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Quantity of Fine
Particulate in PRB Coal
This is the characteristic most identifiable with PRB coal; the quantity of fine dust. Obviously the whole purpose of the system is to collect and filter this dust.
Again, the impact is to the dust collector, duct design, and dust disposal method. The dust collector must handle a continuous dust load of 10 grains per cubic foot of air and a peak of 20 grains per cubic foot of air. We have employed an integral mechanical (cyclonic) pre-separation in route to the filter media. The degree of pre-separation has a direct impact on the filtering velocity. A low concentration of dust may permit a velocity of 6 FPM through the media where as a high concentration would require a velocity in the range of 4 FPM.
The ductwork hood design should capture only the fine particulate. Poorly designed hoods and capture velocities will collect larger particle sizes, ultimately vacuuming the coal off the conveyor belt. This will result in overloading both the dust collector and the disposal method.
Once the dust is collected, the removal of dust from the filter and disposal will determine the amount of re-entrainment which occurs at the next transfer point in the coal handling system. If the dust is directly deposited on top of the coal on the conveyor belt at the first transfer point, the next transfer point will see the same dust plus whatever additional quantity that is being generated in the handling. In this scenario, the more handling, the greater the dust quantity. The plant silos at the combustor are generally the last transfer point. Directly spouting the dust into the silo or multiple silos will eliminate re-entrainment. The second problem of returning dust directly to the conveyor belt is during the purge cycle. The only period of time we want to stop the airflow is after there is no more coal on the conveyor belt. Unfortunately, the surge of dust from the purging of the dust filter cake will be discharged rapidly by the airlock onto an empty belt conveyor. This invariably results in a cloud of airborne dust and increases housekeeping. The solutions to this problem and for other conveyor transfers is solved best by the following two methods:
METHOD 1: Pneumatic Transfer
METHOD 2: Agglomeration