In situand wide range quantification of hydrogen sulfide in industrial gases by means of photoacoustic spectroscopy

This paper describes a photoacoustic spectroscopy-based detector of hydrogen sulfide (H 2 S) in biogas, natural gas and oil process technology. The instrument is capable of measuring H 2 S concentrations over four orders of magnitude (from a few ppm level up to several per cent) in changing gas mixtures. Problems caused by harsh industrial circumstances, contamination and widely varying composition of gases can be overcome by optimizing wavelength modulation, resonance frequency tracking and an easy-to-use method enabling in situ monitoring calibration. A diode laser emitting around 1.57 µm served as the excitation source; at this wavelength spectral overlap between H 2 S and CO 2 is substantial. Spectral interference was eliminated by optimizing the amplitude of wavelength modulation; furthermore, a simplified calibration method was implemented taking advantage of a nearby absorption line of CO 2 providing fast and economical measurements. Frequency dependence of the photoacoustic signal was determined by two methods to ensure accuracy. For 10 s integration time and 6800 Hz modulation frequency, the minimum detectable concentration was 6 ppm (3σ).