The aim of the presented project Biogas + ASH Processing (BioGAP, 2012-2015) was to investigate the option to purify biogas from small-scale biogas plants by entrapping CO2 and H2S with regionally available biomass ash. A crotch was constructed at the existing Neustift (Tyrol) biogas plant to enable a splitting of the biogas stream, and connected with a speed-regulated pump. A cubical steel tank (1.6 m * 1.0 m * 0.7 m, h/w/d) was constructed where wood ash was placed on a gas distribution layer. Within this tank, the wood ash was used as a “filter” for removal of CO2 and H2S from biogas. Results were: With the process conditions chosen, an early breakthrough of CO2 was observed, leading to rapidly increasing CO2 concentrations after a few hours and a rather short peak in CH4. Contrary, a long-lasting removal of H2S (up to 34 d) was observed. The cumulative CO2 uptake ranged between 35 kg CO2/t WS and 135 kg CO2/t WS and the cumulative H2S uptake was 0.56 kg H2S/tWS or 1.25 kg H2S/t WS. The pH of the ash and the leachability of Pb and Ba were reduced by contact with biogas. It was concluded that wood ash (and similar biomass ashes) may be used as a “filter” for removal of hydrogen sulphide from biogas in small and medium biogas plant, although the economy depends on transport distances for the ash and other regional circumstances.
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