Digester Gas Upgrading to Synthetic Natural Gas in Solid Oxide Electrolysis Cells

This work focuses on the process design and performance of an innovative plant for digester gas upgrading to synthetic natural gas (SNG). The differences and advantages over traditional upgrading processes are discussed. The main strength of digester gas upgrading via high-temperature electrolysis concerns its higher synthetic natural gas productivity for a given raw digester gas feed. Electrolysis is performed through a solid oxide electrolysis cell (SOEC) system, which is fed with demineralized water and purified digester gas (made up of methane and carbon dioxide). Surplus electricity from intermittent renewable energy sources is used to supply the energy required for the SOEC stacks. The resulting methane-rich syngas is reacted in a series of methanators to yield a high CH4 content output stream. The steam reforming reaction is promoted by means of a nickel catalyst in the cathode (fuel) electrode, which reduces the methane fraction: hence, sulfur, which is present in several types of digester gas (e.g., from sewage or landfills) in the form of hydrogen sulfide, has been identified as a possible inhibitor for this reaction. However, it is also well-known that sulfur is responsible for the deterioration of the electrochemical performance of a stack. Therefore, its effect on the system has been modeled for different thermodynamic conditions. This study analyses the electrochemical and energy performance of the integrated process through which all the carbon contained in digester gas is converted/upgraded to methane-rich gas. The electrochemical dissociation of the CO2 contained in the digester gas to CH4 (with the addition of external demineralized water) is one way of cleverly exploiting the carbon content in digester gas when poor quality or limited biological substrates are available for anaerobic digestion. Finally, a comparison with other commercial digester gas upgrading techniques has been made.