(247u) Developing Reaxff Force Field to Study Syngas Combustion Kinetics and Extending It for Larger Hydrocarbons

Syngas (Synthesis Gas); a mixture of CO and H₂gas, has shown a great potential to solve future energy crisis. However, a detail chemical kinetic study on syngas combustion is required to ensure quality combustion and identify the key reactions. Although numerous kinetic models are proposed, many of the reaction pathways and their relative importance are yet to be known. To address this issue, we developed a ReaxFF interatomic potential to describe the C/H/O system particularly for smaller hydrocarbons, as the previous ReaxFF combustion force field [1]while successful for large hydrocarbon pyrolysis and combustion, was less accurate for C1-chemistry.

Molecular Dynamics (MD) simulations performed on a syngas system using this force field can capture many more reaction channels than previous force field, specially the reactions involving HCO radicals. HCO is believed to be the most important intermediate in syngas combustion and adding the reactions involving HCO with H₂-O₂system kinetics can completely define the kinetic model for syngas. Previous ReaxFF combustion force field estimated a high-energy barrier for hydrogen atom and CO combination reaction, leaving this specie and the associated reaction channels absent in the dynamics-in contrast to DFT-results that predict a low barrier for this reaction. Our current force field resolves this issue and our result demonstrate the consumption of HCO radicals through a couple of well-defined chain terminating reactions. Overall, the simulation results can describe the syngas combustion kinetics that is in good agreement with literature.

Once the force field to capture the C₁chemistry in combustion kinetics is completely validated, it will be integrated with the existing ReaxFF combustion force field for larger hydrocarbons [2]–[5], which has been immensely successful in recent years. This will result a fully transferable ReaxFF C/H/O description, applicable for small to large-scale combustion of wide variety of hydrocarbons.

Reference:

[1]  Chenoweth K., et al. , “Development and Application of a ReaxFF Reactive Force Field for Oxidative Dehydrogenation on Vanadium Oxide Catalysts,” J. Phys. Chem. C, 112(37), pp. 14645–14654 (Sep. 2008).

[2]       Castro-Marcano,  F., et al., “Combustion of an Illinois No. 6 coal char simulated using an atomistic char representation and the ReaxFF reactive force field,” Combust. Flame, 159(3), pp. 1272–1285 (Mar. 2012).

[3]       Qian, H.-J., et al., “Reactive Molecular Dynamics Simulation of Fullerene Combustion Synthesis: ReaxFF vs DFTB Potentials,” J. Chem. Theory Comput., 7(7,) pp. 2040–2048 (Jul. 2011).

[4]       Liu, L., et al., “Mechanism and Kinetics for the Initial Steps of Pyrolysis and Combustion of 1,6-Dicyclopropane-2,4-hexyne from ReaxFF Reactive Dynamics,” J. Phys. Chem. A, 115(19), pp. 4941–4950 (May 2011).

[5]       Chenoweth, K., et al., “Initiation Mechanisms and Kinetics of Pyrolysis and Combustion of JP-10 Hydrocarbon Jet Fuel,” J. Phys. Chem. A, 113 (9), pp. 1740–1746 (Mar. 2009).