Electric Power Generation : Concepts

Introduction of Generation of Electric Power:

Energy exists in various forms like mechanical energy, electrical energy, thermal energy and so on. One form of energy can be converted into another form by suitable arrangements. Out of these forms, electrical energy is preferred due to the following reasons.
(i) It can be easily transported from one form to another,
(ii) Losses in energy transportation are minimum,
(iii) It can be easily controlled and regulated to suit requirements,
(iv) It can be easily converted into other forms of energy particularly mechanical and thermal,
(v) It can be easily sub-divided.

SOURCES OF ENERGY:

Some of the sources of energy are
1. Fuels:
(a) Solid fuels. wood, charcoal, peat, coke, pulverized coal, pulverized coal.
(b) Liquid fuels petroleum based like petrol, diesel, etc. Non-petroleum fuels like ethyl alcohol, methyl alcohol, benzene etc.
(c) Gaseous fuels producer gas, blast furnace gas, coke oven gas, water gas.

2. Energy of water as used in hydroelectric plants where potential energy of water is converted into electrical energy

3. Nuclear energy.
4. Solar energy.
5. Wind power.
6. Wave power.
7. Tidal power.
8. Geothermal energy.
9. Fuel cells.
10. Thermo electric power.
11. Thermionic converter.
12. Magneto hydrodynamic (MHD) generator.

In all power stations, electric energy is generated from other forms of energy e.g.,
(i) Chemical energy of fuel (thermal energy),
(ii) Energy of falling water (hydraulic energy),
(iii) Atomic energy (nuclear energy).

Accordingly power stations are classified as :

Thermal power stations:

Those power stations which convert chemical energy of fuel (coal, diesel etc.) into electrical energy are called thermal power stations. The fuel used in thermal power stations may be solid fuel (coal) or liquid fuel (diesel).

The chemical energy of fuel is used to run the prime mover to which is coupled the alternator (A.C. generator). Thus electrical energy is obtained from the alternator.

According to the prime-mover employed for driving the alternate, thermal power stations may be broadly divided into the following two important types :

(a) Steam power stations:

Steam power stations employing steam engine or turbine as the prime-mover. Coal is used fuel.

(b) Diesel powe

Electric Power Generation: Steam Power Station:

Here steam turbine or engine is used as a prime-mover.

Fuels used in Steam Power Station:

Fuels for thermal power plants should have sufficient calorific value, easy availability in large quantities, ease of handling, storing and transporting. Common fuel for most of the plants is coal. Several varieties of coal are available - charcoal, lignite, peat, coke, briquetted coal, bituminous coal etc. A coal may have :

Fixed carbon	35 to 45 percent
Volatile matter	25 to 35 percent
Ash	8 to 22 percent
Moisture	10 to 18 percent
Heating value	5000 to 6500 kcal/kg.

There are several considerations in the selection of coal for a power plant. However, ash content is a major problem which needs careful consideration.
Coal is classified by:

1. Rank degree of metamorphism :

Rank degree of metamorphism or progressive alteration, in the natural series from fignite (inferior quality) to anthracites (superior quality).

2. Grade quality :

Grade quality determined by designation, calorific value, ash content, ash softening temperature and sulphur.

3. Type of coal :

Coal based on original plant ingredients and subsequent alterations e.g. boghead coal, cannel coal, split coal, common bonded coal, etc.

The pulverization of coal is done in coal crushers and ball mills. Pulverized coal is transferred to bankers and then to burners where coal dust along with air is fed to the boiler through burners.

Stokers in steam power station :

Stokers In this coal is fired as received from the mines, without any major preparation. A stoker continuously feeds fuel to the boiler furnace. In over feed stokers, coal is supplied on the top of the gate. As the coal burns, ash and clinkers are formed which collect at the bottom. Primary air is supplied under pressure from the bottom of the gate which gets heated up while passing through the layer of ash. Secondary air is supplied from the sides. However in case of underfeed stokers primary as well as secondary air is supplied through grate.

Travelling Grate Stoker

(Traveling Grate Stoker)

(Spreader Stoker)

Pulverized coal system in Steam Power Station :

The firing of coal in pulverized form involves primarily the function of breaking coal into line panicles, mixing of coal with air, delivery of coal air mixture to the burners and combustion.

(Pulverized Coal System)

The coil is crushed in a bowl mill or ball mill (using impact of steel balls). A classifier is used to regulate the fineness of particles.

Water treatment in steam power station:

Water is one of the most important raw materials for steam power plants. However impurities from water must be removed before feeding to the boiler. The impurities may be in the form of:

(i) Visible impurities : like algae, bacterial slimes, turbidity etc.
(ii) Dissolved gases : like carbon dioxide oxygen, nitrogen, methane, hydrogen, sulphide, etc.
(iii) Minerals and salts : of iron, manganese, sodium, potassium, etc.

Summary of Water Treatment Processes in steam power station:

Impurity	Trouble	Limit of tolerance in feed water	Kind of treatment
Oxygen	Corrosion	30 cc per kilo liter	chemical de-oxidation thermal deaeration
Carbon dioxide, Calcium, Magnesium salts	Corrosion Scale	3 parts per million, 20 ppm for low pressure boilers ;2 ppm for high pressure boilers, as CaCO₃	thermal dearation external softening internal softening
SiO₂	Carry over, scale	40 ppm for low pressure and 5 ppm for High pressure boilers	deionization
Na alkalinity	Embrittlement, Foaming	5 ppm as CaCO₃ for HP boilers. Variable for LP boilers	acid neutralizer
Total dissolved solids	Priming foaming	as specified	de-concentration
Turbidity	Sludge sediments	5 ppm or less	coagulation sedimentation filtration
Oil	Foaming	5 ppm or less	coagulation, filtration surface blow off

Condenser in steam power station :

Thermal efficiency of a steam power plant can be improved by reducing the turbine exhaust pressure. Low pressure at the exhaust can be maintained, by condensing material discharged from the turbine. In a way condenser is a heat exchanger in which heat from the exhaust steam is transferred to circulating boiler. There are two types of condensers - jet and surface type. Surface condensers are commonly used in power plants. A condenser needs huge quantity of cooling water. For this purpose cooling towers are used so that water can be re-circulated through the condenser.

Ash Handling in steam power station:

Usually 10 to 20 percent of the coal field in station is released in the form of ash thus a plant burning 50 tonnes of coal per hour will have 5 to 10 tonnes of ash produced which must be are moved immediately for the continuous operation of the plant.

The problems in ash handling are due to high temperature of ash and its contents which range from fine particles of fly as to large clinkers.

Most of the plants have arrangements for quenching ash with water and then carrying it through pipe lines to distant places used as pumps. Motive power may be provided by steam or pumps. For arresting fly ash, electrostatic precipitators are used, before flue gases are laid to chimney.

from ur's -- Bellapuri saikumar
( www.facebook.com/saikumar544)

r stations:

Diesel power stations employing diesel engine as the prime-mover.

Monday, 15 June 2015