Note: Descriptions are shown in the official language in which they were submitted.
~8~2~Z
TWO STAGE CIRCULATING FLUIDIZED BED REACTOR
AND METHOD OF OPERATING THE REACTOR
,i BACKGROUND OF THE INVENTION
j The present invention relates to an improved circulating,
¦ i.e., fast, fluidized bed reactor having two stages, namely, a
circulating fluidized bed reaction stage and a cyclonic reaction
stage downstream of the fluidized bed; and to a method of operat-
ing the reactor. More particularly, the invention relates to a
two stage circulatinq fluidized bed reactor in which the size of
the fluidized bed reaction chamber and the cyclonic reaction ves-
sel are substantially reduced.
The present invention has specific application, inter alia,
to adiabatic fluidized bed combustors, fluidized bed boilers, and
compressed hot air generators. As used herein, and in the accom-
panying claims, "adiabatic combustorn denotes a fluidized bed
combustor that does not contain internal cooling means, and
~boiler" denotes a fluidized bed combustor that contains internal
heat absorption means, in the form of boiler, s~perheater,
evaporator, and/or economizer heat exchange surfaces. The tem-
perature of adiabatic fluidized bed combustors is typically con-
trolled by the use of pressurized air in substantial excess of
the stoichiometric amount needed for combustion. On the other
hand, fluidized bed boilers require very low e~cess air, so that
heat absorption means are required in the fluidized bed. Flu-
idized bed gasifiers, in contrast, utilize less than sto-
ichiometric amounts of air.
1--
,. . ~ . .
3L28~ 72
The state of fluidi7ation in a fluidized ~ed of solid parti-
~cles is primarily dependent upon the diameter of the particles
and the fluidizing gas velocity. At relatively low fluidizing
gas velocities exceeding the minimum fluidizing velocity, the bed
of particles is in what has been termed the "bubbling" regime.
Historically, the term ''fluidized bed" has denoted operation in
the bubbling regime. This fluidization mode is generally charac-
terized by a relatively dense bed having an essentially distinct
upper bed surface, with little entrainment, or carryover, of the
bed particles (solids) in the flue gas, so that recycling the
solids is generally unnecessary. At higher fluidizing gas
velocities, above those of the bubbling regime, the upper surface
of the bed becomes progressively diffuse and carry-over of the
solids increases, so that recirculation of solids using a
particulate separator, e.g., a cyclone separator, becomes neces-
sary in order to preserve a constant solids inventory in the bed.
The amount of solids carry-over depends upon the fluidizing
gas velocity and the distance above the bed at which the
carry-over occurs. If this distance is above the transfer
disengaging height, carry-over is maintained at a constant level,
as if the fluidizing gas were "saturated" with solids.
If the fluidizing gas velocity is increased above that of
the bubbling regime, the bed then enters what has been termed the
"turbulent" regime, and finally, the "fast," i.e., "circulating"
,
12~0~72
'il '
regime. If a given solids inventory is maintained in the bed,
and the fluidizing gas velocity is increased just above that of
the turbulent regime, the bed density drops sharply over a narrow
velocity range. Obviously, if a constant solids inventory is to
be preserved in the bed, the recirculation, or return, of solids
must equal the carry-over at "saturation.''
At fluidizing gas velocities below those associated with the
aforementioned sharp drop in bed density, the effect upon bed
density of returning solids to the fluidized bed at a rate well
above the "saturation" carry-over is not marked. The addition of
solids to a bed~fluidized in either the bubbling or turbulent re-
gime at a ra~e above the saturation carry-over will simply cause
the vessel containing the fluidized bed to fill up continually,
while the fluidized density will remain substantially constant.
However, at the higher fluidizing gas velocities associated with
the circulating regime, the fluidized density becomes a marked
function of the solids recirculation rate.
Circulating fluidized beds afford intimate contact between
the high velocity fluidizing gas and a large inventory of solids
surface per unit bed volume. Additionally, slip velocity (l.e.,
solids-fluidizing gas relative velocity) is relatively high in
circulating fluidized beds, when compared with that in ordinary
fluidized beds. Consequently, there is generally a very high
level of particulate loading in the combustion gases exiting from
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!
.,
~Z8~Z72
circulating fluidized bed combustors. The combustion process
which takes place in a circulating fluidized bed combustor is
also generally more intense, having a higher combustion rate than
that occurring in traditional fluidized bed combustors. Further-
more, as a result of the high solids recirculation rate in
circulating fluidized beds, the temperature is essentially uni-
form over the entire height of such combustors.
Conventional circulating fluidized bed combustors operate at
gas superficial velocities many times higher than the terminal
velocity of the fluidized bed mean particle. Consequently, there
is a very high particulate loading in the combustion product
gases exiting from the combustor and entering the downstream cy-
clone particle separator. Such conventional cyclone particle
separators typically have a height which is roughly three times
their diameter, so that separators having a large diameter de-
signed to remove the entrained solid$ from circulating fluidized
bed combustors are typically quite tall and bulky. Such large re-
fractory coned cyclone particle separators constitute a signifi-
cant portion of the total cost of conventional circulating flu-
idized bed combustor systems.
Notwithstanding the many advantages offered by conventional
circulating fluidized bed reactors, as enumerated above, the high
cost of constructing and maintaining the extremely large cyclone
particle (gas-solids) separators re~uired for recirculation of
~Z~3027~
tl the entrained solids at the rate necessary to maintain the bed in
the circulating fluidization regime constitutes a severe economicl
, impediment to widespread commercial utilization of such reactors.i
! Prior art circulating fluidized bed combustor boilers are
known which employ vertical heat exchanger tube-lined walls in
the entrainment region of the combustor (l.e., parallel to the
flow). Such combustors rely primarily on the transfer of heat
from gases which typically are heavily laden with solids, and re- i~
quire an extremely large internal volume to accomodate the large
heat transfer surface required.
The tube-lined wall heat transfer surface installed in the
free board region in conventional fluidized bed combustors neces- ¦
- sarily possesses a significantly lower heat transfer coefficient
than that of a heat transfer surface fully immersed in the flu-
idized bed. Furthermore, its heat transfer coefficient is depen-
dent primarily on two parameters: (a) fluidizing gas velocity,
and (b) particle concentration in the flue gases, i.e., particle
loading. The latter parameter is, in turn, strongly dependent on
the fluidizing gas velocity and the mean particle size of the
fluidized bed material. The concentration of particles in the
ascending gas flow in a conventional circulating fluidized bed
¦combustor is directly proportional to the gas velocity to the
3.5-4.5 power, approximately, and inversely proportional to the
fluidized bed mean particle diameter to the 3.0 power,
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.
~ I .
.,
I1 ~28~)Z~7Z
approximately. The strong effect of, and careful attention to,
these two parameters on the concentration of particles in the
ascending gas flow helps to achieve a reasonable heat transfer
coefficient for conventional tube-lined wall heat transfer sur-
faces in the free board region and facilitates the control of
combustion temperature at nominal and reduced boiler capacity.
Nevertheless, there is a need in the art for a fluidized bed
combustor boiler having a reasonable heat transfer coefficient
and permitting control of combustion temperature at nominal and
reduced capacity without being so strongly dependent on flu- !
idizing gas velocity and fluidized bed mean particle diameter.
The height of the free board region of a conventional
circulating fluidized bed combustor boiler having a tube-lined
wall heat transfer surface as described above is directly propor- i
tional to the superficial gas velocity to the 0.5 power and in-
versely proportional to the surface's heat transfer coefficient.
Also, it can be shown that the particle loading and heat transfer
coefficient are directly proportional to any change in the super-
ficial gas velocity. The latter fact means that, for instance, a
reduction of the superficial gas velocity will require an incease
- in the free board height for such a conventional combustor of a
given capacity. Similarly, it can be shown that in order to in-
crease the capacity of such a combustor, the free board height
must be increased, thereby significantly increasing the cost of
constructing such a higher capacity combustor.
