2. What is Biogas ?
◦ Biogas is a type of renewable energy that is produced by breaking down
organic matter, such as animal manure, food waste or plant material, in the
absence of oxygen. (Anaerobic Digestion)
◦ AD typically takes place in environments such as swamps, marshes, and the
digestive tracts of animals.
◦ During anaerobic digestion, the microorganisms consume the organic matter
and produce a mixture of gases (BIOGAS) including methane and carbon
dioxide. This mixture can be used as a fuel for heating, cooking, or generating
electricity.
◦ It is clean and sustainable source of energy that can help reduce
greenhouse gas emissions and dependence on fossil fuels.
◦ It also provides a way to manage organic waste, reducing the need for landfills
and their associated environmental problems.
3.
4. In biogas production systems, the process is carried out in an enclosed container called a bioreactor or
digester,
where organic waste is added and then sealed to create an oxygen-free environment.
The waste is then left to decompose, with the help of naturally occurring microorganisms or inoculated
bacteria, which break down the organic matter and produce biogas.
5. The anaerobic digestion process occurs in four
stages:
A. Hydrolysis: In this stage, large molecules of organic matter, such as proteins,
fats, and carbohydrates, are broken down into smaller molecules by enzymes
produced by the microorganisms.
B. Acidogenesis: In this stage, the smaller molecules are converted into organic
acids, such as acetic acid, butyric acid, and propionic acid, by acid-
forming bacteria.
C. Acetogenesis: In this stage, acetogenic bacteria convert the organic acids
produced in the previous stage into hydrogen gas and carbon dioxide.
D. Methanogenesis: In this final stage, methanogenic bacteria convert the acetic
acid and hydrogen gas produced in the previous stage into methane gas,
which is the primary component of biogas.
The biogas produced can then be collected and used as a source of renewable
energy. The leftover material, called digestate, can be used as a nutrient-rich
fertilizer for plants.
6. Constituents of Biogas
Biogas is a mixture of different components and the composition varies depending upon the characteristics of
feed materials, amount of degradation, etc.
The energy content of biogas depends on the amount of methane it contains.
Name of the gas Composition in biogas (%)
Methane (CH4) 50-70
Carbon dioxide (CO2) 30-40
Hydrogen (H2) 5-10
Nitrogen (N2) 1-2
Water vapour (H2O) 0.3
Hydrogen sulphide (H2S) Traces
Properties Range
Net calorific value (MJ/m3) 20
Air required for combustion (m3/m3) 5.7
Ignition temperature (0C) 700
Density (kg/m3) 0.94
7. Biogas plant and its components
A physical structure designed to carry out anaerobic digestion of organic materials is called “Biogas
plant”.
Mixing tank: Cow dung is collected from the shed and mixed with the water in equal proportion
(1:1) to make a homogenousmixture (slurry) in the mixing tank
Feed inlet pipe/tank: The homogenous slurry is let into the digester through the inlet pipe (KVIC
biogas plants)/tank (Janatha biogasplants)
Digester: The fed slurry is subjected to anaerobic fermentation with the help of microorganisms
inside the digester
Gas holder: As a result of anaerobic fermentation, gas produced is stored in gas holder (Drum
in the case of KVIC and dome in the case of fixed dome biogas plants)
Slurry outlet tank/pipe: The digested slurry is let out from the digester through slurry outlet
pipe (KVIC biogas plants)/tank (Janathabiogas plants)
Gas outlet pipe: Stored gas is released and conveyed through the gas outlet pipe present at the top
of gas holder.
8.
9. Classification of Biogas Plants
1} Batch Type:
The organic waste materials to be digested under anaerobic condition are charged only
once into a reactor-digester.
The feeding is between intervals, the plant is emptied once the process of digestion is
complete.
Retention time usually varies from 30 to 50 days.
The gas production in it is intermittent (Not continuous or steady).
These plants are well suited for fibrous materials.
This type of plant needs addition of fermented slurry to start the digestion process
Not economical to maintain which is considered to be the major drawback.
10. Semi- Continuous type
A predetermined quantity of feed
material mixed with water is
charged into the digester from one
side
Feed material is charged at
specified interval of time; (say once
a day)
The digested material (effluent)
equivalent to the volume of the
feed, flows out of the digester from
the other side (outlet).
11. Continuous type
The feed material is continuously charged to the digester with simultaneous
discharge of the digested material (effluent).
The waste material is first converted to fluid form generally by adding the
appropriate amount of water.
Biogas production is better and uniform in continuous fed plants than the
batch plants.
Today, nearly all industrial/commercial biogas plants are operating on a
continuous mode
The main features of this type of plants are continuous gas production,
requires small digestion area, lesser period for digestion, less maintenance,
etc.
