U.S. patent application number 09/739281 was filed with the patent office on 2002-06-20 for oxygen separation and combustion apparatus and method.
Invention is credited to Bool, Lawrence E. III, Kobayashi, Hisashi.
Application Number | 20020073938 09/739281 |
Document ID | / |
Family ID | 24971599 |
Filed Date | 2002-06-20 |
United States Patent
Application |
20020073938 |
Kind Code |
A1 |
Bool, Lawrence E. III ; et
al. |
June 20, 2002 |
OXYGEN SEPARATION AND COMBUSTION APPARATUS AND METHOD
Abstract
An oxygen separation and combustion apparatus such as a boiler
or a nitrogen generator in which a plurality of fluid passages and
oxygen transport membranes are located within a combustion chamber.
The oxygen transport membranes separate oxygen from an oxygen
containing gas, thereby to provide the oxygen within the combustion
chamber to support combustion of a fuel and thereby to generate
heat. The fluid passages are positioned to allow a portion of the
heat to be transferred from the combustion to the oxygen transport
membranes to heat said oxygen transport membranes to an operational
temperature and a further portion of the heat to be transferred
from the combustion to the fluid, thereby to heat the fluid and
also, to help stabilize the operational temperature of said oxygen
transport membranes. Fuel is introduced into the combustion chamber
by injection or as a mixture with circulated flue gas. The fuel is
introduced into the combustion chamber and the combustion products
are discharged so that said combustion products flow in a direction
predominantly parallel to said membranes.
Inventors: |
Bool, Lawrence E. III;
(Hopewell Junction, NY) ; Kobayashi, Hisashi;
(Putnam Valley, NY) |
Correspondence
Address: |
PRAXAIR, INC.
LAW DEPT - M1557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
24971599 |
Appl. No.: |
09/739281 |
Filed: |
December 19, 2000 |
Current U.S.
Class: |
122/451.1 ;
122/488; 122/489; 95/234; 95/243 |
Current CPC
Class: |
B01B 1/005 20130101;
C01B 13/0251 20130101; B01D 63/06 20130101; C01B 2210/0046
20130101; B01J 8/009 20130101; B01J 8/067 20130101; C01B 3/384
20130101; B01J 2208/00309 20130101; C01B 2203/0233 20130101; B01D
53/22 20130101; C01B 2203/0811 20130101 |
Class at
Publication: |
122/451.1 ;
122/488; 122/489; 95/234; 95/243 |
International
Class: |
F22B 037/26 |
Claims
We claim:
1. A oxygen separation and combustion apparatus comprising: a
combustion chamber; a plurality of parallel oxygen transport
membranes located within said combustion chamber to separate oxygen
from an oxygen containing gas, thereby to provide the oxygen within
the combustion chamber to support combustion of a fuel and thereby
generate heat; a plurality of fluid passages passing through said
combustion chamber; said fluid passages positioned so that a
portion of the heat is transferred from the combustion to said
oxygen transport membranes to heat said oxygen transport membranes
to an operational temperature and a further portion of the heat is
transferred from the combustion to said fluid passages to provide
heat to heat fluid and to promote stabilization of the operational
temperature of said oxygen transport membranes; at least one inlet
for introducing at least the fuel into said combustion chamber; and
an exhaust from said combustion chamber to discharge combustion
products arising from combustion of the fuel; the exhaust and said
at least one inlet spaced apart from one another so that said
combustion products flow in a direction predominantly parallel to
said oxygen transport membranes.
2. The apparatus of claim 1, wherein said oxygen transport
membranes and said fluid passages are of tubular configuration.
3. The apparatus of claim 2, wherein said direction is
countercurrent to gas flow of the oxygen containing gas within said
oxygen transport membranes.
4. The apparatus of claim 2, wherein said direction is co-current
to gas flow of the oxygen containing gas within said oxygen
transport membranes.
