Note: Descriptions are shown in the official language in which they were submitted.
FLUIDIZED ~ED HEAT EXCHANGER
HAVING SEPARATING DRAIN AND METHOD O~ OPERATION THER~OF
The subject matter of this application is closely associated
with tha-t of the ap~licant's pen~ing application Canadian Serial
No. 398,098, filed March ]1, 1982.
BACKGRO~ND OF THE INVENTION
This invention relates to a fluidized bed heat
exchanger and, more particularly, to a heat exchanger in which
heat is generated by the combustion of particulate fuel in a
fluidized bed and a method o~ operation thereof.
The use of iluidized beds has long been recognized
as an attractive way o~ generatiny heat. In a normal fluid-
ized bed arrangement, air is passed through a perforated plate
or grid supporting particulate material which usually includes
a mixture of a fuel material, such as high sulfur bituminous
coal, and an adsorbent material for adsorbing the sulfur
released as a result of the combustion of the coal. As a
result of the air passing through the bed, the bed behaves
like a boiling liquid which promotes the combustion of the
fuel. The basic advantages of such an arrangement include a
relatively high heat txansfer rate, substantially uniform bed
temperature, combustion at relatively low temperatures, ease of
~andling the coal, a reduction in corrosion and boiler fouling
and a reduction in boiler size.
In the fluidized bed combustion process, the coal
and adsorbent are continuously introduced into the bed by
suitable feeders, injectors, or the like, and coal ash and
adsorbent are discharged from the lo~/er portion of the bed,
usually through a gravity drain pipe having an entrance
re~istering ~ith a discharge opening formed through the
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perforated support plate and a distal end communicating with
a screw cooler, a conveyor belt, or the like. However, in
arrangements in which the size of the coal extends over a
relatively large range, relatively heavy pieces of coal ash
tend to migra~e to an area above the drain pipe and form a
dense area that is difficult, if not impossible, to fluidize.
As a result, the heavy pieces of coal ash do not drain, but
rather cause a clogging of the drain pipe and an attendant
severe curtailment in the operating efficiency of the bed.
Furthermore, in the operation of the fluidized bed,
in order to maximize heat transfer efficiency, it is desirable
to maintain close control over the level of material in the
bed. Precise control is difficult to achieve in a fluidized
bed in which new material is continuously being introduced, if
the drain tends to become clogged. Moreover, it is desirable
to not only maintain a continuously controllable discharge
through the drain, but it is also desirable to retain the
relatively light adsorbent material particles in the fluidized
bed, while permitting only the relatively heavy coal ash
particles to discharge through the drain. In this manner, the
adsorbent material is retained in the fluidized bed for a longer
time to adsorb more sulfur from the combustion of the coal and,
as a result, less new adsorbent material need be continuously
introduced. There is an acceptable loss or attrition of
adsorbent material in the normal operation of the bed by the
reduction of the adsorbent material to fine particle size due
to the boiling action of the bed and the grinding of the
particles against one another, and by the entrainment of the
fine adsorbent material particles in the fluidizing gas, by
which they are carried out through the flue.
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SU~lMARY OF THE INVENT~:ON
It is therefore an object of the present invention
to provide a fluidized bed heat exchanger and method of opera-ting
the same in which all of the coal is fluidized and in which
the coal ash is prevented from cloggin~ the drain pipe.
It is another object of the present invention to
provi.de a fluidized bed heat exchanger and method o~ operation
in which the coal ash is permitted to discharge thxough the
drain pipe, but the adsorbent material is prevented from
doing so.
It is still another object of the present invention
to provide a fluidized bed heat exchanger and method of
operation in which the level of the fluidized bed i5 precisely
controlled.
