Coal Mill Rejects Handling System Using Dense Phase Pneumatic Conveying

Coal Mill Rejects Handling System: An Overview

Every coal pulverizer generates mill rejects—materials that cannot be ground to the required fineness. These rejects typically include iron pyrites, tramp metal, stones, and other hard or oversized materials.

A reliable coal mill rejects handling system is designed to continuously remove these materials from the coal mill and transfer them safely to a mill rejects bunker or storage silo without disrupting normal mill operation.

Dense phase pneumatic conveying is an established approach for handling coal mill rejects. In this type of system, rejects are collected in a pyrite hopper below each coal mill and transported in controlled batches through enclosed conveying pipelines to a storage bunker.

Unlike open mechanical conveying systems, pneumatic mill rejects handling uses compressed air to transport the material through a closed pipeline, which travels overhead to the bunker top. This helps minimize spillage, reduce floor-space requirements and provide an automated method of mill rejects removal.

A typical coal mill rejects handling system generally includes a pneumatically operated knife edge gate valve, maintenance knife edge gate valve, pyrite hopper with sizing grid, dense-phase conveying vessel, Dome Valve®, conveying pipeline, instrumentation, PLC-based control system, air compressor and mill rejects storage bunker.

What Are Coal Mill Rejects?

Coal mill rejects are non-grindable materials separated from coal during the pulverization process. Common coal mill rejects include:

• Iron pyrites
• Tramp metal
• Stones and other hard mineral matter
• Oversized or difficult-to-grind coal particles

These materials are generally dense, abrasive and irregular in shape and can potentially damage coal mill internals if they are allowed to accumulate.

For this reason, an efficient coal mill rejects system requires a reliable evacuation route. The primary objective is to remove rejects from beneath the pulverizer while allowing the coal mill to continue operating normally.

How Does Dense Phase Pneumatic Conveying Handle Mill Rejects?

A dense phase pneumatic conveying system transports coal mill rejects in controlled batches rather than continuously suspending the material in a high-velocity air stream.

The typical mill reject handling process involves the following steps:

1. Collection in the pyrite hopper:

Reject material falls from the coal mill through a pneumatically operated knife gate valve into the pyrite hopper.

2. Oversize separation:

A sizing grid helps prevent material exceeding the specified size from entering the conveying vessel.

3. Batch loading:

When triggered by a level signal or programmed operating sequence, the inlet valve opens and rejects enter the conveying vessel.

4. Vessel sealing:

The Dome Valve closes and seals the conveying vessel.

5. Pneumatic conveying:

Compressed air enters the vessel and pushes the batch of rejects through the conveying pipeline.

6. Discharge:

The conveyed rejects enter the mill reject bunker or storage silo.

7. Cycle reset:

After conveying is completed, the vessel returns to its normal condition, and the next conveying cycle begins.

Depending on the system design, conveying may be initiated by a hopper-level signal or programmed timer. PLC-based automation and monitoring can be used to control conveying cycles, valves, pressure conditions and system interlocks.

Why Use Dense Phase Pneumatic Conveying for Coal Mill Rejects?

Dense phase conveying offers several potential advantages for coal mill rejects handling in power plants.

1. Lower Conveying Velocity

Dense phase systems transport material at comparatively low velocity. This is particularly useful for abrasive materials such as pyrites and mineral rejects because lower material velocity can help reduce erosive wear on conveying pipelines and components.

2. Enclosed Material Handling

The conveying route remains enclosed from the collection hopper to the storage bunker. This helps minimize reject spillage and reduce housekeeping requirements compared with open mechanical handling routes.

3. Reduced Floor-Space Requirement

Pneumatic conveying pipelines can be routed overhead, leaving valuable floor space around coal mills available for operator access, maintenance and plant movement.

This can be particularly beneficial where the space surrounding existing coal mills is limited.

4. Independent Mill Operation

A mill rejects handling system for a power plant can be configured with dedicated conveying arrangements for individual mills. This allows reject removal from individual mills to operate independently rather than relying entirely on one common mechanical conveying route.

