Training Report

Published on February 2017 | Categories: Documents | Downloads: 38 | Comments: 0 | Views: 338
of 69
Download PDF   Embed   Report

Comments

Content

TRAINING REPORT
N.T.P.C. Badarpur Thermal Power Station

Submitted By:Rohan Rustagi (10BEE029) B.Tech.(E.E.) Institute of Technology, Nirma University.

INDEX
1. INTRODUCTION 2. GENERAL OVERVIEW OF A THERMAL POWER PLANT 3. RANKINE CYCLE 4. SWITCHGEAR 5. MOTORS 6. COAL HANDLING PLANT 7. ASH HANDLING PLANT 8. WATER TREATMENT PLANT 9. GENERATOR 10. TRANSFORMER 11. SWITCHYARD 12. TURBINE

INTRODUCTION
The year 1975 witnessed the birth of an organization that went on to achieve great feats in performance in a sector that was then, characterized largely by lack of investment, severe supply shortages and operational practices that mad the commercial viability of the sector unsustainable. NTPC symbolized hop of the country suffering from crippling power black-outs, the Government of India, which was trying to pull an ailing, economy back on the track and he World Bank, which was supporting the country in many development initiatives. Thus, NTPC was created not only o redraw the power map of India but also excel in is performance and se benchmarks for others to follow. It succeeded on both counts. Today with an installed capacity of 39,174 MW, NTPC contributes one fourth of the Nations Power generation, with only one fifth of India total installed capacity. An ISO 9001:2000 Certified company, it is world world`s 10th largest power generation in the world, 3rd largest in the Asia. NTPC is #1 independent Power Producer (IPP) IN THE WORLD. Also it is 384 th largest company in he world (FORBES 2011). It is one of the largest Indian companies in terms of market cap. The corporation recorded a generation of 222.07 billion unit (BUS) IN 2011-2012; through 16 coal based and 7 gas based power plant spread all over the country and also has 07 plants in joint venture. Rated as one of the best company to work for in India, it has developed into a multi-location and multi-fuel company over the past three decades.

POWER GENERATION
Presently, NTPC generates power from Coal and Gas. With an installed capacity of 39,174 MW, NTPC is the largest power generating major in the country. It has also diversified into hydro power, coal mining, power equipment manufacturing, oil and gas exploration, power trading and distribution. With an increasing presence in the power value chain, NTPC is well on its way to becoming an ―Integrating Power Major.‖

INSTALLED CAPACITY
Be it the generating capacity or plant performance or operational efficiency, NTPC’s Installed Capacity and performance depicts the company’s outstanding performance across a number of parameters.

POWER STATIONS IN INDIA

ABOUT BPTS
The Badarpur Thermal Power Plant has an installed capacity of 705 MW. The main plant equipment was supplied by M/S. BHEL. The boiler of Stage - 1 (3×95) MW units are of CZECHOSOLOVAKIAN design and that of 210 MW units are of COMBUSTION ENGINEERING design. The Turbo-alternators, supplied by M/S BHEL, are of RUSSIAN design and Control and Instrumentation for Stage-1 (3×95) and Stage-2 units are mostly of RUSSIAN design and for Stage-3 are of KENT design and supplies by M/S Instrumentation Ltd., KOTA.

STRATEGIES

JOURNEY OF NTPC

General Overview of a Thermal Power Plant:Though each plant is unique in itself in terms of specific features and functionalities, still there is a broad outline to which all thermal power plants confirm to and in this article we will study about the general layout of a typical power plant. There are four main circuits in any thermal power plant and these are Coal & Ash Circuit – this circuit deals mainly with feeding the boiler with coal for combustion purposes and taking care of the ash that is generated during the combustion process and includes equipment and paraphernalia that is used to handle the transfer and storage of coal and ash. Air & Gas Circuit – we know that air is one of the main components of the fire triangle and hence necessary for combustion. Since lots of coal is burnt inside the boiler it needs a sufficient quantity of air which is supplied using either forced draught or induced draught fans. The exhaust gases from the combustion are in turn used to heat the ingoing air through a heat exchanger before being let off in the atmosphere. The equipment which handles all these processes fall under this circuit. Feed Water & Steam Circuit – this section deals with supplying of steam generated from the boiler to the turbines and to handle the outgoing steam from the turbine by cooling it to form water in the condenser so that it can be reused in the boiler plus making good any losses due to evaporation etc. Cooling Water Circuit – this part of the thermal power plant deals with handling of the cooling water required in the system. Since the amount of water required to cool the outgoing steam from the boiler is substantial, it is either taken from a nearby water source such as a river, or it is done through evaporation if the quantity of cooling water available is limited.

 Boiler
Boiler or Steam generator helps to burn the coal and transfer the heat to water, thereby generating steam at the required pressure and temperature. Coal is fed to the Raw Coal Bunkers that is sufficient to store one day requirement. Then, the coal is pulverised into a fine powder in the pulverisers or mills and carried away into the boiler by the hot air at 60 – 70 supplied by Primary Air Fans. The fuel/air mixture is fed in the four corners of the boiler in a corner fired boiler, in four or five elevations. For complete combustion of coal in the furnace, secondary air is supplied by the Forced Draft fans. Feed water is supplied to the boiler drum situated at the top of the boiler from the economiser which make use of the heat in flue gas to preheat the feed water to increase the boiler efficiency. Water converts into steam in the furnace area and the dry and saturated steam is collected in the top portion of the drum. The saturated steam is then superheated superheaters. The superheated steam is called main steam that leaves the boiler at rated pressure and temperature.

 Turbine
Turbine is a high speed rotating machine that converts the kinetic energy and pressure energy of the steam into useful work. A turbine generally has three stages viz. High Pressure, Intermediate Pressure and Low Pressure. The main steam enters into HP turbine and after expansion in the turbine the pressure and temperature fall down. The main steam is returned to the boiler for reheating in the Reheater. The Hot Reheated steam is admitted in the intermediate pressure turbine IPT after expansion in the IPT, steam enters into the Low pressure turbine. When the useful work is extracted from the steam, the pressure falls below the atmospheric pressure. Vacuum is maintained in the condenser to create steam flow by means of vacuum pump or steam jet ejectors. Cooling Water flows in the tubes of condenser to cool the steam. The condensate is pumped to the deaerator by condensate extraction pump (CEP) through Low Pressure Heaters where the temperature gain in the condensate is achieved from the heat of extraction steam from the turbine. The deaerator helps to remove the oxygen in the feed water as dissolved oxygen enhances corrosive action. The feed water from deaerator is pumped to the boiler drum, by Boiler Feed Pump (BFP) through HP heaters and economiser. Boiler Feed Pump is the heart of a thermal power station as it supplies feed water to the boiler continuously.

 Generator
Turbine is coupled with the TurboGenerator that normally spins at 3000 rpm in countries with 50 Hz supply frequency or at 3600 rpm in countries with 60 Hz supply frequency. The generated voltage is stepped up in Generator Transformer and the power is evacuated through transmission line feeders.

 Handling of Fly Ash
After complete combustion of coal in the furnace, the heat in the flue gas is utilised to preheat the water in the economiser and primary and secondary air in AirPreHeaters. The fly ash laden gas is evacuated through chimney by Induced Draft fans through Electro Static Precipitators (ESP). Electro Static Precipitators are devices that separate fly ash from the flue gas and thus Solid Particulate Matter in the exit gas is controlled. Apart from fly ash, bottom ash is also collected in the bottom of the furnace and is disposed off in the form of ash slurry in ash dyke. The hot water discharge from condenser is cooled in cooling towers of Natural Draft or Induced Draft type in closed circuit cooling system. In open circuit cooling system, the hot water is discharged into sea or perennial river in such a way that it does not affect the flora / fauna of the ecosystem. Cooling Water Pumps (CW pumps) pump the cold water stream back to the condenser.

 Environmental issues
Though thermal generation is the back bone of our country, the Carbon-di-oxide emission amounts to Green House Gas (GHG) playing a vital role in Global Warming. United Nations Framework Convention of Climate Change (UNFCCC) is working on emission reduction. New technologies like Oxyfuel combustion, Integrated Gasification Combined Cycle (IGCC), CO2capture in flue gas etc., are on the way to mitigate CO2 emission related problems.

THE RANKINE CYCLE
Any steam powered power plant employs the Rankine thermodynamic cycle. To avoid transporting and compressing two-phase fluid (as in Carnot cycle), we can try to condense all fluid exiting from the turbine into saturated liquid before compressed it by a pump.

