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United States Department of Agriculture

Agricultural Research Service

Title: Sensing Pulverized Material Mixture Proportions by Resonant Cavity Measurements

Authors
item Nelson, Stuart
item Kraszewski, Andrzej

Submitted to: Institute of Electrical and Electronics Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 23, 1998
Publication Date: N/A

Interpretive Summary: In order to prevent explosions in coal mines, Federal regulations require that the dust in the mines must contain at least 65% noncombustible material. Therefore, pulverized rock dust is distributed in the mines to mix with the coal dust to meet this requirement. Currently, the noncomustible content is determined in a testing laboratory by heating processes that burn the coal without decomposing the rock dust. A rapid, reliable test for the proportion of coal and rock dust is needed for guidance on amounts of rock dust to be added for safety. Research on measuring the electrical properties of coal and rock, known as dielectric properties, has shown that the microwave dielectric properties of the coal and limestone rock are sufficiently different that a suitable electronic measurement can be expected to differentiate the two materials. Tests made on mixtures of powdered coal and limestone with a microwave resonant cavity yoperating at a frequency of 2.5 GHz have shown that the proportion of coa in mixtures ranging from 10% to 60% coal can be determined by microwave measurements within about 2%. This research has demonstrated the feasibility of the technique, and the accuracy can be improved with further research. The development of instruments for rapid measurement of the coal-rock dust proportions in coal mine dust would improve the safety and efficiency of operations for coal mines.

Technical Abstract: Based on differences in the permittivity of coal and limestone rock, a resonant cavity measurement technique is presented for the rapid determination of the proportions of coal and limestone in powdered mixtures. The ratio of the resonant frequency shift and the change in the transmission factor when mixed coal and limestone samples are inserted into othe cavity provides a means for estimating the percentage of coal in the mixture relatively independent of the bulk density of the mixture. The technique might be developed for rapid tests of dust in coal mines to determine whether the 65% noncombustible content requirement is being met for prevention of coal-mine explosions.

Last Modified: 4/18/2014
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