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.

Description

FIELD OF INTEREST

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

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.

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.

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_x reduction. U.S. Pat. No. 5,454,712 discusses an apparatus for staged combustion to reduce NO_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_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_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_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_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° 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.

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_x emissions . . . ”. NO_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.

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.

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”)

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.)

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_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_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_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

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.

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° 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

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:

FIG. 1 is a schematic process flow diagram of present invention;

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.

FIG. 3 is a plot of Mass of Reactants vs. Oxygen Enrichment Level, while maintaining a steady fuel flow rate.

FIG. 4 is a plot of Mass of Fuel vs. Oxygen Enrichment Level, while maintaining a steady mass flow rate for reactants.

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

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.

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.

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.

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 O₂/N₂ selectivity. Ambient air contains 21% oxygen by volume.

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.

The burner assembly 8 contains a means of atomizing the fuel, an igniter, and a flow-altering device for inducing turbulent burning/mixing.

The combustion chamber 9 contains the hot exhaust gases.

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.

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.

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.

With reference to FIG. 2, the constant pressure adiabatic flame temperature of a diesel/air mixture is calculated using the chemical formula C_10.8H_18.7 for diesel fuel (Turns, S. R.). Though not shown, a similar relationship exists when using JP-8 fuel.

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.

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.

1.21% Oxygen Enriched (Ambient) Air: aC_10.8H_18.7+b(O₂+cN₂)→dCO₂+eH₂O+fN₂

Reaction coefficients are determined as follows:

a=1 (1 mole fuel used as baseline)

d=10.8

e=18.7/2=9.35

b=(2d+1e)/2=(21.6+9.35)/2=15.475

c=(1−X_o2)/X_o2=(1−0.21)/0.25=3.76

f=b*c=58.186

Restating the reaction C_10.8H_18.7+15.475(O₂+3.76N₂)→10.8CO₂+9.35H₂O+58.186N₂

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

2. 30% OEA: aC_10.8H_18.7+b(O₂+cN₂) →dCO₂+eH₂O+fN₂

The reaction coefficients are determined as follows:

a=1

d=10.8

e=18.7/2=9.35

b=(2d+1e)/2=(21.6+9.35)/2=15.475

c=(1−X_o2)/X_o2=(1−0.30)/0.30=2.33

f=b*c=36.11

Restating the reaction, C_10.8H_18.7+15.475(O₂+2.33N₂)→10.8CO₂+9.35H₂O+36.11N₂

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

3. 50% OEA: aC_10.8H_18.7+b(O₂+cN₂)→dCO₂+eH₂O+fN₂

The reaction coefficients are determined as follows:

a=1

d=10.8

e=18.7/2=9.35

b=(2d+1e)/2=(21.6+9.35)/2=15.475

c=(1−X_o2)/X_o2=(1−0.50)/0.50=1.0

f=b*c=15.475

Restating the reaction, 1.53C_10.8H_18.7+23.62(O₂+1.0N₂)→16.52CO₂+14.31H₂O+15.475N₂

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

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.

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_10.8H_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.

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.

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.