Techniques for transformation of biogas to biomethane

Biogas from anaerobic digestion and landfills consists primarily of CH4 and CO2. Trace components that are often present in biogas are water vapor, hydrogen sulfide, siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen. In order to transfer biogas into biomethane, two major steps are performed: (1) a cleaning process to remove the trace components and (2) an upgrading process to adjust the calorific value. Upgrading is generally performed in order to meet the standards for use as vehicle fuel or for injection in the natural gas grid.Different methods for biogas cleaning and upgrading are used. They differ in functioning, the necessary quality conditions of the incoming gas, the efficiency and their operational bottlenecks. Condensation methods (demisters, cyclone separators or moisture traps) and drying methods (adsorption or absorption) are used to remove water in combination with foam and dust.A number of techniques have been developed to remove H2S from biogas. Air dosing to the biogas and addition of iron chloride into the digester tank are two procedures that remove H2S during digestion. Techniques such as adsorption on iron oxide pellets and absorption in liquids remove H2S after digestion.Subsequently, trace components like siloxanes, hydrocarbons, ammonia, oxygen, carbon monoxide and nitrogen can require extra removal steps, if not sufficiently removed by other treatment steps.Finally, CH4 must be separated from CO2 using pressure swing adsorption, membrane separation, physical or chemical CO2-absorption.

► Biogas can be cleaned to remove trace components : review of techniques.
► Biogas can be upgraded to adjust the calorific value : review of techniques.
► Conclusions are focused on removal of water, H2S and CO2.