Low emission combustor having tangential lean direct injection

Lean direct injection is used in a gas turbine combustor to reduce NO.sub.x emissions. The combustor has a plurality of fuel jets for tangentially injecting fuel and a plurality of air jets for tangentially injecting air therein. The fuel jets and the air jets are preferably disposed in a common cross-sectional plane, although additional groups of fuel and air jets in other planes can be provided. The jets are all evenly spaced and alternate between fuel and air jets. All of the jets preferably point in the same circumferential direction. Alternatively, the jets can be arranged so that all fuel jets are located in a first cross-sectional plane, and all air jets are located in a second cross-sectional plane. Preferably, the fuel jets point in one circumferential direction while the air jets point in the opposite circumferential direction.

BACKGROUND OF THE INVENTION 
This invention relates generally to combustors for gas turbines and more 
particularly concerns a combustor using lean direct injection for reduced 
NO.sub.x emissions. 
Traditional gas turbine combustors use nonpremixed ("diffusion") flames in 
which fuel and air freely enter the combustion chamber separately. Typical 
diffusion flames are dominated by regions which burn at or near 
stoichiometric conditions. The resulting flame temperatures can exceed 
3900.degree. F. Because diatomic nitrogen rapidly disassociates at 
temperatures exceeding about 3000.degree. F. diffusion flames typically 
produce unacceptably high levels of NO.sub.x emissions. One method 
commonly used to reduce peak temperatures (and thereby reduce NO.sub.x 
emissions) is to inject water or steam into the combustor, but this 
technique is expensive in terms of process steam or water and can have the 
undesirable side effect of quenching CO burnout reactions. 
Lean premixed injection is a potentially more attractive approach to 
lowering peak flame temperature than water or steam injection. In lean 
premixed combustion, fuel and air are premixed in a premixer section, and 
the fuel-air mixture is injected into a combustion chamber where it is 
burned. Due to the lean stoichiometry resulting from the premixing, lower 
flame temperatures, and therefore lower NO.sub.x emissions, are achieved. 
However, the fuel-air mixture is generally flammable, and undesirable 
flashback into the premixer section is possible. Furthermore, gas turbine 
combustors utilizing lean premixed combustion typically require some 
conversion from premixed to diffusion operation at turndown conditions to 
maintain a stable flame. Such conversion capability introduces design 
complexities and generally raises costs. 
Accordingly, there is a need for a dry low NO.sub.x combustion system which 
does not require premixing of fuel and air prior to combustion. 
SUMMARY OF THE INVENTION 
The above-mentioned need is met by the present invention which employs lean 
direct injection for obtaining low NO.sub.x emissions. Lean direct 
injection is defined herein as an injection scheme which separately 
injects fuel and air directly into the combustion chamber of a combustor 
with no external premixing. The fuel and air are injected in controlled 
amounts so as to produce a lean fuel-air equivalence ratio which produces 
low NO.sub.x emissions. Since there is no premixing region with lean 
direct injection, concerns of flashback are eliminated, and complex 
conversion capability is not needed for turndown because the separate 
injection of fuel and air is similar to diffusion operation. In addition, 
a lean direct injection combustor is likely to be more compact and lighter 
than a lean premixed combustor because any premixing section or sections 
are eliminated. 
Specifically, the present invention provides a lean direct injection 
combustor comprising a housing having a combustion chamber formed therein. 
A plurality of fuel jets are provided for tangentially injecting fuel into 
the combustion chamber and a plurality of air jets are provided for 
tangentially injecting air into the combustion chamber. The fuel jets and 
the air jets are preferably disposed in a common cross-sectional plane, 
although additional groupings of fuel and air jets in other planes can be 
provided. The jets are all evenly spaced about the periphery of the 
housing so that each one of the fuel jets is between two air jets and each 
one of the air jets is between two fuel jets. The jets also all point in a 
single circumferential direction at a given plane. 
In another embodiment, a first group of jets is located in a first 
cross-sectional plane, and a second group of jets is located in a second 
cross-sectional plane, the second plane being located downstream of the 
first plane. The first group comprises all air jets, and the second group 
of jets comprises all fuel jets, or conversely, the first group comprises 
all fuel jets, and the second group of jets comprises all air jets. In any 
event, the jets of the first group point in one circumferential direction 
while the jets of the second group point in the opposite circumferential 
direction. 
