Gas turbine catalytic combustor with preburner and low NO.sub.x emissions

During low-load operating conditions, preburner combustion products are supplied with a chemical reactant to reduce NO.sub.x. The preburner products of combustion are mixed with a hydrocarbon fuel in the presence of a combustion catalyst to ignite and initiate a catalytic combustion reaction. The preburner is then shut down. The fuel/air mixture supplied the catalytic reactor bed during the mid-load operating range of the turbine is sufficiently lean to produce a combustion reaction temperature too low to produce thermal NO.sub.x. Thus, at low-load conditions, preburner combustion occurs with NO.sub.x reduction by chemical reactant, while the catalytic combustion occurs at mid-range operating conditions at temperatures too low to produce NO.sub.x. For high-load operating conditions, the catalytic combustion occurs as previously described and additional lean fuel/air mixture is supplied the reaction zone whereby thermal NO.sub.x is likewise avoided.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to apparatus and methods for reducing 
NO.sub.x emissions from a gas turbine and particularly relates to 
apparatus and methods for reducing NO.sub.x in a gas catalytic combustion 
system with preburner over the entire operating range of the gas turbine. 
As set forth in my prior U.S Pat. No. 4,845,952, the objectives of many 
manufacturers of gas turbines include operating at high efficiency without 
producing undesirable air-polluting emissions. Conventional fuels normally 
used in gas turbines, when burned, typically produce oxides of nitrogen, 
carbon monoxide and unburned hydrocarbons. 
NO.sub.x compounds are produced by the reaction of nitrogen in the air at 
the elevated temperatures conventionally found in combustors of gas 
turbines. NO.sub.x formation can be reduced by reducing maximum flame 
temperature in the combustor, for example, by introduction of steam. 
However, penalties to thermodynamic efficiency and increased capital costs 
are incurred. It is known to use a combustion catalyst in the reaction 
zone of a gas turbine combustion system to promote complete combustion of 
lean pre-mixed fuel and air to minimize the level of air-polluting 
emissions. Catalytic combustion occurs at a relatively low temperature 
insufficient to generate NO.sub.x from nitrogen and oxygen reactions which 
occur at higher temperatures. It will be appreciated, however, that when 
combustor inlet air temperature and temperature rise across the combustion 
system are too low to support catalytic combustion, a diffusion flame 
preburner may be used to obtain catalytic reactor ignition. That is, 
catalytic combustion alone cannot be used over the entire operating range 
of the gas turbine because the inlet air temperature and temperature rise 
across the combustion system are too low to initiate and sustain pre-mixed 
catalytic combustion during gas turbine ignition, acceleration and 
operating at the low end of the gas turbine load range. 
When using a diffusion flame preburner, however, significant amounts of 
NO.sub.x emissions are generated. Prior catalytic combustion system 
designs do not include methods for reducing the preburner NO.sub.x 
emission. Consequently, while low NO.sub.x emissions are obtained over the 
mid-operating range of the gas turbine combustion system, prior catalytic 
combustion system designs do not include any method of reducing NO.sub.x 
emissions from the preburner. Accordingly, a purpose of the present 
invention is to provide a catalytic combustion system and method of 
preburner NO.sub.x abatement such that the catalytic combustion system may 
operate with extremely low NO.sub.x emissions over the entire operating 
range of the gas turbine. 
