Note: Descriptions are shown in the official language in which they were submitted.
~3~.308i!3
A STEA~ GENERATOR AND METHOD OF OPERATING SAME
UTILIZING SEPARATE FLUID
AND COMBINED_ AS FLOW CIRCUITS
-~ackqround of_the Invention
5This invention relates to a steam generator and a
method of operating same in which heat is generated by the
combustion of fuel in a plurality of fluidized heds.
Steam generating systems utilizing fluidized beds as
the primary source of heat generation are well known. In
these arrangements, air is passed through a bed of
particulate material, including a fossil fuel such as coal
and an adsorbent for the sulfur generated as a result of
combustion of the coal, to fluidize the bed and to promote
the combustion of the fuel at a relatively low
temperature. The heat produced by the fluidized bed is
: utilized to convert water to steam which results in an
~ attractive combination of high heat release, high sulfur
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adsorption, low nitrogen oxides emissions and fuel
flexibility.
The most typical fluidized bed combustion system is
commonly referred to as a bubbling fluidized bed in which
a bed of particulate materials is supported by an air
distribution plate, to which combustion-supporting air is
introduced through a plurality of perforations in the
plate, causing the material to e~pand and take on a
suspended, or fluidized, state. In a steam generator
environment, the walls enclosing the bed are formed by a
plurality of heat transfer tubes, and the heat produced by
combustion within the fluidized bed is transferred to
water circulating through the tubes. The heat transfer
tubes are usually connected to a natural water circulation
circuitry, including a steam drum, for separating water
from the steam thus formed which is routed to a turbine or
to another steam user.
In an effort to extend the improvements in combustion
efficiency, pollutant emissions control, and operation
turn-down afforded by the bubbling bed, a fluidized bed
reactor has been developed utilizing a circulating
fluidized bed process. According to this process,
fluidized bed densities between 5 and 20% volume of solids
are attained which is well below the 30% volume of~solids
_3_ ~3~3~88
typical of the bubbling fluidized bed. The formation of
the low density circulating fluidized bed is due to its
small particle size and to a high solids throughput, which
require high solids recycle. The velocity range of a
ci~culating fluidized bed is between the solids terminal,
or free fall, velocity and a velocity beyond which the bed
would be converted into a pneumatic transport line.
The high solids circulation required by the
circulating fluidized bed makes it insensitive to fuel
heat release patterns, thus minimizing the variation of
the temperature within the steam generator, and therefore
decreasing the nitrogen oxides formation while increasing
sulfur dioxide adsorption. Also, the high solids loading
results in an increase in sulfur adsorbent and fuel
residence time, which reduces the adsorbent and fuel
consumption.
However the circulating fluidized bed process is not
without problems, especially when used in a steam
generation environment. For example, it normally lacks a
method of independently controlling the outlet temperature
of the reheat as compared to the temperature of the main
steam and~or superheat, especially when it is necessary to
heat both of these fluids to temperatures of 950F or
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higher and maintain these temperature levels over a wide
control range without excessive attemperation.
United States Patent Number 4,665,864 dated May 19,
1987 addresses this problem by providing a steam generator
and a method of operating same in which a flow circuit is
provided for the reheat steam which is independent of the
circuitry for the other steam stages. In this
arrangement, an independently fired fluidized bed is
provided to directly control the temperature of the reheat
steam, and separate fluidized beds are provided for
controlling the steam generation rate and the temperature
of the superheat steam.
Summary~of the Invention
The object of the present invention is to provide a
steam generator and method which further improves the
steam generator and method disclosed in the
above-identified patent by reducing the amount of heat
exchange surface required in the circulating fluidized bed
for a given heat input to that bed by removing heat from
the recycled fly ash.
It is another object of the present invention to
provide an improved steam generator and method of the
above type in which a greater percentage of combustion
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13~L3~)8~3
takes place in the circulating bed portion of the
generator to allow a reduction in the firing rate of the
bubbling bed section to take advantage of the better fuel
combustion and limestone utilization inherent in the
circulating bed.
It is a further object of the present invention to
provide an improved steam generator and method of the
above type in which the overall immersed tubing required
b~ the fluidized bed is reduced by utilizing an unfired
bed operating at a low fluidizing velocity.
