Methane (CH₄) and ethylene (C₂H₄) are important products of coal pyrolysis in coal chemical processes. Understanding their pyrolysis evolution mechanisms is crucial for comprehending the thermal decomposition of coal, optimizing fuel utilization efficiency, and reducing harmful gas emissions. This study develops an online monitoring device capable of simultaneously detecting CH₄ and C₂H₄ volume fractions, which combines tunable diode laser absorption spectroscopy (TDLAS), wavelength modulation, and time-division multiplexing techniques. The system is successfully applied to gas analysis during coal pyrolysis.

The system utilizes distributed feedback (DFB) lasers as its light sources, with central wavelengths of 1653 nm and 1620 nm for CH₄ and C₂H₄, respectively. The laser beams are directed into a Herriott-type high-temperature multipass gas cell with an effective optical path length of 15 m, which enhances the sensitivity of the detection system. Through the combination of hardware circuitry and specifically designed software programs, the system achieves precise modulation, demodulation, and volume fraction inversion of CH₄ and C₂H₄ spectral signals. Calibration of the system is carried out using standard gases with varying volume fractions. The results demonstrate excellent linearity, with correlation coefficients of 0.999 and 0.998 for CH₄ and C₂H₄, respectively, underscoring the reliability and accuracy of the device in measuring these gas volume fractions.

To further evaluate the system performance, continuous measurements are conducted using standard gases with volume fractions of 200×10?? and 100×10?? for CH₄ and C₂H₄, respectively. Over a duration of 1000 s, the maximum volume fraction fluctuations are found to be 2×10?? for CH₄ and 6×10?? for C₂H₄. The standard deviations of the measurements are 0.7 and 2.1 for CH₄ and C₂H₄, respectively, indicating the high stability of the system measurements. To further examine the stability and precision of the measurement system, Allan variance analysis is performed. The minimum detection limits derived from this analysis are 0.025×10?? for CH₄ and 0.133×10?? for C₂H₄, demonstrating the system exceptional sensitivity for trace gas detection. The system dynamic response time is also evaluated through gas-switching experiments. By alternating between nitrogen and a standard CH₄ gas with a volume fraction of 200×10??, the system dynamic response time is determined to be 17.8 s. This response time is considered suitable for real-time monitoring of gas volume fractions in coal pyrolysis processes, highlighting the system practical applicability in industrial scenarios. To validate the system performance under real-world conditions, it is employed to monitor gas emissions during the pyrolysis of Shandong bituminous coal. The device successfully measures the volume fraction release curves of CH₄ and C₂H₄ over the course of the pyrolysis process. By altering experimental parameters such as heating rates, coal sample particle sizes, and oxygen volume fractions, the relationships between these experimental conditions and the release profiles of CH₄ and C₂H₄ are systematically investigated. The findings reveal the release characteristics of both gases during coal pyrolysis, thus providing important insights into their evolution mechanisms. These results serve as a critical foundation for optimizing coal pyrolysis processes, improving fuel utilization efficiency, and controlling the emission of harmful gases in industrial applications.

This study successfully develops an advanced online monitoring system for the simultaneous detection of CH₄ and C₂H₄ volume fractions using TDLAS. The system demonstrates high sensitivity, excellent linearity, and outstanding stability, making it a reliable tool for gas analysis in coal pyrolysis processes. Moreover, its application in real-world scenarios provides valuable data for understanding the release mechanisms of CH₄ and C₂H₄ under different experimental conditions. These findings contribute significantly to the optimization of coal pyrolysis techniques and the development of more efficient and environmentally friendly fuel utilization strategies.