Fluid Dynamics of the Transport Zone of Circulating Fluidized Beds - with Application to Boilers

Doktorsavhandling, 1995

The particle suspension flow structure in circulating fluidized beds (CFB) was studied with emphasis on the layer of downflowing particles at the wall. The main experiments were carried out in the furnace of the 12 MWth CFB boiler of 1.7x1.7 m cross-section, and in a two-dimensional CFB cold bed of 0.1x0.7 m cross-section at Chalmers University of Technology. Optical-fibre probes were used in determining the local, time-averaged, particle concentration in CFB cold beds and in studying the time-dependent characteristics of the wall-layer in the boiler furnace. In view of the rough environment in the furnace, a particle flux probe and a suspension flow momentum probe were developed to determine the local particle velocity and concentration. A detailed theoretical analysis on the momentum probe is given. The average particle concentration, based on an integration of the concentration profile measured by the probes, is consistent with that obtained by pressure-drop measurements. The flux probe works through non-isokinetic sampling. It is shown to give reliable results by a good mass flow balance in the cold bed. It is found that the core/annulus structure of the flow pattern found in many small-scale beds with a circular cross-section is also characteristic of large-scale CFB boilers, but is more pronounced. There is a strong particle downflow in the region close to the furnace walls with the falling velocity about 0.9-1.6 m/s. The distribution of particle velocity and concentration is flat over the core region. The particle flow pattern is developed in small-scale risers but is developing both down along the walls and up through the core in the large-scale CFBs. This situation is described by a simple mathematical model which is focused on a coefficient of mass transfer from the core to the wall. The expressions of the particle downflow at the wall, the local and average concentrations, as well as the wall-layer thickness as a function of the bed height, were obtained from this model. The local particle concentration in the transport zone, i.e. the main extension of a CFB, is found to be directly proportional to bed bulk density as obtained by pressure drop measurements. Empirical correlations describing this self-similarity relation are obtained. The particle flux measurements show that the downflow is unevenly distributed around the bounding walls. In the rectangular cold bed, most particles tend to go down and accumulate along the two wide walls. In the furnace of the boiler, the strongest downflow was detected in the corners. It was found that the formation and development of the particle wall-layer are closely related to the exit configuration. The wall-layer at lower levels of the furnace is strongly affected by the secondary air penetration. The configuration of the boiler membrane-tube wall enhances the particle accumulation in the region close to the fins between the tubes of the membrane-tube wall. A diagnostic measurement on the fluctuation characteristics in the boiler furnace was carried out by simultaneously sampling the signals of both the local fluctuation of particle downflow and the global fluctuation of pressure drops in the upper zone, in the bottom zone and from the air plenum to atmosphere. Spectrum and probability analysis on the signals were performed, which give a detailed insight into the time-dependent particle flow pattern and fluid-dynamics in the furnace of CFB boilers.