This document discusses biomass energy and biogas production. It describes photosynthesis, biomass fuels, and factors that affect biogas generation such as pH, temperature, loading rate, and retention time. It also discusses types of biogas plants and the advantages of anaerobic digestion which include producing a stable sludge and reducing pathogens. Biogas can be used to reduce coal consumption and air pollution. The document classifies biogas plants as either continuous or batch systems and describes common dome and drum reactor designs.

1. UNIT - 6
BIOMASS ENERGY: Introduction, Photosynthesis
process, Biomass fuels, Biomass conversion
technologies, Urban waste to Energy Conversion,
Biomass Gasification, Biomass to Ethanol Production,
Biogas production from waste biomass, factors
affecting biogas generation, types of biogas plants –
KVIC
and Janata model; Biomass program in India. 6 Hours

2. • Biomass is organic matter produced by
plants both terrestrial and aquatic.
Solid mass
Liquid
Gas

3. Photosynthesis
• Photosynthesis is a process used by plants and other
organisms to convert light energy, normally from the light,
into chemical energy
• This chemical energy is stored in t carbohydrate
molecules, molecules, such as sugars, which are
synthesized from carbon dioxide and water hence the name
photosynthesis,
• 2n CO2 + 4n H2O + photons → 2(CH2O)n + 2n O2 + 2n
H2O carbon dioxide + water + light energy →
carbohydrate + oxygen + water

4. Factors affecting Bio digestion
• pH or hydrogen ion concentration.
• Temperature.
• Total solid content of the feed material.
• Loading rate.
• Seeding.
• Uniform feeding.
• Diameter to depth ratio.
• C to N2 ratio.
• Nutrients.
• Mixing or stirring or agitation of the content of the digester.
• Retention time or rate of feeding.
• Type of feed stock .
• Toxicity due to end product.
• Pressure.
• Acid accumulation inside the digester.

5. 1) pH or hydrogen ion concentration
• pH of the slurry changes at various stages of
the digestion. In the initial acid formation
stage in the fermentation process, pH is
around 6 or less and much of CO 2 is given
off. In the latter 2-3 weeks time, pH
increases as the voltaic acid & N 2
compounds are digested & CH 4 is
produced.
• To maintain a constant supply of gas, it is
necessary to maintain a suitable pH range in

6. • The digester is usually buffered if the pH is
maintained b/n 6.5 to 7.5. In this pH range, the
micro organisms will be very active and bio
digestion will be very efficient. If the pH range is
4 to 6, it is acidic. b/n 9 & 10- alkaline. Both these
are determinental to methanogenic
• ( methane production) organisms. Any sudden
upset in pH by the addition of any material which
is likely to cause an un balance in the bacterial
population

7. 2) Temp
• Best at 35 -380 C. The fall in production
• 20 0C. and stops at 10 0C. Other weather
conditions like wind velocity, sun shine and
type of feed also influence temp.

8. 3) Total solid content
• The cow dung is usually mixed with water
(1:1) to bring solid content to 8-10%. Raw
cow dung contains 80-82% moisture. The
balance 18-20% is solid. Adjustment of this
helps in increasing the rate of gas
formation. Also decides th quantity of
mixing of other types of stocks( Crop
residues, weeds, plants etc).

9. 4) Loading rate
• Amount of raw material usually in kgs fed to the
digester per day per volume. Most municipal
sewage treatment at a loading rate of 0.5 to 1.6 Kg
/day/3.
• If a digester is loaded with too much raw material
at a time, acids will accumulate and fermentation
will stop.
• The main advantage of higher loading rate is that
by stuffing a lot into a little space , the size &
therefore the cost of the digester can be reduced

10. 5) Seeding
• Although the bacteria required for acid
fermaentation and methane fermentation are
present in the cow dung , their numbers are not
large.
• While the acid formers proliferate fast and
increase in numbers , the methane formers
reproduce and multiply slowly.
• It is reqd to increase the no. of methane formers
by artificial seeding with a digested sludge that is
rich in methane formers.
• Beyond a certain seed concentration , the gas
production will decrease, due to reduction of raw
material ( cow dung) solids fed to the digester.

11. 6) Uniform feeding
• Reqd so that micro organisms are kept in a
relatively constant organic solid
concentration at all time.
• Therefore the digester must be fed at the
same time everyday with a balanced feed of
the same quantity and quality.

