Supercritical Power Plants
Why a supercritical power plant?
- Supercritical Power plants operate at temperatures resulting in higher efficiencies
– up to 46 percent for supercritical plants – and lower emissions than traditional
(subcritical) coal-fired plants. The "efficiency" of the thermodynamic
process of a coal-fired power describes how much of the energy that is fed into
the cycle is converted into electrical energy. The greater the output of electrical
energy for a given amount of energy input, the higher the efficiency.
- A supercritical power plant uses a boiler/turbine system that operates at 1075
degrees F; subcritical plants operate at 850 degrees F. A supercritical plant is
much more efficient than a subcritical plant, producing more power from the less
coal and with lower emissions.
- Benefits of advanced supercritical power plants include:
- Reduced fuel costs due to improved plant efficiency.
- Significant reduction in CO2 emissions.
- Excellent availability, comparable with that of an existing sub-critical plant.
- Plant costs comparable with sub-critical technology and less than other clean
coal technologies.
- Much reduced NOx, SOx and particulate emissions.
- Compatible with biomass co-firing.
- Can be fully integrated with appropriate CO2 capture technology.
- In summary, highly efficient plants with best available pollution control technology
will reduce existing pollution levels by burning less coal per megawatt-hour produced,
capturing the vast majority of the pollutants, while allowing additional capacity
to be added in a timely manner.
- Today’s state-of-the-art, supercritical coal-fired power plants provide efficiencies
that exceed 45 percent. This benefit will significantly increase the kWh produced
per pound of coal burned, with fewer emissions. In addition to using less coal,
lower emission levels for supercritical plants are achieved using well-proven emissions
control technologies:
- NOx emissions: Nitrogen oxide emissions are reduced using a combination of low
NOx burners and selective catalytic reduction technology.
- SOx and SO2 emissions: Sulfur oxide and sulfur dioxide are captured
using wet limestone-gypsum flue gas desulphurization (FGD). The product, gypsum,
can be recycled for use in products such as wallboard, plaster and fertilizer.
- Particulate emissions: More than 99% of particulate dust is removed via an electrostatic
precipitator (ESP).
Typical Supercritical Power Plants
The proposed 850-MW advanced supercritical pulverized clean coal plant (SCPC) will
use the latest and most advanced technology to improve operating efficiency and
control emissions.
- Advanced SCPC technology is proven
- More than 400 SCPC plants are operating successfully worldwide, including 25 with
the advanced SCPC technology Power4Georgians plans to use
- Most new coal plants planned or currently under construction in the United States
use SCPC technology
SCPC systems operate at higher temperatures and greater steam pressures than conventional
systems. They require less coal per megawatt-hour, leading to lower emissions per
megawatt (including carbon dioxide and mercury), and lower fuel costs per megawatt,
leading to higher efficiency and lower fuel costs.
In short, SCPC provides the best overall balance in performance, reliability, lower
emissions and cost for the company’s customers.
High Performance Coal-Fired Plants Are Cleaner
Many regions of the US are experiencing fast growing electricity demand. Permitted
emissions from power plants have been reduced to meet air quality standards. About
50 percent of the electricity generated in the US comes from coal. Coal is an abundant
fuel resource in the US and forecasts show that it is likely to remain a dominant
fuel for electricity generation for many years to come.
Power plant suppliers have invested heavily in generation technologies that produce
power more efficiently. Enhanced plant reduces emissions of CO2 and all
other pollutants by using less fuel per unit of electricity generated. Modern subcritical
cycles have attained efficiencies close to 40% lower heating value (LHV). Further
improvement in efficiency can be achieved by using supercritical steam conditions.
Current supercritical coal fired power plants have efficiencies above 45% (LHV).
A one percent increase in efficiency reduces by two percent, specific emissions
such as CO2, NOx, SOx and particulates (See
Figure 1.
What is Supercritical?
There is nothing "critical" about supercritical. "Supercritical"
is a thermodynamic expression describing the state of a substance where there is
no clear distinction between the liquid and the gaseous phase (i.e. they are a homogenous
fluid). Water reaches this state at a pressure above 22.1 megapascals (MPa). (See
Figure 2).
The "efficiency" of the thermodynamic process of a coal-fired power describes
how much of the energy that is fed into the cycle is converted into electrical energy.
The greater the output of electrical energy for a given amount of energy input,
the higher the efficiency. If the energy input to the cycle is kept constant, the
output can be increased by selecting elevated pressures and temperatures for the
water-steam cycle.
