What are the different types of coal pulverizing mills?

Coal pulverizing mills are essential equipment designed to crush solid coal into fine particles by applying external forces to overcome the coal’s internal cohesive strength, transforming large lumps into small granules. This process combines principles of both crushing and grinding machinery. Primarily, coal pulverizers fall into three main categories based on their operating speed and design: Low-Speed Mills, Medium-Speed Mills, and High-Speed Mills.

1. Low-Speed Mills (Ball/Tube Mills)

Low-speed mills, commonly known as ball mills or tube mills, operate at rotational speeds typically between 15 to 25 RPM. They are characterized by a large, horizontally mounted, rotating cylindrical shell. This shell, lined with wear-resistant steel plates, is partially filled with grinding media – traditionally hardened steel balls, but sometimes long cylindrical steel rods (hence “tube mills” or “rod mills”).

The fundamental principle of operation relies on the cascading and cataracting action of the grinding media. As the mill rotates, the media are lifted by the shell’s liner plates due to friction and centrifugal force. Upon reaching a certain height where gravity overcomes the centrifugal force, the balls or rods fall or cascade down onto the coal charge at the bottom of the mill. The impact and attrition (rubbing action) between the falling media, the coal particles themselves, and between the media and the liner plates effectively crush and grind the coal. The tumbling action continuously exposes fresh coal surfaces to the grinding forces.

The ground coal particles are swept out of the mill by a stream of air (in dry systems) or water (in wet systems) entering through a hollow trunnion at one end. The finer particles are carried out via the opposite trunnion, while oversized particles remain for further grinding. Ball mills are highly versatile, capable of handling a wide range of coal types, including very hard coals and those with variable moisture or grindability. However, they tend to be large, heavy, require significant foundations, consume relatively high energy, generate noticeable noise, and have higher steel consumption due to media and liner wear compared to other mill types. They are often favored in smaller to medium-sized plants due to their robustness and wide fuel flexibility.

2. Medium-Speed Mills (Vertical Spindle Mills)

Medium-speed mills operate at speeds ranging from approximately 30 to 100 RPM. The most prevalent type is the Vertical Spindle Mill, often referred to as a roller-race mill or bowl mill. This design is highly efficient and widely used in power generation due to its compactness, lower power consumption, lower noise levels, and reduced steel wear compared to ball mills.

Vertical spindle mills are particularly well-suited for grinding bituminous coal, sub-bituminous coal, and lignite, especially when the coal has moderate abrasiveness, a Hardgrove Grindability Index (HGI) between 35 and 100, and surface moisture content below 25%. Their core components include:

• Grinding Table (Bowl/Race): A rotating, horizontal or slightly angled table (often segmented with replaceable liners) that holds the coal bed.
• Grinding Rollers (Tires): Typically three large, spring- or hydraulically-loaded rollers (arranged 120 degrees apart) that roll on the coal bed atop the rotating table. The rollers apply significant pressure, crushing the coal primarily by compression.
• Classifier: Located at the top of the mill housing. It uses adjustable vanes and centrifugal force to separate particles. Oversized particles are rejected back onto the grinding table for further pulverization, while fine particles pass through.
• Primary Air System: Hot primary air is introduced tangentially below the grinding table through a “throat” or nozzle ring. This air performs three critical functions:
  1. Drying: Evaporates surface moisture from the coal.
  2. Transport: Carries the ground coal upwards through the mill body towards the classifier.
  3. Classification Assist: Creates a vortex aiding the separation of coarse and fine particles.

Raw coal is fed centrally onto the rotating table. Centrifugal force throws it outwards under the heavy rollers. The coal is crushed between the rollers and the table liner. The primary air, flowing upwards through the annular space around the table (the throat), fluidizes the crushed coal particles. This air stream lifts the particles past the classifier. Coarse particles, being heavier, fall back onto the table for regrinding. Finely pulverized coal meeting the size requirement exits the mill outlet with the air stream and is transported directly to the burners. The precise control of grinding force (roller pressure), air flow, and classifier settings allows for excellent control over product fineness and mill performance. Their modular design also facilitates easier maintenance.

3. High-Speed Mills (Fan Mills)

High-speed mills, predominantly represented by Fan Mills (or Beater Wheel Mills), operate at very high rotational speeds, often exceeding 400 RPM. They are uniquely designed for high-moisture, low-abrasion coals, particularly brown coals and lignites with moisture contents ranging from 25% up to a remarkable 70%.

