CONTROLS OF SUPER CRITICAL BOILERS

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CONTROLS OF SUPER CRITICAL BOILERS
Presentation By AJAY SHUKLA Sr.Faculty PMI,
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Boiler Turbine Control
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Boiler Following Mode
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Turbine Following Mode
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Coordinate Mode Control
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Coordinate Mode Control
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Drum and OT Control Comparison
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Drum and OT Control Comparison
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OT Control Overview
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Cycle of Supercritical Power Plant
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Supercritical Power Plant
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OT Control Overview
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OT Start up Control mode
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Super Critical Units Controls Mode
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Flushing Mode
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Flushing Mode
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Start-Up System with Recirculation
Start-Up System Recirculation Pump in Main
Bypass Line
SH
Separator
Flash Tank
WW
C
ECO
HWL
To Condenser
C
Deaerator
HPH
C
BFP
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Start UP System
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START-UP

• If the water system of the boiler is empty
  (economizer, furnace walls, separators), then the
  system is filled with approximately 10 TMCR feed
  water flow.
• When the level in the separator reaches
  set-point, the WR valve will begin to open.
• When the WR valve reaches gt30 open for
  approximately one minute, then increase feed
  water flow set-point to 30 TMCR. As the flow
  increases, WR valve will reach full open and ZR
  valve will begin to open.
• The water system is considered full when
• The separator water level remains stable for
  two(2) minutes
• and
• The WR valve is fully opened and ZR valve is gt15
  open for two(2) minutes.

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SEPARATOR STORAGE TANK LEVEL CONTROL

• Separator level is maintained by the combined
  action of a separator storage tank level feed
  water demand and the positioning of WR and ZR
  drain valves.
• Feed water demand is developed in response to
  separator storage tank level error and total fuel
  flow so as to prevent tank level from dropping
  too low.
• The WR and ZR valves are controlled in a split
  range manner to maintain the liquid level once
  the level reaches a high limit.
• The WR valve will respond first and then the ZR
  when the WR exceeds its linear operating range.
• Tank geometry is such that fluctuations in tank
  level are very dynamic, for this reason, only
  proportional control action established through
  the WR/ZR function curves is used to position
  these valves in response to level error.

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UG VALVE CONTROL

• Control objective
• Maintain minimum economizer inlet flow.
• Control action
• The boiler circulating pump is started following
  the start of a turbine-driven feed water pump and
  the final clean-up cycle. This pump circulates
  feed water from the evaporator outlet back to the
  economizer inlet.
• Located at the outlet of this pump is the UG
  valve which controls economizer inlet flow during
  the start-up phase of operation. Demand for this
  recirculation control valve is established based
  on measured economizer inlet flow compared to a
  minimum boiler flow set point.

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Separator water circuit of Super Critical
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FEEDWATER CONTROL LOOP

• Control objective
• Develop total unit feed water demand as required
  to support unit load.
• Adjust feed water demand to maintain desired
  separator outlet temperature.
• Adjust separator outlet temperature set point as
  required to maintain acceptable platen superheat
  spray control range.
• Incorporate separator storage tank level (wet
  mode) feed water demand.
• Maintain minimum required economizer inlet flow.
• .

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Feedwater Control
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Feed water firing rate ratio control
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Feed water firing rate ratio control
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Feed Forward with model
Process
Setpoint
Feed Forward
f(x)
S
P /-
C -/
D
K
?
C /-
P /-
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Feed Forward
process
FF
setpoint
PROCESS
FF
f(x)
P /-
C -/
P /-
C -/
D
D
K
K
C /-
P /-
C /-
P /-
X
S
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Model predictive control

MW
_
Fuel with model
Without model
Turbine setpoint
Water wall Temperature
C
tsec
0
60
180
240
300
120
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Firing Rate Master
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Fuel Rate Master
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Feed Forward Firing Rate Control
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Feed Forward Firing Rate Control
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ANY QUESTIONS PLEASE
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Yuhuan 4x1000MW Preparation of light off
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Multi-combusting Nozzles with Separated Overfire
Air Damper
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START-UP contd..

• Water flows through the economizer and
  evaporator, and discharges the boiler through
  the WR valve to the flash tank and via connecting
  pipe to the condenser.
• Start BCP and open the UG valve to establish
  minimum water wall flow
• at 30 TMCR.
• As the pressure is raised, first the WR and
  then the ZR valves will open when
• separator water level increases due to boiler
  water swell. As pressure further
• increases, the WR and ZR valves will start to
  close as the water level decreases.
• The steam temperature at the separator inlet will
  reach a stable superheated
• condition at app. 30 TMCR, causing the level in
  the separator to decrease and
• eventually disappear. The boiler is now in
  once-through mode (dry mode). The
• BCP (Boiler Circulating Pump) will be stopped
  automatically.
• It is extremely important that minimum water wall
  flow be maintained at all times when firing the
  boiler to prevent tube damage due to overheating.
  

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FEEDWATER CONTROL LOOP contd..

• Demand for feed water is established
  predominately by the Boiler Master demand.
• This signal, processed though a boiler transfer
  function provides the feed forward component of
  the total feed water demand.
• The boiler transfer function is a tunable
  dynamic element providing a means to dynamically
  match the feed water feed forward demand to
  actual evaporator heat transfer.
• Optimization of the feed forward in this manner
  minimizes temperature fluctuations that may
  otherwise result from varying dynamic response
  between the firing and feed water control systems
  (as they relate to evaporator heat transfer)
  thereby lessening the dependence on feedback
  correction.

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FEEDWATER CONTROL LOOP contd..

• The first controller acts on a load dependent
  average platen spray differential temperature.
• Its output represents the required adjustment to
  evaporator heat transfer/steam generation to
  maintain both the steam conditions and flue gas
  temperatures entering the platen superheat
  section so as to ensure adequate platen spray
  control range.
• A second controller acts on a load dependent
  separator outlet temperature set point corrected
  by the platen spray differential temperature
  controllers output.
• This controller acts to adjust feed water in
  response to firing system disturbances and the
  relatively fast effect they have on separator
  outlet steam temperatures.
• The overall combined feed water feedback control
  action is such that feed water demand is
  responsive to changes in the overall unit heat
  transfer profile.

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FEEDWATER CONTROL LOOP contd..

• The combined feed forward/feedback demand signal
  is subject to a minimum economizer inlet flow set
  point (wet mode) activated if the boiler
  circulation pump is not in service and the unit
  is being fired.
• This ensures the minimum economizer inlet cooling
  flow is maintained by the feed water supply
  system in the event the start-up system is not
  available.
• The feed forward/feedback demand signal is
  subject to a second wet mode feed water demand
  developed to support separator storage tank level
  control.
• The resulting demand provides the set point to a
  feed water master controller.
• The fuel/feed water ratio protection logic
  provides overriding control of individual feeder
  speed demands in the event of an excessively
  high fuel to feed water ratio.

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