Kinetic characterization for dilute sulfuric acid hydrolysis of timber varieties and switchgrass

Hydrolysis of four timber species (aspen, balsam fir, basswood, and red maple) and switchgrass was studied using dilute sulfuric acid at 50 g dry biomass/L under similar conditions previously described as acid pretreatment. The primary goal was to obtain detailed kinetic data of xylose formation and degradation from a match between a first order reaction model and the experimental data at various final reactor temperatures (160–190 °C), sulfuric acid concentrations (0.25–1.0% w/v), and particle sizes (28–10/20 mesh) in a glass-lined 1 L well-mixed batch reactor. Reaction rates for the generation of xylose from hemicellulose and the generation of furfural from xylose were strongly dependent on both temperature and acid concentration. However, no effect was observed for the particle sizes studied. Oligomer sugars, representing incomplete products of hydrolysis, were observed early in the reaction period for all sugars (xylose, glucose, arabinose, mannose, and galactose), but were reduced to low concentrations at later times (higher hemicellulose conversions). Maximum yields for xylose ranged from 70% (balsam) to 94% (switchgrass), for glucose from 10.6% to 13.6%, and for other minor sugars from 8.6% to 58.9%. Xylose formation activation energies and the pre-exponential factors for the timber species and switchgrass were in a range of 49–180 kJ/mol and from 7.5 × 104 to 2.6 × 1020 min−1, respectively. In addition, for xylose degradation, the activation energies and the pre-exponential factors ranged from 130 to 170 kJ/mol and from 6.8 × 1013 to 3.7 × 1017 min−1, respectively. There was a near linear dependence on acid concentration observed for xylose degradation. Our results suggest that mixtures of biomass species may be processed together and still achieve high yields for all species.