U.S. patent application number 17/527335 was filed with the patent office on 2022-07-28 for fuel-flexible combustor.
This patent application is currently assigned to Aspen Products Group, Inc.. The applicant listed for this patent is Aspen Products Group, Inc.. Invention is credited to Decio H. Coutinho, Mark D. Fokema, Craig D. Thompson, Sai C. Yelishala.
Application Number | 20220235931 17/527335 |
Document ID | / |
Family ID | 1000006329423 |
Filed Date | 2022-07-28 |
United States Patent
Application |
20220235931 |
Kind Code |
A1 |
Fokema; Mark D. ; et
al. |
July 28, 2022 |
Fuel-Flexible Combustor
Abstract
A liquid-hydrocarbon fuel is used to produce thermal energy by
introducing the liquid-hydrocarbon fuel and air to a vaporizer. The
liquid-hydrocarbon fuel is vaporized in the vaporizer to produce
hydrocarbon-fuel vapor, and the hydrocarbon-fuel vapor and air are
blended to form a hydrocarbon-fuel-vapor-and-air mixture. Then,
hydrocarbon-fuel-vapor-and-air mixture is introduced to a catalytic
combustor including a catalyst, wherein the catalyst promotes
oxidation of the hydrocarbon-fuel vapor to form a carbon-dioxide-
and water-vapor-containing exhaust and to generate thermal energy.
The carbon-dioxide- and water-vapor-containing exhaust and air is
then introduced to a recuperator, wherein the recuperator transfers
thermal energy from the carbon-dioxide- and water-vapor-containing
exhaust to the air to produce heated air.
Inventors: |
Fokema; Mark D.;
(Northborough, MA) ; Coutinho; Decio H.;
(Marlborough, MA) ; Thompson; Craig D.; (Sudbury,
MA) ; Yelishala; Sai C.; (Westborough, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Aspen Products Group, Inc. |
Marlborough |
MA |
US |
|
|
Assignee: |
Aspen Products Group, Inc.
Marlborough
MA
|
Family ID: |
1000006329423 |
Appl. No.: |
17/527335 |
Filed: |
November 16, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
63139587 |
Jan 20, 2021 |
|
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F23L 15/04 20130101;
F23D 2900/3102 20210501; F23D 11/443 20130101; F23D 2900/00001
20130101 |
International
Class: |
F23D 11/44 20060101
F23D011/44; F23L 15/04 20060101 F23L015/04 |
Goverment Interests
GOVERNMENT SUPPORT
[0002] This invention was made with government support under
contract number W911QX-17-P-0157 awarded by the United States Army.
The government has certain rights in the invention.
Claims
1. A method for converting a liquid-hydrocarbon fuel into thermal
energy, the method comprising: introducing the liquid-hydrocarbon
fuel and air to a vaporizer; vaporizing the liquid-hydrocarbon fuel
in the vaporizer to produce hydrocarbon-fuel vapor and blending the
hydrocarbon-fuel vapor and air to form a
hydrocarbon-fuel-vapor-and-air mixture; then introducing the
hydrocarbon-fuel-vapor-and-air mixture to a catalytic combustor
including a catalyst, the catalyst promoting oxidation of the
hydrocarbon-fuel vapor to form a carbon-dioxide- and
water-vapor-containing exhaust and generate thermal energy; and
then introducing the carbon-dioxide- and water-vapor-containing
exhaust and air to a recuperator, wherein the recuperator transfers
thermal energy from the carbon-dioxide- and water-vapor-containing
exhaust to the air to produce heated air.
2. The method of claim 1, wherein the air introduced to the
vaporizer is heated air produced by transferring thermal energy
from the carbon-dioxide- and water-vapor-containing exhaust in the
recuperator.
3. The method of claim 1, wherein the vaporizer includes a porous
structure and the method further comprises flowing the air through
the porous structure, wherein the porous structure promotes wicking
of the liquid-hydrocarbon fuel, evaporation of the
liquid-hydrocarbon fuel into the air to produce
evaporated-hydrocarbon fuel, and mixing of the
evaporated-hydrocarbon fuel into the air.
4. The method of claim 1, wherein the catalyst is selectively
coated on selected areas of interior surfaces but not on other
areas of interior surfaces of the catalytic combustor so as to
selectively generate thermal energy at the selected areas of
interior surfaces of the catalytic combustor.
5. The method of claim 4, wherein the amount of the catalyst on the
selected areas of interior surfaces of the catalytic combustor is
varied so as to maintain a constant specific hydrocarbon-oxidation
rate.
6. The method of claim 1, wherein thermal energy is transferred
from the exhaust to the hydrocarbon fuel and air within the
vaporizer to form a heated hydrocarbon-fuel vapor and air
mixture.
