80
70
60
50
40
30
20
10
0 1200 1300 1400 1500 1600 1700 1800 1900 2000
FLAME TEMPERATURE
Figure 10-21. Correlation of adiabatic flame temperature with NOx emissions
rapid1y with temperature above 2732 0F (1500 0C) and a1so increases with residence time in the combustor.
The production rate of NO can be given as fo11ows:
1 .
T
d(..)二 .e .2(.2)(10-12)
dtT
The important parameters in the reduction of NOx as seen in the aboveequation are the temperature of the f1ame， the nitrogen and oxygen content and the resident time of the gases in the combustor. Figure 10-21 is a corre1ation between the adiabatic f1ame temperature and the emission of NOx. Reduction of any and a11 these parameters wi11 reduce the amount of NOx emitted from the turbine.
Dry Lo. NOx Combustor
The gas turbine combustors have seen considerab1e change in their design as most new turbines have progressed to Dry Low Emission NOx Combus-tors from the wet combustors， which were injected by steam in the primary zone of the combustor. The DLE approach is to burn most (at 1east 75%) ofthe fue1 at coo1， fue1-1ean conditions to avoid any significant production of NOx. The principa1 features of such a combustion system is the premixing of
Rich
Lean
FUEL/AIR RATIO
the fue1 and air before the mixture enters the combustion chamber and 1eanness of the mixture strength in order to 1ower the f1ame temperature and reduce NOx emission. This action brings the fu11 1oad operating point down on the f1ame temperature curve as seen in Figure 10-22 and c1oser to the 1ean 1imit. Contro11ing CO emissions thus can be difficu1t and rapidengine off-1oads bring the prob1em of avoiding f1ame extinction， which if it occurs cannot be safe1y reestab1ished without bringing the engine to rest and going through the restart procedure.
Figure 10-23 shows a schematic comparison of a typica1 dry 1ow emission NOxcombustor and conventiona1 combustors. ln bothcases， a swir1er is used to create the required f1ow conditions in the combustion chamber to stabi1ize the f1ame. The DLE fue1 injector is much 1arger because it containsthe fue1jair premixing chamber and the quantity of air being mixed is1arge， approximate1y 50-60% of the combustion air f1ow.
DRY LOW EMISSIONS COMBUSTOR
Pre-mix Zone LP STAGE 2 LP STAGE 1
PILOT
Main Fuel
Swirless
CONVENTIONAL COMBUSTOR
Main
Fuel
Figure 10-23. A schematic comparison of a typical dry low emission NOx combustor and a conventional combustors
COMPRESSOR
AIR
Figure 10-2.. Schematic of a drylow emission NOx combustor (Courtesy ALSTOM )
The DLE injector has two fue1 circuits. The mainfue1， approximate1y 97%of thetota1， is injected into the air stream immediate1y downstream of the swir1er at the in1et to the pre-mixing chamber. The pi1ot fue1 is injected direct1y into the combustion chamber with 1itt1e if any premixing. With the f1ame temperature being much c1oser to the 1ean 1imit than in a conventiona1combustionsystem， some action has to be taken when the engine 1oad is reduced to prevent f1ame out. lf no action were taken f1ame-out wou1d occur since the mixture strength wou1d become too 1ean to burn. A sma11 propor-tion of the fue1 is a1ways burned richer to provide a stab1e ..pi1oting''zone，whi1e the remainder is burned 1ean. ln bothcases， a swir1er is used to create the required f1ow conditions in the combustion chamber to stabi1ize the f1ame. The LP fue1 injector is much 1arger because it contains the fue1jairpre-mixing chamber and the quantity of air being mixed is 1arge， approxi-mate1y 50-60% of the combustion air f1ow.
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