Thermal Power Plant

Introductory 

In a coal based power plant coal is transported from coal mines to the power plant by railway in wagons or in a merry-go-round system. Coal is unloaded from the wagons to a moving underground conveyor belt. This coal from the mines is of no uniform size. So it is taken to the Crusher house and crushed to a size of 20mm. From the crusher house the coal is either stored in dead storage( generally 40 days coal supply) which serves as coal supply in case of coal supply bottleneck or to the live storage(8 hours coal supply) in the raw coal bunker in the boiler house. Raw coal from the raw coal bunker is supplied to the Coal Mills by a Raw Coal Feeder. The Coal Mills or pulverizer pulverizes the coal to 200 mesh size. The powdered coal from the coal mills is carried to the boiler in coal pipes by high pressure hot air. The pulverized coal air mixture is burnt in the boiler in the combustion zone. 

Generally in modern boilers tangential firing system is used i.e. the coal nozzles/ guns form tangent to a circle. The temperature in fire ball is of the order of 1300 deg.C. The boiler is a water tube boiler hanging from the top. Water is converted to steam in the boiler and steam is separated from water in the boiler Drum. The saturated steam from the boiler drum is taken to the Low Temperature Superheater, Platen Superheater and Final Superheater respectively for superheating. The superheated steam from the final superheater is taken to the High Pressure Steam Turbine (HPT). In the HPT the steam pressure is utilized to rotate the turbine and the resultant is rotational energy. From the HPT the out coming steam is taken to the Reheater in the boiler to increase its temperature as the steam becomes wet at the HPT outlet. After reheating this steam is taken to the Intermediate Pressure Turbine (IPT) and then to the Low Pressure Turbine (LPT). The outlet of the LPT is sent to the condenser for condensing back to water by a cooling water system. This condensed water is collected in the Hotwell and is again sent to the boiler in a closed cycle. The rotational energy imparted to the turbine by high pressure steam is converted to electrical energy in the Generator.·    

   

  Coal Preparation

i)Fuel preparation system: 

In coal-fired power stations, the raw feed coal from the coal storage area is first crushed into small                         pieces and then conveyed to the coal feed hoppers at the boilers. The coal is next pulverized into a very fine powder, so that   coal will undergo complete combustion during combustion process.

Pulverizer is a mechanical device for the grinding of many different types of materials. For example, they

are used to pulverize coal for combustion in the steam-generating furnaces of fossil fuel power plants.

Types of Pulverisers: Ball and Tube mills; Ring and Ball mills; MPS; Ball mill; Demolition.

ii)Dryers:  they are used in order to remove the excess moisture from coal mainly wetted during transport. As the presence of moisture will result in fall in efficiency due to incomplete combustion and also result in CO emission.

iii)Magnetic separators: coal which is brought may contain iron particles. These iron particles may result in wear and tear. The iron particles may include bolts, nuts wire fish plates etc. so these are unwanted and so are removed with the help of  magnetic separators.

            The coal we finally get after these above process are transferred to the storage site.
            Purpose of fuel storage is two –

·         Fuel storage is insurance from failure of normal operating supplies to arrive.

·         Storage permits some choice of the date of purchase, allowing the purchaser to take advantage of seasonal market conditions. Storage of coal is primarily a matter of protection against the coal strikes, failure of the transportation system & general coal shortages.

There are two types of storage:

1.  Live Storage(boiler room storage): storage from which coal may be withdrawn to supply combustion equipment with little or no remanding is live storage. This storage consists of about 24 to 30 hrs. of coal requirements of the plant and is usually a covered storage in the plant near the boiler furnace. The live storage can be provided with bunkers & coal bins. Bunkers are enough capacity to store the requisite of coal. From bunkers coal is transferred to the boiler grates.

2.  Dead storage- stored for future use. Mainly it is for longer period of time, and it is also mandatory to keep a backup of fuel for specified amount of days depending on the reputation of the company and its connectivity.There are many forms of storage some of which are –

1.  Stacking the coal in heaps over available open ground areas.

2.  As in (I). But placed under cover or alternatively in bunkers.

3.  Allocating special areas & surrounding these with high reinforced concerted retaking walls.

·         Boiler and auxiliaries

A Boiler or steam generator essentially is a container into which water can be fed and steam can be taken out at desired pressure, temperature and flow. This calls for application of heat on the container. For that the boiler should have a facility to burn a fuel and release the heat. The functions of a boiler thus can be stated as:-

1.  To convert chemical energy of the fuel into heat energy

2.  To transfer this heat energy to water for evaporation as well to steam for superheating.

The basic components of Boiler are: -

1.  Furnace and Burners

2.  Steam and Superheating

a. Low temperature superheater

b. Platen superheater

c. Final superheater

·         Economiser

It is located below the LPSH in the boiler and above pre heater. It is there to improve the efficiency of boiler by extracting heat from flue gases to heat water and send it to boiler drum.

Advantages of Economiser include

1) Fuel economy: – used to save fuel and increase overall efficiency of boiler plant.

2) Reducing size of boiler: – as the feed water is preheated in the economiser and enter boiler tube at elevated temperature. The heat transfer area required for evaporation reduced considerably.

·         Air Preheater

The heat carried out with the flue gases coming out of economiser are further utilized for preheating the air before supplying to the combustion chamber. It is a necessary equipment for supply of hot air for drying the coal in pulverized fuel systems to facilitate grinding and satisfactory combustion of fuel in the furnace

·         Reheater

Power plant furnaces may have a reheater section containing tubes heated by hot flue gases outside the tubes. Exhaust steam from the high pressure turbine is rerouted to go inside the reheater tubes to pickup more energy to go drive intermediate or lower pressure turbines.

·         Steam turbines

Steam turbines have been used predominantly as prime mover in all thermal power stations. The steam turbines are mainly divided into two groups: -

1.  Impulse turbine

2.  Impulse-reaction turbine

The turbine generator consists of a series of steam turbines interconnected to each other and a generator on a common shaft. There is a high pressure turbine at one end, followed by an intermediate pressure turbine, two low pressure turbines, and the generator. The steam at high temperature (536 ‘c to 540 ‘c) and pressure (140 to 170 kg/cm2) is expanded in the turbine.

·         Condenser

The condenser condenses the steam from the exhaust of the turbine into liquid to allow it to be pumped. If the condenser can be made cooler, the pressure of the exhaust steam is reduced and efficiency of the cycle increases. The functions of a condenser are:-

1) To provide lowest economic heat rejection temperature for steam.

2) To convert exhaust steam to water for reserve thus saving on feed water requirement.

3)  To introduce make up water.

We normally use surface condenser although there is one direct contact condenser as well. In direct contact type exhaust steam is mixed with directly with D.M cooling water.

·         Boiler feed pump

Boiler feed pump is a multi stage pump provided for pumping feed water to economiser. BFP is the biggest auxiliary equipment after Boiler and Turbine. It consumes about 4 to 5 % of total electricity generation.

·         Cooling tower

The cooling tower is a semi-enclosed device for evaporative cooling of water by contact with air. The hot water coming out from the condenser is fed to the tower on the top and allowed to tickle in form of thin sheets or drops. The air flows from bottom of the tower or perpendicular to the direction of water flow and then exhausts to the atmosphere after effective cooling.

The cooling towers are of four types: -

1. Natural Draft cooling tower

2. Forced Draft cooling tower

3. Induced Draft cooling tower

4. Balanced Draft cooling tower

·         Fan or draught system

In a boiler it is essential to supply a controlled amount of air to the furnace for effective combustion of fuel and to evacuate hot gases formed in the furnace through the various heat transfer area of the boiler. This can be done by using a chimney or mechanical device such as fans which acts as pump.

i) Natural draught

When the required flow of air and flue gas through a boiler can be obtained by the stack (chimney) alone, the system is called natural draught. When the gas within the stack is hot, its specific weight will be less than the cool air outside; therefore the unit pressure at the base of stack resulting from weight of the column of hot gas within the stack will be less than the column of extreme cool air. The difference in the pressure will cause a flow of gas through opening in base of stack. Also the chimney is form of nozzle, so the pressure at top is very small and gases flow from high pressure to low pressure at the top.

ii) Mechanized draught

There are 3 types of mechanized draught systems

1)                  Forced draught system

2)                  Induced draught system

3)                  Balanced draught system

Forced draught: – In this system a fan called Forced draught fan is installed at the inlet of the boiler. This fan forces the atmospheric air through the boiler furnace and pushes out the hot gases from the furnace through superheater, reheater, economiser and air heater to stacks.

Induced draught: – Here a fan called ID fan is provided at the outlet of boiler, that is, just before the chimney. This fan sucks hot gases from the furnace through the superheaters, economiser, reheater and discharges gas into the chimney. This results in the furnace pressure lower than atmosphere and affects the flow of air from outside to the furnace.

Balanced draught:-In this system both FD fan and ID fan are provided. The FD fan is utilized to draw control quantity of air from atmosphere and force the same into furnace. The ID fan sucks the product of combustion from furnace and discharges into chimney. The point where draught is zero is called balancing point.

 ·         Ash handling system

The disposal of ash from a large capacity power station is of same importance as ash is produced in large quantities. Ash handling is a major problem.

i) Manual handling: While barrows are used for this. The ash is collected directly through the ash outlet door from the boiler into the container from manually.

ii) Mechanical handling: Mechanical equipment is used for ash disposal, mainly bucket elevator, belt conveyer. Ash generated is 20% in the form of bottom ash and next 80% through flue gases, so called Fly ash and collected in ESP.

 iii) Electrostatic precipitator: From air preheater this flue gases (mixed with ash) goes to ESP. The precipitator has plate banks (A-F) which are insulated from each other between which the flue gases are made to pass. The dust particles are ionized and attracted by charged electrodes. The electrodes are maintained at 60KV.Hammering is done to the plates so that fly ash comes down and collect at the bottom. The fly ash is dry form is used in cement manufacture.

 ·         Generator

Generator or Alternator is the electrical end of a turbo-generator set. It is generally known as the piece of equipment that converts the mechanical energy of turbine into electricity. The generation of electricity is based on the principle of electromagnetic induction.

Advantages of coal based thermal Power Plant

·         They can respond to rapidly changing loads without difficulty

·         A portion of the steam generated can be used as a process steam in different industries

·         Steam engines and turbines can work under 25 % of overload continuously

·         Fuel used is cheaper

·         Cheaper in production cost in comparison with that of diesel power stations

Disadvantages of coal based thermal Power Plant

·         Maintenance and operating costs are high

·         Long time required for erection and putting into  action

·         A large quantity of water is required

·         Great difficulty experienced in coal handling

·         Presence of troubles due to smoke and heat in the plant

·         Unavailability of good quality coal

·         Maximum of  heat  energy lost

·         Problem of ash removing

Power Plant Boiler

The boiler generates high pressure steam by transferring the heat of Combustion in various heat transfer sections. This part of the article series briefly describes the flow and arrangement of the heat transfer sections in a boiler. In line diagrams help make the concept clear.

 Basics

Volume of one unit mass of steam is thousand times that of water, When water is converted to steam in a closed vessel the pressure will increase. Boiler uses this principle to produce high pressure steam.

Conversion of Water to Steam evolves in three stages.

• Heating the water from cold condition to boiling point or saturation temperature – sensible heat addition.
• Water boils at saturation temperature to produce steam - Latent heat.addition.
• Heating steam from saturation temperature to higher temperature called Superheating to increase the power plant output and efficiency.

Sensible Heat Addition

Feed Water Pump.

The first step is to get a constant supply of water at high pressure into the boiler. Since the boiler is always at a high pressure. ‘Boiler feed water pump’ pumps the water at high pressure into the boiler from the ‘feed water tank’. The pump is akin to the heart in the human body.

Pre-Heating

'Feed water heaters’, using extracted steam from the turbine, adds a part of the sensible heat even before the water enters the boiler.

economizer.

 Most of the sensible heat is absorbed in the Economizer. These are a set of coils made from steel tubes located in the tail end of a boiler. The hot gases leaving the boiler furnace heat the water in the coils. The water temperature is slightly less than the saturation temperature. From the economizer the water is fed to the 'drum'.

Latent Heat Addition

Drum.

The drum itself a large cylindrical vessel that functions as the storage and feeding point for water and the collection point for water and steam mixture. This is the largest and most important pressure part in the boiler and weighs in the range 250 Tons for 600 MW power plant.

Water Walls

Boiling takes place in the ‘Water Walls’ which are water filled tubes that form the walls of the furnace. Water Walls get the water from the ‘down comers’ which are large pipes connected to the drum. The down comers and the water wall tubes form the two legs of a water column.

As the water heats up in the furnace a part of the water in the water-wall tubes becomes steam. This water steam mixture has a lower density than the water in the downcomers. This density difference creates a circulation of water from the drum, through the downcomers, water walls and back to the drum. Steam collects at the upper half of the drum. The steam is then sent to the next sections.

The temperature in the drum, downcomers and water wall is at the saturation temperature.

WaterWalls

Materials and Thermal Efficiency of a Power Plant

What are the limitations imposed by materials on thermal power plants achieving the highest efficiency and output at the lowest cost? How to get over these limitations? Read on...

Laws of Thermodynamics

1. You cannot win, you can only break even.
2. You can only break even at absolute zero.
3. You cannot reach absolute zero.

The thermodynamic cycle used in a Thermal Power plant utilises steam at high temperatures and pressures. Increasing the upper temperature and pressure limits of the thermodynamic cycle increases the efficiency of the cycle. Power Plants operating with steam parameters of 540 °C and 170 bar pressure have an efficiency of 38 % while Ultra super critical power plants with steam parameters of 300 bar and 620 °C can have efficiencies of 48 %. This increase in efficiency is a direct emissions reduction apart from the cost savings.

Why are the older power plants operating at a lower temperature and pressure? Why are power plants with higher temperatures greater than 615 °C not made? This is because of the limitations imposed by the materials used for making the tubes, drums, and pipes which contain and transport the steam.

Limitations

The limitations in material are due to

• Reduction in strength.
  
  
• The mechanical strength properties of steel drastically reduce with increased temperatures. This means to withstand the higher pressure, the thickness of the tubes and pipes have to be increased.
• Added to this the continued operation at high pressure and temperatures leads to creep or slow degradation in the mechanical strength properties.
• Increased thickness means higher thermal stresses which imposes severe limitations on the design engineers. Also increased thickness means higher weight, meaning more structures and foundations, all leading to design limitations and higher cost.
• Oxidation.
  
  
• At higher temperatures, due to oxidation, scales form on the tube material. This in the continuous operation effect the life of the plant. Oxidation limit for Carbon steels is around 425 °C.As the steam and gas temperatures increase above the this limit, special alloy steels have to be used to prevent oxidation.

To overcome these limitations newer and newer materials are developed. Two decades ago the tube material for carrying the Superheated steam was grade T22 or P22 which had an allowable stress value of 50 N/mm² at 570 °C. At 600 °C it reduces drastically to 34 N /mm². Today we use grade T91 or P91 that has strength of 78 N/mm² at 570 °C and 60 N/mm² at 600 °C. This relates to a 40 % reduction in thickness at 570 °C.

The difference is the addition of alloying elements to the basic Carbon Steel. Grade P22 has 2.0 % Chromium and 1 % Molybdenum whereas Grade P9 has 9 % Chromium and 1 % Molybdenum, Nickel, and Vanadium.

For the high temperature application these special steels are called creep resistant steels. These are derived from the normal Carbon steels by adding alloy elements that increase the mechanical strength and heat resistant properties.

The five criteria that the industry is looking for in developing new materials are:

• Mechanical strength of the material should be available at higher temperatures.
• Mechanical strength properties should be consistent throughout the life of the plant at these elevated operating conditions or it should be creep resistant.
• The materials should be easily produced and available.
• The materials should be easy for fabrication and construction.
• All this at a reasonable cost for investment.

But the most important thing is that this allows the power plants to operate at higher temperatures and pressure which means higher efficiency and lower emissions.

91 or T91 grade material

For the last two decades the power industry standard material for high temperature applications is the P91 or T91 grade material. What is this material? What are its benefits? What are the precautions to be taken during construction?

The steam leaving the super heater of a modern large capacity boiler is in the order or 570 °C to 600 °C and at pressures ranging from 170 bar to 230 bar. This means the last stages of the super heater and the pipes carrying the steam to the turbine should withstand these extreme conditions. This requires this material should have very high strength properties, which do not deteriorate with time, and should be creep resistant.

Advantages of P91

SA 213 T91 or SA 335 P91 is such a ferritic alloy steel that meets this condition. This material has been in use for the last two decades successfully in power plant service. It is also called 9 Cr 1 Mo steel based on its composition.

Compared to its predecessor, the T22 or P22 grade, grade 91 exhibits high strength up to temperatures in the range of 600 °C. Also the oxidation temperature limits are higher. This allows the power plant designers to engineer components, superheater coils, headers and steam piping, with less thickness. This contributes to a higher thermal fatigue life of almost ten times. This allows them to increase the operating temperature to a higher level, increasing the efficiency of the power plant.

This makes it ideally suitable for plants that operate on a cyclic basis like combined cycle plants. Also the reduction in thickness suits HRSG designers since in an HRSG the temperature head is limited and locating the coils in the heat transfer path is very critical.

Why is P91 different ?

What makes this steel different is the addition of a high amount of Chromium. Grade 91 contains 9 % Chromium and 1 % Molybdenum compared to 2.5 % Chromium in the next best P22 grade. Chromium improves high temperature strength and increases oxidation resistance. Molybdenum increases the creep resistance. Also present are smaller quantities of Nickel and Manganese which increase the hardenability of the steel.

More important than the alloying elements is the formation of this alloy steel. The steel is formed by normalizing at 1050 °C, air cooling down to 200 °C. It is then tempered by heating to 760 °C. The temperatures and the cooling rates are very important. This produces the microstructure that results in the high creep strength properties.

