April 11, 2010

Power Generation in Typical Coal Based Power Plant

- Er. Sandip De MBA, B.E. Mechanical

Different systems The Process Different Stages – Project, Commissioning, Operations and Maintenance. O&M vs. Non-O&M Coal systems Water systems Furnace systems TG systems Ash systems Power evacuation systems Support systems

--Technical and

--Non-technical Differences in Gas based and Hydro power projects

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POWER STATION

1. Coal-how much and how 21. Rotor 2. CHP 22. Stator 3. Coal Conveyor 23. Generator Transformer 4. Boiler Coal Bunker 24. Condenser 5. Bucket Wheel Machine 25. Condensate Extraction Pump 6. Coal Feeder 26. Low Pressure Feed Heat7. Pulverizing Mill 27. Deaerator 8. Primary Air Fan 28. Boiler Feed Pump 9. Boiler light-up 29. High Pressure Feed Heaters

10. Boiler 30. Economizer 11. Forced Draught Fan 31. Steam Drum 12. Air Heater 32. Cooling Tower 13. Electrostatic Precipitator 33. Circulating Water Pumps 14. Induced Draught Fan 34. Circulating Water Make-Up Pumps 15. Main Chimney 35. FGD 16. Super heater 36. Finally..

17. High Pressure Turbine 18. Boiler Reheater19. Intermediate Pressure Turbine20. Low Pressure Turbine

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• Coal is transported from the mine to loading place• Loading place into train• Train to the location• Unloaded at the location (Wagon Tripler)• Transported through conveyors into bunkers• From bunkers into mills (Pulverizes)• Powdered, put into furnace and burnt• The heat generated is used to heat water, steam, air

1. Coal- How much and how • Coal required for a 100 MW plant per year

= Power Plant Wattage x Coal unit/ Hr x Hr/ Yr

= 100 x (Per unit Heat Required/ Coal Heat Value) x (24 x 365)

= 100 x (10.765/ 20) x 8760 kg/ Yr

= 471.51 x 1000 kg/ Yr

= 471.51 mT/ Yr

2. CHP – Coal handling plant • At the receiving location stock yard

or sent to coal bunkers• To make sure to generate electricity

when you want to, you have to make sure the coal is in the right place at the right time

• Typically 15 days stock is maintained at site

• The stock yard stocks these and helps in times of lean supply from the mines or when transportation is not available

3. Coal Conveyor • Coal conveyors are used to move

coal around efficiently. Coal arriving by train can be stocked for later use or taken straight to the coal bunkers

• CHP control room with remote control system helps to ensure that the conveyors take the coal to the right bunkers

4. Boiler Coal• Coal bunker supplies coal to pulverizing

fuel mills. Each bunker can hold 1,000 tonnes of coal, and there may be six to eight bunkers per unit

• Power station coal is not as lumpy as coal used in the home. Typically around half of it is less than 12.5 millimeters across and 95% is less than 50 millimeters

• This when powdered is called “200 mesh” cleared. That is the powdered coal passes through a sieve with so many holes in square inch area. It is better than the face powder in terms of size

5. Stack and Reclaim • Machines are used to put coal out to the

stockpile and reclaim coal from the stockpile

• Water is sprayed on coal to stop them from getting burnt when in storage yard due to internal heat up or sun heat

• Coal when powdered is heated and being lifted by hot air that is sent into the “mills”

• This goes and burns inside the furnace producing ash and converting the water in the pipelines into steam

6. Coal Feeder • The variable speed coal feeder feeds

coal from the bunkers to the mill • It uses a conveyor to move coal

through a fixed gap at a precisely controlled speed

• Varying the speed controls the amount of coal supplied to the boilers

• These are precision bits of equipment that have to move exact amounts of coal.

• They can move 40 tonnes of coal in an hour

7. Pulverisor or Mill • Each of unit may have six to eight

pulverizing fuel mills, each capable of pulverizing 40 tonnes of coal per hour

• Inside the mills, ten giant hollow steel rollers crush the coal into a fine powder

• Crushing the coal into a fine powder makes it easier to burn it more completely

• In other words, “due to the increased surface area, the combustion efficiency increases”

8. Primary Air Fan • Air to blow the coal from the mill to

the boiler, called the primary air, is supplied by a large fan driven by a variable speed motor

• When mixed with a stream of air the powdered coal behaves more like a gas than a solid

