Boiler of 500 MW

BOILER  AND  AUXILIARIES

KORBA SIMULATOR

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KORBA SIMULATOR

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BOILER AND AUXILIARIES SALIENT FEATURES OF 500 MW BOILER.

 With increase in demand of power in India, new power projects are being constructed  with higher capacity and advanced technology for the better economy and reliability of  operation. Compared to other lower capacity Boilers supplied by BHEL, these 500 MW capacity   boiler have incorporated certain special technical features which are detailed here under: CONTROLLED CIRCULATION SYSTEM

 This is achieved by three numbers of glandless pump and wet motor installed in the downcomer line after the suction manifold. These pump motor assemblies have single suction and double discharge introduction of these pumps in the boiler system have led to the designing of a furnace with lesser diameter tubes and high parameters operating characteristics.  The advantages of the controlled circulation boiler over natural circulation boiler are given below: •

Uniform drum cooling and heating. In controlled circulation boilers this is possible because of arrangement of relief tubes inlets to the drum and the internal baffles of the drum from both sides. The internal base plates are arranged in such a way that it guides the steam water mixture from the relief  tubes along the whole circumference of the drum. The drum is therefore uniformly heated and cooled.

 Whereas in Natural Circulation Boiler, the arrangement of relief tubes and baffle plates is only on one side of the drum and this imposes a constraint on uniform heating of drum. Similar arrangement of relief as in controlled circulation boiler does not exist in natural circulation (NC) boiler because in that case the rel ief required to be taken over the drum and fed from both sides. This shall increase the pressure losses in the riser tubes and also the hot static head requirement for start up. Since the available head in NC Boiler is very less; efforts are always made to reduce the pressure loss and improve the circulation. Second reason is to commence flow in the riser  tubes immediately after light up hot static head is kept as minimum as possible. •

Rapid heating & cooling (start up & shut down): As mentioned in Para 1, the controlled circulation boiler does not impose any thermal constraints on the drum and hence rapid cooling and heating of the boiler is possible. In NC  boiler, rise in saturation temperatures is limited to maximum of 110 OC/hr. Hence, the controlled circulation boiler can be started at a rate two to three times faster than NC boilers.

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•

•

Better cleaning of boiler: For effective acid washing, the acid has to be kept at certain temperature uniformly through the system. This is possible with the assistance of controlled circulation. Uniform expansion of pressure part and lower metal temperature:  This means lesser thermal stresses on the tubes. Because of controlled circulation, lower diameter tubes are used, which result in high mass flow rate thereby preventing departure from nucleate boiling (DNB) maintains a lower  metal temperature.

USE OF RIFLE TUBES FOR FURNACE CONSTRUCTION

 This is one of the extraordinary features of 500 MW capacity boilers. Because of the excessive heat release in the burner zone of the furnace, the metal tubes constituting the furnace at that zone are exposed to the maximum temperature. This being a   water-cooled furnace, the steam water mixture inside the tubes should effectively  carry the heat from the burner zone of the furnace. In this zone, the tubes have an internally cut spiral like a rifle bore so that when water  flows through the tubes, due to hot static heat, it takes a screwed path and attains a  certain degree of spin by which the watness of the tube is always maintained. This prevents the tubes form departure from Nucleate boiling under all operating condition of the boiler and increases the circulation ratio. OVER FIRE AIR SYSTEM FOR NOX (OXIDES OF NITROGEN) CONTROL 

Industrial growth in the recent years has necessitated the need to have a cleaner and pollution free atmosphere, by controlling the production of industrial wastes with the application of improved technology. Power plants are the major sources of the industrial pollution by virture of the stanch emission in the atmosphere. These emissions contain mostly gases and dust particles, which have ill effect on the ecological system. In the 500 MW capacity boiler design, this aspect has been given due importance and certain technical improvements have been incorporated. These are tilting tangential firing and over fire air system. Tangential firing helps in keeping the temperature of the furnace low so that NOX emission is reduced considerably. In addition to the above the over fire air is provided which is used as combustion process adjustment technically for keeping the furnace temperature low and thereby  low Nox formation. Each corner of the burner windbox is provided with two numbers of separate over fire air compartments, kept one above the other and the over fire air is admitted tangentially into the furnace.

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 The over fire air nozzles has got tilting arrangement and compartment flow control dampers for working in unison with the tilting tangential type burner system for  effective control of Nox formation.  AIR PRE-HEATER SYSTEM

 As compared to trisector air pre-heater in 200 MW units, 500 MW units have been incorporated with bisector air pre-heaters. This has been done for optimum utilisation of space and also improved system layout. This has resulted in the flexibility and efficient operation and maintenance of the air pre-heaters and the boiler as a whole. PRIMARY AIR SYSTEM

 The primary air system delivers air to the mills for coal drying and transportation of  coal powder to furnace. The 500 MW units have two stage axial flow primary fans as compared to radial fans in 200 MW units. By introducing axial flow fans, the system efficiency has gone up as the axial flow fans consistently high efficiency at all operating loads. MILLING SYSTEM

