Designing a coal mill involves several critical considerations to ensure efficient, safe, and reliable operation. Below is a structured approach to designing a coal mill for power plants or industrial applications:
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1. Design Requirements
– Coal Type: Bituminous, sub-bituminous, lignite, or anthracite (affects grindability and moisture content).
– Capacity: Throughput (tons/hour) based on plant demand.
– Fineness: Required particle size (e.g., 70% passing 200 mesh for pulverized coal).
– Moisture Content: High moisture may require drying systems.
– Grindability: Hardgrove Grindability Index (HGI) influences mill selection.
– Safety: Explosion prevention (inerting, pressure relief).
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2. Mill Type Selection
Common coal mill types include:
– Vertical Spindle Mills (Bowl Mills):
– Suitable for medium-fineness grinding.
– Handles high moisture with hot air drying.
– Ball Tube Mills (Tumbling Mills):
– For harder coals; slower response but robust.
– Roller Mills (e.g., Loesche, MPS):
– Energy-efficient with adjustable grinding pressure.
– Impact Mills (Hammer Mills):
– For softer coals; simpler but higher wear.
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3. Key Components
# (a) Grinding Zone
– Rollers/Balls/Pendulums: Apply force to crush coal.
– Grinding Table/Bowl: Rotates to distribute coal.
– Classifier: Separates fine particles from coarse ones (returns oversize for regrinding).
# (b) Drying System
– Hot primary air (~300°C) dries coal before grinding.
– Air-to-coal ratio optimized to avoid fire/explosion risks.
# (c) Drive System
– Motor, gearbox, and auxiliary drives (e.g., hydraulic system for roller pressure).
# (d) Coal Feed S
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– Gravimetric feeders ensure consistent flow.
# (e) Rejects Handling
– Pyrites and tramp material removal.
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4. Design Calculations
# (a) Grinding Power
\[ P = C \times W_i \times \left( \frac{1}{\sqrt{d_p}} – \frac{1}{\sqrt{d_f}} \right) \]
Where:
– \(P\) = Power (kW)