Boilers in Thermal Power Stations

The booklet describes the boilers used in Thermal Power Station specially 210 and 250 MU units. This booklet is for the students of 1 year PG course in Power Plant Engineering.

1. totaloutput power solutionsManohar Tatwawadi Page1
BOILERS IN THERMAL POWER STATIONS
INDEX
S.N.TOPICS PAGE NOS
1BOILER TECHNOLOGIES02 14
2TYPES OF BOILERS IN BIG
THERMAL POWER STATIONS
15 20
3GENERAL DESCRIPTIONOF
STEAM GENERATOR250 MW
21 23

2. totaloutput power solutionsManohar Tatwawadi Page2
BOILERTECHNOLOGIES
Existing and emerging trends
Boiler technology the world over has evolved vastly over the years. From the conventional
pulverized coal boilers to fluidised bed combustion technology and multi-fuel firing boilers, the
industry has indeed come a long way.
This write-up describes the available and emerging technology options, their benefits and
limitations.
CURRENT TECHNOLOGIES
Pulverised fuel boilers
Pulverised fuel boiler is the most commonly used method in thermal power plants, and is based
on many decades of experience. Units operate at close to atmospheric pressure, simplifying the
passage of materials through the plant.
Most coal-fired power station boilers use pulverised coal, and many of the larger industrial
watertube boilers also use this fuel. This technology is well developed, and there are thousands
of units around the world, accounting for well over 90 per cent of coal-fired capacity.
Thecoal is ground (pulverized) to a fine powder so that less than 2 per cent is +300 micro metre
(?m) and 70-75 per cent is below 75 microns, for bituminous coal. The pulverized coal is blown
with part of the combustion air into the boiler plant through a series of burner nozzles.
Secondary and tertiary air may also be added. Combustion takes place at temperatures from
1,300 to 1,700
o
C, depending largely on coal grade. Particle residence time in the boiler is
typically two to five seconds, and the particlesmust be small enough for complete combustion
to have taken place during this time.
This system has many advantages such as the ability to fire varying qualities of coal, quick
responses to changes in load, use of high preheat air temperatures, etc. Pulverised coal boilers
have been built to match steam turbines, which have outputs of between 50 and 1,300 Mwe. In
order to take advantage of the economies of scale, most new units are rated at over 300 Mwe,
but there are relatively few really large ones with outputs from a single boiler-turbine

3. totaloutput power solutionsManohar Tatwawadi Page3
combination of over 700 Mwe. This is because of the substantial effects such units have on the
distribution system if they should trip out for any reason, or be unexpectedly shut down.
Fluidised bed combustion
Fluidized bed combustion has emerged as a viable alternative and has significant advantages
over the conventional firing system and offers multiple benefits. Some of the benefits are
compact boiler design, fuel flexibility, higher combustion efficiency and reduced emission of
noxious pollutants such as SO
x
and NO
x
. The fuels burnt in these boilers include coal, washery
rejects, rice husk, bagasse and other agricultural waste. Fluidised bed boilers have a wide
capacity range from 0.5 T per hour to over 100 T per hour.
There are three basic types of fluidized bedcombustion boilers:
" Atmospheric classic fluidized bed combustion system (AFBC).
" Atmospheric circulating (fast) fluidized bed combustion system (CFBC)
" Pressurised fluidized bed combustion system (PFBC).
AFBC/ Bubbling bed
In AFBC, coal is crushedto a size of 1-10 mm depending on the rank of coal, and type of fuel fed
into the combustion chamber. The atmospheric air, which acts as both the fluidisation air and
combustion air, is delivered at a pressure and flows through the bed after being preheated by
the exhaust flue gases. The velocity of fluidizing air is in the range of 1.2 to 3.7 m per second.
The rate at which air is blown through the bed determines the amount of fuel that can be
reacted.
Almost all AFBC/ bubbling bed boilers use in-bed evaporator tubes in the bed of limestone,
sand and fuel for extracting the heat from the bed to maintain the bed temperature. The bed
depth is usually 0.9 m to 1.5 m and the pressure drop averages about 1 inch of water per inch
of bed depth. Very little material leaves the bubbling bed-only about 2 to 4 kg of solids are
recycled per kg of fuel burnt.
The combustion gases pass over the superheater sections of the boiler, flow past the
economiser, the dust collectors and the air preheaters before being exhausted to the
atmosphere.

