A coal mill pulverizer in a thermal power plant is a mechanical device that grinds raw coal into a fine powder, which is then blown into the boiler furnace for combustion.

Pulverizing the coal increases its surface area, enabling faster and more complete burning, which is essential for efficient steam generation and power output.

Without pulverization, thermal power plants would struggle to maintain consistent flame temperatures, heat transfer rates, and energy efficiency.

Why Pulverization is Essential

Coal used in thermal power stations must be ground to a fine consistency for several critical reasons:

  • Improved Combustion Efficiency: Finely ground coal burns more completely, reducing fuel consumption.
  • Stable Flame Control: Pulverized coal ensures consistent flame shape and temperature in the boiler furnace.
  • Enhanced Heat Transfer: Fine coal particles improve heat absorption in the boiler tubes.
  • Reduced Emissions: Complete combustion helps lower unburned carbon and harmful gases.
  • Efficient Load Management: Pulverized coal allows rapid adjustment to varying power demands.

Types of Coal Mill Pulverizers

Several types of coal pulverizers are employed in thermal power plants based on the design of the boiler and load requirements. The most common types include:

1. Ball Tube Mill

Overview:

A ball tube mill is a slow-speed horizontal rotating cylinder filled with steel balls. It is one of the oldest types used in base-load thermal power plants.

Working Principle:

  • Coal enters the rotating cylinder through a central feed pipe.
  • Steel balls inside the drum tumble and crush the coal by impact and attrition.
  • As the mill rotates, the ground coal is carried out by hot primary air and directed to the classifier.
  • The fine powder is sent to the boiler while oversized particles are returned for regrinding.

Application:

  • Best suited for large, steady-load boilers.
  • Common in older thermal power stations.

2. Bowl Mill (Vertical Spindle Roller Mill)

Overview:

The bowl mill, a subtype of the vertical spindle roller mill, is a medium-speed pulverizer and the most commonly used in modern thermal power plants.

Working Principle:

  • Raw coal from feeders is dropped onto a rotating grinding table (bowl).
  • Heavy rollers press the coal against the bowl and crush it.
  • Hot primary air flows upward through the mill, drying the coal and carrying it to the classifier.
  • Properly sized particles are transported to the furnace; others are returned to the grinding zone.

Advantages:

  • High grinding efficiency
  • Stable output under varying load conditions
  • Compact and less space-intensive

Application:

  • Ideal for mid- to large-capacity power stations.
  • Widely used with supercritical and ultra-supercritical boilers.

3. Ball and Race Mill

Overview:

A ball and race mill is another medium-speed type that uses a rotating bottom ring and stationary top ring, with steel balls in between.

Working Principle:

  • Raw coal is fed into the mill center.
  • The balls, supported by the rotating ring, grind the coal as it is compressed between the rotating and stationary rings.
  • Pulverized coal is separated by a dynamic or static classifier and sent to the furnace.

Application:

  • Used in base-load thermal plants.
  • Suitable for moderate capacity boilers.

4. Hammer Mill Pulverizer

Overview:

Though not as common in large-scale thermal stations, hammer mill pulverizers are used in smaller auxiliary systems or older plants.

Working Principle:

  • Coal is fed into a chamber with rapidly rotating hammers.
  • The hammers impact and shatter the coal against breaker plates.
  • Sized particles pass through a screen; oversized ones are recirculated.

Application:

  • Low-capacity plants or as auxiliary equipment.
  • Simpler and more rugged design for coarse grinding.

5. Impact Mill and Attrition Mill (Less Common)

While not widely used in the primary systems of large thermal plants, these pulverizers combine impact and grinding mechanisms.

Key Points:

  • They provide finer coal for precise combustion control.
  • More common in industrial boilers than utility-scale power plants.

Components of a Coal Mill in Thermal Power Plants

Understanding the key components of a coal mill enhances comprehension of its function and maintenance requirements:

  • Grinding Table/Bowl – The rotating surface where coal is crushed.
  • Grinding Rollers/Balls – Heavy-duty elements that grind the coal.
  • Feeder – Introduces raw coal into the mill.
  • Classifier – Separates fine coal from coarse particles.
  • Primary Air Fan – Supplies hot air for drying and carrying the pulverized coal.
  • Drive Motor & Gearbox – Powers and controls mill rotation.
  • Exhaust System – Ensures proper airflow and pressure control.

