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Biogas upgrading

Biogas upgrading and the production of biomethane nowadays is a state-of-the-art-process of gas separation. A number of different technologies to fulfil the task of producing a biomethane stream of sufficient quality to act as a vehicle fuel or to be injected into the natural gas grid are already commercially available and have proven to be technically and economically feasible. Nevertheless, intensive research is still in progress to optimise and further develop these technologies as well as to apply novel technologies to the field of biogas upgrading. All technologies have their own specific advantages and disadvantages and this review shows, that no technology is the optimal solution to each and every biogas upgrading situation. The right choice of the economically optimal technology is strongly depending on the quality and quantity of the raw biogas to be upgraded, the desired biomethane quality and the final utilisation of this gas, the operation of the anaerobic digestion plant and the types and continuity of the used substrates as well as the local circumstances at the plant site. This choice is to be made by the planner and future operator based on a broad assessment of data and knowledge.

Figure: Basic flowsheet of biogas upgrading for terminology purposes

As already mentioned, biogas upgrading is a gas separation task where the raw biogas basically is split into two gas streams: the methane-rich biomethane stream and the carbon-dioxide-rich offgas stream. As no separation technology is perfect, this waste-gas stream still contains a certain amount of methane depending on the methane recovery of the applied technology. Whether this gas stream is legally permitted to be vented to the atmosphere or has to be further treated is depending on the methane content, on the methane slip of the upgrading plant (amount of methane in the offgas related to the amount of methane in the raw biogas) and on the legal situation at the plant site.

Figure: The most important steps during biogas upgrading

The task of biogas upgrading is to produce a methane-rich product gas stream with a certain specification. Depending on the raw biogas composition this separation task comprises the separation of carbon dioxide (and thus increasing the heating value and Wobbe-Index), the drying of the gas, the removal of trace substances like oxygen, nitrogen, hydrogen sulphide, ammonia or siloxanes as well as the compression to a pressure needed for the further gas utilisation. Furthermore, tasks like odorisation (if injected to a local low-pressure natural gas grid) or adjustment of the heating value by propane-dosing might have to be performed.

Figure: The most important process steps of biogas upgrading