Agathe Legendre

NaTran R&I launched the CARABIO project in 2016 to characterize biomethanes from methanization injected into the national gas network, aiming at addressing the knowledge gap in the composition of trace compounds. Between 2016 and 2023, 75 biomethane sampling and analysis campaigns were conducted at 40 injection plants, reflecting the variety of feedstocks and upgrading technologies in France. The investigations carried out required a unique analytical development work.Across 75 Gas Chromatography-Mass Spectrometry screenings, 530 trace compounds were semi-quantified. Of these, 100 were detected at least ten times, while 207 appeared only once. On average, biomethane contained 48 trace compounds, predominantly hydrocarbons. For organo-oxygenated compounds, terpenes, sulfur compounds, organo-halogens, siloxanes and amines, 87% of average concentrations were below 0.1 mg/m³(n) by compound, with maximum 0.35 mg/m³(n).Quantitative analyses were performed using micro-Gas Chromatography – Thermal Conductivity for monoaromatic hydrocarbons and terpenes, Gas Chromatography-Pulsed Flame Photometry for sulfur compounds, absorption spectroscopy for ammonia, Inductively Coupled Plasma-Mass Spectrometry for metals, and atomic fluorescence spectrometry for mercury. A statistical assessment seeking correlations between trace compounds, feedstocks, and upgrading processes revealed no clustering, indicating good overall homogeneity of biomethane quality.Levels for comparable compound families were lower than those in natural gas, and no trace compound reached concentrations that could pose risks to gas infrastructures or end-users, confirming biomethane’s suitability as a renewable energy source.

In the context of the energy transition, European countries pursue the common goal of increasing the share of renewable gases (from anaerobic digestion, pyrogasification, and hydrothermal gasification for instance) in the gas mix. Although produced gases are mainly composed of methane after upgrading, impurities of various natures and quantities may also be present in the produced raw gases and still after upgrading, including volatile organic compounds (VOCs) at trace levels that may have an impact on different stages of the gas chain even at low concentrations. These new renewable and/or low‐carbon gases imply the need to develop new analytical tools to deeply characterize them, and thus fully manage their integration into the gas value chain. In this study focused on VOC analysis, a sampling approach using pre‐concentration on an adsorbent phase was developed for gas sampling to ensure rapid collection, requiring only a small volume (≈ 100 mL). Traces were then analyzed by thermal desorption hyphenated to a comprehensive two‐dimensional gas chromatography time‐of‐flight mass spectrometer. After identifying the compounds, a semi‐quantification method was developed to provide an estimation of the concentration of VOCs for each chemical family. The method was first developed on a biomethane sample before being successfully applied to a raw gas produced by hydrothermal gasification, in which around 250 compounds were detected at a trace level (total concentration equivalent at 200 µg/L).

In the context of the energy transition and in order to fully manage the integration of renewable and/or low-carbon gases into the gas mix, these new renewable gases need to be characterized, including volatile organic compounds (VOCs) at trace levels that may have an impact on different stages of the gas chain event at low concentrations. This study focuses on sampling because it is the first step in any method for analyzing trace VOCs and its careful execution is essential to ensure reliable results even if the on-site conditions can be variable, such as the external temperature. The stabilization time, the effect of external temperature, and the impact of using an intermediate sample cylinder prior to transfer to tubes were hence studied in the laboratory using a standard gas mixture representative of renewable gases. The latter was also studied using a real sample. To perform this study, VOCs were sampled in Tenax® TA tubes and then analyzed by thermal desorption hyphenated to comprehensive two-dimensional gas chromatography–time-of-flight mass spectrometry. The results showed that 45 min were required to stabilize the sampling system and that external temperature had little influence, limited to the heaviest compounds. Finally, the use of a cylinder to temporarily collect gas prior to transfer to tubes led to a loss of compounds, particularly when the cylinder was stored before the transfer.

The development of renewable and low‐carbon gases for injection into the gas grid obtained by different processes such as anaerobic digestion, pyrogasification, hydrothermal gasification, and methanation, followed by upgrading steps, increases the demand for analysis and characterization in order to fully manage their integration into the gas value chain. If the analysis of the main compounds (methane, carbon dioxide, hydrogen, and carbon monoxide) is well described, the analysis of impurities in renewable gases remains more challenging due to their various natures and quantities.After a brief description of renewable and low‐carbon methane production processes, the review focuses on the methods used for the analysis of the different compounds in renewable gases, from the main ones to impurities at ppb_v levels. Gas chromatography (GC), coupled with different detectors, is the preferred technique, enabling the analysis and quantification of siloxanes, terpenes, oxygenates, and sulfur compounds. Recently, comprehensive two‐dimensional GC has been applied to renewable gases, increasing the number of compounds detected. Non‐chromatographic techniques are also reviewed. As sampling is of major importance in the search for reliable analyses, a whole section is devoted to this aspect. Among the available methods, pre‐concentration on adsorbent tubes emerges as the most relevant solution.

In order to quantify organic impurities in gas produced from renewable sources, thermal desorption coupled with GC-MS or GC×GC-MS is very useful. However, the preparation of the standard tubes appears not to be trivial. For that, different strategies, based on commercial setups, have been developed. The goal of this study was to compare the classical manual deposit of a liquid standard solution with other commercial methods such as gas stream assisted deposit and vaporization followed by adsorption assisted by gas stream. A standard mixture of 48 compounds from different families was used for the comparison of the performances of the three strategies using the accuracy profile methodology. A global validation score was attributed to each strategy as well as a score according to family of compounds and boiling point range, in order to provide a detailed comparison of the techniques. On the set of studied molecules, commercial setups were found to be more efficient than the manual deposit.