The present study provides a deeply analysis of the flow behavior of bubbling fluidized beds with fine particles in two- (2D) and three-dimensional (3D) conditions, and computational fluid dynamics (CFD) simulations of agglomerates fluidization are carried out coupled with the modified agglomerate-force balance model, correspondingly. The experimental results indicate that the fluidized bed can be divided into bottom unfluidized, middle ascending fluidized, and upper descending back-mixing sections. The local solids volume fraction value ranges from 0.11 to 0.30, which depends on the interaction between bubble phase (εs = 0–0.04) and emulsion phase (εs = 0.26–0.30). The wall effect appears to be weakened, and the cohesive particles fluidize more uniformly in 3D fluidized beds. The simulations are in reasonable agreement with the experimental findings. However, at the top region of the bed the predicted solids holdup slightly deviates from experimental measurement. The vector plots of computed agglomerates velocity support the central and wall falling down-both sides rising up flow pattern of solids, two core-annular flows exist in the bed, which can be also observed experimentally.

•We investigated the fluidization/defluidization behavior of carbon-coated iron ore.•The Ccritical value increased with increasing the temperature.•The Ccritical value was little affected by the gas composition.•Gas composition showed obvious effects on the stability of the deposited carbon.•The carbon content in DRI was reduced to < 5 wt.% by process optimization.To reduce the carbon content in the direct reduction iron (DRI) obtained via the two-step fluidized bed reduction process, we investigated the fluidization/defluidization behavior of the carbon-coated iron ore under different gas compositions and temperatures. It was found that the critical carbon content value (Ccritical) needed to prevent defluidization increases obviously with increasing fluidization temperature, but is little affected by the gas composition, while the stability of the deposited carbon is highly dependent on the gas composition, i.e. when the H2 mole fraction is greater than a certain critical value, the deposited carbon is unstable and will be constantly consumed during reduction, leading to the defluidization. Through the process optimization, the carbon content in DRI decreases to < 5 wt.%, which is also much lower than those reported values of 16.5–22.3 wt.% in literature.

•A new drag model for Geldart-B particles TFM simulation in bubbling fluidized beds was proposed.•The new model incorporated the contribution of meso-scale structure effects.•Simulated results by the new model showed better agreement with experiments than Gidaspow model.In this work, a new drag model for TFM simulation in gas–solid bubbling fluidized beds was proposed, and a set of equations was derived to determine the meso-scale structural parameters to calculate the drag characteristics of Geldart-B particles under low gas velocities. In the new model, the meso-scale structure was characterized while accounting for the bubble and meso-scale structure effects on the drag coefficient. The Fluent software, incorporating the new drag model, was used to simulate the fluidization behavior. Experiments were performed in a Plexiglas cylindrical fluidized bed consisting of quartz sand as the solid phase and ambient air as the gas phase. Comparisons based on the solids hold-up inside the fluidized bed at different superficial gas velocities, were made between the 2D Cartesian simulations, and the experimental data, showing that the results of the new drag model reached much better agreement with experimental data than those of the Gidaspow drag model did.

The present work focuses on a fully statistical analysis of bubbling behavior in the two-dimensional (2D) fluidized beds with cohesive particles. Various significant bubble properties such as bubble size, rising velocity, aspect ratio, bed expansion and bubble hold-up, etc., were investigated. An equation for bubble diameter is developed, db= 0.21(ug−umb)0.49(h+4A0)0.48/g0.2, and the observed bubbles are generally smaller than the ones generated in the beds with A or B type powders. Both the average bubble size and rising velocity initially increase with the elevation above the distributor and keep constant beyond certain heights. The bubbles exhibit oblong with the most density aspect ratio (β) equal to 0.7. In addition, the bubble rising velocity coefficient ranges from 0.8 to 1.5. Two core-annular flows form in the large diameter, shallow fluidized bed used in this experiment.The bubbling behavior of the 2D fluidized beds with cohesive particles has been studied experimentally by using the digital image analysis technique. Various significant bubble properties such as bubble size, rising velocity, aspect ratio, bed expansion and bubble hold-up were investigated. The observed bubbles are generally smaller than the ones generated in the beds with A or B type powders.Highlights► The bubbling behavior of cohesive particles is fully analyzed for the first time. ► The bubble diameter can be described by db= 0.21(ug−umb)0.49(h+4A0)0.48/g0.2. ► The bubbles are smaller than the ones generated in the beds with A or B type powders. ► The bubble rising velocity coefficient ranges from 0.8 to 1.5.

Along with the fast development of computer technology and measurement techniques, fundamental research on fluidization is faced with both new challenges and opportunities. Among others, great attention should be focused on the meso-scale structure of fluidized beds, to study the quantitative prediction theory and optimum control method for the meso-scale structure of fluidized beds, and to establish the modeling of the relationship between meso-scale structure and momentum transfer, heat transfer, mass transfer, and chemical reaction. These efforts, combined with advanced computer simulation, are expected to solve difficult problems in optimum control and scale-up of fluidized bed processes and equipment.Fundamental research on fluidization should focus on the meso-scale structure of fluidized beds, to establish the relationship between meso-scale structure and momentum, heat and mass transfer, and chemical reaction. These efforts, combined with advanced computer simulation, are expected to solve difficult problems in optimum control and scale-up of fluidized bed processes and equipments.

Dense–dilute phase structure is a common feature of gas–solids flow in circulating fluidized bed. This paper presents an experimental study of dense–dilute phase characteristics in a high-density downer (5.6 m tall, 80 mm ID), in which solids holdup as high as 15% has been achieved. The quantitative characteristics of dense–dilute phase, such as time fraction, duration time, frequency, have been investigated to show the effects of operating conditions, radial position and axial elevation. Preliminary comparison of the phase structure in a high-density downer to that in a high-density riser has been carried out in this work.

Kinetics parameters of iron oxide reduction by hydrogen were evaluated by the isothermal method in a differential micro-packed bed. Influence of external diffusion, internal diffusion and heat transfer on the intrinsic reaction rate was investigated and the conditions free of internal and external diffusion resistance have been determined. In the experiments, in order to correctly evaluate the intrinsic kinetics parameters for reducing Fe2O3 to Fe3O4, the reaction temperatures were set between 440 °C and 490 °C. However, in order to distinguish the reduction of Fe3O4 to FeO from that of FeO to Fe, the reaction temperature in the experiment was set to be greater than 570 °C. Intrinsic kinetics of iron oxide reduction by hydrogen was established and the newly established kinetic models were validated by the experimental data.

•Cluster phase plays a key role in gas-solid transport phenomena in CFB.•Mass transfer in CFB is dominated by mass exchange between meso-scales.•The MSMT model can capture the effect of meso-scale cluster in CFB.Catalytic oxidation of carbon monoxide over a Pt catalyst was employed as a model reaction to investigate the effect of meso-scale structure on gas-solid mass transfer in a circulating fluidized bed (CFB). Both experimental and theoretical analyses were performed to determine the conditions under which the reaction process was dominated by mass transfer. The experimental works involved the measurements of the axial distribution of carbon monoxide concentration, whereas the theoretical analyses were on the use of an Multi-Scale Mass Transfer (MSMT) model with considering effects of clusters on gas-solid transport phenomena in the CFB. The results of MSMT model show a good agreement with experimental data, suggesting that the particle clusters play an important role in the gas-solid momentum transfer and mass transfer in CFBs.