Two mill systems are employed for most coal grinding applications in the cement industry. These are, on the one hand, vertical roller mills (VRM) that have achieved a share of almost 90% and, on the other hand, ball mills whose share decreased to just over 10%.
Gebr. Pfeiffer SE supplies stand-alone MPS coal grinding mills as well as complete coal grinding-drying systems. Both mill and grinding system can be operated under pressure or suction as well as under air or inert gas atmosphere. System configurations cover grinding plants with temporary storage of the pulverized coal in silos (for cement plants, or in blast-furnaces for steel plants, etc.) and installations with direct injection into the combustion system (for power generating plants). For over 40 years, the MPS mills for power stations have been manufactured under license of Hitachi Power Europe (formerly Babcock Borsig AG), eadquartered in Oberhausen, Germany.
Depending on the material to be ground, throughput rates of 5-200 t/h can be chieved in a fineness range between 1% R0.063 mm and 25% R0.090 mm. Thus, the MPS coal grinding mill device for fresh material is arranged in this area where the fresh feed is also mixed with the grits rejected from the classifier. The main difference of an MPS mill for solid fuels to other MPS mills lies in its pressure-shock resistance.
Mill and classifier housing, feed unit and expansion joints are designed to be pressure-shock resistant. To avoid accumulations of coal dust as a source of spontaneous combustion all surfaces in the grinding and classifying zone are vertically oriented or at least steeply inclined.
The most reliable and comprehensive basis for layout and design is provided by pilot-scale grinding tests of the project-relevant materials. At the GPSE test station, extensive test series with the semi-industrial mill type MPS 40 B are conducted for the grinding of different solid fuels to determine the basic rating data. With this mill, a pilot plant is available that can be used for the determination of solid fuel material characteristics and project-related data, i.e. specific power consumption, gas volume requirements and specific wear rate. Significant differences occur in material properties and specific mill performance data between the different solid fuels such as anthracite, hard coal, lignite and petcoke. Even within a single fuel type the deviation is quite high for grindability, ash content, volatiles, moisture and abrasiveness. The layout and design of a grinding system for solid fuels with low to medium moisture contents is generally determined by grinding, whereas the mill rating for lignite with feed moistures of up to 45% is determined by drying.