Other objects and advantages of the present invention will become apparent 
upon reading the following detailed description and the appended claims 
with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings wherein identical reference numerals denote the 
same elements throughout the various views, FIGS. 1-3 show a lean direct 
injection combustor 10 of the present invention. The combustor 10 
comprises a housing 12 which has an open interior defining a combustion 
chamber 14 therein. The housing 12 is shown in the form of a cylindrical 
tube but is not necessarily limited to this shape. The combustion chamber 
14, which is where fuel is burned, may be protected with a liner (not 
shown) in some cases. The flow of combustion products exiting the 
downstream end of the combustion chamber 14 is utilized to drive a 
turbine. 
A plurality of jets or inlets is formed in the cylindrical wall of the 
housing 12 near the upstream or head end of the combustor 10. The jets are 
divided into two types: fuel jets 16 and air jets 18. As used herein, the 
term "jet" refers to an opening from which a stream of fluid is 
discharged. Thus, by definition, the fuel jets 16 and the air jets 18 
discharge fuel and air, respectively, into the combustion chamber 14. The 
fuel jets 16 and the air jets 18 function independently of one another. 
That is, fuel and air are injected separately into the combustion chamber 
14 without any premixing of fuel and air outside of the combustion chamber 
14. 
As best shown in FIG. 1, the fuel jets 16 and the air jets 18 are all 
oriented tangentially to the outer wall of the housing 12 so that air and 
fuel are tangentially injected into the combustion chamber 14. The 
tangential injection produces swirl which acts to stabilize the flame. All 
of the jets 16,18 are preferably disposed in a common plane which is 
perpendicular to the longitudinal axis of the housing 12 and is thus 
referred to herein as a cross-sectional plane of the combustor 10. As an 
alternative to fully lying in a cross-sectional plane, each jet 16,18 can 
be arranged at an acute angle to a cross-sectional plane (while still 
being oriented tangentially to the outer wall of the housing 12) so as to 
partially point downstream. Fuel and air will thus be injected in a 
downstream direction as well as tangentially. In this case, the points at 
which the jets 16,18 intersect the housing 12 will preferably be disposed 
in a common cross-sectional plane. 
The jets 16,18 are all arranged to point in the same circumferential 
direction, i.e., either all counter-clockwise, as shown in FIG. 1, or all 
clockwise. The jets 16,18 are evenly spaced about the periphery of the 
combustor housing 12 and alternate between fuel jets 16 and air jets 18. 
That is, each one of the fuel jets 16 is between two air jets 18 and each 
one of the air jets 18 is between two fuel jets 16. The alternating 
injection of fuel and air in the same cross-sectional plane is believed to 
contribute to quick and intense mixing within the combustion chamber 14. 
The even spacing of the fuel jets 16 and the air jets 18 about the 
periphery of the combustor 10 facilitates mixing of the fuel and air in 
the combustion chamber 14, thereby improving overall efficiency. 
Fuel is delivered to the fuel jets 16 from an external source of fuel 20 
via fuel lines 22 shown schematically in FIG. 2. Air is delivered to the 
air jets 18 from a source of air 24, which is typically a compressor, via 
air lines 26 shown schematically in FIG. 3. Although shown in FIG. 3 as 
being directly connected to the air jets 18, the air lines 26 can be 
configured so that the inlet air is first passed over the outer surface of 
the combustion liner before being injected into the combustion chamber 14 
via the air jets 18. Thus, the relatively cool compressor air will provide 
backside cooling to the liner as is generally known in the art. 
FIGS. 1-3 show two fuel jets 16 and two air jets 18 formed in the housing 
12. However, the number of fuel jets 16 and air jets 18 is not restricted 
to this number. There can more or less than the total of four jets in one 
cross-sectional plane as long as there is an adequate number to provide 
sufficient amounts of fuel and air to the combustion chamber 14. 
Preferably, there will be equal number of fuel jets 16 and air jets 18 to 
permit the alternating distribution of the different types of jets 16,18. 
In any event, the number of air jets 18 relative to the fuel jets 16 must 
be sufficient to ensure that fuel and air are injected in the proper 
proportions for lean combustion. The respective diameters of the jets 
16,18 also affects the ratio of fuel and air injected into the combustion 
chamber 14. Accordingly, the diameter (and thus the cross-sectional area) 
of the fuel jets 16 is generally 10 smaller than that of the air jets 18 
to ensure proper proportions of fuel and air as well as to accommodate 
pressure drops typical to gas turbines. 