12. Popular Designs of Biogas Plants
Some of the most common biogas plants that are recognized by the MNRE (Ministry of
New and Renewable Energy). Govt. of India is the following :
1. Floating-drum plant with a cylinder digester (KVIC model).
2. Fixed-dome plant with a brick reinforced, moulded dome (Janata model).
3. Floating-drum plant with a hemisphere digester (Pragati model).
4. Fixed-dome plant with a hemisphere digester (Deenbandhu model).
5. Floating-drum plant made of angular steel and plastic foil (Ganesh model).
6. Floating-drum plant made of pre-fabricated reinforced concrete compound units.
7. Floating-drum plant made of fibre glass reinforced polyester.
13. Applications of Biogas
1.Cooking: Biogas can be used as a fuel for cooking, either in a gas stove or a biogas burner. Biogas burns
cleanly and efficiently, making it a good alternative to traditional fuels such as wood, charcoal or kerosene.
2.Lighting: Biogas can be used to generate electricity, which can be used for lighting. Biogas-powered lamps
are an affordable and sustainable lighting option in rural areas with limited access to electricity.
3.Heating: Biogas can be used to provide heat for space heating, water heating, or industrial processes.
Biogas-powered boilers or furnaces are a cost-effective and environmentally friendly alternative to fossil
fuels.
4.Transportation: Biogas can be used as a fuel for vehicles, either in compressed natural gas (CNG) or
liquefied natural gas (LNG) form. Biogas-powered vehicles have lower emissions and are more
environmentally friendly than traditional gasoline or diesel vehicles.
5.Electricity generation: Biogas can be used to generate electricity in a biogas power plant. The biogas is
burned to heat water and produce steam, which drives a turbine to generate electricity. Biogas power
plants are an important source of renewable energy and can help to reduce greenhouse gas emissions.
6.Fertilizer production: The digested slurry from biogas production is a valuable fertilizer that can be used to
improve soil fertility and crop yields. The slurry contains nutrients such as nitrogen, phosphorus, and
potassium, which are essential for plant growth.
Overall, biogas has many versatile applications, and its use can help to promote sustainable and
environmentally friendly practices in various sectors.
14. Multiple batch/pit type plant
• It is used for continuous
gas supply which cannot be
achieved in single batch
digesters.
• The waste material was
filled and removed
alternatively to digesters.
• The advantage of steady
gas supply was ensured in
multiple batch digesters.
• The removal of messy
digested material by labor
was still a problem in these
systems.
15. Floating drum biogas plant (KVIC) • It consists of a cylindrical or dome-shaped digester
fitted with a moving, floating gas-holder, or drum.
• The gas-holder move up and down on a guide
frame which floats either directly in the
fermenting slurry or in a separate water jacket.
• The biogas produced in digester is stored in floating
drum.
• If gas is consumed, the gas-holder sinks back due to
its weight.
• Animal and human wastes are normally digested.
• The daily input amount of slurry is fed once or twice
a day.
• The capacity of 2 to 20 m3 plants is used by farmers
for their domestic needs.
• The bigger plants are of capacity 25 to 140 m3 are
constructed and used by institutions/industries.
• The digester is usually made of brick, concrete or
quarry-stone masonry with plaster.
• The gas drum normally consists of 2.5 mm steel
sheets for the sides and 2 mm sheets for the top.
16. ◦ Suitable coating products are oil paints, synthetic paints and bitumen paints should be
used.
◦ The gas drum is having a slightly sloping roof to protect from rusting in rainy season.
◦ Floating drums made of glass-fiber reinforced plastic and high-density polyethylene
have been used successfully, but the construction costs are higher compared to using steel.
Advantages
1. Easy in construction and operation.
2. Provide gas at a constant pressure.
3. Gas-volume can be estimated easily with drum height with respect to its rest position. ·
4. Less chances of leakage of gas.
Disadvantage
1. Transportation to remote villages is difficult and expensive. ·
2. Cost of steel drum is high. ·
3. Corrosion problem occurs after some time, so maintenance is high. ·
4. Life of drum is short in comparison to masonry digester
18. ◦ The plant is constructed underground and helps in dampening the temperature
fluctuations too.
The complete digester system required several components :
1. Digester
2. Inlet mixing Tank outlet Tank
3. Gas outlet, Pipe, Pipe lines and Vales
4. Manure collection pit
5. Gas utilization Appliances
6. Water Removal
◦ The fixed dome plant consists of a dome shaped or spherical shape digester with fixed
gas holder and displacement pit, also named 'compensation tank’.
◦ When gas production commences, the slurry is displaced into the compensating tank. Gas
pressure increases with the volume of gas stored.
◦ The digesters of fixed-dome plants are usually masonry structures.
19. Advantages ·
Cost is low.
Life span is long.
The design is compact and saves space and well insulates due to earth cover.
Disadvantages
Highly Skilled Mason is required.
Excavation can be difficult and expensive in bedrock.
Fluctuating gas pressure decrease utilization efficiency