-
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~a~Z~
In contrast to most conventional circulating fluidized bed
combustors, the combustor disclosed in U.S. Patent No. 4,469,050
to Korenberg (assigned to a common assignee herewith) does not
provide for transferring the entrained granular bed material,
unburnt fuel, ash, gases, etc. directly into a cyclone particle
separator. Rather, the entrained solids and gases are carried
upward into a cylindrically shaped upper region of the combustor
chamber, i.e., an extended free board region, where further com-
bustion takes place. Vertical rows of tangential nozzles are
built into and evenly spaced over this cylindrical upper free
board region. This tangentially fed secondary air is supplied at
a sufficient velocity, and the geometric characteristics of the
`cylindrical upper region are adapted, to provide a 5wirl number
(S) of at least about 0.6 and a Reynolds number (Re) of at least
about 18,000 within such upper region, which are required to cre-
ate a cyclone of turbulence.
This cyclone of turbulence enables the combustor shown in
U.S. Patent No. 4,~6g,050 to achieve specific heat releases
higher than 1.5 million Kcal per cubic meter per hour, thereby
,significantly increasing the rate of combustion. As a direct
result, the "vessel~ size of this combustor is significantly
smaller than other prior art combustors. Essentially, compared
to its downstream cyclone particle separator, the combustor ves-
sel appears like a refractory-lined duct.
. l .
~8~;Z7Z
!
The relatively large size of the cyclone particle separator
compared to the combustor vessel produced an incentive for
improving this system by eliminating the cyclone particle separa-
tor. This was achieved in the circulatin~ fluidized bed
combustor disclosed in U.S. Patent No. 4,457,289 to Korenberg
(assigned to a common assignee herewith) by eliminating the'en-
tire external solids recirculation loop and utilizing "internal
recirculation." To achieve this, a ''throat" was inserted at the
top of the cylindrical upper region of the combustor and the ex-
ternal cyclone separator was eliminated.
The combustor disclosed in U.S~. Patent No. 4,457,289 is sig-
nificantly less expensive to construct than the combustor dis-
closed in U.S. Patent No. 4,469,050, and other prior art
circulating fluidized bed combustors, since it does not require a
separate cyclone particle separator. However, it has demon-
strated a somewhat reduced particulate capturing efficiency com-
pared to such other combustors, particularly when burning solid
coal particles. Furthermore, the combustor disclosed in U.S.
Patent No., 4,457,289 provides a residence time for solid coal
particles and conventional sulfur absorbents which, in some
cases, may be less than optimum for capturing any sulfur in the
coal.
In conventional, non-circulating and circulating fluidized
bed reactors for combusting particulate material, the material to
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~L~80Z72,
be combusted is fed in or over a bed of granular material, usu-
ally fuel ash, sulfur absorbents such as limestone, and/or sand.
SUMMARY OF THE INVENTrON
The present invention, in a radical departure from the con-
ventional circulating fluidized bed reactors discussed above, has
overcome the above-enumerated problems and disadvantages of the
prior art by providing a two stage circulating fluidized bed re-
actor having a fluidized bed reaction (e.g., combustion) stage
followed by a cyclonic reaction (e.g., cyclonic combustion)
stage. A minor portion of the reaction gases (e.g., air) is
supplied beneath the fluidized bed as fluidizing gases, while a
major portion of the gases is supplied into the cyclonic reaction
stage. Such major portion of the gases is fed tangentially into
an upright cylindrically shaped cyclonic reaction vessel so as to
create a cyclone of high turbulence, whereby the reaction takes
place in both the fluidized bed and the cyclonic reaction vessel
at a significant~y increased rate. The solids entrained in the
fluidized bed stage are carried over into the cyclonic reaction
vessel where they are separated from the gases therein and
recycled back into the fluidized bed.
It is an objec~ of the invention to provide a circulating
fluidized bed reactor utilizing a cyclonic reaction stage which
provides a cyclone of turbulent gases having a Swirl number of at
least about 0.6 and a Reynolds number of at least about 18,000 in
~28C)~72
'la cylindrically shaped, refractory lined cyclonic reaction vessel¦
¦¦downstream of the fluidized bed, thereby providing a signifi- I
cantly improved reaction rate and requiring a significantly lowerj
,Ivolume of gases and solids circulating from the fluidized bed to
the cyclonic reaction vessel. Consequently, the size of the re-
actor of the present invention is significantly smaller than
prior art circulating fluidized bed reactors. Specifically, the
height and internal diameter of the free board region of the flu-
idized bed and the height and internal diameter of the cyclonic
reaction vessel of the present invention are significantly re-
duced, compared to the fluidized bed free board region and cy-
~clone particle separator, respectively, of a conventionalcirculating fluidized bed reactor having the same reactor capac-
lty.
A further object is to provide a reactor having a shorterfluidizing gas residence time required to complete the reaction
to the desired level. Specific heat releases in excess of about
1.5 million Kcal per cubic meter per hour are believed to be
obtainable according to the present invention.
The foregoing advantages will permit a significant reduction
in the size and, thus, the cost of constructing the circulating
fluidized bed reactor of the present invention. This will be
true in adiabatic combustor and boiler applications of the
invention. It is anticipated, for example, that several times
.
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.i .
~1 lX~ 2
less internal volume will be required for a combustor constructed¦
in accordance with the present invention, and for boiler applica-
tions, at least about 3-5 times less heat transfer surface area
will be needed for the combustion stage.
Still another object of the invention is to proYide an im-
proved boiler system having a high turndown ratio and easier
start-up than prior art systems. It is an additional object o~
the invention in this regard to provide a separate cooling flu-
idized bed adjacent to the circulating fluidized bed for removing
heat from the combustion stage by cooling the solids in the cool-
ing fluidized bed and then recycling them back to the combustion;
stage. The cooling fluidized bed is preferably fluidized in the
bubbling regime and contains evaporator, superheater and~or econ-
omizer coils immersed in the bubbling fluidized bed with the fur-
ther objective of significantly reducing the heat exchanger sur-
face area required for effective heat transfer. In such an
overall system (circulating fluidized bed reactor with adjacent
bubbling fluidized bed heat exchanger), it is a further objective
to eliminate the vertical heat exchanger tube-lined walls previ-
ously utilized in the upper region (vapor space) of prior art
circulating fluidized bed reactors, thereby considerably reducing
the cost of constructing such a syst.em.
To achieve the objects and in accordance with the purposes
of the invention, as embodied and broadly described herein, a
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1 ~Z8[)272
., ;
method of operating a circulating fluidized bed combustion reac- j
tor according to the invention comprises: (a) providing a sub-
stantially enclosed combustion reactor containing a fluidized bed.
of granular material, the reactor comprising a substantially up-
right combustion chamber and a substantially upright and cylin-
drical cyclonic combustor vessel adjacent to the chamber, the re-
spective upper regions of the chamber and the vessel being
connected via a conduit and the respective lower regions of the 1-
chamber and the vessel being operatively connected, the vessel
having a cylindrically shaped e~it throat aligned substantially
concentrically with, and at the top of, the vessel; (b) feeding
combustible matter into the combustion chamber; (c) supplying the !
first stream of pressurized air to the reactor through a plurali-
ty of openings at the bottom of the combustion chamber at a suf-
ficient velocity to fluidize the granular material and the matter
in the circulating regime for combusting a minor portion of the
matter in the chamber, whereby a substantial portion of the gran-
ular bed material, combustion product gases and uncombusted mat-
ter are continually entrained out of the chamber and into the
cyclonic combustor vessel via the conduit; (d) tangentially sup-
plying a second stream of pressurized air into the reactor
:through a plurality oE openings in the cylindrically shaped inte-
rior side wall of the vessel for cyclonic combustion of a major
portion of the combustible matter in the vessel, the second
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. , ,
~ ~L280Z7Z
stream being supplied, and the vessel being constructed and oper-
~ated, so as to produce a Swirl number of at least about 0.6 and a
Reynolds number of at least about 18,000 within the vessel for ¦,
creating a cyclone of turbulence therein having at least one in-
ternal reverse flow zone, thereby increasing the rate of combus-
tion therein; (e) permitting the combustion product gases gener-
ated in the reactor to exit from the reactor via the exit throat
in the cyclonic combustor vessel, while retaining substantially
all of the granular material and uncombusted matter within the
reactor; (f) collecting the qranular bed material and any
uncombusted matter in the lower region of the cyclonic combustor
vessel and returning it to the lower region of the combustion
chamber; and (g) controlling the combustion process in the reac-
tor by controlling the flow of the first and second streams of
air into the combustion chamber and the cyclonic combustor ves-
sel, respectively, and by controlling the flow of granular bed
material and matter to be combusted in the chamber and the ves-
sel.
