Note: Descriptions are shown in the official language in which they were submitted.
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F1eld o~ the Invention
This invention re]ates to burning and desulfurizing
coal.
Background of the Invention
Coal burners with fluid bed desulfurization using
sorbe~ts such as limestone are known in the prior art. However,
these prior art devices have not been adequately satisfactory.
They have been characterized by inefficient use of sorbents, in-
adequate removal of surfur except at the expense of uneconomical
inefficient use of sorbents, and inability to make use of many
available sorbent materials. Using softer limestones resulted
in fragm.entation and blowing away before adequately reacting;
using harder limestones resulted in sulfate coating that reduced
reactivity. Also, the large particle size limestone dis-
charged in p~ior art devices has caused de.signers to use in-
creased steam tube spacing, with consequent i.ncreased boiler
cost~.
Efforts to burn coal in fluid bed Gombustors have
~let with problems both in turndown (burning at less than
maximum capaci.ty) and startup preheat~ as a number of those
in the art have recognized. One approach to turndown has
been to use a multiplicity of combustor units and to totally
shut down one or more of them. Another approach has been
simply to turn a single combustor off and on fairly frequently.
Such approaches have ~equired a good bit of hardware, or
been cl~msy, or made provisions for ~ery little modulation
between fu.ll-burning and non-burning conditions, or been
characterized by an undesirably low turndown ratio, or made
impractical turnoff except at the cost of time-consuming
restarts, or several of these. The prior art has also taught
away from using e~cess air for the purpose of bed cooling
~L
during turndown. - 2 -
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Summclr~ of the In~entic,n
. . . _-- ... . _ . . .. _ _ _ .... _
We have di~cov~red that a gas (such a5 the products
of coal combustion) may be reacted with a solid material (such
as limestone) in a fluid bed device with great efficiency if
the gas as it enters the fluid bed is baffled to such an extent
that the entering gas does not by impact upon the solid fracture
it, if the solid is chosen so that it has an ade~uately large
~by adequately large,we mean a microchip generation rate greater
than the rate of buildup of coating, e.g., calcium sulfate)
lC in-bed attrition rate, if there is included in the fluid bed
enough material ~"ballast") which does not break up easily
so as to maintain desired bed height, if overflow means
are provided to regulate the depth of the bed, so that attrition
is by in-bed scrubbing movement3 to produce a constant shedding
of very small particles ("microchips"), and if solid is
added at a rate calculated to deal with the amount of reacting
desired. Particles formed by in-the-bed attrition are extremely
fine (e.g., diameter 3 microns), produced by a surface rubbing
effect, as noted in the prior axt ("~xperimental and Engineering
?~ support of the FBC program," DL Kearns, et al., Westinghouse
Research Labs, Pittsburgh, PA, monthly report No. 14 to
the United States E.P.A., contract 68-02-2132, Jan~ '77).
(Gas emitted at non-baffled distributor holes is travelling
fast enough to smash the entrained particles into each other,
shattering them, to produce a w:ide range of particle sizes,
many of which are too small to stay in the bed, but too
large to effectively absorb SO2 of their way out.)
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We have discovered that very effective turndown
can be achieved in a fluidized combustor, free from all
the above-mentioned prior art disadvantages, if two zones
in which portions of bed materials may be stored are provided,
one portion being a lower part of the actual fluidized bed
burning zone, below cooling tubes in the upper part of the
burning zone and a separate storage zone to and from which
bed material may be transferred from and to the first zone
to respectively drop levels of materials below the cooling
0 tubes or elevate the level into the cooling tubes, adequate
cooling of the bed being achieved, when its level is below
the tubes, by blowing therethrough excess air, the depth
below said tubes being such that the first storage zone
may be operated as a fluid bed combustor even though the
depth is not greater. Our invention has the additional
advantage of greatly improving preheating, a further requirement
that has caused difficulty (use of larger and more expensive
burners and undesirable production of soot and unburned
hydrocarbons), as well as making practical preheating with
various energy sources (including gas, liquid, solid, and
electrical).
We have discovered that by providing for combustion
(ordinarily in oxygen from primary air) without sorbent
present in an upstream fluidized (slow, matrix material
not leaving the bed) zone, and by providing for treatment,
with sorbent (e.g., limestone) and otherwise, in a downstream
fluidized zone, it is possible to provide important improved
flexibility of optimization. The burner zone may be operated,
with many coals, at higher temperatures, providing increased
fuel and system efficiency, and also without (owing to the
absence of limestone) causing undue fireside corrosion of
water tubes. This also makes possible optimizing fuel-
to-air ratios and the partic~lar solids content (material
and size) chosen for use in each zone. Burning in the manner
just described also makes possible optimizing removal of
carbon monoxide and oxides of nitrogen. Combustion efficiency
is maximized when the combustion bed is operated at the
highest possible temperature below ash fusion and in the
presence of an excess of air at or downstream of the combustion
bed; NO~ formation is minimized by operating the combustion
bed at a similarly high temperature, and also by permitting
the nitrogen oxide associated with the volatiles to be chemically
reduced to molecular nitrogen in the downstream bed in the
presence of char and a small excess of oxygen; carbon monoxide
formation is minimized by combustion with an excess of air,
at a temperature above 1430~; sulfur dioxide removal is
maximized by the operation of the downstream bed at 1550F
~ 50 in the presence of an excess of oxygen; the presence
of the char required to reduce the NOX associated with the
volatiles is promoted by the combustion of coal in a reducing
atmosphere in the combustion bed; omitting sorbent from the
burn-zone permits heating tubes to higher temperatures
without undue fireside corrosion; and the increased heated
fluid temperatures permitted thereby result in additional
system efficiency.
Brief Descr ptlon of the Drawinqs
Fig. 1 is a diagrammatic view, mainly in section,
of the most preferred embodiment of the invention,
Fig. 2 is a sectional view of a nozzle and portion
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of a steam tube used in the embodlment oE Fig. l;
Fig. 3 is a sectional view of a portion of a bubble
cap installed in the middle distributor assembly of the
embodiment of Fig. l;
Fig. ~ is a side elevation view, partly in sec-tion,
of a bubble cap and portion of the upper distributor assembly
of the embodiment of Fig. l;
Fig~ 5 is a sectional view of an expandable joint
used in mounting the lower distributor plate in the embodiment
of Fig. l;
Fig. 6 is a diagrammatic view of the water circulation
system of the embodiment of Fig. l;
Fig. 7 is a side eleva~ion view, partly in section
and partly broken away, of the apparatus for drying and
crushing coal and transporting it to the em~odiment of Fig.
-
Fig. 8 is a side elevation view, mainly in section,
of a limestone pot for removing limestone from the embodiment
of Fig. l; and
Fig. 9 is a schematic view of a coal burning and
desulfuxizing system.
Description of the Preferred Embodiment
We turn to description of the structure and then
operation of the embodiment of Fig. 1.
Structure
There is shown in Fig. 1, diagrammatically, a
three-bed, all fluidized, combustor-desulfurizer, indicated
generally at 200. A metal housing 202 surrounds layers
20~ and 206 of lower and higher density refractory, respectively,
to enclose the entire unit~ which rests on supports 208.
Metal distributor plates 210, 212, and 2l4 ex-tend across
the housing interior to define the bo-ttoms, respectively,
of three fluidized beds--a lower bed for storing sand, a
middle combustor bed, and an upper desulfurizer bed.
The lower sand sto~age bed has under it a plenum
fed by a windbox 216, through which fluidizing air enters
the bed underneath distributor plate 210. A multiplicity
of bubble caps 218 extend through plate 210 (over which
extends an insulating board, not shown, to avoid hot spots),
which is held in place by expandable joints 219. Extending
through the housing wall above caps 218 is coarse-ash disposal
pipe 220, which carries away to a baghouse hopper excess
bed material.
The middle combustor hed has under it plenum 227
for supplying fluidizing combustion air to the middle bed.
A multiplicity of bubble caps 224 extend through distributor
plate 212 and water jacket 225, which ser~es to cool plate
212 to prevent it from buckling. A layer of insulation
228 rests on plate 212 surrounding each of caps 224, and
a layer of stones 230 (actually coarse quartz in a mix of
si~es from 3/8" to 1" in diameter) covers insulation 228.
A similar layer of insulation 229 is secured (by means not
shown) to the bottom of water jacket 225. The insulation
serves to cut heat loss to the water in jacket 225. Above
bubble caps 224 is coal feed pipe 232, which deposits coal
at the bottom of the combustor bed, just above bubble caps
224. (Under-the-bed feeding of the coal allows the use
of coal fines in the feed which would otherwise, i.e., with
over-the-bed feeding, be blown out of the bed without combusting.
