Thermal Power Plants: Components & Working Principle

Working Principle of Thermal Power Plants

Thermal power station’s working principle is “Heat released by burning fuel which produces (working fluid) (steam) from water. Generated steam runs the turbine coupled to a generator which produces electrical energy in Thermal Power Plants.

General layout of Thermal Power Plant:

Working Principle of a Thermal Plant

The working fluid is water and steam. This is called feed water and steam cycle. The ideal Thermodynamic Cycle to which the operation of a Thermal Power Station closely resembles is the RANKINE CYCLE.

In a steam boiler, the water is heated up by burning the fuel in the air in the furnace, and the function of the boiler is to give dry superheated steam at the required temperature. The steam so produced is used in driving the steam Turbines. This turbine is coupled to synchronous generator (usually three-phase synchronous alternator), which generates electrical energy. The exhaust steam from the turbine is allowed to condense into the water in steam condenser of turbine, which creates suction at very low pressure and allows the expansion of the steam in the turbine to very low pressure. The principal advantages of the condensing operation are the increased amount of energy extracted per kg of steam and thereby increasing efficiency, and the condensate which is fed into the boiler again reduces the amount of fresh feed water.

The condensate along with some fresh makeup feed water is again fed into the boiler by a pump (called the boiler feed pump). In the condenser, the steam is condensed by cooling water. Cooling water recycles through the cooling tower. This constitutes a cooling water circuit.

The ambient air is allowed to enter the boiler after dust filtration. Also, the flue gas comes out of the boiler and gets exhausted into the atmosphere through stacks. These constitute air and flue gas circuit. The flow of air and also the static pressure inside the steam boiler (called draught) is maintained by two fans called Forced Draught (FD) fan and Induced Draught (ID) fan.

The total scheme of a typical thermal power station along with different circuits is illustrated below.

Inside the boiler there are various heat exchangers, viz. Economiser, Evaporator (not shown in the fig above, it is basically the water tubes, i.e. downcomer riser circuit), Super Heater (sometimes Reheater, air preheater are also present).

In Economiser the feed water is heated to a considerable amount by the remaining heat of flue gas. The Boiler Drum maintains a head for natural circulation of a two-phase mixture (steam + water) through the water tubes.

There is also Super Heater which also takes heat from flue gas and raises the temperature of steam as per requirement.

You may want to read: Gas turbine power plants and their functioning.

Principal Components of a Thermal Power Plant

• Boiler (1)
  • A huge boiler acts as a furnace transferring heat from the burning fuel to row upon row of water tubes that entirely surround the flames.
  • Water is kept flowing through the tubes by a pump P1
• Drum (2)
  • It contains water and steam at high pressure and produces a stream for the turbine.
  • It also receives water delivered by boiler-feed pumpP2 
• High-pressure (HP) Turbine (3)
  • Converts thermal energy into mechanical energy by letting the steam expand as it moves through turbine blades.
  • The steam is then passed through a reheater S3 in order to raise the thermal efficiency and to prevent premature condensation. 
• Medium-pressure(MP) turbine (4)
  • It is similar to HP turbine, except that it is bigger so that the steam ma expand still more. contains water and steam at high pressure and produces stream for the turbine.
• Low-pressure(LP) Turbine (5)
  • It is composed of two identical sections and it removes the remaining available energy from the steam. The steam turbine can be of impulse or reaction turbine and sometimes combination of both.

• Condenser (6)
  • It causes the steam to condense by letting it flow over cooling pipes S4.
  • Coldwater from outside sources flowing through pipes carries away the heat. The temperature of cooling water increases by 5oc to 10oc as it flows through the condenser tubes. The condensed steam has a temperature of between 27oc and 33oC
  • It is condensing steam that creates near-vacuum pressure of 5kPa.
  • A condensate pump P2 removes the lukewarm condensed steam and drives it through reheater (7) toward a feedwater pump (8).
• Reheater (7)
  • Heat exchanger receives hot steam, bled off from high-pressure turbine, to raise the temperature of the feedwater. Thermodynamic studies show that when some steam is bled off this way, the overall efficiency increases.
• Burner (9)
  • Burners supply and control the amount of gas, oil, or coal injected into boiler. Coal is pulverized before it is injected. Similarly, heavy bunker oil is preheated and injected as an atomized jet to improve surface contact with the surrounding air.
• Forced-draft (10)
  • It furnishes the enormous quantities of air needed for combustion.
• Induced-draft fan(10)
• It carries gases and other products of combustion toward the cleansing apparatus and from there to the stack and the outside air.

• Generator (G)
• Connected to three turbines converts’ mechanical energy into electrical energy.

• Cooling Towers
  • Whenever one percent of a body of water evaporates, the temperature of the remaining water drops to 5.8 ^o C. To cool the condenser, the phenomenon of evaporation is used to produce the cooling effect.
  • Evaporation is produced by exposing large surface of water to surrounding air. The simplest way to do this is to break up the water into small droplets and blow air through this artificial rain.

Warm cooling water flows out of the condenser is piped to the top of the cooling tower where it is broken up into small droplets. As the droplets fall toward the open reservoir below, evaporation takes place and the droplets are chilled.

