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Dear GF,<br>
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On 12/30/2010 10:49 PM, GF wrote:
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cite="mid:8CD76CDFFD681E4-1318-B88@webmail-stg-m02.sysops.aol.com"
type="cite">
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-----Original Message-----
From: GF <a class="moz-txt-link-rfc2396E" href="mailto:gfwhell@aol.com"><gfwhell@aol.com></a>
To: <a class="moz-txt-link-abbreviated" href="mailto:gasification@lists.bioenergylists.org">gasification@lists.bioenergylists.org</a>
[...Regarding biogas] production,... I've always wanted to ask:
Is it possible to get more gas production from "Microbes" if they are working in a pressurized environment, say , 20 atmospheres?
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The short answer is: very likely not, or certainly not where
reasonable limits are placed. Consider in the first instance
reasonable limits on research. For example, there may be candidate
organisms in an around deep oceanic thermal vents, but who will
gather them, culture the separate species (if that can be done; some
organisms require complex symbioses) and do enough [expensive]
research to learn enough about their requirements to make their
cultivation and use practical? And secondly, one must consider
reasonable limits on energy inputs. For example, how much energy
would be required to achieve and sustain 20 atm pressure? Not that
such energy must be subtracted not from the energy produced, really,
but rather from the <i>marginal increase</i> in energy produced (if
any) which would be realized by the higher pressure.<br>
<br>
Regardless, even more generally, I would tend to doubt that
increased pressures in that range would offer increased production.
Methanogens can produce methane under relatively high pressures--
where a digester is 33+ ft deep, 2 atm absolute is obtained, and
such digesters appear to produce biogas at a rate similar to those
operated under the same conditions, fed the same materials, and
having a comparable volume, but which are not as deep. Even so,
methanogens are fragile and slow-growing because the energy
available from anaerobic metabolism is relatively limited: but most
especially they are sensitive to lower pH. At higher pressures, more
CO2 is dissolved in the slurry, and this can have an impact on pH,
depending on the buffer system present in the digester. The whole
chemical/biochemical/ecological picture would no doubt be quite
complex, but I know of no reason to assume that such high pressures
would be of any benefit to the methanogens. Indeed, while it is
difficult to find research that shows much about the correlation
between moderately differential pressures and production, what
little there is tends to point in the opposite direction. That is,
digestion under a vacuum has been shown in some studies to increase
production. <br>
<br>
Again, however, the more energy required to run the digester, the
less energetic sense it may make. (The economics may of course
provide a different picture.) And not to speak heresy on this list,
but in some studies, and in terms of net energy per unit land,
biogas has been shown to out-produce ethanol and biodiesel by as
much as 8 times, and (for some feedstocks) to produce the near
equivalent to the energy provided by combustion et al, with lower
GHGs. (No doubt someone will point out to me that GHG emissions
depend on the design of the gasifier or what-have-you, and of course
I would agree. At the same time, a reduction in GHG emissions will
likely be accompanied by a reduction in usable net energy, and of
course you would agree.)<br>
<br>
<br>
<br>
d.<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 href="www.completebiogas.com">www.completebiogas.com</a></code><br>
<em>Vahid Biogas</em>, an alternative energy consultancy <code><a
href="www.vahidbiogas.com">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 class="moz-txt-link-freetext" href="http://bahai.us/">http://bahai.us/</a></div>
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