Thermal Power Plant

July 11, 2020
what is thermal power plant

What is a thermal power plant? A thermal power plant or thermal power station uses the energy of heat to produce electric power. It is the most convenient source of electric power generation. Also known as steam power stations because it uses steam to run a steam turbine.

The burning of fuels such as oil, coal, or liquefied natural gas produces high temperature and high-pressure steam. We use this steam to turn a steam turbine, which drives an electrical generator that produces electrical energy.

Besides using thermal stations to generate electrical power, it can produce heat energy for industrial purposes, district heating, and water desalination.

We need to understand how thermal plants work, and that’s what we will do. But we first have to know the history of the thermal power station.

History of the Thermal Power Station

To talk about a thermal power plant, we will start from the 18th century when James Watt designed a reciprocating steam engine to produce mechanical power.

The first commercial electrical power station powered by a reciprocating steam engine was built at the pearl street station in New York and Holborn Viaduct power station in London in 1882.

In 1884 a steam turbine was developed to provide a large and more efficient design for central generating stations. Then in 1892, the steam turbine became an alternative, which made the turbine offered higher speeds. Additionally, it was more compact machinery and produced stable speed regulation.

Then again, in 1905, the large central power stations used the reciprocating engines entirely.

Steam Power Plant Working Principle

The thermal power plant work depends on the Rankine cycle principle, a closed cycle that repeatedly uses the same fluid.

Firstly we fill the water into the boiler until we fill the entire surface area of heat transfer. Secondly, we use hot gases of combustion fuel with air to heat the water in the boiler. So, the water turns into vapor. Then we take this produced steam with the needed pressure and temperature to produce the turbine’s mechanical power.

Next, we cool the steam out of the turbine into a condenser using cooling water turned into water. And we reused the condensate water as boiler feedwater. This cycle goes on and repeats many times.

But it doesn’t stop here as we use the turbine’s rotation to turn a generator directly linked to the turbine. Hence, the turbine rotates the generator output terminals to generate electricity.

Remember, although this process is a closed cycle process, there would be a reduction of water as the water may decrease due to an intentional or unintentional lake of the water.

Thermal Power Plant Diagram

Here we will have a look at the layout of the thermal power plant. This illustrates how coal converts into electricity. Let’s see:

Coal

We start from coal transported from coal mines through trains, trucks, or ships to the generating station that needs to be store in the bunker house. It has the capacity to run a plant for 8-12 Hours.

We usually use bituminous or brown coal after cleaning it in a magnetic cleaner to filter it from any iron particles.

Boiler

At this stage, we mix the air with the pulverized coal and then burn them in the combustion zone. This produces a large fireball at the boiler center, that we use to convert water into high-pressure and high-temperature steam.

The flow gases go out of the boiler to superheater, reheater, economizer, evaporator, air preheater. Then from the chimney to the atmosphere.

Superheater

We hang the superheater tubes at the hottest part of the boiler. Subsequently, after the steam saturates in the boiler, we superheat it in the superheater to about 540ᵒ. Then this high-pressure superheated steam is fed to the steam turbine.

Economizer

It is simply a feedwater heater where we heat the water before we supply it to the boiler.

Air Preheater

Firstly, we use a primary air fan to take air from the atmosphere. Then we pass it to the air preheater injected with coal in the boiler to be warmed. This is to help improve coal combustion.

Steam Turbine

Here we feed the high-pressure superheated steam to cause a rotation of the turbine blades. So, mechanical energy is produced, which makes the steam turbine acts as a prime mover. Additionally, this makes the pressure and temperature of the steam fall to a lower value.

And when it passes through the turbine, it extends in volume. Then this extended low-pressure steam is exhausted in the condenser.

Condenser

We use cold water circulation to condense the low pressure exhausted steam, which starts to lose its pressure and temperature until it is converted back into the water. Also, it is essential for increasing the efficiency of the cycle.

Alternator

We couple the steam turbine to an alternator, so when the turbine rotates, the alternator also rotates to generate the electrical energy. Then we use a transformer to step-up this generated electric voltage to be transmitted to where it’s utilized.

Feedwater Pump

We re-feed the condensed water to the boiler using the feedwater pump. Also, we may use an external water source to supply the water lost during this working cycle.

Components of a Thermal Power Plant

We mention that we heat the water in the boiler, and the steam produces mechanical energy in the turbine. So we need to know all the components of the thermal power plant. They are:

Boiler

It is an enclosed vessel that provides combustion heat to transfer into the water until it becomes steam. Again, we use this steam under pressure for transferring the heat to a process, and this boiler contains:

Feedwater system: It provides water to the boiler and regulates it to meet the steam demand.

Steam system: It collects and controls the steam produced in the boiler.

Fuel system: It includes the equipment used to provide fuel to generate the heat needed.