This steel is not tolerant to variations in its microstructure, unlike P22 grade or other grades.

The steel has to be from manufacturers who strictly and precisely follow the heat treatment requirements. Many cases have been reported of failures of the base materials in the early stages of usage.

After the steel is worked, proper and precise heat treatment is required to reinstate the microstructure back to its original conditions. If this is not done the steel has properties that are much lower than its predecessor P22. Many failures have resulted because of this. In the case of P22 and other low alloy steels, the effect of variations in heat treatment on the properties is not as vehement as in P91.

During the fabrication and construction phase, any process that affects the micro structure has to be reversed by a precise heat treatment. This brings back the microstructure back to original.

Welding P91

Welding is one process that is widely used during the construction. This affects the microstructure. Preheating, maintaining inter-pass temperatures, and post-weld heat treatment procedures are very critical for P91 grade. Failure to follow the procedures will result in catastrophic failures.

For thick walled pipes, the use of an induction heating system is the ideal method. This gives better control, and uniform heating between the inner and outer diameters. In induction heating the coils themselves do not heat up. This is ideal for maintaining the inter-pass temperatures and carry out the welding. This is a more worker friendly heating process. This is also ideal for complex shapes likes weldolets and tees.

The Nickel and Manganese content, even though in smaller percentages, have profound effects on the critical temperatures, which decides the heat treatment temperatures and the cooling rates. Because of this, the composition of the welding electrodes used should be in line with the parent material.

Effect of Water

The un-heat treated steel has great affinity to Hydrogen. Hydrogen can cause stress corrosion cracking. Pre-heating has to be done properly to remove any moisture. The post weld heat treatment has to be done as quickly as possible to avoid any contact with water likely from moisture condensation, rainfall, etc. Great care has to be taken to see that all joints are post-weld heat treated prior to hydro test.

Dissimilar weld joints especially at complicated geometries can result in the heat treatment not having the desired effect throughout the cross sections. This can also lead to failures. Great care has to be taken to avoid such design flaws.

As the industry accepts these practices of constructions, the use of 91 grade steel continues to its successful journey.

PIPING IN POWER PLANT

HP PIPING
 1. MAINSTEAM
 2. COLD REHEAT (CRS)
 3. HOT REHEAT(HRH)
 4. HIGH PRESSURE FEED WATER (HPFW)
 5. FEED WATER RECIRCULATION
 6. LP BYPASS
 7. HP BYPASS 8. DESUPERHEATING

  IP/LP PIPING
 1. Condensate piping
2. Condensate desuperheating
3. Condensate discharge
4. Hot well drain
5. Condensate storage tank
6. No.1 extraction
7. No.2 extraction
8. No.3 extraction
9. No.4 extraction
10. No.5 extraction
 11. No.6 extraction
12. LP feed steam
13. HP feed steam
 14. Turbine house auxiliary steam
15. Auxiliary interior steam
16. LP feed water 1
7. IP feed water
 18. Instruments air
19. Service air
20. Auxiliary main stream piping
21. Deacration & hp heater vent piping
22. LH heaters vent piping
23. LP heater drain
24. HP heater drain
25. Deaerator drain & over flow piping
26. Miscellaneous piping
27. Cccw
28. Occcw
 29. Rubber ball piping
30. Nitrogen piping
31. Drain flash tank piping
32. Main drain tank

OHM'S LAW IN AC CIRCUITS

Many ac circuits contain resistance only. The rules for these circuits are the same rules that apply to dc circuits. Resistors, lamps, and heating elements are examples of resistive elements. When an ac circuit contains only resistance, Ohm's Law, Kirchhoff's Law, and the various rules that apply to voltage, current, and power in a dc circuit also apply to the ac circuit. The Ohm's Law formula for an ac circuit can be stated as

Remember, unless otherwise stated, all ac voltage and current values are given as effective values. The formula for Ohm's Law can also be stated as

The important thing to keep in mind is: Do Not mix ac values. When you solve for effective values, all values you use in the formula must be effective values. Similarly, when you solve for average values, all values you use must be average values. This point should be clearer after you work the following problem: A series circuit consists of two resistors (R1 = 5 ohms and R2 = 15 ohms) and an alternating voltage source of 120 volts. What is Iavg?

The alternating voltage is assumed to be an effective value (since it is not specified to be otherwise). Apply the Ohm's Law formula

The problem, however, asked for the average value of current (I avg). To convert the effective value of current to the average value of current, you must first determine the peak or maximum value of current, Imax.