Composition of Biogas
Biogas is a colourless, odourless, inflammable gas, produced by
organic waste and biomass decomposition (fermentation). Biogas can be produced
from animal, human and plant (crop) wastes, weeds, grasses, vines, leaves,
aquatic plants and crop residues etc. The composition of different gases in
biogas :
Methane (CH4) : 55-70%
Carbon Dioxide (CO2) : 30-45%
Hydrogen Sulphide (H2S) : 1-2%
Nitrogen (N2) : 0-1%
Hydrogen (H2) : 0-1%
Carbon Mono Oxide (CO) : Traces
Oxygen (O2) : Traces
3.9.4 Property of Biogas
Biogas burns with a blue flame. It has a heat value of 500-700 BTU/Ft3
(4,500-5,000 Kcal/M3) when its methane content is in the range of 60-70%. The
value is directly proportional to the amount of methane contains and this
depends upon the nature of raw materials used in the digestion. Since the
composition of this gas is different, the burners designed for coal gas, butane
or LPG when used, as ‘biogas burner’ will give much lower efficiency.
Therefore specially designed biogas burners are used which give a thermal
efficiency of 55-65%.
Biogas is a very stable gas, which is a non-toxic, colourless, tasteless and
odourless gas. However, as biogas has a small percentage of Hydrogen Sulphide,
the mixture may very slightly smell of rotten egg, which is not often noticeable
especially when being burned. When the mixture of methane and air (oxygen) burn
a blue flame is emitted, producing large amount of heat energy. Because of the
mixture of Carbon Dioxide in large quantity the biogas becomes a safe fuel in
rural homes as will prevent explosion.
A 1 M3 biogas will generate 4,500-5,500 Kcal/m2 of heat energy, and when burned
in specifically designed burners having 60% efficiency, will give out effective
heat of 2,700-3,200 Kcal/m2. 1 Kcal is defined as the heat required to raise the
temperature of 1 kg (litre) of water by 1 degrees Celsius. Therefore this
effective heat (say 3,000 Kcal/m2 is on an average), is sufficient to bring
approx. 100 kg (litre) of water from 20 degrees Celsius to a boil, or light a
lamp with a brightness equivalent to 60-100 Watts for 4-5 hours.
3.9.5 Mechanics of Extraction of Biogas
The decomposition (fermentation) process for the formation of methane from
organic material (biodegradable material) involves a group of organisms
belonging to the family- ‘Methane Bacteria’ and is a complex biological and
chemical process. For the understanding of common people and field workers,
broadly speaking the biogas production involves two major processes consisting
of acid formation (liquefaction) and gas formation (gasification). However
scientifically speaking these two broad process can further be divide, which
gives four stages of anaerobic fermentation inside the digester-they are (i)
Hydrolysis, (ii) Acidification, (iii) Hydrogenation and (iv) Methane Formation.
At the same time for all practical purposes one can take the methane production
cycle as a three stage activity- namely, (i) Hydrolysis, (ii) Acidification and
(iii) Methane formation.
Two groups of bacteria work on the substrate (feedstock) inside the
digester-they are (i) Non-methanogens and (ii) Methanogens. Under normal
conditions, the non-methanogenic bacteria (microbes) can grow at a pH range of
5.0-8.5 and a temperature range of 25-42 deg. ;whereas, methanogenic
bacteria can ideally grow at a pH range of 6.5-7.5 and a temperature range of
25-35 degrees Celsius. These methanogenic bacteria are known as ‘Mesophillic
Bacteria’ and are found to be more flexible and useful incase of simple
household digesters, as they can work under a broad range of temperature, as low
as 15 degrees Celsius to as high as 40 degrees Celsius. However their efficiency
goes down considerably if the slurry temperature goes below 20 degrees Celsius
and almost stop functioning at a slurry temperature below 15 degrees Celsius.
Due to this Mesophillic Bacteria can work under all the three temperature zones
of India, without having to provide either heating system in the digester or
insulation in the plant, thus keeping the cost of family size biogas plants at
an affordable level.
There are other two groups of anaerobic bacteria-they are (i) Pyscrophillic
Bacteria and (ii) Thermophillic Bacteria. The group of Pyscrophillic Bacteria
work at low temperature, in the range of 10-15 degrees Celsius but the work is
still going on to find out the viability of these group of bacteria for field
applications. The group of Thermophillic Bacteria work at a much higher
temperature, in the range of 45-55 degrees Celsius and are very efficient,
however they are more useful in high rate digestions (fermentation), especially
where a large quantity of effluent is already being discharged at a higher
temperature. As in both the cases the plant design needs to be sophisticated
therefore these two groups of Bacteria (Pyscrophillic & Thermophillic) are
not very useful in the case of simple Indian rural biogas plant.
3.9.6 Biogas Plant (BGP)
Biogas Plant (BGP) is an airtight container that facilitates fermentation of
material under anaerobic condition. The other names given to this device are ‘Biogas
Digester’, ‘Biogas Reactor’, ‘Methane Generator’ and ‘Methane
Reactor’. The 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. Thus the BGP also acts as a miniature Bio-fertilizer
Factory hence some people prefer to refer it as ‘Biogas Fertilizer Plant’ or
‘Bio-manure Plant’. The fresh organic material (generally in a homogenous
slurry form) is fed into the digester of the plant from one end, known as Inlet
Pipe or Inlet Tank. The decomposition (fermentation) takes place inside the
digester due to bacterial (microbial) action, which produces biogas and organic
fertilizer (manure) rich in humus & other nutrients. There is a provision
for storing biogas on the upper portion of the BGP. There are some BGP designs
that have Floating Gasholder and others have Fixed Gas Storage Chamber. On the
other end of the digester Outlet Pipe or Outlet Tank is provided for the
automatic discharge of the liquid digested manure.
3.9.6.1 Components of Biogas Plant (BGP)
The major components of BGP are - (i) Digester, (ii) Gasholder or Gas Storage
Chamber, (iii) Inlet, (iv) Outlet, (v) Mixing Tank and (vi) Gas Outlet Pipe.
