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Potential for Spontaneous
Combustion in PRB Coal
The relative difference in the propensity for spontaneous combustion between Eastern or Midwestern bituminous and PRB coal is the greatest threat to system safety. A combination of high moisture and more volatiles results in this condition. This propensity impacts the coal dust system in three areas:
(a) Collector housing
(b) Dust disposal
(c) Duct design
First, the collector housing must be vented for a rapid rise in internal pressure, the National Fire Protection Association (NFPA) Guide 68, will provide a method to size the vent area.
The use of pressure relief vents assumes the collector is located outdoors, unfortunately many dust collectors on existing power plants are not. The NFPA Guide 68 does recommend an outdoor location, however if indoor, the pressure vents must be ducted to the exterior of the building. It is our experience this is impractical if the pressure vent duct distance is greater than 3 meters because of insufficient internal pressure capability with most existing dust collector housings to withstand the back pressure during venting, and is also cost prohibitive on new units. This requirement is the single major reason most existing coal dust systems must have the system filter replaced and located outdoors.
Another important dust collector feature is complete static electrical grounding of the housing and especially the filter media and its wire supports (cages). The passing of airborne coal dust over a polyester filter media has a sufficient triboelectric gradient to induce a charge. NFPA 77 illustrates a method of insuring an electrical ground for static electricity. Since most filter tube sheets are coated or corrode, a positive method of grounding is imperative.
If the dust collector is going to be shut down, it is necessary to initiate a purge herein called, the “end of run purge”. A dust collector functions by developing a dust cake on the filter media. It is held onto the media by the airflow through the media. Upon shut down of the system fan producing airflow, the dust cake will be discharged to the hopper in greater quantities than experienced during normal continuous operation. This dust cake must be removed from the media by the extended operation of the media pulse cleaning system. All coal dust must be removed from the hopper and dust disposal equipment, before complete shut down of all system components is completed. This is necessary to avoid hopper fires.
The third area is duct design covered by NFPA Standard 120. It requires a minimum acceptable design velocity of 4,500 FPM. Most existing systems do not have duct velocity sufficient to meet this standard. Lower velocities will result in potential accumulations, especially when more moisture is present and the dust becomes "sticky". In addition to velocity, any practice in the design of transitions or tapers, clean outs, dead end caps, or hoods which allow an accumulation to occur should be re-engineered. It has been our experience that only a small amount of dust, with the right conditions, is required to promote spontaneous combustion.
A fire system will detect trouble via thermal sensors. Earlier detection can occur with a CO Monitor, which detects the presence of carbon monoxide from combustion. As the carbon monoxide migrates through the ductwork system, it can be detected by this instrument and alert plant personnel before becoming a fire in the dust collector.
Another ductwork feature is to equip the filter inlet with a backdraft damper. The main purpose of this device is to minimize propagation of a rapid fire in the dust collector through the inlet ductwork in reverse flow. Since this rapid fire will generate substantial volume and pressure quickly, this pressure will easily overcome fan induced flow and travel back to the hoods. It is especially important to avoid propagation to enclosed buildings. Another benefit to the backdraft damper is to minimize reverse airflow with dust during the purging cycle (when the fan is shutdown) from migrating to the hoods. This occurs predominately when the enclosed boiler forced draft fans or other ventilation fans have caused a negative pressure in the building, drawing airflow back to the hoods.