Flow patterns in the square cross-section riser of a circulating fluidised bed and the effect of riser exit design

Solids flow patterns were observed and measured in the square cross-section riser of a laboratory circulating fluidised bed (CFB). Measurements in the 0.14 m square riser used a sampling probe, of internal diameter 3.4 mm, to measure upwards and downwards solids fluxes throughout the cross-section. Interpolation gave complete solids flux profiles over the entire 0.14 x 0.14 m cross-section. Integration of the solids flux profile gave the external solids circulation rate, in reasonable agreement with measurements external to the riser. Special features of the solids flux profiles are (i) much higher down flux in the corners of the square section and (ii) slightly higher down flux half-way between the corners, near the walls. These features may be due to secondary flow in the square, section duct and to enhanced sheltering provided by the two side walls. There is qualitative similarity between these solids flux profiles in the 0.14 m square riser and the profiles measured by Werdermann ((1992, Feststoffbewegung und Warmeubergang in zirkulierenden Wirbelschichten von Kohlekraftwerken. Dr-Ingenieur dissertation, Technical University Hamburg-Harburg) for large industrial. combustors. But it is clear that there is a scale-up effect: the film, i.e. the cross-sectional area of duct containing the downflow (adjacent to the walls), is a much smaller proportion of the total area in the large units as compared with the laboratory units. The detail design of the riser exit or outlet bend at the top of the riser has a profound effect upon solids flux profiles in the riser and in particular on the reflux ratio defined as k(m) = (solids downward flow)/(external circulation rate). Measured values of k(m) varied by a factor of about 20 according to outlet bend design. These measurements were mostly near the top of the riser, but indications are that k(m) is affected, throughout the riser, by the design of outlet bend, a phenomenon previously observed in a CFB of circular cross-section by Brereton and Grace ((1994). In A. A. Avidan, Circulating Fluidized Bed Technology IV (pp. 137-144). New York: AIChE. (C) 2000 Elsevier Science Ltd. All rights reserved.