The heart of a fan mill is a large, high-speed impeller (beater wheel) resembling a heavy-duty fan, mounted horizontally on a shaft. This wheel is equipped with multiple radially arranged hammers or beaters (often replaceable wear plates). The mill housing typically forms a scroll or volute, similar to a fan casing.

Raw coal, often pre-crushed, is fed into the center of the rapidly spinning beater wheel. The beaters impact the coal with tremendous force, shattering it against breaker plates lining the mill casing. This impact crushing is the primary size reduction mechanism. Crucially, the rotating beater wheel acts as a powerful exhauster fan. It draws hot drying gas (flue gas or hot air, often at very high temperatures – 600°C to 900°C or more) through the mill. This high-velocity gas stream serves multiple purposes:

1. Intense Drying: The high gas temperature and turbulent flow rapidly evaporate the large amounts of surface and inherent moisture present in brown coal/lignite.
2. Particle Transport: Sweeps the crushed coal particles through the mill.
3. Classification: Carries the particles to an integrated classifier (usually a simple inertial separator or cyclone within the mill outlet duct). Coarse particles, being heavier, are thrown outwards by the rotating wheel and gravity, falling back to the center for re-crushing. Fine particles remain entrained in the gas flow.
4. Pneumatic Transport: Provides the motive force to convey the pulverized coal and gas mixture directly to the furnace burners. The mill essentially acts as its own primary air fan.

Fan mills are exceptionally effective for very wet coals due to their high gas flow rates and temperatures enabling rapid drying simultaneously with grinding. They offer a simple, robust design. However, their application is limited to low-abrasion, high-moisture coals. Wear on the beaters and liners can be significant with abrasive contaminants, and they generally produce a coarser product fineness compared to medium-speed mills. Specific models exist for different lignite types (e.g., S-type for older, harder lignites/brown coals up to ~40% moisture; N-type for younger, softer lignites with 40-70% moisture).

Other Notable Grinding Technologies

While Low, Medium, and High-Speed mills dominate coal pulverization, other grinding principles exist, sometimes finding niche applications:

• Autogenous Mills (AG Mills): These mills grind material using the material itself as the grinding media. Large lumps of coal impact and abrade smaller lumps. They eliminate the need for steel balls/rods but require specific feed characteristics (a mix of lump sizes) and are less common for dedicated coal pulverization compared to minerals processing. Types include “Dry” (Aerofall) and “Wet” (Cascade) designs.
• Vibration Mills: These employ a chamber filled with grinding media (balls, rods) that is subjected to high-frequency vibrations. The intense vibratory motion causes the media to impact and shear the coal particles, excelling at fine and ultra-fine grinding. They achieve high grinding intensity due to very high media filling ratios (65-85%) and high surface contact area but are typically lower capacity and used more for specialized materials than bulk coal firing.
• Ring-Roller Mills (Raymond Mills / Pendulum Rollers): These are a type of medium-speed mill where spring-loaded rollers rotate against a stationary grinding ring. Coal fed into the center is ground as it moves outwards under the rollers. Ground coal is lifted by an air stream for classification. While historically significant and still used in some industries, they are less common for large-scale modern power plant coal pulverization compared to the dominant vertical spindle roller-race designs.

Conclusion

The selection of a coal pulverizing mill is a critical decision, primarily driven by the characteristics of the coal fuel (moisture content, abrasiveness, grindability/HGI, volatile matter), the required product fineness for efficient combustion, plant capacity, operational costs (power consumption, maintenance, wear part replacement), and capital expenditure. Low-speed ball/tube mills offer broad fuel flexibility but higher operating costs. Medium-speed vertical spindle mills provide an excellent balance of efficiency, control, and cost for a wide range of bituminous and sub-bituminous coals. High-speed fan mills are the specialized solution for efficiently drying and grinding challenging high-moisture lignites and brown coals. Understanding the distinct operating principles, strengths, and limitations of each mill type is fundamental to optimizing coal preparation for efficient and reliable power generation or industrial processes.

Food Producing Process Flow:

Ultrafine grinding is one of the very key procedure in whole production line. It depends how many mesh that customer need to produce. That’s to say this machine can decide what kind of material will get finally before mxing.

Coarse Crusher Machine —>Grinding Machine —> Vibrating Table Sifter Machine—>Mixing Machine—>Packing Machine —> (Printed Packaging Film Roll or bags ) —> Filling, sealing machine, capping and Labeling Machine —>Pack into Box.

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