The four processes in a simple Rankine cycle are: isentropic compression of the liquid from 1 to 2, isobaric heating of the liquid, boiling and super-heating the vapour (from 2 to 3), isentropic expansion from 3 to 4, and isobaric heat rejection until full condensation of the vapour. The Rankine cycle is less efficient than the Carnot cycle (Fig. 17.9), but it is more practical since the compression is not in the two-phase region (see Chapter 6) and only requires a small work, and the expansion is mainly in the gaseous phase (high-speed droplets erode turbine blades). Water is not the ideal working substance because it changes phase at relatively low temperatures (below the critical point at 647 K), generating a lot of entropy in the heat transfer from typical high-temperatures heat-sources: 1000 K in nuclear reactors up to 2000 K in conventional combustion plants. Nevertheless, water is practically the only working substance used, because of its good thermal properties and availability. A caution note is that the right-hand-side end of the two-phase region, the saturated vapour line, that for water in the Ts diagram has the shape shown in Fig. 17.9, may be more vertical and even have negative slope at low temperatures for heavier molecular substances, naturally avoiding the problem of wet -vapour at the turbine.

Since, at ambient temperature (the heat sink), water change phase at lower -than-atmospheric pressure (e.g. 5 kPa at 33 ºC) the condenser must operate under vacuum and a so-called 'deaerator' is needed to remove non-condensable gasses from the feed water or infiltration; moreover, removal of oxygen and carbon dioxide in feed water is always desirable to avoid corrosions in the circuit. Gas solubility in a liquid decreases near the pure-liquid vapour saturation curve, thus deaeration may be achieved by

heating the liquid at constant pressure (e.g. adding some vapour) or by making vacuum at constant temperature (e.g. with a small jet of vapour by Venturi suction). Maximum temperature in a steam power plant is limited by metallurgical constraints to less than 900 K (some 600 ºC), and the maximum pressure depends on the variations to the simple Rankine cycle used, with typical values of 10 MPa (supercritical Rankine cycles surpass 22 MPa).



Thermal efficiency can be improved by – – – (a) Lowering the condensing pressure (lower condensing temperature, lower T L) (b) Superheating the steam to higher temperature ( c) Increasing the boiler pressure (increase boiler temperature, increase T H)

REHEATING
• The optimal way of increasing the boiler pressure but not increase the moisture content in the exiting vapor is to reheat the vapor after it exits from a first-stage turbine and redirect this reheated vapor into a second turbine.

Fig:- Reheating  In this reheat cycle, steam is expanded isentropically to an intermediate pressure in a highpressure turbine (stage I) and sent back to the boiler, where it is reheated at constant pressure to the inlet temperature of the high-pressure turbine. Then the steam is sent to a low-pressure turbine and expands to the condenser pressure (stage II) When the number of the reheat stages increases, the expansion and reheat processes approach an isothermal process at the maximum temperature. But using more than two stages is not practical.



REGENERATION
From 2-2’, the temperature at 2 is very low, therefore, the heat addition process is at a lower temperature and therefore, the thermal efficiency is lower. Why? Use regenerator to heat up the liquid (feedwater) leaving the pump before sending it to the boiler, therefore, increase the averaged temperature (efficiency as well) during heat addition in the boiler.

Regenerative Cycle:• • • Improve efficiency by increasing feedwater temperature before it enters the boiler. Open feedwater: Mix steam with the feedwater in a mixing chamber. Closed feedwater: No mixing.

Diagram of a typical coal-fired thermal power station

In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating this steam is taken to the Intermediate Pressure Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high pressure steam is converted to electrical energy in the Generator.

SWITCHGEAR
 Switchgear is one that makes or breaks the electrical circuit.  It is a switching device that opens & closes a circuit that defined as apparatus used for switching, Lon rolling & protecting the electrical circuit & equipments.  The switchgear equipment is essentially concerned with switching & interrupting currents either under normal or abnormal operating conditions.  The tubular switch with ordinary fuse is simplest form of switchgear & is used to control & protect& other equipments in homes, offices etc.  For circuits of higher ratings, a High Rupturing Capacity (H.R.C) fuse in condition with a switch may serve the purpose of controlling & protecting the circuit.  However such switchgear cannot be used profitably on high voltage system (3.3 KV) for 2 reasons.  Firstly, when a fuse blows, it takes some time to replace it & consequently there is interruption of service to customer.  Secondly, the fuse cannot successfully interrupt large currents that result from the High Voltage System.  In order to interrupt heavy fault currents, automatic circuit breakers are used.  There are very few types of circuit breakers in B.P.T.S they are VCB, OCB, and SF6 gas circuit breaker.  The most expensive circuit breaker is the SF6 type due to gas.  There are various companies which manufacture these circuit breakers: VOLTAS, JYOTI, and KIRLOSKAR.  Switchgear includes switches, fuses, circuit breakers, relays & other equipments.  In low tension switch gear thermal over load relays are used whereas in high tension 5 different types of relays are used.

EQUIPMENTS THAT NORMALLY FALL IN THIS CATEGORY
ISOLATOR  Isolator cannot operate unless breaker is open  Bus 1 and bus 2 isolators cannot be closed simultaneously  The interlock can be bypass in the event of closing of bus coupler breaker.  No isolator can operate when the corresponding earth switch is on SWITCHING ISOLATOR  Switching isolator is capable of:  Interrupting charging current  Interrupting transformer magnetizing current  Load transformer switching. Its main application is in connection with the transformer feeder as the unit makes it possible to switch gear one transformer while the other is still on load. CIRCUIT BREAKER  One which can make or break the circuit on load and even on faults is referred to as circuit breakers. This equipment is the most important and is heavy duty equipment mainly utilized for protection of various circuits and operations on load. Normally circuit breakers installed are accompanied by isolators. LOAD BREAK SWITCHES  These are those interrupting devices which can make or break circuits. These are normally on same circuit, which are backed by circuit breakers EARTH SWITCHES  Devices which are used normally to earth a particular system, to avoid any accident happening due to induction on account of live adjoining circuits. These equipments do not handle any appreciable current at all. Apart from this equipment there are a number of relays etc. which are used in switchgear.

LOW TENSION SWITCHGEAR
MAIN SWITCH  Main switch is control equipment which controls or disconnects the main supply. The main switch for 3 phase supply is available for the range 32A, 63A, 100A, 200Q, 300A at 500V grade.

FUSES  With Avery high generating capacity of the modern power stations extremely heavy carnets would flow in the fault and the fuse clearing the fault would be required to withstand extremely heavy stress in process. It is used for supplying power to auxiliaries with backup fuse protection. With fuses, quick break, quick make and double break switch fuses for 63A and 100A, switch fuses for 200A,400A, 600A, 800A and 1000A are used.

CONTACTORS  AC Contractors are 3 poles suitable for D.O.L Starting of motors and protecting the connected motors. OVERLOAD RELAY  For overload protection, thermal overload relay are best suited for this purpose. They operate due to the action of heat generated by passage of current through relay element. AIR CIRCUIT BREAKERS  It is seen that use of oil in circuit breaker may cause a fire. So in all circuits breakers at large capacity air at high pressure is used which is maximum at the time of quick tripping of contacts. This reduces the possibility of sparking. The pressure may vary from 50-60kg/cm^2 for high and medium capacity circuit breakers.

HIGH TENSION SWITCHGEAR
MINIMUM OIL CIRCUIT BREAKER  These use oil as quenching medium. AIR CIRCUIT BREAKER  In this the compressed air pressure around 15 kg per cm^2 is used for extinction of arc caused by flow of air around the moving circuit . The breaker is closed by applying pressure at lower opening and opened by applying pressure at upper opening. When contacts operate, the cold air rushes around the movable contacts and blown the arc SF6 CIRCUIT BREAKER  The principle of current interruption is similar to that of air blast circuit breaker. It simply employs the arc extinguishing medium namely SF6. When it is broken down under an electrical stress, it will quickly reconstitute itself. VACUUM CIRCUIT BREAKER  It works on the principle that vacuum is used to save the purpose of insulation and. In regards of insulation and strength, vacuum is superior dielectric medium and is better that all other medium except air and sulphur which are generally used at high pressure.

OIL CIRCUIT BREAKERS

AIR CIRCUIT BREAKERS

SF6 CIRCUIT BREAKERS

Coal Handling Plant (C.H.P.):The coal handling plant (CHP) in a thermal power station covers unloading of coal, its crushing, storage and filling of boiler bunkers. The planning and design of the CHP is site specific and depends on the following factors: i) Station capacity ii) Coal source and quality iii) Coal transportation mode iv) Topography and geometry of the area for coal handling system

 Station capacity
Station capacity determines the quantum of coal to be handled by coal handling plant and thus the capacity of coal unloading system, crushers, coal conveying system etc. Generally for unit size of 500 MW and above, one coal handling plant is provided to cater for two units. Coal conveying system may cater to maximum three units to limit the outage of units in the event of failure of coal handling plant. Provision of interconnection between separate CHP’s may also be provided. If a plant consists of nonidentical units (in terms of size), then separate CHPs may be necessary to cater to different bunker floor levels due to non-uniformity of unit sizes. Alternatively, plants with non-identical units can have some facilities like unloading, crushing, storage in common and separate conveyors may be used to feed different bunker floors.

