Steam Power Plant PDF

Learning Module 3 Steam Power Plant Learning Packet 1 STEAM POWER PLANT (Theories and Elements)  Learning Packet 2 Steam Power Plant Introduction The course packet deals with the basic components and principles in the operation of a steam powe...

Learning Module 3 Steam Power Plant Learning Packet 1 STEAM POWER PLANT (Theories and Elements)  Learning Packet 2 Steam Power Plant Introduction The course packet deals with the basic components and principles in the operation of a steam power plant. Objectives Understand the basic principles in the operation of steam power plant Understand coal handling in the operation of steam power plant Learning Management System Google Classroom Duration Topic 01: Components of Steam Power Plant --------1 hour Topic 02: Coal Handling and Maintenance ------ - 1 hour Delivery Mode Synchronous Mode Assessment with Rubrics Theoretical questions and Solving Problems Readings 1. https://youtu.be/IdPTuwKEfmA Introduction Steam is an important medium of producing mechanical energy. Steam has the advantage that, it can be raised from water which is available in abundance it does not react much with the materials of the equipment of power plant and is stable at the temperature required in the plant. Steam is used to drive steam engines, steam turbines etc. Steam power station is most suitable where coal is available in abundance. Thermal electrical power generation is one of the major methods. For a thermal power plant, the range of pressure may vary from 10 kg/cm² to super critical pressures and the range of temperature may be from 250°C to 650°C. Essentials of Steam Power Plant Equipment  1. A furnace to burn the fuel. 2. Steam generator or boiler containing water. Heat generated in the furnace is utilized to convert water in steam. 3. Main power unit such as an engine or turbine to use the heat energy of steam and perform work. 4. Piping system to convey steam and water In addition to the above equipment the plant requires various auxiliaries and accessories depending upon the availability of water, fuel and the service for which the plant is intended. The flow sheet of a thermal power plant consists of the following four main circuits: 1. Feed water and steam flow circuit 2. Coal and ash circuit 3. Air and gas circuit 4. Cooling water circuit. A steam power plant using steam as working substance works basically on Rankine cycle. Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser and fed into the boiler again. The different types of systems and components used in steam power plant are as follows: 1. High pressure boiler 2. Prime mover 3. Condensers and cooling towers 4. Coal handling system 5. Ash and dust handling system 6. Draught system 7. Feed water purification plant 8. Pumping system 9. Air preheater, economizer, super heater, feed heaters. Steam condensing system consists of the following: 1. Condenser 2. Cooling water 3. Cooling tower 4. Hot well 5. Condenser cooling water pump 6. Condensate air extraction pump 7. Air extraction pump 8. Boiler feed pump 9. Make up water pump. Power Station Design Power station design requires wide experience. A satisfactory design consists of the following steps: 1. Selection of site 2. Estimation of capacity of power station. 3. Selection of turbines and their auxiliaries. 4. Selection of boilers, and their auxiliaries. 5. Design of fuel handling system. 6. Selection of condensers. 7. Design of cooling system. 8. Design of piping system to carry steam and water. 9. Selection of electrical generator. 10. Design and control of instruments. 11. Design of layout of power station. Quality of coal used in steam power station plays an important role in the design of power plant. The various factors to be considered while designing the boilers and coal handling units are as follows: 1. Slagging and erosion properties of ash. 2. Moisture in the coal. Excessive moisture creates additional problems particularly in case of pulverized fuel power plants. 3. Burning characteristic of coal. 4. Corrosive nature of ash. Characteristics of Steam Power Plant The desirable characteristic for a steam power plant are as follows: 1. Higher efficiency. 2. Lower cost. 3. Ability to burn coal especially of high ash content, and inferior coals. 4. Reduced environmental impact in terms of air pollution. 5. Reduced water requirement. 6. Higher reliability and availability. Coal delivery equipment is one of the major components of plant cost. The various steps involved in coal handling are as follows: a. Coal Delivery. The coal from supply points is delivered by ships or boats to power stations situated near to sea or river whereas coal is supplied by rail or trucks to the power stations which are situated away from sea or river. The transportation of coal by trucks is used if the railway facilities are not available. b. Unloading. The type of equipment to be used for unloading the coal received at the power station depends on how coal is received at the power station. If coal is delivered by trucks, there is no need of unloading device as the trucks may dump the coal to the outdoor storage. Coal is easily handled if the lift trucks with scoop are used. c. Preparation. When the coal delivered is in the form of big lumps and it is not of proper size, the preparation (sizing) of coal can be achieved by crushers, breakers, sizers driers and magnetic separators. d. Transfer. After preparation coal is transferred to the dead storage by means of the following systems : 1. Belt conveyors. 2. Screw conveyors. 3. Bucket elevators. 4. Grab bucket elevators. 5. Skip hoists.  