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1.1 Basic Rankine Cycle:
The Rankine cycle is the oldest functional heat cycle utilized by man. The
Rankine cycle is the very a basic vapor power cycle which is adopted in all the
thermal power plants. It is a four step process (Figure 1.1) which involves the
heating of the working fluid to its saturation temperature and vaporizing it
isothermally, expanding the vapor on a turbine (work cycle), condensing the
steam isothermally to the liquid phase and pumping it back to the boiler.
Figure 1.1.1 Basic Rankine Cycle
Figure 2 represents the temperature-entropy diagram for the simplest version of
the Rankine cycle. Although this simple version is rarely used it gives a very clear
and simple picture on the working of the cycle.
Process 1-2 is the pumping of the working fliud (water) into the boiler
drum. The power required is derived from the overall power developed. Process
2-3 is the heating of the water upto its saturation temperature (100°C at 1 atm
pressure for water) is reached and then isothermal heating of the water where the
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phase change from liquid to vapor occurs. Points 3 lie on the saturated vapor line.
The steam here is completely dry. Process 3-4 is the adiabatic expansion of the
vapor/steam on the turbine to obtain mechanical work. It is an isentropic process.
The temperature of the steam is reduced and it falls below the saturated vapor
line. The dryness fraction is reduced to less than one and a mixed liquid vapor
phase is present. Process 4-1 is the condensation process. This mixture is
condensed in a condenser isothermally and brought to the liquid phase back to the
FIGURE 1.1.2 Temperature vs. Entropy diagram for Rankine cycle
The steam is however, usually, superheated so as to obtain more work output.
Increasing the superheat to greater extent would lead to more work output.
However the energy spent in superheating the fuel is also high. The overall effect
is an increase in the thermal efficiency since the average temperature at which the
heat is added increases. Moisture content at the exit of the steam is decreased as
seen in the figure 1.3.
Superheating is usually limited to 620°C owing to metallurgical considerations.
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Figure1.1.3 Rankine cycle with superheating
1.2 Energy Analysis of the Rankine Cycle:
All four components in the Rankine Cycle (pump, boiler, turbine and
condenser) are steady flow devices and thus can be analyzed under steady flow
processes. K.E and P.E changes are small compared to work and heat transferred
and is thereby neglected.
Thus the steady flow equation (per unit mass) reduces to:
Q+hini = W+hfinal
Boiler and condenser do not involve any work and pump and turbine are assumed
to be isentropic. The conservation of Energy relation for each device is expressed
As the expansion is adiabatic (Q=0) and isentropic (S3=S4), then,
W3-4=Wturbine= (h3-h4) kJ/kg
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Heat rejected in the condenser, Q4-1+h4=h1+W4-1
Since W4-1=0, Q4-1=h1-h4
Work required to pump water:
Wpump=h1-h2 kJ/kg (-ve work)
Heat added in boiler:
Q2-3=h3-h2 kJ/kg=h3-h1-Wpump kJ/kg
Thus, the Rankine Efficiency=Work done/Heat added
= (h3-h4-Wp) / (h3-h1-Wp)
Neglecting feed pump work as it is very small compared to other quantities, the
efficiency reduces to:
ηrankine= (h3-h4) / (h3-h1).
1.3 Factors increasing the Rankine Efficiency:
i. Lowering the condenser pressure:
Lowering the condenser pressure would lead to the lowering of
temperature os steam. Thus for the same turbine inlet state, more work is obtained
at lower temperatures.
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This method though cannot be extensively used as it reduces the dryness
fraction x of the steam. This is highly undesirable as it decreases the turbine
efficiency is reduced due to excessive erosion of the turbine blades.
ii. Superheating the steam to high temperature:
There is an increase in the work output if superheating of steam is done. It
increases the thermal efficiency as the average temperature at which heat is added
There is also another benfit of superheating; the steam at the exit of the
turbine is drier than in case of non superheated steam.
iii. Increasing the boiler pressure:
Increasing the boiler pressure raises the average temperature at which heat
is added and thereby increases the theramal efficiency. However the dryness
fraction decreases for the same exit temperature of the boiler. This problem can be
solved by employing reheating procedure. If however the boiler pressure is raised
to supercritical point greater efficiency is obtained as the latent heat absorbed
during phase change is reduced to zero.