The method of the present invention may be performed in an
adiabatic mode, in which the total pressuri~ed air supplied is in
excess of the stoichiometric amount needed for combustion; or in
a non-adiabatic mode in which a heat exchange surface is provided
in the fluidi~ed bed for removing heat from the bed.
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~2~3~)272
A method of operating a circulating fluidized bed combustionl
reactor according to a further embodiment of the invention com- ¦
prises: (1) providing a substantially enclosed combustion reactor¦
comprising: (a) a substantially upright combustion chamber con- ¦
taining a fluidized bed of granular material fluidized in the
circulating regime, (b) a first cooling chamber adjacent to the
combustion chamber and having a first heat exchange surface, (c)
a second cooling chamber having a second heat exchange surface,
the first and second cooling chambers having a common bubbling
fluidized bed in their bottom regions, and (d) a substantially
upright and cylindrical cyclonic combustor vessel adjacent and
operatively connected to the second cooling chamber and opera-
tively connected to the combustion chamber, the vessel having a
cylindrically shaped exit throat aligned substantially concen-
trically with, and at the top of, the vessel; (2) permitting sol-
ids from the bubbling fluidized bed to flow into the circulating
fluidized bed in the combustion chamber for controlling the tem-
perature of the latter bed; (3) feeding combustible matter into
the combustion chamber; (4) supplying a first stream of pressur-
ized air to the reactor through a plurality of openings at the
bottom of the combustion chamber at a sufficient velocity to flu-
idize the granular material and the matter in the circulating re-
gime for combusting a minor portion of the matter in the combus-
tion chamber, whereby a substantial portion of the
'i
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I
l ~ 2~3~2~ 1
granular bed material, combustion product gases and uncombusted
matter are continually entrained upward and out of the chamber
into the first cooLing chamber; (5) passing the product gases and
l entrained solids downward through the first cooling chamber and
i removing heat therefrom via the first heat exchange surface, and
permitting the entrained solids to enter the bubbling fluidized
! bed; (6) then passing the gases from the first cooling chamber to
,I the second cooling chamber and permitting the gases to ascend
¦ through the second cooling chamber while removing heat therefrom
via the second heat exchange surface; (7) entraining the solids
I. containing the uncombusted matter in the ascending gases in the
;jsecond cooling chamber and passing the gases and entrained solids
out of the second cooling chamber and into the upper region of
the cyclonic combustor vessel; (8) tangentially supplying a sec-
ond stream of pressurized air into the reactor through a plurali- .
, I
. ty of openings in the cylindrically shaped interior side wall of
the vessel for cyclonic combustion of a major portion of the com-
bustible matter fed to the reactor in the vessel, the second
stream being supplied, and the vessel being constructed and oper-
j~ated, so as to produce a SwirL number of at least about 0.6 and a
!~ Reynolds number of at least about 18,000 within the vessel forcreating a cyclone of turbulence therein having at least one in-
,¦ternal reverse flow zone, thereby increasing the rate of combus-
¦!tion therein, (9) permitting the combustion product gases
:j
1 5
B~:'72
il
generated in the reactor to exit from the reactor via the exit
throat in the cyclonic combustor vessel, while retaining substan-
tially all of the granular material and uncombusted matter within
the reactor; (lO) collecting the granular bed material and any
uncombusted matter in the lower region of the cyclonic combustor
vessel and returning it to the combustion chamber; and (ll) con-
trolling the combustion process in the reactor by controlling the
flow of the first and second streams of air into the combustion
chamber and the cyclonic combustor vessel, respectively, and by
controlling the flow of granular bed material and matter to be
combusted in the combustion chamber, the first and second cooling
chambers, and the vessel.
In addition to the above-described methods, the present
invention is also directed to a circulating fluidized bed reactor
comprising: (a) a substantially enclosed combustion reactor for
containing a fluidized bed of granular material, the reactor com-
prising a substantially upright combustion chamber and a substan-
tially upright and cylindrical cyclonic combustor vessel adjacent
to the chamber, the respective upper regions of the chamber and
the vessel being connected via a conduit and the respective lower
regions of the chamber and the vessel being operatively con-
nected; (b) means for feeding combustible matter into the combus-
tion chamber; (c) means for supplying a first stream of pressur-
ized air to the reactor through a plurality of openi~gs at the
-16-
80Z7~
bottom of the combustion chamber at a suf~icient velocity to flu-~
¦l idize the granular material and the matter in the circulating re-
,¦ gime for combusting a minor portion of the matter in the chamber,~
¦ whereby a substantial portion of the granular bed material, com-
',, bustion product gases and uncombusted matter are adapted to be
' continually entrained out of the chamber and into the cyclonic
j combustor vessel via the conduit; (d) means for tangentially sup-
plying a second stream of pressurized air into the reactor , !-
! through a plurality of openings in the cylindrically shaped inte-
rior side wall of the vessel for cyclonic combustion of a major
~ilportion of the combustible matte~r in the vessel, the vessel being
! constructed for producing a Swirl number of at least about 0.6
land a Reynolds number of at least about 18,000 within the vessel
: for creating a cyclone of turbulence therein having at least one
internal reverse flow zone, thereby increasing the rate of com-
bustion therein; (e) a cylindrically shaped exit throat aligned
substantially concentrically with, and at the top of the vessel
-for permitting the combustion product gases generated in the re-
;actor to exit from the reactor, while retaining substantially all
,of the granular material and uncombusted matter within the reac-
¦tor; and (f) means for collecting the granular bed material and
lany uncombusted matter in the lower region of the cyclonic
Icombustor vessel and returning it to the lower region of the com-
lbustion chamber.
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~ lZ8027'~
The accompanying drawings, which are incorporated in and
~Iconstitute a part of this specification, illustrate several em-
ilbodiments of the invention and, together with the description,
!~ serve to explain the principles of the invention.
¦ BRIEF DESCRIPTION OF THE DRAWINGS
', FIG. 1 is a diagrammatic vertical section view of an
,¦ adiabatic circulating fluidized bed reactor constructed in accor-
dance with the present invention.
i FIG. 2 is a diagrammatic vertical section view of a
icirculating fluidized bed reactor constructed in accordance with
the invention.
`¦ FIG. 3 is a diagrammatic plan cross sectional view A-B-C-D
of the circulating fluidized bed reactor depicted in FIG. 2.
FIG. 4 is a diagrammatic vertical section view of a
circulating fluidized bed reactor according to a further embodi-
ment of the invention.
FIGS. 5, 6 and 1 are further diagrammatic vertical section
views of the circulating fluidized bed reactor depicted in FIG.
4.
FIG. 8 and 9 are diagrammatic front section and top section
views, respectively, of an alternative heat exchanger tube
i arrangement suitable for use in the reactor shown in FIGS. 4-7.
