DETAILS OF COAL PULVERISERS FOR POWER PLANTS

DETAILS OF COAL PULVERISERS FOR POWER PLANTS

Types of Pulverisers

Coal pulverisers may be classified by speed, as follows:

• Low Speed
• Medium Speed
• High Speed

Low Speed

Ball and Tube Mills

A ball mill is a pulveriser 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 slime (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 only, people call it as Tube Mill and due to use of Grinding Balls for crushing, it is called Ball Mill. Hence, is the name Ball Tube Mill. These Mills are also designated as BBD 4772; where, B - Broyer (Name of Inventor). B - Boulet (French word for Balls). D - Direct firing. 47 - Diameter of shell (in Decimetre) i.e. 4.7 metres is the shell diameter. 72 - Length of shell (in Decimetre) i.e. 7.2 metres is the length of shell.

By the name the grinding in the ball and tube mill is produced by rotating quantity of steel balls by their fall and lift due to rotation of tube. 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 outlet of pulverized fuel from same end simultaneously. This helps in reducing the number of installations per unit.

Mill Internals and Other Details

A ball tube mill may be described as a cylinder made of steel plate having separate heads or trunion attached to the ends with the trunion resting on suitable bearings for supporting the machine. The trunion 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, 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 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 metres. The liners are fastened to the inner side of mill shell (cylindrical part) to protect the shell from the impact of steel balls. There are 600 Nos. of liners of ten variants in each shell weighing around 60.26 MT. The original lift value of the liners is around 55 mm and the minimum allowable lift is 20 mm.

Working Principles

Primary air in the case of Tube Mill has dual function to perform. It is used as drying as well as transporting media and by regulating the same, the Mill output is regulated. Governed by the pulverized fuel outlet temperature requirement, the combination of cold air and hot air dampers are suitably regulated to have proper primary air temperature. Apart from raising the coal temperature inside the Mill for drying and better grinding, the same air works in tandem as a carrying media for pulverised coal through annular space between fixed trunion tube and rotating hot air tube on way to classifier. Coal-laden air passing through double cone static classifiers with adjustable classifier vanes for segregation into pulverised fuel of desired fineness and coarse particles continues its journey towards nozzle of coal burners for combustion. Coarse particles rejected in classifier find their way back to mill for another cycle of grinding. In order to avoid excess sweeping of coal from Mill only, part of the primary air, directly proportional to the boiler load demand is passed through Mill. Further to ensure and maintain sufficient velocity of pulverized fuel and to avoid settling in pulverised fuel pipes, an additional quantity of primary air is fed in to mixing box on raw coal circuit. This by-pass air tapped from the primary air duct going in to Mill makes appreciable contribution for drying of raw coal by flash drying effect in addition to picking up the pulverised fuel from Mill outlet for its transportation towards classifiers. Tube mill output while responding to boiler load demand is controlled by regulating primary air-flow. Such regulation by sweeping away of pulverized fuel from Mill being very fast rather well comparable with oil firing response, needs coal level to be maintained in the Mill. Mill level control circuit sensing the decreased coal level in Mill automatically increases the speed of raw coal feeder and vice versa. Maintaining the coal level in Mill offers built-in-capacity cushion of pulverised fuel to take care of small interruption in raw coal circuit. The mill is pressurized and the tightness is ensured by plenum chambers around the rotating trunion filled with pressurized seal air. Bleeding seal air from plenum chamber to Mill provides air cushion between pulverized fuel in the Mill and the outside atmosphere. Inadequacy or absence of seal air will allow escape of pulverized fuel into atmosphere creating nuisance. On the other hand excess of seal air leaking into Mill will affect the desired Mill outlet temperature. As such the seal air is controlled by a local control damper by maintaining just sufficient differential pressure for sealing of pulverized fuel.

Medium Speed

Ring and Ball Mill/Ball and Race Mill

This type of mill consists of two rings separated by a series of large balls, like a thrust bearing. The lower ring rotates, while the upper ring presses down on the balls via a set of spring and adjuster assemblies, or nitrogen gas pressurised rams/loading cylinders. The material to be pulverized is introduced into the centre or side of the pulveriser (depending on the design). As the lower ring rotates, the balls to orbit between the upper and lower rings, and balls roll over the bed of coal on the lower ring. The pulverized material is carried out of the mill by the flow of air moving through it. The size of the pulverized particles released from the grinding section of the mill is determined by a classifier separator - if the coal is fine enough to be picked up by the air, it is carried through the classifier. Coarser particles return to the grinding zone for further pulverization.

Vertical Spindle Roller Mill

Similar to the ring and ball mill, this mill uses large "tyres" to crush the coal. These are usually found in utility plants.

Raw coal is gravity-fed through a central feed pipe to the grinding table where it flows outwardly by centrifugal action and is ground between the rollers and table. Hot primary air for drying and coal transport enters the wind box plenum underneath the grinding table and flows upward through a swirl ring having multiple sloped nozzles surrounding the grinding table. The air mixes with and dries coal in the grinding zone and carries pulverized coal particles upward into a classifier.

Fine pulverized coal exits the outlet section through multiple discharge coal pipes leading to the burners, while oversized coal particles are rejected and returned to the grinding zone for further grinding. Pyrites and extraneous dense impurity material fall through the nozzle ring and are ploughed, by scraper blades attached to the grinding table, into the pyrites chamber to be removed. Mechanically, the vertical roller mill is categorized as an applied force mill. There are three grinding roller wheel assemblies in the mill grinding section, which are mounted on a loading frame via pivot point. The fixed-axis roller in each roller wheel assembly rotates on a segmentally-lined grinding table that is supported and driven by a planetary gear reducer directly coupled to a motor. The grinding force for coal pulverization is applied by a loading frame. This frame is connected by vertical tension rods to three hydraulic cylinders secured to the mill foundation. All forces used in the pulverizing process are transmitted to the foundation via the gear reducer and loading elements. The pendulum movement of the roller wheels provides a freedom for wheels to move in a radial direction, which results in no radial loading against the mill housing during the pulverizing process.

Depending on the required coal fineness, there are two types of classifier that may be selected for a vertical roller mill. The dynamic classifier, which consists of a stationary angled inlet vane assembly surrounding a rotating vane assembly or cage, is capable of producing micron fine pulverized coal with a narrow particle size distribution. In addition, adjusting the speed of the rotating cage can easily change the intensity of the centrifugal force field in the classification zone to achieve coal fineness control real-time to make immediate accommodation for a change in fuel or boiler load conditions. For the applications where a micron fine pulverized coal is not necessary, the static classifier, which consists of a cone equipped with adjustable vanes, is an option at a lower cost since it contains no moving parts. With adequate mill grinding capacity, a vertical mill equipped with a static classifier is capable of producing a coal fineness less than 50 mesh up to 99.5 percent or more and less than 200 mesh up to 80 percent or more, while one equipped with a dynamic classifier produces coal fineness less than 100 mesh up to 100 percent and less than 200 mesh up to 95 percent or more.

In 1954, a Jet Pulveriser was developed which operates like a Vertical Pulverizer; the coal is pulverized by the high speed air action only; forcing/pressurizing coal against coal. 

Bowl Mill

Similar to the vertical roller mill, it also uses “tyres” to crush coal. There are two types, a deep bowl mill, and a shallow bowl mill.

High Speed

Attrition Mill

Rotor, Stationary Pegs

Beater Wheel Mill

Hammer Mill

It is used in farms for grinding grain and chaff for feed.

Demolition Pulveriser

An attachment fitted to an excavator. Commonly used in demolition work to break up large pieces of concrete.

Posted in Blog on 22 January, 2013