Initial Pyrolysis Mechanism of Oil Shale Kerogen with Reactive Molecular Dynamics Simulation

Molecular dynamics (MD) simulations using a reactive force field (ReaxFF) method for a Green River oil shale model demonstrate that the thermal decomposition of the oil shale molecule is initiated with the cleavage of the oxygen bridge (C–O bond), and the first product is formaldehyde (CH2O). The simulation results show that the C–O bond is weaker than the other bonds, agreeing with its smaller bond dissociation energy (BDE). The ring-opening position of the aliphatic ring is usually determined by the stability of free radicals formed in this process. For aromatic hydrocarbons, the long-chain substituents are found to be easier to leave and the cleavage of C–C bonds leads to a series of chain reactions and the formation of small fragments, such as ethylene and propylene. The bond cleavages are almost in accordance with the minimum bonding energy rule. NVT simulations show that the pyrolysis process progresses in two stages: the decomposition of kerogen into heavy (C40+) species and then the generation of light compounds. Recombinations and rearrangements of different fragments are also observed via MD simulations. The main hydrocarbon fragments of C10–C20 are regarded as the component or precursor of diesel oil. The formation pathways of typical aromatic components are analyzed by tracking the motion trajectories of relevant structures. The intermediates and products in MD simulations are found to be similar to the gas chromatography–mass spectrometry (GC–MS) results from previous experiments.