1.
INTRODUCTION
1.1
DEFINITION A steam power plant converts the chemical energy of the fossil
fuel into the heat energy and then to the mechanical energy. Raising the temperature and then expanding it in turbine achieve this. There are two following purpose by the in the power plants. 1.
To produce electric power.
2.
To prod produc uce e ste steam am for for ind indus ustr tria iall app appli lica cati tion ons. s.
Central Stations: The electricity generated is meant for general sale and it can be purchased. Generally these stations are condensing type. This is done in which steam is expanded to generate power. Industrial stations: The manufacturing companies run these types and the power thus generated is utilized for their own purposes. Normally these are of condensing type and the steam produced is sent into the atmosphere directly directly without without condensi condensing. ng. In condens condensing ing type there are followin following g advantages:
The amount of energy extracted per Kg of steam is increased.
The condensed steam can again be re-circulated to the boiler with the help of pumps.
1.2 1.2 LAYO LAYOUT UT OF POW POWER PLAN PLANT T The layout of steam power plant has the following four main Circuits and they are 1.
Coal and Ash circuit
2.
Feed water and flow circuit
3.
Air and Gas circuit
4.
Cooling water ci circuit
1
2
1.2.1 2.1
Coal Co al and Ash Ci Circui rcuit t
The coal is supplied to the mills with the help of belt conveyers loaded with bucket elevators. The coal then flows from mills to boilers in the the pulv pulver eris ised ed form form with with the the prim primar ary y and and is made made to burn burn and and releases heat. Ash is formed as bottom ash and fly ash. The bottom ash is only 20% remaining is fly ash and is collected in the precipitator.
1.2. 1.2.2 2
Feed Feed Water ater and and Circ Circui uit t
Water is supplied from the de-mineralizing de-mineralizing plant to the boilers for the generation of power and this steam is sent turbines to expand and produce work out-put. The steam expanded is condensed in the condenser and then make up water is added to it to compensate the evaporative losses. losses. The circulation of water is due to the difference in the density. The The stea steam m is pass passed ed thro throug ugh h the the supe superr heat heater er to incr increa ease se the the temperature of steam and to give more work.
1.2.3 2.3
Air Air an and Gas Gas Circ Circui uit t
The fresh air from atmosphere is forced in to furnace by means of a forced draught fan and it is heated in air pre-heater. In air preheater the heat is transferred from flue gas to the air, and the flue gases formed are made to pass through the super heater, economiser and air pre-heater to transfer the heat to water and primary air. At last flue gases pass through the chimney.
1.2. 1.2.4 4
Cool Co olin ing g Wat Water er Circ Circui uit t
The cooling water is used to cool the components of steam power plant and this water is circulated from a natural source like a lake, river if there is plenty of water. Normally the cooling towers are used to cool the components. And at RTPP one cooling tower for each unit is used to supply the cooling water to the 4 x 210 MW plant.
3
1.3
ENERGY CONVERTION This part of the lesson outlines the various conversion process
which are carried out a power station indicating, where possible, the efficiency of the conversion process or in other words, indicating how successful the operation is of converting one form of energy into another. Remember, energy cannot be destroyed, but its conversion incurs difficulties, which result in not all the energy being usefully converted.
1.3.1
1.
Chemical energy to the heat energy
2.
Heat energy to mechanical energy
3.
Mechanical energy to electrical energy
Chemical energy to the Heat energy Briefly, the process is as follows. Chemical energy stored in coal
is converted into heat energy by the process known as combustion. The carbon and other combustion elements in coal are made to unite with oxygen of the air in the furnaces of boilers. This process converts chemical energy into heat energy. 1.3.2
Heat energy to Mechanical energy The superheated steam passes to the turbine, which is an engine
designed to convert the heat energy in the steam to mechanical energy in rotary motion. To do this, steam is admitted at high pressure and temperature and as it passes through the turbine its heat is taken out and converted to mechanical energy. It is however only possible to extract the heat energy while steam remains as a vapour. Remember, as it losses heat the steam will revert back to water, and this is harmful to the blades of turbine. Thus the modern highly efficient turbine accepts steam at a high pressure and temperature as the metal of the blades withstand in this stage is highly charged with heat energy, and gives up heat right down
4
to the point at which the steam cannot remain a vapour any longer but is reverting to water. The lowest point at which the steam remains as a vapour dictates the lower limit. Earlier it was said that the boiling point of water was dependent upon pressure; the higher the pressure, the higher the boiling point. However, the opposite is true also, the lower the pressure, the lower the boiling point. If then the pressure at the end of the turbine can be reduced, the steam can be kept as a vapour for a long period. The boiling point of water is only 101.7 0 F at 1 lb/in 2. Thus, if the pressure at the outlet end of the turbine can be reduced to 1 lb/in2 steam will remain as a vapour from the super heated inlet temperature at which it was delivered from the boiler house right down to 1100 F before changing to water. By this means, the steam turbine can extract heat right down to the saturation temperature corresponding to the absolute pressure at the back end or outlet of the turbine. Further more, turbine designers, in an endeavourer to get everything possible from the steam, extract even more heat at the expanse of marking the steam wet. However as water droplets are detrimental to the turbine blades there is a limit to the amount of wetness that can be tolerated, in fact about 1/10 th. Engineers speak of this point 10% wetness, or 90% dryness of steam, and these are one and the same point.
1.3.3
Mechanical Energy To Electrical Energy
The rotating shaft of the generator converts the heat energy, which is successfully converted into mechanical energy. This is highly efficient machine for converting the mechanical energy into electrical energy for the use of the generating board’s consumer in the form of light, heat or power.
5
6
1.4 ABOUT RAYALASEEMA THERMAL POWER PLANT (R.T.P.P)
1.4.1 LOCATION: The Rayalaseema Thermal Power Plant (RTPP) is located at Kalamalla village that is 12 km from Proddatur in Kadapa Dist. RTPP is spread in a wide area of about 2800 acres. Rayalaseema thermal power plant is one of the major power generation facilities developed in Andhra Pradesh to meet the growing demand for power. The project envisages the installation of 2x210 MW thermal generating units under stage-1. Recently one more 2x210MW unit is incorporated under stage-2 taking ultimate capacity of the project to 840MW. The cost of project is Rs.850 crores for stage-I (unit 1&2) and Rs.1600 crores for stage-II (Unit 3&4). Further erection of another 210 MW (Unit 5), and 600MW (Unit 6) is planned for next three years. Rayalaseema is in the southern part of the state, most of the generating facilities are in southern part except two major hydel power stations in the central part. The Rayalaseema gets its power needs through EHT lines and frequently faces low voltage problem particularly during summer when the hydro power station generations goes down. The area is a draught prone area and has to depend on industrial growth for its economic development. Priority is therefore given for industrial development and power being the basic infrastructure, it is necessary to ensure proper power supplies. It is in this context that Rayalaseema thermal project is taken up for not only to improve the
7
base load thermal capacity of the grid but also to ensure proper voltage profile in the area under all conditions.
1.4.2. AVAILABILITY OF COAL: Steam power plant using coal as fuel requires huge amounts of coal per annum. Normally a plant of 210MW capacity needs 3300 to 3500 tons of coal per day. And this coal supplied continuously to have constant generation of power. So the coal must be stored for a certain period. Here at RTPP the coal is stored for 10 days. The required coal is supplied from Singareni mines, Manugoor and Talcher mines. The coal is downloaded from the rail wagons directly with the help of wagon tipplers and from there they are transported in the conveyors to the storage yard. 1.4.3. AVAILABILITY OF WATER: The requirement of water is obtained from Mylavaram Reservoir, this is on the Penna River. The flow is sufficient to meet the requirement through out the year for the project. The requirement is being released through a 21 km long steel pipeline. The total requirement of water is 80 cusecs/day (For 4 X 210 MW). Water is the working fluid in all the steam power plants. There are some evaporative losses in the steam power plant and this will be compensated by adding make up water. The condensers also need water and this water is taken from the ponds constructed near the boiler. The water needs no treatment. But the water used for the generation of steam needs treatment. The water is treated in the De-mineralizing plant.
8
1.4.4. TRANSPORTATION FACILITIES: Transportation facilities also play a major role in the site selection of a steam power plant. RTPP has got both road and rail facilities.
1.4.5. ASH DISPOSAL FACILITIES: The ash coming out of the plant is hot and corrosive and there is a problem of handling the ash. If it is sent into the atmosphere it will affect the human life. So the fly ash formed is collected in the ESP’s and the impurities are also deposited. The bottom ash is collected in the hoppers provided at the bottom of the boilers. The hydraulic ash disposal system is used to send this out.
1.5 THE PROCESS AND EQUIPMENT Clarified water
DM water
1. Ph
7.4 – 7.9
6.8 – 7.2
2.SIO2
12 ppm
< 10 ppm
3. Micro organisms
>500 micro Siemens
>1 micro Siemens
4. Purpose
Used for drinking,
For Steam
cooling water
generation
Coal is fed to the crusher whom it is crushed into a size of 20mm. The coal stored in the coal yard is taken to the plant by means
9
of conveyor belts at the same rate. The coal is fed into the conveyor belts with the help of stacker reclaimer. The coal that is fed to the burner should be in the powdered form, but the coal in the coal in the coalbunkers is of granular size. Therefore, coal that is fed to the burner should be powdered and the mills accomplish this process. The coal from bunker is fed to the mill via feeder, in order to avoid the coal to be jammed in the mill. The function of feeder is to control the flow of coal to pulverize, to meet the load demand. The coal flows onto the moving belt and the speed of the belt is varied to control
the
coal
flow.
The
coal
allowed
falling
into
mills
for
pulverization. In mill the coal from each feeder is ground into fine power in the pulverizer and carries into the coal burners by primary air fans. The PA fan is used to dry the coal by mixing the box that is located between feeder and mill. The pulverize coal –air mixture, while moving to the burners is allowed to pass through the classifier, which filters out any waste materials, heavy coal pieces and are again sent back to the mill. The pulverized coal and primary air mixtures conveyed to the burners by coal piping. The burner arrangement is arranged inside the boiler furnace. Here, we have tangential fired furnaces to which fire is given from four corners of the furnace. The combustion of fuel inside the furnace gives out heat gives out heat energy is used to convert the water into steam where the water tubes are arranged to the walls of furnace. The heating up of boiler feed water is done by the economiser by absorbing the heat energy from the gases and is fed to the boiler through the boiler feed pumps. Primary air required for transferring the coal powder to the burners and secondary air, which is supplied to the wind box are heated up by using the heat energy from flue gases. Then the flue gases are directed towards the chimney with the help of induced draught fans. These fans exhaust combustion products
10
from the steam generators. The flue gas temperature is maintained at 1400 C. The combustion of fuel inside the furnace results in two products:
Water to steam conversion
Combustion waste
After this the process is carried out in two parallel paths:
Ash disposal
Power generation
The ash is disposed as bottom ash, wet ash and dry ash. The power generation path includes boiler-turbine generator. generator. The power power gener generati ation on proce process ss is by the modif modified ied sub-c sub-crit ritica icall Ranking cycle with one re-heat, five regenerative feed water-heating stages (2 HP heaters, 3 LP heaters and one dearator). Besides there is grand steam condenser. The boile boilerr of single single pass pass tower tower type type with with draina drainable ble ho horiz rizont ontal al heat heatin ing g eleme lement nts, s, limi limiti ting ng the the flue flue gas gas velo veloci citi ties es to 6-8 6-8 m/s m/s to minimize erosion of pressure parts due to use of high ash (45-60%). The capacity of each boiler is 690 T/hr of steam at a pressure of 155Kg/cm2 and 5400 C. The coal is pulverized in three horizontal tube mills each having capacity of 105 T/hr, two is in service for one unit and one is standby mode. To achieve total pollution control, 6 field ESP having a capacity of 13,82,000 m2 /hr and 99.89% efficiency are installed. Induced draught system incorporates incorporates 2 radial ID fans. The height of chimney is 220m. The The stea steam m turb turbin ines es are are 3-cy 3-cyli lind nder er reac reacti tion on type type turb turbin ines es.. Microprocessor Microprocessor based automatic turbine run up system is envisaged. The generators have hydrogen cooled stator coils and they are rate rated d for for 210M 210MW W with with a term termin inal al vo volt ltag age e of 15.7 15.75K 5KV. V. Fast Fast ac acti ting ng brus brushe hes s type type exci excita tati tion on syst system em is envi envisa sage ged d to main mainta tain in stea steady dy generator terminal voltage under variable load conditions.
