GAS Turbine Combustion DLN-2

In the late 1980s, a new technology emerged in the gas turbine sector known as the F-class. It was known for its high efficiency, capable of generating 200 MW with a 50% efficiency rating in Combined Cycle Duty Application. This technology also boasted high firing temperatures, exceeding 1100°C. To keep emissions within acceptable limits, around 25 ppmvd, studies were conducted to determine the type of DLN combustor needed for these machines. These studies led to the redesign of DLN-1combustor, resulting in the DLN-2 combustor, which became the standard system for 6FA, 7FA, and 9FA machines.

DLN-2 Combustion System

The studies found that air usage in the combustor, aside from mixing with fuel, needed to be minimized, especially for cooling purposes. A proposed design suggested repackaging DLN-1 premixing technology but removing components like the venturi and centerbody assemblies, which require cooling air.

The DLN-2 combustion system shown in Figure, is a single-stage dual-mode combustor that can operate on both gaseous and liquid fuel. On gas, the combustor operates in a diffusion mode at low loads (< 50% load), and a premixed mode at high loads (> 50% load). Liquid fuel operation on this combustor is in the diffusion mode across the entire load range, with diluent injection used for NOx control.

Each DLN-2 combustor system has a single burning zone (one stage) formed by the combustor liner and the face of the cap.

In low emissions operation, 90% of the gas fuel is injected through radial gas injection spokes in the pre-mixer, and combustion air is mixed with the fuel in tubes surrounding each of the five fuel nozzles. The pre-mixer tubes are part of the cap assembly. The fuel and air are thoroughly mixed, flow out of the five tubes at high velocity and enter the burning zone where lean, low-NOx combustion occurs.

Flame stabilization is achieved through mechanisms such as the vortex breakdown from the swirling flow exiting the pre-mixers, along with the sudden expansion in the liner.

DLN-2 fuel nozzle.

Each combustor has 5 nozzle assemblies located on the head end of combustor.

The DLN-2 fuel nozzle tube arrangement is similar in design and technology to the secondary nozzle of a DLN-1.

As noted, the nozzle has passages for diffusion gas, premixed gas, oil, and water. When mounted on the end cover, as shown in Figure

Working principle

As was explained previously, the DLN_2 combustor has five fuel nozzles, four nozzles out of five are located outboard and one nozzle in the middle.

• the diffusion passages of four of the fuel nozzles are fed from a common manifold, called the primary,
• The premixed passages of the same four nozzles are fed from another internal manifold called the secondary.
• The pre-mixed passages of the remaining nozzle are supplied by the tertiary fuel system.
• The remain diffusion passage of that remaining nozzle is always purged with compressor discharge air and passes no fuel.

A quaternary fuel manifold is located on the circumference of the combustion casing to bring the remaining fuel flow to casing injection pegs located radially around the casing.

Primary fuel: fuel gas entering through the diffusion gas holes in the swirler assembly of each of the outboard four fuel nozzles

Secondary fuel: premix fuel gas entering through the gas metering holes in the fuel gas injector spokes of each of the outboard four fuel nozzles

Tertiary fuel: premix fuel gas delivered by the metering holes in the fuel gas injector spokes of the inboard fuel nozzle

The quaternary system: injects a small amount of fuel into the airstream just up-stream from the fuel nozzle swirlers

Operation modes

The DLN-2 combustion system can operate in several different modes during the load change in function of turbine combustion reference temperature.

Primary: Fuel only to the primary side of the four fuel nozzles; diffusion flame. Primary mode is used from ignition to 81% corrected speed.

Lean-Lean: Fuel to the primary (diffusion) fuel nozzles and single tertiary (premixing) fuel nozzle. This mode is used from 81% corrected speed to a pre-selected combustion reference temperature. The percentage of primary fuel flow is modulated throughout the range of operation as a function of combustion reference temperature.

If necessary, lean-lean mode can be operated throughout the entire load range of the turbine.

Premix Transfer: Transition state between lean-lean and premix modes. Throughout this mode, the primary and secondary gas control valves modulate to their final position for the next mode. The premix splitter valve is also modulated to hold a constant tertiary flow split.

Piloted Premix: Fuel is directed to the primary, secondary, and tertiary fuel nozzles.

• This mode exists while operating with temperature control off as an intermediate mode between lean-lean and premix mode.
• This mode also exists as a default mode out of premix mode and, in the event that premix operating is not desired, piloted premix can be selected and operated to baseload.

Primary, secondary and tertiary fuel split are constant during this mode of operation.

Premix: Fuel is directed to the secondary, tertiary and quaternary fuel passages and premixed flame exists in the combustor.

• The minimum load for premixed operation is set by the combustion reference temperature and IGV position. It typically ranges from 50% with inlet bleed heat on to 65% with inlet bleed heat off.

Mode transition from premix to piloted premix or piloted premix to premix, can occur whenever the combustion reference temperature is greater than 1204 C. Optimum emissions are generated in premix mode.

Tertiary Full Speed No Load (FSNL):

Initiated upon a breaker open event from any load > 12.5%. Fuel is directed to the tertiary nozzle only and the unit operates in secondary FSNL mode for a minimum of 20 seconds, then transfers to lean-lean mode.

Fuel control of DLN-2

The fuel distribution to each fuel manifold system is a function of combustion reference temperature and IGV temperature control mode.

Operation modes are established by changing the distribution of fuel flow in the combustor. The gas fuel consists of the gas fuel stop-ratio valve, primary gas control valve, secondary gas control valve premix splitter valve and quaternary gas control valve. The stop-ratio valve is designed to maintain a predetermined pressure at the control-valve inlet. The primary, secondary and quaternary gas control valves regulate the desired gas fuel flow delivered to the turbine in response to the fuel command from the SPEEDTRONIC? controls.

The premix splitter valve controls the fuel flow split between the secondary and tertiary fuel system

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conclusion:

Manufacturers are continuously enhancing Lean Premix (LPM) gas turbine combustion as the preferred strategy to meet upcoming emission regulations. From a cost perspective across the lifecycle, it has been demonstrated that preventing the formation of pollutants is more economically viable compared to employing post-combustion techniques. Ongoing efforts are utilizing the latest experimental and analytical tools to improve the emissions control and operational flexibility of LPM systems.