Electric Power Generation : Concepts - 3

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.

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