Method and apparatus for sequentially staged combustion using a catalyst

A low NOx generating combustor in which a first lean mixture of fuel and air is pre-heated by transferring heat from hot gas discharging from the combustor. The preheated first fuel/air mixture is then catalyzed in a catalytic reactor and then combusted so as to produce a hot gas having a temperature in excess of the ignition temperature of the fuel. Second and third lean mixtures of fuel and air are then sequentially introduced into the hot gas, thereby raising their temperatures above the ignition temperature and causing homogeneous combustion of the second and third fuel air mixtures. This homogeneous combustion is enhanced by the presence of the free radicals created during the catalyzation of the first fuel/air mixture. In addition, the catalytic reactor acts as a pilot that imparts stability to the combustion of the lean second and third fuel/air mixtures.

BACKGROUND OF THE INVENTION 
The present invention relates to a combustor. More specifically, the 
present invention relates to a combustor, such as a combustor for use in 
gas turbines and the like, in which combustion is sequentially staged 
using a catalyst. 
In a gas turbine, fuel is burned in compressed air, produced by a 
compressor, in one or more combustors. Traditionally, such combustors had 
a primary combustion zone in which an approximately stoichiometric mixture 
of fuel and air was formed and burned in a diffusion type combustion 
process under essentially homogeneous conditions. Additional air was 
introduced into the combustor downstream of the primary combustion zone. 
Although the overall fuel/air ratio was considerably less than 
stoichiometric, the fuel/air mixture was readily ignited at start-up and 
good flame stability was achieved over a wide range in firing temperatures 
due to the locally richer nature of the fuel/air mixture in the primary 
combustion zone. 
Unfortunately, use of such approximately stoichiometric fuel/air mixtures 
resulted in very high local temperatures in the primary combustion zone. 
Such high temperatures promoted the formation of oxides of nitrogen 
("NOx"), considered an atmospheric pollutant. It is known that combustion 
at lean fuel/air ratios (sometimes referred to as "lean pre-mix 
combustion") reduces NOx formation by reducing the maximum local gas 
temperatures. However, such lean mixtures are difficult to ignite and 
exhibit poor flame stability. Although ignition and stability can be 
improved by the use of diffusion type combustion from a centrally disposed 
pilot burner, the use of such a pilot will increase the NOx generation 
and, thereby, provide a lower limit on the NOx generation. 
Accordingly, it has been proposed to employ a combustion catalyst in a 
heterogeneous combustion process. The creation of free radicals in the 
catalyzed fuel/air mixture has the effect of reducing the activation 
energy associated with the combustion reactions. Consequently, the 
combustion temperature can be more readily maintained below the level at 
which molecular nitrogen will be converted to NOx (sometimes referred to 
as "thermal NOx"). Moreover, even at temperatures favoring the formation 
of thermal NOx, the NOx generation rate will be decreased because the 
catalyzation of the fuel/air mixture will enhance the ability of the fuel 
to compete with nitrogen for the available oxygen. 
Catalytic combustion has been proposed in various modes, including flowing 
all of the fuel/air mixture through a catalytic combustor preceded by a 
pilot burner or operating a catalytic combustor in parallel with a lean 
pre-mix combustor. In any case, the catalytic process requires that the 
fuel/air mixture be preheated, generally to a temperature of at least 
400.degree. C. Unfortunately, the compressed air produced by a gas turbine 
compressor may be lower than 400.degree. C. Therefore, a diffusion type 
pre-heating burner is often required. However, as in a lean pre-mix type 
combustor, the pre-heating burner can create sufficient NOx to undermine 
the ability of the catalytic combustor to meet ultra-low emission 
requirements. 
It is therefore desirable to provide a combustor capable of stable 
combustion with very lean mixtures of fuel and air, so as to reduce the 
formation of NOx, without the use of a NOx generating pilot. 
