(266d) Milling, Crystallinity and Pyrolysis Rates for Cellulose and Whole Biomass

The effect of milling and cellulose crystallinity on pyrolysis rates of microcrystalline cellulose (MCC) and two forms of whole biomass (switchgrass and tall fescue) were studied using a combined experimental and computational approach. The crystallinity and particle size of milled samples were carefully determined along with the temperature at which the peak rate of decomposition occurs (T^DTG_max). A statistical analysis of the data shows that over a broad range of particle sizes from about 15 to 750 µm, only the crystallinity has a statistically significant effect on the T^DTG_max. Mirrored real-time high-temperature X-ray (RTHT-XRD) diffraction and thermalgravimetric analysis (TGA) show that while the onset of crystallinity loss is abrupt, it tends to lag the onset of weight loss as a function of temperature. The RTHT-XRD data shows that crystallinity of gently milled samples decrease by as much as 10% between room temperature and about 100 ^oC. Thereafter, crystallinity remains about constant until at between 200 and 250 ^oC a quantifiable recrystallization occurs followed by the sharp onset of crystallinity loss, lagging weight loss by as much as 25 ^oC. The notable recrystallization suggests high molecular mobility and the possibility of a liquid phase. Careful mirrored experiments using MCC suggest that crystallinity is retained until between 30 and 40% of the mass has been volatilized. Molecular dynamics simulation for cellulose deploymerization to form levoglucosan (LVG) assuming random dissociation of the β-glycosidic bond and bond rearrangement are consistent with the experimental observations. Upon removal of between 20 to 30% of the thus transformed glucose units, the simulated cellulose structure abruptly transitions from crystalline to amorphous. Notably, the retention of at least a portion of the formed LVG (as a condensed phase) appears to help extend crystallinity until somewhat higher levels of conversion.