Note: Descriptions are shown in the official language in which they were submitted.
lOq~7~7
The present invention relates generally to so-called
fluidized bed combustion and, more particularly, to certain
new and useful improvements in fluidized bed boiler apparatuses
and methods for carrying out and operating same.
Combustible materials, such as a variety of fossil
fuels, have been combusted in fluidized bed apparatuses, such
as boilers, to generate heat by passing a heat-exchange fluid,
such as water, in a heat-exchange relationship through the
fluidized bed to obtain steam from the water. Fluidized bed
combustion is well-known and is and has been the subject of
extensive development (see "Fluidized-Bed Combustion Review"
by H. Nack et al., Batelle, Columbus, Ohio, presented at
International Conference on Fluidization June 1975, Asilomar,
California.
Typically, a fluidized bed boiler has a combustion
chamber in which a particulate fuel, such as coal, is intro-
duced to a fluidized bed of particulate material by passing
air through the bed to promote the combustion of the fuel.
The fluidized bed generally includes inert particulate parti-
cles, and may also include dolomite, limestone or othermaterials which serve to absorb or to react with the sulfur
or other undesirable additives in the fuel to be combusted.
The fluidized bed is often ~uite shallow and has heat-exchange
tubes immersed in the fluidized bed to effect heat transfer
from the heated particles to the water passing through the
heat exchange tubes. In some cases, additional tubes with
water are placed in the space above the bed.
Fluidized bed combustion generally operates at
temperatures of 1500 to 1700F, which are milder conditions
than those encountered in conventional nonfluidized bed
boilers, and thus nitrous oxide gases are considerably reduced.
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Such fluidized bed boilers are quite useful, particularly with
solid fuel, such as coal, since they eliminate the premixing
of the air in the fuel and lead to lower temperature condi-
tions, thus, eliminating slag deposits on the cooling surface,
and at the same time providing for very high heat-transfer
-rate to the surface.
In the operation of various prior art fluidized bed
combustion apparatuses, such as incinerators and boilers, a
portion of the fluidized bed has been recirculated by various
techniques to improve heat output or apparatus performance,
such as, for example, the recirculation of a bed within an
incinerator as set forth in U.S. Patent 3,702,595, and the
recirculation of a fluidized bed comprising a coal- and solid-
absorbent material in a stacked fluidized bed arrangement, as
set forth, in example, in U.S. Patent 3,905,336, and the
recirculation internally within a fluidized bed apparatus as,
for example, in U.S. Patent 3,910,235.
However, there are certain problems associated with
known fluidized bed boilers and the pressure conditions under
which they operate, particularly where heat exchange tubes
are inserted into the fluidized bed. Such heat -exchange
tubes often obstruct the flow of the fluidized bed particles,
present difficult support problems at the temperature condi-
tions employed, and restrict fluidization of the bed by the
creation of dead spots therein. Fluidization in such systems
has been improved by increasing the air velocity through the
bed of solids, but this creates a further problem in that the
finer bed particles tend to be carried out of the fluidized
bed. To prevent excess fluidized bed particle carryover,
larger-size particles in the fluidized bed have been employed
that is particles with an average particle size of about 500
67~7
microns, and typically 300 to 450 microns in size, in compari-
son to the usual average particle range of about 100 to 150
microns. The employment of larger particles, however, reduces
the heat-transfer rate due to the reduced surface area per
given weight of the larger-size particles, and also creates
unsteady flow conditions in the bed, resulting in large bubble
formations, thereby reducing the efficiency of contact and
efficiency of combustion. Therefore, in most fluidized bed
boilers, a compromise is employed wherein velocity, particle -^
size, operating pressure and the use of the form, number and
shape of the heat exchange tubes placed in the bed are balanced
to arrive at a compromise on the heat efficiency desired in
the particular boiler.
Another drawback of prior art fluidized bed boilers
is the relatively low number of control points for enabling
various parameters, such as temperature and particularly,
operating capacity, to be controlled. In addition, current
proposals for increasing overall capacity include stacking
several fluidized beds over each other, while slumping some
of the beds to control capacity. However,not only is such
configuration severely limited by vertical dimension con-
straints but there will also be structural support restric-
tions, particularly in view of the operating temperatures,
all of which serve to limit the practicality of this approach.
Another proposal involves passing fluidized bed
particles to a chamber having heat exchange tubes positioned
therein. However, the tubes tend to obstruct flow and, more
importantly, a gas bubble-like layer builds up on the surface
of the tubes rather than a continuous mixture of solids and
gas, thereby substantially lowering heat transfer efficiency
since much greater heat transfer is provided by solids contact
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than gas at the same temperature.
Furtheremore, prior art devices are generally
incapable of utilizing the "Fines", or finely ground solid
fuel (e.g., coal) particles, produced during the crushing
operation. Thus, efficiency is further reduced to the extent
the "Fines" are lost with the flue gas or are not used at all.
Accordingly, it is an object of the present invention
to provide a new and improved fluidized bed boiler apparatus
and method for carrying out and operating of same. Another
object of the invention is to provide a new and improved
fluidized bed boiler apparatus and method for carrying out and
operating same, capable of relatively high efficiency heat
transfer, yet operating at relatively low temperatures to limit
production of nitrous oxide and enabling suitable reaction
for removing S02.
It is also an object of the present invention to
provide a new and improved fluidized bed boiler apparatus and
method for carrying out and operating same, capable of rela-
tively high heat transfer with no slagging of the ash.
It is an additional object of the invention to
provide a new and improved fluidized bed boiler apparatus and
method for carrying out and operating same, which enables
finely ground solid fuel particles otherwise too small to
burn in a fluidized combustion bed, to be completely com-
busted and the heat generated thereby to be efficiently
utilized.
It isstillanother object of the instant invention
to provide a new and improved fluidized bed boiler apparatus
and method for carrying out and generating same, which includes
a relatively large number on control points for enabling cer-
tain operating parameters to be controlled, particularly the
operating capacity of steam generation.