Toward the fulfillment of these and other objects,
the fluidized bed heat exchanger of the present invention
includes a perforated plate supporting a fluidized bed of
particulate material and a drain pipe to which a source of
compressed air is connected to f].ow upwardly through the drain
pipe and into the material of the fluidized bed above the drain
pipe, thereby preventing the heavy pieces of coal ash from
accumulating. The upward flow of air also results in a low
density area in the fluidized bed in a generally conical
region above the inlet to the drain pipe, thereby providing
less support for the particulate material in the region above
the drain pipe. Therefore, the heavier particles of the
fluidized bed tend to migrate toward the low density region
and to sink into the drain pipe. The flow of compressed air is
selected so that it comprises a separating air screen by which
the relatively light particles of adsorben-t materi.al are buoyed
and lifted upwardly, while the heavier coal ash particles are
pulled by gravity down through the upwardly flowing compressed
air into the drain pipe. Thus, the area in the fluidized bed
aro~nd the inlet to the drain pipe i5 kept free of any accumu-
lation of material, and the light adsorbent material particles
are retained in the fluidized bed, while the heavier coal ash
particles are allowed to continuously and freely discharge
through the drain pipe. Since the coal ash particles drain freely,
they discharge at a relatively constant rate, so that the rate
of particulate material flowing into the fluidized bed can be
adjusted, whereby the level of the fluidized bed can be pre-
cisely controlled~
In accordance with the principal object, the invention
contemplates a fluidized bed heat exchanger having a perforated
support plate to support a bed of particulate material including a
fuel material and an adsorbent material, and a means for intro-
d~ing air through the perforations to fluidize the particulate
material. ~ means starts combustion in the bed by which relatively
heavy fuel ash particles are formed, and a means introduces
additional particulate material to the bed. A drain having an
inlet in the bed discharges spent particulate material from the
bed. The improvement comprises a separating means for retaining
the adsorbent material particles in the fluidized be~ and for
permitting the relatively heavy fuel ash particles to fall into
the drain. The separating means comprises a means for providing
a flow of air through the drain and into the bed sufficier.t to
retain the adsorbent material particles in the fluidized bed
but insufficient to prevent the relatively heavy fuel ash
particles from falling into the drain. The air flow provides
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a means including an element connected to and annularly
arranged with respect to the drain. The element has at least one
perfora~ion in communication with the drain directing the flo~7
of air through the drain, and a means allows the relatively
heavy fuel ash particles to pass.
The invention also encompasses the novel method of
operating a fluldized bed having a perforated support plate
and a drain and comprises supplying particulate material to
the fluidized bed, including a fuel material and an adsorbent
1~ material. Fluidizing air is supplied to the fluidized bed
through the perforated support plate. Combustion is started
in the fluidized bed by which relatively heavy fuel ash parti-
cles are formed. An annular element is provided which has a
central aperture and at least one perforation in communication
with the drain. The fuel ash particles are discharged through
the drain while retaining the adsorbent material in the
~luidized bed, the discharging step comprises providing a
flow of air into the drain through the perforation through the
drain and into the fluidized bed sufficient to retain the
adsorbent material in the fluidized bed but insufficient to
prevent the relatively heavy fuel ash particles from falling
into the drain.
BRIEF DESCRIPTION OF THE DRAWIN~S
The above brief description, as well as further objects,
features and advantages of the present invention will be more
fully appreciated by reference to the following detailed
description of the presently pr~ferred but nonetheless illus-
trative embodiment in accordance with the present invention
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when taken in connection with the accompanying drawinys in
which:
Fig. 1 is a vertical sectional view of -the fluidized
bed heat exchanger of the present invention; and
Fiy. 2 is an enlarged cross-sectional view of the
drain pipe of Fig. 1.
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DETAILED DESCRIPTION OF THE P~EFERRED EMBODIMENT
As is illustrated in Fig. 1 of the drawings, the
reference numeral 6 refers in general to an enclosure forming
a major portion of a fluidized bed hea-t exchanger which may
be in the form of a boiler, a combustor, a process reactor
or any similar device. The enclosure 6 comprises a ~ront
wall 8, a rear wall 10, and two sidewalls, one of which is
shown by the reference numeral 12. Each wall is formed by a
plurality of vertically extending tubes 1~ disposed in spaced,
parallel relationship and connected together by a plurality of
elongated fins 16 extending for the entire lengths of the
tubes 14 and connected to diametrically opposed surfaces of
the tubes in a conventional manner. The upper portion of the
enclosure 6 is not shown for the convenience of presentation,
it being understood that it comprises a convection section, a
roof and an outlet for allowing the combustion gases to discharge,
also in a conventional manner.
A bed of particulate material, shown in general by
the reference numeral 18,is disposed within the heat exchanger
6 and rests on a plate 20 extending horizontally in the lower
portion o the heat exchanger and having a plurality of
perforations 21. The bed 18 can comprise a mixture of discrete
particles of fuel material, such as bituminous coal, and an
adsorbent, such as limestone, for adsorbing the sulfur released
by the combustion of the fuel material.