DIGESTER
It is either an under ground Cylindrical-shaped or Ellipsoidal-shaped structure
where the digestion (fermentation) of substrate takes place. The digester is
also known as ‘Fermentation Tank or Chamber’. In a simple Rural Household
BGP working under ambient temperature, the digester (fermentation chamber) is
designed to hold slurry equivalent to of 55, 40 or 30 days of daily feeding.
This is known as Hydraulic Retention Time (HRT) of BGP. The designed HRT of 55,
40 and 30 days is determined by the different temperature zones in the country-
the states & regions falling under the different temperature zones are
already defined for India. The digester can be constructed of brick masonry,
cement concrete (CC) or reinforced cement concrete (RCC) or stone masonry or
pre-fabricated cement concrete blocks (PFCCB) or Ferro-cement (ferroconcrete) or
steel or rubber or bamboo reinforced cement mortar (BRCM). In the case of
smaller capacity floating gasholder plants of 2 & 3 M3 no partition wall is
provided inside the digester, whereas the BGPs of 4 M3 capacity and above have
been provided partition wall in the middle. This is provided for preventing
short-circuiting of slurry and promoting better efficiency. This means the
partition wall also divides the entire volume of the digester (fermentation
chamber) into two halves. As against this no partition wall is provided inside
the digester of a fixed dome design. The reason for this is that the diameter of
the digesters in all the fixed dome models are comparatively much bigger than
the floating drum BGPs, which takes care of the short-circuiting problems to a
satisfactory level, without adding to additional cost of providing a partition
wall.
GAS HOLDER OR GAS STORAGE CHAMBER
In the case of floating gas holder BGPs, the Gas holder is a drum like
structure, fabricated either of mild steel sheets or ferro-cement
(ferroconcrete) or high density plastic (HDP) or fibre glass reinforced plastic
(FRP). It fits like a cap on the mouth of digester where it is submerged in the
slurry and rests on the ledge, constructed inside the digester for this purpose.
The drum collects gas, which is produced from the slurry inside the digester as
it gets decomposed, and rises upwards, being lighter than air. To ensure that
there is enough pressure on the stored gas so that it flows on its own to the
point of utilisation through pipeline when the gate valve is open, the gas is
stored inside the gas holder at a constant pressure of 8-10 cm of water column.
This pressure is achieved by making the weight of biogas holder as 80-100
kg/cm2. In its up and down movement the drum is guided by a central guide pipe.
The gas formed is otherwise sealed from all sides except at the bottom. The scum
of the semidried mat formed on the surface of the slurry is broken (disturbed)
by rotating the biogas holder, which has scum-breaking arrangement inside it.
The gas storage capacity of a family size floating biogas holder BGP is kept as
50% of the rate capacity (daily gas production in 24 hours). This storage
capacity comes to approximately 12 hours of biogas produced every day.
In the case of fixed dome designs the biogas holder is commonly known as gas
storage chamber (GSC). The GSC is the integral and fixed part of the Main Unit
of the Plant (MUP) in the case of fixed dome BGPs. Therefore the GSC of the
fixed dome BGP is made of the same building material as that of the MUP. The gas
storage capacity of a family size fixed dome BGP is kept as 33% of the rate
capacity (daily gas production in 24 hours). This storage capacity comes to
approximately 8 hours of biogas produced during the night when it is not in use.
INLET
In the case of floating biogas holder pipe the Inlet is made of cement concrete
(CC) pipe. The Inlet Pipe reaches the bottom of the digester well on one side of
the partition wall. The top end of this pipe is connected to the Mixing Tank.
In the case of the first approved fixed dome models (Janata Model) the inlet is
like a chamber or tank-it is a bell mouth shaped brick masonry construction and
its outer wall is sloppy. The top end of the outer wall of the inlet chamber has
an opening connecting the mixing tank, whereas the bottom portion joins the
inlet gate. The top (mouth) of the inlet chamber is kept covered with heavy
slab. The Inlet of the other fixed dome models (Deenbandhu and Shramik Bandhu)
has Asbestos Cement Concrete (ACC) pipes of appropriate diameters.
OUTLET
In the case of floating gas holder pipe the Outlet is made of cement concrete
(CC) pipe standing at an angle, which reaches the bottom of the digester on the
opposite side of the partition wall. In smaller plants (2 & 3 M3 capacity
BGPs) which has no partition walls, the outlet is made of small (approx. 2 ft.
length) cement concrete (CC) pipe inserted on top most portion of the digester,
submerged in the slurry.
In the two fixed dome (Janata & Deenbandhu models) plants, the Outlet is
made in the form of rectangular tank. However, in the case of Shramik Bandhu
model the upper portion of the Outlet (known as Outlet Displacement Chamber) is
made hemi-spherical in shape, designed to save in the material and labour cost.
In all the three-fixed dome models (Janata, Deenbandhu & Shramik Bandhu
models), the bottom end of the outlet tank is connected to the outlet gate.
There is a small opening provided on the outer wall of the outlet chamber for
the automatic discharge of the digested slurry outside the BGP, equal to
approximately 80-90% of the daily feed. The top mouth of the outlet chamber is
kept covered with heavy slab.
MIXING TANK
This is a cylindrical tank used for making homogenous slurry by mixing the
manure from domestic farm animals with appropriate quantity of water. Thoroughly
mixing of slurry before releasing it inside the digester, through the inlet,
helps in increasing the efficiency of digestion. Normally a feeder fan is fixed
inside the mixing tank for facilitating easy and faster mixing of manure with
water for making homogenous slurry.
GAS OUTLET PIPE
The Gas Outlet Pipe is made of GI pipe and fixed on top of the drum at the
centre in case of floating biogas holder BGP and on the crown of the fixed dome
BGP. From this pipe the connection to gas pipeline is made for conveying the gas
to the point of utilisation. A gate valve is fixed on the gas outlet pipe to
close and check the flow of biogas from plant to the pipeline.
