Heat and Mass Transfer to a Single Particle in Fluidized Bed
Sammanfattning: Heat and mass transfer to/from a single active particle in bubbling and circulating fluidized beds have been studied with application to fluidized bed combustion. A method of calculation of the heat and mass transfer coefficients in a bubbling fluidized bed (BFB) has been developed. The flow conditions in the emulsion phase, characterised by the Archimedes number, and the active-to-inert particle size ratio are the main parameters to determine the heat transfer intensity. The method accounts for the effect of temperature and fluidization velocity. The feasibility of the method is validated for a wide range of the particle characteristics and temperatures by comparison with all the experimental data available.The heat transfer coefficients in the wall layer of a circulating fluidized bed (CFB) boiler have been measured with dark and light calorimeters. A model of heat transfer to an active particle in a CFB has been developed. The model accounts for the effect of turbulence and the difference between velocities of the active particle, bed particles and gas. The model was used to explain the extremely high excess temperatures of the burning particles measured in a CFB furnace at a rather low oxygen concentration. A model of turbulence based on bubbling behaviour in large-scale CFBs has been developed. Order of magnitude estimates of turbulence characteristics on different scales were made. The estimates agree reasonably well with available data on dispersion coefficients of gas and solids. The gas-convective heat transfer coefficients in the freeboard of a BFB and in the upper part of CFBs have been found to be 50 % higher than expected in one-phase non-turbulent flow. Such an augmentation could be caused by turbulence with intensity larger than 10 % and a characteristic scale comparable to the size of the probe. Estimates of Taylor-scale characteristics close to those mentioned were obtained for CFBs. An analytical model of coupled drying and devolatilization of a single biomass particle has been developed and applied to the two-dimensional geometry. The model distinguishes between different thermal conductivity of virgin biomass and char and takes into account the effect of thermal radiation on the effective thermal conductivity.
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