Supercritical Technology

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Supercritical Technology

Supercritical Technology

The term "supercritical" refers to main steam operating conditions, being above the critical pressure of water (221.5 bar). The significance of the critical point is the difference in density between steam and water. Above the critical pressure there is no distinction between steam and water, i.e. above 221.5 bar, water is a fluid.
 
 
Supercritical steam cycle with one reheat.
 
a − b : Condensate cycle up to Deaerator
 
      c : Boiler feed pump discharge
 
c − d : Feed water heating
 
d − e : Main steam generation
 
e − f  : Expansion in turbine
 
f − g  : Reheat steam generation
 
g − h : Expansion in turbine
 
In supercritical cycle, equipment is designed to operate above the critical pressure of water. Supercritical boilers are once-through where in the feedwater enters the economiser and flows through one path and main steam exits the circuit. Typically current supercritical units operate at 242 bar main steam pressure, 565ºC main steam temperature and 593ºC reheat steam temperature.
 
 History
 
 
Supercritical technology has evolved over the past 30 years. Advancements in metallurgy and design concepts have made supercritical technology units extremely reliable and highly efficient. Modern supercritical technology is largely available in Japan and Europe for Boilers & Turbines ranging upto 1000 MW.
 
Advantages of Modern Supercritical Technology
 
 Higher Efficiency
 
Supercritical steam conditions improve the turbine cycle heat rate significantly over subcritical steam conditions. The extent of improvement depends on the main steam and reheat steam temperature for the given supercritical pressure. A typical supercritical cycle having turbine throttle pressure of 242 bar with temperatures for main steam and reheat steam as 565ºC and 593ºC respectively, will improve station heat rate by more than 5%. This results in fuel savings to the extent of 5%. Overall supercritical power plant efficiency of 42% is achievable with current supercritical parameters.
 
 
 Emissions
 
Improved heat rate results in 5% reduction in fuel consumption and hence 5% reduction in CO2 emissions per MWh energy output. Typically for 800 MW supercritical unit the annual reduction in CO2 emission will be about 725,000 tonnes of CO2 with respect to baseline emission established by CEA for 2008 – 2009.
 
Supercritical technology based thermal power project is a potential candidate to avail the benefits under Clean Development Mechanism (CDM) established by United Nations Framework Convention on Climate Change (UNFCCC).
 
 
 Operational Flexibility
 
Supercritical technology units also offer flexibility of plant operation such as:
 
Shorter start-up times
 
Faster load change flexibility and better temperature control
 
Better efficiency even at part load due to variable pressure operation and
 
High reliability and availability of power plant