3.9.7 Functioning of a Simple India Rural Household Biogas Plants (BGPs)
The fresh organic material (generally in a homogenous slurry form) is fed into
the digester of the plant from one end, known as Inlet. Fixed quantity of fresh
material fed each day (normally in one lot at a predetermine time) goes down at
the bottom of the digester and forms the ‘bottom-most active layer’, being
heavier then the previous day and older material. The decomposition
(fermentation) takes place inside the digester due to bacterial (microbial)
action, which produces biogas and digested or semi-digested organic material. As
the organic material ferments, biogas is formed which rises to the top and gets
accumulated (collected) in the Gas Holder (in case of floating gas holder BGPs)
or Gas Storage Chamber (in case of fixed dome BGPs). A Gas Outlet Pipe is
provided on the top most portion of the Gas Holder (Gas Storage Chamber) of the
BGP. Alternatively, the biogas produced can be taken to another place through
pipe connected on top of the Gas Outlet Pipe and stored separately. The Slurry
(semi-digested and digested) occupies the major portion of the digester and the
Sludge (almost fully digested) occupies the bottom most portion of the digester.
The digested slurry (also known as effluent) is automatically discharged from
the other opening, known as Outlet, is an excellent bio-fertilizer, rich in
humus. The anaerobic fermentation increases the ammonia content by 120% and
quick acting phosphorous by 150%. Similarly the percentage of potash and several
micro-nutrients useful to the healthy growth of the crops also increase. The
nitrogen is transformed into Ammonia that is easier for plant to absorb. This
digested slurry can either be taken directly to the farmer’s field along with
irrigation water or stored in a Slurry Pits (attached to the BGP) for drying or
directed to the Compost Pit for making compost along with other waste biomass.
The slurry and also the sludge contain a higher percentage of nitrogen and
phosphorous than the same quantity of raw organic material fed inside the
digester of the BGP.
3.9.7.1 Type of Digestion
The digestion of organic materials in simple rural household biogas plants can
be classified under three broad categories. They are (i) Batch-fed digestion
(ii) Semi-continuous digestion and (iii) Semi-batch-fed digestion.
BATCH-FED DIGESTION
In batch-fed digestion process, material to be digested is loaded (with seed
material or innouculam) into the digester at the start of the process. The
digester is then sealed and the contents left to digest (ferment). At completion
of the digestion cycle, the digester is opened and sludge (manure) removed
(emptied). The digester is cleaned and once again loaded with fresh organic
material, available in the season.
SEMI-CONTINUOUS DIGESTION
This involves feeding of organic mater in homogenous slurry form inside the
digester of the BGP once in a day, normally at a fixed time. Each day digested
slurry; equivalent to about 85-95% of the daily input slurry is automatically
discharged from the outlet side. The digester is designed in such a way that the
fresh material fed comes out after completing a HRT cycle (either 55, 40 or 30
days), in the form of digested slurry. In a Semi-continuous digestion system,
once the process is stabilized in a few days of the initial loading of the BGP,
the biogas production follows a uniform pattern.
SEMI-BATCH FED DIGESTION
A combination of batch and semi-continuous digestion is known as Semi-batch fed
Digestion. Such a digestion process is used where the manure from domestic farm
animals is not sufficient to operate a plant and at the same time organic waste
like, crop residues, agricultural wastes (paddy & weed straw), water
hyacinths and weeds etc, are available during the season. In as Semi-batch fed
Digestion the initial loading is done with green or semi-dry or dry biomass
(that can not be reduced in to slurry form) mixed with starter and the digester
is sealed. This plant also has an inlet pipe for daily feeding of manure slurry
from animals. The Semi-batch fed Digester will have much longer digestion cycle
of gas production as compared to the batch-fed digester. It is ideally suited
for the poor peasants having 1-2 cattle or 3-4 goats to meet the major cooking
requirement and at the end of the cycle (6-9 months) will give enriched manure
in the form of digested sludge.
3.9.7.2 Stratification (Layering) of Digester due to Anaerobic Fermentation
In the process of digestion of feedstock in a BGP many by-products are formed.
They are biogas, scum, supernatant, digested slurry, digested sludge and
inorganic solids. If the content of Biogas Digester is not stirred or disturbed
for a few hours then these by-products get formed in to different layers inside
the digester. The heaviest by-product, which is Inorganic Solids will be at the
bottom most portion, followed by Digested Sludge, and so on and so forth as
shown in the three diagrams for three different types of digester.
BIOGAS
Biogas is a combustible gas produced from the anaerobic digestion of organic
matter. Comprising 55-70% Methane, 30-45% Carbon Dioxide, 1-2% of Hydrogen
Sulphide and traces other gases.
| LAYERING | USEFUL FRACTIONS | |
| Gas | BIOGAS | Combustible gas |
| Fibrous | SCUM | Fertilizer |
| Liquid | SUPERNATANT | Biologically Active |
| Semi Solid | DIGESTED SLUDGE | Fertilizer |
| Solid | INORGANIC SOLIDS | Waste |
| LAYERING | USEFUL FRACTIONS | |
| Gas | BIOGAS | Combustible gas |
| Fibrous | SCUM | Fertilizer |
| Liquid | DIGESTED SLURRY | Fertilizer |
| Liquid | SLURRY IN DIFFERENT STAGES OF FERMENTATION | Biologically Active |
| Solid | INORGANIC SOLIDS | Waste |
| LAYERING | USEFUL FRACTIONS | |
| Gas | BIOGAS | Combustible gas |
| Fibrous | SCUM | Fertilizer |
| Liquid | DIGESTED SLURRY (EFFLUENT) | Fertilizer |
| Liquid | MIXTURE OF SUPERNATANT AND SLURRY IN DIFFERENT STAGES OF FERMENTATION | Biologically Active |
| Semi solid | DIGESTED SLUDGE | Fertuilizer |
| Solid | INORGANIC SOLIDS | Waste |
SCUM
Mixture of coarse fibrous and lighter material that separates from the manure
slurry and floats on the top most layer of the slurry is called Scum. The
accumulation and removal of scum is sometimes a serious problem. In moderate
amount scum can’t do any harm and can be easily broken by gentle stirring, but
in large quantity can lead to slowing down biogas production and even shutting
down the BGPs.
SUPERNATANT
The spent liquid of the slurry (mixture of manure and water) layering just above
the sludge, in case of Batch-fed and Semi Batch-fed Digester, is known as
Supernatant. Since supernatant has dissolved solids, the fertiliser value of
this liquid (supernatant) is as great as that of effluent (digested slurry).
Supernatant is a biologically active by-product; therefore must be sun dried
before using it in agricultural fields.
DIGESTED SLURRY (EFFLUENT)
The effluent of the digested slurry is in liquid form and has its solid content
(total solid-TS) reduced to approximately 10-20% by volume of the original
(Influent) manure (fresh) slurry, after going through the anaerobic digestion
cycle. Out of the three types of digestion processes mentioned above, the
digested slurry in effluent-form comes out only in semi-continuous BGP. The
digested slurry effluent, either in liquid-form or after sun drying in Slurry
Pits makes excellent bio-fertilizer for agricultural and horticultural crops or
aquaculture.
SLUDGE
In the batch-fed or semi batch-fed digester where the plant wastes and other
solid organic materials are added, the digested material contains less of
effluent and more of sludge. The sludge precipitates at the bottom of the
digester and is formed mostly of the solids substances of plant wastes. The
sludge is usually composted with chemical fertilizers as it may contain higher
percentage of parasites and pathogens and hookworm eggs of etc., especially if
the semi-batch digesters are either connected to the pigsty or latrines.
Depending upon the raw materials used and the conditions of the digestion, the
sludge contains many elements essential to the plant life e.g. Nitrogen,
Phosphorous, Potassium plus a small quantity of Salts (trace elements),
indispensable to the plant growth- the trace elements such as boron, calcium,
copper, iron, magnesium, sulphur and zinc etc. The fresh digested sludge,
especially if the night soil is used, has high ammonia content and in this state
may act like a chemical fertiliser by forcing a large dose of nitrogen than
required by the plant and thus increasing the accumulation of toxic nitrogen
compounds. For this reason, it is probably best to let the sludge age for about
two weeks in open place. The fresher the sludge the more it needs to be diluted
with water before application to the crops, otherwise very high concentration of
nitrogen my kill the plants.
INORGANIC SOLIDS
In village situation the floor of the animals shelters are full of dirt, which
gets mixed with the manure. Added to this the collected manure is kept on the
unlined surface which has plenty of mud and dirt. Due to all this the feed stock
for the BGP always has some inorganic solids, which goes inside the digester
along with the organic materials. The bacteria can not digest the inorganic
solids, and therefore settles down as a part of the bottom most layer inside the
digester. The Inorganic Solids contains mud, ash, sand, gravel and other
inorganic materials. The presence of too much inorganic solids in the digester
can adversely affect the efficiency of the BGP. Therefore to improve the
efficiency and enhance the life of a semi-continuous BGP it is advisable to
empty even it in a period of 5-10 years for thoroughly cleaning and washing it
from inside and then reloading it with fresh slurry.
3.9.8 Classification of Biogas Plants (BGPs)
The simple rural household BGPs can be classified under the following broad
categories- (i) BGP with Floating Gas Holder, (ii) BGP with Fixed Roof, (iii)
BGP with Separate Gas Holder and (iv) Flexible Bag Biogas Plants.
3.9.8.1 Biogas Plant with Floating gas Holder
This is one of the common designs in India and comes under the category of
semi-continuous-fed plant. It has a cylindrical shaped floating biogas holder on
top of the well-shaped digester. As the biogas is produced in the digester, it
rises vertically and gets accumulated and stored in the biogas holder at a
constant pressure of 8-10 cm of water column. The biogas holder is designed to
store 50% of the daily gas production. Therefore if the gas is not used
regularly then the extra gas will bubble out from the sides of the biogas
holder.
3.9.8.2 Fixed Dome Biogas Plant
The plants based on Fixed Dome concept was developed in India in the middle of
1970, after a team of officers visited China. The Chinese fixed dome plants use
seasonal crop wastes as the major feed stock for feeding, therefore, their
design is based on principle of ‘Semi Batch-fed Digester’. However, the
Indian Fixed Dome BGPs designs differ from that of Chinese designs, as the
animal manure is the major substrate (feed stock) used in India. Therefore all
the Indian fixed dome designs are based on the principle of ‘Semi
Continuous-fed Digester’. While the Chinese designs have no fixed storage
capacity for biogas due to use of variety of crop wastes as feed stock, the
Indian household BGP designs have fixed storage capacity, which is 33% of the
rated gas production per day. The Indian fixed dome plant designs use the
principle of displacement of slurry inside the digester for storage of biogas in
the fixed Gas Storage Chamber. Due to this in Indian fixed dome designs have ‘Displacement
Chamber(s)’, either on both Inlet and Outlet sides (like Janata Model) or only
on the Outlet Side (like Deenbandhu or Shramik Bandhu Model). Therefore in
Indian fixed dome design it is essential to keep the combined volume of Inlet
& Outlet Displacement Chamber(s) equal to the volume of the fixed Gas
Storage Chamber, otherwise the desired quantity of biogas will not be stored in
the plant. The pressure developed inside the Chinese fixed dome BGP ranges from
a minimum of 0 to a maximum of 150 cm of water column. And the maximum pressure
is normally controlled by connecting a simple Manometer on the pipeline near the
point of gas utilisation. On the other hand the Indian fixed dome BGPs are
designed for pressure inside the plant, varying from a minimum of 0 to a maximum
of 90 cm of water column. The Discharge Opening located on the outer wall
surface of the Outlet Displacement Chamber and automatically controls the
maximum pressure in the Indian design.
