The Rise of Buoyant Fuel-Particles in a Slugging Gas-Fluidized Combustor

The rise-times, from just above the distributor to the top of a bed, of single particles of various buoyant fuels, were measured in a gas-fluidized combustor (i.d. 25.5 mm) operated in slug flow. Three sizes of sand, of mean diameters 252, 777 and 1543 μm, were used. The excess gas velocity, U − U mf, varied between 101 and 709 mm s −1; the depth of the bed at incipient fluidization was 400 or 600 mm. The bed was electrically heated and held at a constant temperature between 700 and 900°C. In beds of the two coarser sands, the rise-times of single fuel-particles, containing large proportions of volatile matter and moisture, were long enough for devolatilization and char burn-out to be complete before reaching the surface. The measured rise-velocities were much lower in these slugging beds than in bubbling beds (Fiorentino et al., 1997; Bruni et al., 2002). The theory of Rees et al. (2005a, b) was used to predict the rise-times of inert particles in a hot slugging bed. The predictions compare favourably with measurements using fuels containing only small amounts of volatile matter and moisture. The experimental results suggest that once a minimum rate of evolution of volatiles and moisture from a fuel-particle is achieved, bubbles form which are large enough to increase significantly the rise-velocity of the particle. For fuels in this category, the rise-time appears to be independent of the size of the fuel-particle.