12. Carbon nitrogen ratio of the input
material
• Besides carbon the quantity of N2 present in the
wastes is a critical factor in production of bio gas.
All living organisms require N2 to form their cell
proteins from a biological view point, a digester is
a culture of C. and N2
• (Protein, ammonia nitrates etc) are the main food
of anaerobic bacteria. C is used for energy & N2
for building the cell structure.

13. • C/N ratio is 30 will permit digestion to proceed at an
optimum rate. When there is too much carbon in the raw
wastes, N2 will be used up first and carbon left over. This
will make the digester slow down & come to a stop. In
this case the bacteria will not be able to use all the carbon
present and the breaking down of the organic matter will
be inefficient.
• On the hand if there is too much N2, the carbon soon
becomes exhausted & fermentation stops. The N2 left over
will combine with H2 to form ammonia. This can kill or
inhibit the growth of bacteria specially the methane
producers.
• Optimum C/ N2 ratio that best suits for maximum micro
biological activity is 30 : 1.

14. Diameter to depth ratio
• Range b/n 0.66 to 1.
• Digester of 16 feet depth & 4 or 5 feet
diameter were reported to work
satisfactorily.
• Since, in a simple unstirred single stage
digester the temp varies at diff depths. The
most actively digesting sludge is in the
lower half of the digester and this is less
affected by change in night & day temp.

15. Nutrients
• The major nutrients reqd by the bacteria in
the digester are C, H2, O2, P &S of these
nutrients N2 & P are always short in supply
& therefore to maintain proper balance of
nutrients an extra raw material rich in P (
night soil) & N2( chopped leguminous
plants ) should be added along with the cow
dung to obtain maximum production of gas

16. Mixing or stirring or agitation of
the content of the digester
• Since bacteria in the digester have very
limited reach to their food, it is necessary
that the slurry is properly mixed and
bacteria get their food supply.
• It is found that slight mixing improves the
fermentation; however a violent slurry
agitation retards the digestion.

17. Retention time or rate of feeding
• The period of retention of the material for bio gas
generation , inside the digester is “ Retention
period”. This period will depend on the type of
feed stocks and the temperature. Normal value is
b/nt30 to 45 days and in some cases 60 days.
• By regulating the daily feed volume, the retention
time can be controlled.
• i) Cow & buffalo dung …50 days.
• ii) Pig dung 20
• iii) Poultry droppings 20 days

18. Type of feed stocks
• When feedstock is woody or containing more of
lignin, then bio digestion becomes difficult.
• Cow & buffalo dung, human excreta, poultry
droppings, pig dung, waste materials of plants,
cobs etc can all be used as feed stocks.
• To obtain an efficient bio digestion, these feed
stocks are combined in proportions. Pre digestion
and finely chopping will be helpful in the case of
some materials. Animal wastes are pre digested.
Plant wastes do not need pre digestion. Excessive
plant material may choke the digester.

19. Toxicity
• The digester slurry if allowed to remain in the
digester beyond a certain time becomes toxic to
the micro organisms and might cause fall in the
fermentation rate.
• Bio logical systems needs some trace elements
like calcium, magnesium, potassium etc.
Production of bio gas is reduced when these
elements are present in higher concentrations.
Synthetic materials are toxic to methanogenic
bacteria. Pesticides and disinfectants from farms
can kill bacteria.

20. Pressure
• Pressure on the surface of slurry also effect
the fermentation. Better at lower pressure.

21. Acid accumulation inside the
digester
• Intermediate products like acetic propionic butyric acids
are produced, during the process of biodigestion. This
causes a decrease of pH , especially when fresh feed
material is added in large amounts.
• These acid may be converted into methane by addition of
neem cake. However the buffering nature of the digester
should not be upset.
• Cow dung operated plant remain well buffered and the
problem of acid accumulation does not arise in the
continuous fermenting systems. Acid accumulation is
usually occurred in batch digestion systems.