3.9.8.3 Biogas Plant with Separate Gas Holder
The digester of this plant is closed and sealed from the top. A gas outlet pipe
is provided on top, at the centre of the digester to connect one end of the
pipeline. The other end of the pipeline is connected to a floating biogas
holder, located at some distance to the digester. Thus unlike the fixed dome
plant there is no pressure exerted on the digester and the chances of leakage in
the Main Unit of the Plant (MUP) are not there or minimised to a very great
extent. The advantage of this system is that several digesters, which only
function as digestion (fermentation) chambers (units), can be connected with
only one large size gas holder, built at one place close to the point of
utilisation. However, as this system is expensive therefore, is normally used
for connecting a battery of batch-fed digesters to one common biogas holder.
3.9.8.4 Flexible Bag Biogas Plant
The entire Main Unit of the Plant (MUP) including the digester is fabricated out
of Rubber, High Strength Plastic, Neoprene or Red Mud Plastic. The Inlet and
Outlet is made of heavy duty PVC tubing. A small pipe of the same PVC tubing is
fixed on top of the plant as Gas Outlet Pipe. The Flexible Bag Biogas Plant is
portable and can be easily erected. Being flexible, it needs to be provided
support from outside, up to the slurry level, to maintain the shape as per its
design configuration, which is done by placing the bag inside a pit dug at the
proposed site. The depth of the pit should as per the height of the digester
(fermentation chamber) so that the mark of the initial slurry level is in line
with the ground level. The outlet pipe is fixed in such a way that its outlet
opening is also in line with the ground level. Some weight has to be added on
the top of the bag to build the desired pressure to convey the generated gas to
the point of utilisation. The advantage of this plant is that the fabrication
can be centralised for mass production, at the district or even at the block
level. Individuals or agencies having land and some basic infrastructure
facilities can take up fabrication of this BGP with small investment, after some
training. However, as the cost of good quality plastic and rubber is high which
increases the comparative cost of fabricating it. Moreover the useful working
life of this plant is much less, compared to other Indian simple Household BGPs,
therefore inspite of having good potential, the Flexible Bag Biogas Plant has
not been taken up seriously for promotion by the field agencies.
3.9.9 Common Indian Biogas Plant (BGP) Designs
The three of the most common Indian BGP design are- (i) KVIC Model, (ii) Janata
Model and (iii) Deenbandhu Model, which are briefly described in the subsequent
paragraphs:
3.9.9.1 KVIC Model
The KVIC Model is a floating biogas holder semi continuous-fed BGP and has two
types, viz. (i) Vertical and (ii) Horizontal. The vertical type is more commonly
used and the horizontal type is only used in the high water table region. Though
the description of the various components mentioned under this section are
common to both the types of KVIC models (Vertical and Horizontal types) some of
the details mentioned pertains to Vertical type only. The major components of
the KVIC Model are briefly described below:
FOUNDATION
It is a compact base made of a mixture of cement concrete and brick ballast. The
foundation is well compacted using wooden ram and then the top surface is
cemented to prevent any percolation & seepage.
Digester (Fermentation Chamber)
It is a cylindrical shaped well like structure, constructed using the foundation
as its base. The digester is made of bricks and cement mortar and its inside
walls are plastered with a mixture of cement and sand. The digester walls can
also be made of stone blocks in places where it is easily available and cheap
instead of bricks. All the vertical types of KVIC Model of 4 M3 capacity and
above have partition wall inside the digester.
GAS HOLDER
The biogas holder drum of the KVIC model is normally made of mild steel sheets.
The biogas holder rests on a ledge constructed inside the walls of the digester
well. If the KVIC model is made with a water jacket on top of the digester wall,
no ledge is made and the drum of the biogas holder is placed inside the water
jacket. The biogas holder is also fabricated out of fibre glass reinforced
plastic (FRP), high-density polyethylene (HDP) or Ferroconcrete (FRC). The
biogas holder floats up and down on a guide pipe situated in the centre of the
digester. The biogas holder has a rotary movement that helps in breaking the
scum-mat formed on the top surface of the slurry. The weight of the biogas
holder is 8-10 kg/m2 so that it can stores biogas at a constant pressure of 8-10
cm of water column.
INLET PIPE
The inlet pipe is made out of Cement Concrete (CC) or Asbestos Cement Concrete
(ACC) or Pipe. The one end of the inlet pipe is connected to the Mixing Tank and
the other end goes inside the digester on the inlet side of the partition wall
and rests on a support made of bricks of about 1 feet height.
OUTLET PIPE
The outlet pipe is made out of Cement Concrete (CC) or Asbestos Cement Concrete
(ACC) or Pipe. The one end of the outlet pipe is connected to the Outlet Tank
and the other end goes inside the digester, on the outlet side of the partition
wall and rests on a support made of bricks of about 1 feet height. In the case
KVIC model of 3 M3 capacity and below, there is no partition wall, hence the
outlet pipe is made of short and horizontal, which rest fully immersed in slurry
at the top surface of the digester.
BIOGAS OUTLET PIPE
The Biogas Outlet Pipe is fixed on the top middle portion of the biogas holder,
which is made of a small of GI Pipe fitted with socket and a Gate Valve. The
biogas generated in the plant and stored in the biogas holder is taken through
the gas outlet pipe via pipeline to the place of utilisation.
3.9.10 Janata Model
The Janata model consists of a digester and a fixed biogas holder (known as Gas
Storage Chamber) covered by a dome shaped enclosed roof structure. The entire
plant is made of bricks and cement masonry and constructed underground. Unlike
the KVIC model, the Janata model has no movable part. A brief description of the
different major components of Janata model is described below:
Foundation
The foundation is well-compacted base of the digester, constructed of brick
ballast and cement concrete. The upper portion of the foundation has a smooth
plaster surface.
