Case Study – Coal Mill Rejects Handling System for PF Boiler

Case Study – Coal Mill Rejects Handling System for PF Boiler



Location of the Plant / project STAGE – II (2 X 250 MW) NTPC SAIL Power Company Ltd
Brief Scope of Work Design, Engg., Manufacturing, Supply, Erection, Testing & Commissioning of complete Mill Rejects Handling System
Installed System Collection of the rejects from 07 mills (working 06 Nos.) and pneumatically conveying to a storage bunker for each boiler unit.



Macawber Beekay have pioneered in employing Denseveyor system for handling ‘difficult’ rejects from pulverizing mills to bunkers in power plants. Conventional conveying systems suffer from many inherent disadvantages leading to frequent breakdowns, burning of belts, spillage of rejects, heavy structural works posing movement restrictions, choking of screws etc. These effect continuous operation of system resulting in production losses and heavy maintenance and replacement costs.

Macawber Beekay’s Denseveyor system with distinct merits such as negligible maintenance and recurring costs, requirement of very less space for installation (conveying pipes travel overhead) thus leaving the plant movement unrestricted. Totally enclosed system ensures no spillage of mill rejects, no excavation works like trenches and civil foundations.There is an individual system for each mill.

Denseveyor system with proven track record for mill rejects handling in power plants is now the established technology and enjoys widespread acceptance of power plant engineers.


The pneumatic cylinder operated plate valve at mill outlet (Pyrite Hopper Inlet) is normally kept open. Mill Rejects from coal mill drop into the Pyrite Hopper through the grid. Oversized material is retained on the grid. Manual Operated Plate Valve on Over-size material discharge chute is operated on need basis. The bypass chute is fitted with a Manual operated plate valve and is opened in case of emergency only. Whenever the Manual operated oversize valve is to be opened, it shall be ensured that Pyrite Hopper inlet valve is closed. The Hand Wheel operated plate valve at Pyrite Hopper Outlet is normally kept in open position as it is a maintenance valve. Denseveyor is installed below the Pyrite Hopper for conveying coal mill rejects to Rejects Bunker through independent material conveying pipeline. On top of the Bunker, a bag filter is provided to vent out clean air. Bunker is also equipped with a manually operated sector gate for discharge of rejects into the trucks.


The Configuration of the Coal Mills of boiler units is in two rows on either side of the boiler with one stream of three mills and another stream of four mills on the other side of boiler.There are seven mills in each unit i.e. total 14 mills in two units. Each mill is provided with collection and conveying equipment as defined above.

Conveying Air System

2×100% (1W+1Sb) reciprocating non-lubricated type air compressors with drive, housed in Compressor Room are provided to supply Compressed Air required for mill rejects conveying. The compressed air system has two air receivers, and necessary pipe work.

Instrument Air System

For equipment operation in the system, instrument air is provided at each stream through distribution pipe work from the specified tapping point to Pneumatic Cylinder Operated Plate Valves, Denseveyor Panel and Bag Filter.

Cooling & Spray Water System

Cooling water requirements of Water-Cooled Top Plate of Dome Valve in Denseveyor Assembly are met from the Service Water circuit. Necessary pipe work is provided in this system. Spray Water for mill rejects quenching is sourced from Service water connection provided at each stream and sprayed in Pyrite Hopper through a solenoid valve operated spray nozzle.