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Research Papers: Combustion of Waste/Fluidized Bed

# 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

[+] Author and Article Information
Prabir Basu

Department of Mechanical Engineering, Dalhousie University, P. O. Box 1000, Halifax, Canada B3J 2X4

Munish Chandel1

Department of Mechanical Engineering, Dalhousie University, P. O. Box 1000, Halifax, Canada B3J 2X4

James Butler2

Greenfield Research Incorporated, P. O. Box 25018, Halifax, Canada B3M 4H4

Animesh Dutta3

Greenfield Research Incorporated, P. O. Box 25018, Halifax, Canada B3M 4H4

1

Present address: Ecole des Mines de Nantes, France.

2

3

J. Energy Resour. Technol 131(4), 041401 (Oct 12, 2009) (8 pages) doi:10.1115/1.4000174 History: Received August 21, 2008; Revised July 07, 2009; Published October 12, 2009

## Abstract

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 $3.2 MWth$ 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.

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## Figures

Figure 1

Loop-seal of a large commercial CFB boiler with a twin recycle chamber

Figure 2

Schematic of the CFB boiler used for the experiment

Figure 3

Side view of the loop-seal of CFB boiler, looking from view port

Figure 4

Front view of loop-seal looking from the furnace (all dimensions are in millimeters)

Figure 5

Loop-seal used in the bench-scale CFB test rig at Dalhousie

Figure 6

Total mass flow through the recycle system using one or both of the recycle chambers (dp=228 μm) plotted against the uncorrected fluidization velocity

Figure 7

Total mass flow through the recycle system using one or both of the recycle chambers (dp=228 μm) after the flow of solids through the single chamber is modified by subtracting the air leakage from the standpipe

Figure 8

Comparison of the present data (twin-exit) on the effect of fluidizing velocity through the recycle chamber on the solids flow through the loop-seal as measured by laboratory units at Dalhousie University (5) and a 65 t/h CFB boiler by Han (1). Here, the solids flux is based on the recycle chamber area.

Figure 9

Experimental data on laboratory scale loop-seal (101×101 mm2) at Dalhousie. Solids flux is expressed per unit cross-sectional area of the passage separating the two chambers. Data are given for three ratios of length to height (L/H) for the passage.

Figure 10

Mass flux (flow per unit cross section of passage) of sand through the recycle system plotted against fluidization number (dp=228 μm)

Figure 11

Effect of particle size on the minimum fluidization velocity for the onset of solids flow through the loop-seal, as measured in the CFB boiler

Figure 12

Effect of particle properties on solids circulation with both recycle chambers in operation: (a) mass flux versus aeration velocity and (b) mass flux versus fluidization number

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