4. totaloutput power solutionsManohar Tatwawadi Page4
The main feature of atmospheric fluidized bed combustion is the constraint imposed by the
relatively narrow temperature range within which the bed must be operated. With coal, there
is a risk of clinker formation in the bed if the temperature exceeds 950
o
C and loss of
combustion efficiency if the temperature falls below 800
o
C. For efficient sulphur retention, the
temperature should be in the range of 800 850
o
C.
Features of bubbling bed boilers
Fluidised bed boilers can operate at near-atmospheric or elevated pressure and have these
essential features:
" Distribution plate through which air is blown for fluidising,
" Immersed steam-raising or water heating tubes which extract heat directly from the bed,
" Tubes above the bed, which extract heat from hot combustion gas before it enters the
flue duct.
Circulating fluidized bed combustion
CFBC technology has evolved from conventional bubbling bed combustion as a means to
overcome some of the drawbacks associated with conventional bubbling bed combustion.
CFBC technology utilizes the fluidised bed principle in which crushed (6-12 mm size) fuel and
limestone are injected into the furnace or combustor. The particles are suspended in a stream
of upwardly flowing air (60-70 per cent of the total air), which enters the bottom of the furnace
through air distribution nozzles. The fluidizing velocity in circulating beds ranges from 3.7 to 9
m per second. The balance of the combustion air is admitted above the bottom of the furnace
as secondary air. The combustion takes place at 840-900
o
C, and the fine particles (<450
microns) are elutriated out of the furnace with flue gas velocity of 4-6 m per second. The
particles are then collected by the solid separators and circulated back into the furnace. Solid
recycle is about 50 to 100 kg per kg of fuel burnt.
There are no steam generation tubes immersed inthe bed. The circulating bed is designed to
move a lot more solids out of the furnace area and to achieve most of the heat transfer outside
the combustion zone convection section, water walls, and at the exit of the riser. Some
circulating bed units even have external heat exchanges.
The particle circulation provides efficient heat transfer to the furnace walls and longer
residence time for carbon and limestone utilisation. Similar to pulverized coal (PC) firing, the
controlling parameters in the CFBC process are temperature, residence time and turbulence.

5. totaloutput power solutionsManohar Tatwawadi Page5
For large units, the taller furnace characteristics of CFBC boilers offer better space sorbent
residence time for efficient combustion and SO
2
capture, and easier application of staged
combustion techniques for NO
x
control than AFBC generators. CFBC boilers are said to achieve
better calcium to sulphur utilisation 1.5 to 1 versus 3.2 to 1 for the AFBC boilers, although the
furnace temperatures are almost the same.
CFBC boilers are generally claimed tobe more economical than AFBC boilers for industrial
applications requiring more than 75-100 T per hour of steam. CFBC requires huge mechanical
cyclones to capture and recycle the large amount of bed material, which required a tall boiler.
At right fluidizing gas velocities, a fast recycling bed of fine material is superimposed on a
bubbling bed of larger particles. The combustion temperature is controlled by the rate of
recycling of fine material. Hot fine material is separated from the flue gas by a cyclone and is
partially cooled in a separate low velocity fludised bed heat exchanger, where the heat is given
up to the steam. The cooler fine material is then recycled to the dense bed.
At elevated pressure, the potential reduction in boiler size is considerable due to the increased
amount of combustion in pressurised mode and high heat flux through in-bed tubes.
A CFBC boiler could be a good choice if the following conditions are met:
" Capacity of boiler is large to medium,
" Sulphur emission and NO
x
control is important,
" The boiler is required to fire low-grade fuel or fuel with highly fluctuating fuel quality.
Pressurised fluid bed combustion
PFBC is a variation of fluid bed technology that is meant for large-scale coal burning
applications. In PFBC, the bed vessel is operated at pressure up to 16 ata (16 kg per cm
2
).
The off-gas from the fluidized bed combustor drives the gas turbine. The steam turbine is
driven by steam raised in tubes immersed in the fluidized bed. The condensate from the steam
turbine is preheated using waste heat from gas turbine exhaust and is then taken as feedwater
for stem generation.
The PFBC system can be used for cogeneration or combined cycle power generation. By
combining the gas and steam turbines in this way, electricity is generated more efficiently than
in the conventional system. The overall conversion efficiency is higher by 5 to 8 per cent.
EMERGING BOILER TECHNOLOGY