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It is also a further object of the invention to
provide a new and improved fluidized bed boiler apparatus and
method for carrying out and generat:ing same, wherein particu-
late matter circulates in heat exchange means for increased
heat transfer.
It is still an additional object of the invention to
provide a new and improved fluidized bed boiler apparatus and
method for carrying out and operating same, which enables
control of the temperature of the fluidized bed.
Objects and advantages of the invention are set
forth in part above and in part below. In addition, these
and other objects and advantages of the invention will become
apparent herefrom, or may be appreciated by practice with the
invention, the same being realized and attained by means of
the instrumentalities, combinations and methods pointed out
in the appended claims. Accordingly, the present invention
resides in the novel partsl constructions, arrangements,
improvements, method and steps herein shown and described.
My invention relates to a fluidized bed combustion
apparatus and the method of carrying out and operating same.
In particular, my method and apparatus is directed to an
improved, compact, fluidized bed boiler and the method of
operation thereof, and more particularly to a fluidized bed
boiler and its method of operation, which involves a high-
solids recirculation system, providing for a compact boiler
of high heat and combustion efficiency. My boiler and method
of operation include the employment of a high-solids recircu-
lation system of the fluidized bed constituents, while the
fluidized bed particulate material, including unburned fuel
therefrom, is moved in an upwardly and then optionally a down-
wardly flow-transfer path, the flow-transfer path being
~Qq67~
substantially free of any flow-transport obstructions therein.
Briefly described, the method and associated appara-
tus according to the present invention, for carrying out
fluidized bed combustion and transferring the resultant heat
to a heat transfer mechanism, includes providing a fluidized
bed of particulate matter and introducing fuel particles into
the fluidized bed, causing a portion of the constitutents of
the fluidized bed to flow into and upwardly through vertically
extending heat exchange means which are separated from the
fluidized combustion bed and which are substantially free of
any obstruction to flow therein, and reintroducing the with-
drawn portion of the fluidized bed constitutents into the
fluidized bed. Advantageously, the means causing transfer of
the portion of the fluidized bed constituents to the heat
exchange means comprises means for controlling the density of
particulate transport flow through the heat exchanger and,
advantageously, also include means for controlling the rate
of particulate flow into the heat exchange means. As prefer-
ably embodied, the transfer causing means comprise a nozzle
valve governing the inlet of the heat exchange means and
adapted to introduce a gas which is at least partially
combustible into the heat exchange means, at the bottom of
the heat exchange means. Also, advantageously, the nozzle
valve is further adapted to introduce additional fluidizable
fuel into the heat exchange means. Also advantageously,
additional heat exchange means, such as superheater for heated
heat exchange fluid and/or feed water heater for heating heat
exchange fluid for the heat exchange means, are positioned
above the fluidized combustion bed.
Also, as preferably embodied, the portion of fluidi-
zed bed constituents flowing into and through the heat ex-
change means flows from a relatively quiescent area adjacent
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the fluidized combustion bed into the heat exchange means.
According to one aspect of the invention, a with-
drawal duct couples the flow between the quiescent zone and
the heat exchange means, and the inlet to the heat exchange
means is controlled by the nozzle valve means which is adap-
ted to vary the rate of solids flow through the inlet to the
heat exchange means and introduce additional fuel along with
the gas into the heat exchange means. Alternatively, the
quiescent area is located directly adjacent the heat exchange
inlet which is controlled by the nozzle valve means.
By the method and apparatus according to the inven-
tion, heated particulate matter from the fluidized bed and
unreacted fuel particles, such as coal, are removed from the
fluidized bed through the use of a pressure difference created
externally of the bed which, preferably, does not have any
heat-exchange cooling surface within it. Removal is accompli-
shed through the introduction of air into the heat exchange
device, at the bottom thereof, to provide a means for carrying
the solids to establish a circulation flow rate and, advanta-
geously,to provide additional air for combustion of unburned
and/or additional fuel particles. The withdrawn fluidized
bed materials are passed upwardly in a first vertical heat
exchange chamber and, advantageously, downwardly in a second,
return, heat exchange chamber, connected by an arch-contacting
chamber, and discharged into the fluidized bed from the exit
of the downward chamber, which is preferably positioned
directly above the fluidized bed, with combustion of unburnt
and/or additional fuel particles carried out in the heat ex-
change chambers.
According to another aspect of the invention, there
is a minimum defined distance between the walls of the heat
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lQ~67~7
exchange chambers, so that, as heat is transferred and combus-
tion proceeds, there is a substantially and generally uniform
temperature profile maintained across the cross-sectional area
at any given point in the heat exchange chambers. As the
fluidized bed particles are moving upwardly in the first up-
ward vertical chamber, the particles are in a denser phase
than while moving downwardly in the second downward chamber.
Thus, by maintaining a high-solids circulation rate through
withdrawal of a high portion of the fluidized bed and passing
the particles from the fluidized bed through an obstruction-
free flow-transfer path, first in an upward vertical direction
and then downwardly in a parallel direction, a relatively com-
pact boiler may be provided and method of operation of same,
with high controllable efficiency obtained.
Since there may be some tendency for the flue gas so
discharged to carry fine solid particles upwardly out of the
bed, the superheater and/or additional heat exchange means
positioned above the bed, serve not only to capture additional
heat but also to provide a particle-impingement barrier to
knock the particles back into the bed in order to maintain
efficient mixing of the cooled and heated particles. A small
portion of finer particles which pass through the impingement
barrier is collected by a particle-collection system, such as
an overhead cyclone, and then is returned by a different
recycle system directly to the fluidized bed.
Accordingly, it will be found that the objects and
advantages specifically described herein are achieved by the
invention as herein disclosed and claimed. Thus, for example,
it will be found that a method and associated apparatus for
carrying fluidized bed combustion and transferring heat
produced thereby to a boiler may be made in accordance with
lQq~7~7
the present invention, enabling substantially highly efficient
utilization of the heat produced by the combustion-boiler
apparatus.