 Coal source and quality
Sources of coal for a thermal power station may vary i.e. indigenous run of mine coal, indigenous washed coal or imported coal. Quality of the coal (GCV, HGI, moisture content etc.) determines the specification of coal handling equipment apart from the quantity of coal to be handled. Presently need for providing facilities for blending of indigenous and imported coal is also being felt in view of the shortage of Indian coal. Some time coal blending may also be resorted for environmental reasons. Blending can be done in many ways. One method is to provide facility in coal handling system to lay indigenous and imported coal in layers on the belts while conveying coal to bunkers. These coal layers would get mixed while falling into bunkers. The other method is to stock indigenous and imported coal inlayers in stockyard. Yet another method in use is dedicating one mill for firing imported coal and then adjust the mill parameters to achieve the optimum heat load of the burners.

 Coal transportation mode
The selection of particular mode of transportation of coal depends on the location of power plant with respect to coal mines/ coal sources and other site conditions. Various transportation means such as rail or other captive systems such as merry go round (MGR), belt conveyors are adopted. For coastal stations, coal is received at ports byships/barges and transported through belt/pipe conveyor system or rail etc. Most of the power stations receive coal through rail. Power stations located near to the indigenous coal source (i.e. mine mouth) receive coal through their own MGR and those located far away (load centre stations) from the coal mines receive coal rakes through Indian Railway network. Conveyor Belt may also be used as an alternative to MGR. This type of transportation system is preferred when the coal mine or port is close to the powerplant. The coal received at power station may be unloaded by means of track hopper or wagon tippler or by combination of both depending on the type of wagons (BOBR or Box-N wagons) in the coal rakes expected to be received at the station. 

Topography and geometry of the area for coal handling system

Layout of coal handling system varies with topography, geometry of the area, coal storage requirements as well as wind direction. No. of transfer points may also vary with topography and geometry of the area.

BRIEF DESCRIPTION OF COAL HANDLING PLANT SYSTEM : Coal unloading system
As mentioned above, the coal received at power station may be unloaded by means of wagon tippler or track hopper or by combination of both depending on the type of coal rakes to be used for transportation of coal to the station. Generally coal rake consists of 59 wagons, each wagon carrying payload of 60 tons. The two unloading systems are briefly described below: Track hopper unloading system The coal received through bottom opening bottom release (BOBR) wagon rakes is unloaded in under ground R.C.C. track hopper. Paddle feeders are employed under track hopper to scoop the coal and feeding onto underground reclaim conveyors. Belt weigh scales are provided on these conveyors for measurement of coal flow rate. Wagon tippler unloading system The coal received from Box-N wagons is unloaded in underground RCC hoppers by means of rota side type wagon tipplers. Side arm chargers are employed for placement of wagons on the tippler table and removal of empty wagon from tippler table after tippling. Apron feeders are employed under each wagon tippler for extracting coal from wagon tippler hopper and feeding onto underground reclaim conveyors. Belt weigh scales are provided on these conveyors for measurement of coal flow rate.

Provision is kept for shunting locomotives for placing the rakes in position for the side arm charger to handle and begin unloading operation.

 Coal crushing
Coal unloaded in the wagon tippler hoppers/track hoppers is conveyed to crusher house by belt conveyors via pent house and transfer points depending on the CHP layout. Suspended magnets are provided on conveyors at pent house for removal of tramp Iron pieces. Metal detectors are also provided to detect non-ferrous materials present in the coal before crushers. In case the sized coal is received, then the coal is sent directly to stockyard and the crusher is by-passed. Conveyors leading to crusher house have facility for manual stone picking, at a suitable location after penthouse. In line magnetic separators are also provided at discharge end of conveyors for removal of remaining metallic ferrous tramp from the coal before it reaches the crushers. Coal sampling unit is provided to sample the uncrushed coal. The size of the coal received is normally (-) 300 mm which may, however, depend on coal tie up. The received coal is sized in crushers (ring granulators) from (-) 300 mm to (-) 20 mm. Screens (vibrating grizzly type or roller screens) provided upstream of the crushers screen out (-) 20 mm coal from the feed and (+) 20 mm coal is fed to the crushers. A set of rod gates and rack & pinion gates is provided before screens to permit maintenance of equipment downstream without affecting the operation of other stream. The crushed coal is either fed to coal bunkers of the boilers or discharged on to conveyors for storage in coal stockyard through conveyors and transfer points.

 Coal Stacking & Reclaiming at Stockyard
Crushed coal is sent to stockyard when coal bunkers are full. Stacking/ reclaiming of coal is done by bucket wheel type stacker-cum- reclaimer moving on rails. The stacker-cum- reclaimer can stack coal on either sides of the yard conveyor. During stacking mode coal is fed from conveyors on boom conveyor and while in reclaim mode, boom conveyor discharges coal on the yard conveyor for feeding coal to bunkers through conveyors and transfer points. The yard conveyor can be reversible type depending on layout requirement. When direct unloading from rakes is not in operation, coal is reclaimed by the stacker –cum-reclaimer and fed to the coal bunkers. Emergency reclaim hopper (ERH) can be provided to reclaim coal by dozers when stacker –cum- reclaimer is not in operation. Emergency reclaim hopper can also be used for coal blending. Coal stockpile is provided with required storage capacity depending on location of plant vis-à-vis coal source. Metal detectors and in-line magnetic separators are also provided before feeding to bunkers for removal of metallic ferrous tramp from reclaimed crushed coal. Coal sampling unit is provided to sample crushed coal of (-) 20 mm size. Belt weigh scales are also provided, on conveyors for measurement of flow rate of as fired coal.

 Dust Control System and Ventilation system
The dust control system is required for control of fugitive dust emissions from dust generation points such as transfer points, feeders, crushers etc. Dust control is achieved by dust suppression and extraction system. Dust suppression is achieved by two methods viz. Plain Water Dust Suppression System and Dry Fog Type Dust Suppression System. Ventilation system is provided for all the working areas/ locations/ buildings/ underground structures of CHP. The required ventilation is achieved by mechanical ventilation system/ pressurised ventilation system depending on the area requirement. The pressurized ventilation system is capable of pressurizing slightly above atmospheric pressure to prevent ingress of dust from outside. The MCC/switchgear room areas of coal handling plant are provided with pressurised ventilation system while other areas have mechanical ventilation. The control rooms, office room and RIO (Remote Input/ Output) room are provided with air conditioning system. The coal handling plant at NTPC-BTPS consists of two plants: Old Coal Handling Plant (OCHP)  New Coal Handling Plant (NCHP) The OCHP supplies coal to Unit- I, II, III & NCHP supplies coal to Unit- IV and V.

The main constituents of CHP plant are: WAGON TIPPLER
Wagon from coal yard come to the tippler and emptied here. There are 2 wagon tipplers in the OCHP.

 CONVEYER
Conveyer belts are used in the OCHP to transfer coal from one place to other as required in a convenient & safe way.

 ZERO SPEED SWITCH
It is used as a safety device for the motor i.e. if the belt is not moving & the motor is ON, then it burns to save the motor. This switch checks the speed of the belt & switches off the motor when speed is zero.

 METAL DETECTOR
As the conveyer belt take coal from wagon to crusher house, no metal piece should go along with coal. To achieve this objective, metal detectors & separators are used.

 CRUSHER HOUSE
Both the plants i.e. OCHP & NCHP use TATA crusher powered by BHEL motor. Crusher is designed to crush the pieces to 20 mm size i.e. practically considered as the optimum size for transfer via conveyer.

 ROTARY BREAKER
If any large piece of metal of any hard substances like metal impurities comes in the conveyer belt which cause load on the metal separator, then the rotary breaker rejects them reducing the load on the metal detector.

 PULL GUARD SWITCH
These are the switches which are installed at every 10m gap in a conveyer belt to ensure the safety of motors running the conveyer belts. If at any time some accident happens or coal jumps from belt and starts collecting at a place, then the switch can be used.

CHP auxillaries:   
   Operational Cycle Wagon Tippler Conveyer Belt System Crushing Mechanism

1. Operational Cycle
Bunkering Cycle Stacking Cycle Reclaiming Cycle

2. Wagon Tippler

 These are used to unload the coal wagons into coal hoppers in very less time (e.g. 20 wagons/hr. or more).  Weighing of coal is carried out at wagon tippler.  Normally 55-60 metric ton of coal come in each wagon.  Wt. of coal = Wt. of loaded wagon – Wt. of empty wagon

Equipments Used in Wagon Tippler System:  Water Sprinkler System Dust Extraction System

3. CONVEYOR BELT SYSTEM: Construction of Conveyor Belt  Conveyor Belt Drive System  Counter Weight  Idlers  Protection System  Interlocking  Magnetic Separator

4. CRUSHING MECHANISM IN CHP:Three Stage Crushing System is used in Plant.



1. Double Roll Crusher



2. Rotary Breaker Crusher



3. Impact Crusher

TRIPPERS :Belt conveyors passing over the top of overhead coal bunkers are fitted with travelling trippers having chutes on one or both sides of the conveyor. These trippers are power propelled and travels on rails. It has been provided with clamping device to prevent it from running away.

