[Digestion] Removal of hydrogen sulphide from biogas

David david at h4c.org
Tue Sep 25 09:20:32 PDT 2012



Dear Isiaka,


On 9/24/2012 3:13 AM, multicareng at yahoo.com wrote:
> I am working on biogas application on farm and low / middle class 
> homes. [...]
>
> I am looking for simplest and easiest way(s) to eliminate hydrogen 
> sulphide from biogas and I think you can be of assistance.

In addition to the excellent advice and information you have already 
gotten, I would mention that there is a low-cost means of measuring 
the amount of H_2 S in the biogas detailed in a freely downloadable 
PDF from The Complete Biogas Handbook website, here 
<http://completebiogas.com/PDFs/BiogasHandbook_A01_analysis.pdf>.

Further, the following is from the same book:

        Henry (1961) claims that at mesophilic temperatures and pH
        5.0, 98% of all of the sulfides dissolved in the slurry show
        up as H_2 S, whereas at pH 7.2, this is reduced to 24% with an
        approximate 50% reduction for each 0.2 pH unit increase
        thereafter (e.g., 12% at 7.4, 6% at 7.6, 3% at 7.8, 1.5 % at
        8.0). Where scrubbing H_2 S is desirable but unrealistic for
        some reason, pH adjustment may be the answer--- but take care
        not to over-adjust. A further idea is the addition of the
        ferric ion of iron; Henry used ferric chloride at the rate of
        about 2 grams per liter. This tied up the sulfide ion and no
        H_2 S was produced in a digester with this high dosage of
        ferric chloride.


For a good recipe for creating a mixture that will assist scrubbing of 
H_2 S-- remembering the caveats mentioned by Alex-- I suggest looking 
at the booklet "Some Conditions Affecting the Usefulness of Iron Oxide 
for City Gas Preparation", by William. A. Dunkley (February, 1921). It 
can be downloaded from here 
<http://ia600400.us.archive.org/14/items/someconditionsaf00dunkrich/someconditionsaf00dunkrich.pdf>.

My final thought is that we may ultimately find the best means of 
scrubbing H_2 S from biogas is by the use of sulfur-loving bacteria. 
However, to the best of my knowledge, no one has yet adapted this 
relatively high-tech process to low-tech-- i.e. village-- use. (One of 
the key hurdles to overcome, for example, is that these bacteria need 
just a hint of oxygen in order to metabolize the H_2 S into elemental 
S.) I would love to hear of anyone that has ideas or sources of 
information on this. Some steps in this direction are indicated (for 
example) by a thesis by Mary Elizabeth Fischer, "Biogas Purification: 
H2S Removal using Biofiltration," available here 
<http://uwspace.uwaterloo.ca/bitstream/10012/5458/1/Mary%20Elizabeth%20Fischer,%202010.pdf>.



d.
-- 
David William House
"The Complete Biogas Handbook" |www.completebiogas.com|
/Vahid Biogas/, an alternative energy consultancy |www.vahidbiogas.com

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