Granular flows are commonly encountered in many industrial processes, but are difficult to characterize due to the opaque nature of the flow. For instance, screw pyrolyzers are being developed for the thermochemical conversion of biomass into bio-oil, but the granular flow and mixing process inside the reactor lacks fundamental understanding. In this study, X-ray particle tracking velocimetry (XPTV) is used to qualitatively and quantitatively characterize the three-dimensional (3D) granular flow structures in a double screw mixer, which geometrically replicates double screw pyrolyzers, by visualizing the position and speed profiles and quantifying the dimension-less pathlength and dimensionless residence time of individual tracer particles. The influence of screw rotation speed, dimensionless screw pitch, screw rotation orientation, and material injection configuration are investigated. Certain operating conditions are shown to significantly influence the granular flow structures and, in some instances, cause the double screw mixer to behave similar to two single screw conveyors. Comparisons with previous granular mixing studies are made to provide a link between granular flow behavior and mixing effectiveness. (C) 2015 Elsevier B.V. All rights reserved.