Raman spectroscopy applied to in-situ natural gas and hydrogen-enriched natural gas composition measurements

The natural gas sector is constantly demanding for non-invasive, self-standing and consumables-free approaches capable to measure the composition and hence the thermodynamics properties of the mixtures flowing in the network. A novel approach is represented by Raman spectroscopy as the only full-optical method able to measure simultaneously the mixture composition, including gases difficult or impossible to be measured with conventional spectroscopy, such as hydrogen and nitrogen.<br/> In this work, the latest developments regarding a novel industrial-grade instrument are presented. The Raman emission is triggered by a broadband solid-state laser diode with emission centered at 455 nm, that is stabilized in temperature by a TEC controller. The laser beam is focused into a gas-cell which can operate with an internal pressure up to 17 absolute bar, compatible with pressures found in the transport network. The Raman scattering is collected at 90&deg; angle by a high-luminosity (<i>f</i>/2.4) objective + spectrometer. The experimental spectra are finally analyzed via a custom-made fitting procedure giving the mixture composition, the Heating Value and the mixture density.<br/> The performance has been characterized in terms of composition measurements and related calculation of the mixture thermodynamics properties over the full required environmental temperature range (from -20&deg;C to 50&deg;C) and at sample pressures up to 17 absolute bars. The results comply the metrological requirements for all mixtures tested: different natural gas mixtures with different content of heavier hydrocarbons and impurities, hydrogen-enriched natural gas mixtures and methane-hydrogen binary mixtures.