6. Flight conveyor e. Outdoor and Covered Storage It is desirable that sufficient quantity of coal should be stored. Storage of coal gives protection against the interruption of coal supplies when there is delay in transportation of coal. The amount of coal to be stored depends on the availability of space for storage, transportation facilities, the amount of coal that will whether away and nearness to coal mines of the power station. Coal required for one month operation of power plant is stored in case of power stations situated at longer distance from the collieries whereas coal need for about 15 days is stored in case of power station situated near to collieries. Storage of coal for longer periods is not advantageous because it blocks the capital and results in deterioration of the quality of coal. The coal received at the power station is stored in dead storage in the form of piles laid directly on the ground. The coal stored has the tendency to whether (to combine with oxygen of air) and during this process coal loss some of its heating value and ignition quality. Due to low oxidation the coal may ignite spontaneously. This is avoided by storing coal in the form of piles which consist of thick and compact layers of coal so that air cannot pass through the coal piles. This will minimize the reaction between coal and oxygen. The other alternative is to allow the air to pass through layers of coal so that air may remove the heat of reaction and avoid burning. In case the coal is to be stored for longer periods the outer surface of piles may be sealed with asphalt or fine coal. The coal is stored by the following methods: 1. Stocking the coal in heats. The coal is piled on the ground up to 10-12 m height. The pile top should be given a slope in the direction in which the rain may be drained off. The sealing of stored pile is desirable in order to avoid the oxidation of coal after packing an air tight layer of coal. Asphalt, fine coal dust and bituminous coating are the materials commonly used for this purpose.  2. Under water storage. The possibility of slow oxidation and spontaneous combustion can be completely eliminated by storing the coal under water. Coal should be stored at a site located on solid ground, well drained, free of standing water preferably on high ground not subjected to flooding. f. In-Plant Handling From the dead storage the coal is brought to covered storage (Live storage) (bins or bunkers). A cylindrical bunker. In plant handling may include the equipment such as belt conveyors, screw conveyors, bucket elevators etc. to transfer the coal. The commonly used methods to weigh the coal are as follows: 1. Mechanical 2. Pneumatic 3. Electronic The Mechanical method works on a suitable lever system mounted on knife edges and bearings connected to a resistance in the form of a spring of pendulum. The pneumatic weight used a pneumatic transmitter weight head and the corresponding air pressure determined by the load applied. The electronic weighing machines make use of load cells that produce voltage signals proportional to the load applied. The important factor considered in selecting fuel handling systems are as follows: 1. Plant flue rate 2. Plant location in respect to fuel shipping 3. Storage area available. Dewatering of Coal Excessive surface moisture of coal reduces and heating value of coal and creates handling problems. The coal should therefore be dewatered to produce clean coal. Cleaning of coal has the following advantages: 1. Improved heating value. 2. Easier crushing and pulverizing 3. Improved boiler performance 4. Less ash to handle. 5. Easier handling. 6. Reduced transportation cost. Fuel is burnt in a confined space called furnace. The furnace provides supports and enclosure for burning equipment. Solid fuels such as coal, coke, wood etc. are burnt by means of stokers whereas burners are used to burn powdered (Pulverized) coal and liquid fuels. Solid fuels require a grate in the furnace to hold the bed of fuel. Type of Furnace According to the method of firing fuel furnaces are classified into two categories: 1. Grate fired furnaces They are used to burn solid fuels. They may have a stationary or a movable bed of fuel. These furnaces are classified as under depending upon the method used to fire the fuel and remove ash and slag. 2. Chamber fired furnaces. They are used to burn pulverized fuel, liquid and gaseous fuels. Furnace shape and size depends upon the following factors: 1. Type of fuel to be burnt. 2. Type of firing to be used. 3. Amount of heat to be recovered. 4. Amount of steam to be produced and its conditions. 5. Pressure and temperature desired. 6. Grate area required. 7. Ash fusion temperature. 8. Flame length. 9. Amount of excess air to be used. Hand and Mechanical Firing Hand Firing is a simple method of firing coal into the furnace. It requires no capital investment. It is used for smaller plants. This method of fuel firing is discontinuous process, and there is a limit to the size of furnace which can be efficiently fired by this method. A hand fired grate is used to support the fuel bed and admit air for combustion. While burning coal the total area of air openings varies from 30 to 50% of the total grate area. The grate area required for an installation depends upon various factors such as its heating surface, the rating at which it is to be operated and the type of fuel burnt by it. The width of air openings varies from 3 to 12 mm. Mechanical Firing (stokers) are commonly used to feed solid fuels into the furnace in medium and large size power plants. The various advantages of stoker firing are as follows: 1. Large quantities of fuel can be fed into the furnace. Thus, greater combustion capacity is achieved. 2. Poorer grades of fuel can be burnt easily. 3. Stoker save labor of handling ash and are self-cleaning. 4. By using stokers better furnace conditions can be maintained by feeding coal at a uniform rate. Stokers save coal and increase the efficiency of coal firing. The main disadvantages of stokers are their more costs of operation and repairing resulting from high furnace temperatures. Types of Stokers A. Chain grate stoker and traveling grate stoker differ only in grate construction. A chain grate stoker consists of an endless chain which forms a support for the fuel bed. B. A spreader stoker. In this stoker the coal from the hopper is fed on to feeder which measures the coal in accordance to the requirements. Feeder is a rotating rum fitted with blades. Feeders can be reciprocating rams, endless belts, spiral worms etc. From the feeder the coal drops on to spreader distributor which spread the coal over the furnace. C. A multi-retort stoker. The coal falling from the hopper is pushed forward during the inward stroke of stoker ram. The distributing rams (pushers) then slowly move the entire coal bed down the length of stoker. The length of stroke of pushers can be varied as desired. Pulverized Coal Coal is pulverized (powdered) to increase its surface exposure thus permitting rapid combustion. Efficient use of coal depends greatly on the combustion process employed. For large scale generation of energy the efficient method of burning coal is confined still to pulverized coal combustion. The pulverized coal is obtained by grinding the raw coal in pulverizing mills. The various pulverizing mills used are as follows: 1. Ball Mill A line diagram of ball mill using two classifiers. It consists of a slowly rotating drum which is partly filled with steel balls. Raw coal from feeders is supplied to the classifiers from where it moves to the drum by means of a screw conveyor. 2. Ball and race Mill In this mill the coal passes between the rotating elements again and again until it has been pulverized to desired degree of fineness. The coal is crushed between two moving surfaces namely balls and races. The upper stationary race and lower rotating race driven by a worm and gear hold the balls between them. The raw coal supplied falls on the inner side of the races. The moving balls and races catch coal between them to crush it to a powder. 3. Shaft Mill In the fuel pulverization with a shaft mill. The fuel from bunker is moved to feeder via automatic balance. Then from duct fuel goes to mill where it is crushed by beaters secured on the spindle of the mill rotor. Ash Disposal A large quantity of ash is, produced in steam power plants using coal. Ash produced in about 10 to 20% of the total coal burnt in the furnace. Handling of ash is a problem because ash coming out of the furnace is too hot, it is dusty and irritating to handle and is accompanied by some poisonous gases. It is desirable to quench the ash before handling due to following reasons: 1. Quenching reduces the temperature of ash. 2. It reduces the corrosive action of ash. 3. Ash forms clinkers by fusing in large lumps and by quenching clinkers will disintegrate. 4. Quenching reduces the dust accompanying the ash. Ash Handling Equipment Mechanical means are required for the disposal of ash. The handling equipment should perform the following functions: 1. Capital investment, operating and maintenance charges of the equipment should be low. 2. It should be able to handle large quantities of ash. 3. Clinkers, soot, dust etc. create troubles, the equipment should be able to handle them smoothly. 4. The equipment used should remove the ash from the furnace, load it to the conveying system to deliver the ash to a dumping site or storage and finally it should have means to dispose of the stored ash. 5. The equipment should be corrosion and wear resistant. Ash Handling and Dust Collections System The commonly used ash handling systems are as follows: a. Hydraulic system b. Pneumatic system c. Mechanical system. The commonly used ash discharge equipment is as follows: a. Rail road cars b. Motor truck c. Barge. Types of Dust Collector The various types of dust collectors are as follows: 1. Mechanical dust collectors. 2. Electrical dust collectors. Mechanical dust collectors may be wet type or dry type. Wet type dust collectors called scrubbers make use of water sprays to wash the dust from flue gases. Mechanical Dust Collector Dry type dust collectors include gravitational, cyclone, louvered and baffle dust collectors. A cyclone dust collector uses a downward flowing vortex for dust laden gases along the inner walls. The clean gas leaves from an inner upward flowing vortex. The dust particles fall to the bottom due to centrifuging action. Fly ash centrifugal scrubber. It is similar to a mechanical ash collector but has a flowing water film on its inner walls. Due to this film, the collected ash is removed more rapidly from the apparatus to the bin and there is less possibility for secondary. Capture of collected dust particles by the gas flow. The degree of ash collection in scrubbers varies from 0.82 to 0.90. The dust laden gas enters through the inlet pipe. Cinder Catcher. Cinder catcher is used to remove dust and cinders from the gas. In this catcher, the dust laden gas is made to strike a series of vertical baffles that change its direction and reduce its velocity. The separated dust and cinders fall to the hopper for removal. Cinder catchers are ordinarily used with stoker firing.  Activity Sheet Theoretical Questions 1. Describe the various types of grates used with hand fired furnaces. 2. Name the various methods of ash handling. Describe the pneumatic system of ash handling. Why it is essential to quench the ash before handling? 3. Name the different types of coal-pulverizing mills. Describe Ball-Mill. 4. Describe the various types of burners used to burn pulverized coal. 5. What is the cause of smoke? State the factors necessary for its prevention. 6. Describe the various methods used to control the degree of superheat. Name the advantages gained by using super-heat steam. 7. What is a steam trap? Where it is located? Describe Ball Float steam trap. 8. What are the requirement of a well-designed pipe line in a steam power plant? Name and describe the various expansion bends used in piping steam. 9. A. How will you classify various types of boilers? B. Write short notes on the following: a. Efficiency of boiler b. Maintenance of boiler c. Accessories of a boiler d. Overall efficiency of steam power plant e. Steam turbine specifications f. Causes of heat loss in boiler. 10. Explain the methods used to increase thermal efficiency of a steam power plant.