5. The apparatus of claim 2, wherein: the oxygen transport
membranes are closed at one end and open at the end to discharge an
oxygen-depleted retentate; and a plurality of coaxial lance tubes
project into open ends of said oxygen transport membranes to supply
the oxygen containing gas thereto.
6. The apparatus of claim 5, wherein said direction is
countercurrent to gas flow of the oxygen containing gas within said
oxygen transport membranes.
7. The apparatus of claim 5, wherein said direction is co-current
to gas flow of the oxygen containing gas within said oxygen
transport membranes.
8. The apparatus of claim 6 or claim 7, wherein said at least one
inlet comprises an inlet to said combustion chamber for introducing
a mixture of the fuel and a flue gas into said combustion
chamber.
9. The apparatus of claim 7, wherein said at least one inlet
comprise fuel nozzles located adjacent to the open ends of said
oxygen transport membranes.
10. The apparatus of claim 8, wherein said fluid is water.
11. The apparatus of claim 9, wherein said fluid is water.
12. The apparatus of claim 10, wherein: said fluid passages are
interspersed between said oxygen transport membranes and said fluid
passages and said oxygen transport membranes are parallel to one
another; said fluid passages communicate between fluid inlet and
outlet manifolds to supply fluid to said fluid passages and to
discharge fluid therefrom, respectively; said oxygen transport
membranes project, from said open end thereof, from a retentate
outlet manifold to discharge oxygen depleted air; and said lance
tubes project from an air inlet manifold.
13. The apparatus of claim 12, wherein: said fluid passages are
interspersed between said oxygen transport membranes and said fluid
passages and said oxygen transport membranes are parallel to one
another; said fluid passages communicate between fluid inlet and
outlet manifolds to supply fluid to said fluid passages and to
discharge fluid therefrom, respectively; said oxygen transport
membranes project, from said open end thereof, from a retentate
outlet manifold to discharge oxygen depleted air; and said lance
tubes project from an air inlet manifold.
14. An oxygen separation and combustion method comprising:
introducing an oxygen containing gas into a plurality of parallel
oxygen transport membranes located within a combustion chamber;
separating oxygen from the oxygen containing gas within the
plurality of parallel oxygen transport membranes, thereby to
provide oxygen within the combustion chamber; introducing fuel into
the combustion chamber; combusting the fuel within the combustion
chamber in the presence of the oxygen to generate heat; passing a
fluid through a plurality of fluid passages located within the
combustion chamber; discharging combustion products from the
combustion chamber; the combustion products being discharged from
the combustion chamber and the fuel being introduced so that the
combustion products flow in a direction predominantly parallel to
said oxygen transport membranes to provide a reactive purge to
promote the separation of the oxygen from the oxygen containing
gas; and the fluid passages being positioned so that a portion of
the heat is transferred from the combustion to said oxygen
transport membranes to heat said oxygen transport membranes to an
operational temperature and a further portion of the heat is
transferred from the combustion to said fluid passages to provide
heat to heat the fluid and to promote stabilization of the
operational temperature of said oxygen transport membranes.
15. The method of claim 14, wherein said oxygen transport membranes
and said fluid passages are of tubular configuration.
16. The method of claim 14, wherein said direction is
countercurrent to gas flow of the oxygen containing gas within said
oxygen transport membranes.
17. The method of claim 14, wherein said direction is co-current to
gas flow of the oxygen containing gas within said oxygen transport
membranes.
18. The method of claim 14 or claim 16 or claim 17, wherein said
fluid is water.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a oxygen separation and
combustion apparatus and method that can be applied to such devices
as a boiler or a nitrogen generator in which oxygen separated from
an oxygen containing gas by oxygen transport membranes supports
combustion of a fuel within a combustion chamber and temperature of
the oxygen transport membranes is controlled by fluid circulating
within fluid passages passing through the combustion chamber.
BACKGROUND OF THE INVENTION
[0002] Growing concerns about environmental issues, such as global
warming and pollutant emissions, are driving industries to explore
new ways to increase efficiency and reduce emissions of pollutants.
