[Gasification] Fwd: Syngas on Wiki_

David david at h4c.org
Tue Jan 4 15:37:05 CST 2011

Dear GF,

On 12/30/2010 10:49 PM, GF wrote:
> -----Original Message-----
> From: GF<gfwhell at aol.com>
> To: gasification at lists.bioenergylists.org
> [...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.)

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)

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