Pyrolysis of forest combustibles is an important research topic in forest fires and is of great significance for early warning and control of forest fires. The pyrolysis of forest combustibles mainly produces carbonaceous gases such as CO, CO2, and CH4. The continuous release of these gases will likely trigger forest fires and aggravate the greenhouse effect. The rapid and accurate detection of the concentrations of these three components is beneficial for early warning of fires and atmospheric environmental protection. In this paper, the concentrations of three CO, CO2, and CH4 components in the pyrolysis gases of six mountain forest tree species samples were accurately measured by frequency division multiplexing combined with tunable diode laser absorption spectroscopy (TDLAS) technique. The applicability of the frequency division multiplexing-TDLAS technique for simultaneous multi-component measurements of pyrolysis gases of forest combustibles is demonstrated. Firstly, the basic principle of the frequency division multiplexing-TDLAS technique is introduced, and the absorption spectra of the three components are determined without interfering with each other and with suitable spectral intensity. Secondly, the characteristics of the second harmonic (2f) signal and the second harmonic/DC (2f/DC) signal are investigated to invert the different concentrations accurately. The difference in the accuracy of inversion of different concentrations using the 2f signal and 2f/DC signal is compared using Simulink simulation, and the results show that the 2f/DC signal has a larger linear interval and is suitable for the measurement of different concentration components in the pyrolysis gas of forest combustibles. Finally, an experimental setup for simultaneous CO, CO2, and CH4 measurements was built using two distributed feedback (DFB) lasers with center wavelengths of 1 580.0 and 1 653.7 nm, respectively. The 2f/DC signals of the three components were measured using a Herriott absorber cell, and the absolute concentrations of the three components in the pyrolysis gas were obtained by establishing calibration models with standard gases. The results showed that the peak 2f/DC signals of the three components satisfied a good linear relationship with the concentrations withlinearity greater than 0.995, and the concentrations of CO in the pyrolysis gases of the six tree samples were significantly higher than those of CO2, and CH4 under the effect of the coke gasification reaction and Boundouard reaction. by analyzing the spectra of Chinese sweetgum leaf samples, it was shown that within the 2 s measurement time, the spectroscopic system for CO, CO2, and CH4 with a minimum detection limit lower than 0.008% and sensitivity better than 0.005%, which meets the demand of forest fire early warning. This study provides a methodological reference for the simultaneous multi-component measurement of pyrolysis gases of forest combustibles and forest fire early warning.
