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SUPERCRITICAL BOILER
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REQUIREMENTS FOR BOILER DESIGN
PHILOSOPHY
Steam cycle parameters: Optimumbest Boiler and auxilieries : Most efficient lowest sp. Coal
consumption i.e. highest High availability Short start-up timeminimum start-up losses e.g. fuel oil
consumption Quick load-following capability Flexible when coal of varying quality or fouling
tendencies Sophisticated firing system minimise NOx emissions
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IMPORTANCE OF EFFICIENCY
The cost of operating the Plant of 3 x 660MW, one percentage point below the design
point, will incur additional fuel cost of Rs. 20Crores in a year.
The indirect costs for Maintenance, Ash Dykeland etc. will give additional burden of Rs. 2
Crores in a year. The life time loss is Rs. 660 Crores forefficiency lower by one percentage point .
At Sipat, we gain an efficiency of 2.5percentage point by adopting super criticaltechnology in 1980 MW station, thus savingRs. 1650 Crores in its life time.
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QUEST FOR EFFICIENCY IMPROVEMENT
Supercritical parameters (Press. above225Kg/cm2 and temperature above374.15 C) is an effort in that direction.
The selected supercritical parameters
for Sipat Boiler are : Pressure : 256 kg/cm2
Temperature : 540 C SH and
568C RH
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RANKINE CYCLE
The Carnot Cycle is theoretically most efficient, but it is
having practical difficulties. For steam power plant, practical thermal cycle was
suggested by Rankine, called Ideal cycle or Rankinecycle.
3-3 BFP raises pressure from p2 to p13-4 Heating In feed heaters & eco
4 -1 Heating In boiler
1-2 Work done in Turbine from p1 to p2
1
23
3
4
T
S
T1
T2
p1
p2
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THERMAL EFFICIENCY OF RANKINECYCLE
Q1-Q2 W Useful work = ------- = --- = ----------------Q1 Q Heat supplied
Rejected Heat = 1 - --------------------
Useful HeatT1 - T2 T2
Carnot = -------- = 1 - ---
T1 T1
To achieve more efficiency T2 should be as low as
possible and T1 should be as high as possible
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CYCLE THERMODYNAMICS ANALYSIS
How?P0T0Reheat one or two , TreheatRegeneration
the best More the better7 to 8 ?
Higher P0(P0-Pc)
8-10 heaters
Pc
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METHODS OF INCREASING RANKINE CYCLEEFFICIENCY
Raising supply temperature by super heating.Increasing the inlet temperature will raise the heatsupply to the cycle more than the heat rejection.
Raising inlet pressure of steam :Increasing the pressure will mean increase insaturation temperature at which steamevaporates thus increasing the average inlettemperature (T1)
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(Contd..)
Dropping the final pressure (or temperature) at which heat is
rejected. Regenerative Heating : Heating the feed water pumped to Boiler
by bleeding steam from turbine.
Reheat Cycle : Reheating of steam in boiler after it has already
expanded in HP Turbine will avoid moisture formation in LT
Turbine. Also, more heat content of steam before IP Turbine,
will improve efficiency.
http://../backup/Our%20Presentation/WORLD%20OF%20SUPER%20CRITICAL_PRESENTATION%20ON%2006.08.04/RANKINE%20CYCLE.ppthttp://../backup/Our%20Presentation/WORLD%20OF%20SUPER%20CRITICAL_PRESENTATION%20ON%2006.08.04/RANKINE%20CYCLE.ppt -
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WHY SUPERCRITICAL PRESSURE
A Boiler operating at a pressure abovecritical point is called SUPERCRITICAL
BOILER
A point where boiling water and drysaturated lines meet so that associated
latent heat is zero, this point is called
Critical Point and occurs at 225 kg/cm2(abs) 374.15 C temperature.
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CRITICAL CONDITION
Definition
CRITICAL is a thermodynamic expressiondescribing the state of a substancebeyond which there is no clear distinctionbetween the liquid and gaseous phase.
The critical pressure & temperature forwater are
Pressure = 225.56 Kg / cm2
Temperature = 374.15 C
TEMP
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0
100
200
300
400
500
600SUPER CRITICAL
BOILER CYCLE
WITH SH, RH &Regeneration
of SIPAT 3 x 660 MW
540C 568C
Steam flow :2225 T/HrSteam temp : 540 cSteam Pres : 256 kg/cmRH pre : 51.6 Kg/cmRH Temp : 568cFeed water Temp : 291c
ENTROPY
TEMP
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SUPERCRITICAL BOILER
Supercritical pressure boiler has no drum and heat
absorbing surface being, in effect, one continuous
tube, hence called once through Supercritical
pressure boilers.
The water in boiler is pressurized by Boiler FeedPump, sensible heat is added in feed heaters,
economizer and furnace tubes, until water attains
saturation temperature and flashes instantaneously
to dry saturated steam and super heatingcommences.
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SUPERCRITICALTHERMAL CYCLE
ADVANTAGES (1)
Improvements in plant efficiency bymore than 2 %
Decrease in Coal Consumption
Reduction in Green House gases. Overall reduction in Auxiliary Power
consumption.
