During normal operation the coal stream is inerted with nitrogen or similar gas. The problem typically occurs during startup, shutdown or failure of the inerting system.
It is not uncommon for tramp metal, railroad spikes, or similar metal objects to be fed into the grinder and thus create sparks. These sparks can travel from the pulverizer through the transport pipe and into the dust collector.
Another source of ignition in the dust collector is static electricity. The atmosphere in the dust collector is typically, at ~200°F with the dry coal dust suspended. These conditions increase the risk of explosions in the dust collector.
The other common ignition source is pyrites that are collected below the pulverizing bed. If there is a failure of the inert gas source then air is introduced, and the pyrites can ignite the coal dust.
Coal is milled to a fine powder in a pulverizer – this increases the surface area of the coal and hence the rate of combustion. The powdered coal is blown into the combustion chamber of the burner nozzle where it is burnt at around 1400°C.
The Mine Safety and Health Administration (MSHA) a branch of U.S. Department of Labor reports that during the past 5 months, eight explosions have occurred, resulting in one fatality and nine nonfatal injuries. Six of these explosions occurred in cement plants.