U.S. patent application number 11/016829 was filed with the patent office on 2006-06-22 for in situ membrane-based oxygen enrichment for direct energy conversion methods.
This patent application is currently assigned to United States of America as respresented by the Department of the Army. Invention is credited to H. Scott Coombe.
Application Number | 20060134569 11/016829 |
Document ID | / |
Family ID | 36596315 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060134569 |
Kind Code |
A1 |
Coombe; H. Scott |
June 22, 2006 |
In situ membrane-based oxygen enrichment for direct energy
conversion methods
Abstract
A method for combusting a diesel or JP-8 fuel at high
temperatures enabling efficiency and power density improvements for
portable direct energy conversion systems such as
thermophotovoltaics and thermionics is provided. Oxygen enriched
air is processed in situ using membrane separation methods. A
blower or pump downstream from the membrane provides oxygen
enriched air to a fuel burner where high temperature oxidation of a
diesel or JP-8 fuel is then enabled in a burner assembly. The hot
combustion gases in the burner heat an emitter specifically
designed for a thermophotovoltaic or thermionic device. A second
blower or pump provides nitrogen enriched air for auxiliary cooling
purposes.
Inventors: |
Coombe; H. Scott; (Fairfax,
VA) |
Correspondence
Address: |
DEPARTMENT OF THE ARMY;CECOM LEGAL OFFICE, FORT BELVOIR
AMSEL-LG-BELV
10235 BURBECK ROAD
FORT BELVOIR
VA
22060-5806
US
|
Assignee: |
United States of America as
respresented by the Department of the Army
|
Family ID: |
36596315 |
Appl. No.: |
11/016829 |
Filed: |
December 21, 2004 |
Current U.S.
Class: |
431/12 |
Current CPC
Class: |
Y02E 20/344 20130101;
Y02E 20/34 20130101; F23L 7/007 20130101; F23M 2900/13004
20130101 |
Class at
Publication: |
431/012 |
International
Class: |
F23N 1/00 20060101
F23N001/00 |
Goverment Interests
GOVERNMENT INTEREST
[0001] The invention described herein may be manufactured, used,
sold, imported, and/or licensed by or for the Government of the
United States of America.
Claims
1. An apparatus for enabling efficiency and power density
improvements for fueled portable direct energy conversion systems
comprising: a membrane separation apparatus used for increasing the
oxygen volume content of intake air through membrane separation
techniques: permeate and feed pumps for providing a variable supply
of oxygen-enriched air to a fuel burner apparatus; a burner
apparatus that atomizes fuel and burns said fuel with
oxygen-enriched air to produce hot exhaust gases, a combustion
chamber/emitter assembly that contains and is heated by the hot
combustion gases, and transfers energy; and a feedback mechanism to
control fuel flow as a function of load demand.
2. The apparatus of claim 1 wherein the permeate and feed pumps are
downstream from the membrane.
3. The apparatus of claim 1 wherein the permeate and feed pumps are
upstream from the membrane.
4. The apparatus of claim 1 wherein the fuel flow, air flow, and
oxygen content is controllable.
5. The apparatus of claim 4 wherein an optimum emitter temperature
is maintained by controlling the fuel flow, air flow, and oxygen
content.
6. The apparatus of claim 4 wherein the oxygen content of intake
air can be increased from 21% to 22-50%.
7. The apparatus of claim 6 wherein the oxygen content is
established through a membrane separation means.
8. The apparatus of claim 1 wherein a nitrogen-enriched retentate
flow is used for auxiliary cooling purposes.
9. The apparatus of claim 1 wherein the energy transfer takes place
through a means selected from the group comprising
thermophotovoltaic means or thermionic means.
10. The apparatus of claim 4 wherein the fuel and air flow are
controlled to achieve a near stoichiometric mixture of fuel &
air with an equivalence ratio of about 1.0.
11. A method for enabling efficiency and power density improvements
for fueled portable direct energy conversion systems comprising the
steps of: using a membrane separation apparatus for increasing the
oxygen volume content of intake air through membrane separation
techniques: providing permeate and feed pumps for providing a
variable supply of oxygen-enriched air to a fuel burner apparatus;
atomizing fuel with a burner apparatus that burns said fuel with
oxygen-enriched air to produce hot exhaust gases, providing a
combustion chamber/emitter assembly that contains and is heated by
the hot combustion gases, and transfers energy; and controlling the
fuel flow with a feedback mechanism such that fuel flow is
controlled as a function of load demand.