WAGON TIPPLER

CRUSHER

MAGNETIC FILTER

TRIPPER

R.C. BUNKER

R.C. FEEDER

FURNANCE

MILL

FIG. 3.1

Fig. Coal Cycle

SEQUENTIAL OPERATION OF OCHP: Unloading the coal  Crushing & storage.  Conveying to boiler bunkers. o Coal arrives to plant via road, rail, sea, and river or canal route from collieries. Most of it arrives by rail route only in railway wagons. Coal requirement by this plant is approximately 10,500 metric ton/day. o This coal is tippled into hoppers. If the coal is oversized (400 mm sq), then it is broken manually so that it passes the hopper mesh where through elliptic feeder it is put into vibrators & then to conveyor belt 1A & 1B. o The coal through conveyor belts 1A & 1B goes to the crusher house. Also the extra coal is sent to stockyard through these belts. o In the crusher house the small size coal pieces goes directly to the belt 2A & 2B whereas the big size coal pieces are crushed in the crusher & then given to the belts 2A & 2B. o The crushed coal is taken to the bunker house via the conveyor belts 3A & 3B where it c can be used for further operations.

SEQUENTIAL OPERATION OF NCHP:o Coal arrives in wagons and tipples into hoppers. o if the coal is oversized (400mm sq), then it is broken manually so that it passes through the hopper mesh. o From hopper it is taken to TP-6 12A & 12B. o Conveyors 12A & 12B take the coal to the breaker house which renders the coal size to be 100 mm sq. o Metal separator & metal detector are installed in conveyor belts 14A/B & 15A/B respectively to remove the metal impurities o Stones which are not able to pass through the 100mm sq mesh of hammer are rejected via 18A & 18B to the rejection house. o Extra coal is sent to the reclaim hopper via conveyor 16A & 16B. o From TP-7, coal is taken by conveyor 14A & 14B to the crusher house whose function is to render size of the coal to 20mm sq.

Pulverisation of Coal :Before being sent to the furnance, crushed coal is pulverized into fine sand like particles in the ball mill and stored in PC(pulverized coal) bunkers. Coal is pulverized (powdered) to increase its surface exposure thus permitting rapid combustion.Efficient use of coal depends greatly on the combustion process employed.For large scale generation of energy the efficient method of burning coal is confined still to pulverized coal combustion. The pulverized coal is obtained by grinding the raw coal in pulverizing mills. The essential functions of pulverising mills are as follows: (i) Drying of the coal (ii) Grinding (iii) Separation of particles of the desired size. Proper drying of raw coal which may contain moisture is necessary for effective grinding. The coal pulverising mills reduce coal to powder form by three actions as follows: (i)Impact (ii) Attrition (abrasion) (iii) Crushing. Most of the mills use all the above mentioned all the three actions in varying degrees. In impact type mills hammers break the coal into smaller pieces whereas in attrition type the coal pieces which rub against each other or metal surfaces to disintegrate. In crushing type mills coal caught between metal rolling surfaces gets broken into pieces. The crushing mills use steel balls in a container. These balls act as crushing elements The type of mill used at BPTS is BALL MILL-

A line diagram of ball mill using two classifiers is shown in Fig. 4.21. It consists of a slowly rotating drum which is partly filled with steel balls. Raw coal from feeders is supplied to the classifier from where it moves to the drum by means of a screw conveyor. As the drum rotates the coal gets pulverized due to the combined impact between coal and steel balls. Hot air is introduced into the drum. The powdered coal is picked up by the air and the coal air mixture enters the classifiers, where sharp changes in the direction of the mixture throw out the oversized coal particles. The over-sized particles are returned to the drum. The coal air mixture from the classifier moves to the exhauster fan and then it is supplied to the burners.

Pulverised coal follows its way into the furnace, where it is fired with excess air to create a ‘fire ball’. It is where water is converted ti high pressure steam. In large plants like BPTS we use water tube boilers.

Pulverised coal firing :Bin or Central System. It is shown in Fig. 4.26. Crushed coal from the raw coal bunker is fed by gravity to a dryer where hot air is passed through the coal to dry it. The dryer may use waste flue gases, preheated air or bleeder steam as drying agent. The dry coal is then transferred to the pulverizing mill. The pulverised coal obtained is transferred to the pulverised coal bunker (bin). The transporting air is separated from the coal in the cyclone separator. The primary air is mixed with the coal at the feeder and the mixture is supplied to the burner.

ASH HANDLING PLANT
Sustainable ash utilization is one of the key concerns at NTPC. The Ash Utilization Division (AUD), set up in 1991, strives to derive maximum usage from the vast quantities of ash produced at its coal based power stations. The AUD proactively formulates policies, plans and programmes for ash utilization. It further monitors the progress in these activities and works for developing new segments of ash utilization. Ash Utilization Cell at each station, handles ash utilization activities. The quality of fly ash produced at NTPC’s power stations is extremely good with respect to fineness, low unburnt carbon and has high pozzolanic activity and conforms to the requirements of IS 3812 2003-Pulverized Fuel Ash for use as Pozzolana in Cement, Cement Mortar and Concrete. The fly ash generated at NTPC stations is ideal for use in manufacture of Cement, Concrete, Concrete products, Cellular concrete products, Bricks/blocks/ tiles etc. To facilitate availability of dry fly ash to end – users, dry fly ash evacuation and storage system have been set up at coal based stations. The various segments of ash utilization currently include Cement, Asbestos – Cement products & Concrete manufacturing industries, Land development, Road embankment construction, Ash Dyke Raising, Building Products such as Bricks/ blocks/tiles, Reclamation of coal mine and as a soil amender and source of micro and macro-nutrients in agriculture.

MoEF Notification on Fly Ash
Ministry of Environment & Forests (MoEF), Govt. of India vide its notification (amendment) dated 3rd Nov 2009 has made it mandatory:



Within 100Km radius of a Thermal Power Plant
1. To use Fly Ash based Building products such as cement or concrete, fly ash bricks, blocks, tiles etc. in all construction projects 2. To use Fly Ash in Road or Flyover Embankment construction 3. To use Fly Ash in Reclamation of low lying areas



Within 50Km of a Thermal Power Plant (By Road)
1. To use Fly Ash in back filling of underground and open cast mines



Financial institutions to include a clause in their loan documents for compliance of this notification

Major Projects where Fly Ash has been utilized
1. 20 lakh Cu.M of Pond Ash from NTPC Badarpur Station has been utilized in Noida - Greater Noida Expressway 2. About 5.0 lakh Cu.M of Pond Ash from NTPC Badarpur Station has been utilized in Yamuna Expressway & Badarpur Flyover

3. More than 15 lakh Cu.M of Pond Ash has been used by Delhi Metro Rail Corporation (DMRC) in their Shastri Park rail car depot from NTPC Badarpur Station.

Brief Description of ASH DISPOSAL SYSTEM at NTPC-BadarpurA large quantity of ash is, produced in steam power plants using coal. Ash produced in about 10 to 20% of the total coal burnt in the furnace. Handling of ash is a problem because ash coming out of the furnace is too hot, it is dusty and irritating to handle and is accompanied by some poisonous gases. It is desirable to quench the ash before handling due to following reasons: 1. Quenching reduces the temperature of ash. 2. It reduces the corrosive action of ash. 3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will disintegrate. 4. Quenching reduces the dust accompanying the ash. Handling of ash includes its removal from the furnace, loading on the conveyors and delivered to the fill from where it can be disposed off.

The commonly used ash handling systems are as follows : (i) Hydraulic system (ii) pneumatic system (iii) Mechanical system. The commonly used ash discharge equipment is as follows: (i) Rail road cars (ii) Motor truck (iii) Barge.

Hydraulic Ash Handling System: The hydraulic system carries the ash with the flow of water with high velocity through a channel and finally dumps into a sump. The hydraulic system is divided into:  Low velocity system  High velocity system In the low velocity system the ash from the boilers falls into a stream of water flowing into the sump. The ash is carried along with the water and they are separated at the sump. In the high velocity system a jet of water is sprayed to quench the hot ash. Two other jets force the ash into a trough in which they are washed away by the water into the sump .Hydraulic Ash handling system is used at the Badarpur Thermal Power station. The slurry is then transported via underground pipes for kilometers. Bottom Ash Collection and Disposal At the bottom of every boiler, a hopper has been provided for collection of the bottom ash from the bottom of the furnace. This hopper is always filled with water to quench the ash and clinkers falling down from the furnace. Some arrangement is included to crush the clinkers and for conveying the crushed clinkers and bottom ash to a storage site.

