For POSOCO Executives
Concept of Sub Critical and SuperCritical Boilers, Parameters, Design,
efficiency and Boiler
( Combustion)
Material compiled by: Mr. V.K.Gupta, EX DGM NPCIL, NPTI
(Nagpur) and CBIP New Delhi. Mo-9555486426 & Email:
vs4951@rediffmail.com
Generation of Electricity from Fossil Fuel
CATAGORY
INSTALLED
GENERATION
CAPACITY(MW)
% of SHARE IN
Total
Electricity generation from Fossil Fuel as on 31.01.2022
data Ministry of Power
Coal
2,03,900
51.6%
Lignite
1.7%
6,620
Gas
Diesel
24,900
510
6.3%
0.1%
Total Fossil Fuel
2,35,929
59.7%
A Sub-critical
or Supercritical
Boiler Lay out
Selective
Catalytic
converter
A schematic layout
of a typical coal
fired supercritical
TPP
The slide shows layout of the Sub-Critical Boiler say below
500MW. It shows how the boiler accessories like Coal
Mills, various Fans, AHP and ESP etc. are located and
connected to boiler or the steam generator what ever you
may call it. Now then, a steam generator has a nest
(banks) of various tubes like Water walls, Economiser,
Superheater and Re-heater tubes. If we look from outside,
boiler is a enclosure of sealed tubes all around. In the first
pass it has water walls, arch tubes, in the horizontal pass
it has tubes nest forming roof or penthouse and in the
second pass it has steam tubes covering it all around up
to the economiser ash hopper. The bottom portion of the
first pass has apertures made by forming water walls at
the four corners to accommodate wind box assemblies or
the burner assemblies. At the bottom it has ash panel and
trough seal.
It is through this opening 20% of the ash and clinkers
from combustion process will be existing through
clinker grinder and to the ash disposal arrangement.
WW, economiser, superheater and reheaters tubes
banks are inner assemblies and their connecting
headers out side the furnace. Only tubes form the
heat absorbing surfaces and headers not.
Water walls are also called riser tubes or the
evaporating tubes because it is here the boiler feed
water is evaporated by absorbing sensible & latent
heat from combustion zone. It is partial in case of
sub-critical boilers and complete (100%) in case of
supercritical boilers.
Modern thermal power plants operate at very high pressures
greater than the Critical pressure of steam. The description
explains how Super Critical power plants are different from
the normal power plants.
Supercritical power plants boiler were in service from the
late fifties. But the technology did not really take off due to
problems of reliability especially from the metallurgical
aspect. The single most important factor that determines the
use of higher and higher pressure and temperatures are the
availability of materials to withstand these conditions.
Increases in operating pressure and temperatures have to
go hand in hand with developments in metallurgy. With more
than 600 boiler units in service the reliability issue seems to
have resolved. Supercritical boiler units are the standard for
future power plants in many countries including India.
A Block diagram of the plant cycle
 Why Supercritical Technology:
 To Reduce emissions for each Kwh of electricity
generated. Adhering to latest environment
emission release norms.
 Efficiency of these units is higher than subcritical.
1% rise in efficiency reduces Co2 emission by 23%. Nox is also reduced.
 The most economical way to enhance efficiency.
 To achieve fuel cost saving : Economical
 Reduce boiler size / MW
 To Reduce Start Up Time
 Understanding sub-critical technology
 Water when heated to subcritical pressure,
temperature increases until it start boiling.
 This temperature called saturation temperature at
that pressure, remains constant until all water is
converted to steam.
 When all liquid is converted to steam then again
temperature starts rising (Superheat).
 Sub-Critical boilers typically have a mean ( Boiler
drum) to separate steam and water.
 The mass of boiler the drum , which limits the
rate at which the sub-critical boiler respond to the
load changes.
 Too great a firing rate will result in high thermal
stresses in the boiler drum.
There are no operational limitations due to oncethrough boilers compared to drum type boilers. In
fact, once-through boilers are better suited to
frequent load variations than drum type boilers,
since the drum is a component with a high wall
thickness, requiring controlled heating. This limits
the load change rate to 3% per minute, while oncethrough boilers can step-up the load by 5% per
minute. This makes once-through boilers more
suitable for fast start up as well as for transient
conditions.