An air plenum 22 is provided immediately below the
perforated plate 20 and an air inlet pipe 24 is provided
through the plenum for distributing air from an e~xternal source
(not shown~ to the plenum under the control of a valve 26. Since
the valve 26 can be of a conventional design, it will not be
described in any further detail. A bed light-off burner 28 is
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mounted through the front wall 18 immediately above the plate
20 for initially lighting off the bed 18 during startup.
Three over~ed feeders 30, 32 and 34 are provided
which extend through a sidewall 12. The feeders 30, 32 and
34 receive particulate coal from inlet ducts or the like,
and are controlled by valves or other flo~ control devices
to feed the coal particles onto the upper surface of the bed
18. The feeders 30, 32 and 34 can operate by gravity discharge
or can be in the form of spreader feeders or any other similar
device. It is understood that feeders identical to the feeders
30, 32 and 34 and controlled by identical devices can also be
provided through one or more of the front wall 8, the rear
wall 10 and the other side wall 12, and that similar feeders
and control devices can also be provided for discharging the
adsorbent onto the bed 18.
A pair of horizontal headers 40 are connected in
fluid communication with the tubes 14 forming the front
wall 8 and the rear wall 10, and another pair of horizontal
headers 42 are connected in fluid corNmunication with the tubes
14 forming the side walls 12. It is understood that headers
similar to the headers 40 and 42 are provided in communication
with the upper ends of the walls 8, 10 and 12. As a result,
a fluid to be heated can be sequentially or simultaneously
passed through the walls 8, 10 and 12 to pick up the heat
from the fluidized bed in a conventional manner.
As can be seen in greater detail in Fig~ 2, a drain
43 extends through the air plenum 22 and includes an outer
pipe 44, an inner pipe 46 defining a throat concentrically
disposed within the outer pipe 44, and a bevelled collar 48
secured between the outer and inner discharge pipes 44 and 46
at their upper ends so that an upper edge of the bevelled
collar 48 is level with the lower surface of the perforated
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plate 20 to form a gradually narrowing inlet for the
particulate coal ash entering the drain 43. The inlet is
positioned at an opening 49 defined in the perforated plate 20.
The bevelled collar 48 may be secured in any suitable manner,
as by threadedly connecting the bevelled collar to th~ e~ternal
surface of the inner pipe 46 and welding the bevelled collar to
the outer pipe ~4. The outer pipe 4~ is supported by a threaded
connection or other suitable connection to an annular flange
S0 depending from the lower surface of the perforated plate
20. The outer and inner pipes 44 and g6 extend downwardly
throuyh a bottom wall 52 o~ the plenum 22 where they are
guided by a collar 54 interposed between the outer pipe 44 and
an annular flange 56 depending from the lower wall 52 of the
plenum 22. The outer and inner pipes 44 and 46 terminate
at lower ends which are welded or otherwise suitably secured
to a flat annular plate 58 which extends radially outward from
the lower end of the inner pipe 46. The flat annular plate 58
includes a plurality of apertures 62 spaced o~tward from the
outer pipe 4~ for receiving fasteners, such as nuts and bolts
63, to connect the flange 58 to a compressed air inlet assembly 64.
The compressed air inlet assembly 64 includes a compres-
sed air inlet pipe 66 having a lateral inlet port 68 which is
connected to a source of compressed air (not shown), the flow of
compressed air to the inlet pipe 66 being controlled by a valve 70.
The compressed air inlet pipe 66 includes a radially extending
upper flange 72 including a plurality of apertures 74 by which the
nuts and bolts 63 can connect the flange 72 to the flange 58. An
inner pipe 76, including at its upper end an outwardly flaring
frustoconical plate 78 having a plurality of perforations 80, is
positioned concentrically within the compressed air inlet pipe 66.
The frustoconical plate 78 includes a central aperture 81 through
which the draining coal ash particles can pass. The upper
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end of -the frustoconical plate 78 includes an outwardly
extending flange 82 which overlies the flange 72 and includes
apertures 84 in alignment with the apertures 74, so that the
flange 82 can be clamped between the flanges 72 and 58 when
the appropriate fasteners are ins-talled. A lower annular flange
86 extends radially outward from the lower end of the inner
pipe 76 beyond the air inlet pipe 66 so as to define, with the
air inlet pipe 66, the inner pipe 76 and the frustoconical plate
78, an annular plenum chamber 88.
