We have been hearing about many companies setting up joint ventures in India to manufacture Supercritical boilers because of higher efficiency and lower carbon (CO2)emissions. GOI has also announced that they would allow only Supercriticaltechnology for new power plants from 2012.
Supercritical is a thermodynamic expression describing the state of a substance where there is no clear distinction between the liquid and the gaseous phase. Water reaches this state at a pressure above 221 bar(22.1 Mpa) and temperature above 3740C.
The “efficiency” of the thermodynamic process depends on how much of the energy fed into the cycle is converted into electrical energy. If the energy input to the cycle is kept constant, selecting elevated pressures and temperatures for the water-steam cycle can increase the output.
Up to an operating pressure of around 190 bar in the evaporator part of the boiler, the cycle is sub-critical. This means, that there is a non-homogeneous mixture of water and steam in the evaporator part of the boiler. In this case a drum-type boiler is used because the steam needs to be separated from water in the drum of the boiler before it is superheated and led into the turbine. Above an operating pressure of 221 bar in the evaporator part of the boiler, the cycle is supercritical.
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Fluid is heated in the supercritical state undergoes a continuous transition from a liquid-like state to a vapor-like state. There is no distinct temperature such as a boiling point in the supercritical state. Literally the word “boiler” should not be used for supercritical pressures and is rather known as steam generator, as no “boiling” actually occurs in this device.
In the 60’s the normal pressure and temperature for supercritical power plants (mainly in US and Japan) were around 245 bar and 540° C respectively, with only a few exceptions. In the 70’s, pressure was slightly increased to 250 bar with reheating temperature of 565° C, in most of the cases. In the 80’s, main steam
temperature itself was increased to 565° C. Since the beginning of the 90’s, steam pressure was substantially increased to 285 bar and temperature was raised in steps from 565° C to 580° C. The steam parameters in the new projects are even higher and exceed 600° C and 620° C with pressure of 300 bar (30MPa). There are few power plants operating at such high temperature/ pressure and are referred to
as Ultra Supercritical (USC) plants. In future further efficiency increase is expected to be achieved principally through the use of USC parameters by achieving live steam conditions of 760°C and 350 bar(35MPa).
Supercritical coal fired power plants with efficiencies of around 45% have much lower emissions than sub-critical plants for a given power output. Today’s state of the art in supercritical coal fired power plants permits efficiencies that exceed 45%, depending on cooling conditions. Options to increase efficiency above 50 % in ultrasupercritical power plants rely on elevated steam conditions as well as on improved process and component quality. An increase in cycle efficiency from 30% to 50% decreases CO2 emissions by more than 30%. This huge effect on environment makes a compelling case for both developed and developing countries to switch to Supercritical & Ultra Supercritical power plants.