Digester
The digester is a cylindrical tank resting on the foundation. The top surface of
the foundation serves as the bottom of the digester. The digester (fermentation
chamber) is constructed with bricks and cement mortar. The digester wall has two
small rectangular openings at the middle, situated diametrically opposite, known
as inlet and outlet gate, one for the inflow of fresh slurry and the other for
the outflow of digested slurry. The digester of Janata BGP comprises the
fermentation chamber (effective digester volume) and the gas storage chamber (GSC).
Gas Storage Chamber (GSC)
The Gas Storage Chamber (GSC) is also cylindrical in shape and is the integral
part of the digester and located just above the fermentation chamber. The GSC is
designed to store 33% (approx. 8 hours) of the daily gas production from the
plant. The Gas Storage Chamber (GSC) is constructed with bricks and cement
mortar. The gas pressure in Janata model varies from a minimum of 0 cm water
column (when the plant is completely empty) to a maximum of up to 90 cm of water
column when the plant is completely full of biogas.
Fixed Dome Roof
The hemi-spherical shaped dome forms the cover (roof) of the digester and
constructed with brick and cement concrete mixture, after which it is plastered
with cement mortar. The dome is only an enclosed roof designed in such a way to
avoid steel reinforcement. (Note: The gas collected in the dome of a Janata
plant is not under pressure therefore can not be utilised. It is only the gas
stored in the Gas Storage Chamber (GSC) portion of the digester and that is
under pressure and can be said as utilisable biogas).
Inlet Chamber
The upper portion of the Inlet Chamber is in the shape of bell mouth and
constructed using bricks and cements mortar. Its outer wall is kept inclined to
the cylindrical wall of the digester so that the feed material can flow easily
into the digester by gravity. The bottom opening of the Inlet Chamber is
connected to the Inlet Gate and the upper portion is much wider and known as
Inlet Displacement Chamber (IDC). The top opening of the inlet chamber is
located close to the ground level to enable easy feeding of fresh slurry.
Outlet Chamber
It is a rectangular shaped chamber located just on the opposite side of the
inlet chamber. The bottom opening of the Outlet Chamber is connected to the
Outlet Gate and the upper portion is much wider and known as Outlet Displacement
Chamber (ODC). The Outlet Chamber is constructed using bricks and cement mortar.
The top opening of the Outlet Chamber is located close to the ground level to
enable easy removal of the digested slurry through a discharge opening. The
level of the discharge opening provided on the outer wall of the outlet chamber
is kept at a somewhat lower level than the upper mouth of the inlet opening, as
well as kept lower than the Crown of the Dome ceiling. This is to facilitate
easy flow of the digested slurry out the plant in to the digested slurry pit and
also to prevent reverse flow, either in the mixing tank through inlet chamber or
to go inside the gas outlet pipe and choke it.
Biogas Outlet Pipe
The Biogas Outlet Pipe is fixed at the crown of the dome, which is made of a
small length of GI Pipe fitted with socket and a Gate Valve.
3.9.10.1 Deenbandhu Model
The Deenbandhu Model is a semi continuous-fed fixed dome Biogas plant. While
designing the Deenbandhu model an attempt has was made to minimise the surface
area of the BGP with a view to reduce the installation cost, without
compromising on the efficiency. The design essentially consists of segments of
two spheres of different diameters joined at their bases. The structure thus
formed comprises of (i) the digester (fermentation chamber), (ii) the gas
storage chamber, and (iii) the empty space just above the gas storage chamber.
The higher compressive strength of the brick masonry and concrete makes it
preferable to go in for a structure that could be always kept under compression.
A spherical structure loaded from the convex side will be under compression and
therefor, the internal load will not have any effect on the structure.
The digester of the Deenbandhu BGP is connected with the Inlet Pipe and the
Outlet Tank. The upper part (above the normal slurry level) of the outlet tank
is designed to accommodate the slurry to be displaced out of the digester
(actually from the gas storage chamber) with the generation and accumulation of
biogas and known as the Outlet Displacement Chamber (ODC). The Inlet Pipe of the
Deenbandhu BGP replaces the Inlet Chamber of Janata Plant. Therefore to
accommodate all the slurry displaced out from the Gas Storage Chamber (GSC), the
volume of the Outlet Chamber of Deenbandhu model twice the volume of the Outlet
Tank of the Janata BGP of the same capacity.
Being a fixed dome technology, the other components and their functions are same
as in the case of Janata Model BGP and therefore are not elaborated here once
again.
3.9.10.2 Shramik Bandhu Model
This new BRCM biogas plant model which is also a semi-continuous hydraulic
digester plant was designed by the author and christened as SHRAMIK BANDHU
(friend of the labour). Since then, three more models (rural household plants)
in the family of SHRAMIK BANDHU Plants have also been developed. The second one,
a semi-continuous hydraulic digester, works on the principle of semi-plug flow
digester (suitable for use as a Night Soil based or Toilet attached plant). The
third one uses simple low cost anaerobic bacterial filters, designed for
possible application as a Low Cost and low Maintenance Wastewater Treatment
System. The fourth one is a semi-batch fed hydraulic digester, ideally suitable
for the regions where plenty of seasonal crop wastes and waste green biomass are
available and population of domestic farm animals are less, for producing the
desired quantity of biogas from it alone. For this reason the first model which
is the simplest and cheapest in the family of Shramik Bandhu plants, is
christened as SBP-I Model. The other three models, yet to be field evaluated,
are, SBP-II, SBP-III and SBP-IV, respectively.
The family of SHRAMIK BANDHU biogas plants designs uses the fixed dome concepts
as in the case of pervious two most popular Indian fixed dome plants, namely,
Janata and Deenbandhu models. In other words, all the four Models of the family
of SHRAMIK BANDHU Plant have both, (i) the Gas Storage Chamber (GSC) and (ii)
the Dome shaped Roof. However, in this section, the description about Shramik
Bandhu plants relates to SBP-I model only.