SUMMARY OF THE INVENTION 
Accordingly, it is the general object of the current invention to provide a 
combustor capable of stable combustion with very lean mixtures of fuel and 
air, so as to reduce the formation of NOx, without the use of a NOx 
generating pilot. 
Briefly, this object, as well as other objects of the current invention, is 
accomplished in a combustor for burning a fuel in air comprising (i) means 
for mixing a first flow of fuel into a first flow of air in a mixing zone, 
thereby producing a first fuel/air mixture, (ii) a combustion catalyst, 
(iii) means for directing the first fuel/air mixture through the catalyst 
so as to produce a catalyzed fuel/air mixture, (iv) a first combustion 
zone in which the catalyzed fuel/air mixture is combusted, thereby 
producing a combusted gas, (v) a second combustion zone disposed 
downstream of the first combustion zone, and means for directing the 
combusted gas from the first combustion zone to the second combustion 
zone, and (vi) means for mixing a second fuel/air mixture into the hot gas 
in the second combustion zone and combusting the second fuel/air mixture, 
thereby producing a further combusted gas. 
In one embodiment of the current invention, the combustor has (i) a third 
combustion zone disposed downstream of the second combustion zone, (ii) 
means for directing the further combusted gas from the second combustion 
zone to the third combustion zone, and (iii) means for mixing a third 
fuel/air mixture into the further combusted gas in the third combustion 
zone and combusting the third fuel/air mixture. 
According to one aspect of the current invention, the combustor has means 
for heating the first fuel/air mixture prior to directing the first 
fuel/air mixture through the combustion catalyst. In this embodiment, the 
means for heating the first fuel/air mixture comprises means for 
transferring heat from the further combusted gas to the first fuel/air 
mixture. 
The current invention also encompasses a method of combusting a fuel in 
air, comprising the steps of (i) mixing a first flow of fuel into a first 
flow of air so as to form a first fuel/air mixture, (ii) flowing the first 
fuel/air mixture through a combustion catalyst, thereby producing a 
catalyzed fuel/air mixture, (iii) combusting the catalyzed fuel/air 
mixture, thereby producing a combusted gas, (iv) mixing a second flow of 
fuel into a second flow of air so as to form a second fuel/air mixture, 
and (v) combusting the second fuel/air mixture by mixing the second 
fuel/air mixture into the combusted gas so as to raise the temperature of 
the second fuel/air mixture above its ignition temperature, thereby 
producing a further combusted gas.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings, there is shown in FIG. 1 a longitudinal 
cross-section through the portion of a gas turbine in the vicinity of the 
combustion section. The gas turbine is comprised of a compressor section 
1, a combustion section 2, a turbine section 3 and a rotor 4 that extends 
through these three sections. The compressor is comprised of a plurality 
of alternating rows of stationary vanes 5 and rotating blades 7. The vanes 
are affixed to a cylinder 9 and the blades are affixed to discs 8 mounted 
on the rotor 4. As is conventional, the combustion section 2 is comprised 
of a cylinder 10 that forms a chamber 27 in which a plurality of 
combustors 15 and ducts 16 are disposed. The turbine section 3 is 
comprised of a plurality of alternating rows of stationary vanes 6 and 
rotating blades (not shown). The vanes 6 are affixed to an inner cylinder 
11 (only a portion of which is shown) that is enclosed by an outer 
cylinder 12. 
During operation, the compressor 2 inducts ambient air and compresses it. 
The compressed air 17 from the compressor 1 is then directed to the 
chamber 27, which distributes it to the combustors 15. In the combustors 
15, a fuel, which may be liquid (such as distillate oil) or gaseous (such 
as natural gas), is burned in the compressed air 17, as discussed further 
below, so as to produce a hot compressed gas 40. The hot gas 40 is 
directed to the turbine section 3 by the ducts 16. In the turbine section, 
the hot compressed gas 40 is expanded, thereby producing power in the 
rotor 4. 
