Combustion in Boilers

Pulverised Coal

Pulverised Coal is coal which has been powdered to a fine size.  Coal is powdered or pulverised by passing into through a pulveriser.  A Pulveriser is a machine which consists of rollers which crush coal into a fine size. Today, almost all coal fired plants used Pulverised Coal. 

The Pulverised coal is mixed with hot air.  This air and coal mixture is fed to a burner in the furnace which ignites it.  Modern pulverisers can grind coal to a very fine size of the order of microns.  When the pulverised coal mixes with air, it flows almost like a fluid stream.  

Pulverised coal burns more efficiently as it has a higher surface area.  Sometimes, pulverised coal is mixed with other fuels such as biomass. 

Cyclone furnaces are used in boilers to burn poor quality coal which are not suitable for normal pulverized coal combustion.  These coal particles require higher temperatures and more oxygen to burn. 

Cyclone furnaces, as the name suggests, are able to provide a cyclone of air which results in greater turbulence which results in better mixing of the coal and air.  The cyclone of air also causes greater exposure of the surface area to the flame. 

The particles are whirled about in the air flow which results in greater exposure of the surface of the coal particle to the atmosphere. 

Stoichiometric Combustion in Boilers

Stoichiometric Combustion is the ideal combustion where at the end of the combustion process no fuel or air is left behind.  Thus all the carbon in the fuel is converted to carbon dioxide and the sulphur is converted to sulphur dioxide.  If residues such as carbon, carbon monoxide or sulphur remain, then the combustion is not stoichiometric.

Stoichiometric combustion cannot be achieved practically.  By adjusting the air/fuel mixture, the actual combustion can be made as close to the stoichiometric combustion as possible. 

Excess Air in Combustion

When fuel is burned in the combustion chamber of the boiler, the calculated amount of air which is required for the fuel to burn is never sufficient.  Excess air needs to be fed to the combustion chamber.  This air which is in excess of the calculated amount is known as excess air.

Excess Air refers to the additional air which is fed to the combustion chamber of the boiler to ensure that the fuel gets burnt properly.  Excess Air is provided by means of blowers.  in the case of supercharged boilers, it is provided by compressors. 

Different types of fuel have different requirements for excess air. Gas may require up to 10% of excess air.  Fuel will require up to 20 % and coal will require large amounts of excess air up to 60 %.

Draft in Combustion

Draft or draught is an important factor in the combustion of fuel in the boiler.  Draft refers to the difference in pressure in the boiler furnace to the pressure on the top of chimney.

This pressure difference is necessary for the flow of fresh air into the boiler and for the removal of flue gases out of the boiler.  The draft should be optimal.  The draft has a direct influence on the fuel/air mixture in the furnace.  A higher draft will result in more air being sucked into the furnace.  This will result in higher combustion. 

A lower draft will not be able to remove the flue gases properly from the furnace.  This will result in less air entering the furnace.  This, in turn, will result in incomplete combustion which affects the efficiency.  Incomplete combustion will also result in more pollution.

Stack Temperature for Boilers

The Stack temperature of the boiler is a very important parameter in Boiler design.  The stack temperature is the temperature of the flue gases when they reach the stack or the chimney.

A low temperature of the stack temperature indicates that little heat is carried away by the flue gases and that the boiler is operating efficiently.  The stack temperature is a very important specification at the time of boiler purchase.

If the temperature is too low, it can result in cold corrosion. 

The Stack temperature will vary with the time of the year as it is also dependent on ambient temperature. 

When the fuel is burnt in the furnace of a boiler, all the energy in the fuel is not available to heat the boiler.  Some of the energy is lost in the form of losses.
The Losses in combustion of a boiler are categorized in to the following types.

Loss in the Flue gases

The heat generated by burning the fuel is present in the flue gases.  When these gases escape into the atmosphere, some amount of heat also escapes with the gases.

Hydrogen Losses

This refers to the heat used in evaporating moisture or water present in the fuel.  This is particularly significant in coal-fired boilers.

Losses due to improper combustion

The improper combustion of fuel due to poor quality or inadequate air also results in loss of potential energy

Losses due to Convection and Radiation

The furnace of the boiler is insulated.  Despite this, some heat escapes from the furnace to the atmosphere.