11
The
generator
transformers
are
2nos
and
of
240M.V.A,15.75/220KV three phase unit step-up transformers one for each unit are installed for evacuation of power at 220KV.
1.6 Details of Power Generation and Consumption of Water and Coal Year
2000-2001
Consumption Consumption Power of Coal of Water Generated (MT) (MT) (MU) 2800010 273942 3475.38
2001-2002
2371652
249587
3400.80
2002-2003
2292237
253771
3488.82
2003-2004
2246752
201012
3401.58
2004-2005
2149622
206170
3353.78
2005-2006
1519053
190547
2370.99
2006-2007
2225399
214382
3293.76
2007-2008
2222250
210254
3146.89
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2.
LITERATURE RE REVIVE
2.1
CARNOT CYCLE
13
This is the ideal cycle used in heat engines; with the efficiency is compared. Fig. Shows a Carnot cycle on T-s and p-V diagrams. It consists; 1. Two constant pressure operations (4-1) and (1-2) 2. Two constant temperature operations (2-3) and (3-4)
Fig 2.1 Carnot Cycle on T-s and p-V diagrams Process (4-1): 1Kg of boiling water at temp T 1 and is heated to form wet steam of dryness fraction x 1. Thus heat is absorbed at constant temp T1 and pressure P 1 during this process. Process
(1-2):
During
this
operation
steam
is
expanded
is
entropically to temp T2 and pressure P 2. The point 2 represents the condition of steam after expansion. Process(2-3): During this operation steam is rejected at constant pressure P2 and Temp T2. As steam is exhausted it becomes wetter and cooled from 2-3. Process(3-4): During this operation the wet steam at 3 is compressed is entropically till the regains its original state of temp T1 and pressure P1. Thus the cycle is completed
14
From T-S diagrams: Heat supplied at constant temperature=T1 (S1-S4) or T1(S2-S3) Heat rejected at constant temperature= T 2 (S1-S3) Network done=Heat supplied-Heat rejected= T 1 (S1-S3)- T2(S2-S3) = (T1- T2)*(S2-S3) Carnot cycle Efficiency=Work done/Heat supplied =
=
(T1- T2)*(S2-S3) ____________ T1 (S2-S3) (T1- T2) ______ T1
2.2 RANKINE CYCLE Rankine cycle is the theoretical cycle on which the steam turbine works. The Rankine cycle is shown in figure .It comprises of following process:
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Fig 2.2 (a) p-V Diagram (b) T-s Diagram (c) h-s Diagram for Rankine cycle Process 1-2: Reversible adiabatic expansion in turbine. Process 2-3: Constant pressure transfer of heat in the condenser. Process 3-4: Reversible adiabatic pumping process in the feed pump. Process4-1: Constant pressure transfer of heat in the boiler.
Fig shows the Rankine cycle on p-V, T-s and h-s diagrams. Considering one Kg of fluid & applying steady flow energy equation to boiler, turbine, condenser and pump. η = h1 - h2 h1-hf4
The Rankine cycle efficiency can be improved by:
•
Increasing the average temperature at which heat is supplied.
16
•
Decreasing or reducing the temperature the conditions of steam at which heat rejected.
This can be achieved by making suitable changes in generation or Condensation, as discussed bellow:
1.
Increasing boiler pressure
2.
Superheating
3.
Reducing condenser pressure
The thermal efficiency of the Rankine cycle is also improved by following methods: •
By regenerative feed heating
•
By reheating of
•
By water extraction
•
By using binary-vapour
Calculation of Efficiency: In order to compare the energy contained in the raw material coal, with the energy as the end product (electricity) it is necessary to have some common unit. Coal can be measured by weight in tons, a smaller unit of weight is lb and 2,240Ibs equal 1 ton. Electricity is measured by board of trade units or “Kilowatt hour” and before assessment of the total or overall efficiency of the convertion process in a power station can be calculated, it is necessary to reduce both the coal burned and the electricity generated to a common unit. And for this British Thermal Unit (BTU) is used The station chemist takes careful samples of the theoretical quantity of heat in BTU per lb contained in the coal. Thus by multiplying the heat input to the station in BTU calculated.
17
2.3
Comparison between Carnot and Rankine cycle 1. Between the some temperature limits Rankine cycle provides a higher specific work output than a Carnot cycle. Rankine cycle requites a smaller steam flow rate resulting in smaller size plant for given power output. 2. Rankine cycle efficiency is lower than that of Carnot cycle.
Fig .2.3Comparison between Carnot and Rankine cycle
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19
3.1 DEFINATION OF STEAM GENERATOR Steam Boiler or generator is defined as a closed vessel in which
the water
is converted
into the
steam
required
pressure and
temperature. Or
20
Steam generator popularly known as boiler made of high quality steel in which steam is generated from water by application of heat.
3.2 STEAM GENERATION 1.
SENSIBLE AND LATENT HEAT In order to understand fully the process of converting water into
steam the simple experiment will be followed by boiling a pound of water (which is a little under a point) in a kettle & observing the various effects of the heat by inserting a thermometer in the water & taking readings periodically as heat is applied. Assume that the temperature of the pound of water as supplied from the tap is 600 F now if heat energy were added to the water the temperature would indicated one of the first effects of this heat, the temperature would rise steadily. The rise would continue until the water reached boiling point when the thermometer would register 212 0 F the water would now be boiling & steam be given off. As more heat was added, it would be noticed that the thermometer reading did not increase but remained at 2120 F the heat now no longer causes the water to get hotter but has another effects. That of changing the state of the water from a liquid to a vapor steam. Thus the heat supplied to the kettle has done two jobs. Firstly, It has raised the temperature of water from 60 0F, to 212 0F that is by 1520F. This change of temperature has been “sensed” by the thermometer & the mercury has risen from the 60 0F point to the 212 0F point. The amount of heat required to do this first part of the job is
21
called “sensible” heat since it is the readily detected by the sence of sight in using the thermometer, or the sence of touch if hand were dipped in the water. (This quantity of heat of heat is spoken of
22
sometimes as “liquid heat” since it is the heat added to water in the truly liquid state). The second job is to convert the boiling water at 212 0F into steam at 2120F this has been done without any rise in temperature & the thermometer will not detect it. The change from water to steam, although not visible, has required quite a lot of heat energy. This undetected heat is called “latent” heat. Thus, the total heat used in converging the 1 Ib of tap water in the kettle into steam consisted of the Sensible Heat which raised the water from 600F to 2312 0F the Latent Heat which did not raise the temperature at all but changed the state from water into steam. Quantity of sensible heat is the weight multiplied by the temperature rise that is:1 Ib X (212-60) 0F Or 1X152=152 Btu Thus, the sensible Heat is 152 Btu. Experiments have proved that the additional quantity of heat which must be added to the 1Ib of water at 22 0F to change its state into 1Ib of same temperature is considerably more: it is 970Btu. Thus latent Heat =970 Btu. Since Total Heat=Sensible heat + latent heat Then the total heat given to the water=152+970 =1,122 Btu. Although our example takes water at 60
0
F it should be clearly
understood that sensible the boiling point of the water which will vary with the pressure.
SATURATED STEAM The word “saturated’’ in this instance, however, indicates that the steam is saturated with heat & no moisture is present. It is called “Dry saturated steam”.
23
From the simple example of the kettle take a slightly more scientific example. Fig. represents a steel container about full of water. It is equipped with a thermometer for measuring the temperature, a Bourdon gauge for measuring pressure, a pressure regulating value
FORMATION OF STEAM IN A BOILER which can be adjusted & a safety valve. Note that this is set up for experimental purpose, power station boilers don’t have their pressure regulated in this way. Now place this equipment over a gentle source of heat and carries our similar experiment to that carried out with the kettle. In order to explain what is going on inside the steel vessel assume that it is provided with a glass window so that the water and the space above it can be observed the test.
24
At first the pressure regulating value will be set in the open position so that the water is exposed to atmospheric pressure only. As heat is added the temperature recorded by the thermometer will rise until at 212 0 F the water beings to boil. Looking through the glass window at the surface of the water it would be seen that has the bubbles of steam break through, they carry with them small droplets of water and in the space immediately above the water would be a layer of steam which held entrained finely divided particles of water not yet fully converted into steam. Although these particles of water have received a full quota of sensible heat and are at temperature of 212 0 F they have not yet taken up enough latent heat to turn them into steam. On the other hand, the steam in which they are carried has received the required amount of both sensible and latent heat. This layer of steam which contains fine particles of water is called wet steam. Assuming that the water is boiling gently, this layer of wet steam would remain fairly close to the water. Above this layer would be steam which contains no fine particles of water. This is steam which has received all water has been fully converted into steam. Steam in this state is called saturated steam because it is in fact, saturated which the full quota of latent heat and is fully formed steam. Not specially that steam which contains moisture is not saturated steam but wet steam because it is not fully formed or heat saturated. It is important to note, also that wet steam does not exist ONLY in association which a water surface; for example, wet steam can also occur in the final stages of the L.P .turbine, as described later in this lesson.
25
3.3 CLASSIFICATION OF BOILERS The boiler is classified as follows: 1. Depending upon the relative position of the hot gases. a. Fire tube boilers b. Water tube boilers 2. Depending upon the pressure of the steam. a. High pressure boiler b. Low pressure boiler 3. Depending upon method of firing. a. Internally fired boiler b. Externally fired boiler 4. Depending upon the method of circulation of water. a. Natural circulation b. Forced circulation 5. Depending upon the nature of draught a. Natural draught b. Artificial draught
FIRE TUBE BOILER: Savory watt and newcomen engines all operated at pressures only slightly above atmospheric pressure. In 1800 the American inventor Oliver Evans built a high pressure steam engine utilizing a forerunner of the fire tube boiler. Evan’s consists of two cylindrical shells; one inside the other, water occupied the region between the shells. The fire grate and flue were housed inside the inner cylinder, permitting a rapid increase in steam pressure. Simultaneously but independently, the British engineer Richard Trevithick developed a similar “Cornish” boiler. The first major improvement over Evans and Trevithick’s boilers was the fire tube “Lancashire Boiler” patented in 1865 by the British engineer Sir William Fairbairn, in which hot
26
combustion gages were passed through tubes inserted into the water container, increasing the surface area through which heat could be transferred. Fire tube boilers were limited in capacity and pressure, and were also some times, dangerously explosive.
WATER TUBE BOILER: Boiler pressures, however, remained limited until the first successful design of water tube boiler, patented in 1867 by the American inventors George Herman Babcock and Stephen Wilcox. In water tube boiler, water flowed through tubes heated externally by combustion gases, and steam was collected above the drum.
This
arrangement used both the conversion heat of the gases and the radiant heat from the fire and the boiler walls. Wide application of the water tube boiler become possible in the 20 th century with such developments as high temperature steel alloys and modern welding techniques, which made the water tube boiler the standard type for all large boilers. Modern water tube boilers can operate at pressure in excess and generate more than 9 million Ib of steam per hour. Because combustion temperatures may exceed 3000 0K, the water flow is controlled by natural or forced circulation. By using super heaters modern boilers can achieve almost 90% efficiency.
3.4 BOILER MOUNTINGS AND ACCESSORIES: MOUNTINGS These are different fittings and devices that are necessary for the operation and safety of boiler. Usually these are mounted over boiler shell. Some of them are: 1.
Safety valve
2.
Water level indicator
3.
Pressure gauge
27
4.
Feed check valve
5.
Blow off cock
6.
Fusible plug
7.
Stop valve
8.
Man hole
9.
Mud holes or sight holes •
Water level indicator: The function of this is to indicate the level of water in
boiler. It is also called as water gauge. •
Safety valve: The function is to release the excess steam when the
pressure of the steam inside the boiler exceed to rated pressure. •
Pressure gauge: The function is this to measure the pressure exerted
inside the vessel. •
Feed Check Valve: To control the supply of water to the boiler and to
prevent the escaping of water from the boiler when the pump pressure is less or pump is stopped. •
Blow off cock:
The functions are: 1.
It may discharge a portion of water when the boiler is in operation to blow out mud, scale sediments periodically.
2.
It may empty the boiler when necessary for cleaning, inspection and repair. •
Fusible plug: The function of fusible plug is to protect the boiler
against the damage due to overheating for low water level. •
Steam stop valve: To
regulate the flow of steam from one pipe to the
28
other (or) from boiler to the steam pipe.
ACCESSORIES: These are auxiliary parts required from steam boiler for their proper operation and for increase the efficiency. Commonly used accessories are:
Feed pumps
Economiser
Air pre-heater
Super heater
Steam separator
1. Feed pump: The function of feed pump is to force feed water into the boiler drum at high pressure. 2. Economizer: In this waste heat of flue gases is utilized for heating the feed water. It is placed in between boiler furnace and air heater. 3. Air preheater: The function of air pre heater is to increase the temp of air, this enters the furnace. It is placed between economiser and chimney. 4. Super heater: The function of super heater is to increase the temp of steam above its saturation temp by utilizing the heat of flue gases. It is placed in the path of flue gases.
3.5 PERFORMANCE OF BOILERS The performance of boiler gives the efficiency of boiler and thus we can calculate the amount of input to the system and output. The following parameters are used to represent the performance of boilers. They are; 1.
Evaporative capacity:
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Evaporative capacity of boiler is the amount of steam generated per hour. It is represented in following units.
Kg of steam/hr
Kg of steam/hr/m 2
Kg of steam/Kg of fuel fired
From this we can calculate the amount of coal used per hour and we can store the amount of coal for a particular period. If the amount of steam generated is known then we can calculate the quantity of water to be supplied in order to maintain the constant level in the boiler to prevent any damages to the boiler components.
2.
Equivalent Evaporation: The amount of steam generated for different boilers is different
and this is not useful in comparing two boilers. To serve that purpose the
term
equivalent
has
been
introduced.
The
pressure
and
temperatures are also different for different boilers. It is imperative to define all the boilers on a common base. The standard condition adopted in this is temperature of feed water at 100 0C the amount of energy needed to water is 225 KJ. The term equivalent evaporation is defined as the amount of water evaporated from water at 100 0C to steam at 1000 C.
3.
Factor of Evaporation: The ratio of heat received by 1 Kg of water under working
conditions to that received by 1 Kg of water evaporated from and at 1000C.
4.
Boiler Efficiency: The ratio of heat actually utilized in generation of steam to
that heat supplied by the fuel in the same period.
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4.1 PULVERISED FUEL HANDLING SYSTEAM Two methods are in general use to feed the pulverized fuel in to the combustion chamber of the power plant. 1. Unit system 2. Central or Bin system The Bin & feeder method was an early form of pulverized fuel firing. Coal was pulverized & stored in a hopper ready to be blown in to the furnace. This pulverized fuel is then distributed to each furnace with the help of high pressure air current. The Unit system, the present
day method of
firing
was
introduced with the improvement of milling plant. Pulverized coal no longer be stored in hopper but can be produced in sufficient quantities by passing raw coal directly through the mill to furnace.
4.2
MILLS A mill is one that grinds down pieces of coal into fine powder,
which is fed to the boiler furnace. The type of mill used in RTPP is Ball mills in Stage-I and Bowl mills in Stage-II.
4.3
ASH HANDLING SYSTEM. The modern ash handling systems are mainly classified into
four groups, they are 1. Mechanical handling system 2. Hydraulic system 3. Pneumatic system 4. Steam Jet system
31
4.4.
ELECTROSTATIC PRECIPIRATORES:
The equipment was first introduced by Dr. F.G. Cottel in 1906 & was first economically used in 1937 for the removal of dust & ash particles carried with the exhaust gases of the thermal power plant. The electrostatic precipratore are extensively used in removal of fly ash. An electrostatic precipratore can be designed to operate at desire efficiency for use as a primary collector or as supplementary unit to a cyclone collector. The dust-laden gas is passed between appositively charged conductors & it becomes ionized as the voltage applied between the conductors is sufficiently large (30000-60000 volts dependent on electrode spacing). As the dust laden gas is passed through this highly charged electrodes, both negative & positive ions are formed, the later being as high as 80% the ionized gas is further passed through the collecting unit which consists of a set of vertical metal plates alternate plates are positively charged & earthen.
As the alternate plates are
grounded, high intensity electrostatic field developed between the plates. When the field is charged, dust particles are passed between the plates the electrostatic field exerts a force on positively charged dust particles & drives them to the ground plates. The deposited dust particles are removed from the plates with the help of shaking motion,
32
collected in the hopper. A care should be taken that dust collected in the hopper should not be entrained in the clean gas.
4.4.1. ADVANTAGES OF ESP: 1. This is more effective to remove very small particles like smoke, mist & fly ash. Range of dust removal is sufficiently large (0.01 meu1.00 meu). These small dust particles below 10 meu cannot be removed with the help of mechanical Separators & scrubbers. 2. This is also most effective for high dust loaded gas. Its efficiency is as high as 99.5%. 3. The draught loss of this separator is least of all forms. ( 1cm of water). 4. The maintenance charges are least among all separators. 5. It provides ease of operation. 6. The dust is collected in dry form &can be removed either Dry / wet.
4.4.2. DISADVANGES OF ESP: 1. The running charges are considerably high as the amount of power required for charging is considerably large. 2. This space required is large than wet system. 3. The efficiency of the collector is not maintained if the gas velocity exceeds than for which the plants is designed. The dust carried with the gas increases with an increase of the gas velocity. The efficiencies decreases from 100 to 80% when the gas flow increases from 1000m3 /min- 60*m3 /min.
The annual generation of fly ash from thermal power plants of APGENCO is presently around 6.8 million tons/annum. APGENCO has been implementing following measures:
1. Dry fly ash is issued free of cost to all consumers. 2. Brick plants are set up VTPS, KTPS & RTPP. 3. Research & field tests are underway through Acharya NG Ranga Agricultural University
(ANGARU) & Department of Agricultural, Govt.
33
of AP to establish benefits with fly ash use for agriculture for all crops in the different agro-climate zones of Andhra Pradesh.
4.5 BURNERS Primary air that carries the powdered coal from mill to the furnace is only about 20% of total air needed for combustion. Before coal enters the furnace it can be mixed with additional air called secondary air. Air burners mounted on the furnace wall. In addition to the prime function of milling, burners also maintain stable ignition of fuel-air mix & travel in the furnace.
Requirements of a burner 1. Coal& air should be handled so that there is stability of ignition. 2. Combustuition is complete. 3. In the flame, the heat is uniformly developed avoiding any superheat spots. 4. Adequate protection against overheating, internal fires excessive abrasive wear.
Pulverized fuel burners: Pulverized fuel burners may be classified as follows: 1.
Long flame burners
2.
Turbulent burners
3.
Tangential burners
4.
Cyclone burners
The one used at RTPP is a tangential burner. These are set as shown in this case. Four burners are
located in the 4 corners of the
furnace & are fired in such a way that four flames are tangential to an imaginary circle formed at the center. The swirling action produces
34
adequate turbulence in the furnace to complete the combustion in a short period & avoids the necessity of producing high turbulence at the burner it self.
Tangential burners serve the following purposes: 1. Parts of burners are well protected. 2. Operation is simple. 3. High heat release with complete & effective utilization of furnace volume is possible. 4. Completeness of combustion
is good
& maximum
degree of turbulence exists throughout the furnace. Good combustion means: 1. Stable combustion. 2. Non-flickering & Non- pulsating flame. 3. Does not require oil support. 4. High efficiency i.e. consuming mechanical &chemical UN burnt. 5. Least erosion & Boiler tube failure. The good combustion is recognized by colour of the flame observed through peepholes. Colour
----------pale orange white coal on firing.
Flame
----------300-400mm away from burner top.
Flame temperature-1050-1150c (measured by optical pyrometer)
4.6 COLLING TOWERS: The cooling towers are used in many applications is engineering. The main applications are in power plants & refrigeration plants. FUNTIONS OF COLLING TOWER: To the hot water from condenser by expanding it to the atmosphere air so that cold water may be used again for circulation, the cooling towers are used in steam power plants where there is
35
limited supply of cooling tower. It is placed at a certain height (at about 9m from the ground level). The hot water falls down in radial sprays from height & atmosphere air enters from the base of the tower. The partial evaporation of water takes place, which reduces the temperature of circulating water. This cooled water is collected in the pond at the base of the tower & pumped into the condenser. Factors affecting the cooling water in cooling tower: 1. Size & height of cooling tower. 2. Arrangement of plates in cooling tower. 3. Temperature of air entering the cooling tower. 4. Humidity of air. 5. Temperature of air. 6. Accessibility of air to all the parts cooling towers. Types of cooling towers. According to type of draught 1. Natural draught 2. Forced draught In natural draught cooling to tower circulation of air produced by the pressure difference of air inside & out side the cooling tower. In forced Draught cooling tower circulation of air produced by means of fans placed at the base of the tower.
4.7 FANS Fans are provided through out the steam generating unit to supply air on to exhaust flue gas to meet the needs of various systems. In addition, fans are used for building heat and cold to prevent contamination of equipment from lubricating. The fans used in steam power plant are
Forced draught fan
Primary air fan
36
Induced draught fan
FD fans supply combustion air to the steam generator. PA fans normally
handle
relatively low
flow
air
at
very high pressure
differentials. ID fan exhaust combustion products from the steam generator.
4.8 TURBINE The turbine function is to convert thermal energy to mechanical energy. Reaction turbines are used in RTPP. The major parts of stationary turbine are: 1. High pressure cylinder 2. Intermediate pressure cylinder 3. Low pressure cylinder Valves are used to give up protection for turbines. The primary function of valves is to regulate the steam flow. The generator generates electricity on the principle of law of electro magnetism.
4.9 EXCITING SYSTEAMS The purpose of all exciting systems is to provide Direct Current to rotating electro magnetic field. The three types of excitation systems are: 1. Stationary transformer-is used when supply is taken from grid and it is used for plant. 2. Power transformer-is used to step up the generated voltage. 3. Unit auxiliary transformer- is used to supply power to the auxiliaries.
37
5.1 ECONOMISER FUNCTION An economizer is a device which extracts a part of heat from the flue gases and uses for heating the feed water. IMPORTANCE OF ECONOMISERS 1.
Use of economizer increases the boiler efficiency, thereby overall efficiency of the plant increases. For every 6 0 C increase in temperature of feed water, boiler efficiency increases by 1%.
2.
The heat of the flue gases which are being wasted can be used effectively.
3.
By using the heat of the flue gases in economizer, the fuel supply to the boiler can be decreased.
DESCRIPTION: The economizer is a feed water heater deriving heat from the flue gases discharged from the boiler. The greatest item in a boiler is the heat carried away by the flue of the chimney or stack. Some of heat being carried away by the flue gases may be recovered and sent back to the boiler. In the economizer if the path of feed water placed in the path of the flue gases in between from the boiler and enters to the chimney, the heat from the flue gases is transferred to the feed water. It has been found that a rise in temperature of feed water by 6 o C improves the boiler efficiency by 1%. LOCATION: Economizer is generally placed between the Reheater and the Air pre heater. In some cases a low temperature economizer is placed
38
after the air the air pre heater, such an economizer is called stack cooler and act as a low pressure feed water heater expect that the heating system is the flue gases instead of steam bud to the turbine.
39
CLASSIFCATION The economizers are into following categories as: 1. Steaming Economizer 2. Non Steaming Economizer Steaming Economizer:In this type of economizer water temperature is raised to the boiling point and partially (10-20%) evaporates. Non Steaming Economizer:In this type of economizer the temperature of water is below the boiling point and feed water passes this section before going to steaming economizer. CONSTRUCTIONAL DETAILS:Economiser is brick work construction, consisting of a large numbers of closely spaced parallel tubes with thin walls and small diameters connected by headers or boxes in order to increase the heat transfer rate. The tubes are provided with fins. The tubes are available up to 3.25m length and having external diameter of 14 cm and thickness of 1.25cm. The tubes are hydraulically pressed into top and bottom headers. The sections are erected side in the economizer. DESIGN REQUIREMENTS The design requirements must satisfy the following conditions; 1. The heat transfer rate should be minimum. 2. It must able to extract maximum possible heat from exhaust gases.
40
3. The height of tube banks should be minimum so that cleaning on load can be done effectively. 4. It must fit dimensionally with preceding unit, usually the prime super heater. 5. There
must be ample allowance for
expansion under all
operating conditions without setting up excessive stresses in any components. 6. There must be uniform water flow to avoid steam formation in the economizer. The pressure loss of water should be minimum to reduce the running expenses of the pump. 7. There must be a connection from the steam and water drum to the economizer Inlet header to permit the free circulation of water around the economizer, to prevent the overheating and boiling during the period where there is no feed water during the early pressure raising are connected to these top and bottom pipes. Once the feed water is pumped into the bottom pipe, it goes into top pipe by passing through all pipes and absorbing heat from the flue gases which flowing from top(i.e., in bottom direction or counter flow direction). 8. The gas pressure loss should be minimum to reduce the running expense of induced draught fans.
