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RESEARCH PAPERS

The Performance of a Loop Seal in a CFB Boiler

[+] Author and Article Information
Andreas Johansson

Department of Energy Conversion, Chalmers University of Technology, SE-412 96 Göteborg, Sweden and University College of Borås, SE-501 90 Borås, Sweden

Filip Johnsson

Department of Energy Conversion, Chalmers University of Technology, SE-412 96 Göteborg, Sweden

Bengt-Åke Andersson

 University College of Borås, SE-501 90 Borås, Sweden; Kvaerner Power AB, SE-402 75 Göteborg, Sweden

J. Energy Resour. Technol 128(2), 135-142 (Jan 31, 2006) (8 pages) doi:10.1115/1.2199567 History: Received June 11, 2004; Revised January 31, 2006

High in-bed heat transfer and low corrosive environment imply that the loop seal of a circulating fluidized bed (CFB) boiler is an advantageous location for superheaters. In order to increase the knowledge on the flow pattern and the heat transfer distribution to the tubes within a loop seal, measurements were performed in the loop seal of a 30MW CFB boiler as well as in a 13 scaled-down seal operated according to simplified scaling laws. The scale model measurements show that the solids recirculation flux can be maintained with a substantial decrease of the fluidization flow in the seal compared to that currently used at full load conditions. It was also possible to significantly decrease the fraction of the bottom of the seal that was fluidized without affecting the solids flux through the seal. A gradient in the solids flow were detected in the vertical direction.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) A schematic sketch of the particle seal. The upper sketch is a side view that shows the location of the tube bundle. Filled symbols show the location of the tubes equipped with thermocouples. The lower sketch indicates the boundaries between the four compartments of the air plenum. (b) A photo of the seal with the tube bundle.

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Figure 2

Side view of the test rig and pressure distribution in the entire CFB loop. Full load case, i.e., the gas velocity in the riser (u0,R) is 2.9m∕s, the fluidization velocity in the loop seal (u0,S) is 0.65m∕s and the windboxes 2 and 3 are in use.

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Figure 3

Heat transfer rate (dT∕dt) in the scaled-down loop seal compared with the heat transfer coefficient α for 100μm glass beads at 25°C measured by Wunder (13) (here divided with 1200 for comparison with data from this work) versus fluidization velocity. The minimum fluidization velocity for the particles used in the scale model (dp=100μm) is 0.01m∕s.

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Figure 4

Tube-temperature time series. The initial linear decrease is used for the calculation of dT∕dt.

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Figure 5

The position of the manhole in the loop seal of the 30MW boiler with the insertion depths of the gas suction probe indicated (Vattenfall AB)

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Figure 6

Pressure drop for the dense bed in the scaled-down loop seal versus the fluidization velocity in the seal (u0,S)

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Figure 7

Solids recirculation flux for different fluidization velocities in the scaled-down loop seal (u0,S) and different fluidized fraction of the bottom area versus the gas velocity in the riser (u0,R)

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Figure 8

Height of solids in the downcomer in the boiler and in the cold unit versus the fluidization velocity. The values from the model are re-calculated according to the difference in scale (rings-cold model, crosses-30MW boiler).

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Figure 9

Standard deviation for temperature measurements versus elevation in the bed of the scaled-down loop seal at different operating conditions. The gas velocity in the riser (u0,R) is 2.5m∕s.

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Figure 10

Heat transfer rate for the whole tube bundle versus the recirculation flux in the scaled-down loop seal. The tube bundle is situated in a fluidized zone and the fluidization velocity in the loop seal is held constant (u0,S=0.65m∕s).

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Figure 11

Heat transfer rate (dT/dt) in the scaled-down loop seal for case (D), i.e., with the area beneath the tube bundle being defluidized, versus the distance from the downcomer. The gas velocities in the riser (u0,R) are in these cases 2.9 and 2.6m∕s.

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Figure 12

Heat transfer rate for the tube bundle located in the loop seal in the 30MW boiler versus boiler load

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Figure 13

Heat transfer rate in the dense bed of the scaled-down loop seal as a function of height

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Figure 14

The heat transfer rate in the scale-model and the CO-concentration in the loop seal of the 30MW boiler as a function of normalized height above the air distributor

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Figure 15

Heat transfer rate for the upper tube row (open symbols) and the lower tube row (filled symbols) for two different fluidization velocities in the scaled-down loop seal (u0,S) versus the horizontal distance from the downcomer

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Figure 16

Solids flow pattern through the dense bed of the loop seal as obtained from the heat transfer rate measurements. The arrows only give the main direction of the solids flow and not the velocity.

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