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

This article discusses measures to improve the efficiency and heat rate of supercritical units, including higher steam parameters, adoption of double reheat cycle, enhancing regenerative feed heating, reduced auxiliary power consumption, and increase in condenser vacuum. It also explains the evolution of higher steam parameters and the advantages of once-through boilers.

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

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  1. 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

  2. Steam generation process Super Critical Boilers

  3. Sub-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) Super Critical Boilers

  4. Increase in Efficiency due to steam parameters Super Critical Boilers

  5. Effect of steam parameters on coal consumption(500 MWe, 6500 h/a, LHV 13917 kJ/kg)Coal consumption in kg/MWh Super Critical Boilers

  6. 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 Super Critical Boilers

  7. Super Critical Boilers

  8. Super Critical Boilers

  9. STEAM TO TURBINE STEAM TO TURBINE SH SH ECO DRUM HEAT HEAT DOWN COMER Water Wall Water Wall ECO ORIFICE CIRC. PUMP 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 Faster Cold Start 4-5 Hours 2 Hours Hot Start 1-2 Hours 0.5 Hours Super Critical Boilers

  10. 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

  11. 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

  12. 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

  13. Once Through Boiler-Concept Once through system Super Critical Boilers

  14. 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

  15. 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

  16. 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

  17. 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

  18. Super Critical Boilers

  19. Furnace Arrangement SPIRAL TYPE VERTICAL TYPE Super Critical Boilers

  20. Once -through Boiler Operating Range Super Critical Boilers

  21. Sliding Pressure Operation Super Critical Boilers

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

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