Biogas or biomethane, specific needs for sampling

At the outlet of the digester, the biogas is mainly composed of NH4 (methane) and CO2but it also contains traces of H2O, H2S, siloxanes, N2, O2, NH3 more or less important depending on the degraded materials.
The biogas produced can be recovered by combustion in the form of heat or electricity.
It can also be purified to the quality of natural gas and then injected into the gas network. This purified biogas is called biomethane..

Whatever the production process used, this purification step is essential to rid the biogas of impurities and undesirable components, in order to respect the physico-chemical characteristics defined by the network operators. Specifications include PCS, Wobbe index, density, water dew point, hydrocarbon dew point, O2, CO2... After being purified, biogas deemed to comply with natural gas specifications becomes biomethane. It contains more than 97% methane.

Principle of methanisation (Source - INSA Rennes)

To be injected, the biomethane is then transported to a injection station where four steps are carried out odorization, quality control, pressure regulation and metering.

The analysis systems present on the biomethane injection stations make it possible to measure the characteristics of the gas supplied by the producer in order to verify its conformity before its injection into the network, to control the correct odour of the gas and to provide information on the gas's PCS. A fraction of the biomethane production is continuously collected for analysis.

As for natural gas sampling, a sampling probe with or without pressure regulation (depending on the application) is used for sample extraction. A separator is usually used to protect the analyzer against liquids in the sample. Traced tubing is recommended if the sampling location is far from the analysis shelter.