Abstract:
Method and apparatus for safely combusting a fuel in such manner that very low levels of NO x  and CO are produced. The apparatus comprises an inlet line (12) containing a fuel and an inlet line (18) containing an oxidant. Coupled to the fuel line (12) and to the oxidant line (18) is a mixing means (11,29,33,40) for thoroughly mixing the fuel and the oxidant without combusting them. Coupled to the mixing means (11,29,33,40) is a means for injecting the mixed fuel and oxidant, in the form of a large-scale fluid dynamic structure (8), into a combustion region (2). Coupled to the combustion region (2) is a means (1,29,33) for producing a periodic flow field within the combustion region (2) to mix the fuel and the oxidant with ambient gases in order to lower the temperature of combustion. The means for producing a periodic flow field can be a pulse combustor (1), a rotating band (29), or a rotating cylinder (33) within an acoustic chamber (32) positioned upstream or downstream of the region (2) of combustion. The mixing means can be a one-way flapper valve (11); a rotating cylinder (33); a rotating band (29) having slots (31) that expose open ends (20,21) of said fuel inlet line (12) and said oxidant inlet line (18) simultaneously; or a set of coaxial fuel annuli (43) and oxidizer annuli (42,44). The means for producing a periodic flow field (1, 29, 33) may or may not be in communication with an acoustic resonance. When employed, the acoustic resonance may be upstream or downstream of the region of combustion (2).

Description:
STATEMENT OF GOVERNMENTAL INTEREST 
     The government has rights in this invention pursuant to contract no. DE-AC-04-76-DP00789 awarded by the U.S. Department of Energy to Sandia Corporation. 
    
    
     TECHNICAL FIELD 
     This invention pertains to the field of combusting fuel in a safe manner, while advantageously minimizing the production of nitrogen gases (NO x ) and carbon monoxide (CO). 
     BACKGROUND ART 
     Keller et. al., &#34;NO x  and CO Emissions from a Pulse Combustor Operating in a Lean Premixed Mode&#34;, Western States Section/The Combustion Institute 1993 Spring Meeting, University of Utah, Salt Lake City, Utah, Mar. 22-23, 1993, discloses portions of the pulse combustor embodiment of the present invention. This paper, however, contains data points for carbon monoxide which are incorrect. A corrected, as yet unpublished, version of this paper is appended to this specification as Appendix A, and is expressly incorporated by reference herein. 
     Keller et al., &#34;Safe and Benign Controlled Premixed Burner Design Resulting in Ultra-Clean Combustion of Gaseous Fuels for Residential, Commercial, Industrial and Utility Applications&#34; is another unpublished paper giving further background and details of the present invention. Said paper is appended to this specification as Appendix B, and is also expressly incorporated by reference herein. 
     Belles et al., &#34;Development and Commercialization of a 5 million BTU/hr Pulse Combustion Commercial/Industrial Steam Boiler with Modulating Capabilities&#34;, Final Report for Gas Research Institute, Contract No. 5087-295-1548, January 1993, relates generally to the subject matter of this patent application. See in particular page 23, which discusses &#34;quasi-premixed operation&#34;, and FIG. 22. In contrast with the present invention, the apparatus shown in FIG. 22 shows separate flapper valves for the air and the gas. These valves do not close at the same time; therefore, it is not possible to control the equivalence ratio as it is in the present invention. 
     Also see U.S. Pat. Nos. 5,118,281; 5,020,987; 4,955,805; 4,938,203; 4,926,798; 4,856,981; 4,752,209; 4,687,435; 4,484,885; 4,309,977; and U.S. Pat. No. 3,667,451. 