6. totaloutput power solutionsManohar Tatwawadi Page6
Up till the 1970s, certain high-grade fuels like oil and better quality coals were utilisedin boilers
for power generation purposes. But with growing awareness of sustainable use of energy,
extensive utilisation of better quality fuels has become a cause for concern.
To conserve fossil sources for purposes other than burning and steam generation, boilers with
better fuel utilisation technology are required in addition to adopting boilers with multi-fuel
burning capability. In this regard, there has been increased interest in areas like supercritical
boiler technology, integrated coal gasification combined cycle and advance PFBC technology, to
name a few. Boilers now also burn a variety of waste fuels such as biomass and bagasse for
cogeneration purposes.
Advanced-PFBC (A-PFBC) system
The A-PFBC (series type) technology, developed in Japan, makes use of the advantageous
conditions of the raised GT temperatures and improved steam conditions while mitigating
developmental loads (there is no need to develop a topping combustor).
In the A-PFBC system, the gas produced in the partial gasifier(syngas) is fed to a high
temperature dry desulphuriser where syngas is desupphurised by using limestone, and then is
cooled by a syngas cooler (SGC). The desulphurisation of the gas prior to cooling makes SGC
atmosphere slow corrosive and enables more sensible heat of the gas to be recovered as high
temperature steam. The cooled gas (450
o
C) is subjected to strict dust removal with a cyclone,
ceramic filter and is then fed to the combustor of the gas turbine to generate power.
The oxidiser plays a role notonly in the combustion of unburnt carbon (char) transferred from
the partial gasifier but also in oxidizing CaS formed in the desulphuriser into gypsum (CaSO
4
).
The high temperature flue gas from the oxidizer is introduced into the partial gasifier; thusthe
heat energy (sensible heat) of the flue gas is effectively used as a heat source for the partial
gasifier.
Integrated coal gasification combined cycle
Integrated coal gasification combined cycle (IGCC) is a new coal-utilised power generation
technology that achieves higher thermal efficiency and better environmental performance for
the next generation.
In Japan, the development of original air-blown IGCC technology has been pushed forward as a
national project.
Like PFBC, the technology is relatively new in connection with power generation. It was only in
the late 1990s that coal-based IGCC plants for power generation started gaining acceptance.

7. totaloutput power solutionsManohar Tatwawadi Page7
IGCC uses a combined cycle format with a gas turbine driven by the combusted syngas, while
the exhaust gases are heat exchanged with water/ steam to generate superheated steam to
drive a steam turbine. Using IGCC, more of the power comes from the gas turbine. Typically 60
70 per cent of the power comes from the gas turbine with IGCC, compared with about 20per
cent using PFBC.
Coal gasification takes place in the presence of a controlled shortage of air/oxygen, thus
maintaining reducing conditions. The process is carried out in an enclosed pressurised reactor,
and the product is a mixture of CO and H
2
(called synthesis gas, syngas or fuel gas). The product
gas is cleaned and then burnt with either oxygen or air, generating combustion products at high
temperature and pressure. The sulphur present mainly forms H
2
S but there is also a little COS.
The H
2
S canbe more readily removed than SO
2
. Although no NO
x
is formed during gasification,
some is formed when the fuel gas or syngas is subsequently burnt.
The IGCC demonstration plants use different flow sheets, and therefore test the practicalities
and economicsof different degrees of integration. As with PFBC, the driving force behind the
development is to achieve high thermal efficiencies together with low levels of emissions.
Supercritical boiler
The earliest supercritical boilers were built in the US in the late 1950s and early 1960s.
Designed to operate above steam s critical pressure of 3208 psi, these early units developed a
reputation for high thermodynamic efficiency (around 35 per cent, based on lower heating
value-LHV) but low reliability.
The materials of that era, plant owners came to realise, were simply not up to the temperature
and pressure challenges, and the North American industry put supercritical technology on the
backburner . The ascent of gas-fired combined cycles continued to suppress interest in the
technology in this region.
In Europe and Asia, however, supercritical technology continued to be pursued, and by the
1990s it had come to dominate new capacity projects. The capital cost of supercritical
technology is slightly higher than subcritical, but fuel savings and environmental advantages
can tip the scale.
Compared to the 1950s designs, steam pressures in most of these units have increased well
into the supercritical range up to 4,500 psig-although steam temperatures were maintained
around the same 1,000
o
F limit. The result was a thermal efficiency of approximately 40 per
cent (LHV).