This is particularly true for fossil fuel fired combustion systems,
which represent one of the largest sources of carbon dioxide and
air pollution emissions. One effective way to reduce emissions and
to increase efficiency is to use oxygen, or oxygen enriched air, in
the combustion process. The use of oxygen or oxygen enriched air
reduces stack heat losses, which increases the system efficiency,
while at the same time reducing NOx emissions. Further, the
concentration of carbon dioxide in the flue gas is higher since
there is little or no nitrogen to act as a diluent. The higher
carbon dioxide concentration enhances carbon dioxide recovery
options. Oxygen use in the prior art has been limited to those
processes with high exhaust temperatures, such as glass furnaces.
In such applications, the fuel savings and the benefits achieved
are greater than the cost of the oxygen. In low exhaust temperature
systems, such as boilers, the reverse is true. In these systems,
the cost of oxygen produced with current technologies is more
expensive than the available fuel savings. This makes oxygen use in
such systems economically unattractive. Moreover, when the energy
required to produce the oxygen is taken into consideration, the
overall thermal efficiency decreases.
[0003] Oxygen transport membranes have been advantageously utilized
in the prior art to produce oxygen for heat consuming oxygen
separation and combustion apparatus and processes in a manner that
results in a savings of energy that would otherwise have to be
expended in the separation of oxygen. Oxygen transport membranes
are fabricated from oxygen-selective, ion transport ceramics in the
form of tubes or plates that are in themselves impervious to the
flow of oxygen and other gases. Such ceramics, however, exhibit
infinite oxygen selectivity at high temperatures by transporting
oxygen ions through the membrane. In oxygen transport membranes,
the oxygen is ionized on one surface of the membrane to form oxygen
ions that are transported through the membrane. The oxygen ions on
the opposite side of the membrane recombine to form oxygen with the
production of electrons. Depending upon the type of ceramic, oxygen
ions either flow through the membrane to ionize the oxygen or along
separate electrical pathways within the membrane, or by an applied
electric potential. Such solid electrolyte membranes are made from
inorganic oxides, typified by calcium-or yttrium-stabilized
zirconium and analogous oxides having fluoride or perovskite
structures.
[0004] In U.S. Pat. No. 5,888,272 oxygen transport membranes are
integrated into a combustion process itself, with all the oxygen
produced going directly into the combustor. The heated flue gases
can then be routed to a process wherein the thermal energy can be
used to heat a fluid or perform useful work. In one embodiment,
flue gases are recycled through a bank of oxygen transport membrane
tubes and enriched with oxygen. Typically the flue gas enters the
bank containing anywhere from 1 to about 3 percent oxygen and
leaves the bank containing from about 10 to about 30 percent oxygen
by volume. The enriched flue gas is then sent to a combustion space
where it is used to burn fuel. In another embodiment, called
reactive purge, the oxygen transport membrane tubes are placed
directly in the combustion space. A fuel diluted with flue gas is
passed through the tubes and combust with the oxygen as it passes
through the tubes. Thus oxygen production and combustion take place
simultaneously.
[0005] As will be discussed, the present invention utilizes oxygen
transport membranes to produce oxygen to support combustion within
a oxygen separation and combustion apparatus such as a boiler in a
manner that inherently reduces the energy expenditures involved in
compressing an incoming oxygen containing feed to the membranes.
The advantages of the present invention will become apparent from
the following discussion.
SUMMARY OF THE INVENTION
[0006] In one aspect, the present invention provides an oxygen
separation and combustion apparatus comprising a plurality of
parallel oxygen transport membranes located within a combustion
chamber. The plurality of parallel oxygen transport membranes serve
to separate oxygen from an oxygen containing gas, thereby to
provide the oxygen within the combustion chamber to support
combustion of a fuel and generate heat. A plurality of fluid
passages pass through the combustion chamber and are positioned so
that a portion of the heat of combustion is transferred from the
combustion to the oxygen transport membranes to heat the oxygen
transport membranes to an operational temperature and a further
portion of the heat is transferred from the combustion to the fluid
passages to provide heat to heat the fluid and to promote
stabilization of the operational temperature of the oxygen
transport membranes. At least one inlet is provided for introducing
at least the fuel into the combustion chamber and an exhaust from
the combustion chamber discharges combustion products arising from
combustion of the fuel. The exhaust and the at least one inlet are
spaced apart from one another so that the combustion products flow
in a direction predominantly parallel to the oxygen transport
membranes.
