Adding gas from biomass to the gas grid - SGC

OxygenOxygen is often present in landfill gas, originating from air extracted together with the gas, but is nota natural component of biogas produced by controlled anaerobic digestion of manure etc. However,oxygen might be present due to either leaks in the production and transport system or because airis supplied to the biogas in e.g. a hydrogen sulphide cleaning process. In order to obtain pipelinequality gas, the oxygen sources must be controlled and minimised. In some cases the upgradingprocess, if e.g. membrane separation or PSA is applied, reduces the oxygen content.Organic silicon compoundsOrganic silicon compounds are occasionally present in biogas, and they can cause severe damageto gas driven engines. They can be removed by absorption in a liquid medium, a mixture ofhydrocarbons with a special ability to absorb the silicon compounds.Carbon dioxideCarbon dioxide is a major component in the raw biogas and the vast majority of it will be removed inthe upgrading process in order to raise the calorific value, Wobbe index etc., see § 4.3. However,typically traces of carbon dioxide will be present in the upgraded gas. The pipeline specifications forcarbon dioxide are varying for different grids. In some grids the natural gas contains severalpercentages carbon dioxide. In case of stringent limits for the carbon dioxide content, an upgradingprocess must be chosen that provides high purity methane, and therefore only a minor fraction ofcarbon dioxide.4.3 UPGRADING4.3.1 Upgrading and carbon dioxide removalThe upgrading process is basically a separation of the methane and carbon dioxide of the biogas, inorder to obtain pipeline gas quality with regards to calorific value, Wobbe index, relative density etc.Figure 2 illustrates how the Wobbe index is increased and the relative density decreases, as themethane content of the upgraded gas is raised (i.e. the carbon dioxide content is reduced). A rawbiogas composition of 65% methane and 35% carbon dioxide is assumed.page: 33