Investigation of components interaction during pressurized pyrolysis of biomass via ReaxFF MD simulation: Free radicals driven synergistic deoxygenation and polymerization reactions

A deeper deoxygenation and upgrading of biomass could be realized through pressurized pyrolysis, whereas the product properties and reaction pathways from different biomass vary significantly, primarily due to components variations and interactions. This study employed reactive force field molecular dynamics (ReaxFF MD) to elucidate co-pyrolysis mechanisms of biomass three components at microcosmic level. The effects of temperatures and pressure on individual component pyrolysis were initially investigated. Results indicated that pressure significantly impacted the pyrolysis of cellulose and hemicellulose, particularly on cellulose, increasing its char yields at 1400–1800 K. The H₂O and CO₂ yields under pressure were markedly higher than atmospheric pressure. Moreover, by comparing calculated and simulated values, the influence of component interactions on product characteristics under atmospheric and pressurized conditions was analyzed. Simulation results showed that pressure enhanced component interactions, particularly between cellulose and lignin (C-L) at temperatures above 1700 K, with a maximum deviation of 13.97 % in char yields. The actual C₂H₂O₂ and C₃H₄O₃ yields were notably higher than calculated at 1800–2300 K, especially under pressure. C-L co-pressurized pyrolysis resulted in more -CO and -COOH groups removal as aldehyde and carboxylic acid small molecule volatiles. In contrast, cellulose-hemicellulose (C-H) interaction mainly occurred during volatiles secondary reaction at elevated temperature and pressure. These results are consistent with our existing experimental data. Ultimately, by tracking C, H, and O elements dynamic migration, a crucial “initiator” role of radicals in synergistic reactions was revealed. The work provides theoretical support for producing high-quality biochar via optimizing pressure conditions and components regulation.