8. totaloutput power solutionsManohar Tatwawadi Page8
More advanced designs introduced in the late 1990s have raised steam temperature as high as
1,150
o
F, achieving efficiencies of 44 per cent. And main steam conditions above 1,200
o
F are
foreseen, which should yield an efficiency approaching 50 per cent. The increase in efficiency
not only reduced fuels cost, but also specific (per MW) emissions of NO
x
and SO
2
, as well overall
emission of CO
2
, compared to sub critical coal-fired boilers.
All supercritical boilers are of a once through arrangement, meaning that water and steam flow
through the boiler circuitry only once. Contrast this with drum boilers, in which water and
steam recirculate through the furnace enclosure. The major difference between the various
once-through boiler technologies in the market is the configuration of the furnace enclosure
circuits and in the system used to circulate the water through those circuits during start-upand
at lower loads.
Reference Book:
Power Line
Volume 8, No. 3,
December 2003

9. totaloutput power solutionsManohar Tatwawadi Page9

10. totaloutput power solutionsManohar Tatwawadi Page10
Subcritical 500 MW Singrauli NTPC

11. totaloutput power solutionsManohar Tatwawadi Page11

12. totaloutput power solutionsManohar Tatwawadi Page12
Once through 500 MW Boiler,Talcher

13. totaloutput power solutionsManohar Tatwawadi Page13

14. totaloutput power solutionsManohar Tatwawadi Page14
Fludised Bed Combustion Boiler

15. totaloutput power solutionsManohar Tatwawadi Page15
TYPES OF BOILERS IN BIG THERMAL POWER STATIONS
HISTORY OF BOILERS:
Boiler means any closed vessel exceeding 22.75 liters in capacity used for steam generation
under pressure. The first Boiler was developed in 1725 & it s working pressure was 6 to 10
kg/cm
2
and was called Wagon Boiler.
TYPES OF BOILERS:There are two types of Boilers:-
1)Fire tube boilers (Carnish & Lauchashire blrs.) developed in the year 1844
2)Water tube boilers developed in the year 1873 .
Water tube Boilers are used in Thermal power stations. These are sub divided according to
watercirculation
1)Natural circulation: Drum to down comers to ring main header to water wall tubes
& back to drum. Due to difference in density of water and steam this type of circulation
takes place.
2)Forced circulation: As operating pressure of theboiler approaches to the critical
pressure, additional pumps are required to install in down comers, because at this
pressure there is no appreciable density difference between water and steam to have a
natural circulation of water.
According to working pressure the Boiler, Boilers are classified as:
1)Drum type sub critical pressure boiler: When working pressure of the boiler is between
130 kg/cm2 and 180 kg/cm2, the boiler is called as, Drum type sub critical pressure
boiler .
2)Critical pressure Boilers :When boiler working pressure is 221.2 kg/cm2, it is termed as,
Critical pressure Boilers .
3)Super critical or drum less once through boilers: When boiler working pressure is 240
kg/cm2, it is called as, Super critical .
All modern Boilers are top slung from steel structures. From the beams a series of slings
take up the boiler loads. Approximately suspended weight of one 210 MW boiler is 3640