22. Advantages of anaerobic digestion:
• Calorific value of gas is high.
• No sludge production- The conversion of organic matter
to methane & CO 2 results in a smaller quantity of
sludge.
• Stable sludge- In the case of municipal digestion the
main reason for their installation was to produce a non-
putrescable and in offensive sludge and in ,many cases
only a proportion of the gas produced was utilized.
• Low running cost- There is no alteration in the anaerobic
treatment naturally in this digestion.
• Low odour- Since the system is enclosed. Compounds
which are responsible for odour are broken down during
digestion. Only slight odour of H 2S is present.

23. • Stability- A well adapted anaerobic sludge can be
presented un fed for a considerable period of time with
out appreciable deterioration.
• Pathogen reduction-Work has shown that passage of the
effluent through the digester reduces the number of
pathogens present , so reducing subsequent disposal
problems.
• Value of sludge- The cases where aerobic sludges are
treated anaerobically the resultant sludge has a higher
nitrogen content giving it increasing value as a fertilizer.
It has also been reported that the sludge acts as a soil
conditioner.
• Low nutrient requirement- As a consequence of low
production of the bacterial solids the nutrient
requirement is also low.

24. • In addition using bio gas in industries will
curtail the consumption of coal.
• If bio gas is used in boilers, it will lessen
the air pollution.

25. Classification of Bio gas Plants
• Continuous & Batch type (As per process)
• The Dome & Drum types
• Different variations in the drum types

26. Continuous & Batch type:
• Continuous Type: There is a single digester in
which raw material are charged regularly and the
process goes on with out interruption except for
repair & cleaning etc.
• In this case raw material is self buffered like cow
dung. Or otherwise thoroughly mixed with
digesting mass where dilution prevents souring
and the bio gas production is maintained.
• The continuous process may be completed in
• i) Single stage or ii) Two stage.

27. Single stage Process

28. • Single digester is reqd.
• This chamber is regularly fed with raw
materials while the spent residues keeps
moving out.
• Serious problems are encountered with
agriculture residues when fermented in a
single stage continuous process

29. Double stage process

30. • The acidogenic stage and methanogenic stages are
physically separated into two chambers.
• Thus the first stage of acid production is carried out
in a separate chamber and only the diluted acids
are fed into the second chamber where
bio methanation takes place.
• The biogas can be collected from the second
chamber. Considering the problems encountered in
fermenting fibrous plant the two stage process may
offer higher potential of success.
• However appropriate technology suiting to rural
area needed, this type is developed.

31. Features of Continuous type
• 1) Produce gas continuously.
• 2) Requires small digestion chambers.
• 3) Needs lesser period for digestion.
• 4) It has low problems compared to batch
type & easier in operation.

32. The Batch Plant
• The feeding is b/n intervals ,the plant is emptied once the
process of digestion is complete.
• In this type, a batch of digesters are charged along with
lime, urea etc and allowed to produce gas for 40 -50 days.
These are charged and emptied one by one in synchronous
manner which maintains a regular supply of the gas
through a common gas holder.
• Some times the freshly charged digester is aerated for few
days after which it is closed to atmosphere.
• The bio gas supply may be utilized after 8 – 10 days. Such
a plant is expensive to install and unless operated on large
scale it would not be economical.

33. Features:
• i) The gas production in it , is intermittent, depending
upon the clearing of digester.
• ii) It needs several digesters or chambers for continuous
gas production, these are fed alternatively.
• iii) batch type are good for long fibrous materials.
• iv) This plant needs addition of fermented slurry to start
the digestion process. There may be a direct change to the
acid phase in absence of fermented slurry, which affects
formation of methane.
• v) This plant is expensive & has problems comparatively;
the continuous plant will have less problems & will be
easy for operation.

34. The Dome & Drum types
• i) The floating gas holder plant
• ii) Fixed dome digester.
• Floating gas holder is KVIC. Fixed dome- Chinese plant.
There are different shapes in both the designs, cylindrical,
rectangular. Spherical etc. Again, the digester may be
horizontal or vertical .
• They can be constructed above or underneath the ground.
The floating gas holder digester may be masonry
construction with gas holder made of MS plates.
• The gas holder is separated from the digester. Rusting of
the gas holder as well as cost of the gas holder are the main
draw backs of the system.

35. In the fixed dome digester, gas holder and the digester are
combined. The fixed dome is best suited for batch process
especially when daily feeding is adopted in small quantities.
The fixed dome digester is usually built below ground level
and is suitable for cooler regions.
Local materials can be used in this construction. The
pressure inside the digester varies as the gas is collected.
This is not causing any serious problems in small plants.