SUB CRITICAL POWER PLANT
Understanding Supercritical Technology
When water is heated at constant pressure above critical
pressure, its temperature never be constant. Because
there is no latent heat requirement. Water straight
flashes into steam and continuing gaining heat to
superheat level.
No Distinction between liquid and Gas , the mass
density of the two phases remain the same.
No stage where water exist as two phases and require
separation. No Drum.
The actual location of the transition from liquid to steam
in a once through supercritical boiler is free to move with
different conditions: Sliding pressure Operation.
For changing boiler load and pressure, the process is
able to optimize the amount of liquid and gas region for
effective heat transfer.
 Spiral Tubes Water Walls Advantages / Disadvantages:
 Advantages:
 Benefits from averaging of heat absorption variation
resulting in less tube failure.
 No individual tube orifice
 Reduced no of evaporator wall tubes and ensure minimum
water flow
 Minimizes peak tube metal temperature
 Minimum Tube to Tube metal temperature difference
 Disadvantages:
 Complex wind box opening
 Complex water wall support system
 Tube leakage identification – a tough task
 More the water wall pressure drop : Increases boiler feed
pump power input
 Adherence of ash on the shell of tube fin
Comparison of Subcritical and Supercritical
Description
Supercritical
Subcritical
Coal and Ash handling
Low
High
02.
Pollution
Low
High
03.
Auxiliary Power
Consumption
Low (6%)
More (7-8%)
Low
More
Sr No.
01.
04.
Specific Enthalpy
05.
Sp. Coal Consumption
Low ( approx. 4% less
than subcritical)
High
06.
Air Flow & Dry Flu Gas
Loss
Low
High
07.
Coal and Ash handling
Low
High
08.
Pollution
Low
High
09.
Auxiliary Power
Consumption
Low (6%)
More (7-8%)
Comparison of Subcritical and Supercritical
Sr No.
Description
Supercritical
Subcritical
10.
Overall Efficiency
High (40-42%)
Low (36-37%)
11.
Total Heating Surface
Required
high (84439m2)
low (71582m2)
12.
Tube Diameter
Low
High
13.
Material Requirement
(Tonnes)
Low 7502 MT
High 9200 MT
14.
Start Up Time
Less
More
15.
Blow Down Loss
Nil
More
16.
Water Consumption
Less
More
17.
Cost Of Generation
Less
More
Current designs of supercritical plants have installation costs that are only 2%
higher than those of subcritical plants. Fuel costs are considerably lower due to
the increased efficiency and operating costs are at the same level as subcritical
plants. Specific installation cost i.e. the cost per megawatt (MW) decreases with
increased plant size.
Justification of Fast start Up & bearing higher load variation
SALIENT FEATURES OF 500 MW Sub-Critical
BOILER
•
•
•
•
•
•
•
•
•
•
TYPE OF BOILER:
Single Drum Top supported,
Fusion welded WW Panel,
Controlled circulation system,
Tangential fired, Radiant
Dry Bottom
Pulverized coal fired outdoor
Balanced Draught
Cut corner
51mm OD WW Tubes at
63.5 mm pitch
• Rap around Buck stay
system
• FIRST PASS:
• Boiler Furnace Depth 15.7972mtr
• Furnace Width 19.177mtr
• Height of Furnace (upto
Radiant roof)64.168mtr.
• SECOND PASS:
• Depth -13.716mtr
• Width -19.177mtr
• Total Height of Boiler90mtr.
Boiler Description:
The boilers of thermal power plants are known as utility
boilers, because they are there for generation of high
pressure and temperature steam which drives steam
turbine and generator to generate electricity.
Thus the utility boilers means which are used for
generation of electricity. A 500 MW STG will
continuously generate 500 MW of electricity. But this
500 MW is a load which is put on the generator and in
turn it will generate 500MW every moment. If the load
on TG is less it will generate less but if load imposed is
more it can not generate more as the maximum
generation limit is 500 MW. This is called rating of
Turbine and generator.
The boiler has to produce superheated steam at rated
pressure and temperature around 1600 Tons / hour.
 All boilers are manufactured as per the rules of IBR (Indian
Boiler Regulation).
 A boiler is defined as the closed vessel having a volume of
more than 22.75 liters and is used for raising steam at
pressure above 1 kg/cm2g. This is because safety is
connected to it.
