NOx suppressant stationary gas turbine combustor

The NOx emissions of a stationary gas turbine are reduced by concentrating a NOx suppressant in the reaction zone of the gas turbine combustor by dividing the flow of air to the reaction zone and the dilution zone of the combustor by means of an air flow splitter and by taking advantage of the radially stratified compressor flow. The air flow to the two zones is preferably separated by a common flow shield.

BACKGROUND OF THE INVENTION 
The abatement of emissions, particularly the oxides of nitrogen (NOx) is 
gaining increasing attention and significant resources are being applied 
to the associated problems. 
It has been found that NOx is formed in the combustors of stationary gas 
turbines through two NOx forming mechanisms. Thermal NOx is formed by 
reaction between the nitrogen and oxygen in the air initiated by the high 
flame temperature and fuel NOx, on the other hand, results from the 
oxidation of organic nitrogen compounds in the fuel. 
Various governmental agencies have proposed or enacted codes for regulating 
the NOx emissions of stationary gas turbines. For example, the United 
States Environmental Protection Agency has proposed a code limiting NOx 
emissions to 75 ppm at 15% oxygen with an efficiency correction. In 
Southern California, the Los Angeles County Air Pollution Control 
District's Los Angeles County Rule 67 limits NOx emissions to 140 lbs. per 
hour. 
It has been found that the NOx emissions of a stationary gas turbine can be 
regulated by the addition of a suitable NOx suppressant fluid to the air 
supply of the gas turbine combustor. One example involves the 
recirculation of exhaust gases from a gas turbine-steam turbine combined 
power plant and is described in more detail in copending application Ser. 
No. 113,635, filed Jan. 21, 1980 of common assignee as the instant 
invention, the disclosure of which is hereby incorporated by reference. 
Another example involves the supply of an oxygen-deficient air mixture 
which is the by-product of an oxygen separation unit in a coal 
gasification plant, the oxygen being used together with coal to generate a 
medium BTU coal gas which is employed as the fuel for the stationary gas 
turbine combustor. The latter arrangement is described in more detail in 
copending application Ser. No. 113,637, filed Jan. 21, 1980 of common 
assignee as the instant invention, the disclosure of which is hereby 
incorporated by reference. Examples of other useful NOx suppressants in 
addition to the above are nitrogen, carbon dioxide and other high specific 
heat gases which are relatively inert. 
When NOx suppressants are used, they are generally added to the air supply 
for the stationary gas turbine compressor. However, commercial gas 
turbines use a portion (15% or more) of the compressor discharge air for 
nozzle and turbine cooling. Since these air flows do not effect NOx 
emissions, adding the NOx suppressants to these flows represents a waste 
of the suppressant. Additionally, a minimum suppressant flow rate is 
desirable and concentrating a fixed amount of suppressant in only the 
combustor air or preferably in the primary reaction zone will produce 
better NOx control.

