Fuel combustion exhibiting low NO.sub.x and CO levels

Method and apparatus for safely combusting a fuel in such manner that very low levels of NO.sub.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).

TECHNICAL FIELD 
This invention pertains to the field of combusting fuel in a safe manner, 
while advantageously minimizing the production of nitrogen gases 
(NO.sub.x) and carbon monoxide (CO). 
BACKGROUND ART 
Keller et. al., "NO.sub.x and CO Emissions from a Pulse Combustor Operating 
in a Lean Premixed Mode", 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 A corrected, as yet unpublished, version of this 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., "Safe and Benign Controlled Premixed Burner Design Resulting 
in Ultra-Clean Combustion of Gaseous Fuels for Residential, Commercial, 
Industrial and Utility Applications" 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., "Development and Commercialization of a 5 million BTU/hr 
Pulse Combustion Commercial/Industrial Steam Boiler with Modulating 
Capabilities", 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 
"quasi-premixed operation", 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.sub.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.

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' and oxidant holes 21' 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' line up (simultaneously with holes 21 
and 21' 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' (and 21 and 21') 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.sub.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 "Kelvin-Helmholtz" 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'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.sub.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. 
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