Adding gas from biomass to the gas grid - SGC

Removal of hydrogen sulphide by oxidation with airThe H 2 S content of the biogas can be reduced significantly by addition of 5 - 10% air to the biogasin a cleaning unit after the biogas production [lit.9,10,11]. The filter vessel is filled with plasticcarriers and a liquid made up from gas condensate and liquid from effluent slurry separation iscontinuously recirculated over the filter. A temperature of approximately 35 °C promotes the processin which the H 2 S is biologically converted to sulphur. The sulphur is retained in the liquid in the filter.After successful full-scale experiments at the Fangel plant in 1993, this method has been utilisedwidely in Danish biogas plants. The utilisation of the method is increasing because it is cheaperthan chemical cleaning. The method is also able to remove the ammonia content of the biogas. Withaddition of air in biological filters the H 2 S content can be reduced from 2000 - 3000 ppm to 50 - 100ppm. In other experiments the H 2 S content has been reduced from 800 ppm to 10 ppm.Limitations:· Aftertreatment might be required to obtain pipeline gas quality.· The addition of air may conflict with pipeline specifications for oxygen (and nitrogen) content.· The addition of 5 - 10% air might reduce the effectiveness or capacity of an upgrading processfollowing the H 2 S cleaning.Removal of hydrogen sulphide by adsorption on activated carbonH 2 S can be adsorbed on activated carbon. The sulphur containing carbon can then either bereplaced with fresh activated carbon or regenerated.The adsorption of hydrogen sulphide on activated carbon is catalytic and the carbon acts ascatalyst. The carbon is often impregnated with potassium iodide (KI) or sulphuric acid (H 2 SO 4 ) toincrease the reaction rate. The chemical reaction is:2H 2 S + O 2 ---> 2S + 2H 2 OOxygen is needed for the reaction and is usually added as air. This results in nitrogen being addedto the gas, but if the H 2 S content is low, only minor amounts of nitrogen will be present in thecleaned gas.If the carbon is regenerated, this is done with hot nitrogen (inert gas) or steam. The sulphur will bevaporised and, after cooling, liquify at approximately 130 °C. Regeneration requires two reactionvessels for continuous running. The most common way for utilising activated carbon adsorption iswithout regeneration of the carbon.page: 29