Prepared by: Ejaz Ali Soomro

2/18/2013

DLN Stands for Dry Low NOx System

Used to improve the combustion system

Reduce emissions level

NOx level 15-9ppm

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The Gas Turbines Control System “Speedtronic” controls the fuel and associated system Dry Low NOx (DLN) is a two-stage premixed combustor designed for operation on natural gas. The combustor operates by premixing the gas fuel with the air in the first stage, and then combusting the mixture in the second stage. The fuel/air mixture flame has more mass than a standard diffusion fuel flame, and so burns colder with less NOx produced. The DLN combustor also operates on #2 distillate liquid fuel, but not with premixing the fuel with air.

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While operating on liquid fuel, water injection is used for

NOx control. DLN operates at a constant flame

temperature, and so has limited turndown in the premix

operation mode. A product called “inlet bleed heat” mixes

compressor discharge air with inlet air to extend turndown

with DLN premix combustion. The DLN combustor has six

individual fuel nozzles in the primary combustion zone and

a single fuel nozzle in the secondary combustion zone.

The DLN combustion system offers lower NOx emissions

levels on gas fuel fired units without the parts life reduction

associated with water or steam injection NOx reduction

systems.

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Primary – Fuel to the primary nozzles only. Flame is in

the primary stage only. This mode of operation is used

to ignite, accelerate and operate the machine over low-

to mid-loads, up to a pre-selected combustion reference temperature. The typical firing temperature TTRF range for this mode is from startup ignition through full speed no load to approximately 1500°F. Lean-Lean – Fuel to both the primary and secondary

nozzles. Flame is in both the primary and secondary

stages. This mode of operation is used for intermediate

loads between two pre-selected combustion reference temperatures. Typical firing temperatures for the Lean-Lean mode are between 1500°F and 1950°F

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Secondary – Fuel to the secondary nozzle only. Flame

is in the secondary zone only. This mode is a transition

state between lean-lean and premix modes. This mode

is necessary to extinguish the flame in the primary

zone, before fuel is reintroduced into what becomes the primary premixing zone. A typical firing temperature for this mode is 1900°F. Premix – Fuel to both primary and secondary nozzles.

Flame is in the secondary stage only. This mode of

operation is achieved at and near the combustion

reference temperature design point. Optimum emissions are generated in premix mode. The typical firing temperature range for this mode of operation is above 1900°F.

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The control variable TTRF1 (Combustion

Reference Temperature) signifies mode transfer points

and fuel split schedules for the DLN system.

Appropriate values of TTRF1 are then stored in the

controller as combustion mode transfers points so that

the combustor may operate properly over its range.

TTRF1 is also used to modulate the fuel system

splits over the load range. It is not used to regulate the

load level of the machine.

TTRF1 is calculated from measurements of

barometric pressure, compressor discharge pressure

and temperature, and turbine exhausts temperature

obtain from the standard turbine instrumentation.

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To ensure acceptable part load turndown behavior,

the database covers a span of ambient temperatures,

and full range modulation of Inlet Guide Vane and Inlet

Bleed Heat. It is targeted to provide a consistent

reference for combustor operating behavior.

The load range associated with DLN modes varies with the

degree of inlet guide vane modulation and, to a smaller

extent, with the ambient temperature. At ISO ambient, the

premix operating range is 50% to 100% load with IGV

modulation down to 42°, and 75% to 100% load with IGV

modulation down to 57°. The 42° IGV minimum requires an

inlet bleed heat system.

If required, both the primary and secondary fuel nozzles can

be dual-fuel nozzles, thus allowing automatic transfer from

gas to oil throughout the load range. When burning natural

gas or distillate oil, the system can operate to full load in the

lean-lean mode . This allows wet abatement of NOx on oil

fuel and power augmentation with water on gas.

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The spark plug and flame detector arrangements in a

DLN-1 combustor are different from those used in a

conventional combustor. Since the first stage must be

re-ignited at high load in order to transfer from the

premixed mode back to lean-lean operation, the spark

plugs do not retract. One plug is mounted near a

primary zone cup in each of two combustors.

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The system uses flame detectors to view the primary

stage of selected chambers (similar to conventional

systems), and secondary flame detectors that look

through the center body and into the second stage.

The primary fuel injection system is used during

ignition and part load operation. The system also

injects most of the fuel during premixed operation and

must be capable of stabilizing the flame.

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DLN 1.0 liquid fuel operation is limited to the Primary

and Lean-Lean modes of combustion. The unit will

startup and load up from full speed no load in

Primary mode. A transition to the Lean-Lean mode

will be made at an approximate firing temperature of

1600°F by diverting a portion of the liquid fuel to the

Secondary combustion zone. Liquid fuel operation

will occur in the Lean-Lean mode with approximately

a 50/50 primary to secondary fuel split distribution

above TTRF= 1600°F until Base Load.

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The following display messages will appear on the

SPEEDTRONIC control panel CRT in order

to inform the operator of the current combustion

mode of operation:

1. Primary Mode

2. Lean-Lean Mode Pos

3. Lean-Lean Mode Neg

4. Lean-Lean Ext. Mode

5. Secondary Transfer

6. Secondary Load Recovery

7. Premix Transfer

8. Premix Steady State

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Operation of the gas turbine with reduced minimum

IGV settings (typically less than 57 Deg) can be

used to extend the Premix operating region to lower

loads. Reducing the minimum IGV angle allows the

combustor to operate at near a constant firing

temperature high enough to support Premix

operation while maintaining a sufficient fuel to air

ratio.

Inlet heating through the use of recirculated

compressor discharge airflow is necessary when

operating with reduced IGV angles in order to

protect the turbine compressor.

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Inlet heating protects the turbine compressor from

stall by relieving the discharge pressure and by

increasing the inlet air stream temperature. Also,

inlet heating prevents ice formation due to increased

drop across the reduced angle IGVs.

The DLN Inlet heating system regulates compressor

discharge bleed flow through a control valve and into

a manifold located in the compressor inlet air

stream. The control valve varies the inlet heating air

flow as a function of the IGV angle.

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At minimum IGV angles the inlet bleed flow is

controlled to a maximum of 5% of the total

compressor discharge flow. As the IGVs are opened

at higher loads, the inlet bleed flow is turned down

linearly until shutoff.

The inlet bleed heat control valve is monitored for its

ability to track the command set point. If the valve

command set point differs from the actual valve

position by a prescribed amount for a period of time,

an alarm will annunciate to warn the operator.

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If the condition persists for an extended amount of

time, the inlet bleed heat system will be tripped and

the IGVs will be reset to their normal schedule.

The inlet bleed heat system also looks to detect a

temperature rise in the compressor inlet airflow as

an indication of flow when the control valve is

opened. Failure to detect a sufficient temperature

rise in a set amount of time will cause the inlet bleed

heat system to be tripped and an alarm annunciated

to alert the operator.