It will be found that by separating the heat ex-
change means of the boiler section from the fluidized combus-
tion bed and providing the heat exchange means substantially
free of any obstructions to fluid flow therein, the combus-
tion/boiler apparatus will provide relatively high efficiency
heat transfer.
It will also be found that by providing means for
controlling the density of transport flow through the heat
exchange means, the operating temperature of the combustion
section can be controlled to maintain a predetermined low
level of nitrous oxide production and prevent any slagging.
sy reintroducing particles which have circulated in
the heat exchange means back into the fluidized bed, it will
be found that the temperature within the fluidized bed is
controllable due to the intimate mixing of the cooled reintro-
duced particles with the particles being heated by combustion
of the fuel.
In addition , by providing a vertically extending
heat exchanger adapted to permit upward flow of a portion of
the fluidized combustion bed, it will be found that the rela-
tively low density of particulate matter with a high percentage
of voids, about 95%, within the heat exchanger enables a por-
tion of the heat associated with red hot particles to be trans-
ferred to the -heat exchanger by radiation, thereby increasing
the amount of heat trasnferred to the boiler section.
By introducing additional fuel and combustible gas
into the heat exchange device, it will be found that other-
wise unusable fuel particles can be efficiently burned in
-- 10 --
1~967~7
accordance with the present invention, In addition, by pro-
viding such combustion within the heat exchange means accord-
ing to the present invention, additional heat may be trans-
ferred thereto by radiation from the burning of the additional
fuel. Moreover, the burning additional fuel traveling through
the heat exchanger helps provide a substantially uniform tem-
perature throughout the flow path of the heat exchanger, i.e.,
from inlet to exit.
It will further be found that by providing a nozzle
valve governing the inlet of the heat exchanger and gas inlets
in both the heat exchanger and the withdrawal duct, a relativ-
ely large number of control points are provided for maintain-
ing substantial control over the operational parameters of the
system. AlSo, by separating the walls of the heat exchange
means by a distance of between about one-half to about four
feet, it will be found that an essentially uniform temperature
profile will be provided in the heat exchange device across
the flow path at any vertical position therein.
By providing additonal heat exchange means above
the fluidized combustion bed, it will be found that not only
is heat associated with escaping flue gases transferred to the
heat exchange fluid in the additional heat exchange means, but
also lighter fuel particles tending to escape with flue gases
will impinge on the additional heat exchange devices to be
prevented from escaping combustion.
My invention will be described for the purpose of
illustration only in connection with my preferred and speci-
fic embodiments. However, as will be recognized by those
persons skilled in the art to which this invention is directed,
various changes and modifications may be made to my illustra-
ted fluidized bed combustion apparatus and method of operation
thereof without departing from the spirit and scope of the
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lQq67~7
invention covered thereby. Further and in particular, in
defining the high-solids recirculation system of my apparatus
and its method, the terminology"transport flow" shall mean
that flow which operates above the choking velocity, that is,
the minimum gas velocity required for vertical flow known in
the art and as set forth more particularly in "Fluidization
Engineering", Daizo, Kunii and Octave, Levenspiel, pages 385-
387, published by John Wiley & Sons, Inc., 1969.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a representational, cross-sectional, eleva-
tion view of a fluidized bed double-boiler apparatus according
to the present invention.
Fig. 2 is a partial, generally schematic view of the
boiler of Fig. 1 illustrating the high-solids recirculation
system and solids flow path according to the present invention.
Fig. 3 is an enlarged, fragmentary, cross-sectional
view along lines 3-3 of the apparatus shown in Fig. 1.
Fig. 4. is a partial elevation view generally simi-
lar to that of Fig. 2, showing another aspect of the present
invention.
Fig. 5. is a partial elevation view generally simi-
lar to that of Fig. 2, showing still another aspect of the
invention.
Fig. 6 is a sectional view taken along section
6-6 of Fig. 1.
Referring now more particularly to the embodiments
of the invention illustrated in the accompanying drawings,
wherein like reference characters refer to like parts through-
out the various views, there is shown in Figs. 1 and 2, an
embodiment of a fluidized bed double boiler (indicated gener-
ally at 10), having boiler or vessel walls 12, with a fuel
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inlet 14 which includes means for the introduction of a fuel.
Fuel inlet device 14 may include any conventional feeder
mechanism for injecting fuels such as a fossil fuel like fuel
oil or a particulate fuel, such as coal particles, or any
combination thereof. Thus, for example, fuel inlet 14 may
include a screw feeder for introducing particulate coal or,
preferably, a spreader stoker which throws the particles for
distribution over the entire fluidized bed.
Boiler 10 also includes air inlet 16 for the intro-
duction of a fluidizing gas having a combustible component(such as an oxygen-containing gas, preferably air), under
pressure, coupled to the combustion chamber for fluidizing
the constituents making up the fluidized bed. To this end,
boiler 10 includes a column chamber 18 formed by floor plate
17, partition members (preferably solid) 19 extending up-
wardly therefrom and a perforated distributor plate 20 suppor-
ted by partition members 19 to form a plurality of air dis-
persion chambers (unnumbered). Distributor plate 20 is here
shown extended across a major part of the lower cross-section
of the boiler, and may be tilted or sloped at one end, as
explained more fully below.
The fluidizing gas in introduced from inlet 16 into
column chamber 18 (through inlet conduits 16a, each leading
to an air dispersion chamber) for dispersion into the fluidi-
zed bed combustion chamber (indicated generally at 22) by
dispersion plate 20 to fluidize the combustion bed constitu-
ents. As here embodied, the fluidized bed may include an in-
ert refractory material, such as sand, and, preferably, a
material which absorbs or reacts with undesirable constituents
(particularly S02) in the fuel, such as dolomite or limestone
particles, or any combination of such constituents, with solid
- 13 -
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particulate or other combustible fuel, such as coal, tar
sands, natural gas, etc. introduced therein to form a fluidi-
zed combustion bed of particulate matter in the fluidized bed
combustion chamber 22.
