Results from the operation of an efficient and flexible large-scale biogas methanation system

This study reports the design and operation of a power-to-gas system producing 240 kW of synthetic natural gas (SNG) from biogas and PV electricity. The system is composed of a solar field, an electrolyser, a plate-type heat exchanger methanation reactor and the required ancillary units. Biogas is cleaned from undesired components (such as H2S) and the raw gas including methane and CO2 is directly processed in the reactor. The process consumes biogas and renewable electricity to produce SNG and high-pressure steam from the methanation waste heat. The process efficiency in this configuration is 76%. The methanation reactor produces grid compliant SNG in all the load cases tested and in all the biogas composition cases. The reactor shows an excellent flexibility at the start-up, as grid-compliant SNG is produced in less than 10 minutes from feed start in hot-standby. Additionally, the reactor adapts in few minutes to load changes. The reactor is modelled to better understand the origin of the excellent performance in the biogas methanation reaction. It was found that the plate-type heat exchanger operated with boiling water as cooling is an ideal solution for the methanation reaction as it approximates well the optimal reaction pathway in terms of temperature and conversion profile. Large cooling is available where needed, preventing the operation at too high temperature. Isothermal conditions are established at the end of the reactor, allowing reaching the required high conversion.