Methane and carbon dioxide mixed gas detection based on sphere–tube coupled photoacoustic cell

Mixed gas detection plays an important role in environmental protection, medical diagnosis, combustion analysis, industrial process control and coal mine gas detection [1], [2], [3], [4], [5], [6], [7], [8], [9]. Coal mine gases mainly include methane (CH4), acetylene (C2H2) et al. CH4 and CO2 are also the main greenhouse gases that cause global warming. At present, gas detection technologies are mainly composed of chemical technologies such as catalytic combustion, electrochemical, and gas chromatography, as well as spectral technologies such as non-dispersive infrared spectroscopy (NDIR), Fourier transform infrared spectroscopy (FTIR), and photoacoustic spectroscopy (PAS). Compared with chemical techniques, spectroscopic techniques have the advantages of flexible sampling, non-destructive measurement, non-contact, multi-component detection.

Photoacoustic spectroscopy has the advantages of high sensitivity, good selectivity, and zero background detections, and has attracted extensive attention in the field of mixed gas detection [10], [11], [12], [13], [14], [15], [16], [17]. Besson et al. used near-infrared lasers and a resonant cylindrical photoacoustic cell to achieve mixed gas detection of HCl, CH4, and H2O, with minimum detection limits of 3 ppm, 0.5 ppm, and 0.2 ppm, respectively [18]. McNaghten et al. used tunable diode lasers and cantilever photoacoustic sensors to detect trace gases, including CO, C2H2, and CH4 in nitrogen-based gases, and the corresponding noise equivalent concentrations were 249.6 ppm, 1.5 ppm, and 293.7 ppm, respectively [19]. Wang et al. designed a quartz-enhanced photoacoustic sensor with time division multiplexing of the driving current to realize 10−6-level detection of CH4 and C2H2 [20]. Chen et al. constructed multi-gas analyzer with a mid-infrared broadband source and a near-infrared laser. The analyzer realized high-sensitivity detection of CO, CO2, CH4, C2H6, C2H4 and H2O gas [21]. Zhang et al. fabricated three kinds of quartz tuning forks with different resonance frequencies for trace gas detection, and realized the three-component gas detection of H2O, CH4 and C2H2 [22]. At present, many scholars have also made contributions to improve the sensitivity of the photoacoustic cell [23], [24], [25]. Since increasing the gas absorption path can improve the photoacoustic signal and reduce the detection limit, some long optical path photoacoustic cells have also been developed, such as [26], [27], [28].

In photoacoustic detection, the intensity of the photoacoustic signal is proportional to the excitation light power. When the optical path length is increased, the effect of an equivalent high-power laser could be achieved. A photoacoustic detection setup for mixed gas of CH4 and CO2 was built with a long optical path sphere–tube coupled photoacoustic cell as the core sensor. Different from the T-type photoacoustic cell which the laser passes through the absorption cell [2], the light beam was reflected multiple times in the spherical diffuse absorption cavity to realize a long absorption path of the measured gas. Acoustic resonance was generated in the tube, further amplifying the photoacoustic signal. Two near-infrared distributed feedback lasers with central wavelengths of 1653 nm and 2004 nm were employed as excitation light sources, and were driven by time division multiplexing technology. The experiment results showed that the setup had good responsivity for the two gases, and the normalized noise equivalent absorption coefficients reached 10−9 level.

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