Coal in Dust Collection System Design and Dust Collector Manufacturing

Coal, being an explosive dust, must be handled with great care.  An accumulation of as little as 1/32″ is enough to cause an area to be classified as hazardous, according to the National Electric Code.  As a layer of coal dust thickens, the potential for self-heating and auto-ignition increases.  Good housekeeping is essential. All areas in the vicinity must be kept clean, particularly checking ledges and flat roofs.  No dust should be allowed to accumulate on hot surfaces, such as electric motors, to prevent self-heating to the ignition temperature.

In a power plant, areas where coal fines are produced, such as at the crushing station, are typically equipped with exhaust hoods and ductwork to convey the fugitive dust to a central dust collector.  This dust collector should be fitted with explosion relief vents, designed and sized according to the rules of NFPA-68.  Equally important is to take as many precautions as reasonably possible to prevent an explosion from happening in the first place.  The following measures should be implemented with a collector handling coal fines:

• Use Grounded Filter Bags

Filter bags can accumulate static electricity, which if not discharged can be a source of ignition.  The least expensive way to provide for bag grounding is to use bags with stainless steel ground wires down their lengths which are attached to studs on the tubesheet.  A disadvantage of this is that static is discharged local to the ground wire, but may not be on parts of the bag opposite of the wire.  The next step up is using epitropic bags, which are embedded with carbon fibers.  The carbon fibers are conductive and static over the entire surface can be discharged equally.  In addition, the fibers contact the tubesheet directly and no secondary grounding connection to the tubesheet is needed.  In the most hazardous situations, bags with 5% stainless steel fibers rather than carbon fibers should be used, due to the greater conductivity.

  • Prevent Build-up in Hoppers and on Surfaces

Be sure that the rotary valve is well-maintained and lubricated and removes dust efficiently.  A good idea would be to install a level detector on the hopper to sound an alarm in case of high dust accumulation.

  • Control Sources of Ignition

Although this should not be assumed to be a complete means of explosion prevention, the control of ignition sources can go a long way towards mitigating the hazards of an explosion.  Open flames from welding or burning operations and sparks from grinding should be eliminated in the presence of coal dust.  Non-sparking materials of construction should be used in rotating equipment.  As much as possible, hard objects and impurities should be removed from the coal feed to prevent their causing friction in the equipment.

  • Avoid Over-cleaning Filter Bags

Cleaning bags too frequently creates a cloud of fine particles which increases the chance of an explosion.  In addition, it removes the underlying dust cake which provides the bag’s efficiency, allowing coal dust to get through the bags immediately following a cleaning pulse (the condition known as “puffing”).  The differential pressure should be monitored carefully so that the collector operates at a stable differential of 2″ to 4″ W.G.

  • Check Ducts and Hoods

Proper hood and duct design is critical to efficient ventilation of a work space and conveyance of fugitive dust to the collector at a velocity to prevent fallout and within the pressure drop limitations of the fan.  According to NFPA-120, the minimum velocity to convey coal dust is 4500 ft/min.  A ducting system can be complex and a good handbook such as published by ASHRAE should be consulted.  The hood should have four times the area of the duct and a minimal number of hoods should be used.  If fugitive dust is not being conveyed properly, try adjusting the damper or the elevation of the hood over the process.