Flow Characteristics of Coal Ash in a Circulating Fluidized Bed

Abstract

Solids flow characteristics of coal ash are investigated with the measurement of the local solids flux, local solids concentration, and spatial distribution of particle size. Two sets of the solids sampling probe are used to quantify the local solids flux at different radial and axial locations. The macroscopic flow structure is presented by analyzing the radial profiles of solids flux and the local particle size distributions, while the mesoscale flow structure is illustrated on the basis of probability density function of instantaneous solids concentration, the intermittency index, and the cluster frequency. It is found that both macroscopic and mesoscopic structures of gas−solid flow of coal ash are different from those of commonly studied fluid catalytic cracking and sand particles. The differences include higher upward and downward solids fluxes in the lower dense region, significant particle segregation, and more uniform annular flow. Furthermore, a semiempirical model taking into consideration the higher slip velocity between gas and solids is developed to account for the radial profiles of the solids flux in the upper dilute region of the riser.

Characterization of Flow Intermittency and Coherent Structures in a Gas–Solid Circulating Fluidized Bed through Electrostatic Sensing

“…They were also found to exhibit symmetric, periodic and apparent flapping motion 15 . Similarly in a riser, it is known that the flow intermittency is strongly dependent on the presence of particle clusters 5,6,9 , which should thus be regarded as coherent structures. In addition, particle vortices are classified as typical coherent structures in a bubbling fluidised bed, as they are generated by velocity gradients and wall shear, carrying flow energy and heterogeneity 17 .…”

“…The higher the index value, the more flow structures (e.g. particle agglomerates, particle clusters) existing at the given location 5,6,9 . However, this index only provides a rough quantification and is in fact insufficient to describe the complex intermittent flow behaviours in a CFB.…”

Computational characterisation of intermittent hydrodynamic behaviours in a riser with Geldart A particles

“…In addition to the PDF of wavelet coefficients derived from the solids fluctuating velocity signals, the PDF of local solids volume fraction provides useful information about the flow pattern and flow structures in a riser [2]. Figure 13 shows the PDF of solids volume fraction at h=3.0 m, Ug=1.52 m/s and Gs=26.9 kg/(m centre.…”

“…Owing to the small sizes, lower apparent densities and high specific surface areas, Geldart A particles are highly prone to form heterogeneous flow structures, typically the particle clusters, under the complex gas-solid interactions during fluidisation. These flow structure s a l o n g w i t h t h e intermittent nature of local solids flow yield an important consequence called flow intermittency, referring to the intermittent occurrence of large-magnitude fluctuations reflected in the flow signals [1,2]. As the flow intermittency is closely related to the heterogeneous distribution of hydrodynamic parameters and the non-equilibrium flow state, which can further result in fluidisation faults such as stagnant zones, hot spots, particle adhesion to the wall, slugs, and even defludisation, it is necessary to understand the hydrodynamic characteristics of the intermittent flow behaviours in a CFB with Geldart A particles.…”

Non-intrusive characterisation of particle cluster behaviours in a riser through electrostatic and vibration sensing

“…Single-value thresholds (e.g. times of the standard deviations above the mean voltages) have been applied to solids holdup signals from capacitance and optical fibre probes [9,27,28]. Considering that such thresholds may lead to dynamic information loss, wavelet transform was then used for the threshold determination of optical fibre signals, because of its capability of demarcating different flow scales (microscale, mesoscale, macroscale) [1,2,29].…”