July 1, 2010

Use Dry Fog to Control Coal Dust Hazards

By Dr. Robert Peltier, PE

Fog System Application Guidelines

Like any technology, fog systems must be properly applied to achieve the desired results. With fog systems, a fuel-handling transfer point is a “scrubbing chamber.” Gilroy says that each chamber must be designed so that airflow is properly managed, such as by having adequately sized conveyor skirt covers at the conveyor load point. This is an important consideration for foggers because air movement must be slow so that the fog droplets injected into the transfer point will have sufficient retention time to attach to the airborne dust particles and settle them out on the belt. The design must have enough cover height for the air to expand and enough length for agglomeration to occur (Figures 1–4).

1. Gravity works. The Dust Solutions Inc. (DSI) dry fogging system agglomerates the airborne dust particles to micron-sized water droplets so that when the particles become heavy enough, they are returned to the product stream by the force of gravity. With conveyor transfer points, there must be sufficient room for the fog to fully develop, particularly in the receiving belt area. In addition, the conveyor skirt boards must be tight, and the inspection and conveyor covers must be in place so air leakage does not affect the efficiency of the system. Courtesy: Dust Solutions Inc.

2. Blanket of fog. The system design is based on a unique nozzle design that can produce a very dense fog of 1- to10-micron-size water droplets that literally blanket the dust source and keep the dust particles from becoming airborne in this rail car unloading system. The DSI nozzle requires a maximum of 15 psi water pressure. Courtesy: Dust Solutions Inc.

3. Droplet size matters. For agglomeration of the dust particles to occur at the dust source point, two conditions must occur: Enough water droplets of the same size as the dust particles must be present, and both dust and water droplets must be constrained to the same volume so that agglomeration can occur. If the water droplet’s size is too large, the dust particle will follow the air streamline and miss the water droplet. Source: Dust Solutions Inc.

4. Micron-scale water particles. An actual high-speed photograph of droplets generated by a DSI fogger designed to produce water droplets less than 10 microns in diameter. The droplets were captured on a greased microscope slide in a chamber that was at 100% relative humidity to eliminate evaporation of the droplets. The small squares are 2 microns across. Any fugitive dust particles encapsulated by such densely packed fog droplets have little or no chance of escaping. Courtesy: Dust Solutions Inc.

In the past few years, controlled-flow chutes have taken these principles one step further by shaping the material flow and reducing material impact within the chute. Gilroy notes that although some manufacturers claim to have a “dustless” transfer chute design, his customers say that controlled flow transfer points have helped reduce dust generation; however, in many cases, that is not enough, and a fog system must be added to achieve the desired results.

Gilroy pointed out an interesting recent project where dry foggers were added to an existing PRB coal-handling system. Dry Fork Mine (part of Basin Electric Power Cooperative) is a PRB coal mine that installed a passive control chute under a primary crusher to achieve the specified amount of dust control. The staff at Dry Fork Mine has long experience with fog systems, having previously replaced their mechanical dust collectors at the silo fill and conveyor transfer points. When the conveying system was designed for supplying coal from the mine to the Dry Fork Power Station now under construction, Basin Electric specified dry fog combined with passive control chutes for optimum control.

Basin Electric is also installing fogging systems at its new 400-MW Dry Fork Station in Gillette, Wyo. Each transfer point will use both a controlled flow chute and a fog system to achieve the highest degree of dust control efficiency.