A framework for automatic discovery of chemically termolecular reactions

Given the large mole fractions of highly reactive species encountered in combustion, there is significant potential for chemically reactive collisions to occur on similar timescales as the energy-transferring collisions responsible for thermalization. Recent work has shown that this can result in chemically termolecular reactions of importance to combustion predictions. This type of reaction, where three reactants are involved in bond breaking and forming, has generally been neglected in previous phenomenological kinetic models. Combinatorially, there are hundreds to thousands of such reactions that are possible. Clearly, it is not feasible to perform detailed master equation analysis for all of these potential pathways, and yet, a priori, it is difficult to know which of these thousands of reactions are sufficiently likely to impact combustion predictions to warrant detailed master equation calculations. This paper presents a theoretical and computational framework that identifies and estimates rate constants for potential chemically termolecular reactions, for which information has been previously limited, based on information already available for known reactions in combustion mechanisms. Applications of this approach to flames, presented herein, reveal a few chemically termolecular reactions that impact flame emissions and flame speeds - including H + N-2 + O = NH + NO, H + CO + H = H-2 + CO, and H + C2H2 + O-2/OH/H. These reactions, therefore, are worthwhile candidates for detailed master equation calculations for improved quantitative predictions. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.