Abstract Mixing characteristics of a novel Mechanically Fluidized Reactor (MFR), a continuous, cylindrical, mechanically mixed reactor developed for biomass pyrolysis in the ICFAR laboratory, have been investigated with the specific objective of selecting the optimal stirrer geometry for fast pyrolysis of biomass. The MFR does not use any fluidization gas and its stirrer provides the required mixing between the injected biomass and the bed material while effectively breaking any possible agglomerate. In addition, good mixing is crucial to achieve effective heat transfer characteristics between the heaters and the bed, and between the bed and the biomass particles. In conventional fluidized beds, the torque required to mix the bed decreases as the superficial velocity of its fluidization gas increases, becoming constant above a critical superficial velocity which is a function of the stirrer characteristics. For the MFR, a method has been developed to monitor the torque and the power required to mechanically mix a bed of low density particles with the natural bed aeration resulting from the formation of gases and vapors during pyrolysis. In this study, three different shaped stirrers were first compared at various rotational speeds by artificially aerating the bed with nitrogen at different superficial velocities to simulate the generation of vapors and gases during pyrolysis. Furthermore, different fluidization gases were used in order to simulate the different characteristics of vapors produced during pyrolysis and, specifically, to evaluate the effects of their density and viscosity. The critical aeration rate at which the stirrer torque becomes constant is similar for all the stirrers. The second part of this study focused on actual wood pyrolysis tests. The reactor was first supplied with nitrogen above the critical aeration rate. Then, the nitrogen was shut off and wood pellets were fed into the reactor. The formation of pyrolysis gases and vapors greatly decreased the power required to mix the bed and this reduction was dependent on the type of stirrer. The stirrers were ranked in terms of their performance in minimizing the pyrolysis time. The vertical blade stirrer resulted in the smallest pyrolysis time and power consumption. Additionally no segregation of the biomass pellets was observed. Hence, gases and vapors were formed at the optimum location for effective aeration. The findings of this project have provided a better understanding of the MFR technology when used for the pyrolytic processing of biomass materials.