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The separation principle of absorption is based on different solubilities of various gas components in a liquid scrubbing solution. In an upgrading plant using this technique the raw biogas is intensively contacted with a liquid within a scrubbing column filled with a plastic packing in order to increase the contact area between the phases. The components to be removed from the biogas (mostly carbon dioxide) are typically far more soluble in the applied liquid than methane and are removed from the gas stream. As a result, the remaining gas stream is enriched with methane and the scrubbing liquid leaving the column is rich in carbon dioxide. In order to maintain the absorption performance, this scrubbing liquid has to be replaced by fresh liquid or regenerated in a separated step (desorption or regeneration step). Currently, three different upgrading technologies embodying this physical principle are available.

The available technologies utilise different scrubbing liquids for absorption resulting in different operating conditions. The following technologies are currently state-of-the-art: