126106198-dln-1-0-ms9001
DESCRIPTION
DLN1 DescriptionTRANSCRIPT
DLN Stands for Dry Low NOx System
Used to improve the combustion system
Reduce emissions level
NOx level 15-9ppm
2/18/2013
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.
2/18/2013
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.