 Now-a-days older thermal power plants of capacity less than
210MW are being gradually phased out. Even at some
places older 210 MW TPS are being discontinued being,
1.Uneconomical
2.They do not meet the environment safety norms.
3. Being replaced by 500 MW and above units and they are
the future.
4. Boiler can be Sub Critical or Super critical. If the steam
pressure and temperature are below 221 kg/cm2 and
temperature below 3740C, they are called subcritical and the
one above are supercritical.
 Vertical Tube Furnace:
 To provide sufficient flow per tube , constant
pressure furnace employ vertically oriented tubes.
 Tubes are appropriately sized and arranged in
multiple passes in the lower furnace where the
burners are located and the heat input is high.
 By passing the flow twice through the lower
furnace periphery (two passes). The mass flow
per tube can be kept high enough to ensure
sufficient cooling.
 In addition the fluid is mixed between passes to
reduce the upset fluid temperature.
Spiral Tube Furnace:
The spiral design on the other hand, utilizes fewer
tubes to obtain the desired flow per tube by
wrapping them around the furnace to create the
enclosure.
This also has the benefit of passing all tubes
through all heat zones to maintain a nearly even
build temperature at the outlet of the lower portion
of the furnace.
Because the tubes are “wrapped” around the
furnace to form the enclosure, fabrication and
erection are considerably more complicated and
costly.
 Spiral Tubes Water Walls Advantages / Disadvantages:
 Advantages:
 Benefits from averaging of heat absorption variation resulting
in less tube failure.
 No individual tube orifice
 Reduced no of evaporator wall tubes and ensure minimum
water flow
 Minimizes peak tube metal temperature
 Minimum Tube to Tube metal temperature difference
 Disadvantages:
 Complex wind box opening
 Complex water wall support system
 Tube leakage identification – a tough task
 More the water wall pressure drop : Increases boiler feed
pump power input
 Adherence of ash on the shell of tube fin
660 MW Supercritical Thermal Power Plant Boiler Operating Parameters
FD FAN
2 No’s (Axial)
11 KVA /1950 KW
228mmwc
1732 T/Hr
PA FAN
2 No’s (Axial)
11 KVA /3920 KW
884 mmwc
947 T/Hr
ID FAN
2 No’s (Axial)
11 KVA /5820 KW
3020 T/Hr
Total Air
2535 T/ Hr
SH OUTLET PRESSURE /TEMPERATURE / FLOW
256 Ksc / 5400C
/2225 T /Hr
SH OUTLET PRESSURE /TEMPERATURE / FLOW
46 Ksc / 5680C /
1742 T/Hr
SEPARATOR OUTLET PRESSURE /TEMPERATURE
277 Ksc / 412 C
ECONOMISER INLET
304 Ksc / 270 C
MILL OPERATION
7 / 10
COAL REQUIREMENT
471 T/Hr
SH / RH Spray
89 / 0.0 T/Hr
Boiler Efficiency
87 %
AIR AND FLUE GAS PASS SYSTEM
Similar to 500 MW units
No of ESP Passes: 6
No of field per passes = 18, 1 to 7 field 70 kv
8 & 9 field 80 Kv
No of Hoppers per pass = 36
Flue Gas Flow per pass = 1058 T / Hr
(Over Heat Degree Superheat)
Over Heat Degree of Super Heat (OHDR)
In Operation of Supercritical Boiler, many important Parameters are
required to be monitored and controlled; Over Heat Degree of Super Heat
(OHDR) is one of them.
Just to simplify, if in a supercritical Boiler, if the evaporator water
temperature is maintained at 405 Deg C at 25 MPA , then it will have a
OHDR of 20 deg C, as the critical temperature as per T-S curve for a
pressure of 25 MPA is 385 Deg C
Generally Boiler Manufacturers specify OHDR around 25 Deg C, and
Water to coal ratio of 6 to 7.
Higher OHDR than 25 Deg C essentially means that the Heat absorbed in
the evaporator section is more than designed as a result of which steam
will be generated in in Straight Water walls , steam being a bad conductor
of Heat will not be able to absorb heat from the Water wall Tubes, as a
result of which the metal temperature of water walls will have a tendency to
shoot up. This may lead to failure of water walls or supply tubes exposed to
high heat radiant portions, but not able to cool down.