FIG. 10 is a diagrammatic vertical section view of a
circulating fluidized bed reactor constructed in accordance with
a further embodiment of the invention.
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i!
'.1
`
. ~L28~2~2
FIGS. 11-13 are graphs plotting particulate loading vs. the ¦
fraction of air supplied as fluidizing air for three combustor
embodiments of the invention.
! DESCR r PTION OF THE PREFERRED EMBODIMENTS
Reference will now be made in detail to the presently pre-
ferred embodiments of the invention, examples of which are illus-
i trated in the accompanying drawings.
One preferred embodiment of the circulating fluidized bed
reactor of the present invention is shown in FIG. 1. As shown,
the reactor of the present invention may comprise, for example, a
combustor, represented generally by the numeral 1. In accordance
with this embodimènt of the invention, the combustor 1 includes a i
fluidized bed combustion chamber 10 containing a fluidized bed of !
granular material in its lower region ll. The granular bed mate-
rial is preferably fly ash, sand, fine particles of limestone
and/or inert materials.
The granular bed material is fluidized in the circulating
fluidization regime with pressurized oxygen-containing gas, for
example, air, which is supplied as a stream through a plurality
;of fluidization nozzles 12 extending through support surface 13.
¦At maximum operating capacity for the combustor, the air supplied
!j through openings 12 preferably constitutes less than about 50~,
¦and still more preferably between about 15-35%, of the total air
,supplied to combustor 1, i.e., the air required for the
-19-
.,
.
I! ,
,
. ' ' :
~28~2~2
combustion process. As will be discussed in detail below, one of
the primary objects of the invention, namely, the significant re-
duction in the size of the combustor 1 relative to conventional
circulating fluidized bed combustors, is achieved primarily by
feeding significantly reduced levels of air to the combustor as
fluidizing air, i.e., through nozzles 12. Thus, although amounts
of air in excess of 50% the total air supplied to combustor 1 can
be fed via fluidization ndzzles 12 in accordance with the present
invention, the extent to which the size of combustor 1 can be re-
duced will be increased proportionately by reducing the amount of
air supplied to combustor 1 as fluidizing air.
A source of pressurized air, e.q., a blower (not shown),
preferably feeds the air to a plenum chamber 15 beneath support
surface 13 or as shown in Fig. 1. Chamber 15 supplies the air
to nozzles 12. A separate conduit (not shown) extends through
support surface 13 for removing refuse, such as tramp material
and/or agglomerated ash, etc., if required, from combustion cham-
ber 10.
Combustor 1 further includes means for feeding combustible
matter to the combustor, preferably to the lower region 11 of
combustion chamber 10. As embodied herein, such means may com-
prise any suitable conventional mechanical or pneumatic feeding
mechanism 17. The combustible matter which may comprise
gases, liquids and/or solid particles, may be introduced into or
-20-
~3L2a~)Z7~
- l above the bed in lower region 11 of combustion chamber 10. The
combustible matter undergoes partial combustion in lower
i ententent to an extent limited by the free oxygen available in
'i the fluidizing gas. The unburnt fuel, any gaseous volatile mat-
ter, and a portion of the granular bed material are carried up-
ward (i.e., entrained) by the fluidizing gas and the flue gases
into an upper region 16 of combustion chamber 10, and exit from
upper region 16 through conduit 14 tangentially into the upper
region 18 of adjacent cyclonic combustor vessel 20.
It is generally known that the quantity of particles trans-
ported by an ascending gas from a circulating fluidized bed is a
function of the gas flow velocity to the third to fourth power.
Thus, greater solids reaction surface can be achieved by: (a)
maintaining maximum solids' saturation in the ascending gas flow,
and (b) increasing the vertical velocity of the fluidizing gas to
a desired level sufficient to provide the desired carry-over into
upper region 18 of cyclonic combustor vessel 20. For any solid
fuel having a given specific ash particle size distribution, this
vertical gas velocity must be sufficiently high, as noted above,
but must not be so high as to cause intensive erosion of the re-
fractory liner in upper region 16 of combustion chamber 10, due
to very high ash concentration in this region, as will be dis-
cussed below.
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OZ72
1~ The interior surface of upper region 18 is cylindrically
¦Ishaped in order to achieve swirling flow in such upper region, as j
discussed more fully below.
¦ In accordance with the invention, means are provided for
tangentially supplying a second stream of pressurized gas, e.q.,
air, to the upper region 18 of cyclonic combustor vessel 20
through openings 19, and preflerably at least two oppositely dis-
posed openings 19. Still more preferably, a plurality of open-
ings 19 are provided at several aqgregate points in upper region
la. As shown in FrG. 1, in one advantageous embodiment the plu-
rality of oppositely disposed openings are vertically aligned and
spaced apart throughout upper region 18. (The cross-sectional
view shown in FIG. 1 necessarily depicts only one vertical row of
openings.)
As embodied herein, a source of pressurized air, e.q., con-
ventional blower (not shown) feeds the second stream of air to,
for example, a conventional vertical manifold (not shown). In
one preferred embodiment of the invention, the second stream of
air constitutes between about 65%-85% of the total air fed to
combustor 1, i.e., the total air flow required for the combustion
process, at maximum combustor capacity.
In accordance with the invention, it is critical that the
secondary air be supplied at a sufficient velocity, and that the
geometric characteristics of the interior surface of upper region
-22-
I` ~Z80272
18 of cyclonic combustor vessel 20 be adapted, to provide a Swirl
number (S) of at least about 0.6 and a Reynolds number (Re) of at
least about 18,000, which are required to create a cyclone of
! turbulence in upper region 18. Preferably, upper region 18 is
I constructed and operated in a manner adapted to yield these mini-
lmum values of Swirl number and Reynolds number when operating at
jmaximum reactor capacity. On the other hand, the Swirl number
,iand Reynolds number must not exceed those values which would re-
,sult in an unacceptable pressure drop through vessel 20.
, This cyclone of turbulence enables combustor 1 to achieve
specific heat release values higher than about 1.5 million Kcal
per cubic meter per hour, thereby significantly increasing the
.I rate of combustion. As a result, the size of the chamber 10 and
vessel 20 of the present invention can be significantly reduced,
compared to the size of a conventional circulating fluidized bed
combustor free board region and hot cyclone separator, respec-
tively.
Cyclonic combustor vessel 20 is provided with a cylindri-
cally shaped exit throat 21 aligned substantially concentrically
, with the cylindrical interior surface of upper region 18. Exit
!I throat 21 and the interior of the upper region 18 of vessel 20
must exhibit certain geometric characteristics, together with the
~applicable gas velocities, in order to provide the above-noted
required Swirl number and Reynolds number. These features are
!
-23-
::'
~2a~72
explained below and are discussed generally in "Combustion in
Swirling Flows: A Review," supra~ and the references noted there-
in, which publications are hereby specifically incorporated here- ,
in by reference. I
The majority of the fuel combustion in combustor l prefer-
ably takes place in the cyclone of turbulence in upper region 18
of cyclonic combustor vessel 20 at a temperature below the fusion
point, which provides a friable ash condition.
The cyclone of turbulence in upper region 18, and the accom-
panying large internal reverse flow zones created therein, when
the cross-sectional area and length of upper region 18, the
cross-sectional area of tangential openings 19, and the diameter
of cylindrical exit throat 21 are properly sized (see below), ef-
fectively prevent all but the smallest solids from exiting from
upper region 18 through exit throat 21.
rn the embodiment shown in FIG. l, the granular bed material
ash and any unburnt fuel are collected in the lower region 22 of
vessel 20 and allowed to descend under the force of gravity
through port 23, re~urning to lower region 11 of combustion cham-
ber lO, thus constantly increasing the height of the bed in lower
region 11, if a fuel having a sensible amount of ash is burned.
As a result, it will be necessary to frequently discharge these
solids. The solids collected and not fluidized in lower region
22 of vessel 20 descend as a gravity bed effectively precluding
any gas flow through port 23.