In the preferred embodiment described, the fluidized
bed combustion chamber 22 does not have any heat exchange tubes
therein, although such tubes may be inserted into the bed if
desired in order to, for example, cool bed 22. According to
one aspect of the invention the fluidized bed has, at the one
end, a quiescent section 24 which is not disturbed by the
introduction of the fluidizing gas through the distributor
plate 20, like that portion of bed constituents directly above
distributor plate 20, as schematically illustrated in Fig. 2.
In addition, an interfacial surface, 26, is defined between
the fluidized portion of the bed and disengaging chamber 28.
Advantageously, disengaging chamber 28 is positioned
directly above the bed 22, with heat exchange tubes, or super-
heater tubes, 30 positioned in chamber 28 in a radiant heat
exchange relationship. Superheater tubes 30 not only contain
steam to be heated by the rising gases and particles from bed
26, but they also serve as an impingement barrier to the up-
ward movement of fine particles carried by the gases. Also
advantageously, the boiler 10 includes upper heat exchange
tubes 32, or feed-water heaters, also positioned in chamber
28 above both the combustion bed and superheaters 30. Tubes
32 typically contain water to be heated in a heat exchange
relationship with the rising gases from bed 22 as explained
more fully below, and,like superheaters 30, provide an addi-
tional impingement barrier to upwardly moving fine particles.
A gas-solids separator 34, such as a gas-solids
cyclonetype separator, is positioned in disengaging chamber 28,
~67~:37
above tubes 30 and 32, to receive very fine upwardly rlowing
solid particles which pass these impingement barriers. Ad-
vantageously, gas-solids separator 34 directs the finely-divi-
ded separated solids so captured through solids return line
36 to an exit 38, whereby the solids are returned directly to
the fluidized bed 22. Further advantageously, the exit of the
solids return line 36 is positioned approximately adjacent to
the fuel inlet 14, to aid in preheating the fuel to be intro-
duced into the fluidized bed 22.
Boiler 10 also includes a steam-water-separator
boiler drum 40, a flue-gas air heater 42 which receives flue
gas from the separator 34 for heating air introduced through
inlet 16, and a flue-gas exit 44, coupled thereto and leading
to the stack and filters (not shown). Exit 44 preferably
includes an air slide 46 to remove fly ash through an ash-
discharge hopper (not shown) before exhausting the flue gases.
The boiler also includes inlet 48 for introducing heat ex-
change fluid, such as feed water, to the boiler system. In-
let 48 is coupled to entrance header 50 of feedwater heater 32
having exit header 52 therefrom, leading to drum 40. Entrance
54 of superheater 30 is coupled to drum 40 and has exit header
56 therefrom, which is the exit for the superheated steam from
the system and leads, for example, to a steam turbine. In
addition, heat exchange fluid line 58 extends from the boiler
drum 40 to a mud drum 60, all as will be explained more fully
below.
As here embodied, the high-solids recirculation
system according to the invention comprises a withdrawal duct
62 which includes a U-bend feed withdrawal duct, the entrance
of which is in direct communication with the quiescentsection
24 of the fluidized bed, as by funnel-shaped inlet member 61,
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so that the particulate material may be drawn from the fluidi-
zed bed into the withdrawal duct directly. Withdrawal duct
62 also includes funnel-shaped outlet member 63 (shown in
Figure 6) for introducing fluidized particles into the boiler
section (indicated generally at 70), as described more fully
below. Advantageously, withdrawal duct 62 is of generally
reduced cross-sectional area relative to the cross-sectional
area (as defined in the plane perpendicular to the plane of
Figs. 1 and 2) of quiescent zone 24 for enabling substantial
control over the circulatory flow of fluidized particles
between the combustion section and the boiler section.
Thus, duct 62 may have a generally circular cross-
sectional area while inlet member 61 is tapered both as indi-
cated in Fig. 2 and in the direction perpendicular thereto to
increase the density of fluidized particulate matter leaving
quiescent zone 24, Outlet 63 is provided with a reverse
taper in the upward flow direction to distribute flow over
perforated distributor plate 68 which is positioned at the
bottom of the boiler section. However, as here embodied, the
upward reverse taper is only in the directions perpendicular
to the plane of Fig. 2, since the boiler section channel can
be about as wide as the diameter of duct 62.
As preferably embodied, boiler 10 also includes
inlet 64 for the introduction of a gas having a combustible
component, such as air, or if desired,a mixture of air and
fuel, such as natural gas or other fuel, as will be explained
more fully below. In addition, an adjustable nozzle 66,
adjustable within the withdrawal duct 62, is operable within
duct 62 to control the rate of solids circulation in the heat
exchange section by vertical movement of the nozzle 66 in
combination with varying the flow of gas through inlet 64.
- 16 -
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Advantageously, additional fuel may be introduced along with
the gas through inlet 64 or through a separate fuel inlet.
As preferably embodied, the nozzle 66 and air inlet
64 are formed as integral adjustable nozzle valve which com-
prises a plug type valve or an injection type cone valve, such
as described in U.S. Patent No. 2,630,352, of which I am a
co-inventor. Accordingly, the valve includes a hollow central
tube, enabling the introduction of the gas and additional fuel
therethrough, as well as a head portion controlling the size
of the opening (63a) leading into upward chamber 72. Alter-
natively, a slide valve could be used instead of nozzle valve
66.
Boiler section 70 comprises vertical, upward flow
chamber 72, an arched roof chamber 74 and a downward flow
chamber 76. Flow chamber 72 and 76 are constructed of standard
boiler web walls or furnace wall panels (indicated generally
at 87) comprising a series of heat exchange tubes 88 joined
together by webs 89. As preferably embodied, a series of
partition walls 86 formed of tubes 88 are positioned at spaced
intervals between and perpendicular to the walls 87 of tubes
88 to define a series of generally parallel slots, or heat
exchange passages, 84, between the walls of each chamber for
the transport flow of the particulate material. Figure 1
shows the schematic view of the partitioned, walled chambers,
AdvantageouSly, the arched roof of the chamber 74 is also
constructed of the joined web-type heat exchange panels 87,
and serves as the heat-exchange fluid connection between the
respective web panels of the chambers 72 and 76. However, as
indicated in Figure 1, the portion of each partition 86
residing within arched chamber 74 is formed with conduits
86a leading to a tube 88 in the wall portion thereof, thereby
;737
defining only a small section which does not include heat
exchange tubes.
