Despite decades of sustained research in the kinetics of combustion, missing reactions are still being actively identified, and these continue to have a noticeable impact on practical simulations. Current widely used kinetics databases such as the NIST Kinetics Database (NIST Standard Reference Database, 2015), NASA-JPL evaluations (JPL Publication, 2019), reviews from Baulch et al. (J. Phys. Chem. Ref. Data, 2005) and Tsang et al. (J. Phys. Chem. Ref. Data, 1986) curate experimental/theoretical rate coefficients of elementary reactions that were already identified in literature studies. These databases are structured typically either by reaction order (unimolecular, bimolecular), reaction classes (addition, abstraction etc.) or a combination of these for specific reactants (i. e. all reactions relevant to CH₄ combustion). As such, these databases provide an excellent summary of information already available but do not provide any guidance on unknown or missing pathways particularly for multi-channel reactions. The present study describes an alternative kinetics database to curate/summarize elementary reactions constrained by species stoichiometry and informed by AutoMech, an open-source software for automated theoretical kinetics calculation (Elliott et al., Proc. Combust. Inst., 2021). First, at a given stoichiometry (i. e. C_xH_yO_z), all the possible molecular structures of wells (C_xH_yO_z) and bimolecular channels (C_x-x’H_y-y’O_z-z’ + C_x’H_y’O_z’) on the C_xH_yO_z potential energy surface (PES) were generated using MOLGEN, which is open-source software for identification of structural isomers. Then, the connectivity between each well and bimolecular channels within this PES was analyzed by using AutoMol within AutoMech. The systematic analysis of these wells and bimolecular channels and their connectivity enables the identification of a complete set of elementary reactions that can occur within the imposed stoichiometric (and energetics) constraints. The proposed systematic framework was demonstrated for generating the list of elementary reactions for species stoichiometries with 0 – 3 heavy atoms (C, H, and O only). The generated list of elementary reactions was compared with the widely used NIST Kinetics database, highlighting specific examples of potential elementary reactions that are newly identified from the proposed framework. The advantages and constraints posed by this alternative method to generate a kinetics database will also be discussed.

Dr. Jaeyoung Cho

Postdoctoral researcher, Argonne National Laboratory

Appearances