According to the present invention, there is provided a catalytic combustor 
with a diffusion flame preburner for a gas turbine system for minimizing 
NO.sub.x emission throughout the operating range of the turbine. Three 
different operating modes for this combustion system are provided over the 
load range of the gas turbine. The first operating mode is a low-load 
operating condition of the gas turbine where only preburner combustion 
occurs with chemical/catalytic NO.sub.x removal, i.e., deNO.sub.x. For 
example, hydrocarbon fuel may be supplied to a preburner start-up fuel 
nozzle and air may be directed to a preburner combustion zone. An 
electrical ignition device, such as a spark or glow plug, ignites the 
fuel/air mixture in the preburner combustion zone with the flame being 
stabilized by vortex recirculation generated by swirl vanes in the 
start-up fuel nozzle. Significant amounts of thermal NO.sub.x are 
generated by this diffusion flame reaction within the preburner combustion 
liner. To reduce this NO.sub.x to molecular nitrogen and water vapor, a 
chemical reactant, such as ammonia, urea, isocyanic acid or the like may 
be injected through the primary injector for the catalytic combustion 
section (used during mid and high-load operating ranges) into the 
preburner products of combustion. Mixing may be promoted by the infusion 
of nitrogen with the chemical reactant. The chemical reactant may also 
include enhancers to accelerate the rate of reaction with NO.sub.x from 
the diffusion flame preburner. The chemical reaction may occur within the 
catalytic reactor assembly liner and the catalytic reactor bed of the 
catalytic combustion zone, including the catalyst, to accelerate the 
deNO.sub.x chemical reactions. 
In a second operating mode characterized as a mid-load operating range for 
the gas turbine, catalytic combustion occurs. To achieve this, fuel is 
supplied by the primary injector and mixed with the preburner products of 
combustion. This mixture enters the catalytic reactor bed which contains a 
combustion catalyst, for example, palladium. This mixture of fuel and 
preburner products of combustion ignites in the presence of the combustion 
catalyst at preburner discharge temperature. Once the combustion reaction 
has been initiated, the preburner may be shut down, with the reaction 
being sustained at compressor discharge air temperature. By introducing a 
lean fuel/air mixture into the catalytic reactor bed, combustion reaction 
temperature is maintained too low to produce thermal NO.sub.x. The 
hydrocarbon fuel oxidation reactions go to completion in the reaction zone 
within the main combustion liner. Thus, the NO.sub.x emissions during low 
and mid-range operating conditions are substantially eliminated or 
minimized to ultra-low emissions. 
At high load operating conditions for the gas turbine, a combination of 
catalytic and pre-mixed combustion is provided. The catalytic reactor 
operates in the same manner previously described as in the second 
operating mode, i.e., mid-range catalytic combustion. A secondary 
injector, however, is provided for mixing hydrocarbon fuel with compressor 
discharge air. This fuel/air mixture enters the reaction zone within the 
main combustion liner and is ignited by the hot products of combustion 
exiting the catalytic reactor bed. Because this fuel/air mixture is lean, 
combustion reaction temperature is likewise too low to produce thermal 
NO.sub.x. In this manner, NO.sub.x emissions are substantially minimized 
or eliminated throughout the entire operating range of the gas turbine. 
In a preferred embodiment according to the present invention, there is 
provided a method of operating a gas turbine catalytic combustion system 
having a preburner section and a catalytic combustion section to minimize 
or eliminate NO.sub.x emissions comprising the steps of combusting a 
fuel/air mixture in the preburner section, reducing the NO.sub.x resulting 
from the combustion of the fuel/air mixture in the preburner section, 
operating the preburner section to obtain catalytic reaction ignition and, 
upon ignition, operating the catalytic combustion section at a combustion 
temperature too low to produce NO.sub.x whereby NO.sub.x emissions from 
the gas turbine operation are substantially minimized or eliminated. 
In a further preferred embodiment according to the present invention, there 
is provided a method of operating a gas turbine catalytic combustion 
system at low-load and mid-load ranges of gas turbine operation wherein 
the combustion system has a preburner section and a catalytic combustion 
section, comprising the steps of, at low-load operation, supplying a 
fuel/air mixture in the preburner section for combustion, reducing the 
NO.sub.x resulting from the combustion of the fuel/air mixture in the 
preburner section, operating the preburner section to obtain catalytic 
reactor ignition in the catalytic combustion section and, upon ignition 
and at mid-load range, operating the catalytic combustion section with a 
lean fuel/air mixture such that the combustion reaction temperature is too 
low to produce thermal NO.sub.x whereby NO.sub.x emissions from gas 
turbine operation at low and mid-load ranges of operation are 
substantially minimized. 