7. A combustion apparatus for converting liquid-hydrocarbon fuels
into thermal energy, the apparatus comprising: a vaporizer coupled
with a source of liquid-hydrocarbon fuel and with a source of air,
the vaporizer comprising a porous structure that promotes
vaporization of the liquid-hydrocarbon fuel and distributes the
vaporized liquid-hydrocarbon fuel into the air to form a mixed
hydrocarbon-fuel-vapor-and-air product; a catalytic combustor
located downstream of the vaporizer, the catalytic combustor
comprising a combustor enclosure that has internal surfaces coated
with a catalyst that can cause a reaction involving the
hydrocarbon-fuel vapor and the air that forms a
carbon-dioxide-and-water-vapor-containing exhaust product and
generates thermal energy; and a recuperator located downstream of
the combustor, wherein the recuperator is configured to transfer
thermal energy from the carbon-dioxide-and-water-vapor-containing
exhaust product to an air stream to form a heated air stream that
can be fed to the vaporizer.
8. The apparatus of claim 7, wherein the vaporizer is located
within the recuperator, and wherein the recuperator is configured
to transfer thermal energy from the
carbon-dioxide-and-water-vapor-containing exhaust product to the
hydrocarbon-fuel vapor and air.
9. The apparatus of claim 7, wherein the porous structure comprises
a reticulated-foam material.
10. The apparatus of claim 9, wherein the reticulated-foam material
defines pores smaller than 0.1 mm and pores larger than 0.5 mm.
11. The apparatus of claim 7, wherein the mass of the catalyst
coating per unit area of the internal surfaces of the combustor
enclosure increases as the distance from the vaporizer
increases.
12. The apparatus of claim 7, wherein the combustor enclosure is
cylindrical and includes a combustor outlet configured to release
the carbon-dioxide-and-water-vapor-containing exhaust product from
the combustor enclosure.
13. The apparatus of claim 12, further comprising a cylindrical
inner liner, comprising an upstream end and a downstream end, and
located within the combustor enclosure that defines an annular
space for flow of the reacting hydrocarbon-fuel vapor and air.
14. The apparatus of claim 13, wherein the cylindrical inner liner
is perforated to define a flow path for the exhaust from the
annular space to the combustor outlet.
15. The apparatus of claim 13, wherein the annular space is defined
by a gap between the combustor enclosure and the cylindrical inner
liner that is less than 3 mm.
16. The apparatus of claim 13, wherein the cylindrical inner liner
includes a catalyst coating that can cause a reaction involving the
hydrocarbon-fuel vapor and the air that forms a
carbon-dioxide-and-water-vapor-containing exhaust product and
generates thermal energy.
17. The apparatus of claim 16, wherein the mass of the catalyst
coating per unit area of the cylindrical inner liner increases as
the distance from the vaporizer increases.
18. The apparatus of claim 13, further comprising a cylindrical
non-metallic heat shield located within the combustor enclosure at
the upstream end of the cylindrical inner liner that defines an
annular space for flow of the hydrocarbon-fuel vapor and air.
19. The apparatus of claim 18, wherein an annular gap across the
annular space between the combustor enclosure and the non-metallic
cylindrical heat shield is smaller than an annular gap across the
annular space between the combustor enclosure and the cylindrical
inner liner.
20. The apparatus of claim 7, wherein the combustor enclosure has
planar surfaces and includes a combustor outlet configured to
release the carbon-dioxide-and-water-vapor-containing exhaust
product from the combustor enclosure.
21. The apparatus of claim 20, further comprising a divider between
the combustor enclosure and the recuperator that defines a planar
space for flow of the reacting hydrocarbon-fuel vapor and air.
22. The apparatus of claim 21, wherein the planar space is defined
by a gap between the combustor enclosure and the divider that is
less than 3 mm.
23. The apparatus of claim 21, wherein the divider includes a
catalyst coating on its combustor-enclosure-facing surface that can
cause a reaction involving the hydrocarbon-fuel vapor and the air
that forms a carbon-dioxide-and-water-vapor-containing exhaust
product and generates thermal energy.
24. The apparatus of claim 23, wherein the mass of the catalyst
coating per unit area of the combustor-enclosure-facing surface of
the divider increases as the distance from the combustor outlet
decreases.
Description
RELATED APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Application No. 63/139,587, filed 20 Jan. 2021, the entire content
of which is incorporated herein by reference.
BACKGROUND
[0003] The ability to efficiently generate thermal energy via
combustion of hydrocarbon fuels and to transfer that thermal energy
in a controlled manner across specific interfaces facilitates the
development of advanced power generation and heating systems.
Power-generation technologies, such as thermoelectrics,
thermophotovoltaics, thermionics, and fuel cells, all benefit from
precise delivery of thermal and radiative energy to defined surface
geometries at specific surface temperatures and heat fluxes.
[0004] For instance, thermoelectric modules, which rely upon the
conduction of thermal energy from the hot side of the module to the
cold side of the module to generate electricity via the Seebeck
effect, typically have an optimum hot-side operating temperature,
below which the power output and efficiency of the thermoelectric
module are reduced and above which the module is damaged.