Higher OHDR also means that as compared to Water flow,
coal consumed will be more, displaying lower Water to Coal
ratio than specified. Low water to coal ratio will also happen
in cases where GCV of coal is much lower requiring much
more coal for generating same quantity of steam.
Lower OHDR than 25 DegC signifies that Water is not able to
absorb required heat, thereby leading to evaporation of water
post evaporator, Water to coal ratio will show higher value ,
establishing that water flow is higher than Coal, this will lead
to drop in Water and eventually steam temperature. Low
OHDR will lead to lowering of Steam generation and
eventually MW generated.
So monitoring and maintaining OHDR and Water to Coal
ratio are significantly important aspects for stable and
efficient Operation of a Supercritical Boiler
Comparison of water chemistry
Steam pressure vs. load:
Constant pressure implies stable pressure of the steam
generator and main steam line over the unit’s load range.
Meanwhile, the basic nature of a simple, rotating turbine is to
require less pressure as load and flow rate are reduced, and if
the main steam pressure is limited to only that required for
each load, this mode is referred to as pure sliding pressure.
Sliding Pressure:
Implies the variable pressure required at the turbine inlet
based on load & steam flow rate . Again the sliding pressure
can be classified as
 pure sliding pressure
 modified sliding pressure
The basic nature of a simple, rotating turbine is to require less
pressure as load and flow rate are reduced, and if the main
steam pressure is limited to only that required for each load,
this mode is referred to as pure sliding pressure.
However, when we speak generally of “sliding pressure” we
often mean “modified sliding pressure”.
This mode has a limited amount of pressure throttling to
provide a modest amount of fast-response load reserve. The
modified sliding pressure operation combines the advantages
of constant-pressure operation with those of the sliding
pressure mode. The ability to activate the storage capacity of
the boiler by opening the throttle valves is combined with the
advantages of low lifetime consumption of the plant and high
part load efficiency.
Kcal /kWh HHV
2376
2232
Spiral Waterwall Tubes:
Among the heat-absorbing surfaces, the furnace walls are
exposed to the highest heat flux. This is because of the
intense radiant heat from the fireball.
Currently, two design variants are used for once-through
units: the spiral furnace tube arrangement and the vertical
tube arrangement. Design choice is governed by furnace size
and customer preference – both variants have advantages,
depending on project drivers.
RIFLE TUBE & PLAIN
TUBE
Start up and low load re-circulation system
One of the critical parameters of a once-through system is the
proper selection of minimum acceptable once-through flow in
the evaporator tubes.
Below a particular load, the water wall flow is kept constant in
order to ensure flow, high enough to cool the tubes. This load
is typically 30 – 40% of BMCR and below this load; the boiler
will operate under the low load re-circulation system
At low loads where the water flow is to be kept constant, a
water-steam separator and a drain water return system are
required. The water separator consists of one or more vertical
vessels with tangential inlets. The separator is in a wet
condition when operating under the low load circulation range.
In the once-through mode, the separator runs dry.
 Steam Separator:
Generally a steam separator and a separator
drain tank were installed to separate the
steam and the water at the furnace outlet
during a low-load recirculation operation. This
design is different from that of a conventional
NC boiler, for which a steam drum is installed
to separate the water from the steam under
all operating loads. The steam drum is
designed to have sufficient water storage
capacity, and usually contains complicated
internal parts, such as steam cyclones,
scrubbers, internal feed pipes, and baffles.
Because of the complex internals, steam
drums require a large amount of maintenance
work during outage periods. However, the
steam separator design is simple in
configuration and has no internal, therefore
significantly less maintenance work is
required.
Steam Drum
Steam Separator
Boiler Start –up systems:
The start-up system in super critical boilers is used to
protect super-heaters from water carry-over by
separating water from steam and re-circulating it
through the evaporator surfaces during start-up, low
load operation and shutdown of the boiler. The required
water flow rate through the evaporator tubes is
therefore maintained greater than the evaporation rate
to protect them against overheating.
The start-up system equipment consists of two steam
water separators, a water collection tank, a boiler
circulating pump and the associated piping and control
valves to return the fluid from the water collection tank
to the economizer inlet.
During start-up, the unit is operated much like a drum boiler
where water is recirculated to maintain a minimum flow through
the furnace equivalent to 30% of full load flow. The system is
similar to a pumped circulation drum boiler with the steam
water separators and the water collection tank functioning like
the steam drum. The water flowing through the furnace is a
combination of water from the water collection tank and boiler
feedwater. The boiler feed pump controls the total flow through
the furnace so the minimum required mass flow is maintained.
