An Investigation Into the Operation of the Twin-Exit Loop-Seal of a Circulating Fluidized Bed Boiler in a Thermal Power Plant and Its Design Implication

Loop-seal is a critical component of a circulating fluidized bed (CFB) boiler, and yet very little information on its working or design is available in published literature. Among the limited available information, none is on twin-exit loop-seal though it is one of the most commonly used loop-seal in large commercial CFB boilers. To circulate larger amounts of solids, a twin-exit loop-seal provides larger solids flow sections. It receives solids from one standpipe but delivers it through two recycle chambers and two delivery pipes. The present research was conducted in a twin-exit loop-seal of a $3.2MWth$ CFB boiler operating in a thermal power plant for cofiring purpose. Data obtained in this industrial unit were supplemented with those collected in a single-exit bench-scale loop-seal in the authors’ laboratory from the single-exit loop-seal in a 65 t/h CFB boiler and a scale model of a 30 MW CFB boiler. The effect of recycle chamber’s aeration on the solids circulation rate was studied for several particle sizes. Results suggest that the total solids circulation rate does not increase proportionately with the increase in loop-seal discharge area provided by the twin-exit loop-seals. The linear horizontal velocities of solids and the minimum aeration in the recycle chambers are comparable to those measured in a single-exit loop-seal. The implication of these new findings on the design procedure of loop-seals is discussed.