 Thermal power plant operate on the principle of Rankine thermodynamic cycle  The Rankine cycle is an idealized thermodynamic cycle of a heat engine that converts heat into mechanical work. The heat is supplied externally to a closed loop, which usually uses water as the working fluid  thermodynamic cycle in which there is net heat transfer to the system and a net work transfer from the system.  Power generation from coal can be estimated by means of its calorific value kCal/kg or kJ/kg.

 Transportation network  Power transmission network  Geology and soil type  Earthquake and geological faults  Topography  Rivers and floodway's  Water resources  Environmental resources  Population centers  Need for power

 Availability of labors.  Availability of utilities (Water, Electricity).  Climate  Land cover  Area size  Distance from airports  Archeological and historical sites

The plant can be divided into four main circuits 1) Fuel and Ash Circuit 2) Air and Gas Circuit 3) Feed Water and Steam Circuit and 4) Cooling Water Circuit

 (a) High pressure boiler  (b) Prime mover  (c) Condensers and cooling towers  (d) Coal handling system  (e) Ash and dust handling system  (f) Draught system  (g) Feed water purification plant  (h) Pumping system  (i) Air preheater, economizer, super heater, feed heaters.

A boiler should fulfil the following requirements :  (a) Safety : The boiler should be safe under operating conditions.  (b) Accessibility : The various parts of the boiler should be accessible for repair and maintenance.  (c) Capacity : The boiler should be capable of supplying steam according to the requirements.  (d) Efficiency : To permit efficient operation, the boiler should be able to absorb a maximum amount of heat produced due to burning of fuel in the furnace.  (e) It should be simple in construction and its maintenance cost should be low.  (f) Its initial cost should be low.  (g) The boiler should have no joints exposed to flames.  (h) The boiler should be capable of quick starting and loading.

 According to flow of Water and hot gasses:  (a) Water tube  (b) Fire tube  Water tube: ◦ Water circulates through the tubes and hot products of combustion flow over these tubes.  It Requires less weight of metal for a given size  Less liable to explosion  It produces high pressure  It is accessible and can respond quickly to change in demand  Tubes and drums of water tube boilers are smaller than that of fire –tube boilers  due to smaller size of drum higher pressure can be developed easily

Water tube:  Water-tube boilers require lesser floor space The efficiency of water-tube boilers is more  Fire tube: ◦ the hot products of combustion pass through the tubes, which are surrounded by water  low initial cost  more compacts  more likely to explosion  water volume is large and due to poor circulation they cannot meet quickly the change in steam demand.  For the same output the outer shell of fire tube boilers is much larger than the shell of water-tube boiler.

 Based on the method of generating heat, furnaces are broadly classified into two types: (i)combustion type (using fuels) (ii) electric type depending upon the kind of combustion it can be broadly classified as oil fired, coal fired or gas fired.  Based on the mode of charging of material, furnaces can be classified as (i) Intermittent or Batch type furnace or Periodical furnace and (ii) Continuous furnace.  Based on mode of waste heat recovery as recuperative and regenerative furnaces.  Another type of furnace classification is made based on mode of heat transfer, mode of charging and mode of heat recovery as shown in the figure below

 Determination of the quantity of heat to be imparted to the material or charge.  Liberation of sufficient heat within the furnace to heat the stock and overcome all heat losses.  Transfer of available part of that heat from the furnace gases to the surface of the heating stock.  Equalization of the temperature within the stock.  Reduction of heat losses from the furnace to the minimum possible extent

 Pulverised coal firing is done by two systems :  (a) Unit System or Direct System.  (b) Bin or Central System.

 The economizer is a feed water heater, deriving heat from the flue gases. The justifiable cost of the economizer depends on the total gain in efficiency. In turn this depends on the flue gas temperature leaving the boiler and the feed water inlet temperature.

 Boilers are provided with economizer and air pre-heaters to recover heat from the flue gases.  An increase of about 20% in boiler efficiency is achieved by providing both economizer and air pre-heaters.  Economizer alone gives only 8% efficiency increase. The feed water from the high pressure heaters enters the economizer and picks up heat from the flue gases after the low temperature super heater.  Economizer can be classified as an inline or staggered arrangement based on the type of tube arrangement.

 The flue gases coming out of the economizer is used to preheat the air before supplying it to the combustion chamber.  An increase in air temperature of 20 degrees can be achieved by this method.  The pre heated air is used for combustion and also to dry the crushed coal before pulverizing.  An air preheater or air heater 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.

 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).

 The use of a condenser in a power plant is to improve the efficiency of the power plant by decreasing the exhaust pressure of the steam below atmosphere.  Another advantage of the condenser is that the steam condensed may be recovered to provide a source of good pure feed water to the boiler and reduce the water softening capacity to a considerable extent  The condenser helps maintain low pressure at the exhaust.

 cooling water from the condenser has to be cooled.  The cooling water after condensing the steam becomes hot and it has to be cooled as it belongs to a closed system.  The Cooling towers do the job of decreasing the temperature of the cooling water after condensing the steam in the condenser.  The evaporation and effective cooling of air is greater when the air outside is warmer and dryer than when it is cold and already saturated.

 Condensers need huge quantity of water to condense the steam.  Water is led into the plants by means of circulating water pumps and after passing through the condenser is discharged back into the river.  If such a source is not available closed cooling water circuit is used where the warm water coming out of the condenser is cooled and reused.  In such cases ponds and cooling towers are used where the water loses heat to the atmosphere.

 Super heated steam is that steam which contains more heat than the saturated steam at the same pressure. The additional heat provide more energy to the turbine hence power out put is more.  Superheated steam causes lesser erosion of the turbine blades and can be transmitted for longer distance with little heat loss  The function of the super heater is to remove the last trash of moisture from the saturated steam.  A superheater may be convention type, radiant type or combination

One of the most important accessories of a boiler is a super heater. It effects improvement and economy in the following ways :  The super heater increases the capacity of the plant.  Eliminates corrosion of the steam turbine.  Reduces steam consumption of the steam turbine

 Plate Super heaters.  Pendant Super heaters.  Radiant Super heaters.  Final Super heaters.

 The recirculation of some percentage of the combustion gases serves to control steam temperature and increase in excess air. By introducing the hot gases below the combustion zone, relatively high efficiency may be maintained.

 The reheater functions similar to the superheater  In addition to super heater modern boiler has reheater also. The function of the reaheater is to superheat the partly expanded steam from the turbine, this ensure that The steam remain dry through the last stage of the turbine.  A reheater may be convention type, radiant type or combination

 The combustion in the boiler requires supply of sufficient quality of air and removal of exhaust gases  The Circulation of air is caused by difference of pressure is known as draught. Thus draught is the differential in pressure between the two points.  A draught tube may be 1. Natural Draught 2. Mechanical Draught

 A natural Draught is provided by the chimney or stack.  Natural draught has its limitation. Modern plants has high rate of heat transfer and Draught losses are very high. in view of this Natural draught is used only for small boilers.

 Modern large size plants use very large size of boilers of capacity above 1000,000 kg per hour. such boiler needs tremendous volume of air (around m3) Per minute. A chimney provide this. Therefore mechanical draught is used.  In a mechanical draught the system the movement air is due to the action of fan. A mechanical Draught consist of forced Draught or induced draught or both.  In forced draught system the fan is installed near the boiler.the fan force the air through the furnace, economizer, air preheater and chimney. The pressure of air, throughout the system, is above atmospheric and air is forced to flow through the system

 In an induced draught system the, the fan is installed near the base of the chimney.The burnt gases are sucked out from the boiler, thus reducing the pressure inside the boiler. to less than atmosphere. this induces fresh air to enter the furnace.  A mechanical Draught need additional capital investment and maintenance.But it required for proper operation of modern power plant. In super thermal power plant, each boiler may used two forced fans and two induced fan.

 An electrostatic precipitator (ESP), or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. the basic idea of an ESP:   Charging   collecting.   removing  Every particle either has or can be given a charge—positive or negative.  we impart a negative charge to all the particles in a gas stream in ESP.  Then a grounded plate having a positive charge is set up.  The negatively charged particle would migrate to the grounded collection plate and be captured.  The particles would quickly collect on the plate, creating a dust layer. The dust layer would accumulate until we removed it.

 The structural design and operation of the discharge electrodes (rigid-frame, wires or plate) and collection electrodes.  tubular type ESP  plate type ESP

 A deaerator is a device that is widely used for the removal of oxygen and other dissolved gases from the feedwater to steam-generating boilers.  dissolved oxygen in boiler feed waters will cause serious corrosion damage in steam systems by attaching to the walls of metal piping and other metallic equipment and forming oxides (rust)

 Steam entering from a small opening attains a very high velocity. The velocity attained during expansion depends on the initial and final content of the steam.  The difference in initial and final heat content represent the heat energy to be converted to kinetic energy.

 To maximize turbine efficiency the steam is expanded, generating work, in a number of stages.  These stages are characterized by how the energy is extracted from them and are known as either impulse or reaction turbines  One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft  typical isentropic efficiencies ranging from 20%-90% based on the application of the turbine

 An impulse turbine has fixed nozzles that orient the steam flow into high speed jets.  These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction  In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles.  This type of turbine makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam is directed onto the rotor by the fixed vanes of the stator

 A feedwater heater is a power plant component used to pre-heat water delivered to a steam generating boiler. Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system

 Feed Water heating improve overall efficiency.  The dissolved oxygen which would otherwise cause boiler corrosion are removed in the feed water heater.  Thermal stresses due to cold water entering the boiler drum are avoided.  Quantity of steam produced by the boiler is increased.  Some other impurities carried by steam and condensate, it causes corrosion in boiler. The impurities are precipitated outside the boiler.

 Use one or more low pressure feed water heaters to raise the temperature of condensate from condensate pump discharge temperature to the de-aerator inlet temperature.  The heater drains are cascaded from the higher pressure heater to the next lower pressure heater with the lowest pressure heater draining to the condenser

 High pressure feed water heaters are used in the feed water system between the boiler feed pump discharge and the boiler, and utilize high pressure turbine extraction steam for heating the feed water  The condensate or feed water temperature increase for each feed water heater will be in the range of 28 to 56 degrees  Use one or more high pressure feed water heaters to raise the temperature of feed water from de-aerator outlet temperature to the required boiler economizer inlet temperature

 Ash is the by-product of combustion  Ash in power plant is about 30-40% of total coal consumption Fly Ash ( Around 80% is the value of fly ash generated) Bottom ash (Bottom ash is 20% of the ash generated in coal based power stations.

Fly Ash (Dry Ash)  Ash generated in the ESP which got carried out with the flue gas is generally called Fly ash. It also consists of Air pre heater ash & Economizer ash (it is about 2 % of the total ash content). Bottom ash  Ash generated below furnace of the steam generator is called the bottom ash.  bottom ash has been handled in a wet condition

 The operation of ash handling plants is…….  Removal of ash from the furnace using ash hoppers  Transfer of the ash to a fill or storage  And disposal of stored ash

 Fuel used is cheaper  Smaller space is required compared to hydro power plant  Economical in initial cost compared to hydro plants and running costs are less compared to gas plants or diesel plants  Thermal plants can be placed near load centers unlike hydro and nuclear plants. Hence transmission of power losses can be minimized  Thermal plants are able to respond to the load demand more effectively and supports the performance of the electrical grid  Steam plants can withstand for overload for certain extent

 Higher maintenance and operational costs  Pollution of the atmosphere  Huge requirement of water  Handling of coal and disposal of ash is quite difficult and requires large area  Gestation period (period for commissioning of plant) takes long time  Efficiency of thermal plant is quite less (30-35%)  Operational cost of thermal plant is more costlier compared to hydro and nuclear plant