Steam generated through the furnace circuits is separated from
the water in the vertical separator, routed to the super heater
and then to either the steam turbine or the turbines bypass
system.
The water from the vertical separator is returned to
the water collection tank and then to the circulating
pump. The 381 valve, located at the discharge of the
circulating pump, controls the flow proportionally to
tank level to maintain the water inventory in the
collection tank. Water is also recirculated from the
pump discharge to the collecting tank to assure that
the minimum flow required through the pump is
maintained.
Above the minimum boiler load the unit switches to oncethrough operation. The circulating pump is taken out of service
but is kept pressurized. A small flow of feedwater from the
economizer outlet is routed to the circulating pump inlet and
back to the separator to maintain the components in the ready
state for use during shutdown.
From the figure below we can understand the complete
system of the start up of the supercritical boiler. If we can see
the start up thoroughly we will find the starting procedure of a supercritical boiler is
similar to the subcritical boiler.











TYPE OF BOILER:
Single Drum Top supported,
Fusion welded WW Panel,
Controlled circulation system,
Tangential fired, Radiant
Dry Bottom
Pulverized coal fired outdoor
Balanced Draught
Cut corner
51mm OD WW Tubes at 63.5 mm pitch
Rap around Buck stay system
Controlled Circulation In Boiler: If the Operating pressure of
boiler is between 180 kg/cm2 to 200 kg/cm2 then circulation
in boiler is to be assisted with mechanical pumps, to maintain
circulation through WW. To regulate the flow through various
tubes, orifice plates are used
WHAT IS A BOILER TURNDOWN RATIO?
A boiler turndown ratio refers to the ratio between
a boiler’s highest fire setting (maximum output) and
lowest fire setting (minimum output).
Turndown ratios vary depending on the boiler. For
example, if your boiler has a 3:1 turndown ratio, that
means that at its lowest fire setting you will be
operating at 33% of the boiler’s full capacity. If you
have a 10:1 turndown, that means that at its lowest
setting, your boiler is operating at 10% of its full
capacity.
WHY IS A BOILER TURNDOWN RATIO IMPORTANT?
Generally, a higher turndown ratio provides more flexibility
and more efficient operations that translate into significant
cost savings over time.
Some facilities need to run their boilers at the highest settings
only on infrequent occasions or during seasonal peak
demands. Even though these occasions are rare, the facility
needs to have enough steam system capacity to meet those
demands when they arise. It doesn’t make sense to run a
boiler at full capacity 24x7x365, which is why it’s crucial for
facilities with fluctuating demands to have a flexible turndown
ratio that allows them to “turn down” the capacity to better
match their actual demand.
If the boiler does not have a high turndown ratio,
facilities don’t have many options for controlling
how much fuel they’re using. Operating a boiler
system with a high turndown ratio offers these
facilities greater efficiency because it can operate
at a lower capacity during low-demand times while
also meeting those peak demands with ease.