Over-the-bed feeding would also make it difficult to operate
the bed in any but its full-on position, i.e., with sand
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covering the top of steam tubes 234. At lower sand levels,
with over-the-bed feeding, the coal would fall onto the
steam tubes, and an agglomeration of unburned coal would
soon build up. The inabillty to operate at reduced sand
levels in the middle bed would eliminate use of the preferred
turndown and startup methods, as wiIl be described.)
Above pipe 232 and extending across the combustor
bed are steam tubes 234, which are mounted at their ends
in tube sheets 235 lone shown in broken lines) that define
manifolds for introducing water into the tubes and removing
water and steam from them. Tubes 234 are spaced and occupy
25% of the housing volume in the zone from the top row of
tubes to the bottom row.
The upper desulfurizer bed has under it apertured
secondary ai~ pipe 236, which has two rows of holes 238
inclined downwardly at 30 for spreading out the secondary
air beneath the upper bed and a third row of holes 240 at
the bottom of the pipe for blowing out any particles that
may have gotten into the pipe. Above pipe 236 is water
jacket 242~ which serves to cool distributor plate 214 to
prevent it from buckling~ Baffles 244 (one shown) serve
t:o kee p the velocity of the circulating cooling water high
enough to avoid local hot spots that might cause damage.
A multiplicity of bubble caps ~46 extend through jacket
242 and plate 214. A layer of insulation 248 rests on plate
214 surrounding each of caps 246, and a layer of stones
250 (the same materials as stones 230) covers insulation
248 and caps 246. A similar layer of insulation 249 is
secured to the bottom of water jacket 242. The insulation
serves the same purposes as that for the middle bed distributor
and water jacket. (The purpose of the stones 250 is to
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allow the gases emerging from bubble caps 246 to spread
laterally over the distributor, allowing them to emerge
into the upper bed at a sufficiently low velocity to avoid
shattering the limestone particles.) Above the upper bed
are three rows of tubes 252 to deflect particles back into
the bed. Each tube in the middle r~w is positioned directly
above a corresponding tube in the bottom row, but each of
the tubes in the top row is positioned halfway between each
adjacent pair of vertical pitch lines for the lower two
rows. This arrangement avoids the possibility of a line
- of sight opening at any angle through the ~u~es so any particle
that is ejected from the bed will solidly contact one of
the tubes before leaving the bed, thereby reducing its speed
and the likelihood of splashing into the freeboard. (A
similar bank of tubes may usefully be placed above the second-
~i.e., combustion--bed.) Tubes 252 are supported near their
ends and at spaced positions longitudinally thereof by apertured
sheets 254 (only ane shown), which are in turn supported
from housing 202 by rods 256. Above tubes 252 extends limestone
feed pipe 258, which deposits limestone in the desulfurizer
bed to a level just above the top row of tubes 252. The
limestone drops from the outlet tee 259 of pipe Z5~ through
a gap (not shown)in the assembly of tubes 252, without this
gap, some limestone particles may be too large to pass through
the tube assembly. Limestone downcomer 260 cooperates with
a limestone pot (not shown in Fig. 3 but chown in Fig. 8)
to maintain the level of limestone just above tubes 252
and to carry away spent limestone. Hot desulfurized gases
leave through smoke pipe 262, through which they can be
transported throu~h a boiler to which they give up their
remaining heat, then to abaghouse for removal of any ash
_ g _
or other particul~tes th~t ~ay escape from the upper bed,
and finally to a stack.
Upcomer assembly 264 and downcomer assembly 266
permit bed material to be moved Erom the lower bed to the
middle bed and vice versa, for preheating and turndown (both
to be discussed in more detail subsequently). Upcomer assembly
264 includes upcomer piping 268, which, when door 270 is
opened by actuator 272 (shown in broken lines because it
is mounted on the exterior of housing 202), permits bed'
material to be taken from the lower bed and blown by air
under presure from tube 274 into the middle bed through
door 276, which is held shut by gravity to prevent filling
up of the upcomer piping with bed material when it is not
in use but which opens in response to bed material forced
up from the lower bed. The normal bed material level fox
...
operating the combustor at 100% of capacity is just above
th~ topmost steam tubes, as shown in Fig. 1. Tee fittings
278 and 279 are used when the bed material makes a sharp
turn, to reduce wear on the piping there.
Downcomer assembly 266 includes downcomer piping
280, which, when door 282 is opened by actuator 284 (shown
in broken lines because it is mounted on the exterior of
the housing), permits bed material that has entered the
piping from the middle bed to be fed with a feed screw into
the lower bed., For normal operation downcomer pipe 280 should
be filled with bed material to act as a pressure seal so
that air from plenum 227 is not able to keep bed material
from coming down the piping. Tee fitting 281 is positioned
where the bed material makes a sharp turn.
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Turning -to particular details of the structure
of Fig. l, each boiler tube 234 has a nozzle 288 (Fig. 2)
fitted into its end connected to the water inlet manifold.
The purpose of the nozzle is to assure that water flows
into and through each steam tube and does not simply flow
through the lowest tubes because the hydrostatic pressure
is greatest for them. If water does not flow through a
tube, the tube will likely overheat. With the nozzle, the
primary resistance to flow is the pressure drop through
the nozzle, which is the same for all the tubes, so that
the lower tubes no longer provide a substantially easier
flow path.
Fig. 3 shows a bubble cap 224 in middle distributor
plate 212. Bubble cap 224 includes nipple 290 and cap 292.
Nipple 290 ls fitted through a hole in plate 212 and through
a tube in water jacket 225 (not shown in Fig. 3; see Fig.
4 for the same construction for a bubble cap in the upper
bed). Just below cap 292 are four radially spaced holes
294 (three are shown) in nipple 290. Cap 292 has an annular
shoulder 296, which acts as an umbrella aver holes 294 to
prevent sand (or, in the upper bed, limestone and sand)
from falling into the holes and "weeping" onto the bed below
when the burner is turned off. Actually, weeping is much
less of a problem in the upper bed because stones 250 protect
against the weeping of sand and limestone. The diameter
of shoulder 296 is chosen so that ang~e ~ measured from the
lower outer edge of the shoulder to the inside bottom surface
of one of holes 294 (Fig. 3) is smaller than the angle of
repose of the sand (or limestone), which is the angle that
a pile of a particular material makes with the base on which
it rests. The angle of repose for the sand used in the
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unit of Fig. 1 is 37. ~n~le (~ is ac-tually computed by
first subtractinq from -the diameter of hole 294 the p~rti-
cle diameter and then choosinq a shoulder diameter that, when
a line is drawn from the shoulder edge to the bottom
of hole 234 less the particle diameter taS indicated in
Fig. 3), will yield an angle smaller than the angle of repose.
Angle ~ can be computed in this way because it has been
found that a single layer of particles will not fall into
one of the holes.
Bubble caps 218 and 246 are similar to bubble
caps 224 except for size. All the bubble caps are removable
from their respective distributor p]ates, to allow easy
replacement. Each upper bed bubble cap 246 is fitted through
a tube 298 (Fig. 4) in water jacket 242 and wedged there
by ferrule 300. Half-coupling 251 is threaded to the bottom
of nipple 290 to secure it in the hole. Except for ferrule
300 and half-coupling 251, bubble cap 246 is kept relatively
isolated from tube 298 and jacket 242 to permit the bubble
cap to become hotter than the distributor, to burn off any
tars or other volatiles that may collect on the bubble c~ps
during start-up. Because this volatile-,burnoff function
shortens the life of the bubble caps, the easy replacement
of the bubble caps without h,aving to replace the distributor
is a significant advantage.
Fig. 5 shows an expandable joint 219 for the lower
distributor plate 210. Joint 219 includes L-shaped member
302, which is loosely bolted to the outer underside of plate
210 by bolts 304 ~one shown). Fastened to housing 202 and
extending inwardly through refractory layers 204 and 206
into the space between plate 210 and member 302 is angle
iron 306. Angle iron 306 extends the length of plate 710
in the direction coming out of the plane of the paper, as
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does member 302. However, spaced sawcuts 308 (one shownin Fig. 5) are made in angle iron 306 to permit lateral
expansion of the angle iron in response to heating, particularly
at the outer edge of the angle iron. When plate 210 expands
in the direction of the arrows in Fig. 5 during burner operation,
room is provided for that expansion because the outer edge
of angle iron 306 is spaced from bolt 304 and the outer
edges of plate 210 and member 302 are correspondingly spaced
from refractory 206. Joints 21~ thus prevent plate 210
from buckling. Bolt 304 fastens plate 210 and me~ber 302
tightly enough to minimize leakage of gases through the
joint but loosely enough to allow free lateral movement
of the plate and L-member with respect to the angle iron.
Joints 219 secure distributor plate 21Q to housing 202 on
at least three sides. Either a further joint 219 or a groove
in the housing wall could be provided on the fourth side.
Fig. 6 shows a portion of the water circulation
system for combustor-desulfurizer 200. Feed water enters
the system at return pipe 328 and is pumped by circulator
pump 312 through piping 314 into the inlet manifold (not
shown because it is on the opposite side of the burner from
that shown in Fig. 6) for steam tubes 234, through steam
tubes 234, through the outlet manifold 316 and back out
piping 318. Steam is separated from water in outlet manifold
316 and leaves through supply pipe 320. Piping 322 carries
a portion of the water from the circulator pump to water
jacket 242 to cool the upper distributor plate. Water leaves
jacket 242 through piping 324, through which it is carried
to ou-tlet manifold 316 and from there back to piping 318.
Although the piping is not shown, a similar arrangement
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exists for water jacket 225 (not shown). Steam drum 326,
a separate unit positioned alongside of combustor-desulfurizer
200, supplies w~ter to the system to make up for the water
that is lost by steam yeneration in the combustor and to
make sure that there is always enough water for pump 312.
Steam drum 326 includes feedwater pipe 328 and outlet pipe
330, the latter for supplying water to pump 312. Float
~alve assembly 332 allows additional water to enter the
steam drum through pipe 328 whenever the water in the drum
is below a certain level. Piping 334 is used to flood the
steam tubes 234 with water if there is an emergency shutdown,
to prevent the tubes from overheating. Solenoid valve 336
automatically turns on this wate~ during such a shutdown.
Piping 338 vents the steam drum to supply pipe 320 to avoid
steam pressure buildup or a steam vacuum in steam drum 326.
The water~ci~cuit for combustor-desulfurizer 200 is in parallel
with the water circuit of the boiler (not shown;, as follows.
The pipe bringing return water to pipe 328 is also connected
to thé water inlet of the boiler, and the steam line to
2~ the load that is connected to pipe 320, is also connected
to th~ steam outlet of the boiler.
Turning to the system for feeding coal to combustor-
desulfurizer 200, Fig. 7 shows variable-speed screw feeder
348l coal drier 340, crusher 342, rotary air lock 368 and
coal pot 344. Coal from coal bin 346 passes through screw
feeder 348 and enters drier 340 through chute 341, from
which it falls into trough 350, partially semicircular in
cross section, in which is positioned revolving shaft 352
carrying at each end a whee~ 353 (one shown) between the
two of which extend bars 354. One of the bars 354 carries
smaller transverse pins 356 (1" in length), which are spaced
with 1/4" gaps therebetween to rake alon~ larger lumps of
coal but not to catch smaller pieces of coal and the fines.
At the base of trough 350 is fitted hot air duct 358, which
introduces hot air l500-700F) into trough 350 through p~rforated
distributor plate 359 to fluidize the smaller particles
and heat by jet impingement the larger ones. This air is
first heated by passing it through a coil (not shown~ in
the combustion bed. Enclosing trou~h 350 is co~er 360.
Vent 361 permits hot air and any gases emitted by the coal
to be vented. The revolving assembly promotes the drying
of coarse lumps of coal, when only large sizes are being
fefl (e.g., stoker coal, sized 1 1/4 x 1/4") by stirring
the lumps, thereby eliminating pockets of moisture that
might otherwise be left between coal pieces. When the drier
is being used to dry coal containing both coarse and fine
coal (e.g., nut and slack coal, sized 1 1/2 x 28 mesh),
the raking motion of pins 356 removes coarse lumps, these
lumps not requiring as much drying as the fines, thus allowing
the remaining coal to be dried in the fluidized state. Coal
mois~re9 of up to 15% can be handled by the drier~ while
using air that is safely below the coal gas explosion point,
because of the relatively long residence time (typically
15 seconds) of the coal in the trough, which permits the
drying air access to a relatively large amount of coal surface,
thereby promoting the transfer of heat to the coal. At
the side of trough 350 opposite screw feeder 348 is outlet
lip 362, which is at the end of a radius 15 below a horizontal
radius, and directs dried coal to piping 364 and from there
to crusher 342 (a crusher designed to cr~sh the coal to
a particle diameter of 95~ -1/2", 50~ +8 mesh) and into
rotary air lock 368. Vent 369 prevents pressure buildup
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upstream of the air lock, and is vented to atmosphere.
Pockets of crushed coal are then dumped from air
lock 368 into coal pot 344. Coal pot 344 has mounted in
it ,a perforated distributor plate 370 below which air inlet
pipe 372 enters the pot to direct fluidizing air through
plate 370. Coal outlet pipe 374 extends from the pot above
plate 370~ and is connected to the pot by reducer 375. Blast
air tube 376 is directed into reducer 375 to assist in carr~ing
coal out through pipe 374 pneumatically to coal feed pipe
232 (Fig. 1) of combustor-desulfurizer 200.
The purpose of the coal pot is to smooth out the
fluctuations in the coal feed rate that would otherwise
occur when the rotary air valve empties each pocket. Such
fluctuations increase the likelihood of a plug in the coal
feed line, thereby reducing the system's reliability, and
also dimi*ishing its ability to tran~port relatively large
coal sizes (of up to 1/2"), in the 1 1/4" d.iameter pipe
374 w~thout plugging; a larger pipe 374 might thus be needed
to accommodate such lumps. But a larger coal pipe reguires
greater amounts of pneumatic transport air, ~hich in turn
adversely affects the combustor's operation by increasing
the likelihood of blowouts through the bed when the bed
height is at its minimum dpeth. These blowouts carry with
them the coal fines (<28 mesh) in the coal supply, and
adversely affect the burner's combustion efficiency. ~luctu-
ations in the coal outlet flow rate are damped as follows.
The coal pot is sized to permit vigorous fluidization; i.e.,
the velocity of the fluidizing air through the pot is designed to be at
leæt three times the im~m fluidization velocity of the ~x~. ~his
causes coal particles to be flung into the space above the coal pot's
bed level; those of the particles that are flung into the vicinity
of the reducer 375 are drawn into the reducer and blown into
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~L~17~0~
the coal pipe 374 and to burner-desulfurizer 200. Withou-t
fluidization, the coal feed rate would change appreciably
with the slight changes in bed height that inevitably occur
with the surges of incoming material, but with fluidization,
the feed rate to the reducer is seen to depend on the splashing
from the bed, which is independent of the surges. The jet
blast tube 376 assists in providing the smoothness of the
flow by eliminating any possible buildùps that might othérwise
occur in the reducer 375.
The coal pot may also be used to split the main
coal stream into a number of smaller, equal feed streams,
each of which feeds an equally-sized bed area. Suah stream
splitting is required for fluid bed combustors whose bed
area exceeds 10 s~uare feet. The coal pot performs the
stream splitting function by placing the required number
of outlets (one per 10 square feet) at the perimeter of
the pot, each identical to the assembly of reducer 375,
pipe 374, and blast tube 376.
(An even more preferred design would move rotary
2~ air lock 368 between feed screw 348 and drier 340, allowing
heated air leaving the drier at vent 361 to be used as coal
transport air, entering the coal pot at pipe 372 before driving
the coal to the fluid bed combustor. This eliminates the
thermal loss and fuel loss associated with venting the drier's
exhaust gases to atmosphere, and eliminates the need for
any particulate collector system that would otherwise be
required with an atmospheric vent system.)
Fig. 8 shows limestone pot 378, which is similar
to coal pot 344. Pot 378 is connected to limestone downcomer
260 through standpipe 380, which extends downwardly centrally
within pot housing 382. Also within housing 382 is a perforated
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distributor plate 384 positioned below the mouth of' thestandpipe 380. Above plate 384 is orifice plate 386, which
is vertically adjustable by handle 388 and threaded shaft
390 to adjust the size of the opening from standpipe 380
into housing 382. At a level above mouth 381 of standpipe
380 is outlet fitting 392, which connects housing 382 to
limestone disposal pipe 394. Blast air tube 396 is directed
to blow air into outlet fitting 392 to assist in pneumatically
transporting spent limestone through pipe 394 to baghouse
hopper Inot shown). Below plate 384 is air inlet pipe 398,
which sends fluidizing air through plate 384. Housing 382
rests on supports 400. The limestone pot has a similar
function as the coal pot, i.e.,,to transport solids in a
jam-resistant fashion and in the smallest practical pipe.
