G'day Takamoto,
You have to look at the tradeoff between cost of a larger digester at
ambient temperature (where you get all the biogas to use!) and the cost
(capital, operational and maintenance!) of installing insulation (loved
by birds and mice?) and a heating system on a smaller digester. Of
course you also have to consider the energy cost of heating the digester
- if you want more gas do you get it by using some of the gas to heat
the digester?
Based on a steady state model (see http://biowattsonline.com/ for a web
version) and a 4 cubic metre digester you should be able to go from 3
beef cattle to 11 beef cattle, so would get about 4 times the gas. My
simple Excel model shows the heater about halves the cost of biogas but
nearly doubles the digester capital cost and uses about 1/12 of the
increased gas production (about half of the ambient gas production -
most of the gas is used to heat the effluent if insulation is 50 mm
thick) - I used 20 ambient and 35 digester temperature.
Happy Digesting,
HOOROO
Mr Paul Harris BEng (Ag) (Melbourne)
Visitor at The University of Adelaide
On 3/10/2012 11:05 PM, Takamoto wrote:
Dear Biogas List,
I have been thinking about the biggest hurdles to producing more gas
from small scale biogas systems (4 cubic meters to 12 cubic meters)
and by far the biggest barrier is heat. From the literature I have
read it seems that if you increase the temperature of the digester
from about 18C (the temperature of our digesters) to 37C you can
nearly double the gas yield per unit of input and nearly halve the
retention time which would reduce the capital costs.
Does anyone know of tests that have been done or ideas that have been
put forth to heat small scale digesters in a controlled manner? (For
the moment assume that such a process could be managed on many
disparate, small scale biogas systems. That is the next challenge.)
The processes I was thinking of were 1.) to heat the biogas system
with biogas from the system itself or 2.) to bubble a very slight
amount of air through the digester so that there was a slight
anaerobic reaction that would produce heat and warm the digester. Or
3.) you could use sunlight to warm the digester if you can warm the
digester and not the gas holder as warming the gas holder will only
cause the gas to expand and no heat will be transferred to the slurry.
These methods are probably most applicable to fixed dome and floating
drum.
Have either of these ideas been tried? Are there other ideas out there?
Cheers,
Kyle
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for more information about digestion, see
Beginner's Guide to Biogas
http://www.adelaide.edu.au/biogas/
and the Biogas Wiki http://biogas.wikispaces.com/