-24-
-` i!
~ Z~ 2
If the upper region 18 of vessel 20 is designed and operated
so as to achieve a Swirl number of at least about 0.6 and a
Reynolds number of at least about 18,000 therewithin, and the
ratio of the diameter of the combustor exit throat 21 (De) to the !
diameter of upper region 18 (Do), i.e., De/Do (defined herein as
jX), lies within the range of from about 0.4 to about 0.7, prefer-
ably about 0.5 to about 0.6, upper region 18 will, during opera-
; tion, exhibit large internal reverse flow zones, with as many asthree concentric toroidal recirculation zones being formed. Such
recirculation zones are known generally in the field of conven-
tional cyclone combustors (i.e., not involving fluidized beds),
~ and reference is made to "Combustion in Swirling Flows: A Re-
! - view", suPra, and the references noted therein, for a general ex- !
;planation of such phenomena. Such cyclonic flow and
recirculation zones in upper region 18 act to separate the solids
from the gases present in upper region 18. The very high level
of turbulence in upper region la results in significantly im-
proved combustion intensity and, as a result of improved
solids-gas heat exchange, a substantially uniform temperature
throughout cyclonic combustor vessel 20.
As mentioned, vessel 20 should be constructed such that the
value of the ratio X lies ~ithin the range of from about 0.4 to
,about 0.7. The greater the value of X, the lesser the pressure
drop through vessel 20 and the greater the Swirl number; so that,
-25-
-- . ~
~LZ80~7~
.1 !
¦I generally, higher values of X are preferred. However, for value
of X in excess of about 0.7, the internal reverse flow zones are
I not formed sufficiently to provide adequate gas-solids separa-
! tion.
Although the fluidized bed reactor of the present invention
is fluidized in the "circulating" or "fast" fluidization regime,
it differs fundamentally from prior art circulating fluidized bed
reactors, in that: (a) it does not require the use of a large cy-
clone particle separator to separate the fluidized solids, e.q~,
the granular bed material, unburnt fuel, ash, etc., from the flue ;
gases, and (b) there is a significantly reduced gas flow through
upper region 16 of combustion chamber 10 and into cyclonic
combustor vessel 20 which, thus, can be of a smaller size. The
elimination of the requirement for large cyclone separators and
the reduced size of chamber 10 and vessel 20 will significantly
reduce the size and the cost of reactor systems constructed in
accordance with the present invention.
In operation, the combustible matter is fed into combustion
chamber 10. Optionally, for gaseous or liquid fuels, all or a
portion of the combustible matter may be fed directly into
cyclonic combustor vessel 20, preferably via tangential openings
19 .
The first stream of pressurized air is supplied to chamber
10 through fluidizing nozzles 12 at a sufficient velocity to
-26-
1~802~2
fluidize the granular bed material and combustible matter in the
circulating regime for combusting a portion of the combustibLe
matter in chamber 10. A substantial portion of the granular bed
material, combustion product gases and uncombusted matter are
continually entrained out of chamber 10 and into cyclonic
combustor vessel 20 via tangential conduit 14.
The second stream of pressurized air is supplied tan-
gentially to vessel 20 through openings 19 in the cylindrically
shaped interior side wall of the upper region 18 of vessel 20 for
cyclonic combustion of a major portion, for example, greater than
about 50% and preferably between about 65% and 85%, of the
,uncombusted matter in vessel 20.
The second stream of air is supplied, and vessel 20 is con-
structed and operated, so as to produce a Swirl number of at
least about 0.6 and a Reynolds number of at least about 18,000
within vessel 20 for creating a cyclone of turbulence therein
having at least one internal reverse flow zone, thereby increas-
ing the rate of combustion in vessel 20.
The combustion product gases generated in reactor 1 e~it
from the reactor via exit throat 21 in cyclonic combustor vessel.
Substantially all of the granular bed material and uncombusted
matter are separated from the combustion product gases and are
retained within vessel 20, collected in lower region 22 and
recycled to lower region 11 of chamber 10; preferably under the
-27-
2~2
.j ,
force of gravity via port 23. Alternately, any conventional sol- ~
ids transfer mechanism capable of preventing flue gases from en- f
tering into vessel 20 from chamber 10 may be used to recycle the
solids back to chamber 10.
A key advantage of the fluidized bed combustor 1 of the
present invention is that the cross-sectional areas of each of
the upper region 16 of chamber 10 and the upper region 18 of ves-
sel 20 are significantly smaller than the corresponding
cross-sectional area of the upper region, i.e., the free board
region, and the cyclone particle separator, respectively, of a
conventional circulating fluidized bed combustor of the same
capacity. This results in a significant savings in construction
costs for the fluidized bed combustor of the present invention.
The above-described size reduction is accomplished by, for
example, applying conventional circulating fluidized bed design
criteria to size combustion chamber 10 and vessel 20 to operate
at, for example, 25% of the desired capacity. That is, upper re-
gion 16 of chamber 10 and upper region 18 of vessel 20 may be
sized to handle only, for example, 2~ of the air flow associated
with a conventional circulating fluidized bed combustor free
board region and cyclone particle separator, respectively, of the
desired capacity. This significant reduction in size is made
possible by using vessel ?0 as both a cyclone particle separator
and a cyclonic combustor. Where, to continue the example,
-28-
~28~ Z'7%
combustion chamber 10 and vessel 20 are reduced in size to handle ¦
only 25% of the conventional air flow, the remaining 75% of the
conventional air flow is supplied as the second stream of air fed
tangentially to cyclone combustor vessel 20 via openings 19 for
¦cyclonic combustion of the major portion of the combustible mat-
ter in vessel 20.
I Thus, by selecting the relative amounts of air supplied to f
icombustor 1 via fluidi~ing air nozzles 12 in combustion chamber
f 10 and via tangential openings 19 in cyclonic combustor vessel
- 20, it is possible, in accordance with the present invention, to
reduce the volume of air flowing through chamber 10 and into ves-
.1 !
sel 20 via tangential conduit 14 and thereby proportionally re-
duce the cross-sectional areas of upper regions 16 and 18, com-
pared to the corresponding cross-sectional areas of the free
board region and the cyclone separator of a conventional
circulating fluidized bed combustor.
As shown, the embodiment depicted in FIG. 1 may comprise an
adiabatic combustor for generation of hot combustion gases, i.e.,
without any heat extraction from combustion chamber 10 or
cyclonic combustor vessel 20. The hot gases may, for example, be
used as process heat supply or supplied to heat a boiler, as
,known in the art. Such an adiabatic combustor operates at high
i! excess air, ~ith the level of excess air depending on the heating
value of the fuel being burned.
i .
'I
.
11~ 1280~:~Z
- ~, The combustion temperature in cyclonic combustor vessel 20
is controlled by controlling the fuel to air ratio. The desired
temperature difference between chamber 10 and vessel 20, which
~will vary from case to case, is controlled by maintaining the
proper mean particle size of the granular bed material and by
controlling the fluidizing air superficial velocity in chamber 10
-. to provide a mean,particle suspension density in chamber 10 and
. vessel 20 sufficient to sustain the desired temperature differ-
ence for the particular fuel being utilized.
FIG. 11 is a graph showing the particulate loading (KG/M3)
of fluidized bed granular material in upper region 16 of combus-
tion chamber 10 and upper region 18 of cyclonic combustor vessel
:20 for combustor 1 shown in FIG. 1 as a function of the fraction
( ~ ) of the total air flow into the combustor that is introduced
as fluidizing air via nozzles 1.2 in the bottom of chamber 10 for
temperature differences (~) between chamber 10 and vessel 20 of
50F t28C), 100F (56C) and 150F (8gC). This graph was pre-
pared based on calculations for Ohio bituminous coal having a low
heating value (LHV) of 6371 KCAL/KG, an air stoichiometric
.coefficient ( ~) of 3.3 and assuming the temperature of the flue
-gases exiting from combustor 1 via exit throat 21 is 1500F for
.the adiabatic combustor of FIG. 1.