Combustion devices employed to provide thermal energy for
thermoelectric-module operation deliver the thermal energy to the
module at the optimum temperature, with as uniform a surface
temperature as possible, and through a surface that matches the
geometry of the hot side of the thermoelectric module to ensure
good thermal contact between the hot combustor surface and the hot
side of the thermoelectric module.
[0005] Similarly, thermophotovoltaic systems, which rely upon the
irradiation of semiconductor cells with infrared energy to generate
electricity via electron/hole pair separation, typically have an
optimum irradiation flux, below which the power output and
efficiency of the semiconductor cells are reduced and above which
the cells are damaged. Combustion devices employed to provide the
radiative energy for thermophotovoltaic cell operation deliver the
radiative energy to the cells at the optimum flux, with as uniform
a surface flux as possible, and in a geometry that maximizes the
fraction of radiation emitted from the combustor surface that is
intercepted by the thermophotovoltaic cells.
[0006] Existing systems and apparatuses for converting hydrocarbon
fuels to thermal energy suitable for use in power generation
systems are typically limited to narrow operating ranges with
regards to parameters, such as equivalence ratio, fuel throughput,
temperature, pressure, type of fuel, etc. Efficient combustion of
liquid fuels (in particular, high-flash-point hydrocarbon fuels) is
particularly challenging, as vaporization and thorough mixing of
the fuel vapor with oxidant is generally required to ensure
complete fuel combustion. Consequently, there is a need for compact
and efficient combustors that convert liquid hydrocarbon fuels into
thermal energy over a wide range of operating conditions that can
be delivered to thermal-to-electric converters in a highly
controlled manner.
SUMMARY
[0007] The following description relates to a method and apparatus
for liquid-hydrocarbon combustion, and more particularly, to an
integration of several components for converting liquid-hydrocarbon
fuels into thermal energy that can be efficiently directed to
thermal-to-electric conversion devices.
[0008] The liquid-hydrocarbon fuel is vaporized into air by passing
the fuel and the air through a vaporizer. After vaporization, the
mixed hydrocarbon-fuel vapor and air passes into a catalytic
combustor that converts the hydrocarbon-fuel vapor and air into a
carbon-dioxide-and-water-vapor-containing exhaust while
simultaneously generating thermal energy and transferring that
energy through the walls of the combustor. The hot exhaust passes
into a recuperator where thermal energy from the exhaust is
transferred to an air stream to produce a heated air stream that
can be fed to the vaporizer.
[0009] Vaporization of the liquid-hydrocarbon fuel is conducted
within a vaporizer that contains a porous structure to aid in
vaporization. The porous structure comprises a random or ordered
three-dimensional array of interconnected pores distributed within
an array of interconnected solids, such as a reticulated foam,
packed fibers, and the like. When the liquid hydrocarbon is brought
into contact with the porous structure, the liquid hydrocarbon is
wicked into the porous structure via capillary action, which is
dependent upon the surface tension of the porous structure, the
surface tension of the hydrocarbon, the pore dimensions of the
porous structure, and the overall porosity of the porous structure.
The liquid hydrocarbon within the porous structure evaporates into
air that simultaneously flows through the porous structure, with
the porous structure providing a large specific surface area from
which mass transfer of the hydrocarbon from the liquid phase to the
vapor phase takes place. Following evaporation, the hydrocarbon
vapor and air are progressively mixed as the stream passes through
the tortuous features of the porous structure.
[0010] Combustion of the hydrocarbon vapor and air mixture is
initiated via contact of the mixture with a catalyst located within
a combustor enclosure. The catalyst promotes heterogeneous
oxidation of the hydrocarbon, resulting in the consumption of
hydrocarbon and oxygen and the production of thermal energy along
with primarily carbon dioxide and water vapor, which are exhausted
from the combustor. The catalyst can be coated on selected surface
areas of the combustor enclosure but not on others in order to
selectively generate thermal energy at those surface areas and to
promote the transfer of that thermal energy out of the combustor
enclosure via those surface areas.
[0011] A recuperator is used to transfer a portion of the thermal
energy retained within the hot combustor exhaust back into the air
stream that is fed to the vaporizer. Efficient transfer of thermal
energy within the recuperator is accomplished by passing the hot
exhaust stream across heat-exchange surfaces, the opposite sides of
which are exposed to the flowing air stream. Routing of
exhaust-stream and air-stream flows within the recuperator may be
accomplished via a variety of heat-exchanger geometries, including
shell and tube, parallel plate, and plate fin, among others.
[0012] Thermal-energy recuperation and hydrocarbon vaporization may
also be conducted simultaneously by locating the vaporizer within
the recuperator to form a single vaporizer/recuperator device in
which thermal energy is transferred from the hot exhaust to the
cooler hydrocarbon and air streams as the liquid hydrocarbon is
evaporated and mixed into the air. In this instance, in addition to
providing evaporation and mixing functionality, the porous
structure within the vaporizer/recuperator also enhances the
transfer of thermal energy into the hydrocarbon-air mixture by
increasing the thermal conductivity on the vaporization side of the
vaporizer/recuperator and by promoting hydrocarbon-air stream
turbulence as the stream flows through the tortuous pathways within
the porous structure.
