The combustion of large particles of char in bubbling fluidized beds: The dependence of Sherwood number and the rate of burning on particle diameter

Particles of char derived from a variety of fuels (e.g., biomass, sewage sludge, coal, or graphite), with diameters in excess of ∼1.5mm, burn in fluidized bed combustors containing smaller particles of, e.g., sand, such that the rate is controlled by the diffusion both of O2 to the burning solid and of the products CO and CO2 away from it into the particulate phase. It is therefore important to characterize these mass transfer processes accurately. Measurements of the burning rate of char particles made from sewage sludge suggest that the Sherwood number, Sh, increases linearly with the diameter of the fuel particle, dchardchar (for dchar>∼1.5mm). This linear dependence of Sh on dchardchar is expected from the basic equation Sh=2εmf(1+dchar/2δdiff)/τSh=2εmf(1+dchar/2δdiff)/τ, provided the thickness of the boundary layer for mass transfer, δdiffδdiff, is constant in the region of interest (dchar>∼1.5mm). Such a dependence is not seen in the empirical equations currently used and based on the Frössling expression. It is found here that for chars made from sewage sludge (for dchar>∼1.5mm), the thickness of the boundary layer for mass transfer in a fluidized bed, δdiffδdiff, is less than that predicted by empirical correlations based on the Frössling expression. In fact, δdiffδdiff is not more than the diameter of the fluidized sand particles. Finally, the experiments in this study indicate that models based on surface renewal theory should be rejected for a fluidized bed, because they give unrealistically short contact times for packets of fluidized particles at the surface of a burning sphere. The result is the new correlationSh=2εmfτ+AcushAchardcharδdiff for the dependence of Sh on dchardchar, the diameter of a burning char particle. This equation is based on there being a gas-cushion of fluidizing gas underneath a burning char particle; the implication of this correlation is that a completely new picture emerges for the combustion of a char particle in a hot fluidized bed.