Assessing the specific energy consumption and physical properties of comminuted Douglas-fir chips for bioconversion

Abstract

Size reduction homogenizes the bulk biomass and facilitates downstream feedstock handling, transportation, and storage. Effects of feeding rate, mill-type (hammer and knife mill), screen size, and moisture content on comminution energy consumption of commercial Douglas-fir (Pseudotsuga menziesii) pulp chips were quantified. The resulting particles were characterized by geometric mean diameter, size distribution, aspect ratio and bulk density. We also employed scanning electron microscopy (SEM) to visualize the dominant fracture surface features. A linear regression was used to describe the relationship between specific energy consumption (SEC) and comminution ratio, which can be used for estimating SEC of various particle sizes of feedstock. Results demonstrated that with a screen size of 3.18mm, the hammer mill consumed 141kJkg−1, while the knife mill consumed 345kJkg−1. Selecting the feeding rate, which allows the machine reach its rated power, can result in lowest total SEC. SEC also increased substantially with an increase in moisture content from 11% to 17% (fraction in total mass basis). Analysis of the particle morphology revealed that the dominant fracture mechanism was across lumen for hammer milling and along middle lamella for knife milling.