Coal received from the mines is stored in the coal yard adjacent to the power plant. It is then conveyed on a daily basis to the boiler and stored in a ‘Coal Silo’. ‘Coal feeders’ continuously feed the required amount of coal to the ‘Coal Pulverisers’.
Coal Pulverisers grind the coal to a very fine powder to make it burn easily. Pulverisers have steel rollers or steel balls which crush the coal between them into a fine powder. This powder is easy to burn. Coal contains moisture. Hot air form the Primary Air Fans dry the coal in the pulverisers. This makes the burning easy and efficient. This air also carries the dry coal powder from the pulverisers to the burners in the boiler furnace.In the burners the coal powder is mixed with the required amount of Combustion air and burned in the furnace.
Coal Flow In a Boiler
Correct amount of air is the most essential ingredient for Combustion. More air or less air both makes the combustion process inefficient.
Forced Draft Fan supplies most of the Combustion air. This fan takes air from the atmosphere and blows it into the furnace through air ducts. The Air Heater heats the air before it enters the Furnace.
Air Heater utilises the heat of the hot flue gases that leave the boiler to heat the combustion air. Hot air improves the efficiency of combustion. The Air Heater works on the regenerative principle. Steel plates alternatively placed in the hot flue gas path and then in the air path heats the cold air entering the Air Heater.
Primary Air Fan supplies the air to the pulverisers for drying and transporting coal. This air called the Primary air also is heated in the Air Heater.
Flue gas system.
Coal burns in the furnace giving out heat and forming flue gases.
Induced Draft Fan. The hot flue gases from the furnace is drawn out by the Induced draft fan. The gases passes through the various heating surfaces of the boiler, the Electrostatic Precipitator and discharges to the atmosphere at the top of the stack. Induced Draft Fan provides the energy for this flow of flue gases. The Induced Draft Fan is normally located adjacent to the Stack.
Electrostatic Precipitators capture the fly ash in the flue gases without letting them out into the atmosphere. High voltage electrodes placed in the gas path ionise the ash particles which collects on collecting electrodes and falls into ash hoppers.
Stack or the Chimney disperses the hot gases and any other particles at a great height. The height enables a very large dispersion area and regulates emission concentrations at ground levels to the level acceptable to humans and vegetation. Stack heights for large power plants are around 250 to 280 meters.
Balanced Draft. The Forced Draft fan and the Induced Draft fan operate in such a way that the air pressure in the furnace is at zero pressure ie: at atmospheric pressure. This is called the ‘Balanced Draft system’.
Air and Flue Gas System In a Boiler
Ash is the inert matter in coal and is the residue after combustion. This has to be collected and disposed off without letting it out into the atmosphere. A part of the ash, around 15 % collects as ‘Bottom ash’ at the bottom of the furnace. The other part collects as ‘Fly ash’ in the Electrostatic Precipitators. The collected ash is then transported to disposal yards or storage silos.
The above are the brief of the main equipment required for the proper combustion of coal in a large power plant boiler.
This post is part of the series: Working Of A Power Plant Boiler
Boiler in a power plant has two functions. The Combustion system converts energy in coal to Heat. Water and steam system converts the heat to steam at high pressures and temperatures. This series explains the working of the boiler in modern power plant.