Electric Power Generation : Concepts - 2

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.

(Traveling Grate Stoker)

(Spreader Stoker)

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