Up to an operating pressure of around 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 led into the turbine. Above an operating
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. Once-through boilers are therefore
used in supercritical cycles.
Advanced Steels
Currently, 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. Figure
3 depicts a supercritical cycle arrangement with steam parameters that
yield high efficiency while allowing the use of well-proven materials.
Steam Conditions
Today’s state of the art in 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%. Manufacturers and operators
are cooperating in publicly sponsored R&D projects with the aim of constructing
a demonstration power plant of this type.
Other improvements in the steam cycle and components can yield a further 3 percentage
points rise in efficiency. 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. However, these technologies are not in widespread use due to
their cost.
Turbine Generator Set
There are several turbine designs available for use in supercritical power plants.
These designs need not fundamentally differ from designs used in subcritical power
plants. However, due to the fact that the steam pressure and temperature are more
elevated in supercritical plants, the wall-thickness and the materials selected
for the high-pressure turbine section need reconsideration. Furthermore, the design
of the turbine generator set must allow flexibility in operation. While subcritical
power plants using drum-type boilers are limited in their load change rate due to
the boiler drum (a component requiring a very high wall thickness), supercritical
power plants using once-through boilers can achieve quick load changes when the
turbine is of suitable design.
High-Pressure Turbine (HPT)
In this section, the steam is expanded from the live steam pressure to the pressure
of the reheat system, which is usually in the order of 4 to 6 MPa. In order to cater
for the higher steam parameters in supercritical cycles, materials with elevated
chromium content, which yield higher material strength, are selected. The wall thickness
of the HP turbine section should be as low as possible and should avoid massive
material accumulation (e.g. of oxides) in order to increase the thermal flexibility
and fast load changes.
Intermediate-Pressure Turbine (IPT)
The steam flow is further expanded in the IP turbine section. In supercritical cycles,
there is a trend to increase the temperature of the reheat steam that enters the
IP turbine section in order to raise the cycle efficiency. As long as the reheat
temperature is kept at a moderate level (approximately 560°C), there is no significant
difference between the IP turbine section of a supercritical plant and that of a
subcritical plant.
Low-Pressure Turbine (LPT)
In the LP turbine section the steam is expanded down to the condenser pressure.
The LP turbine sections in supercritical plants are not different from those in
subcritical plants.
Once-Through Boiler
Apart from the turbine generator set, the boiler is a key component in modern, coal-fired
power plants. Its concept, design and integration into the overall plant considerably
influence costs, operating behavior and availability of the power plant.
Once-through boilers have been favored in many countries, for more than 30 years.
They can be used up to a pressure of more than 30 MPa without any change in the
process engineering. Wall thicknesses of the tubes and headers are designed to match
the planned pressure level. At the same time, the drum of the drum-type boiler,
which is very heavy and located on the top of the boiler, can be eliminated. Since
once-through boilers can be operated at any steam pressure, variable pressure operation
was introduced into power plants at the start of the 1930s to make the operation
of plants easier.
Once-through boilers have been designed in both two-pass and tower type design,
depending on the fuel requirements and the manufacturers general practice. For the
past 30 years, large once-through boilers have been built with a spiral shaped arrangement
of the tubes in the evaporator zone. The latest designs of once-through boilers
use a vertical tube arrangement.
Other Cycle Components
A comparison of the water-steam cycle equipment in subcritical and supercritical
coal fired power plants shows that the differences are limited to a relatively small
number of components i.e. to the feed water pumps and the equipment in the high
pressure feed water train i.e. downstream of the feed water pumps. These components
represent less than 6% of the total value of a coal-fired power plant.
High Efficiency and More Reliability
Operational Issues
More than 400 supercritical power plants are operating in the US, in Europe, Russia
and in Japan. Availability of supercritical plants is equal or even higher than
those of comparable subcritical plants.
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.
Fuel Flexibility is not Compromised in Once-Through Boilers
All the various types of firing systems (front, opposed, tangential, corner, four
wall, arch firing with slag tap or dry ash removal, fluidized bed) used to fire
a wide variety of fuels have already been implemented for once-through boilers.
All types of coal as well as oil and gas have been used. The pressure in the feed
water system does not have any influence on the slagging behavior as long as steam
temperatures are kept at a similar level to that of conventional drum type boilers.
Life Cycle Costs of Supercritical Coal Fired Power Plants
Current designs of supercritical plants have installation costs that are only 2%
higher than those of subcritical plants. Fuel costs are considerably lower due to
the increased efficiency and operating costs are at the same level as subcritical
plants. Specific installation cost i.e. the cost per megawatt (MW) decreases with
increased plant size.
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