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.