     DISCLOSURE OF INVENTION 
     The present invention is a method for safely combusting fuel while achieving low levels of NO x  and CO. The method comprises the steps of thoroughly mixing a fuel and an oxidant without combusting them. The mixed fuel and oxidant are injected (4) into a region (2) where combustion occurs. The injected mixture has the form of a large-scale fluid dynamic structure (8). This enables macroscopic mixing of the fuel and the oxidant, as created by the injection profile and the associated geometry. The flow field within the combustion region (2) is time-varied in order to temporarily control the mixing characteristics of the premixed reactants (fuel and oxidant) with the ambient fluid in the combustion chamber (2). Controlling the rate and character of the rapid and thorough mixing of the ambient fluid with the premixed reactants allows the combustion characteristics to be modified to optimize a desired process. One such process is to minimize the emission of harmful pollutants without the sacrifice of efficiency; another is to maximize oxidation of hazardous organic compounds. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a pulse combustor 1 using the present invention. 
     FIG. 2 is an enlarged side cross-sectional view of a portion of FIG. 1. 
     FIG. 3 is a side cross-sectional view of a flapper valve 11 used in conjunction with the present invention, in which valve 11 is closed. 
     FIG. 4 is a side cross-sectional view of a flapper valve 11 used in conjunction with the present invention, in which valve 11 is partially open. 
     FIG. 5 is a side cross-sectional view of a flapper valve 11 used in conjunction with the present invention, in which valve 11 is open. 
     FIG. 6 is an end view of a flapper 16 used in conjunction with flapper valve 11. 
     FIG. 7 is an end view of a backer plate 17 used in conjunction with flapper valve 11. 
     FIG. 8 is a side view of rod 10 used in conjunction with the present invention. 
     FIG. 9 is an end view of a first embodiment of a stagnation plate 7 used in the present invention. 
     FIG. 10 is an end view of a second embodiment of a stagnation page 7 used in the present invention. 
     FIG. 11 is an isometric view of a second embodiment of a one-way valve 29 used in the present invention. 
     FIG. 12 is a side cross-sectional view of a second embodiment of an acoustic resonance 32 used in the present invention. 
     FIG. 13 is an end view of a valve seat 15 used in flapper valve 11. 
     FIG. 14 is a cross-sectional side view of an alternative embodiment of a mixing means 40 of the present invention. 
     These and other more detailed and specific objects and features of the present invention are more fully disclosed in the following specification, reference being had to the accompanying drawings, in which: 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of the present invention using a Helmholtz-type pulse combustor 1. A fuel and an oxidant are combusted within combustion chamber 2. The gaseous flow field within chamber 2 is time-varied by pulsing the combustion. A tailpipe 9 has a smaller cross-section than that of chamber 2. A contraction section 5 transitions between chamber 2 and tailpipe 9. Expansion terminator 39 has a larger cross-section than that of tailpipe 9. Gases are allowed to expel through cooling exhaust pipe 6. 
     In the present invention, mixing chamber 3 of pulse combustor 1 is not used. Rather, premixed fuel and oxidant are fed into combustion chamber 2 via intake port 4. The mixing is accomplished by providing a one-way valve comprising a barrier. Upstream of the barrier, the fuel and the oxidant do not mix. Downstream of the barrier, the fuel and the oxidant are allowed to mix in a mixing region that is upstream of chamber 2. FIG. 1 illustrates the embodiment of the present invention in which the one-way valve is a flapper valve 11 inserted axially along the intake port 4. The geometry does not have to be axi-symmetric. The gaseous flow fields are time-varying all the way from valve 11 to expansion terminator 39. Periodic pulsing of the combustion combined with careful selection of the geometry of the components within pulse combustor 1 creates a condition of acoustic resonance within combustor 1, i.e., a pattern of oscillatory standing waves of the gases within combustor 1. This advantageously increases the rates of heat, mass, and momentum transfer. 
     The fuel is introduced through fuel port 12. Oxidant is introduced through oxidant port 18. The fuel can be any gaseous fuel such as methane, natural gas, or propane. The oxidant can be air. A lean fuel/oxidant equivalence ratio is used. 
