3.1.2.
Biogas
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.
Thus the BGP also acts as
mini bio-fertilizer factory; hence some people refer it as ‘Biogas fertilizer plant’ or ‘Bio-manure
plant’. For example, in a semi-continuous BGP, the fresh organic
material (in homogenous slurry form) is fed into the plant from one end
known as ‘inlet pipe’ or ‘inlet tank’. Decomposition
(fermentation) takes place inside the digester as a result of bacterial
(microbial) action, which produces biogas and organic fertilizer (manure)
rich in humus and other nutrients. There is a provision for storing biogas
on the upper portion of the BGP. Some BGP designs have floating gasholder
while 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.
Domestic biogas plants installed in India and other
South Asian countries use bovine (cattle or/and buffalo) dung mixed with
equal quantity of water to maintain 8- 12% (average 10%) total solids
(TS) concentration in the influent slurry. Whereas, the effluent has
9.5- 9.75% TS (average) discharged from the plants is, in general, collected
into the slurry pits or spread onto the ground for drying before transportation
to fields for use as organic manure.
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. Waste from the kitchen, human and animal waste, indigenous
plants or residues from agriculture crops can be used for the production
of biogas.
Applications:
- Replacement of firewood
as the main fuel for cooking. It may also be used for direct lighting
using especially
designed biogas lamps,
where there is no electricity.
- Generation of electricity
using internal combustion engines or gas turbines.
Advantages:
- As a replacement
for firewood the biogas reduces pressure on forests, and can aid
to re-forestation projects.
- Biogas provides safe
and environmentally sound way to dispose off a variety
of organic wastes, thereby improving
local health and sanitation.
- Solid residues from biogas
production can be used in compost making.
Disadvantages:
- The
system (especially batch-fed digesters) must be maintained and
cleaned regularly with periodic removal of solid residues.
- Usual
safety precautions must be observed for any gas distribution system.
A household digester unit or family size biogas plant normally has the
gas production capacity to meet all the cooking and 2-4 hours of lighting
(using biogas lamps with single mental) needs of a family. In India 1
m3 biogas plant refers to the rated capacity of that particular unit
which has been designed to produce 1 m3, 1000 lt. or 35 ft3 gas per day
under optimal conditions.
SL. No |
Plant Capacity |
Av. Daily Fresh Bovine Dung
and Slurry (ratio of 1 Kg. Dung :1 Lt. Water) Requirement |
Approx. No. of Av. Size
Cattle |
No. of family members (cooking
and lighting requirement) |
Average cost as on Sept
1, 2006 |
|
|
|
Fresh dung |
Fresh Slurry |
|
|
|
|
(M3) |
(Ft3) |
(Kg) |
(Lt.) |
(Nos.) |
(Nos.) |
(Indian Rs.) |
(a) |
(b) |
(c) |
(d) |
(e) |
(f) |
(g) |
|
1 |
1 |
35 |
|
50 |
2-3 |
3-4 |
10000 |
2 |
2 |
70 |
50 |
100 |
4-6 |
6-8 |
12000 |
3 |
3 |
105 |
75 |
150 |
6-9 |
9-12 |
15000 |
4 |
4 |
140 |
100 |
200 |
8-12 |
12-16 |
18000 |
5 |
6 |
|
150 |
300 |
12-18 |
18-24 |
24000 |
Table 3.2: Capacity,
daily requirements of cattle dung and biogas produced,
average cost of fixed dome biogas
plant in India. Smallest size BGP having 1 m3 capacity can meet the cooking and lighting
needs of generally a small family of 3-4 members. Whereas, the biggest
family size of 6-m3 capacity plant can fully meet domestic needs of
a comparatively large joint family (18-24 members).
Capacity
rating of biogas plant- Indian Vs Chinese & International
Terms
It is very important to know how a biogas plant is rated in different
countries, other wise the confusion may arise among the practitioners
and implementers of biogas technologies, especially for new people
entering in to this field. This aspect is clarified further in the
subsequent paragraphs, by giving examples of two main countries (India
and China) involved in the promotion and implementation of simple
household biogas models, especially for rural applications.
