Coal fired power plants

The usage of renewable energy is increasing day by day. But still it is inadequate compared to the demand side. Also some unpredictable renewable energy sources are quite tough to maintain the daily load demand. Therefore, the contribution of fossil fuel based power plants can not be denied. Fossil fuels are also known as hydrocarbon based fuels due to having large carbon and hydrogen chains in its chemical structure.

These types of fuels have negative environmental impact and can produce greenhouse gases. As we all know, these greenhouse gases are responsible for warming the earth's atmosphere. Among the various hydrocarbon based fuels, coal is the one who emits greater greenhouse gases compared to oil and natural gas. Thus coal power plants have the highest impact on the environment which should be controll by applying the latest technology. Modern coal  fired power plants are capable of minimizing these issues at their lowest point. 

Coal fired power plants use coal as fuel. Coal can be of various forms such as bituminous, lignite, brown coal etc. according to its calorific values. The more calorific values it has, the more thermal energy can be gain by the process of combustion of these coal. Lower grade of coal can be use in this type of plant by means of pulverizing. The pulverizers are use to crush the raw coal to finest power so that they can be easily burned to achieve the highest thermal energy. For this reason coal fired power plants are also call pulverized fuel fired plants or pulverized fuel firing systems.  

Basically a coal fired power plant consists of following components-

1.  Coal pile

2. Pulverizer

3. Chimney 

4. Water reservoir

5. Furnace or combustion chamber

6. Steam Turbine

7. Generator

8. Step-up transformer  

Components:

Let us discuss the components one by one according to their implements.

1.Coal pile: It is a kind of coal reservoir where preliminary coal is store. Coal piles can be various forms and can have the ability to store coal for a couple of days for continuing the production. From the coal pile, it is throw into the grate which is the previous stage of combustion.

2. Pulverizer: As described earlier, pulverizers are use to crush the coal to get powder so that it can be efficiently burn. Pulverizer is a kind of plant accessory that increases the efficiency of the plant.

3. Chimney: Burnt gas contains carbon dioxide, carbon monoxide and other greenhouse gases are not suppose to free away to the environment. Chimney is work as a filter to confine the fly ash and other pollutant air particles.

4. Water reservoir: Water is need to get the super heated steam to run the steam generator. Combustion process helps to the steam by burning the coal after leaving the pulverizer chamber. 

5. Combustion chamber: It is the place where coal is burnt to get heat so that sufficient amount of steam can be produce to run the steam generator. It can be in various forms.

6. Steam Turbine: Steam turbines are use to produce mechanical energy from the high pressure steam and to deliver it to the generator.  

7. Generator: The purpose of a generator is to produce electrical energy from the mechanical energy. The produced power is then fed to the step up transformer.  

8. Step-up transformer: Before transporting the power to the transmission line, the voltage must be higher. In order to do that, a step up transformer is need before delivering to the transmission lines.  

Working principle:

First, coal is reduce to a fine powder with the help of a grinding mill and then projected into the combustion chamber with the help of hot air current.  The amount of air (secondary air) required to complete the combustion is supply separately to the combustion chamber. The resulting turbulence in the combustion chamber helps for uniform mixing of fuel and air. The amount of air which is used to carry the coal and dry it before entering into the combustion chamber is known as primary air. The efficiency of the pulverized fuel firing system mostly depends upon the size of the powder. After that, it is delivered to the combustion chamber where burning takes place. Water is converted to steam with the help of this combustion process. Steam is deliver to a turbine that rotates the generator to produce electricity.   

Advantages

• Any grade of coal can be use since coal is powder before use

• The rate of feed of the fuel can be regulate properly resulting in the economy

• Since there is almost complete combustion of the fuel there is increase rate of evaporation and higher boiler efficiency

• Greater capacity to meet peak loads

• The system is practically free from sagging and clinkering troubles

• No standby losses due to banked fires

• Practically no ash handling problems

• No moving parts in the furnace is subject to high temperatures

• This system works successfully with or in combination with gas or oil

• Much smaller quantity of air is require as compared to that of stoker firing

• Practically free from clinker troubles

• The external heating surfaces are free from corrosion

• It is possible to use highly preheated secondary air which helps for rapid flame propagation

• The furnace volume required is considerably less

Disadvantages:

• High capital cost

• Lot of fly ash in the exhaust, which makes the removing of fine dust uneconomical

• The possibility of explosion is more as coal burns like a gas

• The maintenance of furnace brick work is costly

• Special equipment is need to start this system

• Skilled operators are require

• A separate coal preparation plant is necessary

• High furnace temps cause rapid deterioration of the refractory surfaces of the furnace• Nuisance is create by the emission of very fine particles of grit and dust

• Fine regular grinding of fuel and proper distribution of burners is usually difficult to achieve

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