     FIG. 2 shows that preferably a stagnation plate 7 is placed within combustion chamber 2 near the entrance 37 thereof. The stagnation plate 7 can be as described in U.S. Pat. No. 5,118,281. Plate 7 is fixedly mounted at the end of an elongated rod 10 that is coaxially disposed within intake port 4. Plate 7 helps to create a large-scale fluid dynamic structure 8 (as this term is conventionally used in the fluid dynamics art) within combustion chamber 2. This large-scale fluid dynamic structure 8 advantageously enhances the rapid macroscopic mixing of the fuel and the oxidant within chamber 2. Preferably, the fluid dynamic structure 8 has the form of a coherent vortex, such as a toroidal vortex. The flow field within combustion region 2 is time-varied in order to mix the premixed fuel and oxidant with the ambient gases, e.g., the products of combustion, to enhance the rate of mixing, advantageously controlling the combustion fluid dynamics to optimize the desired process. The time-varying may be periodic, i.e., oscillatory. 
     FIGS. 3-7 and 13 illustrate a first embodiment of the mixing means in which the mixing means is a one-way flapper valve 11. A flapper 16 is free to move axially between a valve seat 15 located upstream of flapper 16 and a backer plate 17 located downstream of flapper 16. Flapper 16 is made of a non-porous material such as Teflon. Backer plate 17 is fixedly spaced apart from the valve seat 15. The distance of this spacing is selected based upon flow rate requirements. Opening the flapper 16 causes fuel holes 20 and oxidant holes 21 to open simultaneously. Closing the flapper 16 causes the fuel holes 20 and the oxidant holes 21 to close simultaneously. The simultaneity feature is important, because it enables the fuel/oxidant equivalence ratio to be precisely controlled. Preferably, there are many fuel holes 20 and many oxidant holes 21, to enhance the mixing process. In the illustrated embodiment, the fuel holes 20 are smaller than the oxidant holes 21, but this is not necessary. FIG. 3 shows flapper valve 11 in the closed position. FIG. 4 shows flapper valve 11 in the partially open position. FIG. 5 shows flapper valve 11 in the open position. 
     Backer plate 17 contains apertures to communicate to flapper 16 pressure information from downstream. The apertures in backer plate 17 do not have to be aligned with the apertures in valve seat 15. Valve seat 15 has a relatively large center aperture 22 to accommodate rod 10. Screws 26 (see FIG. 3) are used to space backer plate 17 apart from valve seat 15. Screws 26 pass through apertures 25 in backer plate 17, apertures 46 in flapper 16, and apertures 23 in valve seat 15 (FIGS. 7, 6, 13, respectively). Rigid pins 24 fixedly mounted on valve seat 15 can also be used for spacing purposes (FIG. 13). 
     Flapper valve 11 may be constructed in two major portions, an upstream housing 13 and a downstream housing 14, for ease of assembly. Tooled within housing 14 is an oxidant manifold 30 and a fuel manifold 19. The purpose of these manifolds 30,19 is to divide the gas flow from the single oxidant input port 18 into many oxidant holes 21, and to divide the gas flow from the single fuel input port 12 into many fuel holes 20, respectively. 
     By using many fuel holes 20 and oxidant holes 21, the mixing of the fuel and the oxidant is advantageously thorough. Just downstream of backer plate 17 and flapper valve 11, the fuel and the oxidant are thoroughly mixed. 
     FIG. 8 shows an exemplary center rod 10. The upstream end 27 of rod 10 may be threaded so as to fit within rod opening 22 within valve seat 15. The downstream end of rod 10 may be a swirl 28. By this device, a series of helical paths is inserted within intake port 4. This advantageously introduces more vorticity in the axial direction, which breaks down the fluid dynamic structure 8 more quickly to enhance the microscopic mixing. Swirl 28 does not need to rotate. Rather than using a static swirl 28, a time-varying (dynamic) swirl could be used within intake port 4. 
     FIGS. 9 and 10 illustrate two embodiments of stagnation plate 7. In FIG. 10, plate 7 has the shape of a flat washer. In FIG. 9, plate 7 has the shape of a star. The number of star points is selected based upon the natural breakdown eigenvalue of the fluid dynamic structure 8. Compared with the FIG. 10 embodiment, the FIG. 9 embodiment breaks down the fluid dynamic structure 8 more rapidly, thereby increasing the rate of microscopic mixing. 