Step-by-Step Working

Here’s how a coal pulverizer functions within a thermal plant setup:

Step 1: Coal Feeding

Raw coal is fed into the coal mill through feeders. Its size and moisture content vary depending on the source.

Step 2: Grinding

The coal enters the grinding zone where balls or rollers crush it into fine powder. The grinding pressure and speed are calibrated for optimal fineness.

Step 3: Drying

Hot primary air is injected into the mill to remove moisture from the coal. This step is essential for improving combustion quality.

Step 4: Separation

A built-in classifier separates fine coal particles from coarse ones. Properly sized coal moves to the burners; the rest returns to the grinding zone.

Step 5: Transport

The pulverized coal is carried by hot air through ducts into the furnace burners for combustion.

Operational Considerations in Power Plants

To maintain high efficiency and safety, the following operational parameters must be closely monitored:

  • Coal Fineness: Measured by residue on a 200‑mesh sieve; optimal fineness is 70–75%.
  • Primary Air Temperature: Should be between 250 °C–300 °C for effective drying.
  • Mill Outlet Temperature: Should not exceed 85 °C to avoid fires or spontaneous ignition.
  • Differential Pressure: Indicates mill performance and blockage risks.
  • Grinding Roll Wear: Worn parts reduce efficiency and must be replaced regularly.

Maintenance Practices for Coal Pulverizers

Preventive and predictive maintenance is essential to reduce unplanned outages and ensure safe, efficient operation:

  • Vibration Monitoring – Detects imbalance or bearing failure early.
  • Periodic Roll/Ball Inspections – Prevents severe wear and surface cracking.
  • Lubrication System Checks – Ensures smooth operation of moving parts.
  • Classifier Calibration – Maintains coal fineness.
  • Seal Air System Inspection – Prevents hot air leakage and backfire.

Challenges in Coal Pulverizer Operations

  • Coal Variability: Changes in coal HGI (Hardgrove Grindability Index) affect mill performance.
  • Fire and Explosion Risk: Accumulated coal dust or improper temperatures can cause serious hazards.
  • Wear and Tear: Continuous grinding leads to material fatigue and failure.
  • Power Consumption: Mills can consume 1‑2 % of the total power plant energy output.

Recent Technological Advancements

To meet evolving efficiency and environmental standards, coal pulverizers in thermal power plants are undergoing improvements:

  • Digital Monitoring: Real‑time sensors for temperature, pressure, and vibration.
  • Advanced Classifiers: Dynamic classifiers ensure consistent coal fineness.
  • Wear‑Resistant Materials: Ceramics and tungsten carbide reduce downtime.
  • Automation and AI Control: Enhances consistency and predictive maintenance.

Conclusion

Coal mill pulverizers are the backbone of thermal power plants. By converting raw coal into fine powder, they enable efficient, controlled combustion, ensuring optimal steam generation and electricity output.

Among the various types, bowl mills and ball tube mills are most commonly used depending on plant size and load profile.

For plant engineers and operators, understanding the types, working principles, and maintenance of pulverizers is essential for ensuring reliability, fuel efficiency, and compliance with environmental standards.

With advancements in automation and materials, the future of coal pulverization continues to evolve, supporting the transition to cleaner and more efficient power generation.

Frequently Asked Questions (FAQs)

Q1. What type of coal mill is most common in thermal plants?
A: The bowl mill or vertical spindle roller mill is the most common due to its efficiency and compact design.

Q2. What are the safety risks with coal mills?
A: The major risks are coal dust explosions, overheating, and mechanical failures. Proper airflow, sealing, and monitoring systems help mitigate these.

Q3. How often should a coal mill be serviced?
A: Regular inspections (weekly/monthly) and major overhauls every 1–2 years are common depending on operating hours and load conditions.