As shown in FIG. 2, each combustor 15 extends into a barrel 13 that extends 
from the cylinder 10 and that is sealed by an end plate 14. The front end 
of the combustor 15 is formed by inner and outer concentric liners 51 and 
52, respectively. The liners 51 and 52 form an annular passage 75 between 
themselves having a circumferential inlet 58 that receives a first portion 
18 of the compressed air 17 from the compressor 1. In the preferred 
embodiment, the compressed air 18 is equal to about 20% of the total 
combustion air for the combustor 15. From the inlet 58, the passage 75 
initially extends radially inward and then turns 90.degree. and extends 
axially. 
A plurality of axially extending fuel supply bars 63 are circumferentially 
distributed in the passage 75 just down stream of the inlet 58. A 
plurality of fuel outlet ports are distributed along the length of each of 
the fuel supply bars 63 that extends in the passage 75. Consequently, the 
fuel supply bars 63 serve to well distribute a first portion 30 of the 
fuel around the passage 75, thereby forming a lean fuel/air mixture 83. 
Swirlers 67, which may be of the vane type, are optionally distributed 
around the passage 75 and serve to aid in the mixing of the fuel 30 and 
compressed air 18. In the preferred embodiment, the flow rate of the fuel 
30 introduced by the fuel supply bars 63 into the compressed air 18 is 
regulated so that an equivalence ratio of less than 0.5 is maintained in 
the passage 75--equivalence ratio being defined as the ratio of the actual 
fuel/air ratio to the stoichiometric fuel/air ratio. 
From the passage 75, the fuel/air mixture 83 enters a pre-heat zone 91. In 
the preferred embodiment, pre-heating of the fuel/air mixture 83 is 
accomplished in two ways. During start-up, a supplemental fuel 31 from 
fuel supply pipe 72 is introduced by a fuel nozzle 50, which may be a 
conventional spray type, into a passage 59 formed by the inner liner 51. 
Air 21 from a conduit 73 is also introduced into the passage 59 so as to 
create a rich fuel/air mixture that is ignited by an ignitor (not shown), 
which may be of the conventional spark type, and burned so as to produce a 
very stable diffusion-type flame. The hot gas produced by the combustion 
of the supplemental fuel 31 mixes with the lean fuel/air mixture 83 from 
the passage 75 in the pre-heat zone 91, thereby raising the temperature of 
the fuel/air mixture 83 into the range suitable for being catalyzed, as 
discussed further below. In the preferred embodiment, the fuel/air mixture 
83 is heated to approximately 400.degree. C. 
In one embodiment of the invention, the combustor 15 is operated 
continuously with the diffusion-type combustion from the fuel nozzle 50 as 
a source of pre-heat. However, the size of the pre-heating flame is much 
smaller than that required for conventional non-catalytic lean premix 
combustors since, according to the current invention, only 20% of the air 
flow must be pre-heated and then only to a much lower temperature. Thus, 
NOx generation is greatly reduced. In another embodiment of the invention, 
after stable operation of the combustor 15 has been achieved, the 
supplemental fuel 31 is shut off. Thereafter, pre-heating is accomplished 
by transferring heat from the hot gas 40, which is being discharged from 
the combustor 15, to the lean fuel/air mixture 83. 
In the embodiment shown in FIG. 2, this heat transfer is accomplished by 
drawing a portion 41 of the hot gas 40 discharging from the combustor 15 
through a duct 80 and then mixing it into the fuel/air mixture 83 in the 
pre-heat zone 91. The duct 80 connects an inlet port 81, formed in a liner 
56, that is in flow communication with a combustion zone 95 (as discussed 
further below, the combustion zone 95 is the third of three combustion 
zones). The duct 80 connects the inlet port 81 to an outlet port 82 formed 
in the inner liner 51. As a result of the high axial velocity of the 
fuel/air mixture 83 flowing around it, the axially extending portion of 
the inner liner 51 forms an eductor 33 that draws the hot gas 41 from the 
third combustion zone 95. 
