Autogenerative high pressure digestion: anaerobic digestion and biogas upgrading in a single step reactor system

Conventional anaerobic digestion is a widely applied technology to produce biogas from organic wastes and residues. The biogas calorific value depends on the CH₄ content which generally ranges between 55 and 65%. Biogas upgrading to so-called ‘green gas’, with natural gas quality, generally proceeds with add-on technologies, applicable only for biogas flows >100 m³/h. In the concept of autogenerative high pressure digestion (AHPD), methanogenic biomass builds up pressure inside the reactor. Since CO₂ has a higher solubility than CH₄, it will proportion more to the liquid phase at higher pressures. Therefore, AHPD biogas is characterised by a high CH₄ content, reaching equilibrium values between 90 and 95% at a pressure of 3–90 bar. In addition, also H₂S and NH₃ are theoretically more soluble in the bulk liquid than CO₂. Moreover, the water content of the already compressed biogas is calculated to have a dew point <−10 °C. Ideally, high-quality biogas can be directly used for electricity and heat generation, or injected in a local natural gas distribution net. In the present study, using sodium acetate as substrate and anaerobic granular sludge as inoculum, batch-fed reactors showed a pressure increase up to 90 bars, the maximum allowable value for our used reactors. However, the specific methanogenic activity (SMA) of the sludge decreased on average by 30% compared to digestion at ambient pressure (1 bar). Other results show no effect of pressure exposure on the SMA assessed under atmospheric conditions. These first results show that the proposed AHPD process is a highly promising technology for anaerobic digestion and biogas upgrading in a single step reactor system.