16. totaloutput power solutionsManohar Tatwawadi Page16
metric tones. Height of Boiler is about 64 meters and Boiler drum is at a height of 52 meters
from the ground.
Boiler design consideration: Following factors are taken into consideration for designing
the modern boiler.
1)Lowest capital cost, ease of construction, simplicity, safety, good working condition,
ease of maintenance.
2) Efficient operation, effective baffling for heat transfer, well insulated casings, ability to
deliver pure steam with effective drum internals to generate steam of fuall capacity.
3) Availability of auxiliaries.
The main parts of Boilers are:
1)Boiler drum Pressure Part
2)Down comers PressurePart
3)Water walls Pressure Part
4)Furnace
5)Platen superheaterPressure Part
6)Reheater Pressure Part
7)Final superheaterPressure Part
8)Primary superheaterPressure Part
9)Economizer Pressure Part
10)Air Pre HeatersRotary / Tubular
11)Burners FO / LDO
12)Ignitors Gas / LDO
1)Boiler drum:
Size:Length : 15.7 meters, ID: 1976 mm, Thickness 132 mm.

17. totaloutput power solutionsManohar Tatwawadi Page17
Location & Purpose: The steam drum is located at the top of the boiler to provide an upper
reservoir for the water covering the generating tube bank. Water is distributed from the
steam drum to the lower headers by pipes called downcomers. Generated steam is also
collectedand is separated from the water in the steam drum. Boilers are also equipped
with safety valves to relieve excessive pressure. The valves are located on the steam drum
and superheater outlet. They are designed to relieve sufficient pressure to safely steamthe
boiler at 120% with boiler steam stop valves closed.
Air vents or air cocks are installed on top of the steam drum to expel air from inside the
steam drum during cold boiler light-off or when filling the boiler. The air vents or air cocks
are shut when the boiler starts generating steam.
The drum is made of special carbon steel plates of SA299 A grade A-1 by fusion welding
(submerged arc welding). Two gauge glasses are provided for level indication. Three safety
valves are provided. Drum vents, chemical dosing live. Emergency blow down line are
provided.
Inside the drum there is a position called separating chamber through which steam enters
from riser tubes and goes through primary separators called turbo separators. Turbo
separators have spinningblades, moisture is separated here and the steam further goes
through secondary separator and finally through drying screens. The drying screens are
located in the upper part of the drum. Water level is maintained 254 mm below the
geometrical centerline ofthe drum, upper part is left to occupy the generated steam.
2)Down comers:Made of SA106 Gr. C material
There are 6 down comers from boiler drum of size 406x32 mm and are joined to ring
main header to provide water to water wall tubes. There are two downcomers of size
323.9x24.4 mm joined to platen water wall header. Platen water wall headers are not
provided to every boiler.

18. totaloutput power solutionsManohar Tatwawadi Page18
3)Water Walls:Made of SA 210 Gr. A1 material, 63.5x6.3 mm, 76.1mm.
The water wall tubes forms membrane panels. The each membrane panel is of 22 tubes
joined by fins welding and having length of 60 to 70 feet each and width of panel is
about 7 feet wide and there are 83 such panels. After getting heated water goes
through these tubes by natural circulation to the drum. The latest designof furnace
walls are fully cooled on all sides by bare tubes. Refractory cover on blocked tube walls
are being abandoned.
Natural Circulation: The water in the flows to the bottom ring headers and receives
heat in the Riser Tubes/ Water Walls. This heatedwater goes back to the boiler drum
alongwith steam. This is achieved because of Density difference. This is known as
Natural Circulation.
Forced circulation: when the pressure developed because of density difference is not
sufficient for the flow in the riser tubes, the same is achieved by means of Hot Water
Pumps. This is known as forced circulation.
4)Furnace Size: Typical-13.868-m width, 10.592-m depth, and 5494m3 volume.
The tall rectangular radiant type furnace has now become a feature of the modern
design of pulverised fuel boiler. The height of modern boiler is increased to lower gas
temperature and reduce slagging. The furnace is of two passes. The 1
st
pass comprises
of main furnace, enclosed by four walls of membrane panels 7feet wide & 60 to 70 feet
in lengths. The firing equipment such as burners, oil guns, igniters are mounted in the
first pass of the furnace, here combustion of fuel takes place and hence this the most
hot zone of the boiler and is called as firing zone. The maximum heat transfertakes
place in furnace only. Temperature of the firing zone is about 1200 to 1400
0
C, where
the heat is generated due to conversion of chemical energy of the fuel. This type of
furnace is called water-cooled furnace, as the membrane panels are made of tubes
through which water is circulating (water wall tubes).
Over the water wall tubes from out side skin welding is done with M.S. sheet and glass
wool lagging of about 100 to 150 mm thick is placed under G.I. sheets to reduce the
radiation losses from furnace. The height of membrane panel is 60 to 70 feet to avoid
joints in firing zone. i.e. A,B,C,D,E and F elevations of the boilers.
The extended furnace is called second pass where primary superheater and economizer,
A.H. is installed.