FLY ASH COLLECTION
Fly ash is the finely divided residue that results from the combustion of pulverized coal and is transported from the combustion chamber by exhaust gases. Fly ash is produced by coal-fired electric and steam generating plants. Typically, coal is pulverized and blown with air into the boiler's combustion chamber where it immediately ignites, generating heat and producing a molten mineral residue. Boiler tubes extract heat from the boiler, cooling the flue gas and causing the molten mineral residue to harden and form ash. Coarse ash particles, referred to as bottom ash or slag, fall to the bottom of the combustion chamber, while the lighter fine ash particles, termed fly ash, remain suspended in the flue gas. Prior to exhausting the flue gas, fly ash is removed by particulate emission control devices, such as electrostatic precipitators or filter fabric baghouses.

Figure 1-1: Method of fly ash transfer can be dry, wet, or both.

Handling: The collected fly ash is typically conveyed pneumatically from the ESP or filter fabric hoppers to storage silos where it is kept dry pending utilization or further processing, or to a system where the dry ash is mixed with water and conveyed (sluiced) to an on-site storage pond. The dry collected ash is normally stored and handled using equipment and procedures similar to those used for handling portland cement:
   

Fly ash is stored in silos, domes and other bulk storage facilities Fly ash can be transferred using air slides, bucket conveyors and screw conveyors, or it can be pneumatically conveyed through pipelines under positive or negative pressure conditions Fly ash is transported to markets in bulk tanker trucks, rail cars and barges/ships Fly ash can be packaged in super sacks or smaller bags for specialty applications

Dry collected fly ash can also be moistened with water and wetting agents, when applicable, using specialized equipment (conditioned) and hauled in covered dump trucks for special applications such as structural fills. Water conditioned fly ash can be stockpiled at jobsites. Exposed stockpiled material must be kept moist or covered with tarpaulins, plastic, or equivalent materials to prevent dust emission.

ELECTROSTATIC PRECIPITATORS (E.S.P.)

Six activities typically take place:

1. 2. 3. 4. 5. 6.

Ionization - Charging of particles Migration - Transporting the charged particles to the collecting surfaces Collection - Precipitation of the charged particles onto the collecting surfaces Charge Dissipation - Neutralizing the charged particles on the collecting surfaces Particle Dislodging - Removing the particles from the collecting surface to the hopper Particle Removal - Conveying the particles from the hopper to a disposal point

Process variables: 1. Gas Flow Rate A precipitator operates best with a gas velocity of 3.5 - 5.5 ft/sec. At higher velocity, particle reentrainment increases rapidly. If velocity is too low, performance may suffer from poor gas flow distribution or from particle dropout in the ductwork. 2. Particle Size A precipitator collects particles most easily when the particle size is coarse. The generation of the charging corona in the inlet field may be suppressed if the gas stream has too many small particles (less than 1 µm). Very small particles (0.2 - 0.4µm) are the most difficult to collect because the fundamental field-

charging mechanism is overwhelmed by diffusion charging due to random collisions with free ions. 3. Particle Resistivity Resistivity is resistance to electrical conduction. The higher the resistivity, the harder it is for a particle to transfer its electrical charge. Resistivity is influenced by the chemical composition of the gas stream, particle temperature and gas temperature. Resistivity should be kept in the range of 108 - 1010 ohm-cm. High resistivity can reduce precipitator performance. For example, in combustion processes, burning reduced-sulfur coal increases resistivity and reduces the collecting efficiency of the precipitator. Sodium and iron oxides in the fly ash can reduce resistivity and improve performance, especially at higher operating temperatures. On the other hand, low resistivity can also be a problem. For example (in combustion processes), unburned carbon reduces precipitator performance because it is so conductive and loses its electrical charge so quickly that it is easily re-entrained from the collecting plate. 4. Gas Temperature The effect of gas temperature on precipitator collecting efficiency, given its influence on particle resistivity, can be significant. 5. Interactions to Consider Particle size distribution and particle resistivity affect the cohesiveness of the layer of precipitated material on the collecting plates and the ability of the rapping system to dislodge this layer for transport into the precipitator hopper without excessive re-entrainment.

Basic Operation: Discharge electrodes (emitter) are charged to a negative potential of 70 kv dc. Ash particles are charged and deposited at the thick metallic plates (collector) that are part of the circuit. The plates are hammered by rapper motors and fly ash is collected at the bottom and finds into the ash hopper. Entire operation is monitored in the control room. At BPTS there are 26 rapper motors and 16 transformers (one for each unit). H.T. supply from plant is converted to dc at 70 kv by means of a pair of SCR’s, bridge rectifier and A.C. voltage controller (P.I.D.) along with several other auxiliaries (shown in fig below).



About Discharge Electrodes:Discharge electrodes emit charging current and provide voltage that generates an electrical field between the discharge electrodes and the collecting plates. The electrical field forces dust particles in the gas stream to migrate toward the collecting plates. The particles then precipitate onto the collecting plates. Common types of discharge electrodes include:  Straight round wires  Twisted wire pairs  Barbed discharge wires  Rigid masts  Rigid frames

 

Rigid spiked pipes Spiral wires

Discharge electrodes are typically supported from the upper discharge frame and are held in alignment between the upper and lower discharge frames. The upper discharge frame is in turn supported from the roof of the precipitator casing. High-voltage insulators are incorporated into the support system. In weighted wire systems, the discharge electrodes are held taut by weights at the lower end of the wires.

About Collecting Plates
Collecting plates are designed to receive and retain the precipitated particles until they are intentionally removed into the hopper. Collecting plates are also part of the electrical power circuit of the precipitator. These collecting plate functions are incorporated into the precipitator design. Plate baffles shield the precipitated particles from the gas flow while smooth surfaces provide for high operating voltage. Collecting plates are suspended from the precipitator casing and form the gas passages within the precipitator. While the design of the collecting plates varies by manufacturer, there are two common designs:  Plates supported from anvil beams at either end.The anvil beam is also the point of impact for the collecting rapper.



Plates supported with hooks directly from the precipitator casing .Two or more collecting plates are connected at or near the center by rapper beams, which then serve as impact points for the rapping system.

Top, center, or bottom spacer bars may be used to maintain collecting plate alignment and sustain electrical clearances to the discharge system.

About Power Supplies and Controls
The power supply system is designed to provide voltage to the electrical field (or bus section) at the highest possible level. The voltage must be controlled to avoid causing sustained arcing or sparking between the electrodes and the collecting plates. Electrically, a precipitator is divided into a grid, with electrical fields in series (in the direction of the gas flow) and one or more bus sections in parallel (cross-wise to the gas flow). When electrical fields are in series, the power supply for each field can be adjusted to optimize operation of that field. Likewise, having more than one electrical bus section in parallel allows adjustments to compensate for their differences, so that power input can be optimized. The power supply system has four basic components:     Automatic voltage control Step-up transformer High-voltage rectifier Sensing device

Automatic voltage control varies the power to the transformer-rectifier in response to signals received from sensors in the precipitator and the transformer-rectifier itself. It monitors the electrical conditions inside the precipitator, protects the internal components from arc-over damages, and protects the transformer-rectifier and other components in the primary circuit. The ideal automatic voltage control would produce the maximum collecting efficiency by holding the operating voltage of the precipitator at a level just below the spark-over voltage. However, this level cannot be achieved given that conditions change from moment to moment. Instead, the automatic voltage control increases output from the transformer-rectifier until a spark occurs. Then the control resets to a lower power level, and the power increases again until the next spark occurs.

