A novel system of two adjacent fluidised beds operating in different gas atmospheres and exchanging solids was developed for the combined flash pyrolysis of biomass and combustion of the produced char. Fluidised sand particles (200 mu m < d(p) < 400 mu m) are transported from the pyrolysis reactor to the combustor through an orifice and recycled by a standpipe, riser and cyclone. Advantages of the new design are its compactness and the high level of heat integration. The solids circulation rate and holdup distribution between the two compartments could be controlled adequately in experiments at room temperature and atmospheric pressure. A model, developed to predict the solids and gas exchange between the two reactor compartments, was validated with experiments in which the three relevant gas flows, the orifice diameter and the particle diameter were varied. (C) 2000 Elsevier Science S.A. All rights reserved.