A still further object of the present invention is to
provide a steam generator and method of the above type in
which the size and firing rate of the bubbling bed section
of the steam generator is reduced so that the generator
has a broader tolerance to small fuel and limestone
particle sizes.
It is a still further object of the present invention
to provide a steam generator and method of the above type
in which a greater percentage of combustion and sulfur
capture occurs in the circulating fluidized bed section of
the steam generator where fuel and limestone fine
particulate can be retained to reduce the
calcium-to-sulphur ratio required to achieve sulphur
dioxide emissions.
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-6- ~3~8~
It is a still further object of the present invention
to provide a steam generator and method of the above type
in which the circulating fluidized bed section is
relatively large to improve the fuel carbon utilization.
It is a still further object of the present invention
to provide a steam generator and method of the above type
in which the overall height of the furnace may be reduced
by virtue of cooling the recycled fly ash in an unfired
fluidized bed.
Toward the fulfillment of these and other objects,
two beds of particulate material containing fuel are
established in a vessel and the fuel is combusted in each
of the beds while air and additional fuel is added to the
beds to fluidize the beds and promote combustion of the
fuel. A mixture of flue gases and the entrained
particulate material from the beds are combined and the
entrained particulate rnaterials are separated from the
flue ~ases. The separated particulate material is passed
to a third bed in the vessel and air is introduced to the
latter bed to fluidize the separated material. This air
along with any immersed heat exchange surface located in
the said third bed cools the separated particulate
material.
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More particularly the invention in one aspect
comprehends a method of operating a steam generator
comprising the steps of forming at least two beds of
particulate material containing fuel in a vessel,
combusting the ~uel in each of said beds, introducing air
and additional fuel into each of said beds to fluidize
said beds to promote the combustion of said fuel,
establishing a first flow circuit for passing water in a
heat exchange relation to at last one of said beds for
converting said water to steam, combining a mixture of
flue gases and the entrained particulate material from
both of said beds, separating said entrained particulate
material from the flue gases of said combined mixture,
passing said separated particulate material to a third
bed in said vessel, and introducing air to said third bed
to fluidize and cool said separated particulate material.
Another aspect of the invention pertains to a steam
generator comprising a vessel, means of forming at least
two beds of particulate material containing fuel in said
vessel, means for introducing air and fuel into ea~h of
said beds to fluidize said beds and combust said fuel,
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- 6B -
first flow circuit means for passing water in a heat
exchange relation to one of said beds for converting said
water to steam, means for combining the flue gases and
the entrained particulate material from both of said
beds, means for separating said entrained particulate
materials from the flue gases of said combined mixture,
means for forming a third bed, means for passing said
separated particulate material into said third bed, means
for introducing air to said third bed to fluidize said
separated parti.culate material, and means for cooling
said separated particulate material.
_7_ ~3~3~
Brief Description of the Drawinqs
The above brief description as well as further
objects, features and advantages of the method of the
present invention will be more fully appreciated by
reference to the following detailed description of
presently preferred but nonetheless illustrative
embodiments in accordance with the present invention when
taken in conjunction with the accompanying drawing in
which:
Fig. l is a schematic side elevation view of a forced
circulation steam generator employing features of the
present invention; and
Figs. 2-~ are schematic plan views of a portion of
the steam generator of Fig. l depicting alternate
embodiments of the present invention, with portions of the
steam generator being deleted for the convenience of
presentation.
Description of the Preferred Embodiment
Referring specifically to Fig. l of the drawing, the
; 20 reference numeral 10 depicts, in general, a forced
circulation steam generator according to the present
invention including a plurality of elongated
vertically-extending steel support columns such as shown
-8- ~3~3~8
by reference numerals 12, 19, and 16 extending from the
floor 18 of the generator to a plurality of spaced
horizontally-extending beams, one of which is shown by the
reference numeral 20, which define the ceiling of the
generator. A plurality of hangers ~2 extend downwardly
from the beam 20 for supporting a steam drum 24 having a
plurality of downcomers extending downwardly therefrom,
one of which is shown by the reference numeral 26. A
plurality of additional hangers 28 extend downwardly from
the beam 20 for supporting a heat recovery portion of the
generator 10 which will be described in detail later.
Two fluidized bed chambers A and B are supported in
the lower portion of the generator 10 by a bottom support
system (not shown) of a conventional design.
