Steam power plant basically operates on the Rankine cycle. Coal is burnt in a boiler, which converts water into steam. The steam is expanded in a turbine, which produces mechanical power driving the alternator coupled to the turbine. The steam after expansion in prime mover (turbine) is usually condensed in a condenser to be fed into the boiler again. In practice, however, a large number of modifications and improvements have been made so as to affect economy and improve the thermal efficiency of the plant.
The working of a modern coal-fired steam power plant can be studied conveniently with the help of various cycles.
The entire arrangement for the sake of simplicity may be divided into four main circuits namely:
1. Fuel and ash circuit
2. Air and fuel gas circuit
3. Feed water and steam circuit, and
4. Cooling water circuit.
1. Fuel and Ash Circuit:
Coal is delivered from the supply points to the storage site by road, rail or water. By road coal is transported in trucks and for small stations such inland transport may be enough. Where power plants are situated close to a waterway, such as a canal, river or sea transportation by boat or ship may be quite effective. However, in most cases transportation of coal by rail, road is the most common.
In the case of small power plants the quantity of coal being small, manual unloading from rail car may be used but for large power stations the unloading from the railway siding is done with the help of wagon tipplers and then the coal is conveyed to the coal handling plant. From the coal handling plant, the coal having been good enough to be burnt into boilers is taken into the boiler bunkers by means of bucket conveyers.
The coal is thus stored in the bunkers from where it falls into the hoppers by gravity and finally the requisite quantity of coal either goes on falling directly on the grate or where the coal spreaders are provided, coal is spread in the grate up to the rear end. When use of spreaders is made, most of the coal burns in air and the remaining falls at the rear end of the grate.
Any unburnt coal particles in the middle of the grate are collected in a pipe and are again refired by cinder-refiring fan. The grate in such types of boilers, where use of spreaders is made move from rear end to front end, and without spreaders, the movement of the grate is from front end to rear end. Combustion is controlled by controlling the grate speed, quantity of coal entering the grate, the damper openings. The ash resulting after complete combustion of fuel collects at the back of the boiler and is removed to the ash storage by means of scrap conveyors.
2. Air and Fuel Gas Circuit:
Air is drawn from the atmosphere by a forced draught fan or induced draught fan through the air preheater, in which it is heated by the heat of flue gases passing to chimney and then admitted to the furnaces. The flue gases after passing around boiler tubes and super heater tubes are drawn by the induced draught fan through dust collector (or precipitator), economiser and air preheater and finally exhausted to the atmosphere through chimney.
3. Feed Water and Steam Circuit:
The steam coming out of the turbine is condensed and the condensate is extracted from the condenser by the condensate extraction pump and is forced to the low pressure feed water heaters (usually three in numbers) where its temperature is raised by the heat from bled steam (steam extracted from the lowest pressure extraction point of the turbine).
The feed water is now pumped through deaerator to high pressure feed water heaters (usually 2 or 3 in numbers) where it gets heated by the heat from bled steam extracted at suitable point of the steam turbine. The function of deaerator is to reduce dissolved oxygen content in the condensate (i.e., in the feed water).
The feed water is then pumped into boiler through economiser in which it is further heated by the heat of flue gases passing through it on the way to chimney. A small part (about 1 per cent) of steam and water in passing through the different components of the system is lost. Therefore, water is added in the feed water system as make-up water. In boiler water is converted into high pressure steam, which is wet. Wet steam is passed through super-heater, where it is dried and further superheated, and then supplied to the steam turbine through the main valve.
After giving out its heat energy to the turbine it is exhausted to the condenser where its latent heat is extracted and steam is converted into feed water. At one or more stages a quantity of steam is bled or withdrawn for heating of feed water. Making-up water for boiler is taken through the evaporator, where it is heated by low pressure steam extracted at suitable point of turbine.
4. Cooling Water Circuit:
Cooling water is supplied from a natural source of supply such as river, canal, sea or lake or cooling towers through screens to remove the matter that might choke the condenser tubes. It is circulated through the condenser for condensing the steam and finally discharged to the suitable position near the source of supply. During the passage its temperature rises and in the case of cooling towers the heat must be extracted before the water is again pumped to the condenser. The circulations of cooling water to the condenser help in maintaining a low pressure in the condenser.