In the 500 MW units at SSTPS, Raymond’s Pressurised bowl mills have been installed.  These are similar to the 200 MW mills except that 500 MW mills have vane wheel surrounding the bowl and external lubrication unit. Introduction of vane wheel has led to uniform distribution of primary air within the mill and less rejects. These mills are also supplied with weld overlay technology, which has increased the minimum  wear life of grinding parts to 6000 hrs. I. D. FANS

Unlike 200 MW units, the 500 MW units have been supplied with radial type I.D. fans.  These fans have a lower speed and are less susceptible to wear and tear due to the abrasive flue gases. The control of the I.D. fans is achieved through a variable speed hydraulic coupling and motorised inlet damper. By introducing variable speed control through a hydraulic coupling the losses in the fan at various load has been minimised and efficiency of the fan has remained high at all operating conditions. ELECTROSTATIC PRECIPITATORS:

Electro static precipitators are installed in the 500 MW units for minimising the particulate emission from the stack flue gases. There are four ESP passes for one unit  of 500 MW and each is independently operated. The emitting electrodes are changed at high-ve voltage DC and the gas while crossing this charged path gets jonised. The ionised ash particles of the gas are attracted towards the collecting electrode, which is maintained at high +ve voltage. The ash collects at the collecting electrode and is periodically tapped to dislodge the accumulated ash. The ash falls into the hopper,  which is evacuated by the ash handling system and taken out as slurry.

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 TECHNICAL SPECIFICATION OF 500 MW BOILER MAIN BOILER GENERAL SPECIFICATION

Manufacturer   Type

:

M/s BHEL (C.E. Design)

:

Balanced

Draft,

Single

drum,

Dry

bottom,

Controlled

Circulation plus.  Type of Firing

:

Minimum load at which the steam generator 

 Tilting Tangential

:

2 Mills at 50%

:

20%

:

50%

can be operated continously with complete flame.

Minimum load at which the steam generator can be operated continuosly with complete flame. Stability with oil support (% MCR) Maximum

load

for

which

individual

mill

 beyond which no oil support is required FURNACE SPECIFICATION

 Wall

:

Bottom

 Water Steam cooled

:

Dry 

:

Membrane

:

+ 660

:

3 second

:

14770

:

63.65

Depth (M)

:

15.289

 Width (M)

:

18.049

:

7610

 Tube arrangement  Explosion/Implosion withstand capacity  (MWG) at 67% yields point. Residence time for fuel particles in the furnace. Effective

volume

used

to

calculate

the

residence time (M 3) Height from furnace bottom ash hopper to furnace roof (M)

Furnace projected area (M 2)

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Furnace volume (M 3)

:

14770

Number 

:

283

OD (MM)

:

51.00

Design thickness (MM)

:

5.19

Pitch (MM)

:

63.5

:

5.6

:

SA 210C

 Total projected surface (M 2)

:

1160

Method of joining long tube

:

Butt weld

:

181000

Design pr. of tubes Kg/cm 2 (ABS)

:

207.3

Max. pressure of tubes Kg/cm 2 (ABS)

:

197.3

Design metal temp OC

:

416

 WATER WALLS FRONT WALLS

 Actual thickness used (MM) Material

 Total wt. of tubes (kgs)

SIDEWALLS, REAR WALLS

Side walls

Rear walls

Roof 

Number 

444

283

142

OD (MM)

51

51

57

Design thickness (MM)

5.19

5.19

5.54

Pitch (MM)

63.5

63.5

127

 Actual thickness used

5.6

5.6

5.7

 Total projected surface area 

1430

930

220

BUTT WELD

BUTT WELD

BUTT WELD

277000

186000

45000

207.3

207.3

204.9

193.3

197.3

192.3

417

417

416

& ROOF 

of tubes (M 2) Method of joining long tubes.  Total wt. of tubes (Kgs) Design Pr. of tubes Kgs/cm 2 (ABS) Max pr. of tubes Kgs/cm 2 (ABS) Design metal Temp o C

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 WATER WALL HEADERS

Lower drum

WW outlet

No. of headers

1

5

Outside Dia (Dia (MM)

914

273

Design thickness (MM)

86

38.5

 Actual thickness (MM)

89

45

 Total wt. of headers (Kgs.)

166000

37300

Design pressure of headers kg/cm 2 (ABS)

207.3

204.9

HEADERS

Lower drum

Max working pressure of headers Kg/cm2

197.3

192.4

SA-299

SA-106 Gr-B

 WW outlet

(ABS) Material specification DRUM

Material specification

:

SA-299

Design pressure Kg/cm 2 (ABS)

:

204.9

Design metal temp OC

:

366

Max operating pressure Kg/cm 2 (ABS)

:

192.4

:

152.4

Overall length of Drum (MM)

:

22070

OD of Drum (MM)

:

2130

Internal dia (MM)

:

1778

Corrosion allowance (MM)

:

0.75

Number of distribution headers

:

6

No. of Cyclone separator 

:

96

No. Of Secondary driers

:

96

Shroud material

:

Carbon Steel

Max permissible temp differential between any 

:

50

:

10

Drum wt. with internals (tonnes)

:

237.00

Drum wt. without internals (tonnes)

:

215.00

 Actual thickness used for dished ends

two parts of the drum ( oC).  Water capacity at MCR conditions (in seconds) between normal and lowest water level permitted (up to LL trip)

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