A study of cellulosic biomass size reduction

Size reduction is an essential operation for preparing biomass for the production of pellets, biofuels and bioproducts. Size reduction ranks second in terms of energy consumption after drying in a pelleting operation. The major challenge in sizing and operating a grinder is the difficulty in predicting the performance of a grinder and the quality of product due to the variability in structure and composition of the biomass. As a result, grinders are often over- designed to handle a wide range of biomass species, leading to disproportionate equipment size and operating costs. This research investigated factors influencing the power requirement for grinding biomass and developed mechanistic model equations to predict energy input to a grinder to achieve a targeted particle size. Two softwood species and three hardwood species were ground in a knife mill and/or a hammer mill. The experimental data consisted of power inputs, mass flow rates, and particle size reduction ratios. The well-known mechanistic model equations: Rittinger, Kick, and Bond, which relate energy input to particle size reduction, were evaluated and the Rittinger equation was found to give the best prediction of the experimental data. Douglas-fir consumed the least specific energy of grinding, 132-178 kJ kg‐¹, followed by aspen, 197-232 kJ kg‐¹, pine, 201-263 kJ kg‐¹, and poplar, 252-297 kJ kg‐¹. Specific surface area (m² kg‐¹) created was largest for aspen and smallest for Douglas-fir. Correspondingly, Douglas-fir consumed the least specific energy and aspen, with the largest specific surface area created, required the highest specific energy. These data suggest that the specific energy has a direct relation with the total surface area created as a result of size reduction, as captured by the Rittinger equation. Ground Douglas-fir and willow were also pelletized in a single pelletization unit. The combined grinding/densification energy input decreased with increasing particle size. The properties most significantly affecting the grinding energy consumption based on the comparison of the Rittinger’s constant, kR, were lignin content, particle density, and fibre length. Woody biomass of a higher lignin content, lower particle density, and longer fibre length requires more energy input to be ground to a targeted size.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat