Power to Gas constitutes a possible form of long-term accumulation and exploitation for excess electricity produced by non-programmable renewable sources. This can be converted into hydrogen via an electrolyser. The subsequent methanation of H2 and CO2 (Sabatier reaction) would immediately allow exploiting the almost infinite storage capacity of the gas network. CO2 can come from pre- or post-combustion capture systems, capture in industrial processes or from syngas or biogas. As the sector of biogas production from anaerobic digestion has developed greatly in Italy and Europe, RSE has focused on the study of a methanation technology which is very similar to anaerobic digestion and which can be easily coupled to it: ex-situ biomethanation (i.e., carried out downstream of the digestion process). In the biomethanation reactor the catalysts are microorganisms belonging to the Archaea domain who are also present in digesters.
RSE focused on trickle bed reactors (TBRs), three-phase reactors in which gas and water pass through a bed of elements that support microorganisms – which is promising in terms of energy efficiency and gas quality, but still little studied. It has created a biological methanation plant capable of accommodating up to 3 reactors and of operating between 30 °C and 70 °C and from 1 to 10 bar a, with the possibility of pH control and nutrient supply. Hydrogen is produced by a PEM electrolyser.
After commissioning the plant, a reactor was started using a thermophilic inoculum: methane production began 70 hours after inoculation. The percentage of methane in the composition of the output gas began to grow, reaching 10% after 120 hours of operation.
In parallel, a modeling activity was carried out. A 2+1-phase Eulerian-Eulerian mathematical model has been defined for the description of the fluid dynamics within a TBR, which incorporates the mass, momentum, and energy transfer phenomena involved in the dissolution of the gaseous phase into the liquid one .
Semi-empirical models relate interphase friction to wetting efficiency, bed geometric characteristics, fill element geometric characteristics, and Ergun parameters. Being developed for the case of filling elements with a pseudo-spherical shape and with regular packing layout, these models must be “calibrated” experimentally for the case of filling elements with complex shapes and irregular arrangement.
Starting from the mathematical model, a numerical method was developed and then implemented in C/C++ in the CFD software “Immerflow” (Optimad), with the possibility of adapting the grid to local flow conditions. The code was implemented for high-performance parallel computing via the distributed memory approach