Effect of torrefaction intensity on the flow properties of lignocellulosic biomass powders

Flowability characterization of milled lignocellulosic biomass is essential for developing viable conveying, storing and handling solutions for gasification processes. This study investigated the effect of torrefaction on particles size and shape obtained after grinding and on flow properties of pulverized wood. Spruce and poplar samples with six torrefaction intensities were knife-milled to obtain biomass powders. Particles size and shape distribution were assessed using a morphological particle size analyser and flowability parameters were determined with a ring shear tester. A more intense treatment produces finer, rounder and more regular particles. Simultaneously, a gradual shifting was observed from a cohesive behaviour for native biomass to a nearly free flowing behaviour for the most intensively treated samples. The trends in flowability cannot be explained by the size reduction nor the increase of distribution width. Instead, the explanation lies in the reduction of shape factor and the sharpness of particle surfaces for treated samples. However, all observations are consistent with the loss of resilience of treated wood. From our results, it is clear that torrefaction, in addition to its interest on reduction of energy consumption of grinding, should also be considered as a pre-treatment step allowing to modify the flow behaviour of biomass powders.