Spontaneous Combustion

Coal should be burn inside furnace but as long the way from stock pile to furnace sometimes get unproperly operational so the coal can be burn spontaneoulsy at the stockpile and the pulverizer.

Stock Pile
The factors which influence spontaneous combustion in stock pile and which can lead to a big fire, are the following:
1. Rank of coal, low rank of coals are most susceptible because of their higher porosity.
2. Amount of surface area exposed to air
3. Ambient temperature, with high solar insulation aiding it.
4. Oxygen content of coal.
5. Configuration of coal stockpile, steep conical piles with coarse coal at the edges and fines near the top are more susceptible because they promote natural convection (chimney effect) and good air flow through the pile to support combustion as it develops.

To prevent spontaneous combustion, it is important to maintain a dry pile and compaction at regular intervals.


Pulverizer
Pulverizer system is defined as the plant bounded by the mouth of the coal silo feeding the pulverizer, the mouth of the PF burners fed from that pulverizer and the points at which the hot air and cold air ducts leave their respective main supply ducts.


Steps work to get fineness of coal by pulverizer.
Grinding: Reduce coal size to allow coal to circulate within the pulverizer.
Drying: Hot air primary air input +residence time in the mill.
Classification: Produce coal so that 70% will pass through a 200 mesh screen. The term is known as coal fineness.

Explosive Ratio
Inerting by steam is first sequence must be done. This is for remove any hot deposit of coal or PF inside pulverizer system which can make spontaneous combustion. The air fuel ratio of the mixture has an explosive range between 4:1 to 50:1. Most reactive within in range 5:1 to 12:1.


















It means when we put primary air in at the first time with the pulverizer has contains hot deposit, and explosive range met, fire or explosive is very possible happen.

From graph, the minimum of primary air flow through pulverizer is 65 T/h(141 lb), less it pulverizer can not start due to potential blocked as long coal pipe to burner. And the minimum coal fed into pulverizer is 20 T/h (44 lb). So its save from the explosive range (3.25:1) but difficult to do.


Usually operator set primary air flow at 75 T/h and feed coal minimum 15 T/h. Explosive ratio always met (5:1). So when starting up or shutting down the pulverizer, Coal Ratio Explosive always met.

To make this save operation by set outlet temperature not more than 65C is very important. After stabilize operation, air fuel ratio is around (2.5:1).