Particle Motion in CFB Cyclones as Observed By Positron Emission Particle Tracking

Abstract

Circulating fluidized bed (CFB) cyclones operate at high solids loadings. The paper presents, for the first time, particle trajectories within a cyclone obtained by positron emission particle tracking (PEPT), as a function of the solids loading (C s ). The pressure drop across a cyclone is a strong function of the solids loading. The objective of this work was to explain this behavior by direct observation of the particle movement. Cyclones normally operate in a stable particle movement mode, always with a spiral motion in the cylindrical part of the cyclone, followed by either a continued spiral in the cone (at low C s ) or by a much denser solids flow near the cone wall at higher C s values. Data are used to obtain the tangential and axial velocity components of a tracer particle, the residence time of the particles in the cyclone, the thickness of the boundary layer in the cylindrical section of the cyclone, and the thickness of the dense wall layer in the conical section. This downward moving layer reduces the effective "free" cross section of the cyclone, thus increasing the air velocity and the pressure drop (∆P), especially for small cyclones, as in the present research. This effect will be negligible for larger cyclones, where the influence of solids film thickness is less important, when compared to cyclone diameter, and ∆P values are expected to remain almost constant with increases in the solids loading.

Fluidized bed waste incinerators: Design, operational and environmental issues

“…Cyclones [89][90][91][92][93][94][95][96] Standpipes [98][99][100][101][102][103][104][105] L-valves [106][107][108][109][110][111][112][113] 3. Stack emissions: Problems and remedies…”

Characteristics and mechanisms of particle adhesion patterns in an aerodynamic cyclone

“…In the experiment, there were five turns of the SPs. Because the particle-particle interaction decreased the number of turns (Chan et al 2008;Chu et al 2011), it was believed that if the CFD simulation considered the particle-particle interaction, the number of turns would be the same as that in the experiment. Moreover, as there was no significant change in the wall shear stress (Section S2) on most parts of the cyclone, the aerodynamic force on the particle near the wall did not change significantly.…”

“…By counting the band of the trajectory, the number of the spiral trajectory turns was 6, which was different from experimental results (5 turns). This difference was supposed due to the particle-particle interaction in the experiment, as the particle-particle interaction was found to reduce the number of turns (Chan et al 2008;Chu et al 2011).…”

Prediction of the maximum-efficiency cyclone length for a cyclone with a tangential entry

“…The friction loss coefficient f 0 is a function of Reynolds number and roughness k (or relative roughness ε = k/a h0 ). Although effect of high solids loadings is not considered in this model, regarding to the information provided in Chan et al [18] the model can be modified.…”