[0013] The apparatus and methods described herein offer many
advantages over existing approaches. The fuel-flexible combustor
apparatus and methods can operate on a variety of liquid
hydrocarbon fuels, such as heating oil, diesel, jet fuel, gasoline,
kerosene, naphtha, alcohols, and the like, over a wide range of
fuel throughputs. The vaporizer readily evaporates the
liquid-hydrocarbon fuel into the air stream at low temperatures,
minimizing the risk of coke and residue formation, and
simultaneously mixes the evaporated-hydrocarbon vapor into the air
stream, reducing the likelihood of coke-forming, fuel-rich pockets
within the combustor. The use of a heterogeneous or
homogeneous/heterogeneous combustion process enables operation over
a much wider range of equivalence ratios than a homogeneous
combustion process. The use of a homogeneous/heterogeneous
combustion process provides dual heat-generation mechanisms that
can be used to control where thermal energy is generated within the
combustor and where it is transferred out of the combustor.
Configuring the vaporizer within the recuperator enables effective
recovery of waste heat from the combustor exhaust while
simultaneously preparing the hydrocarbon-fuel-air mixture for
efficient combustion within the combustor. The combustor apparatus
can also be light-weight.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] In the accompanying drawings, described below, like
reference characters refer to the same or similar parts throughout
the different views. The drawings are not necessarily to scale,
emphasis instead being placed upon illustrating particular
principles of the methods and apparatus characterized in the
Detailed Description.
[0015] FIG. 1 is a conceptual drawing showing the relationship
between selected elements of an embodiment of the apparatus
[0016] FIG. 2 is another conceptual drawing showing the
relationship between selected elements of an embodiment of the
apparatus.
[0017] FIG. 3 is a cross-sectional side view of an embodiment of
the apparatus.
[0018] FIG. 4 is a cross-sectional side view of another cylindrical
embodiment of the apparatus.
[0019] FIG. 5 is a cross-sectional side view of another planar
embodiment of the apparatus.
[0020] FIG. 6 is a magnified photographic image of an
exemplification of the porous structure.
DETAILED DESCRIPTION
[0021] The foregoing and other features and advantages of various
aspects of the invention(s) will be apparent from the following,
more-particular description of various concepts and specific
embodiments within the broader bounds of the invention(s). Various
aspects of the subject matter introduced above and discussed in
greater detail below may be implemented in any of numerous ways, as
the subject matter is not limited to any particular manner of
implementation. Examples of specific implementations and
applications are provided primarily for illustrative purposes.
[0022] Unless otherwise herein defined, used or characterized,
terms that are used herein (including technical and scientific
terms) are to be interpreted as having a meaning that is consistent
with their accepted meaning in the context of the relevant art and
are not to be interpreted in an idealized or overly formal sense
unless expressly so defined herein. For example, if a particular
composition is referenced, the composition may be substantially
(though not perfectly) pure, as practical and imperfect realities
may apply; e.g., the potential presence of at least trace
impurities (e.g., at less than 1 or 2%) can be understood as being
within the scope of the description. Likewise, if a particular
shape is referenced, the shape is intended to include imperfect
variations from ideal shapes, e.g., due to manufacturing
tolerances. Percentages or concentrations expressed herein can be
in terms of weight or volume. Processes, procedures and phenomena
described below can occur at ambient pressure (e.g., about 50-120
kPa--for example, about 90-110 kPa) and temperature (e.g., -20 to
50.degree. C.--for example, about 10-35.degree. C.) unless
otherwise specified.
[0023] Although the terms, first, second, third, etc., may be used
herein to describe various elements, these elements are not to be
limited by these terms. These terms are simply used to distinguish
one element from another. Thus, a first element, discussed below,
could be termed a second element without departing from the
teachings of the exemplary embodiments.
[0024] Spatially relative terms, such as "above," "below," "left,"
"right," "in front," "behind," and the like, may be used herein for
ease of description to describe the relationship of one element to
another element, as illustrated in the figures. It will be
understood that the spatially relative terms, as well as the
illustrated configurations, are intended to encompass different
orientations of the apparatus in use or operation in addition to
the orientations described herein and depicted in the figures. For
example, if the apparatus in the figures is turned over, elements
described as "below" or "beneath" other elements or features would
then be oriented "above" the other elements or features. Thus, the
exemplary term, "above," may encompass both an orientation of above
and below. The apparatus may be otherwise oriented (e.g., rotated
90 degrees or at other orientations) and the spatially relative
descriptors used herein interpreted accordingly.
[0025] Further still, in this disclosure, when an element is
referred to as being "on," "connected to," "coupled to," "in
contact with," etc., another element, it may be directly on,
connected to, coupled to, or in contact with the other element or
intervening elements may be present unless otherwise specified.