12. The method of claim 11 wherein the permeate and feed pumps are
downstream from the membrane.
13. The method of claim 11 wherein the permeate and feed pumps are
upstream from the membrane.
14. The method of claim 11 wherein the fuel flow, air flow, and
oxygen content is controllable.
15. The method of claim 14 wherein an optimum emitter temperature
is maintained by controlling the fuel flow, air flow, and oxygen
content.
16. The method of claim 14 wherein the oxygen content of intake air
can be increased from 21% to 22-50%.
17. The method of claim 15 wherein the oxygen content is
established through a membrane separation means.
18. The method of claim 11 wherein a nitrogen-enriched retentate
flow is used for auxiliary cooling purposes.
19. The method of claim 11 wherein the energy transfer takes place
through a means selected from the group comprising
thermophotovoltaic means or thermionic means.
20. The apparatus of claim 14 wherein the fuel and air flow are
controlled to achieve a near stoichiometric mixture of fuel &
air with an equivalence ratio of about 1.0.
Description
FIELD OF INTEREST
[0002] The invention relates to oxygen enriched combustion of
diesel and JP-8 fuels. In particular, the oxygen enrichment of the
fuel raises combustion temperatures beyond that which is possible
with ambient air, thereby enabling various improvements and design
flexibility for soldier-portable direct energy conversion devices
such as Thermionic and Thermophotovoltaic power systems.
BACKGROUND OF THE INVENTION
[0003] The U.S. Army has invested considerable research in the area
of direct energy conversion. The appeal of these technologies is
that they are characterized by solid-state designs and they utilize
burner systems that can be readily adapted to logistics fuels,
diesel or JP-8. Further, these technologies offer lower noise and
vibration than conventional reciprocating internal combustion
engines. To date, however, these technologies have not reached
suitable levels of fuel-to-electric efficiency or power density to
be attractive for the military. These two technical challenges are
the focus of this patent application.
[0004] Thermophotovoltaics and thermionic converters are
particularly attractive in the soldier-portable power range of up
to 1000 W, principally because no suitable power source
alternatives exist in the commercial or military marketplace. A gap
exists between soldier-portable batteries and soldier-portable
fueled power sources. On the low end, batteries are not practical
for long-term continuous power needs above 50 W. On the high end,
the smallest standard fueled power source available in the military
is the 2 kW Military Tactical Generator, a single-cylinder,
diesel-engine driven system. Though widely used in the military,
its noise level is 79 dB(A) at 7 meters distance (ref
MEP-HDBK-633), therefore it is not considered for applications
where low noise is paramount.
[0005] There are many related patents, however none describe the
proposed invention, and none are focused on the overall claims
submitted herein. U.S. Pat. No. 4,537,606 teaches an oxygen
enriched gas supply arrangement for combustion using membrane
materials, but the scope is limited only to the oxygen enrichment
apparatus. U.S. Pat. No. 4,931,013 involves a high temperature
burner and teaches that oxygen enrichment leads to higher flame
temperatures, more complete combustion, and increased burner
efficiency. The use of substantially pure oxygen is discussed. U.S.
Pat. No. 5,051,114 discusses Perfluorodioxole membranes for high
flux air separation. U.S. Pat. No. 5,147,417 teaches an air intake
system for mobile engines using polymer membranes for oxygen or
nitrogen enrichment. U.S. Pat. No. 5,248,252 discusses an enhanced
radiant output burner based on gaseous fuel preheating to cause
soot formation. U.S. Pat. No. 5,302,112 teaches the method of
operation for a gaseous fuel burner apparatus for NO.sub.x
reduction. U.S. Pat. No. 5,454,712 discusses an apparatus for
staged combustion to reduce NO.sub.x in an air-oxy-fuel burner
system. U.S. Pat. No. 5,593,480 describes the use of a zeolite
ceramic material for the oxygen enrichment of air. U.S. Pat. No.
