Characterization of biogas and biomethane trace compounds: A critical review of advances in in situ sampling and preconcentration techniques

Abstract

Worldwide, the valorization of biogas, landfill gas and biomethane is gaining momentum as circular economies and energy transitions are triggered. Nevertheless, to sustainably integrate those gases into today's energy mix, their quality must be controlled regarding their major, minor and trace constituents to preserve the integrity of infrastructures wherein they are burned, transported or stored. Field gas sampling is the first and most critical step in the analytical chain to characterize the composition of such gases. A large array of gas sampling techniques is available, yet choosing the most suitable technique is complex, especially when targeting trace compounds (<ng·Nm^-3 to mg·Nm^-3) which often require a preconcentration step to be detectable. Sampled trace compounds must be kept stable (no loss, degradation or contamination can occur) during the storage phase between sampling and analysis, and all materials in the sampling chain that contact the gas potentially influence this stability. This paper aims to review the available gas sampling and preconcentration techniques for determining trace compounds in biogas, landfill gas and biomethane. Techniques reviewed include 1) whole gas sampling methods (gas sampling bags, gas cylinders, canisters) and 2) gas sampling methods with preconcentration on solid media (sorbent tubes for physisorption or chemisorption, amalgamation, solid phase microextraction); preconcentration in liquid media (absorption in impingers); and cryogenic preconcentration. These techniques are reviewed for the sampling of nonmetal(loid) volatile organic trace compounds (aliphatic, aromatic, halogenated and oxygenated species; organic silicon compounds (siloxanes, silanes), and (in)organic sulfur compounds) as well as for volatile (in)organic metal(loid) compounds. The suitability of all presented sampling and preconcentration methods for given families of trace compounds regarding storage stability issues, is discussed as well as considerations regarding the ease of field implementation, advantages and disadvantages. This review highlights the intricate complexity of sampling trace compounds in biogas, landfill gas and biomethane. The different trace compounds indeed display extremely diverse physicochemical properties (volatility, polarity, reactivity …) which results in different stabilities in given sampling units so that no sampling method along can trap and recover all families of trace compounds. The review finishes with a list of recommendations to select proper sampling units, materials and parameters and to apply suitable sample transport and storage conditions to safeguard the integrity of samples.

Keywords: Biogas and biomethane; Preconcentration; Trace compounds; Volatile metal(loid) compounds; Volatile organic compounds; Whole gas sampling.