Steam Power Plant_mdm.pdf

POWER PLANT BY MADHUMITA MANDAL ASSISTANT PROFESSOR ELECTRICAL ENGINEERING DEPT JADAVPUR UNIVERSITY

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Structure of Power System – Generation, transmission and distribution. Power generating stations – different types. Steam power stations: Main parts and working, types of boilers and their characteristics. Characteristics of steam turbines and alternators. Main flow circuits of steam power station. Power station auxiliaries, cooling system of alternators. Starting up and shut down procedures of thermal units. Gasturbine power stations- Main parts, plant layout and Bryton cycle operation. Combined cycle generation & Co-generation. Nuclear power stations- Layout of nuclear power station, types of power reactors, main parts and control of reactors, nuclear waste disposal, radioactivity and hazards. Hydroelectric stations: Arrangement and location of hydroelectric stations, principles of working, types of turbines and their characteristics, Pumped storage plants. Coordination of operation of different power stations.

• COs • Idea of power system hierarchy • Concepts of different types of electrical power generations and concept of substations • Idea of electrical wiring and installation • Knowledge of economic and electrical considerations of power distribution system

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Introduction Basically there are four different parts of a power systems:1. Generation 2. Transmission 3. Distribution 15KV

15/400KV

33-400 KV

400/33KV

33/11KV load

Basic Single line diagram of a power system

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Introduction • A power plant is assembly of systems or subsystems to generate electricity, i.e., power with economy and requirements. The power plant itself must be useful economically and environmental friendly to the society. • A power plant may be defined as a machine or assembly of equipment that generates and delivers a flow of mechanical or electrical energy. The main equipment for the generation of electric power is generator. When coupling it to a prime mover runs the generator, the electricity is generated. The type of prime mover determines, the type of power plants.

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Classification of power plants

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Classification of power plants • The major power plants, which will be discussed in the following lectures are 1. Steam power plant 2. Gas turbine power plant 3. Nuclear power plant 4. Hydro electric power plant • The Steam Power Plant, Gas Turbine Power Plant together with Diesel Power Plant are called THERMAL POWER PLANT, because these convert heat into electric energy.

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Energy Sources • The conversion into electrical energy is usually made by using one or more of the following sources:1. Fuels/Fossil Fuel :- Fuels may be either solid or liquid(liquid and gaseous). 2. Flowing water creates energy that can be captured and turned into electricity. This is called hydroelectric power or hydropower. 3. Nuclear Energy 4. Wind Energy 5. Solar Energy The last 2 are basically renewable energy sources.

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Energy Sources The advantages and disadvantages of solid fuels over others are : Advantages: i. Cheaper ii. No danger of explosion. iii. No serious problem in operation even under cold climate iv. Capital investment for solid fuel based power plant is less as compared to other plants. Disadvantage:i. Handling of solid fuel is always a difficult task. ii. Its storage requires more space. iii. Heat production rate by burning solid fuels only cannot be changed quickly. Hence it is incapable of meeting quick variation in load demand. iv. Fuel waste(ash) handling and removal is always a difficult and expensive job. MdM, EE, JU

COAL AS FUEL:• Since the advent of industrialization coal has been most common source of energy. • Coal is a complex mixture of compounds of carbon, hydrogen and oxygen. Small amounts of nitrogen and Sulphur compounds are also present in coal. • It is mainly available in Bihar, West Bengal, Orissa and Madhya Pradesh. • The big coal mines in our country are at Jharia and Bokaro in Bihar and at Raniganj in West Bengal. • It is considered as the backbone of the energy sector for its use in industry, transportation and electric power generation.

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Classification of Coal

1. Peat: the peat is the first stage in the process of transformation of buried vegetation into coal. It contains high percentage of moisture and small percentage of volatile matter and carbon. 2. Lignite: It is the next stage in the development of coal.Contain high percentage of moisture (30- 45 %) but can be dried just by exposing to air, can be used as fuel in pulverised form. 3. Bituminus: most popular form of coal. It has low moisture content. Ash content varies from 6 -12 % and ash fusion temperature is usually 1093 deg centigrade. 4. Anthracite:- It is the last stage in the process of transformation. Contain highest percentage of carbon and the percentage of voltaile matter is below 8 %. Pulverisation is very difficult and MdM, EE, JU costly .

STEAM POWER PLANTS

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Steam power Plants • Steam is an important medium of producing mechanical energy. Steam has the advantage that, it can be raised from water which is available in abundance it does not react much with the materials of the equipment of power plant and is stable at the temperature required in the plant. Steam is used to drive steam engines, steam turbines etc. Steam power station is most suitable where coal is available in abundance. • Thermal electrical power generation is one of the major method. Out of total power developed in India about 60% is thermal. For a thermal power plant the range of pressure may vary from 10 kg/cm2 to super critical pressures and the range of temperature may be from 250°C to 650°C. • In steam engines, the spent steam is discharged to atmosphere. In case of steam turbines, after steam passes through the turbine, it is usually condensed in a condenser. However in smaller units < 10 MW, steam is usually discharged into atmosphere, or, sometimes, utilized in process plants such as paper mills, textile mills, sugar mills, etc. This process is known as co-generation. The operation of a steam engine or turbine is based on Rankine cycle. MdM, EE, JU

Rankine cycle

• There are four processes in the Rankine cycle. These states are identified by numbers (in brown) in the above T-s diagram. • Process 1-2: The working fluid is pumped from low to high pressure. As the fluid is a liquid at this Stage, the pump requires little input energy. • Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapour. The input energy required can be easily calculated graphically, using an enthalpy-entropy chart (aka h-s chart or Mollier diagram), or numerically, using steam tables. • Process 3-4: The dry saturated vapour expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur. The output in this process can be easily calculated using the chart or tables noted above. • Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure to become a saturated liquid. MdM, EE, JU

Basic concept:-

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Steam power plant are of two types:A.

B.

Condensing type:- exhaust steam is discharged into a condenser, which creates suction at very low pressure and allows the expansion of steam in the turbine at low pressure & hence increase efficiency. These type of power plants are generally used as central power plants to generate electrical energy to supply different consumers .Steam is condensed into water in condenser and re-circulated to boiler pumps. Non condensing type:- in this type exhaust steam from turbine is discharged at atmospheric pressure or at a pressure greater than atmospheric pressure. In this continuous supply of fresh water is required. The process industries requiring low pressure steam for process purpose are of this type. They use the exhaust steam from turbine for process purpose. Industrial turbo generator plant, known as captive power plant, are usually of small capacity (upto 10MW).

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CHARACTERISTICS OF STEAM POWER PLANT 1. 2. 3. 4. 5. 6.

Higher efficiency. Lower cost. Ability to burn coal especially of high ash content, and inferior coals. Reduced environmental impact in terms of air pollution. Reduced water requirement. Higher reliability and availability.

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Selection of site for a thermal station:1. 2. 3. 4. 5.

Cost of land should be reasonable and further extensions should be possible. Abundant quantity of cooling water for the condenser should be available. Facilities should exist for the transport of fuel. If the station is located near the center of distribution, the distribution costs are low. Site should be away from populated area to avoid pollution hazards.

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Efficiency •

Huge amount of heat losses occur in a thermal power plant. The approximate figures are as follows: • Boiler house losses: Dry flue gas : 5% Moisture in gas: 5% Ash and un-burnt carbon: 1% Radiation and leakage: 2.5% Stray losses: 2.5% Overall 16% • Turbine/Generator losses: Heat rejected to condenser 54% Losses in alternator 1% • It is thus seen that the most of the losses in the plant occur at the condenser. This loss is unavoidable and can not be converted into mechanical form. The thermal efficiency of the turbine and the power plant mainly depends upon the temperature and pressure of the steam at the inlet of the turbine, and the back pressure at the condenser. It increases with increase in operating pressure and temperature, reduction of back pressure at the condenser, which is typically about 0.04 kg/mm2 . MdM, EE, JU

Efficiency Continued • The thermal efficiency of a conventional thermal power station, considered as the ratio of heat equivalent of the mechanical power transmitted to the turbine shaft to the heat combustion, is low, typically in the range 30 to 48%. No heat engines, by the laws of thermodynamics can be more efficient than the ideal heat engine operating on Carnot cycle. The overall efficiency is obtained by multiplying the thermal efficiency with generator efficiency and is in the range 29% to 46%. The rest of the energy must leave the plant in the form of heat. This waste heat can be disposed of with cooling water or in cooling towers. If the waste heat is instead utilized it is called cogeneration. • Lot of developments has taken place in the design of boiler, turbine and generator. The modern trend is to use high pressure and high temperature steam at the turbine input. Increase of operating temperature has more influence on efficiency than pressure. The present practice is to employ operating temperature of 480 K and pressure of 6.5 - 10 N/mm2 are typical values. • Different units for measurement of pressure • 1 atm. (1 bar) = 76 cm of Hg = 13.6 x 76 =1034 gm/cm2= 1.034 kg/cm2 = 101400 Pascal ( N/m2) • 1 N/mm 2 = 1 MPa ≈ 10 bar, 1kg/mm2 ≈ 10 N/mm2 • Typical Boiler pressure: 10N/mm2 =10 Mpa ≈ 100 (bar) atm. MdM, EE, JU

BASIC LAYOUT OF STEAM POWR PLANT:-

Circuits in Steam Power Plants The flow sheet of a thermal power plant consists of the following four main circuits : (i) Coal and ash circuit (ii) Air and gas circuit (iii) Feed water and steam flow circuit (iv) Cooling water circuit. • A steam power plant using steam as working substance works basically on Rankine cycle. • Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser and fed into the boiler again.

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Coal and ash circuit: • Coal arrives at the storage yard and after necessary processing passes into the furnace through fuel feeding devices. • Ash formed after combustion collects in the bottom ash collector hopper and removed to ash storage yard using ash-handling equipment.

Air and gas circuit: •

Air is drawn from the atmosphere through forced draught (FD) fan and passes through the air pre-heater. A part of this hot air passes through the coal mill and carries pulverized coal into the furnace through coal burners and the other part enters the furnace directly to support combustion of coal. • The resulting hot gases heat the water tubes, superheater tubes, the reheater tubes, economizer, air pre-heater and finally to the chimney after passing over dust collecting devices. An induced draught (ID) fan, installed between the dust collector and the chimney maintains a negative pressure within the furnace. MdM, EE, JU

Feed-water and steam flow circuit: • The condensate leaving the condenser is first heated by bled steam in a closed feed-water heaters. The de-aerator(removes corrosive oxygen dissolve into water) maintains a negative pressure to extract any dissolved gas from the condensate. The condensate then passes through a few more water heaters and the temperature gradually rises before being pumped into the boiler-drum by a boiler-feed pump through the economizer, where the feed water attains temperature nearly as high as that of the boiler. In the boiler drum and tubes water circulates due to convection current. • Wet steam passes through separators in the boiler drum to separate water droplets from steam. • The steam is further heated up in the super-heater before being supplied to the high-pressure turbine. After expansion the spent steam is taken to the reheat zone of the furnace, where further heat is imparted to the steam at constant pressure, before being passed over to IP stages of the turbine. • The steam from the IP turbine passes over the LP turbine blades to deliver further kinetic energy to the prime mover. From there, it is exhausted to the condenser. Some part of the steam is utilized in heating feed-water. • The part of the steam lost due to leakage and other reasons, is made up by additional water from a water treatment plant which receives raw water from river and purifies it before MdM, injecting EE, JU into the feed water circuit.

COOLING WATER CIRCUIT:• The cooling water supply to the condenser is needed to maintain a low back pressure by condensing all the steam that enters there. The water may be taken from a nearby natural source such as river, lake etc and the water from the condenser is discharged into the river. • The flow of water through the condenser should be at such a rate that the hot water at the outlet is not unsafe for the aquatic creatures. In case of scarcity of natural sources, the heated water is cooled in a spray pond or cooling tower and re-circulated into the condenser.

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Different Components of Steam Power Plants

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BOILER:-

• Boiler is a closed vessel in which water is converted into steam by the application of heat. Usually boilers are coal or oil fired. • The boiler has to produce steam at high purity, pressure and temperature required for the steam turbine that drives the electrical generator. The typical boiler pressure is in the range 80-160 bar, steam temperature of the order of 540 oC and superheated steam temperature ranges from 600900 oC. • A boiler should fulfill the following requirements:(i) Safety. The boiler should be safe under operating conditions. (ii) Accessibility. The various parts of the boiler should be accessible for repair and maintenance. (iii) Capacity. The boiler should be capable of supplying steam according to the requirements. (iv) 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. (v) The boiler should be capable of quick starting and loading. • The performance of a boiler may be measured in terms of its evaporative capacity also called power of a boiler. It is defined as the amount of water evaporated or steam produced in kg per hour. It may also be expressed in kg per kg of fuel burnt or kg/hr/m2 of heating surface. MdM, EE, JU

Boiler Contd….. According to flow of water and hot gases. 1. Water tube. 2. Fire tube. • In water tube boilers, water circulates through the tubes and hot products of combustion flow over these tubes. • In fire tube boiler the hot products of combustion pass through the tubes, which are surrounded, by water.

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Fire Tube Boilers Advantages of it: 1. Low cost 2. Fluctuations of steam demand can be met easily 3. It is compact in size. There are various types Fire tube boilers. They are as follows:1. Cochran Boiler. 2. Lancashire Boiler. 3. Locomotive Boiler. We will be studying only the Cochran Boiler.

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COCHRAN BOILER The features of this fire tube boiler are listed below: 1. It is very compact and requires minimum floor area. 2. Any type of fuel can be used with this boiler. 3. It is well suited for small capacity requirements.

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Water tube Boilers Various advantages of water tube boilers are as follows. (i) High pressure of the order of 140 kg/cm2 can be obtained. (ii) Heating surface is large. Therefore steam can be generated easily. (iii) Large heating surface can be obtained by use of large number of tubes. (iv) Because of high movement of water in the tubes the rate of heat transfer becomes large resulting into a greater efficiency

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BABCOCK WILCOX BOILER The features of this water tube are : 1. The evaporative capacity of this boilers is high compared with other boilers (20,000 to 40,000 kg/hr). 2. The defective tubes can be replaced easily. 3. The entire boiler rests over an iron structure, independent of brick work, so that the boiler may expand or contract freely. The brick walls which form the surroundings of the boiler are only to enclose the furnace and the hot gases.

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MERITS OF WATER TUBE BOILERS OVER FIRE TUBE BOILERS 1. Generation of steam is much quicker due to small ratio of water content to steam content. This also helps in reaching the steaming temperature in short time. 2. Its evaporative capacity is considerably larger and the steam pressure range is also high 200 bar. 3. Heating surfaces are more effective as the hot gases travel at right angles to the direction of water flow. 4. The combustion efficiency is higher because complete combustion of fuel is possible as the combustion space is much larger. 5. The thermal stresses in the boiler parts are less as different parts of the boiler remain at uniform temperature due to quick circulation of water. 6. The boiler can be easily transported and erected as its different parts can be separated. 7. Damage due to the bursting of water tube is less serious. Therefore, water tube boilers are sometimes called safety boilers. 8. All parts of the water tube boilers are easily accessible for cleaning, inspecting and repairing. 9. The water tube boiler's furnace MdM, areaEE, JU can be easily altered to meet the fuel requirements.

DEMERITS OF WATER TUBE BOILERS OVER FIRE TUBE BOILERS 1. It is less suitable for impure and sedimentary water, as a small deposit of scale may cause the overheating and bursting of tube. Therefore, use of pure feed water is essential. 2. They require careful attention. The maintenance costs are higher. 3. Failure in feed water supply even for short period is liable to make the boiler over-heated.

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HIGH PRESSURE BOILERS • In all modern power plants, high pressure boilers (> 100 bar) are universally used. In order to obtain efficient operation and high capacity, forced circulation of water through boiler tubes is found helpful. • The Advantages are: 1 . The efficiency and the capacity of the plant can be increased as reduced quantity of steam is required for the same power generation if high pressure steam is used. 2. The forced circulation of water through boiler tubes provides freedom in the arrangement of furnace and water walls, in addition to the reduction in the heat exchange area. 3. The tendency of scale formation is reduced due to high velocity of water. 4. The danger of overheating is reduced as all the parts are uniformly heated. 5. The differential expansion is reduced due to uniform temperature and this reduces the possibility of gas and air leakages. MdM, EE, JU

LA MONT BOILER • The feed water from hot well is supplied to a storage and separating drum (boiler) through the economizer. Most of the sensible heat is supplied to the feed water passing through the economizer. • A pump circulates the water at a rate 8 to 10 times the mass of steam evaporated. This water is circulated through the evaporator tubes and the part of the vapor is separated in the separator drum. • The large quantity of water circulated prevents the tubes from being overheated. • The distribution headers distribute the water through the nozzle into the evaporator. • These boilers have been built to generate 45 to 50 tones of superheated steam at a pressure of 120 bar and temperature of 500°C.

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BENSON BOILERS • The main difficulty experienced in the La Mont boiler is the formation and attachment of bubbles on the inner surfaces of the heating tubes. • The attached bubbles reduce the heat flow and steam generation as it offers higher thermal resistance compared to water film. • These difficulties were removed by the use of “SUPERCRITICAL STEAM GENERATORS (BENSON BOILERS) ”. • Supercritical steam generators (also known as Benson boilers), in contrast to a "subcritical boiler", operates at such a high pressure (over 3200 PSI, 22 MPa, 220 bar) that actual boiling ceases to occur, and the boiler has no water - steam separation. • There is no generation of steam bubbles within the water, because the pressure is above the "critical pressure" at which steam bubbles can form. Steam passes below the critical point as it does work in the high pressure turbine and enters the generator's condenser. This is more efficient, resulting in slightly less fuel use. MdM, EE, JU

BENSON BOILERS Contd…. • To increase the efficiency of steam power plants the basic method is to improve the thermal efficiency by increasing the operating pressure. • To understand how a Supercritical power plant works we have to understand the basics of steam generation. What happens when you heat water at normal atmospheric pressure? • As you go on heating the water, the temperature of water increases till it reaches 100 deg C. This is the Sensible Heat addition. • Further heating does not increase the temperature; instead small bubbles of steam start to form. The temperature remains constant at 100 deg C till all the water becomes steam. The water absorbs the heat without temperature change for conversion to steam. At atmospheric pressure the Latent Heat of vaporization is 2256 kJ/kg. • Further heating called superheating will increase the temperature of the steam. How high one can go depends on the withstanding capacity of the vessel.

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BENSON BOILERS Contd…. • What happens when the water is at a higher pressure, say, at 100 bar? Then the boiling takes place at 311 deg C and the latent heat of vaporization is 1318 kJ/kg. • If the water pressure is 200 bar then the boiling takes place at 366 deg C and the latent heat of vaporization is 584 kJ/kg. • As the pressure increases the boiling temperature increases and the latent heat of vaporization decreases. • A further increase in pressure and temperature leads us to a point at which the latent heat of vaporization is zero, or there is no boiling. Water directly becomes steam. This is the Critical Pressure and the Critical Temperature. For steam this occurs at 374 deg C and 220.6 bar. • Conventional steam power plants operate at a steam pressures in the range of 170 bar. These are Subcritical power plants. The new generation of power plants operate at pressures higher than the critical pressure. These are Supercritical power plants. The operating pressures are in the range of 230 to 265 bar. • The efficiency of the Rankine cycle depends on the pressure at which it operates. Higher pressure and temperature increase the efficiency of the thermal cycle and power plant. This is the reason for operating at higher steam pressures. MdM, EE, JU

Features of Benson Boiler 1.

2. 3.

4.

5.

6.

The boiler pressure was raised to critical pressure (225 atm.), so that the steam and water would have the same density and therefore the danger of bubble formation can be completely taken care of. The Benson boiler can be erected in a comparatively smaller floor area. The space problem does not control the size of Benson boiler used. The Superheater in the Benson boiler is an integral part of forced circulation system, therefore no special starting arrangement for Superheater is required. The Benson boiler can be operated most economically by varying the temperature and pressure at partial loads and overloads. The desired temperature can also be maintained constant at any pressure. Sudden fall of demand creates circulation problems due to bubble formation in the natural circulation boiler which never occurs in Benson boiler. This feature of insensitiveness to load fluctuations makes it more suitable for grid power station as it has better adaptive capacity to meet sudden load fluctuations. Explosion hazards are not at all severe as it consists of only tubes of small diameter and has very little storage capacity compared to drum type boiler. MdM, EE, JU

SUPERHEATER AND REHEATER • The function of the superheater is to remove the last traces of moisture from the saturated steam leaving the boiler tube and also to raise the temperature of this steam. • The heat of the flue gases from the furnace is used for superheating. They are classified as convection, radiant or combination of these. • A convection superheater is placed somewhere in the gas stream and receives most of the heat by convection. A radiant superheater is placed in or near the furnace where it receives most of its heat by radiation. A combination superheater has both convection and radiation sections. The steam leaving the boiler drum first passes through the convection section then through the radiation section and then to the steam header. • Convection superheaters are most commonly used because radiant superheater gives drooping characteristics i.e the temp. of superheat falls with increase in steam output because with increase in steam o/p, the furnace temp. rises is at a much less rate than steam o/p and the radiant heat transfer being a function of furnace temperature increase slowly with steam flow and thus the steam temp. falls. MdM, EE, JU

SUPERHEATER AND REHEATER • In addition to this , modern superheaters have reheaters also. • The function of them is to superheat the partly expanded steam from the turbine. This ensures that the steam remains dry through the last stages of turbine. The steam is superheated to the highest economical temp. not only to increase the efficiency but also to have following advantages:1. Due to high steam temperature and pressure, steam has high internal energy , therefore for same output less quantity of steam is required and therefore boiler size is reduced. 2. Since the dry superheated steam flows over the turbine blades therefore mechanical resistance to the turbine blades is less which results in high efficiency. 3. Due to dry steam blade corrosion of the turbine blades is avoided.

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ECONOMIZER:- (600-350 DEG CENTIGRADE) • Flue gases coming out of the boilers carry a lot of heat. An economiser extracts a part of the heat from the flue gases and uses it for feeding heat water. • The use of economiser results in saving coal consumption and higher boiler efficiency but needs extra investment and increases maintenance costs and floor area required for the plant. • In an economiser, a large number of small diameter thin walled tubes are placed between two ends. Feed water enters the tubes through one end and leaves through the other end. • The economizer chamber should be leak proof against air infiltration, otherwise boiler draught may be adversely affected.

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AIR PREHEATER(350°C – 250 °C):• After the flue gas leaves the economiser, some more heat can be extracted from them and used to heat the incoming air for combustion. Cooling of flue gases by 200 C raises the plant efficiency by 1 %. • Air preheater may be plate type or tubular type or regenerative type. • A plate air preheater has alternate narrow lanes for gas and air passages and the two fluids (gas and air) flow in opposite directions. • In tubular air preheaters, the gases flow inside the tubes and the air over the tube exteriors. • A regenerative air preheater uses a cylindrical rotor made of corrugated steel plates. The rotor is fixed on a shaft which rotated at a slow speed of 2 to 4 rpm. As the rotor rotates, it alternately passes through the flue gases and air zones. The rotor elements are heated by the flue gases in their zone and the transfer heat to the air when they are in the air zone.

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Electrostatic Precipitators • It consists of two sets of collecting and emitting electrodes. • At high voltage, an electric field exists between these electrodes, which ionizes the gas molecules which in turn after cohesion imparts the charge to the dust particles • The charged dust particles are collected by the collecting electrodes. • The high voltage system maintains a negative potential of 40 to 80 KV DC between the collecting electrodes. The collecting electrodes have a large surface area. Accumulated dust falls off the electrodes when it is tapped mechanically.

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CONDENSER • The condenser does the job of condensing the steam exhausted from the turbine. • Thus it helps in maintaining low pressure (below atmospheric) at the exhaust, thereby permitting expansion of steam in the turbine to a very low pressure. • The exhaust steam is used as feed water for the boiler. • Modern power plants mostly use surface condenser. A surface condenser consists of an air-tight cylindrical shell having a chamber at each end. • Water tubes extend between the chambers. The shell is made of welded steel plate construction and the tubes are made of copper zinc alloy. • Cooling water flows through the tubes. The steam is admitted from the top and gets condensed due to contact with the tube surfaces. The condensate leaves from the bottom. For efficient operation , the temperature rise in cooling water passing through the condenser should be around 10⁰ C.

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Boiler Feed pump and Condensate Pump • The discharge pressure of condensate pump is less than the boiler pressure. Thus boiler feed pump is used to raise the pressure of the condensate and send it to the boiler. Condensate pump takes its suction from the hotwell of the condenser.

Feed Water Heater • The steam coming out of the turbine is condensed and the condensate is fed back to the boiler as feed water. Before feeding this water into the boiler it is necessary to heat it due to the following reasons: 1. Feed water heating improves overall plant efficiency. 2. The dissolved oxygen and carbon dioxide which would otherwise cause boiler corrosion are removed from feed water. 3. Thermal stresses due to cold water entering the boiler drum are avoided. 4. Quantity of steam produced by the boiler is increased. MdM, EE, JU

Feed Water Heater Contd…. • In large power plants bleed steam from the main turbine is used for feed water heating. • The feed water is thus heated put under pressure and then further heated so that its temperature approaches and pressure exceeds that of water in the boiler.

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Deaerator • A steam generating boiler requires that the boiler feed water should be devoid of air and other dissolved gases, particularly corrosive ones, in order to avoid corrosion of the boiler tubes. • Generally, power stations use a deaerator to provide for the removal of air and other dissolved gases from the boiler feedwater. A deaerator typically includes a vertical, domed deaeration section mounted on top of a horizontal cylindrical vessel, which serves as the deaerated boiler feedwater storage tank. • There are many different designs for a deaerator and the designs will vary from one manufacturer to another. The diagram below depicts a typical conventional trayed deaerator. If operated properly, most deaerator manufacturers will guarantee that oxygen in the deaerated water will not exceed 7 ppb by weight (0.005 cm³/L).

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Boiler make-up water treatment plant and storage • 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 so as to maintain the desired water level in the boiler steam drum. • For this, continuous make-up water is added to the boiler water system. The impurities in the raw water input to the plant generally consist of suspended particles, clay and dissolved calcium and magnesium salts, which impart hardness to the water. Suspended particles and clay are removed in filter beds. • 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 a water demineralising plant (DM). • A DM plant generally consists of cat-ion, an-ion and mixed bed exchangers. The final water from this process consists essentially of hydrogen ions and hydroxyl ions which is the chemical constituent of pure water. MdM, EE, JU

• The DM water, though very pure, absorbs oxygen from the atmosphere because of its high affinity for oxygen and becomes highly corrosive. • The capacity of the DM plant is dictated by the capacity of the plant, 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 usually made from PVC, a material which is not affected by corrosive water. • The piping and valves are generally made 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 atmospheric air.

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DRAUGHT To burn a fuel completely four basic condition must be fulfilled (1) Supply of enough air. (2) Thorough mixing of fuel and air. (3) Sufficiently high furnace temperature. (4) Enough surface volume to provide sufficient time for completing the combustion reaction. • The burnt product must also be removed quickly. To facilitate the flow of air and remove the burnt products, a pressure difference is required. This difference in pressure is known as draught. • When the draught is produced with the help of chimney only, it is known as natural draught and when the draught is produced by any other means except chimney, it is then called artificial draught.

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CHIMNEY DRAUGHT • The draught produced by the chimney is due to the temperature difference of hot gases in the chimney and cold air outside the chimney. • Let the height of the chimney above the grate level is ‘H’. • The pressure acting on the grate from the chimney side is given by , P1 = Pa + wg*H . • The pressure acting on the grate at atmospheric side , P2 = Pa + wa*H . Where, Pa = atmospheric pressure. wa = weight density of atmospheric air. wg = weight density of hot gases passing through chimney. Net acting pressure on the grate: P2 – P1 = H (wa – wg) kgf/cm2

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CHIMNEY DRAUGHT • this difference in pressure is responsible for causing the flow of air through the combustion chamber. • The acting pressure can be increased either by increasing the height of the chimney or by reducing the density of hot gases by allowing the hot gases to go out of the boiler at higher temperature. Advantages of chimney draught:1) No external power is required 2) Less capital investment 3) Release flue gases at a certain height, therefore contamination of air is less. Limitations:1) Available draught decreases with increase in outside air temp. resulting, in the loss of overall plant efficiency. 2) As there is no thorough mixing of fuel and air in the combustion chamber due to low velocity of air , combustion is very poor. This increases fuel consumption. MdM, EE, JU

ARTIFICIAL DRAUGHT

INDUCED DRAUGHT

FORCED DRAUGHT

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BALANCED DRAUGHT

Induced draught • In induced draught a blower is installed near the base of the chimney. The air is sucked in the system by reducing the pressure through the system below atmosphere. • The suction head produced by I.D fan depends upon the draught requirement. • After the drop in the flue, furnace and economizer etc, finally a suction head should be available to draw in air through the coal bed. • Draught is independent of the hot column of air in the chimney. Therefore gases may be let out as cold as possible(of course much above the dew point when condensation may start) after recovering heat as much as possible. • The function of the chimney in this arrangement is to dispose of the hazardous smoke and gases high up in the atmosphere as rules pertaining to smoke nuisance require. • Its primary purpose is not for developing draught , therefore height of the chimney is not very much. MdM, EE, JU

INDUCED DRAUGHT

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FORCED DRAUGHT • In F.D system , a blower or fan is installed near the base of the boiler and air is forced to pass through the furnace, economizer, air – preheater and finally to the stack (or chimney). • this draught system is also called positive draught system because the pressure of air throughout the system is above atmospheric pressure air is forced to flow through the system.

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BALANCED DRAUGHT • If the F.D is used alone then the furnace cannot be opened either for firing or inspection because high pressure air inside the furnace will try to blow out suddenly and there is every chance of blowing out the fire completely and furnace stops. • If the I.D is used alone then also furnace cannot be opened either for firing or for inspection because the cold air will try to rush in to the furnace as the pressure inside the furnace is below atmospheric pressure. this reduces the effective draught and dilutes combustion. • To overcome both the difficulties mentioned above either using FD or ID alone, a balanced draught is always preferred. • The force draught overcomes the resistance of the fuel bed therefore sufficient air is supplied to the feed bed for proper and complete combustion. • Induced draught fan removes the gases from the furnace maintaining the pressure in the furnace just below the atmospheric pressure. this helps to prevent blow off of flame when the furnace doors are opened as the leakage of air is inward. The pressure inside the furnace is near atmospheric but not atmospheric hence MdM, EE, JU there is no danger of blow out of flames.

BALANCED DRAUGHT • The pressure below the grate is above atmospheric and it helps for proper and uniform combustion. • The pressure above the grate is below atmospheric and it helps to remove the exhaust gases as quick as possible from the combustion zone.

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BALANCED DRAUGHT • The FD fans push atmospheric air through the air pre-heater, dampers, various ducts and burners into furnace . • The ID fans pull the combustion gases from the furnace, throughout heat transfer surfaces in the super heater, re-heater, economizer and gas side air pre heater and into the stack(chimney). • The stack, because of its height, adds a natural driving pressure of its own. • The furnace in this case is said to operate with balanced draught meaning that the pressure in it is approximately atmospheric pressure. • Actually it is kept at a slightly negative gas pressure to ensure that any leakage would be inward. • In general configurations fans with backward – curved blades are used for FD fans and with flat or forward curved blades for ID fans.

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STEAM TURBINE:• The superheated steam from the super heater is fed to the steam turbine via a controllable inlet valve .The expansion of steam in the turbine causes its heat energy to be converted into mechanical energy at the turbine shaft. • The exhausted steam is condensed in the condenser which is kept cool by circulating water. Keeping the condenser cool helps to condense the exhausted steam thereby creating a pressure & temp. difference , both of which will help to improve the conversion ratio. • Steam turbines are of two types:1. Impulse turbines:- here the steam expands in stationary nozzles, attains high –velocity and impinges impulsive force on the moving blades to let the turbine shaft rotate. They have high speed. 2. Reaction turbine:- here the steam is partially expanded in the stationary nozzles and the remaining expansion is over the moving blades. Thus there will be reaction force on the moving blades to set the turbine shaft rotating. They are characterized by low rpm. MdM, EE, JU

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Lubricating System • Lubricating system does not allow the turbine shaft to come to direct contact with the bearings and remove quickly the heat generated during the operation. • Properties of lubricating oil are liable to change under high temperature and this should be maintained at a reasonable limit. Oil lubrication should be continuous , under pressure, cool and free from injurious foreign matters. • Lubricating system uses following pumps for its operation: 1. Jacking pump: used at a time of starting of the turbo alternator. 2. Flushing Pump: used during slow turning period. 3. Auxiliary Pump: supplies lubricating oil during starting or shutting down of the turbo alternator. 4. Main Oil Pump: supplies oil to the bearing when the turbo alternator reaches its synchronous speed.

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Functions of Lubricating System: • A high pressure oil system operating at a pressure of about 1000-2000 psi supplies oil to the turbine and alternator bearings. • A high pressure oil system lifts the rotor shaft from the bearings and establishes an oil field between them before the barring gear is put into operation and so prevents the damage under slow running conditions. A motor driven high pressure oil pump known as the jacking oil pump (JOP) is installed for this purpose. • Another auxiliary oil pump known as the flushing oil pump (FOP) is provided to supply the oil for lubricating purposes during slow turning period. This pump can also be arranged to serve as a standby turbine driven auxiliary oil pump. • Auxiliary oil pump (AOP) is driven by a small turbine and is used when the turbine is rotated and it automatically cuts of operation when the turbo alternator reaches rated speed and the main pump is supplying oil at the proper pressure. This pump supplies oil to the bearing while starting up or shutting down of the turbo alternator. Arrangement is made for its automatic operation in the event of failure of the main oil pump. MdM, EE, JU

Functions of Lubricating System: • The main oil pump (MOP) which is directly driven through gears from the turbine shaft takes its supply of oil from the oil tank through strainers incorporated in the tank. • The pump discharge pressure is about 50 psi and the oil flowing through the bearings is usually throttled to a pressure of about 5 to 7 psi. • The oil at the pump discharge pressure (50 psi) is also used in the governing system to act as the motive medium for opening and closing the valves. • If the oil system fails due to any reason the steam supply to the turbine is cut off immediately.

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Barring Gear • This is a turning or rolling gear for slowly rotating the shaft of the turbo alternator. The functions of the barring gear are: 1. To start the turbine from rest without use of steam. 2. To rotate the shaft slowly when the steam has been cut-off thus preventing any permanent set or bending due to uneven contraction which may take place if the turbo alternator is allowed to cool under stationary conditions. 3. It minimizes the possibility of shaft distortion at starting due to unequal heating caused by sudden inrush of steam. 4. It maintains an oil film between shaft journals and bearings. 5. It helps to run the motor for inspection purposes. • Barring Speed: is normally in the region of 1.5 to 2 rpm. It trips out automatically when a speed of 20 to 30 rpm is reached.

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Barring gear contd…. • 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 revolution per minute) by the barring gear until it has cooled sufficiently to permit a complete stop.

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Governing System • All turbo alternators are required to operate at constant speed. Governing system is therefore made speed responsive admitting more steam when the turbine speed drops and vice versa. • In a fly ball governor, when the turbine speed exceeds rated speed, the fly balls move apart due to increase in centrifugal force and thereby operates a pilot valve through a linkage which in turn admits or releases high pressure oil to an operating piston which actually tends to close the main steam valve so as to minimize the supply of steam to the turbine. On the other hand when the turbine speed falls, the fly balls move closer and thus increase the opening of the main steam valve so as to admit more steam to the turbine. • If the governing system fails for any reason steam to the turbine would be immediately cut-off.

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Cooling System Air Cooling : • Turbo alternators are high speed machines having high axial length and small rotor diameter. The heat reducing surfaces are relatively small. Large quantity of air must therefore be forced through the air gap and the vent holes of the machine at high speed for effective cooling. • The quantity of air required is so large that air without filtering cannot be used for cooling. The cooling air must be clean and usually a closed circuit cooling system is used. • In this system, air is passed through the machine and back, through a surface cooler over and over again so that there is least chance of dirt and dust to get inside the cooling system. • This method reduces temperature and noise of the turbine rotor.

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Cooling System Contd…. Hydrogen Cooling: • Hydrogen as a coolant has numerous advantages over air. The ability of a gas to absorb heat is proportional to its heat capacity which is (specific heat × density). Heat capacity of both hydrogen and air are same. So , a given volume of hydrogen and air can absorb the same amount of heat under same temperature and pressure conditions. • The advantages of using hydrogen as a coolant are: a) A low density of hydrogen reduces windage loss to 7 to 8 % of that due to air and reduces noise also. b) Hydrogen has a thermal conductivity 7 times as great as air and approximately the same as that of the most winding insulation. Due to this high thermal conductivity hydrogen is able to carry away heat across small spaces in the insulation and between laminations more readily than air. c) On account of total absence of dirt , moisture and oxygen , life of insulation is increased and maintenance expenses are decreased. d) Hydrogen does not support combustion hence much less damage would MdM, EE, JU result in case of internal short circuit.

Other Systems:Monitoring and alarm system • Most of the power plant operational controls are automatic. However, at times, manual intervention may be required. Thus, the plant is provided with monitors and alarm systems that alert the plant operators when certain operating parameters are seriously deviating from their normal range. Battery supplied emergency lighting and communication • A central battery system consisting of lead acid cell units is provided to supply emergency electric power, when needed, to essential items such as the power plant's control systems, communication systems, turbine lube oil pumps, circuit breaker trip coil and emergency lighting. This is essential for a safe, damage-free shutdown of the units in an emergency situation.

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ELECTRICAL EQUIPMENTS •Alternator •exciter •voltage regulator •reactor, transformer •circuit breaker and isolator •switch yard

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ALTERNATORS • The alternator is driven by the prime mover, a steam, water or gas turbine, and runs at synchronous speed ( Ns=120 f/P). Turbo alternators have 2 pole cylindrical rotors, and run at 3000 rpm. • The rotor produces the magnetic flux. It is supplied with dc excitation from exciter which is mounted on the same turbine shaft. • The stator carries the armature winding where the emf ( 11 / 18/ 33 kV) is induced. • The alternator is connected to the unit transformer through bus-ducts. Circuit breakers and isolators are provided on the H.T. side of the transformer to connect the alternator to the power station bus bars. • Smaller alternators are air-cooled, but as the size increases, more elaborate cooling arrangement becomes a necessity. Sealed type units, with circulating hydrogen gas is employed for the purpose of cooling. Still larger units employ both hydrogen and water cooling. Stator conductors are built with hollow strands, through which purified water is circulated to keep the temperature low. MdM, EE, JU

Exciter: • DC excitation system is needed to feed the necessary field current to the rotor winding of the alternator. • Modern power plants require smart excitation system that has fast response and capable of varying excitation over a wide range. • The reliability of the excitation system is of utmost importance as the loss of excitation would lead to serious disturbance in the power system. It has a strong impact on generator dynamic performance, like availability, quality of generated voltage, active and reactive power. • Following types are common: a) Conventional Excitation system: The alternator rotor is fed from a dc compound generator, called exciter, mounted on the same shaft, through brushes and slip rings. The field of the exciter is supplied from a pilot exciter, again mounted on the same shaft, through automatic field regulator. b) Static excitation systems (SES), feeding rotor through brushes and slip rings directly from thyristor bridges energised from station bus bars.

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Exciter Contd: c)

• 1. 2. 3. 4. 5.

Brushless excitation systems, in which, the field winding of the main alternator is supplied directly, from the exciter, which is an inverted alternator, via rotating rectifier bridge, all mounted on the same shaft. This eliminates the need for separate slip rings and brush system. The exciter has a static field winding and fed from a permanent magnet alternator, called pilot exciter, again mounted on the same shaft, through a thyristor bridge. The output of the bridge is controlled by an Automatic Voltage Regulator (AVR). Main functions of excitation system are to provide variable dc current for controlling the terminal voltage with suitable accuracy, to ensure stable operation with network and / or other machines, to enhance transient stability subsequent to a fault, to communicate with the power plant control system and to keep the machine within permissible operating range.

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Voltage Regulator • The voltage regulator controls the current delivered by the excitation system so as to maintain the terminal voltage of the alternator within specified limits.

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Reactor: • Reactors are some-times connected in series with an alternator to limit the short circuit current for preventing damage to the equipments due to large short circuit currents. • The reactors should be designed to withstand strong mechanical forces and not to get saturated when carrying high fault currents. • It must have small resistance to minimize copper losses since it may have to carry continuously the normal load currents. • To minimize the size, some times iron core with air gap is utilized.

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Transformer: • The unit transformer steps up the generator voltage to transmission level ( 132 / 220 / 400 kV). For transmission at or above 400 kV, the phases are housed in individual tanks as in single phase transformer. There are other auxiliary transformers to supply power to the station auxiliaries.

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Circuit breaker and isolator: • For switching the alternator in or out of the system a circuit breaker (CB) with an isolator on its either side is employed. The CB is capable of making and breaking an electrical network during normal operation. It can switch off a circuit automatically in the event of a fault. It is a very costly piece of apparatus. Different types of breakers are in use, but in the present day EHV system, SF6 circuit breakers have become more popular. • The isolator operates under no-load condition only, i.e., it has to be closed before the associated CB is closed and opened only after the CB has opened the circuit. This requires proper interlocking to prevent wrong operations. Isolators physically show the status of the circuit, whether connected or disconnected, and is meant for isolating a circuit during the period of maintenance.

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Switch yard • Switch yard is the place adjacent to the power house where the transformers, circuit breakers, isolators, bus bars, CT & PTs, lightning arrestors, current limiting reactors and other equipments are installed. • At a thermal power plant, out door type switch yard is employed. • Some times, in hydel power plants, all these equipments may be of indoor type.

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• Over all efficiency of a steam station=  generator  thermal  boiler        efficiency  efficiency  efficiency 

• heat produced per hr = Coal consumption/hr x heating value of coal • unit generated per hr =Heat produced per hr x overall efficiency

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Steps Of Cold Starting of Turbines(Shafts at rest):1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18.

Cooling water pump is put in service. Lubricating oil system is put on. Lubricating oil purifier is started. Jacking oil pump is started. Turbine shaft is put on turning gear. Start auxiliary oil pump for power oil – check emergency stop valve in fully closed position. Check governor regulator hand-wheel at lowest position. Set emergency trip plunger. Check governor valve open. check operation of trip gear. Reset trip plunger and governor valve open. Charge up the main steam line and raise vacuum. Check steam temperature and pressure within recommended range. Open slowly emergency stop valve. Generator exciter circuit switched on. Check terminal voltage. If voltage and frequency are approximately same, switch on the synchroscope and synchronize. Adjust governor to regulate steam flow. MdM, EE, JU Start loading the generator in recommended rates.

Steps of Shutting Down of a Turbo –generator:1. Start unloading the set at the recommended rate. 2. When load falls to 20 % of rated value, ensure that condensate recirculating valve opens to provide adequate cooling water. 3. Check that steam drain valve open when bled steam temperature reaches 107 deg C. 4. Check that lubricating oil temperature controller opens up cooling water supply to lower oil temperature. 5. Shut off the steam supply to the air ejectors after closing the air suction valves. 6. When the speed of the unit falls to 400 rpm start jacking oil pump. 7. When the turbine shafts have stopped turning , check that the meshing motor and turning gear motor start automatically. 8. When the turbine shafts are on turning gear, shut gland steam supply. 9. Shut down the auxiliary oil pump. 10.Shut down the steam supply to auxiliary steam system. 11.Circulating water should be kept on the auxiliaries for at best one hour after the machine has been shut down. MdM, EE, JU

Recommended staring procedure of a Turbo- alternator:Type of start

Cold

Warm

Hot

After Trip

Estimated period of Shutdown in hours

> 120

36 -120

8-36

<8

Initial turbine blade temperature in deg C

< 100

100-300

300-400

> 400

Maximum back pr. before steam admission in Atm.

0.5

0.25

0.15

< 0.15

Initial steam pressure in Atm.

50

50

50

60-90

Initial steam temp. in deg C

275- 325

325-400

400- 500

> 500

Run to rated speed time in min

60

15

5

3.5

Initial load of on synchronizing in MW

3

5

10

> 10

Loading rate in MW/min

1/3

1

3

4-6

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Fuel handling:1. 2. 3. 4. 5. 6. 7. 8. 9.

Fuel delivery(truck , boat , rail) Unloading (by cranes , rotary car dumpers , wagon tripplers) Preparation – (i) crusher (ii) sizer (iii) dryers (iv) magnetic separator. Transfer – (i) belt conveyors (ii) bucket elevators . Outdoor (Dead)storage – this is an insurance against complete shut down of power plant which may arise from failure of normal coal delivery. Indoor (Live) storage – coal storage for a day , live storage can be provided with bunker and coal bins. In plant handling – belt conveyors /bucket conveyors to boiler furnace. Weighing and measuring Furnace firing.

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Methods of Fuel Firing:1. Solid fuel firing 2. Pulverized fuel firing. Again solid fuel firing is divided into (i) Hand firing (ii) stoker/mechanical firing.

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•

Hand Firing:- suitable for small plants. Adjustment of supply air is required, every time green coal is fed in furnace. • Stoker firing (mechanical firing):- large quantity of fuel can be uniformly burnt. (a) overfeed stoker :- coal is fed on the grate above the point of admission of air.

Air

Grate:when air moves through grate air gets heated up and grate is cooled

A layer of ash

Incandesce nt coke ProductsCO,CO2 and H2

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Green coke + secondary air is feed for complete combustion

Boiler CO2, N2,H2O, O2 and CO (if combustion is incomplete )

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•

Underfeed Stoker:- in this type of stoker fuel (coal ) is fed into the grate below the point of admission of primary air.

Primary air

Green coal + secondary air

Incandescent coke

Ash

boiler

Bituminous and semi-bituminous coal with small ash content and fusing temperature above 1300 deg C can be burnt very effectively in this burners. MdM, EE, JU

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Pulverized fuel firing :• Coal is pulverized to increase its surface exposure to fire & therefore to increase the combustion rate. • The pulverized coal is obtained by grinding raw coal in pulverizing mills. • The essential functions of these mill are: 1) dry of coal 2) grinding. 3) separation of particles. 4) forming proper air fuel ratio. The various types of pulverizing mills are: 1. Ball mill 2. Ball and rice mill. 3. Ball and hammer mill. • Coal gets pulverized due to the combined impact between coal and steel balls(within the drum of pulverizing mill) • For pulverizing firing systems and pulverized fuel burners follow any standard text book. MdM, EE, JU

Advantages of pulverized fuel firing:1. Surface area of fuel is increased in almost the ratio 400:1. therefore rapid combustion is possible. 2. Due to increase in surface area of fuel, small quantity of air is needed for complete combustion. 3. Low grade coal can be used since it is used in powdered form. 4. The rate of feed of fuel(coal) can be regulated properly resulting in fuel economy. 5. The rate of evaporation is increased. 6. Increase in boiler efficiency. 7. Firing of boiler is easy. 8. Fluctuation of load can be meet easily . 9. Ash removing troubles are reduced.

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Disadvantage of pulverized fuel firing:1. 2. 3. 4.

Extra cost for pulverizing mills. High combustion temperature cause high thermal losses in flue gases. There is a danger of explosion as coal is burnt like gas. Operating cost of pulverization plant is also high from energy utilization point of view (loss through flue gases) 5. The furnace temperature , un-burnt fuel and effect of ash etc deteriorate the refractory material of furnace. 6. Pulverizing coal firing produces fly ash (fine dust) which requires a separate fly ash removal equipment. Therefore it involves extra cost.

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Fluidised Bed Combustion • Fluidised Bed Combustion (FBC) is a very flexible method of electricity production – most combustible material can be burnt including coal, biomass and general waste. • FBC systems improve the environmental impact of coal-based electricity, reducing SOx and NOx emissions by 90%. • In fluidised bed combustion, coal is burned in a reactor comprised of a bed through which gas is fed to keep the fuel in a turbulent state. This improves combustion, heat transfer and recovery of waste products. The higher heat exchanger efficiencies and better mixing of FBC systems allows them to operate at lower temperatures than conventional pulverized coal combustion (PCC) systems. By elevating pressures within a bed, a highpressure gas stream can be used to drive a gas turbine, generating electricity.

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Fluidised Bed Combustion Contd… • FBC systems fit into two groups, non-pressurised systems (FBC) and pressurised systems (PFBC), and two subgroups, circulating or bubbling fluidised bed. • Non-pressurised FBC systems operate at atmospheric pressure and are the most widely applied type of FBC. They have efficiencies similar to PCC – 30-40%. • Pressurised FBC systems operate at elevated pressures and produce a high-pressure gas stream that can drive a gas turbine, creating a more efficient combined cycle system – over 40%. • Bubbling uses a low fluidising velocity – so that the particles are held mainly in a bed – and is generally used with small plants offering a nonpressurised efficiency of around 30% • Circulating uses a higher fluidising velocity – so the particles are constantly held in the flue gases – and are used for much larger plant offering efficiency of over 40% • The flexibility of FBC systems allows them to utilise abandoned coal waste that previously would not be used due to its poor quality. MdM, EE, JU

Ash Handling of thermal power plant:• Dust problem is maximum in pulverized fuel firing . • Dust is separated from flue gases before it exhausts through the chimney. • Dust is separated by electrostatic precipitator. In this dust of flue gases are electrically attracted to the metal tubes placed in the path of flue gases. • Ash handling comprises of following operation:1. Removal of ash from furnace by ash hoppers. 2. Transfer of this ash to a fill or storage. 3. Disposal of stored ash. Ash can be disposed off in following ways:1. Waste land site may be filled. 2. Building contractor may utilize it. Etc

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Calorific value of fuel • Amount of heat produced due to combustion of 1kg of solid or liquid or 1 cum of gaseous fuel. • Determined experimentally by calorimeter. • Knowing the amount of different constituents of the fuel and calorific value of each of their constituents. • The hydrogen in fuel combines with oxygen to form steam. In various combustion system this steam escapes into outside atmosphere. • Latent heat of the steam is lost with the products of combustion • The calorific value of fuel which includes the latent heat of any steam formed is known as higher calorific value. • higher calorific value – latent heat = lower calorific value. MdM, EE, JU

Air fuel ratio • The quantity of air necessary for burning 1khg of fuel is called chemically correct quantity(weight) of air. The ratio of this chemically correct quantity of air to the weight of fuel is known as theoretical air fuel ratio or stochiometic air fuel ratio. • In practice this air is not sufficient for rapid & complete combustion of fuel. For instance, in a boiler the actual quantity of air required is 50-100% more than the calculated quantity. • The ratio of actual amount of air required in a furnace to the theoretical required quantity is called excess air coefficient. MdM, EE, JU

Air fuel ratio… • It depends on the kind of fuel used and the furnace design. • The excess air coefficient ranges from 1.3 for gaseous fuel to 1.5 for grate fired solid fuel. • This portion of air does not take part in combustion process but only helps in complete combustion of fuel supplied. The excess air, in fact, carries away a part of heat produced during combustion.

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Air required for complete combustion of solid fuel • Combustion is high temp. oxidation of combustible elements in fuel with intensive release of heat. The combustible elements in coal are C, H & S.

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