Cu- AND Mg-MODIFIED ACTIVATED CARBON MONOLITHS FOR H2S REMOVAL FROM BIOGAS

The purification of reformate, natural gas and raw biogas represents a mandatory step before their use in downstream processes. One of the main impurities is H2S which is toxic, corrosive for engines and fuel processing devices and poisonous to catalysts even in small quantities. Adsorption is an effective technique for H2S removal because it is a simple, highly efficient and low cost process. Activated carbons (AC) are good sorbents due to their surface chemistry (often enhanced by dispersion of metal nanoparticles) and high internal surface area and porosity. ACs are generally in the form of powders or granules that limit their applicability when high pressure drops must be avoided. ACs in structured form are commercially available but their production generally requires the use of binders which, despite offering good mechanical properties, reduce the adsorption capacity due to the partial blockage of porosity. In this work, copper and magnesium oxides were dispersed onto commercial AC honeycombs (400cpsi) to increase the H2S capture capacity at room temperature in the presence of H2O and O2. The adsorption performance varied significantly depending on the nature of the metal. Addition of magnesium provided a higher capture capacity than copper for the same load level. Copper promoted an initial fast mechanism, associated to the formation of sulfates, substituted by a slower mechanism involving the formation of elemental sulfur. Magnesium activated a mechanism occurring through the dissociation of H2S into HS- and H+ promoted by the basic character of MgO. A thermal treatment up to 620°C under inert gas of spent sorbents restored the porous structure of both Cu- and Mg-modified AC monoliths due to the decomposition of sulfate species at lower temperature and the evaporation of elemental sulfur. Therefore, the sorbents can operate several capture cycles without detectable loss of capture capacity. This makes these structured metal-modified ACs very good candidates for H2S capture process at room temperature for natural and raw biogas purification