Ken Birchett looks at a new technology for use in coal pulverisation which is claimed to be the next frontier of material advancement in severe duty applications.
For more than 10 years Xwin technology - from Magotteaux - has been applied to numerous vertical roller mill (VRM) applications throughout the world in aggregate, cement, mining and power. For power generation, Xwin VRM grinding elements have been implemented on all types of coal pulverisation from brown coals (lignite) to anthracite and Petcoke.
Xwin is a technology in which granularised ceramic, with a hardness of 2100 Vickers, is combined with a high chrome alloy in a unique casting process. This combination of metal and ceramic, in a granular form, creates a metal matrix composite (MMC) called Xwin. Within the MMC layer the granularised ceramic is encapsulated by the high chrome metal. The result is an Xwin casting that provides the extreme hardness of ceramic with the durable reliability and ductility of a high chrome alloy.
Xwin castings are no more susceptible to breakage than a solid high chrome casting whether it be from the subjection of rock or tramp metal entering the mill or from operational excursions of the mill. No special handling, operating or maintenance is required with Xwin castings. Xwin is a direct replacement to the existing grinding elements whether it is tires, rolls, tables or segments. Xwin is applicable for the grinding elements on all types and styles of vertical mills such as Alstom/CE, BHEL, B&W - MPS (Pfeiffer), EVT, FWEC - MBF, IHI, MHI, SHMP, Stein or any pendulum style roller mill such as Williams or Raymond.
The hardness of Xwin grinding elements, at 2100 Vickers, can be compared with that of high chrome or hard-faced high chrome grinding elements at 800-950 Vickers. The resulting product being an Xwin casting that is much harder and longer lasting. Additionally, Xwin is not susceptible to potential cracking or spalling that is sometimes encountered with hard-surfaced grinding elements particularly those castings that are hard-surfaced more than once.
The aggressiveness of coal is primarily determined by the silica content of the ash within the coal. Silica (Quartz) having a general hardness range of 1700-1800 Vickers is much harder than the high chromium and high chromium hard-faced materials (800-950 Vickers). However, the Xwin MMC layer at 2100 Vickers is harder than the quartz and will not be adversely affected by the quartz content of the ash as compared with those grinding elements composed of materials softer than quartz.
Vertical Roller Mills are commonly referred to as attrition mills because the material, being fed (under pressure) between the grinding elements by means of centrifugal force imparted by the rotating table or by a rotating pendular shaft, is being crushed against itself. As such the primary wear component in the crushing zone is compression which is a direct force that acts 'normal' to the grinding face. The VRM also utilises air to dry, classify and convey the pulverised coal from the mill. The portions of the grinding elements that are exposed to the pulverised coal/air mixture from the pulverised coal being suctioned off the bowl are also subjected to abrasion or erosion. Erosion is a velocity related wear component in which the force of erosion acts at an acute angle to the grinding face. The erosion component can be quite severe to the grinding elements particularly if high amounts of silica are present. Xwin, being much harder than the materials being crushed, can resist the components of wear whether it is in the form of compression or erosion.
Numerous operating experiences have demonstrated that Xwin grinding elements can extend the lifetime of the grinding elements by at least 2x for MPS and MBF type mills, 2-3x+ for Alstom/CE type pulverisers and 3x for Pendulum style roller mills. This increase in running life, extends the maintenance cycles (Time Between Outages), reduces the frequency for routine maintenance (ie, roll/ring adjustments) and improves mill availability. Xwin eliminates the grinding elements as being the weak link to the maintenance cycle and reduces the operating and maintenance cost of the mill.
Xwin reduces the wear velocity of the grinding elements. This reduction of wear velocity directly correlates to the ability to maintain a consistent grinding profile. Because the Xwin wear velocity is greatly reduced, the grinding profile and the grinding efficiency of the mill is maintained for a much longer duration allowing optimum pulveriser efficiency and achievement of acceptable fineness, capacity and assistance in maintaining furnace performance (ie, maximised performance coupled with optimum maintenance cycles). The ability to maintain mill efficiency at rated capacity correlates to the ability to better maintain downstream furnace and back-end performance.m
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Ken Birchett is Product Manager North America for Vertical Mills for Cement and Utilities, Magotteaux International S A, Vaux-sous-Chèvremont, Belgium. www.magotteaux.com
