super critical mw

7/31/2019 Super Critical mw

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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.
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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

super critical mw

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