Pulverizer

Basically, pulverizing is done by subjecting the coal to the grinding force between a rotating shallow bowl and three stationary roll wheel assemblies. Since the roll wheels do not touch the bowl surface, both crushing and attrition take place by attrition of coal against coal.

The capacity or output of any pulverizer, in general, depends on three factors. These are grindability, moisture content and fineness desired.

Grindability is not an inherent quality of coal like moisture content or heating value. The grindability index indicates the comparative value and ease with which specific coal can be pulverized.

On the Hardgrove Scale, a fifty Hardgrove Grindability Index (HGI) is the standard from which base or nominal capacities are calculated. A high grindability index indicates the coal is relatively easy to grind and, therefore, increases pulverizer capacities. Likewise, coal with a lower grindability index is harder to pulverize and decrease pulverizer capacities


Moisture content of the coal has an effect on pulverizer capacity, but is not as significant as grindability. Very high moisture coal tends to reduce pulverizer capacities. This is due to the fact that the higher moisture coals react differently and tend to resist the grinding process.

Pulverizer capacity is greatly affected by the fineness requirement of the pulverized coal. In simple terms, more work is required to achieve a greater fineness of the coal leaving the pulverizer. Therefore, without changing the work input to the pulverizer, its output will be reduced if fineness of the product is increased.
The fineness of the pulverized coal is expressed in terms of the percentage that will pass through a two hundred-mesh screen. The method of arriving at this value is as follows:
A fifty gram sample of pulverized coal obtained from a composite collection taken after each pulverizer discharge valve in the pulverized coal system is put on top of a series of three screens. The first screen is called a fifty mesh because it has fifty openings to the inch, or two thousand five hundred divisions per square inch. The next screen is the one hundred mesh. The final screen is the two hundred mesh, which is the screen with the smallest openings. To be exact, two hundred openings per inch for a total of four thousand per square inch.


Basic Principles of Operation
The continuous output of a pulverizer is determined by the amount of coal fed into the pulverizer. Primary air flow transports the pulverized coal through the mill, dries the coal for optimum combustion, and maintains coal circulation within the pulverizer.

Raw coal from the feeder is fed through a downspout in the center of the pulverizer and flows radially outward due to centrifugal force where it is pulverized between rollers and grinding segments. The mechanism of the particle size reduction is typically particle-to-particle attrition within the material layer interposed between the grinding ring segments and the rotating roller elements. This type of grinding mechanism produces low wear rates on the grinding elements.
The automatic spring pressure adjusting system installed on the pulverizer is designed to provide a means of varying the roll wheel spring loads in relationship to the coal flow.

The primary air flow carries the finely pulverized coal upward toward the top of the mill. Coal particles, which have not been sufficiently pulverized and are too large to be carried by the air stream, fall back into the grinding zone to be reground. The smaller particles are carried to the top of the mill and into the classifier by the air stream.

The classifier acts as a cyclone separator returning heavy particles to the grinding zone for further pulverization while allowing particles of sufficient fineness to pass to the mill outlet.

The coal-air mixture temperature leaving the pulverizer is held constant by regulating the inlet air temperature to the mill. This is done by regulating the hot air and tempering air damper positions. Changes in the fuel-air ratio and moisture content of the raw coal will cause the air temperature entering the pulverizer to vary for a constant outlet temperature.

Foreign particles in the raw coal feed, such as tramp iron, rock, and pyrites, which are too heavy to be carried by the air stream fall through the throat. Pyrite plows (attached to the yoke) continuously bring these foreign particles around to the discharge chute where they fall into the pyrite box. The material is removed from the pyrite box by way of a pyrite sluice system or manually at regular intervals of time


The trend beside is describe about pulverizer got choke. It shouldn't be happened if we take action early when the differential reading was tend to increase, reduce classifire speed, increase hydraulic roll wheel pressure or reduce feeder speed.


Source:
Babcock & Wilcox training.
PF Code of Paiton Power Station Operational.

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