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Circulating fluidised bed fluid and particle mechanics: modelling and experimental studies with application to combustion

University of British Columbia

The fluid and particle mechanics of circulating fluidised beds (CFBs) was studied in a series of theoretical and experimental investigations, leading to the development of two riser models. Though the study focussed primarily on CFB combustors, most of the results apply to all CFB applications. Experimental tests were performed in a 9.3 m high, 152 mm ID transparent cold model riser. The effects of varying the riser base section geometry, riser exit geometry, secondary air injection, particle size distribution (PSD) and particle density were investigated. Local solids concentrations within the riser were measured by a ueedle capacitance probe, and axial suspension density profiles were estimated from measured differential pressures along the riser length. All parameters investigated influenced the solids flow and distribution in the riser. In particular, changing the base section of the riser from a cylindrical to conical geometry significantly influenced suspension densities in the lower part of the riser. PSD affected solids hold-np within the riser when there was downflow of particle sheets or “streamers” at the wall. Radial particle size segregation was detected in tests with wide PSD particles. Experimental results were also obtained from a 7.3 m high, 152 mm x 152 mm square pilot-scale CFB combustor. Axial suspension density profiles were recorded for typical CFB combustor operating conditions. Wear patterns on erosion probes and results from high tem perature capacitance probe traverses indicated a core-annulus solids distribution, similar to that observed in cold unit risers. Detailed analyses of likely gas-particle and particle-particle interactions within the riser were performed. An extension to existing methods for estimation of the response of discrete particles to gas turbulence was derived that allowed for particle inertia and “crossing trajectory” effects. Based on these analyses, a comprehensive model for dilute gas-particle suspension flow was developed. Both particle collisions and particle-turbulence interactions were considered. The turbulence was represented by energetic eddies of characteristic size and decay time. The particle phase was discretised into multiple size/density fractious, and ensemble average r.m.s. fluctuating and mean velocity components were assigned to each fraction. Particle fraction mass, momentum and fluctuating kinetic energy balances were derived. An additional energy balance for the modulation of the gas turbulence intensity by particles was included. A fully developed flow version of the model was coded, and simulations of riser flows were performed. Model simulations predicted particle fluctuating velocities that were similar in magnitude to reported values measured in pilot-scale tests. Trends were consistent with those observed in the cold unit PSD tests. In simulations with small FCC catalyst particles, gas turbulence was predicted to significantly influence the particle motion. In contrast, turbulence was of secondary importance in simulations with larger particles used in CFB combustors. Greater reductions in gas turbulence intensity, due to modulation by the particles, were predicted in larger diameter and elevated temperature risers, than in cold unit plot-scale units. A semi-empirical predictive model was also developed, based on a core-annulus two-zone approach. A mechanistic equation for entrainment of particles from wall streamers into the riser core flow was proposed. “Exit effects” observed in the cold unit tests, due to the riser exit geometry, were characterised by an exit “reflection coefficient.” Constants needed for the model were obtained by fitting to results from the pilot-scale combustor. Good agreement between model predictions and data from a larger prototype CFB combustor was achieved using these constants, except that a different “reflection coefficient” was used, which was consistent with the prototype unit exit geometry. A mechanism for the formation of wall streamers was proposed, supported by calculations of discrete particle trajectories in the region of steep gas velocity gradient near the riser wall. The estimated shear-flow-induced lift force on the particles in this region can be significant. This investigation suggested that “non-continuum effects” in the particle phase may exist in these layers. These effects are not allowed for in two-fluid model formulations.

Thesis/Dissertation

application/pdf

2008-12-17

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http://hdl.handle.net/2429/3055

Chemical and Biological Engineering

University of British Columbia

UBCV

DSpace