[0026] The terminology used herein is for the purpose of describing
particular embodiments and is not intended to be limiting of
exemplary embodiments. As used herein, singular forms, such as "a"
and "an," are intended to include the plural forms as well, unless
the context indicates otherwise. Additionally, the terms,
"includes," "including," "comprises" and "comprising," specify the
presence of the stated elements or steps but do not preclude the
presence or addition of one or more other elements or steps.
[0027] "Air" typically refers to a gas comprising approximately 78%
nitrogen, approximately 21% oxygen, and approximately 1% argon, and
other components in lesser amounts. As used herein, "air" may more
generally refer to any gas in which nitrogen and oxygen are the two
most prevalent components.
[0028] Now, referring to FIGS. 1-6, features and details of the
fuel-flexible combustor apparatus and method are described. The
same numeral present in different figures represents the same item.
Particular embodiments are detailed, below, for the purpose of
illustration and not as limitations of the invention.
[0029] Selected components of the apparatus are illustrated and
labeled in FIGS. 1 and 2. These FIGURES offer a simplified view of
the more-detailed illustration of the cylindrical exemplification
presented in FIG. 3 and are provided for ease of illustrating
broader aspects of the apparatus and method. The apparatus in FIG.
1 includes the principal components through which the
liquid-hydrocarbon fuel 12 (and, later, its oxidation products) is
passed. Those components, through which the liquid-hydrocarbon fuel
12 (and, later, its oxidation products) passes in sequence, are the
vaporizer 100, combustor 102 and recuperator 101. The vaporizer 100
and recuperator 101 are combined in FIG. 2 because the structure of
the recuperator 101 is intertwined with the vaporizer 100 in
various embodiments of the apparatus, as shown in the other
FIGURES. In the various embodiments, the recuperator 101 includes
the structures that transfer heat from the combustor 102 to the
vaporizer 100.
[0030] In the illustrated embodiment shown in FIG. 3, the
hydrocarbon fuel 12 enters the vaporizer 100 through the fuel inlet
tube 104 and wicks into the porous structure 106. Air 14 enters the
vaporizer 100 through the air inlet tube 108 and flows through the
porous structure 106. The recuperator 101 includes the concentric
annular channel 122 formed by the cylindrical shell 123 encircling
the inner channel that contains the porous structure 106 and
through which the fuel 12 and air 14 flow. Exhaust 16 from the
combustor 102 flows through the annular channel 122, allowing heat
from the exhaust 16 to flow through the wall separating the annular
channel from the inner channel, into the air 14 and fuel 12
mixture, and into the porous structure 106 in the vaporizer
100.
[0031] The hydrocarbon fuel 12 and air 14 flowing through the
porous structure 106 form a well-mixed hydrocarbon-vapor/air stream
that flows to the combustor 102 via the hydrocarbon/air transfer
line 110. The hydrocarbon-vapor/air stream flows around the heat
shield 112 and into the annular combustion zone 114 defined by the
walls of the inner liner 116 and the combustor 102. The walls of
the inner liner 116 and combustor 102 typically comprise a
high-temperature metallic alloy, such as stainless steel, an
austenitic nickel-chromium-based alloy (available as INCONEL alloy
from Special Metals Corporation), a nickel-molybdenum alloy
(available as HASTELLOY alloy from Haynes International), an
iron-chromium alloy (available as FECRALLOY alloy from Goodfellow),
etc. A combustion catalyst 118 is present on the interior surface
of the combustor 102 within the annular combustion zone 114.
Exemplary combustion catalysts 118 that may be used include
noble-metal (such as Pt, Pd, Rh, etc.) or transition-metal-oxide
(such as Mn.sub.2O.sub.3, Fe.sub.2O.sub.3, NiO, Co.sub.3O.sub.4,
etc.) species supported on high-surface-area inorganic supports
(such as Al.sub.2O.sub.3, SiO.sub.2, etc.).
[0032] The combustion catalyst 118 can be coated on selected
surface areas of the combustor enclosure 103 but not on others in
order to selectively generate thermal energy at those surface areas
and to promote the transfer of that thermal energy out of the
combustor enclosure 103 via those surface areas. Because the
thermal energy is simultaneously generated and transferred out of
the combustor at these surface areas, the surface temperature is
much lower than the adiabatic flame temperature of the
hydrocarbon-air mixture. The surface temperature can be readily
controlled by manipulating the catalytic oxidation rate (for
instance, by manipulating the hydrocarbon-air equivalence ratio,
the combustor pressure, the rate of hydrocarbon and air mass
transfer to the combustor-enclosure surface, the catalyst activity,
the catalyst loading on the combustor enclosure surface, etc.) and
manipulating the surface heat-transfer rate (for instance, by
manipulating the composition of the combustor-enclosure wall, the
thickness of the combustor-enclosure wall, the emissivity of the
combustor-enclosure surface, the properties of the fluid external
to the combustor-enclosure wall, etc.). A high catalytic-oxidation
rate and a low heat-transfer rate will produce a higher surface
temperature, while a low catalytic-oxidation rate and a high
heat-transfer rate will produce a lower surface temperature.
