Study on multi-species hydrocarbon detection using tunable diode laser absorption spectroscopy

Tunable diode laser absorption spectroscopy (TDLAS) is a widely used diagnostic technique due to its high sensitivity, fast response, low cost, and other merits. Hydrocarbon detection is a field of great interest in the application of tunable diode lasers as hydrocarbons are fundamental molecules in many industrial processes. Many tunable diode lasers are only suitable for single species detection due to the short scanning range and in real situations. However, different hydrocarbon species tend to exist simultaneously.
Here we present a laser system based on the difference-frequency generation (DFG) method for simultaneous hydrocarbon mixtures detection. The direct absorption spectra of different hydrocarbons covering various groups (e.g., alkane, olefin, and aromatic) were measured. The measurements of the concentration dependence of absorbance for each molecule were carried out. The R2 values were larger than 0.997, which demonstrated the system can measure hydrocarbons covering different molecular classes accurately. The mixture components were identified using the independent component analysis and quantitative analysis were performed using the classical least-squares method. The analysis of different types of mixture measurements proved the capability of multi-species hydrocarbon detection.
Two actual validation experiments were also performed. In the low temperature coal pyrolysis experiment, the coal sample was heated up to 350℃ and the recorded spectra were analyzed by comparing with the pure hydrocarbon spectra database. A least-squares fitting was performed to quantitatively determine the concentration of each component within the mixture. Totally eleven different species were identified and the R2 values close to 1 indicates the variance between measured and fitted data was small. In the engine exhaust experiment, the spectra of engine exhaust were recorded and analyzed using the same method. Hydrocarbon from C3-C8 and small amount of methane were recognized. The concentration variation with time was also observed.
Overall, the proposed system is a good candidate for industrial hydrocarbon detection by overcoming the limitation of other tunable diode lasers designed for hydrocarbon measurement while maintaining its advantages. The two actual experiments proved the proposed laser system can detect multiple hydrocarbons simultaneously in actual situations.

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