In this article we will discuss about:- 1. Steam Power Plants 2. Efficiency of Steam Power Plants 3. Merits and Demerits 4. Layout 5. Steam Power Plant Auxiliaries 6. Turbo-Alternators.
Introduction to Steam Power Plants:
The fact that thermal energy is the major source of power generation itself shows the importance of thermal power generation in India more than 50 per cent of electric power is produced by steam plants in India. This position is likely to continue due to large pit head plants being set up. Larger sizes of units due to overall increase in demand for power and because of necessity of keeping down the cost of power generation with increasing fuel prices are the developing trends in large steam power plants.
As on July 31, 2010, and as per the Central Electricity Authority the total installed capacity of Coal and Lignite based power plants in India is 87,093.38 MW.
In steam power plants, the heat of combustion of fossil fuels (coal, oil or gas) is utilised by the boilers to raise steam at high pressure and temperature. The steam so produced is used in driving the steam turbines or sometimes steam engines coupled to generators and thus in generating electrical energy. Steam turbines or steam engines used in steam power plants not only act as prime movers but also as drives for auxiliary equipment, such as pumps, stokers fans etc.
Steam power plant may be installed either to generate electrical energy only or generate electrical energy along with the generation of steam for industrial purposes such as in paper mills, textile mills, sugar mills and refineries, chemical works, breweries, plastic manufacture, food manufacture etc. The steam for process purposes is extracted from a certain section of turbine and the remaining steam is allowed to expand in the turbine. Alternatively the exhaust steam may be used for process purposes.
Steam power plants may be either condensing or non-condensing type. In condensing type power plants the exhaust steam is discharged into a condenser, which creates suction at very low pressure and allows the expansion of steam in the turbine to a very low pressure and thus increases the efficiency. In non-condensing type power plants, the steam exhausted from the turbine is discharged at atmospheric pressure or at a pressure greater than atmospheric.
The principal advantages of condensing operation are the increased amount of energy extracted per kg of steam and the greater amount of power developed by a given size of turbine or engine. Moreover, in non-condensing type power plants, a continuous supply of fresh feed water is required whereas in condensing type power plants steam condensed into the water in the condenser can be re-circulated to the boilers with the help of pumps. This point becomes very important at places where there is a shortage of pure water.
According to use such plants can be classified into:
(1) Industrial power plants or captive power plants, and
(2) Central power plants.
1. Industrial Power Plants:
The industries requiring steam for process purposes may use steam turbines for generation of electrical energy also for its own use. The steam for process purposes may be tapped from extraction bleeding of the turbines at a pressure of 1.4-7 kg/cm2. Such plants are usually of non-condensing type. Industrial turbo-generator plants are usually of small capacity (say up to 10 MW).
Industrial turbo-generator plants, also known as captive power plants, otherwise also becoming more and more popular nowadays. The reasons for it are poor reliability of electric supply from the grid, frequent upward revision of power tariff charged by electric utilities and long power cuts. In the recent past many state governments have revised their policies as regards captive power generation and are encouraging setting up of captive power plants.
2. Central Power Plants:
Steam power plants are mainly used as central plants to generate electrical energy for supply to various consumers (industrial, agricultural, commercial, domestic etc.). Such plants are usually of condensing type. The size of the generating sets may be from 10-1,000 MW or even higher. When such power plants are used in a system having both hydro and steam power plants they can be used to supply load economically and to ensure reliability of power supply at all times.
Steam power plant for generation of electrical power is preferred, where large amount of power is required to be generated and financial, climatic, and geographical conditions do not permit the installation of hydroelectric power plants and coal is available in plenty.
Efficiency of Steam Power Plants:
The thermal efficiency of steam power plants, defined as the ratio of the heat equivalent of mechanical energy transmitted to the turbine shaft and the heat of combustion is quite low (about 30%). Overall efficiency of the power plant, defined as the ratio of heat equivalent of electrical output to the heat of combustion, is about 29 per cent.
The overall efficiency is determined by multiplying the thermal efficiency of power plant by the efficiency of generation (or electrical efficiency). In case of most modern supercritical pressure steam plants employing many heat saving devices, the plant overall efficiency may reach the value of 50 per cent.
Losses occurring in steam power plants may be summarized as follows:
(a) Boiler House Losses:
(i) To dry flue gases – 5%
(ii) To moisture in gases – 5%
(iii) To ash and unburnt carbon – 1.0%
(iv)To radiation and leakage – 2.5%
(v) Unknown loses – 2.5%
Total = 16.0%
(b) Turbine Losses:
Heat rejected to condenser – 54%
Alternator losses – 1%
Thus output is about – 29%
From the above mentioned figures of various losses occurring in steam power plants it is obvious that more than 50 per cent of total heat of combustion is lost as heat rejected to the condenser. This loss of heat energy is unavoidable as heat energy cannot be converted into mechanical energy without a drop in temperature, and the steam in condenser is at the lowest temperature.
The thermal efficiency of the plant mainly depends upon the following factors:
(i) Pressure and
(ii) Temperature of the steam entering the turbine and the pressure in the condenser.
The thermal efficiency increases with the increase in temperature and pressure of the steam entering the turbine. For this reason high pressures and temperatures are used. The thermal efficiency is effectively increased by decreasing the pressure in the condenser. Pressure in the condenser is kept very low usually 0.04 kg/cm2.
The thermal efficiency also increases by reheating the steam between turbine stages, but is somewhat inconvenient. Bleeding of steam also affects the thermal efficiency.
The overall thermal efficiency of 1st steam plant (50 MW units at Bokaro) commissioned in year 1952-53 in India was 28% while for the largest and most efficient thermal power plant having unit size of 160 MW at Trombay commissioned in 1965 it was 37% with heat rate 2,330 kcal/kWh of electrical energy produced. Each of the three major elements of a thermal power plant—the boiler, the turbine and the alternator have undergone intensive development and as a result the efficiency of electric generation has gone up to nearly 42% in 1980’s.
Choice of Steam Pressure and Temperature:
The modern trend is towards high pressure and temperature, but the choice should be economical one. The effects of increased pressure and temperature on the efficiency and cost of the plant are illustrated in Figs. 3.2 (a) and 3.2 (b).
From curves shown in Figs. 3.2 (a) and 3.2 (b), it is obvious that the efficiency follows the law of diminishing return with the increase in pressure, but with the increase in temperature, the efficiency follows the straight line law. So use of highest possible temperature is desirable.
The highest temperature is limited due to the strength of material and beyond 480°C, the change in physical properties of the material is very rapid and the problem becomes complicated. With the increase in pressure, the degree of superheat is to be reduced so as to keep within limits the total temperature. The present practice is to use steam pressures of about 6.5 N/mm2 for entirely new plant.
At a temperature of about 600°C and pressure of 30 N/mm2, water enters a supercritical phase and has properties between those of liquid and gas. Water in supercritical stage can dissolve a number of organic compounds and gases and on addition of hydrogen peroxide and liquid oxygen combustion process starts. The steam power plants operating on this principle are called supercritical plants.
The advantages of such plants are that low grade fossil fuels (e.g., lignite) can be used, NO2 emissions are completely eliminated and SO2 emissions are reduced and complete burning of coal occurs. So the plant has no need of desulphurisation and denitrification equipment and soot collector.
With this system the cost of processing flue gas emissions (electrostatic precipitator etc.) is eliminated and cooling water requirements are also reduced, so the system becomes more economical and efficient. Supercritical power plants, these days have an overall efficiency of just over 40%. With the use of temperatures around 700°C (known as ultra-supercritical condition), the overall efficiency of the system may be improved to around 50%.
Merits and Demerits of Steam Power Plants:
The steam power plants have the following merits and demerits over other plants:
(i) Fuel used is cheaper.
(ii) Less space is required in comparison with that for hydroelectric plants.
(iii) Cheaper in initial cost in comparison with other types of power plants of same capacity.
(iv) Cheaper in production cost in comparison with that of diesel power plants.
(v) Such plants can be installed at any place irrespective of the existence of fuels, while hydroelectric plants can be developed only at the source of water power.
(vi) Such plants can be located near the load centres, while the hydroelectric plants have essentially to be installed at source of water power which is usually isolated from urban areas, transmission costs are, therefore, reduced.
(vii) Able to respond to rapidly changing loads without difficulty.
(viii) Steam engines and turbines can work under 25% of overload continuously.
(ix) A portion of steam raised can be used as process steam in various industries (paper mills, textile mills, sugar mills, refineries, chemical works etc.)
(i) High maintenance and operating costs.
(ii) Pollution of atmosphere due to fumes and residues from pulverised fuels.
(iii) Requirement of water in huge quantity.
(iv) Handling of coal and disposal of ash is quite difficult.
(v) Troubles from smoke and heat from the plant.
(vi) With the increase in the operating temperature and pressure, the plant cost increases.
(vii) Requires long time for erection and put into action.
(viii) Efficiency falls rapidly below 75% of full load.
(ix) Costlier in operating cost in comparison with that of hydro and nuclear power plants.
Steam Power Plant Controls:
Until few years back, even in case of large power plants the various controls used to be accomplished manually on the basis of instrument readings.
However, now the various controls involved in the power plant operation have been completely automated resulting in:
(i) Increased labour productivity,
(ii) Improvement in the safety of operation and reliable functioning of the various instruments and equipment.
A number of controls, such as the boiler, turbine and generator unit are provided in a steam power plant so as to maintain the best condition at all loads. Turbine governing is affected by throttling the steam at the main valve (thus reducing the steam pressure and, hence mass flow also) or by reducing only the steam mass flow by cutting off one or more nozzles through which the seam enters the blades.
The first method of governing, known as throttle governing or qualitative governing, is used in case of small turbines and the second method of governing, known as nozzle governing or cut-off governing or quantitative governing is used for large turbines.
Maintenance of proper vacuum in the condenser, enough circulating water, a number of pumps, oil pressure for control of circuits, steam bleeding if any and the heater and feed-water control are other requirements for the turbine.
In case of an isolated generating unit, increase in load causes reduction in the speed of the unit and hence reduction in frequency. However, in case of generator connected to infinite bus-bars the load shared by the unit can be adjusted by adjusting the turbine speed. In this case frequency remains constant.
In general, centralized control is employed for modern steam power plants, the boiler and turbine control being at one place in the turbine room and the generator and feeder controls in the control room, in some cases all controls are centralized in one room, called the control room.
Layout of Steam Power Plant Main Building:
The layout of equipment in the boiler house and the machine room in the main building should be such that the following needs are met with:
1. Convenient erection, maintenance and supervision of the equipment and auxiliaries.
2. Efficient performance and minimum construction cost.
3. Good illumination and ventilation of the workplaces.
4. Dependable and trouble-free operation of the plant.
5. Provision for future expansion.
In modern steam power plants, a closed equipment layout is used. The boiler room and the machine room (where the turbo-generators are installed) adjoin each other and are arranged in parallel. In the boiler room, the boilers are usually installed in one row.
In a perpendicular arrangement, the turbo generators are installed perpendicular to the centre line of the machine room. With such an arrangement, the span of the machine room is increased but the room length is reduced. Also, the throttle valve of each turbine can be fitted near the partition wall. This reduces the steam pipe length. This arrangement is common practice when unit operation (one boiler for each turbine) is envisaged. The boilers can be installed on the turbine centre line.
The alternative arrangement is parallel arrangement where turbo-generators are placed parallel to the partition wall. Here, the turbine sets are placed much close together. The width of the machine room is less as compared to perpendicular arrangement. Also, more turbo-generators can be installed without increasing the width of the machine room.
The large capacity fuel bunkers holding enough fuel for two or three shifts are arranged either in a separate building or between the boiler and machine rooms. In the former case, the distance between the boiler units and turbines gets reduced significantly, but the crossing of the gas flues with pulverised-fuel ducts becomes rather involved. The ash room usually accommodates the coal pulverising mills and the other associated equipment.
The auxiliary equipment of boiler room and the machine room is sometimes installed in a special multi-storey building constructed between them. The draught equipment and the ash collectors are usually located on the level of zero datum.
Steam Power Plant Auxiliaries:
The equipments which help in the proper functioning of the plant are called plant auxiliaries. The various plant auxiliaries can be grouped under the subheadings of boiler auxiliaries, coal and ash auxiliaries, turbo-alternator auxiliaries and miscellaneous ones.
The boiler auxiliaries depend upon the type of fuel firing employed. In the case of stoker firing, stoker drives are essential along the forced and induced draught fans, boiler feed pumps, secondary air fans, air preheaters, soot blowers etc. In the case of pulverized fuel firing, the various auxiliaries are primary air fans, forced and induced draught fans, feed pumps, pulverized fuel conveyors and feeders, pulverizing mills, exhausters, air heaters and soot blowers etc. For oil firing the auxiliaries are fuel oil pumps and associated auxiliaries.
Coal and ash auxiliaries include wagon tipplers, elevators, skip hoists, conveyors, cranes, pumps, exhausters etc.
Turbo-alternator auxiliaries include circulating water pumps, condensate extraction pumps, governor control, evaporator, sludge and distillate pumps, ventilating fans, oil pumps, oil purifier, exciter, exciter field rheostat, turning gear etc.
Miscellaneous auxiliaries include air compressors, water and fire service pumps, workshop machinery and equipment.
All the above plant auxiliaries can be divided into two categories namely essential or continuous auxiliaries and nonessential or non-continuous auxiliaries. The essential auxiliaries are those which are associated with the running of a unit and whose loss would cause an immediate reduction in the output of the unit. The non-essential auxiliaries are those which may be put out of operation for some time.
