Extension of the ReaxFF Combustion Force Field toward Syngas Combustion and Initial Oxidation Kinetics

Abstract

A detailed insight of key reactive events related to oxidation and pyrolysis of hydrocarbon fuels further enhances our understanding of combustion chemistry. Though comprehensive kinetic models are available for smaller hydrocarbons (typically C or lower), developing and validating reaction mechanisms for larger hydrocarbons is a daunting task, due to the complexity of their reaction networks. The ReaxFF method provides an attractive computational method to obtain reaction kinetics for complex fuel and fuel mixtures, providing an accuracy approaching ab-initio-based methods but with a significantly lower computational expense. The development of the first ReaxFF combustion force field by Chenoweth et al. (CHO-2008 parameter set) in 2008 has opened new avenues for researchers to investigate combustion chemistry from the atomistic level. In this article, we seek to address two issues with the CHO-2008 ReaxFF description. While the CHO-2008 description has achieved significant popularity for studying large hydrocarbon combustion, it fails to accurately describe the chemistry of small hydrocarbon oxidation, especially conversion of CO from CO, which is highly relevant to syngas combustion. Additionally, the CHO-2008 description was obtained faster than expected H abstraction by O from hydrocarbons, thus underestimating the oxidation initiation temperature. In this study, we seek to systemically improve the CHO-2008 description and validate it for these cases. Additionally, our aim was to retain the accuracy of the 2008 description for larger hydrocarbons and provide similar quality results. Thus, we expanded the ReaxFF CHO-2008 DFT-based training set by including reactions and transition state structures relevant to the syngas and oxidation initiation pathways and retrained the parameters. To validate the quality of our force field, we performed high-temperature NVT-MD simulations to study oxidation and pyrolysis of four different hydrocarbon fuels, namely, syngas, methane, JP-10, and n-butylbenzene. Results obtained from syngas and methane oxidation simulation indicated that our redeveloped parameters (named as the CHO-2016 parameter set) has significantly improved the C chemistry predicted by ReaxFF and has solved the low-temperature oxidation initiation problem. Moreover, Arrhenius parameters of JP-10 decomposition and initiation mechanism pathways of n-butylbenzene pyrolysis obtained using the CHO-2016 parameter set are also in good agreement with both experimental and CHO-2008 simulation results. This demonstrated the transferability of the CHO-2016 description for a wide range of hydrocarbon chemistry.

Exploring the Chemistry of Low‐Temperature Ignition by Pressure‐Accelerated Dynamics

“…[66][67][68][69][70][71][72][73][74] For all these cases, "generally a good agreement with experimental results in terms of the initiation mechanism and barriers were observed". [52] Comparison of the bond dissociation energies of the species used in the training data (ReaxFF bond dissociation energies vs. B3LYP bond dissociation energies, cf. Chenoweth et al [55] ) shows that we have to expect average errors of more than 10 kcal mol À 1 in bond dissociation energies.…”

“…With a second set of pAD simulations at different temperatures, extrapolation of the kinetics is possible without the need of reoptimizations. Apart from the periodically high concentrations in the nanoreactor, equilibrium simulations with high concentrations have already been successfully used to accelerate hightemperature oxidation and pyrolysis in ReaxFF studies, [52] the oxidation of cyclohexane and other systems using metadynamics and the GFN2-xtb hamiltonian. [53] Bal and Neyts [54] use the CVHD acceleration method and ReaxFF to investigate the oxidation processes of n-dodecane.…”

Trajectories of Graphitizable Anthracene Coke and Non-Graphitizable Sucrose Char during the Earliest Stages of Annealing by Rapid CO2 Laser Heating

“…A time step of 0.1 fs ensures efficient coverage of the phase space and allows the reaction of occur smoothly. The force field used here is an extension [23] of the original CHO force field [18]. thermostat [22].…”

Molecular dynamics simulation of soot formation during diesel combustion with oxygenated fuel addition

“…5,10,16 The improved CHO2016 parameters in ReaxFF have been successfully used to investigate the combustion kinetics of conventional fuels like syngas, ethanol, JP-10, and toluene. [19][20][21] Most importantly, the CHO2016 description is also trained from C2013 parameters that are able to describe the chemistry of aromatic hydrocarbons properly in condensed and gaseous phases simultaneously. 16,19,22 Therefore, we perform ReaxFF MD simulation in a single force field without any predefined radicals or reaction pathways to reveal the role of fuel molecular structure on soot formation, which was previously inaccessible using existing computational chemistry methods.…”