[0007] The oxygen transport membranes and the fluid passages can be
of tubular configuration. The direction of flow of the combustion
products can either be countercurrent or co-current to gas flow of
the oxygen containing gas within the oxygen transport membranes.
Preferably, the oxygen transport membranes are closed at one end
and open at the end to discharge an oxygen-depleted retentate and a
plurality of coaxial lance tubes project into open ends of the
oxygen transport membranes to supply the oxygen containing gas
thereto. The at least one inlet can comprise an inlet to the
combustion chamber for introducing a mixture of the fuel and a flue
gas, if flue gas is required, into the combustion chamber.
Alternatively, in case of open ended, tubular oxygen transport
membrane units, the at least one inlet can comprise fuel nozzles
located adjacent to the open ends of the oxygen transport
membranes.
[0008] The fluid can be water and thus, the fluid heater can be a
boiler. In such case, the fluid passages are interspersed between
the oxygen transport membranes and the fluid passages and the
oxygen transport membranes are parallel to one another. Preferably,
the fluid passages communicate between fluid inlet and outlet
manifolds to supply the fluid to the fluid passages and to
discharge steam therefrom, respectively. In such case, the oxygen
transport membranes project, from the open end thereof, from a
retentate outlet manifold to discharge oxygen depleted air and the
lance tubes project from an air inlet manifold.
[0009] In another aspect, the present invention provides an oxygen
separation and combustion method in which an oxygen containing gas
is introduced into a plurality of parallel oxygen transport
membranes located within a combustion chamber. Oxygen is separated
from the oxygen containing gas within the plurality of parallel
oxygen transport membranes, thereby to provide oxygen within the
combustion chamber. A fuel is introduced into the combustion
chamber and the fuel is combusted within the combustion chamber in
the presence of the oxygen to generate heat. The fluid is passed
through a plurality of fluid passages also located within the
combustion chamber and combustion products are discharged from the
combustion chamber. The combustion products are discharged from the
combustion chamber and the fuel is introduced so that the
combustion products flow in a direction predominantly parallel to
the oxygen transport membranes to provide a reactive purge to
promote the separation of the oxygen from the oxygen containing
gas. The fluid passages are positioned so that a portion of the
heat is transferred from the combustion to the oxygen transport
membranes to heat the oxygen transport membranes to an operational
temperature and a further portion of the heat is transferred from
the combustion to the fluid passages to provide heat to heat the
fluid and to promote stabilization of the operational temperature
of the oxygen transport membranes. The fluid can be water that is
heated.
[0010] In either of the foregoing aspects of the present invention,
and as used herein and in the claims, the term "heated" means
transferring heat to the fluid and thereby raising its temperature.
Further, the term, "water" encompasses both water in liquid form
and steam or a two-phase mixture of water and steam. Thus, as used
herein and in the claims, the term, "heated" when used in
connection with water means raising the temperature of the water by
any amount. As such, the temperature rise of water may or may not
be sufficient to raise steam and if the water enters the heat
transfer passages as steam, the steam will become superheated.
[0011] The integration of the oxygen transport membranes and the
combustion system described above dramatically reduces the power
requirement for oxygen production. The oxygen flux through an
oxygen transport membrane is approximately proportional to the log
of the partial pressure ratio between the source side and the
product side, if mass transfer rate is controlled by the membrane
itself. For example, to produce pure oxygen at one atmosphere
absolute, the air must be compressed to about fifteen atmospheres.
