# Methods for Sampling Biogas and Biomethane on Adsorbent Tubes after Collection in Gas Bags
## 1 Introduction  
Biogas can be produced from different substrates (inclusive food wastes, sludge from water treatment plants, manure, etc.) and consists mainly of methane and carbon dioxide. However, biogas also contains many other gaseous substances called VOCs (volatile organic compounds) [1; 2], but in much lower concentrations. Some of these substances have undesirable odours and may damage the process and/or the equipment and can be harmful for the environment and/or human health. It has been shown that the contaminants that form in the biogas depend on the substrate and the control parameters [3]. Terpenes and, specifically, p-cymene and D-limonene have repeatedly been in biogas samples originating from plants where household waste is digested. Alkanes (linear and aromatic) and siloxanes occur in significant quantities in biogas from wastewater treatment plants. Ketones and sulphur compounds are usually found in biogas produced by the digestion of energy crops and by-products from industry. Biogas has many applications; especially applications already using natural gas. Worldwide, biogas is currently used primarily for electricity production; however, biogas is also widely used after upgrading to biomethane as vehicle fuel. If biogas is to be injected into the natural gas grid or used directly as a vehicle fuel, carbon dioxide must be removed so the methane fraction is typically increased to above 96%. Depending on the upgrade method (water scrubber, amine scrubber, pressure swing adsorption, membranes, etc. ), some of the contaminants present in the biogas may still be present in the upgraded gas. For other applications, biogas shall also be upgraded because some of the compounds found in the biogas are harmful to the infrastructure. Therefore, depending on the application, several specifications related to biogas and biomethane have been adopted. It is then important to control thequality of the product (biogas or biomethane) against these specifications to insure the integrity of the end-user's appliances [4; 5]. But before this assessment can be made, biogas and/or biomethane need to be sampled and transported to the laboratory. During this step, which can take from some days to up to a week, the composition of the gas collected shall not be altered [6]. This requires accurate and representative sampling. One risk is the loss of impurities by absorption onto the inner surfaces of the sampling vessel. It is therefore necessary to choose the sampling vessel after having fully assessed the short-term stability of the impurities of interest in the vessels. However, the impurities of interest have different properties (with regards to their boiling points, polarities, water solubilities and reactivities). Recently, a literature survey [7] pointed out that it may be impossible to find a vessel that will be suitable for all impurities and thus, combining several vessels will probably be required. There are two main methods of sampling. Sampling without enrichment is mostly performed in cylinders, canisters or bags. Sampling with enrichment is mostly performed by adsorption onto adsorbent tubes or by adsorption in a solution using impingers. The sampling of biogas in bags has been widely investigated. Most of the studies have used Tedlar bags for this application [8; 9; 10; 11; 12; 13]. Other bags have also been tested in some studies (Nalophan [14], Teflon [15] or so-called Plastigas [16]). Sampling bags present the advantage to be easy to use; the sampling is simple and fast, do not require skilled personnel or a lot of additional equipment. For instance, no coolants or flowmeters are required. The disadvantages are that the method does not allow enrichment while, with some analytical instruments, the detection limit requires a larger volume of gas to be sampled. Moreover, studies [6; 10; 17] have shown that a reduction in the concentration may occur when the gas is stored in a bag for a few days.