Reduction in requirement of Ash dykeLand & Consumptive water.
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SUPERCRITICAL ADVANTAGES (2)
Sliding pressure operation because of Once
through system .
Even distribution of heat due to spiral wall
arrangement leading to less Boiler tube
failure, thereby improving system continuity
and availability of the station.
Low thermal stress in Turbine .
The startup time is less for boiler.
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SUPERCRITICAL DISADVANTAGES
Higher power consumption of BFP
Higher feed water quality required.
More complex supporting and framing inBoiler due to Spiral Wall tubes.
Slight higher capital cost.
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COMPARISION OF THERMAL CYCLEEFFICIENCIES.
OPEN CYCLE EFFICIENCY - 14.68 %
WITH CONDENSER - 26.2 %
WITH SUPER HEAT 30.75 TO 34.15 %
WITH REHEAT - 34.2 TO 36.6 %
WITH SUPER CRITICAL PARAMETERS36.0 TO 39.15 %
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INCREASE IN PLANT EFFICIENCY by SUPERCRITICAL PARAMETERS
1.5
0.90.6
3.2
167 bar
538/538c
250 bar
538/538250 bar
540/560c
250 bar
580/600c
250bar
566/566c
1
2
3
4
5
6
.
Efficiency Increase
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CopyrightALSTOM2
005
Well suited for < 2100 psi cycles
Natural Circulation System
To Superheater
Risers
Furnaceor
CombustorEcon
Drum
Downcomers
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CopyrightALSTOM2
005
Optimum solution for reliable high pressure subcritical operation
Controlled Circulation System
To Superheater
Furnace
Econ
Drum
Downcomers
BWCPs
(Orifices)
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CopyrightALSTOM2
005
Suitable for Sub and Supercritical Cycles
Once-Through Circulation System
Separator
Furnaceor
Combustor
Econ
Circ.
Pump Distribution Header
To Superheater
SLIDING PRESSURE
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SLIDING PRESSUREOPERATION(SPO)
Natural S.P.O.(NSPO) Turbine inlet valves :fully open during
normal operation
P live steam steam flow
Sliding Pressure Supercritical
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CopyrightALS
TOM2
005
Sliding Pressure SupercriticalOperation
Pressure operation mode at boiler outlet
4350
3625
2900
2175
1450
725
0
psig)
1
2
3 1. Constant Pressure Operation
2. Modified Sliding Pressure Operation
3. Pure Sliding Pressure Operation
S P O ADVANTAGES
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S.P.O. - ADVANTAGES
1. No additional pressure loss between boiler and turbine cycle within (Pb.o. Pc)2. Boiler Pr. at low loads : low FW pump auxiliary power
consumption : low At full load : same for both Operating modes
E.x. : 50% load , Power consumption in fixed Pr.operation (f . p. o) is double as in S.P.O
3. Lower Pr. at low loads less fatigue of Pr. partcomponents e.g. boiler , turbine piping longer life
of all components, Less wear of components LessMaintenance
N C B with F P O :
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N.C.B. with F.P.O :
Pr. loss at turbine inlet for f.p.o significanttemp
Higher Pr. loss higher temp
Different loads : Pr. loss = var, temp = var Each load change Tt.i. = var
Permissible temp change at t.i. : LMITEDload change in f.p.o : LIMITED
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O.T.B with S.P.O
Much larger permissible load transients (twice as highas in natural circulation boilers)
A boiler requirement : maintain T0 = T0 ratedwithin widest load change
Special methods employed : Flue gasrecirculation, Attemperators, Special heatexchangers
In drum boilers: T0 at loads < (50 60 )%In supercritical boilers : T0= T0 rated in entire
controlled load range
Start up time and start up
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Start up time and start uplosses :
Thick walled drum of n. c. boiler (thickness 200mm ,P > 180bar ) : permits only small temp . gradients and a small no. ofsharp changesThin walled separating vessels of once through boilers : largergradients and a small no. of sharp changes
This is increasingly more important as Tstart up E.g. 48 hrs after shut down : boiler relatively cold : O.T.B start up 80 min N.C.B start up 210 min
Disadvantage of NSPO:
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Disadvantage of NSPO:
Storage effect of boiler >>that of turbineThis slows down load changes
Turbine inlet valve opening = const
Pr before turbine must first be raised or lowered .The same for the complete boiler.
Requirements of electrical grid can hardly befulfilled using NSPO
REMEDY? Modified S.P.O. (MSPO)
Modified SPO (MSPO)
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Modified SPO (MSPO)
With load variation admission crosssection at turbine : altered briefly
Accumulated steam in boiler : discharged
at once Dynamic response of boiler : improved
substantially
Advantages of SPO: lost partly
WHY NOT SLIDING Pr. OPERATION IN
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NATURAL CIRCULATION BOILERS?
P steam formation in down comers instabilities incirculation system P water surface in drum disturbed drum level
control problems feed water control problems Drum : most critical thick walled component
Tin drum = Ts = f(P)
In S.P.O ,P= var Drum under highest thermal stresses
Note: almost all natural circulation boilers operated in fixed
Pr. mode and mostly for base load operation
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THANK YOU