This results in a net energy requirement of approximately 160
kW/ton assuming expansion of the oxygen depleted air. Although this
power requirement is less than conventional equipment, which is
closer to 200 KW/ton, integrated the oxygen transport membrane unit
with the combustion system of the boiler reduces this energy
requirement still further by providing a reactive purge to consume
the oxygen as it passes through the membrane. Such oxygen
consumption produces an oxygen concentration on the product side of
the oxygen transport membrane that is therefore always near zero.
This provides such a large driving force that requires only minimal
compression, typically just enough to move the air through the
oxygen transport membrane. This can be accomplished with a blower
instead of a more expensive compressor.
[0012] In a tubular membrane, since the largest driving force for
oxygen separation occurs at the entry point of the air or other
oxygen containing gas to the membrane, a countercurrent flow of
combustion products provides a more fuel-rich and therefore oxygen
lean conditions at the opposite end of the membrane, where less
driving force is present, to further enhance the effect of the
reactive purge.
[0013] Since fuel, flue gas and combustion products exist as a
mixture within the combustion chamber, the fuel is diluted so that
the driving force of the diffusion of the fuel to the surface of
the oxygen transport membrane is reduced. At the same time, the
oxygen flux through the membrane is low enough that, by in large,
fuel rich conditions are encountered. Therefore, combustion of the
fuel can be said to take place at or near the surface of the
membrane. This of course depends on the degree of dilution.
[0014] The result of the location of the combustion in apparatus
and methods in accordance with the present invention produces a
heat of combustion that can cause a thermal runaway of the oxygen
transport membrane resulting in damage and premature failure. In
the present invention, the heat transfer passages, which can be
interspersed steam tubes, act to withdraw the heat and thereby help
stabilize the operational temperature of the oxygen transport
membranes.
[0015] A further advantage that may be obtained from the present
invention is a potential for a high degree of integration. Since
the oxygen is produced at the point of use, no oxygen-safe piping
is required. Further the energy required to heat the air and the
fuel-flue gas mixture to the optimal operation temperature of the
oxygen transport membrane comes directly from the oxygen transport
membrane without concomitant heat loses that would otherwise occur
with external piping. The integration also minimized the
boiler/heater size and complexity. Since oxygen is produced in the
unit, no other space is required for an on-site conventional air
separation system. The location of the oxygen transport membranes
and heat transfer passages within a combustion chamber also helps
to minimize the overall footprint of a fluid heater of the present
invention.
[0016] Another major benefit that may be obtained from the present
invention is that high purity nitrogen can be produced as a
byproduct. The high driving forces for oxygen transport allow for
the production of such nitrogen with little or no oxygen.
Furthermore, a fluid heater in accordance with the present
invention will produce very little NOx since combustion takes place
in the presence of oxygen instead of air. Since the oxygen is
gradually added to the fuel-flue gas mixture as it passes through
the combustion chamber, the combustion takes place under fuel rich
conditions. Hence, the combustion is inherently staged with a long
residence time in the fuel-rich regime, and with slow transition
from fuel rich to fuel lean combustion to also lessen the
possibility of NOx formation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Although the present invention concludes with claims
distinctly pointing out the subject matter that Applicants regards
as their invention, it is believed that the invention will be
better understood when taken in connection with the accompanying
drawings in which:
[0018] FIG. 1 is a schematic illustration of a boiler in accordance
with the present invention;
[0019] FIG. 2 is a is a schematic illustration of an alternative
embodiment of a boiler in accordance with the present
invention;
[0020] FIG. 3 is a schematic illustration of a further alternative
embodiment of a boiler in accordance with the present invention;
and
[0021] FIG. 4 is a graphical illustration of an example in
accordance with the present invention showing the requisite ratio
of oxygen transport membrane area to steam tube area for thermal
control of the membranes.
[0022] In order to avoid repetition of explanation of elements
serving the same function within the various embodiments of the
present invention, the same reference numerals are used in the
figures where such elements are illustrated.
