[Digestion] Removal of hydrogen sulphide from biogas

[David]

Alex,

On 9/25/2012 1:53 PM, alexanderb.eaton at gmail.com wrote:
> David,
>
> Some really interesting points here. For example, any idea how much 
> ferric oxide would make sense to add to a digester? How would that 
> effect the effluent as a fertilizer? 

Well of course the question before your question has been mentioned 
already, which is whether one needs to do anything at all. I don't 
think it makes sense to do a preemptive strike on H_2 S, but rather 
find out if it will be a problem, then if it is, to deal with it. And 
of course whether it may be a problem depends on a number of factors. 
And remembering that the original question was about small-scale, farm 
or household digestion, of the millions of Chinese- and Indian-style 
digesters extant, I don't know of any significant use of scrubbing H_2 
S that is used. Your thought appears to be the same since you say "In 
the SNV program they essentially use no filtration, and they really do 
not have issues."

That said, using one of the methods mentioned by Henry of adding a 
source of the ferric ion, based on the figures he presents-- and they 
were minimal in his paper-- one would need 2 g Fe_2 O_3 /l of slurry. 
That can end up being quite a lot of material.

But it is important to note that Henry does not assert that this is a 
well-tested dosage. In fact, according to his paper, he tried this 
technique only twice-- not with a range of dosages, in a series of 
digesters with varying amounts of sulfides, but only two times, once 
each in two test digesters. He says that

        "It can be seen that the amount of sulfide in the sludge in a
        digester is not a measure of the hydrogen sulfide in the
        digester gases. The sulfide may be tied, as in this case, with
        iron [DWH: i.e. often by adding it]. The iron content, of
        course, is not an exact figure. Any small bit of rust would
        have shown up too." 


So even the 2g/l figure is "more or less", and really this comes down 
to a sort of general guideline: if one has "too much" H2S in the 
biogas, then try adding "some" rust (or perhaps a bit of 
iron-containing soil, such as hematite) to the slurry.


> My sense is that the sulpher and iron would actually work well as 
> micro nutrients, as both would be present in high concentrations. We 
> could make a powdered mixture with cal (lyme?) To both raise pH and 
> add the iron. Both materials are really cheap and effective in small 
> volumes. 

S (more likely) and Fe (less likely) might be micro-nutrients, but 
that would depend on the soil to which the effluent is being added. 
And even cheap materials, given that one would have to mix and add 
them, have a marginal cost in labor and complexity, so again either 
additive should earn its place following some degree of proof that it 
was actually needed, or sufficiently beneficial to warrant inclusion.


> Regarding the biological filtration, I think the thesis you posted 
> talks about running the gas through a drier mix of cow manure. 

The cow manure filtration actually is mentioned in a different paper, 
although at the moment I don't have the time to track it down. The 
thesis I mentioned offers a lot of information that might be adapted 
to lower-tech biofiltration of H_2 S.


> I have seen the industrial systems (RCM International a really 
> impressive model), but I have not been able work through the pumping 
> of substrate and air at a small scale.

Any digester evolving biogas will provide enough pressure to cause the 
biogas to flow. The problem, as you indicate, would be getting enough 
but not too much air into the gas stream just before the point of 
exposure to whatever media was supporting the sulfur-loving bacteria.


> Aside from engines, the question that SNV made me ask was: why 
> filter for small systems? If the combustion is complete the sulphur 
> becomes elemental, with a slight odor but not dangerous, or at least 
> how I understand it. We have never registered any dangerous sulphur 
> compositions in a kitchen. Thoughts? 

Actually when H_2 S is burned, it produces SO_2 , which is quite 
irritating (that concentrations of three to five PPM are readily 
detectable). Further, in the presence of water (and when the exhaust 
gases cool, water will be present), SO_2 combines to produce sulfurous 
acid (H_2 SO_3 ), which can corrode appliances, flues, and the like. I 
use sulfur to control gophers on my farm, for example, by putting some 
elemental sulfur in a run that I've dug up, then burning it into the 
burrow with a propane torch. Believe me, it's sufficiently deadly.

I think, in sum, that the reason so little scrubbing is done is 
because few substrates contain sufficient sulfur to be of much 
concern, and those which do contain sulfur in some form (pig manure, 
for example) are generally digested at a pH sufficient to reduce the 
production of H_2 S (and/or there is sufficient ferric ion present-- 
widely available in soil and water-- to suppress the production of H_2 
S) that it simply is not a problem in most situations.



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

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