It also serves to cool the sorbent from its bed temperature
~ca. 155aF)~to one low enough (under 400F) to safely transport
the spent sorbent to the baghouse. It also serves to use
the dipleg 380 feeding it as a pressure seal, thereby allowing
the pressure in the limestone pot to build up sufficiently
(typicall~ to 1 psi~ to transp~xt the spent sorbent to the
baghouse hopper, thus eliminating the need for a mechanical
seal in the high-temperature dipleg. (According to Walker
et al., Combustion; Feb. '79, p. 31, no device for performing
these functions, referred to as a bed letdown cooler, is
currently available.)
Dimensions and other specifications in the embodiment
shown~ave been selected to give an energy input of 500,000
BTU/hour/sq. ft., and are as follows. Steam pressure is
15 psig or less. The horizontal area of middle distributor
plate 210 is 20 square feed (3 ft., 2" by 6 ft., 4"), for
10,000,000 BTU/hour energy input. The lower bed when filled
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is 7" deep (when settled) and uses sand with an average
particle size of 20 mesh (850 ~). The middle bed varies
from a depth of 6" to 11.5"and uses the same size sand as
the lower bed. The upper bed is 6" deep and uses 90% 6
mesh, 50~ ~10 mesh sand and -5/8 +5/16 limestone chips.
The limestone is type 1360 that is available from the Monmouth
Stone Co., Monmouth, Ill. The coal is Peggs Run bituminous
stoker coal, sized 1 1/4" x 1~4", containing 3 1/4% sulfur
and having 13,750 BTU/lb., available from Peggs Run Coal
Co., Shippingport, Pennsylvania. The distance between
the top of bubble caps 224 and the bottom of steam tubes
234 is six inches. (This permits the burner to operate
without blowout at the coal feed pipe. Blowout occurs in
this embodiment when the bed material in the middle bed
is less than three inches deep, as measured from the top
...
of bubble caps 224. The provision to make the middle bed
six inches deep, rather than the minimum of three inches,
is to provide a safety factor against blowout due to the
inadvertent loss of bed material during operation.)
The middle bed freeboard is 30", and the upper
bed has a freeboard of 18". These freeboard dimensions
where chosen to prevent significant loss of sand and sorbent,
respectively. The o~erall height of combustor-desulfurizer
200 is 100". Bubble caps 218, 224, and 246 are all on 3"
centers to give 16 per square foot. They are positioned
closer to the walls than shown in Fig. 3, for uniform fluidization
throughout the bed. Nipples for the bubble caps on the
upper bed are 1" IIPS) and the eaps are 1" NPT; each is
1/2" in the two lower beds. Shoulder 296 is 1.9" in diameter.
The radial gap 291, between the nipple 293 of the upper
bed and the surrounding tube 298 is appro~imately 0.1 inch.
-- 19 --
Holes 294 in the middle distributor are .192" in diameter;
the equivalent holes in the upper distributor are 0.400"
in diameter. The ~uartz rocks 250 are piled to a depth
of two inches over the top of caps 246. Although only one
coal feed pipe 232 is shown, two are actually used, each
centrally positioned over a bed area of ten square feet.
Feed pipe 232 is l l/4" IPS, steam tubes 234 are l l/2"
OD, and secondary air pipes 236 (only one shown) are 4"
OD. Holes 238 are l/2" along the two sides, on 7" centers,
and holes 240 are l/4" on 3.5" centers. The steam tubes
are spaced with a 6.5~ pitch in the horizontal direction,
and a 2.8" pitch in a direction 30 from the horizontal.
Limestone feed pipe 258 is l l/2" OD. Tubes 252 are 3/4"
OD~ The centerlines of tubes 252 are spaced l 3/8" apart,
in rows whose centerlines are also l 3/8" apart. Upcomer
piping 268, downcomer piping 280, and limestone downcomer
260 are 2" IPS. The turndown ratio is 30 to l. Preheater
223 i8 a propane-fired duct heater and has 15% of the heating
capacity of combustor~desulfurizer 200. Refractor~ 204 has
a ~ity o~ 50 lhs/cu.ft~ and i5 2.5" thick, and refractory 204 has a
density o~ 140 1bs./cu`.~t~ and is 1 l/2" thick. ~r~ng the cn~l nnt,
air inlet pipe 372 is 1 V2'1 IPS, blast air tube 376 is a 3/8" tube. outlet
fitting 375 is a 2 1/2" x 1 i/4" eccentric reducer. As to the limes~e
air inlet PiPe 398 is 1 1~2" IPS, blast air tube 396 is a 3t8"
tube, outlet fitting 392 is a 2 1/2" x 1 l/2" eccentric
reducer, and outlet pipe 394 is 1 l/2" IPS.
As to materials, upcomex piping 268, downcomer
piping 280, limestone downcomer 260, and wavebreaker tubes
252 are made of 316 stainless steel. The secondary air
pipes 236, coal feed pipe 232, and the nipples for the bubble
caps are made of 304 stainless steel. Housing 202, boiler
tubes 234, water jackets 225 and 242, all three distributor
- 20 -
~7~
plates, coal pot 394, and limestone pot 378 are made of
carbon steel. Caps for the bubble caps are made of 304
stainless steel. Blast air tubes 376 and 396 are made of
copper.
Operatlon
Sand is supplied to fill the middle bed to a depth
-of about 11.5". Type 1360 limestone crushed to a mean particle
diameter of 20 mesh (850 ~) is supplied through feedpipe
258 to fill the upper bed to a depth of about 6".
Start-up of a cold combustor requires preheating
as follows. Fluidizing air is supplied from a blower (not
shown) through windbox 216, and the middle bed, assuming
that it has been previously filled with bed material, is
emptied via downcomer assembl~ 266 until the bed level is
below the inlet to the downcomer so that boiler tubes 234
are no longer covered with bed material (remaining material
i9 about 6" deep)~ Air from windbox 216 passing through
bubble caps 218 acts to spread out the bed material deposited
by the downcomer, and directed through the storage bed when
either the upcomer or downcomer is in operation, to keep
the lower bed material uni~oxmly spread out. When the bed
level in the middle bed is down to 6 inches, the fluidizing
air is turned off. The water circulator pump 312 is turned
on. Preheater 223, which is spaced below distributor 212
to provide uniform heating of the middle bed, is then turned
on. Flames generated in the preheat burner are cooled to
approximately 1700F by secondary air before they emerge
from the burner, to avoid overheating bubble caps 224. Hot
gases emerging from the preheat burner 223 heat the material
in the middle bed to about 1000F in about an hour, following
which coal is added for a minute with fluidization (to assist
- 21 -
'4~
further preheating), following which preheating is resumed15 minutes or so, until the bed reaches about 1350 Fahrenheit.
Because the boiler tubes are not in contact with material
in the middle bed, they do not draw heat from the bed material,
and because the bed material is heated when it is not being
fluidized (i.e., as a fixed bed), the surface area for heat
loss from the bed material is reducèd, so ~hat the bed material
can be heated with a fairly small preheater.
When the middle bed has reached 1350F, the propane-
fired preheater is turned off. Fluidizing combustion air
from the blower is supplied through windbox 222 and through
bubbble caps 224 to fluidize the middle bed. The fluidizing
combustion air is controlled by a valve (not shown) to provide
an airflow of llO scfm per square foot of bed area, which
produces a superficial velocity of approximately 7 1/2 ft/sec.
in the upper bed at 1550F. The coal feed screw 348 and
transport air compressor (not shown) feeding air to the
coal pot at inlet pipe 372, and to the limestone rotary
feeder outlet ~not shown) are then started, and coal is
fed from bin 346 through screw feeder 348, drier 340, crusher
342, rotary air lock 368, coal pot 344, and to the middle
bed through pipe 374. The coal mixes with the hot bed material
and burns. Fluidization causes the coal to be distributed
away from the coal feed pipe and become mixed throughout
the bed. The heat released from the burning coal heats
the bed, until the middle-bed approaches the desired temperature
of 1800F. (A lower setting may be required to avoid clinkering
when coals with low-ash-fusion-points are used, and a higher
setting may be used with hard-to-~urn , unreactive fuels
with high-ash-fusion-points). The middle bed is kept from
reaching a hotter temperature in part by the cooling effects
- 22 -
~:~L7~
of the steam tubes 234, which are being splashed by the
fluidized bed material, and in part by the effect of the
control thermocouple in the middle bed, which serves to
control the middle-bed's temperature by adjusting the speed
of the screw feeder 348, thereby affecting the fuel/air
ratio in the middle bad. Typically the burner will be oper-
ating at an excess air of 100~ at this condition.