19. totaloutput power solutionsManohar Tatwawadi Page19
5)Superheaters, Reheaters and Economisers:The Superheater material should be
suitable for the transient high metal temp. During the start up condition superheater
receives relatively high heat input & there is low steam flow through it. steam is
superheated in the super heaters.
i)Primary superheaterorlow temperature superheater (LTSH): From drum steam comes
to LTSH this is in two stages called lower bunch & upper bunch. Typically there are 134
assemblies in each bunch at 102-mm pitch. The material used are SA209T, SA210 Gr. A,
SA 213 T11. The size of tubes is 44.5x4.5 mm & temperature range is 450
0
C to 480
0
C.
Soot blowing steam is taken from LTSH outlet before attemperation.
ii)Platen superheater: It is situated in furnace vertically. It s headers are in pent house.
There are 29 assemblies at pitch of 457 mm. The pitch is more in comparison to others
to avoid choking or fouling. From LTSH the steam comes to platen superheater after
attemperation. The material used is alloy steel as SA 213 T11, SA 213 T22. SA 213 to 347
H and it stands pto 580
0
C. The size of tubes are 51x7.1 mm & 51x8.6mm.
iii)Final superheater: Its headers are in pent house header no 13 & 14. it is situated
vertically behind reheater. It is having 119 assembly at a pitch of 114 mm and size of
tubes are 51x7.6 mm the materials are SA213 T22 alloy steel and stands up to 580
0
C
(alloy steel)
iv)Reheater: The materials SA213 T11 alloy steel & stands upto 550
0
C. The reheater tube
size is 54x3.6 mm and are placed behind the hotter section of superheater. This is in
general givesadequate protection. Temperature control of superheater is achieved by
burner tilt mechanism and this mechanism also controls the temperature of reheat
steam. If reheaters are located close to furnace can receive too much heat for initial
steam flow causing an excessive rise in reheat steam temp. The steam, which is coming
from HP turbine, is heated up in R.H. to its normal temp. of 540
0
C and used in IP turbine.
Reheater is in two parts called front and rear. In front R.H. there are 59 assemblies at a
pitchof 229 mm and at rear there are 89 assemblies at a pitch of 152 mm.
v)Economizer: It is placed between LTSH and Air Heater in second pass of the furnace for
utilization of heat of flue gas for heating feed water, other wise the heat which is
received by theeconomiser may go waste if it is not utilised in this way. The feed water
after HP heaters passes through economizer and is heated by flue gas. After passing
through the economiser feed water reaches to boiler drum. Economiser is in two
bunches called lower bunch & upper bunch. There are 270 assemblies at a pitch of 102
mm, the material used are carbon steel of SA 210 Gr. A1 stands up to 450
0
C, size of the
tubes are 44.5x4.5 mm.