An alternate embodiment for pre-heating the fuel/air mixture 83 using an 
indirect heat exchanger 100--that is, a heat exchanger in which heat 
transfer is accomplished without contact between the hot gas 40 and the 
fuel/air mixture 83--is shown in FIG. 4. The heat exchanger 100 is formed 
by a shell 104 that encircles a portion of the liner 56' that encloses the 
combustion zone 95, thereby forming annular axially extending passages 102 
and 103. A plurality of radially extending heat transfer fins 101 extend 
into the passages 102 and 103. A portion 21 of the compressed air 17 from 
the compressor 1 is drawn through the passages 102 and 103. As it flows 
through the passages 102 and 103, the compressed air 21 receives heat 
transferred to the fins 101 and the liner 56' from the hot gas 40, thereby 
raising the temperature of the air. From the passages 102 and 103, the 
heated air 25 enters a circumferentially extending manifold 74 that 
encircles the shell 104. From the manifold 74, the heated air 25 flows 
into the inlet of the conduit 80. 
As previously discussed, the eductor 33, shown in FIG. 2, could be utilized 
to induce the flow of compressed air 24 through the heat exchanger 100. 
Alternatively, as shown in FIG. 4, an eductor 34 could also be 
incorporated into the duct 80. A portion 22 of the compressed air 17 from 
the compressor 1 is further pressurized by a boost compressor 42. A high 
velocity jet of this further pressurized air 25 is then directed into the 
duct 80 by the eductor 34, thereby drawing the compressed air 24 through 
the heat exchanger 100. 
In the embodiments discussed above, pre-heating is achieved by pre-heating 
the fuel/air mixture 83 itself. Alternatively, pre-heating can be 
accomplished by pre-heating the compressed air 18 prior to mixing it with 
the fuel 30. Such air pre-heating could be accomplished by utilizing only 
radially extending fuel supply bars 63' (indicated in phantom in FIG. 2) 
located downstream of the passage 75 so that only air 18 flows through the 
passage 75. After the compressed air 18 has been pre-heated in the 
pre-heat zone 91, it would then be mixed with the fuel 30 so as to form a 
fuel/air mixture. 
Regardless of the manner of pre-heating, from the pre-heat zone 91, the 
pre-heated fuel/air mixture 89 is then directed by a diverging liner 57 to 
flow through a catalytic reactor 86 containing a combustion catalyst, as 
shown in FIG. 2. The liner 57 serves to support the catalytic reactor 86 
and, at its aft end, encloses two combustion zones, as discussed further 
below. 
In the preferred embodiment, the catalytic reactor 86 comprises a 
monolithic substrate, which may be formed from a metallic or ceramic 
material, having a honeycomb structure impregnated with one or more 
catalytically active materials. The honeycomb structure forms a large 
number of passages through which the pre-heated fuel/air mixture 89 flows, 
thereby bring the mixture into surface contact with the catalytic 
material. Alternatively, other types of catalytic reactors, such as a 
packed bed type, could also be utilized, provided that they did not impose 
too great a pressure drop on the gas flow. A variety of combustion 
catalyst material may be used, such as platinum, palladium or nickel, 
depending on the type of fuel being combusted. Suitable catalytic reactors 
are disclosed in U.S. Pat. No. 3,928,961 (Pfefferle) and U.S. Pat. No. 
4,072,007 (Sanday), each of which is hereby incorporated by reference in 
its entirety. 
As shown in FIGS. 2 and 3, middle and outer cylindrical liners 53 and 54 
encircle the liner 57. An outer annular passage 77 is formed between the 
middle and outer liners 53 and 54 and an inner annular passage 76 is 
formed between the middle liner 53 and the liner 57. A portion 19 of the 
compressed air 17 from the compressor 1 enters the inner passage 76 via an 
inlet 78. Another portion 20 of the compressed air 17 enters the outer 
passage 77 via an inlet 79. In the preferred embodiment, about 80% of the 
combustion air for the combustor 15 flows through passages 76 and 77. 