The SHRAMIK BANDHU Plant is made of Bamboo Reinforced Cement Mortar (BRCM),
by pre-fabricated bamboo shells, using the correct size mould for a given
capacity SBP-I model- Thus, completely replacing the bricks. These bamboo shells
are made by weaving bamboo strips (weaving of which can be done in the village
itself) for casting a BRCM structure. The BRCM structures on the one hand are
used for giving the right shape to this plant, while on the other hand acts as
the reinforcement to the cement mortar plaster as it is casted more or less like
the ferro-cement structure. In order to protect the bamboo strips from microbial
attack, they are pre-treated by immersing them in water mixed with prescribed
ratio of Copper Sulphate (CuSO4) for a minimum of 24 hours before weaving of
shell structure is done. As a further safety measure DPC powder in appropriate
quantity is mixed while doing second layer (coat) of smooth plastering on the
Main Unit of the Plant (MUP), Outlet Chamber (OC); as well as other BRCM
components and sub-components, to make the entire structure of SBP-I moisture
proof. The Shramik Bandhu plant made from BRCM would be much stronger because it
has both higher tensile, as well as compressive strength, as compared to either
First Class Bricks or Cement Concrete (CC) or Cement Mortar (CM), when used
alone. The reason for this is that the bamboo shell structures used (for both
reinforcement and shape of the plant) for the construction of Shramik Bandhu
plant is made by weaving strips [only the outer harder surface (skin) and not
the softer inner part of bamboo] from seasoned (properly cured) bamboo.
Therefore, the entire structure (body) of the SBP-I model would be very strong,
durable and have long useful working life. The two previous fixed dome models,
namely Janata and Deenbandhu model have no reinforcement and are made of Bricks
and Cement Mortar only, therefore, while they are very strong under compression
but cannot withstand high tensile force. The hemi-spherical shell shaped
(structure) of SHRAMIK BANDHU (SBP-I) model loaded from top on its convex side
will be under compression. However, due to comprehensive strength provided by
both cement mortar, as well as the reinforcement provided by the woven bamboo
shell will ensure that the internal and external load will not have any residual
effects on the structure. The bamboo reinforcement will provide added strength
(both tensile and compressive) to make the entire structure of SHRAMIK BANDHU (SBP-I)
model very sound, as compared to the previous two fixed dome Indian models (Janata
& Deenbandhu), referred above.
The digester of SBP-I model is connected to the slurry mixing tank with inlet
pipe made of 10 cm or 100 mm (4”) diameter Asbestos Cement Concrete (ACC)
pipe, for feeding the slurry inside the plant.
The Outlet Displacement Chamber (ODC) is designed to accommodate the slurry to
be displaced out of the digester with the generation & accumulation of
biogas. The Outlet Displacement Chamber (ODC) of SBP-I model is also kept
hemi-spherical in shape to reduce it’s surface area for a given volume (to
save in building materials and time taken for construction)- The ODC is also
made of BRCM, using a hemi-spherical shaped woven bamboo shell structure.
A Manhole opening of about 60 cm or 600 mm (2.0 Ft) diameter is provided on the
crown of the hemi-spherical shaped ODC. The Manhole is big enough for one person
to go inside and come out, at the same time small enough to be able to easily
close it by a same size Manhole Cover, which is also made of BRCM.
COMPONENTS OF SHRAMIK BANDHU (SBP-I MODEL) BIOGAS PLANT (BGP)
The Shramik Bandhu (SBP-I) Model is made of two major components and several
minor components and sub-components. They are categorized as, (a) Main Unit OF
The Plant (MUP), (b) Outlet Chamber (OC) and (c) Other Minor Components. These
major and minor components are further divided into sub-components, as given
below:
Main Unit Of the plant (MUP)
The Main Unit of the Plant (MUP) is one of the major components of Shramik
Bandhu (SBP-I) Model. The MUP has following six main “Sub-Components”:
(i). Digester {or Fermentation Chamber (FC)}
(ii). Gas Storage Chamber (GSC)
(iii). Free Space Area (FSA), located just above the GSC
(iv). Dome (Roof of the Plant-entire area located just above the FSA); and
(v). The following three other sub-components:
[{(e)-(i) the Foundation of the MUP & (e)-(ii)} the Ring Beam for MUP (these
two have also been considered here as the two sub-components of the MUP} and
{the third is (e)-(iii) the Gas Outlet Pipe (GIP), for better explanation &
understanding of the constructional aspects of SBP-I Plant].
Outlet Chamber
The Outlet Chamber (OC)) is the second major component of Shramik Bandhu (SBP-I)
Model. The OC has the following four main “Sub-Components”:
(i). Outlet Tank (OT)
(ii). Outlet Displacement Chamber (ODC)
(iii). Empty Space Area (ESA) above the ODC- though for all practical purpose
the ODC includes the Empty Space Area (ESA) above it; however, from the
designing point of view, the effective ODC of SBP-I model is considered up to
the starting of discharge opening located on its outer wall
(iv). Discharge Opening (DO)
Minor Components of the SBP-I Plant
The Minor Components of the Shramik Bandhu (SBP-I) Model are as follows:
(i). Inlet Pipe (IP)
(ii). Outlet Gate (OG)
(iii). Mixing Tank (MT) or Slurry Mixing Tank (SMT)
(iv). Short Inlet Channel (SIC)
(v). Gas Outlet Pipe (GOP)
(vi). Grating (made of Bamboo Sticks)
(vii). Manhole Cover (MHC) for ODC
Being a fixed dome technology, the components and their functions are same as in the case of Janata and Deenbandhu Model BGP and therefore not elaborated here once again.
3.10 Conversion of biomass into electricity
Historically one of the earliest alternatives to fossil fuels is a wood fired
boiler producing steam which powers an engine driving a generator. This,
unfortunately is about the only advantage. But the steam power has all the
disadvantages of an engine/generator and even several more. The wood must be
chopped and carried, cured, split, and fed, just as for any wood stove. Ashes
must be handled and hauled. The entire installation requires constant control
while it is running. Due to compounds in some of the feedstocks, “slagging and
fouling” can occur. Slagging is accumulation of solid residues on parts
of the combustion system. Fouling is simply the accumulation of liquid or
semi-liquid residue. This is an important aspect of plant operation and
operators need to understand how biomass differs from more commonly used fuels.
