Thermal Power Generation Plant or Thermal Power Station

What is a Thermal Power Plant?

A thermal power generation plant or thermal power station makes electric power from heat. In the usual coal-fired plant, steam from a boiler drives a steam turbine coupled to a generator.

The sections below follow that steam cycle, then efficiency and siting.

Theory of Thermal Power Station

Thermal power stations raise steam in a high-pressure boiler and pass it through a turbine coupled to an alternator.

Indian plants mainly fire bituminous coal in the boiler. Some also use lignite. The 8 to 33% volatile-matter and 5 to 16% ash figures are for selected bituminous coal. Much Indian thermal coal has higher ash. Coal is usually pulverised before firing.

In a coal thermal power plant, pulverised coal is burned in the steam boiler furnace. The steam is then superheated.

That steam enters the turbine and turns the blades. The turbine shaft is coupled to an alternator so the rotor turns with it.

As the steam expands through the turbine, its pressure falls and its volume rises.

After giving up energy to the rotor, the steam leaves the turbine and enters the condenser.

Circulating cooling water in the condenser turns the low-pressure exhaust steam back into water.

That condensate is heated in a low-pressure heater by bled steam, then heated further at higher pressure before it returns to the boiler.

The main steps in a thermal power station are:

1. First, the pulverized coal is burnt into the furnace of steam boiler.
2. High-pressure steam is produced in the boiler.
3. This steam is then passed through the superheater, where it further heated up.
4. This super heated steam is then entered into a turbine at high speed.
5. In turbine, this steam force rotates the turbine blades that means here in the turbine the stored potential energy of the high pressured steam is converted into mechanical energy.

Line Diagram of Power Plant

1. After rotating the turbine blades, the steam has lost its high pressure, passes out of turbine blades, and enters into a condenser.
2. In the condenser, the cold water is circulated with help of a pump which condenses the low-pressure wet steam.
3. This condensed water is then further supplied to a low-pressure water heater where the low-pressure steam increases the temperature of this feed water, it is then again heated in a high-pressure heater where the high pressure of steam is used for heating.
4. The turbine in the thermal power station acts as a prime mover of the alternator.

Overview of Thermal Power Plant

A typical plant follows the cycle below.

Water and steam are the working fluid. That loop is the feed-water and steam cycle. The nearest ideal model for a Thermal Power Station is the rankine cycle.
Fuel burned in air heats water in the boiler to dry superheated steam at the set temperature. That steam drives the steam Turbines.

The turbine is coupled to a synchronous generator, usually a three-phase alternator.

Exhaust steam is condensed in the steam condenser of turbine. That low pressure lets the steam expand further in the turbine.

Condensing extracts more energy from each kilogram of steam and returns condensate to the boiler so less fresh make-up water is needed.

A boiler feed pump returns that condensate, plus make-up water, to the boiler.

Cooling water takes heat from the condenser and is itself cooled in a cooling tower. This loop is the cooling-water circuit.

Filtered air enters the boiler. Flue gas leaves through the stack. Those paths are the air and flue-gas circuits.

A Forced Draught (FD) fan and an Induced Draught (ID) fan keep air flow and furnace draught.

The plant circuits are shown below.

Inside the boiler the heat exchangers include an Economizer, an Evaporator (the water tubes, i.e. downcomer riser circuit, not drawn above), a Super Heater and often a Reheater and air preheater.

The economiser uses leftover flue-gas heat to raise feed-water temperature.

The boiler drum gives the head for natural circulation of steam and water in the tubes.

The superheater takes more flue-gas heat and raises steam temperature to the required value.

The efficiency of Thermal Power Station or Plant

Overall efficiency is electrical-output heat equivalent divided by heat of coal fired. For a thermal power station the 20% to 26% band below is an older overall figure that rises with plant size. Large supercritical units can convert a larger share of heat than that band. The table is the page’s original capacity grouping.

Installed plant capacity	Average overall thermal efficiency
upto 1MW	4%
1MW to 10MW	12%
10MW to 50MW	16%
50MW to 100MW	24%
above 100MW	27%

Thermal Power Plant Location

Fuel, cooling water, land and ash disposal decide whether a thermal plant can be built and run at a reasonable cost. Location therefore matters.

A Power generation plant is cheaper to run if it sits where fuel, water and load fit the network.
One teaching method for a load-centre site is the graphical construction below,

A site at the centre of gravity of the loads shortens the power generation plant’s power transmission lines and can cut line cost.
Take X and Y as reference axes.
Let Q₁(x₁, y₁), Q₂(x₂, y₂), Q₃(x₃, y₃), Q₄(x₄, y₄),……………………………………….and Q_n(x_n, y_n) be n load centres.

The load centre of gravity is then Q(x, y) where

That CG is a teaching ideal. A city-centre load peak often cannot host a coal plant.

The load CG is often in a city, so fuel, water, land, ash and chimney limits usually decide the site.

1. The power plant should be built where the land is affordable.
2. The land should be such that the acquisition of private property must be minimum.
3. A large quantity of cooling water is required for the condensers etc of thermal power generation plant, hence the plant should preferably be situated beside the big source of a natural water source such as a big river.
4. Availability of a huge amount of fuel at a reasonable cost is one of the major criteria for choosing a plant location.
5. The plant should be established on plane land.
6. The soil should be such that it should provide a good and firm foundation of plants and buildings.
7. The thermal power plant location should not be very nearer to the dense locality as there is smoke, noise steam, water vapors, etc.
8. There must be ample scope of development of future demand.
9. A place for an ash handling plant for thermal power stations should also be available very nearby.
10. A very tall chimney of power station should not obstruct the traffics of airships.

Advantages of Thermal Power Station

Advantages often claimed for a thermal power station are:

1. Economical for low initial cost other than any generating plant.
2. Land required less than hydropower plant.
3. Since coal is the main fuel and its cost is quite cheap than petrol/diesel so generation cost is economical.
4. Maintenance is easier.
5. Thermal power plants can be installed in any location where transportation and bulk of water are available.

Disadvantages of Thermal Power Station

Drawbacks of a thermal power station include:

1. The running cost for a thermal power station is comparatively high due to fuel, maintenance, etc.
2. A large amount of smoke causes air pollution. The thermal power station is responsible for Global warming.
3. The heated water that comes from the thermal power plants has an adverse effect on the aquatic lives in the water and disturbs the ecology.
4. The overall efficiency of the thermal power plant is low like less than 30%.