Two air distribution plates 30 and 31 extend
horizontally through the entire width of each of
chambers A and B, respectively. Air plenums 32 and 39
extend immediately below the chambers A and B,
respectively, for introducing air upwardly through the
corresponding portions of the air distribution plates 30
and 31, respectively, into the chambers.
The chamber A is defined by the air distribution
plate 30, a pair of vertically-extending spaced walls 36
-9- ~L313~8~
and 38 and a diagonally-extending roof 40 while the
chamber B is defined by the air distribution plate 31, the
wall 38, and a vertically-extending wall 42 disposed in a
spaced relation to the wall 38.
A third chamber C is defined by a vertical wall 44
extending in a spaced relation to the wall 42, a
distribution plate 45 and an upper, diagonal roof 46. An
air plenum 47 extends below the plate 45 for directing air
upwardly through the plate into the chamber C. It is
understood that a pair of spaced sidewalls (not shown) are
provided which, together with the walls 36, 38, 42, and 44
form an enclosure, and that these sidewalls, along with
the latter walls are formed by a plurality of
vertically-extending waterwall tubes connected in an
air-tight relationship.
Bundles of heat exchange tubes 48 and 50 are provided
in the chambers A and C, respectively for circulating
Eluid through the chambers as will be described in detail
later.
The walls 38 and 42 extend for substantially the
entire height of the generator 10 and an opening 52 is
provided through the wall 38 in order to permit the flue
gases from the chamber A to flow to the chamber B where
- 1 o - ~313~8 !3
they mix with those from the chamber B. The flue gases
then pass upwardly in the chamber B before passing through
one or more openings 53 provided in the wall 92 and into a
cyclone separator 54 disposed above the chamber C. The
separator 54 includes a funnel portion 56 having a
dipleg 58 connected to its lower outlet end and extending
through the roof 46 and into the lower portion of the
chamber C for reasons to be described later. It is
understood that additional separators 54 can be provided
as needed.
A heat recovery area, shown in general by the
reference numeral 64, is disposed adjacent the upper
portion of the chamber B on the side thereof opposite that
of the cyclone separator 54. The heat recovery area 64 is
defined by a vertical wall 66 extending in a spaced
relationship to the wall 38 and a substantially horizontal
roof 68 which spans the heat recovery area, the chamber B,
and the cyclone separator 54.
A wall 69 extends across the top of the cyclone
separator 54 and the top of the chamber B and, together
with the wall 68, defines a duct for passing gases from
the cyclone separator 54 to the heat recovery area, as
will be described later. The walls 66, 68, and 69 may
~3~3~)~8
also be formed by a plurality of waterwall tubes connected
in an air tight manner.
A gas control damper system 70 is disposed in the
lower portion of the heat recovery area 64 and controls
the flow of gas throuqh the heat recovery area in a manner
to be described, before the gas passes over a tube
bundle 72 and exits from a flue gas duct 74 to an air
heater (not shown) in a manner also to be described in
detail later.
A pump 76 is connected to the lower portion of the
downcomers 26 of the steam drum 24. Since more than one
downcomer 26 and pump 76 can be provided, manifolds 78 are
connected to the inlet and-outlet of the pump(s) 76 for
supplying water from the steam drum 24 to a plurality of
substantially horizontally and vertically extending water
lines, one of each of which are shown by the reference
numerals 80 and 82.
A plurality of vertical feeders 83, one of which is
shown in the drawing, extend from the water lines 80 and
are connected to a header 84 which supplies water to a
water tube wall 85 disposed in the heat recovery area 64,
it being understood that other vertical feeders are
connected to the water lines 80 for supplyinq water to the
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sidewalls (not shown) of the heat recovery area 64.
A plurality of feeders 86 extend from the water
lines 80 and are connected to headers (not shown) forming
portions of a pair of seal assemblies 88 associated with
each wall 38 and 42. The seal assemblies 88 function to
accommodate relative differential expansion between the
lower portion of the steam generator 10 supported from the
floor 18 and the upper portion of the steam generator
top-supported by the hangers 22 and 28. Since the seal
assemblies 88 are fully disclosed in co-pending
CDN. Patent Application Serial No. 499,894 filed on Jan-
uary 20, 1986 and assigned to the same assignee as the
present invention, they will not be described in any
further detail. It is understood that the headers
associated with the seal assemblies 88 supply water to the
waterwall tubes forming the upper portions of the walls 38
and 42.
