Focus on Coal
Coal pulverisation with vertical roller mills
Ken Birchett looks at the essential requirements of pulverisation. W
ithin the power industry, vertical roller mills (VRMs) are the predominant equipment used for coal pulverisation. Te essential requirements of pulverisation are
grinding, heating, classification and transport of the pulverised coal from the pulveriser. Te grinding elements coupled with proper alignment and compression are critical to coal pulverisation in the lower mill body (grinding zone) while the classification of the pulverised coal, discharge and recirculation, is critical in the upper mill body (classification zone). Te proper metering, distribution and control of
air flow through the mill, called primary air, is critical throughout the pulveriser and is the bridge between the lower grinding zone and the upper classification zone. Te coal pulveriser is the critical component of the fuel delivery system and is a major influence of combustion efficiency in the burner zone as well as the performance and efficiency of the furnace itself. As such the coal pulveriser is truly the ‘heart’ of the combustion process for a coal-fired power station.
Coal pulveriser design In general, coal pulverisers are designed to achieve the maximum rated capacity grinding a design coal with a grindability of 55 HGI and 8-12 per cent 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. Te 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.
Wear mechanisms in a coal pulveriser Compression, abrasion and impact are the three wear mechanisms that act upon the grinding elements and the pulveriser internals. Compression is a wear component that results from
the crushing of the material upon itself (attrition) between the grinding elements, ie between the tyres/rolls and the table. Compression is a force that acts normal (perpendicular) to the surface of the grinding elements. Abrasion (also called erosion) is a velocity driven
wear component that results when the crushed material that is suctioned off the bowl and entrained in the primary air rushes past the grinding elements. Abrasion is a shearing force that acts upon the grinding elements at an acute angle. Te severity of abrasion is a function of the shear angle, the primary air velocity and the abrasiveness of the entrained material. Impact is a momentum driven wear component that is a function of the density and the force by which the component being crushed or impacted makes contact with the crushing surface. Te primary source of impact wear in VRMs is generally from undesirable dense rock or tramp metal that enters the pulveriser with the coal. Compression and abrasion are the primary wear mechanisms that affect the grinding elements in a coal pulveriser.
Below: Welded HiCr 903 Roll 9.5 Mths/190 000 tons Wear Gap: 11 mm.
Xwin 903 Roll; 1 194 000 tons Wear Gap: 15.5 mm; Xwin Performance Superiority 4x.
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Xwin 903 Table; 1 194 000 tons; Wear Gap: 6.6 mm Xwin Performance Superiority 5x.
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