20. totaloutput power solutionsManohar Tatwawadi Page20
6)Windbox:The wind box is situated at 11 m level of Boiler it is in two parts one is on LHS
and other is on RHS of Boiler. There are thirteen compartments in it on each corner out
of which 3 for oil burners, 6 for coal mills, 4 for auxiliary air. These compartments are
connected to burner tilt mechanism which is operated +/-30
0
as per requirement
according to final temperature of steam. The secondary air after air preheater comes to
wind box and is given to furnace along with fuel for complete combustion of fuel as per
requirement.
7)Burners:Coal is used as a primary fuel and oil as secondary fuel during start up of Boiler
and for flame stability at low loads & during other transient operating conditions.
Burner is to atomise fuel, penetrate & mix with proper proportions for complete
combustion. The burners are situated at 3 elevations called AB,CD,EF. At every elevation
there are four burners. FO/LSHS can be fired at all three elevations but LDO can be
taken at AB elevation only for start up of Boiler. For every burner whether LDO/FO there
is one igniter to ignite the burner. Now igniters are being changed form HSD/LDO to HEA
(High energy arc igniters, purely electrical)
8)Soot Blowers:About 78 soot blowers are provided at different zones to remove the
accumulated soot on boiler tubes for effective heat transfer. They are of three types.
a) Wall Soot blowers: These are situated on the furnace and are 56 in numbers. These are
driven by electric motors. Super heated steam is blown through them to clean the
designated area of the water wall.
b)L.R.S.B.: Long Retractable Soot Blowersare 20 in numbers. These are used to clean S.H.
and R.H. and are located in 2
nd
pass of the furnace.
c)Two soot blowers are located on Air Heater to clean the baskets of A.H. Steam from P.
R. D. S. is taken for this purpose.
******

21. totaloutput power solutionsManohar Tatwawadi Page21
GENERAL DESCRIPTIONOF STEAM GENERATOR250 MW.
The steam generator is a radiant reheat, natural circulation, single drum, semi outdoor type
unit, designed for 100% coal firing which is the principal fuel.
The complete furnace section is of welded wall type, arranged as a gas andpressure tight
envelope. The extended side wall section(where the reheaters are located) is also covered
with water cooled section. The circulation system is complete with the six number of
unheated down comers, supply and riser piping.
The superheated steam system has mainly three sections. The low temperature superheater
section (arranged in the second pass of the unit), the radiant platen superheater (arranged at
the outlet section of the furnace) and the final superheater (arranged after the reheater).
Two number desuperheaters have been provided in between the LTSH and the platen
superheater for controlling the superheated steam temperature. The complete second pass
of the boiler (upto the economiser) has been covered with the steam cooled superheater
wall sections.
The complete reheater has been arranged in one section in the horizontal pass of boiler in
between the radiant platen superheater and the final superheater section. Two number
reheater desuperheaters have also been envisaged in the cold reheat steam piping at the
inlet of the reheater. To minimise unbalance in Reheater outlet temperature, two nos.
reheater outlet headers have been provided.
The location and selection of the superheater and the reheater sections have been so chosen
that the rated temperatures of superheat and reheat are anticipated to be achieved from
60% TMCR to 100% BMCR load of the boiler. The steam generator is also capable of
operating on sliding pressure mode.
The boiler includes bare tube economiser made of seamless steeltubes arranged in between
the LTSH section and the regenerative airpreheaters.
The boiler has two number regenerative air pre-heaters of tri-sector type, as the last stage of
heat recovery. The tri-sector air-heater has separate sections for heating the primary air
(required for the pulverisers) and the secondary air, with the flue gases.
The fuel preparation section has primarily six numbers Bowl Mills XRP 943. Each mill has got
separate raw coal feeder. The pulverised fuel from top of mill is taken through pulverised
fuel piping, one each to the four corners of the furnace. The burner wind box section is
located in the four corners of the furnace and arranged for tilting tangential firing.

22. totaloutput power solutionsManohar Tatwawadi Page22
The complete boiler is of top supported type and is provided withall the required structural
supporting steel work, galleries and staircases.
The boiler has two numbers radial primary air fans and two numbers forced draught fans of
axial reaction type for supplying the required primary/secondary air for the boiler.
Thedust collection system for the boiler includes four numbers of electrostatic precipitators.
The boiler is equipped with two numbers of radial ID fans with VFD, arranged after the dust
collecting system,
The complete integral piping, valves & fittings, allair and gas ducting with dampers/gates are
provided. Necessary refractory materials are also be provided, The details of the various
sections of the boiler are elaborated in the following pages.
Layout
The boiler and auxiliaries layout is of "Cold PrimaryAir System with Rear Mill Arrangement.
The arrangement is such that the boiler with furnace and second-pass with the regenerative
tri-sector air heater, followed by Bowl mill bay, ESP, ID fan and the common stack.
***********

23. totaloutput power solutionsManohar Tatwawadi Page23
FIGURE No.1