Modeling Nitrogen Species as Pollutants: Thermochemical Influences

Experimental and Kinetic Study of the Effect of Nitrogen Dioxide on Ethanol Autoignition

The contribution of NO₂ to the ethanol ignition delay time was investigated behind reflected shock waves. The experiments were performed at a pressure of 0.20 MPa, temperature range of 1050-1650 K, equivalence ratio of 0.5/1.0/1.5, and ethanol/NO₂ mixing ratios of 100/0, 90/10, and 50/50. The experimental results showed that the addition of NO₂ decreased the ignition delay time and promoted the reactivity of ethanol under all equivalence ratios. With an increase in NO₂ blending, the effect of equivalence ratio on the ethanol ignition delay time decreased, and with an increase in temperature, the effect of NO₂ in promoting ethanol ignition weakened. An updated mechanism was proposed to quantify NO₂-promoted ethanol ignition. The mechanism was validated based on available experimental data, and the results were in line with the experimental trends under all conditions. Chemical kinetic analyses were performed to interpret the interactions between NO₂ and ethanol for fuel ignition. The numerical analysis indicated that the promotion effect of NO₂ is primarily due to an increase of the rate of production and concentration of the radical pool, especially the OH radical pool. The reaction NO + HO₂ ⇔ NO₂ + OH is key to generating chain-initiating OH radicals.

Non-physical Species in Chemical Kinetic Models: A Case Study of Diazenyl Hydroxy and Diazenyl Peroxide

Predictive chemical kinetic models often consider hundreds to thousands of intermediate species. An even greater number of species are required to generate pressure-dependent reaction networks for gas-phase systems. As this immense chemical search space is being explored using automated tools by applying reaction templates, it is probable that non-physical species will infiltrate the model without being recognized by the compute or a human as such. These non-physical species might obey chemical intuition as well as requirements coded in the software, e. g., obeying element electron valence constraints, and may consequently remain unnoticed. Non-physical species become an acute problem when their presence affects a model observable. Correcting a pressure-dependent network containing a non-physical species may significantly affect the computed rate coefficient. The present work discusses and analyzes two specific cases of such species, diazenyl hydroxy (⋅N=NOH) and diazenyl peroxide (⋅N=NOOH), both previously suggested as intermediates in nitrogen combustion systems. A comprehensive conformational search did not identify any non-fragmented energy well, and energy scans performed for diazenyl peroxide (⋅N=NOOH), at DFT and CCSD(T) show that it barrierlessly decomposes. This work highlights a broad implication for future automated chemical kinetic model generation, and provides a significant motivation to standardize non-physical species identification in chemical kinetic models.

RMG Database for Chemical Property Prediction

The Reaction Mechanism Generator (RMG) database for chemical property prediction is presented. The RMG database consists of curated datasets and estimators for accurately predicting the parameters necessary for constructing a wide variety of chemical kinetic mechanisms. These datasets and estimators are mostly published and enable prediction of thermodynamics, kinetics, solvation effects, and transport properties. For thermochemistry prediction, the RMG database contains 45 libraries of thermochemical parameters with a combination of 4564 entries and a group additivity scheme with 9 types of corrections including radical, polycyclic, and surface absorption corrections with 1580 total curated groups and parameters for a graph convolutional neural network trained using transfer learning from a set of >130 000 DFT calculations to 10 000 high-quality values. Correction schemes for solvent-solute effects, important for thermochemistry in the liquid phase, are available. They include tabulated values for 195 pure solvents and 152 common solutes and a group additivity scheme for predicting the properties of arbitrary solutes. For kinetics estimation, the database contains 92 libraries of kinetic parameters containing a combined 21 000 reactions and contains rate rule schemes for 87 reaction classes trained on 8655 curated training reactions. Additional libraries and estimators are available for transport properties. All of this information is easily accessible through the graphical user interface at https://rmg.mit.edu. Bulk or on-the-fly use can be facilitated by interfacing directly with the RMG Python package which can be installed from Anaconda. The RMG database provides kineticists with easy access to estimates of the many parameters they need to model and analyze kinetic systems. This helps to speed up and facilitate kinetic analysis by enabling easy hypothesis testing on pathways, by providing parameters for model construction, and by providing checks on kinetic parameters from other sources.

Comparison of detailed reaction mechanisms for homogeneous ammonia combustion

Ammonia is a potential fuel for the storage of thermal energy. Experimental data were collected for homogeneous ammonia combustion: ignition delay times measured in shock tubes (247 data points in 28 datasets from four publications) and species concentration measurements from flow reactors (194/22/4). The measurements cover wide ranges of temperature T, pressure p, equivalence ratio φ and dilution. The experimental data were encoded in ReSpecTh Kinetics Data Format version 2.2 XML files. The standard deviations of the experimental datasets used were determined based on the experimental errors reported in the publications and also on error estimations obtained using program MinimalSplineFit. Simulations were carried out with eight recently published mechanisms at the conditions of these experiments using the Optima++ framework code, and the FlameMaster and OpenSmoke++ solver packages. The performance of the mechanisms was compared using a sum-of-square error function to quantify the agreement between the simulations and the experimental data. Ignition delay times were well reproduced by five mechanisms, the best ones were Glarborg-2018 and Shrestha-2018. None of the mechanisms were able to reproduce well the profiles of NO, N2O and NH3 concentrations measured in flow reactors.

Gas-to-Liquid Phase Transition of PAH at Flame Temperatures

Significant evidence has shown that soot can be formed from polycyclic aromatic hydrocarbon (PAH) in combustion environments, but the transition of high molecular PAH from the gas phase to soot in a liquid or solid state remains unclear. In this study, the relationships between the boiling points of various planar PAHs and their thermodynamic properties are systematically investigated, to find a satisfactory marker for the phase transition event. Temperature-dependent thermodynamic properties, including entropy, specific heat capacity, enthalpy, and Gibbs free energy, are simultaneously calculated for PAHs, using density functional theory and three composite compound methods. Comparison of the results indicates that the individual G3 method, plus an atomization reaction approach, produces the most accurate thermochemistry parameters. Compared to entropy, enthalpy, and Gibbs free energy, the specific heat capacity at 298 K is found to be a better marker for the boiling point of PAHs due to the observed linear correlation, predictable characteristics, and fidelity of accuracy as a function of temperature. The correlation equation Y = 10.996X + 122.111 is proposed (where Y is the boiling temperature (K) and X is C_p at 298 K (cal/K/mol)). The standard deviation is as low as 16.7 K when comparing the calculated boiling points and experimentally determined values for 25 different aromatic species ranging from benzene to ovalene (C₃₂H₁₄). The effects of carbon number, structural arrangement, and partial pressure on the boiling point of large planar PAH are discussed. The results reveal that the carbon number in large planar PAH is the dominant factor determining its boiling points. It is shown that PAHs containing about 60-65 carbon atoms are likely to exist as liquids in flames, although the partial pressure of such species is very low.