A suitable device, such as a rotary feeder 90, is
secured at the lower end of the inner pipe 76 to control the
discharge of the coal ash. Although a rotary feeder has been
indicated in the drawings, other suitable discharge devices,
such as screw feeders, can be employed.
In operation, the valve 26 associated with the air
inlet pipe 24 is opened to allow air to pass up through the
plenum 22 and through the perforations 21 in the perforated
plate 20. The light-off burner 28 is then fired to heat the
material in the bed until the temperature of the material
reaches a predetermined level, whereby combustion is started
and relatively heavy coal ash particles begin to form, at
which time particulate fuel is discharged from the feeders 30,
32 and 34, and adsorbent material is discharged from other
feeders (not shown) onto the upper surface of the bed 18 as
needed.
After the bed 18 has been fluidized and has reached
a predetermined elevated temperature in accordance with the
foregoing, the light-off burner 28 is turned off while the
feeders 30, 32 and 34 continue to distribute particulate fuel
to the upper surface of the bed in accordance with predetermined
feed rates. ~luid, such as water, to be heated is passecl into
the headers 40 and 42 where it passes simultaneously, or in
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sequence, th~ough the tubes 14 forming the walls 8, 10 and 12
to add heat from the fluidized bed to the fluid before it is
passed to external apparatus for further processing.
Compressed air is admitted to the annular plenuJn
chamber 88 through the ccmpressed air inlet port 68 by the
manipulation of the valve 70. The compressed air flows
through the perforations 80 in the frustoconical plate 78 and
increases in veloci~y when it enters the throat defined by the
inner drain pipe 46, from which it flows di~ectly upward
through the fluidized bed 18, creating a generally conical
low density region in the fluidized bed above the drain 43.
There is a greater volume of compressed air flowing upwardly
over the drain 43 than in any other region of the fluidized bed
18. As a result, the bed material over the drain ~3 is
prevented from accumulating around the inlet to the drain. In
addition, the material of the fluidized bed 18 tends to migrate
toward the low density region over the drain 43. Furthermore,
the diameter of the inner drain pipe 46 and the flow of air
from the compressed air source are selected so that the air
flowing up through the drain 43 defines an air screen separating
the relatively llghtweight adsorbent material particles from the
heavier coal ash particles. Thus, when the particulate material
of the fluidized bed 18 moves into the low density region over
the drain 43, the flow of compressed air from the drain forces
the relatively lightwei~ht adsorbent material particles upward,
but permits the heavier coal ash particles to sink into the drain
43, from which they are discharged by the rotary feeder 90 or
other suitable discharge device.
There is an inherent rate of attrition of the
adsorbent particles due to their reduc-tion to fine size by the
collisions and abrasions of the boiling action of the fluidized
bed 18 and the resultant entrainment of the fine adsorbent
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~articles by -the fluidizing air, in which they are carried out
through the flue. Thus, the adsorbent particles are normally
elimina-ted in the flue yas, the coal ash particles are
continuously discharged through the drain 43 and additional
adsorbent and coal is continuously supplied to the top of
the fluidized bed 18 to maintain a continuous circulation of
material and a constant level in the fluidized bed 18. Since
the flow of coal ash particles into the drain 43 occurs freely,
rather than being unpredictably restricted or blocked by
accumulations around the entrance to the drain 43, it occurs
at a relatively constant rate, so that the rate of particulate
material being fed to the fluidized bed 18 can be adjusted,
whereby the level of the fluidized bed 18 can be precisely
controlled. If it is desired to increase the rate of removal
of the adsorbent material from the fluidized bed 18, the flow
of compressed air through the drain pipe 43 can be reduced so
that the lighter adsorbent material particles will fall through
the drain pipe 43 along with the heavier coal ash particles.
By adjusting the amount of compressed air flowing through the
drain 43 into the fluidized bed 18 the size of particles which
will be allowed to fall through the drain 43 can be controlled.
It is understood that other modifications, changes
and substitutions are intended in the foregoing disclosure and,
in some instances, some features of the invention will be
employed without a corresponding use of other features. According-
ly, it is appropriate that the appended claims be construed
broadly and in a manner consistent with the spirit and scope of
the invention.
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