DETAILED DESCRIPTION
[0023] With reference to FIG. 1 a boiler 1 in accordance with the
present invention is illustrated. It is to be noted that although
the present invention is discussed in connection with a boiler, the
present invention is not so limited. A boiler is but a single
application of an oxygen separation and combustion apparatus in
accordance with the present invention. Other fluids could be heated
such as petroleum products or the fluid passages could contain
methane, steam and a suitable steam reforming catalyst. The object
of an oxygen separation and combustion apparatus of the present
invention might not be to heat a fluid, but rather to generate a
nitrogen product. In such case any suitable heat transfer fluid
might be utilized.
[0024] Boiler 1 is provided with a combustion chamber 10 and a
plurality of parallel oxygen transport membranes 12, 14 and 16
located within combustion chamber 10. A plurality of parallel fluid
passages 18, 20, 22, and 23 pass through combustion chamber 10.
Combustion of fuel, for instance, methane or natural gas, in the
presence of oxygen produced by oxygen transport membranes 12, 14,
and 16 produces heat to heat water circulating within fluid
passages 18, 20, 22, and 23.
[0025] Oxygen transport membranes 12, 14 and 16 are preferably in
the form of open ended tubes connected to a retentate outlet
manifold 24 having an outlet 26. Lance tubes 28, 30 and 32 project
into open ends of oxygen transport membranes 12, 14 and 16,
respectively, to supply the oxygen containing gas thereto. In this
regard lance tubes 28, 30 and 32 are connected to an air inlet
manifold 34 having an inlet 36. Heated air enters inlet 36 and air
inlet manifold 34 then distributes the air to lance tubes 28, 30
and 32. The air flows from the closed ends of oxygen transport
membranes 12, 14 and 16 towards the open ends thereof as indicated
by arrow head A. Oxygen in the form of oxygen ions permeate through
oxygen transport membranes 12, 14 and 16 and is discharged into
combustion chamber 10.
[0026] Although tubular oxygen transport membranes 12, 14 and 16
are illustrated plate-like elements could be substituted.
Additionally, although fluid passages 18, 20, 22, and 23 are also
illustrated as being parallel to one another and to oxygen
transport membranes 12, 14, and 16, other configurations are
possible. For instance, fluid passages 18, 20, 22, and 23 could be
at right angles to their illustrated orientation or possibly spiral
around respective oxygen transport membranes 12, 14, and 16.
[0027] A mixture of fuel and flue gas is introduced into combustion
chamber 10 by means of a fuel inlet 38. The fuel combusts at the
surfaces of oxygen transport membranes 12, 14 and 16 to produce
heat and combustion products to form the flue gas. The resultant
heat, heats oxygen transport membrane elements 12, 14 and 16 to
their operational temperature while at the same time also supplying
heat to fluid passages 18, 20, 22, and 23 which are connected at
opposite ends to fluid inlet and outlet manifolds 46 and 48. Heated
water is introduced into an inlet 50 of fluid inlet manifold 40.
Water then passes through fluid passages 18, 20, 22, and 23 to
generate steam that is expelled from an outlet 52 of fluid outlet
manifold 48.
[0028] The flue gas is discharged from combustion chamber 10
through a flue gas outlet 54. Although not illustrated, part of the
flue gas discharged from flue gas outlet 54 is cooled, circulated
by a blower and then mixed with the fuel. The mixture is then
introduced into inlet 38. The spacing between inlet 38 and flue gas
outlet 54 cause the flue gas and therefore the combustion products
to pass in any direction parallel to oxygen transport membranes 12,
14 and 16.
[0029] Although only a single row of fluid passages 18, 20, 22, and
23 and a single row of oxygen transport membranes 12, 14 and 16 are
illustrated, it is particularly advantageous that a plurality of
such rows be supplied so that each of the oxygen transport
membranes 12, 14 and 16 is surrounded by fluid passages such as 18,
20, 22, and 23 to conduct the heated combustion to the fluid
passages and help stabilize the operational temperature of the
oxygen transport membranes 12, 14 and 16.
