Metrology for biomethane conformity assessment: measure trace gas impurities in biomethane

Abstract

To facilitate the use of biomethane in existing transmission and distribution infrastructures, CEN/TC 408 developed specifications (EN 16723) for injecting biomethane into the natural gas grids and using it as a transport fuel. Currently, the test methods cited in EN 16723 lack metrological aspects and have not been specifically developed for biomethane. To address this need, ISO/TC193/SC1/WG25 “Biomethane” has been created to work on standardized methods. To assess conformity of biomethane with the specification and to provide essential input to WG25, test methods are being developed in this research for a group of impurities such as siloxanes, halogenated volatile organic compounds, hydrogen chloride, hydrogen fluoride. A further objective of this research is to develop fit-for-purpose measurement standards for these parameters, to enable SI-traceable calibration and facilitate accurate measurement results. An overview of the progress made with respect to the development of measurement standards and test methods for trace level concentrations of impurities is presented, with a focus on static measurement standards of siloxanes and halogenated volatile organic compounds, dynamic gas standards of HCl and HF, as well as corresponding test methods based on gas chromatography and spectroscopic techniques. The work presented is pivotal for the development of metrological infrastructure for biomethane conformity assessment.

Bilateral comparison VSL-NPL on primary gas standards for the siloxane content in biomethane

“…Prevalent silicon-containing substances include trimethyl silane, trimethylsilanol and the siloxanes hexamethyldisiloxane (L2), octamethyltrisiloxane (L3), hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). This bilateral comparison concludes over a decade of work invested in the preparation of PSMs for the measurement of the siloxane concentration (or amount fraction) in methane and biomethane [4][5][6]. The said PSMs enable the dissemination of metrological traceability by, e.g., calibration and the provision of CRMs for silicon in biomethane, thereby supporting the implementation of the European specification for biomethane EN 16723 [1,2].…”

“…For the analysis of Mixture 1 at VSL one PSM (VSL347730) was used. The PSM was prepared gravimetrically according to ISO 6142-1:2015 [11,16]. The mixture was prepared in three stages, as a parent mixture from the pure liquids.…”

“…These measurement or test methods require metrologically traceable calibration of the instrumentation used, for which CRMs are needed. Substantial work in the development of the primary standard gas mixtures (PSMs), needed to facilitate the production and supply of CRMs, has been undertaken [4][5][6][7]. As the specification for the silicon concentration in biomethane is very low (down to 0.1 mg m −3 [8]), and siloxanes (and other silicon-containing volatile organic compounds (VOCs)) are reactive, it is challenging to develop and maintain such PSMs.…”

Accurate analysis of HCl in biomethane using laser absorption spectroscopy and ion-exchange chromatography

“…There is an increased need from the European Union (EU) to diversify its energy supply, thereby targeting renewable sources such as biomethane. 1,2 Biomethane (composed mostly of methane, nitrogen and carbon dioxide produced by upgrading biogas, i.e. removing molecules such as carbon dioxide, hydrogen sulfide and water from biogas) is a renewable energy gas that can be used for equipment and applications designed for natural gas, and can be injected into the natural gas network if certain specifications are met.…”

“…[7][8][9][10][11][12][13][14] So far, reports on laser spectroscopic measurements of HCl in biomethane, addressing traceability of the results to the international system of units (SI), are lacking. Direct tunable diode laser absorption spectroscopy (dTDLAS) and wavelength modulation spectroscopy ((WMS) with comparably higher sensitivity 2,9 ) specifically as demonstrated in other applications 2,8,9,11 are good candidates for the development of test methods for HCl concentration measurements in biomethane. dTDLAS is a variant of TDLAS that combines this spectroscopic technique with a special, first principles data evaluation approach to directly extract absolute species amount fraction (concentration) without calibration of the sensor with a calibration gas mixture, under the provision that a metrologically traceable line strength parameter is available.…”