1.INTRODUCTION.
Pressurized water scrubbing.
DMT Environmental Technology has developed a process for water scrubbing of biogas, landfill gas and sour natural gas, for the treatment of both hydrogensulphide and carbon dioxide. Water scrubbing is generally used to remove carbon dioxide but also hydrogen sulphide from biogas since these gases are more soluble in water than methane. The absorption process is purely physical. Usually the biogas is pressurised and fed to the bottom of a packed column where water is fed on the top and so the absorption process is operated counter currently.
Water scrubbing can be used for selective removal of both carbon dioxide and hydrogensulphide since this is more soluble in water than carbon dioxide. The water which exits the column with absorbed carbon dioxide and hydrogen sulphide can be regenerated and circulated back into the absorption column. The regeneration is made by de-pressurising or by stripping with air in a similar column. Stripping with air is not recommended when high levels of hydrogen sulphide are handled since the water will soon be contaminated with elemental sulphur which could cause operational problems. Besides this fact one has to deal with emissions from hydrogensulphide to the atmosphere by odour abatement. The most cost effective method is not to circulate the water if cheap water can be used, for example, outlet water from a sewage treatment plant.
This process can be economically applied in the purification of gases with daily sulphur production rates up to more than 10 tons. Examples of gas streams that can be treated with this process are :
•    Landfill gas
•    Biomass digester gases
•    Natural gas
•    Industrial waste gases
•    Flare gases

The selective absorption process has several key advantages compared to alternative processes available on the market :
•    Low energy consumption
•    No consumption of chemicals
•    Selective towards H₂S and CO₂
•    Simple process scheme and process control
•    Flexible towards feed gas fluctuations
•    Odour free cleaned gas ( H₂S < 1 ppmv is possible

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