While FIGS. 1-3 show one group of jets 16,18 in a common cross-sectional 
plane, one or more additional groups of tangential fuel and air jets 
disposed in additional cross-sectional planes may be provided. The 
additional cross-sectional planes are located slightly downstream from the 
first cross-sectional plane. As before, the fuel and air jets of each 
additional group preferably point in the same circumferential direction, 
are evenly spaced about the periphery of the housing, and alternate 
between fuel and air jets. 
FIGS. 4 and 5 show a lean direct injection combustor 110 which represents a 
second embodiment of the present invention. The combustor 110 comprises a 
housing 112 which has an open interior defining a combustion chamber 114 
therein. A plurality of jets is formed in the outer wall of the housing 
112 near the upstream or head end of the combustor 110. The jets are 
arranged into two groups: a group of four fuel jets 116 and a group of 
four air jets 118. While each of the two groups is shown to have four 
jets, the present invention is not so limited. There can more or less than 
four jets in each group as long as sufficient amounts of fuel and air are 
injected into the combustion chamber 114. There need not be an equal 
number of fuel jets 116 and air jets 118, although this is generally 
preferred. In any event, the number of air jets 118 relative to the fuel 
jets 116 must be sufficient to ensure that fuel and air are injected in 
the proper proportions for lean combustion. Moreover, the diameter (and 
thus the cross-sectional area) of the fuel jets 116 is generally smaller 
than that of the air jets 118 to ensure proper proportions of fuel and air 
as well as to accommodate pressure drops typical to gas turbines. 
The fuel jets 116 and the air jets 118 are all oriented tangentially to the 
outer wall of the housing 112 so that air and fuel are tangentially 
injected into the combustion chamber 114, thereby producing swirl which 
acts to stabilize the flame. As best seen in FIG. 5, the air jets 118 are 
all disposed in a first cross-sectional plane, and the fuel jets 116 are 
all disposed in a second cross-sectional plane, located slightly 
downstream from the first plane. Conversely, the fuel jets 116 could be 
located upstream from the air jets 118. In addition to being oriented 
tangentially to the outer wall of the housing 112, each jet 116,118 can be 
arranged to either lie in the respective cross-sectional plane or be at an 
acute, downstream angle thereto. In either case, the points at which the 
jets 116,118 intersect the housing 12 will preferably be disposed in a 
common cross-sectional plane. 
The fuel jets 116 and the air jets 118 are preferably, but not necessarily, 
arranged to point in opposite circumferential directions. That is, the 
fuel jets 116 all point clockwise, and the air jets 18 all point 
counter-clockwise as shown in FIG. 4, although these directions could be 
reversed. The fuel jets 116 and the air 118 are evenly spaced about the 
periphery of the combustor housing 112 in their respective cross-sectional 
planes. The even spacing of the jets 116,118 about the periphery of the 
combustor 10 facilitates mixing of the fuel and air in the combustion 
chamber 114, thereby improving overall efficiency. 
The concept of the present invention was tested on a laboratory-scale 
device simulating the lean direct injection combustors of the present 
invention. The experiments were performed under atmospheric pressure with 
no preheating of air and used methane for fuel. The results are shown in 
FIG. 6 which is a graph plotting NO.sub.x emissions in parts per million 
against the fuel-air equivalence ratio. Curve A shows premixed combustion 
data derived by tangentially injecting premixed fuel and air into the 
combustion chamber of the laboratory-scale device. Curve B shows 
combustion data derived from a laboratory-scale device simulating the 
combustor of FIGS. 1-3. Curves C and D show combustion data derived from a 
laboratory-scale device simulating the combustor of FIGS. 4 and 5; the 
data of curve C being collected using two fuel jets and two air jets, the 
data of curve D being collected using four fuel jets and four air jets. 
The results show the NO.sub.x emissions to be below 20 ppm for a wide 
range of lean equivalence ratios. The lean direct injection of the 
embodiment of FIGS. 1-3 (curve B) compares quite favorably to that of the 
lean premixed combustion. 
The foregoing has described a lean direct injection combustor which can 
provide low NO.sub.x emissions without premixing air and fuel outside of 
the combustion chamber. While specific embodiments of the present 
invention have been described, it will be apparent to those skilled in the 
art that various modifications thereto can be made without departing from 
the spirit and scope of the invention as defined in the appended claims.