SUPER-CRITICAL FLUID:
A substance above its critical
points at certain temperature & pressure. It do not have liquid- gas phase
boundary. There is no surface tension in supercritical fluid. Above critical point in small increase in pressure causes a large increase
in the density of the super critical phase. It
has low viscosity and high diffusion.
1.
NO Gas –Liquid Phase boundary.
2.
NO Surface Tension
3.
low
viscosities and high diffusivities
4.
capability to reduce particles up to a range of 5-2000 nm
5.
Less reaction time
Using water as the working fluid, this takes it into
supercritical conditions. Efficiencies can be raised from about 39% for
subcritical operation to about 45% using current technology
Tc (mix) = (mole fraction A) x Tc A + (mole fraction B) x Tc B
WHAT
IS SUPER-CRITICAL?
Supercritical is a thermodynamic expression
describing the state of a substance where there is no clear distinction between
the liquid and the gaseous phase, as it is a Homogenous fluid.
Water reaches supercritical state at a pressure above
22.1 (MPa).
Operating pressure approx. at 19 MPa in the evaporator
part of the boiler, the cycle is subcritical.
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 fed into the turbine.
Above pressure
of 22.1 MPa in the evaporator part of the Boiler, the cycle is supercritical. The cycle medium is a
single-phase fluid with homogeneous Properties and there is no need to separate
steam from water in a drum.in supercritical cycle
once-through boiler is
required.
For once-through boilers,
operating pressures up to 30 MPa represent the state of The art. However,
advanced steel types must be used for components such as the boiler and the
live steam and hot reheat steam piping that are in direct contact with steam
under elevated conditions. Therefore, a techno-economic evaluation is the basis
for the selection
Of the appropriate cycle parameters.
As the fluid pressure increases, Latent Heat (Enthalpy of
Steam –Enthalpy of Water) reduces.
At Critical point it becomes zero.
In physical terms at this pressure water transforms to steam
spontaneously.
SUPER
CRITICAL STEAM PARAMETERS:
STEAM PRESSURE
|
STEAM TEMPERATURE
|
> 221.2 BAR
|
> 374.15 Deg C
|
Supercritical coal fired power plants permits efficiencies that exceed
45%, depending on cooling conditions. Options to increase the efficiency above
50 % in ultra-supercritical power plants rely on elevated steam conditions as
well as on improved process and component quality.
Steam conditions up to 30 MPa/600°C/620°C are achieved using
steels with 12 % chromium content. Up to 31.5 MPa/620°C/620°C is achieved using
austenite, which is a proven, but expensive, material.
Nickel-based alloys, e.g. Inconel, would permit 35
MPa/700°C/720°C, yielding efficiencies up to 48%. Other improvements in the
steam cycle and components can yield a further 3 %.
Most of these technologies, like the double reheat concept
where the steam expanding through the steam turbine is fed back to the boiler
and reheated for a second time as well as heat extraction from flue gases have
already been demonstrated.
There are no operational limitations due to once-through
boilers compared to drum type boilers. In fact, once-through boilers are better
suited to frequent load variations than drum type boilers, since the drum is a
component with a high wall thickness, requiring controlled heating. This limits
the load change rate to 3% per minute, while once-through
boilers can
Step-up the load by
5% per minute.
This makes once-through boilers more suitable for fast startup
as well as for transient conditions.
Current designs of super-critical plants have installation costs that are only 2% higher than
those of sub-critical plants. Fuel costs are considerably lower due to the
increased efficiency & operating costs are at the same level as sub-critical
plants. Specific installation cost i.e. Cost per megawatt (MW) decreases with
increased plant size.
Reason behind need of super critical Power Plant:
1.
Demand for power is growing in fast pace with scarcity of Land,water & fuel.
2. With tariff
based bidding gaining ground, utilities have to bring down their cost of generation to remain competitive.
3. Requirement of
Environment Green and Clean Technologies.
4. CDM (Clean Development mechanism)
opportunities arising out of global warming concerns - by selling officially
sanctioned certified emission reduction (CER) credits (Kyoto Protocol).
Benefits of advanced
supercritical power plants include:
1. Reduced fuel costs
due to improved plant efficiency.
2. Significant reduction
in CO2 emissions.
3. Excellent
availability, comparable with that of an existing sub-critical plant.
4. Plant costs
comparable with sub-critical technology and less than other clean coal technologies.
5. Reduced NOx, SOx and dust emissions.
6.
Integrated with appropriate CO2 capture
technology.
SUB-CRITICAL
STEAM GENERATOR SYSTEMS
Water when heated at sub-critical pressure (< 22.1 MPa)
increases in temperature until it starts to boil. While the water is boiling it
exists as two phases, liquid and gas that have different mass densities, and
remains at a constant temp. Known as the saturation temperature for the given pressure.
Once all of the liquid has boiled off to steam (Evaporated) the temperature of
the steam will continue to rise, at constant pressure, and is then referred to
as superheated steam.
Sub-critical boilers typically have a means of separating
the two phases, liquid and steam, to allow the process to be continuous. The
separated liquid is recirculated through the evaporating section of the boiler
and steam passes through to the superheating section.
This separation typically occurs in the boiler drum, a heavy
thick walled steel pressure vessel with a series of cyclones and baffles to
separate liquid from steam.
It is the mass of this boiler drum which limits the rate at
which a sub-critical boiler can be brought on line and how well it responds to
load changes which results in fuel being consumed for no energy compared with a
more responsive boiler.
SUPER-CRITICAL
STEAM GENERATOR SYSTEMS
When water is heated at a constant pressure above the
critical pressure its temperature is Never constant and no distinction between
gas and liquid can be made, the mass density of the two phases is the same.
Properties of the water in the super-critical boiler continuously
Change from liquid to gas (steam).
i.e. Temperature rises steadily, Specific heat and rate of
rise changes considerably
Liquid in the super-critical boiler is assumed to have
changed to steam after the critical temperature for the super-critical
pressure, as the steam is heated further it continues to gain temperature in a
super heated state.
With the super-critical boiler there is no stage where the
water exists as two phases and requires separation, so the boiler is
constructed without a drum. Typically super-critical boilers are once through
boilers where water pumped in at pressure by the Boiler Feed Pump passes
progressively through the heating stages of the boiler and is delivered to the Turbine
at final temperature with no re-circulation.
The actual location of the transition from liquid to steam
in a once through super-critical Boiler is free to move with differing
conditions. This means that for changing boiler loads & pressures the
process is able to optimize the amounts of liquid and gas regions for efficient
heat transfer keeping the high boiler efficiency over a wider range than
sub-critical
Boilers
with drums.
TYPICAL BLOCK DIAGRAM FOR SUB-CRITICAL
TURBINE CYCLE
TYPICAL BLOCK DIAGRAM FOR SUPER
CRITICAL TURBINE CYCLE
