DESIGN, FABRICATION AND OPTIMIZATION OF A SMALL- SCALE ADSORPTION PROCESS REACTOR FOR BIOGAS PURIFICATION

Abstract

Biogas is a renewable energy resource. Emissions from biogas can cause serious damage to the human health and environment due to presence of the contaminants. The aim of this study was to evaluate biogas generation from cattle dung and developing a purification system for biogas in small-scale farms before utilization. Currently, biogas purification occurs in large scale plants. Biogas which is produced in an anaerobic digestion process consists of methane (CH4), carbon dioxide (CO2), small amounts of water vapour together with traces of hydrogen sulphide (H2S) and other impurities. The presences of H2S and CO2 have detrimental effects on health, burning apparatus and the calorific value of biogas. Reducing these impurities will significantly improve the quality of the gas. Biogas samples were collected in small bags from three digesters located in Ongata Rongai division of Kajiado County about 20 km west of Nairobi City. It was analysed in the laboratory for the concentrations of CH4, CO2, H2S, oxygen (O2) and nitrogen (N2). Investigations were performed using chemical adsorption for removal of H2S and H2O and chemical absorption for CO2. The purification system comprised three columns charged with ferric oxide, calcium hydroxide solution and silica gel to scrub H2S, CO2 and water (H2O) respectively. The biogas was passed through the charged columns and the operating parameters mainly contact time and flow rate studied for the contaminants removal from the biogas stream in each column separately. The results show that the initial average concentration 0.0052 ± 0.02% H2S was reduced to 0.0012 ± 0.01% when a flow rate of 20 litres/min of biogas is maintained after passing biogas through the derived ferric oxide adsorbent material. The concentration of CO2 in the biogas for the same flow rate was reduced from an average of 46% to 30 ± 2%. The CH4 concentration realized at saturation of calcium hydroxide (Ca(OH)2) solution was 60 ± 4%. This represented a 20 ± 3% improvement in the CH4