FUDAMENTALS OF THERMAL POWER PLANT

RESEARCH REPORT

(INFORMATION, SEARCH, ANALYSIS & PRESENTATION LABORATORY)

SUBMITTED BY

RAGHAVENDRA.K

UNDER THE GUIDANCE OF

Mrs.REKHA

SJM POLYTECHNICCHALLAKERE, CHITRADURGA

FANDAMENTL OF THERMAL POWER PLANTS

A power plant is a facility for the generation of electric power. The term is also used to refer to the engine in ships, aircraft and other large vehicles.

At the center of nearly all power stations is a generator, a rotating machine that converts mechanical energy into electrical energy by creating relative motion between a magnetic field and a conductor. The energy source harnessed to turn the generator varies widely from installation to installation.

In thermal power plants mechanical power is produced by a heat engine which transforms thermal energy, often from combustion of a fuel, into rotational energy. All thermal energy cannot be transformed to mechanical power, according to the second law of thermodynamics. Therefore, thermal power plants also produce low-temperature heat. If no use is found for the heat, it has to be rejected. If reject heat is employed as useful heat, the power plant is referred to as a cogeneration power plant or CHP (combined heat-and-power) plant.

Thermal power stations are often easily identified by cooling towers, huge cylindrical chimney-like structures that release the waste heat to the atmosphere.

Other power stations use the energy of water (waves, tides, or rivers confined by hydroelectric dams), wind, or sunlight.

GENERAL LAYOUT OF THE 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.

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.

The above breakdown of the plant would give you a clear idea about the components of the plant but a complete picture shown below would be more useful in getting an idea how these circuits are integrated together to form the complete power plant.

THE REQUIREMENTS FOR THE SITE

As the name implies the power plant is meant for generating power which obviously means that it will consume huge quantities of fuel. The exact quantity would depend on the size of the plant and its capacity but it is a general fact that ample quantities of fuel must be available either in the vicinity or it should be reasonably economical to transport the fuel till the power plant. Since most thermal power plants use coal (they can use other fuels as well) it must be ensured that sufficient coal is available round the clock. Just to give you a rough idea a power plant with 1000 MW capacity approximately would require more than ten thousand tons of coal per day hence the necessity for continuous supply and storage capability of coal in the power station.

Ash if the main byproduct of combustion and since the amount of coal used is huge, you can intuitively imagine the amount of ash generated and it is certainly in the region of thousand tons per day. Ash is much more difficult to handle as compared to coal since it comes out hot from the boiler and is very corrosive in nature. Disposing of such huge quantities of ash requires a large amount of empty space where it can be safely dumped.

There must be ample space for the storage of coal, disposal of ash, building of the power plant, and residential colony of workers, markets and so forth. An approximate analysis suggests that for every MW of power generated there must be at least 3 acres of land available for the purpose. Hence the power plant site needs to have good amount of land and this land should have good bearing capacity in order to survive the static and dynamic loads during the operation of the plant.

As we saw in the previous article of this series, large amount of water is required for cooling purposes in the power plant hence it is better if such a source is available nearby in the form of rivers etc.

Apart from these major requirements there are also other requirements which are equally important such as the availability of skilled people to work

for the plant and good transport facilities in the vicinity.

Hence we see that setting up a thermal power plant requires a lot of factors to be considered simultaneously.

ASSOSARIES ARE USED IN COAL BASED POWER PLANT

Alternator Turbine Boiler Furnace Economizer Air pre heater Super heater Deaerator

ALTERNATOR

The alternator is universally used in automotive applications. It converts mechanical energy into electrical energy, by electro-magnetic induction.

In a simple version, a bar magnet rotates in an iron yoke which concentrates the magnetic field. A coil of wire is wound around the stem of the yoke. As the magnet turns, voltage is induced in the coil, producing a current flow. When the North Pole is up, and South is down, voltage is induced in the coil, producing current flow in one direction.

As the magnet rotates, and the position of the poles reverses, the polarity of the voltage reverses too, and as a result, so does the direction of current flow.

Current that changes direction in this way is called alternating current, or AC. The change in direction occurs once for every complete revolution of the magnet.

Theory of operation

Alternators generate electricity by the same principle as DC generators. When magnetic field lines cut across a conductor, a current is induced in the conductor. In general, an alternator has a stationary part (stator) and a rotating part (rotor). The stator contains windings of conductors and the rotor contains a moving magnetic field. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.

The rotor magnetic field may be produced by induction (in a "brushless" generator), by permanent magnets, or by a rotor winding energized with direct current through slip rings and brushes. Automotive alternators invariably use brushes and slip rings, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor but are restricted in size owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator. Brushless AC generators are usually larger machines than those used in automotive applications.

Alternator protection

Over current protection:

Every alternator has an over current protection. With the help of this trip, the alternator and distribution system can be protected from various faults but the main thing to be considered in this method is to maintain power to the distribution system till the time the alternator trips on any other protection devices.

For this reason, the protection device has been designed in such a way that in case the over current is not high enough, a time delay provided by

an inverse definite minimum time (IDMT) relay occurs, which prevents the alternator from tripping in case the over current values reduces back to normal within the IDMT characteristics

But in case of a major fault such as short circuit, the alternator will trip instantaneously without any delay, protecting all devices on the distribution system. Overload of alternator is caused either due to increased switchboard load or serious fault causing very high current flow.

If sudden over load occurs then, the load is reduced with the help of preferential trips which removes non essential load such as of air conditioning, ventilation fans etc., from the switchboard. These preferential trips are operated by relays which are set to about 110% of the normal full load of alternator.

Reverse power protection:

There is not much difference between an alternator and electric motors from the engineer's perspective. They are both based on similar principles. So just imagine what would happen if an alternator suddenly would act as a motor. This is only possible in systems where two or more generators are running in parallel,

Hence this type of protection system is used only if there is more than one alternator on board a ship. The system is designed in such a way that it will release the breaker and prevent motoring of alternator if a reversal of power occurs. This protection device is also used to prevent damage to the prime mover, which might be stopped due to some fault. Though it is extremely difficult to detect reverse current with an alternating current system, reverse power can be detected and protection can be provided by reverse power relay.

STEAM TURBINE

A steam turbine is a mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. Its modern manifestation was invented by Sir Charles Parsons in 1884.

It has almost completely replaced the reciprocating piston steam engine primarily because of its greater thermal efficiency and higher power to weight ratio. Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 80% of all electricity generation in the world is by use of steam turbines. The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the ideal reversible process.

The steam turbine on a page about engines? Maybe you wondered about it. It is hardly used in means of transport, but the steam turbine has many things in common with the combustion engine. The steam turbine changes chemically stored energy into mechanical energy, too. It's called a heat-force engine. The steam turbine works similarly to the water turbine, which is known probably by everyone. Not water, but steam is used as working medium

Turbine

when people began to use water power to win mechanical work, they looked first for the best forms of impellers. Three types were established thereby and variations of them are used today in various applications, among other in steam turbines in power stations, as marine propellers, as compressors in gas turbines etc. These three types are introduced here.

The pelton turbine

The pelton turbine (also free-jet turbine) was invented 1880 by L.A. Pelton. It possesses spoon-shaped shovels, the jet hits the impeller tangentially, gets divided by the two shovels and transfers an impulse. The pelton turbine is used in storage power stations with downward gradients up to 2000 meters and can contain up to 6 nozzles.

The Francis turbine

he reaction turbine invented by J.B. Francis 1849 is hit by the jet almost axially (toward the axle) and radially (away from the center). The rotor blades can be adjusted, in order to ensure an even run. It looks similar to the type shown below as Steam turbine.

The Kaplan turbine

The Kaplan turbine, developed around 1915 by the Austrian V.Kaplan, looks like a marine propeller. The jet is led thereby axially on the freely adjustable shovel pages.

EXCITER

Exciter is providing DC supply to AC Gen Rotor; the DC supply is improving the magnetic flux density in AC Generator. So your Generating voltage is raised, which is up to your rated voltage.

GOVERNOR

The control of a turbine with a governor is essential, as turbines need to be run up slowly, to prevent damage while some applications (such as the generation of alternating current electricity) require precise speed control. Uncontrolled acceleration of the turbine rotor can lead to an over speed trip, which causes the nozzle valves that control the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaks apart, often spectacularly. Turbines are expensive to make, requiring precision manufacture and special quality materials. During normal operation in synchronization with the electricity net power plants are governed with a five percent droop speed control. This means the full load speed is 100% and the no load speed is 105%. This is required for the stable operation of the network without hunting and dropouts of power plants. Normally the changes in speed are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the spring pressure on a centrifugal governor. Generally this is a basic system requirement for all power plants because the older and newer plants have to be compatible in response to the instantaneous changes in frequency without depending on outside communication

BARRING GEAR

Barring gear (or "turning gear") is the mechanism provided to rotate the turbine generator shaft at a very low speed after unit stoppages. Once the unit is "tripped" (i.e., the steam inlet valve is closed), the turbine coasts down towards standstill. When it stops completely, there is a tendency for the turbine shaft to deflect or bend if allowed to remain in one position too long. This is because the heat inside the turbine casing tends to concentrate in the top half of the casing, making the top half portion of the shaft hotter than the bottom half. The shaft therefore could warp or bend by millionths of inches.

This small shaft deflection, only detectable by eccentricity meters, would be enough to cause damaging vibrations to the entire steam turbine generator unit when it is restarted. The shaft is therefore automatically turned at low speed (about one percent rated speed) by the barring gear until it has cooled sufficiently to permit a complete stop.

BOILER

A boiler is a closed vessel in which water under pressure is transformed into steam by the application of heat. In the boiler furnace, the chemical energy in the fuel is converted into heat, and it is the function of the boiler to transfer this heat to the contained water in the most efficient manner. The boiler should also be designed to generate high quality steam for plant use.

A boiler must be designed to absorb the maximum amount of heat released in the process of combustion. This heat is transferred to the boiler water through radiation, conduction and convection. The relative percentage of each is dependent upon the type of boiler, the designed heat transfer surface and the fuels.

Types

Two principal types of boilers are used for industrial applications:

1. Fire tube boilers-Products of combustion pass through the tubes, which are surrounded by water.

2. Water tube boilers- Products of combustion pass around the tubes containing water. The tubes are interconnected to common channels or headers and eventually to a steam outlet for distribution to the plant system.

Utilization

The boiler house or steam generation facility within any given plant is frequently referred to as the heart. In the event this system shuts down for unexpected reasons or for plant turnaround, most processes within the plant will not be operable. For this reason, very conservative treatment measures are used in the boiler. Operating personnel can be reluctant to change treatment programs if the one currently in use is deemed successful. On the other hand, if a treatment program is linked to a boiler failure, change usually comes quickly.

Steam Utilization

Steam is generated for the following plant uses:

1. Turbine drive for electric generating equipment, blowers and pumps

2. Process for direct contact with products, direct contact sterilization and noncontact for processing temperatures

3. Heating and air conditioning for comfort and equipment

The efficiency achievable with steam generation relies heavily on the system's ability to return condensed steam to the operating cycle. Many of the systems described above return a significant portion of the condensed steam to the generation cycle.

The overall functioning of steam-generating equipment is governed by thermodynamic properties of the working fluid. By the simple addition of heat to water in a closed vessel, vapor is formed which has greater specific volume than the liquid, and can develop an increase of pressure to the critical value of 3208 psia (22.1 mega Pascal’s absolute pressure). If the generated steam is discharged at a controlled rate, commensurate with the rate of heat addition, the pressure in the vessel can be maintained at any desired value, and thus be held within the limits of safety of the construction.

Vacuum

Vacuum is maintained at outlet of the turbine called exhaust steam. Assuming you are referring to a steam turbine, the lower the vacuum that can be maintained at the turbine exhaust the lower the temperature of the exhaust steam. This increases the Carnot cycle (theoretical maximum) efficiency of the turbine and results in a higher practical thermal efficiency.The surface condenser enables the steam vacuum to be maintained, by cooling the exhaust steam to below 100 C, and also enables the resulting water condensate to be recycled to the steam boiler.

FEED WATER CONTROL STATION

When the boiler is in service, feed water must be continuously supplied to maintain near- normal water level in the steam drum. It is unsafe to operate the boiler at lower water levels. The Feed water Control station installed in the feed water line maintains the water level in the boiler steam drum. Water flows from the flow control station through the economiser before entering into steam drum. The feed water is taken from the deaerator plant.

BOILER MAKE-UP WATER TREATMENT

Since there is continuous withdrawal of steam and continuous return of condensate to the boiler, losses due to blow down and leakages have to be made up to maintain a desired water level in the boiler steam drum. For this, continuous make-up water is added to the boiler water system. Impurities in the raw water input to the plant generally consist of calcium and magnesium salts which impart hardness to the water. Hardness in the make-up water to the boiler will form deposits on the tube water surfaces which will lead to overheating and failure of the tubes. Thus, the salts have to be removed from the water, and that is done by water demineralising treatment plant (DM). A DM plant generally consists of cation, anion, and mixed bed exchangers. Any ions in the final water from this process consist essentially of hydrogen ions and hydroxide ions,

which recombine to form pure water. Very pure DM water becomes highly corrosive once it absorbs oxygen from the atmosphere because of its very high affinity for oxygen.

The capacity of the DM plant is dictated by the type and quantity of salts in the raw water input. However, some storage is essential as the DM plant may be down for maintenance. For this purpose, a storage tank is installed from which DM water is continuously withdrawn for boiler make-up. The storage tank for DM water is made from materials not affected by corrosive water, such as PVC. The piping and valves are generally of stainless steel. Sometimes, a steam blanketing arrangement or stainless steel doughnut float is provided on top of the water in the tank to avoid contact with air. DM water make-up is generally added at the steam space of the surface condenser (i.e., the vacuum side). This arrangement not only sprays the water but also DM water gets deaerated, with the dissolved gases being removed by an air ejector attached to the condenser.

FURNACE

The part of the boiler that actually creates the heat is called the furnace. The furnace creates heat, usually by burning a fuel such as natural gas, coal or oil.

A furnace or direct fired heater is equipment used to provide heat for a process or can serve as reactor which provides heats of reaction. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. However, all furnaces have some common features. Basically, fuel flows into the burner and is burnt with air provided from an air blower. There can be more than one burner in a particular furnace which can be arranged in? Cells? Which heat a particular set of tubes? Burners can also be floor mounted as in the picture above, wall mounted or roof mounted depending on design. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace

Known as the radiant section. In the chamber where combustion takes place, known as the firebox, the heat is transferred mainly by radiation to tubes around the fire in the chamber. The heating fluid passes through the tubes and is thus heated to the desired temperature. The gases from the combustion are known as flue gas. After the flue gas leaves the firebox, most furnace designs include a convection section where more heat is recovered before venting to the atmosphere through the stack

Insulation

Insulation is an important part of the furnace since is it keeps heat generated inside the furnace and so prevents excessive heat loss. In used in the furnace can be firebrick, cast able refractories, ceramic fiber, etc. The floor of the furnace is normally cast able since it has to be hard enough to walk on during maintenance. Ceramic fiber is commonly used for the roof and wall of the furnace and is graded by its density and then its maximum temperature rating.

Soot blowers

Soot blowers utilize flowing media such as water, air or steam to remove deposits from boiler tubes. There are several different types of soot blowers used. Wall blowers are used for furnace walls and have a very short lance with a nozzle at the tip. The lance has holes drilled into it at intervals so that when it is turned on, it rotates and cleans the deposits from the wall in a circular pattern. It after it has turned a predetermined number of rounds, the soot blowing is completed and stops. Below is a convection section soot blower utilizing medium pressure (10-12bar) steam?

FUEL PREPARATION SYSTEM

In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into small pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder. The pulverizes may be ball mills, rotating drum grinders, or other types of grinders.

Some power stations burn fuel oil rather than coal. The oil must kept warm (above its pour point) in the fuel oil storage tanks to prevent the oil from congealing and becoming unpumpable. The oil is usually heated to about 100 °C before being pumped through the furnace fuel oil spray nozzles.

Boilers in some power stations use processed natural gas as their main fuel. Other power stations may use processed natural gas as auxiliary fuel in the event that their main fuel supply (coal or oil) is interrupted. In such cases, separate gas burners are provided on the boiler furnaces.

Old is gold goes the equally old and wise saying and this is applicable to the black diamond as well, in the perspective of thermal power plants. Well if you are confused let me tell you that I am referring to coal which is one of the oldest used fuels in the world. Though technology has gone skywards in the previous couple of centuries, yet this source continues to occupy an important position in the energy production scenario worldwide.

The four circuits of the thermal power plant make a complete picture when put together helping to generate electricity out of fuels such as coal which is the most widely used fuel. The calorific value of coals depends on the quality of the coal and the place from where it is mined.

Let us perform a simple calculation regarding the amount of coal required in a power plant.

Let us assume an imaginary thermal power plant which has a capacity of 1000 MW and try to find the amount of coal required for its consumption. Also assume that the boiler operates at an efficiency of 75% and the heat supplied per kg of steam is around 500 kcal per kg and that the amount of

steam required per kWh is nearly 5 kgs. Further let us assume that the type of coal used in the plant has a calorific value of 5000 kcal/kg

Then the quantity of coal required per hour would be given by

Weight of Coal Required ==> Capacity * Steam Requirement * Heat Delivered/Calorific Value of Coal * Efficiency of Boiler

==> {1000 * 1000 * 5 * 500}/{5000 * 0.75 * 1000} = 666 tons/hr

Normally it is a practice to store coal for up to one month usage in case the power plant is situated at a sufficient geographical distance from the coal source so that in case of any disruption of the transportation system, the region is not immediately affected. You can calculate that in case the above plant requires such a facility, we would require space to store and handle nearly 480, 000 tons of coal.

Coming back to the actual operation this coal is then fed to the combustion chamber of the boiler where steam is generated giving rise to hot exhaust gases and ash which are handled by their respective circuits. The steam turbine is driven by the steam which converts this thermal energy into the mechanical energy and is coupled with an electric generator to convert it to electrical energy.

The electricity generated through the generator coupled to the turbine is then fed to the main grid via a system of transformers and other electrical equipment and is usually taken to far off places via high voltage transmission lines before it is actually supplied in the domestic or industrial sectors at their respective voltage levels.

Hence we see that how the energy hidden within the ordinary coal is harnessed through the use of a thermal power plant to light our homes and industries.

Coal – Use in Electricity Generation

In the earlier days coal might have been used for providing heat for making food or in the blacksmiths furnace but as technology made strides, so did the level and importance of coal in the energy production arena went upwards. Currently thermal power plants produce hundreds of megawatts of electricity from burning coal.

Despite the hue and cry of environmental concerns, which are of course true to a certain extent, various other parameters have still kept coal as one of the most important sources of power generation in thermal power plants.

The very first parameter is the ample abundance of coal in most parts of the world including the United States. Estimates suggest that the US has reserves of coal which could last more then two centuries even at the current rate of consumption. Apart from the reserves found on land there is also presence of coal layers beneath the sea although it is difficult to commercially extract it from there, but there might be a technology for this in the future.

The next factor is directly related to the above factor and is that of cost. As you know cost is closely associated with availability and more abundant any commodity is, the lesser will be its price and vice versa which is a law of economics. Hence producing electricity through the use of coal is much cheaper than other non-conventional forms of energy such as say wind, nuclear and so forth.

Disadvantages of Coal

Of course all is not green in the literal sense in the use of coal in power plants. Environment and health hazards are one of the most prominent reasons why many groups are against the use of coal for power production. Disposal of large quantities of ash could pose problems in the coming years if the heaps continue to grow.

Another major factor to be kept in mind is that despite the abundant supply of coal it is still a non renewable source of energy which was formed

through a complex process lasting thousands of years and hence cannot be formulated at a short notice.

Despite the disadvantages, coal is still very popular in its use as power plant fuel and continues to provide electricity to this power hungry planet.

ANALYSIS OF COAL

Any substance can be analyzed in different ways such as proximate analysis, chemical analysis and so forth. We will carry out the proximate analysis of coal which gives the different categories of compound present in the substance. Apart from carbon which is an obvious constituent the other constituents are as follows.

Ash –

This is an undesirable constituent of coal which is contained within the coal in two forms namely fixed ash and free ash. Fixed ash is inherent in the coal due to the formation process from vegetable matter and it is not possible to remove it except that it gets separated on burning. The free ash is removable via processes such as washing and screening though they will not be described in detail here.

Like I said earlier it is undesirable to have ash in the coal but normally different types of coal could have ash content anywhere between say around 2% to 30% which is an unnecessary burden adding to transportation costs, lowering of heat value of coal and producing large amounts of corrosive waste which needs to be disposed off in a proper manner. Another disadvantage is that inside the boiler combustion chamber if the ash gets subject to very high temperatures it can form clinkers which could choke the passages and decrease efficiency of the boiler. It is best to use such coal in the powdered form if such a possibility exists.

Hydrocarbons & Gases –

Coal contains a variety of combustible gases such as hydrogen, methane and non-combustible gases such as carbon dioxide etc. This volatile content could be as low as 3% or even as high as nearly 50%. These constituents especially the non-combustible gases are just a waste as far as heat value of the fuel is concerned and an unnecessary burden which needs to be transported around and stored without any useful value in return.

Moisture –

The moisture content of coal could vary from just over 1% to nearly 30-40% and just like ash it has two forms – inherent moisture and free moisture. The former is not easily removable as it is a costly process whilst the latter can be removed by normal drying using slightly heated air. Obviously since coal is used in the boiler combustion chamber any excess moisture would interfere with the combustion lowering the actual heat available to generate steam.

There is another method of analyzing coal in terms of its various chemical constituents such as carbon, hydrogen, sulphur and so forth but that might be taken up in a different set of articles.

CONTROLLING DRAFT

Most boilers now depend on mechanical draft equipment rather than natural draft. This is because natural draft is subject to outside air conditions and temperature of flue gases leaving the furnace, as well as the chimney height. All these factors make proper draft hard to attain and therefore make mechanical draft equipment much more economical.

Induced draft:

This is obtained one of three ways, the first being the "stack effect" of a heated chimney, in which the flue gas is less dense than the ambient air surrounding the boiler. The denser column of ambient air forces combustion air into and through the boiler. The second method is through use of a steam jet. The steam jet oriented in the direction of flue gas flow induces flue gasses into the stack and allows for a greater flue gas velocity increasing the overall draft in the furnace. This method was common on steam driven locomotives which could not have tall chimneys. The third method is by simply using an induced draft fan (ID fan) which removes flue gases from the furnace and forces the exhaust gas up the stack. Almost all induced draft furnaces operate with a slightly negative pressure.

Forced draft:

Draft is obtained by forcing air into the furnace by means of a fan (FD fan) and ductwork. Air is often passed through an air heater; which, as the name suggests, heats the air going into the furnace in order to increase the overall efficiency of the boiler. Dampers are used to control the quantity of air admitted to the furnace. Forced draft furnaces usually have a positive pressure.

Balanced draft :

Balanced draft is obtained through use of both induced and forced draft. This is more common with larger boilers where the flue gases have to travel a long distance through many boiler passes. The induced draft fan works in conjunction with the forced draft fan allowing the furnace pressure to be maintained slightly below atmospheric.

Chimney

A vertical hollow structure of masonry, steel, or reinforced concrete, built to convey gaseous products of combustion from a building or process facility. A chimney should be high enough to furnish adequate draft and to discharge the products of combustion without causing local air pollution. The height and diameter of a chimney determine the draft. For adequate draft, small industrial boilers and home heating systems depend entirely upon the enclosed column of hot gas. In contrast, stacks, which are chimneys for large power plants and process facilities, usually depend upon force-draft fans and induced-draft fans to produce the draft necessary for operation, and the chimney is used only for removal of the flue gas.

FLY ASH COLLECTION

Fly ash is captured and removed from the flue gas by electrostatic precipitators or fabric bag filters (or sometimes both) located at the outlet of the furnace and before the induced draft fan. The fly ash is periodically removed from the collection hoppers below the precipitators or bag filters. Generally, the fly ash is pneumatically transported to storage silos for subsequent transport by trucks or railroad cars.

BOTTOM ASH COLLECTION & DISPOSAL

At the bottom of the furnace, there is a hopper for collection of bottom ash. 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.

SUPER HEATER

Whatever type of boiler is used, steam will leave the water at its surface and pass into the steam space. Steam formed above the water surface in a shell boiler is always saturated and cannot become superheated in the boiler shell, as it is constantly in contact with the water surface.

If superheated steam is required, the saturated steam must pass through a super heater. This is simply a heat exchanger where additional heat is added to the saturated steam.

In water-tube boilers, the super heater may be an additional pendant suspended in the furnace area where the hot gases will provide the degree of superheat required (see Figure 3.4.4). In other cases, for example in CHP schemes where the gas turbine exhaust gases are relatively cool, a separately fired super heater may be needed to provide the additional heat.

If accurate control of the degree of superheat is required, as would be the case if the steam is to be used to drive turbines, then an attemperator (Desuperheater) is fitted. This is a device installed after the super heater, which injects water into the superheated steam to reduce its temperature.

DEAERATOR

Deaerators are usually installed in steam power plants and are used to treat feed water delivered to steam generators and supplementary water delivered to the heating network. In the absence of deaeration the corrosive active gases usually dissolved in water (oxygen and free carbon dioxide), which are liberated in a steam generator or heating network, would cause corrosion of metals. Deaerators can be classified according to their principle of operation as thermal (the most widely used type), desorption, or chemical types.

There are two basic types of Deaerators,

the tray-type

spray-type

Tray-type

Deaeration section mounted on top of a horizontal cylindrical vessel which the tray-type (also called the cascade-type) includes a vertical domed serves as the deaerated boiler feed water storage tank.

Spray-type

The spray-type consists only of a horizontal (or vertical) cylindrical vessel which serves as both the deaeration section and the boiler feed water storage tank.

STEAM CONDENSOR & COOLING SYSTEM

The steam cycle power plants are equipped with cooled condensers where exhaust steam is condensed under vacuum. The operating condenser pressure is 0.03 to 0.4 bars and depends on the cooling system and medium temperature.

Three different steam condensers are used in two fundamentally different cooling systems:

1 .Direct Cooling Systems :a) One through cooling in surface condenser

b) Dry cooling air condenser

2 .Indirect Cooling Systems :a) Wet cooling tower and surface condenser

b) Dry cooling tower and direct contact condenser

1 . Direct Cooling Systems

a) Once Through Cooling in Surface Condenser :This open loop system has been achieved using water from a river, a stream or seawater. The cold water is pumped through the condenser tubes and the warm water is discharged back to the water source. Surface

condenser is explained in wet cooling system .

b) Direct Dry Cooling, Air Cooled Steam Condenser :Another form of condensing system is the air-cooled condenser. They are more environmentally acceptable forms of condensing steam. The process is similar to that of a radiator and fan. Exhaust steam from the low pressure section of a steam turbine runs through the condensing tubes. The heat transferred from the process steam to the cooling air via extended surfaces or tubes. The tubes are usually finned and ambient air is pushed through the fins with the help of a large fan. The steam condenses to water to be reused in the water-steam cycle.

The performance of dry cooling systems is primarily dependent on the ambient temperature of the dry air. Since the ambient dry air temperature is higher than the wet air temperature, dry cooling systems are less efficient

than wet cooling tower design .In dry cooling systems, the turbine exhaust is connected directly to the air cooled steam condenser (direct cooling system). The steam exhaust duct has a large diameter and is usually as short as possible to reduce pressure losses. An optimum fin tube geometry which gives the highest heat transfer

for the minimum amount of metal should be selected .

Advantages of dry cooling Disadvantages of dry cooling No water required Duct pressure losses, less efficient Can be located at fuel source Large plot area required No impact on environment Generated more noiseless permitting required-

The condensation temperature in the condenser section is by 2-4 °K lower than the exhaust steam temperature, due to the steam pressure drop

through the distributing duct and the heat exchanger tubes .

2 . Indirect Cooling Systems :

a) Indirect Wet Cooling System, Surface Condenser:

The need to reduce the amount of water requires a closed loop or wet cooling system. In a wet cooling system, water is circulated to condense the steam in the surface condenser. The warm water, instead of being rejected to the water source, is cooled in cooling tower using air as cooling medium. The wet cooling tower based on principle of evaporation.

The heated cooling water coming out of the surface condenser is cooled as it flows through a cooling tower, where air is forced through the tower by either natural draft or mechanical. The exhaust steam is condensed at the outside of the surface condenser tubes. Using cold water coming from the

cooling tower .Part of the cooling water is evaporated in the cooling tower, and a continuous source of fresh water (make-up water) is required to operate a wet cooling tower. The Make-up requirements for a cooling tower consists

of the summation of evaporation loss, drift loss and blow-down .

Estimation of the evaporation loss :Meva = 0.0017 x Mcw x dTcool (dT in °K)

Drift is entrained water in the tower discharge vapors. Drift loss is a function of the drift-eliminator design. And a typical value is 0.005% of the cooling

water flow rate .

Surface Condenser :

Water cooled condenser used in once through cooling system and in wet cooling system. The steam condenser is a major component of the steam cycle in steam power and combined cycle power plants. It is a necessary

component of the steam cycle for two reasons : -It converts the used steam back into feed water for return to the boiler .

-It increases the cycle´s efficiency by allowing the cycle to operate with the largest possible Temperature and pressure difference between the boiler

and the condenser .

Design principle: There are different condenser designs which are defined by suppliers. Condenser tubes are arranged as tube bundles in condenser shell with a single-pass or two-pass. The bundle shape and air cooler

location are optimized by the supplier .The design of single-pass condenser provides cooling water flow through straight tubes from the inlet water box on one end, to outlet water box on the other end. The design of two-pass condenser provides cooling water flow through straight tubes from the inlet water box, reversed in the return water box to the outlet watebox on the same end of Inlet water box.

In the condenser several thousand tubes are placed at low tube pitch in order to get acceptable dimensions. It is not favorable to increase friction losses in the steam flow; therefore the number of tube rows along the steam flow is limited. The separation between the water box areas and the steam condensing area is accomplished by two tube sheet to which the cooling water tubes are attached. The cooling water tubes are supported

within the condenser by the tube support plates .

The condenser tubes are made of brass or stainless steel to resist corrosion from either side. Nevertheless they may become internally fouled during operation by bacteria or algae in the cooling water or by mineral scaling, all of which inhibit heat transfer and reduce thermodynamic efficiency. Many plants include an automatic cleaning system that

circulates sponge rubber balls through the tubes to scrub them clean without the need to take the system off-line.

AIR PRE HEATER

Air preheaters are classified as recuperative or feed-water types according to the principle of operation. In recuperative air preheaters the heat exchange between the heat carrier and the air to be heated takes place continuously through the walls of the heating surfaces that separate them; in feed-water air preheaters the heat exchange is accomplished by the alternate heating and cooling of the metallic or ceramic nozzles of the fixed or rotating heating surfaces of the pre-heater. Tubular (steel or cast-iron) recuperative air preheaters, and less frequently rotating feed-water preheaters, are used mainly at thermal electric power plants. Periodic-operation feed-water air preheaters with a ceramic nozzle are widely used in the metallurgical industry. The air can be heated to 450°-600° C with metal units and to 900°-1200° C with ceramic-nozzle units.

An air preheated (APH) is a general term to describe any device designed to heat air before another process (for example, combustion in a boiler) with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.

In particular, this article describes the combustion air preheaters used in large boilers found in thermal power station producing electric power from e.g. fossil fuels, biomasses or waste.

The purpose of the air preheater is to recover the heat from the boiler flue gas which increases the thermal efficiency of the boiler by reducing the useful heat lost in the flue gas. As a consequence, the flue gases are also sent to the flue gas stack (or chimney) at a lower temperature, allowing simplified design of the ducting and the flue gas stack. It also allows control over the temperature of gases leaving the stack (to meet emissions regulations, for example).

ECONOMIZER

Modern-day boilers, such as those in coal-fired power stations, are still fitted with economizers which are descendants of Green's original design. In this context they are often referred to as feed water heaters and heat the condensate from turbines before it is pumped to the boilers.

Economizers are commonly used as part of a heat recovery steam generator in a combined cycle power plant. In an HRSG, water passes through an economizer, then a boiler and then a super heater. The economizer also prevents flooding of the boiler with liquid water that is too cold to be boiled given the flow rates and design of the boiler.

A common application of economizers in steam power plants is to capture the waste heat from boiler stack gases (flue gas) and transfer it to the boiler feed water. This raises the temperature of the boiler feed water thus lowering the needed energy input, in turn reducing the firing rates to accomplish the rated boiler output. Economizers lower stack temperatures which may cause condensation of acidic combustion gases and serious equipment corrosion damage if care is not taken in their design and material selection.

THERMAL POWER PLANT IN INDIA

Moreover, if talk about upcoming projects then 4620MW in Mundra, Gujarat by ADANI Power is the largest coal based thermal power plant in India, followed by 4000MW in same location by TATA Power 

Currently work is going on at Chandrapoor super thermal power plant , Maharashtra for 2x 500 MW units (Unit 8 and 9). Current capacity is 2340 MW and will be enhanced to 3340 MW in near future. Along with this work for Solar Photo Voltaic plant is in progress. 

India’s total thermal power generation is 86397MW

Singrauli Uttar Pradesh 2000MW

Korba Chhattisgarh 2100MW

Ramagundam Andhra Pradesh 2600MW

Vindhyachala Madhya Pradesh 3260MW

NCTPP Dadri Uttar Pradesh 2310MW

Talcher Orissa 3000MW

Udupi power corporation Karnataka 1250MW

RTPC Raichur Karnataka 1720MW

JSW Bellary Karnataka 600MW

Vedanta Orissa 2400MW

CTPS Maharashtra 2340MW

Jindal Chhattisgarh 1000MW

THERMAL EFFICENCY OF POWER PLANT

Coal the primary energy source consists mainly of Carbon. During the combustion process the Carbon in the coal combines with Oxygen in the air to produce Carbon dioxide producing heat. The high heating value, the energy available in the coal, is in the range of 10,500 kJ/kg to 27,000 kJ/kg.

For example, consider a coal with a high heating value of 20,000 kJ/kg. Theoretically this is equivalent to 5.56 kwhr of electrical energy. Can we get all of this as electric power? No. In practice the effective conversion is only around one third of the theoretically possible value.

Why is it so?

The first process of energy conversion is the combustion where the potential energy in coal is converted to heat energy. The efficiency of this conversion is around 90 %. Why?

Due to practical limitations in heat transfer, all the heat produced by combustion is not transferred to the water; some is lost to the atmosphere as hot gases.

The coal contains moisture. Also coal contains a small percent of Hydrogen, which also gets converted to moisture during combustion. In the furnace, moisture vaporizes taking Latent heat from the combustion heat and exits the boiler along with the hot gases.

Improper combustion of coal, hot ash discharged from the boiler and radiation are some of the other losses.

The second stage of conversion is the thermodynamic stage. The heat from combustion is transferred to the water to produce steam. The energy of the steam is converted to mechanical rotation of the turbine. The steam is then condensed to water and pumped back into the boiler for re-use. This stage works on the principle of the Rankine cycle. For plants operating with steam at subcritical pressures (less than 221 bar) and steam temperatures of 570 °C, the Rankine cycle efficiency is around 43 %. For the state of the art plants running at greater than supercritical pressure and

steam temperatures near to 600 °C, the efficiency is around 47 %. Why is it so low?

The steam is condensed for re-use. During this process the latent heat of condensation is lost to the cooling water. This is the major loss and is almost 40 % of the energy input.

Losses in the turbine blades and exit losses at turbine end are some of the other losses.

The Rankine cycle efficiency is dictated by the maximum temperature of steam that can be admitted into the turbine. Due to metallurgical constraints steam temperatures are at present limited to slightly more than 600 °C.

The third stage converts the mechanical rotation to electricity in a generator. Copper, magnetic and mechanical losses account for 5 % loss in the Generator. Another 3 % is lost in the step-up transformer which makes the power ready for transmission to the consumer.

To operate the power plant it is required to run various auxiliary equipment like pulverizes fans, pumps and precipitators. The power to operate these auxiliaries has to come from the power plant itself. For large power plants around 6 % of the generator output is used for internal consumption.

This brings the overall efficiency of the power plant to around 33.5 %. This means we get only 1.9 kwhr of electrical energy from one kg of coal instead of the 5.56 kwhr that is theoretically available in the coal.

The efficiency or inefficiency of power plants is something that we have to live with for the present till technology finds away out.

BOILER FITTING AND ACCESSORIES

Safety valve : It is used to relieve pressure and prevent possible explosion of a boiler

Water level indicators : They show the operator the level of fluid in the boiler, also known as a sight glass, water gauge or water column is provided

Bottom blow down valves : They provide a means for removing solid particulates that condense and lie on the bottom of a boiler. As the name implies, this valve is usually located directly on the bottom of the boiler, and is occasionally opened to use the pressure in the boiler to push these particulates out.

Continuous blow down valve : This allows a small quantity of water to escape continuously. Its purpose is to prevent the water in the boiler becoming saturated with dissolved salts. Saturation would lead to foaming and cause water droplets to be carried over with the steam - a condition known as priming. Blow down is also often used to monitor the chemistry of the boiler water.

Automatic Blow down/Continuous Heat Recovery System : This system allows the boiler to blow down only when makeup water is flowing to the boiler, thereby transferring the maximum amount of heat possible from the blow down to the makeup water. No flash tank is generally needed as the blow down discharged is close to the temperature of the makeup water.

Flash Tank : High pressure blow down enters this vessel where the steam can 'flash' safely and be used in a low-pressure system or be vented to atmosphere while the ambient pressures blow down flows to drain.

Hand holes : They are steel plates installed in openings in "header" to allow for inspections & installation of tubes and inspection of internal surfaces

Steam drum internals : A series of screen, scrubber & cans (cyclone separators

Low- water cutoff : It is a mechanical means (usually a float switch) that is used to turn off the burner or shut off fuel to the boiler to prevent it from running once the water goes below a certain point. If a boiler is "dry-fired" (burned without water in it) it can cause rupture or catastrophic failure.

Surface blow down line : It provides a means for removing foam or other lightweight non-condensable substances that tend to float on top of the water inside the boiler.

Top feed : A check valve (clack valve) in the feed water line, mounted on top of the boiler. It is intended to reduce the nuisance of lime scale. It does not prevent lime scale formation but causes the lime scale to be precipitated in a powdery form which is easily washed out of the boiler.

Feed water check valve or clack valve : A non-return stop valve in the feed water line. This may be fitted to the side of the boiler, just below the water level, or to the top of the boiler.

Circulating pump : It is designed to circulate water back to the boiler after it has expelled some of its heat.

Desuperheater tubes or bundles : A series of tubes or bundles of tubes in the water drum or the steam drum designed to cool superheated steam. Thus is to supply auxiliary equipment that doesn't need, or may be damaged by, dry steam.

Chemical injection line : A connection to add chemicals for controlling feed water PH.