As shown in FIGS. 2 and 3, a plurality of radially extending fuel supply 
bars 64 are circumferentially distributed around the passages 76 and 77 
just down stream of the inlets 78 and 79, respectively. A circular 
manifold 70 distributes fuel 32 from a fuel supply pipe 71 to the various 
supply bars 64. As shown in FIG. 3, a plurality of fuel outlet ports are 
distributed along the portion of the length of each of the supply bars 64 
that extends in the passages 76 and 77. Consequently, the fuel supply bars 
64 serve to well distribute the fuel 32 around the passages, thereby 
forming lean fuel/air mixtures 84 and 85 in the inner and outer passages 
76 and 77, respectively. Preferably, mixing devices are incorporated into 
the passages 76 and 77 to aid in the mixing of the fuel 32 and compressed 
air 19 and 20. These mixing devices may be swirl vanes, such as those 
utilized in passage 75, as previously discussed. Alternatively, radially 
extending baffles 69 may be distributed around the passages 76 and 77, as 
shown in FIG. 2. In the preferred embodiment, the flow rate of the fuel 32 
introduced by the fuel supply bars 64 into the compressed air 19 and 20 is 
regulated so that an equivalence ratio of less than 0.5 is maintained in 
the passages 76 and 77. 
As shown in FIG. 2, the liner 57 encloses a combustion zone 93 that is 
formed immediately downstream of the catalytic reactor 86. A plurality of 
outlet ports 87 are circumferentially distributed around an outward 
flaring section of liner 57 that forms an end wall for the inner passage 
76. The outlet ports 87 direct the fuel/air mixture 84 into a second 
combustion zone 94 enclosed by the aft end of the liner 53. A plurality of 
outlet ports 88 are circumferentially distributed around an outward 
flaring section of liner 53 that forms an end wall for the outer passage 
77. The outlet ports 88 direct the fuel/air mixture 85 into a third 
combustion zone 95 enclosed by a liner 56. The outlet ports 87 and 88 
serve to create pressure drops that prevent the gas in the combustion 
zones 94 and 95 from flowing backward into the passages 76 and 77. 
Preferably, the inner surface of the liner 56 is coated with a ceramic 
material 90 to allow it to better withstand the heat generated in the 
combustion zone 95. 
In the catalytic reactor 86, the pre-heated fuel/air mixture 89 comes into 
contact with the catalyst's surface. On this surface, fuel is more readily 
oxidized than in the gas stream because the catalyst alters the reaction 
mechanism--i.e., reduces the activation energy for the reaction. This 
oxidation of fuel on the catalyst's surface generates heat that is 
liberated into the gas stream, increasing its temperature. As its 
temperature is increased, the rate of oxidation in the gas increases until 
eventually, downstream of the catalyst bed, it becomes self-sustaining. In 
addition to the effect of increasing the temperature of the gas, the 
catalyst injects into it free radicals that accelerate oxidation. 
Depending on the free radicals, this effect may accelerate oxidation 
reaction involving fuel to a greater degree than those involving nitrogen, 
thereby decreasing the proportion of NOx generated. For example, accepted 
reaction schemes have hydroxyl, OH, and atomic oxygen, O, participating in 
many reaction steps with methane or its fragments, while these oxidizing 
radicals participate in fewer elementary reactions with nitrogen. Thus, if 
the catalyst injects O or OH into the gas stream, one would anticipate 
that the rate of oxidation of fuel would be more greatly accelerated than 
that of nitrogen. 
As a result of the catalyzation of the fuel/air mixture 83, as described 
above, upon exiting the catalytic reactor 86, its temperature is rapidly 
raised above that required for ignition, typically approximately 
550.degree. C. Consequently, homogenous gas phase combustion begins in the 
combustion zone 93 almost immediately after the fuel/air mixture 89 exits 
the reactor. In the preferred embodiment, the temperature of the fuel/air 
mixture 89 exiting the catalytic reactor is approximately 480.degree. C. 
