During the thermochemical valorization of biomass feedstock, the assessment of biomass powder flowability is essential for the design of feeding, storage, and handling systems. Biomass handling in valorization processes require powders to be characterized for both consolidated and static conditions, encountered in storage facilities, or under free-surface dynamic flow conditions, present in feeding systems. This paper evaluated the flow properties of two different sizes of raw, mildly and intensively torrefied poplar powders, using shear tests and the evaluation of avalanching behaviors, in a rotating drum. The size and shape descriptors showed that the particle size distributions and particle morphologies of the six samples were determined by the coupled effects of torrefaction intensity and the sieving stage. According to the shear test results, although flowability trends are stress-dependent, coarse powders have better flow properties than fine powders. A flowability ranking was established, based on the parameters extracted from the flow dynamics inside of a rotating drum, including the tendency to form particle clumps, upper angle of stability, irregularity of avalanches, and the fraction of revolution for avalanches. The relationships among the macroscopic flow parameters, obtained from both characterization techniques and the powder characteristics (particle mean size, shape, and polydispersity) were examined using adjusted correlations. Overall, intense torrefaction appears to improve the flow behaviors of powders, under both free and consolidated flow conditions.