[Digestion] Scrubbing biogas into biomethane article

[Duncan Martin]

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
================================






----- Original Message ----- 
From: "Warren Weisman" <weiswar at yahoo.com>
To: <digestion at listserv.repp.org>
Sent: Thursday, August 30, 2007 7:13 PM
Subject: [Digestion] Scrubbing biogas into biomethane article


>
> Here's an article that covers a number of different
> technologies to scrub biogas into biomethane.
>
> The simple, low-cost method of just bubbling your
> biogas through a solution of lime and water is not
> mentioned, but is in use in many countries.
>
> http://www.westernuniteddairymen.com/Biogas%20Fuel%20Report/Chapter%203.pdf
>
> Warren Weisman
> USA
>
>
>
> ____________________________________________________________________________________
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> Digestion mailing list
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> Beginner's Guide to Biogas
> http://www.adelaide.edu.au/biogas/
> http://info.bioenergylists.org
>
>
>
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