Workflow and performance of Coal Based Thermal Power Plants

Workflow and performance of Coal Based Thermal Power Plants

Workflow and performance of Coal Based Thermal Power Plants

A coal based thermal power plant converts the chemical energy of the coal into electrical energy through the medium of thermal power plant components.

Major Thermal Power Plant Components -
Below are the major Thermal Power Plant Components required to generate electricity
  1. Boiler and auxiliaries system
  2. Turbine and auxiliaries system
  3. Generator and auxiliaries system
  4. Electrical system
  5. Coal handling plant
  6. Ash handling plant
  7. Water treatment plant
  8. Fuel oil handling plant
  9. Circulating water system 
Boiler and Auxiliaries System: 
Boiler and Auxiliaries System is required to produce electrical power and providing steam for a various process in the thermal power plant. 
Salient features of 500MW boiler of BHEL make are as follows
The boiler is radiant reheat, controlled circulation, single drum, dry bottom type unit
The boiler is designed for maximum continuous rating for the following parameters
S/H Outlet – 1725 tons/ hr
R/ H Outlet – 1530 tons/ hr
Working pressure – 178 kg/cm2
S / H outlet temp. – 540 degree
R /H outlet temp. – 540 degree
R / H Inlet temp. – 344 degree
R / H Inlet pressure – 42.85 kg/cm2
R/H Outlet pressure – 43.46 kg/ cm2
Feed water temp. at economizer out let – 256 degree
 
 
Each boiler corner is fitted with tangential burner boxes comprising of four high energy arc igniters, four light-heavy oil burners, and eight pulverized coal burners. Angle of burner tilt is plus minus 30degree.
Feed water to boiler drum passes through L.P. Heaters, H.P. Heaters, Economizer
Water from drum flows into water wall circuit via three boiler circulating pumps, returning steam/water mixture to boiler drum
Steam from top of drum enters the superheater circuit. The superheater temp. is maintained by spray attemperator provided between LTSH outlet header and superheater divisional panel header.
 
Reheater temperature is maintained by burner tilt or spray attemperator installed in the first stage of reheat

Boiler Auxiliary System:
Boiler Auxiliary System is mostly used during the startup of any thermal power plants for many purposes like heating of fuel oil, steam blowing, and initial gland sealing of turbine glands. 
Pulverized Coal System: 
  • The crushed coal is fed to each coal mill by its coal feeder from coal bunker. Primary air is supplied from primary air fans to coal mills which dries the coal and transports the pulverized coal through coal piping system to coal nozzles in wind box assemblies.
  • Pulverized coal and air streams entering the furnace are ignited by suitable ignition source at coal nozzle exit.
Primary Air System: 
  • Primary air system supplies hot air to coal mills to dry and convey pulverized coal to coal burners.
  • The system consists of two primary air fans, two steam coiled air preheater and two regenerative type primary air preheaters. SCAPH are provided in P.A. fan discharge duct to avoid cold end corrosion of air preheater. Each fan is sufficient to cater the need of 60% MCR load.
Secondary Air System: 
  • Secondary air system supplies balance air required for pulverized coal combustion.
  • The system consists of two forced draft fans, two steam coiled air preheaters, two regenerative type air preheaters. SCAPH are provided in F.D. fan discharge duct to avoid cold end corrosion of air preheater.
  • Each fan is sufficient to cater the need of 60% M.C.R. load.
Flue Gas Handling System
  • Flue gas handling system consists of inducing draft fans to draw the flue gases from furnace and discharge them to atmosphere through 275meter height chimney. During the passage flue gases passes on Super heater, Re heater, LTSH, Economizer, Air pre heaters to improve the boiler efficiency and passes through Electro static precipitators to keep emissions through chimney in prescribed limit of 50 mg/m3 set by MOEF.
  • Two induced draft fans are provided. Each fan is sufficient to cater the need of 60% MCR load.
Furnace SafeGuard Supervisory System (FSSS)
  • Furnace safeguard supervisory system is provided for logical sequence and optimum boiler operation. It also provides full on line firing safety in boiler operation.
Turbine and Auxillary System:
Turbine and Auxillary System is a mechanical device that is used to extract thermal energy from the pressurized steam water boiler. 
Turbine: 
  • 500MW BHEL make turbine is three cylinder, re heat, condensing, single shaft machine with separate High pressure, Intermediate pressure, and Low pressure turbine. H.P. Turbine is a single flow with 17 reaction stages while I.P. and L.P. Turbines are double flow with 12 and 6 reaction stages per flow. The individual turbines and generator are coupled by rigid couplings.
  • Steam flow for H.P. Turbine is controlled by 2 main combined stop and control valves and steam flow for I.P. Turbine is controlled by 2 Re heat stop and control valves. I.P. Exhaust is connected to L.P. Turbine without any valve.
  • L.P. Turbine exhaust is connected to the condenser. The purpose of the condenser is to create sub atmospheric pressure (vacuum) so that steam can expand to the lowest possible pressure and higher output can be obtained. Condensation of steam in to water in the condenser is achieved by removing the latent heat of steam by circulating cooling water in the condenser.    
  • On H.P. Exhaust lines swing check valve is provided to prevent entry of hot re heat steam flowing back in H.P. Turbine.
  • H.P., I.P., L.P. Turbine and Generator rotors are supported on six bearings. H.P. Turbine rear bearing is combined journal and thrust bearing. Other bearings are journal bearings.
Turbine Lubrication Oil System:
  • Turbine lubricating oil system lubricates and cools the bearings. The main oil pump mounted on turbine shaft in front pedestal draws oil from lubricating oil tank, circulates it through bearings and oil coolers and returns back to main oil tank.
  • Auxiliary lubricating oil pumps are provided for lubricating the bearings during start up and shut down.
Turbine Governing System: 
  • Turbine is provided with electro hydraulic governing system.
  • Electric system measures and controls the speed of the turbine by operating the control valves hydraulically in conjunction with electro hydraulic convertor.
Hydraulic Turning Gear:
  • Hydraulic turning gear is provided to rotate the turbine at low speed during start up or shut down to avoid distortion of the shaft due to irregular heating up or cooling down.
  • Jacking oil pump provides high pressure oil under turbine bearings and lifts the turbine shaft to prevent failure of boundary lubrication of bearings while the turbine is rotating at slow speed.