Turbine

It’s a rotary engine used to convert the energy of a moving stream of water, steam, or gas into mechanical energy. Then, we transfer this energy through a driven shaft to operate a machine, compressor, electric generator, or propeller.

And turbines are classified as hydraulic turbines, water turbines, steam turbines, or gas turbines.

Deaerator

It’s a widely used device to remove air and dissolved gasses from the feed water to steam-generating boilers.

Heat Exchangers

They are equipment that transfers heat from one medium to another. The proper design, operation, and maintenance of the heat exchangers will minimize energy losses and make the process energy efficient.

And heat exchangers may be classified according to the flow arrangement to:

Firstly, in parallel-flow heat exchangers.

Secondly, in counter-flow heat exchangers.

Lastly, in a cross-flow heat exchanger.

Superheater

It is a device on a steam engine that we use to re-heat the steam generated by the boiler to increase its thermal energy again and decrease the likelihood of condensations inside the engine.

We use the superheater to increase the efficiency of the steam engine as they are widely adopted.

Economizers

It is a mechanical device used to reduce energy consumption or perform useful functions like preheating fluids. Hence, we can say that the economizer is a heat exchanger.

Condenser

The condenser consists of a shell where the exhaust steam from the low-pressure turbine enters to be cooled and converted. It condensates by flowing into the other component of the condenser and the tube heat exchanger.

We take care to keep the condenser’s temperature as low as practical to achieve the lowest possible pressure in the condensing steam to have the best efficiency.

Feedwater Heater

It is the device through which the condensate pumps the condensate water. It then raises the water temperature by utilizing the extraction of steam from various stages of the turbine.

We use the feedwater heater to improve the system’s thermodynamic efficiency as it reduces the irreversibilities involved in steam generation. By the way, it reduces plant operating costs and helps to avoid thermal shock to the boiler metal in any case.

Electrical Generator

It is an electrical device that converts mechanical energy into electrical energy using electromagnetic induction. This generator forces the electric charges to move through an external electrical circuit but doesn’t create electricity.

And it is somehow analogous to a water pump, which creates a water flow but doesn’t create the water inside.

Location of Thermal Power Plant

The most important factor which affects the economy of the thermal station is the location. Graphical methods determine this. The plant’s ideal location is the center of gravity of the load, as it will make the length of the power transmission network minimum.

This will reduce the capital cost of the system. But it isn’t available all the time to establish the thermal station at the center of gravity.

So, we consider other points to decide the best location of the plant, as:

Load Center Determination

Firstly, the electric power plant must be where the land cost is quite reasonable. The electric power plant would be built beside large sources of water, such as the river. This is because the condensers need a large quantity of cooling water.

Secondly, the availability of a large amount of fuel at a reasonable cost is a must.

The thermal plant would also cause smoke, noise, steam, water vapors, etc. so we shouldn’t build it near to dense locality.

They should also be very near a place for ash handling plant.

Types of Thermal Power Generation:

Steam Power Generation plant

In this plant, we burn fuels as heavy oil, liquefied natural gas, and coal in a boiler to generate a high temperature, high-pressure steam. Then we use this steam to rotate the impeller of the steam turbine. So the power generators are connected to the turbine-driven to generate electricity.

And this steam power plant has a thermal efficiency of about 42% to 46% of the middle load supply.

Combined Cycle Power Generation Plant:

In this method, we incorporate a gas turbine to reuse the waste heat to drive a steam turbine. Then again, we power this gas turbine with high-temperature combustion gas. This produces a sufficient temperature and pressure steam, which drives a steam turbine and generates electricity.

This combined-cycle plant has high thermal efficiency, and it also has a lower cost than the oil-fired thermal power.

Gas Turbine Power Generation Plant:

It’s a special type of thermal power plant as it burns fuels like liquefied natural gas (LNG) or Kerosene to produce high-temperature combustion gas with sufficient energy to rotate the gas turbine, which then produces electricity.

The Efficiency of the Thermal Power Plant:

After knowing approximately everything about thermal power plants, we have to know the most important thing: efficiency.

Efficiency is the ratio of heat equivalent of electrical output to the heat of the combustion of coal. The overall efficiency of thermal power plants varies from 20% to 26%, depending on its capacity.

Thermal Power Plant Efficiency Diagram
Thermal Power Plant Efficiency Diagram

Advantages of Thermal Power Station:

The thermal power plant has great advantages as:

  • The low initial cost.
  • It requires less space of land.
  • The maintenance is easier than for other stations.
  • The fuel cost is economical.
  • We can build in any location as transportation is effortless.

Disadvantages of Thermal Power Station:

But unfortunately, we may meet some drawbacks as:

  • The comparatively high running cost.
  • The low overall efficiency, which may be 30%.
  • It causes a large amount of smoke, which populates the atmosphere.
  • The heated water produced by the thermal station disturbs the ecology and affect aquatic life.
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