To achieve in-situ online detection of hydrocarbon gases at various temperatures, we explore the use of tunable diode laser absorption spectroscopy (TDLAS) for measuring hydrocarbon gas content. A difference frequency generation (DFG) laser, with a scanning range of 3250?3400 nm, is used to construct a hydrocarbon gas detection system. The 3 μm wide-band scanning provides a significant number of characteristic absorption peaks and reveals the absorbance line shapes of the absorption band. This makes hyperspectral fitting feasible and enhances the system’s versatility.

First, the DFG-TDLAS system and hydrocarbon gas detection experimental setup are established. The DFG-TDLAS system includes a 1060 nm ECDL, a 1582 nm DFB laser, a WDM fiber coupler, and a PPLN module. This system can achieve 3250?3400 nm laser output with a resolution of 0.01 cm^-1. The hydrocarbon gas detection experimental system includes a high-temperature absorption chamber and a heating chamber. The absorbed laser signal is collected from the high-temperature chamber. Samples are heated in the chamber, and the gas produced during the decomposition process is collected. Second, the DFG-TDLAS output laser is employed to measure the absorption spectra of seven hydrocarbon molecules, including C1?C4 hydrocarbons and benzene. Standard hydrocarbon gases are measured at different temperatures and volume fractions to establish the absorption spectra database. The coal is heated to 623 K in the heating chamber, and the resulting gases from thermal decomposition are measured in the high-temperature chamber. The DFG-TDLAS system and hydrocarbon absorption spectra databases are used for qualitative analysis.

During the detection of standard hydrocarbon gases, the pressure in the high-temperature absorption chamber is maintained at 0.1 MPa, and the detection temperatures are 298, 423, 523, 623, and 723 K (Fig. 3). Hydrocarbon gas absorption is also measured at different volume fractions at 298 K. The experimental results demonstrate that the fitting R² values for the characteristic absorption peak intensity of hydrocarbon gases in relation to temperature and concentration are shown in Figs. 4 and 5. As the number of carbon atoms increases (e.g., in propane, propylene, butane, and benzene), the absorption lines increase, and the absorption intensity at any given wavelength is enhanced by the superposition of adjacent spectral lines. This results in a continuous wide-band absorption range. As shown in Fig. 5, in the detection of mixed gas produced by coal pyrolysis, a strong absorption is observed at 3325?3400 nm due to the absorption of C5 (or higher) hydrocarbon macromolecules. Based on existing literature, 15 possible hydrocarbon gases and water vapor absorption spectra are used for non-negative least squares spectral fitting, followed by qualitative analysis. The detected contents are primarily methane and propane, with a lesser amount of ethane. The components identified through spectral fitting align with the principles of chemical reactions.

For methane and ethane, multiple relatively independent high-intensity absorption lines are present in the 3250?3400 nm range, making them suitable for identification. Ethylene, propane, propylene, and butane each exhibit a distinct high-intensity characteristic absorption peak. Due to the more complex molecular structure of benzene, its absorption lines merge into a continuous curve within the 3250?3400 nm range, influenced by the broadening effects of adjacent lines. Identifying the components and calculating volume fractions requires establishing a database and using line shape functions. In the standard hydrocarbon gas experiments, the fitting R2 values for the temperature and volume fraction change curves of the characteristic absorption peaks of hydrocarbon gases are all above 0.99, indicating high accuracy. Methane, ethane, ethylene, propane, and propylene molecules can be identified through characteristic absorption peaks in the detection of hydrocarbon mixed gases generated by coal pyrolysis. Spectral fitting and qualitative analysis reveal that methane and propane are the primary hydrocarbon components detected, with lower volume fractions of other components like ethane. These results are consistent with the expected formation patterns of hydrocarbon molecules during coal decomposition, confirming the feasibility of 3 μm hydrocarbon gas detection based on DFG-TDLAS technology.