     An alternative embodiment of the one-way valve mixing means is the mechanical means illustrated in FIG. 11. In this embodiment, the mixing means is a rotating band 29 having many elongated slots 31 cut therefrom. Band 29 is rotated by a motor (not illustrated). A fuel manifold 19 transitions the single fuel input line 12 into several fuel holes 20. Similarly, an oxidant manifold 30 transitions the single oxidant input line 18 into several oxidant holes 21. A large number of holes 20, 21 advantageously increases the amount of mixing, as does alternating fuel holes 20 with oxidant holes 21. Holes 20, 21 are fixedly positioned just inside slots 31 as slots 31 rotate past holes 20, 21. As a result, the fuel and oxidant are simultaneously injected into intake port 4 each time a slot 31 passes over the series of holes 20,21. The pulsing can easily be made to be periodic, by rotating band 29 at a constant speed and by providing an equal spacing between slots 31. A periodic pulse rate combined with a proper selection of geometry of the components within combustor 1 can be used to set up a condition of acoustic resonance. This embodiment illustrates that the time-varying flow field can be created upstream of the combustion chamber 2, as well as downstream as with the conventional pulse combustor 1. 
     FIG. 12 illustrates another embodiment of the present invention in which the flow field is time-varied upstream of the combustion chamber 2. Alternatively, the flow field may be time-varied downstream of the combustion chamber 2. In the embodiment illustrated in FIG. 12, an acoustic chamber 32 is positioned upstream of the combustion chamber 2. Chamber 32 can be dimensioned to create a condition of acoustic resonance, and can be pressurized to enhance the resonant effect. Chamber 32 contains a fixed outer cylindrical sleeve 34 containing fuel holes 20 and oxidant holes 21. Fitting within outer cylindrical sleeve 34 is a rotating inner cylindrical sleeve 33 containing fuel holes 20&#39; and oxidant holes 21&#39; that are longitudinally aligned with holes 20 and 21, respectively. Preferably, a plurality of fuel holes 20 (produced by a fuel manifold) and a plurality of oxidant holes (produced by an oxidant manifold) are utilized, to enhance the mixing process. Sleeve 33 is rotated by a motor (not illustrated). When holes 20 and 20&#39; line up (simultaneously with holes 21 and 21&#39; lining up), the fuel and oxidant are simultaneously passed from acoustic resonator 32 into an upstream-extending zone 38 of combustion chamber 2, and are mixed in this zone 38. The fuel and oxidant are further mixed by stagnation plate 7. When holes 20 and 20&#39; (and 21 and 21&#39;) are not aligned, the fuel/oxidant mixture is not introduced into the combustion chamber 2. This pulsing of the fuel and oxidant time-varies the gaseous flow fields. As with all the other embodiments illustrated herein, these pulses are advantageous because the combustion time is shortened, which tends to reduce the levels of thermal NO x . Also, the pulsing strengthens the fluid dynamic structure 8. This advantageously enhances mixing. The speed of rotation of inner cylinder 33, as well as the geometry of the acoustic resonator 32 and the cylinders 33, 34, can be matched so as to create a condition of acoustic resonance. In this case, the acoustic resonance occurs upstream of the combustion chamber 2. 
