Till date in India, coal based power plants contribute maximum power to the power system grid. In coal based thermal power plant, we use coal for heat generation. Coal stores chemical energy. When coal burns, it releases heat. This heat produces steam in the boiler at high pressure and high temperature. This steam then flows through a steam turbine. The turbine converts this energy into mechanical energy. We usually use an axial-flow type turbine. The steam turbine works as a prime mover. It drives the electric generator (alternator).
A simple diagram of a coal-fired thermal power plant is shown in figure 1, below.
Boiler (Heat Addition)
Water enters the boiler through oiler feed pump. Coal burns and produces heat in the boiler furnace. Then the furnace supplies heat to the boiler. Water inside the boiler absorbs this heat. As a result, it becomes high-pressure, high-temperature steam.
Turbine (Expansion)
Then, steam enters and flows into the turbine. Here, the steam expands and loses pressure. Because of this, the turbine rotates. Here, heat energy converts into mechanical energy.
Condenser (Heat Rejection)
After that, steam leaves the turbine and enters into the condenser. Here, the steam removes heat. As a result, steam becomes condensed and converts back into water. The condenser uses cooling towers for the cooling purpose of the steam.
Boiler Feed Pump (Pressurization)
Then this condensed water is pumped back to boiler through a boiler feed pump. So, the cycle repeats again.
Coal Burning Cycle
There is another cycle that runs parallel to the steam cycle of a thermal power plant. This is a coal-burning cycle. First, we feed coal to the pulverizing plant through conveyor belts. The pulverizing plant crushes the coal into fine dust. Pulverization increases the calorific efficiency of coal as a fuel.
This fine coal dust then enters the furnace. It burns and releases a large amount of heat energy. We also use draft fans to force hot air from the furnace into the boiler. This forced air improves the efficiency of the furnace system. It spreads heat uniformly around the furnace and helps better heat transfer inside the boiler. After circulating inside the boiler, the hot gases move out toward the electrostatic precipitator.
Electrostatic Precipitator
The plant releases air pollutants through exhaust gases. These pollutants include dust particles and harmful gases like NOx, CO, CO₂, and SOx. So, we send the exhaust air and gases through an electrostatic precipitator. The electrostatic precipitator reduces air pollution by using scrubbers. These devices remove harmful particles from exhaust gases. Then sends the filtered exhausts to chimney.
Arrangements for Enhancing Efficiency of a Thermal Power Plant
In addition to the basic requirements of a thermal power plant, we use some extra arrangements to increase its thermal efficiency. There are mainly three arrangements for this purpose.
Super Heating
First, the boiler produces high-pressure steam. But we must heat this steam beyond its saturation temperature. Otherwise, moisture in the steam can cause corrosion in the turbine and steam pipelines. So, we superheat the steam. The temperature of steam increases from about 100°C to around 550–650°C. Now the steam becomes dry and highly energetic. The super heating is done in the boiler section of the plant.
This superheated steam flows through the pipeline and enters the turbine. Inside the turbine, the steam expands from high pressure to low pressure. Due to this expansion, the steam releases mechanical energy and rotates the turbine. This expansion, reduces the temperature of the steam. Then we send this exhaust steam back to the boiler section for reheating.
Reheating
After reheating, the steam again goes to another part of the turbine. This is the low pressure part of the turbine. This is actually another sets of turbine blades mounted on the same turbine shaft. This process increases the efficiency of the cycle. Finally, the steam from the low-pressure turbine goes to the condenser. In the condenser, the steam loses heat and converts back into water.
Feed Water Pre Heater
Next, we can the heat of the exhaust steam before condensation. This steam passes through pipelines around the feedwater line. It transfers heat to the feedwater. As a result, the feedwater temperature increases before entering the boiler. This reduces the amount of coal required for combustion. By using pre heating of feed water, we improve the thermal efficiency of the plant.