Effect of Ball Milling on Switchgrass, Tall Fescue, and Microcrystalline Cellulose: Characterization and Slow Pyrolysis Analysi

Biomass pyrolysis offers an opportunity to produce renewable chemicals, fuels and energy through the use of raw, abundantly available energy crops. Potential herbaceous biomass energy crops, switchgrass and tall fescue, grow in harsh environments making them universally accessible as feedstock. Raw biomass is generally milled to reduce heat and mass transfer limitations and consequently pyrolysis process temperatures. In addition to reducing the particle size, milling also reduces the crystallinity of the cellulose fraction. Both particle size and cellulose crystallinity effect pyrolysis rates. Slow pyrolysis was used to investigate the relative importance of particle size and cellulose crystallinity on pyrolysis rates. The crystallinity and extent of pyrolysis were studied for microcrystalline cellulose, switchgrass and tall fescue using mirrored real-time high-temperature X-ray diffraction (HT-XRD) and thermogravimetric analysis (TGA). For particles between about 15 and 750 mum in nominal size, crystallinity was found to be the only statistically significant factor effecting the temperature at which the peak rate of pyrolysis occurs. And, while the three biomass types show a similar relationship between thermally-induced crystallinity loss and extent of pyrolysis, the rate of onset and onset temperatures are different. Finally, evidence for thermally-stimulated recrystallization of highly amorphous cellulose was noted between 200 °C and 250 °C for all biomass types suggesting a high degree of molecular mobility and possibly the presence of a liquid phase.