Photoacoustic system for on-line process monitoring of hydrogen sulfide (H2S) concentration in natural gas streams

A dual-channel hydrogen sulfide (H2S) concentration measuring system based on photoacoustic spectroscopy is described. The system uses a single-mode, fiber-coupled, room-temperature-operated, telecommunication-type diode laser with a wavelength of 1574.5 nm and an output optical power of 40 mW and two identical resonant photoacoustic cells to achieve minimum detectable H2S concentration at 0.5 ppm (3 sigma) in both measured natural gas streams. The instrument features excellent long-term stability and unattended automatic on-line monitoring even when operated in harsh industrial environments. The potentially deteriorating effect of temporal variation in the natural gas composition was successfully suppressed by applying a spectral baseline correction method and by introducing an additional measurement phase with measurement of a reference gas from which the H2S has been removed. Various tests of the instrument demonstrate its reliable performance and suitability for process-control application.