Abstract
Abstract. The measurement of low methane (CH4) concentrations is a key objective for safety of industrial and public infrastructures and in environmental research. Laser spectroscopy is best suited for this purpose because it offers high sensitivity, selectivity, dynamic range, and a fast measurement rate. The physical basis of this technique is infrared absorption of molecular gases. Two detection schemes – direct absorption spectroscopy (DAS) and photoacoustic spectroscopy (PAS) – are compared at three wavelength regions in the near-infrared (NIR), mid-wavelength (MWIR), and long-wavelength (LWIR) infrared ranges. For each spectral range a suitable semiconductor laser is selected and used for both detection techniques: a diode laser (DL), an interband cascade laser (ICL), and a quantum cascade laser (QCL) for NIR, MWIR and LWIR, respectively. For DAS short absorption path lengths comparable to the cell dimensions of the photoacoustic cell for PAS are employed. We show that for DAS the lowest detection limit can be achieved in the MWIR range with noise-equivalent concentrations (NECs) below 10 ppb. Using PAS, lower detection limits and higher system stabilities can be reached compared to DAS, especially for long integration times. The lowest detection limit for PAS is obtained in the LWIR with a NEC of 7 ppb. The different DAS and PAS configurations are discussed with respect to potential applications.
Highlights
Natural gas is one of the most important energy sources and is needed for many products
The influence of the laser power is clearly demonstrated by inserting neutral density (ND) filters into the quantum cascade laser (QCL) beam
If the QCL intensity is reduced by a factor of 100, the signal of 10 ppm CH4 is similar to the resonance signal of the zero-gas background of N2 obtained with full QCL power
Summary
Natural gas is one of the most important energy sources and is needed for many products. The main constituent of natural gas is methane (CH4). Methane is explosive as well as a greenhouse gas with significant global warming potential (EPA and OA, 2016). Due to these facts, sensor concepts to determine the concentration of methane are highly required and demanded (Frost and Sullivan, 2015). Methane detection can be realized in a large variety of different concepts. A laser-based solution should principally aim at a selective, sensitive and fast detection, otherwise the relative costly laser is probably not a well-suited choice. An optical and a non-optical laser-based detection scheme are investigated with respect to a potentially suited methane detection application. The two different detection schemes are based on tuneable diode laser absorption spectroscopy
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