Innovative high-pressure water scrubber for biogas upgrading at farm-scale using vacuum for water regeneration

Biogas upgrading is becoming essential in the valorization path for biogas. Biomethane from anaerobic digestion is identified to be one of the main resources to decarbonize the energy requirement. However, small-scale (especially farm-scale) applications suffer from a lack of implementation worldwide, mainly limited by the cost of the upgrading process. As one of the most used technologies, High Pressure Water Scrubbing presents a great margin for improvement. The most expansive items in this technology remain the packing column used for absorption and the stripping column used for water regeneration. In this work, the stripping column is replaced by a flash tank submitted to a rough vacuum (up to 0.2 bar in absolute pressure) associated to a static mixer to promote the gas desorption and thus the complete regeneration of the water. The implementation of an anisotropic packing, with properties evolving with the height of the column and adapted to the reduction of the gas flowrate due to absorption, is also investigated in order to reduce the size of the absorption column. A full-scale prototype was developed and implemented on a farm anaerobic digester. The range of application is from 20 to 40 Nm3/h of raw biogas. These field experiments associated to a complete description of the gas fluxes allowed to characterize the device in terms of carbon dioxide absorption efficiency, biomethane purity and methane recovery. Influences of six parameters (absorption pressure, desorption pressure, temperature, water flowrate, biogas flowrate, column height) were elucidated regarding those performances. Results highlight the great improvement of the water regeneration conducted under rough vacuum. From 0.8 to 0.2 bar in absolute pressure, the biomethane purity increased by 10%, the carbon dioxide elimination rate by 14%, and the methane recovery by 19%. With an increase in the packing height using an anisotropic configuration (modification of the packing properties with the column height) suited to the gas flowrate decrease into the column, a biomethane purity over 97% was obtained. The energy consumption associated was measured at 0.8 kWh/Nm3 of raw biogas but can be reduced to 0.3 kWh/Nm3. The overall energy efficiency for biogas upgrading can reach up to 91%. These results could pave the way for gas grid injection and vehicle fuel applications for small-scale anaerobic digestion.