An additional feeder 94 extends from each of the
water lines 80 and supplies a header 96 for circulating
water through a water tube wall 98 which, together with
the walls 42 and 69, and the sidewalls (not shown),
enclose the cyclone separator 59.
-13- ~3~3088
The vertical water lines 82 are respectively
connected to horizontal water conduits 100 each of which
has a plurality of vertically-extending feeders 102
extending therefrom which are connected to headers 104
connected to the lower ends of the walls 36, 38 and 42 for
supplying water to the wall 36 and the lower portions of
the walls 38 and 42. Additional feeders 106 supply water
from the water conduits 100 to corresponding headers 108
for the bundle of water tubes 48 in the chamber A.
A pipe 110 extends from a boiler feed pumping and
preheating system (not shown) to an inlet header 112 for
the tube bundle 72. The outlet of the tube bundle 72 is
connected, via a header 114, a transfer line 116, and an
inlet header 118 to a bundle of water tubes 120 disposed
within the heat recovery area 64 and functioning as a
economizer. The outlet of the tube bundle 120 is
connected, via a header 122 and a transfer line
conduit 124, to the inlet of the steam drum 24. Water
flow through the circuit of the present invention is
established from the pumping and preheating system into
and through the tube bundle 72, the tube bundle 120, and
into the steam drum 24. This water is mixed with the water
separated from the water/steam mixture supplied to the
.
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-14- ~313~8~
drum 24 and the resulting water passes through the
downcomer 26 and, via the pump(s) 76, into the
manifold 78. The water then passes from the manifold 78
through the water lines 80, the feeders 83 and 94, and to
the waterwalls 66, 85, 38, 42, and 98. The water lines 82
supply water, via the conduits 100 and the feeders 102 and
106 to the walls 36, 38, and 42, and to the tube bundle 48.
The reference numeral 130 refers to a plurality of
headers disposed at the upper end portions of the
10 walls 66, 85, 38, 42, and 98, it being understood that the
side walls associated with the heat recovery area 64, the
chamber B and the cyclone separator 54 would have similar
type headers. A plurality of risers 132 extend upwardly -
from the headers 130 and connect with a conduit 133 which
15 extends from the endmost riser 132 to the steam drum 24 to
transfer the fluid from the various headers in the wall
into the steam drum.
The water passing through the walls 36 and 38 is
partially converted to steam and passed to a pair of
headers 134 while the water passing through the tube
bundle 98 is also partially converted to steam and passed
to a plurality of outlet headers, one of which is shown by
-15- 13~3~
the reference numeral 135. The steam from the header 135
passes into a conduit 136 and the steam from the
headers 134 is passed, via conduit 137, to the
conduit 136. The conduit 136 is connected to the steam
drum 24 so that the steam from the headers 134 and 135
mixes with the steam entering the steam drum 24 from the
conduit 133 in the manner described above.
The superheat circuitry includes a bundle of
tubes 140 functioning as a primary superheater disposed in
the heat recovery area 64 and having an inlet header 142
connected, via a conduit 149, to the outlet of the steam
drum 29. After passing through the tube bundle 140 the
superheated steam exits, via a header 146. Although not
shown in the drawing it is understood that a spray
attemperator, or the like, is connected to the header 146
to reduce the temperature of the steam as necessary before
the steam is introduced to a finishing superheater.
Although not shown in the drawings in the interest of
clarity, it is understood that the finishing superheater
may be formed by an additional tube bundle in the
chamber A or by a tube bundle in the chamber C. ~fter
passing through this superheater, the steam is then passed
to the inlet of a turbine. Thus the finishing superheater
-16- ~3~3088
circuit is independent of the steam generating circuit
described above.
A plurality of tubes forming bundles 150 and 162 are
provided in the heat recovery area 64 and each bundle
functions as a reheater. One or two conduits, one of
which is shown by the reference numeral 1~4, extend from
the high pressure turbine (not shown) and is connected to
an inlet header 166 which is connected to the tubes
forming the tube bundles 160 and 162. After passing
through the tube bundles 160 and 162 the reheated steam is
passed to an outlet header 172 which, in turn, is
connected, via one or two conduits 174, to a low pressure
turbine (not shown). It is noted that this reheat flow
circuitry is entirely independent from the steam
generating flow circuitry and superheat circuitry
described above.
