Improvement of the water scrubbing technology for biogas upgrading.

Abstract

Biogas is a renewable energy resource comprising of mainly methane, carbon dioxide, and hydrogen sulphide. Its range of uses can be increased by removing hydrogen sulphide and enhancing methane content by removal of carbon dioxide. The presence of carbon dioxide reduces the heat value of biogas and also makes it expensive to transport. Additionally, the presence of hydrogen sulphide and its oxides react with water to form corrosive acids. One of the cheapest and simplest biogas upgrading technologies for removal of carbon dioxide and hydrogen sulphide from biogas is water scrubbing. The research reported in this thesis aimed at improving an existing water scrubbing system that could only raise methane content in biogas from 55% to 65% to a system that could upgrade biogas to a methane content of 80% and reduce the hydrogen sulphide to a recommended value of 20ppm for most biogas electricity generation devices. By operating at a maximum raw biogas pressure of 1.0589 bar and varying the new water scrubbing system operating conditions of packing material type, packing depth, water and gas flow rates, appropriate parameters for upgrading biogas from an initial composition of 71.5ppm hydrogen sulphide, 43% carbon dioxide and 55.85% methane to non-detectable hydrogen sulphide and 80% methane were identified. Results show that for a column packed with steel-wire mesh to a depth of 0.4m, it is possible to increase the volumetric percentage of CH4 in biogas to 80% for water to gas flow rate ratios of 1.9 and above. Increasing the packed depth to 0.8m increases volumetric percentage of CH4 in upgraded biogas to 80% at a lower ratio of 0.7. This increase in packed depth resulted in an improvement from 1800 litres to 700 litres of water for every m3 of raw biogas upgraded. However, to achieve 80% CH4 in marble packed columns of similar depths, the water to gas flow rate ratio has to be raised above 2. These results imply that although at low biogas pressure methane content could be raised to 80%, the benefit comes at a very high water cost. Water scrubbing should therefore be considered in areas where there are free or very low cost water sources.