Characteristics of gas mixing in a circulating f luidised bed J. Sterne ´us, F. Johnsson * , B. Leckner Department of Energy Conversion, Chalmers University of Technology, S-412 96 Go ¨teborg, Sweden Received 1 March 2001; received in revised form 1 January 2002; accepted 29 January 2002 Abstract Horizontal gas dispersion is studied by tracer-gas measurements and smoke visualisation in the riser of a 1/9-scale model of the Chalmers’ circulating fluidised-bed (CFB) boiler, with complementary measurements in the boiler. The model was designed and operated according to fluid dynamic scaling-laws. The results are based on the time-average horizontal dispersion of gas, dynamics of the measured concentration of tracer gas and smoke visualisation of the gas flow. The fluidisation velocity in the boiler ranged from 1.2 to 4.3 m/s and the bed material was silica sand with an average particle diameter of 0.3 mm. When the cold scale-model was operated to resemble the conditions of the tracer-gas measurements in the full-scale boiler, the results from the two units were similar (scale model operated with a fluidisation velocity of 0.4 to 1.4 m/s and with 0.06-mm bronze particles). The scale model also covers conditions that cannot be reached in a full-scale boiler. For instance, changes in the characteristics of the gas mixing during a gradual increase of particle concentration in the riser, starting from an empty riser, could be studied. The gas flow is characterised by a combination of small-scale motion (widening of a gas plume) and large-scale motion (a meandering motion of the whole plume). Both types of motion contribute to the mixing of gas. The large-scale motion in the transport zone is linked to the bubble eruptions in the bottom bed and the horizontal dispersion is higher in the bottom- and splash zones than in the transport zone. The dispersion rate is higher at low gas velocities than at high velocities because of a great contribution from the large-scale motion generated by bubbles at low particle circulation rates. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Fluidisation; Two-phase flow; Gas mixing; Dispersion; Tracer; Turbulence 1. Introduction In a circulating fluidised-bed (CFB) boiler, the charac- teristics of the gas flow are important for both fluidisation and combustion. The bottom part of boilers and (narrow) laboratory risers (such as those used in most of the pre- vious gas mixing studies) differ in character, e.g. Refs. [1,2], and this may also give differences in gas dispersion in the remaining parts of the riser. An important feature of CFB boilers is that, with a sufficient amount of bed material in the system, a bottom bed can be maintained also under circulating conditions up to velocities which are several times the terminal velocity of an average bed particle. A large part of the fluidisation gas passes the bottom bed through the so-called exploding bubbles, which erupt at the surface of the bed in different lateral positions. Above the bottom bed, which is up to about a meter high, there is a splash zone with a strong solids backmixing and a pronounced decay in the solids concentration with height. The transport zone, following the splash zone, is dilute (the solids volume concentration, c v < 0.01) and has a core/ wall-layer flow structure, with the solids backmixing mainly at the furnace walls [3]. The profile of the solids flux is flat in the core [4], which indicates that the time average profile of the (vertical) gas velocity is also flat in the region. A previous publication [5] on the mixing rate of gas in a CFB boiler (the Chalmers’ 12 MW th boiler) summarised the gas-mixing investigations carried out in the CFB risers (e.g. Refs. [6–9]) where, as well as in the present work, the mixing rate was determined by the dispersion of a tracer gas, continuously injected from a point source. The disper- sion of gas is quantified by a dispersion coefficient ex- pressing the resulting (time and spatial average) horizontal dispersion between the injection and sampling levels. Pre- vious work has focused on the influence of various opera- tion variables on the horizontal spread of gas in the core of the transport zone. The mechanisms of mixing have only been studied to a limited degree. 0032-5910/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0032-5910(02)00039-6 * Corresponding author. Tel.: +46-31-772-1449; fax: +46-31-772-3592. E-mail address: fijo@entek.chalmers.se (F. Johnsson). www.elsevier.com/locate/powtec Powder Technology 126 (2002) 28– 41