Now Days Coal Filled power plants operate at very high pressures greater
than the Critical pressure of steam. Here We Will learn how Super Critical
power plants are different from the normal power plants?:)
Supercritical power plants were in service from the
late fifties. But the technology did not really take off due to problems of
reliability especially from the metallurgical aspect.
The single most important factor that determines the use of
higher and higher pressure and temperatures are the availability of materials
to withstand these conditions. Increases in operating pressure and temperatures
have to go hand in hand with developments in metallurgy.
With more than 600
units in service the reliability issue seems to be resolved. Supercritical
units are the standard for future power plant in many countries including
China.
EFFICIENCY OF SUCH TYPES OF POWER PLANTS
The main advantage
and the reason for a higher pressure operation is the increase in the
thermodynamic efficiency of the Rankine cycle.:)
Large Subcritical
thermal power plants with 170 bar and 540 / 540 ° C (SH / RH) operate at an
efficiency of 38 %. Supercritical units operating at 250 bar and 600/615 ° C
can have efficiencies in the range of 42 %.
Ultra supercritical
units at 300 bar and 615 / 630 °C will still increase the efficiency up to 44
%.
Operational flexibility Of Such Types of power plants
Most of the Supercritical units use the once through technology.
This is ideal for sliding pressure operation which has much more flexibility in
load changes and controlling the power grid.:)
However this also requires more sensitive and quick responding
control systems.
Evaporation end point of these plants
In subcritical units
the drum acts as a fixed evaporation end point. The furnace water walls act as
the evaporator.
Not so in the case of a supercritical unit. The evaporation end
point can occur in various levels of the furnace depending on the boiler load
The percentage of
Superheat in supercritical units is higher than subcritical units.
Because of this the
furnace tubes act more as superheaters than waterwalls.
Heat transfer area
Higher steam temperatures in supercritical units results in a
lesser differential temperature for heat transfer. Because of this heat
transfer areas required are higher than subcritical units.
WATER CHEMISTRY:
In supercritical units the water entering the boiler has to be
of extremely high levels of purity.
Now Days Coal Filled power plants operate at very high pressures greater
than the Critical pressure of steam. Here We Will learn how Super Critical
power plants are different from the normal power plants?:)
Supercritical power plants were in service from the
late fifties. But the technology did not really take off due to problems of
reliability especially from the metallurgical aspect.
The single most important factor that determines the use of
higher and higher pressure and temperatures are the availability of materials
to withstand these conditions. Increases in operating pressure and temperatures
have to go hand in hand with developments in metallurgy.
With more than 600
units in service the reliability issue seems to be resolved. Supercritical
units are the standard for future power plant in many countries including
China.
What are the key
differences between the subcritical units and the Supercritical unit
EFFICIENCY OF SUCH TYPES OF POWER PLANTS
The main advantage
and the reason for a higher pressure operation is the increase in the
thermodynamic efficiency of the Rankine cycle.:)
Large Subcritical
thermal power plants with 170 bar and 540 / 540 ° C (SH / RH) operate at an
efficiency of 38 %. Supercritical units operating at 250 bar and 600/615 ° C
can have efficiencies in the range of 42 %.
Ultra supercritical
units at 300 bar and 615 / 630 °C will still increase the efficiency up to 44
%.
Increase in
efficiency directly lead to reductions in unit cost of power and CO2 emissions.:
Operational flexibility Of Such Types of power plants
Most of the Supercritical units use the once through technology.
This is ideal for sliding pressure operation which has much more flexibility in
load changes and controlling the power grid.:)
However this also requires more sensitive and quick responding
control systems.
Evaporation end point of these plants
In subcritical units
the drum acts as a fixed evaporation end point. The furnace water walls act as
the evaporator.
Not so in the case of a supercritical unit. The evaporation end
point can occur in various levels of the furnace depending on the boiler load
The percentage of
Superheat in supercritical units is higher than subcritical units.
Because of this the
furnace tubes act more as superheaters than waterwalls.
This necessitates the use of higher grade of materials like alloy
steels in the furnace.
Heat transfer area
Higher steam temperatures in supercritical units results in a
lesser differential temperature for heat transfer. Because of this heat
transfer areas required are higher than subcritical units.
Higher Superheat steam temperatures entering the HP turbine also
mean higher reheater inlet temperatures which again results in a higher heat
transfer areas.
WATER CHEMISTRY:
In supercritical units the water entering the boiler has to be
of extremely high levels of purity.
Supercritical boilers do not have a steam drum that separates
the steam and the water.:)
If the entering water quality is not good, carry over of
impurities can result in turbine blade deposits.
Materials:-
Super critical power plants use special high grade materials for
the boiler tubes.
The turbine blades are also of improved design and materials.
In fact, the very increase in higher pressure and temperatue
designs are dependendent on the development of newer and newer alloys and tube
materials.
The main advantage is of
industries to achieved efficiency about 50%
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