5,723,074 discusses an oxygen ion-conducting dense ceramic for
oxygen enrichment. U.S. Pat. No. 5,914,154 discusses a non-porous
gas permeable membrane for oxygen separation. U.S. Pat. No.
5,942,203 teaches a process for producing and utilizing an oxygen
enriched gas for gas turbine applications and the conversion of
fuels to synthetic gases, etc. U.S. Pat. No. 5,944,507 describes an
oxy/oil swirl burner for liquid fuels to reduce NO.sub.x. Pure
oxygen is used for the oxidant. U.S. Pat. No. 5,960,777 describes a
combustion engine air supply for oxygen or nitrogen enriched
applications based on membrane use. U.S. Pat. No. 6,055,808
describes a method and apparatus for reducing particulates and
NO.sub.x emissions from diesel engines using oxygen enriched air
(OEA). U.S. Pat. No. 6,126,438 discusses preheating fuel and
oxidants in a combustion burner application. Natural gas is the
preferred fuel, and oxygen is used as the oxidant. U.S. Pat. No.
6,126,721 discusses an OEA supply apparatus for portable breathing
applications. U.S. Pat. No. 6,286,482B1 describes a premixed charge
compression ignition engine with optimal combustion and NO.sub.x
control. U.S. Pat. No. 6,406,517B1 discusses designed selectivity
gas permeable membranes, and provides a Robeson plot of
oxygen/nitrogen selectivity for several materials. U.S. Pat. No.
6,523,349B2 discusses clean air engines for transportation and
other power applications using membrane based oxygen separation. A
Clean Air Engine (CLAIRE) is discussed. NO.sub.x reduction is
discussed. A means to compress oxygen and fuel before entry into
the combustion device is discussed. The use of the water byproduct
of combustion for steam generation is discussed. The use of
intercooling is also mentioned. U.S. Pat. No. 6,596,220 teaches a
method for oxy-fueled combustion to achieve 4500.degree. F. flame
temperatures. An external supply of oxygen is needed. U.S. Pat. No.
6,685,464B2 describes high velocity injection of enriched oxygen
gas for furnace and boiler applications. Oxygen enriched air is
injected downstream. The limit of oxygen enrichment is 23% by
volume.
[0006] U.S. Pat. No. 5,356,487 describes a thermally amplified and
stimulated emission radiator fiber matrix burner. The description
differs from the invention proposed herein, however, in a number of
ways. U.S. Pat. No. 5,356,487 discusses the combustion of natural
gas (and other low-molecular-weight gaseous fuels including oil
aerosols) whereas the invention described herein is focused
specifically on diesel and JP-8 fuels. The molecular weight of
diesel fuel, for example, is 148.6 g/mol whereas the molecular
weight of methane is 16.043 g/mol (Turns, Stephen, "An Introduction
to Combustion . . . "). In addition, U.S. Pat. No. 5,356,487 notes
that "there is a need for a high-energy density burner that
produces low NO.sub.x emissions . . . ". NO.sub.x is not a
consideration in the invention proposed herein. In the description
of FIG. 8, U.S. Pat. No. 5,356,487 notes that "the air may be
replaced with or enriched with oxygen . . . ". Also, in the
description of FIG. 10, U.S. Pat. No. 5,356,487 notes again that
"oxygen may be substituted for air". The invention described herein
specifies a volume content of oxygen in the range of 22-50%.
Further, the only reference to oxygen enrichment in the claims for
U.S. Pat. No. 5,356,487 is the phrase in claim 19, ". . . an oxygen
enrichment means to heat the fibers . . . ". Nothing is mentioned
regarding the method for achieving oxygen enrichment. The invention
herein promotes the use of either a polymer or ceramic
membrane-based oxygen enrichment method. U.S. Pat. No. 5,356,487
does not mention the application for Thermionics contained within
the invention proposed herein.
[0007] Three different technologies can be employed for air
separation: cryogenic distillation, ambient temperature adsorption,
and membrane separations. Organic polymer membrane technology is
economical for the production of nitrogen and oxygen-enriched air
(up to about 40% oxygen) at small scale. Adsorption technology
provides 85-95% oxygen at flow rates up to 100 tons/day. The
cryogenic process can generate oxygen or nitrogen at flows of 2500
tons/day from a single plant (Robert M. Thorogood, "Air
separation"). Membrane separation is the only one of the three
currently adaptable to small portable applications as discussed
here.
