Drying Question 1
Presently we use coal from China and South Africa, maintaining volatile matter in fine coal below 32 per cent. We use cooler exhaust gas (ambient) for coal mill drying. We want to use coal from Indonesia having about 42 per cent volatile matter. Is it safe to grind in the coal mill having hot gases from cooler? We have CO2 inertisation system in coal mill circuit. What precautions are required to be taken? How much percentage of high VM coal can be used? How long we can store this type of coal in the yard?
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There are well established guidelines for the safety operation of coal grinding equipment. These fall into two broad categories:
(i) explosion prevention, and
(ii) explosion protection.
For prevention the best solution is to ensure that there is less than 12 per cent oxygen in the atmosphere. You cannot do that when cooler exhaust air is used for coal drying. As a general guideline drying with cooler exhaust air is suitable with direct firing systems. Other prevention measures are to ensure there are no fine coal dust accumulations in the system and no possible sources of ignition. These are determined by the design of the coal milling and storage system.
With your non-inert coal grinding system you will have to have rigourous explosion protection designed into the equipment. The mill, raw coal feeder and ductwork must be capable to withstand a pressure of 9 bar. The dust filter, any cyclones and fine coal hoppers, must be fitted with explosion relief doors or rip-foil sides. Ducts entering these containers must be have explosion relief ahead of the container is they are longer than 5x their diameter.
You can see that there is no simple answer to your question. A full inspection and audit would be required before anyone could certify that it is safe to grind the Indonesian coal in your system. I would recommend modifying the system to dry the coal with inert preheater exhaust gases.
Regarding storage it is not desireable to store high volatile coal in large stockpiles. However, you will have to import large shipments to make it economic. The stockpiles need to be compacted to minimise air ingress into the piles. You should also limit the height of such piles. With 42 per cent volatiles the height of the piles should be limited to 4m.