Simultaneously with the coal feed, the limestone
feed to the upper bed is started, at a predetermined Ca~S
ratio, as explained hereinafter. The limestone, -5/8 +
5/16" chips, flow from a limestone bin (not shown~ through
the rotary feeder and are pneumatically con~eyed to the
upper bed through pipe 258. The rate at which the limestone
is fed is determined by the speed of the limestone rotary
feeder, whose speed is slaved to the coal screw feeder 348
in order to provide the predetermined Ca/S ratio.
Gases emerging from the middle bed pass through
bubble caps 246 and the upper bed, and leave combustor-
desulfurizer 200 at pipe 262. As the temperature of the
upper bed reaches the upper-bed set point of 1550F, wh.ich
is the temperature at which desulfurization efficiency is
best, a thermocouple in the upper bed causes a modulating
valve (not shown) at the combustion-air blower to open,
causing secondary air to flow into the middle-bed freeboard
237 through secondary air pipe 236. The seocndary air flow
rate is modulated continuously to maintain the upper-bed
temperature at 1550F.
Solids too small to remain in the middle bed,
including ash, sorbent, and small particles of carbon, are
blown through bubble caps 246 and trapped in the upper bed,
where combustion of the small bits of carbon is continued
- 23 -
~or a few moments., he:Eore being blown f.rom the upper bed
and out of the burner at duct 262.
Particles too coarse to blow out of the upper
bed will cause the upper bed level to rise, causing excess
material to be removed at limestone overflow pipe 260.
If, after the bed has reached its desired bed
temperature, the system steam pressure is beneath the required
amount, the burner automatically arranges to increase its
heat output, as follows. Bed material from the lower bed
0 i5 blown into the middle bed, by operating the upcomer 264.
This slowly increases the contact of the bed material with
steam tubes 234, thus increasing the heat loss from the
bed, caUsing the middle bed temperature to drop momentarily.
The middle-bed temperature sensor causes the coal feed screw
348 to increase the coal feed rate, in order to maintain
, .
the feed temperature at its former set point. This process
continues, whereby the bed level is increased along with
the coal feed rate, until the steam tubes are completely
covered. At this point, appxoximately 2/3 of the heat of
~0 comhustion is removed at the steam tubes 234, with the remaining
heat removed at the boiler, and the excess air at the middle
bed is 5 to 30~O The upcomer must blow fed material slowly
enough t.o avoid quenching of the middle bed, i.e, to allow
the coal feed rate to increase fast enough to overcome the
cooling effects of the incoming material.
The burner-desulfurizer 200 continues to operate
at this, i.ts maximum, capacity, until the steam pressure
reaches a set point P2, causing downcomer 266 to be activated
and the material in the middle bed to be transferred slowly
to the storage bed, thereby reducing the heat transfer to
steam tubes 234 and eventually by response of the middle
~79~
bed thermocouple, the coal feed rate. Reduced heat transfer
to steam tubes 234 in turn causes the steam pressure to
drop, eventually, below set point P2. At this, downcomer
266 is inactivated. Under most circumstances, the burner
is now at equilibrium: the coal feed rate, bed depth, and
steam output all remain constant.
Under some circumstances, as for example when
there is a large amount of stored steam in the system, so
that the steam pressure changes very slowly in response
to variations of the firing rate, the downcomer may have
been activated too long, causing too much bed material to
be transferred to the storage bed, and eventually causing
the steam pressure to fall below a lower set point Pl, which
is less than P2. At this, the upcomer 264 is activated
until, eventually, the middle-bed level is such that the
steam output matches the steam requirement, and the steam
pre~sure remàins between Pl and P2. At this condition,
again, no further adjustments to the middle-bed level are
re~uired, as long as the steam demand remains constant.
If the steam demand changes, however, causing the steam
pressure to pass either of the set points, the upcomer or
downcomer are acti.vated until the system is again in equilibrium.
In this manner, the burner-desulfurizer is continuously
modulated from full capacity to about 50% of full capacity.
Operation in this manner is called the modulating mode.
If the steam demand drops below the 50% level,
a further reduction of the materal depth in the middle bed
would be incapable of producing further reductions in steam
output and, in fact, cannot be achieved insofar as the
entrance of downcomer 266 prevents removal of middle-bed
material below the six inch depth. In this case, the steam
- 25 -
pressure continues to rise, eventually exceeding set point
P4 (higher than P2), which causes the burner-desulfurizer
200 to be shut off. Shutoff consists of turning off coal-
feeder 348, drier 340, crusher 342, rotary air lock 368,
and the limestone feeder. After 15 seconds, enough time
to clear away the solids in coal pipe 374 and limestone
pipe 258, the transport air compressor and combustion air
blower are turned off. The burner-desulfurizer 200 is left
off until the steam pressure drops below set point P3 (between
P2 and P4), where it is turned on, in the reverse sequence
from which it was turned off. As long as steam capacity
is below 50~ of rated capacitv, the burner-desulfurizer
200 will continue to cycle on an~ off as steam pressure
fluctuates between P3 and P4. This mode of operation is
called the cycling mode. In the cycling mode, the burner-
desulfurize~ may be left off for periods of up to an hour,
before it cools below the temperature at which coal is readily
ignited. About three minutes' operation is required to
heat the bed back to its set point, at a coal feed rate
o~ two-thirds of the full-capacity rate. By this means,
the overall turndown of 30-to-l is achieved. If the stream
demand averages less than 1130 of full capacity for an hour
or more, the middle-bed temperature sensor observes that
the bëd is blow the reignition point, and prevents the feeding
of coa:L to the unit until the preheater has been used to
return the bed to its minimum ignition temperature. Greater
turndown than 30-to-l could be achieve, if necessary, by
the use of more extensive insulation all around the middle
bed.
An alternative mode of operation in the modulating
mode causes the combustion air flow to be slaved to the
- 26 -
coal feed rate. This increases the system's thermal efficiency by minimizing
the excess air, and thereby the ther~a] stack losses, but re~lires a more
complex control than does the previously described method by which the air-
fl~ remains constant throushout.
Another mDde of operation, called the low-nitric-oxide mode,
arranges to have the combustion bed operated substoichio~etrically (typi-
cally at an equivalence ratio of .85), while the upper bed is operated
at a slight exoe ss-air level (typically, 3%), and to have tertiary air
added above the desulfurizing ked through an aerodynamic mixer tnot shown)
lQ to create an atmosphere containing 20-30~ air at the burner-desulfurizer's
outlet. The freeboard above the upper bed will need to be increased,
to allow unburned hydrocarbons, including carbon monoxide, to be ade-
quately combusted. m e purpose of this method is to minimize the nitric
oxide (NO) emissions from the ~urner, while still achieving good ccmbustion
efficiency and pollution characteristics with regard to S02, 00, and other
hydrocarbons. Operation of the co~bu~stion bed at substoichiometric condi-
tions reduces the rate of oo~bustion of the coal particles, thereby increas-
ing the carbon content of both the combustion bed and the desulfurizing bed.
Previous investigators (Beer, et al. "NO Reduction by Char in Fluidized
Combustion", Proc. of the 5th Conf. on Fluidized Bed Ccmbustion, Washington,
_ _ _ _ .
DC, Dec. '77) have sh~n that the pxesence of small am~unts of carbon in
a bed is sufficient to drasti~lly reduce the NO level emitted from a
fluid bed. Other investigatvrs (Horio, et al., "A Mbdel S~udy of the
Development of Lcw NO~ Fluidized-Bed Coal co~bustors", Proc. o the 5th
Con~. on Fluidized Bed Co~bustion, Washington, DC, Dec. '77) have taught
_ _
that a bwo-stage fluidized bed oo~bustor is particularly effective at
mixing the NO with ~le carbon, thereby chemically reducing the NO to form
lecular nitrogen. The oxygen level in the upper bed m~st be optimized
to meet both the requirements ,of desulfurization, which is favored by a
high-oxygen atmosphere, and af NO reduction, which is favDred by a low-
oxygen atmDsphere, although the presence of small amounts of oxygen
ttypicallY, 3% excess air), are acceptable to the NO reduction process.
Since it is impossible to operate the system with
low excess air in the cycling mode, the attainment of low
NO levels by this method is not achievable in the cycling
mode.
Periodically, depending on the rate of depletion
or accumulation of solids in the system, various parts of
the system must be checked for the amount of solids inventory,
and appropriate measures must be taken, as follows:
The accum~lation of coarse particles in the middle
bed is not normally observed, unless large particles of
mineral matter are fed with the coal. In such a case) the
~urner is periodically turned down, whereby the downcomer
266 i.s activated, and all hed material in excess of 6" in
the upper bed is blown into the lower bed, and all material
in ex~ess of 7" in the lower bed is then removed at overflow
220.~ The..coarse material may be screened out, and the remaining
material be returned to the bed, or the entire excess may
be dumped into the baghouse hopper for disposal. Use of
the storage bed as A removal site permits the excess material
2~ to be cooled from the bed temperature (about 1800F) to
a safe temperature for transporting and storage (under 400F)
by allowing fluidi2ing air to enter the burner-desulfurizer
at windbox 216 until the lower bed is at the low temperature,
at which point the valve in overflow pipe 220 is opened.
