Successful feeding is critical to biomass utilization processes, but trouble-free delivery is difficult due to particle. heterogeneity, physical characteristics and moisture content: A model, extended from previous models in the literature, is proposed to delineate what limits screw feeding in terms of the mechanisms of blockage and to predict torque requirements for biomass materials. Predictions on feeding of wood pellets, sawdust, hog fuel and wood shavings in a screw feeder/lock hopper system are in good agreement with our earlier experimental findings. The choke section and seal plug are shown to play important roles in determining the torque requirements for biomass screw feeding, although they are rarely mentioned in previous research on screw feeding of other materials. (c) 2007 Elsevier B.V. All rights reserved.