[Digestion] Small Scale Digester Heating

Thu Oct 4 22:08:16 CDT 2012

Dear Biogas List,

There
are various methods used by several researchers in increasing the digester
temperature and ultimately the gas production especially in winter months in
the developing countries, which are summarized below.

Insulation
Heating
requirements or heat losses can be minimized by using insulation. Insulation is
very straightforward; it comes down to the selection of construction material
with a low heat transfer coefficient which is both affordable and available to
retain more heat. In conclusion, the extent of which biogas plants are exposed
to the low ambient temperature in winter time needs to be limited, and a
digester under the ground surface is preferable over a digester on top of the
ground. In addition, insulation needs to be applied on the upper part of the
digester.

Heat losses
happen when heat is transferred from a warm body to a cold body. In our case, heat
will transfer from the digester to the colder surroundings (soil and ambient
air) in the winter. This flux of heat needs to be minimized to retain the heat
in the digester. To minimize heat losses, we take the following factors into
account: a. Surface volume ratio, b. Insulation, c. Location of digester.

Composting on top of the dome

One
of the most important factors affecting biogas is temperature. When slurry
temperature is low, the gas production is greatly reduced. A compost pile can
generate significant amount of heat from decomposition of organic materials.
Decomposition can be accelerated by the addition of water with effluent from
the plant. The effect of compost for heat generation greatly varies with the
height of the compost pile and the time it takes to decompose. The height
should be not less than a meter or so. The compost should be piled on the top
of the dome for heat generation. The compost is made with straw, grasses, and
effluent from the plant itself. After three to four months the compost is ready
for feeding flora. Based on the study carried out in a 10 cum plant installed
in R &D office of GGC office at Butwal and a control plant in Kalikanagar
in 1982 the slurry temperature variation was significant (Devkota GP 1984).The
slurry temperature inside the digester of both plants were measured and it was
found that on an average the temperature on the compost plant was found 2.03 °C  higher than the control. The composting
technology can be applied to each dome plant, and it can increase inside slurry
temperature by 2 °C which results in 22.3% gas production increment. The same
experiment was conducted in Kathmandu and the gas production was about 52%
higher than that of control. It shows that the effect of composting is great in
colder areas such as Kathmandu than in (Terai) Butwal of Nepal.

The
compost pile should be kept moist by watering; a slight concave at top prevents
runoff of water. Decomposition can be accelerated by adding manure or effluent
from the plant. Composting can be facilitated by inoculation with lignin
degrading fungi such as Trichoderma viride and the use of cellular enzymes.

Solar radiation
The slurry temperature can be reduced if cold water is
used to mix the input. The mixing water can be left in the sun for a few hours
in a black painted drum or else the slurry can be solar heated after it is
mixed. If the slurry is held in a shallow mixing pit (about 7 cm deep), which
is covered by a plastic sheet during the hottest part of the day, its
temperature can rise as much as 9 °C (4.5 °C on a cloudy day) (Fulford D 1986).

Passive
solar heating of influent at the inlet can be used as an inexpensive means of
heat input into the plant. Since radiation can only penetrate a small distance
in slurry, the depth of slurry in the inlet pit should be shallow. This is
achieved by building shallower inlet pit of larger surface area. To prevent
heat losses by forced convection and back radiation the inlet pit should be
covered by transparent materials such as plastic or glass. Plastic is preferred
since it is easier and cheaper to replace when broken.
Experiments
were performed in Butwal, Nepal to determine the effect of plastic cover, the
optimal retention time of slurry in the inlet pit and the depth of penetration
of solar radiation. Having determined the later, the inlets could be modified
by making them shallower but with a larger surface area.

Hope this will help.

With best wishes,

Govinda

________________________________
 From: Takamoto <kyle at takamotobiogas.com>
To: For Discussion of Anaerobic Digestion <digestion at lists.bioenergylists.org> 
Cc: Paul Muthui <paul at takamotobiogas.com> 
Sent: Thursday, October 4, 2012 8:10 PM
Subject: Re: [Digestion] Small Scale Digester Heating

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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to Send a Message to the list, use the email address
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to UNSUBSCRIBE or Change your List Settings use the web page
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for more information about digestion, see
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http://www.adelaide.edu.au/biogas/
and the Biogas Wiki http://biogas.wikispaces.com/
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