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Sustainable Energy Solutions to Reduce Poverty in South Asia
- MANUAL-


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

25

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

210

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).

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