About Rapping Systems
Rappers are time-controlled systems provided for removing dust from the collecting plates and the discharge electrodes as well as for gas distribution devices (optional) and for hopper walls (optional). Rapping systems may be actuated by electrical or pneumatic power, or by mechanical means. Tumbling hammers may also be used to dislodge ash. Rapping methods include:

 Electric vibrators  Electric solenoid piston drop rappers  Pneumatic vibrating rappers  Tumbling hammers  Sonic horns (do not require transmission assemblies) 1. Discharge Electrode Rapping In general, discharge electrodes should be kept as free as possible of accumulated particulate. The rapping system for the discharge electrodes should be operated on a continuous schedule with repeat times in the 2 - 4 minute range, depending on the size and inlet particulate loading of the precipitator. 2. Collecting Plate Rapping Collecting plate rapping must remove the bulk of the precipitated dust. The collecting plates are supported from anvil beams or directly with hooks from the precipitator casing. With anvil beam support, the impact of the rapping system is directed into the beams located at the leading and/or trailing edge of the collecting plates. For direct casing support, the impact is directed into the rapper beams located at or near the center of the top of the collecting plates. The first electrical field generally collects about 60-80% of the inlet dust load. The first field plates should be rapped often enough so that their precipitated layer of particulate is about 3/8 - 1/2" thick. There is no advantage in rapping more often since the precipitated dust has not yet agglomerated to a sheet which requires a minimum layer thickness. Sheet formation is essential to make the dust drop into the precipitator hopper without re-entrainment into the gas stream. Rapping less frequently typically results in a deterioration of the electrical power input by adding an additional resistance into the power circuit. Once an optimum rapping cycle has been found for the first electrical field (which may vary across the face of a large precipitator), the optimum rapping cycles for the downstream electrical fields can be established. The collecting plate rapping system of the first field has a repeat time T equal to the time it takes to build a 3/8 - 1/2"layer on the collecting plates. The plates in the second field should have a repeat time of about 5T, and the plates in the third field should have a repeat time of 25T. Ideally, these repeat times yield a deposited layer of 3/8-1/2" for the plates in all three fields. Adjustment may be required for factors such as dust resistivity, dust layer cohesiveness, gas temperature effects, electrical field height and length, and the collecting area served by one rapper. 3. Gas Distribution Plate and Hopper Wall Rapping The gas distribution plates should also be kept free of excessive particulate buildup and may require rapping on a continuous base with a cycle time in the 10-20 minute range, depending on the inlet particulate loading of the precipitator and the nature of the particulate. Gas distribution plates in the outlet of the precipitator may be rapped less often (every 30 - 60 minutes). 4. Improving Rapping System Performance All precipitator rapping systems allow adjustment of rapping frequency, normally starting with the highest frequency (the least time between raps), progressing to the lowest frequency. The times that are actually available may be limited. Rapping systems with pneumatic or electric actuators allow variations of the rapping intensity. Pneumatic or electric vibrators allow adjustments of the rapping

time. State-of-the-art rapper controls allow selection of rapping sequences, selection of individual rappers, and provide anti-coincidence schemes which allow only one rapper to operate at a given time.

Rapping systems can be optimized for top precipitator performance using precipitator power input and stack opacity as criteria. Optimization of the rapping system starts with the discharge electrode rapping system operating on its own time schedule, for example with repeat times of 2 - 4 minutes. The rapping system for the gas distribution screens in the inlet and outlet of the precipitator should then be operated with repeat times of 2-3 minutes for the inlet and 2 - 3 hours for the outlet screens. The only rapping system requiring optimization is the collecting plate rapping system. The optimization should start with the Collecting Plate Rapping Schedule determined above. Next, the rapping frequency of the inlet field should be increased or decreased until the electrical power input of the inlet field remains constant. Next, the rapping frequency of the other fields should be adjusted in sequence until their electrical power inputs remain constant. If the stack opacity trace shows rapping spikes, the rapping intensity should be reduced while observing the electrical power input of the precipitator. The adjustment of the rapping system for optimum precipitator performance is a slow process. It requires a substantial amount of time for stabilization after each adjustment.

About Hoppers
Precipitator hoppers are designed to completely discharge dust load on demand. Typically, precipitator hoppers are rectangular in cross-section with sides of at least 60-degree slope. These hoppers are insulated from the neck above the discharge flange with the insulation covering the entire hopper area. In addition, the lower 1/4- 1/3 of the hopper wall may be heated. Discharge diameters are generally 8" - 12". 1. Insulation Insulation provides protection for facility personnel as well as working to retain as much hopper wall temperature as possible. Hopper wall temperature retention discourages condensation on the inside of the hopper. Heaters are added to ensure hot metal surfaces immediately above the fly ash discharge. 2. Facilitating hopper discharge Hopper discharge problems are caused by compaction of the fly ash in the hopper. Compaction characteristics are affected by moisture content, particle size and shape, head of material, and vibration. The flow of fly ash out of the hopper can be facilitated by the use of external vibrators. These can operate on the outside wall of the hopper or on an internal hopper baffle. 3. Hopper fluidizers Hopper fluidizers have a membrane that permits air flow to the fly ash directly above. This air flow fills the voids between the fly ash particles at a slight pressure, changes the repose angle of the particles, and promotes gravity flow. 4. Ash handling system The fly ash handling system evacuates the fly ash from the hoppers, and transports the fly ash to reprocessing or to disposal. The ash handling system should be designed and operated to remove the collected fly ash from the hoppers without causing re-entrainment into the gas flow

through the precipitator. The design of the ash handling system should allow for flexibility of scheduling the hopper discharges according to the fly ash being collected in these hoppers. Either the precipitator hopper or the feeder hopper is used for temporarily storing material prior to discharge. Three types of handling systems are in use: Negative pressure or vacuum system Connects to the hopper by a simple discharge valve 2. Positive pressure dilute phase system Uses an airlock-type feeder; the feeder is separated from the hopper by an inlet gate and from the conveying line by a discharge gate 3. Positive pressure dense phase system Connects to the hopper with an airlock type feeder.
1.

There are other aspects also like-Gas Distribution Systems, Ductwork, Gas Velocity Distribution, Reentrainment, Corona Power.

WATER TREATMENT PLANT
As the types of boiler are not alike their working pressure and operating conditions vary and so do the types and methods of water treatment. Water treatment plants used in thermal power plants are designed to process the raw water to water with very lowin dissolved solids known as "dematerialized water". No doubt, this plant has to be engineered very carefully keeping in view the type of raw water to the thermal plant, its treatment costs and overall economics Actually, the type of demineralization process chosen for a power station depends on three main factors: •The quality of the raw water. •The degree of de-ionization i.e. treated water quality •Selectivity of resins.
Water treatment process which is generally made up of two sections: •Pre-treatment section •Demineralization section

Pre-treatment section
Pretreatment plant removes the suspended solids such as clay, silt, organic and inorganic matter, plants and other microscopic organism. The turbidity may be taken as of two types of suspended solids in water. Firstly, the separable solids and secondly the non separable solids (colloids). The coarsecomponents, such as sand, silt etc, can be removed from the water by simple sedimentation. Finer particles however, will not settle in any reasonable time and must be flocculated to produce the large particles which are settling able. Long term ability to remainsuspended in water is basically a function of both size and specificgravity. The settling rate of the colloidal and finely divided (approximately 001 to 1 micron) suspended matter is so slow that removing them from water by plain sedimentation is tank shavingordinary dimensions is impossible. Settling velocity of finelydivided and collide particles under gravity also are so small that ordinary sedimentation is not possible. It is necessary, therefore, to use procedures which agglomerate the small particles into larger aggregates, which have practical settling velocities. The term"Coagulation" and "flocculation" have been used indiscriminately to describe process of turbidity removal. "Coagulation" means to bring together the suspended particles. The process describes the effect produced by the addition of a chemical Al (SP) g to acolloidal dispersion resulting in particle destabilization by areduction of force tending to keep particles apart. Rapid mixing is important at this stage to obtain. Uniform dispersion of the chemical and to increase opportunity for particles to particle contact. This operation is done by flash mixer in

thec1ariflocculator. Second stage of formation of settle able particlesfrom destabilized colloidal sized particles is termed a"flocculation". Here coagulated particles grow in size by attaching to each other. In contrast to coagulation where the primary force is electrostatic or intrinsic, "flocculation" occurs by chemical bridging. Flocculation is obtained by gentle and prolonged mixingwhich converts the submicroscopic coagulated particle intodiscrete, visible & suspended particles. At this stage particles are large enough to settle rapidly under the influence of gravity anomaly be removed
If pre-treatment of the water is not done efficiently then consequences are as follows:

  

Si02 may escape with water which will increase the anion loading. Organic matter may escape which may cause organic fouling in the anion exchanger beds. In the 'pre-treatment plant chlorine addition provision is normally made to combat organic contamination. Cation loading may unnecessary increase due to addition of Ca (OH)2 in excess of calculated amount for raising the pH of the water for maximum floe formation and also AKOrDgmay precipitate out. If less than calculated amount of Ca (OH)2 is added, proper pH flocculation will not be obtained and silica escape to demineralization section will occur, thereby increasing load on anion bed.

