[Digestion] Scrubbing biogas

[adkarve]

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
> ================================