Controlling dust in the tripper room is a difficult task due to several factors. Most notability, the coal has been handled and worked several times before arriving at the tripper floor. If no active dust collection has been applied to the stream, the coal will contain a large volume of fines when it arrives.
Tripper Room Dust Control
Coal Chute Design
Introduction
Lift Conveyor to Tripper Conveyor Transfer Point
If no active dust collection is installed on the tripper-deck or servicing the transfer of the lift conveyor to the tripper conveyor, the plant might consider applying dry fog (air atomized water application) to control dust through the transfer.
The application of dry fog requires proper sealing of the head chute and transfer chutes, along with properly sized stilling chambers to reduce air velocities. The reduction of air velocities is required to allow the applied moisture to achieve its suppressing effect. If new transfer systems are installed, the positive effect of dry fog may be fully realized if these criteria are maintained.
This effort may not be sufficient for complete dust control and additional measures, such as active, wet collection may need to be applied in combination with Bunker dust control (see below).
Bunker Filling Dust Control
While wetting agents can be successful at controlling dust through transfer points, they have little to no effect on coal falling from the tripper car conveyor to the heap in the bunker. This fall is of varying height, changing as the bunker fills. While the dust liberated during the conveyor to conveyor transfer may have been treated with a wetting agent, it only represents a small fraction of the dust that is contained in the material stream. The bulk of the dust is not treated before being discharged from the tripper car conveyor to the bunker. This untreated dust will now be liberated in the free fall to bunker heap level. A large portion of this newly liberated dust will become airborne in the bunker and may remain airborne for quite some time, depending on its particle size.
In addition to liberating dust, the falling coal will create a positive pressure within the bunker through the release of entrained and induced airflow. The airflow will be proportional to the open area on the head chute of the tripper car conveyor. It is imperative that the car is sealed as tightly as possible.
Air introduced into the bunker area will seek an escape path. Holes in the tripper room floor, openings in the bunkers, failures of the tripper floor sealing system, etc, will all become paths for dust escape. Air liberated from these areas typically carries the smallest particles of dust with it, as they are the easiest to transport. This sub-10-micron-sized dust becomes ‘float dust’ in the tripper room, typically settling on beams and flat surfaces from the floor to the ceiling. It is this dust that, when (1) accumulating in this fashion and (2) in this specific area, seems to be the main focus of Combustible Dust Management Policies. In addition, from a risk management standpoint, dusting in this area is of high concern due to its impact on personnel and business interruption should an event occur.
The control of dusts liberated during bunker filling requires the use of active dust collection. Acensium typically recommends that plants consider wet dust collection to service the coal bunkers and the lift to tripper conveyor transfer. Wet dust collection does not concentrate the hazard (coal dust) in a hopper, but rather mixes the dust-laden air stream with water, creating a slurry that can be discharged to a floor drain. The unit can be contained within the building, eliminating a need for ductwork insulation. No explosion relieving construction is necessary as an explosive environment does not exist by design. Ductwork to the bunkers shall be designed with minimum transport velocities to satisfy NFPA requirements and prevent dust fall out during transportation.
The units are typically located towards the end of the tripper car conveyor run. Their suction line will penetrate the concrete bunker roof in one or more locations, as dictated by the length of the bunker, the impediment of coal heaps to airflow and the required negative pressure at the most remote point of the bunker. The units will discharge air back to the tripper room, with no discharge of air outside of the building. Slurry water will be discharged to floor or dedicated drains.
These units have been installed at plants burning western coal for several years. While they were plagued with a few early operational problems, solutions have been applied and no issues remain unresolved.
Tripper Cars
Acensium observes that most tripper cars lack any sealing against induced air being allowed into the bunker during filling operation. It is most imperative that the cars be completely sealed between the belt line/coal flow and the car. Head chute sealing on the tripper cars is the easiest and most cost-effective way to control induced air in the bunkers and therefore reduce the size of the dust collection necessary as described.
Acensium also typically suggests a rework of the discharge chute from the tripper car conveyor to the bunker to smooth material flow, reduce fines generation and extend chute wall/wear-liner life. Most tripper seal-belts have been opened up to allow a light to shine into the bunker and bunker fill levels to be observed. This is an escape point for dust and positive pressure to be released from the bunker.
Material Handling Dust Control
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