Theoretical evaluation and optimization of a cryogenic technology for carbon dioxide separation and methane liquefaction from biogas

The transport sector represents one of the major cause of air pollution and greenhouse gases emissions. For this reason, several measures have been taken: penalty and incentive schemes have been introduced worldwide to increase renewable energy sources and alternative fuels use. It is widely accepted that natural gas (NG) is a good alternative fuel, especially in its liquid form (LNG), and it can be produced using biogas as methane source to obtain renewable liquefied biogas (LBG). However raw biogas contains several impurities and it needs to be pre-treated. At present, several technologies allow biogas purification. In particular cryogenic separation represents a promising solution for simultaneous purification and liquefaction. It can achieve impurities removal and methane liquefaction by means of a single plant. In this work, a novel cryogenic separation process, based on CO2 cold recovery, is presented. The proposed plant is optimized and a sensitivity analysis is also performed. Results indicate that the proposed plant represents a valid option for LBG production. Indeed the specific energy consumption is 1.574 kWh kg−1 for a CO2 content in inlet biogas of 50%. Furthermore the energy use and operative costs are compared with those of standard technologies and they result respectively 23% and 22% lower, taking into account the influence of CO2 as by-product.