Demineralization
This filter water is now used for demineralising purpose and is fed to cation exchanger bed, but enroute being first dechlorinated, which is either done by passing through activated carbon filter or injecting along the flow of water, an equivalent amount of sodium sulphite through some stroke pumps. The residual chlorine which is- maintained in clarification plant to remove organic matter from raw water is now detrimental to action resin and must be eliminated before its entry to this bed. Normally, the typical scheme of demineralization up to the .mark against average surface water is three bed system with a provision of removing gaseous carbon dioxide from water before feeding to Anion Exchanger. Now, let us see, what happens actually in each bed when water is passed from one to another. Resins, which are built on synthetic matrix of a styrene divinely benzene copolymer, are manufactured in such a way that these have the ability to, exchange one ion for another, hold it temporarily in chemical combination and give it to a strong electrolytic solution. Suitable treatment is also given to them in such a way that a particular resin absorbs only a particular group of ions. Resins, when absorbing and releasing cationic portion of dissolved salts, is called cation, exchanger resin and when removing anionic portion is called anion exchanger resin. Preset trend is of employing 'strongly acidic cation exchanger resin and strongly basic anion exchanger resin in a DM Plant of modernthermal power station. We may see that the chemically activegroup in a cationic resin is SOx-H (normally represented by RH) and in an anionic resin the active group is either tertiary amine or quaternary ammonium group (normally the resin is represented by ROH. The water from the ex-cation contains carbonic acid also sufficiently, which is very weak acid difficult to be removed by strongly basic anion resin and causing hindrance to remove silicate ions from the bed. It is therefore a usual practice to remove carbonic acid before it is led to anion exchanger bed. The excation water is trickled in fine streams from top of a tall tower packed with, rings, and compressed air

is passed from the bottom. Carbonic acid breaks into C03 and water mechanically (Henry's Law) with the carbon dioxide escaping into theatmosphere. The water is accumulated in suitable storage tank below the tower, called degassed water dump from where the same is led to anion exchanger bed, using acid resistant pump. The ex-anion water is fed to the mixed bed exchanger containing both cationic resin and anionic resin. This bed not only takes care of sodium slip from cation but also silica slip from anion exchanger very effectively. The final output fromthe mixed bed is Exira -ordinarily pure water having less than0.2/Mho conductivity 7.0 and silica content less than 0.02 pm. Any deviation from the above quality means that the resins in mixed bed are exhausted and need regeneration, regeneration of themixed bed first calls for suitable, back washing and settling, so that he two types of resins are separated from each other. Lighter anion resin rises to the top and the heavier cation resin settles to the bottom. Both the resins are then regenerated separately with alkali and acid, rinsed to the desired value and air mixed, to mix the resin again thoroughly. It is then put to final rinsing till the desired quality is obtained. It may be mentioned here that there are two types of strongly basic anion exchanger. Type II resins are slightly less basic than type I, but have higher regeneration efficiency than type I. Again as type II resins are unable to remove silica effectively, type I resins also have to be used for the purpose. As such, the general condition so far prevailing in India, is to employ type II resin in anion exchangers bed and type I resin in mixed bed (for the anionic portion).It is also a general convention to regenerate the above two resins under through fare system i.e. the caustic soda entering into mixed bed for regeneration, of type I anion resin, is utilized to regenerate type II resin in anion exchanger bed. The content of utilizing the above resin and mode of regeneration is now days being switched over from the economy to a higher cost so as to have more stringent quality control of the final D.M water.

INTERNAL TREATMENT
This final D.M effluent is then either led to hot well of the condenser directly as make up to boilers, or being stored in D.M. Water storage tanks first and then pumped for makeup purpose to boiler feed. There are five D.M.Tank: three tanks of 500 metric ton for three units Of 95 MW and other two of 600 metric ton for two units of 210 MW. The purpose of an internal water treatment program is: 1. To react with incoming feed water hardness and prevent it from precipitating on the boiler metal as scale 2. To condition any suspended matter such as hardness sludge in the boiler and make it non adherent to the boiler metal 3. To provide antifoam protection to permit a reasonable concentration of dissolved and suspended solids in the boiler water without foaming 4. To eliminate oxygen from the feed water 5. To provide enough alkalinity to prevent boiler corrosion 6. To prevent scaling and protect against corrosion in the steam-condensate systems.

D.M. Tank

AC MOTORS
 Squirrel cage motor  Wound motor  Slip ring induction motor In modern thermal power plant three phase squirrel cage induction motors are used but sometime double wound motor is used when we need high starting torque e.g. in ball mill. THREE PHASE INDUCTION MOTOR  Ns (speed) =120f/p  Stator can handle concentrated single layer winding, with each coil occupying one stator slot

 The most common type of winding are:  DISTRIBUTED WINDING : This type of winding is distributed over a number of slots.  DOUBLE LAYER WINDING : Each stator slot contains sides of two different coils. SQUIRREL CAGE INDUCTION MOTOR  Squirrel cage and wound cage have same mode of operation. Rotor conductors cut the rotating stator magnetic field. an emf is induced across the rotor winding, current flows, a rotor magnetic field is produced which interacts with the stator field causing a turning motion. The rotor does not rotate at synchronous speed, its speed varies with applied load. The slip speed being just enough to enable sufficient induced rotor current to produce the power dissipated by the motor load and motor losses. BEARINGS AND LUBRICATIONS A good bearing is needed for trouble free operation of motor. Since it is very costly part of the motor, due care has to be taken by checking it at regular intervals. So lubricating plays an important role. Two types of lubricating are widely used  Oil lubrication  Grease lubrication  Insulation INSULATION Winding is an essential part so it should be insulated.

INSTRUMENTS SEEN MICROMETER This instrument is used for measuring inside as well as outside diameter of bearing. MEGGAR This instrument is used for measuring insulation resistance. VIBRATION TESTER It measures the vibration of the motor. It is measured in three dimensions-axial, vertical and horizontal. Ther are 2 departments L.T. motors-for ratings 230-440 V.  H.T. motors-for ratings 6.6 KV.

GENERATORS
 The generator works on the principle of electromagnetic induction. There are two components stator and rotor. The rotor is the moving part and the stator is the stationary part. The rotor, which has a field winding, is given a excitation through a set of 3000rpm to give the required frequency of HZ. The rotor is cooled by Hydrogen gas, which is locally manufactured by the plant and has high heat carrying capacity of low density. If oxygen and hydrogen get mixed then they will form very high explosive and to prevent their combining in any way there is seal oil system. The stator cooling is done by de-mineralized (DM) water through hollow conductors. Water is fed by one end by Teflon tube. A boiler and a turbine are coupled to electric generators. Steam from the boiler is fed to the turbine through the connecting pipe. Steam

drives the turbine rotor. The turbine rotor drives the generator rotor which turns the electromagnet within the coil of wire conductors.  Carbon dioxide is provided from the top and oil is provided from bottom to the generator. With the help of carbon dioxide the oil is drained out to the oil tank.

RATINGS
  TURBO GENERAOR 100MW TURBO GENERATOR 210 MW

The 100 MW generator generates 10.75 KV and 210 MW generates 15.75 KV. The voltage is stepped up to 220 KV with the help of generator transformer and is connected to the grid. The voltage is stepped down to 6.6 KV with the help of UNIT AUXILLARY TRANSFORMER (UAT) and this voltage is used to drive the HT motors. The voltage is further stepped down to 415 V and then to 220 V and this voltage is used to drive Lt Motors. 100 MW TURBO GENERATOR          MAKE CAPACITY POWER STATOR VOLTAGE STATOR CURRENT SPEED POWER FACTOR FREQUENCY EXCITATION BHEL, Haridwar 117,500 KVA 100,000 KW 10,500 V 6475 A 5000rpm 0.85 50 HZ 280 V

210 MW TURBO GENERATOR        MAKE CAPACITY POWER STATOR VOLTAGE STATOR CURRENT SPEED POWER FACTOR BHEL, Haridwar 247,000 KVA 210,000 KW 15,750 V 9050 A 5000 rpm 0.85

  

FREQUENCY EXCITATION GAS PRESSURE

50 HZ 310 V 3.5 kg/cm

TRANSFORMERS
It is a static machine which increases or decreases the AC voltage without changing the frequency of the supply. It is a device that:  Transfer electric power from one circuit to another.  It accomplishes this by electromagnetic induction.  In this the two electric circuits are in mutual inductive influence of each other.

WORKING PRINCIPLE: It works on FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION (self or mutual induction depending on the type of transformer). COOLING As size of transformer becomes large, the rate of the oil circulating becomes insufficient to dissipate all the heat produced & artificial means of increasing the circulation by electric pumps. In very large transformers, special coolers with water circulation may have to be employed.

