[Digestion] Fate of CO2

[Duncan Martin]

Lou and all

The principle of sequestration is fine - but I very much doubt that the 
economics would work for AD.

You have obviously grasped one of the major problems, that of scale, when 
you suggest locating the biogas plant near a carbon sequestration site 
(which would certainly have to be near a major source, such as a large power 
plant or a large sink, such as an oilfield). Such installations are not 
generally in the same areas as anaerobic digesters.

 The energy used to haul the digester feedstock to the biogas plant and to 
haul the digestate back to wherever it is to be used would add substantially 
to the operating costs of the biogas plant, the economics of which are 
generally fairly marginal at best.

 At the scale of many biogas plants, a more realistic option might be to 
supply it to glasshouses, where it would be sequestered by boosting plant 
growth. At least one Scotch whisky distillery was doing this 30 years ago, 
although the CO2 came from the fermentation process, not from a biogas 
plant.

(The Scots might be a more cheerful race, if it were not for the fact that 
only 50% of the carbohydrate that goes into a distillery comes out as 
ethanol!! The remainder, of course, is lost as CO2. However, they can 
console themselves with a per capita production rate of ethanol about 10x 
higher than their nearest competitor! 20x would be nice, of course - but why 
be greedy?)

An interesting alternative might be to release it in an unconfined growing 
environment, such as a biomass plantation or forest. I do not know how 
efficient the capture of CO2 would be but my guess would be that a dense 
plantation or forest canopy might extract quite a lot of the CO2, at least 
in summer, provided it was released at ground level and well away from the 
edges of the planted area. An efficiency well below 100% would still make a 
useful contribution, given that the energy cost and transport costs would be 
almost zero.

Otherwise, I do not think there is usually much alternative to venting the 
CO2 to atmosphere. It might be argued that this is equally good, since it 
will sooner or later be taken up by plant growth anyway. It follows that the 
above 'forest' proposal would only be useful if there was a significant 
improvement in the biomass growth rate -- which there might be.

Duncan J Martin

Chair
Republic of Ireland Centre
Chartered Institution of Wastes Management

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Duncan J Martin, PhD, CEng, CSci, MIEI, MCIWM, MIChemE
24 Townsfield, Cloughjordan, N Tipperary, Ireland
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Email: duncanjmartin at eircom.net
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----- Original Message ----- 
From: "Lou Dobb" <congoagriculture at yahoo.com>
To: <Digestion at listserv.repp.org>
Sent: Tuesday, September 11, 2007 6:14 PM
Subject: [Digestion] What happens to CO2 when scrubbed out of biogas?


Okay, I'm a bit of a lay man, but I wonder what happens to the CO2 if it is 
scrubbed out of biogas by water washing.

And, importantly, wouldn't it be possible to store this CO2 underground (as 
in carbon capture and storage systems; in depleted oil fields)? This would 
make biogas carbon-negative.

In principle you could make ultra-clean biogas near a carbon sequestration 
site (independently of power plants), store the carbon there, and export the 
carbon-negative biomethane which is now upgraded to natural gas quality.

You would get a premium for this carbon-negative gas (carbon currently costs 
â,¬20 per ton, but this is expected to increase significantly).

So has anyone thought about coupling biogas production to CCS?


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