Paul,
On 11/20/2012 12:43 AM, Paul Muthui wrote:
G'day All,
How much energy is given out in the digesters as a result of
anaerobic reaction? I can't seem to get any publication with enough
details on this. I would like to know the temperature rise that
would be caused by the reaction.
Dr. Fulford is correct, in essence, in that according to ordinary
experience, no functional metabolic heat is available from anaerobic
digestion, as contrasted to the very considerable metabolic heat
available from aerobic digestion (at about 40% of the energy provided
as methane in the former case). However:
Self-heating of anaerobic digesters using energy crops
<http://ebookbrowse.com/self-heating-of-anaerobic-digesters-using-energy-crops-pdf-d61331597>
H. Lindorfer, R. Kirchmayr, R. Braun
Water Sci Technol. 2006;53(8):159-66.
Abstract
With the increasing application of energy crops in
agricultural biogas plants and increasing digester volumes,
the phenomenon of self-heating in anaerobic digesters appeared
in some cases. Until now this development was just known from
aerobic systems. To get an idea of the thermodynamics inside
an anaerobic digester, a detailed analysis of all heat fluxes
in a full scale agricultural biogas plant was carried out.
Several experiments were realised to quantify the influences
of different internal and external energy sources. To estimate
the impact of self-heating in anaerobic systems, data of other
full scale agricultural biogas plants in Austria were
collected. Alternatives to the cooling of the digesters are
discussed basing on individual experiences of several plants.
A connection between carbohydrate rich substrates, especially
with high starch contents, and the self-heating could be
shown. But from the results it can be assumed that heat
enthalpy due to anaerobic microbial metabolism plays a key
role in self-heating, which is in contrast to the current
thermodynamic knowledge.
And Prof. Dr. Christof Schütte
<http://www.matheon.de/about_us/people/list_members.asp?action=detail&id=812>--
a mathematician, to be sure, but as applied to biological systems--
goes so far as to claim
<http://www.matheon.de/research/show_project.asp?id=134> that
"Knowledge of the net energy production of anaerobic
fermenters is important for reliable modelling of the
efficiency of anaerobic digestion processes. Furthermore
energy balancing is crucial, because present biogas plants
still suffer from severe instabilities caused by self-heating
and full-scale plants typically have to be cooled down."
These studies and statements, however, have a context, which is the
Austrian/German biogas industry, where digestion is a high-tech (some
might say gold-plated) affair in large part because of the previous
regime of above-market price guarantees for electricity from biogas.
That is to say, the digesters being analyzed and discussed above are
very likely extremely well insulated.
But as far as I know for the kinds of digesters with which you are
working-- underground masonry, if I'm not mistaken-- I believe you can
assume that the contents of the digester will be at ambient
temperature six feet deep.
d.
--
David William House
"The Complete Biogas Handbook" |www.completebiogas.com|
/Vahid Biogas/, an alternative energy consultancy |www.vahidbiogas.com
|
|
"Make no search for water. But find thirst,
And water from the very ground will burst."
(Rumi, a Persian mystic poet, quoted in /Delight of Hearts/, p. 77)
http://bahai.us/
|
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