Figure 3 shows the solids concentration distribution measured by high-temperature ECT. The images of solids concentration distribution were obtained by the projected Landweber iteration algorithm with an optimal step length. As shown in Figure 3, when the superficial gas velocity increases gradually, the occurrence of bubbles can be clearly distinguished, i.e. the transition to bubbling fluidization at different temperature can be identified. A careful check with the snapshots of solids concentration distribution at different temperature suggests that the appearance of first bubble occurs at superficial gas velocity of 3.6, 3.4, 3.2 and 2.9 cm/s for T= 20, 200, 400, and 600°C, respectively.