Re: DIG-L: Digester out put the Same as Compost?

["renertech" <renertech@xtra.co.nz>]

Tom,
I am reluctant to start entering into this debate as I am sure that there
are a lot of people more qualified and experienced than me to comment, but
this whole question of the heat energy output from a compost heat and from a
biogas digester  fills me with a dread foreboding.   Anaerobic digestion
produces very very little output as heat. The beauty of A.D. is that the
potential energy  in the substrate is channeled into a combustible gas which
can be burnt at a high temperature and thereby give a high delta T. for good
heat transfer characteristics.   O.K.  compost heaps do heat up,  but only
if they are stacked up and covered in such a way as to conserve the heat
they produce.  Why do you think you have to turn a compost heap once or
twice and put all the outer edges back into the middle?    To get good
breakdown of materials, you have to get the thermophylic bacteria working as
well as the mesophylics, and that needs temperatures of at least blood heat,
certainly no less than.  If you start trying to pull heat out of a compost
heap and you will find that there is not a great reserve of  energy to
maintain a high temperature, and the moment that the temperature does start
to drop, you will be in trouble with your composting operation.  The biggest
problem however, is that  at the best of times you are only working at
40-50oC.   Trying to move a large slug of thermal energy accross from one
place to another, when you have only a few degrees of temperature difference
to push the transfer operation, is a lot harder and a lot more expensive
than you might think.   Thats my two cents worth!  Over to someone else to
put numbers to all this.  Sincerely,  Ken Calvert.  renertech@xtra.co.nz  at
www.coffee.20m.com





----- Original Message -----
From: "Tom Hall" <tomhall@alum.mit.edu>
To: "Digestion Group" <digestion@crest.org>
Sent: Friday, July 26, 2002 10:01 AM
Subject: DIG-L: Digester out put the Same as Compost?


> We are developing a large food green house system where normally all of
> the waste biomass would be composted for use in the green houses.  How
> similar is the solids from a methane digester to compost?  Can it be
> converted to be useable in the green house?  I want to use the green
> house wastes for energy production if the farmers can be convinced.
>
> Tom Hall
> Chamber of Sustainable Utilities and Infrastructure, Vedic City, Iowa
>
> --
> ----------------------------
> Tom Hall, B.S.E.E., MS
> 1140 Beebe Lane
> Williston, VT 05495
> 802-482-2439
> tomhall@alum.mit.edu
>
>
>
>
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Digestion List Archives:
http://www.crest.org/discussion/digestion/200202/

Digestion List Moderator:
Paul Harris, paul.harris@adelaide.edu.au
http://www.roseworthy.adelaide.edu.au/~pharris
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Other Digestion Events and Information:
http://www.bioenergy2002.org
http://crest.org/discussiongroups/resources/biomass/biogas/BIOGASMK.pdf
http://www.crest.org/articles/static/1/1010424940_7.html

Beginners Tour of Biogas
http://WWW.roseworthy.adelaide.edu.au/~pharris/biogas/beginners