DIG-L: re: Gas treatment inquiry

[Harmssen@t-online.de]

Hello,
1. Removing H2S:
The easiest way is to add some air to the biogas. 
The H2S will be oxidized to elementaric Sulfur or sometimes SO4. 
This is done by bacteria, which will settle on the walls of the gas-
part of the digester as soon as you add some air and H2S is 
present. They live on this chemical reaction. The sulfur (a yellowish 
sludge) will fall back into the digester, when it is too heavy. 
Precautions: 
a:  Do not add more than 5% of the biogas-production. The best is 
to take a little pump like it is used for aquariums.
b:  The pH in the lower layers of the formed sulfur sludge may be 
very low becaus of the formation of SO4 (sulfuric acid). Material 
must stand this. Black iron and some concrete will not, wood will.
c: Sulfur sludge may form  heavy layers. The construction inside 
the digester must stand this weight.

CO2:
Removal of CO2 is not so easy. It is possible with water, but this 
will be saturated very quickly. The more technical way to remove 
the CO2 from the water is to wash the biogas with water under 
pressure (10 bar or so, may be use the first step of the 
compression) and release the CO2 from the water under normal 
pressure; water is reused. 
Another way to remove the CO2 from the water to have always 
water which can solve CO2, may be to use an algae pond. The 
biogas is bubbled near the ground into the water, the CO2 
dissolves in the water and is used by the algae and the methane is 
catched again by a type of upside down funnels near the surface.
As a side-effect you can use the algae.
Best Greetings
Henrik



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