[Digestion] Reaction Kinetics for AD

[David]

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

|
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"Make no search for water.   But find thirst,
And water from the very ground will burst."
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