As can be seen from FIG. 11, for ~ = 0.25, a temperature
difference of 100F or 150~F can be maintained between chamber 10
-30-
1l~ lZ80Z7Z
il and vessel 20 by maintaining the particulate loading at about 31
KG/M3 and 21 KG/M3, respectively, using conventionally known
j techniques, for example, by controlling mean particle size and
fluidizing air superficial velocity.
The method of the present invention can also be used for
boiler applications which, from an economic standpoint, require
¦ low excess air for combustion and, therefore, heat absorption in
the fluidized bed. In one embodiment of the invention, such heat
absorption is accomplished by installing a heat exchange surface
in upper region 16 of combustion chamber 10. As shown, for exam-
ple, in the dashed lines in FIG. 1, the heat exchange surface may ¦
comprise a heat exchanger tube arrangement 25. The tube arrange- ¦
ment may be of any 5uitable size, shape and alignment, including
a vertical tube wall, as is well known in the art. Preferably,
heat exchanger tube arrangement 25 will be operatively connected
to a process heat supply or to a conventional boiler drum (not
shown) for boiler applications. The heat exchanger cooling media
may comprise any suitable conventional liquid or gaseous media,
such as, for example, water or air.
In boiler applications, the exhaust gases exiting from
combustor 1 (FIG. 1) are preferably fed to the boiler convective
¦ tube bank in a conventionally known manner.
In the embodiment of FIG. 1, if heat exchanger tube arrange-
ment 25 is provided in upper region 16 of chamber 10, the
-31-
~2~3~Z7~
,1
combustion temperature in cyclonic combustor vessel 20 is con-
trolled by controlling the fluidizing air flow rate through ple-
num 15 at a given tangential air flow rate in upper region 18 of
cyclonic combustor 20. This, in turn, controls the amount of
solid particulate carryover from upper region 16 to upper region .
18 via tangential conduit 14 and, consequently, the heat transfer
coefficient of heat exchanger tube arrangement 25 is changed.
In the embodiment shown in FIG 1 utilizing optional heat
exchanger tube arrangement 25, combustor capacities below 100~
are achieved by sequentially reducing the tangential air flow in
vessel 20 and then reducing the fluidizing air flow through noz-
zles 12 in chamber 10.
FIG. 12 is a graph showing the temperature difference in de-
grees Celslus ( ~T) between vessel 20 (essentially the tempera-
ture of the flue gases exiting via throat 21) and chamber 10,
(essentially the temperature in upper region 16) as a function of
the particulate loading ~XG/M ) of fluldized bed granular mate-
rial in the flue gases in the upper region 16 of chamber 10, for
the FIG. 1 embodiment utilizing heat exchanger tube arrangement
25. This graph was prepared based on calculations for Ohio bitu-
minous coal having a L~V of 6371 KCAL/KG, an c~ of 1.25 and
assuming the temperature of the flue gases exitlng via exit
throat 21 is 1550F for the combustor of FIG. 1 with heat ex-
changer tube arrangement 25 installed.
-32-
~8~;~t7Z
ll i
`il As can be seen from FIG. 12, a very wide range of tempera- !
Iture differences between chamber 10 and vessel 20, 25C (45F) to
,1184~C (150F), can be achieved if the particulate loading is var-
jied between 50 ~G/M3 and 15 KG/M3, respectively. Such tempera-
'ture differences do not depend upon the value of ~ , the frac-
¦tion of the total air flow that is introduced as fluidizing air
(as described above), but rather, depend upon the particulate
! loading Z. Consequently, such a combustor can be designed with
~ 25% and a relatively low air superficial velocity in chamber
10, provided the particulate loading is maintained at least at 15 i
XG/M3, for example, a temperature difference ( ~ S) limit of 150F
~for a given combustor design.
Turning now to FIGS. 2 and 3, these figures illustrate an
~embodiment of the invention particularly suitable for use in
boiler applications in which a high boiler turndown ratio is de-
sired. Like reference numerals have been used in FIGS. 2 and 3
to identify elements identical, or substantially identical, to
those depicted in FIG. 1, and only those structural and opera-
tional features which serve to distinguish the embodiment shown
in FIGS. 2 and 3 from those shown in FIG. 1 will be described
below.
! In particular, the embodiment shown in FIGS. 2 and 3 in-
cludes a cooling fluidized bed 40 (with a heat exchanger) situat-
ed immediately adjacent to region 11 of combustion chamber 10 and
-33-
` I ~ zaoZ~Z
¦~separated therefrom by a partition 30 having an opening 41
¦communicating with lower region 11. Cooling fluidized bed 40
¦comprises an ordinary (i.e., bubbling) fluidized bed of granular
! material, and includes a heat exchange surface, e.q., shown here
as heat exchanger tube arrangement 42, which contains water or
another coolant fluid, such as, for example, steam, compressed
air, or the like. The bed 40 is fluidized by tertiary pressur- I
ized air supplied from a plenum 43 through openings 44 in a SUp- j
port surface. As shown, these openings may take the form of noz- j
zles.
Fluidized bed 40 is comprised of the granular material and
other solids flowing from lower region 11 into bed 40 through
opening 41, as will be explained below by referring to both FIG.
2 and FIG. 3. Combustion also takes place in fluidized bed 40.
~eat exchanger tube arrangement 42 functions as a cooling coil to
cool fluidized bed 40. The cooled solids and combustion gases
leave bed 40 through openings 45 and 46, respectively, in parti-
tion 30 which separates bed 40 from the circulating fluidized bed
contained in lower region 11, and re-enter lower region 11 of re-
actor chamber 10. The solids are again fluidized therein. The
,¦ fluid passing through tube arrangement 42 is preferably supplied
from, for example, a conventional boiler drum (not shown) and
after being heated and partially vaporized, is returned to the
boiler drum. The fluid passing through tube arrangement 42 may
-34-
lZ~OZ72
also typically comprise steam for superheating or air for genera- ¦
tion of compressed air.
The movement of solids from the bubbling fluidized bed 40 to
the circulating fluidized bed in lower region ll of combustion
chamber lO is preferably motivated by specially designed solids
reinjection channel 47 (see FIG. 3) having a high solids
reinjection rate capability for reinjection of solids back into
lower region ll via port 48. Reinjection channel 47 has sepa-
rately fed fluidizing nozzles (not shown) beneath it, with the
solids reinjection rate being controlled by controlling the
amount of air fed through these nozzles.
Fluidized bed 40 may optionally consist of two or more sepa-
rate beds which may be interconnected or not, as desired, with
each having a separate tube arrangement.
For a better understanding of how this boiler embodiment
functions to improve the turndown ratio, a preferred procedure
for initially placing it into operation from the cold condition
to a full load and then turn it down to a desired level will be
explained.
An ignition burner (not shown), which may be located above
or under the fluiudized bed level in lower region 11, is turned
on along with the first (fluidizing) air stream (nozzles 12),
with the second air stream (nozzles l9), the cooling bed flu-
idizing air stream (nozzles 44) and the solids reinjection air
-35-
~%B0272
;l :
stream being shut off. When the combustor's refractory in cham-
ber 10 and its internal volume temperature exceed the solid fuel
ignition temperature, the uel is fed into combustion chamber 10. ,
After the solid fuel is ignited and, consequently, the
combustor's exit gas temperature has risen to the design level,
the ignition burner is turned off, and from this moment an
adiabatic fluidized bed combustor scheme is in operation at a
high excess air and having a capacity lower than the minimum de-
signed capacity.