After completion of the excess-material removal,
the burner resumes its normal operation.
In the upper bed, whenever the bed height is observed
to drop below the minimum allowable level of four inches,
the bed is filled of the optimum depth of six inches by
the addition of coarse sand, sized 90% minus 6 mesh, 50~
plus 10 mesh. Also, if the upper bed height is observed
- 28 -
~3~ 7~0S
to exceed a maximum allowable depth of nine inches, transport
air is allowed to enter the limestone pot 378 at pipe 398,
thereby fluidizing the solids resting on distributor plate
384, permitting the material which has entered the limestone
pot to be blown out of the limestone pot and into the baghouse.
Orifice 386 is adjusted to restrict the rate at which bed
material is allowed to enter the limestone pot, thereby
preventing the flow of excessively hot materials from the
limestone pot, and also preventing the buildup of excessive
pressures in the limestone pot that would destroy the pressure
sealing effect of the material located in the downcomer
260 and standpipe 380. When the upper bed depth has been
reduced to its desired height, fluidizing air to the limestone
pot is turrled off, and the standpipe 380, as well as downcomer
260, are allowed to remain full, or to be refilled, after
which no further material is removed from the upper bed,
until the cycle needs to be repeated.
Other areas requiring periodic attention include
the bags in the baghouse, which must be cleaned of dust
accumulations by any one of several standard procedures
for cleaning the bags. Preferably, the burner is shut down
momentarily during baghouse cleaning; even at the full-
bed conditions, baghouse cleanup can be accomplished before
the middle bed has been cooled below the coal ignition point.
Also, periodically, the baghouse hopper, where residue is
collected, must be emptied, and settling chambers within
the boiler may have to be cleaned out.
29 -
Shutdown of the burner for prolonged periods,
i.e., more than an hour, should be started with the middle-
bed operating at its set-point temperature, whereby the
coal feed screw is simply turned off, allowing the coal
in the bed to burn off. Failure to burn off the coal may
result in clinkering upon subsequent restart. The combustion
air and the circulating water pump 312 are left on until
the burner is cool enough to avoid warping steam tubes 234
when the coolant flows are turned off.
The calcium/sulfur ratio must be made sufficiently
large/ by proper adjustment of the limestone feed rate,
to produce at least enough microchips, ~measured in mass
per unit time) to absorb all of the SO2 generated by combustion;
otherwise, the bed wilts. Wilting is said to occur when
sulfation hardening of the particles occurs at a faster
rate than attrition can wear away the particlesl surfaces;
the surfate hardening causes a reduction in the attrition ratej which
causes more sulfate ha~ ng~ creating a cycle that ends only when attri-
tion virtually stops Onoe a bed has wilted, its scrubbing efficlency
~0 is greatly redu~l, unless the Ca/S ratio is greatly increased.
Once a bed has wilted, it ls best to replace it
with a fresh bed of sorbent, and start again with a higher
Ca/S feed ratio.
The onset of wilting is accompanied by a xeduction
of the density of the cloud of microchips at the bed's outlet,
and a simultaneous increase in the rate of buildup of the
upper bed's depth. The observation of either effect may
be used to prevent wilting by increasing the sorbent feed
- 30 -
~'7~g~05
rate. Altèrnatively, the Ca/S may merely be set at a sufficiently
high rate to provide a safe margin beyond the minimum requirement,
thereby avoiding wilting without any measurement of the
attrition rate.
Ideally, very soft sorbents are employed, so that
none of the particles ever become sulfate hardened, instead
continuing to attrit until they are too small to remain
` in the bed. Increasing the particle size of the sorbent
feed promotes attrition, and allows the use of sorbents
that might otherwise not be useful. An upper limit on sorbent
size occurs when sorbent particles are so large, and thus,
so few, that gaps occur in the bed where no sorbent is located;
S2 fumes would be expected to escape from such gaps, thereby
reducing the scrubbing efficiency.
Harder sorbent particles may not attrit fully,
but eventuall~ sulfate harden, particularly as they become
smaller. The hardened particles remain in the bed, absorbing
the conventional amount of S02 (typically, a third of the
stoichiometric amount) before leaving the bed at overflow
260 The SO~ removed by these particles directly reduces
the requixed amount of microchip production needed to avoid
wilting, although the total limestone requirement is higher
than if the very soft limestone were used, where no particles
experience sulfate hardening.
Under circumstances when harder sorbent is used,
thus, the sorbent itself may serve as both sorbent and ballast.
- 31
os
r~odifications and Varlations
Regarding modifications and variations to the most
preferred embodiment, refractory layers 204 and 206 may be
replaced by a single refractory of intermediate density, or
in large capacity unlts, by water-cooled walls. Secondary air
pipe 236 may be designed to reinject solids, e.g., unburned
carbon collected from a cyclone before the baghouse. The
stones 250 may be replaced hy other materials, such as stain-
less steel s~heres, or by other constructions, such as in-
verted channels located over the bubble caps, that serveto reduce the velocity of the gases entering the bed to a
sufficiently low value that particulate shattering is avoided.
The baffle tubes 252 above the upper bed may be deleted, par-
ticularly if some additional freeboard is emploved. The same
is true of the baffle tubes above the middle bed. Other con-
figurations of baffles that also prevent a line of-sight open-
ing may be used in place of the preferred configuration.
Downcomer 266 may be replaced by a vertical stand-
pipe that passes through the middle-bed distribu-tor assembly,
and terminates in a tric~le valve just above the storage bed's
highest bed level. To actuate the downcomer, the trickle
valve is opened a predetermined araount, high enough for the
middle bed material to flow into the storage bed at a desired
~peed by gravity, but less than the speed at which the stand-
pipe might be starved for bed material, thereby losing its
pressure-sealing characteristic. When the downcomer is in-
activated, the trickle valve is allowed to return to its
closed position. Use of the alternate downcomer mechanism
eliminates the need for the screw feeder shown in the pre-
ferred downcomer assembly, but requires a more careful
- 32 -
adjustment of the tricl~le valve o~ening to operate correctly,and is somewhat more subject to malfunction due to the pre-
sence of large particulates in the middle hed.
The bubble cap design of the preferred embodiment
is just one of many that might be considered; there are many
bubble cap designs shown in the prior art that serve all of
the functions identified; the preferred design has the ad-
vantage of being fabricated readily from commonly available
com~onents.
Lower-distributor Plate 210 may be made of the
water-cooled construction as are the upper distributor plates,
or other mechanisms for the prevention of buckling, such as
the use of bellows at the edge of the distributor plate, may
be used to prevent buckling. The distributors may be de-
signed to extend beyond casing 20~, and be sealed against
leakage by flanges attached to the ou~side of casing 202,
instead of being attached to anglè irons inside the casing,
as shown in Figure 1.
When the burner-desulfurizer 200 i.s used to generate
high pxes~ure steam, alternate methods may be used to cool
the distributors. For example, a separate cooling loop may
be used, whereb~ water is circulated with a separate pump
through the distxibutors, to a water-to-water heat exchanger,
and back to the distributors~ The other side of the water-
to-water heat exchanger is cooled by boiler feedwater. This
method avoids the overheating of the distributors, thereby
minimizing their thermal expansion and tendency to buckle,
and also minimizes the pressure levels and corresponding
structural requirements for the distributors, thereby reducing
0 their weight and cost. The steam drum 326 is optional;
- 33 -
s
alternatively, the inlet pipe 328 ancl float valve assembly332 may he attached directly to the casing o, the outlet
manifold 316. Superheater tubes, if any, are located in the
freeboard of the upper bed, if a small degree of superheat is
required, and in a separate module similar to Figure 1, if
extensive superheating is required. In the latter case, the
superhea~ tubes are located in the same position, relative
to the distributor, as are tubes 2340
Other types of coal feeders may be used, such as
weigh-belt feeders. The coal drier may be eliminated if coal
of moderate moisture is used, in conjunction with an air-swept
crusher design that permits drying air to dry the coal within
the crusher. The stirrer of coal drier 340 may be of a
different design than that shown; for example, a spiral or
pair of spirals may be used~ instead of ~ransverse pins 354.
