To investigate the biomass mixing in the bubbling fluidized bed gasifier, a digital image processing based technique is developed in the paper. Red wooden balls are used as tracer particles to study biomass mixing in a two-dimensional fluidized bed gasifier. The experimental results show a high degree of biomass mixing in the vertical direction in the bed, whereas horizontal mixing in the bottom zone is found to be relatively limited. Along the vertical direction of the bed, a more uniform distribution of biomass particles is found at a low superficial velocity than at a high superficial velocity. With respect to lateral mixing, the biomass concentration in the center increases with the superficial velocity, whereas it decreases in the wall region. The biomass concentration in the bottom region decreases with the superficial velocity, but increases in the upper region of the bed. (c) 2006 Elsevier Ltd. All rights reserved.