     An alternative embodiment 40 of the mixing means and injecting means of the present invention is shown in FIG. 14. A co-axial injection system 40 is comprised of a solid rod 10 placed on the centerline. Rod 10 protrudes beyond the exit plane 41 of the co-axial fuel and oxidizer delivery system 40. Attached to the end of this rod 10 is a stagnation plate 7. Flow past this plate 7 deposits vorticity with a radial component into the flow, creating a coherent toroidal vortex 8. The strength of this toroidal vortex 8 will be, in part, determined by the axial position of the stagnation plate 7. The oxidizer/fuel delivery system 40 is configured as a system of coaxial annular delivery tubes 42,43,44 around the central rod 10. The fuel is delivered by an annular tube 43, with the oxidizer 42,44 existing on both sides of the annular fuel jet 43. The cross-sectional area of each annular jet 42,43,44 is designed so that injection velocity for the oxidizing stream and for the fuel stream are not equal. The injection velocity for both the fuel and oxidizing stream may or may not be periodic in time. This creates a free shear at each interface 45 between the air and oxidizer due to &#34;Kelvin-Helmholtz&#34; instabilities, coherent vertical structures aligned in the azimuthal direction (a radial component of vorticity). These vortex structures 45 entrain the fuel and the oxidizer from each side of the layer into the center. The number and size of these annular injection streams 42,43,44 are determined by the size of the burner, the natural shedding frequency of these structures, the growth characteristics of the shear layer, and its strength. (The design for these co-axial annular jets 42,43,44 can be readily determined from the power output, and fuel type specified by a specific application.) This stratified annular flow is injected past stagnation plate 7. Vorticity with a radial component is shed in the streamwise direction, resulting in a large coherent toroidal vortex 8. The high strain rates created as the reactants accelerate past the stagnation plate 7 suppress the reaction due to fluid dynamic stretch. These high strain rates are a result of large velocity gradients and exist spatially in regions of intense fluid dynamic mixing. The axi-symmetric toroidal vortex 8 that is created by the streamwise deposition of vorticity can be caused to go unstable, resulting in a cascade of energy from large scale to fine scale motion, providing further microscopic mixing of the fuel and oxidizer. This can be induced by one or more of the following mechanisms: 1) Enhancing the natural eigenvalue breakdown mode of the toroidal vortex 8 by creating spatially uniform lobes in the toroidal vortex 8 equal in number to the eigen breakdown value (i.e., making stagnation plate 7 starred, with 6 to 8 lobes). 2) Causing the axial deposition of vorticity to be of unequal strengths in the azimuthal direction (for example, making an elliptically shaped stagnation plate 7). This will cause the axial vorticity to compete in the azimuthal direction. The vortex will undulate in the radial direction, systematically changing the azimuthal orientation of the major and minor axis. 3) Introducing a radial component of vorticity in the flow (i.e., placing swirl generators 28 upstream of the stagnation plate 7). Accelerating the swirling flow past the stagnation plate 7 stretches the radial vorticity component and, due to conservation of angular momentum, the rotational velocity will increase and the spatial region of influence will decrease. This is the identical phenomenon to an ice skater increasing the rate of spin by placing the body&#39;s extremities near the axis of rotation. The increase in local circulation results in a more efficient transfer of energy from the large scale to the fine scale, causing a more rapid and thorough microscopic mixing of the fuel and oxidizer. 
     The present invention does not require an acoustic resonant condition, either upstream or downstream of the combustion region 2. An acoustic resonance may be advantageous in some applications and disadvantageous in other applications. The FIG. 1 embodiment advantageously uses a flow in acoustic resonance to drive the periodic injection process. In the mechanically driven injection embodiments (FIGS. 11 and 12), an acoustic resonance may or may not be employed. If employed, it may be located either upstream or downstream of the combustion region 2, depending upon the application. The same mixing characteristics can be created with or without flows in acoustic resonance. 
     Safety is preserved in the present invention by mixing the reactants in a fluid dynamic flow field incapable of supporting combustion. Thorough mixing of the reactants with any ambient fluid (acting as a diluent, dropping temperature, and/or as an ignition source), controls the combustion fluid dynamics, optimizing the combustion process of choice. One important result is to optimize for the minimum emission of harmful pollutants (NO x  and CO). In the case of flashback (an undesired condition from a safety standpoint), a benign diffusion flame is stabilized at the face of the mixing valve 11, 29, 33 without the possibility of an explosion, because a minimum of premixed reactants are present at any one given time. 
     The above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. The scope of the invention is to be limited only by the following claims. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the spirit and scope of the invention. ##SPC1##