Coal mills, also referred to as coal pulverizers or coal crushers, are a type of equipment used to reduce coal to a powder for use in further processing. By pulverizing the coal, it is crushed into a fine powder that allows it to be burned in much the same way as a gaseous substance, thus allowing combustion to be very efficient.
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Pulverized coal, which is produced through coal mills, is often used for burning in the power generation industry, particularly in fossil fuel power plants; however, coal mills are also utilized in a broad spectrum of industries including cement production, aerospace, defense, electric utility, construction, automotive and electronics for many processing applications. Although coal mills offer increases in processing efficiency, some possible disadvantages include significant vibrations that lead to damage and disturbance to machinery in the vicinity, which can result in the interruption of plant operations. However, coal mill manufacturers have developed coal mill design features that limit vibrations and significantly reduce such disadvantages. Another way to limit the force of vibrations is to use proper lubrication; in fact, the bearings in coal mills often require re-lubrication and should be frequently monitored. While pulverizers often use pressure or collision-wrought force to crush materials, coal mills often utilize grinding to achieve the same effect.
There are four commonly utilized types of coal mills: ball-tube mills, impact mills, vertical roller mills and ring-roll and ball-race mills. Ball-tube mills are constructed mainly of a rotating tube that contains cast alloy balls. The raw coal is inserted into either side of the mill through two hollow trunnions. In order to pulverize the coal, the tube is rotated and the balls are used as grinding media in order to deform and/or break apart the coal through the use of friction. An impact mill is a screenless, high-speed type of coal mill. In impact mills, the coal is fed by means of an inlet box at the top of the mill. Crushing occurs in two steps: first, the coal is crushed using primary beater tools at the top of the rotor, then grinding tools are used to crush the coal again at the side of the rotor. A vertical roller mill utilizes rollers that have been hydraulically-loaded in order to crush the coal by means of a rotating table that passes underneath of the rollers. In this type of coal mill, hot air is introduced through an opening at the bottom of the mill and is used to transport the pulverized mill to a burner. A ring-roll and a ball-race mill are nearly the same type of coal mill, except that a ring-roll mill uses rollers and rings and a ball-race mill uses balls and races or raceways. In this type of coal mill, the ring or race is rotating while the ball or rollers are stationary, and springs force the two together to crush the coal.
