H2S removal from biogas on Cu- and Mg- modified activated carbon honeycomb monoliths at low temperature

The purification of reformate gas, natural gas and raw biogas from hydrogen sulfide is a mandatory step before their use as fuels as it is a toxic compound, corrosive for engines and fuel treatment devices and poisonous to catalysts also in small concentrations. Due to its easy management, high efficiency and low cost, adsorption is one of the most effective techniques for H2S removal. In particular, Activated Carbons (ACs) are excellent candidates as good H2S absorbents due to their surface chemistry, high internal surface and porosity. ACs are generally used in powder form, however, as high pressures are required, their application in structured form is preferred. Production of ACs in structured form generally requires the use of binders which, while improving the mechanical properties of the sorbents, reduce the adsorption capacity due to the partial blocking of porosity. Otherwise, the introduction of suitable metals can lead to the improvement of the original properties of the AC. In this work, commercial honeycomb ACs were functionalized with copper and magnesium oxides both separately and mixed in order to improve H2S capture capacity under different operating conditions. The sulfur capture capacity and the different oxidation reactions by varying the two metals were evaluated through dynamic catalytic tests (100ppmv H2S) at room temperature in the presence of humidity and O2. Depending on the nature of the metal used, the performance of the catalyst was significantly different. The presence of basic MgO promotes the formation of elemental sulfur through a faster path than the deeper oxidation mechanism promoted by CuO, which leads to the formation of sulfates / sulfuric acid in addition to elemental sulfur. Mg allows easier dissociation of H2S into HS- and H+ which improves the capture capacity of ACs in the presence of wet streams compared to Cu-modified ACs for the same load level. The mixed Cu-Mg sorbents therefore inherited the fast kinetic characteristics of MgO, promoted by the presence of water vapor, and the oxidation activity of CuO, also exercised in dry conditions. Adsorption of N2 at -196 ° C, PSD, XPS, TG-MS have been performed on honeycomb catalysts, both fresh and spent, in order to study the specific nature of the S species formed during the catalytic adsorption. In particular, TG-MS experiments from exhausted sorbents allowed the speciation of adsorbed sulfur species.The original porous structure and the performance of the modified AC monoliths was restored through a thermal treatment up to 620°C under a flow of inert gas which led to the decomposition of sulfate species at a lower temperature and the evaporation of elemental sulfur at higher temperature allowing a complete regeneration of the spent sorbents. Therefore these structured AC monoliths functionalized with copper and/or magnesium can operate several capture cycles without detectable loss of capture capacity releasing only SO2 and S and are excellent candidates for the H2S capture process at room temperature for biogas purification.