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Higher Moisture Content in PRB Coal
PRB coal has generally between 20% and 30% water content compared to 10% for a midwestern bituminous coal. This moisture is continually liberated once the coal is mined. The presence of water in vapor form will pose the dust collection system little problems, but once a dew point is reached, more problems begin.
Dew points in the dust collection airstream generally occur because of changing weather conditions, space heating of coal handling structures or differences in temperature of the coal versus the ambient air. For example, an underground coal pile reclaim tunnel is warmed in the fall season by the coal pile and earth. The air will be moist especially if the conveyor system is washed with water. This ambient air in the tunnel is above freezing whereas outdoors, the dust collector is below freezing. Not only is there going to be a dew point, but ice formation in the dust filter. A second example is where the coal is conveyed to the plant silos, which are heated indoors, and the dust collector is outside on the roof. A third example is a crusher house where moisture is liberated at larger quantities because more surface area is being created by the coal crushing operation.
Our experience has been extensive in designing for these conditions as virtually every system incurs some problem. First, the filter is sized with a lower filtering velocity to account for the loss of bag porosity due to the droplets of moisture. A velocity of 6 FPM where few problems are anticipated to a low of 4 FPM where substantial moisture is present. There are also filter media considerations, which can alter the effect of moisture. Secondly, if the outdoor temperature is below freezing, ice will form on the collector walls. This is not as much of a problem as sublimation from air movement will remove it if the dust filter remains operational.
However, ice on the hopper slopes will impede flow and cause dust accumulations. We recommend enclosing all outdoor hopper surfaces and indirectly heating the hopper slope by heating the space around the hopper. Examples of these types of enclosures are in figure 3 for a rotary car dump dust collection system. (The smaller dust collector has an integral insulated enclosure; the larger dust collector has a separate site assembled insulated enclosure around a structural steel frame. Both types provide a heated enclosure to approximately 70 º F).
The enclosure for hopper space heating has further benefits to the dust disposal. The rotary airlock below the hopper is where a moisture problem with PRB coal first manifests itself. First, the coal dust becomes wet and sticks into the vane pockets, reducing the valve capacity to discharge coal dust from the dust collector hopper. The best solution is providing a rotary airlock with substantially more capacity and a large diameter rotor so the vane pocket spacing is wide to minimize the pocket-filling tendency. The second rotary airlock problem arises when the moist coal dust in the vane pocket freezes and the vanes freeze to the valve housing. Thus the enclosure for the filter hopper with space heating eliminates this second difficulty completely.
Our experience in hopper heating with direct contacts and heat tracing of the dust disposal has been unsatisfactory. The heating can inadvertently be operational in warm weather promoting the spontaneous combustion tendency of PRB coal. The indirect space heating avoids this type of operation.
Ductwork is also affected by the presence of moisture. The ductwork routing should be designed so as to be self-draining towards the hoods, where possible, or the dust collector.