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David and listers, Hello,<br>
<br>
The microbes do not seem to be affected significantly by pressure,
at least up to the 1 atmosphere found in small dome plants as used
in India, Nepal and China. In the 1970s there were reports of some
earlier work in India that suggested less gas was generated at
higher pressures; but gas analysis demonstrated that the reduction
in volume at higher pressures was mainly due to carbon dioxide
dissolving in the slurry. It is suggested that slurry from a dome
plant will release carbon dioxide when it enters the reservoir
chamber from the bottom of the digestion chamber, thus reducing its
proportion in the gas chamber. <br>
<br>
This early work in India led to attempts to create negative relative
pressure by using floating gas drums suspended from cables with
counter-weights. This occasionally allowed air to get in and mix
with the gas. Many years ago, a colleague (long since dead,
unfortunately) told me the story of a farmer in India trying to
demonstrate biogas production to some visitors by lighting the gas
from the outlet. The explosion made the gas drum jump upwards,
spraying slurry all over the visitors. <br>
<br>
A quick look at the chemistry of how carbon dioxide dissolves in
slurry suggests that the process is fairly complex. Carbon dioxide
forms carbonic acid with water, which then forms carbonates and
bicarbonates with alkalis in solution. This is affected by the
volatile fatty acids produced by the digestion process. A biogas
plant is self-buffering; i.e. it controls its own pH as carbon
dioxide is absorbed or released. It is possible that the pH is
slightly decreased at higher pressures, as more carbon dioxide is
dissolved, which might reduce biogas production, but I have never
seen this reported. So, the answer is that the relationship between
pressure and biogas production seems rather complicated.<br>
<br>
Regards,<br>
<br>
David<br>
<br>
On 19/11/2010 22:31, David wrote:
<blockquote cite="mid:4CE6FAB0.7060603@h4c.org" type="cite">
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<div class="moz-text-html" lang="x-western"> <font face="Georgia"><br>
Dear friends,<br>
<br>
Excellent discussion lately. I was particularly impressed by
the article by Dr. Fulford about biogas burners. Best
information I've seen on this subject.<br>
<br>
<br>
I wanted to ask two questions just now, and I'll therefore be
sending another message to the list quite soon. Separating the
questions will allow the development of two different threads,
if there is much discussion about either question.<br>
<br>
The first question concerns pressure. Obviously AD is able to
generate biogas at pressure. Any vessel 33 feet deep will have
two atmospheres of pressure at the bottom, and there are a
number of digesters which are at least that deep. The gas
produced in deeper vessels-- if they are not stirred-- tends
to have a bit less carbon dioxide, because that gas dissolves
rather easily as compared with methane.<br>
<br>
But I am not familiar with any work that has been done
regarding the relationship between pressure and the rate of
production. That is, one would expect that at increasing
pressure, less biogas (measured, of course, at STP) would be
produced. But what is the relationship? How dramatic is the
reduction in the rate of production with deeper vessels?<br>
<br>
<br>
d.<br>
</font><br>
<div class="moz-signature">-- <br>
<div style=""><span style="font-size: 110%;">David William
House<br>
</span>
<div style="padding-left: 3em; font-size: 80%;">"The
Complete Biogas Handbook" <code><a moz-do-not-send="true">www.completebiogas.com</a></code><br>
<em>Vahid Biogas</em>, an alternative energy consultancy <code><a
moz-do-not-send="true">www.vahidbiogas.com</a><br>
<br>
</code></div>
<div style="padding-left: 2em;">"Make no search for water.
But find thirst,<br>
And water from the very ground will burst."
<div style="padding-left: 2em; font-size: 80%;">(Rumi, a
Persian mystic poet, quoted in <em>Delight of Hearts</em>,
p. 77) <br>
<br>
<a moz-do-not-send="true" class="moz-txt-link-freetext"
href="http://bahai.us/">http://bahai.us/</a></div>
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_______________________________________________
Digestion mailing list
to Send a Message to the list, use the email address
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to UNSUBSCRIBE or Change your List Settings use the web page
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for more information about digestion, see
Beginner's Guide to Biogas
<a class="moz-txt-link-freetext" href="http://www.adelaide.edu.au/biogas/">http://www.adelaide.edu.au/biogas/</a>
and the Biogas Wiki <a class="moz-txt-link-freetext" href="http://biogas.wikispaces.com/">http://biogas.wikispaces.com/</a>
</pre>
</blockquote>
<br>
<div class="moz-signature">-- <br>
<p align="center">********************************************************************<br>
Dr David Fulford CEnv MEI, 15, Brandon Ave, Woodley, Reading RG5
4PU <br>
<a href="mailto:d.j.fulford@btinternet.com">d.j.fulford@btinternet.com</a>,
Tel: +44(0)118 326 9779 Mob: +44(0)7746 806401 <br>
Kingdom Bioenergy Ltd, <a href="http://www.kingdombio.com">www.kingdombio.com</a>,
<a href="mailto:davidf@kindombio.com">davidf@kindombio.com</a> </p>
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