Exergy and energy analysis of three biogas upgrading processes

The aim of this work was to provide a complete exergy and energy analysis of three biogas upgrading technologies: amine scrubbing, water scrubbing and membrane separation processes. Biogas production and treatment represents a key-process for the application of Circular Economy principles, since allows to reuse/reconvert industrial by-products or agro-industrial waste in a product that can be used in different energy demanding sectors, after proper cleaning and upgrading processes. The three technologies here reported have been implemented in Aspen Plus flowsheets, and were used to upgrade a biogas to biomethane, meeting the UNIT/TS 11537:2019 standards for Biogas to be injected in the gas grid. Each units of all the simulated processes have been analysed calculating total exergy feed, total exergy produced and exergy loss, distinguishing that lost for irreversibility and as waste. Water scrubbing was characterized by the highest values of exergy efficiency (94.5%) and methane recovery (99%), whereas the lowest exergy efficiency belonged to membrane separation (90.8%) that returned also the largest specific energy consumption (0.94 kWh/m3 STP). Conversely, amine scrubbing was characterized by the lowest specific energy consumption value (0.204 kWh/m3 STP) but by an exergy efficiency of 91.1%.