A pulverizer or grinder is a mechanical device for the grinding of many different types of materials. For example, a pulverizer mill is used to pulverize coal for combustion in the steam-generating furnaces of coal power plants.
Types of coal pulverizers
Coal pulverizers may be classified by speed, as follows: Low Speed Medium Speed High Speed
Low speed
Ball and tube mills
A ball mill is a pulverizer that consists of a horizontal rotating cylinder, up to three diameters in length, containing a charge of tumbling or cascading steel balls, pebbles, or rods.
A tube mill is a revolving cylinder of up to five diameters in length used for fine pulverization of ore, rock, and other such materials; the material, mixed with water, is fed into the chamber from one end, and passes out the other end as a slurry.
Both types of mill include liners that protect the cylindrical structure of the mill from wear. Thus the main wear parts in these mills are the balls themselves, and the liners. The balls are simply "consumed" by the wear process and must be re-stocked, whereas the liners must be periodically replaced.
The ball and tube mills are low-speed machines that grind the coal with steel balls in a rotating horizontal cylinder. Due to its shape, it is called a tube mill and due to use of grinding balls for crushing, it is called a ball mill, or both terms as a ball tube mill.
These mills are also designated as an example size, BBD-4772,asdadasd
B – Broyer (Name of inventor). B – Boulet (French word for balls). D – Direct firing. 47 – Diameter of shell (in decimeters) i.e. 4.7 m diameter. 72 – Length of shell (in decimeters) i.e. 7.2 m length.
The grinding in the ball and tube mill is produced by the rotating quantity of steel balls by their fall and lift due to tube rotation. The ball charge may occupy one third to half of the total internal volume of the shell. The significant feature incorporated in the BBD mills is its double end operation, each end catering to one elevation of a boiler. The system facilitated entry of raw coal and exit of pulverized fuel from same end simultaneously. This helps in reducing the number of installations per unit.
Mill construction details
A ball tube mill may be described as a cylinder made of steel plates having separate heads or trunions attached to the ends with each trunion resting on suitable bearings for supporting the machine. The trunions are hollow to allow for the introduction of discharge of the materials undergoing reduction in size. The mill shell is lined with chilled iron, carbon steel, manganese steel, or high chrome liners attached to shell body with counter sunk bolts. These liners are made in different shapes so that the counter inside surface of the mill is suited for requirement of a particular application.
The shells are of three pieces. The intermediate shell connects to the end shells by flange joints and the total length of shell is 7.2 m. The liners are fastened to the inner side of mill shell (cylindrical part) to protect the shell from the impact of the steel balls. There are 600 liners of ten variants in each shell weighing 60.26 tonnes. The original lift value of the liners is 55 mm. and the minimum lift allowed is 20 mm.
Operation
The primary air input to a ball tube mill performs a dual function. It is used for drying and as the fuel transport medium, and by regulating it the mill output is regulated. Governed by the pulverized fuel outlet temperature requirement, the cold air and hot air dampers are regulated to achieve the correct primary air temperature. In addition to raising the coal temperature inside the mill for drying and better grinding, the same air works as the transport medium to move the pulverized coal out of the mill: it travels through the annular space between the fixed trunnion tubes and the rotating hot air tube onwards to the classifier. Coal-laden air passes through double cone static classifiers, with adjustable classifier vanes, for segregation into pulverized fuel of the desired fineness, and coarse particles. The pulverised fuel continues its journey towards the coal burners for combustion. The coarse particles rejected in the classifier are returned to the mill for another cycle of grinding.