[Digestion] Woody Biomass digestion via compost

[David Fulford]

Abel,

This does sound interesting. The problem of wood, as far as anaerobic 
digestion is concerned, is that the bacteria do not degrade lignin. 
However, the cellulose within the lignin structure can be degraded, given 
the right conditions. I have seen plants in Sri Lanka that generate biogas 
from straw, which gives a similar problem. The straw is packed tightly into 
a tank and sprayed with liquor from a working digester. The straw is only 
wetted, not flooded. The tank is sealed and produces gas after a week or so 
(Sri Lanka has tropical temperatures). The biogas is generated for a month 
or two. Once generation drops off, the tank is opened and emptied and refilled.

The first stage of digestion is hydrolysis. The hydrolysing bacteria are 
facultative, i.e. they can work in oxygen or without it. A water saturated 
compost heap will have an anaerobic center, with aerobic activity around 
the edges. It is possible that the aerobic bacteria (which are more 
aggressive) will break down the cellulose in the wood into soluble acids 
that can be used by the methanogens in the center to generate biogas. The 
Sri Lankan approach would probably work with wood shavings as well as it 
does with straw. It might be worth a try. The wood could be aerobically 
composted for a while first. I am not sure when the cellulose gets to the 
point when the anaerobic bacteria can use the fatty acids. If you leave it 
too long, the acids degrade to carbon dioxide and water. If you do not 
leave it long enough, the cellulose has not broken down enough.

Just some thoughts,

David F.

At 18:47 26/11/2006 -0800, Wake Robin Design WRD wrote:
>Greetings,
>   I was wondering if any readers have come across the work of Jean 
> Pain.  The man who converted overgrown hardwood forests in southern 
> france into healthy woodlands while simultaneously producing methane, 
> methanol, hot water, and compost from the digestion process?  I am trying 
> to track down his book or a book on the subject which illuminates the 
> specifics of this method.
>
>   What I understand is that instead of creating a slurry, he built piles 
> of wood shavings saturated by water.  I am guessing that by composting 
> the debris he was able to heat the inside of the pile enough to produce 
> methane.  It seems like a simpler approach than heating a tank filled 
> with a slurry.  Does this seem possible?
>
>   I want to eventually apply this method in the Pacific Northwest region 
> of the USA.  What is the feasability of using wood shavings from 
> northwest connifers like douglas fir and western red cedar?  Does anyone 
> have any specifics on the amount of energy produced per ton using this 
> particular material?  How hot do you need to keep it?  Does the 
> collection energy justify the energy retrieved by digestion?
>
>   I am also interested in learning more about the way gas is collected 
> from a compost pile.  This particular bit of information has not been 
> revealed by any articles on Pain
>
>   Finally, any ideas on some good chipper shredders in the PNW?
>
>   If anyone has any thoughts on any of these questions I would be grateful,
>
>   Abel Kloster
>
>
>
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*** Dr David Fulford, MSc Renewable Energy, Engineering Building ***
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