However, as a result of the homogeneous combustion in first combustion 
zone 93, the temperature of the gas 38 entering the second combustion zone 
94 is likely in excess of 1050.degree. C. and preferably, approximately 
1230.degree. C. 
As a result of the high temperature of the gas 38, the temperature of the 
fuel/air mixture 84 formed in the inner passage 76 is rapidly raised above 
its ignition temperature when it enters combustion zone 94 from the outlet 
ports 87, thereby causing a homogeneous combustion reaction that, in the 
preferred embodiment, maintains the temperature of the combined gas flow 
39 exiting from the second combustion zone 94 above the fuel ignition 
temperature. Similarly, as a result of the high temperature of the hot gas 
94, the temperature of the fuel/air mixture 85 formed in the outer passage 
77 is rapidly raised above its ignition temperature as it enters the third 
combustion zone 39, thereby causing a further homogeneous combustion 
reaction sufficient to raise the temperature of the hot gas 40 to the 
desired value for expansion in the turbine section 3. 
In the preferred embodiment, the fuel 32 introduced into the inner and 
outer passages 76 and 77 comprises approximately 80% of the fuel burned in 
the combustor 15. By introducing this additional fuel and its associated 
combustion air 19 and 20 sequentially into the combustion products of the 
catalytic reactor 86 in two stages, quenching of the combustion gas, which 
could reduce its temperature below that required for ignition, is 
prevented. Although, for purposes of simplicity, only two stages are 
shown, it should be understood that three or more stages of combustion may 
be utilized downstream of the catalytic reactor 86. In any case, due to 
the absence of locally high temperatures as a result of the lean 
combustion of the fuel/air mixtures 84 and 85, the generation of NOx is 
minimized. 
As the foregoing indicates, by staging the combustion according to the 
current invention, with the homogeneous combustion of the second and third 
fuel/air mixtures 84 and 85 occurring in series with and subsequent to the 
catalytic combustion of the first fuel/air mixture 83, the generation of 
NOx is further reduced over that associated with prior lean combustion 
processes. The catalytic reactor 86 acts as a pilot for the combustion of 
the lean fuel/air mixtures 84 and 85, thereby allowing operation at very 
lean fuel/air ratios while providing adequate stability. It is to be noted 
that this stability is achieved without the generation of additional NOx 
as a result of the use of diffusion type pilot burners. 
In addition to permitting lean combustion without the use of NOx generating 
pilot burners, it is suspected that the free radicals created by the 
catalyzation of the fuel/air mixture 83 reduces the NOx generation as a 
result of the homogenous combustion of the fuel/air mixtures 84 and 85 in 
another way as well. The free radicals enhance the ability of the fuel to 
compete with nitrogen for the available oxygen, as previously discussed, 
so that the NOx generated by the combustion of the fuel/air ratios 84 and 
85 is reduced over that associated with non-catalytic combustion even at 
similarly lean fuel/air ratios. The free radicals also allow combustion to 
occur at leaner fuel/air ratios than would otherwise be possible. 
As previously discussed, preferably pre-heating of the fuel/air mixture 83 
is accomplished by heat transfer from the hot gas 39 so as to avoid the 
generation of additional NOx. However, the staging of the combustion so 
that not all of the fuel and air flow through the catalytic reactor 86 
allows lower values of NOx to be achieved even if the fuel nozzle 50 were 
used continuously for preheating. This is so because, according to the 
current invention, the majority of the fuel and air do not flow through 
the catalytic reactor 86 and, hence, need not be pre-heated. Thus, the 
amount of fuel burned in the high NOx generating fuel nozzle 50 would be 
lower than in processes in which all of the fuel and combustion air had to 
be pre-heated. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification, as indicating the scope of the invention.