The tubes are thus occupied about 10–45% of the riser height from the top. Sand was used as the bed material with mean diameter of 143 μm, 256 μm and 363 μm with respective densities of 2631, 2564 and 2740 kg/m3. The fluidizing air velocity and solid recycle flux were varied in the range of 4–8 m/s and 21–72 kg/m2 s respectively resulting in the average cross-sectional suspension density of 10–42 kg/m3, which is the range usually obtained in commercial boilers.