[0033] Beyond the selective application of the catalyst 118 on
selected surface areas of the combustor enclosure 103, as described
above, varying the degree of catalyst loading (i.e., the mass of
catalyst per unit area of surface) across the surface areas is a
particularly useful tool for more-precisely controlling the
magnitude and spatial distribution of the surface temperature, as
the loading of the catalyst 118 can be increased (i.e., from
left-to-right in the configuration and orientation of FIG. 3) as
the hydrocarbon is depleted from the hydrocarbon-air mixture in
order to maintain a constant specific hydrocarbon oxidation rate
(i.e., moles of hydrocarbon oxidized per unit time per unit area of
surface) along the catalyst-coated surface.
[0034] In addition to catalytic hydrocarbon oxidation at the
catalyst-coated combustor surface areas, at higher temperatures,
homogeneous (non-catalytic, flame) hydrocarbon combustion may also
occur within the internal volume of the combustor 102. Thus, at
these higher temperatures, the generation of thermal energy
proceeds via mixed homogeneous-heterogeneous oxidation processes.
In this case, the location of thermal-energy generation within the
combustor is governed by both the location of the catalytic coating
on the combustor surfaces and the location at which the homogeneous
combustion flame is stabilized within the combustor volume. The
location at which the homogeneous combustion flame is stabilized
within the combustor enclosure 103 may be controlled by
manipulating the geometry of the combustor 102, the location of
flame stabilizing elements within the combustor 102, and the
location of the catalyst 118, among other features.
[0035] The combustion catalyst 118 can also be coated on selected
surface areas of the inner liner 116 but not on others in order to
generate additional thermal energy and thereby increase the
combustor temperature.
[0036] In the annular combustion zone 114, the hydrocarbon vapor is
oxidized to generate thermal energy that is transferred out of the
combustor along with a hot combustion gas that exits the annular
combustion zone 114 through an outlet 120. Exemplary reactions that
may occur within the annular combustion zone 114 include
C.sub.nH.sub.m+(m/4+n) O.sub.2.fwdarw.n CO.sub.2+(m/2) H.sub.2O,
C.sub.nH.sub.m+(m/4+n/2) O.sub.2.fwdarw.n CO+(m/2) H.sub.2O, and
CO+(1/2) O.sub.2.fwdarw.CO.sub.2.
[0037] The hot combustion gas enters the annular channel 122 that
forms the recuperator 101 where the hot combustion gas provides
thermal energy to the hydrocarbon vapor-air stream within the
porous structure 106. The combustion gas exhaust 16 exits through
the exhaust tube 124. Thermal insulation 126 is positioned to
minimize thermal-energy transfer to the environment from surfaces
other than those surfaces adjacent to the annular combustion zone
114, while the heat shield 112 is positioned to minimize thermal
energy transfer from the annular combustion zone 114 to other
upstream components within the combustor 102 and vaporizer 100.
[0038] A heater 128 is positioned within the volume confined by the
inner liner 116 to provide thermal energy during combustor startup.
The heater 128 is typically an electrically-resistive heating
element. Thermal energy is transferred from the heater 128 to the
walls of the inner liner 116 and to the combustion catalyst 118 to
heat the combustion catalyst 118 to a temperature at which
heterogeneous oxidation of the hydrocarbon vapor may proceed.
Simultaneously, thermal energy is transferred from the heater 128
to the porous structure 106 via air flowing from the outlet 120 to
the exhaust tube 124 to assist with hydrocarbon fuel
vaporization.
[0039] Another cylindrical embodiment of the apparatus is shown in
FIG. 4. In this embodiment, the vaporizer 100 is located adjacent
to the combustor 102 in order to increase thermal integration and
reduce the size and mass of the cylindrical fuel-flexible
combustor. In this exemplification, the annular channel 122 serves
as the recuperator 101, facilitating the transfer of heat from the
exhaust 16 to the fuel/air mixture 12/14 before the fuel/air
mixture 12/14 flows into the combustor 102. Heat shield 112 is
positioned to minimize thermal-energy transfer from the annular
combustion zone 114 to the hydrocarbon/air transfer line 110 and
modulate thermal-energy transfer to the vaporizer 100.
