[Digestion] Scrubbing biogas

Sat Sep 1 16:50:07 EDT 2007

G'day All,

As I said recently in relation to CO2, you need to look at the whole
system when considering H2S scrubbing. If the H2S level is such that it
halves engine life or causes other major problems it is probably worth
the effort of scrubbing, but if it just means a slight reduction in
engine life I would suggest leaving it in the biogas (it s a fuel, after
all) and doing away with all the messing around. This "engineering"
approach amy not satisfy the purists, but most of us want minimum work.

Happy digesting,
HOOROO

adkarve wrote:
> 
> Dear Biogas Workers,
> All over the world, efforts are on to use biogas as fuel in an internal
> combustion engine. We were told by an expert that there was no need to
> remove CO2 from the biogas. I would like to know, how important it was to
> remove H2S from biogas. In my system, where we produce biogas from food
> waste, we have only traces of H2S in our biogas. Due to oxidation, it is
> supposed to get converted into S02, and by combining with water vapour,
> perhaps into H2SO3. This is not as strong an acid as H2SO4. The cylinder,
> piston, piston rings and valves of an internal combustion engine are
> supposed to be coated by a film of lubricating oil. So how much is the
> danger of corrosion of the engine due to sulphurous acid? There is also
> nitrogen in the air, which goes into the engine, and a part of it gets
> converted into NOx.  Does this compound get converted into nitrous and
> perhaps also into nitric acid? Don't these acids harm the engine?  I shall
> be grateful to get answers to my questions.
> Yours
> A.D.Karve
> 
> ----- Original Message -----
> From: Duncan Martin <duncanjmartin at eircom.net>
> To: Warren Weisman <weiswar at yahoo.com>; <digestion at listserv.repp.org>
> Sent: Friday, August 31, 2007 3:39 AM
> Subject: Re: [Digestion] Scrubbing biogas into biomethane article
> 
> > Whatever method you think of using, do reflect first on the advice given
> by
> > Paul Harris the few hours ago to consider very carefully why you should
> want
> > to produce pure methane.
> >
> > Unless you want it for some kind of chemical feedstock, you presumably
> want
> > it as a fuel.
> >
> > If you want it as a transport fuel, it is true that storage capacity would
> > be enhanced by the use of pure methane.  However, it would not even be
> > doubled, so the cost would have to be very low to make this worthwhile.
> >
> > Pure methane would be more energy efficient in any combustion device BUT
> the
> > energy required to purify it might well exceed any efficiency gain -- and
> > that is before you even consider the cost.
> >
> > Bear in mind too that the diluent effect of carbon dioxide in combustion
> is
> > vastly exceeded by the diluent effect of the nitrogen in the combustion
> air.
> >
> > Consider the stoichiometry of combustion.
> >
> > With pure gases:
> > CH4 + 2O2 = CO2 + 2H2O
> >
> > If the CH4 comes with some unwanted CO2 (lets say 2:1 for simplicity), the
> > eqn becomes
> > (CO2 + 2CH4) + 4O2 = 3CO2 + 4H2O
> >
> > Now the extra CO2 is indeed a 'passenger' in any boiler or gas engine,
> > adding to the volume of the  hot gases that escape via the flue or exhaust
> > pipe -and thus to the energy lost in them.
> >
> > BUT remember that the oxygen comes with a lot of unwanted gas too -
> > nitrogen, in a 4:1 ratio, so the true equation for the combustion process
> > is:
> > (C02 + 2CH4) + (4O2 + 16N2) = 3CO2 + 4H2O + 16N2
> >
> > The nitrogen is just as much a passenger as the carbon dioxide is -- and
> > there is 16 times more of it (by volume). We all know that it is very
> rarely
> > economic to supply an engine or any boiler with pure oxygen instead of air
> > (unless you work for NASA!), so this should be a warning that the removal
> of
> > carbon dioxide is unlikely to make sense either. Of course, it might be
> > economic if the removal of carbon dioxide was very much cheaper than the
> > removal of nitrogen. However, there is little point in considering one
> > without considering the other, since for all you know you might be
> > considering the less economic option.
> >
> > Food for thought?
> >
> >
> > Best regards
> >
> > Duncan J Martin
> >
> > Chair
> > Republic of Ireland Centre
> > Chartered Institution of Wastes Management
> >
> > ================================
> > CONTACT DETAILS
> > Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
> > 24 Townsfield, Cloughjordan, N Tipperary, Ireland
> > Mobile: +353 86 8377 906
> > Home: +353 505 42087
> > Email: duncanjmartin at eircom.net
> > ================================
> 
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-- 
Mr. Paul Harris
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Faculty of Sciences,
The  University of Adelaide, Roseworthy Campus, AUSTRALIA 5371
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