3.10.1 Gasification
Usually, electricity from biomass is produced via the condensing steam turbine,
in which the biomass is burned in a boiler to produce steam’ which is expanded
through a turbine driving a generator. The technology is well-established,
robust and can accept a wide variety of feedstocks. However, it has a relatively
high unit-capital cost and low operating efficiency with little prospect of
improving either significantly in the future. There is also the inherent danger
in steam. Steam occupies about 1200 times the volume of water at atmospheric
pressure (known as “gage” pressure). Producing steam requires heating water
to above boiling temperature under pressure. Water boils at 100° C at sea
level. By pressurizing the boiler it is possible to raise the boiling
temperature of water much higher. Elevating steam temperature has to be done to
use the generated steam for any useful work otherwise the steam would condense
in the supply lines or inside the cylinder of the steam engine itself.
Gasification is the newest method to generate electricity from biomass.
Instead of simply burning the fuel, gasification captures about 65-70% of the
energy in solid fuel by converting it first into combustible gases. This
gas is then burned as natural gas is, to create electricity, fuel a vehicle, in
industrial applications, or converted to synfuels-synthetic fuels. Since
this is the latest technology, it is still under development.
A promising alternative is the gas turbine fuelled by gas produced from biomass
by means of thermochemical decomposition in an atmosphere that has a restricted
supply of air. Gas turbines have lower unit-capital costs, can be considerably
more efficient and have good prospects for improvements of both parameters.
Biomass gasification systems generally have four principal components:
(a) Fuel preparation, handling and feed system;
(b) Gasification reactor vessel;
(c) Gas cleaning, cooling and mixing system;
(d) Energy conversion system (e.g., internal-combustion engine with generator or
pump set, or gas burner coupled to a boiler and kiln).
When gas is used in an internal-combustion engine for electricity production
(power gasifiers), it usually requires elaborate gas cleaning, cooling and
mixing systems with strict quality and reactor design criteria making the
technology quite complicated. Therefore, “Power gasifiers world-wide have had
a historical record of sensitivity to changes in fuel characteristics, technical
hitches, manpower capabilities and environmental conditions”.
Gasifiers used simply for heat generation do not have such complex requirements
and are, therefore, easier to design and operate, less costly and more energy-
efficient.. All types of gasifiers require feedstocks with low moisture and
volatile contents. Therefore, good quality charcoal is generally best, although
it requires a separate production facility and gives a lower overall efficiency.
In the simplest, open-cycle gas turbine the hot exhaust of the turbine, is
discharged directly to the atmosphere. Alternatively, it can be used to produce
steam in a heat recovery steam generator. The steam can then be used for heating
in a cogeneration system; for injecting back into the gas turbine, thus
improving power output and generating efficiency known as a steam-injected gas
turbine (STIG) cycle; or for expanding through a steam turbine to boost power
output and efficiency - a gas turbine/steam turbine combined cycle (GTCC)
(Williams & Larson, 1992). While natural gas is the preferred fuel, limited
future supplies have stimulated the expenditure of millions of dollars in
research and development efforts on the thermo-chemical gasification of coal as
a gas-turbine feedstock. Much of the work on coal-gasifier/gas-turbine systems
is directly relevant to biomass integrated gasifier/gas turbines (BlG/GTs).
Biomass is easier to gasify than coal and has a very low sulphur content. Also,
BIG/GT technologies for cogeneration or stand-alone power applications have the
promise of being able to produce electricity at a lower cost in many instances
than most alternatives, including large centralized, coal-fired, steam-electric
power plants with flue gas desulphurization, nuclear power plants, and
hydroelectric power plants.
Gasifiers using wood and charcoal (the only fuel adequately proved so far) are
again becoming commercially available, and research is being carried out on ways
of gasifying other biomass fuels (such as residues) in some parts of the world.
Problems to overcome include the sensitivity of power gasifiers to changes in
fuel characteristics, technical problems and environmental conditions. Capital
costs can still sometimes be limiting, but can be reduced considerably if
systems are manufactured locally or use local materials. For example, a
ferrocement gasifier developed at the Asian institute of Technology in Bangkok
had a capital cost reduced by a factor of ten. For developing countries, the
sugarcane industries that produce sugar and fuel ethanol are promising targets
for near-term applications of BIG/GT technologies.
Gasification has been the focus of attention in India because of its potential
for large scale commercialization. Biomass gasification technology could meet a
variety of energy needs, particularly in the agricultural and rural sectors. A
detailed micro- and macroanalysis by Jain (1989) showed that the overall
potential in terms of installed capacity could be as large as 10,000 to 20,000
MW by the year 2000, consisting of small-scale decentralized installations for
irrigation pumping and village electrification, as well as captive industrial
power generation and grid fed power from energy plantations. This results from a
combination of favourable parameters in India which includes political
commitment, prevailing power shortages and high costs, potential for specific
applications such as irrigation pumping and rural electrification, and the
existence of an infrastructure and technological base. Nonetheless, considerable
efforts are still needed for large- scale commercialization.
3.10.2 CO-FIRING
Co-firing of biofuels (e.g. gasified wood) and coal seems to be the way how to
reduce emissions from coal firing power plants in many countries. In 1999 a new
co-firing system - biomass and coal - started its operation in Zeltweg
(Austria). A 10 MW biomass gasification unit was installed in combination with
an existing coal fired power station. The gasifier needs 16 m3 woody biomass
(chips and bark) per hour. The calorific value of the gas ranges between 2,5 - 5
MJ/Nm3. The project named “Biococomb” is an EU demonstration project. It was
realised by the “Verbund” company together with several other companies from
Italy, Belgium, Germany and Austria and co-financed by the European Commission.