[0040] A planar embodiment of the apparatus is shown in FIG. 5. In
this embodiment, the recuperator 101 is located adjacent to the
combustor 102 in order to provide a planar fuel-flexible combustor
in which the divider 132 separates the recuperator 101 from the
combustor 102. The recuperator 101 includes, in particular, the
exhaust chamber 134, the lower wall/boundary of which is adjacent
the porous structure 106, facilitating the transfer of heat from
exhaust 16 flowing through the exhaust chamber 134 (before its
discharge) to the porous structure 106 and to the mixture of fuel
12 and air 14 flowing therethrough. In the illustrated embodiment,
the hydrocarbon fuel 12 enters the vaporizer 100 through the fuel
inlet tube 104 and wicks into the porous structure 106. Air 14
enters the vaporizer 100 through the air inlet tube 108 and flows
through the porous structure 106. The hydrocarbon fuel 12 and air
14 flow through the porous structure 106 to form a well-mixed
hydrocarbon-vapor/air stream that flows to the combustor 102 via
the hydrocarbon/air transfer line 110. The hydrocarbon-vapor/air
stream flows into the planar combustion zone 130 defined by the
walls of the divider 132 and the combustor 102. The walls of the
divider 132 and combustor 102 are typically composed of a
high-temperature metallic alloy, such as stainless steel, an
austenitic nickel-chromium-based alloy (available as INCONEL alloy
from Special Metals Corporation), a nickel-molybdenum alloy
(available as HASTELLOY alloy from Haynes International), an
iron-chromium alloy (available as FECRALLOY alloy from Goodfellow),
etc. A combustion catalyst 118 is present on the interior surface
of the combustor 102 within the planar combustion zone 130.
Exemplary combustion catalysts 118 that may be used include
noble-metal or transition-metal-oxide species supported on
high-surface-area inorganic supports. The combustion catalyst 118
can also be coated on selected surface areas of the divider 132 but
not on others in order to generate additional thermal energy within
the planar combustion zone and thereby increase the combustor
temperature.
[0041] In the planar combustion zone 130, the hydrocarbon vapor is
oxidized to generate thermal energy that is transferred out of the
combustor 102 along with a hot combustion gas that exits the planar
combustion zone 130 through an outlet 120. Exemplary reactions that
may occur within the planar combustion zone 130 include
C.sub.nH.sub.m+(m/4+n) O.sub.2.fwdarw.n CO.sub.2+(m/2) H.sub.2O,
C.sub.nH.sub.m+(m/4+n/2 O.sub.2).fwdarw.n CO+(m/2) H.sub.2O, and
CO+(1/2) O.sub.2.fwdarw.CO.sub.2.
[0042] The hot combustion gas enters the planar exhaust chamber 134
of the recuperator 101 where it transfers thermal energy to the
hydrocarbon vapor-air stream within the porous structure 106 of the
vaporizer 100. The combustion gas exhaust 16 exits through the
exhaust tube 124. Thermal insulation 126 is positioned to minimize
thermal-energy transfer to the environment from surfaces other than
those surfaces adjacent to the planar combustion zone 130.
[0043] A heater 128 is positioned adjacent to the divider 132 to
provide thermal energy during combustor startup. Thermal energy is
transferred from the heater 128 to the divider 132 and to the
combustion catalyst 118 to heat the combustion catalyst 118 to a
temperature at which heterogeneous oxidation of the hydrocarbon
vapor may proceed. Simultaneously, thermal energy is transferred
from the heater 128 to the porous structure 106 via air flowing
from the outlet 120 to the exhaust tube 124 to assist with
hydrocarbon-fuel vaporization.
[0044] An exemplary method for operating the fuel-flexible
combustor includes feeding a liquid hydrocarbon fuel 12 into the
fuel inlet tube 104 and air 14 into the air inlet tube 108. The
liquid hydrocarbon fuel 12 and air 14 are fed to the fuel-flexible
combustor at an equivalence ratio between about 0.3 and 1.0. The
liquid hydrocarbon fuel in this and other exemplifications can
comprise any of the following: heating oil, diesel, jet fuel,
gasoline, kerosene, naphtha, alcohols, and the like.
[0045] The porous structure 106 distributes the liquid hydrocarbon
fuel and assists with evaporation of the liquid hydrocarbon and
mixing of the hydrocarbon vapor with the air. The porous structure
106 comprises a metal (stainless steel, aluminum, copper, etc.) or
ceramic (alumina, zirconium oxide, silicon carbide, etc.)
reticulated foam or fibrous network that operates at a pressure
between about 0 kPa and 200 kPa and a temperature between about
0.degree. C. and 600.degree. C. The reticulated foam or fibrous
network comprises two scales of porosity, a finer scale, comprising
pores with average dimensions less than about 0.3 mm--e.g., less
than about 0.1 mm, and, in more-particular embodiments, less than
about 0.05 mm that retain liquid hydrocarbons under the influence
of capillary pressure and that promote the spread of the liquid
hydrocarbons out across the surface of the porous structure 106,
and a coarser scale, comprising interconnected pores with average
dimensions greater than about 0.5 mm--e.g., greater than about 1
mm, and in more-particular embodiments, greater than about 2 mm
that provide a pathway for the flow of air and hydrocarbon vapor
through the porous structure 106. A magnified image of a porous
structure is presented in FIG. 6, showing both finer-scale pores
202 and coarser-scale pores 204.
[0046] A reticulated foam or fibrous network comprising of two
scales of porosity may be formed by a number of methods. For
example, a reticulated foam may be formed by coating a reticulated
polyurethane foam with a ceramic or metal composition, followed by
burning out the polyurethane substrate. The resulting ceramic or
metal foam possesses coarser pores derived from the pores of the
original reticulated foam and finer pores derived from voids formed
upon polyurethane burnout. A fibrous network may be formed by using
a knitted or woven fabric. The coarser pores arise from the spaces
between the knitted or woven yarn, while the finer pores arise from
voids located between the interlocked fibers of the yarn. Numerous
other methods may also be used to form a reticulated foam or
fibrous network comprising two scales of porosity.
