Removal of sulfur compounds and siloxanes by physical and chemical sorption

The removal of sulfur compounds and siloxanes, which are major impurities in the biogas produced from the anaerobic digestion of sewage sludge, was studied using a bench-scale adsorptive gas purification experimental setup. The main impurities are hydrogen sulfide (H2S), carbonyl sulfide (COS), carbon disulfide (CS2), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). The commercially available adsorbents iron oxide (IO), iron oxide hydroxides (IH, IHS), activated carbon (AC), impregnated activated carbon (IAC), silica gels (A2 and NS10) and molecular sieves (5A and 13X) were first extensively characterized using scanning electron microscopy (SEM), X-ray fluorescence (XRF), and BET surface area measurements. IHS, comprising mainly 42% iron oxide hydroxide, 11% silica gel, and 10% activated carbon, exhibited the best adsorption capacities for H2S (539 mg/g) and COS (32 mg/g) among the adsorbents studied, as well as relatively good adsorption capacities for siloxanes D4 and D5. AC and IAC showed the greatest CS2 removal efficiency. A2 demonstrated extremely high adsorption capacities for siloxanes D4 and D5, namely 1055 and 1968 mg/g, respectively.