The Natural Gas (NG) sector is looking at the Hydrogen-enriched Natural Gas mixtures (H-NG) with growing interest. Hydrogen injections in the NG networks are expected to increase in the next years. Simultaneously, there is the requirement to constantly sample the composition and the thermodynamics properties of combustible blends. Raman spectroscopy is an intrinsic non-invasive approach for gas analysis. In addition, this technique is able to provide multiple-species analysis in a simultaneous way. An industrial-grade instrument, designed to operate directly on-site, has been developed. Its aim is to determine the NG and H-NG blends composition in an accurate and repeatable way, by referring to the OIML R 140 standard. The system is going forward in its industrialization by applying all the engineering steps useful to make it robust and easily replicable. The system laser source is a broadband multi-mode diode centered at 447nm with an optical power of 2W. The scattered radiation is collected by an appositely designed diffraction grating spectrometer and acquired by a 2D uncooled camera. The spectrometer guarantees Raman Stokes acquisition of the entire spectral region of interest without any mechanical movement. Three typical NG and one H-NG certified mixtures has been measured by placing the system in a climatic chamber. The results obtained during this validation show a high accuracy and repeatability in the overall temperature range by requiring only one calibration set carried out at room temperature. The calorific value, calculated by the measured gas mixture concentration, results within ±0.5% error in the full temperature range.
The combustible gas sector requires for instrumentation capable to determine the composition and the quality of the gas mixtures present in the transport and distribution networks. The gas parameters need to be monitored in a wide interval, since mixtures are found within an extremely variable range. A compact, fast and highly sensitive instrument based on Raman spectroscopy has been developed with the specific aim to operate directly on-line. This approach is intrinsically non-invasive, since it needs a laser beam passing through the gas, and multi-species sensitive, since the different components of the gas mixture are simultaneously detected. The Raman scattering is stimulated by a laser diode centered at 455 nm with multi-mode emission and 1.5 W optical power. The laser is focused on a gas cell through a window, the Raman emission is collected by a grating spectrometer and finally acquired by a 2D camera. The measured spectra are fitted with the calibration dataset acquired at room temperature to achieve the mixture composition. The system is able to determine the main components of the natural gas: methane, heavier hydrocarbons, nitrogen, carbon dioxide and hydrogen. The Heating Value (HV) is finally calculated using the ISO6976:2016 standard. Several certified gas mixtures have been tested with the instrument operated at different temperatures in the range from -20°C to 50°C, to prove the capability to operate in a wide industrial temperature range. Each measure requires less than 25 seconds, with a sample pressure of 6 bars. The calculated HV value lies in the ±0.5% error range.
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