BIO MASS ENERGY
Classification of Biogas Plants: Biogas plants are mainly classified as:
(1) Continuous and batch types (as per the process).
(2) The dome and the drum types.
(3) Different variations in the drum type.
Continuous and batch types
(a) Continuous plant. There is a single digester in which to material are charged regularly
and the process goes on without ruption except for repair and cleaning etc. In this case the
raw material is self buffered (like cow dung) or otherwise thoroughly mixed with digesting
mass where dilution prevents souring and the biogas production is maintained. The
continuous process may be completed in a singe stage or separated into two stages.
Single stage process. The entire process of convers complex organic compounds into biogas
in completed in a single chamber. This chamber is regularly fed with the raw materials while
the spent residue keeps moving out. Serious problems are encountered with agricultural
residues when fermented in a single stage continuos process.
Double stage process. The acidogenic stage and methanogenic stage 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 and the biogas can be collected from the second chamber.
Considering the problems encountered in fermenting fibrous plant waste materials the two
stage process may offer higher potential of success. However, appropriate technology
suiting to rural India is needed to be developed based on the double stage process.
The main features of continuous plant are that:
(1) It will produce gas continuously;
(2) It requires small digestion chambers
(3)It needs lesser period for digestion
(4) It has less problems compared to batch type and it is easier in operation
(b) The batch Plant. The feeding is between intervals, the plant is emptied once the process
of digestion is complete. In this type, a battery 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 a synchronous manner which maintains a regular supply of the gas through a
common gas holder. Sometimes the freshly charged digester is aerated for a few days after
which it is closed to atmosphere. The biogas supply may be utilized after 8-10 days.
Obviously such a plant would be expensive to install and unless operated on large scale it
would not be economical. Such systems have been generally installed in European
countries. Their installation and operation being capital and labour intensive. They are
totally unsuitable for Indian conditions, except when it is taken as a commercial venture.
The main features of the batch plant are:
1)The gas production in it, is intermittent, depending upon the clearing of the digester.
2) It needs several digesters or chambers for continuous gas production, these are fed
alternately.
3)Batch plants are good for long fibrous materials.
4) This plant needs addition of fermented slurry to start the digestion process.
The dome and the drum types.There are numerous modela of a biogas plant mainly
two main types are usually used.
1)The floating gas holder plant
2) Fixed dome digester.
The floating gas holder digester which is used in India is know an KVIC plant. The fixed dome
digester is called the Chinese plant. There are different shapes in both the designs,
cylindrical rectangular, spherical etc. Again, the digester may be vertical or horizontal. They
can be constructed above or underneath the ground the floating gasholder digester
developed in India in of masonry construction with the gas holder made of M.S. plates. The
gas holder is separated from the digester. Rusting of the gas holder as well as the cost of the
gas holder are the main drawbacks of this system.
In the fixed dome digester, the 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 found to cause any serious problems in small plants.
Different variations in the drum type. There are two main variations in the floating
drum design. One with water seal and the other 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 clayey soils and sandy tracks.
Gas holders are also manufactured out of ferro cement, as M.S. sheets get corroded.
Polyethylene is also used in the construction of gas holder. The latest design uses fibre glass
reinforecd plastic.
he horizontal plants are suited for high ground water rocky areas. These are not
recommended when retention period is 30 days. Cylindrical shape of the digester is
preferred because cylinder no corners and so that there will be no chances of cracks due to
faulty construction. This shape also needs smaller surface area per mit volume, which
reduces heat losses also. Moreover the scum formation may be reduced by rotating gas
holder in the cylindrical digester
TYPES OF BIOGAS PLANTS
FLOATING DRUM(CONSTANT PRESSURE) TYPE
It has an inverted mild steel drum to work as gasholder. This is the most expensive
component of the plant. The drum floats either direct on fermentation slurry or in water
jacket of its own. Most Indian plants now have a pair of central guide pipes. The gasholder is
free to rise or fall depending on the production and use of gas. It also free to rotate on its
axis. Gasholder rotation will also be useful as a device to break the scum in the digester. A
flexible hosepipe is attached at the top of the gasholder for gas dispersion. The gas passes
through a moisture trap before supplying to the utility/house.
The digester is a deep circular pit or a well, built of bricks, mortar and plaster. with a
partition wall. The bifurcation of digestion chamber through partitioning wall provides
optimum conditions for growth of acid formers and methane formers as the requirement of
ph values for these bacteria are different. Therefore, this plant operates very well with good
biogas yield. The underground structure helps minimize the heat loss from the plant and the
cylindrical shape has better structural strength.
FIXED DOME (CONSTANT VOLUME) TYPE
This plant is more economical as only masonry work is required. Gas pressure in the dome
varies depending on the production/consumption rate. By construction a dome Structure is
very strong for outside pressures but a weak one for inner pressures. As gas pressure is
exerted from inside out, the dome structure may fail if proper care is not taken in its
construction. The dome is constructed underground to maintain pressure over it. Skilled
masonry workmanship is required for construction of dome. In case of any leakages/cracks
the plant may fail. A typical family size Chinese model (Janata Model) is shown in Fig. 8.13.
Many variations of this basic design are developed to reduce the cost by making use of
different materials to suit local conditions.
The slurry enters from inlet and the digested slurry is collected in a displacement tank
Stirring is required if raw material is crop residue. There is no bifurcation in the digester
chamber and therefore the gas production is somewhat less as compared floating point
design. The gas produced is stored in the dome and displaces the liquids in inlet and outlet,
often leading to gas pressure as high as 100 cm of water. The gas occupies about 10 per cent
of the volume of the digester. As complete plant is constructed underground, the
temperature tends to remain constant and is often considerably higher than ambient
temperature in winter.
Many variations of the above two basic models are developed keeping in view the
portability, ease of installation and maintenance, local availability of material and cost, etc.
Some of them are: plastic bag reactor, prefabricated steel/bamboo/plastic reactors (both
are portable models) and horizontal type reactors, etc.
Comparison of Floating Drum and Fixed Dome Type Plants
s.n Features Floating drum Fixed dome
o
1 Cost More(due to steel drum) Less
2 Corrosion Yes(likely in steel drum) No
3 Maintenance More(1)drum requires painting Less(1)No steel part is
once or twice a year(2)flexible gas used (2)gas pipe is a fixed
pipe requires replacement GJ pipe.
4 Thermal Bad (heat loss due to steel drum) Good (temperature will be
insulation constant due to complete
underground construction
5 Scum trobles Less likely (as solids are constantly more likely
submerged)
6 Gas production High (due to bifurcation, both acid Low
per unit volume and methane formers find better
of digester environment for growth)
7 Scum braking By rotation of drum (no stirrer External stirrer is required
required)
8 Leakage less More
9 Danger of mixing No More likely
with oxygen due
to leakage/cracks
10 Gaspressure constant Variable
11 Masonry Avearge skill pecialized, skilled masonry
workmanship work required
12 Repairing of Easy Difficult
defects in gas
holder
13 Choice of feed Can process dung and night soil It It can easily modified for
material can be easily adapted/mod slurry. use of other materials
Other organic materials can clog the along with dung slurry
inlet pipe
SELECTION OF SITE FOR A BIOGAS PLANT
Distance: The distance between the plant and site of gas consumption should be less in
order to achieve economy in pumping of gas and minimizing gas leakage.
Minimum gradient: For conveying the gas a minimum gradient of 1% must be made
available for the line.
Open Space: The sunlight should fall on the plant as temperature between 15°C to 30°C is
essential for gas generation at good rate.
Water table: The plant is normally constructed underground for ease of charging the feed
and unloading slurry requires less labour In such cases care should be taken to prevent the
seepage of water and plant should not be constructed if the water table is more than 10 ft.
Seasonal run off:Proper care has to be taken to prevent the interference of run off water
during the monsoon. Intercepting ditches or bunds may be constructed.
Distance from wells: The seepage of fermented slurry may pollute the well water. Hence a
minimum of 15 m should be maintained from the wells.
Space requirements: Sufficient space must be available for day to day operation and
maintenance. As a guideline 10 to 12 m² area is needed per m of the gas.
Availability of water: Plenty of water must be available as must be the cow dung slurry with
a solid concentration of 7% to 9% is used.
Source of cow dung materials for biogas generation. The distance between the material for
biogas generation and the gas plant site should be minimum to economise the
transportation cost.
