Biogas upgrade using modified natural clay

Abstract

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 CO2 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 CO2 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 (SiO2/Al2O3) ratio from 11.2 to 8.5 and enhanced the CO2 uptake by more than 5 folds. The equilibrium and the kinetics data of CO2 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 CO2 uptake capacity for modified clay was 5.72 mmol/g. Furthermore, complete removal of hydrogen sulfide was achieved. The CO2 uptake decreased on increasing the ratio of adsorbent mass to biogas volume. However, the CO2 removal efficiency increased to maximum of 93.8% at 7 g/l ratio. Therefore, modified clay is a potential candidate for biogas purification.

Keywords

• Activated adsorbent
• Adsorption isotherm
• Biogas upgrade
• Breakthrough curve
• Carbon dioxide removal
• Greenhouse emissions