A novel autothermal reactor, named internally interconnected fluidized beds (IIFB), was developed to convert biomass into liquid fuels and other valuable chemicals by fast pyrolysis. The IIFB reactor has a pyrolysis bed directly connected to a combustion bed to perform continuous and simultaneous operations of biomass pyrolysis and energy recovery from coke burning. The current study focuses on the hydrodynamics of the IIFB reactor, which can be used for identifying the optimal operating conditions for the complex biomass fast pyrolysis process. A visible plexiglass IIFB (internal diameter, 0.1 m: height, 1.5 m) system was built to investigate the flow behaviors of gas and solid in the reactor. The effects of spouting and fluidizing gas, static bed height, particle size and exit resistance on solid circulation rate and gas bypassing between the pyrolysis bed and combustion bed were studied. The results show that the solid circulation could be adjusted by changing spouting gas and fluidizing gas velocities. The highest solid circulation rate is about 121 kg/m(2) s (213.1 kg/h). In addition, lower static bed height or smaller particle diameter leads to higher solid circulation rate, while lower resistance at pyrolysis bed outlet results in higher circulation rate. The gas bypassing characteristics has also been investigated. The results show that the gas bypassing fraction from combustion bed to pyrolysis bed can be controlled below 5%, while that from pyrolysis bed to combustion bed can be controlled below 15%. (C) 2011 Elsevier B.V. All rights reserved.