The essential auxiliaries include forced and induced draught fans, feed water pumps, circulating water pumps, condensate pumps, auxiliary oil pump, ventilating fans for alternators, lighting and switchgear tripping circuits. In addition to these essential auxiliaries, the other essential auxiliaries required depend upon the type of boiler firing employed such as stoker drive in case of stoker firing, primary air fans and pulverized fuel feeders in case of central system of pulverized fuel firing, mills and feeders in case of unit system of pulverized fuel firing while fuel oil pumps and associated auxiliaries with oil fuel firing.
Non-essential or service auxiliaries include coal and ash handling plants, service pumps, oil pumps, exciter field rheostats, hoists and overhead cranes, valves, governor control, evaporator auxiliaries etc.
Auxiliaries in a steam power plant account for 4 to 8 per cent of station load.
Auxiliary Supply Features:
There should be two main sources of supply:
(i) From the grid via the station transformers, and
(ii) From the main generators through the unit transformers.
The essential auxiliaries should be supplied from the unit transformers while nonessential auxiliaries from the station transformers. Each generating unit should have a separate supply and failure of one unit should not affect the other. The unit board and station board should be interconnected for starting purposes and to provide a supply in case of failure of unit supply. A standby supply should also be provided so that a unit can be brought back to service in the event of failure of one of the components in any of the normal supply circuits.
For a 200 MW unit, the rating of unit transformer is about 12.5 MVA and the rating of station transformer is about 20 MVA. Such a station transformer can supply starting load of one unit (about 8 MVA), essential auxiliary load of one unit (about 8 MVA) and non-essential auxiliary load of about 4 MVA.
In central power stations, the steam turbine and alternator are directly coupled to avoid transmission losses. Turbo-alternators are high speed machines (3,000 or 1,500 rpm) or 50 Hz systems. These machines have horizontal configurations and smooth cylindrical (or non-salient pole) type field structure wound usually for 2 or 4 poles. To reduce the peripheral speed (maximum peripheral speed should not exceed 175 m/s) the diameter of the rotor is kept small and axial length is increased. The ratio of diameter to axial length ranges from 1/3 to 1/2.
Due to high peripheral speed, the rotating part of the turbo-alternator is subjected to high mechanical stresses. As a result the rotor of large turbo-alternator is normally built from solid steel forging. Chromium-nickel steel or special chrome-nickel-molybdenum steel is used for rotors of turbo- alternators. The forging has radial slots in which the field copper, usually in the form of strips, is placed. The coils are held in place by steel or bronze wedges and the coil ends are fastened by metal rings.
Normally two-thirds of the rotor is slotted for the field winding and one-third is left without slots so as to form the pole faces. Rectangular slots with tapered teeth are milled out in the rotor so that rectangular conductors can be used for the field windings. Each slot is provided with a ventilation hole at the bottom. To reduce harmful tooth ripples, either stator slots or poles are skewed.
The non-salient field structure used in turbo-alternators has the following special features:
(i) They are of smaller diameter (maximum 1 m in 2- pole machine) and of very long axial length.
(ii) Robust construction and noiseless operation.
(iii) Less windage (air-resistance) loss.
(iv) Better in dynamic balancing.
(v) High operating speed (3,000 or 1,500 rpm).
(vi) Nearly sinusoidal flux distribution around the periphery, and therefore, gives a better emf waveform than obtainable with salient pole field structure.
(vii) There is no need of providing damper windings (except in special cases to assist in synchronising) because the solid field poles themselves act as efficient dampers.
500 MW units generally use hollow stator conductors. The short-circuit ratio is 0.4 to 0.6.
Turbo-alternators are usually rated at 11 kV with 3-phase star-connection of stator windings. Standard frequency of generation is 50 Hz. Generally turbo-alternators are rated at 0.8 power factor lagging.
Main exciters are dc compound generators of 125 or 250 V rating capable of supplying required excitation for main generators on full load and with an overload of about 20% at rated power factor. Minimum of 10% overload capacity under normal operating conditions is provided for in the case of turbo-generators. Static exciters are also now in much use.
The small machines (of rating up to 50 MW) are air cooled. However, hydrogen cooling is invariably used for both stator and rotor of medium and large size turbo-generators. There are several advantages of using hydrogen as coolant in place of air in closed circuit ventilation system such as improved efficiency, increased output rating, increased life, elimination of fire hazard, less noise, smaller size of coolers etc.
The machines are built in size from 10 MW to over 1,500 MW. 500 MW units have been installed at Singrauli, Anpara, Chandarpur, Ramagundem, Trombay, Farakka and korba.