SUPERCRITICAL BOILER :Supercritical steam generator (Benson boiler) is frequently used for the production of electric power. they operate at “supercritical pressure”. In contrast to a “supercritical boiler”,a supercritical steam generator operates at such a high pressure ( over3200psi,22mpa,220 bar)that actual boiling ceases to occur, and the boiler has no water –steam separation. There is no generation of steam bubbles within the water, because the pressure above the “ critical pressure” at which the steam bubbles can form. It passes below the critical point as it does work in the high pressure turbine and enters the generator’s condenser. This is more efficient, resulting in slightly less fuel use and therefore less greenhouse gas production. The term boiler should not be used for a supercritical pressure steam generator, as no “boiling” actually occurs in this device.

Supercritical pressure boiler has no drum and heat absorbing surface being, in effect, one continuous tube, hence called “once through supercritical pressure boiler”.

The water in boiler is pressurized by boiler feed pump ,sensible heat is added in feed heaters ,economizer and furnace tubes, until water attains saturation temperature and flashes instantaneously to dry saturated steam and super heating commences.

CONTENTS :

Why supercritical

What is supercritical boiler

Advantages &disadvantages of supercritical boiler

Sipat supercritical boiler

Comparison with 500mw boiler

Latest development

Importance of efficiency

WHY SUPERCRITICAL PRESSURE :

A point where boiling water and dry saturation lines meet so that associated latent heat is zero, this point called “critical point”.

A boiler operating at a pressure above the critical point is called

“supercritical boiler”.

CRITICAL CONDITION

“Critical” is a thermodynamics expression describing the state of a substance beyond which there is no clear distinction between the liquid and gaseous phase.

The critical pressure for water: 225.56kg/cm2 . The critical temperature for water: 374.150c .

SUPERCRITICAL BOILER :

Supercritical steam generator (Benson boiler) is frequently used for the production of electric power. they operate at “supercritical pressure”. In contrast to a “supercritical boiler”, a supercritical steam generator operates at such a high pressure (over3200psi, 22mpa and 220 bar) that actual boiling ceases to occur, and the boiler has no water –steam separation. There is no generation of steam bubbles within the water, because the pressure above the “critical pressure” at which the steam bubbles can form. It passes below the critical point as it does work in the high pressure turbine and enters the generator’s condenser. This is more efficient, resulting in slightly less fuel use and therefore less greenhouse gas production. The term boiler should not be used for a supercritical pressure steam generator, as no “boiling” actually occurs in this device.

Supercritical pressure boiler has no drum and heat absorbing surface being, in effect, one continuous tube, hence called “once through supercritical pressure boiler”.

The water in boiler is pressurized by boiler feed pump ,sensible heat is added in feed heaters ,economizer and furnace tubes, until water attains saturation temperature and flashes instantaneously to dry saturated steam and super heating commences.

ADVANTAGES :

1. SUPERCRITICAL THERMAL EFFICIENCY

Improvements in plant efficiency. Decrease in coal consumption. Reduction in green house gases. Overall reduction in auxiliary power consumption. Reduction in requirement of ash dyke land & consumptive water.

2. SUPERCRITICAL

Sliding pressure operation because of once through system.

3. LOWER PART SPIRAL WALL

Lower thermal stress in turbine. The start up time is less for boiler.

DISADVANTAGES :

Higher power consumption of bfs. Higher feed water quality required . More complex supporting and faming in boiler due to spiral wall tube. Slighty higher capital cost. Increased dependancy for spares and services.

SIPAT SUPERCRITICAL BOILER :

The boiler has two pass arrangement , once-through and tangential firing unit.

Pendent super heater surfaces, widely spaced arrangement can be arranged in the upper part of furnece for high heat absorption in super heaters.

Proper heat absorption distribution can be achived in accordance with the steam cycle requirements.

Flue gas temperature entering the closer spaced convection heating surface can be contrrolled to the proper level.

The gas velocity is independent of the furnace cross section and can be optimized for any convection section in the boiler.

The height of the unit is less which results in advantage in structural steel design, perticularly with high seismic and wind loads.

LATEST DEVELOPMENTS :

Ultra super critical boiler (6000 0c , 305 bar).

Two times reheating to increasing the efficiency and to reduce the wetness of exhaust from the low pressure turbine.

For use boiler high temperature steels like p91 and p92 have been developed for commertial use.

PARTS OF BOILER :

1. SEPARATOR :

A device is used to separate water and steam from mixture formed through a furnace wall at the supercritical boiler. It used only when boiler starts , stops and operates in low loads. When the boiler operates in normal, separator is alike a header.

2. HEADER AND NOZZLE :

Water and steam are heated through the economiser , water wall, superheater and reheater. The headers gather water and steam, and redistribute them to the next stage equally.

3 . HIGH EFFICIENCY COAL PULVERIZER :

Coal pulverizer pulverizes and dries coal for higher combustion efficiency. The pulverizer is used by adjusting the temperature of hot air passing from the air preheater .

4. DUCT :

A pipe, tube, channel or any other unit necessary for conveying gases, combustion air from one point to another.

5. FEED WATER HEATER :

It increases the efficiency of power plants by heating and supplying the condensed feed water to the boiler. Based on its flawless technology in design, manufacturing, installation and repair services, doosan has been supplying highly efficient and reliable feed water heaters to the global power industries.

6. DEAERATOR :

It is a system that removes uncondensed gas like dissolved oxygen and increases the temperature of feed water. Based on its flawless technology in design, manufacturing, installation and repair services, doosan has been supplying quality deaerators to the global power industries.

7. ECONOMISER :

A common application of economizers in steam power plants is to capture the waste heat from boiler flue gases and transfer it to the boiler feed water. This raises the temperature of the boiler feed water thus lowering the needed energy input, in turn reducing the firing rates to accomplish the rated boiler output. Economizers lower stack temperatures which may cause condensation of acidic combustion gases and serious equipment corrosion damage if care is not taken in their design and material selection.

8. PRIMARY AIR FAN :

Air to blow from the mill to the boiler called the primary air, is supplied by a large fan driven by a 3300 volts variable speed motor.

9. FORCED DRAUGHT FAN :

Each unit has two forced draught fans. The fans draw warm air from the top of the boiler house through large air heaters becoming the primary and secondary air used for the boiler combustion process. The air heater warms the incoming air by transferring heat energy from the outgoing flue gases.

CONCLUSION :

In India, NTPC- sipat is the first station towards the journey of super critical technology.

Efforts are on in NTPC also to go for more efficiency and eco friendly and clean technologies.

To improve the efficiency, further stress on R&D is required for high strength materials.