BOILER EXPANSION DETAILS
FIRST PASS:
• Vertically Downward Expansion at Ring Header
(10.250mtr elev.) : 330mm
• Horizontal Expansion at Ring Header
(10.250mtr elev.) : 36mm
SECOND PASS :
• Vertically Downward Expansion at Ring Header
(44.060mtr elev.) : 174mm
• Horizontal Expansion at Ring Header
(44.060mtr elev.) : 58mm
BOILER EXPANSION DETAILS
• BOILER DRUM EXPANSION :
• Vertical Downward Expansion at 75.333mtr elev.=29mm
• Horizontal Expansion at 75.333mtr elev.= 53mm
BOILER PARAMETERS (BMCR) / 500 MW
• STEAM FLOW AT SH OUTLET • PRESSURE OF SUPERHEATED STEAM • TEMP.OF SUPERHEATED STEAM -
1625 t/h
178 kg/sq.cm
540 0C
•
•
•
•
•
1392.4 t/h
45.04 kg/sq.cm
337 0C
42.39kg/sq.cm
540 0C
STEAM FLOW AT REHEATER OUTLET ENTRY PRESSURE OF REHEATED STEAM ENTRY TEMP. OF REHEATED STEAM EXIT PRESSURE OF REHEATED STEAM ENTRY TEMP. OF REHEATED STEAM -
• FEED WATER INLET TEMP. FEED WATER TEMP BEFORE ECO. FEED WATER TEMP AFTER ECO. -
254.5 0C
283 0C
357 0C
Boiler Parameters
• TEMP.AT LTSH OUTLET • TEMP.AT S/H DIVISIONAL OUTLET • TEMP.AT FINAL S/H OUTLET -
436 0C
488 0C
540 0C
DESIGN PARAMETERS OF BOILER
•
•
•
•
•
•
BOILER DRUM PRESSURESH OUTLET PRESSURE FINAL SH TEMP. REHEATER OUTLET PRESSURE BOILER HYD. TEST PRESSURE REHEATER HYD. TEST PRESSURE -
209 kg/sq cm
191 kg/sq cm
540 0C
52.4 kg/sq cm
313.5 kg/sq cm
78.6 kg/sq cm
HEATING SURFACE OF BOILER
•
•
•
•
•
•
•
•
•
ECONOMISER WATER WALL STEAM COOLED WALL LTSH SH DIVISIONAL PANELS PLATEN SH (FINAL SH) SH ROOF RH FRONT COILS RH REAR COILS -
20000 sq mtr
7000 sq mtr
2010 sq mtr
8500 sq mtr
2025 sq mtr
2200 sq mtr
830 sq mtr
4128 sq mtr
3500 sq mtr
• TOTAL HEATING SURFACE AREA = 50193 sq mtr
WATER HOLDING CAPACITY OF BOILER
•
•
•
•
•
BOILER DRUM (FULL) :
CIRCULATION SYSTEM :
ECONOMISER
:
SUPERHEATER(LTSH,SH Div. Panel, Final SH):
REHEATER :
• TOTAL VOLUME :
60 cubic mtr
170 cubic mtr
150 cubic mtr
130 cubic mtr
105 cubic mtr
615 cubic mtr
• DM WATER REQUIRED FOR HYD. TEST
(DRAINABLE)
:
420 cubic mtr
SOOT BLOWER DETAILS
• WALL SOOT BLOWERS : 88 Nos. (FUTURE - 16 Nos.)
(FRONT & REAR AT EACH ELEVATION - 6 Nos. EACH, RHS &LHS -5 Nos. EACH, TOTAL 4 ELEVATIONS)
• LRSB
• APH SOOT BLOWERS
: 40 Nos. (FUTURE - 4 Nos.)
:2 Nos.
COMPARISON BETWEEN NATURAL & ASSISTED CIRCULATION BOILERS
S/H
S/H
DRUM
DRUM
BOILER
WALLS
ECO
ECO
BOILER
WALLS
CC PUMP
FEED LINE
FEED LINE
RING HEADER
NATURAL CIRCULATION BOILER
(SUB CRITICAL BOILER 30MW-250MW)
DRUM CG. 26 MTR – 58 MTR
FORCE CAUSING FLOW OF
WATER THROUGH TUBES
F = WCOLD X H1 - W HOT X H2
RING HEADER
ASSISTED CIRCULATION BOILER
(500MW) DRUM CG – 75.33 MTRS
CRITICAL BOILERS OR ONCE TROUGH OR MONOTYPE BOILER
STEAM
SEPERATOR
BOILER WALLS
S/H
WATER
ECO
DRAIN
FEED PUMP
IMPORTANT FEATURES OF 500 MW BOILER
•
•
•
•
•
•
•
•
•
•
•
•
•
•
C.C. PUMPS
ORIFICE PLATES IN BOTTOM RING HEADER
RIFLED TUBES IN WATER WALLS
DIVISIONAL SUPERHEATERS
ECO LINK CONNECTIONS FROM BOTTOM OF BOILER DRUM
DOWN COMMERS CONNECTION TO ONLY BOTTOM RING HEADER.
ECO RECIRCULATION CONNECTION TO BOTTOM RING HEADER
REAR.
2 NOS. TRISECTOR TYPE AIR PREHEATERS.
8 NOS. BOWL MILLS ( XRP 1043 TYPE) & GRAVEMETRIC 36” SIVE
FEEDER ( FEEDER LOCATION 21 MeL )
4 NOS OIL ELEVATION & 8 NOS. COAL ELEVATION.
2 NOS. I.D FANS, 2 NOS. F.D.FANS, 2 NOS. SEAL AIR FAN, 11
SILENCERS ,
No of SAFETY VALVES - 20
HEA IGNITERS.
