Adding gas from biomass to the gas grid - SGC

4.3.4 Upgrading by absorption of carbon dioxide without chemical reactionRemoval of carbon dioxide by water wash with regenerationCarbon dioxide can be absorbed in a physical absorption process. The carbon dioxide is dissolvedin the absorbent without any chemical reaction taking place. Regeneration of the solvent is madeeither by flashing (decreased pressure), by heating a combination of both. Water is the mostcommonly used solvent for absorbing carbon dioxide from biogas. The solubility for carbon dioxidein water increases with increasing pressure. Water can either be regenerated and reused forabsorption in a circulation system or used for absorption in a single pass flow.Carbon dioxide is absorbed in water at elevated pressure. Cleaned biogas is compressed and fedinto the bottom of an absorption column. Water is fed from the top of the column to achieve a gasliquidcounter flow. The column is equipped with random packing to give a large surface for gasliquidcontact and internals for the collecting and redistribution of water. Carbon dioxide is absorbedby the water and the gas leaving the top of the column is stripped from most of the carbon dioxidecontent.Methane is partly soluble in pressurised water and therefore some methane will be removed withthe water. To minimise the losses of methane, the water is depressurised in a flash tank afterleaving the absorption column. The released gas mixture is rich in methane and is recirculated tothe compressor inlet.Water from the flash tank is depressurised to atmospheric pressure and fed into the top of adesorption column. Air is blown from the bottom of the desorption column to remove dissolvedcarbon dioxide from the water. The desorption column is equipped with internals and packing in thesame way as the absorption column. The regenerated liquid is cooled in a heat exchanger tomaintain the absorption temperature and recycled to the absorption column.The vent gas is either released to the atmosphere or treated in some kind of gas filter to removeodours.Hydrogen sulphide is absorbed together with carbon dioxide in the absorption column. The solubilityfor hydrogen sulphide in water is rather high and all hydrogen sulphide cannot be removed in thedesorption column. Furthermore, part of the hydrogen sulphide is oxidised with air to elementarysulphur in the desorption column. This sulphur is accumulated in the system and can causeproblems with plugging and fouling after some period of operation. It is therefore desirable that highcontents of hydrogen sulphide are removed before the absorption. The product gas is saturated withwater and has to be dried.page: 38