Torrefaction can upgrade the properties of biomass, making it similar to coal. This study develops an innovative indirectly heated two-stage rotating drum-type torrefier where biomass moves due to the rotation of inclined cylindrical reactors. The rotary reactors consist of three rectangular flights placed axially along the reactor. This paper mainly investigates the mean residence time and filling factor of reactors operated at steady and plug flow conditions. Semi-empirical expressions using dimensionless correction constants for the mean residence time and the filling factor were proposed. Experiments keeping a constant solid flow rate at the dryer inlet were conducted in order to measure filling factor and mean residence time at different angular speeds and inclinations of reactors (dryer and torrefier). Mean residence times of 5.8 min at 8 rpm and 38, and of 27.5 min at 4 rpm and 18 were found in the dryer (diameter: 64 mm, length: 610 mm), whereas they were 4.7 and 21.4 min in the torrefier (diameter: 100 mm, length: 762 mm) at respective operating conditions. Filling factors of 3.0 % at 8 rpm and 38, and 14.3 % at 4 rpm and 18 were found in the dryer, whereas they were 0.8 % and 3.6 % in the torrefier at the corresponding angular speed and inclination. Measured and predicted mean residence times were also compared and validated against those of other investigators. The proposed expressions, which include the effects of different operating and geometrical parameters, have strong predictive power and may be useful in scaling-up a rotary torrefier.