In a still further preferred embodiment according to the present invention, 
there is provided a gas turbine catalytic combustion system with low 
NO.sub.x emissions comprising a preburned section, means for introducing 
fuel and air into the preburner section, an igniter in the preburner 
section for combusting the fuel/air mixture and means for reducing the 
NO.sub.x in the products of combustion of the preburner section. A 
catalytic combustion section is provided having a catalytic reactor bed 
having a catalyst and a reaction zone. Means are provided for introducing 
a lean mixture of fuel and air into the catalytic combustion bed with 
catalytic combustion occurring at least initially from ignition by the 
preburner products of combustion in the presence of the catalyst in the 
bed. Means are also provided for mixing compressor discharge air and fuel 
and supplying the mixture to the reaction zone of the combustion section 
for ignition by the hot products of combustion exiting the catalytic 
reactor bed. 
Accordingly, it is a primary object of the present invention to provide 
novel and improved apparatus and methods for operating a catalytic 
combustion gas turbine system which achieves ultra-low NO.sub.x emissions 
over the entire operating range of the gas turbine.

DETAILED DESCRIPTION OF THE DRAWING FIGURES 
Reference will now be made in detail to the present preferred embodiment of 
the invention, an example of which is illustrated in the accompanying 
drawings. 
As well known, a gas turbine includes a compressor section, a combustion 
section and a turbine section. The compressor section is driven by the 
turbine section through a common shaft connection. The combustion section 
typically includes a circular array of a plurality of circumferentially 
spaced combustors. A fuel/air mixture is burned in each combustor to 
produce the hot energetic flow of gas which flows through a transition 
piece for flowing the gas to the turbine blades of the turbine section. 
Thus, for purposes of the present description, only one combustor is 
illustrated, it being appreciated that all of the other combustors arrayed 
about the turbine are substantially identical to the illustrated 
combustor. 
Referring now to FIG. 1, there is shown generally at 10, a combustor for a 
gas turbine engine and including a preburner section 12, a catalytic 
reactor assembly 14, a main combustion assembly 16 and a transition piece 
18 for flowing hot gases of combustion to the turbine blades not shown. 
The preburner assembly 12 includes a preburner casing 20, an end cover 22, 
a start-up fuel nozzle 24, a flow sleeve 26 and a pre-combustion liner 28 
within sleeve 26. An ignition device, not shown, is provided and may 
comprise a spark or glow plug. Combustion in the preburner assembly 34 
occurs within the combustion liner 28. Preburner combustion air is 
directed within liner 28 via flow sleeve 26 and enters the combustion 
liner through a plurality of holes formed in the liner. The air enters the 
liner under a pressure differential across liner 28 and mixes with fuel 
from fuel nozzle 24 within liner 28. Consequently, a diffusion flame 
combustion reaction occurs within liner 28, releasing heat for purposes of 
driving the gas turbine. 
The catalytic combustion zone includes the reactor assembly 14 and 
combustion assembly 16. In that zone, there is provided an annular support 
ring 30 which supplies hydrocarbon fuel to an injector 32. For example, 
this might take the form of the multiple Venturi tube gas fuel injector 
described and illustrated in my U.S. Pat. No. 4,845,952, the disclosure of 
which is incorporated herein by reference. Thus, the mixture of 
hydrocarbon fuel and preburner products of combustion enters the catalytic 
reactor bed via the catalytic reactor assembly liner 36. The catalytic 
reactor bed 34 is generally cylindrical in shape and may be formed from a 
ceramic material or substrate of honeycombed cells coated with a reaction 
catalyst on their surfaces. The reaction catalyst may, for example, 
comprise palladium. The structure of the catalytic reactor bed 34 may be 
as described and illustrated in my U.S. Pat. No. 4,794,753, the disclosure 
of which is incorporated herein by reference. Thus, the mixture of fuel 
and preburner products of combustion ignites in the presence of the 
combustion catalyst at preburner discharge temperature. The fuel/air 
mixture entering catalytic reactor bed 34 is very lean and the hydrocarbon 
fuel oxidation reactions go to completion in the reaction zone within the 
main combustion assembly 16. 
