Coal Dust Explosion

 Coal Dust Explosion 

Definition 

An explosion is the Sudden combustion process of great intensity characterised by mechanical destructive effects through pressure and heat. For coal dust to ignite, combustible dust may be present in form of thick clouds. 

Thick cloud of coal dust + O2 + ignition source (flame) ==> initiates combustion of coal

Stages of Coal dust combustion

stage 1 

Ignition often starts as a puff, i.e. sudden combustion of part or whole of the air-borne dust with high temperature, but no marked effects.

stage 2

The transition from Puff to Explosion; during this stage Pressure in burning layers increases as the heat of combustion, is transferred to surrounding unburnt layers due to radiation.

stage 3

full fledge explosion; when the pressure value exceeds a certain value, a full-fledged explosion in which combustion occurs at a very rapid rate as a result heat is transferred totally rather than lost to its surrounding. 

once the pressure wave is formed it stirs up the deposited dust forming new dust clouds. 

Factors affecting coal dust Explosion

The flammability of coal dust depends upon:

1. particle size: 

• 150 to 1000 µm participates in explosion depending upon rank, the intensity of ignition source, O2 Concentration, etc.
• The most vulnerable particle size to explosion is 10 to 100 µm.
• particles of less than 10 µm sizes tend to agglomerate.

1. dustiness: 

• all dust particles may not be airborne.
• if a dust concentration of 100 to 120 gm/m3 is present, explosion may happen.
• if the concentration is 300 to 400 gm/m3, violent explosion will happen.

1. Volatile matter 

• flammability of coal dust increases if the combustible volatile matter content increases.
• if volatile matter content is < 10 % then non-fllammable 
• if volatile matter = 10 - 14 % then its less-flammable 
• if Volatile matter content is > 14 % then its highly flammable 
• In general if volatile content is less than 20 % then explosion does not happen.

1. percentage of ash:

• Increase in ash content or presence of inert material in dust reduces its flammability due to absorption of heat.

1. Oxygen (O2) concentration:

• if O2 content is less, then requirement of ignition energy is greater and ignition temperature is also greater. mean if less oxygen is present then coal dust is unlikely to get ignited easily. 

coal dust ignition stages