The document provides information on supercritical Rankine cycles and supercritical boilers. Some key points: 1) A supercritical Rankine cycle operates above the critical point of the working fluid, where it behaves as a supercritical fluid with properties between a liquid and gas. This improves efficiency over subcritical cycles. 2) Supercritical boilers operate at pressures above 221.2 bar and temperatures above 374°C for water. Special high-temperature alloys are needed to withstand these conditions. 3) Boiler design considerations include symmetric shapes for uniform temperatures, downfired burners, and optimized dimensions. Materials like Inconel 740 are commonly used in supercritical boiler components.

1. INTRODUCTION
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 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 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

2. 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 pump.
FIGURE 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.
2

3. Figure1.3 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 follows:
 Steam turbine:
As the expansion is adiabatic (Q=0) and isentropic (S3=S4),
then,
W3-4=Wturbine= (h3-h4) kJ/kg
3

4.  Condenser:
Heat rejected in the condenser, Q4-1+h4=h1+W4-1
Since W4-1=0, Q4-1=h1-h4
Thus,
Q4-1=-(h4-h1) kJ/kg
 Pump:
Work required to pump water:
Wpump=h1-h2 kJ/kg (-ve work)
 Boiler:
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.
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.
4

5. 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 increases.
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.
5

6. 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 Phase diagram of water
6

7. 2.2 Efficiency:
The Rankine cycle can be greatly improved by operating in the
supercritical region of the coolant. Most modern fossil fuel plants employ the
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)
2.3 Definition:
Figure 2.2 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.
7

8. 2.4 Material Concerns:
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 used.
The metallurgical challenges faced and solutions:
 Normal Stainless steel proves of absolutely no use in building SC and USC
Boilers.
 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
challenge.
8

9. CHAPTER 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 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
9

10. lowest values for C indicate the higher degree of homogeneity. Thus the symmetrical
boiler seems to be the most suitable design.
A B C
Peak Temperature (K) 2618 2437 2106
Burnout % 97 99 100
Standard deviation of 375 238 90
temperature (K)
Standard deviation of 3 5 1
O2 concentration %
Table 3.1 Results of the boiler shape determination
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.
Upfired Downfired
Heat transfer rate, kW/m2 220 261
Outlet temperature, K 1722 1568
Table 3.2 Results of the burner location determination
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
10

11. enthalpy cannot be efficiently used. On the contrary, in the large boiler
although the heat fluxes are uniform, 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
Firing Density 774 238 89
3
kW/m
Outlet temperature, 1805 1558 1299
K
Table3.3 Results of the boiler size determination
3.2 Working:
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.
11

12. 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.
12

13. Heat surface Tube material Header material
Economiser SA-210 C SA-106 C
Furnace Walls SA-213 T12 SA-106 C
Super SA-213 T12 SA-335 P12
heater/Reheater SA-213 T23 SA-335 P91
SA-213 TP 304H SA-335 P911
SA-213
TP347HFG
SUPER 304H
Steam Piping SA 335 P91
Table 4.1 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)
13

14. Component 1,050° F 1,150 °F 1,300° F 1,400 °F
Casings CrMoV (cast) 9–10% Cr (W) CF8C-Plus CCA617
(shells, valves, 10CrMoVMb 12CrW (Co) CCA617 Inconel 740
steam chests,
nozzles) CrMoWVNbN Inconel 625 CF8C-Plus
Nimonic
263
Bolting 422 9–12% CrMoV Nimonic Nimonic
105 105
9–12% CrMoWVNbN
CrMoV Nimonic Nimonic
115 115
Nimonic 80A
Waspaloy U700
Rotors/Discs 1CrMoV 9–12 % CrWCo CCA617 CCA617
12CrMoVNbN 12CrMoWVNbN Inconel 625 Inconel 740
Nozzles/Blades 422 9–12% CrWCo Wrought Wrought
Ni-based Ni-based
10CrMoVNbN 10CrMoVCbN
Table 4.2 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
14

15. 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 The Lagisza 300 MWe plant in Poland
15

16. The following table gives a detail report on the performance of the Lagisza 800MWe
Power plant.
Parameters Values
Steam Conditions
Main Steam Pressure(barg) 315
Main steam Temperature(°C) 604
Reheat Steam temperature(°C) 615
Feed Water temperature(°C) 289
Emissions
SO2 mg/NM3 111
NOx mg/NM3 100
Power generation
Gross power MWe 805
Net power MWe 739
Net Plant Efficiency % 40.7
Table 5.1 Performance Parameters
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
boilers.
16

17. Technology Efficiency (%) Steam Typical emissions
pressure/temperature
Ultra Supercritical >242 bar and SO2-0.408
593.33°C kg/MHh
33–35 NOx-0.286
kg/MWh
CO2-0.96 T/MWh
Supercritical >221.2 bar and SO2-0.431
537°C kg/MHh
36–40 NOx-0.304
kg/MWh
CO2-1.02 T/MWh
Subritical 165 bar 537°C SO2-0.445
kg/MHh
42–45
NOx-0.31 kg/MWh
CO2-1.02 T/MWh
Table 5.2 Comparison of sub and supercritical boilers
17

18. ADVANCE IN SC TECHNOLGY AND FUTURE IN INDIA
6.1 Natural Gas Production with a Supercritical Geothermal Power Application By-
product:
In many cases, the source for Natural Gas production is geothermally heated
brine. In these cases, the brine temperatures range from 240 °F (390 K) up to 360 °F
(455 K) depending linearly upon well depth. In this temperature range, the brine
passes through a heat exchanger to feed a supercritical Rankine cycle for Propane,
which has a critical temperature of 206 °F (370 K) and a critical pressure of 616 psia.
Theoretically, the brine may be able to run the cycle directly but there are too many
contaminants and compositional variations for this to be feasible. If a power cycle like
this were employed, the sites producing Natural Gas could potentially generate power
for Grid use or, at a minimum, generate the majority of the plant’s electrical
Requirements. In the United States, there are several areas along the Texas and the
Louisiana Gulf Coast where this type of power cycle is feasible. The benefit to this
cycle is that it is extremely simple in terms of system components. The system
Requires only a single phase heat exchanger, a turbine, an air cooled condenser, and a
pump. The nominal operating pressure of this system is approximately 1000 psia,
which suggests that all of the support piping and equipment is commercially
available. Given a 15 Million BTU/hr brine source, this system could generate
approximately 400 kW net powers with a thermal efficiency of 9%. Additionally, the
system can be built to be self regulating by using the power grid as a dynamic brake
for the turbine-generator set. In effect, this acts as a speed control for the turbine
during slight variations in system demand under normal operating conditions.
Additional controls can be implemented to automate the system based on brine
temperature and flow rates, all of which minimize the need to have a full-time
operator, thus reducing operational costs.
18

19. 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
consideration
19

20. CONCLUSION
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
availability.
20

21. REFERENCES
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 February 2010.
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; 2003.
6. “Steam Turbine Design Considerations for Supercritical Cycles”;
Justin Zachary, Paul Kochis, Ram Narula; Coal Gen 2007
Conference;1-3 August 2007.
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 2004.
9. “http://en.wikipedia.org/wiki/Boiler#Supercritical_steam_generator”
21