Dear GF,
On 12/30/2010 10:49 PM, GF wrote:
-----Original Message-----
From: GF<[email protected]>
To: [email protected]
[...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?
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 /marginal increase/ in energy produced (if any) which
would be realized by the higher pressure.
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.
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.)
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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