7/28/2019 How Coal Power Plants Produce Electricity.docx

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How Coal Power Plants Produce Electricity

The conversion from coal to electricity takes place in three stages.

Stage 1

The first conversion of energy takes place in the boiler. Coal is burnt in the boiler furnace to produce heat.

Carbon in the coal and Oxygen in the air combine to produce Carbon Dioxide and heat.

Stage 2

The second stage is the thermodynamic process.

1. The heat from combustion of the coal boils water in the boiler to produce steam. In modern power plant, boilers

produce steam at a high pressure and temperature.

2. The steam is then piped to a turbine.

3. The high pressure steam impinges and expands across a number of sets of blades in the turbine.

4. The impulse and the thrust created rotates the turbine.

5. The steam is then condensed and pumped back into the boiler to repeat the cycle.

Stage 3

In the third stage, rotation of the turbine rotates the generator rotor to produce electricity based of Faradays Principle o f

electromagnetic induction.

Check out this series describing thelayout of thermal power plants.

Key Facts About Coal-Fired Electricity Production

In practice to effect these three stages of conversion, many systems and sub systems have to be in service. Also involved are

different technologies, like combustion, aerodynamics, heat transfer, thermodynamics, pollution control, and logistics.

As an example consider these facts for typical coal fired power plant of capacity 500 MW.

Around 2 million tons of coal will be required each year to produce the continuous power.

Coal combustion in the boiler requires air. Around 1.6 million cubic meter of air in an hour is delivered by air fans into

the furnace.

The ash produced from this combustion is around 200,000 tons per year.

Electrostatic precipitators capture almost all of this ash without dispersing this to the atmosphere.Pollutants from coal

power plantslike carbon dioxide, sulphur dioxide, and nitrogen oxide can also affect the environment. Thermal power

plants are the biggest producers of Carbon Dioxide.

The boiler for typical 500 MW units produces around 1600 tons per hour of steam at a temperature of 540 to 600

degrees Centigrade. The steam pressures is in the range of 200 bar. The boiler materials are designed to withstand these

conditions with special consideration for operational safety.

Heat transfer from the hot combustion gases to the water in the boiler takes place due to Radiation and convection.

The Electrical generators carry very large electric currents that produce heat and are be cooled by Hydrogen and water.

The steam leaving the turbine is condensed and the water is pumped back for reuse in the boiler. To condense all the

steam it will require around 50,000 cubic meter per hour of cooling water to be circulated from lakes, rivers or the sea.

The water is returned to the source with only an increase of 3 to 4 degrees centigrade to prevent any effect to the

environment.

Apart from the cooling water the power plant also requires around 400 cubic meter per day of fresh water for making

up the losses in thewater steam cycle.

Details of Generating Electricity from Coal

These are some of the facts to highlight the complexities of the working of a Coal Fired Power Plant generating Electricity.

For more details, discoverhow coal is blendedto the right mix to maximize energy production or learn about the

specificcaloric energies of coaland how moisture in the coal can affect a power plant's efficiency. Also learn how thecoal isprepared to be fired in the boiler.

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Coal, oil and gas are used to make thermal electricity. They all work basically the same way (with a fewexceptions: for example, in an oil- or gas-fired plant, fuel is piped to the boiler).

1. Coal supply After haulers drop off the coal, a set of crushers and conveyors prepare and deliver thecoal to the power plant. When the plant needs coal, coal hoppers crush coal to a few inches in size andconveyor belts bring the coal inside.

2. Coal pulverizer The belts dump coal into a huge bin (pulverizer), which reduces the coal to a finepowder. Hot air from nearby fans blows the powdered coal into huge furnaces (boilers).

3. Boiler

The boiler walls are lined with many kilometres of pipe filled with water. As soon as the coalenters the boiler, it instantly catches fire and burns with high intensity (the temperatures inside the furnacemay climb to 1,300 C). This heat quickly boils the water inside the pipes, changing it into steam.

4. Precipitators and stack As the coal burns, it produces emissions (carbon dioxide, sulphur dioxide andnitrogen oxides) and ash.The gases, together with the lighter ash (fly ash), are vented from the boiler upthe stack. Huge air filters called electrostatic precipitators remove nearly all the fly ash before it isreleased into the atmosphere. The heavier ash (bottom ash) collects in the bottom of the boilers and isremoved.

5. Turbine and generatorMeanwhile, steam moves at high speed to the turbines, massive drums withhundreds of blades turned at an angle, like the blades of a fan.As jets of high-pressure steam emergefrom the pipes, they propel the blades, causing the turbine to spin rapidly. A metal shaft connects theturbine to a generator. As the turbine turns, it causes an electro-magnet to turn inside coils of wire in thegenerator. The spinning magnet puts electrons in motion inside the wires, creating electricity.

6. Condensers and cooling water system Next, the steam exits the turbines and passes over cool

tubes in the condenser. The condensers capture the used steam and transform it back to water. Thecooled water is then pumped back to the boiler to repeat the heating process. At the same time, water ispiped from a reservoir or river to keep the condensers constantly cool. This cooling water, now warm fromthe heat exchange in the condensers, is released from the plant.

7. Water purificationTo reduce corrosion, plants purify water for use in the boiler tubes. Wastewater isalso treated and pumped out to holding ponds.

8. Ash systemsAsh is removed from the plant and hauled to disposal sites or ash lagoons. Ash is alsosold for use in manufacturing cement.

9. transformer and transmission linestransformers increase the voltage of the electricity generated.transmission lines then carry the electricity at high voltages from the plant to substations in cities andtowns.