Accordingly, from the heat exchange fluid line 58,
heat exchange fluid passes through one of mud drums 60 (adap-
ted to separate solids from the fluid circulating through
tubes 88), then through the tubes 88 forming the walls 87 of
chambers 72, 74 and 76 to one of the headers 78 which carries
the heated fluid to the boiler drum 40. To this end, a mud
drum 60 extends under all the parallel extending heat exchange
tubes 88 in each wall 87, in fluid communication therewith, to
feed heat exchange fluid thereinto. In addition, cross-over
connectors 90 and 92 provide fluid communication, respectively,
between the tubes 88 of the inner wall 87 of upward chamber 72
and the inner wall of downward chamber 76, and between the
inner and outer walls of chamber 76 so that the heat exchange
fluid flows upwardly in chambers 72 and 76 and into separator
drum 40 through one of the headers 78. It will be understood
that two headers 78 may be used to prevent too many holes (to
allow entry from each tu~e 88 coupled thereto)therein to
weaken the header. However, one large header 78 may be used
to accommodate all the ~ubes without risking structural weak-
nesses.
Although a natural circulation of heat exchange
fluid will occur through the conduits described immediately
above, pumps 59 may be placed in fluid line 58 to assist in
the flow, particularly during start-up. In addition, cross-
over members 90 (which are aligned with and equal in number
to the partitions 86) advantageously lead into header 94 at
the bottom of and coupled to all the tubes 88 of the inner
wall 87 of chamber 76, and, crossovers 92 (also aligned with
and equal in number to partitions 86) lead from header 94 to
- 18 -
1~676~7
header 96 at the bottom of and coupled to all the tubes 88 of
outer wall 87 of chamber 76.
The flow chamber 76 has a solids exit (indicated
generally at 80) which discharges into the disengaging chamber
28, preferably directly above the fluidized bed 22, whereby
the particles circulating within the withdrawal duct and slot-
ted chambers 72, 74 and 76 may be discharged downwardly into
and returned to the fluidized bed 22. Although the exit 80
shown in Figures 1 and 2 is shown positioned over quiescent
zone 24, it will be understood that the particles in the exit-
ingflow will tend to fall into bed 22 due to the intermixing
of exiting flow with the gases rising from bed 22 and quies-
cent zone 24. However, as preferably embodied, exit 80 is
positioned generally over bed 22 as indicated in Figure 4.
As the various gases (i.e., the flue gases flowing
in and the gas introduced into boiler section 70) and the
particulate matter travel through the chambers 72, 74 and 76,
particulate matter adjacent the surface of partition walls 86
will tend to circulate thereat due to the combined effect of
the drag (i.e., friction) from the wall and the downward pull
of gravity as well as the upward "push" of the gases. Thus,
for example, the particulate matter may travel at an average
velocity of about 7ft/sec while the superficialvelocity of
the gas may be at about twice, or more, that rate, thereby
providing an efficient heat transfer from the fluidized
combustion constitutents to the heat exchanger tubes 88.
As preferably embodied, the withdrawal duct 62 also
includes a fluidizing or an aeration air inlet 82 which aids
in fluidizing (i.e., controlling the density of) the solid
particulate material from the fluidized bed 22 by providing
an entry for air flow into the withdrawal duct 62. It will be
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1~67~7
und2rstood that inlet 82 provides an addi~ional control point
for enabling control of the density of the fluidized particu-
late material flowing into flow chamber 72.
Advantageously, in order to facilitate flow of
fluidized bed constituents towards the quiescent zone 24 and
thence into boiler 70, distributor plate 20 may be sloped a
slight degree downwardly toward quiescent zone 24 particu-
larly for relatively shallow beds (less than about 6 inches).
Alternatively, distributor plate 20 may be level but formed
with one or more grooves which slope downwardly towards
quiescent zone 24. However, it will be understood that for
most moderately deep beds (i.e. about 6 inches or more), the
top surface of the bed will find its own gradient, or ang~le
of repose towards quiescent zone 24 where distributor plate
20 is not sloped.
Fig. 3 shows in more detail the plan view of the
high-solids concentration recirculation system wherein the
upward (72) and downward (76) flow-transfer chambers comprise
a plurality of heat exchange passages 84, each being about
two feet thick (i.e., the distance between opposite walls 87)
and about six feet wide (i.e., the spacing between partition
walls 86) to effect a heat exchange relationship between the
combustion flue gases and particulate matter moving upwardly
in chamber 72, laterally in the chamber 74 and downwardly in
chamber 76. The upward, top and downward flow-transfer path
and chambers are shown and illustrated as a plurality of
parallel aligned, flow passages. However, it will be under-
stood that chambers of various dimensions, shapes and sizes
may be employed, so long as such flow-transfer path in the
chambers are free of any flow obstructions therein.
_ 20 -
67~'7
Also, advantageously and according to another aspect
of the invention, cllambers ~4 should not have a cross-section-
al area so large as to allow an extensive heat gradient across
the cross-section of a heat exchange passage (i.e., between
the center of the passage and the adjacent wall) at any point
in chambers72, 74 or 76. Accordingly, the cross-sectional
area should be limited to permit the maintenance of a sub-
stantially uniform high temperature profile across the cross-
section (i.e., the thickness) at any given point in each
chamber during the high-solids circulation flow. Cross-sec-
tional areas of too large a dimension will prevent the main-
tenance of a substantially uniform temperature profile,
because there will be insufficient contact of the fluidized
particles in the center section of the flow-transfer path to
permit efficient heat transfer.
Accordingly, as here embodied, the thickness of
each heat exchange passage 84 is at least about one-half
foot (1/2') but no more than about four feet (4') throughout
the three chamber sections, 72, 74 and 76. In addition, the
width of each passage 84 is no more than about six feet (6').
