Biogas Digester Final

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BIOGAS
Biogas
•Biogas is a mixture of methane and carbon dioxide.
•The properties of biogas are similar to the ones of natural
gas.
•Biogas is the common name for the mixture of gases
released from anaerobic digestion.
50 to 75 %
•Typically biogas is composed of: Methane (CH4)
Carbon Dioxide (CO2)
Hydrogen (H)
Nitrogen (N2)
Hydrogen sulphide (H2S)
25 to 50 %
5 to 10 %
1 to 2 %
Traces
•Methane is the valuable part of the biogas. Biogas that
contains about 60 to 70 % of CH4 has a calorific value of
about 6 kWh/m3 what corresponds to about half an L of
diesel oil.
I)
2
Possible Benefits for Users:
– Social:
• Improved sanitation: reduction of pathogens, worm eggs and flies
• Reduction of workload: less firewood collection, better cooking
performance
• Improved indoor air quality: less smoke and harmful particle
emission of biogas stove compared to wood or dung fuels;
– Environmental
• Production of green energy
• Reduction of greenhouse gas emission
• Organic fertilizer production
– Economical:
• Better Health more work capacity
• Fertilizer, better crop yields, better Health
• Fuel substitution
3
History of Biogas
1808 – Sir
Humphrey Davy
found that
methane was
present in the
gases that is
formed by the
Anaerobic
Digestion of
manure
1895 – Biogas is
used to light up
the streets in
Exeter, England
1884 – Louis
Pasteur student,
Ulysse Gayon,
performed the
anaerobic
fermentation of
manure and
water at 35ºC
and obtained
100 liters of
Biogas per cubic
meter of
Manure.
1957 – A British
Inventor, Bates,
modifies his car
to run on Biogas
produced from
pig manure.
2005 – The
Biogas Support
program in
Nepal wins the
Ashden Reward
for installing
over 150,000
Biogas Plants in
rural areas. And
a Biogas
powered train
starts it's service
in Sweden
Anaerobic Digestion
Degradation of organic material by bacteria. In the
absence of air (anaerobic). Four stages:
–Hydrolisis
• Cleavage of a chemical compound through the reaction with water.
• Insoluble complex molecules are bracken down to short sugars, fatty
acids and amino acids.
–Fermentation (Acidogenesis)
• Products from hydrolysis are transformed into organic acids, alcohols,
carbon dioxide (CO2), hydrogen (H) and ammonia (NH3).
–Acetogenesis
• Organic acids and alcohols are converted into hydrogen (H2), carbon
dioxide (CO2) and acetic acid (CH3COOH). Therefore, oxygen is
consumed and anaerobic conditions are created
–Methanogenesis
• Methanogenic bacteria (methanogenesis), transform the acetic acid,
carbon dioxide and hydrogen into biogas.
5
Anaerobic Digestion
D. SPUHLER (2010)
6
Anaerobic Digestion
7
ANAEROBIC DIGESTION
• Types of Biogas Plants
– There are various types of biogas plants available in the
world and they are classified mainly based on feeding
method and construction.
– Based on the feed method, three different forms can be
distinguished:
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
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Batch plants,
Continuous plants,
Semi-batch plants.
– Based on the construction, two main types of simple
biogas plants can be distinguished:


Fixed-dome plants,
Floating-drum plants.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Batch type digesters.


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In batch type plants, materials fed into the digester at a time and
sealed only allowing the gas to exit and then emptied completely
after a fixed retention time.
Each design and each fermentation material is suitable for batch
filling, but batch plants require high labor input.
The major disadvantage of this type is unsteady gas-output.
3
1
2
1 - Digester.
2 - Gasholder.
3 - Gas pipe.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Continuous type digesters.
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

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Once the process started, regular quantity of waste are fed and
regular quantity of material discharged, continuously.
They empty automatically through the overflow whenever new
material is filled in.
Therefore, the substrate must be fluid and homogeneous. This
technology is suitable for both medium and large scale waste
treatment and large scale biogas production.
Advantages of this type are constant and higher gas production
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Semi-batch type digesters.


If the two materials which have completely different digestion
rates (such as straw and dung) are to be digested together, a
biogas plant can be operated on a semibatch basis.
The slowly digested straw-type material is fed in about twice a
year as a batch load. The dung is added and removed regularly.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Fixed Dome type digesters.

A fixed-dome plant comprises of a closed, dome-shaped digester
with an immovable, rigid gas-holder and a displacement pit.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Fixed Dome type digesters.
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
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The gas is stored in the upper part of the digester.
When gas production commences, the slurry is displaced into the
displacement tank.
Gas pressure increases with the volume of gas stored, i.e. with
the height difference between the two slurry levels.
If there is little gas in the gasholder, the gas pressure is low.
The digesters of fixed-dome plants are usually masonry
structures, structures of cement and ferro-cement exist.
Main parameters for the choice of material are technical
suitability (stability, gas- and liquid tightness), cost-effectiveness,
availability in the region and transport costs and availability of
local skills for working with the particular building material.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Floating drum type digesters.

Major difference between fixed dome and floating drum type plant
is, a floating-drum plant consists of a floating gas-holder, or drum.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Floating drum type digesters.



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This floats either directly in the fermenting slurry or in a separate
water jacket.
The drum in which the biogas collects has an internal and/or
external guide frame that provides stability and keeps the drum
upright. If biogas is produced, the drum moves up, if gas is
consumed, the gasholder sinks back.
Floating-drum plants are used mainly in continuous feed mode of
operation.
They are used most frequently by small- to middle-sized farms
(digester size: 5-15m3) or in institutions and larger agro-industrial
estates (digester size: 20-100m3)
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Advantages of fixed dome type.




Produce just as much gas as floating-drum plants, if they are
gas-tight.
Low cost operation
Simple design
Long life of the plant (20 years or more)
– Disadvantages of fixed dome type.


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Utilization of the gas is less effective as the gas pressure
fluctuates substantially.
Labor-intensive design
Not easy to build.
Difficult to achieve gas tightness.
ANAEROBIC DIGESTION
• Types of Biogas Plants
– Advantages of floating drum type.


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Simple operation
Provide gas at a constant pressure
Stored gas-volume is immediately recognizable by the position of
the
– Disadvantages of floating drum type.
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
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The steel drum is relatively expensive and maintenanceintensive.
Removing rust and painting has to be carried out regularly.
The life-time of the drum is short (up to 15 years; in tropical
coastal regions about five years).
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