Comparison at laboratory scale of different types and configurations of commercial molecular sieves for one-step direct upgrading of biogas produced by anaerobic digestion

In the present study we investigated, at laboratory scale, the possibility of upgrading biogas with a single step, without involving any pre- or post-treatment or multiple stage recirculation. This would allow small-scale AD plants to achieve convenient in-situ biomethane production entirely in accordance with REPowerEU objectives. In this respect, three commercial molecular sieves, i.Honeywell 13X, 4A, and 5A, were tested in two configurations in series (4A/13X and 4A/5A) in 25/75 v/v relative amounts. A control trap containing only the 4A molecular sieve using the same amount as the in series configuration was also used. Real biogas fed directly from the production reactors at atmospheric pressure was used in the experiments. The ability of the sieves to adsorb CO2 in the proposed operational conditions was tested and proved. In particular, sample 4A was highly efficient in biogas upgrading (CO2 retention rate of 42.4 gCO2/kg, equivalent to 0.96 mmolCO2/g) but its activity was limited to 18 days, while increasing the amount of material and combining different types of molecular sieves in series improved the performance. The possibility to regenerate and reuse the sieves was also proven.