Condensate Extraction Pump & Boiler Feed Pump:

  • Condensate from the condenser is pumped through low-pressure heaters to de-aerator where air is removed.
  • Boiler feed pump suction is taken from de-aerator and its discharge is passed through high-pressure heaters and economizer to boiler drum.
Generator and Auxiliary Systems:
  • Generator is a 2 pole machine which is rotated by the turbine at 3000rpm speed to generate electricity.
  • The generator consists of a Stator frame, stator core, stator winding, end shields, rotor, rotor winding, and field connections

Following additional auxiliaries are required for generator operation

  • Excitation system
  • Seal oil system
  • Hydrogen gas system
  • Primary water system
Excitation system:
  • Excitation system consist of three phase pilot exciter, three phase main exciter and rectifier wheels
  • Pilot exciter is a permanent magnet three phase generator. Output voltage is fed to A.V.R. where it is rectified and controlled. This variable D.C. current is used for excitation of three phase main exciter.
  • In main exciter field winding is arranged on stator and 3 phase A.C. voltage is induced on rotor. This voltage is fed to rectifier wheels and is then feed to generator field winding through D.C. leads arranged in generator rotor shaft.
Seal Oil System:
  • Generator shaft seals are provided to prevent hydrogen losses at generator shaft and to prevent air ingress in to a generator. Generator shaft seals are supplied with pressurized seal oil. This system consists of two A.C. Seal oil pump and one D.C. Seal oil pump, seal oil tank, seal oil coolers.
Hydrogen Gas System: 
  • Hydrogen is circulated in a generator in a closed circuit by multi stage axial fan located at turbine end to cool the generator. The fan draws hot gases and delivers it to coolers and re-circulates the same.
  • The system comprise of hydrogen cylinder rack and carbon dioxide rack.
  • To prevent hydrogen coming in contact with air which causes an explosion, carbon dioxide is used . Air in a generator is purged with co2 and then co2 is purged with hydrogen so direct contact of air and hydrogen is avoided.
Primary Water System:
  • Primary water system is provided to continuously remove the heat produced in stator winding by circulating low conductivity D.M. water through hollow conductors of the stator winding.
  • The system consist of a primary water tank, primary water pump and primary water cooler
Electrical System: 
Power transformers:
  • Generator transformer shall be provided to step up generating a voltage for connection to grid.
  • Unit auxiliary transformer shall be provided to meet the unit load requirement during normal running of the unit
  • Station transformer shall be provided to cater the start up power requirement, balance of plant station load requirement during normal operation and in case of UAT outage
H.T. and L.T. switch gears:
  • H.T. switch gears (11/6.6/3.3 KV) are provided to supply power to run various auxiliaries of main plant and balance of plant having 160 kw or more motor rating
  • L.T. switch gears (415 V) are provided to supply power to run various auxiliaries in the plant having less than 160 kw motor rating 
Diesel Generator set:
  • D.G. set shall be provided for feeding emergency loads such as turbine lubricating oil pump, hydrogen seal oil pump, jacking oil pump, Air pre heater motor, emergency lighting etc.
D.C. System:
  • Standard voltage levels of 220/48/24 volt D.C. batteries and battery charger system is provided for the control and protection system of the unit
Electric Earthing System:
  • All electric equipments shall have to be ensured that they are properly earthed as per IS standard to avoid an accident
Illumination system:
 
Adequate illumination shall be provided as per IS standard. Emergency A.C. and D.C. lighting shall be provided at important places such as P.C.R. , Turbine floor, emergency escape route etc.
 
