Silicate minerals for CO2 scavenging from biogas in Autogenerative High Pressure Digestion

• Application of mafic silicate minerals in Autogenerative High Pressure Digestion.
• Wollastonite additions buffer acidification caused by glucose overload.
• Mineral surface area and pH can be used to estimate Ca2+-leaching rate in AHPD.
• Subsequent CO2 scavenging improved biogas composition to over 85% CH4.
• Calcium carbonate precipitated in the sludge.

Autogenerative High Pressure Digestion (AHPD) is a novel concept that integrates gas upgrading with anaerobic digestion by selective dissolution of CO2 at elevated biogas pressure. However, accumulation of CO2 and fatty acids after anaerobic digestion of glucose resulted in pH 3–5, which is incompatible with the commonly applied high-rate methanogenic processes. Therefore, we studied the use of wollastonite, olivine and anorthosite, with measured composition of CaSi1.05O3.4, Mg2Fe0.2Ni0.01Si1.2O5.3 and Na0.7Ca1K0.1Mg0.1Fe0.15Al3.1Si4O24, respectively, to scavenge CO2 during batch AHPD of glucose. Depending on the glucose to mineral ratio the pH increased to 6.0–7.5. Experiments with wollastonite showed that Ca2+-leaching was caused by volatile fatty acid (VFA) production during glucose digestion. At 1, 3 and 9 bar, the CH4 content reached 74%, 86% and 88%, respectively, indicating CO2 scavenging. Fixation of produced CO2 by CaCO3 precipitation in the sludge was confirmed by Fourier Transferred-InfraRed, Combined Field emission Scanning Electron Microscopy-Energy-dispersive X-ray spectroscopy and Thermogravimetric Analysis-Mass Spectroscopy.

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