Biogas deep clean-up based on adsorption technologies for Solid Oxide Fuel Cell applications

• A biogas deep treatment was installed and operated at pilot-scale in a WWTP in Spain.
• The H2S adsorption mechanism on the iron adsorbent was oriented to oxidation to S(s).
• Siloxane D4 was responsible of the adsorption breakthrough on the activated carbon.
• The integration of biogas treatment and fuel cell systems is technically possible.
• Cascade configuration reduces operation cost compared to stand alone sorption systems.

Biogas from anaerobic digestion is a powerful renewable fuel that can be used as a feedstock for fuel cell systems. A biogas deep treatment was installed and operated at pilot plant level in a Waste Water Treatment Plant (WWTP) in Spain in order to demonstrate the integration opportunities with Solid Oxide Fuel Cell (SOFC) technologies. The three-stage polishing system based on adsorption consisted of: (i) a regenerable iron-based adsorbent unit to remove H2S, (ii) a biogas drying unit to remove moisture and (iii) an activated carbon unit to remove the remaining trace components (siloxanes, linear and aromatic hydrocarbons). The biogas entering the polishing system was previously treated in a biotrickling filter for primary H2S abatement. Removal efficiencies on the iron-based adsorbent were over 99% and adsorption capacity was calculated to be of 21%wt. An adsorption mechanism for H2S chemisorption oriented to oxidation to elemental sulphur rather than to crystalline FeS(s) was proposed and could explain the low efficiency of the regeneration process. The remaining contaminant traces were efficiently removed in the drying and activated carbon unit and concentration levels below 0.1 mg/Nm3 were obtained. A roll-up phenomenon with siloxane D4, which was responsible of adsorption breakthrough on the activated carbon filters, was postulated; and leaded to an overall adsorption capacity of 2%wt. The economic assessment concluded that the cascade configuration of an upstream H2S abatement followed by downstream adsorption technologies, compared to stand-alone adsorption systems, divides the overall treatment cost by three; increasing the profitability of biogas-powered fuel cell projects.