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

Super Critical Boilers Steam generation process

Super Critical Boilers Sub-Critical vs Super Critical DescriptionSub CriticalSuper Critical Full Load Pressure<190 atm>240 atm Flow in Water wall2-Phase1-Phase Cycle efficiencyBase % Approx Fuel savingBase % Approx Boiler weightBase+ 10 % Stages of reheat12 Preferred Preference on dateMaintainingCyclic (Yet to start in India)

Super Critical Boilers Increase in Efficiency due to steam parameters

Super Critical Boilers Effect of steam parameters on coal consumption (500 MWe, 6500 h/a, LHV kJ/kg) Coal consumption in kg/MWh

Super Critical Boilers Evolution of Higher steam parameters 1980s:Pressure increased from bar to 225 bar ;temp mostly around 540 deg C 1990s:Pressures raised to 285 bar; temp raised to Deg C Today 255 bar 568/568 Deg C commonly used 300 bar & 620 Deg C under development

Super Critical Boilers

DRUM vs ONCE THROUGH PressureSub criticalSub & super Critical Steam SeparationDrumSeparator (Low loads) TypesNatural / Assisted (Sulzer) / (Benson) Burner PanelStraight tubeSpiral Tube / Straight (MHI) Load ChangeBaseFaster Cold Start4-5 Hours2 Hours Hot Start1-2 Hours0.5 Hours SH STEAM TO TURBINE HEAT DOWN COMER DRUM ECO Water Wall ORIFICE CIRC. PUMP SH STEAM TO TURBINE ECO HEAT Water Wall

Super Critical Boilers 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).

Super Critical Boilers 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.

Super Critical Boilers 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

Super Critical Boilers Once Through Boiler-Concept Once through system

Super Critical Boilers 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

Super Critical Boilers Once -thru Boiler Advantages: 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

Super Critical Boilers 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.

Super Critical Boilers 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

Super Critical Boilers

Furnace Arrangement VERTICAL TYPE SPIRAL TYPE

Super Critical Boilers Once -through Boiler Operating Range

Super Critical Boilers Sliding Pressure Operation

Super Critical Boilers Advantages of sliding pressure Higher efficiency Less auxiliary power Less fatigue of pressure parts