Encyclopedia of Environmental Science and Engineering, Volume I and II

FLUIDIZED BED COMBUSTION 409

Other problems associated with bubbling beds are scale-

up and turn-down. Scale-up is limited because of the feed

distribution problem and turn-down is usually more frequent

because of the erosion problem. Incidentally, it should have

been mentioned before, that, in general, FBC’s take longer

to start up and turn down than conventional boilers because

of the large amounts of bed material which must be heated

or cooled.

The major advantage which PFBC’s hold over AFBC’s

is that NO x , SO 2 , and CO are weakly linked. Thus, 60 mg/

MHJ NO x can be attained while at the same time only 50 mg/

MJ SO 2 (or less) and 10 mg/MJ (or less) CO are produced.

Another advantage of PFBC’s is that the waste contains

negligible lime, sulfi des, and sulfi tes. The decreased lime

concentration makes the waste less reactive and probably

renders it nonhazardous. The decreased sulfi tes makes the

Concentration (wt percent)

Component Test no.1a Text no. 2a Test no. 3a Baghouseb Full-scale residues

CaSO 4 26.1 31.6 21.7 27.7 26.1

CaS 0.8 0.6 0.7 5.2 0.45

Free CaO 24.8 28.3 24.7 15.7 23.5

CaCO 3 3.4 3.2 5.2 6.8 4.6

Fe 2 O 3 10.7 9.5 9.1 10.6 15.9

Other mainly SiO 2 and C 27.3 20.8 33.8 19.5 24.2

LOIc (corrected) 11.4 5.4 5.1 8.0 4.3

Sum 104.5 99.4 100.3 — 99.0

a Composite pilot-scale residues.

b Calculated from TGA and other analyses.

c LOI indicated loss on ignition.

TABLE 1

Major chemical components of composite residues: pilot and full-scale CFBC units

Pilot-scale composite

residues

Pilot-scale baghouse

residue Full-scale residue

Specific gravity 2.83–3.07 2.58 2.95

Mean size, D 50 mm 0.2 0.04 0.04

Optimum water content percenta 14.5–17.5 32 26.5–30.5

Unconfined compressive strength, kPab

Curing period, days

0 230–360 — —

3 — — 2470

7 150–290 — 4120

8 — 309 —

10 260–425 385 —

12 — 461 —

28 — — 4660

Freeze/thaw cycles

4 — 201 —

6 540–1020 Sample destroyed —

7 — — 3200

15 — — 880

a As determined by standard Proctor test.

b Samples were cured at 100 percent relative humidity at 23 ± 2
C for periods shown.

TABLE 2

Geotechnical properties of the pilot-scale rig composite and baghouse samples and full-scale unit residues

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