5. Suitability for Difficult Materials

Dense-phase pneumatic conveying is suited to difficult-to-handle bulk solids. Dedicated mill rejects systems can be designed for particle sizes up to approximately 40 mm, depending on the specified system and equipment configuration.

Main Components of a Coal Mill Rejects Handling System

A typical coal mill rejects handling system consists of several interconnected components designed to collect, convey, control and store mill rejects.

1. Pyrite Hopper

The pyrite hopper receives rejects discharged from the coal mill. Depending on the system design, it may incorporate:

• Sizing grid
• Inspection arrangement
• Level instrumentation
• Temperature instrumentation
• Oversize discharge
• Auxiliary equipment

The hopper provides temporary storage before the rejects enter the pneumatic conveying system.

2. Knife Gate Valves

Pneumatically operated knife gate valves isolate different sections of the system and control the movement of reject material from the coal mill and pyrite hopper.

3. Dense Phase Conveying Vessel

The dense phase conveying vessel receives a batch of coal mill rejects before compressed air is introduced.

Once loaded and sealed, the vessel acts as the pressure vessel from which the reject material is conveyed through the pipeline.

4. Dome Valve

The Dome Valve (https://www.mbl.in/dome-valve/) is used as the inlet valve on dense-phase conveying vessels.

Its inflatable-seat design is intended to provide an airtight seal while operating in abrasive conveying applications. According to the source material, the valve is designed specifically for dense-phase applications and has one moving part.

5. Conveying Pipeline

The conveying pipeline transports each batch of rejects from the conveying vessel to the mill reject bunker or storage silo.

Wear-resistant bends and fittings may be selected according to the material characteristics and conveying conditions because changes in material direction can create localized wear.

6. PLC and Instrumentation

PLC-based controls coordinate:

• Valve operation
• Conveying pressure
• Hopper-level signals
• Conveying cycles
• Equipment status
• System interlocks
• Alarms

Automation allows the mill rejects handling system to operate with limited manual intervention and provides operators with information about system status.

7. Conveying Air Compressor

Compressed air provides the motive force for transporting rejects through the conveying pipeline.

Compressor capacity should be selected according to factors such as:

• Rejects loading
• Conveying distance
• Pressure requirements
• Pipeline configuration
• System operating philosophy

8. Mill Rejects Bunker or Storage Silo

The mill rejects bunker or storage silo receives and temporarily stores the conveyed rejects before they are removed from the plant for disposal or further handling.

Dense Phase vs. Mechanical Mill Rejects Handling

Mechanical conveying can also be used for mill rejects. However, dense phase pneumatic conveying offers advantages when abrasive and irregular materials must be transported through areas with restricted floor space.

The selection between pneumatic and mechanical conveying ultimately depends on rejects characteristics, conveying distance, coal mill arrangement, available space, operating philosophy and maintenance strategy.

The Role of the Dome Valve in a Mill Rejects Handling System

The inlet valve of a dense-phase conveying vessel has an important function: it must allow the vessel to fill with abrasive material and then provide a reliable seal before conveying begins.

The Dome Valve is designed for dense-phase conveying applications. Its inflatable seat allows the valve to seal against the valve dome rather than relying on a conventional rigid valve element moving through the material stream.

For coal mill rejects handling, valve reliability is particularly important because conveying cycles are repeated throughout plant operation.

How to Improve Coal Mill Reject Handling System Reliability

A reliable mill rejects handling system depends not only on equipment selection but also on appropriate operation, inspection and maintenance.

Key areas to monitor include:

• Dome Valve seal condition
• Pyrite hopper sizing grid
• Knife gate valve operation
• Conveying vessel pressure
• Pipeline bends and wear points
• Hopper level and temperature instrumentation
• Compressed-air pressure and availability
• Mill rejects bunker or silo level
• PLC alarms and system interlocks

Pipeline bends and other locations where reject particles change direction can experience higher wear and should therefore be included in the inspection program.

Compressed-air capacity should also match the actual conveying duty. Insufficient air pressure or capacity can affect conveying cycles and system reliability.

