Detailed chemical kinetic modelling of vapour-phase cracking of multi-component molecular mixtures derived from the fast pyrolysis of cellulose

摘要

The kinetics of vapour-phase cracking of nascent volatiles generated from the fast pyrolysis of cellulose was studied both experimentally and numerically. A two-stage tubular reactor (TS-TR) was developed for evaluating the reaction kinetics of secondary vapour-phase cracking of cellulose while minimising volatile-char interactions. The TS-TR was divided by a filter into two zones, one for the rapid pyrolysis of cellulose and the other one located downstream for the secondary pyrolysis of the nascent volatiles in the gas phase. Variations in gas compositions during secondary pyrolysis were monitored at a residence time of up to 6 s and a temperature ranging from 973 to 1073 K. These experiments were numerically simulated using a detailed chemical kinetic model that comprises more than 500 species and around 8000 elementary step-like reactions. Exhaustive comparisons between experimental data and numerical predictions were conducted for more than 20 species to critically evaluate the kinetic model. The model predictions generally agreed for the experimental concentration profiles of major species such as H_2, CO, CO_2, CH_4, and C_2H_4. Agreements for minor products such as acetaldehyde, acetic acid, acetone, hydroxyl acetone, furan, benzene, and toluene were fair, though further efforts are needed to improve predictions for concentration profiles of compounds such as methanol and C_3 hydrocarbons. Reaction pathway analysis was also conducted for first aromatic species such as benzene in order to assess mechanistically how tarry materials (i.e., aromatic hydrocarbons) are generated from cellulose, which is originally free of aromatic structures.