[Digestion] Small Scale Digester Heating

[Paul Harris]

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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