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This work presents long time ReaxFF MD simulations of fuel-rich combustion for up to 10 ns to explore the initial mechanism of soot nanoparticle formation. A 24-component rocket propellant 1 (RP-1) model based on the major components of RP-1 fuel was employed. Simulations were performed by GPU-accelerated code GMD-Reax, and reactions therein were revealed with the aid of VARxMD. Simulated evolution of physical and chemical properties of the largest molecule exhibits the overall structural transitions of three stages for incipient ring formation, nucleation, and graphitization from fuel molecules to the formation of a single soot nanoparticle. The incipient ring formation takes place in stage 1 by large ring generation from activated aliphatic polyyne-like chains, ring number increase from internal bridging between carbon atoms of large rings, and consequent formation of PAH-like molecules with aliphatic side chains. Nucleation of a nanoparticle in stage 2 is the result of coalescence of PAH-like molecules, accompanied by ring closure reactions that occurred at side chains of the PAH-like molecules, and following formation of internal bridged bonds. Graphitization of the nanoparticle in stage 3 is dominated by the transformation from C5/C7 rings to C6 rings with C3 rings as intermediates. This work demonstrates that ReaxFF MD simulation might be a promising approach for qualitatively characterizing morphological evolution and the underlying chemical complexity of soot formation using multicomponent fuel models.

Deep understanding of the detailed coal pyrolysis process is very important for clean coal utilization. The overall stages in coal pyrolysis were investigated by ReaxFF MD simulations of large-scale coal models combined with reaction analysis of a cheminformatics approach. Analysis of slow heat-up ReaxFF molecular dynamics (MD) simulations shows that the Liulin coal pyrolysis process can be divided into four stages based on the thermal cleavage of bridge bonds: the activation stage of the coal structure (Stage-I), the primary pyrolysis stage (Stage-IIA), the secondary pyrolysis stage (Stage-IIB), and the recombination dominated stage (Stage-III). The transition from the dominant cleavage of the ether bridged bond into breaking of the aliphatic bridged bonds corresponds to the transition of Stage-IIA to Stage-IIB in Liulin bituminous coal pyrolysis. Further investigation of the relationship between radicals and gas production suggests that temperatures for the transition of gas generation rates can be used as indicators for pyrolysis stage transitions, namely H 2 O for Stage-I and Stage-IIA, and CH 4 for the primary and secondary pyrolysis reactions, provided such production rate transitions could be detected experimentally. In addition, the compromise between the competition reactions of decomposition and recombination as well as radical generation and consumption plays a significant role along the entire pyrolysis process, and the slight differences of the reactions in competition determine the yield, species, and distribution of final pyrolyzates, which seems consistent with the mesoscale structure theory.

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The high-temperature reaction pathways of bio-oil oxidation were investigated by simulations of a 24-component bio-oil model using reactive force field (ReaxFF) molecular dynamics. Evolution profiles of fuel, O2, and major products, including radicals, with time and temperature during the initial stage of bio-oil oxidation were obtained. Major products generated during the simulations are consistent with observations reported in the literature. A kinetic model obtained from the simulated bio-oil oxidation is able to predict a long-time evolution trend of fuel consumption. Reaction networks of five representative components of the bio-oil model were revealed. The bio-oil oxidation is initiated by a series of homolysis and H-abstraction reactions and then propagation reactions involving H-shift, H-abstraction, and β-scission reactions. Oxidation of the unsaturated C–C bond, ring reduction of the phenolic radical, and abscission of the −CO structure (decarbonylation) appear frequently. Reaction pathways obtained from the comprehensive observations of simulation results employing VARxMD are in broad agreement with the literature. This work demonstrated a methodology that ReaxFF molecular dynamic simulations combined with the capability of VARxMD for reaction analysis can provide useful insights into the reaction pathway of bio-oil combustion.