Also, transverse pins 356 are optional, particularly with coal
that contains few fines below 1/4 inch. The temperature
of the air entering the drier may be thermostatically con-
trolled to create the optimum coal outlet temperature of
about 200 degrees F, regardless of the moisture. This avoids
the possibi1ity of overheating the coal, and starting a fire
in the coal drier during shutdowns. It also allows the drying
of coals whose moisture content is greater than 15%, bv in-
creasing the drier's air inlet temperature above 700 degrees F.
The heat for the drier may come from a separately-fueled
duct burner, rather than from a pipe in the middle bed as de-
scribed in the preferred emboidment.
In the limestone pot, a rotary valve mav be placed
in standpipe 380, which prevents the escape of transport air
0 into the upper bed, instead of to the baghouse hopperl, i`Ls
- 34 -
intended dcstinatioll. The rotary valve replaces -the pressure
sealing effect of material in the overflow pipe 260 and
standpipe 380, makina the system less tbuchy and also
allowing the use of ~reater ~ressures in the limestone pots,
with correspondingly greater allowable distances between the
limestone pot and the baghouse hopper.
Blast air pipes 396 and 376 are oDtional. Coal pot
344 is optional, and may be replaced by a simple transition
pan, into which coal falls and from which coal is blown into
the combustor-desulfurizer 200. ln systems employing more
than one coal feed pipe, the transition piece is made circular
in cross-section and has a number o coal outlet pipes at its
periphe~y, which are fed by a rota~ing spreader located at
the center.
The burner-desulfurizer 200 of the preferred embodi-
ment has~a capa~ity of 10,000,000 BTU/hr. Actually, the design
is suitable for a wide array of sizes ranging from the commer-
cial sizes of 1 million BTU/hour to the electric utility
8ize rated at up to 10 billion BTU/hour. The steam conditions
at steam tubes 234 depend upon the application. In electrical
utility generation, where the highest steam temperature and
presæure3 are desired because of the effect on thermal effi-
ciency, steam temperatures as high as 1200 degrees F may be
obtained, at supercritical pressures. These exceed the tem-
peratures~ by about 200 degrees F, of both conventional oil
and coal-fired hoilers and o conventional fluidized bed com-
bustors, and may reduce the fuel consumption of such systems
bv 5 to 10 percent, as well as prolonging boiler life and im-
proving maintainability. As has been taught elsewhere, (John
Stringer, "Materials for Fluidized Bed Combustors," 3rd Annual
- 35 -
~onference on Materials for Coal Conversion and Utilization,
____ _ __ _ __
NBS, Gaithersburg, Maryland, October 1978, p. 154, et al.~,
the mechanism for fireside corrosion that had formerly limited
the maximum steam temperatures of utility boilers is not pre-
sent in fluidized-bed combustors, primarily because the alkalis
released in conventional high temperature combustors are not
released in the lower-temperature fluidized bed systems. But
fluidi2ed bed combustors do experience fireside corrosion,
due to the presence of calcium sulfate that coats the steam
tubes. mhis form of corrosion is eliminated by the use of
sand, rather than sorbent, adjacent the steam tubes in the
combustor~desulfurizer 200, thereb~ eliminating the corrosion~
producing sulfate coating, and allowing the boiler to be
operated at temperatures determined by factors other than fire~
side corrosion. In the larger systems, the burners would be
buiIt in modules to avoid the over-stressing of the steam
tubes and distributors due to excessively large spans. To
increase the capacity of the system, the cross-sectional area
o~ the beds must be increased by one square foot per 500,000
BTU/hour. The burner height must be increased only to the ex-
tent that lateral v010cities in the plenums 227, 237, and in
the freeboard above the upper bed are kept low enough to avoid
entrainment of bed material. Pipe diameters are increased pro-
portionally to the capacity of the burner. One exception is
the coal feed pipes: these are always about 1-1/4 inches in
diameter, with one pipe required per 10 square fee~ of bed area.
?~bove-the~bed feeding may be used, with larger coal particles
and larger coal feed pipes, or spreaders, if the steam pipes
are arranged with a sufficiently wide space at the plane of
coal entry to allow the coal particles to be injected directly
- 36 -
~7~
into the bed without hitting the pipes. The bubble cap dimen-
sions and also the boiler tubes are not increased in diameter,
although steam tubes' diameters may be increased somewhat for
greater strength when they are used to span greater spans than
those of the unit in Figure 1.
Any type of coal may be used, of any rank, sulfur
content, moisture, caking characteristic, or ash content.
Waste fuels, containing large amounts of inert material, such
as culm or shale, may also be burned, as may unreactive fuels
such as coke breeze. In the former case, some or all of the
steam tubes may have to be moved out of the splash zone of the
middle bed; this is achieved by removing tubes 234, or raising
them far enough to avoid their being splashed by bed material
when the bed depth i~ at its minimum value. Other fuels in-
cluding wood chips, as well as oil and natural gas, may also
be used. Fo~ these fuels, a method of injecting the fuel into
the bed at close intervals ls required; such methods have been
described in the prior art. Provisions for these fuels would
allow the unit to be operated whenever changes in fuel prices
or unavailability of supplies indicates a switch in the fuel
type. Burning of oil in the combustor-desulfurizer 200 may
also be preferrable to the burning of this- fuel in conventional
boilers, particularly if the oil contains an objectionably
high sulfur level; the levels of nitric oxide may also be re-
duced without the use of a scrubber, and the maximum steam
temperature that may be achieved without the occurrence of fire-
side corrosion, may also be increased.
Burner-desulfurizer 200 may be located within the
firebox of a boiler, or constructed as a separate unit (a so-
called "dutch oven") as shown in Figure 1. In this case,
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~7~0~
multifuel capability of the burner-boiler system may also be
provided, by operatinq the hurner-desulfurizer 200 only with
solid fuels and removing the smokepipe 262 from the boiler and
installing in its place a gas or oil burner, whenever these
fuels are required. This approach achieves an extra level of
reliability for the overall system, though it reduces the
steam capacity and thermal efficiency in comparison with the
configuration where gas ~r oil are added at the middle bed.
A wide variety of limestone types, including dolomites,
are suitable; approximately half of all calcium carbonates appear
suitably attrition~prone to be useful. Synthetic sorbents,
consisting of lime that has been ground to 10 microns or under,
and cemented with a pozzolanic material, mav also be useful.
Nothing about the dimensions of the beds, steam tube
geometry, bubble cap spacing, or materials usec~ in the burner-
desulfurizer, is critical, although certain trends cause a
deterioration of performance as the optimum conditions are
passed. In particular the use of greater middle bed depths
increases the violence of the bubbling and with it, the need
for for provisions to improve combustion efficiency, such as
increased freeboard heights, fly ash reinjection or the use of
a separate carbon burn-up cell, and combustion air preheating.
The us¢ of greater hed depths in the desulfurizing bed has
less serious effects and may, in fact, improve desulfurization
and denitrification; as long as splash-preventing baffles are
used ahove the upper bed, freeboard requirements are not greatly
increased.
Startup may be accomplished by leaving the propane
burner on until the middle bed reaches a temperature of 1100
3~ degrees F, at which point the preheat burner is extinguished
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~:~L'7~?C~
and normal coal-fired operation commences; the step of adding coal
for a minute, to assist in preheating, may be dispensed with. -
The coarseness of sand and sorbent may be varied, solong as fluidization and mixing are sufficient under all condi-
tions to avoid hot spots or clinkering. The heat release per unit
area may also be varied, by * 25%, or more, if adequate precau-
tions are taken to prevent defluidization, at low velocities, or
excessive carbon losses at the high extreme. Fly ash reinjection
may be employed to improve carbon utilization and help desulfuriza-
tion, the reinjected particles being injected into either the pri-
mary or secondary air streams.
Coal sizes other than those indicated may be used~ al-
though an excess of fines, of under 28-mesh, will cause increasing
difficulties in achieving high combustion efficiencies, any may
cause the plugging of the upper distributor.
Burner-desulfurizer 200 may be used to heat other fluids
in tubes 234, such as air that may be used in ovens, or compressed
air, that may be used to drive gas turbines in combined cycles
that are used to generate electricity, or in cogeneration units,
that produce both electricity and heat.
Another variation in operation is to use the combustor-
-desulfurizer of Fig. 1 as the char burner in U.S. Patent No.
4,051,791, entitled "Coal Burning Arrangement", granted October 4,
1977. Fig. 9 shows this use of the combustor-desulfurizer in a
system including pyrolyzer 400, char cyclone 402, combustor-
-desulfurizer 404, ash cyclone 406, and afterburner 408. Coal and
limestone are pneumatically fed through line 412 to pyrolyzer 400.
