Shield dust has emerged as a bigger portion of the total dust generated on the longwall face, as benchmarked against NIOSH surveys completed in the 1980’s.

“Operators have substantially increased the amount of air supplied to their longwalls to control dust and methane levels resulting from improved production. For some mines with high levels of methane, the air velocity on the face is now around 8m per second. These mines have raised concerns of entraining more dust than they are diluting with the additional air they are supplying on the face,” NIOSH dust researcher Jay Colinet said.

To address this problem, NIOSH is conducting research to evaluate the impact of high velocity airstreams on dust entrainment and dilution. Testing is being conducted in a wind-tunnel to quantify the amount of dust that is entrained in velocities ranging from 2 to 8 meters per second. Feed dust consisting of a mix of coal dust (less than 50 microns) and chunks of coal up to 0.6 cm was dropped into the airstream to simulate dust dropping from the shield canopies during shield advance.

Initial test results showed that the amount of dust in both the respirable and total dust size ranges increased significantly as the air velocity was increased.

The percentage of respirable dust in the feed mix was altered to evaluate the impact on airborne respirable dust levels and similar increases were observed. This testing led to the hypothesis that the respirable particles were adhering to one another and to oversize material and the energy associated with higher air velocities was able to overcome these adhesion forces resulting in an increase in respirable dust.

Another series of tests were completed by using compressed air to inject the coal dust through an eductor. This resulted in a cloud of dust being injected into the wind tunnel and in these tests dust levels were diluted as air velocity was increased.

For both series of tests, the moisture content of the feed material was relatively dry at 1%. Future tests will examine the impact on airborne dust levels of increasing the moisture on the coal.

These results indicate that higher air velocities have the potential to increase airborne respirable dust levels from shield movement when the product on the shields is relatively dry. Unfortunately, this may be the case in many operating longwalls since limited controls are available for reducing shield dust liberation.

Shields can be equipped with sprays mounted in the canopies to wet the material on top of the shield. Typically, four spray nozzles are mounted on each shield and directed toward the debris on the top of the shield. However, the effectiveness of these spray systems for controlling dust liberation has not been documented.

“There is a lot of material above the shields that needs to be wetted and how well the water is going to be distributed from a limited number of sprays raises a question. Will these sprays wet the material at the edges of the shield before this material drops into the airstream?” said Colinet.

“Also, if each shield is equipped with four sprays and there are 200 shields on the face, this requires operators to try and maintain 800 sprays. If you operate a longwall with 800 shield sprays that need maintained, you would like to know that you are helping yourself and that it is beneficial to commit the resources to maintain that spray system,” he said.

“Hopefully, the ongoing research should provide insight into the benefits derived from adding moisture in the high velocity airstreams.”