[Digestion] Digester Heating by a thermosiphon system

govinda devkota govindadevkota at yahoo.com
Tue Nov 20 01:14:59 PST 2012

Hi Paul,

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 like Nepal, which are summarized below.
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
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
Composting on top of the dome
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.
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).
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.
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.

Use of waste heat from power generation

heat exchanger was applied in a 500 cf. steel drum plant at Bhutaha of
Nawalparasi district Nepal, the gas of which was used in 7 HP engine for
agro-processing on experimental basis. The gas production was increased by
about 37 percent in winter, assuming that only 50 percent gas could be produced
in a similar plant of the same capacity without having heat exchanger. Similar
experiments were conducted in R&D unit of GGC at Butwal as well.

engine installed was of water cooling system. One heat exchanger assembly is
made up of concentric GI pipes connecting between the engine and heat exhaust
silencer and uses waste heat from the exhaust gas to heat water. The other part
of the heat exchanger was placed one foot above the digester base and heats the
slurry in the digester. A valve can adjust the amount of cooling water flowing
through the engine. Maximum heat was generated when the water was flowing at
the rate of 2 lit. /min. as the flow rate increased the temperature decreased
and vice versa.

Govinda P. Devkota

 From: David <david at h4c.org>
To: For Discussion of Anaerobic Digestion <digestion at lists.bioenergylists.org> 
Sent: Saturday, November 17, 2012 8:46 AM
Subject: Re: [Digestion] Digester Heating by a thermosiphon system


On 11/15/2012 12:03 AM, Paul Muthui wrote:

Good Day All,
>I am looking into ways if heating my biogas digester to increase
      specific gas yield. I have thought of using a thermosiphon,
      whereby I heat my primary liquid and then pass it through coils
      inside the digester.
>Has anyone done this before and what would be the expected
      efficiency of such? Is it an idea worth pursuing? 
There have been a number of efforts at solar heating digesters, such
    as the BARC (Bhabha Atomic Research Centre) digester (picture here), as mentioned on Dr. Fulford's site (here). That digester appears to couple passive solar hot water with an active system for circulating the water in the digesters. BARC has a design which it licenses to other many companies, but it appears to be a design that emphasizes effluent safety rather than biogas production. 

Jaime Martí Herrero in Bolivia has built a lot of passively solar
    heated digesters in the high cold altiplano, information about which
    can be found here. (Information about his efforts and designs is available in a very good publication-- in Spanish-- found here.) There is a good deal of excellent information about low-cost, found materials solar greenhouses 

There are some ideas mentioned in the article "Biogas production in
    climates with long cold winters" (here) for using solar heating, and a small but growing number of other articles.

As Paul indicated, the implication of having a completely passive thermosiphon system is that whatever you want to heat-- in this case, ultimately, the contents of the digester-- will need to be above the collector. Thus to have a truly passive heating system, either you would put the digester at the top of a south-facing slope, or raise it relative to the collector in some other way. It should be possible in the right circumstances to produce a concentrating collector that injects steam into the digester, but it would require a clever design to make such a heater largely or completely passive, if indeed such a thing is possible. 

Finally, you may wish to find out more about the Larkin thermosiphon
    (or thermosyphon) design, intended to avoid reverse thermosiphon
    (night sky cooling) without the use of valves, i.e. in a completely
    passive manner.

As far as the efficiency of any of these approaches, that is a very
    complex question, and in any case cannot be answered without
    specifying some further parameters. Are Jaime's passive and very
    low-tech designs "efficient"? Surely it depends on what is of
    greatest value (which we then efficiently conserve), which in turn
    is a matter of culture, situation, and similar circumstances and
    idiosyncratic choices.

But ultimately yes, solar heating for increased biogas is worth



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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