Air from one or more forced draft fans 180 is passed,
via a duct 182 and a plurality of vertical ducts 184 to
the plenums 32, 34 and 47 e~tending below the chambers A,
B, and C, respectively. Although omitted from the
drawings in the interest of clarity, it is understood that
a bed of particulate material is disposed in each of the
chambers A, B, and C which is fluidized in respons~e to the
-17- ~3~
air passing upwardly from the plenums 32, 34 and 47,
respectively, through the air distribution plates 30, 3L,
and 45 and into the latter chambers. It is also
understood that each chamber A, B, and C may be subdivided
by partitions, or the like (not shown), into segments that
are used during start-up and for load control of the steam
generator 10.
The fluidizing velocity of the air introduced into
the bed in the chamber A is regulated in accordance with
the size of the particles in the bed so that the
particulate material in the chambers A and B is fluidized
in a manner to create a "bubbling" bed with a minimum of
particles being entrained by the air and gases passing
through the bed. The velocity of the air introduced into
the chamber B relative to the particle size in the bed is
such that a "circulating" bed is formed, i.e. a bed in
which the particulate material in the bed is fluidized to
an extent that the combustion gas is very near saturation
solids capacity for the entire length of the chamber B.
Chamber C is an inactive, or unfired, bed in that there is
no introduction o particulate fresh fuel or adsorbent
into the bed and no ignition of the fuel so that the air
passing through the bed serves to cool the particulate
material forming the bed, as will be explained.
-18- ~3~ 8
The fuel introduced to the beds in the chambers A and
B is ignited and additional fuel and adsorbent is added to
the beds by conventional feeders (not shown). The
resulting flue gases, which include the gaseous products
of combustion and the air passing through the beds,
entrain a small portion of the relatively fine particulate
material in the latter chambers. The resulting mixture of
flue gases and particulate material in the chamber A
passes through the opening 52 in the wall 38 and into the
chamber B where it combines with a similar mixture in the
latter chamber. As indicated above, the velocity of the
air passing, via the plenum 34, into the chamber B is such
relative to the size of the particles in the latter
chamber such that the particles are suspended in the flue
gases and eventually transported up~ardly through the
length of the chamber B. The flue gases with the
entrained particles exit through the opening 53 formed in
the upper portion of the wall 42 before passing into the
cyclone separator 59. It is noted that, by virtue of the
fact that chamber B is located between the chambers A and
C, the fluidized bed in the chamber A is thermally
isolated from the fluidized bed in the chamber C.
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The particulate material is separated from the gases
in the cyclone separator 54 and the gases pass upwardly
into the conduit defined between the walls 68 and 69,
through openings formed in the walls 42 and 38 and into
the heat recovery area 64. A portion of the gases in the
heat recovery area 64 passes through the wall 85 which has
openings formed therein for this purpose, before the gases
pass over the tube bundles 140 and 120 forming the primary
superheater and the economizer, respectively. The
remaining gases pass over the tube bundles 162 and 160
forming the reheaters.
The gases passing through the heat recovery area 64
in the foregoing manner then pass through the damper
system 70, which can be adjusted as necessary to control
the fraction of the total gas flow that is to flow across
the tube bundle 140 forming the primary superheater and
the tube bundle 120 forming the economizer. The gases
then pass across the tube bundle 72 and through the outlet
duct 74. It is understood that the duct 74 can be
connected to an air heater (not shown) where the gases
give up heat to the air from the forced draft fan 180
before the gases exit to a dust collector, induced draft
fan, and/or stack (not shown). The air from the fan 180
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is thus preheated before passing to the duct 182, as
described above.
The solid particulate material separated in the
cyclone separator 54 falls into the funnel portion 56 of
the separator before discharging, via the dipleg 58, into
the chamber C. Thus, a bed of the relative hot separated
particulate material from the sepafator 54 is formed in
the chamber C and is cooled by the cooling air passing
from the plenum 47 through the plate 95 and fluidizing the
material. Also, water from an external source, water from
the previously mentioned boiler feed pumping system or
steam from the superheat circuitry, is circulated through
the tubes 50 to further cool the separated particulate
material in the chamber C.
