Biogas purification and upgrading for vehicular fuel application

M.Tech. (Chemical Engineering)Abstract: Energy is used in the form of electricity, heat, and fuel for lighting, cooking, transportation, and manufacturing of different kind of products. South Africa has faced a long-term struggle on energy shortages which may be related to the 20% growth in the country’s electricity consumption and underinvestment on energy. Thus, the need for producing clean energy from alternative renewable and sustainable energy sources remains an attractive technology. Biogas is energy produced by anaerobic digestion of biodegradable organic waste such as garden waste, food waste, municipal waste, industrial waste and sewage sludge. However, it may not be fully exploited because it contains impurities which limit its application. It can be purified and upgraded using techniques such as high-pressure water scrubbing, pressure swing adsorption, membrane separation, activated carbon sieve, cryogenic separation, chemical absorption. This study focuses on chemical absorption as it can yield biogas with over 95 % CH4 by volume. The substrate used to produce biogas in this study was grass inoculated with cow dung. Sodium hydroxide and acetic acid were used to adjust the pH of the feedstock. The chemicals absorbents of focus are sodium hydroxide, potassium hydroxide, ammonia solution and monoethanolamine. Two kinds of setups were used for this study; biochemical methane potential (BMP) assay tests using the Bioprocess Control AMPTS II and a custom made setup using Buchner flasks. Gas Chromatography was used for biogas analysis. The raw biogas contained on average 52% CH4 by volume which improved to over 80% CH4 after CO2 absorption. The CO2 removal efficiency improved from 22% to 66% for the alkalis and was also quite comparable for MEA and NH3 as it increased from 51% to 67%. The removal efficiency for NH3 and MEA increased from 69% to 79% on average with CH4 concentration reaching over 85% volume at 40 °C. The achievable calorific value for this study ranged from 25 to 33.5 MJ/Nm3