Opto-Dielectrometric Sensor for Measuring Total Incombustible Content in Underground Coal Mines

Abstract

Coal dust produced during underground coal-mining, i.e., float dust, which deposits throughout the coal mine, can be feedstock for coal dust explosions. To prevent these explosions, inert rock dust (limestone dust) is applied to roof, floor, and rib areas of a coal mine. The ratio of incombustible mass (rock dust + incombustible content of coal dust) divided by total mass of the deposited dust is defined as the total incombustible content (TIC) of the deposited dust within the mine. Regulations require that a minimum TIC ratio (80%) to be maintained for safe working conditions inside the mine. This paper presents design, fabrication, and experimental results for a real-time sensing module, which uses continuous optical and dielectrometry methods to measure the TIC of the deposited float dust/rock dust. The optical sensor determines the TIC of the deposited dust based on optical reflection, which is described by modified Beer Law. We present an extension of the Bouguer-Beer–Lambert Law to find the relation between the reflectivity of a layer of known thickness (obtained by interdigital dielectrometry sensor) of a dust mixture to the ratio of each constituent. We also present the experimental results from testing the sensor prototypes in a realistic laboratory test bed that is subjected to the deposition of the coal dust/rock dust mixture. The sensor performance and stability at different humidity levels is evaluated and the accuracy of the results is compared with the currently established best practices for measuring TIC in underground coal mines.