Biogas upgrade using modified natural clay

Abstract

• The biogas upgrade using clay slurry resulted in pH decrease from 9.2 to 6. • Activation of clay with NAOH increased CO 2 uptake by more than 5 folds. • The highest CO 2 capacity achieved was 5.72 mmol/g activated clay. • The breakthrough curves for column adsorption fitted best into Yan Model. • H 2 S was eliminated from biogas using clay slurry. The pollutants in biogas reduce its calorific value and limit its application in machines by causing corrosion and wear in metallic parts. The commercial technologies of biogas upgrade are expensive for small digesters. Natural adsorbents present cost-effective and locally available materials for purification of biogas but the process has not been satisfactorily investigated for most materials. The current study for the first time compared upgrading of biogas by removing CO 2 contaminants with clay through dry adsorption and wet carbonation processes. Natural and modified clay was characterized using X-ray fluorescence. Biogas composition was analysed using gas chromatography and digital biogas analyser. The adsorption equilibrium, kinetics isotherm and breakthrough curves of carbon dioxide removal were investigated using CAVS adsorption software. The optimum slurry temperature and clay/water ratio for CO 2 uptake by carbonation process were 75 °C and 1:3 respectively. The carbonation process resulted in pH decrease from 9.2 to 6. Furthermore, activation of clay using 2 M NaOH decreased the silica to alumina (SiO 2 /Al 2 O 3 ) ratio from 11.2 to 8.5 and enhanced the CO 2 uptake by more than 5 folds. The equilibrium and the kinetics data of CO 2 removal by carbonation process fitted best into Freundlich isotherm and pseudo-first order equation respectively. In addition, the breakthrough curves for column adsorption fitted best into Yan model. The highest CO 2 uptake capacity for modified clay was 5.72 mmol/g. Furthermore, complete removal of hydrogen sulfide was achieved. The CO 2 uptake decreased on increasing the ratio of adsorbent mass to biogas volume. However, the CO 2 removal efficiency increased to maximum of 93.8% at 7 g/l ratio. Therefore, modified clay is a potential candidate for biogas purification.