At the top, a feed check valve is provided to control the flow of feed water from the economizer to the boiler and it controls the rate of discharge.
The
tubes
in
the
economizer
must
be
capable
of
withstanding the high pressure in the boiler system and should not cause any effects like scaling, corrosion.
TYPES OF ECONOMISERS There are three main types 1. Plain steel tube economizer.
41
2. Gilled tube economizer. 3. Fabricated steel tube economizer.
(a)
Plain tube economizer
Plain tube types are generally used in Lancashire boiler working under natural draught. The tubes are made of cast iron to resist corrosive action of the flue gases and their ends are pressed into top and bottom headers. Plain tube economizer consists of a group of tubes located in the main flue between the boiler and the chimney. The waste flue gases flow outside the economizer tubes and heat is transferred to the feed water inside the tubes. The external surface is continuously cleaned by soot scrapers moving up and down. High efficiency of economizer can be maintained by preventing the soot deposition which is bad conductor of heat. The height of each is limited to not more than 12 m to enable fuel soot blower penetration. The tube can be either in lined or staggered. Staggered tube type induces more turbulence in the gas than inline construction. This gives higher rate of heat transfer and requires less surface but higher draught.
(b)
Gilled tube type economiser
A reduction in economizer size together with increase in heat transmission can be obtained by casting rectangular grills on bare tube walls made of cast iron. Gilled economizer can be used up to 50 bar working pressure and such economizers are indigenously available. At high pressure (>50bar) steel tubes are used instead of cast iron. Economiser also may have bare or finned tubes. Bare tubes are specified for dirty fins but the use of finned tubes in high fouling fuel applications has increased significantly. Finned tubes are constructed from carbon steels and high grade resistant alloys 150 to 200 fins per meter
are
used
on
the
economizer
tubes
used
in
clean
fuel
42
applications. So fins per meter are used when dry solid fuels are used and 120 fins per meter are used when oil fuels are used. Fin thickness ranges from 0.5 mm to 5 mm. One advantage of this construction is that the gas velocity through the unit is usually sufficient to carryall the dust, but this type is may be more difficult to clean if not swept properly designed sootblowers.
(c ) Fabricated steel tubes: These come between the two having an extended surface which save space and particularly suitable for large re-heat units where space is limited.
Comparison
of
plain
tube
Economiser
with
Gilled
tube
Economiser: In
comparison
with
plain
tube
economizer,
steel
finned
economizers occupy less space for the same material, thermal efficiency and draught loss. The reduction in tube length for similar tube diameter and pitches is usually around 4 to 7. This results in similar casing less structural steel work to support the reduced weight. Fewer weights giving a saving in overall cost.
FAILURES The main kinds of failure of economizer observed at thermal Power plants are:
Tube blisters in most heavily heated section.
Water side corrosion.
Abrasion wears of tubes by ash.
Fire side sulphur content.
Cracks in welded joint of tubes.
Water leakage from economizer hoppers.
43
Prevention of failures: The corrosion and its prevention is very important for safe and efficient working of economizer. Internal corrosion and external corrosion are the major problems caused by dissolved oxygen and carbon dioxide. Internal corrosion can be prevented by following: 1. A properly designed aerator, combined with water treatment plant.
Virtually
eliminated
internal
corrosion
into
the
economizer. 2. Carbon dioxide forms carbonic acid when it dissolves with water. The H2CO3 is the only that exerts gas pressure; there CO2 must be deareated at low pH values. 3. NH gas forms NH4OH ionizes NH3+ ions and OH- ions. Therefore NH4OH is responsible for exerting gas pressure and it must be removed by deareation at higher pH values. 4. The pH values of water passing through economizer should maintain between 8 and 9 to reduce acetic acid. 5. An oxygen scavenger such as hydrazine sodium sulphate, used to provide total protection against inside pitting of tubes. External corrosion of the economizer tubes is very serious when high sulphate content wet fuels are used in boiler furnaces as the changes of forming H 2SO4 is more. Sulphurous acid is created when SO 2 is dissolved in free moisture in the flue gas. Sulphur trioxide may combine with super heated vapor to form H2SO4 at acid dew point. If low sulphur content coals are used, the chances of carrying SO 3 with gases are less and the inlet feed water temperature may much lower provided an adequate margin is held above the acid dew point.
44
External cleaning of Economizer Soot Blowers: The fuels used in steam power plants create soot and this is deposited on the boiler tubes, economizer tubes and air preheater. The soot deposited on these heat exchangers drastically reduces heat transfers. Soot blowers control the build up to soot and shy deposits that create corrosive environment. Economizer soot blowers are of several types: 1. Multi-nozzle non retractable soot blowers. 2. Long-lance rotating soot blowers. 3. Semi or port retractable soot blowers. 4. Rake soot blowers. Out of above four types of soot blowers the most efficient is semi or part retractable soot blowers and is extensively used in economizer. Again these semi or part soot blowers are classified into various types depending upon the width of economizer bank. 1. One-half retractable soot blower 2. One-quarter retractable soot blower. 3. One-sixth retractable soot blower. Advantages: 1. Overall efficiency of the plant can be increased. 2. Evaporative
capacity
of
the
boiler
capacity
can
be
increased. 3. If the boiler is fed with cold water it may result in chiller the boiler metal hence hot feed water checks it. 4. The actual fuel saving will be greater than theoretically calculated.
45
5. The temperature range between various parts of the boiler can be reduced which results in reduction of stresses due to unequal expansion. 6. When the feed water is not pure, the temporary hardness is deposited as a soft sludge can be blow down through the economizer through blow-off valve instead of forming a hard scale inside the boiler.
ECONOMISER USED IN RTPP:Type: The economizer used in RTPP is gilled bare tube horizontally spaced. Location: The economizer is located at the top of the boiler at a height of 56.025m to 62.265m from the ground floor. Economizers are generally placed between the last super heater, reheater and air preheater. Working of Economizer: The economizer consists of two sections. The lower section is known as non-Steaming economizer and the upper section is known as steaming economizer. The economizer is placed above the reheater. The hanger plates are used to support economizer tubes. First the feed water from hot well is sent through the header to lower section of the economizer in which the number of tubes is 100. The feed water temperature rises from 2300C to 2940C.
The flue gases at a
temperature of 418 0C give heat from economizer to water. The water from upper section is sent to the boiler drum where the steam and water is separated. Specification of Economizer in RTPP as per manual: Type: Gilled bare tube horizontally spaced economizer. Total heat surface area
:
7911 m2
46
Number of Blocks
:
2
Volume of economizer
:
25 m 3
Soot Blower used in RTPP: One-half retraceable soot blower is operated 3 times a day. In soot blower station one safety valve is provided. The pressure in soot blower is maintained 30Kg/cm 2. If less pressure is maintained, tubes will undergo corrosion and if high pressure is maintained tubes will undergo erosion.
5.2 SUPER HEATER FUNCTION The function of super heater is 1. To increase the temperature of the steam above the saturation temperature, said to be superheated steam. 2. To remove the lost traces of moisture (1-2%) from the saturated steam coming out the boiler.
NEED OF SUPER HEATER The temperature of the steam at the super heater outlet may be several hundred degrees above the saturation temperature of the steam if sufficient heat is supplied in the super heater. The difference between the temperature of the superheated steam and the saturation temperature at the pressure of the steam is called degree of superheat . The high degree of superheat used in
steam turbines, with a
resulting increasing in the average efficiency of the turbine of about 1% for each 10 deg of superheat. The elimination of entrained moisture reduces erosion of turbine blading. USE OF SUPER HEATED STEAM IN STEAM ENGINES It effects a saving in fuel by:
47
1. Reducing condensation steam-pipe lines, and engine cylinder. 2. Reducing the friction of the steam in the steam ports while entering and leaving the cylinder. 3. Producing more work per pound of steam used.
DESCRIPTION The steam produces in the boiler is nearly saturated. This is as such should not used in the turbine because the dryness fraction of steam leaving the boiler will be low, which causes the corrosion of turbine blades due to the presence of moisture. So as to raise the temperature of steam, super heater is used. The use of superheated steam increases the turbine efficiency. The super heating of steam raises overall efficiency as well as avoids too much condensation in the last stages of the turbine which Also avoids blade erosion. The heat of combustion gases from the furnace is utilized for the removal of moisture from steam to become super heated steam. A super heater consists of inlet and outlet headers which are connected by small diameter tubes. In modern boilers the super heater is divided into two sections. 1. Primary section. 2. Secondary section The steam from boiler drum passes through the primary section first. This arrangement simplifies the control of steam temperature. The primary section in the super heater is arranged in counter flow as heat from hot gases to the vapour in the super heater is transferred at high temperatures, and secondary section in parallel flow to reduce the temperature stressing of the tube wall.
REGIONS IN SUPER HEATER:The super heater basically consists of four sections. They are: i.
Hanger plates
48
ii.
Low temperature super heater (LTSH)
iii.
Intermediate Temperature super heater (ITSH)
iv.
High Temperature super Heater(HTSH)
Location of each section
The hanger platens are located directly above the furnace.
Low Temperature Super Heater (LTSH) is arranged in counter flow and is located between the Low Temperature Reheater and High Temperature Reheater.
Intermediate Temperature Super Heater (ITSH) is arranged in parallel flow and is located between the High Temperature Reheater and hanger platens.
High Temperature Super Heater (HTSH) is arranged in counter flow and is located between the Low Temperature Super Heater and High Temperature Reheater.
METALS USED IN SUPER HEATER TUBES:The metals used for super heater must have a. High temperature strength. b. High creep strength. c. High resistance to oxidation. The metals used for super heater tubes are 1. Carbon steel(5100C) 2. Molybdenum alloys (6500C) 3. Low-chrome molybdenum steel(still high temperatures) DESIGN REQUIREMENTS Effective
super
heater
design
must
consider
the
several
parameters including;
The stem temperature specified.
49
The range of boiler load over which steam temperature is to be controlled.
The super heater surface area required to give this stem temperature.
The gas temperature zone in which the surface is to be located.
The type of steel, alloy or the other material best suited for the surface and supports.
The rate of steam flow through the tubes, which is limited by the permissible steam pressure drop, but in turn exerts a dominant control over the tube metal temperature.
The arrangements of surface to meet the characteristic of the anticipated fuels with particular reference to the spacing of the tubes to prevent the accumulation of ash and stag or to provide for the easy removal of these formation in their early stages.
A change in any of these items may require a counter balancing change in some or all other items. The steam temperature desired in power station design is typically maximum for which the super heater designer can produce an economical component. After the amount of surface necessary to compare several arrangements to obtain optimum combination that:
Requires an alloy of less cost.
Envies a more reasonable steam pressure drop without jeopardizing the tube temperature.
Gives a higher steam ass flux or velocity to lower tube temperature.
Give the tube spacing that minimizes ash accumulations with various types of flues.
It is possible to achieve a practical design with optimum economic and operational characteristics.
50
CLASSIFICATION OF SUPER HEATER Super heaters are classified as I.
According to the mode of reception A. Convective super heater These receive heat by direct contact with the hot products of
combustion which flow around the tubes. The heat of combustion gases is transferred to the surface of super heater tubes by convection. B. Radiant super heater It is located in furnace walls where they see the flame and absorb heat by radiation with a minimum of contact with the hot gases. II.
According to the position of heater tubes A. Horizontal type B. L—shaped type
III. According to the arrangement of super heater tubes in the boiler A. Over deck The sectional-header boilers are located in the gas path above the top row of the boiler tubes. It receives most of its heat by convection. B. Inter deck The sectional- or Box-header boilers are located near the furnace above the shallow bank of boiler tubes with this type two headers are normally used, one header connected to wet steam being supplied by the outlet boiler drum and the second header acting as a super heater steam supply header. C. Inter Tube
51
For bent tube boilers has a super heater between the tubes of the first pass. These requires less heating surface for a given amount of super heat.
Convective super heater The heat of combustion gases is transferred to the surface of the super heater by convection. This is usually divided into two sections. a. Primary (Lower temperature) b. Secondary (Higher temperature) The heat transfer rate in convective zone is no longer largely controlled by the emissive of the flue gases but depend on the following factors. 1. The temperature of the flue gases is more precisely the potential gradient available between the hot gas and the cold item but tube metal interposed between the two fluids. 2. Gas velocity, heat transfer rate increases with mass flow and to give convection type heat transfer the gas should actually impinge on or scrub, the surfaces of the tubes. For this reason a high degree of turbulence is necessary is convective zones.
PRINCIPLE The absorption of heat by water in boiler generating tubes from the gases
sweeping over them and consequently
raise the
temperature of the steam. The magnitude of steam depends upon the exit gas temperature leaving the super heater and gas velocity. The convective heat transfer may be set according to the arrangement of super heater tubes as 1. Inter pass super heater
is placed in a space over the water
tubes. It receives most of its heat by convection. As the steam
52
temperature varies with the boiler load, the range of control is limited. 2. Inter Deck super heater is located between two sets of steam generating tubes. This allows a certain amount of Radiant heat absorption and so extends the load range over which a steady final temperature is obtained. With this type normally two headers are used, one heater is connected to the wet steam being supplied by the boiler outlet drum and the second header acting as a super heater steam supply header. 3. Inter Tube super heaters are arranged with tube elements so spaced that they may be inserted between the boiler tubes, headers being located in the open space. These are suited to boilers having water walls absorbing a high proportion of radiant heat.
The convective path of super heater is located close to the path of hot combustion gases where the temperature exceeds steam temperature. The design of convective heating surface will also depend upon the desired steam gas side heat transfer and acceptable pressure drop.
DESCRIPTION Saturated steam from the boiler is supplied to the upper or inlet header of the super heater by a single pipe or by a group of circulator tubes. Steam flows at a high velocity from the inlet header to the outlet header through a large number of parallel tubes. Nipples protect the headers from temperature stresses. With the convection type, due to gas flow and temperature, and consequently the heat transmission increasing at a higher rate then the steam flow, the super heater temperature thus increases as the load increases.
53
Radiant Super heater
Radiant Super heater The heat of combustion gases is transferred to the surface of super heater tubes by radiation. The radiant super heaters are placed in one or more walls of the furnace of a steam generator, where the super heater tubes receive heat by direct radiation from the fire.
54
It was normal in earlier designs to supply the radiant super heater with saturated steam taken direct from boiler drum. The flow of steam was downwards in one section and upwards in another. This type of installation caused operational problems, particularly the drainage difficulties during lightning.
The main steam flow path is as follows: a) The temperature of the flue gas, or more precisely gradient available between the hot gas and cold steam is the tube metal interposed between the two fluids. b) Gas velocity, heat transfer increases with main steam flow and to give good convection type heat transfer the gas should actually impinge on or scrub the surface of up and low conditions. It is now in common practice to install a primary super heater before the radiant super heater. This is to prevent water logging of lower super heater headers during the critical low period, it can be improved still further when primary super heater is located with the reheater by pass dampers are open. FOULING AND CORROSION: The fouling is a major phenomenon for convective super heater as they are situated in the flow path of hot gases. Reasons for Fouling The alkali metals like sodium and potassium in the coal are volatized in the combustion process are generally when temperature in the
combustion
chamber
exceeds
15000
C
to
16500
C.
This
temperature generally occurs in pulverized fuel banners and condenses as sticky substance at temperature corresponding to those of super heater tubes. The layer may contain the sulphate of sodium and potassium pyrosulphates.
55
Conditions Favorable Alkali deposits, combined with high temperatures favorable produce more severe super heat fouling where fouling conditions are favorable for alkali deposits, which can increase tube temperature to allow sintering process to commence. In this way the fouling keeps on increases continuously and keeps the super heater in danger conditions as such fouling cannot be removed by soot blowers.
Factors responsible I.
Sulphur content of fuel can produce hard acid sulphate type fouling when the coal contains the alkalis.
II.
High chlorine content in pulverized coal (70.5%) forms severe type of fouling particularly when the coal ash contains less than 14% ash.
III.
High temperature of the gases entering into the super heater zone aggravates the severity of trouble with bonded deposits. The temperature below 10000C is considered safer when high chlorine content coals are used.
IV.
Fouling by sodium and vanadium even at lower temperature 6000C is experienced when oils is used as fuel instead of coal.
V.
High velocity of gas also increases the fouling rate because of higher impingement rate.
Corrosion: The super heater tubes are subjected to corrosion when they are exposed to oxidizing and reducing conditions are alternatively. This destroys the protective oxide films exposes the metal surface open to further corrosion. The alkali deposits formed also have corrosion effect on steel depending upon its temperature and composition. Low
56
chromium ferrite steel confers some corrosion resistance but marked resistance is obtained by the use of austenite alloys.
SUPER HEATER USED IN RTPP Type: The super heater used in RTPP is convective type, horizontal super heater. Location The super heaters in different locations are listed below: Stage 1: Low Temperature Super Heater LSTH is located between the low temperature reheater and high temperature super heater at a height between 49.716m to 51.229m from the ground floor. Stage 2: High Temperature Super Heater HTSH is located between high temperature reheater and low temperature super heater at a height in between 44.94m to 48.61m from the ground floor. Stage 3: Intermediate Temperature Super Heater ITSH is located between high temperature reheater and Hanger platens at a height in between 38.162m to 40.618m from the ground floor. Working: The steam formed in the boiler drum is sent to Low Temperature Super Heater to super heat the steam. The steam generated in the boiler drum is wet. This steam is passed to LTSH. The low temperature
57
super heater consists of 100 tubes made of carbon steel, which is having high thermal conductivity. The steam absorbs heat from the flue gases which is passed over the LTSH. The flue gases before LTSH have 9000 C after passing over LTSH the flue gas having a temperature of 7570C. This drop in temperature (143 0C) results increase in the temperature of steam from 373 0C to 4250C at a pressure 162 kg/cm 2. In this stage the quality of steam will improve that means the dryness fraction will increase. After this stage the steam passes through ITSH. This is located below the low temperature reheater. The flue gases before ITSH have a temperature 7600C and after passing ITSH on which the flue gases having a temperature of 637 0C. The steam absorbs the heat energy (120 0C) and its temperature will increase from 3950C to 4730C. The number of tubes maintained in this stage is 16 to maintain uniform heat transfer and to prevent the over heating of tube, Water is sprayed on the super heater. The steam from ITSH is sent to HTSH. In HTSH, the flue gases temperature drops from 748.9 0C to 6390C and this temperature drop (1100C) will increase the stem temperature from 469.90C to 5400C at pressure of 152 kg/cm 2. The dryness fraction of steam is 1.3 to 1.5.
This super heated steam is
sent for expansion in high pressure turbine where mechanical work is obtained. Specifications of RTPP as per Manual: Heating surface area: LTSH=3055m 2 ITSH=1435m 2 HTSH=2715m 2 Description of the Tube Economiser upper 100 Assy at 114.3mm as center distance & 120.65mm as end space. Economiser upper 100 Assy at
Outer Diameter X thickness(mm x mm)
Material
44.5 x 4.5
SA210GrA1
44.5 x 4.5
SA210GrA1
58
114.3mm as center distance & 120.65mm as end space. LTSH, 100 Assy at 114.3mm as center distance & 120.65mm as end space. ITSH, 16 Assy at 685.8mm as center distance & 635mm as end space. HTSH, 50 Assy at 228.6mm as center distance & 177.8mm as end space.
44.5 x 4.5 44.5 x 4.0 44.5 x 6.3 38 x 4.5 38 x 5.7 38 x 4.0 44.5 x 4.0 44.5 x 8.0 44.5 x 5.0 44.5 x 5.6 44.5 x 5.6
SA209T1 SA209T1 SA213T11 SA213T22 SA213TP304H SA213TP304H SA213TP304H SA213T22 SA213TP304H SA213T22 SA213TP304H
5.3 AIR PREHEATER FUNCTION Air heater is the heat transfer surface in which air temperature is raised by transferring heat from other media such as flue gas. Hot air is necessary for rapid and efficient combustion in the furnace and also drying coal in the milling plant, so this became an essential boiler accessory. Air heaters are used in most steam generating plants to the combustion air and enhance the combustion process. Most frequently the flue gas is the source of energy and the air heater serves as a heat trap to collect the useful waste heat from the gas stream.
59
AIR PRE HEATER
LOCATION Air heaters are typically located directly behind the boiler where they receive hot flue gas from the economizer and cold combustion air from forced draught fans. It has been found that an increase of 20 0C in the air temperature increase the boiler efficiency by 1%. Air heaters recycled the waste heat back into boiler and play a very effective role of moderating ESP performance as well. Air preheater Used in RTPP (210MW power station): As the name implies, the trisector pre heater design has three sectors. One for the flue gas, one for primary air (used for drying and transport of coal through mill to the burners). These helps in avoiding wastage of heat pick up by air due to airflow and also help in selecting
60
different temperatures for primary and secondary air. What ever is not utilized in primary air can be picked up by secondary air stream. The Ljungstrom trisector air preheater absorbs waste heat from flue gas and transfers this heat to incoming cold air by means of continuously rotating heat transfer element of specially formed metal sheets. Thousands of these high efficiency elements are compactly arranged with in 12 sector shaped compartments for heater sizes from 24.2 to 27 inches, and 24 shaped compartments for heater sizes from 28 to 33, of radially divided cylindrical shell called rotor . The housing surrounding the rotor is provided with duct connections at both ends and is adequately sealed by radial and axial sealing members forming an air passage through one half of the air preheater and the gas passage through the other. As
the
rotor
slowly
revolves
the
mass
of
the
elements
alternatively through the air and gas passages, heat is absorbed by the element surface passing through the air stream, they releases the stored up heat thus increasing the temperature of incoming air. The Ljungstrom air preheater is more widely used than any other type of heat exchanger for comparable service. The reasons for worldwide acceptance are proven for its performance and reliability, effective leakage control, and its adaptability to most any fuel burning process. It is both designed and built to operate over extended periods.
5.4 REHEATER Function and Working Reheater is a boiler accessory which is used for improving overall cycle efficiency. In order to take full advantage of improved cycle efficiency offered by high steam pressure and to avoid the formation of water at the exhaust end which would reduce efficiency and aggravate water cutting of the exhaust blades, the steam is with drawn from the turbine, re-heated and returned to the machine for further expansion.
61
Increasing the initial pressure would serve the same purpose but re heating is the more practicable because more available heat per Kg may be added with in the temp limits set by the physical properties of economical high-Temp alloys. With out the use of reheater excessive wetness will damage the turbine blades and steam is withdrawn from the turbine at some point during expansion and returned to the boiler for passage through separate super heater where it is re-super heated. After reheating it is brought back to the turbine where it can be expanded down to a very low exhaust pressure without any danger of excessive wetness. Location Reheater is composed of two stages of section•
Low temp reheater (LTR)
•
High temp reheater(HTR)
Assemblies
are
located
in upper
side of
the
boiler. LTR
assemblies are provided below the economizer assemblies and above the LTSH assemblies and HTR assemblies are provided below the HTSH and above the ITSH assemblies. Reheater assemblies are horizontal banks and penetrating to front/rear wall to join with headers. These assemblies are supported by steam cooled hangers.
Steam flow path in Reheater After passing through the high pressure stages of the turbine, steam is returned to the reheater via cold reheat lines. The reheat de super heater is located in between the LTR and HTR. Reheat flow through the unit is as follows: LTR low temperature reheat-DESH-HTR high temperature reheat. After reheating to higher temperature the reheated steam is returned to the low- pressure section of the turbine through the hot reheat lines.
62
5.5
ARRANGEMENT OF ACCESSORIES The following fig shows the arrangement of Accessories in RTPP
63
6.
GRAPHS
6.1
Effect of flue gas temperature entering the Economiser on
fuel saving:
64
Fig. 6.1 Effect of flue gas temperature entering the Economiser on fuel saving:
Importance of Economiser: •
Use of economizer increases the boiler efficiency, for every 60C increase in temperature in the feed water, boiler efficiency increased by1%.
•
Due to increase in boiler efficiency by installing economizer overall thermal plant efficiency increases.
•
Heat of flue gases which are being wasted can be utilized to more extent by usage of economiser.
•
Heat carried with the flue gases are partly recovered in the economiser reduces the fuel supplied to the boiler.
6.2 Effect of Load on super heater temperature of convection and radiation super heaters:
65
Fig.6.2Effect of Load on super heater temperature of convection and radiation super heaters
Importance of super heater: •
Due to installation of super heater thermal efficiency of plant increases.
•
It avoids too much condensation of steam in the last stages of turbine due to which erosion of blade is reduced.
•
It removes the moisture content of steam coming out of boiler and to increase its temperature sufficiently above saturation temperature.
•
Heat of flue gases is maximum utilized.
66
6.3
Effect between Saturation temperature and pressure of
steam
Fig.6.3 Saturation temperature and pressure of steam
If heat is transferred to the system and the discharge valve is partially closed so as to restrict the escape of saturated steam to the atmosphere, the pressure will rise, and the temperature of the saturation steam will also increase. It will be observed that at each pressure there is only one temperature of the saturated steam. Plotting the saturation temperature against saturation pressure the graph is obtained:
67
6.4 Temperature of products of combustion vs Surface required
Fig.6.4 Temperature of products of combustion vs Surface required
Graph shows the relative amount of boiler heat transfer surface required to cool the products of combustion from 1500F to lower temperatures, based on saturated water in the boiler tubes at 544.61F. It will be noted that, as the temperature difference decreases, each increment of added surface becomes less effective and the amount of surface required to cool the gases from 700 to 600 F is about 60% of that required to cool gases from 1500 to 700F.
68
6.5 Efficiency V s. Boiler pressure
Fig.6.5 Efficiency V s. Boiler pressure
It has been observed that by increasing the boiler pressure (other factors remaining the same) the cycle tends to rise and reaches a maximum value at a boiler pressure of about 166 bar.
69
6.6
Efficiency vs. Degree of super heat
Fig 6.6 Efficiency vs. Degree of super heat
All other factors remaining the same if the steam is superheated before allowing it to expand the cycle efficiency may be increased. The use of superheated
steam also ensures longer turbine blade life
because of absence of erosion from high velocity water particles that are suspended in wet vapour.
70
6.7
Efficiency vs. Condenser pressure
The thermal efficiency of the cycle can be amply improved by reducing the condenser pressure (hence by reducing the temperature at which heat is rejected), especially in high vacuums. But the increase in efficiency is obtained at the increased cost of condensation apparatus.
7.
PERFORMANCE ANALYSIS AND DATA COLLECTIONS
71
7.1
NOTATIONS AND NOMENCLATURE I. Boiler: 1.
Mc
=
Mass flow rate of coal, TPD
2.
Cv
=
Calorific value of coal, KJ/KG
3.
Hi
=
Heat input to boiler, KW
4.
Ho
=
Heat output (or) Heat required to raise the Steam temperature, KW
5.
Mw
=
Mass flow rate of feed water TPH
6.
Ne
=
Boiler efficiency,%
II. Economiser 1.
M
2.
T1
w
=
Mass of feed water evaporated /Kg of fuel, TPH
=
Temperature of feed water entering economizer, 0C
3.
T2
=
Temperature of feed water leaving economizer, 0C
4.
Cpw
=
Specific heat of feed water, KJ/Kg0K
5.
Cpf
=
Specific heat of flue gas, KJ/Kg0K
6.
Mf
=
Mass of flue gases /Kg of fuel, TPH
7.
Tf1
=
Temperature of flue gas entering the economizer, 0C
8.
Tf2
=
Temperature of flue gas leaving the economizer,0C
=
Temperature of air supplied to boiler,0C
9.
Ta
10.
Ecoeff =
Efficiency of economizer,%
11.
Qeco
=
Heat transfer rate of economizer, KW
12.
Cv
=
Calorific value of coal, KJ/Kg
13.
Mc
=
Mass flow rate coal, TPD
14.
PHUE =
Percentage Heat Utilised in Economiser,%
15.
Qc
=
Heat gained by steam, KW
16.
Qh
=
Heat lost by flue gas, KW
72
17.
Q
=
Average heat transfer rate, KW
18.
Ao
=
Area of outlet, m2
19.
n
=
Number of tubes in economizer
20.
Do
=
Out side diameter of the tube,
21.
l
=
Length of tube, m
22.
Uo
=
External overall heat transfer coefficient, KW/m2 oK
23.
LMTD =
Logarithm mean temperature difference, 0c
24.
DTi
=
Inlet temperature difference of economizer, 0c
25.
DT0
=
Outlet temperature difference of economizer, 0c
26.
Cc
=
Heat capacity of cold fluid, KW/ 0K
=
Heat capacity of hot fluid, KW/0K
=
Effectiveness of economizer
27.
Ch
28.
E
29.
NTU =
Number of Transfer Units
30. NTU- E =
Effectiveness of economizer by NTU method
31.
R
Ratio of heat capacity
32.
Cmin =Minimum heat capacity of fluid, KW/ 0 K
33.
Cmax =
=
Maximum heat capacity of fluid, KW/0K
III. Super Heater 1.
Tsl1, Tsh1, Tsi1=
Temperature of steam entering LTSH, HTSH, ITSH respectively , 0C
2.
Tsl2,Tsh2,Tsi2 =
Temperature of steam leaving LTSH, HTSH,ITSH respectively, oC
3.
Tfl1,Tfh1,Tfi1 =
Temperature of flue gas entering LTSH, HTSH, ITSH respectively, oC
4.
Tfl2,Tfh2,Tfi2
=
Temperature of flue gases leaving LTSH, HTSH, ITSH respectively, oC
5.
Ta
=
6.
Cpsl,Cpsh,Cpsi=
Temperature of air supplied to boiler, oC
Specific heat of steam in LTSH, HTSH,
73
ITSH respectively, oC 7.
Cpfli Cpfh, Cpfi=
Specific heat of flue gas in LTSH,HTSH, ITSH
8.
Msl Msh, Msi =
Mass flow rate of steam in LTSH, HTSH, ITSH respectively, oC
9.
Mf
=
Mass flow rate of steam in LTSH, HTSH, ITSH respectively, TPH
10.
H1 & H2
=
Enthalpy of steam entering and leaving, KJ/KG
(Read
from
tables
for
corresponding values of temperature and pressure) 11.
Cv
=
Calorific value of coal, KJ/KG
12.
Mc
=
Mass flow rate of coal, TPD
13.
n
=
Number of tubes
14.
Do
=
Out side diameter of the tube, m
15.
l
=
Length of tube, m
16.
QLTSH
=
Heat transfer rates in LTSH, KW
QHTSH
=
Heat transfer rates in HTSH, KW
QITSH
=
Heat transfer rates in ITSH, KW
17.
PHUS
=
% Heat Utilised in Super heater,%
18.
E
=
Effectiveness of super heater
19.
NTU _E
=
Effectiveness of super heater by NTU method
20. EFFI _LTSH, EFFI_HTSH, EFFI _ITSH =
Efficiency of LTSH, HTSH, ITSH respectively, oC
21.
Uo
=
External over all heat transfer coefficient, KW/m2K
22.
Ao
=
Outlet area,m2
23.
Qc
=
Heat gained by steam, KW
24.
Qh
=
Heat lossed by flue gases, KW
74
25.
Q
=
average heat transfer rate, KW
26.
Cc
=
Heat capacity of cold fluid, KW/oK
27.
Ch
=
Heat capacity of hot fluid, KW/oK
28.
NTU
=
Number of Transfer Units
29.
R
=
Ratio of heat capacity
30.
LMTD
=
Logarithm mean temperature difference
31.
Cmin
=
Minimum heat capacity of fluid, KW/oK
32.
Cmax
=
Maximum heat capacity of fluid, KW/oK
33.
DTi
=
Inlet temperature difference of Super heater, oC
34.
DTo
=
Outlet temperature difference of Super heater, oC
7.2 FORMULAE: 1.
BOILER:
a)
Heat Input: Heat input =
(mass of coal* CV of coal)
Hi
b)
Heat output: Heat output
= =
Heat in steam raised (mass of steam raised per Kg of steam* Enthalpy of steam)
Ho c)
Boiler Efficiency: Boiler efficiency
=
(heat in steam raised)/(heat input)
η
=
(Ho /Hi)* 1000
boiler
75
II.
ECONOMISER: a)
Efficiency of Economiser:
η
eco
=
Mw * Cpw *(T2 –T1)* 100 Mf * Cpf *(Tf1 –Ta)
b)
Heat transfer rate of Economiser:
Q
eco
=
Mf * Cpf *(Tf1 –Ta)*1000 3600
c)
Percentage of heat utilization in Economiser:
PHUE
=
Mw * Cpw *(T2 –T1)* 24 Mc * CV
d)
Effectiveness of Economiser: •
Heat gained by steam(Qc) =
Mw * Cpw *(T2 –T1)* 1000 3600
•
Heat lossed by flue gas(Q h) =
Mf * Cpf *(Tf1 –T2)*1000 3600
•
Average,
Q = Qc+Qh /2
•
Area of Outlet,
Ao
=
n*∏*Do*l
76
•
External overall heat transfer coefficient U0
=
Q/A0 * LMTD
Where, LMTD
= Logarithmic Mean Temperature Difference, Difference, DTi –DT0 / ln (DT /DT i /DT0 ) DTi = Inlet Inlet temperat temperature ure differ difference ence (T (Tf1 – T1 ) ) DT o = Outlet temperature temperature difference (T f2 ) ) f2 – T 1
•
Heat capacity of hot fluid, Ch =
Mf * Cpf * 1000/3600
•
Heat capacity of cold fluid, Cc =
Mw * Cpw * 1000/3600
•
If Ch < Cc then then effe effect ctiv iven enes ess, s, E=
(Tf1 – Tf2)/ (Tf1 – T1 ) )
•
Otherwise,
(T2 – T 1 )/ )/ (T f1 ) ) f1 – T 1
E
=
By NTU Method: If Ch < C c c then If C min = C h and C max = C c c Otherwise C
min
= Cc and C
max
= Ch
Number of transfer units, •
NTU =
Uo * Ao / C
•
Ratio of heat capacity, R = C min / C max
•
NTU_E
=
min
(1-(exp(-NTU*(1-R)))) (1-R*(exp(-NTU*(1-R))))
III.
SUPER HEATER: •
η
LTSH
=
Msl * Cpsl *(Tsl2 –Tsl1)* 100 Mf * C
pf1
*(Tf1 –Ta)
•
QLTSH
=
Mf * Cpfl *(Tfl1 –Tfl2)* 1000/3600
•
PHUS
=
(M sl )*24*100))/ )*24*100))/ M c sl * (H 2 – H 1 c *CV
•
Qc
=
Msl * Cpsl *(Tsl2 –Tsl1)* 1000/3600
•
Qh
=
Mf * Cpfl *(Tfl –Ta)*1000/3600
•
Q
=
(Qc + Qh )/2 )/2
•
Ao
=
n*∏*Do*l
77
•
External overall heat transfer coefficient U0
=
Q/(A0 * LMTD)
Where, LMTD
= Logarithmic Mean Temperature Difference, Difference, (DTi –DT0 )/ ln (DTi /DT /DT0 ) DTi = Inlet Inlet temperat temperature ure differ difference ence (T (Tfl1 – Tfl2) DTo = Outlet temperature temperature difference (T fl1 – Tsl1)
•
Heat capacity of hot fluid, Ch =
Mf * Cpf * 1000/3600
•
Heat capacity of cold fluid, Cc =
Mw * Cpw * 1000/3600
•
If Ch < Cc then then effe effect ctiv iven enes ess, s, E=
(Tfl1 – Tfl2)/ (Tfl1 – Tsl1)
•
Otherwise,
(Tsl2 – Tsl1)/ (Tfl1 – Tsl1)
E
=
By NTU Method: If Ch < Cc then If C Otherwise C
min
min
= Ch and C
= Cc and C
max
Uo * Ao / C
max
= Cc
= Ch
•
NTU =
•
Ratio of heat capacity, R = C min / C max
•
NTU_E
=
min
(1-(exp(-NTU*(1-R)))) (1-R*(exp(-NTU*(1-R))))
7.3
HEAT EXCHANGER ANALYSIS “Heat exchanger is equipment which transfers the energy from a
hot fluid with maximum rate and minimum investment and minimum inve invest stme ment nt and and runn runnin ing g co cost st”. ”. Econ Econom omis iser er and and Su Supe perr heat heater er are are counter flow type heat exchangers. For designing the performance of heat exchanger it is necessary that the heat transfer may be related with its governing factors. 1. Over Over all coe coeffi fficie cient nt 2. Total Total heati heating ng surfac surface e area. area.
78
3. Inlet Inlet and outlet outlet temper temperature ature of of fluids. fluids. 4. Effe Effect ctiv iven enes ess. s. Over all heat transfer coefficient “Heat transmitted transmitted per unit area area time per unit degree degree temperature temperature difference between between the bulks of fluids on each side of metal.”
Calculation of ‘U’ Experimentally Heat given up by hot flue gases Q h =mh Cph(Th1-Th2) Heat picked up by the feed water Q c =mcCpc(Tc2-Tc1) Average heat transfer Q=(Q h+Qc)/2 Total heat transfer rate Q=U*A*LMTD Where
U = over all heat transfer coefficient
LMTD=Logarithamic mean temperature difference
Calculation of LMTD LMTD LM TD is defi define ned d as that that temp tempe eratu rature re diff differ eren ence ce whic which, h, if constant, would give the same rate heat transfer as actually occurs under variable condition of temperature difference.
Heat Exchanger Effectiveness and number of transfer units A heat exchanger can be designed by LMTD when inlet and outlet cond co ndit itio ions ns are are spec specif ifie ied d on only ly.. Ho Howe weve ver, r, when when the the prob proble lem m is to deter determin mine e the inlet inlet and outlet outlet tempe temperat rature ures s for a partic particula ularr heat heat exchanger (Economiser, (Economiser, Super heater), The analysis is performed more easily, by using the method based on effectiveness. effectiveness. The heat exchanger effectiveness ‘є’ ‘ є’ is defined as a ratio of actual heat transfer Q to the maximum possible heat transfer Q max. Є Q
= Q/ Qmax = mh Cph(Th1-Th2)=mc Cpc(Tc2-Tc1)
The product of mass flow rate and specific heat is defined as fluid capacity.
79
MhCph=Ch=Hot fluid capacity McCpc=Cc=Cold fluid capacity The maximum heat transfer would occur if the outlet temperature of the fluid with smaller values of C h and Cc.
Qmax=Ch(Th1-Tc1)= Cc(Th1-Tc1) Qmax is the minimum of these two values Qmax=Cmin(Th1-Th2)
Number of transfer units NTU is defined as it is dimension less expression i.e., UA/C min. NTU is a measure of effectiveness of the above equation ‘ є’ is a function of variables and such it is inconvenient to combine them in a graphical or tabular form. However by compiling a non- dimensional grouping, ‘ є’ can be expressed as a function of three dimensional parameters. This method approach facilitates the comparison between the various types of heat exchanger which may be used for a particular application.
Effectiveness for counter flow: ‘є’
=
(1-(exp(-NTU*(1-R)))) (1-R*(exp(-NTU*(1-R))))
The
grouping
of
the
terms
UA/C min.NTU
is
dimensionless
expression.
Cmin /Cmax is the second dimensionless parameter called capacity ratio (R).
The last dimensionless parameter is the flow arrangement i.e., parallel flow, counter flow or cross flow.
80
7.4 (A)
DATA COLLECTION: Boiler: Mass flow rate of coal,(M c)
=
3422 TPD
Calorific value of coal, (CV)
=
14179 KJ/Kg
Mass flow rate of feed water, (M w) = 694 TPH
(B)
Economiser
Cold fluid data: Mass of feed water evaporated /Kg of fuel, (M w)= 694 TPH Specific heat of water, (Cp w)= 4.949KJ/KGoK Temp. of feed water entering economizer, (T 1) =242oC Temp. of feed water leaving economizer,(T2)= 296oC
Hot fluid data: Mass of flue gases/Kg of fuel, (Mf) =766 TPH Specific heat of flue gases,(Cpf )
=1.151KJ/KgoK
Temperature of flue gases,(T f1)
=425.9oC
Temperature of air supplied to boiler,(T a)=30oC
Fuel data: Calorific value of coal,(CV)=14179 KJ/Kg Mass flow rate of coal, (M c)=3422 TPD Temp of flue gases leaving the economizer (Tf 2)=3370C Number of tubes in economiser (n)
= 100
Outside diameter of the tube (D o ) = 0.0445 m Length of the tube
(l)
= 3.25 m
81
( C)
Super heaters Super heater was three stages namely
1)
LTSH(Low temperature super Heater)
2)
HTSH (High Temperature Super Heater)
3)
ITSH (Intermediate Temperature Super Heater)
(1)
LTSH Temperature of steam entering LTSH((T sl1)=367.5oC Temperature of steam leaving LTSH (T sl2)=423 oC Temperature of flue gas entering LTSH (T fl1) =684 oC Temperature of flue gas leaving LTSH (T fl2) =537 oC Temperature of air supplied to the boiler (T a) =30oC Specific heat of steam, (Cp sl) = 2.075KJ/KgoK Specific heat of flue gases (Cp fl) =1.239KJ/KgoK Mass flow rate of steam (Msl) =653 oC Mass flow rate of flue gases (M fl) =768.7 oC Specific enthalpy of steam entering LTSH (H 1) =2807.34 KJ/Kg (From steam tables for temperature 367.5 oC &163 bar) Specific enthalpy of steam leaving LTSH (H 2) =3043.08 KJ/Kg (From steam tables for temperature 423 oC & 156 bar) Mass flow rate of coal (M c) =3422 TPD Calorific value of coal (CV) =14179 KJ/Kg Number of tubes (n)
= 3 x100
Outside diameter of the tube (D o ) = 0.038 m Length of the tube
(l)
= 85.30 m
82
(2)
HTSH Temperature of steam entering HTSH (T sh1) =427 oC Temperature of steam leaving HTSH (T sh2) =543 oC Temperature of flue gas entering HTSH (T fh1) = 752 oC Temperature flue gas leaving HTSH ( T fh2) = 694 oC Temperature of air supplied to the boiler (T a) =30oC Specific heat of steam (Cp sh) =2.141 KJ/Kg oK Specific heat of flue gas (Cp fh) =1.324 KJ/Kg oK Mass flow rate of steam (Msh) =653.3 TPH Mass flow rate of flue gas (Mf ) = 768 TPH Specific enthalpy of steam entering HTSH (H 1) =3056.63 KJ/Kg
o
K (from steam tables for temp427 oC & 154 bar) Specific enthalpy of steam leaving HTSH(H2) = 3421.35 KJ/Kg oK (From steam tables for temp 543 oC &152 bar) Mass flow rate of coal (M c) =3422 TPD Calorific value of coal (CV) =14179 KJ/Kg Number of tubes (n)
= 8x50
Outside diameter of the tube (D o ) = 0.038 m Length of the tube
(l)
= 13.72 m
83
(3)
ITSH Temperature of steam entering ITSH (T si1) = 398 oC Temperature of steam leaving ITSH (T si2) =470 oC Temperature of flue gas entering ITSH (T fi1) =835 oC Temperature of flue gas leaving ITSH (T fi2) = 751 oC Temperature of air supplied to the boiler (T a) = 30oC Specific heat of steam (Cp sh) = 2.094 KJ/Kg oK Specific heat of flue gas (Cp fh) =1.325 KJ/Kg oK Mass flow rate of steam (Msh) = 653.3 TPH Mass flow rate of flue gas (M f ) = 768 TPH Specific enthalpy of steam entering ITSH (H 1) =2965.2 KJ/Kg (From steam tables for temp 398 0C & 154 bar) Specific enthalpy of steam leaving ITSH(H2) = 3180.6 KJ/Kg oK (From steam tables for temp 470 oC &153 bar) Mass flow rate of coal (M c) =3422 TPD Calorific value of coal (CV) =14179 KJ/Kg Number of tubes (n)
= 16 x 26
Outside diameter of the tube (D o ) = 0.038 m Length of the tube
(l)
= 28.895 m
84
7.5
RESULTS & CALCULTIONS
1.
BOILER:
a)
Heat Input: Heat input =
(mass of coal* CV of coal)
Hi = (3422 * 14179 * 1000)/(24 * 3600) = 561580.30 KW
b)
Heat output: Heat output
= =
Heat in steam raised (mass of steam raised per Kg of steam* Enthalpy of steam)
Ho
c)
=
(694 * 1000 * 2442)/ 3600
=
470763.33 KW
Boiler Efficiency: Boiler efficiency
=
(heat in steam raised)/(heat input)
η
=
(Ho /Hi)* 1000
=
(470763.33 /561580.30)*100
=
83.83 %
boiler
RESULTS a)
Heat Input:
Hi
=
561580.30 KW
b)
Heat output:
Ho
=
470763.33 KW
c)
Boiler Efficiency:
η
=
83.83 %
boiler
85
II.
ECONOMISER: a)
Efficiency of Economiser: η
eco
=
Mw * Cpw *(T2 –T1)* 100 Mf * Cpf *(Tf1 –Ta)
= 694 *4.949 *(296-242) * 100 766 * 1.151 * (425.9-30) = 53.14 % b)
Heat transfer rate of Economiser: Q
eco
=
Mf * Cpf *(Tf1 –Ta)*1000 3600
=
766 *1.151 *(425.9-30)*1000 3600
= c)
96958.77 KW
Percentage of heat utilization in Economizer:
PHUE
=
Mw * Cpw *(T2 –T1)* 24
* 100
Mc * CV =
694 *4.949*(296-242)*24
*100
3422 * 14179 = d)
9.17 %
Effectiveness of Economizer: •
Heat gained by steam(Qc)=Mw * Cpw *(T2 –T1)* 1000 3600 =
694 *4.949*(296-242)*1000 3600
=
51519.09 KW
86
•
Heat lossed by flue gas(Q h)= Mf * Cpf *(Tf1 –T2)*1000 3600 =
766 *1.151 *(425.9-337)*1000 3600
=
•
Average,
21772.25 KW
Q = Qc+Qh 2 =
51519.09 + 21772.25 2
= •
•
Area of Outlet,
36645.67 KW Ao
=
=
100 * ∏* 0.0445 *3.25
=
45.435 m2
DTi = Inlet temperature difference (Tf1 – T1) = 425.9 – 296
•
=
129.9 0C
DTo = Outlet temperature difference (T f2 – T1) = 337 – 242
•
n*∏*Do*l
LMTD
=
95 0C
= Logarithmic Mean Temperature Difference, (DT i –DT 0 ) ln (DTi /DT0 ) = (129.9 – 95) ln (122.9/95) = 111.540C
•
External overall heat transfer coefficient U0
=
Q/A0 * LMTD
87
= 36645.67 45.435 * 111.54 7.231KW/ m2 0C
=
•
Heat capacity of hot fluid, Ch = =
M f * Cpf * 1000/3600
766 *1.151 *1000 3600
= •
Heat capacity of cold fluid, Cc = = =
•
244.907KW/0 C M w * Cpw * 1000/3600
694 *4.949*1000 3600 954.057KW/0 C
If Ch < C c then effectiveness, E= =
(T f1 – T f2 )/ (T f1 – T 1 )
425.9-337 425.9-242
=
0.4834
By NTU Method: If Ch < Cc then If C
min
= Ch and C
max
= Cc
Otherwise C min = C c and C max = C h Here C
min
= Cc = 244.907KW/0 C
C max = C h = 954.057KW/ 0 C Number of transfer units, •
NTU = =
Uo * Ao / C min
7.231 * 45.435 244.907
= •
1.3415
Ratio of heat capacity, R = C =
min
/ C max
244.907 954.057
=
0.2567
88
•
NTU_E
=
(1-exp-NTU*(1-R)) (1-(R* exp-NTU*(1-R) ))
=
(1-exp-1.3415*(1-0.2567)) (1-(0.2567* exp-1.3415*(1-0.2567)))
= III.
0.78385
SUPER HEATER: Super Heater has 3 stages namely 1.
LTSH
2.
