The coal pulverzing system ina fossil power plant is considered the final stage of the fuelsupply. Pulverizing equipment reduces the coal to the requiredfineness before it enters the boiler furnace. Different types ofpulverizing units are used for reducing the coal to the requiredfineness, such as:

(1) The "impact mill”which hammers the coal in to fine particles by means of hammersmounted on a rotating shaft.

(2) The "ball mill"where the coal is crushed by heavy steel balls in a rotating drum.

Pulverizing units must bedesigned so that leakage of coal fuel is reduced to a minimum.

The most commonly usedpulverizer fuel system is the direct fire unit in which the fuel ispulverized near the point of use. At this point, the pulverized coalhas reached a mixture of fine particles of coal and is then air sweptthrough pipes to the furnace.

Coal dust or the gasesreleased from freshly crushed coal is hazardous when present in theair in sufficient quantity. If a dangerous combustible mixture iscreated in the atmosphere, an explosion will result if the cloud ofdust is ignited only when it is above the LEL. Newly installed dusttight  pulverizer equipment may or may not allow the escape ofcombustible coal dust to the atmosphere in quantities sufficient toproduce a hazardous condition, which means it is below the LEL. itseems , therefore, that a location im which pulverizer units are usedis safe and could be classified nonhazardous because of the lack ofexplosive coal dust in the atmosphere. This is not so. Even whenpulverizer units are considered dust tight, they eventually may beginto leak. As explained before, during the first year of operation thepulverizer equipment may maintain its dust tightness, but in theyears following, the equipment may start to leak due to wear andhandling. pipe connections and other points in the pulverizer fuelsystem may eventually break down and coal dust or gases may escapethrough cracks, wear or misalignment, eventually, coal dust leakingfrom dust tight pulverizer units may create a dangerous condition,either by producing ignitable amounts of coal dust in the atmosphereor by coal dust deposits growing to amounts that will interfere withsafe operation of electrical equipment which means insufficient heatdissipation. These coal dust deposits may also be thrown insuspension in the air by sudden air movements forming an ignitabledust cloud. a coal dust cloud becomes ignitable when it has reachedits LEL.