Single-particle behaviour in circulating fluidized beds

This paper describes an experimental investigation of single-particle behaviour in a cold pilot-scale model of a circulating fluidized bed combustor (CFBC). In the system, sand is recirculated by means of air. Pressure measurements along the riser are used to determine the suspension density. A radioactive tracking facility, which detects single radioactive particles, is developed and applied to determine the dynamic picture of the particle trajectories in the simulated boiler. The tracer particles are observed to move between the zone above and below the secondary air inlet with a mean frequency of about 1 Hz under the present operating conditions. This relatively high frequency is due to the fact that most of the particle trajectories take place just around the secondary air inlet. It is found that the upward particle velocity in the upper dilute transport zone decreases with the particle size or density, which results in a decreased number of particle observations for the larger particles with the riser height. The experiments show that the mean particle residence times in the zones above and below the secondary air inlet are almost independent of the particle characteristics. The overall mean particle residence time in the riser is proportional to the magnitude of the internal particle recirculation, which increases with the particle size.