B URNING V ELOCITY
5.2. RATIONALE FOR LEAN COMBUSTION IN GAS TURBINES
5.2.6. T URN D OWN
Up to this point, the issues described are applicable for any given operating point for the engine. However, when consideration is given to the entire operating range of the engine (i.e.,“turn-down”), the situation becomes much more complex. Since the gas turbine thermodynamic cycle dictates that pressure ratio change as load changes, the fuel/air ratio will vary over the load range. Due to the different relationships between output power and gas flow (squared) and output power and fuel flow (linear), operating lean over the full load range while maintaining stable combustion requires
“staging”of some form. To illustrate this, Figure 5.15 presents the relative fuel and air flows and the associated fuel/air ratio for a small gas turbine engine. As shown, the overall fuel/air ratio drops as load decreases. Consequently, if the system is optimized to
operate at minimum fuel/air ratio at full load (e.g., to minimize reaction temperatures), it would not be possible to reduce the load of the system since the flame would reach its lean blow-off limit. In response to this problem, lean premixed combustion systems are often staged, with multiple fuel injection points that can be operated sequentially. This allows the local fuel/air ratio for each point to be tailored while allowing the overall fuel/air ratio for the engine to vary as needed to accomplish the turndown desired.
To illustrate this in the context of emission performance, Figure 5.16 compares how staging allows the combustion system to stay within a lean combustion regime (locally for each fuel injection point), whereas the conventional non-staged engine must operate with combustion taking place over a wider range of equivalence ratios and inevitably ending up having to operate at conditions producing high NOx emissions for at least some part of the load range. How the staging is accomplished differs substantially for stationary and aviation engines due to the differing drivers and constraints. Each is briefly reviewed here.
5.2.6.1. Stationary
Staging in stationary engines has evolved into a highly sophisticated engineering task. Consider the two engines originally shown as examples in Figure 5.2. Figure 5.17
NO [ ]
1.0 f
T HC Soot CO
NO
Idle Full
Conventional
Staged:
pilot Staged:
pilot + main
Figure 5.16 Comparison of staged and conventional combustion strategies idle and full power points shown for conventional strategy.
0 200 400 600 800 1000 1200 1400
0 20 40 60
Load (kW)
Mass flow of air (lbm)
0 20 40 60 80 100 120 140
Mass flow of fuel (lbm) F/A*100
Air fuel F/A * 100
Figure 5.15 Example of air and fuel flows for a 3.5:1 pressure ratio gas turbine engine.
shows the combustion strategy used in the engine in Figure 5.2a. Even though this is only a 60 kW engine, it makes use of a five-stage fuel injection strategy. In the first plane of injection (two injectors), both injectors are operated full time and are used to cover the startup and low power conditions. However, in the second plane, anywhere from 1 to 4 injectors are operated as the load ramps up. Within each stage, the equivalence ratio increases as load increases, but with staging, the overall equivalence ratio stays within the low emissions window. The first plane is not operated at extreme lean conditions since it is serving to ensure the reaction remains stable. The resulting NOx emission profile is shown in Figure 5.18 and illustrates clearly where the stage points occur. It also shows that the NOxemissions can be“tuned”for a given load point by altering the staging strategy. This staging point flexibility also provides a powerful
“knob”for avoiding combustion oscillations.
The staging strategy taken in the engine shown in Figure 5.2b is illustrated in Figure 5.19. The GE DLN (Dry Low NOx) strategy features more of a“can”style combustion chamber with traditional swirl, but the use of multiple discrete fuel injection points remain. In this system, four different fuel circuits are used featuring differing combin- ations of primary, secondary, premixed primary, and quaternary circuits. These are operated in different combinations as load changes. The resulting impact on NOx
emissions is shown in Figure 5.20. This engine can achieve very low emissions from 35% to 100% load using lean combustion.
A A
B B
Inner liner
Turbine inlet Outer liner
Dilution air Dilution air
Dilution zone Primary
zone
Outer liner
Inner liner
Igniter
Injector
Recuperator wall
Figure 5.17 Capstone C-60 combustor.
Outer casting Primary
fuel nozzles (6)
Secondary fuel nozzle
(1)
Secondary zone Dilution zone Centerbody
End cover
Venturi Flow sleeve
Lean and premixing primary zone
Figure 5.19 GE DLN fuel injection cross section.
0 10 20 30 40 50 60 70 80 90 100
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Load (%) NOx (ppm @ 15%O2)
Figure 5.18 Staging influence on NOxemissions for Capstone C-60 (Phiet al., 2004).
Figure 5.20 Staging influence on NOxemissions for GE DLN 2.6 injector.
5.2.6.2. Aviation
Stationary engines are not the only devices that need to use staging. Aviation engines are also capable of being staged. For example, Figure 5.21 illustrates the GE concept of using“dual annular”combustors for emissions reduction. The details regarding geom- etry modifications to accommodate the shape can be noted, although examples have been implemented in which the staged combustor can be retrofitted into an engine with a single annular design. However, this approach is not optimum and greater flexibility in design is generally preferred. The manner in which utilization of the dual annular concept impacts NOx emissions is shown in Figures 5.22 and 5.23. The staging approach is beneficial from an operability standpoint as well. For example, the pilot stage can be optimized for ignition and low power operation while the main stage can be optimized for low emissions at high power. It is worth noting that the fuel injection approach used in both single and dual annular configurations features some degree of premixing prior to the reaction zone. In order to operate with low emissions, good mixing is essential for lean operation.
Figure 5.21 Comparison of (A) a single and (B) a dual annular combustor (GE CFM-56).
0
0 100 200 300 400 500 600 700 800 900 1000 1100 30
20
10 EINO
x, g/kg fuel
Pilot only operation
Staging
30% 85%100% power
6%
4%
SLS standard day
Combustor inlet temperature, ⬚F
Staged operation at 40/60
Figure 5.22 Example of dual annular staging influence on NOxemissions.