Accurate measurement of trace H<sub>2</sub>S concentration based on cavity ring-down absorption spectroscopy under CO<sub>2</sub>/CO disturbance

Abstract

Since H<sub>2</sub>S is a corrosive and toxic gas pollutant, the accurate measurement of its concentration is significant. However, in the practical industrial processes, it is difficult to implement because of the disturbance caused by other emissions such as CO<sub>2</sub> and CO. Therefore, in this work, the concentration of H<sub>2</sub>S, CO<sub>2</sub> and CO are measured simultaneously based on cavity ring-down spectroscopy (CRDS) as a viable alternative to measure the concentration of H<sub>2</sub>S accurately when CO<sub>2</sub> and CO exist. First, the wavelength of mixed gas within a range of 6336–6339 cm<sup>–1</sup> is selected as the target region where the spectral line intensity of H<sub>2</sub>S is stronger than 10 times that of CO<sub>2</sub> or CO and the water absorption is extremely weak. Second, the influence of the sampling length (Tm) on the accuracy of the ring-down time is analyzed by evaluating average (accuracy), standard deviation (precision) and consumption time (speed). Third, the experiments are carried out at different pressures in order to obtain the optimal pressure condition. Fourth, the concentration of trace H<sub>2</sub>S is measured when the disturbances caused by CO<sub>2</sub> or CO are added, and the error of the measured concentration is analyzed. Finally, the detection limit of CRDS-based system is calculated to be 6.9 ppb by analyzing the SNR of four groups of low concentration H<sub>2</sub>S spectra, while the lower limit of detection of CRDS-based system is calculated to be 2 ppb by analyzing the Allan variance of long-term data. The measured concentration and its desired value show a good linearity at different dilution ratios. Both the high linearity and the low detection limit of H<sub>2</sub>S indicate the effectiveness of the CRDS-based measurement system to measure H<sub>2</sub>S when CO<sub>2</sub> and CO exist. The successful application of the CRDS-based system to the measurement of H<sub>2</sub>S shows its promising prospect in gas concentration measurement for practical industrial processes.