36. Different variations in the drum
type.
• Types:
• i) water seal
ii) Without water seal
• Water sealing makes the plant completely anaerobic and
corrosion of the gas holder drum is also reduced.
• The other variations are of materials used both in
construction of the digester and the gas holder. Bricks and
stones are the commonly used materials. Ferro cement
rings are also used in the construction of digester, which
are best suited for clays soils and sandy tracks.
• Gas holders are also manufactured out of ferro cement, as
MS sheets get corroded.
• Polyethylene is also used in the construction of gas holder.
The latest design uses fibre glass reinforced concrete.

37. • The horizontal plants are suited for high ground
water level or rocky glass.
• These are not recommended when retention period
is 30 days.
• Cylindrical shape of the digester is preferred
because cylinder has no corners and so that there
will be no chances of cracks due to faulty
construction.
• This shape also needs smaller surface area per unit
volume, which reduces heat losses. Scum
formation is reduced by rotating gas holder in
digester.

38. Advantages & Disadvantages of
Floating Drum Plant
• Advantages: It has less scum troubles because solids are
constantly submerged.
• No separate pressure equalizing device needed when fresh
waste is added to the tank or digested slurry is with drawn.
• In it, the danger of mixing O2 with the gas to form an
explosive mixture is minimized.
• Higher gas production per m3 of the digester volume is
achieved.
• Floating drum has welded braces, which help in breaking
the scum ( floating matter) by rotation.
• No problem of gas leakage.
• Constant gas pressure

39. Disadvantages:
• 1) Higher cost as its construction is dependent on
steal & cement.
• 2) Heat is lost through the metal holder, hence it
troubles in colder regions & periods.
• 3) Gas holder requires painting once or twice a
year depending on the humidity of the location.
• 4) Flexible pipe joining the gas holder to the main
gas pipe requires maintenance, as it is damaged by
UV rays in the sun. It may also be twisted, with
the rotation of the drum for mixing or scum
removal.

40. Advantages & Disadvantages of
Fixed Dome Type Plant
• Advantages: 1) It has low cost compared to
floating drum type, as it uses only cement & no
steel.
• 2) No corrosion trouble.
• 3) Heat insulation is better as construction is
beneath the ground. Temp will be constant.
• 4) Cattle & human excreta and long fibrous stalks
can be fed.
• 5) No maintenance.

41. Disadvantages:
• 1) Needs services of skilled masons.
• 2) Gas production per m3 of the digester
volume is also less.
• 3) Scum formation is a problem as no
stirring arrangement.
• 4) It has variable gas pressure.

42. Types of Bio gas Plants
• Floating gas holder & Fixed dome Digester:
In floating gas holder plant , the gas holder is
separate from the digester. In fixed holder gas
holder & digester are combined. KVIC (Khadi
Village Industries Commission) is floating gas
holder & Janta model (China) is fixed dome
type.
• KVIC plant is steel drum type or floating gas
holder design , in which the digestion takes
place in a masonry well and the drum floats as
the gas collects & is taken out from the top

44. The Janta model ( Chinese) or
fixed Dome digester

45. • Is a drum less type similar in construction to
KVIC model except that drum is replaced by a
fixed dome roof of masonry construction.
• Gas holder is MS. Drum in the KVIC model is
costliest component and its life is less.
• The dome roof of Janta model requires pecialized
design and skilled masonry construction. A poorly
constructed roof generally leads to leakage from
top and junction of the roof with digester wall,
thereby causing drop in the gas yield.

46. • In addition to the cost & construction material
problems, there are constructional problems which
the farmers or beneficiaries face.
• The construction of bio gas plants specially in
Janta type needs the services of skilled masons. It
is observed that plants constructed by unskilled
masons or untrained workers have structurally
failed or unable to retain dung slurry, gas or even
both while failure of such plants adversely effect
plant owners.
• Besides constructional problems, there are some
operational & maintenance problems also.

47. Flexible Bag Digester

48. • The Digester is made of plastic material and
can be easily installed. Short life of material
due to the effect of UV rays is a main draw
back.

49. Nepal ; Taper digester with
floating Gas holder
• Suitable for high water table. The digester
diameter below the gas holder is increased
so that total depth can be reduced