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2 SUPERCRITICAL RANKINE CYCYLE
2.1 Supercritical technology:
When temperature and pressure of live steam are increased beyond the
critical point of water, the properties of steam will change dramatically. The
critical point of water is at 374 °C and 221.2 bar (218 atm), Figure 2.1, and it is
defined to be the point where gaseous component cannot be liquefied by
increasing the pressure applied to it. Beyond this critical point water does not
experience a phase change to vapor, but it becomes a supercritical fluid.
Supercritical fluid is not a gas or liquid. It is best described to be an intermediate
between these two phases. It has similar solvent power as liquid, but its transport
properties are similar to gases.
Figure 2.1.1 Phase diagram of water
The Rankine cycle can be greatly improved by operating in the
supercritical region of the coolant. Most modern fossil fuel plants employ the
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supercritical Rankine Steam Cycle which pushes the thermal efficiency of the
plant (see equation 4) into the low to mid 40% range.
ηsupercritical = (h2-h1-h3+h4 )/( h2-h1) -(eqn 4)
Figure 2.2.1 T-S diagram for supercritical Rankine cycle
For water, this cycle corresponds to pressures above 221.2 bar and
temperatures above 374.15°C (647.3 K). The T-S diagram for a supercritical cycle
can be seen in Figure 6. With the use of reheat and regeneration techniques, point
3 in Figure 2.1, which corresponds to the T-S vapor state of the coolant after it has
expanded through a turbine, can be pushed to the right such that the coolant
remains in the gas phase. This simplifies the system by eliminating the need for
steam separators, dryers, and turbines specially designed for low quality steam.
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The primary concern with this cycle, at least for water, is the material
limits of the primary and support equipment. The materials in a boiler can be
exposed to temperatures above their limit, within reason, so long as the rate of
heat transfer to the coolant is sufficient to “cool” the material below its given
limit. The same holds true for the turbine materials. With the advent of modern
materials, i.e. super alloys and ceramics, not only are the physical limits of the
materials being pushed to extremes, but the systems are functioning much closer
to their limits. The current super alloys and coatings are allowing turbine inlet
temperatures of up to 700°C (973 K). the fourth generation super alloys with
ruthenium mono-crystal structures promise turbine inlet temperatures up to
1097°C (1370 K). Special alloys like Iconel 740, Haynes 230, CCA617, etc. are
The metallurgical challenges faced and solutions:
Normal Stainless steel proves of absolutely no use in building SC and USC
The high temperature and pressure in the boiler induce huge amount of stresses
and fatigue in the materials. Also chances of oxidation are very high at such high
temperature and pressure.
To resist these stress levels and oxidation different advanced materials and alloys
should be introduced.
Also they should me machinable and weldable. This is a great metallurgical
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3 DESIGN AND WORKING
3.1 Boiler Design:
The design of Super and Ultra supercritical boilers (also called as Benson
Boiler) is very critical as the working pressures of these boilers are very high. The
boiler shells, the economizer unit, super heaters, air preheaters are specially
designed. Their location is also of great significance.
i. Boiler shell:
As shown in the figure 3.1 the geometry of the boilers and the
configuration of the inlets determine the recirculation pattern inside boiler. The
intensive recirculation created in the symmetric boiler results in a more uniform
temperature field, lower temperature peaks, moderate oxygen concentration and
complete burnout of the combustible gases and char
Fig 3.1.1 Predicted Recirculation inside the combustion chamber
Table 3.1 lists the peak temperatures and burnout for designs A, B and C. the table also
lists the standard deviations of the predicted temperature and oxygen fields. The lowest
values for C indicate the higher degree of homogeneity. Thus the symmetrical boiler
seems to be the most suitable design.
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ii. Location of burners:
The number of burners in the boiler shell is also of prime importance.
Amongst all of them the downfired boilers are most suitable and advantageous.