[0008] The working principle for polymer membrane-based separation
is that oxygen permeates faster than nitrogen through many organic
polymers based on partial pressure differential. A typical membrane
separator for industrial applications contains small hollow polymer
fibers 100-500 micrometers in diameter and 1-3 m (3-10 ft) in
length. These are assembled in bundles of 0.1-0.25 m (0.3-0.8 ft)
diameter. Polymer fibers used in commercial separators have very
thin dense polymer layers as small as 35 nm that are supported on
thicker porous walls. Commercial polymers have permeation rate
selectivities of about 6 for oxygen over nitrogen. Examples of
polymers in use are polysulfone, polycarbonate, and polyamides.
(Robert M. Thorogood, "Air separation")
[0009] For ceramic membranes, oxygen permeates through a nonporous
surface essentially through one of the following two driving
forces: solid diffusion within the membrane, or interfacial oxygen
exchange on either side of the membrane (Gellings et al.)
[0010] A significant point to be made here is that much of the
oxygen and nitrogen enrichment research work performed to date is
for the purpose of controlling exhaust emissions. For example,
research performed at Argonne National Lab showed a substantial
increase in NO.sub.x with higher oxygen content (Poola, R. B. et
al., "Study of Using Oxygen-Enriched Combustion Air for Locomotive
Diesel Engines"). Later, Argonne studied Nitrogen-enrichment
instead and concluded that this approach can reduce NO.sub.x, but
at the penalty of reducing adiabatic flame temperatures (Nemser et
al., "Nitrogen Enriched Intake Air Supplied by High Flux Membranes
for the Reduction of Diesel NO.sub.x Emissions"). Though the EPA
strictly regulates most "engines", the EPA does not plan to
regulate the emissions from direct energy conversion power systems
such as those described here (US EPA, 2003, "Proposed Tier 4
Emissions Standards") and (e-mail correspondence between author and
Mr. Alan Stout, U.S. EPA Office of Transportation and Air
Quality).
SUMMARY OF THE INVENTION
[0011] Accordingly, one object of the present invention is to
provide an improved fuel-to-electric efficiency by increasing
combustion efficiency. Combustion efficiency generally increases
with higher combustion temperatures (see U.S. Pat. No. 4,931,013).
Oxygen enrichment provides the means to directly achieve higher
flame and exhaust gas temperatures (FIG. 2). In addition, power
density can be increased through oxygen enriched combustion. Oxygen
enrichment reduces the overall mass flow necessary for oxidation of
a given mass flow of fuel (reduction in nitrogen), thereby enabling
burner volume reductions. Alternatively, for a given burner volume
and total mass flow, oxygen enrichment allows increased fuel flow,
leading to a higher Heat of Combustion and higher power density.
See the description of FIGS. 3-5 for detailed analysis.
[0012] An additional benefit of oxygen enriched combustion is the
flexibility to operate through a wider temperature range. The
adiabatic flame temperature rises by over 500.degree. C. when
increasing the percentage of oxygen from 21 to 30% (FIG. 1). This
is important for thermophotovoltaics, because they operate most
efficiently in the 1200-1700 K range (see U.S. Pat. No. 6,177,628),
and thermionics which typically operate with emitter temperatures
of 1600-2500 K (Elias P. Gyftopoulos et al., "Thermionic power
generator"). These temperatures are sometimes difficult to attain
without oxygen enrichment, with or without a recuperator, due to
losses from heat transfer and incomplete combustion. The present
invention addresses this problem.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] These and other objects of the invention will become readily
apparent in light of the Detailed Description Of The Invention and
the attached drawings wherein:
[0014] FIG. 1 is a schematic process flow diagram of present
invention;
[0015] FIG. 2 is a plot of theoretical constant pressure adiabatic
flame temperature of diesel fuel combustion in air with varying
oxygen content. Equivalence ratio used is 1.0.
[0016] FIG. 3 is a plot of Mass of Reactants vs. Oxygen Enrichment
Level, while maintaining a steady fuel flow rate.
