Biogas purification with biomass ash

The aim of the study was to investigate the option to purify biogas from small-scale biogas plants by entrapping CO₂ and H₂S with regionally available biomass ash. Connected to the existing biogas plant Neustift (Tyrol) wood ash placed in a 1 m³ container was used as a trap for CO₂ and H₂S in the biogas. With the process conditions chosen, for a period of a few hours CO₂ was trapped resulting in pure methane. The removal of H₂S was much longer-lasting (up to 34 d). The cumulative H₂S uptake by the biomass ash ranged from 0.56 to 1.25 kg H₂S per ton of ash. The pH of the ash and the leachability of Lead and Barium were reduced by the flushing with biogas, however toxicity towards plants was increased thus reducing the potential of ash use in agriculture. It can be concluded that biomass ash may be used for removal of hydrogen sulphide from biogas in small and medium biogas plants. The economic evaluation, however, indicated that the application of this system is limited by transport distances for the ash and its potential use afterwards.