TYPES OF COOLING: Air cooling Air Natural (AN) Air Forced (AF)

Oil immersed cooling Oil Natural Air Natural (ONAN) Oil Natural Air Forced (ONAF) Oil Forced Air Natural (OFAN) Oil Forced Air Forced (OFAF)

Oil immersed Water cooling Oil Natural Water Forced (ONWF) Oil Forced Water Forced (OFWF)

MAIN PARTS OF TRANSFORMER

1. Secondary Winding 2. Primary Winding. 3. Oil Level 4. Conservator 5. Breather 6. Drain Cock 7. Cooling Tubes.

8. Transformer Oil. 9. Earth Point 10. Explosion Vent 11. Temperature Gauge. 12. Buchholz Relay 13. Secondary Terminal 14. Primary Terminal

GENERATOR TRANSFORMER(125 MVA UNIT 1 & 3 )

 RATING  TYPE OF COOLING  TEMP OF OIL  TEMP WINDING  KV (no load)

125MVA OFB 45^C 60^C HV-233 KVA LV-10.5 KVA

 LINE AMPERES

HV-310 A LV-6880

 PHASE  FREQUENCY  IMPEDANCE VOLTAGE  VECTOR GROUP  INSULATION LEVEL

THREE 50 HZ 15% Y DELTA HV-900 KV LV-Neutral-38

 CORE AND WINDING WEIGHT  WEIGHT OF OIL  TOTAL WEIGHT  OIL QUANTITY

110500 Kg 37200 Kg 188500 Kg 43900 lit

GENERATOR TRANSFORMER(166 MVA UNIT 4 )

 RATING  TYPE OF COOLING  TEMP OF OIL  TEMP WINDING  VOLTS AT NO LOAD

240MVA ON/OB/OFB C C HV-236000 LV-A5750

 LINE AMPERES LV-8798  PHASE  FREQUENCY  IMPEDANCE VOLTAGE  VECTOR GROUP  CORE AND WINDING WEIGHT  WEIGHT OF OIL  TOTAL WEIGHT  OIL QUANTITY  GUARANTEED MAX TEMP  DIVISION  YEAR

HV-587 A

THREE 50 HZ 15.55% Y DELTA 138800 Kg 37850 Kg 234000 Kg 42500 lit C KERELA 1977

UNIT AUXILIARY TRANSFORMER (UAT)
Unit I & V- 12.5 MVA The UAT draws its input from the main bus-ducts. The total KVA capacity of UAT required can be determined by assuming 0.85 power factor & 90% efficiency for total auxiliary motor load. It is safe & desirable to provide about 20% excess capacity then circulated to provide for miscellaneous auxiliaries & possible increase in auxiliary.

STATION TRANSFORMER
It is required to feed power to the auxiliaries during startups. This transformer is normally rated for initial auxiliary load requirements of the unit in typical cases; this load is of the order of 60% of the load at full generating capacity. It is provided with on load tap change to cater to the fluctuating voltage of the grid.

NEUTRAL GROUNDED TRANSFORMER
This transformer is connected with supply coming out of UAT in stage-2. This is used to ground the excess voltage if occurs in the secondary of UAT in spite of rated voltage.

SWITCHYARD
As we know that electrical energy can’t be stored like cells, so what we generate should be consumed instantaneously. But as the load is not constants therefore we generate electricity according to need i.e. the generation depends upon load. The yard is the places from where the electricity is send outside. It has both outdoor and indoor equipments.

OUTDOOR EQUIPMENTS  BUS BAR.  LIGHTENING ARRESTER

 WAVE TRAP  BREAKER  CAPACITATIVE VOLTAGE TRANSFORMER  EARTHING ROD  CURRENT TRANSFORMER.  POTENTIAL TRANSFORMER  LIGHTENING MASK

INDOOR EQUIPMENTS  RELAYS.  CONTROL PANELS  CIRCUIT BREAKERS BUS BAR Bus bars generally are of high conductive aluminum conforming to IS-5082 or copper of adequate cross section. Bus bar located in air insulated enclosures & segregated from all other components .Bus bar is preferably cover with polyurethane.

BY PASS BUS This bus is a backup bus which comes handy when any of the buses become faulty. When any operation bus has fault, this bus is brought into circuit and then faulty line is removed there by restoring healthy power line. LIGHTENING ARRESTOR It saves the transformer and reactor from over voltage and over currents. It grounds the overload if there is fault on the line and it prevents the generator transformer.

WAVE TRAP WAVETRAP is connected in series with the power (transmission) line. It blocks the high frequency carrier waves (24 KHz to 500 KHz) and let power waves (50 Hz - 60 Hz) to pass-through. BREAKER Circuit breaker is an arrangement by which we can break the circuit or flow of current. A circuit breaker in station serves the same purpose as switch but it has many added and complex features. The basic construction of any circuit breaker requires the separation of contact in an insulating fluid that servers two functions:

 extinguishes the arc drawn between the contacts when circuit breaker opens.  It provides adequate insulation between the contacts and from each contact to earth.

EARTHING ROD Normally un-galvanized mild steel flats are used for earthling. Separate earthing electrodes are provided to earth the lightening arrestor whereas the other equipments are earthed by connecting their earth leads to the rid/ser of the ground mar.

CURRENT TRANSFORMER It is essentially a step up transformer which step down the current to a known ratio. It is a type of instrument transformer designed to provide a current in its secondary winding proportional to the alternating current flowing in its primary. POTENTIAL TRANSFORMER It is essentially a step down transformer and it step downs the voltage to a known ratio.

RELAYS Relay is a sensing device that makes your circuit ON or OFF. They detect the abnormal conditions in the electrical circuits by continuously measuring the electrical quantities, which are different under normal and faulty conditions, like current, voltage frequency. Having detected the fault the relay operates to complete the trip circuit, which results in the opening of the circuit breakers and disconnect the faulty circuit. There are different types of relays:  Current relay  Potential relay  Electromagnetic relay  Numerical relay etc.

AIR BREAK EARTHING SWITCH The work of this equipment comes into picture when we want to shut down the supply for maintenance purpose. This help to neutralize the system from induced voltage from extra high voltage. This induced power is up to 2KV in case of 400 KV lines.

MAIN TURBINE

CONSTRUCTIONAL DETAILS:
H.P. CYLINDER: 12 stages (1st is governing stage) each stage Consists of a diaphragm & a set of moving Blades connected on a disc. BODY: In two valves made of Creep Resistance(Cr-Mo-V) steel STUDS & NUTS: High Creep Resistance (Cr-Mo-V) steel Forgings NOZZLE & STEAM CHEST: 4 Nos (2 on Top & 2 on sides) made of High Creep resisting (Cr-Mo-V)Steel casting I .P. CYLINDER: 11 stages BODY: 2 parts (a ) Pressure part made of Creep Resisting (Cr-Mo-V) steel

MAIN COMPONENTS OF TURBINE:  EMERGENCY STOP VALVE Steam from the boiler is supplied to the turbine through two emergency stop valves. The emergency stop valve operated by hydraulic servomotor shuts off steam supply to the turbine when the turbo set is tripped. The emergency stop valves connected tothe four control valves through four flexible loop pipes of Chromium-Molybdenum-Vanadium steel.  H.P. CYLINDER It is made of creep resisting Cr-Mo-V steel casting made of two halves joined at the horizontal plane. The horizontal joint is secured with the help of stud sand nuts made of high creep resisting Cr-Mo-V steel forgings. To ensure H.P. tightness the studs are tightened by heat to a predetermined temperature with the help of electric heater. H.P. ROTOR The H.P. rotor has discs integrally forged with the shaft sand is mechanical forming single Cr-Mo-V steel forging. A special process to prevent abnormal rotor deflection thermally stabilizes the rotor forging. L.P. ROTOR It consists of shrunk fit discs on a shaft. The shaft is a forging of Cr-Mo-V steel while the discs are of high strength Ni steel forging. The H.P. rotor is connected by rigid couplings whole the I.P rotor and L.P. rotor are connected by semi-flexible lens type coupling. The rotors are dynamically balanced to a very precise degree TURBINE BEARINGS The three turbine rotors are supported on fine bearings. The second bearing from pedestal side is a combined radial thrust bearing while all others are journal bearings. THRUST BEARINGS It is Mitchell type with bearing surface distributed over a number of bearing surfaces. They are pivoted in housing on the side of I.P. rotor thrust collar. During operation on oil film is forced between pads and thrust collar and there is a no metal-to-metal contact. A second ring of pads on opposite side of thrust collar takes the axial thrust as may occur under abnormal conditions.

L.P. HEATERS Turbine is provided with non-controlled extractions which are utilized for heating the condensate from turbine bleedingsystem. There are four L.P. heaters. They are equipped withnecessary safety valves in steam space level indicator for visual Mauges are present for measurement of steam pressure. GLAND STEAM COOLER Gland steam cooler has been provided to suck and cool the air steam mixture from the gland seats. It employs a small ejector for which the working medium is steam of low parameters, which can be taken either from the deaerator or auxiliary source. The pressure and temperature of this steam should of this steam is retrieved to the fullest possible extent as the gland steam cooler is also interposed in the condensate heating cycle thereby improving overall efficiency of the cycle. CONDENSATE PUMPS The function of these pumps is to pumps out the condensate to the desecrator through ejectors, gland steam cooler, and L.P. heaters. These pumps have four stages and since the suction is at a negative pressure, special arrangements have beenmade for providing sealing. This pump is rated generally for 160m3 hr. at a pressure 13.2 Kg/cm2.

Sponsor Documents

Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close