Enhanced Biogas Purification Using Low-Cost Adsorption and Absorption Techniques

The effectiveness of biogas as a source of energy is hampered by low calorific value and equipment degradation due to corrosive impurities in raw biogas. Current purification techniques are tailored for large scale applications; they use a lot of energy, which are expensive and require high operating skills. In this study, adsorption and absorption at ambient conditions were explored for the removal of CO2 and H2S from biogas using NaOH and Ca(OH)2. Activated carbon (AC) from coconut shells and iron oxide from waste iron fillings were used as adsorbents. These waste-derived adsorbents are more affordable and easily accessible, in addition, their use contributes to effective waste management. The effect of biogas concentration, absorbent concentration, and adsorbent mass was evaluated. Synergistic effect of adsorption and absorption; recovery, regeneration and reuse of the adsorbents were assessed. The adsorption capacity was 125.5 mg CO2/g AC and 58.2 ppm H2S/g iron oxide. The combined systems attained CO2 and H2S removal efficiencies of 98% and 100%, respectively using iron oxide-NaOH; and >95% and 100%, respectively for AC-iron oxide combination. The spent adsorbents were regenerated and reused at least twice. The CO2 adsorption was best described by the Yoon-Nelson model throughout the entire breakthrough profile with an R2 value of 0.98 using AC. The low-cost sorbents used have great potential for biogas purification for small scale systems.