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Synthetic natural gas production from biogas in a waste water treatment plant

Guilera, Jordi, Andreu, Teresa, Basset, Núria, Boeltken, Tim, Timm, Friedemann, Mallol, Ignasi, Morante, Joan Ramon
Renewable energy 2020 v.146 pp. 1301-1308
anaerobic digestion, biogas, carbon dioxide, catalytic activity, condensation (phase transition), electrolysis, gas production (biological), heat transfer, hydrogen, methane, methane production, natural gas, tap water, wastewater treatment
The technical feasibility of synthetic natural gas production from biogenic source was evaluated under industrially relevant conditions. The biogas produced from anaerobic digestion in a waste water treatment plant was considered as renewable carbon dioxide source and renewable hydrogen was produced by alkaline electrolysis (37 kWhe) of tap water. In the present work, the catalytic methanation of biogas, partially upgraded biogas and carbon dioxide released by a membrane upgrading unit was performed through innovative micro-structured heat-exchange reactors. A 2-step methanation process, including gas pre-heating, catalytic reaction and water condensation was implemented. Experimentation revealed that, through this process strategy, the outlet gas mixture (CH4 ≥ 95%, H2 ≤ 5% and CO2 ≤ 2.5%) fulfils the requirements for gas grid injection. It was observed that most reaction occurred in the first reactor (T ≈ 400 °C), while the second reactor (T ≈ 300 °C) was necessary to assure a high methane content. The introduction of methane together with carbon dioxide was found to be positive, namely biogas or partially upgraded biogas instead of pure carbon dioxide. The methanation of biogas reduced the reaction hot-spots and increased the methane content at the outlet. Finally, a proper balance between injection requirements and costs was found at 4–5 bar (g).