Table 3.2 gives a clear idea.
iii. Boiler dimensions:
One of the most important advantages of HTAC applications are high heat
fluxes. Thus, compact combustion chambers can be built and the investment costs
can be lowered. The fourth calculation series was carried out in order to find the
combustion chamber dimensions which can, on one hand, ensure an efficient heat
exchange between combustion gas and water/steam mixture and on the other
hand, ensure high values of firing density. Three different sizes are tested and
they are named in as the small boiler, the medium size boiler and the large boiler
.It has been observed (see Table 3.3) that the small boiler is too short. At the top a
region of high temperatures exists and its enthalpy cannot be efficiently used. On
the contrary, in the large boiler although the heat fluxes areuniform, they are two
times lower than in the medium size boiler. Therefore, the medium size boiler
configuration is chosen for further investigations.
Small boiler Medium size boiler Large boiler
774 238 89
1805 1558 1299
Table3.1.1 Results of the boiler size determination
As already discussed, the working of Supercritical Boilers is similar to the
working of sub-critical boilers. It works on the supercritical rankine cycle. Most
supercritical boilers are being run at operating pressures above of 235 bars. The
working of ultra supercritical boilers has operating pressures above 273 bars
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4 MATERIAL SELECTION
4.1 Metallurgical Problems:
The available materials today like stainless steel which are usually used
for boiler parts are not suitable for SC and USC boilers. They do not have the
enough creep strength to resist the high pressure. Also there is high rate of
oxidation at such high temperature and pressures which are beyond the capability
of these materials to resist. Capable, qualified materials must be available to the
industry to enable development of steam generators for SC steam conditions.
Major components, such as infurnace tubing for the waterwalls, superheater/
reheater sections, headers, external piping, and other accessories require
advancements in materials technology to allow outlet steam temperature increases
to reach 760°C (1400F). Experiences with projects such as the pioneering Philo
and Eddystone supercritical plants and the problems with the stainless steel steam
piping and superheater fireside corrosion provided a valuable precautionary
lesson for SC development. Industry organizations thus recognized that a
thorough program was required to develop new and improved materials and
protection methods necessary for these high temperature steam conditions.
4.2 Materials used:
The materials used should be sustainable to the very high pressure being
developed and should not get oxidized due to the very high temperature. Different
high temperature materials are being used like 9 to 12% ferritic steels T91/P91,
T92/P92, T112/P122 steel, Advanced Austenitic alloys TP347, HFG, Super 304,
Nickel and chrome-nickel super alloys like Inconel 740.
Table 4.2 gives a very brief idea about the boiler materials used for
different parts of the boiler.
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Heat surface Tube material Header material
Economiser SA-210 C SA-106 C
Furnace Walls SA-213 T12 SA-106 C
SA-213 TP 304H
Steam Piping SA 335 P91
Table 4.2.1 Materials for different boiler parts
The materials for the other parts of the power plant (like turbine) also must be
sustainable for the super critically heated steam. The following table gives a detail idea
on the turbine materials of a plant operating on a supercritical cycle. (Table 4.3)
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Table 4.2.2 Materials for other parts
The following figures show some of the materials used for SC and USC boilers. Iconel
740 is widely used for steam pipings in almost all of them.
Figure 4.1 TP347HFG Figure 4.2 Iconel 740
Component 1,050° F 1,150 °F 1,300° F 1,400 °F
9–10% Cr (W)
9–12 % CrWCo
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5 SUPERCRITICAL BOILERS
5.1 A typical Supercritical Boilers:
Largest CFB and first supercritical CFB sold to date is the Lagisza 460 MWe
unit Ordered by Poludniowy Koncern Energetyczny SA (PKE) in Poland. The design is
Essentially complete with financial closing expected in the first quarter of 2006 at
which time Fabrication and construction will commence. The largest capacity units in
operation today are the two (2) 300 MWe JEA repowered units which were designed to
fire any Combination of petroleum coke and bituminous coals. The physically largest
Foster Wheeler boilers in operation are the 262 MWe Turow Units 4, 5, and 6 which
were designed to fire a high moisture brown coal. The design and configuration of
these units with Compact solids separators and INTREX™ heat exchangers were used
as the basis for the Lagisza design as well as for this study. The Lagisza design was
adjusted to accommodate a typical bituminous coal and the steam cycle.
Figure 5.1.1 The Lagisza 300 MWe plant in Pola
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5.2 Super and Sub Critical Boilers (comparative study):
There are many advantages of super critical boilers over normal
subcritical boilers, the prime advantage being the increased efficiency and reduced
emissions. There are many more advantages like no need of steam dryers, higher
operating pressures leading to more work output etc.