The method of the present invention provides several
advantages. For example, the reheat circuitry is entirely
independent of the steam generating circuitry and the
superheat circuitry depicted. Moreover, the use of the
three separate fluidized beds in the chambers A, B and C
enables the temperatures in each bed to be controlled
independently. Also, by removing heat from the separated
particulate material in the unfired bed C, the amount of
heat exchange surface required in the chamber B per unit
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-21- 13~3~8
heat input to chamber B is reduced. ~urther, since the
amount of heat exchange surface per unit heat input to
chamber B is reduced, it becomes practical to increase the
firing rate of chamber B. This allows a reduction in the
firing rate, area and surface of the bubbling fluidized
bed in chamber A. Further, the use of the unfired bed of
particulate material in chamber C operating at a
relatively low fluidizing velocity and consisting of
relatively fine particles of the separated particulate
material, results in a much higher in-bed tube heat
transfer coefficient, thus reducing the overall immersed
tubing required by the fluidized beds in chambers A and
C. Also, since the reaction performance of the bubbling
fluidized bed in chamber A is more sensitive to small
particle sizes, the reduction in area and firing rate for
this chamber enables the steam generator of the present
invention to be more tolerant to small fuel and limestone
particle sizes.
Still further, by allowing a greater percentage o
combustion to occur in the circulating fluidized bed in
chamber B, where relatively fine particulate fuel material
can be used, the e~ficiency of sulfur capture by adsorbent
: is increased to reduce the percentage of sulphur dioxide
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emissions. Also, the method of the present invention
improves the fuel carbon utilization as a result of the
larger circulating fluidized bed area in chamber B.
Finally, the overall height of the furnace can be reduced
by virtue of the cooling of the separated particulate
material in bed C.
Figures 2-4 depict alternate embodiments of the
present invention and, for the convenience of
presentation, only top plan views showing the chambers A,
B and C, the funnel portion 56 of the cyclone separator 54
and the dipleg 58 are shown. According to the embodiment
of Fig. 2, the dipleg 58 of the funnel portion 56 of the
cyclone separator 54 tnot shown) is located in a corner of
the chamber C. A dipleg 58' of another funnel portion 56'
of another separator is located in another corner of the
chamber C to provide for the introduction of the separated
particulate material from two cyclone separators.
According to the embodiment of Fig. 3, chambers A and
B are in a side-by-side relationship as in the embodiment
of Figs. 1 and 2, but the unfired chamber C is located in
a plane behind that of chamber B. As in the previous
embodiment, the two diplegs 58 and 58' of the funnel
portions 56 and 56' of two cyclone separators are located
in two corners of the chamber C.
1313~8~3
-23-
According to the embodiment of Fig. 4, there are two
unfired chambers Cl and C2 disposed to the side of chamber
B and spaced apart in a front-to-rear relationship as
shown. The funnel portion 56 of the cyclone separator 54
is positioned over the chamber Cl with its dipleg 58
extending slightly in front thereof. A funnel portion 56'
of another cyclone separator is provided with its dipleg
58' extending between the two chambers, and still another
separator and funnel portion 56" is disposed over the
chamber C2 with its dipleg 58" located slightly to the
rear of chamber C2.
It is understood that the funnel portions 56, 56',
and 56" are located relative to the opening 53 in the wall
~2 and to the walls 68 and 69 so that the mixture of flue
gases and particulate material in the chamber B passes
into each of their corresponding separators and so that
the separated gases pass from the separators to the
conduit defined between the walls 68 and 69, as discussed
above.
It is understood that several other variations rnay be
made in the foregoing without departing from the scope of
the invention. For example, the cyclone separator 54 can
extend between the heat recovery area 64 and the chambers
A, ~ and C, and the diplegs 58 do not have to extend
-2q- ~3~3~38
directly into the chamber C but can go through a
conventional seal pot or the like disposed externally of
the chamber C and another dipleg can be provided that
extends from the seal pot into the chamber C.
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. Accordingly, it is
appropriate that the appended claims be construed broadly
and in a manner consistent with the scope of the invention
therein.