[0017] FIG. 4 is a plot of Mass of Fuel vs. Oxygen Enrichment
Level, while maintaining a steady mass flow rate for reactants.
[0018] FIG. 5 is a plot of Heat of Combustion vs. Oxygen Enrichment
Level, while maintaining a steady mass flow rate for reactants.
DETAILED DESCRIPTION OF THE INVENTION
[0019] With reference to FIG. 1, ambient air 1 flows into a
membrane apparatus 2, wherein a portion of the flow permeates a
membrane 3 as oxygen enriched air 4. Oxygen enriched flow 4 from
the membrane apparatus enters an adjustable flow blower/pump 5
where it is fed to a diesel/JP-8 burner assembly 8. Diesel/JP-8
fuel 6 is fed through an adjustable flow fuel pump 7 to the same
diesel/JP-8 burner assembly 8. The fuel is ignited in the burner
assembly 8. The flame and hot combustion gases flow through the
combustion chamber 9 while heating an emitter surface 10. Energy 11
(electromagnetic or electric) is transmitted to surface 12. All
components are housed in enclosure 13.
[0020] Though not shown, nitrogen enriched flow 5 from the membrane
can be used for cooling the thermophotovoltaic cells, cooling the
hot exhaust surfaces, or for other cooling needs.
[0021] Membrane apparatus 2 can consist of a single membrane
element or more than one membrane elements configured in series,
parallel, or a combination thereof as desired to achieve the
desired oxygen content and pressure drop.
[0022] Membrane 3 can be selected from one of a variety of
materials and designs. Polymeric membrane materials can be utilized
to attain 40% oxygen content. If levels above 40% are required,
ceramic membrane materials can be employed (K. Stork and R. Poola.
"Membrane-Based Air Composition Control . . . ", pg. 34). Ceramic
membrane materials typically require elevated operating
temperatures for optimum O.sub.2/N.sub.2 selectivity. Ambient air
contains 21% oxygen by volume.
[0023] Air blower/pump 5 preferably provides variable flow of
oxygen enriched air (OEA) to the fuel burner assembly. Fuel pump 7
preferably provides a variable flow of diesel or JP-8 fuel to the
burner assembly depending on load demand.
[0024] The burner assembly 8 contains a means of atomizing the
fuel, an igniter, and a flow-altering device for inducing turbulent
burning/mixing.
[0025] The combustion chamber 9 contains the hot exhaust gases.
[0026] In one embodiment of the invention, a high temperature
thermophotovoltaic emitter 10 transmits electromagnetic energy
(photons) 11 to an array of thermophotovoltaic cells 12. Of course,
this is a rudimentary depiction--typically thermophotovoltaic
systems are comprised of an emitter, prisms and/or filters,
thermophotovoltaic cells and other components.
[0027] In another embodiment, a high temperature thermionic emitter
10 radiates electrons 11 to the cool collector 12. Though not
shown, inclusion of a recuperator can be employed for maximizing
efficiency for either the thermophotovoltaic or thermionic
application.
[0028] Enclosure 13 is a schematic representation of a militarized
enclosure designed for a tactical environment. It houses items
1-12, and is designed for the environments typically encountered
during a military mission. Though not shown, enclosure 13 provides
a system for performing typical user control actions.
[0029] With reference to FIG. 2, the constant pressure adiabatic
flame temperature of a diesel/air mixture is calculated using the
chemical formula C.sub.10.8H.sub.18.7 for diesel fuel (Turns, S.
R.). Though not shown, a similar relationship exists when using
JP-8 fuel.
[0030] FIG. 2 shows that the theoretical adiabatic flame
temperature rises significantly with oxygen content, from 2230 K
(21%) to 2810 K (30%) and 3820 K (50%). Note that 2230 K is the
maximum constant pressure flame temperature attainable when burning
diesel fuel with ambient air with no losses. Of course, the emitter
temperature would be lower than the flame temperature in practice
due to heat transfer and combustion efficiency losses.
[0031] FIG. 3 plots the Mass Flow of Reactants vs. Oxygen
Enrichment level. The mass flow is calculated below for three
oxidation reactions (21%, 30%, and 50% oxygen content). Dry air, no
dissociation, and a stoichiometric mixture of reactants are
assumed.
