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Hydrogen sulfide is a poisonous, colorless chemical compound. It has a very typical smell of rotten eggs and can be smelled at concentrations as low as 0.5 ppb. However, at concentration higher than 100 ppm, it blocks the nerves in the nose so it can no longer be smelled. Breathing H2S at a concentration higher than 500 ppm can be lethal after only a few breaths. H2S is present, sometimes at very high concentration, in volcanic gases, in hot springs and in natural gas. It is also produced in human and animal wastes and by different human industrial activities, such as rayon textiles manufacture, pulp and paper mills, oil re?nery, natural gas treatment, etc.

Measurement of hydrogen Sulfide is very important for the natural gas industry, because H2S can be extremely corrosive for the gas pipeline system especially when the natural gas also contains water vapor in high concentrations. When H2S is burned, environmentally harmful sulfur-dioxide (SO2) is generated. Therefore the concentration of hydrogen-sulfide in natural gas is a crucial quality parameter, and as such, it is strictly regulated and controlled. Various methods exist for the measurement of H2S concentration but yet up to now none of them is completely accepted by the natural gas industry. A diode laser based photoacoustic system for on-line process monitoring of hydrogen sulfide (H2S) concentration in natural gas streams was constructed (WaSul). The WaSul systems are commercially available from Hilase Ltd. As a light source, a fiber-coupled distributed-feedback (DFB) diode laser with an emitted light power of 40 mW operating at 1574 nm was applied. The wavelength of the laser was selected to minimize the spectral interference of other compounds of natural gas. Since the system has to be operated at natural gas plants at a potentially explosive area, therefore it is equipped with the necessary safety features. The system possesses the certificate of the BKI (Hungarian Approval Service for Ex-proof Electrical Equipments). Our system can measure H2S concentration in the range of 0-1000 ppm with a precision of about 0.5 ppm. Our systems are working at natural gas plants already for several years, where they proved to have highly reliable operation. One special feature of the system is that they can measure H2S concentration continuously in up to four gas streams in parallel. Therefore they are ideally suited for process control applications, e.g. at a natural gas sweetening unit. With a WaSul system it is possible to measure the H2S concentration in the incoming (so called sour) and in the processed (so called sweetened) gas in parallel and the system provides control signals with which the operation parameters of the sweetening unit, such as the amount of added de-sulfurization chemical can be controlled. Another application of our system is to use it at gas transfer or sales points, where it can monitor continuously whether the concentration of H2S is below the amount set by domestic and international standards, or by sales contracts.