Reutilization of Ashes from the Solid Fraction of Digestate as Sorbent Materials in Pressure Swing Adsorption for Biogas Upgrading to Biomethane

The quantity of the solid fraction of digestate (SFD) from anaerobic digestion is expected to reach 177 million tons of dry material in the EU by 2050, and therefore requires sustainable management aligned with circular economy principles. Thermochemical valorization is a possible solution, even though it produces ashes as a by-product. This work experimentally investigates the reuse of SFD ashes to obtain recovered sorbents for CO2 capture in biogas upgrading via pressure swing adsorption (PSA). Five ash-based sorbent materials were synthesized: (i) SFD ash calcinated at 700 °C (ASFD700); (ii) SFD ash calcinated at 900 °C (ASFD900); (iii) ASFD700 washed with an HCl solution (AWSFD700); (iv) ASFD900 washed with an HCl solution (AWSFD900); (v) AWSFD900 modified by zeolitization (ZSFD900). These materials were tested for CO2/CH4 separation by PSA. XRF, XRD, BET-BJH, PSD, SEM-EDS, and FT-IR were used to characterize the solid samples. PSA experiments were performed at industrially relevant pressures (5–9 bara) and room temperature. AWSFD700, AWSFD900, and ZSFD900 had the highest CO2 sorption capacities, reaching up to 1.06 ± 0.25 mmolCO2 g−1 at 9 bara for AWSFD900, while comparable performances were observed for AWSFD700 and ZSFD900; CO2/CH4 selectivity decreased with increasing pressure from approximately 1.4–1.8 mmolCO2 mmolCH4−1 at 5 bara to approximately 0.9–1.0 mmolCO2 mmolCH4−1 at 9 bara, due to competitive adsorption effects. The obtained zeolitic phase in ZSFD900 was analcime, which has a narrow pore size, a key limitation for its application in PSA. Overall, SFD-derived ashes showed potential to form alternative sorbents for biogas upgrading.