Gas Mixing in a Circulating Fluidised Bed Boiler

Doctoral thesis, 2001

The present thesis is a study on the gas-mixing behaviour in a circulating fluidised bed (CFB) boiler. The aim is to describe how gas is spread in different regions of a CFB furnace and to explain variations in the mixing rate under different operating conditions. The results are based on pressure measurements, tracer-gas measurements and smoke visualisation. Experiments have been carried out in the Chalmers 12 MWth CFB boiler and in two experimental rigs, of which one is a scale model of the boiler. The units were operated under both non-circulating and circulating conditions. In the boiler, the superficial gas velocity ranged from 1.2 to 4.3 m/s and the bed material was silica sand with an average particle diameter of 0.32 mm. The mixing of gas is characterised by fluctuations of various scales. Gas, traced from a point source, flows in a plume that gets wider by small-scale fluctuations at the same time as the whole plume changes direction in a large-scale motion. The large-scale motion is due to large velocity fluctuations created by the bubble flow in the bottom-bed and by the explosions and collapses of bubbles at the surface of the bottom bed. Consequently, the dispersion is high in the bottom part of the riser with a maximum dispersion just above the surface of the bottom bed. The large-scale fluctuations in the gas-phase prevail, but with decreased amplitude, all the way up to the upper part of the riser. A similar trend in mixing rate (during operation with a bottom bed) is found in the core of the transport zone in the scale model and in the boiler. At low velocities, below circulating conditions, the mixing rate is high but falls steeply with increased gas velocity and levels out when circulating conditions are reached. The mixing rate during circulating conditions is found to be in the same range as previous literature data on gas mixing in CFB-risers, but at low velocities the mixing rate in the present units is higher because of the great contribution from large-scale motion under non-circulating conditions. In the transport zone, the dispersion of gas is influenced by several variables in a rather complex way.