• Primary air does two jobs – heating the coal powder and secondly lifting it into the furnace through pipelines

9. Boiler light-up• Spark plug provides the initial

ignition. Light Diesel oil is then fed to the burner and it catches fire

• This is followed by heavy furnace oil (HFO)

• Once a stable flame is established the coal/air mix is blown through the burner where it lights spontaneously

• The oil are then shut off. Burner position, coal flow and air flow are controlled to achieve desired output of temperature, pressure and flow and hence the electricity

• At full output 4,000 MW power station can burn more than 50,000 tons of coal a day

10. Boiler • To produce steam each boiler converts

energy, in the form of coal, into steam • The boiler is lined with steel tubing in

which pure boiler feed water is turned to steam by the heat created from the burning of coal

• Each boiler is as high as 60 mts and weighs about 40,00,000 kg (4000 T)

• Inside the boiler there is enough steel tubing to stretch the 500 kilometres and they are joined together by about 20,000 joints

• Pressure inside the tubes could be about hundred times that of car’s wheel pressure

11. Forced Draught (FD) Fan • Each unit shall have two forced

draught fans • The fans draw warm air from the top

of the boiler house through large air heaters becoming the primary and secondary air used for the boiler combustion process

• The air heater warms the incoming air by transferring heat energy from the outgoing flue gases

12. Air Pre-Heater (APH) • The air heaters use the remaining

heat energy in the flue gas to heat up the combustion air for the boiler

• Efficiency is increased by using this heat that would otherwise go up the chimney. The air temperature leaving the air heaters is at 300°C

• The air heaters use the remaining heat energy and efficiency is increased by using this heat that would otherwise go up the chimney

13. Electro-Static Precipitator • Each boiler has 4 passes with 7 fields each

containing high voltage electrodes• These attract the dust or ash from the flue

gases • At regular intervals the electrodes are

rapped with motor-driven hammers and the PFA falls into hoppers below

• In a year 1,000 MW station may generate 1.5 million ton of ash

• This is one of the ways to clean up the flue gases or smoke sent up the chimney

• Secondly this ash is used by construction industry for use in building materials (bricks !!, Cement Fillers)

14. Induced Draught (ID) Fan • Two induced draught fans draw

gases out of the boiler• The gas has already passed through

the air heaters and precipitators before it has reached these fans

• The heat from the flue gases or smoke is used in the air heaters to heat up the primary and secondary air

15. Chimney • The chimney is 275 meters' high and

50,000 tonnes of reinforced concrete were used to make it

• It consists of flues each of which serve typically two or three boilers (two units)

16. Super heater • The steam produced in the boiler

goes to the steam drum and is then piped through the primary, platen and final super-heaters where it reaches the outlet temperature of 560°C and 160 ksc pressure

• At this point in the process they have now turned the water into a very powerful source of energy

• This rotates the turbine to which generator is on the other end

• From rotating generator electricity is produced

17. High Pressure Turbine • High pressure steam at 560°C and

160 ksc pressure passes through the high pressure turbine. The exhaust steam from this section is returned to the boiler for reheating before being used in the next section of the turbine set.

• The blades in the high pressure turbine are the smallest of all the turbine blades, this is because the incoming steam has very high energy and occupies a low volume. The blades are fixed to a shaft and as the steam hits the blades it causes the shaft to rotate

18. Boiler Reheater • After expanding through the high

pressure turbine the exhaust steam is returned to the boiler at 360°C and 40 ksc pressure for reheating before being used in the intermediate pressure turbine

• The Reheater reheats the steam from a temperature of 360°C back to 560°C

19. Intermediate Pressure Turbine • On leaving the boiler Reheater,

steam enters the intermediate pressure turbine at 560°C and 40 ksc pressure (1 ksc = 14.22 psi )

• From here the steam goes straight to the next section of the turbine set

• The steam has expanded and has less energy when it enters this section, so here the turbine blades are bigger than those in the high pressure turbine

• The blades are fixed to a shaft and as the steam hits the blades it causes the shaft to rotate

20. Low Pressure Turbine • From the intermediate pressure

turbines, the steam continues its expansion in the three low pressure turbines. The steam entering the turbines is at 300°C and 6 ksc pressure

• To get the most work out of the steam, exhaust pressure is kept very low, just 50 mille-bar above a complete vacuum

• The tip speed of the largest blades with the shaft spinning at 3,000 revolutions per minute is 2,000 kmph

