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  In situ membrane-based oxygen enrichment for direct energy conversion methodsUSPTO Application #: 20060134569Title: 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. (end of abstract) Agent: Department Of The Army Cecom Legal Office, Fort Belvoir - Fort Belvoir, VA, US Inventor: H. Scott Coombe USPTO Applicaton #: 20060134569 - Class: 431012000 (USPTO) Related Patent Categories: Combustion, Process Of Combustion Or Burner Operation, Controlling Or Proportioning Feed The Patent Description & Claims data below is from USPTO Patent Application 20060134569. Brief Patent Description - Full Patent Description - Patent Application Claims
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. Continue reading... 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