Why Dense Phase Is Suitable for Power Plant Mill Rejects

Coal mill rejects differ from ordinary bulk solids. They may contain abrasive mineral matter, tramp metal, pyrites and irregular sized particles, creating demanding conditions for material-handling equipment.

A dense-phase pneumatic coal mill rejects handling system addresses these challenges through a combination of:

• Low-velocity conveying
• Batch-wise material transport
• Enclosed pipelines
• Automated controls
• Dedicated mill-level rejects removal
• Compact installation
• Pneumatic transfer to centralized storage

Dense-phase mill rejects systems are used in power-plant applications, including documented installations for large generating units. The source material also references an NTPC technical specification for a Super Thermal Power Project describing a dense-phase pneumatic system with dedicated conveying vessels associated with mill rejects pyrite hoppers and pipelines leading to storage silos.

Frequently Asked Questions About Coal Mill Reject Handling Systems

Q. What are coal mill rejects?

A. Coal mill rejects are non-grindable materials separated from coal during pulverization. They commonly include iron pyrites, tramp metal, stones and other hard or oversized particles.

Q. Why is dense phase pneumatic conveying used for mill rejects?

A. Dense phase conveying transports mill rejects at comparatively low velocity and in controlled batches. This can help reduce erosive wear, minimize spillage and reduce the space required for conveying equipment.

Q. What size material can a coal mill rejects handling system handle?

A. Dense-phase systems designed for coal mill rejects can typically handle material mix with up to approximately 40 mm, depending on the equipment design and application.

Q. What is a pyrite hopper in a coal mill rejects system?

A. A pyrite hopper is a collection hopper installed below a coal mill. It receives and temporarily holds mill rejects before they are transferred to the pneumatic conveying vessel.

Q. What is a Denseveyor?

A. Denseveyor is a proprietary dense-phase pneumatic conveying system developed by Macawber Beekay. The source material describes it as a low-velocity dense-phase transporter for difficult-to-handle materials using compressed gas for enclosed conveying.

Q. What does a Dome Valve do in a mill rejects system?

A. The Dome Valve is an OFF-ON valve which controls material entry into the Denseveyor vessel and provides the pressure seal required before pneumatic conveying begins.

Q. Does each coal mill need a separate rejects handling system?

A. A dedicated system can be provided for each coal mill, allowing individual mill rejects removal to operate independently. The exact arrangement depends on the plant configuration and engineering requirements.

Q. How often should a mill rejects handling system be inspected?

A. There is no single inspection interval suitable for every power plant. The maintenance schedule should consider operating hours, conveying cycles, rejects characteristics, equipment condition and the manufacturer’s recommendations. The user must follow the “project specific” Operation & Maintenance Manuals as provided by the supplier.

Particular attention should be given to valves, hopper components, instrumentation and pipeline bends exposed to abrasive material.

Q. Can inadequate compressed-air capacity affect mill rejects system performance?

A. Yes. The conveying-air system must be correctly sized for the rejects loading, conveying distance, pipeline configuration and required conveying pressure. Insufficient air capacity or pressure can affect conveying cycles and overall system reliability.

Conclusion

A coal mill rejects handling system using dense phase pneumatic conveying provides a compact, enclosed and automated method for removing pyrites, tramp metal, stones and other non-grindable materials from coal pulverizers.

By collecting rejects in a pyrite hopper and transferring them in controlled batches through a pressurized conveying vessel (Denseveyor) and enclosed pipeline, the system can help reduce floor-space requirements, minimize spillage and limit conveying velocity for abrasive materials.

Key components such as the Dome Valve, Denseveyor vessel, PLC controls, conveying-air compressor, material conveying pipeline and mill rejects bunker work together to provide reliable rejects evacuation.

For power plants evaluating a new installation or modernization of an existing coal mill rejects handling system, the major design considerations include reject size and characteristics, conveying distance, coal mill configuration, compressed-air availability, storage capacity, wear protection, automation and maintenance requirements.

“Looking for a reliable coal mill rejects handling system for your power plant? Contact Macawber Beekay to discuss your application.”