SUMMARY OF THE INVENTION 
This invention relates to a NOx suppressant stationary gas turbine 
combustor and more particularly to such a combustor where an air flow 
splitter divides the flow of air to the reaction zone and the dilution 
zone of the combustor so that NOx suppressant can be concentrated in the 
reaction zone by injection at a suitable point to take advantage of the 
radially stratified compressor flow. 
DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 are schematic representations of a conventional reverse air 
flow stationary gas turbine combustor which has been modified to include 
the present invention. It should be noted that although the invention is 
described with respect to a reverse air flow combustor, other combustor 
configurations may obviously be used without departing from the spirit and 
scope of the present invention. 
The conventional stationary gas turbine combustor contains a combustion 
liner 1 which encloses, in the direction of flow, a reaction zone, a 
dilution zone and a transition zone leading to the gas turbine. A fuel 
nozzle 2, usually axisymmetrically disposed, introduces a suitable gaseous 
or liquid fuel through liner 1 into the reaction zone. Suitable means for 
introducing combustion air through liner 1 into the reaction zone, such as 
air entry ports 3 and suitable means for introducing a supply of air for 
dilution into the dilution zone, such as air entry port 4, are provided. 
Combustion liner 1 is encased within an outer casing 5. An air channel 6 
carries compressed air from the stationary gas turbine air compressor to 
the combustor and communicates with the channel 7 formed between outer 
casing 5 and combustion liner 1. It is conventional to arrange the 
connection of air channel 6 with channel 7 such that the flow of fluids 
within channel 7, i.e., between outer casing 5 and combustion liner 1, is 
opposite the flow of fluids within combustion liner 1 to provide for 
surface cooling of liner 1. 
In accordance with the present invention, provision is made for splitting 
the flow of air between the air which is intended to be utilized within 
the reaction zone for combustion purposes and the remainder of the air 
which is destined for use as a diluent in the dilution zone or for 
surface-cooling the dilution zone and possibly the transition zone. This 
is accomplished by imposing an annular flow shield 8 within the channel 7 
defined by combustion liner 1 and outer casing 5. At one end of its 
longitudinal length, flow shield 8 joins combustion liner 1 at about the 
dividing point between the reaction zone and dilution zone. The other end 
of flow shield 8 usually extends to near the junction of channels 6 and 7. 
The flow in a gas turbine axial compressor is predominantly in the axial 
direction and therefore a radially stratified inlet flow remains 
segregated at the compressor exit. By selecting the proper location at the 
compressor inlet or downstream of the compressor for injecting NOx 
suppressants, the suppressants can be concentrated in the combustion air 
which thereby maximizes NOx reduction. Thus, injecting NOx suppressant in 
discrete locations at the compressor inlet provides lower NOx emissions 
than homogeneously mixing the flows upstream of the compressor inlet. The 
turbine cooling flow rates are not altered and they do not contain 
significant amounts of NOx suppressant. The injection of the NOx 
suppressant is represented in FIGS. 1 and 2 by suppressor injector 9. 
The flow of air in air flow channel 6 is preferably longitudinally along 
the channel 6 with little transverse component and is divided into two 
paths by air flow splitter 10 which is preferably in the form of an 
aerodynamically curved baffle shield or scoop. By appropriate construction 
of flow shield 8 and air flow splitter 10, the flow of air in channel 7 
adjacent the transition zone is either in common with the flow of air 
adjacent the dilution zone or the reaction zone while the air flows to the 
latter two zones remain segregated. 
In FIG. 1, the flow of air to the reaction zone is isolated from both the 
flow of air to the dilution zone and adjacent transition zone. Thus, in 
this embodiment air flow splitter 10 is connected to flow shield 8. The 
flow of air to the reaction zone is through the path 11 defined by air 
flow splitter 10, flow shield 8, outer casing 5 and that portion of 
combustion liner 1 which is adjacent the reaction zone. The path 12 for 
the flow of air to the dilution zone is defined by air flow splitter 10, 
flow shield 8, outer casing 5 and that portion of combustion liner 1 which 
is adjacent to both the dilution zone and the transition zone. In FIG. 1, 
the positioning of suppressant injector 9 represents the injection of the 
NOx suppressant at the tips of the blades of the air compressor. As a 
result of such positioning and the radially stratified compressor air 
flow, there will be substantially parallel flows in channel 6 with most of 
the NOx suppressant entering path 11 leading to the reaction zone. 
In FIG. 2, the dilution zone is isolated from the reaction zone and the 
transition zone. This is effected by connecting the aerodynamically curved 
baffle shield 10 to combustion liner 1 instead of the flow shield 8 as in 
FIG. 1. Thus, in FIG. 2 the path 13 to the dilution zone is defined by air 
splitter 10, flow shield 8 and that portion of combustion liner 1 adjacent 
the dilution zone. The air flow path 14 to the reaction zone is defined by 
that portion of combustion liner 1 adjacent the reaction zone and the 
transition piece, flow shield 8, outer casing 5 and air splitter 10. The 
positioning of suppressant injector 9 in FIG. 2 represents the injection 
of the NOx suppressant at the roots of the inlet blades of the air 
compressor for the gas turbine and as a result of the radially stratified 
compressor air flow, the NOx suppressant will be concentrated in the air 
flow to the reaction zone. 
Based on data collected in connection with NOx reduction by the injection 
of an oxygen-deficient air mixture as described in the above referenced 
application Ser. No. 113,637, it has been determined that when the 
oxygen-deficient air mixture is mixed homogeneously with the combustion 
and cooling air flows, about a 55% reduction of NOx can be realized. By 
operating in accordance with the present invention, that is, by 
introducing the oxygen-deficient air mixture through NOx suppressant 
injection ports in the combustion air to concentrate the mixture, an 
estimated 63% reduction can be achieved. Since the easy methods of NOx 
reduction have already been identified and commercially adopted and the 
additional increments in NOx reduction are extremely difficult to achieve, 
the additional NOx reduction realized with the present invention 
represents a significant advance and can mean the difference between 
complying and not complying with proposed governmental regulations 
concerning emissions. 
In practicing the present invention in the embodiments illustrated in FIGS. 
1 or 2, the NOx suppressant is concentrated in the combustion flame zone 
and the 55% NOx reduction could be achieved using only about 30% of the 
NOx suppressant flow required above. Alternately, using the same NOx 
suppressant flow rate, larger NOx reductions are possible, but the total 
reduction will ultimately be limited by flame stability criteria. 
Various changes and modifications can be made in the present invention 
without departing from the spirit and scope thereof. For example, in 
conventional multi-combustor arrays, each combustor can be provided with 
an air splitter or a common air splitter/manifold arrangement can be used. 
The various embodiments which have been disclosed herein were for the 
purpose of further illustrating the invention but were not intended to 
limit it.