The gas distribution between the dilute (bubble) and dense (emulsion) phases of a fluidized bed is studied locally and globally in the bubbling regime for mixtures composed of sand and different weight fractions of biomass (2-16%). The dilute phase has been characterized by analyzing the pressure and voidage signals. A suite of pressure transducers was used to measure pressure fluctuations at different locations along the bed. A reflective optical probe measured local voidage signals and was placed at different radii (0 < r/R < 0.87) at a height of h=175 mm above the distributor plate. The mean voidage of the bed is increased with higher biomass loading, primarily because of dilution of the emulsion phase. Changing the quantity of biomass in the bed does not significantly affect the voidage of the bubble and emulsion phases. The void (bubble) fraction increases at the center of the bed, whereas it decreases and then increases at the wall region with increasing weight fraction of biomass. Higher quantities of biomass reduce the mean bubble size and boost the bubble frequency at the center of the bed. The core-annulus structure of the bed is intensified for mixtures with relatively low quantities of biomass, while increasing the biomass load leads to a more uniform distribution of small bubbles across the bed improving the fluidization quality. (C) 2013 Elsevier Ltd. All rights reserved.