[0030] With reference to FIG. 2, a boiler 2 in accordance with the
present invention is illustrated. The difference between boiler 2
and boiler 1 is that the inlet 38 to the combustion chamber and the
exhaust 54 have been reversed as inlet 38' and exhaust 54' so that
now the flue gas predominately moves in a direction indicated by
arrow head "B" which is countercurrent to the direction of the air
(arrow head "A") within oxygen transport membrane elements 12, 14
and 16. As such, at the closed end of the tubular oxygen transport
membrane elements 12, 14 and 16, there exists the highest oxygen
concentration and therefore the highest driving force in the air
itself. As air travels in the direction of arrow head "A", towards
retentate outlet manifold 26, the oxygen concentration within each
oxygen transport membrane unit 12, 14 and 16 is progressively less.
However, fuel is entering at the open end of oxygen transport
membranes 12, 14 and 16 where the least driving force is provided.
However, at such point, the combustion is fuel rich and therefore
the greatest driving force is provided by the reactive purge at
such location.
[0031] With reference to FIG. 3, the fuel is introduced into
combustion chamber 10, separately from the flue gas, by a series of
fuel inlets is provided by a fuel inlet manifold 56 having a fuel
inlet 57 and fuel injectors 58, 60, 62, 64, 66, and 68 connected
thereto. Fuel is sprayed into the combustion chamber 10 in the
countercurrent direction (arrowhead "B") to provide the greatest
reactive purge effect at the open end of the oxygen transport
membrane elements, 12, 14 and 16 where the least amount of driving
force toward the separation exists within the particular membranes.
Flue gas is introduced into combustion chamber 10 through flue gas
inlet 38" and is discharged from exhaust 54". Although not
illustrated, part of the flue gas discharged from exhaust 54" can
be circulated back to flue gas inlet 38" by use of a high
temperature blower.
[0032] In many type of oxygen transport membranes, the flux of
oxygen through the membrane increases as the membrane temperature
increases. The combustion reaction at the surface, and therefore
the heat release at the surface, is therefore limited by the oxygen
flux through the membrane. However, poor temperature control can
lead to catastrophic thermal runaway of the membrane. As the
temperature increases more oxygen passes through the membrane
leading to higher combustion rates at the surface and still higher
membrane temperatures until the temperature limitations of the
membrane is exceeded.
[0033] In any configuration of oxygen transport membranes,
involving the combustion of fuel at or near the surface of a
membrane, the dominant form of heat transfer resulting from the
combustion will be by radiation. The arrangement of fluid passages
and oxygen transport membranes must be designed and employed so
that the fluid passages will be capable of sufficiently absorbing
the radiant heat that thermal runaway is prevented and therefore,
the desired membrane operational temperature is maintained.
[0034] With reference to FIG. 4, a calculated example is shown of
an oxygen transport membrane of tubular form surrounded by six
fluid passages containing water. For purposes of the example, the
oxygen transport membrane was assumed to have an oxygen flux of 20
scfh/ft.sup.2 throughout the optimum operating range. Both the
fluid passages and the oxygen transport membrane acted as black
bodies with the field of view between the oxygen transport
membranes and the surrounding fluid passages estimated by the
crossed string method. The combustion flux for the membrane was set
at 9000 BTU/ft.sup.2 and the fluid passage temperature was fixed at
400.degree. F. The upper limit of the operating range of the
membrane is that temperature at which the membrane will fail. The
lower limit is the temperature at which the membrane will cease to
function. As illustrated, the fluid passages must constitute at
least about 58% of the total surface area of the membranes and the
fluid passages to prevent the membrane from overheating. At the
other extreme, a ratio of greater than about 85% leads to excessive
cooling of the membranes.
[0035] Although the present invention has been described with
reference to preferred embodiments, as will occur to those skilled
in the art, numerous changes and omissions may be made without
departing from the spirit of the present invention.
* * * * *