The effects of the kind of biomass (woody and herbaceous) and their physical conditions (moisture content and particle size and shape) on handling behavior are analyzed. Comminuted poplar and corn stover for energy use are studied at several moisture contents (W-H2O approximate to 7% and 28%) and particle sizes (d(p) < 5 mm and 2 mm). Angle of internal friction, effective angle of friction, cohesion, unconfined yield strength, major consolidation stress and loose and compressed bulk densities are obtained for each physical condition of both biomass materials. The underlying mechanisms that govern inter-particle friction, cohesion, arches or ratholes and particle rearrangement inside the bed material are analyzed. Wall friction tests with four surfaces (roughened carbon steel, carbon steel, HMPE-500 and HMPE-1000) are performed. Surface material has proven to be very relevant for biomass flowability, in particular, the high molecular weight polyethylene (HMPE) shows a noticeable wall friction reduction in practical applications when wet biomass is used. (C) 2013 Elsevier B.V. All rights reserved,