[0047] The well-mixed hydrocarbon vapor-air stream is fed to the
combustor 102 via the hydrocarbon-air transfer line 110 that
operates at a pressure between about 0 kPa and 200 kPa and a
temperature between about 0.degree. C. and 600.degree. C.--e.g.,
about 100 to 400.degree. C., and, in more-particular embodiments,
about 200 to 300.degree. C. Within the combustor 102, the
well-mixed hydrocarbon-vapor/air stream contacts the combustion
catalyst 118 at a pressure between about 0 kPa and 200 kPa and at a
temperature above about 200.degree. C., which is sufficient to
initiate heterogeneous oxidation of the hydrocarbon vapor with
oxygen contained in the air at the surface of the combustion
catalyst 118 and generate hot combustion gas. Homogeneous oxidation
of the hydrocarbon vapor with oxygen contained in the air may also
be initiated within the combustor along surfaces with temperatures
greater than about 600.degree. C. The surfaces defining the annular
combustion zone 114 or the planar combustion zone 130 operate at a
temperature between about 200.degree. C. and 1500.degree. C. and
are typically composed of a high-temperature metallic alloy, as
discussed above.
[0048] The distance between the surfaces defining the annular
combustion zone 114 or the planar combustion zone 130 is typically
about 0.5 to 5 mm--e.g., about 1 to 4 mm, and, in more-particular
embodiments, about 2 to 3 mm. Smaller distances will tend to
promote heterogeneous oxidation processes over homogeneous
oxidation processes, as the surface-area-to-volume ratio of the
combustion zone 130 increases as the distance decreases.
[0049] The distance between the surface of the heat shield 112 and
the wall of the combustor 102 in the exemplification of FIG. 3 is
typically smaller than the distance between the surfaces defining
the annular combustion zone 114 in order to prevent propagation of
the homogeneous oxidation process into the hydrocarbon/air transfer
line 110. The distance between the surface of the heat shield 112
and the wall of the combustor 102 is typically about 0.2 to 4
mm--e.g., about 0.5 to 3 mm, and, in more-particular embodiments,
about 1 to 2 mm.
[0050] The hot combustion gas is fed to the recuperator/vaporizer
100 at a pressure between about 0 kPa and 200 kPa and at a
temperature between about 200.degree. C. and 1500.degree. C. Within
the recuperator/vaporizer 100, thermal energy is transferred from
the hot combustion gas to the hydrocarbon-vapor/air stream, and the
combustion gas exhaust 16 exits through the exhaust tube 124 at a
pressure between about 0 kPa and 200 kPa and at a temperature
between about 100.degree. C. and 800.degree. C.
[0051] In describing embodiments of the invention, specific
terminology is used for the sake of clarity. For purposes of
description, each specific term is intended to at least include all
technical and functional equivalents that operate in a similar
manner to accomplish a similar purpose.
[0052] Additionally, in some instances where a particular
embodiment of the invention includes a plurality of system elements
or method steps, those elements or steps may be replaced with a
single element or step; likewise, a single element or step may be
replaced with a plurality of elements or steps that serve the same
purpose. Further, where parameters for various properties are
specified herein for embodiments of the invention, those parameters
can be adjusted up or down by 1/100.sup.th, 1/50.sup.th,
1/20.sup.th, 1/10.sup.th, 1/5.sup.th, 1/3.sup.rd, 1/2, 2/3.sup.rd,
3/4.sup.th, 4/5.sup.th, 9/10.sup.th, 19/20.sup.th, 49/50.sup.th,
99/100.sup.th, etc. (or up by a factor of 1, 2, 3, 4, 5, 6, 8, 10,
20, 50, 100, etc.), or by rounded-off approximations thereof,
unless otherwise specified. Moreover, while this invention has been
shown and described with references to particular embodiments
thereof, those skilled in the art will understand that various
substitutions and alterations in form and details may be made
therein without departing from the scope of the invention. Further
still, other aspects, functions and advantages are also within the
scope of the invention; and all embodiments of the invention need
not necessarily achieve all of the advantages or possess all of the
characteristics described above. Additionally, steps, elements and
features discussed herein in connection with one embodiment can
likewise be used in conjunction with other embodiments. The
contents of all references, including reference texts, journal
articles, patents, patent applications, etc., cited throughout this
application are hereby incorporated by reference in their entirety.
All appropriate combinations of embodiments, features,
characterizations, components and methods of those references and
the present disclosure may be selected for inclusion in embodiments
of the invention. Still further, the components and methods
identified in the Background section are integral to this
disclosure and can be used in conjunction with or substituted for
components and methods described elsewhere in the disclosure within
the scope of the invention.
* * * * *