Comprehensive insights into cellulose structure evolution via multi-perspective analysis during a slow pyrolysis process

Comprehensive understanding of cellulose pyrolysis is critical to valuable chemical and liquid fuel generation in biomass-based biorefining processes. Herein, a specific tubular reactor was designed to observe the cellulose appearance alteration corresponding to the chemical structure evolution in various pyrolysis stages. The appearance of cellulose changed from white powder to light brown powder below 300 °C as its crystal lattice thermal expansion and the degree of crystallinity decrease. Cellulose began to depolymerize at 300 °C accompanied with the dehydration of pinacol and primary product levoglucosan generation. 325–350 °C was the critical pyrolysis stage for cellulose. The intermediate state was formed accompanied with a series of thermochemical reactions. All the typical signals from FTIR, 13C CP/MAS NMR disappeared when the temperature was up to 350 °C. The percentages of C, H, and O conversion to the volatile products were up to 35%, 55% and 60%, respectively, along with β-1,4-glycosidic bond cleavage, decarboxylation, and decarbonylation reactions. Meanwhile, CO2, CO, and CH4 were measured using a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer (TG-FTIR) in the slow pyrolysis process. The increasing C/H and C/O ratios in the carbonation stage enhanced the aromaticity of biochar. Furthermore, electron paramagnetic resonance (EPR) analysis indicated that it tended to form carbon-centered radicals.