COAL CONSUMPTION – 8400 T/HR.
NUMBER OF TUBES & DIAMETERS
DESCRIPTION
NO. OF TUBES / PANELS
DIAMETER OF TUBE
IN MM
68 NOS.
159
FRONT-269 NOS., REAR-269 NOS,
LHS-212 NOS, RHS-212 NOS.
51
HANGER TUBES
44 NOS.
63.5
SCREEN TUBES
177 NOS.
63.5
REAR ARCH
177 NOS.
63.5
LHS-37 NOS.& RHS-37 NOS.
63.5
ECONOMISER
92 X 3 BANKS
38.1
LTSH
62 X 2 BANKS
47.63
6 ASSY. & 8 PANELS i.e. 48 NOS.
44.5
25 COILS
51
FRONT-74 NOS. & REAR 74 NOS.
54
RADIENT ROOF 1ST PASS
150 NOS.
63.5
SUPER HEATER CONNECTION
TUBES(DRUM TO RADIENT ROOF )
21 NOS.
159
B.P. SIDE WALL ( FRONT )
60 NOS.
63.5
B.P. SIDE WALL ( REAR )
31 NOS.
51
B.P. ROOF TUBES
126 NOS.
51
B.P.FRONT SCW TUBE
126 NOS.
51
B.P.REAR WALL TUBE
137 NOS.
51
RISER TUBES( W/W O/L HDR. TO
DRUM)
WATER WALL ( FUSION WELDED )
EXTENDED SIDE W/W PANEL
DIVISIONAL PANELLETE
FINAL SUPERHEATER
REAHEATER
SAFETY VALVES LOCATION
M.S.LINE HRH LINE CRH LINE
SAFETY
VALVE
2
4
2
2
ERV
(ELECTROMATIC
RELIEF VALVE)
4
DRUM
6
What are the key differences between the subcritical
boiler units and the Supercritical boiler units?
1. Boiler Efficiency
The main advantage and the reason for a higher
pressure operation is the increase in the
thermodynamic efficiency of the Rankine cycle.
Large Subcritical thermal power plants with 170 bar
and 540 / 540 ° C (SH / RH) operate at an efficiency
of 38 %. Supercritical boiler units operating at 250
bar and 600/615 ° C can have efficiencies in the
range of 42 %.
Ultra supercritical boiler units at 300 bar and 615 /
630 °C will still increase the efficiency up to 44 %.
Increase in efficiency directly lead to reductions in
unit cost of power and CO2 emissions.
2. Operational Flexibility:
Most of the Supercritical boiler units use the
once through technology. This is ideal for
sliding pressure operation which has much
more flexibility in load changes and controlling
the power grid. However this also requires
more sensitive and quick responding control
systems.
3. Evaporation End Point:
In subcritical boiler units the drum acts as a fixed
evaporation end point. The furnace water walls act
as the evaporator. Not so in the case of a
supercritical unit. The evaporation end point can
occur in various levels of the furnace depending on
the boiler load. The percentage of Superheat in
supercritical boiler units is higher than subcritical
boiler units. Because of this the furnace tubes act
more as superheaters than waterwalls. This
necessitates the use of higher grade of materials like
alloy steels in the furnace.
4. Heat transfer Area:
Higher steam temperatures in supercritical
boiler units results in a lesser differential
temperature for heat transfer. Because of this
heat transfer areas required are higher than
subcritical boiler units. Higher Superheat steam
temperatures entering the HP turbine also
mean higher reheater inlet temperatures which
again results in a higher heat transfer areas.
5. Water chemistry:
In supercritical boiler units the water entering the
boiler has to be of extremely high levels of purity.
Supercritical boilers do not have a steam drum that
separates the steam and the water. If the entering
water quality is not good, carry over of impurities can
result in turbine blade deposits.
6. Materials:
Supercritical power plants use special high grade
materials for the boiler tubes. The turbine blades are
also of improved design and materials. In fact, the
very increase in higher pressure and temperature
designs are dependent on the development of
newer and newer alloys and tube materials. The aim
of the industry is to achieve power plant efficiencies
in the range of 50 %.
Thankyou