21. Rotor • The shaft that runs through the

turbines is coupled to the rotor, which is a large electromagnet inside a cylinder of copper windings called the stator

• The rotor weighs 100 tonnes and rotates at 3,000 revolutions per minute

22. Stator • As the electromagnet rotates inside

the copper windings, a magnetic field is created which induces a three phase alternating electric current (AC) in the stator windings

• Together the rotor and stator are known as the generator. The stator weighs 300 tonnes

• Electricity is generated at over 80 times the voltage in our homes

• This is “stepped up” to about 4,00,000 volts and then “transmitted”

23. Generator Transformer • From the generator the electricity

then goes to a “transformer” where the voltage is increased to 4,00,000 volts before sending it via cables to the “Grid” for distribution

• Each 1 MW generates about 8 million units and gives about Rs 2 crores revenue every year

• This generates enough electricity to power around 5,000 avg. homes

24. Condenser • With its useful energy spent in the

turbines the steam then passes to condensers

• Here it is condensed back into water and pumped back to the boiler

• This happens via a series of low pressure and high pressure feed heaters

25. Condensate Extraction Pump • The condensate water is drawn from

the condenser by the extraction pump and sent to the low pressure feed heaters

26. Low Pressure Feed Heaters • Feedwater from the condensate

extraction pumps passes through low pressure feed heaters. Steam is used to heat the feedwater

• After the last feedheater, the feedwater is at around 160°C.

27. Deaerator • From the low pressure feed heaters

the water passes through the deaerator before going to the high pressure (HP) feed heaters.

28. Boiler Feed Pump • The boiler feed pump pumps water

into the boiler, overcoming the boiler pressure of 160 bar to achieve it

• The pump is driven by a steam turbine or an electric motor

• It runs at 7,000 revolutions per minute

29. High Pressure Feed Heaters • With a similar purpose to the low

pressure feed heaters, the high pressure feed heaters are the last stage of feedwater heating before the feedwater enters the boiler system at the economizer

• Feedwater leaving these heaters is at 250°C

30. Economizer • Flue gases leaving the superheater

and reheater still contain useful energy

• Water from the high pressure feed heaters is heated in the economiser from 250°C to 290°C before it continues to the steam drum

• Having given up its last heat in the boiler, the flue gases move on to the air heater

31. Drum • After leaving the economizer, the

feedwater reaches the drum, which is a cylindrical vessel at the top of the boiler

• From here the water flows by natural circulation through downpipes into the boiler

• Saturated steam collects here ready to go to the superheater

• “Drum” does the important function of “separating steam from a mixture of steam and water”

32. Cooling Tower • The warm river water is taken from

the condenser tubes to about a quarter of the way up the 100 metre high cooling tower where it is dropped through honeycombed plastic packing

• This breaks the water up into a very fine spray, increasing the surface area of the water droplets making it easier to cool

• The cooling tower is designed as a natural draught chimney, drawing cold air from outside through the falling water

• Cool water is collected in pond at the bottom of the cooling tower

• From here it is pumped back to the condensers

33. Circulating Water Pumps • The circulating water pumps are

used to circulate the water from the cooling tower to the condenser and back again

34. Circulating Water Make-Up Pumps

• These pumps are used to supply water for make-up purpose

• Before going to the cooling Tower the silt is removed in large sedimentation tanks

35. FGD• After passing through the electrostatic precipitators, the boiler flue gas is

increased in pressure and then cooled from between 115°C-130°C to 80°C• It enters the lowest part of the absorber and is further cooled by water used to

wash the inlet duct to prevent a build up of solids• The main SO2 absorption process, and the washing out of any remaining

pulverised fuel ash, occurs as the gas is ‘scrubbed’ by the re-circulating limestone slurry

• This is taken from the bottom of the absorber and is sprayed downwards from nozzles arranged at five separate levels in the absorber tower

• As a result of the process chemistry, the recirculating slurry becomes predominantly gypsum and a portion is continuously pumped away for gypsum separation and the removal of water using a hydro-cyclone system

• A waste water treatment plant ensures any water from the FGD process returned to the river meets quality standards set by the regulatory authority

Finally..

• The cleaned flue gas is discharged up the 275 metre high chimney which has been lined with steel plates/brick lining.

• The generated power stepped upto 4 lakh volts is transmitted and handed over to

distributors at lower voltages

• Finally it is supplied to households at 230 volts and to industries at little higher voltages..

THANK YOU