HTSH
3. ITSH 1. Low Temperature Super Heater (LTSH) a)
Efficiency of LTSH: η
LTSH
=
Msl * C psl *(Tsl2 –Tsl1)* 100 Mf * C
=
pf1
*(Tf1 –Ta)
653 *2.075 *(423-367.5)
* 100
768.7 * 1.239 * (684-30) = b)
12.073 %
Heat transfer rate of LTSH: QLTSH
=
Mf * C
=
768.7 *1.239 *(684-537)*1000
pf1
*(Tfl1 –Tfl2)* 1000/3600
3600 = c)
39640.87 KW
Percentage of heat utilization by LTSH: PHUS
=
(M sl * (H 2 – H 1 )*24*100))/ M c *CV
=
653 *(3043.08-2807.34)*24
*100
3422 * 14179 = d)
7.6 %
Effectiveness of LTSH: •
Heat gained by steam(Qc)= Msl*Cpsl*(Tsl2 –Tsl1)* 1000/3600 =
653 *2.075*(423-367.5)*1000
89
3600 =
•
20889.198 KW
Heat lossed by flue gas(Q h)= Mf * Cpfl *(Tfl1 –Tfl2)*1000 3600 =
768.7 *1.239 *(684-537)*1000 3600
=
•
Average,
38890.45 KW
Q = Qc+Qh 2 =
20889.198 + 38890.45 2
= •
•
Area of Outlet,
29889.824 KW Ao
=
=
3 *100 *∏* 0.038 *85.30
=
3054.945 m2
DTi = Inlet temperature difference (Tfl1 – Tsl1) = 684 – 423
•
=
261 0C
DTo = Outlet temperature difference (T fl2 – Tsl1) = 537 – 367.5
•
n*∏*Do*l
LMTD
=
169.5 0C
= Logarithmic Mean Temperature Difference, (DT i –DT 0 ) ln (DTi /DT0 ) = (261 – 169.5) ln (261/169.5 ) = 211.97 0C
90
•
External overall heat transfer coefficient U0
=
Q/A0 * LMTD
= 29889.824 3054.945 * 211.97 0.0462KW/ m2 0C
=
•
Heat capacity of hot fluid, Ch = =
M f * Cpfl * 1000/3600
768.7 *1.239 *1000 3600 264.56KW/0 C
=
•
Heat capacity of cold fluid, Cc = =
653 *2.075*1000 3600 376.382KW/0 C
= •
Msl * Cpsl * 1000/3600
If Ch < Cc then effectiveness, E= =
(Tfl1 – Tfl2)/ (Tfl1 – Tsl1 )
684-537 684-367.5
=
0.4645
By NTU Method: If Ch < Cc then If C Otherwise C Here C
min
C
max
min
min
= Ch and C
= Cc and C
max
max
= Cc
= Ch
= Cc = 264.56KW/0 C = Ch = 376.382KW/0 C
Number of transfer units, •
NTU = =
Uo * Ao / C
min
0.0462 * 3054.945 264.56
91
= •
0.5331
Ratio of heat capacity, R = C =
min
/ C max
264.56 376.382
=
•
0.7029
NTU_E
=
(1-exp-NTU*(1-R)) (1-(R* exp-NTU*(1-R)))
=
(1-exp-0.5331*(1-0.7029)) (1-(0.7029* exp-0.5331*(1-0.7029)))
=
0.3662
2. High Temperature Super Heater (HTSH) a)
Efficiency of HTSH: η
HTSH
=
Msh * Cpsh *(Tsh2 –Tsh1)* 100 Mf * Cpfh *(Tfh1 –Ta)
=
653.3 *2.141 *(543-427)
* 100
768 * 1.324 * (752-30) = b)
22.10 %
Heat transfer rate of HTSH: QHTSH
=
Mf * Cpfh *(Tfh1 –Tfh2)* 1000/3600
=
768 *1.324 *(755-674)*1000 3600
= c)
22878.72 KW
Percentage of heat utilization by HTSH: PHUS
=
(M sh * (H 2 – H 1 )*24*100))/ M c *CV
=
653.3 *(3421.35-3056.63)*24
*100
3422 * 14179 =
11.786 %
92
d)
Effectiveness of HTSH: •
Heat gained by steam(Qc)=Msh*Cpsh*(Tsh2 –Tsh1)* 1000 3600 = 653.3 *2.141*(543-427)*1000 3600 =
•
45069.715 KW
Heat lossed by flue gas(Q h)= Mf * Cpfh *(Tfh1 –Tfh2)*1000 3600 =
768 *1.324 *(752-694)*1000 3600
=
•
Average,
16382.29 KW
Q = Qc+Qh 2 =
45069.715 + 16382.29 2
= •
•
Area of Outlet,
30726.004 KW Ao
=
=
8 *50 *∏* 0.038 *13.72
=
655.16 m2
DTi = Inlet temperature difference (Tfh1 – Tsh1) = 752 – 540
•
=
209 0C
DTo = Outlet temperature difference (T fh2 – Tsh1) = 694 – 427
•
n*∏*Do*l
LMTD
=
267 0C
= Logarithmic Mean Temperature Difference, (DT i –DT 0 ) ln (DTi /DT0 ) = (267 – 209) ln (267/209 )
93
= 236.820C •
External overall heat transfer coefficient U0
=
Q/A0 * LMTD
= 30726.004 655.16 * 236.82 0.19803KW/ m2 0C
=
•
Heat capacity of hot fluid, Ch = =
M f * Cpfh * 1000/3600
768 *1.324 *1000 3600 282.45KW/0 C
= •
Heat capacity of cold fluid, Cc = =
653.3 *2.141*1000 3600 388.53KW/0 C
= •
Msh * Cpsh * 1000/3600
If Ch < Cc then effectiveness, E= =
(Tfh1 – Tfh2)/ (Tfh1 – Tsh1 )
752-694 752-427
=
0.1785
By NTU Method: If Ch < Cc then If C Otherwise C Here C
min
C
max
min
min
= Ch and C
= Cc and C
max
max
= Cc
= Ch
= Cc = 282.45KW/0 C = Ch = 388.53KW/0 C
Number of transfer units, •
NTU = =
Uo * Ao / C
min
0.19803 * 655.16 282.45
=
0.4593
94
•
Ratio of heat capacity, R = C =
min
/ C max
282.45 388.53
= •
0.72697
NTU_E
=
(1-exp-NTU*(1-R)) (1-(R* exp-NTU*(1-R)))
=
(1-exp-0.4593*(1-0.72697)) (1-(0.72697* exp-0.4593*(1-0.72697)))
=
0.32896
3. Intermediate Temperature Super Heater (ITSH) a)
Efficiency of ITSH: η
ITSH
=
Msi * Cpsi *(Tsi2 –Tsi1)* 100 Mf * Cpfi *(Tfi1 –Ta)
=
653.3 *2.094 *(470-398)
* 100
768 * 1.325 * (835-30) = b)
12.018 %
Heat transfer rate of ITSH: QITSH
=
Mf * Cpfi *(Tfi1 –Tfi2)* 1000/3600
=
768 *1.325 *(835-751)*1000 3600
= c)
23744 KW
Percentage of heat utilization by ITSH: PHUS
=
(M si * (H 2 – H 1 )*24*100))/ M c *CV
=
653.3 *(3180.6-2965.2)*24
*100
3422 * 14179 = d)
6.96 %
Effectiveness of ITSH: •
Heat gained by steam(Qc)=Msi*Cpsi*(Tsi2 –Tsi1)* 1000 3600 = 653.3 *2.094*(470-398)*1000
95
3600 = •
27360.204 KW
Heat lossed by flue gas(Q h)= Mf * Cpfi *(Tfi1 –Tfi2)*1000 3600 =
768 *1.325 *(835-751)*1000 3600
=
•
Average,
23744 KW
Q = Qc+Qh 2 =
27360.204 + 23744 2
= •
•
Area of Outlet,
25552.102 KW Ao
=
=
16 *26 *∏* 0.038 *28.895
=
1434.99 m2
DTi = Inlet temperature difference (Tfi1 – Tsi1) = 835 – 470
•
=
365 0C
DTo = Outlet temperature difference (T fi2 – Tsi1) = 751 – 398
•
n*∏*Do*l
LMTD
=
353 0C
= Logarithmic Mean Temperature Difference, (DT i –DT 0 ) ln (DTi /DT0 ) = (365 – 353) ln (365/353 ) = 358.97 0C
•
External overall heat transfer coefficient U0
=
Q/A0 * LMTD
96
= 25552.102 1434.99 * 358.97 0.0496KW/ m2 0C
= •
Heat capacity of hot fluid, Ch = =
M f * Cpfi * 1000/3600
768 *1.325 *1000 3600 282.67KW/0 C
= •
Heat capacity of cold fluid, Cc = =
653.3 *2.094*1000 3600 380KW/0 C
= •
Msi * Cpsi * 1000/3600
If Ch < Cc then effectiveness, E= =
(Tfh1 – Tfh2)/ (Tfh1 – Tsh1 )
835-751 835-398
=
0.1922
By NTU Method: If Ch < Cc then If C Otherwise C Here C
min
C
max
min
min
= Ch and C
= Cc and C
max
max
= Cc
= Ch
= Cc = 282.67KW/0 C = Ch = 380KW/0 C
Number of transfer units, •
NTU = =
Uo * Ao / C
min
0.0496 * 1434.99 282.67
= •
0.2518
Ratio of heat capacity, R = C =
min
/ C max
282.67 380
= •
0.7439
NTU_E
=
(1-exp-NTU*(1-R)) (1-(R* exp-NTU*(1-R)))
97
=
(1-exp-0.2518*(1-0.7439)) (1-(0.7439* exp-0.2518*(1-0.7439)))
=
0.2064
98
RESULTS:
Heat
Percentage
Efficiency
Transfer
of heat
by NTU
%
rate (KW)
utilized %
method
1. Economiser
53.14
96958.77
9.17
0.4834
0.7839
2. LTSH
12.01
39640.87
7.60
0.4645
0.3662
3. HTSH
22.10
22878.72
11.79
0.1785
0.3289
4. ITSH
12.02
23744.00
6.96
0.1922
0.2064
Description
Effectiveness
Effectiveness
99
8.1 CONCLUSIONS:
1. By using Economizer By installing the economizer in the plant, the plant Efficiency can be increased by 10%.
2. By using Super Heater By implementing the super heater the efficiency can be increased by 25-30 %, and 8-10 % in each stage of super heater.
3. By using High grade coal the following parameters can be improved. •
Due to high calorific value of coal the heat liberated by burning coal in combustion chamber will be more.
•
The fly ash in flue gas is less.
•
The ash content in high grade fuel is 20-25 % only.
•
Mechanical maintenance can be reduced.
•
The coal mill output can be increased by using high grade coal.
•
The mass of fuel gas is required by the coal be reduced by removal of moisture in coal & power required by this can by reduced.
•
The fuel consumption for same power generation can be reduced.
•
The load factor on power plant can be improved.
•
Increase in overall efficiency of the plant.
•
The percentage of pollutants contained in the flue gasses coming out of chimney is less.
100
8.2 MAINTENANCE AND SUGGESSIONS In comparison with plane tube economiser, steel finned Economiser occupies less space for same thermal performance and draught loss. The reduction in tube length for similar tube diameter is usually around 4-7. These results in smaller casing, less structural steel work to support the reduced weight, fewer bends, and fewer welds so there are less losses.
Economiser needs less maintenance for effective running.
The phase change in economiser is not accepted.
Generally economiser tubes are made by carbon steel (k=120 to 170 w/mk) if the tubes are made of aluminium steels (k=205 w/mk) causes improvement in thermal properties.
If the feed water contains minerals & impurities it causes corrosion and scale formation in the economiser tubes. This corrosion and scaling layers are bad conductors of heat and electricity and the heat transfer rate will decrease in the economiser. So the feed water should be de-mineralized to free from impurities and foreign matters.
The pH of feed water should be maintained between 8 & 9.
The feed water leakage at the pipe joints is avoided by using gaskets.
Acoustic steam leakage detectors and micro phones are arranged for detection of leakage in LTSH, ITSH, and HTSH in 8 elevations.
The rate of air supply should be maintained constant for efficient heat transfer.
In order to increase the life span of furnace tubes they should be made by special materials like Monel mixed with
101
stainless
steel
which
prevents
leakages,
hence
the
efficiency of boiler.
9.0
BIBLIOGRAPHY
1. A Course in Power Plant Engineering -By Arora & Dom Kundwar
2. A Course in Thermal Engineering -By Arora & Dom Kundwar
3. Power Plant Engineering
-By P.K.Nag
4. Thermal Engineering
-By P. L. Bhallaney
5. Operations and Maintenance of Boiler Accessories -By BHEL Manual
6. Elementary Steam Power Engineering -By Mac Naughton. E
7. Thermal Engineering
-By H.L. Solberg
8. Steam Generation
-By J.N. Williams
9. Progress in Energy Auditing and Conversion -By
M.P.Muragai
&
Ram
Chandra
102
