Experimental Study On The Flow Characteristics Of Particles In Fluidized Bed

Biomass fast pyrolysis is one of the effective ways for utilizing solid biomass and fluidized bed is a typical fast pyrolysis reactor. The solid particles of quartz sand or sand, which is stable in physical properties, is used as heat carrier for heat transfer from sand to biomass in the fluidized state. Biomass fast pyrolysis process is very complex. The flow characteristics of particles directly determine the heat transfer process between particles, and the heat transfer between the particles also affect the biomass pyrolysis process. It has an important significance to understand the flow characteristics of particles in the fluidized bed for biomass fast pyrolysis.A transparent fluidized bed test apparatus was designed and fabricated for simulating the cold state mixed flow of quartz sand and biomass powder under the action of air flow in the biomass pyrolysis process. Particle Image Velocimetry(PIV) and high speed photography techniques were used to study the motion characteristics of particles under different conditions. High speed photography experiment was conducted in the transparent fluidized bed test apparatus using quartz sand as the bed material with the particle size of 20-40 and 40-60 meshes.The static bed heights were 50 mm, 80 mm and 110 mm.The fluidization state parameters including the bed pressure drop, the height of dilute phase zone,bubble size change were measured. The study found that the overall fluidization effect is the most appropriate in the 80 mm bed height.The flow field in the bed was measured using quantitative PIV technique with80 mm static bed height. Corn stalk powder with particle size of 20-30 mesh was used as the experimental material, the ratios of quartz sand to biomass were 20:1,30:1, 40:1, and the operating wind speeds were 0.56m/s, 0.69m/s, 0.82m/s. The velocity distribution under different conditions, the transverse velocity and longitudinal velocity distribution were determined. The following conclusions were concluded from the experimental results:(1) with the increase of the apparent wind speed, the axial velocity increased significantly while the transverse velocity of mixed particles increased slowly. The center particle axialvelocity increased more obviously than that of side wall. When the fluidization velocity is 0.56m/s, the mixed particle velocity is 0.4m/s; When the fluidization velocity is 0.69m/s, the mixed particle velocity is 0.5m/s; When the fluidization velocity is 0.82m/s, the mixed particle velocity is 0.65m/s;(2) with the decrease of sand to biomass ratio, the axial velocity increased significantly while the transverse velocity of mixed particles increased slowly. The axial velocity distribution is more scattering than that of the transverse velocity;(3) with the decrease of the quartz sand diameter, the axial velocity in the central area increased significantly, whereas, the transverse velocity of mixed particles decreased significantly.