Super Critical Boilers Measures to improve Plant Efficiency And / or Heat Rate of Supercritical Units Turbine side measures : Higher steam parameters

Super Critical Boilers

Measures to improve Plant Efficiency And / or Heat Rate of Supercritical

Units

Turbine side measures :• Higher steam parameters (Pr. & Temp.)• Adoption of double reheat cycle• Enhancing Regenerative feed heating• Reduced Auxiliary power consumption• Increase in condenser vacuum

Boiler design to accommodate the first four requirements


Steam generation process


Sub-Critical vs Super CriticalSub-Critical vs Super Critical

Description Sub Critical Super Critical

Full Load Pressure <190 atm >240 atm

Flow in Water wall 2-Phase 1-Phase

Cycle efficiency Base + 2 - 3 % Approx

Fuel saving Base +8 - 10 % Approx

Boiler weight Base + 10 %

Stages of reheat 1 2 Preferred

Preference on date Maintaining Cyclic(Yet to start in India)


Increase in Efficiency due to steam parameters

SHO/RHOTempPressure

538 /538

538 /565

565 /565

580 /600

600 /620

175bar Base +0.75% +1.47% +2.76% +3.74%

235bar +1.72% +2.57% +3.36% +4.72% +5.69%

300bar +2.42% +3.37% +4.26% +5.74% +6.77%


Effect of steam parameters on coal consumption (500 MWe, 6500 h/a, LHV 13917 kJ/kg)

Coal consumption in kg/MWh

SH/RH-Temp

538 /538

538 /565

565 /565

580 /600

600 /620

175bar 567 572 579 583 587

235bar 556 561 568 572 576

300bar 551 556 563 567 571


Evolution of Higher steam parameters

• 1980s: Pressure increased from 175-180 bar to 225 bar ;temp mostly

around 540 deg C

• 1990s: Pressures raised to 285 bar; temp raised to 565-580-600 Deg C

• Today 255 bar 568/568 Deg C commonly used

300 bar & 620 Deg C under development




DRUM vs ONCE THROUGH DRUM vs ONCE THROUGH

Pressure Sub critical Sub & super Critical Steam Separation Drum Separator (Low loads) Types Natural / Assisted (Sulzer) / (Benson)

Burner Panel Straight tube Spiral Tube / Straight (MHI)Load Change Base FasterCold Start 4-5 Hours 2 HoursHot Start 1-2 Hours 0.5 Hours

SH

STEAM TOTURBINE

HEATDOWN

COMER

DRUMECO

Water Wall

ORIFICECIRC. PUMP

SH

STEAM TO TURBINE

ECO

HEAT

Water Wall


Drum type boiler

• Steam generation essentially takes place in furnace water walls

• Fixed evaporation end point - the drum• Steam -water separation takes place in the

drum• Separated water mixed with incoming feed

water and fed back to ww lower drum by means of boiler water circulating pumps (BWCPs).


Drum type boiler

• Natural Circulation Boiler Circulation through water walls by thermo-siphon effect

• Controlled Circulation Boiler At higher operating pressures (just below critical pressure levels), thermo-siphon effect supplemented by pumps to ensure safety of furnace wall tubes.


THE CONCEPT

The mass flow rate thru’ all heat transfer circuits

from Eco. inlet to SH outlet is kept same except at

low loads wherein recirculation is resorted to

protect the water wall system


Once Through Boiler-Concept

Once through system


Once Through Boiler

• Once -through forced flow through all sections of boiler (economiser, water walls & superheater)

• Feed pump provides the driving head

• Suitable for sub critical & super critical pressures


Once -thru BoilerAdvantages:• Quick response to load changes• Shorter start up time• Better suited for sliding pressure operation• Steam temperature can be maintained over wider

load range under sliding pressure• Higher tolerance to varying coal quality• Suitable for sub critical & super critical pressures


Once -through Boiler

Requirements :• Stringent water quality • Sophisticated control system• Low load circulation system • Special design to support the spiral furnace wall

weight• High pressure drop in pressure parts

• Higher design pressure for components from feed pump to separator.


Once -through Boiler

Characteristics :• Provides Quicker response to TG load changes• Supports achievement of better heat rate at lower

loads• Higher furnace wall pressure drop and consequent

higher feed pump auxiliary power consumption• Needs ultra pure quality feed water - Cannot

operate under conditions of condenser leak



Once -thru Boiler

Disadvantages :

• Higher feed pump auxiliary power consumption

• Cannot operate under conditions of condenser leak


Furnace ArrangementFurnace Arrangement

VERTICAL TYPE

SPIRAL TYPE


Once -through Boiler Operating Range


Sliding Pressure Operation


Advantages of sliding pressure

• Higher efficiency

• Less auxiliary power

• Less fatigue of pressure parts

Documents

Super Critical Boilers Measures to improve Plant Efficiency And / or Heat Rate of Supercritical Units Turbine side measures : Higher steam parameters