In India, where the biogas is mainly generated using cattle manure
(dung), the capacity of RHh or FSBG plants is defined as the quantity
of biogas produced from it in a day (24 hours) and measured in terms
of cubic meter (cum or m3) or liters (lt.) or cubic feet (cft or
ft3). Thus, in India a 1 m3 biogas plant refers to the rated capacity
of that particular unit which has been designed to produce (generate)
1 m3 (or 1000 lt. or 35 ft3) per day (24 hours) under optimum conditions.
This daily biogas generation is the average quantity of biogas, worked
out on the basis of annual gas production data for a given temperature
zone and corresponding HRT (and does not relate to either the maximum
or minimum biogas production on any season or a given day).
Similarly, 2, 3, 4 & 6 m3 (cum), and so on implies
that these biogas units have the rated (designed) capacity
to
generate an
average daily production of 2 m3, 3 m3, 4 m3 and 6 m3
biogas per day etc.,
respectively, and so on, under optimum conditions. This
is by and large accepted by individual and groups involved
in
the development,
promotion, financing including the end users (rural people)
of BGPs in India.
In China, the size (capacity) of biogas units is commonly referred
in terms of digester volume. This is most appropriate as the Chinese
use different types of crop wastes and green biomass etc. available
in the season. They also use combination (mixture) of crop residues,
which are not likely to give uniform gas production. Therefore, in
China it is more logical to refer the capacity of biogas units in
terms of digester (fermentation chamber) volume.
These two examples from India and China point out that one has to
be careful in talking (referring) about the capacities of biogas
units as it can cause lots of confusion.
As far as, the capacities of biogas plants are concerned, the most appropriate
(and perhaps the most scientific too) way would be to refer the capacity of plant
in terms of digester volume. The reason for this is that the volume of a given
digester is always fixed but even for animal manure and night soil, the average
biogas production [in a simple rural house-hold (family size) unit] always varies
and have rarely found to be close to the rated (designed) capacity. The rural
peasants in South Asian countries, operate their plants under varied field conditions
and most of the times do not follow even some of the simple norms for plant operation
on day-to-day basis, hence there is always a variation in biogas production as
compared to its rated (designed) capacity, hence there is a case to redefine
the norm for common International term for referring capacity of biogas plant,
to be followed by all the countries.
|
Components of a simple household
biogas plant (BGP)
Indian Semi-continuous-flow hydraulic digester household biogas plants
are the most popular and common biogas plants built in rural India, which
are floating gasholder model (KVIC) and fixed dome model (e.g. Janata
and Deenbandhu model). So, the major components of only SCFHD biogas
plants are: (i) Digester, (ii) Gas-holder or Gas storage chamber, (iii)
Inlet, (iv) Outlet, (v) Mixing tank, and (vi) Gas outlet pipe.
Classification of Biogas Plants (BGPs)
1. Floating
Gas Holder Biogas Plant:
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 normally designed to store 50% of the daily gas
production.
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 feedstock
for feeding; therefore, their design is based on principle of ‘semi-batch-fed
digester’. 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’. 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 is located
on outer wall surface of the outlet displacement chamber and it spontaneously
controls the maximum pressure.
3. Flexible
Bag Biogas Plant:
Main Unit of the Plant (MUP) including the digester is fabricated by
using rubber, high strength plastic, neoprene or red mud plastic. The
inlet and outlet are made of heavy duty PVC tube. A small pipe of same
PVC tube is fixed on top of the plant as Gas Outlet Pipe. Flexible bag
biogas plant is portable and can easily be erected. It requires 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
on site. The depth of the pit should in proportion to height of the digester
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 utilization. Advantage of flexible bag plant is that
the fabrication can be centralized for mass production. Individuals or
agencies having land and basic infrastructure 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. Moreover, the working
life of this plant is also much less compared to other Indian household
BGPs.
There are other plants available in India but are not very popular as
Tunnel Shaped Biogas Plant and Split Design Biogas Plant (With Separate
Gas Holder).
Back
to the Contents
|