characterized. The aim of this work was therefore to investigate numerically the effect of the presence of impurities in realistic syngas compositions on some fundamental combustion properties of premixed systems such as laminar flame speed and ignition delay time, at realistic engine operating conditions. To perform this study, a state-of-the-art C0–C3 detailed kinetics mechanism was used. This mechanism was combined with recent, optimized sub-mechanisms for impurities which can impact the combustion properties of the syngas such as nitrogenous species (i.e., N2O, NO2, NH3, and HCN) and sulfur-based species such as H2S, SO2 and COS. Several temperatures, pressures, and equivalence ratios were investigated. The results of this study showed that the addition of some impurities modifies notably the reactivity of the mixture. The ignition delay time is decreased by the addition of NO2 and H2S at the temperatures and pressures for which the HO2 radical dominates the H2 combustion. However, while NO2 has no effect when OH is dominating, H2S increases the ignition delay time in such conditions for pressures above 1 atm. The amplitude of these effects is however dependent on the impurity concentration. Laminar flame speeds are not sensitive to NO2 addition but they are to NH3 and HCN, inducing a small reduction of the laminar flame speed at fuel rich conditions. H2S exhibits some inhibiting effects on the laminar flame speed but only for high concentrations. The inhibiting effects of NH3, HCN, and H2S are due to the OH radical consumption by these impurities, leading to the formation of radicals that are less reactive.© 2014 ASME