For operating at high-load conditions for the gas turbine, there is 
provided a secondary fuel injector 40 comprised of a plurality of Venturi 
tubes for mixing hydrocarbon fuel and compressor discharge air flow 
thereto from a plenum formed by compressor discharge casing 42 and 
combustion wrapper 44. This secondary fuel/air mixture enters the reaction 
zone 16 and is ignited by the hot products of combustion exiting the 
catalytic reactor bed 34. 
In operation of the gas turbine, there are three distinct operating modes 
depending upon the load range on the gas turbine. The first operating mode 
is at low turbine loads and during initial start-up. In this mode, 
hydrocarbon fuel is supplied to start-up fuel nozzle 24 and preburner 
combustion air is provided to liner 28 through the plurality of liner 
openings for mixing with the fuel from the start-up fuel nozzle. A 
diffusion flame combustion reaction occurs within the preburner combustion 
liner 28 which is initiated by a spark or glow plug. To reduce the 
significant amount of thermal NO.sub.x generated in the preburner 
combustion liner 28, a chemical reactant, for example, ammonia, urea or 
isocyanic acid, is provided for injection by and through the primary 
injector 32. The primary injector 32 mixes the chemical reactant with the 
preburner products of combustion. Mixing may be promoted by using an inert 
carrier gas, such as nitrogen, with the chemical reactant. The chemical 
reactant may also include enhancers to accelerate the rate of chemical 
reaction with the NO.sub.x from the diffusion flame preburner assembly. 
The deNO.sub.x chemical reaction then occurs within the catalytic reactor 
assembly liner and the catalytic reactor bed 34 which may include a 
catalyst to accelerate those reactions. Consequently, significantly 
reduced NO.sub.x emissions obtain from operation of the preburner at 
low-load operating conditions. 
At mid-range operating conditions, hydrocarbon fuel is supplied to injector 
32. The injector 32 mixes the hydrocarbon fuel with the preburner products 
of combustion and this mixture enters the catalytic reactor bed 34 via the 
catalytic reactor assembly liner 36. The mixture of fuel and preburner 
products of combustion ignites in the presence of the combustion catalyst. 
Once the combustion reaction has been initiated, the preburner may be shut 
down, with the reaction being sustained at compressor discharge 
temperatures. Because the fuel/air mixture entering the catalytic reactor 
bed 34 is lean, the combustion reaction temperature is too low to produce 
thermal NO.sub.x. The hydrocarbon fuel oxidation reactions go to 
completion in the reaction zone within the main combustion assembly liner 
16. Thus, during mid-range load conditions, the temperature of the 
combustion reaction is too low to produce NO.sub.x. 
Under high-load conditions, catalytic combustion is carried on as described 
above. Additionally, hydrocarbon fuel is supplied the secondary injector 
40. Injector 40 mixes the fuel with the compressor discharge air contained 
in the plenum formed between the discharge casing 42 and the combustion 
wrapper 44. This fuel/air mixture enters the reaction zone within the main 
combustion liner 16 and is ignited by the hot products of combustion 
exiting the catalytic reactor bed 34. Because the fuel/air mixture 
entering the main combustion liner 16 is lean, the combustion reaction 
temperature is likewise too low to produce thermal NO.sub.x. 
Consequently, it will be appreciated that NO.sub.x emissions are 
substantially minimized or eliminated throughout the entire operating 
range of the gas turbine. This has been accomplished simply and 
efficiently and by a unique cooperation of essentially known gas turbine 
elements. Importantly, the NO.sub.x emissions have been minimized or 
eliminated at the low end of the operating range, i.e., when using only 
the preburner. Also, it has been accomplished using elements, i.e., the 
primary injector, extant in gas turbines of this type. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiment, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims.