Ee2252-Power Plant Engineering

PRCET/EEE/IV SEMESTER/EE2252-POW SEMESTER/EE2252-POWER ER PLANT ENGINEERING/NOTES ENGINEERING/NOTES

PONNAIYAH RAMAJAYAM COLLEGE OF ENGINEERING & TECHNOLOGY Thanjavur – 613403

DEPARTMENT OF EEE

II YEAR / IV SEMESTER

EE2252- POWER PLANT ENGINEERING

e-Content

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EE2252

POWER PLANT ENGINEERING

LTPC 3 1 0 4

AIM:

Expose the students to basics of various power plants so that they will have the comprehensive idea of power system operation. OBJECTIVE:

To become familiar with operation of various power plants. UNIT I - THERMAL POWER PLANTS

Basic thermodynamic cycles, various components of steam power plant-layout-pulverized coal burners- Fluidized bed combustion-coal handling systems-ash handling systems- Forced draft and induced draft fans- Boilers-feed pumps-super heater- regenerator-condenser- dearearatorscooling tower UNIT II - HYDRO ELECTRIC POWER PLANTS

Layout-dams-selection of water turbines-types-pumped storage hydel plants UNIT III - NUCLEAR POWER PLANTS PLANTS

Principles of nuclear energy- Fission reactions-nuclear reactor-nuclear power plants UNIT IV - GAS AND DIESEL POWER PLANTS

Types, open and closed cycle gas turbine, work output & thermal efficiency, methods to improve performance-reheating, intercoolings, regeneration-advantage and disadvantages- Diesel engine power plant-component and layout UNIT V NON-CONVENTIONAL POWER GENERATION

Solar energy collectors, OTEC, wind power plants, tidal power plants and geothermal resources, fuel cell, MHD power generation-principle, thermoelectric power generation, thermionic power generation L = 45 T = 15 TOTAL = 60 PERIODS TEXT BOOKS

1. A Course in Power Plant Engineering by Arora and Domkundwar, Dhanpat Rai and Co. Pvt. Ltd., New Delhi. 2. Power Plant Engineering by P.K. Nag, Tata McGraw Hill, Second Edition , Fourth reprint 2003. REFERENCES

1.Power station Engineering and Economy by Bernhardt G.A.Skrotzki and William A.VopatTata McGraw Hill Publishing Company Ltd., New Delhi, 20 th reprint 2002. 2. An introduction to power plant technology by G.D. Rai-Khanna Publishers, Delhi110 005. 3. Power Plant Technology, M.M. El-Wakil McGraw Hill 1984. 2


EE2252

POWER PLANT ENGINEERING

LTPC 3 1 0 4

AIM:

Expose the students to basics of various power plants so that they will have the comprehensive idea of power system operation. OBJECTIVE:

To become familiar with operation of various power plants. UNIT I - THERMAL POWER PLANTS

Basic thermodynamic cycles, various components of steam power plant-layout-pulverized coal burners- Fluidized bed combustion-coal handling systems-ash handling systems- Forced draft and induced draft fans- Boilers-feed pumps-super heater- regenerator-condenser- dearearatorscooling tower UNIT II - HYDRO ELECTRIC POWER PLANTS

Layout-dams-selection of water turbines-types-pumped storage hydel plants UNIT III - NUCLEAR POWER PLANTS PLANTS

Principles of nuclear energy- Fission reactions-nuclear reactor-nuclear power plants UNIT IV - GAS AND DIESEL POWER PLANTS

Types, open and closed cycle gas turbine, work output & thermal efficiency, methods to improve performance-reheating, intercoolings, regeneration-advantage and disadvantages- Diesel engine power plant-component and layout UNIT V NON-CONVENTIONAL POWER GENERATION

Solar energy collectors, OTEC, wind power plants, tidal power plants and geothermal resources, fuel cell, MHD power generation-principle, thermoelectric power generation, thermionic power generation L = 45 T = 15 TOTAL = 60 PERIODS TEXT BOOKS

1. A Course in Power Plant Engineering by Arora and Domkundwar, Dhanpat Rai and Co. Pvt. Ltd., New Delhi. 2. Power Plant Engineering by P.K. Nag, Tata McGraw Hill, Second Edition , Fourth reprint 2003. REFERENCES

1.Power station Engineering and Economy by Bernhardt G.A.Skrotzki and William A.VopatTata McGraw Hill Publishing Company Ltd., New Delhi, 20 th reprint 2002. 2. An introduction to power plant technology by G.D. Rai-Khanna Publishers, Delhi110 005. 3. Power Plant Technology, M.M. El-Wakil McGraw Hill 1984. 2


UNIT I - THERMAL POWER PLANTS INTRODUCTION : In this lesson a brief idea of a modern power system is outlined. As consumers, we use electricity for various purposes such as: 1. Lighting, heating, cooling and other domestic electrical appliances used in home.

2. Street lighting, flood lighting of sporting arena, office building lighting, powering PCs etc. 3. Irrigating vast agricultural lands using pumps and operating cold storages for various agricultural products. 4. Running motors, furnaces of various kinds, in industries. 5. Running locomotives (electric trains) of railways. BASIC IDEA OF POWER GENERATION: Prior to the discovery of Faraday‟s Laws of electromagnetic discussion, discussion, electrical power

was available from batteries with limited voltage and current levels. Although complicated in construction, D.C generators were developed first to generate power in bulk. However, due to limitation of the D.C machine to generate voltage beyond few hundred volts, it was not economical to transmit large amount of power over a long distance. For a given amount of power, the current magnitude ( I I = P/V ), ), hence section of the copper conductor will be large. Thus generation, transmission and distribution of d.c power were restricted to area of few kilometer radius with no interconnections between generating plants. Therefore, area specific generating stations along with its distribution networks had to be used. CHANGEOVER FROM D.C TO A.C: In later half of eighties, in nineteenth century, it was proposed to have a power system

with 3-phase, 50 Hz A.C generation, and transmission and distribution networks. Once a.c system was adopted, transmission of large power (MW) at higher transmission voltage becomes a reality by using transformers. Level of voltage could be changed virtually to any other desired level with transformers – which were hitherto impossible with D.C system. Nicola Tesla suggested that constructional simpler electrical motors (induction motors, without the complexit y of commutates segments of D.C motors) operating from 3-phase a.c supply could be manufactured. In fact, his arguments in favor of A.C supply system own the debate on switching over from D.C to A.C system. VARIOUS COMPONENTS OF STEAM POWER PLANT AND LAYOUT: We have seen in the previous section that to generate voltage at 50 Hz we have to run the

generator at some fixed rpm by some external agency. A turbine is used to rotate the generator. 3


Turbine may be of two types, namely steam turbine and water turbine. In a thermal power station coal is burnt to produce steam which in turn, drives the steam turbine hence the generator (turbo set). In figure 2.2 the elementary features of a thermal power plant is shown. In a thermal power plant coil is burnt to produce high temperature and high pressure steam in a boiler. The steam is passed through a steam turbine to produce rotational motion. The generator, mechanically coupled to the turbine, thus rotates producing electricity. Chemical energy stored in coal after a couple of transformations produces electrical energy at the generator terminals as depicted in the figure. Thus proximity of a generating station nearer to a coal reserve and water sources will be most economical as the cost of transporting coal gets reduced. In our country coal is available in abundance and naturally thermal power plants are most popular. However, these plants pollute the atmosphere because of burning of coals. Stringent conditions (such as use of more chimney heights along with the compulsory use of electrostatic precipitator) are put by regulatory authorities to see that the effects of pollution is minimized. A large amount of ash is produced every day in a thermal plant and effective handling of the ash adds to the running cost of the plant. Nonetheless 57% of the generation in out country is from thermal plants. The speed of alternator used in thermal plants is 3000 rpm which means 2-pole alternators are used in such plants. SUBSTATIONS: Substations are the places where the level of voltage undergoes change with the help of

transformers. Apart from transformers a substation will house switches (called circuit breakers), meters, relays for protection and other control equipment. Broadly speaking, a big substation will receive power through incoming lines at some voltage (say 400 kV) changes level of voltage (say to 132 kV) using a transformer and then directs it out wards through outgoing lines. Pictorially such a typical power system is shown in figure 2.6 in a short of block diagram. At the lowest voltage level of 400 V, generally 3-phase, 4-wire system is adopted for domestic connections. The fourth wire is called the neutral wire (N) which is taken out from the common point of the star connected secondary of the 6 kV/400 V distribution transformer. SOME IMPORTANT COMPONENTS / EQUIPMENTS IN SUBSTATION: As told earlier, the function of a substation is to receive power at some voltage through

incoming lines and transmit it at some other voltage through outgoing lines. So the most important equipment in a substation is transformer(s). However, for flexibility of operation and protection transformer and lines additional equipments are necessary 4

PRCET/EEE/IV SEMESTER/EE2252-POWER PLANT ENGINEERING/NOTES

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Suppose the transformer goes out of order and maintenance work is to be carried out. Naturally the transformer must be isolated from the incoming as well as from the outgoing lines by using special type of heavy duty (high voltage, high current) switches called circuit breakers. Thus a circuit breaker may be closed or opened manually (functionally somewhat similar to switching on or off a fan or a light whenever desired with the help of a ordinary switch in your house) in substation whenever desired. However unlike a ordinary switch, a circuit breaker must also operate (i.e., become opened ) automatically whenever a fault occurs or overloading takes place in a feeder or line. To achieve this, we must have a current sensing device called CT (current transformer) in each line. A CT simply steps down the large current to a proportional small secondary current. Primary of the CT is connected in series with the line. A 1000 A/5 A CT will step down the current by a factor of 200. So if primary current happens to be 800 A, secondary current of the CT will be 4 A. Suppose the rated current of the line is 1000 A, and due to any reason if current in the line exceeds this limit we want to operate the circuit breaker automatically for disconnection. The basic scheme is presented to achieve this. The secondary current of the CT is fed to the relay coil of an over current relay. Here we are not going into constructional and operational details of a over current relay but try to tell how it functions. Depending upon the strength of the current in the coil, an ultimately an electromagnetic torque acts on an aluminum disc restrained by a spring. Spring tension is so adjusted that for normal current, the disc does not move. However, if current exceeds the normal value, torque produced will overcome the spring tension to rotate the disc about a vertical spindle to which a long arm is attached. To the arm a copper strip is attached as shown figure 2.8. Thus the arm too will move whenever the disk moves The relay has a pair of normally opened (NO) contacts 1 & 2. Thus, there will exist open circuit between 1 & 2 with normal current in the power line. However, during fault condition in the line or overloading, the arm moves in the anticlockwise direction till it closes the terminals 1 & 2 with the help of the copper strip attached to the arm as explained pictorially in the figure 2.8. This short circuit between 1 & 2 completes a circuit comprising of a battery and the trip coil of the circuit breaker. The opening and closing of the main


contacts of the circuit breaker depends on whether its trip coil is energized or not. It is interesting to note that trip circuit supply is to be made independent of the A.C supply derived from the power system we want to protect. For this reason, we expect batteries along with battery charger to be present in a substation. Apart from above there will be other types of protective relays and various meters indicating current, voltage, power etc. To measure and indicate the high voltage (say 6 kV) of the line, the voltage is stepped down to a safe value (say 110V) by transformer called potential transformer (PT). Across the secondary of the PT, MI type indicating voltmeter is connected. For example a voltage rating of a PT could be 6000 V/110 V. Similarly, across the secondary we can connect a low range ammeter to indicate the line current.

DISTRIBUTION SYSTEM:

Till now we have learnt how power at somewhat high voltage (say 33 kV) is received in a substation situated near load center (a big city). The loads of a big city are primarily residential complexes, offices, schools, hotels, street lighting etc. These types of consumers are called LT (low tension) consumers. Apart from this there may be medium and small scale industries located in the outskirts of the city. LT consumers are to be supplied with single phase, 220 V, 40 Hz. We shall discuss here how this is achieved in the substation receiving power at 33 kV

Power receive at a 33 kV substation is first stepped down to 6 kV and with the help of underground cables (called feeder lines), power flow is directed to different directions of the city. At the last level, step down transformers are used to step down the voltage form 6 kV to 400 V. These transformers are called distribution transformers with 400 V, star


connected secondary. You must have noticed such transformers mounted on poles in cities beside the roads. These are called pole mounted substations. From the secondary of these transformers 4 terminals (R, Y, B and N) come out. N is called the neutral and taken out from the common point of star connected secondary. Voltage between any two phases (i.e., R-Y, Y-B and B-R) is 400 V and between any phase and neutral . Residential buildings are supplied with single phase 230V, 50Hz. So individual are to be supplied with any one of the phases and neutral. Supply authority tries to see that the loads remain evenly balanced among the phases as far as possible. Which means roughly one third of the consumers will be supplied from R-N, next one third from Y-N and the remaining one third from B-N. The distribution of power from the pole mounted substation can be done either by (1) overhead lines (bare conductors) or by (2) underground cables. Use of overhead lines although cheap, is often accident prone and also theft of power by hooking from the lines takes place. Although costly, in big cities and thickly populated areas underground cables for distribution of power, are used. Draught.

Draught is defined as the difference between absolute gas pressure at any point in a gas flow passage and the ambient (same elevation) atmospheric pressure.

Purpose of Draught.

To supply required amount of air to the furnace for the combustion of fuel. The amount of fuel can be burnt per square foot of grate depends upon the quantity of air circulated through fuel bed. To remove the gaseous products of combustion Classification of Draught

The following flow chart gives the classification of draughts


Artificial draught

If the draught is produced by steam jet or fan it is known as artificial draught. Induced draught

The flue is drawn (sucked) through the system by a fan or steam jet. Forced draught

The air is forced into the system by a blower or steam jet. Merits of Natural Draught

No external power is required for creating the draught Air pollution is prevented since the flue gases are discharged at a higher

 

level   

Maintenance cost is practically nil since there are no mechanical parts Its has longer life, Capital cost is less than that of an artificial draught.

De-merits of Natural Draught

Maximum pressure available for producing draught by the chimney is less,  Flue gases have to be discharged at higher temperature since draught increases with the increase in temperature of flue gases.  Heat cannot be extracted from the fluid gases for economizer, superheater, air preheater, etc. Since the effective draught will be reduced if the temperature of the flue gases is decreased Merits of steam Jet draught 

  

This system is very simple and cheap in cost, Low grade fuel can be used Space required is less

De-merits at steam jet draught

PRCET/EEE/IV SEMESTER/EE2252-POWER PLANT ENGINEERING/NOTES o o

It can be operated only when the steam is raised The draught produced is very low

Condenser

A condenser is a device in which the steam is condensed by cooling it with water. The condensed steam is known as condensate. Essential elements of a steam condensing plant o o o o

A closed vessel in which the steam is condensed. A pump to deliver condensed steam to the hot well from the condenser. A dry air-pump to remove air and other non-condensable gases, A feed pump to deliver water to the boiler from hot well.

Sub division of jet condensers   

Low level counter flow jet condenser High level (or) Barometric jet condenser Ejector condenser.

Surface condenser

Down flow condenser  Central flow condenser  Evaporative condenser Advantages of surface condenser 

The condensate can be used as boiler feed water  Cooling water of even poor quality can be used because the cooling water does not come in direct contact with steam  High vacuum (about 73.5 cm of Hg) can be obtained in the surface condenser. Disadvantages of surface condenser 

  

The capital cost is more, The maintenance cost and running cost of this condenser is high, It is bulky and requires more space.

Heat saving devices used in a thermal power plant

Air pre heater  Economizer Thermal power plant Layout of steam power plant: Introduction: Steam is an important medium for producing mechanical energy. Steam is used to drive steam engines and steam turbines. Steam has the following advantages. 1. Steam can be raised quickly from water which is available in plenty. 2. It does not react much with materials of the equipment used in power plants. 3. It is stable at temperatures required in the plant. 

Equipment of a Steam Power Plant:


1. 2. 3. 4. 5.

A steam power plant must have the following equipment. A furnace for burning the fuel. A steam generator or boiler for steam generation. A power unit like an engine or turbine to convert heat energy into mechanical energy. A generator to convert mechanical energy into electrical energy. Piping system to carry steam and water.

Figure: shows a schematic layout of a steam power plant. The working of a steam power plant can be explained in four circuits. 1. Fuel (coal) and ash circuit 2. Air and flue gas circuit 3. Feed water and steam flow circuit 4. Cooling water flow circuit

1. Coal and Ash circuit: This includes coal delivery, preparation, coal handling, boiler furnace, ash handling and ash storage. The coal from coal mines is delivered by ships, rail or by trucks to the power station. This coal is sized by crushers, breakers etc. The sized coal is then stored in coal storage (stock yard). From the stock yard, the coal is transferred to the boiler furnace by means of conveyors, elevators etc. The coal is burnt in the boiler furnace and ash is formed by burning of coal, Ash coming out of the furnace will be too hot, dusty and accompanied by some poisonous gases. The ash is transferred to ash storage. Usually, the ash is quenched to reduced temperature corrosion and dust content. There are different methods employed for the disposal of ash. They are hydraulic system, water jetting, ash sluice ways, pneumatic system etc. In large power plants hydraulic system is used. In this system, ash falls from furnace grate into high velocity water stream. It is then carried to the slumps. A line diagram of coal and ash circuit is shown separately in figure.


Figure: Layout of a steam power plant

2. Water and Steam circuit It consists of feed pump, economizer, boiler drum, super heater, turbine condenser etc. Feed water is pumped to the economizer from the hot well. This water is preheated by the flue gases in the economizer. This preheated water is then supplied to the boiler drum. Heat is transferred to the water by the burning of coal. Due to this, water is converted into steam.

Figure: Fuel (coal) and ash circuit


The steam raised in boiler is passed through a super heater. It is superheated by the flue gases. The superheated steam is then expanded in a turbine to do work. The turbine drives a generator to produce electric power. The expanded (exhaust) steam is then passed through the condenser. In the condenser, the steam is condensed into water and recirculated. A line diagram of water and steam circuit is shown separately in figure.

Figure: Water and Steam circuit

3. Air and Flue gas circuit It consists of forced draught fan, air pre heater, boiler furnace, super heater, economizer, dust collector, induced draught fan, chimney etc. Air is taken from the atmosphere by the action of a forced draught fan. It is passed through an air pre-heater. The air is pre-heated by the flue gases in the pre-heater. This pre-heated air is supplied to the furnace to aid the combustion of fuel. Due to combustion of fuel, hot gases (flue gases) are formed.


Figure: Air and flue gas circuit

The flue gases from the furnace pass over boiler tubes and super heater tubes. (In boiler, wet steam is generated and in super heater the wet steam is superheated by the flue gases.) Then the flue gases pass through economizer to heat the feed water. After that, it passes through the air pre-heater to pre-heat the incoming air. It is then passed through a dust catching device (dust collector). Finally, it is exhausted to the atmosphere through chimney. A line diagram of air and flue gas circuit is shown separately in figure. 4. Cooling water circuit: The circuit includes a pump, condenser, cooling tower etc. the exhaust steam from the turbine is condensed in condenser. In the condenser, cold water is circulated to condense the steam into water. The steam is condensed by losing its latent heat to the circulating cold water.

Figure: Cooling water current

Thus the circulating water is heated. This hot water is then taken to a cooling tower, In cooling tower, the water is sprayed in the form of droplets through nozzles. The atmospheric air enters the cooling tower from the openings provided at the bottom of the tower. This air removes heat from water. Cooled water is collected in a pond (known as cooling pond). This cold water is again circulated through the pump, condenser and cooling tower. Thus the cycle is repeated again and again. Some amount of water may be lost during the circulation due to vaporization etc. Hence, make up water is added to the pond by means of a pump. This water is obtained from a river or lake. A line diagram of cooling water circuit is shown in figure separately. Merits (Advantages) of a Thermal Power Plant


1. The unit capacity of a thermal power plant is more. The cost of unit 2. 3. 4. 5. 6.

decreases with the increase in unit capacity. Life of the plant is more (25-30 years) as compared to diesel plant (2-5 years). Repair and maintenance cost is low when compared with diesel plant. Initial cost of the plant is less than nuclear plants. Suitable for varying load conditions. No harmful radioactive wastes are produced as in the case of nuclear plant.

7. Unskilled operators can operate the plant. 8. The power generation does not depend on water storage. 9. There are no transmission losses since they are located near load centres. Demerits of thermal power plants

1. 2. 3. 4. 5. 6. 7. 8. 9.

Thermal plant are less efficient than diesel plants Starting up the plant and bringing into service takes more time. Cooling water required is more. Space required is more Storage required for the fuel is more Ash handling is a big problem. Not economical in areas which are remote from coal fields Fuel transportation, handling and storage charges are more Number of persons for operating the plant is more than that of nuclear plants. 10. For large units, the capital cost is more. Initial expenditure on structural materials, piping, storage mechanisms is more The type of Basic Boilers thermodynamic cycles and process of the Rankine cycle BOILER CYCLES In general, two important area of application for thermodynamics are: 1. Power generation 2. Refregeration Both are accomplished by systems that operate in thermodynamic cycles such as: a. Power cycles: Systems used to produce net power output and are often called engines. b. Refrigeration cycles: Systems used to produce refregeration effects are called refregerators (or) heat pumps. Cycles can further be categorized as (depending on the phase of the working fluid) 1. Gas Power cycles In this cycle working fluid remains in the gaseous phase throughout the entire cycles. 2. Vapour power cycles In this case, the working fluid exists in the vapour phase during one part of the cycle and in the liquid phase during another part. Vapour power cycles can be categorized as


a. b. c. d. e.

Carnot cycle Rankine cycle Reheat cycle Regenerative cycle Binary vapour cycle

Steam cycles (Ranking cycle) The Rankine cycle is a thermodynamic cycle. Like other thermodynamic cycle, the maximum efficiency of the Ranking cycle is given by calculating the maximum efficiency of the carnot cycle.

Process of the Rankine Cycle

Figure: Schematic representation and T-S diagram of Rankine cycle.

There are four processes in the Rankine cycle, each changing the state of the working fluid. These states are identified by number in the diagram above. Process 3-4: First, the working fluid (water) is enter the pump at state 3 at saturated liquid and it is pumped (ideally isentropically) from low pressure to high


(operating) pressure of boiler by a pump to the state 4. During this isentropic compression water temperature is slightly increased. Pumping requires a power input (for example, mechanical or electrical). The conservation of energy relation for pump is given as Wpump = m (h4 - h3) Process 4-1: The high pressure compressed liquid enters a boiler at state 4 where it is heated at constant pressure by an external heat source to become a saturated vapour at statel‟ which in turn superheated to state 1 through super heater. Common heat source for power plant systems are coal (or other chemical energy), natural gas, or nuclear power. The conservation of energy relation for boiler is given as Qin =m (h1 - h4) Process 1 – 2: The superheated vapour enter the turbine at state 1 and expands through a turbine to generate power output. Ideally, this expansion is isentropic. This decreases the temperature and pressure of the vapour at state 2. The conservation of energy relation for turbine is given as Wturbine = m (h1 – h2) Process 2 – 3: The vapour then enters a condenser at state 2. At this state, steam is a saturated liquid- vapour mixture where it is cooled to become a saturated liquid at state 3. This liquid then re- enters the pump and the cycle is repeated. The conservation of energy relation for condenser is given as Qout = m (h2 – h3) The exposed Rankine cycle can also prevent vapour overheating, which reduces the amount of liquid condensed after the expansion in the turbine

Regenerative Ranking Cycle

The regenerative Ranking cycle is so named because after emerging from the condenser (possibly as a sub cooled liquid) the working fluid heated by steam tapped from the hot portion of the cycle and fed in to Open Feed Water Heater(OFWH). This increases the average temperature of heat addition which in turn increases the thermodynamics efficiency of the cycle.


Principles of Fluidized Bed Combustion Operation:

A fluidized bed is composed of fuel (coal, coke, biomass, etc.,) and bed material (ash, sand, and/or sorbent) contained within an atmospheric or pressurized vessel. The bed fluidized when air or other gas flows upward at a velocity sufficient to expand the bed. The process is illustrated in figure. At low fluidizing velocities (0.9 to 3 m/s). relatively high solids densities are maintained in the bed and only a small fraction of the solids are entrained from the bed. A fluidized bed that is operated in this velocity range is refered to as a bubbling fluidized bed (BFB). A schematic of a typical BFB combustor is illustrated in figure.

Figure: Basic fluid bed Systems


Figure: Atmospheric bubbling bed combustor:

As the fluidizing velocity is increased, smaller particles are entrained in the gas stream and transported out of the bed. The bed surface, well-defined for a BFB combustor becomes more diffuse and solids densities are reduced in the bed. A fluidized bed that is operated at velocities in the range of 4 to 7 m/s is referred to as a circulated fluidized bed, or CFB. A schematic of a typical CFB combustor is illustrated in figure

Figure: Circulating bed combustor. Advantages of fluidized bed combustion The advantages of FBC in comparison to conventional pulverized coal-fueled units can be summarized as follows: 1. SO2 can be removed in the combustion process by adding limestone to the fluidized bed, eliminating the need for an external desulfurization process. 2. Fluidized bed boilers are inherently fuel flexible and, with proper design provision, can burn a variety of fuels. 3. Combustion FBC units takes place at temperatures below the ash fusion


temperature of most fuels. Consequently, tendencies for slagging and fouling are reduced with FBC. 4. Because of the reduced combustion temperature, NOx emissions are inherently low. Fuel Handling System Coal delivery equipment is one of the major components of plant cost. The various steps involved in coal handling are as follows: 1. Coal delivery. 2. Unloading

3. 4. 5. 6. 7. 8. 9.

Preparation Transfer Outdoor storage Covered storage Inplant handling Weighing and measuring Feeding the coal into furnace.

Figure: Steps involved in fuel handling system

i)

Coal delivery The coal from supply points is delivered by ships or boats to power stations situated near to sea or river whereas coal is supplied by rail or trucks to the power stations which are situated away from sea or river. The transportation of


coal by trucks is used if the railway facilities are not available. ii) Unloading The type of equipment to be used for unloading the coal received at the power station depends on how coal is received at the power station. If coal delivered by trucks, there is no need of unloading device as the trucks may dump the coal to the outdoor storage. Coal is easily handled if the lift trucks with scoop are used. In case the coal is brought by railways wagons, ships or boats, the unloading may be done by car shakes, rotary car dumpers, cranes, grab buckets and coal accelerators. Rotary car dumpers although costly are quite efficient for unloading closed wagons. iii) Preparation When the coal delivered is in the form of big lumps and it is not of proper size, the preparation (sizing) of coal can be achieved by crushers, breakers, sizers, driers and magnetic separators. iv) Transfer After preparation coal is transferred to the dead storage by means of the following systems. a. Belt conveyors b. Screw conveyors

c. Bucket elevato d. Grab bucket elevators e. Skip hoists f. Flight conveyor Belt Conveyor

Figure: Belt Conveyor

Figure shows a belt conveyor. It consists of an endless belt moving over a pair of end drums (rollers). At some distance a supporting roller is provided at the centre. The belt is made up of rubber or canvas. Belt conveyor is


suitable for the transfer of coal over long distances. It is used in medium and large power plants. The initial cost of system is not high and power consumption is also low. The inclination at which coal can be successfully elevated by belt conveyor is about 20. Average speed preferred than other types. Advantages of belt conveyor

1. 2. 3. 4.

Its operation is smooth and clean It requires less power as compared to other types of systems Large quantities of coal can be discharged quickly and continuously. Material can be transported on moderate inclines. 2. Screw Conveyor It consists of an endless helicoid screw fitted to a shaft (figure). The screw while rotating in a trough transfers the coal from feeding end to the discharge end.

Figure: Screw conveyor

This system is suitable, where coal is to be transferred over shorter distance and space limitations exist. The initial cost of the consumption is high and there is considerable wear o screw. Rotation of screw varies between 75-125 r.p.m

3. Bucket elevator It consists of buckets fixed to a chain (figure). The chain moves over two wheels. The coal is carried by the bucket from bottom and discharged at the top.


Figure: Bucket elevator

4. Grab bucket elevator It lifts and transfers coal on a single rail or track from one point to the other. The coal lifted by grab buckets is transferred to overhead bunker or storage. This system requires less power for operation and requires minimum maintenance. The grab bucket conveyor can be used with crane or tower as shown in figure . Although the initial cost of this system is high but operating cost is less.

Storage of Coal It is desirable that sufficient quantity of coal should be stored. Storage of coal gives protection against the interruption of coal supplies when there is delay in transportation of coal or due to strike in coal mines. Also when the prices are low, the coal can be purchased and stored for future use. The amount of coal to be stored depends on the availability of space for storage, transportation facilities, the amount of coal that will whether away and nearness to coal mines of the power station. Usually coal required for one month operation of power plant is stored in case of power stations are situated at longer distance from the collieries whereas coal need for about 15 days is stored in case of power station situated near to collieries. Storage of coal for longer periods is not advantageous because it blocks the capital and results in deterioration of the quality of coal. pulverized coal storage in Bunker


Periodically a power plant may encounter the situation where coal must be stored for sometimes in a bunker, for instance during a plant shut down. The bunker, fires can occur in dormant pulverized coal from spontaneous heating within 6 day of loading. This time can be extended to 13 days when a blanket of CO2 is piped into the top of the bunker. The perfect sealing of the bunker from air leakage can extend the storage time as two months or more. The coal in the bunker can be stored as long as six months by expelling air from above the coal with the use of CO2 and then blanketing of all sources of air. A control system used for storing the pulverized fuel in bunker is shown in figure.

Figure : Control system used for storing the pulverized coal with the use of CO. Pulverized Fuel Handling System:

Two methods are in general use to feed the pulverized fuel to the combustion chamber of the power plant. First is „Unit System‟ and second is „Central or Bin System. In unit system, each burner of the plant is fired by one or more pulverizers connected to the burners, while in the central system, the fuel is pulverized in the central plant and then disturbed to each furnace with the help of high pressure air current. Each type of fuel handling system consists of crushers, magnetic separators, driers, pulverizing mills, storage bins, conveyors and feeders.


Figure: Pulverized coal handling plant showing all required equipment for unit and central system The arrangement of different equipment required in both systems is shown in figure. With the help of a block diagram. The coal received by the plant from the mine may vary widely in sizes. It


is necessary to make the coal of uniform size before passing the pulverizer for efficient grinding. The coal received from the mine is passed through a preliminary crusher to reduce the size to allowable limit (30 mm). The crushed coal is further passed over magnetic separator which removes pyrites and tramp iron. The further equipment through which coal is passed before passing to pulverizer are already shown in figure Ball Mill pulverizing A line diagram of ball mill using two classifiers is shown in figure. It consists of a slowly rotating drum which is partly filled with steel balls. Raw coal from feeders is supplied to the classifiers from where it moves to the drum by means of a screw conveyor. As the drum rotates the coal get pulverized due to the combine impact between coal and steel balls. Hot air is introduced into the drum. The powdered coal is picked up by the air and the coal air mixture enters the classifiers, where sharp changes in the direction of the mixture throw out the oversized coal particles. The over-sized particles are returned to the drum. The coal air mixture from the classifier moves to the exhauster fan and then it is supplied to the burners

Figure: Ball mill

Ball and Race Mills pulverizing Figure: shows a ball and race mill


In this mill the coal passes between the rotating elements again and again until it has been pulverized to desired degree of fineness. The coal is crushed between two moving surfaces, namely, balls and races. The upper stationary race and lower rotating race driven by a worm and gear hold the balls between them. The raw coal supplied falls on the inner side of the races. The moving balls and races catch coal between them to crush it to a powder. The necessary force needed for crushing is applied with the help of springs. The hot air supplied picks up the coal dust as it flows between the balls and races and then enters the classifier. Where oversized coal particles are returned for further grinding. Where as the coal particles of required size are discharged from the top of classifier.

Advantages: i) Lower capital cost ii) Lower power consumption iii) Less space required. iv) Less weight Ash handling system Boilers burning pulverized coal (PC) have bottom furnaces. The large ash particles are collected under the furnace in a water-filled ash hopper, Fly ash is collected in dust collectors with either an electrostatic precipitator or a baghouse. A PC boiler generates approximately 80% fly ash and 20% bottom ash. Ash must be collected and transported from various points of the plants as shown in figure. Pyrites, which are the rejects from the pulverizers, are disposed of with the bottom ash system. Three major factors should be considered for ash disposal systems.


1. Plant site 2. Fuel source 3. Environmental regulation Needs for water and land are important considerations for many ash handling systems. Ash quantities to be disposed of depend on the king of fuel source. Ash storage and disposal sites are guided by environmental regulations

Hydraulic System

In this system, ash from the furnace grate falls into a system of water possessing high velocity and is carried to the sumps. It is generally used in large power plants. Hydraulic system is of two types, namely, low pressure hydraulic system used for intermittent ash disposal figure. Figure shows hydraulic system.

In this method water at sufficient pressure is used to take away the ash to sump. Where water and ash are separated. The ash is then transferred to the dump site in wagons, rail cars to

trucks. The loading of ash may be through a belt conveyor, grab buckets. If there is an ash basement with ash hopper the ash can fall, directly in ash car or conveying system Water-Jetting System


Water jetting of ash is shown in figure. In this method a low pressure jet of water coming out of quenching nozzle is used cool the ash. The ash falls into trough and is then removed Pneumatic System

In this system ash from the boiler furnace outlet falls into a crusher where a lager ash particles are crushed to small sizes. The ash is then carried by a high velocity air or steam to the point of delivery. Air leaving the ash separator is passed through filter to remove dust etc. So that the exhauster handles clean air which will protect the blades of the exhauster.

Classification of boilers:

The steam boilers are classified according to the following conditions 1. According to the relative position of water and hot gases a. Fire tube boiler [Cochran Boiler] b. Water tube boiler [ Babcock – Wilcox Boiler] 2. According to the axis of shell a. Vertical boiler [Cochran Boiler] b. Horizontal boiler [Lancashire Boiler] 3. According to the position of boiler a. Internally fired boiler [ all fire tube boilers] [Cochran Boilers] b. Externally fired boiler [ all water tube boilers] [Babcock and Wilcox


Boilers] 4. According to the pressure developed

a. Low pressure boiler [Pressure less than 80 bar] b. High pressure boiler [Pressure greater than 80 bar] 5. According to the method of water circulation a. Natural circulation [all low pressure boilers] [ Cochran Boiler] b. Forced circulation [all high pressure boilers] [ LaMont Boiler] 6. According to the use of the boiler a. Stationary boiler [Cochran Boiler] b. Mobile boiler [Locomotive Boiler] 7. According to the number of drums a. Single Drum b. Multi Drum 8. According to the nature of draught a. Natural Draught b. Forced Draught Difference between water tube and fire tube boilers S.No

1. 2. 3.

Fire tube boiler

Hot flue gases flow inside the tubes

Evaporation takes place slowly. Failure in water supply may not overheat the boiler. 4. Removal of impurities is difficult. 5. The failure of fire tube (explosion), causes a very serious problem. 6. Here the furnace is fitted inside the water space. 7. Inspection is not easy. 8. Efficiency is less. 9. Construction and design is rigid, compact and simple. Cochran Boiler:

Water tube boiler

Water flows inside the tubes. Evaporation takes place quickly. Failure in water supply may overheat the boiler. Removal of impurities is easy. The failure of water tube does not cause any serious problem. Here the furnace is fitted outside the water tube. Inspection is easy. Efficiency is high. Construction and design is complex.

The Cochran boiler is one of the most popular type of vertical, multi tubular, fire tube boilers. Figure shows the Cochran boiler which is made in sizes up to 2.75 metre diameter and metre height. It has an evaporative capacity of 3640 kg of steam per hour when burning 568 kg of coal per hour, for working pressure of 20 bar.


Locomotive boiler:

The locomotive boiler is a horizontal fire tube boiler having an internal firebox.


It consists of a cylindrical shell having a rectangular firebox at one end a smoke box at the other end. The firebox forms the chamber within which the fuel is burnt on the grate which is supported in the firebox at the bottom. The firebox is connected to the smoke box by a number of horizontal smoke tubes. The hot gases from the furnace pass through these tubes into the smoke box and are then discharged from the furnace pass through these tubes into the smoke box and are then discharged from the short chimney. The grate of the boiler is inclinded. A steam dome is placed on the top of the shell and in front of the firebox. A stop valve called the regulator is placed in the steam dome. The steam is taken from the elevated dome to the engine cylinder so that it contains as few water particles as possible. The steam pipe from the regulator leads to a superheater placed in the smoke box and from the superheater, the steam is sent to the cylinder by pipes passing out from the smoke box to the cylinder. The necessary draught is obtained by the steam exhausted from the engine cylinder which is discharged through the blast pipe placed in the smoke box to the chimney. A movable cap is attached to the mouth of blast orifice. A steam blower is also provided for use when the steam supply to the engine is shut – off.

Advantages of locomotive boilers:

a. b. c. d.

Compactness High steaming capacity Portability Fair economy

Disadvantages of locomotive boilers:

a. Large flat surface requires sufficient supporting. b. Corrosion in the water legs. c. It is difficult to clean inside. Lancashire boiler:

It is a stationary, fire tube, internally fired boiler. The size is approximately from 7 – 9 metres in length and 2 – 3 metres in diameter. Description:

It consists of 1. Cylindrical shell 2. Furnace tubes bottom flue and side flues 3. Grate 4. Fire bridge 5. Dampers Cylindrical shell:

It is placed in horizontal position over a brick work. It is partly filled up


with water. The water level inside the shell is well above the furnace tubes. Furnace tubes, bottom flue and Side flues:

Two large internal furnace tubes (flue tubes) extend from one end to the other end of the shell. The flues are built up of ordinary bricks lined with fire bricks. One bottom flue and two side flues are formed by the brick setting, as shown in the figure. Grate:

The grate is provided at the front end of the main flue tubes. Coal is fed to the grate through the fire hole. Fire bridge:

A brickwork fire bridge is provided at the end of the grate to prevent the flow of coal and ash particles into the interior of the furnace (flue) tubes. Otherwise the coal and ash particles carried with gases form deposits on the interior of the tubes and prevents the heat transfer to the water. Dampers:

Dampers in the form of sliding doors are placed at the end of the side flues to control the flow of gases from side flues to the chimney flue. Working:

Coal is fed to the grate through the fire hole and is burnt. The hot gases leaving the grate move along the furnace (flue) tubes up to the back end of the shell and then in the downward direction to the bottom flue. The bottom of the shell is thus first heated. The hot gases, passing through the bottom flue, travel up to the front end of the boiler, where they divide into two streams and pass to the side flues. This makes the two sides of the B o i l e r shell to become heated. Passing along the two side flues, the hot gases travel up to the backend of the boiler to the chimney flue. They are then discharged into the atmosphere through the chimney. With the help of this arrangement of flow passages of the hot gases, the bottom of the shell is first heated and then its sides. The heat is transferred to water through the surfaces of the two flue tubes (which remain in water) and the bottom and sides of the shell. This arrangement of flues increases the heating surface of the b oiler to a large extent. Dampers control the flow of hot gases and regulated the combustion rate as well as stream generation rate. The boiler is fitted with necessary mountings. Pressure gauge and water level indicator are provided at the front. Safety valve, steam stop valve, low water and high steam safetly valve and man – hole are provided on the top of the shell. High steam low water safety valve:

It is a combination of two valves. One is lever safety valve, which blows –


off steam when the working pressure of steam exceeds. The second valve operates by blowing – off the steam when the water level falls below the normal level.

Blow – off cock:

It is situated beneath the front portion of the shell for the removal of mud and sediments. It is also used to empty the water in the boiler during inspection. Fusible plug:

It is provided on the top of the main flues just above the grate. It prevents the overheating of the boiler tubes by extinguishing the fire when the water level falls below a particular level. A low water level alarm is mounted in the boiler to give a warning when the water level falls below the present value. Salient features:

1. The arrangement of flues in this boiler increase the heating surface of the shell to a large extent. 2. It is suitable where a large reserve of steam and hot water is needed. 3. Its maintenance is easy. 4. Superheater can be easily incorporated into the system at the end of the main flue tubes. Thus overall efficiency of the boiler can be increased.

Water tube Boilers: Babcock and Wilcox boiler:

It is a water tube boiler used in steam power plants. In this, water is


circulated inside the tubes and hot gases flow over the tubes. Description:

The Babcock and Wilcox boiler consists of 1. Steam and water drum (Boiler shell) 2. Water tubes 3. Uptake – header and down – comer 4. Grate 5. Furnace 6. Baffles 7. Superheater 8. Mud box 9. Inspection doors 10. Damper

1. Steam and Water drum (Boiler Shell) One half of the drum which is horizontal is filled up with water and steam remains on the other half. It is about 8 metres in length and 2 metres in diameter. 2. Water tubes Water tubes are placed between the drum and the furnace in an inclined position (at an angle of 100 to 150) to promote water circulation. These tubes are connected to the uptake – header and the down – comer as shown. 3. Uptake – Header and Down – comer (or Down take – Header) The drum is connected at one end to the uptake – header by short tubes and at the other end to the down – comer by long tubes.


Figure: Babcock and Wilcox boiler Grate:

Coal is fed to the grate through the fire door. Furnace:

Furnace is kept below the uptake – header. Baffles:

The fire – brick baffles, two in number, are provided to deflect the hot flue gases. Superheater:

The boiler is fitted with a superheater tube which is placed just under the drum and above the water tubes. Mud box:

Mud box is provided at the bottom end of the down – comer. The mud or sediments in the water are collected in the mud box and it is blown – off time by means of a blow – off cock. Inspection doors:

Inspection doors are provided for cleaning and inspection of the boiler. Working principle:

Coal is fed to grate through the fire door and is burnt. Flow of flue gases:

The hot flue gases rise up ward and pass across the left – side portion of the water tubes. The baffles deflect the flue gases and hence the flue gases travel in a zig – zag manner (i.e., the hot gases are deflected by the baffles to move in the upward direction, then downward and again in the upward direction) over the water tubes and along the superheater. The flue gases finally escape to the atmosphere through the chimneA continuous circulation of water from the drum to the water tubes and water tubes to the drum is thus maintained. The circulation of water is maintained by convective currents and is known as “natural circulation Economizer: Function: An economizer pre – heats (raise the temperature) the feed water by the exhaust flue gases. This pre – heated water is supplied to the boiler from the economizer Location:

An economizer is placed in the path of the flue gases in between the boiler and the air pre – heater or chimney


Construction:

An economizer used in modern high pressure boilers is shown by a line sketch. It consists of a series of vertical tubes. These tubes are hydraulically pressed into the top and bottom headers. The bottom header is connected to feed pump. Top header is connected to the water space of the boiler. It is provided with a safety valve which opens when water pressure exceeds a certain limit. To keep the surface of the tubes clean from soot and ash deposits, scrapers are provided in the tubes. These scrapers are slowly moved up and down to clean the surfaces of the tubes. The action of adjacent pairs of scraper is in opposite direction. i.e., when one scraper moves up, the other moves down. Economizers may be parallel or counter-flow types. When the gas flow and water

flow are in the same direction, it is called parallel flow economizer. In counter-flow, the gas flow and water flow are in opposite direction.

Fig. Economizer Working

The feed water is pumped to the bottom header and this water is carried to the top header through a number of vertical tubes. Hot flue gases are allowed to pass over the external surface of the tubes. The feed water which flows upward in the tubes is thus heated by the flue gases. This pre-heated water is supplied to the boiler. Advantages 1. Feed water to the boiler is supplied at high temperature. Hence heat required in the boiler is less. Thus fuel consumption is less. 2. Thermal efficiency of the plant is increased. 3. Life of boiler is increased. 4. Loss of heat in flue gases is reduced. Steaming capacity is increased Air pre-heater Function

Air pre-heater pre-heats (increases the temperature) the air supply to the


furnace with the help of hot the gases. Location

It is installed between the economizer and the chimney. Construction

A tubular type air pre-heater is shown in figure. It consists of a large number of tubes. Flue gases pass through the tube. Air flows over the tubes. Baffles are provided to pass the air number of times over the tubes. A soot hopper is provided at the bottom to collect the soot

Figure: Air pre-heater Working

Hot flue gases pass through the tubes of air pre-heater after leaving the boiler or economizer. Atmospheric air is allowed to pass over these tubes. Air and flue gases flow in opposite directions. Baffles are provided in the air preheater and the air passes number of times over the tubes. Heat is absorbed by the air from the flue gases. This pre-heater air is supplied to the furnace to air combustion Advantages

1. Boiler efficiency is increased.


2. Evaporative rate is increased. 3. Combustion is accelerated with less soot, smoke and ash. 4. Low grade and inferior quality fuels can be used. Superheater Function It superheats the steam generated by the boiler and increases the temperature steam above saturation temperature at constant pressure. Location

Superheaters are placed in the path of flue gases to recover some of their heat. In bigger installations, the superheaters are placed in an independently fired furnace. Such superheaters are called separately fired or portable superheaters.

There are many types of superheaters. A combination type of radiant and convective superheater is shown in figure. Both these superheaters are arranged in series in the path of flue gases. Radiant superheater receives heat from the burning fuel by radiation process. Convective superheater is placed adjacent to the furnace wall in the path of flue gases. It receives heat by convection. Working

Steam stop valve is opened. The steam (wet or dry) from the evaporator drum is passed through the superheater tubes. First the steam is passed through the radiant superheater and then to the convective superheater. The steam is heated when it passes through these superheaters and converted into superheated steam. This superheated steam is supplied to the turbine through a valve. Applications

This type of superheaters are used in modern high pressure boilers.


Advantages of superheated steam (super heaters)

1. Work output is increased for the same quantity of steam. 2. Loss due to condensation of steam in the steam engine and is the steam mains is minimized. 3. Capacity of the plant is increased. 4 . Thermal efficiency is increased since the temperature of superheated steam is high Injector Function An injector lifts and forces water into a boiler which is operating under pressure. Construction

In consists of a converging nozzle, mixing chamber, divergent tube, steam valve and a non- return valve. A steam injector is shown in figure. Working

The steam passes through the converging nozzle through a valve. Steam expands through the nozzle. The pressure drops and consequently velocity of steam increases. This steam mixes with water in the mixing chamber. In the mixing chamber steam condenses and vacuum is created. Due to this vacuum, more water is sucked into the mixing chamber. The jet water enters divergent tube. In the divergent tube kinetic energy of water is converted into pressure energy. Due to this increased pressure, feed water is forced into the boiler through feed check valve.

Figure: Steam injector Application

They are commonly used in vertical and locomotive boilers. 3. Feed pump Function: It delivers feed water into the boiler drum

Location

It is placed in between boiler and water supply source (hot well).


Construction

The feed pumps used may be of reciprocating type or rotary type (centrifugal pump). The reciprocating pump may use plunger or piston. It is driven by a steam engine or electric motor. The piston rod of the steam engine is connected directly with the piston rod of the pump (figure). Working

When the piston moves to the right, vacuum is created in the left side of the piston. The water from the hot well is forced into the cylinder through the left side suction valve. When the piston returns (moves to the left), vacuum is created in the right side of the piston. The liquid from the well is sucked into the cylinder through the right side suction valve. At the same time, the liquid in the left side of the piston is forced out through the left side delivery valve into the delivery pipe. The operations are repeated. During each stroke, suction takes place on one side of the water is delivered continuously in the boiler.

Figure: Feed pump (reciprocating type).

4.

Steam Sepeartors It separates water particles from steam before it is supplied to a steam engine or turbine. Thus it prevents the damaging of turbine blades due to moisture present in steam Location: It is located in the supply line near the turbine or engine

Construction

There are different types of steam separators. A separator with baffle plates is shown in figure. It consists of a cylindrical vessel. The vessel is fitted with baffle plates. A water gauge is fitted to indicate the water collected in the separator to drain away to separated water. Working

The steam is allowed into the separator. The steam strikes the baffle plates and the direction of the flow is changed. As a result, heavier water particles in steam falls down to the bottom of the separator. The separated steam is free from water particles. It is passed to the turbine or engine through the outlet pipe.


5.Steam Seperator Steam trap In any steam system, water may be formed due to partial condensation of steam in the piping system. This may cause water hammer and reduction in efficiency. A steam trap removes the condensed water, without allowing the steam to escape out.


UNIT II HYDRO POWER PLANT LAYOUT – DAM: In a hydel power station, water head is used to drive water turbine coupled to the

generator. Water head may be available in hilly region naturally in the form of water reservoir (lakes etc.) at the hill tops. The potential energy of water can be used to drive the turbo generator set installed at the base of the hills through piping called pen stock . Water head may also be created artificially by constructing dams on a suitable river. In contrast to a thermal plant, hydel power plants are eco-friendly, neat and clean as no fuel is to be burnt to produce electricity. While running cost of such plants are low, the initial installation cost is rather high compared to a thermal plants due to massive civil construction necessary. Also sites to be selected for such plants dep end upon natural availability of water reservoirs at hill tops or availability of suitable rivers for constructing dams. Water turbines generally operate at low rpm, so number of poles of the alternator is high. For example a 20-pole alternator the rpm of the turbine is only 300 rpm. Up stream ater level Water head H

3-phase A.C

Dam Electric power Water Generator Turbine Discharge of water in down stream Potential energy Kinetic Electrical of water energy

energy


Basic components of a hydel generating unit. Dam A dam is a barrier to confine or raise water for storage or diversion to create a hydraulic head WATER TURBINE SELECTION:

A turbine converts energy in the form of falling water into rotating shaft power. The selection of the best turbine for any particular hydro site depends on the site characteristics, the dominant ones being the head and flow available. Selection also depends on the desired running speed of the generator or other device loading the turbine. Other considerations such as whether the turbine is expected to produce power under part-flow conditions, also play an important role in the selection. All turbines have a power-speed characteristic. They will tend to run most efficiently at a particular speed, head and flow combination. A turbine design speed is largely determined by the head under which it operates. Turbines can be classified as high head, medium head or low head machines. Turbines are also divided by their principle way of operating and can be either impulse or reaction turbines. The rotating element (called `runner') of a reaction turbine is fully immersed in water and is enclosed in a pressure casing. The runner blades are profiled so that pressure differences across them impose lift forces, like those on aircraft wings, which cause the runner to rotate. In contrast, an impulse turbine runner operates in air, driven by a jet (or jets) of water. Here the water remains at atmospheric pressure before and after making contact with the runner blades. In this case a nozzle converts the pressurised low velocity water into a high speed jet. The runner blades deflect the jet so as to maximise the change of momentum of the water and thus maximising the force on the blades. Impulse turbines are usually cheaper then reaction turbines because there is no need for a specialist pressure casing, nor for carefully engineered clearances. However, they are only suitable for relatively high heads.


IMPULSE TURBINES:

Impulse turbines are generally more suitable for micro-hydro applications compared with reaction turbines because they have the following advantages: greater tolerance of sand and other particles in the water, better access to working parts, no pressure seals around the shaft, easier to fabricate and maintain, better part-flow efficiency.

The major disadvantage of impulse turbines is that they are mostly unsuitable for low-head sites because of their low specific speeds. The crossflow, Turgo and multi-jet Pelton are suitable at medium heads. A Pelton turbine consists of a set of specially shaped buckets mounted on a periphery of a circular disc. It is turned by jets of water which are discharged from one or more nozzles and strike the buckets. The buckets are split into two halves so that the central area does not act as a dead spot incapable of deflecting water away from the oncoming jet. The cutaway on the lower lip allows the following bucket to move further before cutting off the jet propelling the bucket ahead of it and also permits a smoother entrance of the bucket into the jet. (see diagrams below) The Pelton bucket is designed to deflect the jet through 165 degrees (not 180 degrees) which is the maximum angle possible without the return jet interfering with the following bucket for the oncoming jet.


RUNNER OF A PELTON TURBINE

PRCET/EEE/IV SEMESTER/EE2252-POWE SEMESTER/EE2252-POWER R PLANT ENGINEERING/NOTES

In large scale hydro installation Pelton turbines are normally only considered for heads above 150 m, but for micro-hydro applications Pelton turbines can be used effectively at heads down to about 20 m. Pelton turbines are not used at lower heads because their rotational speeds become very ver y slow and the runner required is very large and unwieldy. If runner size and low speed speed do not pose a problem for a particular installation, then a Pelton turbine can be used efficiently with fairly fairly low heads. If a higher running speed and smaller runner are required then there are two further options: REACTION TURBINES: The reaction turbines considered here are the Francis turbine and the propeller turbine. A special case of the propeller turbine is the Kaplan. In all these cases, cases, specific speed is high, i.e. reaction turbines rotate faster than impulse turbines given the same head and flow conditions. This has the very important important consequences in that a reaction turbine can often be coupled directly to an alternator without requiring a speed-increasing drive system. Some manufacturers make combined combined turbine-generator sets of this sort. Significant Significant cost savings are made in eliminating the drive and the maintenance of the hydro unit is very much simpler. The Francis turbine is suitable for medium heads, while the propeller is more suitable for low heads. On the whole, reaction turbines require more sophisticated fabrication than impulse turbines because they involve the use of larger and more intricately profiled blades together with carefully profiled casings. Francis turbines can either be volute-cased or open-flume machines. The spiral casing is tapered to distribute water uniformly around the entire perimeter of the runner and the guide vanes feed the water into the runner at the correct angle. The runner blades are profiled in a complex manner and direct the water so that it exits axially from the centre of the runner. In doing so, the water imparts imparts most of its pressure pressure energy to the runner before leaving the turbine via a draft tube. The Francis turbine is generally fitted with adjustable guide vanes. These regulate the water flow as it enters the runner and are usually linked to a governing system which matches flow to turbine loading in the same way as a spear valve or deflector plate in a Pelton turbine. When the flow is reduced the efficiency of the turbine falls away.


PROPELLER TURBINE:

The basic propeller turbine consists of a propeller, similar to a ship's propeller, fitted inside a continuation of the the penstock tube. The turbine shaft passes out of the tube at the point where the tube changes direction. The propeller usually has three to six blades, three in the case of very low head h ead units and the water flow is regulated b y static blades or swivel gates ("wicket gates") just just upstream of the propeller. This kind of propeller turbine turbine is known as a fixed blade axial flow turbine because the pitch angle of the rotor blades cannot be changed. The part-flow efficiency of fixed-blade propeller turbines tend to be very poor.


KAPLAN TURBINE:

Large scale hydro sites make use of more sophisticated versions of the propeller turbines. Varying the pitch of the propeller blades together with wicket gate adjustment, enables reasonable efficiency to be maintained under part flow conditions. Such turbines are known as variable pitch or Kaplan turbines. Centrifugal pumps can be used as turbines by passing water through them in reverse. Research is currently being done to enable the performance of pumps as turbines to be predicated more accurately. accurately. Elements of Hydel Power Plant:


1. Water reservoir, 2. Dam, 3. Spillway, 4. Pressure tunnel, 5. Penstock, 6. Surge tank, 7. Water turbine, 8. Draft tube, 9. Tail race, 10. Step-up transformer,Power house Advantages of Hydro-electric power plants Water is a renewable source of energy. Water which is the operating fluid, is neither consumed nor converted into something else, Water is the cheapest source of energy because it exists as a free gift of nature. The fuels needed for the thermal, diesel and nuclear plants are exhaustible and expensive.

There is no ash disposal problem as in the case of thermal power plant. Hydraulic turbines are classified as follows: 1) According to the head and quantity of water available, 2) According to the name of the originator, 3) According to the action of water on the moving blades, 4) According to the direction of flow of water in the runner, 5) According to the disposition of the turbine shaft, According to the specific speed N Comparison of Impulse and reaction turbine S.No Impulse turbine 1. Head: The machine is suitable for high installation. installation. (H=100 + 200 m). 2. Nature of input energy to the runner: The nozzle converts the entire hydraulic energy into kinetic energy before water strikes the runner.

3.

4.

Method of energy transfer: The buckets of the runner are so shaped that they extract almost all the kinetic energy of the jet. Operating pressure: The turbine works under atmospheric pressure. Which is the difference between the inlet and exit points of the runner.

Reaction turbine The machines can be used for medium heads (H=50 to 500 m) and low heads (less

The head is usually inadequate to produce high velocity jet. Hence water is supplied to the runner in the forms of both pressure and kinetic energy. The wicket gates accelerate the flow a little and direct the water to runner vanes to which energies of water are transferred. transferred. The runner works is a closed system under the action of reaction pressure.


5.

Admission of water to the wheel: Only a few buckets comprising a part of the wheel are exposed to the water jet.

The entire circumference of the wheel receives water and all passages between the runner blades are always full of water.

6.

Discharge: They are essential low discharge turbines.

Since power is a product of head and weight of the rate of flow, these turbines consume large quantities of water in order to develop a reasonable power under a relatively low

7.

Speed of operation: The speed are invariably high.

Although the specific speeds of these turbines is high, their actual running speeds are comparatively low.

8.

Size : These are generally small size.

The turbines sizes is much larger than impulse wheels, in order to accommodate heavy discharge.

9.

Casing: It prevents splashing of water. It has no hydraulic function to serve.

The spiral casing has an important role to play; it distributes water under the available pressure uniformly around the periphery of the runner.

10.

Turbine setting: The head between the wheel and race is lost.

The draft tube ensures that the head of water below tail race level is not lost.

ADVANTAGES OF WATER TURBINES ENVIRONMENTAL BENEFITS:

Environmentalists are quick to point out that water turbines produce no carbon as they generate power. Not only do they not emit carbon, but they do not react chemically with the water at all; no water is destroyed in the process of creating electricity. While some critics point out that turbines may alter fish migrations, there are a number of turbines that operate like waterwheels, which do not affect any wildlife. RELIABILITY:

The greatest advantage of water turbines is their reliability. The tidal nature of oceans and the steady flow of rivers means power can be produced around the clock, while wind turbines remain motionless on calm days. Their blades continue to turn on cloudy days that prohibit solar panels from harvesting the energy of protons, as well as after the sun goes down in the evening. The amount of water flowing through a river is nearly always predictable, allowing their gearboxes to keep the speed of the blades at a safe and


productive level. Even in shoreline areas, jetties can be constructed to regulate the flow of water. POTENTIAL:

According to green-trust.org, only 2,400 of the country's 80,000 dams are equipped to generate hydroelectricity. This means much of the construction necessary for large-scale hydroelectric production has already been complet ed, and only a retrofit is necessary to begin to produce power. As fossil fuels become more scarce, engineers have been working to find sources of power, such as the Gulf Stream and various river deltas, where long-term solutions can be developed. PUMPED STORAGE HYDEL PLANT :

Pumped-storage

hydroelectricity (PSH)

is

a

type

of hydroelectric power

generation used by some power plants for load balancing. The method stores energy in the form of potential energy of water, pumped from a lower elevation reservoir to a higher elevation. Low-cost off-peak electric power is used to run the pumps. During periods of high electrical demand, the stored water is released through turbines to produce electric power. Although the losses of the pumping process makes the plant a net consumer of energy overall, the system increases revenue by selling more electricity during periods of peak demand , when electricity prices are highest. Pumped storage is the largest-capacity form of grid energy storage available, and, as of March 2012, the Electric Power Research Institute (EPRI) reports that PSH accounts for more

than

99% [1]

127,000MW. [1][2][3][4]

80%,

of

bulk

storage

capacity

worldwide,

representing

around

PSH reported energy efficiency varies in practice between 70% and

with some claiming up to 87%


At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine, generating electricity. Reversible turbine/generator assemblies act as pump and turbine (usually a Francis turbine design). Nearly all facilities use the height difference between two natural bodies of water or artificial reservoirs. Pure pumped-storage plants just shift the water between reservoirs, while the "pump-back" approach is a combination of pumped storage and conventional hydroelectric plants that use natural stream-flow. Plants that do not use pumped-storage are referred to as conventional hydroelectric plants; conventional hydroelectric plants that have significant storage capacity may be able to play a similar role in the electrical grid as pumped storage, by deferring output until needed. Taking into account evaporation losses from the exposed water surface and conversion losses, approximately 70% to 85% of the electrical energy used to pump the water into the elevated reservoir can be regained.

[6]

The technique is currently the most

cost-effective means of storing large amounts of electrical energy on an operating basis, but capital costs and the presence of appropriate geography are critical decision factors. The relatively low energy density of pumped storage systems requires either a very large body of water or a large variation in height. For example, 1000 kilograms of water (1 cubic meter) at the top of a 100 meter tower has a potential energy of about 0.272 kW·h (capable of raising the temperature of the same amount of water by only 0.23 Celsius = 0.42 Fahrenheit). The only way to store a significant amount of energy is by having a large body of water located on a hill relatively near, but as high as possible above, a second body of water. In some places this occurs naturally, in others one or both bodies of


water have been man-made. Projects in which both reservoirs are artificial and in which no natural waterways are involved are commonly referred to as "closed loop". This system may be economical because it flattens out load variations on the power grid, permitting thermal power stations such as coal-fired plants and nuclear power plants and renewable energy power plants that provide base-load electricity to continue operating at peak efficiency

(Base load power plants), while reducing the need for "peaking" power plants that use the same fuels as many base load thermal plants, gas and oil, but have been designed for flexibility rather than maximal thermal efficiency. However, capital costs for purpose-built hydro storage are relatively high. Along with energy management, pumped storage systems help control electrical network frequency and provide reserve generation. Thermal plants are much less able to respond to sudden changes in electrical demand, potentially causing frequency and voltage instability. Pumped storage plants, like other hydroelectric plants, can respond to load changes within seconds. The first use of pumped storage was in the 1890s in Italy and Switzerland. In the 1930s reversible hydroelectric turbines became available. These turbines could operate as both turbine-generators and in reverse as electric motor driven pumps. The latest in largescale engineering technology are variable speed machines for greater efficiency. These machines

generate

in

synchronization

with

the

network

frequency,

but

operate asynchronously (independent of the network frequency) as motor-pumps. The first use of pumped-storage in the United States was in 1930 by the Connecticut Electric and Power Company, using a large reservoir located near New Milford, Connecticut, pumping water from the Houstatonic River to the storage reservoir 230 feet [7]

above.

A new use for pumped storage is to level the fluctuating output of intermittent energy sources. The pumped storage provides a load at times of high electricity output and low

electricity

demand,

enabling

additional

system

peak

capacity.

In

certain


jurisdictions, electricity prices may be close to zero or occasionally negative (Ontario in early September, 2006), on occasions that there is more electrical generation than load available to absorb it; although at present this is rarely due to wind alone, increased wind generation may increase the likelihood of such occurrences. It is particularly likely that pumped storage will become especially important as a balance for very large scale photovoltaic generation

UNIT III - NUCLEAR POWER PLANTS NUCLEAR PLANT – LAYOUT: As coal reserve is not unlimited, there is natural threat to thermal power plants based

on coal. It is estimated that within next 30 to 40 years, coal reserve will exhaust if it is consumed at the present rate. Nuclear power plants are thought to be the solution for bulk power generation. At present the installed capacity of unclear power plant is about 4300 MW and expected to expand further in our country. The present day atomic power plants work on the principle of nuclear fission of

235

U. In the natural uranium,

only 0.72% and remaining parts is constituted by 99.27% of 234

U. The concentration of

enriched

235

U. When

235

235

238

235

U constitutes

U and only about 0.05% of

U may be increased to 90% by gas diffusion process to obtain

U is bombarded by neutrons a lot of heat energy along with

additional neutrons are produced. These new neutrons further bombard

235

U producing more heat and more neutrons.

Thus a chain reaction sets up. However this reaction is allowed to take place in a controlled manner inside a closed chamber called nuclear reactor. To ensure sustainable chain reaction, moderator and control rods are used. Moderators such as heavy water (deuterium) or very pure carbon

12

C are used to reduce the speed of neutrons. To control the number

neutrons, control rods made of cadmium or boron steel are inserted inside the reactor. The control rods can absorb neutrons. If we want to decrease the number neutrons, the control rods are lowered down further and vice versa. The heat generated inside the reactor is taken out of the chamber with the help of a coolant such as liquid sodium or some gaseous fluids. The coolant gives up the heat to water in heat exchanger to convert it to steam as shown in


figure 2.4. The steam then drives the turbo set and the exhaust steam from the turbine is cooled and fed back to the heat exchanger with the help of water feed pump. Calculation shows that to produce 1000 MW of electrical power in coal based thermal plant, about 6  6

10 Kg of coal is to be burnt daily while for the same amount of power, only about 2.5 Kg of

235

U is to be used per day in a nuclear power stations.

Coolant Control rods 3-phase A.C Electric power

Steam r h e c g x n E a

Reactor

Turbin e

Generator

Fuel rods Exhausted steam from turbine Moderator Condenser Water feed Coolant circulating pump

pump

Nuclear power generation. The initial investment required to install a nuclear power station is quite high but

running cost is low. Although, nuclear plants produce electricity without causing air pollution, it remains a dormant source of radiation hazards due to leakage in the reactor. Also the used fuel rods are to be carefully handled and disposed off as they still remain radioactive. The reserve of

235

U is also limited and can not last longer if its consumption continues


at the present rate. Naturally search for alternative fissionable material continues. For 239

example, plutonium (

233

Pu) and (

available. Absorbing neutrons,

238

U) are fissionable. Although they are not directly

U gets converted to fissionable plutonium

239

Pu in the

atomic reactor described above. The used fuel rods can be further processed to extract

239

Pu

from it indirectly increasing the availability of fissionable fuel. Effort is also on to convert thorium into fissionable

233

U. Incidentally, India has very large reserve of thorium in the

world. Total approximate generation capacity and Contribution by thermal, hydel and nuclear generation in our country are given below.

Method of generation Thermal Hydel Nuclear Total generation

in MW

% contribution

77 340

69.4

29 800

26.74

2 720

3.85

1 11 440

-

PRINCIPLES OF NUCLEAR ENERGY: FISSION REACTION AND FUSION REACTION: NUCLEAR FUSION:

nuclear reaction in which two or more atomic nuclei collide at very high speed and join to form a new type of atomic nucleus (e.g. The energy that the Sun emits into space is produced by nuclear reactions that happen in its core due to the collision of hydrogen nuclei and the formation of helium nuclei). During this process, matter is not conserved because some of the mass of the fusing nuclei is converted to photons which are released through a cycle that even our sun uses. Fusion is the process that powers active stars. The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy, while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission. This means that fusion generally occurs for lighter elements only, and likewise, that


fission normally occurs only for heavier elements. There are extreme astrophysical events that can lead to short periods of fusion with heavier nuclei

NUCLEAR FISSION:

Nuclear reaction or

fission is

either

a nuclear

a radioactive

decay process

in which the nucleus of

an atom splits

into smaller parts

(lighter nuclei),

often

producing

free neutrons and photons (in the form of gamma rays), and releasing a very large amount of energy, even by the energetic standards of radioactive decay. The two nuclei produced are most often of comparable but slightly different sizes, typically with a mass ratio of [1][2]

products of about 3 to 2, for common fissile isotopes.

Most fissions are binary fissions

(producing two charged fragments), but occasionally (2 to 4 times per 1000 events), three positively charged fragments are produced, in a ternary fission. The smallest of these fragments in ternary processes ranges in size from a proton to an argon nucleus. Fission as encountered in the modern world is usually a deliberately produced manmade nuclear reaction induced by a neutron. It is less commonly encountered as a natural form of spontaneous radioactive decay (not requiring a neutron), occurring especially in very high-mass-number isotopes. The unpredictable composition of the products (which vary in a broad probabilistic and somewhat chaotic manner) distinguishes fission from purely quantum-tunneling processes such as proton emission, alpha decay and cluster decay, which give the same products each time. Nuclear

fission

of

heavy

elements

was

discovered

in

1938

by Meitner, Hahn and Frisch, and named by analogy with biological fission of living cells. It

is

an exothermic

reaction which

can

release

large

amounts

of energy both


as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). In order for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom. Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes possible a self-sustaining nuclear chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon. The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very dense source of energy. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over the destructive potential of nuclear weapons may counterbalance the desirable qualities of fission as an energy source, and give rise to ongoing political debate over nuclear power. Nuclear

fission

of

heavy

elements

was

discovered

in

1938

by Meitner, Hahn and Frisch, and named by analogy with biological fission of living cells. It

is

an exothermic

reaction which

can

release

large

amounts

of energy both

as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place). In order for fission to produce energy, the total binding energy of the resulting elements must be greater than that of the starting element. Fission is a form of nuclear transmutation because the resulting fragments are not the same element as the original atom.

Nuclear fission produces energy for nuclear power and to drive the explosion of nuclear weapons. Both uses are possible because certain substances called nuclear


fuels undergo fission when struck by fission neutrons, and in turn emit neutrons when they break apart. This makes possible a self-sustaining nuclear chain reaction that releases energy at a controlled rate in a nuclear reactor or at a very rapid uncontrolled rate in a nuclear weapon. The amount of free energy contained in nuclear fuel is millions of times the amount of free energy contained in a similar mass of chemical fuel such as gasoline, making nuclear fission a very dense source of energy. The products of nuclear fission, however, are on average far more radioactive than the heavy elements which are normally fissioned as fuel, and remain so for significant amounts of time, giving rise to a nuclear waste problem. Concerns over nuclear waste accumulation and over the destructive potential of nuclear weapons may counterbalance the desirable qualities fission as an energy source, and give rise to ongoing political debate over nuclear power. NUCLEAR REACTOR:

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Nuclear reactors are used at nuclear power plants for generating electricity and in propulsion of ships. Heat from nuclear fission is passed to a working fluid (water or gas), which runs through turbines. These either drive a ship's propellers or turn electrical generators. Nuclear generated steam in principle can be used for industrial process heat or for

district

heating.

Some

reactors

are

used

to

produce

isotopes

for medical and industrial use, or for production of plutonium for weapons COMPONENTS OF A NUCLEAR REACTOR: There are several components common to most types of reactors: FUEL:

Uranium is the basic fuel. Usually pellets of uranium ox ide (UO2) are arranged in tubes to form fuel rods. The rods are arranged into fuel assemblies in the reactor core. MODERATOR :

Material in the core which slows down the neutrons released from fission so that they cause more fission. It is usually water, but may be heavy water or graphite. CONTROL RODS :

These are made with neutron-absorbing material such as cadmium, hafnium or boron, and are inserted or withdrawn from the core to control the rate of reaction, or to halt


it. In some PWR reactors, special control rods are used to enable the core to sustain a low level of power efficiently.

COOLANT:

A fluid circulating through the core so as to transfer the heat from it. In light water reactors the water moderator functions also as primary coolant. Except in BWRs, there is secondary coolant circuit where the water becomes steam. (see also later section on primary coolant characteristics). PRESSURE VESSEL OR PRESSURE TUBES :

Usually a robust steel vessel containing the reactor core and moderator/coolant, but it may be a series of tubes holding the fuel and conveying the coolant through the surrounding moderator. Part of the cooling system where the high-pressure primary coolant bringing heat from the reactor is used to make steam for the turbine, in a secondary circuit. Essentially a heat exchanger like a motor car radiator*. Reactors may have up to four 'loops', each with a steam generator. CONTAINMENT:

The structure around the reactor and associated steam generators which is designed to protect it from outside intrusion and to protect those outside from the effects of radiation in case of any serious malfunction inside. It is typically a metre -thick concrete and steel structure. REACTIVITY CONTROL:

The power output of the reactor is adjusted by controlling how many neutrons are able to create more fissions. Control rods that are made of a neutron poison are used to absorb neutrons. Absorbing more neutrons in a control rod means that there are fewer neutrons available to cause fission, so pushing the control rod deeper into the reactor will reduce its power output, and extracting the control rod will increase it. At the first level of control in all nuclear reactors, a process of delayed neutron emission by a number of neutron-rich fission isotopes is an important physical


process. These delayed neutrons account for about 0.65% of the total neutrons produced in fission, with the remainder (termed "prompt neutrons") released immediately upon fission. The

fission

products

which

produce

delayed

neutrons

have

half

lives

for

their decay by neutron emission that range from milliseconds to as long as several minutes. Keeping the reactor in the zone of chain-reactivity where delayed neutrons are necessary to achieve a critical mass state, allows time for mechanical devices or human operators to have time to control a chain reaction in "real time"; otherwise the time between achievement of criticality and nuclear meltdown as a result of an exponential power surge from the normal nuclear chain reaction, would be too short to allow for intervention. In some reactors, the coolant also acts as a neutron moderator. A moderator increases the power of the reactor by causing the fast neutrons that are released from fission to lose energy and become thermal neutrons. Thermal neutrons are more likely than fast neutrons to cause fission, so

more neutron moderation means more power output from the reactors. If the coolant is a moderator, then temperature changes can affect the density of the coolant/moderator and therefore change power output. A higher temperature coolant would be less dense, and therefore a less effective moderator. In other reactors the coolant acts as a poison by absorbing neutrons in the same way that the control rods do. In these reactors power output can be increased by heating the coolant, which makes it a less dense poison. Nuclear reactors generally have automatic and manual systems to scram the reactor in an emergency shutdown. These systems insert large amounts of poison (often boron in the form of boric acid) into the reactor to shut the fission reaction down if unsafe conditions are detected or anticipated.

[6]

Most types of reactors are sensitive to a process variously known as xenon poisoning, or the iodine pit. Xenon-135 produced in the fission process acts as a "neutron poison" that absorbs neutrons and therefore tends to shut the reactor down. Xenon-135 accumulation can be controlled by keeping power levels high enough to destroy it as fast as it is produced. Fission also produces iodine-135, which in turn decays (with a half-life of under seven hours) to new xenon-135. When the reactor is shut down, iodine-135 continues


to decay to xenon-135, making restarting the reactor more difficult for a day or two. This temporary state is the "iodine pit." If the reactor has sufficient extra reactivity capacity, it can be restarted. As the extra xenon-135 is transmuted to xenon-136 which is not a neutron poison, within a few hours the reactor experiences a "xenon burn off (power) transient". Control rods must be further inserted to replace the neutron absorption of the lost xenon135. Failure to properly follow such a procedure was a key step in the Chernobyl disaster. Reactors used in nuclear marine propulsion (especially nuclear submarines) often cannot be run at continuous power around the clock in the same way that land-based power reactors are normally run, and in addition often need to have a very long core life without refueling. For this reason many designs use highly enriched uranium but incorporate burnable neutron poison directly into the fuel rods.

[7]

This allows the reactor to

be constructed with a high excess of fissionable material, which is nevertheless made relatively more safe early in the reactor's fuel burn-cycle by the presence of the neutronabsorbing material which is later replaced by naturally produced long-lived neutron poisons (far longer-lived than xenon-135) which gradually accumulate over the fuel load's operating life.

Figure : Nuclear Power Plant


ELECTRICAL POWER GENERATION:

The energy released in the fission process generates heat, some of which can be converted into usable energy. A common method of harnessing this thermal energy is to use it to boil water to produce pressurized steam which will then drive a steam turbine that generates electricity.

UNIT IV - GAS AND DIESEL POWER PLANTS Heat engine

Any type of engine or machine which derives heat energy from the combustion of fuel or any other source and converts this energy into mechanical work is termed as a heat engine. Essential components of a diesel power plant are:

(i) Engine



 

(ii)Air intake system (iii)Exhaust system (iv)Fuel system (v) Cooling system (vi) Lubrication system (vii) Engine starting system (viii) Governing system. Commonly used fuel injection system in a diesel power station: Common-rail injection system Individual pump injection system Distribution system.

OPEN AND CLOSED CYCLE:


A

closed-cycle

air, nitrogen, helium, argon,

gas [1][2]

turbine is

etc.)

for

a turbine that

the working

uses

fluid as

thermodynamic system. Heat is supplied from an external source.

a

part [3]

gas of

(e.g. a closed

Such re circulating

turbines follow the Brayton cycle. The initial patent for a closed-cycle gas turbine was issued in 1935 and they were first used commercially in 1939. built in Switzerland and Germany by

1978.

[2]

[3]

Seven CCGT units were

Historically, CCGTs found most use

as external combustion engines"with fuels such as bituminous coal, brown coal and blast furnace gas" but were superseded by open cycle gas turbines using clean-burning fuels (e.g.

"gas or light oil"), especially in highly-efficient combined cycle systems. CCGT systems have demonstrated very high availability and reliability.

[6]

[3]

Air-based

The most notable

helium-based system thus far was Oberhausen 2, a 50megawatt cogeneration plant that operated from 1975 to 1987 in Germany.

GAS TURBINE:

A gas turbine, also called a combustion turbine, is a type of internal combustion engine. It has an upstream rotating compressor coupled to a downstream turbine, and a combustion chamber in-between. The basic operation of the gas turbine is similar to the of the steam power plant except that air is used instead of water. Fresh atmospheric air flows through a compressor that brings it to higher pressure. Energy is then added by spraying fuel into


the air and igniting it so the combustion generates a high-temperature flow. This high temperature high-pressure gas enters a turbine, where it expands down to the exhaust pressure, producing a shaft work output in the process. The turbine shaft work is used to drive the compressor and other devices such as an electric generator that may be coupled to the shaft. The energy that is not used for shaft work comes out in the exhaust gases, so these have either a high temperature or a high velocity. The purpose of the gas turbine determines the design so that the most desirable energy form is maximized. Gas turbines are used to power aircrafts, trains, ships, electrical generators, or even tanks. Gas turbines are also used in many liquid propellant rockets, the gas turbines are used to power a turbo pump to permit the use of lightweight, low pressure tanks, which saves co nsiderable dry mass. TURBO PROP ENGINES:

A turboprop engine is a type of turbine engine which drives an external aircraft propeller using a reduction gear. Turboprop engines are generally used on small subsonic aircraft, but some large military and civil aircraft, such as the Airbus A400M, Lockheed L188 Electra , have also used turboprop power. AERODERIVATIVE GAS TURBINES:

Aero derivatives are also used in electrical power generation due to their ability to be shut down, and handle load changes more quickly than industrial machines. They are also used in the marine industry to reduce weight. The General Electric LM2500, General Electric LM6000, Rolls-Royce RB211 and Rolls-Royce Avon are common models of this type of machine. AMATEUR GAS TURBINES:

Increasing numbers of gas turbines are being used or even constructed by amateurs. In its most straightforward form, these are commercial turbines acquired through military surplus or scrap yard sales, then operated for display as part of the hobby of engine [9][10]

collecting.

In its most extreme form, amateurs have even rebuilt engines beyond

professional repair and then used them to compete for the Land Speed Record. The

simplest

form

of

self-constructed

gas

turbine

employs

an

automotive turbocharger as the core component. A combustion chamber is fabricated and plumbed between the compressor and turbine sections.


More sophisticated turbojets are also built, where their thrust and light weight are sufficient to power large model aircraft. The Schreckling design constructs the entire engine from raw materials, including the fabrication of a centrifugal compressor wheel from plywood, epoxy and wrapped carbon fiber strands. Several small companies now manufacture small turbines and parts for the amateur. Most turbojet-powered model aircraft are now using these commercial and semicommercial micro turbines, rather than a Schreckling-like home-build. ADVANTAGES OF GAS POWER PLANT: 1.The three most obvious pros of using natural gas as a fuel to power your generators is that

it is cleaner, less expensive than other non-renewable fuels, and is considerably efficient. 2. In comparison to oil and coal, the emissions of sulfur, nitrogen, and carbon dioxide (a greenhouse gas) are considerably lower. Hence, n atural gas is one of the cleanest fossil fuels when it burns. 3.Another advantage of natural gas generators is that natural gas does not produce a pungent odor, which is fairly common in generators powered by oil or diesel. 4. Natural gas generators are also effective in reducing costs when used to power homes. This is because electricity from the main utility source is a far more expensive alternative. 5. Apart from being cleaner and cheaper, natural gas is also readily available in large cities since it is delivered directly through pipelines. Hence, when using natural gas powered generators, storage of fuel becomes redundant. DISADVANTAGES :

1. When it comes to the cons of natural gas generators, one of its advantages can also be regarded as a disadvantage. Since natural gas need not be stored as it is supplied through gas pipelines, at times of natural calamities the supply of n atural gas is disrupted. You may find yourself facing a lack of fuel when you need to operate your generator the most. 2. Apart from this, natural gas is extremely explosive and can be a serious fire hazard should the pipeline burst. 3. In comparison to diesel generators, natural gas g enerators are: 4.More expensive to run 5. Emit more carbon dioxide, which is a greenhouse gas.


COMBINED LAYOUT OF GAS AND DIESEL POWER PLANT:

ENGINE STRATING SYSTEM: This includes air compressor and starting air tank. The function of this system is to

start the engine from cold supplying compressed air. FUEL SYSTEM:

Pump draws diesel from storage tank and supplies it to the small day tank through the filter. Day tank supplies the daily fuel need of engine. The day tan is usually placed high so that diesel flows to engine under gravity. Diesel is again filtered before being injected into the engine by the fuel injection pump. The fuel is supplied to the engine according to the load on the plant. AIR INTAKE SYSTEM:

Air filters are used to remove dust from the incoming air. Air filters may be dry type, which is made up of felt, wool or cloth. In oil bath type filters, the sir is swept over a bath of oil so that dust particles get coated. EXHAUST SYSTEM:

In the exhaust system, silencer (muffler) is provide to reduce the noise. ENGINE COOLING SYSTEM:

The temperature of burning gases in the engine cylinder is the order of 1500 to 2000‟C. to keep the temperature at the reasonable level, water is circulated inside the engine in water jackets which are passage around the cylinder, piston, combustion chamber


etc. hot water leaving the jacket is sent to heat exchanger. Raw water is made to flow through the heat exchanger, where it takes up the

heat of jacket water. It is then cooled in the cooling tower and re circulates again. ENGINE LUBRICATION SYSTEM:

It includes lubricating oil tank, oil pump and cooler. Lubrication is essential to reduce friction and wear of engine parts such as cylinder walls and piston. Lubricating oil which gets heated due to friction of moving parts is cooled before recirculation. The cooling water used in the engine is used for cooling the lubricant also. ADVANTAGES OF DIESEL POWER PLANT:

1. Plant layout is simple. Hence it can be quickly installed and commissioned, while

the

erection and starting of a steam power plant or hydro-plant takes a fairly long time. 2. Quick starting and easy pick-up of loads are possible in a very short time. 3. Location of the plant is near the load center. 4. The load operation is easy and requires minimum labors. 5. Efficiency at part loads does not fall so much as that of a steam plant. 6. Fuel handling is easier and no problem of ash disposal exists. 7. The plant is smaller in size than steam power plant for same capacity. 8. Diesel plants operate at high overall efficiency than steam. DISADVANTAGES OF DIESEL POWER PLANT:

1. Plant capacity is limited to about 50 MW of power. 2. Diesel fuel is much more expensive than coal. 3. The maintenance and lubrication costs are high. 4. Diesel engines are not guaranteed for operation under continuous, while steam can work under 25% of overload continuously.


UNIT V NON-CONVENTIONAL POWER GENERATION TIDE

The periodic rise and fall of the water level of sea which are carried by the action of sun and moon on water of the earth is called the tide. In a single basin arrangement power can be generated only intermittently. THE CONSISTENCIES OF ‘SOLAR FARM’ AND ‘SOLAR TOWER’

The solar farm consists of a whole field covered with parabolic trough concentrators and a ‘solar tower’ consists of a central receiver on a tower and a whole field of tracking. SEE BECK EFFECT

‚If two dissimilar materials are joined to form a loop and the two junctions maintained at different temperatures, an e.m.f. will be set up around the loop‛. This is called Seeback effect. WORKING PRINCIPLE OF THERMIONIC


A thermionic converter works because of the phenomenon of ‘thermionic emission’. PHOTO VOLTAIC EFFECT

‘Photovoltaic effect’ is defined as the generation of an electromotive force as a result of absorption of ionizing radiation. MHD – generator:

‘MHD generator’ is a device which converts heat energy of a fuel directly into electrical energy without a conventional electric generator. FUEL CELL’

A ‘fuel cell’ is an electrochemical device in which the chemical energy of a conventional fuel is converted directly and efficiently into low voltage, direct current electrical energy. The various non-conventional energy sources are as follows: Solar energy Wind energy  

        

Energy from biomass and biogas Ocean thermal energy conversion Tidal energy Geothermal energy Hydrogen energy Fuel cells Magneto-hydrodynamics generator Thermionic converter Thermo-electric power.

CHARACTERISTIC’S OF WIND ENERGY

1. 2. 3. 4.

Wind-power systems do not pollute the atmosphere. Fuel provision and transport are not required in wind-power systems. Wind energy is a renewable source of energy. Wind energy when produced on small scale is cheaper, but competitive with conventional power generating systems when produced on a large scale. Wind energy entails following short comings/problems: 1. It is fluctuating in nature. 2. Due to its irregularity it needs storage devices. 3. Wind power generating systems produce ample noise.


THE TYPES OF WIND MILLS

1. Multiple blade type 2. Savonius type 3. Darrieus type SOLAR ENERGY COLLECTORS:

Solar energy, radiant light and heat from the sun, has been harnessed by humans since ancient times using a range of ever-evolving technologies. Solar energy technologies include solar

heating, solar

photo

voltaic, solar

thermal

electricity,

solar

architecture and artificial photosynthesis, which can make considerable contributions to solving some of the most urgent energy problems the world now faces. Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air. Solar collectors can be used in a large variety of applications. The main areas of applications include:

1. Solar water heating, which includes thermosyphon, integrated collector storage systems, air systems, direct circulation and indirect water heating systems. 2. Solar space heating systems, which includes both water and air systems. 3. Solar refrigeration, which includes both adsorption and absorption s ystems. 4. Industrial process heat systems, which include both low temperature (air and water based) applications and solar steam generation systems. 5. Solar desalination systems, which include both direct (solar stills) and indirect systems (conventional desalination equipment powered by solar collectors). 6. Solar thermal power generation systems, which include the parabolic trough systems, the power tower or central receiver systems and the parabolic dish systems (dish / Stirling engine).


OCEAN THERMAL ENERGY CONVERSION (OTEC) It uses the temperature difference between cooler deep and warmer shallow or

surface ocean waters to run a heat engine and produce useful work, usually in the form of electricity. However, the temperature differential is small and this impacts the economic feasibility of ocean thermal energy for electricity generation. The most commonly used heat cycle for OTEC is the Rankine cycle using a low pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle engines use a working fluids that are typically thought of as refrigerants such as ammonia or R134a. Open-cycle engines use vapour from the seawater itself as the working fluid. OTEC can also supply quantities of cold water as a by-product. This can be used for air conditioning and refrigeration and the fertile deep ocean water can feed biological technologies. Another by-product is fresh water distilled from the sea. Demonstration plants were first constructed in the 1880s and continue to be built, but no large-scale commercial plants are in operation. CLOSED SYSTEM:

Closed-cycle systems use fluid with a low boiling point, such as ammonia, to power a turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger to vaporize the fluid. The expanding vapor turns the turbo-generator. Cold water, pumped through a second heat exchanger, condenses the vapor into a liquid, which is then recycled through the system. In 1979, the Natural Energy Laboratory and several private-sector partners developed the "mini OTEC" experiment, which achieved the first successful at-sea [12]

production of net electrical power from closed-cycle OTEC.

The mini OTEC vessel was

moored 1.5 miles (2.4 km) off the Hawaiian coast and produced enough net electricity to illuminate the ship's light bulbs and run its computers and television.

OPEN SYSTEM:

Open-cycle OTEC uses warm surface water directly to make electricity. Placing warm seawater in a low-pressure container causes it to boil. In some schemes, the


expanding steam drives a low-pressure turbine attached to an electrical generator. The steam, which has left its salt and other contaminants in the low-pressure container, is pure fresh water. It is condensed into a liquid by exposure to cold temperatures from deepocean water. This method produces desalinized fresh water, suitable for drinking water or irrigation. In other schemes, the rising steam is used in a gas lift technique of lifting water to significant heights. Depending on the embodiment, such steam lift pump techniques generate power from a hydroelectric turbine either before or after the pump is used. In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory) developed a vertical-spout evaporator to convert warm seawater into low pressure steam for open-cycle plants. Conversion efficiencies were as high as 97% for seawater-to-steam conversion (overall efficiency using a vertical-spout evaporator would still only be a few per cent). In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment. This broke the record of 40 kW set by a Japanese system in 1982. HYBRID SYSTEM:

A hybrid cycle combines the features of the closed- and open-cycle systems. In a hybrid, warm seawater enters a vacuum chamber and is flash-evaporated, similar to the open-cycle evaporation process. The steam vaporizes the ammonia working fluid of a closed-cycle loop on the other side of an ammonia vaporizer. The vaporized fluid then drives a turbine to produce electricity. The steam condenses within the heat exchanger and provides desalinated water. WIND POWER PLANT:

Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electrical power, windmills for mechanical power, wind pumps for water pumping or drainage, or sails to propel ships. Large wind farms consist of hundreds of individual wind turbines which are connected to the electric power transmission network. Offshore wind farms can harness more frequent and powerful winds than are available to land-based installations and have


less visual impact on the landscape but construction costs are considerably higher. Furthermore, offshore poses problems when

considering accessibility for maintenance issues. Small onshore wind facilities are used to provide electricity to isolated locations and utility companies increasingly buy surplus electricity produced by small domestic wind turbines. Wind power, as an alternative to fossil fuels, is plentiful, renewable, widely distributed, clean, produces no greenhouse gas emissions during operation and uses little land. The effects on the environment are generally less problematic than those from other power sources. As of 2011, Denmark is generating more than a quarter of its electricity from wind and 83 countries around the world are using wind power on a commercial basis. In 2010 wind energy production was over 2.5% of total worldwide electricity usage, and growing rapidly at more than 25% per annum. The monetary cost per unit of energy produced is similar to the cost for new coal and natural gas installations. Wind power is very consistent from year to year but has significant variation over shorter time scales. The intermittency of wind seldom creates problems when used to supply up to 20% of total electricity demand, but as the proportion increases, a need to upgrade the grid, and a lowered ability to supplant conventional production can occur. Power management techniques such as having excess capacity storage, geographically distributed turbines, dis patchable backing sources, storage such as pumped-storage hydroelectricity, exporting and importing power to neighboring areas or reducing demand when wind production is low, can greatly mitigate these problems. In addition, weather forecasting permits the electricity network to be readied for the predictable variations in production that occur.

Types of Wind Machines

Wind machines (aerogenerators) are generally classified as follows: 1. Horizontal axis wind machines. 2. Vertical axis wind machines. Horizontal axis wind machines. Figure shows a schematic arrangement of horizontal axis machine. Although the common wind turbine with horizontal axis is simple in principle yet the design of a complete system, especially a large


one that would produce electric power economically, is complex. It is of paramount importance‟s that the components like rotor, transmission, generator and tower should not only be as efficient as possible but they must also function effectively in combination.

Figure: Horizontal axis wind machine. Vertical axis wind machines. Figure shows vertical axis type wind machine. One of the main advantages of vertical axis rotors is that they do not have to be turned into the windstream as the wind direction changes. Because their operation is independent of wind direction, vertical axis machine are called panemones


Figure: Vertical axis wind machine

Magneto Hydro Dynamic (MHD) Generator Magnetohydrodynamics (MHD) is power generation technology in which the electric generator is static (nonrotating) equipment. In the MHD concept, a fluid conductor flows through a static magnetic field, resulting in a dc electric flow perpendicular to the magnetic filed. MHD/steam combined cycle power plants have the potential for very low heat rates (in the range of 6,500 Btu/kWh). So2 and NOx emission levels from MHD plants are projected to be very low.

GEOTHERMAL ENERGY:

Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. The Geothermal energy of the Earth's crust originates from the original formation of the planet (20%) and [1][2]

from radioactive decay of minerals (80%).

The geothermal gradient, which is the

difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface. The adjective geothermal originates from the Greek roots γη (ge), meaning earth, and (thermos), meaning hot.


At the core of the Earth, thermal energy is created by radioactive decay and temperatures may reach over 5000 degrees Celsius (9,000 degrees Fahrenheit). Heat conducts from the core to surrounding cooler rock. The high temperature and pressure cause some rock to melt, creating magma convection upward since it is lighter than the solid rock. The magma heats rock and water in the crust, sometimes up to 370 degrees Celsius (700 degrees Fahrenheit). The Earth's geothermal resources are theoretically more than adequate to supply humanity's energy needs, but only a very small fraction may be profitably exploited. Drilling and exploration for deep resources is very expensive. Forecasts for the future of geothermal power depend on assumptions

about technology, energy prices, subsidies, and interest rates. Pilot programs like EWEB's customer opt in Green Power Program show that customers would be willing to pay a little more for a renewable energy source like geothermal. But as a result of government assisted research and industry experience, the cost of generating geothermal power has decreased by 25% over the past two decades. In 2001, geothermal energy cost between two and ten cents per kwh. A geothermal power plant uses its geothermal activity to generate power. This type of natural energy production is extremely environmentally friendly and used in many geothermal hot spots around the globe.

To harness the energy, deep holes are drilled into the earth (much like when drilling for oil) until a significant geothermal hot spot is found.

When the heat source has been discovered, a pipe is attached deep down inside the hole which allows hot steam from deep within the earth crust to rise up to the surface.

The pressurized steam is then channeled into a turbine which begins to turn under the large force of the steam. This turbine is linked to the generator and so the generator also


begins to turn, generating electricity. We then pump cold water down a new pipe which is heated by the earth and then sent back up the first pipe to repeat the process.

The main problems with geothermal energy is that firstly, you must not pump too much cold water into the earth, as this could could cool the rocks too much, resulting in your geothermal heat source cooling down. secondly, geothermal power plants must be careful of escaping gases from deep within the e arth.

We suggest if you would like to learn more on this topic, you take a look at our advantages of geothermal energy, and our disadvantages of geothermal energy articles.

A very good way of thinking about geothermal energy is remembering that all our continents lie on molten rock deep within the earth, this rock produces tremendous levels of heat that we are able to extract, just think of your nation lying on a bed of fire.

Geothermal power is one of the most renewable energy sources that exists on our planet today, the earth will contain this heat for our lifetime. If this heat disappears, our planet will become too cold to survive on.

MHD POWER GENERATION:

The MHD (magnetohydrodynamic) generator transforms thermal energy and kinetic energy directly into electricity. MHD generators are different from traditional electric generators in that they operate at high temperatures without moving parts. MHD was developed because the hot exhaust gas of an MHD generator can heat the boilers of a steam power plant, increasing overall efficiency. MHD was developed as a topping cycle to

increase

the

efficiency

of electric

generation,

especially

when

burning coal or natural gas. MHD dynamos are the complement of MHD propulsors, which have been applied to pump liquid metals and in several experimental ship engines. An MHD generator, like a conventional generator, relies on moving a conductor through a magnetic field to generate electric current. The MHD generator uses hot


conductive plasma as the moving conductor. The mechanical dynamo, in contrast, uses the motion of mechanical devices to accomplish this. MHD generators are technically practical for fossil fuels, but have been overtaken by other, less expensive technologies, such as combined cycles in which a gas turbine's or molten carbonate fuel cell's exhaust heats steam for steam turbine. Natural MHD dynamos are an active area of research in plasma physics and are of great interest to the geophysics and astrophysics communities, since the magnetic fields of the earth and sun are produced by these natural dynamos.

MHD concept

The fundamental MHD concept is illustrated in figure. The fluid conductor is typically an ionized flue gas resulting from combustion of coal or another fossil fuel. Potassium carbonate, called ‚seed,‛ is injected during the combustion process to increase fluid conductivity. The fluid temperature is typically about 2,480 C to 2,650C.


Figure: Basic magnetohydrodynamic concept

The conductive fluid flows through the magnetic fields, inducing an electric field by the Faraday effect. The electric field is orthogonal to both the fluid velocity and magnetic field vectors. As a result, a potential difference is developed between the two walls of the duch as shown in figure. The direct current (dc) generated is converted to alternating current (ac) by a solid-state inverter. CONSTRUCTION AND WORKING PRINCIPLE

The planned application of the MHD concept for utility scale electric power generation uses MHD as a topping cycle combined with a steam bottoming cycle, as shown in figure. The topping cycle consists of the coal combustor, nozzle, MHD channel, magnet, power conditioning equipment (inverter) and a diffuser. The bottoming cycle consists of a heat recovery/seed recovery unit, a particulate removal system, a steam turbine-generator system, cycle compressor,

seed

regeneration plant, and for some concepts, an oxygen p lant. The combustor burns coal to produce a uniform product gas with a high electrical

conductivity (about 10 mho/m). A typical MHD plant requires

combustion gases of about 2,650C at a pressure of 5 to 10 atmospheres. The goal is to remove a large portion (50% to 70%) of the slag (molten ash) formed in the combustion process in the combustor. High ash carryover inhabits efficient seed recovery later in the process. Oxygen enriched air is used as the oxidant to achieve high flue gas temperatures. Commercial-scale MHD plants will use superconducting magnets. Magnetic fields must be in the range of 4.5 to 6 tesla. To achieve superconducting properties, the magnets must be cooled to about 4K.


Figure: Layout of coal-fueled magnetohydrodynamics system.

In addition to converting direct current to alternating current, the power conditioning system consolidates power from the electrode pairs and controls the electric field and current. Commercial power conditioning systems will use existing linecommutated solid state inverter technology. The diffuser is the transition between the topping cycle and the bottoming cycle. The diffuser reduces the velocity of the hot gases from the MHD channel, partially converting kinetic energy into static pressure. The

heat

recovery/seed

convective heat transfer

recovery

unit

consists

of

radiative

and

surfaces to generate and super heat steam. It also

removes slag and the seed from the flue gas. In addition, the heat recovery/seed recovery preheats the oxidant supply NO2 control may be achieved within the recovery unit by a second stage of combustion. The first stage of combustion within the MHD combustor is conducted in a fuel-rich environment. The second stage of combustion within the recovery unit takes place at a temperature above 1.540C with a residence time and cooling rate such that NOX decomposes into N2 and O2. Control of SOX is intrinsic with the removal of the potassium seed from the flue gas. The potassium seed combines with the sulfur to form potassium sulfate, which condenses and is removed downstream by the particulate removal system. The recovered potassium sulfate is converted to potassium seed


in the seed regeneration unit. Thermo Electric Conversion System:

The quest for a reliable, silent, energy converted with no moving parts that transforms heat to electrical power has led engineers to reconsider a set of phenomena called the Thermoelectric effects. These effects, known for over a hundred years, have permitted the development of small, self contained electrical power sources

Seebeck (thermoelectric) effect: The German Scientist Seebeck (in 1822) discovered that if two dissimilar material are joined to form a loop and the two junctions maintained at different temperatures, and e.m.f will be set up around the loop. The magnitude of e.m.f. will be E = T where T is the temperature difference between the two junctions and



is the Seebeck co-efficient.

This effect has long been used in thermocouples to measure temperatures

THERMO ELECTRIC AND THERMIONIC POWER GENERATORS:

Thermoelectric generators (also called See beck generators) are devices which convert heat (temperature differences) directly into electrical energy, using a phenomenon called the "See beck effect" (or "thermoelectric effect"). Their typical efficiencies are around 5-8%. Older See beck-based devices used bimetallic junctions and were bulky. More

recent

devices

use

semiconductor p-n

junctions

made

from bismuth

telluride (Bi2Te3), lead telluride (PbTe), calcium manganese oxide, or combinations there of, depending on temperature. These are solid state devices and unlike dynamos have no moving parts, with the occasional exception of a fan or pump. Radioisotope

thermoelectric

generators can

provide

electric

power

for

spacecraft. Automotive thermoelectric generators are proposed to recover usable energy from automobile waste heat. A thermionic

converter consists

of

a

hot

electrode

which thermionically

emits electrons over a potential energy barrier to a cooler electrode, producing a useful

Ee2252-Power Plant Engineering

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