Coal is pyrolyzed in pyrolyzer 400 to form finely sized char
- 39 -
(minus four mesh) and to produce volatiles including hydrogen
sulfide. Limestone is calcined in the pyrolyzer to form lime
(Ca~), which scrubs the hydrogen sulfide in the volatiles
stream by the reaction:
H2S + CaO ~ CaS + H2O (1)
The calcium sulfide is then ducted with the char and volatiles
by line 414 to char cyclone 402, where gases are separated
from solids, and then by line 416 to the combustor stage 418
of combustor-desulfurizer 404 (which could be the combustor-
desulfurizer of Fig. 1 with the storage fed and the steam tubes
234 removed). Gases from char cyclone 402 are carried to
afterburner 408 by line 420. Char is then ~urned in the com-
bustor stage 418, which is oper,ated from 1800 to 2000F,
(preferably at 1900F). Calcium sulfide is oxidized back to
li~e by the excess air in the combustor by the reaction:
, C~S + 3/202 ~ CaO ~ SO2 (2)
The sulfur dioxide emitted by this regeneration of lime and
the sulfur dioxide released by char combustion are then scrubbed
by lime in desulfurizer stage 422 (operated at from 1450 to
just over 1600F, preferably at 1550P~; SO2 scrubbing efficiency
drops off rapidly above 1600F) of combustor desu-lfurizer 404:
CaO + SO2 ~ 1/2 2 ~>CaSO4 (3)
The lime in reaction (3) is added to desulfurizer stage 422
through line 424 as limes-tone and is in proportion to the
amount of sulfur originally in the coal and the degree of de-
sulfurization reyuired. The spent sorbent too coarse to be
blown ou~ of the combustor-desulfurizer 404 is removed from
desulfurizer 422 through line 426 to a dis~osal site or ash
hopper (not shown). The spen~ sorbent will be a mixture of
CaO and CaSO4, an inert material that ma,v be disposed of with
little or no further treatment.
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1~i7~
The remainjllg sorbent is elutriated from bed 422,
and is removed from the system at ash cyclone 406 or at a
particle collector located downstream of the furnace (not
shown) to which afterburner 408 is attached.
The material in the combustor bed 418, consisting
mostly of CaO, is recycled to the pyrolyzer through line 428
and is there converted to CaS by reaction (1). Thus fresh
limestone is fed through line 412 to the pyrolyzer only to
replenish the amount of limestone that has become unreactive
after a number of cycles through ~he combustor bed and pyrolyzer.
~his replenishment would be zero if the limestone were to re-
tain its reactivity, insofar as the circulating stream acts
as a sulfur carrier and does not actually remove sulfur from
the system~ On the other hand, sorbent added to the desul-
furizer bed 422 actually removes sulfur from the system, ac-
cording to reaction (3). The partially-spent sorbent in the
combustor bed is not wasted, but is disposed through line
430 to the upper desulfurizer bed, where it is used to absorb
S2 before being disposed of through line 426.
Hot combustion gases and ash are transported from
the deculfurizer bed through line 432 to ash cyclone 406,
where the ash is removed, and the gases then go via line 434
to afterburner 408, where they are mixed with the volatiles
from char cyclone 402 and are burned at very high temperatures
(e.g., 3000F).
One problem with operating the combustor bed at
temperatures in the vicinity of 1900F is that the sorbent may
sinter, reducing its reactivity after a number of cycles, and
clinkerincJ of the ash from some types of coal may occur. Ac-
cordingly ~he combustor bed can be operated at a lower tempera-
- 41 -
ture range from 1650 to 1800F to avoid these problems (and
to give added adt~antages of reduced emission of alkalis and an
easing of fireside corrosion problems), ~ut the efficiency
conversion of calcium sulfide to lime in the combustor bed may
also fall at these lower temperatures, although enough should
be converted to return to the pyrol~zer. Another problem is
that if the temperature is too low, CaS may be converted to
CaSO4 instead of CaO and the CaSO4 may form a eutectic with
CaS to produce aclinker o avoid having to dispose of a
large amount of CaS, which may cause formation of H2S at the
disposal site, a post-treatment station 436 is added to the
system. Instead of disposing partially-spent sorbent to the
desulfurizer bed by line 430, the sorbent is transported by
line 438 to station 436, where it is treated (e.g., by wash-
ing in hot carbanated water, as taught by G.P. Curran et al.
in "The ~onoco Process for Hot Desulfurization of Fuel Gas:
A Progress Report," Conoco Coal Development Co., as reported
in "Proceedings of the Four~h International Conference on
Fluidized-Bed Combustion," December 9 - 11, 1975, held at
20 Mitre Corp., McLean, Virginia, pp. 239-263~ to yield acceptably
CaS-free stone. Because the quantity of sorbent undergoing
post-treatment is relatively small, post-treatmen~ should be
an economically viable alternative to high-temperature regen-
eration.
Combustor-desulfurizer 404, whether or not a post-
treatment is used, is preferably operated as an adiabatic
fluid-bed combustor in that the~combustor will be cooled
with excess air ~typi.cally, 150~) added to the co~bustor air
(which enters through line 440) rather than by indirect cool-
0 iny with immersed tubes. Secondary air, added at line 442,
- 42 -
~'7~905
cools the gases emerging rom combustor bed 418 to the temperatureat which the desulfurizer bed best operates.
Further information regarding regeneration of sorbent
can be found in Moss U.S. Patent No. 3,870,480.
One modification of combustor-desulfurizer 404 or the
system of Fi~. 9 is to use immersed tubing in combustor bed 418 to
carry cooling air instead of operating the bed as an adiabatic
fluid-bed combustor with excess air added to the combustion air.
This modification would enable the char burner 404 and ash cyclone
406 to be ~ubstantially reduced in size. However, choice o~ this
modification must take into account the fact that the tubes may be-
come very hot and thereby be subject to corrosion.
- Another modification eliminates the recirculation pipe
428, whereby the limestone is fed through the system on a once-
-through basis. This simplifies the material handling problem,
but increases the amount of residue that must be treated at post-
-treatment reactor 436 if such a reactor is required.
In each of the above versions of Fig. 9, an attrition-
-resistant sorbent is required; otherwise, the sorbent will be
elutriated from combustion zone 418, or even be small enough to by-
pass char cyclone 402, in which case the sorbent may pass as CaS
through the afterburner 408 and associated furnace, eventually
contaminating the solid residue collected at the furnace's outlet
and perhaps making it unsuitable for landfill without further
treatment. Use of attrition-resistant sorbents may limit the
availability of suitable materials, and increase their cost. If
the same sorbent is used at both lines 412 and 424, as would com-
monly be done, the attrition-resistance of the
- 43 -
¢~
sorbent in desulfurizer bed 422 would cause that bed to ~ilt,
thereby increasing its sorbent consumption rate while reducing
its scrubbin~ efficiency, as has been described with combustor-
desulfurizer 200 when used to generate steam.
These problems may be avoided, and the use of
attrition~prone sorbents be used in a manner similar to their
use in combustor-desulfurizer used to generate steam, if a
separate desulfurizing bed (not shcwn) is interjected into line
420. Sorbent would now be added to the separate desulfurizer,
instead of at pyrolyzer 400, from which spent sorbent would
flow to burner-desulfurizer 404, entering it at line 416.
In the separate desulfurizer~ vo]atiles leaving char cyclone
402 would enter through a distributor plate at the vessel's
bottom, and leave at an exit port at the vessel's top. Sor-
bent flow would be in the opposite direction, to provide
counterflow reaction. The desulfurizer would be sized lar~e
enough to provide fixed-bed conditions, thereby minimizing
attrition wi~hin the reactor.
Other combinations will be obvious to those skilled
in the art. For example, in the system incorporating the
~eparate de~ulfurizer in line 420, it may be useful to add a
third distributor and bed to combustor 404. Char would be
combusted in the lower bed, as in Figure 9, and sorbent (mostly
Ca/S) would be co~verted to CaO in the middle bed, while the
removal of SO2 (reaction 3~ would be accomplished in the upper-
most bed. With this method, the fireside corrosion of air
ubes in the combustion bed would be minimized, because it
would remove the sorbent ~rom the vicinity of the tubes that
has heen observed to promote fireside corrosion, as has been
described.
- 44 -
The subj~ct matter claimed in this application is
also useful in single-bed combustors. In such devices, the
fluidizing velocity should be in the range of 5 to 10 times
minimum fluidization velocity (whereas in two-bed designs
fluidlzing velccity should be from 2 to 4 times minimum flui.di-
zation velocity).
Certain subject matter claimed in this application
is also useful in combustors wi.thout a separate desulfurization
bed, or to burn other than coal.
Claims
What is claimed is:
., .
- ~5 -
