Study on the oxidation and release of gases in spontaneous coal combustion using a dual-species sensor employing laser absorption spectroscopy

Coal spontaneous combustion (CSC) is a common hazard in coal mines and a significant reason for the loss of coal in stockpiles and mines. To investigate the oxidation and release of gas due to of CSC, a miniature purpose-built laser-based sensor system capable of precise gas measurement and data processing was configured. A distributed feedback (DFB) diode laser with a central wavelength of 2.33 mu m, in conjunction with a Herriot-type cell and wavelength modulation spectroscopy (WMS), was used for the simultaneous detection of methane (CH4) and carbon monoxide (CO) during coal combustion. The second-harmonic of WMS was applied to ensure sensor robustness and sensitivity with long-term performance for the system being evaluated by calculating the Allan-Werle deviations for the concentrations of CH4 and CO. The limits of detection (LoDs) for CH4 and CO were 0.05 ppm based on integration times of 120 s and 70 s, respectively. The dual-species sensor was employed for real-time and in situ investigations of coal samples for temperatures ranging from 85 degrees C to 200 degrees C and was considered suitable for integration into field monitoring equipment, especially for online early warning forecasting applications in CSC.