[0032] 1.21% Oxygen Enriched (Ambient) Air:
aC.sub.10.8H.sub.18.7+b(O.sub.2+cN.sub.2).fwdarw.dCO.sub.2+eH.sub.2O+fN.s-
ub.2
[0033] Reaction coefficients are determined as follows:
[0034] a=1 (1 mole fuel used as baseline)
[0035] d=10.8
[0036] e=18.7/2=9.35
[0037] b=(2d+1e)/2=(21.6+9.35)/2=15.475
[0038] c=(1-X.sub.o2)/X.sub.o2=(1-0.21)/0.25=3.76
[0039] f=b*c=58.186
[0040] Restating the reaction
C.sub.10.8H.sub.18.7+15.475(O.sub.2+3.76N.sub.2).fwdarw.10.8CO.sub.2+9.35-
H.sub.2O+58.186N.sub.2
[0041] Therefore, the molecular weight of reactants is,
MW=12(10.8)+1(18.7)+15.475[(16)(2)+(3.76)(14)(2)]=2272.71 g
[0042] 2. 30% OEA: aC.sub.10.8H.sub.18.7+b(O.sub.2+cN.sub.2)
.fwdarw.dCO.sub.2+eH.sub.2O+fN.sub.2
[0043] The reaction coefficients are determined as follows:
[0044] a=1
[0045] d=10.8
[0046] e=18.7/2=9.35
[0047] b=(2d+1e)/2=(21.6+9.35)/2=15.475
[0048] c=(1-X.sub.o2)/X.sub.o2=(1-0.30)/0.30=2.33
[0049] f=b*c=36.11
[0050] Restating the reaction,
C.sub.10.8H.sub.18.7+15.475(O.sub.2+2.33N.sub.2).fwdarw.10.8CO.sub.2+9.35-
H.sub.2O+36.11N.sub.2
[0051] Therefore, the molecular weight of reactants is,
MW=1(12)(10.8)+1(18.7)+15.475[(16)(2)+(2.33)(14)(2)]=1653.09 g
[0052] 3. 50% OEA:
aC.sub.10.8H.sub.18.7+b(O.sub.2+cN.sub.2).fwdarw.dCO.sub.2+eH.sub.2O+fN.s-
ub.2
[0053] The reaction coefficients are determined as follows:
[0054] a=1
[0055] d=10.8
[0056] e=18.7/2=9.35
[0057] b=(2d+1e)/2=(21.6+9.35)/2=15.475
[0058] c=(1-X.sub.o2)/X.sub.o2=(1-0.50)/0.50=1.0
[0059] f=b*c=15.475
[0060] Restating the reaction,
1.53C.sub.10.8H.sub.18.7+23.62(O.sub.2+1.0N.sub.2).fwdarw.16.52CO.sub.2+1-
4.31H.sub.2O+15.475N.sub.2
[0061] Therefore, the molecular weight of reactants is,
MW=1(12)(10.8)+1(18.7)+15.475[(16)(2)+(1.0)(14)(2)]=1076.8 g
[0062] Since the mass of reactants decreases with increasing oxygen
enrichment (FIG. 3), we now show how to capitalize on this
phenomenon to improve heat flow through a combustor. FIG. 4 plots
Fuel in Reactants vs. Oxygen Enrichment Level, while maintaining a
steady mass flow through the combustor. A significant increase in
fuel mass flow is shown, 37% with 30% OEA and 53% with 50% OEA.
[0063] As a direct result of the increased fuel mass flow made
possible by oxygen enrichment, the corresponding heat of combustion
rises with increased oxygen enrichment levels, as shown in FIG. 5.
A value of 180.97 kJ/mol is used for the heat of combustion for
diesel fuel, C.sub.10.8H.sub.18.7 (ref Turns, S. R.) FIG. 5
suggests the potential for power density increases up to 37% with
30% OEA and up to 53% with 50% OEA.
[0064] As may be appreciated by those skilled in the art, while the
present invention has been described with reference to preferred
embodiments, numerous additions, omissions and changes may be made
without departing from the spirit and scope of the present
invention as set forth in the appended claims.
* * * * *