Gas mixing in circulating fluidised-bed risers

Steady-state tracer-gas measurements were carried out in a circulating flui dised-bed (CFB) boiler and in a cold experimental (CFB) unit. The main focu s was on the radial spread of gas in the core region of the transport zone under different operating conditions. Helium (He) was used as tracer gas, a nd the local He-concentration was determined by a mass spectrometer. The fl uidisation velocity ranged from 1.2 to 4.3 m/s in the boiler and from 1 to 6 m/s in the cold unit. The bed material was silica sand with an average pa rticle diameter of 0.32 mm in both units. Measurements were also carried ou t in the bottom bed and splash zone of the boiler for comparison with those of the transport zone. In the cold experimental unit, a greater range of o perating conditions could be covered than what was possible in the boiler, including a case without solids. In the core of the transport region of the boiler, there is no backmixing of gas and the horizontal mixing of injecte d tracer gas is characterised by an axi-symmetric spread of tracer gas arou nd the centre line of injection. The horizontal gas-mixing rate is quantifi ed by a dispersion coefficient, which is above 0.01 m(2)/s in the boiler an d around 0.005 m(2)/s in the cold unit. The difference in dispersion coeffi cients of the boiler and the cold unit is due to the difference in size. Th e present results were compared with literature data of dispersion obtained with and without solids in CFB risers. (C) 1999 Elsevier Science Ltd. All rights reserved.