Coal vertical mills are designed to achieve the maximum rated capacity grinding a design coal with a grindability of 55 HGI and 8-12 percent moisture and achieving a discharge fineness of 70 per cent passing a 200 mesh screen (74 micron) and 99.5 per cent passing a 50 mesh screen. Variation of the coal hardness and/or moisture content will effect, up or down, the discharge capacity or the discharge fineness.

To achieve these design considerations it is essential that the grinding elements are in good condition and they maintain a consistent profile. The loss of the grinding profile results in a loss of grinding efficiency, meaning the pulveriser must work harder to achieve an equal result.

Ultimately a point will be reached in which either a loss of capacity or a loss of fineness will result under uniform conditions. Longer lasting, more consistent grinding elements are crucial to pulveriser operation yielding longer operating campaigns, reduced maintenance and a lower overall invested cost.
 

Remarks: Raw coal hastelloy coefficient 50-90

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 micrometer-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 micrometer-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 up to 99.5% or higher<50 mesh and 80% or higher <200 mesh, while one equipped with a dynamic classifier produces coal fineness levels of 100% <100 mesh and 95% <200 mesh, or better.