BIOGAS PLANT

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BIOGAS PLANT

• Biogas plant is an airtight container that
  facilitates fermentation of material under
  anaerobic condition.
• Other names given to this device are Biogas
  Digester, Biogas Reactor, Methane Generator
  and Methane Reactor
• Recycling and treatment of organic wastes
  (biodegradable material) through anaerobic
  digestion (fermentation) technology not only
  provides biogas as a clean and convenient fuel
  but also an excellent and enriched bio-manure.
• The BGP also acts as mini bio-fertilizer factory
  hence some people refer it as Biogas fertilizer
  plant or Bio-manure plant
• Anaerobic digestion of organic matter produces a
  mixture of methane (CH4) and carbon dioxide (CO2)
  gas that can be used as a fuel for cooking,
  lighting, mechanical power and the generation of
  electricity, or a replacement for other fuels.

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CLASSIFICATION
Classification of biogas plants depends upon the
plants design and mode of working. One common way
to classify them is
BIOGAS PLANT
Continuous type plant
Batch type plant
Fixed Dome Biogas Plant
Floating Gas Holder Biogas Plant
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BATCH TYPE BIOGAS PLANT

1. Batch type biogas plants are appropriate where
   daily supplies of raw waste materials are
   difficult to be obtained.
2. Batch type plant is charged at 50-60 day
   intervals
3. Once charged, it starts supplying the gas after
   8-10 days and continuous to do so for about 40-50
   days till the process of digestion is completed.
   Afterwards it is emptied and recharged.
4. Gas production in batch type is uneven.
5. Several digesters occupy more space.
6. This type of plants require large volume of
   digester, therefore, initial cost becomes high.
7. Such plants are installed in european countries.
8. Donot suit the conditions in indian rural areas

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CONTINOUS TYPE BIOGASS PLANT

• In continuous type biogas plant, the supply of
  the gas is continuous and the digester is fed
  with biomass regularly.
• Plant operates continuously and is stopped only
  for maintenance or for sludge removal.
• The gas produced is stored in the plant or in a
  separate gas holder.
• The period during which the biomass remains in
  the digester is known as The retention period
• The thin dry layer often formed at the top of the
  slurry is known as scum.
• This type of plant are very popular in India and
  China.
• Other features
• Retention period is less 
• Less problems as compared to batch type.
• Small digestion chambers are required

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OPERATIONAL PARAMETERS OF A BIOGAS PLANT

• Operation of a bio gas plant is affected by a no.
  of factors-
• Temperature
• Methane- forming bacteria works best in temp.
  ranges 20-55 C
• Digestion at higher temp. proceeds more rapidly
  than at lower temp.
• The gas production decreases sharply below 20 C
  and almost stops at 10 C
• Pressure
• A minimum pressure of 6-10cm of water column
  i.e, 1.2 bar is ideal for proper functioning.
• It should never be alllowed to exceed 40-50 cm of
  water column.
• Excess pressure inhibits release of gas from
  slurry and leakage in masonry.

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• Solid to moisture ratio in the biomass
• If water content is too high, the mean slurry
  temp. and gas production drops.
• If water content is too low, acids accumulate and
  hinder fermentation process.
• pH value
• In initial acid forming stage, pH value may be
  around 6 or less.
• During methane forming stage, pH value 6.5 to 7.5
  is maintained.
• Feeding rate
• Faster feeding rate will not help in increase gas
  production.
• At higher feeding rate the retention period will
  be less and undigested Slurry may come out.
• So optimum feed rate should be maintained.

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• F. Carbon to nitrogen ratio
• The optimum C/N ratio is 301 for maximum
  microbiological activity.
• G. Seeding of biomass with bacteria
• To start and accelerate the fermentation process,
  a small amount of
• digested slurry containing a methane forming
  bacteria is added to the
• freshly charged plant. This process is known as
  seeding.
• Seeding helps to accelerate the starting of the
  digestion process.
• H. Mixing or stirring
• Mixing
• Maintains uniformity in substrate
  concentration,temp,other environmentel factors
• Minimises formation of scum
• Prevents the deposition of solids at the bottom

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• Retention time
• Retention time should be optimum to obtain 70-80
  complete
• Digestion.

