Adding gas from biomass to the gas grid - SGC

600.9500.840Wobbe index [MJ/m 3 n]Relative density [-]0.7300.6200.5Carbon dioxide [vol-%]100.400.365 70 75 80 85 90 95 100Methane content in upgraded gas [vol-%]Figure 2: Wobbe Index and relative density as function of the carbon dioxide concentration inmethane/carbon dioxide mixtures.The following upgrading processes are described:· Membrane separation· Pressure Swing Adsorption (PSA)· Absorption without chemical reaction· Absorption with chemical reaction· Cryogenic removal of carbon dioxide· Adding propane (supplementary upgrading)4.3.2 Upgrading by membrane separationThe principle of membrane separation is that some components of the raw gas are transportedthrough a thin membrane (< 1 mm) while others are retained. In practice all components will betransported through the membrane in some extent. The transport of each component is driven bythe difference in partial pressure over the membrane and is highly dependent on the permeability ofthe component in the membrane material. In order to obtain high methane purity and recovery, theselectivity of the membranes, i.e. the difference in permeability of methane and carbon dioxiderespectively, must be high. The permeability is a direct function of the chemical solubility of thetarget component in the membrane.Solid membranes can be constructed as e.g. hollow fibre modules, which give a large membranesurface per volume and hence compact units. Solid membranes can be constructed from severalpolymers as e.g. acetate-cellulose. In this material the permeability of CO 2 is 20 times higher thanCH 4 , and the permeability of H 2 S is 60 times higher than for methane [lit.15,16].page: 34