[Digestion] Small Scale Digester Heating

Thu Oct 4 09:25:53 CDT 2012

Dear Biogas List,

Thank you all for the advice and encouragement. It can be done which is good news. 

Mr. Karve, could you explain your concept a little bit more? Is it passive or active (requires a pump)? and what is the name of the sponge rubber insulation material?

Manuel, you mention that electricity is required for control. It seems that a battery could be used for control and last for several years. Is this true? At most you could use a tiny solar panel to charge a battery for all of the control power you need. How much stirring is actually required? If you are heating the bottom of the digester then there should be some convection in the tank just from thermal differences. Even so, I think I have seen that stirring only increases gas production by around 20%.

Peter, how much power did your system require for pumping? Is there a way to do away with the pump? How much did a cloudy day affect the heating system? How much power was required to run your control system (if any)? It is good to hear that your mesophilic digester was able to produce better fertilizer than the ambient temperature digester. I have seen a website for a company in Australia called biobowser. Is that the same company? It looks like a beautiful technology all packaged into one easy (?) to install modular unit. Is there a reason why their smallest unit is for 100kg per day? Could their system be scaled down?

Paul, to better understand the rough spreadsheet you made, you found that one twelfth of the additional biogas was used for heating so the net effect was a significant increase in biogas to the user. Have you had bad experience with animals in your insulation? Would you recommend avoiding some kinds of insulation? Are there some insulations that a good for underground installations?

One of the ideas I proposed was to bubble a small amount of air through the digester so that there was a slight aerobic reaction that generates heat and keeps the digester warm. Is there any validity to this idea? Has it ever been done? Or would the amount of oxygen required to raise the temperature of a digester by 17C degrees kill the methanogenic bacteria?

All the best wishes,

Kyle

I built a 1000L small scale digester about 10 years ago which incorporated chamber heating via solar hot water circulation as well as a biogas fired heat-exchanger as a back-up for cold nights.
The system could readily achieve blood temperature and maintain this with minimal management and a small amount of electricity to drive the circulation pump intermittantly.
A solar powered pump would have been sufficient.
I was not so interested in the biogas component at the time as the unit was readily able to produce 2 cubic meters of up to 90% methane per day and I had no other use for this gas other than to flare it off.
I used the gas volume as an indicator of digestation reactivity and a determinant for retention time.
My interest was in the spent digestate, particularly the colloidal nutrient contained within it. 
The bio-gas was a bonus, the fertilizer was beyond compare.
I split the digestate into colloidal liquid and pelletized the remainder.
The colloids were sprayed as a foliar feeder and the pellets fed the soil and plant roots. Carbon content of the soil was raised considerably. Plant growth was exceptional and the fertilizer would enable the growth of vegetation on long term bare ground (at least 20 years barren) which would not previously support any growth at all. The full-scale plant (38,000L) was reproduced in India and achieved 22% better output results than I managed with the prototype. They called it the BioBowser.
Peter Allison. 

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

Dear biogas technologists,
you can heat a biogas digester using a tube which forms a circle at the bottom of the biogas digester. Circulate hot water through this tube. You can use a relatively small wood burning stove to heat a water pot, from which the hot water is tapped for circulation through the tube. For a small biogas plant, this is quite doable. Insulate the digester with sponge rubber blanket, so that it remains warm. You can keep the biogas digester indoors, with the water heating stove outside the house.
Yours
A.D.Karve

not only they have been tried but are used from many many years ago
your numbers are real but as you point it these kind of digester are
of bigger costs because of the technology involved

the most important point is the insulation used, bigger insulation
thickness demands less energy but costs are higher...
you can heat it with solar heater and the reminder energy if needed with biogas
but you cannot provide this kind of digester to a small farm house
without electric supply, for example for control and mixing
porpuses.....
these "high tech" digester are suitable for certain houses /locations/
with all basic requirement satisfied.......

i work with these kind of digesters .....
if i can help you contact me

SKYPE manuel.jimenezt

Manuel

On Oct 4, 2012, at 5:37 AM, Paul Harris wrote:

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 aerobic 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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