Energy Flow Diagram

Modern boilers furnish steam at a temperature of 550^o C and pressure of 16.5 MPa. The overall efficiency (Electrical output/thermal input) is then about 40 %.

Using previous model, we can estimate the characteristics of a 480 MW plant. The cooling tower would have to evaporate q = 2% X 14400 = 288Kg/s of cooling water

 Steam turbines and turbo generator are virtually unchanged over the last 100 years

Advanced Coal-fired Power Plant   

Super critical (SC):                                 540-580 ^oC    and    22.1 – 25 Pa

Ultra super critical (USC):                    580-620 ^oC    and    22 -25 pa

Advanced super critical (A- USC):      700-725 ^o C    and     25-35 Pa

Combined Heat and Power (CHP)

In this heat and electricity is simultaneously produced.  Electrical power is consumed inside the plant of the CHP facility, although any surplus or deficit is exchanged with the utility distribution system. 

• Ayslesford Power plant:  220 MW heat and 98 MW electrical energy
• Lindesy oil refinery co generation power plant: 118 MW heat and 38 MW electrical energy

Power from waste

Different waste material like pages, solid waste, plastic all can be used to produce through following techniques

• Traditional Combustion Plants
• Gasification and pyrolysis

Geothermal Power Plant

The heat generated from earth can also be used to produce power. Hot springs and geysers have been used for at least 10,000 years. Environmentally clean and requires less space compared to hydro and solar power. As we go deeper into the earth, temperature increases in gradient, so for this hydrothermal reservoir is also to be found. 

The geothermal pump is also used domestically to heat water in the winter season mostly in Europe. Geothermal power is dependent on the site also because we have to find that place where temp increase is high per km depth. According to natural resources and energy, the USA has the high number of sites for geothermal power.

Herber geothermal power station:                52   KW

Thermal Power Plant based on Solar Energy

From concentrating solar power, a standard turbine/generator arrangement can make electrical power. 

• Power tower: In this different concave solar mirrors are used to reflect the sun rays on to the tower to heat the fuel (water), in this way steam is produced and then rest of the stuff to produce the electricity. 
• 1.291 mirrored heliostats and a 54 story high tower the World’s largest solar power tower plant near Seville in Spain generating 20 megawatts (MW) of electricity, enough to supply 10,000 homes.

Ocean Thermal Energy Conversion (OTEC)

It is still in the developmental stage. It exploits the temperature difference between warm surface water and the cold water at depth to run a “heat engine”. The temperature difference must be at least 40^oF (22^oC) year round, which is only found near the equator. 

Pakistan: Electricity from Bagasse

Pakistan is the fifth largest producer of sugarcane with the production of 50 million tones of sugarcane annually yielding 10 million tones of bagasse. The country has 87 sugar mills with a generation capacity of 3000 megawatts from bagasse in the winter. Currently, 7 sugar mills sell surplus power to government. Energy generated from bagasse is renewable energy just like wind energy as the carbon emitted after combustion has already been captured by sugarcane plants during their growth. So, the net carbon output of bagasse based power plants is effectively zero.

o 21 MW Tracy Biomass Plant

Impact of Thermal Power Plants on Environment

Carbon dioxide, sulphur dioxide, and water are the main products of combustion when oil, coal, or gas are burned. SO₂ creates substances that give rise to acid rain. Dust and fly ash are other pollutants that may reach atmosphere. Controls are in effect for each of these in new plants, however older plants spew thousands of pounds of each of these into the atmosphere every year. 

In every winter, fog is the major problem in Pakistan which is produced by the thermal power plants. India also admits that its coal plants causing fog inside the Pakistan. Nuclear Power plant can produce radioactive radiations in the environment which are harmful for livings. In the same way, oil and coal plants can pollute the environment. 

o Thermal Power station Guddu (Pakistan)                                    1655 MW  o Thermal Power station Jamshoro (Pakistan )                              850 MW

Pakistan Coal Reserves

o Azad Jamu Kashmir 9 million tonnes

Different CPEC energy projects are commissioned to produce enough power in Pakistan. But due to political issues, many fruitful power projects are still not commissioned which can fulfill energy requirements in Pakistan. 

Thar Coal Plant

It is among the biggest coal reserves in the world having capacity 9.75 billion tons of coal. The underground gas is to be converted into coal gas by a process known as Underground Coal Gasification (UCG). It can produce 50,000 MW of electricity for decades and 100 million barrels of oil for 500 years. 

It can be a game-changer for Pakistan and the electricity produced will be cheaper. But due to political concerns and commission mafias, this project is still having not been commissioned. Pakistan had been facing severe energy crisis is the past decade, Thar coal can be a solution to alleviate Pakistans’s energy shortage.

Advantages of Thermal Power Stations:

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

Disadvantages of Thermal Power Plants

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 a thermal power plant has an adverse effect on the aquatic lives in the water and disturbs the ecology.
4. The overall efficiency of the thermal power plants is low like less than 30%.

Final Words

I hope you’ve liked our comprehensive article on the working principle and operation of a thermal power plant. You may also like the components of a wind power plant and 10 must-learn software for an electrical power engineer.

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