Biogas upgrade using natural and modified adsorbents

Abstract

Biogas is an alternative energy source that is generated from anaerobic digestion. However, the biogas contaminants affect its performance by reducing the energy content. The general objective of the study was to upgrade biogas using natural adsorbents including; clay, zeolite, and wood ash through dry adsorption and carbonation method. The specific objectives were; to characterize the chemical composition of the adsorbents, to determine the effect of adsorption process parameters (adsorbent/water ratio, the slurry temperature, adsorbent mass to biogas volume ratio, and biogas flow rate) and to evaluate the adsorption equilibrium isotherms, kinetics models, and breakthrough curves of carbon dioxide (CO 2 ) removal from biogas. The chemical composition of the adsorbents was characterized using X-ray fluorescence (XRF). The biogas composition was analyzed using gas chromatography and a digital hand gas analyzer. The natural adsorbent mass to water ratios used were; 1:1, 1:2, 1:3, 1:4 and 1:5 while the slurry temperature was varied from 18 o C to 80 o C on carbonation process. The experiments were carried out at biogas flow rates; 45, 100, 150, and 250ml/min. The adsorbent mass to biogas volume ratios tested ranged from 0.5 to 7g/l. The Computer Adsorption Valuation Software was used to study equilibrium isotherms, kinetics models, and breakthrough curves for CO 2 removal. The analysis of the chemical composition of natural adsorbents showed that the calcium oxide (CaO) content in wood ash, clay, and zeolite was 46%, 2.21% and 1.47% respectively. The silica/alumina (SiO 2 /Al 2 O 3 ) ratio of activated clay and zeolite was 8.5% and 15.4% respectively. The wood ash to water ratio of 1:4 gave the highest methane increment of 70.4%. The best biogas flowrate for wood ash slurry was 100ml/min which improved methane by 70.4% while the slurry temperature of 75 o C produced the highest methane enhancement of 88%. Furthermore, the highest uptake capacity of wood ash slurry was 2.30mmol/g at optimum condition. In addition, the high pH value of 10.9 for slurry was found to favor the purification of biogas. In dry adsorption process, the activated clay produced the highest adsorption capacity of 5.72mmol/g. Moreover, an increase in adsorbent mass to biogas volume ratio from 0.5 to 7g/l decreased CO 2 uptake capacity from 5.72 to 2.89mmol/g but the removal efficiency of CO 2 increased from 13.66 to 93.79%. For activated clay, the data of CO 2 removal well fitted in Freundlich isotherms (R 2 =95.9), pseudo-first order kinetics model (R 2 =99.2) and Yan breakthrough curve model (R 2 =99.5). Conclusively, wood ash/water slurry carbonation method and dry activated clay in fixed-bed adsorption are good alternative media for biogas upgrading. Their performance depends on CaO contents and SiO 2 /Al 2 O 3 ratio respectively. This study recommends activated clay and wood ash slurry as potential media for purifying biogas.