To reduce the high excess air to the design level, the fuel
feed rate is increased, and to maintain the combustion tempera-
ture at a constant level, the cooling bed fluidizing air and the
solids reinjection air flow through channel 47 are turned on and
are kept at the required rate. From this moment the combustor
is in operation at its minimum designed capacity with the corre-
sponding design parameters.
To increase the unit's capacity, at this time the air flow
in the second stream (nozzles 19) is gradually increased, with a
simultaneous increase in the solid fuel feed rate, and a corre-
sponding increase in the solids reinjection air flow rate through
;channel 47 to maintain the combustion temperature constant. When
the second stream flow rate achieves its maximum design level,
the combustor can be considered as having its full load (100%
capacity).
-36-
280Z7Z
At this moment, if the gas exit temperature is at the de-
sired, i.e., design, level, the second stream air flow and fuel
rate are not increased any further, and are then maintained in
accordance with the fuel-air ratio required to obtain the most
economical fuel combustion.
The minimum capacity of the reactor, i.e., desired turndown
ratio, can be obtained if the sequence of operations outlined
!
above is followed in reverse order, until the point where the ig-
nition burner is shut off. Namely, while maintaining the desired
fuel-air ratio, the second stream air flow (nozzles 19) is re-
~'duced until it is completely shut off. At the same time, thesolids reinjection air is decreased proportionately to maintain
the combustion temperature at a constant level. As a result, the I
solids' circulation through cooling fluidized bed 40 is reduced
to a minimum corresponding to the combustor's minimum designed
capacity, and likewise the heat exchange process between bed 40
and heat exchanger tubes 42 is reduced.
In brief review, the key feature, in terms of obtaining a
high turndown ratio according to the embodiment depicted in FIG.
2, is the fact that the cooling fluidized bed heat exchange sur-
face 42 may be gradually pulled out (but not physically) from the
combustion process so as to keep the fuel-air ratio and combus-
tion temperature at the required levels.
--37--
, . . .
~28~Z72
!
¦ Furthermore, the above-desired boiler turndown ratio im-
provement has an additional advantage over known circulating flu-
idized bed boilers. Specifically, it requires less than one-half
! the heat exchange surface to absorb excessive heat from the
circulating fluidized bed, due to the following: (a) the tubular
surface 42 immersed in fluidized bed 40 is fully exposed to the
heat exchànge process, versus the vertical tube-lined walls in
the upper region of the combustion chamber of prior art
circulating fluidized bed boilers, in which only 50~ of the tube
surface is used in the heat exchange process; (b) the fluidized
bed heat exchange coefficient in such a system is higher than
that for gases, even heavily loaded with dust, and vertical
tube-lined walls con~ining the combustion chamber of prior art
circulating fluidized bed boilers. The latter resuLts, in part,
from the fact that it is possible, by using a separate fluidized
bed 40, to utilize the optimum fluidization velocity therein, and
the fact that fluidized bed 40 is comprised of small particles,
for example, fine ash and limestone.
FIG. 13 is a graph showing the particulate loading (KG/M3)
of fluidized bed granular material in upper region 16 of combus-
tion chamber lO and upper region 18 of cyclonic combustor vessel
20 for combustor l shown in FIG. 2 as a function of the fraction
of the total air flow into the combustor that is introduced
as fluidizing air via nozzles 12 and 44 in the bottom of chamber
-38-
1280272
¦~10 for temperature differences between chamber 10 and vessel 20
f 45F (20C), 90F (50C) and 150F (34=C). ~his graph was
prepared based on calculations for Ohio bituminous coal having an
¦¦LHV of 6371 KCAL/KG, an C~' of 1.25 and assuming the temperature
of the flue gases exiting from combustor 1 via exit throat 21 is
1550oF.
As can be seen from FIG. 13, for ~ = 0.25, a temperature
difference of 90F or 150F can be maintained between chamber 10
and vessel 20 by maintaining the particulate loading at about 75
KG/M3 and 44 KG/~3, respectively, using conventionally known
techniques as described previously.
In another embodiment of the invention, heat absorption from
the fluidized bed through the use of an adjacent cooling flu-
idized bed 40 (FIG. 2) and by additionally installing a heat ex-
change surface in upper region 16 of combustion chamber 10. As
shown, for example, in the dashed lines of FIG. 2 (indicating its
optional nature), the heat exchange surface may comprise a heat
exchanger tube arrangement 25. The constructional and operation-
al features of tube arrangement 25, as well as its interaction
with the other features of combustor 1 are the same as discussed
previously in connection with FIG. 1.
FIGS. 4-? illustrate a further embodiment of the present
invention for achieving high capacity without requiring an exces-
sively tall or otherwise large unit. This embodiment provides
-39-
~2~3~27~
¦more heat transfer than the other embodiments discussed previous- j
!¦ ly. Like reference numerals have been used to identify elements
¦identical, or substantially identical, to those depicted in FIGS.
l and ~.
In this embodiment, combustion chambe~ 10 is constructed and
functions virtually identical:Ly to chamber 10 in the other em-
bodiments of the invention. Preferably, no heat exchange surface
is present in chamber 10 and conduit 14 extends from upper region
16 into the top of a substantially upright, cooling chamber 50
containing a heat exchange surface. As shown, the heat exchange
surface preferably comprises conventional heat exchanger tube
lined walls 51. Inlet headers 52 and outlet headers 54 are pro-
vided for tube lined walls 51. Optionally, upper region 16 of
; chamber 10 may also contain similar heat exchanger tube lined
walls (not shown).
The co~bustion product gases and the granular bed material
and unburnt combustible matter entrained therein exit chamber l0
through conduit 14 and descend along with the flue gases, through
second chamber 50. At the bottom of chamber 50 is a fluidized
bed 60 fluidized in the bubbling, i.e., non-circulating, regime.
Tube lined walls 80 preferably surround and serve to contain flu-
idized bed 60.
i As shown best in FIG. 4, fluidized bed 60 is in solids, but
not in gas communication with the circulating fluidized bed in
'
I 1 ~280272
1,
chamber 10 through the overflow opening (denoted by the arrow A
in FIG. 4) between chamber 10 and chamber 50. 3y controlling the !
¦ vertical height of fluidized bed 60, which is accomplished by
i controlling ~he fluidizing air flow through nozzles 91 beneath
bed 90, varying amounts of bed material from bed 60 can be made
'to overflow wall 62 into lower region 11 of chamber 10. As a
Iresult of the heat exchange that takes place as the combustion
product gases, granular bed material and unburnt combustible mat-
ter pass through cooling chamher 50, the solids overflowing wall
. 62 into lower region 11 will have a lower temperature than the
l solids in chamber 10. Consequently, the temperature in chamber
10 can be regulated in part by controlling the amount of solids
overflowing wall 62 into chamber 10.
Immediately adjacent to the cooling chamber 50 is a substan-
tially upright second cooling chamber 70.. Chambers 50 and 70
share a common, interior tube lined wall 51A. Wall 51A is prefer-
able constructed as a tube sheet having fins extending between
the tubes to render the tube sheet substantially impervious from
its uppermost point downward to a height just above the top of
;fluidized bed 60 where there are no fins between the tubes, thus
.¦permitting pass-ge of gases from the lower region of chamber 50
linto the lower region of second cooling chamber 70. Thus, in the
,lower region of cooling chamber 50, above fluidized bed 60, the
: gases descending through chamber 50 effectively make a U-turn,
i
,. -41-
.!
~ Z8~);27~
entering second cooling chamber 70 above fluidized bed 60 at the
I¦ bottom of chamber 70.
,1 In second cooling chamber 70, combustion product gases flow
! upward and then out from the upper region of chamber 70 via tan-
gential conduit 71 into the upper region 18 of a cyclonic
combustor vessel 20. Vessel 20 is constructed and functions vir-
tually identically to vessel 20 in the other embodiments of the
invention previously discussed, with the solids collected at the
bottom of vessel 20 being recycled under the force of gravity
through port 23 into the lower region 11 of chamber 10 (see FrGs.
5 and 5). Alternatively, any similar conventional device, such
~as, for example, a non-mechanical sluice, may also be used.
An upflow channel 72 is created within or adjacent chamber
70. As embodied herein, channel 72 is formed by providing an
inner wall 51B (FIGS. 5 and 6), which preferably comprises a tube
lined wall as shown. Wall 51B is open at its upper end and con-
tains a lower opening for permitting fluidized bed solids,
including the granular bed material and unburnt combustible mat-
ter, to enter channel 72 (as shown by arrow B in FIG. 5). At
the bottom of channel 72 are fluidization gas nozzles 73 for flu-
idizing in the pneumatic transport regime. The solids in channel
72 are thus entrained upwardly in the fluidization gases and exit
from the open upper end of channel 72 into the upper region of
chamber 70 (as shown by arrow C in FIG. 5~. At this point, these
.
-42-
~2~302~72
elevated solids are entrained by the ascending gases in chamber
70 and are carried out of chamber 70 via conduit 71. The veloci-
ty of the ascending gases must, thus, be sufficiently high to
! permit such carryover of the solids issuing from the top of chan-
¦nel 72. Preferably, such velocity is sufficiently high, and
! channel 72 is constructed and operated, so as to provide a rate
of particulate solids entry into cyclonic combustor vessel 20 via
tangential conduit 71 substantially equal to, or greater than,
,the rate of particulate solids exiting from combustion chamber 10
,via conduit 14.
The internal cross-sectional area of combustion chamber 10
. can be significantly~smaller than the free board region of a con-
ventional circulating fluidized bed combustor; typically 4 to 5
times smaller, with respect to its cross-sectional area.
In operation of the embodiment depicted in FIGS. 4-7, the
superficial gas velocity is very high in chamber 10 for providing
the desired particulate solids loading in the combustion product
gases exiting via conduit 14. The downward superficial gas ve-
locity in first cooling chamber 50, which is less than that in
combustion chamber 10, is not high enough to cause damaging ero-
¦sion of tube lined walls 51A, 80 or any other heat transfer sur-
! face installed in cooling chamber 50. The same is true for the
upward superficial gas velocity in second cooling chamber 70.
. .
-43-
1~8C~Z'7~:
'I
I The combustion product gases entering first cooling chamber
! 50 via conduit 14 are very heavily laden with solid particles
(i.e., high particulate solids loading), thereby providing a high I
heat transfer coefficient in conjunction with tube lined walls
51A, 80 despite the somewhat lower gas velocity than in combus-
tion chamber 10.
The combustion product gases flowing upward through second
cooling chamber 70 have a sufficient velocity to provide the de-
sired particulate solids loading for the gases entering cyclonic
combustor vessel 20 via tangential conduit 71, i.e., loading se-
lected to maintain the desired combustion temperature in vessel
20. Such loading is controlled by the velocity of upwardly flow-
ing gases in chamber 70 and the amount of particulate solids
exiting from the top of channel 72, as described previously.
A portion of the solids carried by the gases in first and
second cooling chambers 50, 70 will separate from the gases and
fall into bubbling fluidized bed 60. Tramp material and ash
building up in the bed is periodically removed via conduits 85
and 100 in a conventionally known manner. The fluidized bed
material inventory in bed 60 is maintained at the desired levels
by overflowing the bed material from bed 60 into the lower region
11 of combustion chamber 10, as previously described.
Combustion takes place in combustion chamber 10 and cyclonic
combustor vessel 20 as described in connection with the
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¦embodiments of FIGS. 1 and 2, with the majority of the combustion I
¦taking place in vessel 20. For example, in one preferred embodi- ¦
ment, in excess of about 70% of the total air fed to combustor 1
¦is fed via tangential air inlets 19 in vessel 20.
The capacity of the combustor shown in FIGS. 4-7 can be
turned down from 100% capacity, and vice-versa, in substantially
the same manner as described previously in connection with the
embodiments of FIGS. l and 2.
As explained above, in the embodiment shown in FIGS. 4-7
the velocity of the combustion product gases in first cooling
chamber 50 is less than the gas superficial velocity in combus-
tion chamber lO. However, the gas velocit~ in chamber 50 is not
high enough to create an erosion problem with any internal heat
transfer surface. In an alternative embodiment of the invention
shown in FIGS. 8 and 9, the heat transfer surface in first cool-
ing chamber 50 comprises both heat exchanger tube-lined walls 80
and serpentine-like tubular heat exchanger coils 81 installed in-
side the chamber. This embodiment permits the height of first
cooling chamber to be reduced and utilizes a more compact heat
transfer surface. Combustion gases heavily laden with the
particulates entrained out of combustion chamber 10 via conduit
!14 flow downward between the serpentinTe coils 81 which are pref-
,¦erably inclined at 12-15 for natural water circulation. This
heat exchanger coil arrangement provides minimum obstruction to
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' ~28~Z72
., ,
gas flow and does not require any practical increase in the cham-
ber's cross-sectional area at a given gas velocity, compared with
an arrangement in which the heat exchanger coils are aligned hor-
izontally. Moreover, such a horizontal tube alignment does not
provide natural water circulation. On the other hand, a strictly
vertical arrangement of coils 81 would require a multiplicity of
tubes and very large headers.
FIG. 10 depicts a further embodiment of the invention having l~.
enhanced particle separation efficiency in the cyclonic combustor
vessel. Except where noted below, the structure and operation of
combustor l are virtually identical to those shown in FIG. l, and I
like reference numerals have been used to identify elements iden- i
tical, or substantially identical, to those depicted in FIG. 1.
As discussed previously, cyclonic combustor vessel 20 also
performs a gas-solids separation function. In particular, the
lower region 22 of vesel 20 has a downwardly converging shape
(e.g., as a hopper) for collecting the particulate solids sepa-
rated from the gases by the spinning flow in upper region 1~.
~he solids slide down the interior surface of vessel 20 as a mass
of bulk material which is discharged via port 23 back into the
fluidized bed in lower region ll of combustion chamber 10.
It is known in the art of cyclone separation that the ef-
fective operation of a conventional cyclone particle separator
can be destroyed by gas (air) leakage upward into the separator
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~l286)27Z
ll through the particle collection hopper at the bottom of the sepa-
,'rator. Such gas leakage into the bottom of the cyclone separator
jcan, if large enough, provide an upwardly moving gas stream in
¦the separator which can reduce the cyclone particle separation
efficiency to zero.
In the combustor of the present invention, such undesirable
gas leakage can also reduce the particle separation efficiency of
;cyclonic combustor vessel 20. The most destructive effect on
separation efficiency is produced by leaked gases which pass up-
wardly through vessel 20 in the central core region of the ves-
sel. To combat the passage of any leaked gases upward through
the central core~region, the embodiment shown in FIG. lO is
equipped with a substantially centrally located, vertically
aligned, refractory column 82 having a diameter approximately
'equal to or somewhat less than that of exit throat 21. Column 82
functions to divert any gases which may leak into the bottom of
vessel 20 away from the central region of the vessel. Column 82
preferably has a top portion which is frusto-conically shaped.
Gas diverter column 82 may obviously be utilized in any of
the embodiments of the invention disclosed here or in the inven-
tion disclosed in my U.S. patent No. 4,457,289. For example, it
may be installed in cyclonic combustor vessel 20 of the embodi-
ment depicted in FIGS. 4-7.
128~2~2
It will be apparent to those of ordinary skill in the art
that various modifications and variations can be made to the
above-described embodiments of the invention without departing
from the scope of the appendecl claims and their equivalents. As
an example, although the invention has been described in the
field of fluidized bed combustors, the invention can be used for
other applications in which fluidized b~d reactors are used, such
as, for example, various chemical and metallurgical processes.
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