Biomass has been used as a primary fuel source ever since fire was discovered, and was later substituted with fossil fuel in recent human history. Researchers today are refocusing on biomass, a clean and renewable energy source which can be transformed into fuel and chemicals and more importantly, mitigate global warming. Most methods of biomass utilization e.g. combustion [1
], gasification [3
] and pyrolysis, have profitable efficiency when conducted in fluidized beds. Fluidized beds have been widely used because of their good performances such as gas-solid contacting, controllable temperatures and high heat transfer.
The properties of biomass particles i.e. low density, large size and irregular shapes would lead to undesirable fluidization. However, good fluidization is the basis of well mixing, heat transfer, and chemical reactions. To optimize the fluidization process, fluidization mediums such as silica sands or calcite are applied. With these inert mediums, the hydrodynamic characteristics of biomass particles are improved and so are the chemical processes. However, the mixing behavior of biomass and inert particles is quite complicated because of the large difference between these two particle types. Many researchers have attempted to understand the fluidization of mixing biomass with inert medium in recent years using both experimental and simulation approaches. Some researchers conducted experiments to analyze the relation of minimum gas velocity and the concentration of biomass particles [4
], while others compared the biomass fluidization process with or without inert particles [8
]. However, the limitation of some experimental technology could affect the accuracy of the results and conclusions. The measurement of gas and particles would disturb the flow process because it is hard to get the particle information during the fluidization process [13
]. Particle sampling size and sampling distribution also influence the evaluation result of particle mixing behavior.
Compared to the experimental method, numerical simulation could avoid these drawbacks and has better performance on elucidating the mechanisms governing mixing [14
]. Coupled computational fluid dynamics (CFD) and discrete element method (DEM) have proven to be a powerful method for realizing the complex hydrodynamics of mixing particles in a fluidized bed. The CFD-DEM method has been widely applied on the simulation of fluidized beds [7
], spouted beds [17
] and circulating fluidized beds [19
]. Fluid motion could be analyzed at mesh size scale by the CFD method in a Euler frame [20
]. Moreover, the movement of individual particles could be investigated at the particle size scale using the DEM method, allowing for complete analysis of the mixing behavior of particles.
Some researchers studied the mixing behavior of same particles: Oke et al. [21
] and Liu et al. [22
] studied particle lateral mixing system in fluidized beds; Amiri et al. [23
] conducted simulation to compare the effect of particle size and density in particle mixing process; Luo et al. [24
] investigated particle mixing characteristics at different gas velocities. There has also been increasing research on the mixing behavior of binary particles by CFD-DEM method gas flow behavior in fluidized beds: Ma et al. [25
] studied the mixture of binary particles in 2-D simulation in which particle shape was considered; Feng [26
] et al. provided a mixture model of particles 1 mm in diameter and 2 mm in diameter and explored the factor of mixing and segregation process. Most of the mentioned studies were conducted in a 2-D model. Gas and particle movement may be different when loss one dimension. Surya et al. [27
] provided a comparison model of 2-D and 3-D and proved that 3-D simulations could predict predict the bubble characteristics far away from the distributor plate. Particle collision and gas-solid interaction force could not be ignored, especially when particles have large differences of size and density. In the Narrower Direction, the relative displacement of the particles is greater in proportion to the distance traveled. It is necessary to build 3-D model for particle mixing behavior investigation. Some research focused on the effect of biomass particle shape. The shape of particles is also crucial and has been studies separately and reported in our former studies [28
]. However, compared to non-spherical shapes, biomass in a spherical shape is widely applied in present simulation studies for further utilization of biomass, so it is necessary to conduct further study using a 3-D model and provide detailed theoretical support.
The objective of this study is to analyze the mixing behavior of biomass particles and sands in a fluidized bed by 3-D modeling in Euler-Lagrange frame. This study focuses on investigating the relationship of mixing quality and fluidized parameters such as gas velocity and particle size. The hydrodynamic characteristics binary particle systems are analyzed. Finally, the mixing mechanism is discussed.