Intrinsically Safe Fiber-Optic Photoacoustic Gas Sensor for Coal Spontaneous Combustion Monitoring

A high-sensitivity fiber-optic photoacoustic (PA) gas sensor has been presented for coal spontaneous combustion monitoring. The gas sensing head is connected with the demodulator by two optical fibers, which are used to transmit the laser light for generating PA signal and the white light for detecting PA pressure, respectively. The gas sensing head itself does not have electronic components, resulting in intrinsically safe. The filtered gas diffuses into the unsealed miniature PA chamber. A 1531.6 nm laser is used to excite acetylene (C₂H₂) gas molecules to produce PA pressure waves. The generated PA signal is detected by a Fabry-Perot (F-P) interferometric cantilever, which is demodulated by a high-speed spectral demodulation method. Theoretical and experimental results both show that high-concentration methane (CH₄) background gas have an impact on the PA response of trace C₂H₂. This is mainly due to changes of the gas specific heat ratio and the cantilever response due to the change of mixed gas concentration. Another near-infrared laser with a wavelength of 1653.7 nm is chosen to measure the concentration of CH₄ in order to eliminate the influence of the high-concentration CH₄ gas. Experimental results indicated that C₂H₂ detection limit can reach 1 ppm with the background CH₄ concentration range of 0-50%.