Reducing the risk of dust explosions

All industrial processes that handle combustible dust assume varying degrees of explosion risk, but what about in cement manufacturing where limestone, sand, and other raw materials are not explosive?

Does that mean the cement manufacturing process is immune to dust explosions? Not so fast. There is one material often used in cement production which is highly combustible – coal.

Coal is used as a fuel for the kiln, which heats the raw material to a partial melt and transforms it into clinker. A coal mill is required to pulverise the raw coal into a powder, which creates an explosive coal dust cloud and is why explosion protection efforts are often focused on this particular stage of the cement manufacturing process.

How dust explosions occur in coal mills

Coal mills are susceptible to deflagrations because they support the following five elements of the ‘Dust Explosion Pentagon’:

Combustible dust

Fine coal dust is explosive. For example, all bituminous coal passing a 200 mesh sieve presents a dust explosion hazard. The minimum explosive concentration for high-volatile bituminous coal is as low as 40 – 100 g/m3 of air.

Oxidant

In most industrial applications such as coal mills, oxygen in the air is the conveyed oxidising agent.

Dispersion of dust particles

For dust to ignite, it must be suspended in the form of a dust cloud, which is ever present in a coal mill during operation.

Confinement of dust cloud

As stated in NFPA 654, all components of an enclosed system handling combustible particulate solids should be designed to prevent the escape of dust; therefore, confined dust clouds are commonplace in industrial processes such as coal mills.

Ignition

Smouldering product in the pyrites chamber, a foreign object such as tramp metal entering the mill and creating sparks, or a number of other ignition sources may be the only missing element required to create a dust explosion.

If an ignition source meets a dust cloud, the following will likely occur:

1. A primary explosion will occur within the volume and will cause an unprotected vessel to deform and rupture if the deflagration pressure exceeds its design pressure.
2. Secondary explosions may occur as the deflagration propagates into connected upstream and downstream processes, consuming additional fuel and strengthening the deflagration pressure and flames. Resting dust outside of the process may also be ignited, resulting in even more devastation.

While many different types of coal mills exist, the ways in which they pulverise coal, convey the product, and support the elements of a dust explosion remain the same. The locations of possible dust hazards in coal mills include:

1. Milling chamber: A continuous combustible dust cloud exists during operation due to the coal dust being held in suspension by the hot airstream. If an ignition source reaches the milling chamber, the dust cloud will ignite.
2. Pyrites chamber: Smouldering product accumulations may collect in this area where the air is hottest and may ignite the dust cloud above.
3. Inlets and outlets: Dust explosions in the coal mill will propagate through the inlets and outlets into connected processes.

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Read the article online at: https://www.worldcement.com/special-reports/17072023/reducing-the-risk-of-dust-explosions/