It is thus very important to have a comparative study of both the
Table 5.2.1 Comparison of sub and supercritical boilers
Technology Efficiency (%) Steam
>242 bar and 593.33°C SO2-0.408 kg/MHh
>221.2 bar and 537°C SO2-0.431 kg/MHh
165 bar 537°C SO2-0.445 kg/MHh
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6 ADVANCE IN SC TECHNOLGY AND FUTURE IN INDIA
6.2 Supercritical Boilers in India:
There haven’t been any supercritical boilers in use in India so far. The European
countries, USA, Japan have been using supercritical technology since the last two
decades. However, there are upcoming projects to build power plants working under
the supercritical technology in India.
The National Thermal Power Corporation (NTPC) had entrusted a techno
economic study to M/s EPDC for super-critical Vs Sub-critical Boilers for their
proposed Sipat STPS (4x500 MW) in Madhya Pradesh.
M/s EPDC has recommended that a first step to the introduction of super-
critical technology, the most proven steam conditions may be chosen and the most
applicable steam conditions in India shall be 246 kg/cm2
, 538° C/566° C. With these
steam parameters, M/s EPDC has estimated that the capital cost for a supercritical
power station (4x500 MW) shall be about 2% higher than that of sub-critical power
plant but at the same time the plant efficiency shall improve from 38.64% to 39.6%.
Being a pit head thermal power project, the saving in fuel charges is not justified by
increase in fixed charges.
Here are some upcoming projects in India:
North Karanpura, Jharkhand – 3x660 MW
Darlipali, Orissa – 4x800 MW
Lara, Chattisgarh – 5x800 MW
Marakanam, Tamilnadu – 4x800 MW
Tanda-II, Uttar Pradesh - 2x660 MW
Meja, Uttar Pradesh - 2x660 MW
Sholapur – 2x660 MW
New Nabinagar-3x660 MW
Many more projects including 800 MW ultra super critical units under
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The supercritical Rankine cycle, in general, offers an additional 30% relative
improvement in the thermal efficiency as compared to the same system operating in the
subcritical region. The cycle has been successfully utilized in fossil fuel plants but the
current available materials prohibit reliable application of the supercritical cycle to
nuclear applications. There is much work to be done in order to advance materials to
the point where they will be able to reliably withstand the stresses of a supercritical
environment inside a nuclear reactor for a designed life span of 60 years.
Supercritical boiler technology has matured, through advancements in design
and materials. Coal-fired supercritical units supplied around the world over the past
several years have been operating with high efficiency performance and high
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1. “Design Aspects of the Ultra-Supercritical CFB Boiler”; Stephen J.
Goidich, Song Wu, Zhen Fan; Foster Wheeler North America Corp.
2. “Novel conceptual design of a supercritical pulverized coal boiler utilizing
high temperature air combustion (HTAC) technology”; Natalia Schaffel-
Mancini, Marco Mancini, Andrzej Szlek, Roman Weber; Institute of
Energy Process Engineering and Fuel Technology, Clausthal University of
Technology, Agricolastr. 4, 38678 Clausthal-Zellerfeld, Germany; 6
3. “Supercritical (Once Through) Boiler Technology”; J.W. Smith, Babcock
& Wilcox, Barberton, Ohio, U.S.A.; May 1998.
4. “Steam Generator for Advanced Ultra-Supercritical Power Plants 700 to
760°C”; P.S. Weitzel; ASME 2011 Power Conference, Denver, Colorado,
U.S.A; July 12-14, 2011.
5. “Supercritical boiler technology for future market conditions”; Joachim
Franke and Rudolf Kral; Siemens Power Generation; Parsons Conference;
6. “Steam Turbine Design Considerations for Supercritical Cycles”; Justin
Zachary, Paul Kochis, Ram Narula; Coal Gen 2007 Conference;1-3
7. “Technology status of thermal power plants in India and opportunities in
renovation and modernization”; TERI, D S Block, India Habitat Centre,
Lodi Road, New Delhi – 110003.
8. “Applied Thermodynamics”; Dr. H.N Sawant; January 1992; revised July