It will be understood that the six foot dimension on the
passage width is normally limited to permit uniform distribu-
tion of the flowing medium from the exit of duct 62 to plate
68 (via funnel outlet 63) which, in turn, distributes the
flowing particles and gases uniformly throughout the heat
exchange passages 84 for optimizing heat transfer.
It will also be understood that the limitation on
the thickness of each passage 84 is particularly useful since
a thickness of less than about one-half foot will not enable
particulate matter to circulate adjacent the heat exchange
walls 86, as the gases flowing through the open channels will
- 21 -
lQq67~7
tend to push all the particles through the boiler section and
overcome the combined gravitational and fric~ional effect at
the walls. Similarly, a width greater than about four feet
will enable the particulate matter adjacent the heat exchange
walls to escape the push of the gas flowing through flow
chambers 72, 74 and 76, thereby to reduce the efficiency of
total heat transfer. Also, as a practical matter, smaller
widths will not enable a person to enter the slots 84 to
effect repair of a broken tube 88, thereby requiring whole
sections to be cut away. It will also be understood that the
maintenance of the uniform temperature profile is particularly
advantageous since the transport flow close to the heat ex-
change surfaces will be at a temperature only slightly lower
than that at the center of the heat exchange passages, allow-
ing greater heat transfer at the surfaces and, therefore,
greater efficiency.
In my apparatus and method, reliance is placed on
efficient heat exchange during the transport flow of the
fluidized bed particulate matter withdrawn from the withdrawal
duct 62 and introduced for flow transfer through the upward,
top and downward chambers 72, 74 and 76, respectively, In
the fluidized bed combustion chamber 22, there is no trans-
port-flow movement of the particles therein, which particles
comprise the fluidizing air or combustible gas, unburnt or
burning fuel particles, as well as optionally inert refrac-
tory particle and absorbent-type particles. In my high-solids
recirculation flow system, there is a continual flow-transfer
condition in thechambers 72, 74 and 76. My boiler has been
illustrated in itspreferred form, showing an arch chamber and
a downward chamber, however, such chambers are not essential,
although a preferred embodiment of my boiler.
- 22 -
l~q67~7
Advantageously, combustion occurs in boiler section
70 due to the burning of unburnt fuel which may be included
with the portionof fluidized bed constituents circulating
therethrough and the additional fuel introduced thereinto, by
virtue of the additonal gas introduced through air inlet 64
and/or air inlet 82. As preferably embodied, the solid fuel
added to section 70 is normally in the form of relatively
small particles to permit rapid combustion in the solids
circulation system. The addition of this smaller size solid
fuel is an advantage of the present invention, in that fines
produced by the grinding of the solid fuel may be efficiently
combusted in the boiler system and heat generated thereby
efficiently transferred to the heat exchange means because of
the greater surface area per unit mass as compared with the
larger particles. It will be understood that these fine
particles are not normally used in present fluidized bed
boilers because they tend to be blown out of the bed before
complete combustion takes place. In addition, the burning
particles traveling through the boiler 70 tend to make a more
uniform temperature throughout the boiler - i.e., from inlet
at 68 to exit 80 - while they radiate heat to the heat exchan-
ger walls to transfer additional heat.
In the embodiment described, heat exchange tubes or
means are not shown disposed in the fluidized bed combustion
chamber 22, since, in the preferred embodiment, this permits
the firing up more rapidly of my boiler. My boiler may be
fired up without circulation, with the solids heating up
quickly without loss of heat through cooling tubes inserted
into the fluidized bed.
My boiler 10 has numerous advantages in addition to
enabling rapid start up or firing of the boiler. My boiler
67~37
permits the controlled rate of circulation of the fluidized
bed solids in a flow-transfer condition throughout the recircu-
lation system by merely varying the adjustable nozzle valve
66 which, thereby, controls the rate of heat transfer to the
partition walls and tubes 88 in the boiler 70, which permits
rapid turndown or increase in boiler capacity. This control
of heat-transfer rate cannot be easily accomplished in fixed
fluidized bed heat exchange boilers, since adjustment must be
made by stopping the flow of air to sections of the fluidized
bed, thereby causing slumping of the bed.
In addition, the height of known fluidized bed
combustion boilers is normally limited due to pressure drop
considerations, thereby restricting the amount of cooling
surface which may be immersed within the bed. However, no
such restriction exits in the boiler according to the present
invention, since the extent of cooling surface exposed to the
heat transfer medium can be varied by changing circulation
velocity and solids concentration in the circulating system
without increasing pressure drop through the system. The
pressure drop throughout high-solids circulation system 70
is no greater than the pressure drop through the fluidized bed
22 and the static head of fluidized particles in the quiescent
section 24 is greater than the pressure drop through the solid
circulation system, thus providing the pressure gradient neces-
sary to cause the desired circulating rate.
In my boiler, a much greater range of particulate
particle sizes may be employed, with the smaller-particle
range particularly preferred due to the greater surface area
and more efficient and higher heat-transfer rate accomplished
thereby, while where dolomite, limestone and other absorbent
or reactant materials are employed, a smaller-particle size is
- 24 -
lQq67~7
more desirable as to effect the reaction rate with fuel conta-
minants. Typical particle sizes may range from as low as
about 40 microns to 450 microns, and in a typical fluidized
bed, often range from about 250 to 450 microns, while my
boiler may operate efficiently and preferably at the range of
from about 40 to 150 microns for mean particle size, which
effects high heat-transfer rates. It will be understood that
exhausted, or completely reacted, active particles will be
withdrawn with the ash particles and fresh active particles
furnished along with fresh fuel.
My boiler also permits a very high heat transfer
efficiency by employing heat exchange means essentially free
of flow obstructions and, advantageous, providing a substan-
tially uniform temperature profile therein, while the gas is
being combusted during transport flow through the recircula-
tion system. For example, where the slotted chamber has a
dimension of six by two feet, the temperature profile, or
difference, will be about 10F or less between the center of
the passageway and the particles close to the wall surface,
where the wall surface would be about 500F and the operating
temperature would be about 1500 to 1700F. This is due to
the high degree of solids circulating along the wall by fric-
tional resistance and gravitational forces opposing the up-
flow of the carrying gases.
