Biomethane liquefaction followed by CO₂ solidification based biogas upgrading process

Energy consumption and climatic changes are challenging issues these days. Unlike fossil fuels, the utilization of renewable fuels such as biogas is a promising option to meet these challenges. Biogas upgraded form (biomethane) is an emerging potential alternative to natural gas. However, biomethane production is itself challenging because of the pros and cons of each biogas upgrading technology. In this study, a cryogenic technology is adopted because of its dual benefits; carbon dioxide (CO₂) removal and biomethane precooling due to its low-temperature operation. The CO₂ is removed from biogas through the CO₂ solidification process. The phase behavior of CO₂ is investigated and the specified conditions for CO₂ solidification (-68°C and 5.17 bar) are applied for biomethane production. After CO₂ removal, biomethane is liquefied. The refrigeration duty for upgrading and liquefaction is provided by a parallel nitrogen expansion cycle adopting pure nitrogen as a refrigerant. Aspen Hysys® v11 is used as a commercial simulator for process simulation and to evaluate CO₂ freezing behavior in the proposed study. The mixed optimization technique is employed to optimize the design variables of the proposed process. The proposed process shows energy and exergy savings of 17.4 and 29.7%, respectively. It is evaluated that the proposed integrated process depicts superior results than the conventional studies.