Motors:
  • All A.C. motors shall be squirrel cage induction type for direct on load starting
  • D.C. motors shall be shunt wound type       
CHP
  • 1000MW (2x500MW) coal based thermal power station requires about 15000 tons of coal per day.
  • Coal is transported from coal mines to the power plant by railway in wagons or if it a pit head station then by merry go round system. Coal is unloaded from wagon to a moving underground conveyor belt. Coal is either stacked in coal stock yard having about 30 days capacity or is taken to coal crusher house with the help of conveyor belt as per CHP layout. Coal from coal mines can never be of uniform size so in coal crusher house, coal is crushed to 20mm size and is fed to coal bunkers located in the boiler house. With the help of raw coal, feeder coal is fed to coal pulverizers.
AHP
  • 1000MW (2x500MW) coal based power plant generates daily about 5000 tons of ash which is  required to be dispose of.  Out of 5000 tons about 20% is bottom ash and 80% is fly ash.
  • Bottom ash is removed from boiler bottom and is crushed with the help of bottom ash crusher which is then mixed with water and this bottom ash slurry is dumped with the help of pump in the ash pond.
  • Fly ash is removed from ESP and is disposed of with the help of high concentrated slurry disposal pump in the ash pond. HCSD pump reduces the water requirement for ash disposal.
Water Treatment Plant: 
  • 1000MW (2X500MW) coal based power plant requires about 84000 meter cube water per day.

Raw Water Pump House:

  • It is located at the source of water from where water is lifted and brought in to the power plant with the help of raw water pumps.
Raw Water Reservoir: 
  • Raw water reservoir is normally built in power station premises to store the 8days water requirement.
Water pretreatment plant:
  • Generally, clarifiers are used to remove dissolved solids, suspended particulate matter from raw water to avoid fouling /scaling in the condenser to achieve desired heat transfer.  About 80 % water out of total water requirement of the plant is for cooling water make up.

D.M. Plant:

  • To avoid the corrosion of boiler tubes and silica deposits on turbine blades very high quality feed water is required in the boiler. Such type of water is obtained from D.M. plant where clarified water is pass through ion exchangers to remove the impurities present in raw water. About 3% water out of total water requirement is needed as feed water make up.
  • Water is also required for auxiliary cooling, ash disposal, fire fighting, coal dust suppression, drinking, general service.
Fuel Oil Handling Plant:
  • Light diesel oil (L.D.O.) is required to start the boiler from cold condition when steam is not available for Heavy furnace oil (H.F.O.) heating.
  • H.F.O. is used for initial firing and coal flame stabilization.
  • H.F.O. may come by Rail tankers or by road tanker. Depending on this unloading facilities are planned.
  • For 1000MW station 2 H.F.O. storage tanks of 2000 KL and 2 L.D.O. storage tanks of 500 KL are provided.
  • Storage tanks are of cone roof, vertical cylindrical, welded tanks provided with level indicators/ level switches. Conical roof is provided to ensure drainage of rain water.
  • Fuel oil pumps are provided to transport oil to boiler burners.
Circulating Water System: 
  • Circulating water system is provided for supplying water to the condenser to condense L.P. Turbine exhaust steam.
  • Heat gained by circulating water in the condenser is removed in cooling tower for re circulating the circulating water.
  • Cooling towers are of the following two types:
  • Induced draft cooling tower (IDCT)
  • Natural draft cooling tower (NDCT)
  • IDCT are preferred at pit head station as cost of power consumed is low. NDCT are preferred at the station which are not pit head as there is no power consumption in NDCT since no rotating equipments are there.
  • For 2x500 MW station, 5 circulating pumps of each 32200m3/hr are provided. (4 working + 1 common stand bye). IDCT 4 numbers (2 per unit) each of 30000m3/hr capacity or 2 NDCT (1 per unit) each of 60000 m3/ hr capacity with a cooling range of 9.5 degree are provided. 

Workflow and Performance of Coal Based Thermal Power Plants | Major Thermal Power Plant Components | Coal Based Thermal Power Plants 

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