Working of Power Plant

Thermal power plants are one of the main sources of electricity in both industrialized and developing countries. The variation in the thermal power stations is due to the different fuel sources (coal, natural gas, naptha, etc). In a thermal power plant, one of coal, oil or natural gas is used to heat the boiler to convert the water into steam. In fact, more than half of the electricity generated in the world is by using coal as the primary fuel.

The function of the coal fired thermal power plant is to convert the energy available in the coal to electricity. Coal power plants work by using several steps to convert stored energy in coal to usable electricity that we find in our home that powers our lights, computers, and sometimes, back into heat for our homes. The working of a coal power plant is explained in brief:

Firstly, water is taken into the boiler from a water source. The boiler is heated with the help of coal. The increase in temperature helps in the transformation of water into steam. The steam generated in the boiler is sent through a steam turbine. The turbine has blades that rotate when high velocity steam flows across them. This rotation of turbine blades is used to generate electricity. A generator is connected to the steam turbine. When the turbine turns, electricity is generated and given as output by the generator, which is then supplied to the consumers through high-voltage power lines.

Apart from thermal power plants, there are other types of energy resources being used to generate electricity. The various types of energy sources include hydro electricity, solar power, wind power, nuclear power, etc.

Hydro electricity

Hydroelectric power or hydroelectricity is electrical power which is generated through the energy of falling water. A hydroelectric power plant uses the force of the water to push a turbine which in turn powers a generator, creating electricity which can be used on-site or transported to other regions. This method of energy generation is viewed as very environmentally friendly by many people, since no waste occurs during energy generation. It is the most widely used form of renewable energy.

Solar Power

Solar power is energy that is derived from the sun and converted into heat or electricity. It is a versatile source of renewable energy that can be used in an amazing number of applications. Energy from the sun can be converted into solar power in two ways. The first way of obtaining solar power involves the use of photoelectric applications. Photoelectric applications use photovoltaic cells in converting energy from the sun into electricity. The second way involves the use of solar thermal applications wherein heating a transfer fluid is done to produce steam to run a generator.

Wind Energy

Wind power is power which is derived from wind. There are a number of ways to collect and use wind power, and wind power is among the most ancient forms of energy used by humans.Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity.

Nuclear Power

Nuclear energy is produced in two different ways. In one method, large nuclei are split to release energy. Here, nuclear energy originates from the splitting of uranium atoms in a process called fission. At the power plant, the fission process is used to generate heat for producing steam, which is used by a turbine to generate electricity. In the other method, small nuclei are combined to release energy.

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Power Plant Design

Coal Combustion Theory

Combustion is a rapid chemical reaction between fuel and oxygen. When combustible elements of fuel combine with O2, heat energy comes out. During combustion combustible elements like Carbon, Sulfur, Hydrogen etc combine with oxygen and produce respective oxides. The source of oxygen in fuel combustion is air. By volume there is 21 % of Oxygen presents in air and by weight it is 23.2 %. Although there is 79 % (by volume) nitrogen in air but it plays no role in combustion. Actually Nitrogen carries heat produced during combustion to steam boiler stack. As per combustion theory the quantity of air required for combustion is that which provides sufficient O2 to completely oxidize combustible elements of fuel. This quantity of air is normally known as STOICHIOMETRIC AIR requirement. This amount of air depends upon the nature of fuel. STOICHIOMETRIC AIR requirements for different fuels are obtained by analysis of fuel and they are given in tabular form below,

FUEL	STOICHIOMETRIC AIR MASS / UNIT MASS OF FUEL
Bituminous Coal	11.18
Anttiasite Coal	10.7
Coke	9.8
Liquite	7.5
Peat	5.7
Residual Fuel Oil	13.85
Distillate Fuel Oil(Gas Oil)	14.48
Natural Gas(Methane Base)	17.3

Combustion of Coal

For sufficient air,

We have already said that mass wise there is 23.2 % O2 presents in air. Hence the amount of air required to provide 2.67 gm of O2 is

As per ideal combustion theory, after combustion of one gm carbon(C), product of combustion contains only 3.67 gm of CO2 and (11.5 – 2.67 =) 8.83 gm of N2 

Coal Combustion for Insufficient Air

By weight, the requirement of air for providing this much O2 is

After combustion of one gm carbon(C), product of combustion contains only 2.33 gm of CO and (5.75 – 1.33 =) 4.42 gm of N2.

From equation (1) and (2) it is clear that due to insufficient air combustion, the heat lose during 1 gm of coal combustion is (33.94 – 10.12) = 23.82 kj 

Combustion of Sulfur

So, air required for 1 gm sulfur combustion, is

So, combustion product, after completing 1 gm of sulfur combustion, contains 2 gm of SO2and (4.31 – 1 = ) 3.31 gm of N2 

Combustion of Hydrogen

From combustion theory of C, S and H2 it is found that 2.67 gm oxygen is required for 1 gm carbon combustion, which implies 2.67C gm oxygen is required for C gm carbon, 1 gm oxygen is required for 1 gm sulfur combustion, which implies S gm oxygen is required for S gm sulfur and 8 gm oxygen is required for 1 gm hydrogen combustion, which implies 8H gm oxygen is required for H gm hydrogen. 

Hence 1 gm of coal (fuel) which contains C gm carbon, S gm sulfur and H gm hydrogen, requires (2.67C + S + 8H) gm of oxygen for efficient combustion.

Some amount of oxygen may be contained in the fuel itself in form of different compounds and it takes part in combustion also. If O is the original weight of the oxygen presents in 1 gm of fuel, net requirement of oxygen for sufficient coal combustion is (2.67C + S + 8H – O) gm.For that the amount of air required is

This above mentioned analysis is called coal analysis for combustion.

Before efficient combustion can take place, several basic requirements must be fulfilled, most important of them are,

a) The combustion must be done with sufficient oxygen

b) There must be sufficient turbulence to promote throughout mixing of combustible and oxygen. 

Coal Content in Proximate Analysis

Moisture = 8 %, volatile material = 20 to 25 %, fixed carbon = 40%, ash = 30%. Fixed carbon’s combustion temperature = 900°C. Basic component of ash is Si, Al and others. Now fusion temperature of Si is 1200°C.

If the furnace temperature raises above 1100°C then Si will be fused and deposited on the tubes, as slag, causing improper heat transfer.

Now to dilute the temperature excess air and complete combustion are required. 

Now, the volatile material plays important role in combustion. Less the volatile material flame will be high which may be chance for flame impingement of S/H coil.

For fulfilling the point some practical steps to taken. In practice it is always necessary to supply more air to the combustion system than it is theoretically required. Reason for that air and fuel mixing process in any combustion system, as it is not possible to ensure complete and intimate mixing of the fuel with the necessary oxygen at the point of injection. So some excess air is required for proper combustion to a reasonable minimum power, stack loss and unburnt carbon in ash. 

Generally 20% excess air is allowed.

% OF EXCESS AIR	UN-BURNT CARBON IN ASH	C.V. LIBERATED IN FURNACE	UN-BURNT GAS LOSS
0 %	10 %	75 %	CO2, O2, N2, H2O, CO, CH4(15 %)
15 %	2 %	97 %	CO2, O2, N2, H2, CO(1 %)
100 %	0.5 %	99.5 %	CO2, O2, N2

Third process is unsatisfactory for extra fan power and convey huge amount of heat.

The coal particles should be at least 74 microns in 200 mesh. So pulveriser is required for

i) better utility of coal

ii) saving of time.

There are mainly three losses occurred during coal combustion,

1) Unburnt gas loss

2) Dry flue gas loss

3) Combustible in ash loss.

Unburnt Gas Loss

Remember the unburnt gas loss is mainly the result of burning carbon to carbon monoxide instead of carbon dioxide. It is seen that heat release in CO reaction is one third of that in CO2 reaction. So adequate supply of oxygen or excess air will quickly reduce this loss to zero. 

Dry Flue Gas Loss

A further loss of heat is that due to dry flue gas. It is often referred to as the stack loss. If more excess air is admitted, this loss increases. 

Combustible in Ash Loss

This loss is very high when there is little or no excess air because mixing of combustible material and oxygen is so poor. As the air quantity is increased, the loss falls rapidly. However it does not reach to “zero” because the loss depends upon two factors firstly on air – coal mixture and secondly on fineness of pulverized coal grain. More fine grain of pulverized coal helps to complete combustion more perfectly and resulting less combustible in ash loss. In practice, though, a stage is reached where it is not worth grinding the coal any finer because it will cost more to grind than the extra heat release. Practically the loss does not reach to zero. generally a high volatile coal is crushed until 75% of its bulk passes through a 200 mesh whereas a low volatile coal is crushed until 80% passes through similar mesh.

The loss gets less as excess air is added, reaches a minimum and then increases as still more excess air is added. Thus there is only one quantity of excess air which will give lower loss for the combustion of a particular fuel. For bituminous coal 15.5% excess air is optimum requirement for Coal Combustion. 

Power Plants & Types of Power Plant

What is Power Plant?

A power plant or a power generating station, is basically an industrial location that is utilized for the generation and distribution of electric power in mass scale, usually in the order of several 1000 Watts. These are generally located at the sub-urban regions or several kilometers away from the cities or the load centers, because of its requisites like huge land and water demand, along with several operating constraints like the waste disposal etc. For this reason, a power generating station has to not only take care of efficient generation but also the fact that the power is transmitted efficiently over the entire distance. And that’s why, the transformer switch yard to regulate transmission voltage also becomes an integral part of the power plant.

At the center of it, however, nearly all power generating stations has an A.C. generator or an alternator, which is basically a rotating machine that is equipped to convert energy from the mechanical domain (rotating turbine) into electrical domain by creating relative motion between a magnetic field  and the conductors. The energy source harnessed to turn the generator shaft varies widely, and is chiefly dependent on the type of fuel used. 

Types of Power Station

A power plant can be of several types depending mainly on the type of fuel used. Since for the purpose of bulk power generation, only thermal, nuclear and hydro power comes handy, therefore a power generating station can be broadly classified in the 3 above mentioned types. Let us have a look in these types of power stations in details. 

Thermal Power Station

A thermal power station or a coal fired thermal power plant is by far, the most conventional method of generating electric power with reasonably high efficiency. It uses coal as the primary fuel to boil the water available to super-heated steam for driving the steam turbine. The steam turbine is then mechanically coupled to an alternator rotor, the rotation of which results in the generation of electric power. Generally in India, bituminous coal or brown coal are used as fuel of boiler which has volatile content ranging from 8 to 33 % and ash content 5 to 16 %. To enhance the thermal efficiency of the plant, the coal is used in the boiler in its pulverized form.

In coal fired thermal power plant, steam is obtained in very high pressure inside the steam boiler by burning the pulverized coal. This steam is then super heated in the super heater to extreme high temperature. This super heated steam is then allowed to enter into the turbine, as the turbine blades are rotated by the pressure of the steam. The turbine is mechanically coupled with alternator in a way that its rotor will rotate with the rotation of turbine blades. After entering into the turbine, the steam pressure suddenly falls leading to corresponding increase in the steam volume. After having imparted energy into the turbine rotors, the steam is made to pass out of the turbine blades into the steam condenser of turbine. In the condenser, cold water at ambient temperature is circulated with the help of pump which leads to the condensation of the low pressure wet steam. Then this condensed water is further supplied to low pressure water heater where the low pressure steam increases the temperature of this feed water, it is again heated in high pressure. This outlines the basic working methodology of a thermal power plant. 

Nuclear Power Station

The nuclear power generating stations are similar to the thermal stations in more ways than one. How ever, the exception here is that, radioactive elements like Uranium and thorium are used as the primary fuel in place of coal. Also in a Nuclear station the furnace and the boiler are replaced by the nuclear reactor and the heat ex-changer tubes.

For the process of nuclear power generation, the radioactive fuels are made to undergo fission reaction within the nuclear reactors. The fission reaction, propagates like a controlled chain reaction and is accompanied by unprecedented amount of energy produced, which is manifested in the form of heat. This heat is then transferred to the water present in the heat exchanger tubes. As a result, super heated steam at very high temperature is produced. 

Once the process of steam formation is accomplished, the remaining process is exactly similar to a thermal power plant, as this steam will further drive the turbine blades to generate electricity. 

Hydro-Electric Power Station

In Hydro-electric plants the energy of the falling water is utilized to drive the turbine which in turn runs the generator to produce electricity. Rain falling upon the earth’s surface has potential energy relative to the oceans towards which it flows. This energy is converted to shaft work where the water falls through an appreciable vertical distance. The hydraulic power is therefore a naturally available renewable energy given by the equation:

Where g = acceleration due to gravity = 9.81 m/sec 2

ρ = density of water = 1000 kg/m 3

H = height of fall of water.

This power is utilized for rotating the alternator shaft, to convert it to equivalent electrical energy. 

An important point to be noted is that, the hydro-electric plants are of much lower capacity compared to their thermal or nuclear counterpart. For this reason hydro plants are generally used in scheduling with thermal stations, to serve the load during peak hours. They in a way assist the thermal or the nuclear plant to deliver power efficiently during periods of peak hours.

Types of Power Generation

As mentioned above, depending on the type of fuel used, the power generating stations as well as the types of power generation are classified. Therefore the 3 major classifications for power production in reasonably large scale are :- 

1) Thermal power generation. 

2) Nuclear power generation. 

3) Hydro-electric power generation. 

Apart from these major types of power generations, we can resort to small scale generation techniques as well, to serve the discrete demands. These are often referred to as the alternative methods of power generation and can be classified as :-

1) Solar power generation. (making use of the available solar energy) 

2) Geo-thermal power generation. (Energy available in the Earth’s crust) 

3) Tidal power generation. 

These alternative sources of generation has been given due importance in the last few decades owing to the depleting amount of the natural fuels available to us. In the centuries to come, a stage might be reached when several countries across the globe would run out of their entire reserve for fossil fuels. The only way forward would then lie in the mercy of these alternative sources of energy which might play an instrumental role in shaping the energy supplies of the future. For this reason these might rightfully be referred as the energy of the future.