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Fixed dome type of biogas plant
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• Raw materials required
• Forms of biomass listed below may be used along
  with water.
• Animal dung
• Poultry wastes
• Plant wastes ( Husk, grass, weeds etc.)
• Human excreta
• Industrial wastes(Saw dust, wastes from food
  processing industries)
• Domestic wastes (Vegetable peels, waste food
  materials)

Principle Biogas is produced as a result of
anaerobic decomposition of biomass in the
presence of water.
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• Construction
• The biogas plant is a brick and cement structure
  having the following
• five sections
• Mixing tank present above the ground level.
• Inlet chamber The mixing tank opens underground
  into a sloping inlet chamber.
• Digester The inlet chamber opens from below into
  the digester which is a huge tank with a dome
  like ceiling. The ceiling of the digester has an
  outlet with a valve for the supply of biogas.
• Outlet chamber The digester opens from below
  into an outlet chamber.
• Overflow tank The outlet chamber opens from the
  top into a small over flow tank

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• Working
• The various forms of biomass are mixed with an
  equal quantity of water in the mixing tank. This
  forms the slurry.
• The slurry is fed into the digester through the
  inlet chamber.
• When the digester is partially filled with the
  slurry, the introduction of slurry is stopped and
  the plant is left unused for about two months.
• During these two months, anaerobic bacteria
  present in the slurry decomposes or ferments the
  biomass in the presence of water.
• As a result of anaerobic decomposition, biogas is
  formed, which starts collecting in the dome of
  the digester.
• As more and more biogas starts collecting, the
  pressure exerted by the biogas forces the spent
  slurry into the outlet chamber.
• From the outlet chamber, the spent slurry
  overflows into the overflow tank.
• The spent slurry is manually removed from the
  overflow tank and used as manure for plants.
• The gas valve connected to a system of pipelines
  is opened when a supply of biogas is required.

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Floating gas holder type of biogas plant
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• The raw materials used and the principle involved
  are common to both the types of biogas plants.
• Construction
• The floating gas holder type of biogas plant has
  the following chambers/ sections
• Mixing Tank - present above the ground level.
• Digester tank - Deep underground well-like
  structure. It is divided into two chambers by a
  partition wall in between.
• It has two long cement pipes
• i) Inlet pipe opening into the inlet chamber for
  introduction of slurry.
• ii) Outlet pipe opening into the overflow tank
  for removal of spent slurry.
• Gas holder - an inverted steel drum resting above
  the digester. The drum can move up and down i.e.,
  float over the digester. The gas holder has an
  outlet at the top which could be connected to gas
  stoves.
• Over flow tank - Present above the ground level.

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• Working
• Slurry (mixture of equal quantities of biomass
  and water) is prepared in the mixing tank.
• The prepared slurry is fed into the inlet chamber
  of the digester through the inlet pipe.
• The plant is left unused for about two months and
  introduction of more slurry is stopped.
• During this period, anaerobic fermentation of
  biomass takes place in the presence of water and
  produces biogas in the digester.
• Biogas being lighter rises up and starts
  collecting in the gas holder. The gas holder now
  starts moving up.
• The gas holder cannot rise up beyond a certain
  level. As more and more gas starts collecting,
  more pressure begins to be exerted on the slurry.

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• The spent slurry is now forced into the outlet
  chamber from the top of the inlet chamber.
• When the outlet chamber gets filled with the
  spent slurry, the excess is forced out through
  the outlet pipe into the overflow tank. This is
  later used as manure for plants.
• The gas valve of the gas outlet is opened to get
  a supply of biogas.
• Once the production of biogas begins, a
  continuous supply of gas can be ensured by
  regular removal of spent slurry and introduction
  of fresh slurry.

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COMPARISON
SN. FEATURE FLOATING DRUM FIXED DOME
1. COST More(due to steel drum) Less
2. CORROSION Yes (likely in steel drum) No
3. MAINTENANCE More Drum requires painting Flexible gas pipe requires replacement Less No steel part is used Gas pipe is fixed type
4. THERMAL INSULATION Bad Good(due to underground construction)
5. SCUM TROUBLES Less likely More likely
6. GAS PRODUCTION PER UNIT VOLUME OF DIGESTER High Low
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7. SCUM BRAKING By rotation of drum External stirrer is required
8. LEAKAGE Less likely More likely
9. DANGER OF MIXING WITH OXYGEN DUE TO LEAKAGE,CRACKS No More likely
10. GAS PRESSURE constant Variable
11. MASONRY WORKMANSHIP Average skill Specialized, skilled masonry work required
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