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

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.

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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

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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.

ii. Superheating the steam to high temperature:

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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.

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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

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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.

2.4 Material Concerns:

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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 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.

CHAPTER 3

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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

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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A B C

Peak Temperature (K) 2618 2437 2106

Burnout % 97 99 100

Standard deviation of

temperature (K)

375 238 90

Standard deviation of

O2 concentration %

3 5 1

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

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

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medium size boiler. Therefore, the medium size boiler configuration is chosen

for further investigations.

Small boiler Medium size boiler Large boiler

Firing Density

kW/m3

774 238 89

Outlet temperature,

K

1805 1558 1299

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.

MATERIAL SELECTION

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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.

Heat surface Tube material Header material

Economiser SA-210 C SA-106 C

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Furnace Walls SA-213 T12 SA-106 C

Super

heater/Reheater

SA-213 T12

SA-213 T23

SA-213 TP 304H

SA-213

TP347HFG

SUPER 304H

SA-335 P12

SA-335 P91

SA-335 P911

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)

Component 1,050° F 1,150 °F 1,300° F 1,400 °F

Casings CrMoV (cast) 9–10% Cr (W) CF8C-Plus CCA617

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(shells, valves, steam chests, nozzles)

10CrMoVMb 12CrW (Co)

CrMoWVNbN

CCA617

Inconel 625

Nimonic 263

Inconel 740

CF8C-Plus

Bolting 422

9–12% CrMoV

Nimonic 80A

9–12% CrMoV

CrMoWVNbN

Nimonic 105

Nimonic 115

Waspaloy

Nimonic 105

Nimonic 115

U700

Rotors/Discs 1CrMoV

12CrMoVNbN

9–12 % CrWCo

12CrMoWVNbN

CCA617

Inconel 625

CCA617

Inconel 740

Nozzles/Blades

422

10CrMoVNbN

9–12% CrWCo

10CrMoVCbN

Wrought Ni-based

Wrought Ni-based

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

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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

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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.

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Technology Efficiency (%) Steam

pressure/temperature

Typical emissions

Ultra Supercritical

33–35

>242 bar and

593.33°C

SO2-0.408 kg/MHh

NOx-0.286

kg/MWh

CO2-0.96 T/MWh

Supercritical

36–40

>221.2 bar and

537°C

SO2-0.431 kg/MHh

NOx-0.304

kg/MWh

CO2-1.02 T/MWh

Subritical

42–45

165 bar 537°C SO2-0.445 kg/MHh

NOx-0.31 kg/MWh

CO2-1.02 T/MWh

Table 5.2 Comparison of sub and supercritical boilers

ADVANCE IN SC TECHNOLGY AND FUTURE IN INDIA

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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.

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

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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

CONCLUSION

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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

availability.

REFERENCES

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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 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”

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