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THERMAL POWER PLANT
1.1 INTRODUCTION:-
Now a day’s the electricity has become an essential commodity rather than luxury. In a state or region thermal power stations will become important, as hydro resources are inadequate. The concept of modern thermal power stations is that it should be situated at such a place that the basic requirements of fuel, water & land should be satisfied. Basically thermal stations are of two types,
1. Pit Head Stations.
2. Load Demand Stations.
Pit head stations are those which are near to the source of fuel and load demand stations are those which are near to the load centers.
The thermal power station is just like any other industry.
The basic requirements are:
a. Supply of raw materials at competitive costs. Coal and oil are the raw materials required for thermal plants.
b. Access to the markets for its products.
c. Labour force of the size and quantity required.
d. Means of disposal for any trade effluents that is by-products.
The other factors to be considered for selecting the site are:
1. Load Demand
2. Land
3. Site Requirements
4. Access for Construction
5. Transmission Lines
6. Clearances
7. Environmental Factors
Generally 1000 MW plant requires 90-200 acre land. The water requirements for thermal stations come under two main groups. The first requirement is for steam generation and the second requirement is for cooling purpose. Water for steam generation is low of the order of 3-4 tones per hour per megawatt, and make up quantity is 2-3% of the same. Whereas amount of water required for condensation is quite high. Coal for power stations in India, the principal source of commercial energy is amounting to 95% of the total primary energy resources of the country. The coal resources existing in the country are of the order of 1,30,000 million tones.
The main areas where the coal mines are located are the eastern region i.e. Bihar, Bengal central region, Singareni coal fields, Tamilnadu, Naively and small resources located in the rest of the country as well.
Other factors like transport, disposal of effluents, transmission, climatic conditions, proximity of air fields, fisheries and marine life, personnel required and amenities are also taken for considerations.
1.2 PRINCIPLE OF OPERATION:-
The fundamental forms of energy with which thermal stations are principally concerned are heat and work. Heat produces work and this work is further converts into electrical energy through a medium i.e. electrical generator. For the purpose of understanding of thermal plants, the phenomenon of thermodynamic vapour power cycles explained here under.
a. Rankine cycle
b. Reheat cycle
c. Regenerative cycle
a. RANKINE CYCLE:-
This is the simplest theoretical vapour cycle which is the basis for operation of a steam plant. Superheated steam from the boiler is fed into the prime mover and is expanded there. After which it enters the condenser emerging as the condensate. With the help of a pump this condensate is again fed into the boiler.
The main purpose of superheating steam and supplying it to the prime mover is to avoid too much wetness at the end of expansion. Moisture content of steam would result in undue blade erosion. The maximum wetness in the final stage of the steam that may be tolerated without any appreciable harm to the turbine blades is about 12%. Also the use of super heater in the boiler helps in reducing the stack temperature by extracting the heat from the flue gases before these are passed out of the chimney.
b. REHEAT CYCLE:-
In its simplest form the cycle involves with drawing the steam from the turbine at some intermediate stage, returning it to the steam generator where a separate super heater is provided in the gas path re-superheating the steam after which it is re introduced into the turbine at the following stage. It reduces the wetness of the steam at final stage and improves the efficiency of the cycle.
c. REGENERATIVE CYCLE:-
This cycle is an attempt to induce reversibility in the ordinary rankine cycle and thus to increase its efficiency. The mixing of coal condensate with the saturated steam and water mixture in the boiler constitute the principal irreversible process of the cycle and generation aims at reducing this irreversibility by heating the feed water near to the saturation temperature through the utilization of heat of the steam which is partially expanded in the prime mover. Since the purpose is the thermal regeneration of the condensate the cycle is known as regenerative cycle.
1.3 COAL TO STEAM PROCESS:-
Coal from mines is brought to plant through wagons and these wagons are unloaded in coal handling plant. The coal is transported to raw coal bunkers with the help of belt conveyors. Coal is then transported to mills through feeders where the coal is pulverized to powder form. This coal powder is lifted to the boiler with the help of primary air fans (PA fan). PA fan takes the air from the atmosphere, a part of which is sent to air pre heater for heating while a part goes directly to the mills for temperature control. Atmospheric air from FD fan heated in the air heaters and sent to the furnace as combustion air.
Water from the boiler feed pumps passes through the economizer and reaches the boiler drum. Water from the drum passes through the down comers and goes to bottom ring header. Water from the ring header is divided to all four sides of the furnace. Due to the heat and density difference the water raises up in the water wall tubes. Water is partially converted to steam as it rises up in the furnace. This steam and water mixture is taken to the boiler drum where the steam is separated from the water with the help of turbo separators. Water follows the same path while the steam is sent to superheaters for superheating. The superheaters are located inside the furnace and the steam is superheated (540o C) and finally it goes to turbine.
Flue gases from the furnace is extracted by the induced draft fans (ID fans), which maintains balance draft in the furnace with forced draft fan. This flue gases emit their heat energy to various superheaters in the pent house and finally passes through air preheaters and goes to electrostatic precipitator, where the ash particles are extracted. Electro static precipitators consist of metal plates, which are electrically charged. Ash particles are attracted on to these plates, so that they do not pass through the chimney to pollute the atmosphere. Regular mechanical hammers blows cause the accumulation of ash to fall to the bottom of the precipitator, where they are collected in a hopper for disposal. This ash is mixed with water to form slurry and is pumped to ash pond.
1.4 STEAM TO MECHANICAL POWER:-
A steam pipe conveys steam to the turbine through stop valve and control valves that automatically regulate the supply of the steam to the turbine. Steam from the control valves enters the high-pressure cylinder of the turbine, where it passes through a ring of stationary blades fixed to the cylinder wall. These act as nozzles and direct the steam into second ring of moving blades mounted on a disc secured to the turbine shaft. This second ring turns the shaft as a result of the force of the steam. The stationary and moving blades together constitute a stage of the turbine and in practice many stages are necessary so that cylinder contain a number of rings of stationary blades with rings of moving blades arranged between them. The steam passes through each stage in turn until it reaches the end of the high pressure cylinder and in its passage some of its heat energy is changed in to mechanical energy.
The steam leaving high pressure cylinder goes back to the boiler for reheating and enters into intermediate pressure cylinder through HRH lines. Hence it passes through another series of stationary and moving blades.
Finally steam is taken to the low pressure cylinders, each of which it enters at the center for following outwards in the opposite direction through the rows of turbine blades- an arrangement is known as double flow to the extremities of the cylinder. As the steam gives up its heat energy to drive the turbine, its temperature and pressure fall and it expands. Because of this expansion the blades are much larger and longer towards the low pressure end of the turbine.