BIOGAS TO LBG VIA CRYOGENIC UPGRADING TECHNOLOGIES

Liquid biomethane (LBM), also referred to as liquid biogas (LBG), is a promising biofuel for transport that can be obtained from upgrading and liquefaction of biogas. With respect to fossil fuels, LBM is a renewable resource, it can be produced almost everywhere, and it is a carbon neutral fuel. With respect to compressed biomethane (CBM), LBM is a 3 times more energy dense fuel and it allows longer vehicle autonomy. With respect to other transport biofuels, LBM has also a higher energy density, it is produced from wastes and recycled material without being in competition with food production, and it assures a high final energy/primary energy ratio. The low temperatures at which LBM is obtained strongly suggest the use of cryogenic technologies also for biogas upgrading. In addition, it is well proved in the literature on natural gas purification (indeed biogas can be considered as a “particular” natural gas with a high CO2 content) that cryogenic technologies and, in particular, cryogenic distillation are less energy consuming when compared to traditional technologies such as amine washing for CO2 removal. Low-temperature purification processes allow the direct production of a biomethane stream at high purity and at low temperature, suitable conditions for the direct synergistic integration with biogas cryogenic liquefaction processes, while CO2 is obtained in liquid phase and under pressure. In this way, it can be easily pumped for transportation, avoiding significant compression costs as for classical CO2 capture units (where carbon dioxide is discharged in gas phase and at atmospheric pressure). In this paper, the three most common natural gas low-temperature purification technologies have been modelled and their performances have been evaluated through energy consumption analysis and comparison, in terms of the equivalent amount of methane required for the upgrading, with the amine washing process, proving the profitability of cryogenic technologies. Specifically, the Ryan-Holmes, the dual pressure low-temperature distillation process and the anti-sublimation process have been considered. It has been found that the dual pressure low-temperature distillation scheme reaches the highest thermodynamic performances, resulting in the lowest equivalent methane requirement with respect to the other configurations. This is mainly due to the distributed temperature profile along a distillation column that differs from a reversible heat exchange process to a lesser extent.