Reactive Molecular Dynamics Simulations of Biomass Pyrolysis and Combustion under Various Oxidative and Humidity Environments

Biomass, as a renewable carbon neutral energy source with abundant reserves, is a good candidate for future energy supplies. In this paper, a simplified biomass model composed of cellulose, hemicellulose, and lignin, described by a carefully selected reactive force field (ReaxFF), is investigated using molecular dynamics (MD) simulations. The pyrolysis and combustion processes of the biomass under different temperatures and oxidative and humidity conditions, are studied. We find that the individual products from the pyrolysis of the three biomass components are similar, including H2O, H2, CO, CO2, and small organic molecules. The calculated activation energies for C–C bond dissociation are 34.53, 26.08, and 16.23 kJ mol–1, respectively, for cellulose, hemicellulose, and lignin, consistent with the trend in experiments. Interestingly, light tar (C5–13) production reaches a maximum under intermediate temperatures, which could be further explored to optimize the production of light tar as liquid fuels. Compared to biomass pyrolysis in vacuum, hydrothermal treatment makes the C–C bonds more difficult to dissociate, but C–O bonds more vulnerable due to stronger attacks from ·H radicals. Higher H2 concentration is produced under the H2O atmosphere, while more CO is formed under the mixed H2O/O2 atmosphere. During biomass combustion, CO2 mainly comes from the cracking and reforming of ·COOH and ·CHO radical groups or directly from CO oxidation. We also observe that during biomass combustion, the formation of CO is facilitated at higher temperatures, whereas CO2 production is favored at lower temperatures. More rapid decomposition and oxidation of biomass during combustion occur under fuel-lean conditions compared to fuel-rich conditions. Finally, more H2O and fewer H2 molecules are generated during the combustion process under the O2/CO2 atmosphere when increasing the concentration of CO2. On the basis of this theoretical study, a better understanding of the radicals, intermediates, products, and reaction kinetics involved in biomass pyrolysis and combustion could be achieved.