Super Critical Boiler – Definition, Advantages and types:
Super critical boiler is a boiler that operates at super critical pressure (high pressure) to increase the efficiency of the plant and to reduce the cost of electricity production. Normally, water tube boilers are generally preferred for high pressure. In this water tube boilers, the water is circulated through tubes and their external surfaces are exposed to the flue gases.
It is the most economical cycle. The working steam pressure range is 125 bar to 300 bar and temperature is 510°C to 600°C. Usually sub-critical boiler consists of three distinct sections as preheater, evaporator and superheater. And in case of supercritical boiler, only preheater and super heater are required.
Generally super critical boilers are used for more than 300 MW power plants.
Advantages of super critical boilers:
- The amount of scale formation is less since the velocity of water through pipes are more.
- In this method, light weight tubes with better heating surface arrangement can be used. It occupies less space. The cost for foundation, time for erection are very less.
- All parts of the system are heated uniformly. So there is no danger of over heating.
- The differential expansion is reduced due to uniform temperature throughout structure. So there is no leakages of gas (or) air
- The flexibility is more.
- The temperature of steam can be raised rapidly without the use of any control devices.
- By increasing the temperature and pressure of the steam, the thermal efficiency of the plant can be increased by 40 to 42%.
- The system can be started from cold condition rapidly.
- The heat transfer rate is more. The steam side heat transfer co-efficient for sub critical boiler is 16,500 kJ/m2-hr°C and the temperature of steam and pressure are 180 bar and 538°C. But in super critical boiler, the heat transfer co-efficient is 2,20,000 kJ/m2hr-°C when the steam is generated at 240°C.
- The turbo generators connected to super critical boilers can generate peak loads by changing the pressure of operation.
Some of the Important Super Critical boilers (High Pressure) are:
- La Mont Boiler,
- Benson boiler,
- Loefler Boiler,
- Velox boiler.