Why Coal Burning in a Power Plant Requires Pulverizing

The coal fired power plant converts energy in coal to electricity. The basic input into the system is the fuel that is coal. All other systems and equipment only aid in this conversion process.

Before going into the details, let us look at the quantity of coal processed in a power plant. A 2000 MW power station could be handling coal in the range of 1600 tons/hr or around 12 million tons of coal annually. A single 500 MW unit consumes in the range of 400 tons/hr of coal.

The coal input to the boiler of the power plant passes through three different phases before combustion in the boiler.

• Receiving, storing, and transporting the coal to the boiler. This is bulk storage and bulk handling.
• Preparing the coal for firing or pulverizing. This is what is special in a coal fired power plant, especially large thermal power plants.
• Burning the coal in the combustion process to release the energy in the fuel.

The basic requirement of coal combustion is that all the carbon particles in coal should get sufficient air to burn and release the heat.

Coal normally is available to power plants in large lumps ranging from 2 mm to 50 mm size. Coal is commercially available in different sizes known as “Run of Mine,” “stoker,” “slack,” etc. depending on their size.

In the olden days, coal burning was done on grates without any change in size or pulverizing. Because of the large size of coal, some of the carbon particles do not come in contact with the air. These unburned carbon particles go out with ash. This is a loss and could be in the range of 5 % or greater in the older grate-fired combustion.

Modern boilers powder the coal to a very fine dust so that while burning, it is almost like a fluid stream. The size of the coal particles is in the range of 75 microns. This means every particle comes into contact with the air. The loss due to unburned carbon is only in the range of 0.5 % or less making the boilers very efficient.

The pulverizer does the powdering or the pulverizing. Pulverizers come in different models and are a very important auxiliary of a power plant.

There is one important characteristic of coal that decides the type, size, and power required for pulverizing. This is the grindability of coal and is denoted by the Hardgrove Grindability Index (or HGI). Grindability in general is a function of the grade of coal.

• Sub-bituminous coal from the Indian sub-continent has an index near fifty. A higher number indicates the coal is soft and easy to grind.
• Anthracitic coals, higher grade coals, have higher hardness and consequentially a lower Grindability Index in the range of forty or less.
• Lignite, the lower grade of coal, on other hand is very soft and easy to pulverize and can have an index of a hundred and greater.

The Grindability Index decides what type of pulveriser to use and the power requirement for pulverizing.