Adding gas from biomass to the gas grid - SGC

4.2 CLEANINGThe main components in biogas are methane and carbon dioxide, normally 90 - 95%. Additionalsubstances can be water, hydrogen sulphide, nitrogen and sometimes oxygen. When biogas isupgraded to natural gas quality, most of these additional substances have to be removed. This canbe done before the upgrading process, in the upgrading process or after the upgrading process.4.2.1 Removal hydrogen sulphideProteins and other sulphur containing materials produce hydrogen sulphide in the digestionprocess. Hydrogen sulphide is poisonous and corrosive, as well as environmentally hazardoussince it is converted to sulphur dioxide by combustion. Furthermore it contaminates some upgradingprocesses. Hydrogen sulphide can be removed either in the digester, from the crude biogas, or inthe upgrading process.In-situ reduction of the hydrogen sulphide concentrationHydrogen sulphide can be treated directly in the digester vessel. The sulphide is either reacted witha metal ion to form an insoluble metal sulphide or oxidised to elementary sulphur. Iron salts are themost used reactants for the in-situ reduction of hydrogen sulphide emissions.Iron, in the form of Fe 2+ , reacts with sulphide ions to form iron sulphide, FeS. The iron ion isnormally supplied as iron chloride, FeCl 2 , which is added to the digester. Hydrogen sulphide levelsof typically 100 to 150 ppm in the gas stream can be reached with this method.The in-situ removal of hydrogen sulphide is included in the turn key biogas plant or installed by theplant owner. The investment costs are rather low since the only equipment needed is a dosingsystem. The operational cost for this method depends on the amount of hydrogen sulphide that isformed by the digestion processes. When using raw materials that are rich in protein and othersulphur containing molecules, this method is rather expensive.Plant examplesParameter Unit Process Plant 1 Plant 2 Plant 3 Plant 4Location/nameLinköpingbiogasLaholmbiogasNSR 2 KalmarbiogasCapacity rangem 3 hydrogen no specificall 100 - 10,000sulphide/yearrangeInput· hydrogen sulphide Ppm500 – 300,000 In situ In situ In situ In situOutput· hydrogen sulphide Ppm50 - 150 50 - 100 100 - 150 15 - 35 115 - 400Process conditionsnormal digesterconditionsDigester40 °CDigester38 °CDigester36 °CDigesterNo temp availTable 7: In-situ reduction of hydrogen sulphide productionpage: 27