In operation, my boiler is started up typically by
the employment of an extraneous-type fuel, such as natural
gas or light fuel, injected into the bed in order to heat up
quickly the inert particulate solids in the fluidized bed 22,
while air is introduced into the air inlet 16 through the
plenum chamber 18 and the distributor plate 20 to form a
heated fluidized bed combustion chamber 22. The corresponding
- 25 -
1~67~'7
fluidized bed dimensions for each six-by-two foot heat ex-
change passage 84 in the boiler section may have a size of
from about six feet in thickness (corresponding to the width
of the passage 84) by fourteen feet wide (the distance from
inlet 14 to quiescent zone 24) and about two feet deep. How-
ever, the bed size and depth may, of course, vary, depending
upon the boiler. The fluidized bed particles may have a
density of approximately 35 to 75 pounds per cubic foot or
generally about 50 pounds per cubic foot, with the pressure
drop in the boiler at about 1 pound per square inch and the
air (preheated by air heater 42 to about 750F) iniroduced
into the air inlet 16 about 1 pound per square inch above
atmospheric pressure.
Once the boiler operation is commenced, air (also
preheated by air heater 42) is introduced through air inlet
82 at approximately 1-1/2 pounds per square inch above atmos-
pheric pressure to fluidize the particulate material from the
quiescent zone 24 and to create a flow of the particles from
quiescent portion 24 through the U-bend withdrawal duct 62,
due to the pressure difference. Additional air (also pre-
heated by heater 42) is also introduced through air inlet 64
through adjustable nozzle valve 66 and the nozzle valve
positioned so as to permit a flow-transfer condition for the
particulate material introduced into the withdrawal duct 62.
As described above, combustion occurs during the
upward movement transport flow, with heat transferred to all
the heat exchange tubes 88. The particulate material moves
upwardly to the top chamber 74, laterally therethrough and
then downwardly through chamber 76 where it enters a lean-
density phase through gravity-acceleration forces, and then is
discharged from exit 80 into the disengaging chamber 28. A
- 26 -
lQ~67;~7
majority of the particles are returned to the fluidized bed
for recirculation, with the pressure in the disengaging cham-
ber typically being about 1/4 to lJ2 pound per square inch
over atmospheric pressure. The particles returned to the
fluidized bed 22 are then recycled back into the high-solids
recirculation system. Rapid circulation, and~ therefore,
control of the heat exchange rate and heat transfer, is per-
mitted through the positioning of the adjustable nozzle valve
and the amount of air flow introduced to control transport
flow; that is, the pressure drop created in the circulation
system.
The majority of the particulate material discharged
into the disengaging chamber 28, drops into the fluidized bed
22, while finely-divided particles move upwardly and strike
superheater 30 or feed water heater 32 as impingement barriers,
and, therefore, drop back to the fluidized bed. Much finer-
divided particles move upwardly through the feed-water heater
32 where they are collected by gas-solids cyclone 34 and are
returned through the solids return line 36 to the fluidized
bed through discharge inlet 38, where they preheat the fuel
to be introduced into the bed 22 by inlet 14. The flue gases
pass through the air heater 42 and flue gas exit 44, after
which they are discharged through the filter and stack.
The heat exchange fluid, initially water (as, for
example, condensed steam from a steam turbine generator or
other steam operated device coupled to boiler fluid exit 56),
introduced through inlet 48 flows through preheater 32 via in-
let 50 and thence into drum 40. Heated water from drum 40
flows through inlet 58 to mud drums 60, up through the numer-
ous heat exchange tubes 88 and back into drum 40 generally asa mixture of water and steam. Steam from drum 40 flows through
inlet 54 to superheater 30 and thence, via outlet 56 to a
; - 27 -
l~q67~
steam turbine or other steam operated device.
The flow-transfer high-solids circulation system is
illustrated schematically in Fig. 2 in which various sections
of my boiler have been designated with alphabetical characters.
As a typical example of an improved boiler, the boiler drum 40
has a steam pressure of 1000 psi and the boiler is rated at a
rate of about 50,000 pounds of steam (at about 100F) per hour
per 2' x 6' heat exchange chamber 84, having a vertical upflow
length of about 30 feet (between distributor plate 68 and arch
section 74) and a vertical downward flow length of about 18
feet (between arch section 74 and exit 80). Circulation of the
entire bed is at the rate of 20 pounds of solids circulated
per-pound of steam generated, with the solids total residence
time in the system rangingfrom about 5 to 60 seconds. The
heat-transfer, the solids-concentration density and the super-
ficial gas velocity are shown more particularly in this boiler
in the following table.
TABLE I
Boiler with Fluidized Bed
Mean Particle Diameter 150 Microns
Heat-Transfer Solids-Concentra- Superficial Gas
Area Rate 2 tion3Density Velocity (V)
(Fig. 2) Btu/hr/Ft /F Lbs/Ft Feet/sec
A 50
B 1.6
C 50 3.4 15
D 68 4,7
E 36 1.1
F 0.3 8
G 0.07
H 0.0015
In the typical operation of my boiler as above,
generating about 1000 pounds of steam per square inch, the
~Qq~7~17
boiler feedwater temperature entering at inlet 48 will be
334F, preheated in exchanger 32 to 485F before entering
drum 40. The water temperature entering the boiler 70 and
in the heat exchange system will be 545F. The steam from
boiler drum 40 is superheated in heater 30 to about 1000F.
The fluidizedbed 22 temperature would be 1550F using high-
sulfur Illinois coal-particle feed, and the exit temperature
at 80 being 1440F, with the inlet ~emperature at plate 18 at
1540F, with an overall temperature profile difference through-
out the boiler of about 100F. The flue gas entering theseparator 34 has a temperature of 952F and the preheated air
at inlets 16 and 64 has a temperature of 750F. These condi-
tions described permit the operation of the boiler at a high
thermal efficiency of over 90%.
A study of circulating gas solids mass velocity
versus the heat-transfer rate finds that the heat-transfer
rate of the upwardly moving particles is increased, while the
heat-transfer rate of the downwardly flowing particles in a
recirculation system is reduced by virtue of the differences
in density occasioned by the downward and upward acceleration
forces, respectively. The arch or roof chamber connecting
the upper ends of the first upwardly and second downwardly
moving chambers also affects the heat-transfer rate due,
apparently, to tangential forces and a concentration of so-
lids therein, but overall, a relatively constant heat-trans-
fer rate will be found to be affected throughout my high-so-
lids recirculation system.
Turning now to Figure 4, there is illustrated ano-
ther aspect of the present invention, wherein exit 80 of
downward chamber 76 is adapted to discharge the exiting trans-
port flow directly into fluidized bed 22, To this end, a
- 29 -
~aq67~7
curved, extended portion (indicated at 98) is formed at the
end of the inner wall 87 of downward chamber 76, with exit 80
thereby defining an exit plane which is vert:ical.
Accordingly, particulate matter exiting from cham-
ber 76 will be discharged horizontally, as indicated in Fig-
ure 4. The heavier particles will, therefore, fall directly
into bed 22, while any lighter particles tending to be car-
ried upwardly by gases rising from bed 22 will be knocked
down by the impingement barriers provided by superheater 30
and feed water heater 32.
Advantageously, and according to another aspect of
the invention, superheater tubes 30 include a close pitch sec-
tion (indicated at 30a) positioned directly above bed 22 and
with the closely spaced heat exchange tubes thereof extending
in the direction parallel to distributor plate 20. Thus, the
closely spaced tubes will serve as a substantial impingement
barrier to the upward flow of particulate matter. In addi-
tion, and as here preferably embodied, close pitch heat ex-
change tubes 30a (which are fed by header 56a into which tubes
30 feed and which lead to exit header 56) are positioned ad-
jacent exit 80. In this way, the horizontally directed flow
from exit 80 will tend to clean any particles which have
built up on the impingement barrier.
Also shown in Figure 4 are alternate means for lead-
ing heat exchange fluid to the walls 87 of upward chamber 76.
As here embodied, conduit 91 directly couples a mud drum 60 to
header 94, thereby obviating cross-over members tapped into the
heat exchange tubes 88 of upward conduit 72, as described above.
Turning now to Figure 5, there is illustrated ano-
ther aspect of the invention, which enables the combustion/-
boiler apparatus to be adapted for pressurized use. To this
end, wall 12 comprises the wall of a pressurizable vessel into
- 30 -
lQ~7~)'7
which fuel and other replenishable fluidized bed constituentsare introduced by conventional means (not shown). According
to the aspect illustrated in Figure 5, a modified quiescent
zone 24' isformed directly under the inlet 63' to upward cham-
ber 72, which is controlled by nozzle valve 66, substantially
as described above. In this way, withdrawal duct 62 is eli-
minated and the mud drums (indicated at 60') and the heat ex
change fluid line (indicated at 58') are located within the
vessel for facilitating operation under pressure conditions.
It will be understood that the elements designated
with primed characters, although somewhat different in appear-
ance, are substantially similar in function to their counter-
parts in Figures 1-5. However, since the flue gas in pressur-
ized boilers is utilized to run, for example, a gas turbine,
the feed water heaters (indicated at 32 in Figures 1, 2 and 4)
are not positioned above the combustion chamber. Rather, they
are located, flow-wise, beyond the device operated by the
flue gas in order to obtain the highest efficiency of opera-
tion. Similarly, the superheater tubes (indicated at 30' in
Figure 5) are positioned only adjacent the outer wall of down-
ward chamber 76, (for additional heating and preventing over-
heating of fluid in chamber 76), while only the close pitch
superheater tubes (indicated generally at 30a') are directly
over the combustion bed to provide the impingement barrier.
Operation is essentially the same as the embodiments
in Flgures 1, 2 and 4 except that particulate matter in bed
22 flows directly through the modified quiescent zone 24' and
into inlet 63'a when air is introduced through inlet 64 and
the nozzle valve 66 is oriented to open inlet 63'a.
In sum, by my method of operation, a relatively con-
stant heat-transfer rate is obtained in the high-solids circu-
- 31 -
lQq67~7
lation system, while adequate control over the operating tem-
perature and capacity is effected through adjustment of the
nozzle valve. Thus, control over the system parameters, par-
ticularly the capacity, is provided principally by controlling
the density of solid particles in the heat exchange chambers,
with control points at the fuel inlet to the fluidized bed,
the valve controlling flow into heat exchange means, the gas
- introduced into the heat exchange means, the fuel introduced
into the heat exchange means and the gas introduced into the
withdrawal duct as well as the fluidizing gas introduced into
the fluidized bed.
It will be readily appreciated by those skilled in
the art that the invention in its broader aspects is not
limited to the specific embodiment herein shown and described.
Thus, for example, the inlet end of the heat exchange means,
including the nozzle valve, can be located directly in the
fluidized bed, with the nozzle valve and air/fuel inlet oper-
able from beneath the bed distribution plate so that particu-
late matter flows directly into the heat exchange passages.
In addition, it will be understood that the method
and apparatus according to the present invention can be ap-
plied to any heat transfer mechanism for transferring heat of
combustion to a fluid in a generally heat exchange manner,
such as in a steam-methane reforming system, an ethylene
cracking system, etc. Moreover, the solids circulation sys-
tem according to the invention may be adapted for use with
any exothermic reaction apparatus, such as the reaction of
carbon monoxide and hydrogen to hydrocarbons.
- 32 -
!~ lQD67~7 1.`
1 Accordingly, i~ will be understood that variations may
2 be made from the specific embodiments disclosed herein, whîch are
3 within the scope o the accompanying cl2ims ~ without departing
4 from the principles of th2 invention and without sacrificing its
chief advan ges.
7 1 :
,21
241
26 .
`281
30 I
~ -33- j
