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Biogas Plant
Introduction
Biogas is a clean fuel produced through anaerobic digestion
of a variety of organic wastes: animal, agricultural, domestic,
and industrial.
Anaerobic digestion comprises three steps.
- Decomposition (hydrolysis) of plant or animal matter to
break down complex organic materials into simple organic
substances
- Conversion of decomposed matter into organic acids
- Conversion of acids into methane gas
As the process temperature affects the rate of digestion, it
should be maintained in the mesophilic range (30oC-40oC) with
an optimum of 35oC. It is also possible to operate plants in
the thermophilic range (55oC-65oC) under controlled conditions.
Apart from temperature, the rate of biogas production also depends
on factors such as the carbon: nitrogen ratio, hydraulic retention
time, solid concentration, and types of feedstock.
Biogas consists of methane, carbon dioxide, and traces of
other gases such as hydrogen, carbon monoxide, nitrogen, oxygen,
and hydrogen sulphide. The gas mixture is saturated with water
vapour and may contain dust particles. The relative percentages
of these gases depend on the quality of feed material and
the process conditions. The percentage of methane in the gas
determines its calorific value, as the other constituents
do not contribute to the energy content. The methane content
of biogas is appreciably high, at 60%. This provides a calorific
value high enough to find use in many energy applications,
including power generation. Table 1 provides a comparison
of the calorific values of various fuels.
Comparison of the calorific values of
various fuels
| Fuel |
Calorific Value (Approximate)
|
| Natural Gas |
8600 kcal per m3 |
| Liquefied petroleum gas
|
10 800 kcal per kg |
| Kerosene |
10 300 kcal per kg |
| Diesel |
10 700 kcal per kg |
| Biogas |
5000 kcal per m3 |
Components of Biogas Plants
- Mixing tank: The feed material (dung) is collected in
the mixing tank. Sufficient water is added and the material
is thoroughly mixed till a homogeneous slurry is formed.
- Inlet pipe: The substrate is discharged into the digester
through the inlet pipe/tank.
- Digester: The slurry is fermented inside the digester
and biogas is produced through bacterial action.
- Gas holder or gas storage dome: The biogas gets collected
in the gas holder, which holds the gas until the time of
consumption.
- Outlet pipe: The digested slurry is discharged into the
outlet tank either through the outlet pipe or the opening
provided in the digester.
- Gas Pipeline: The gas pipeline carries the gas to the
point of untilization such as a stove or lamp.
Types of Biogas plants 
The fixed-dome biogas plant consists of one lower segment
(for the digester) and a hemisphere over it (for both digester
and gas holder). The mixing tank is connected to the digester
by a 15-cm asbestos cement pipe. Through the outlet hole provided
in the digester, the slurry is pushed into the outlet tank
and overflows through another hole provided in the outlet
tank.
Floating-drum type
 The
floating-drum biogas plant consists of a deep well-shaped
underground digester connected by inlet and outlet pipes.
A mild-steel gas storage drum, inverted over the slurry, xrises
and falls around a guide pipe corresponding to the accumulation
and withdrawal of gas.
 Bag
type
Made of rubberized nylon fabric, the bag-type biogas plant
is a portable unit, which can conveniently be placed at any
location.
The appropriate model is selected on the basis of technical
requirements such as location,distance between kitchen and
cattle shed, availability of dung and water, preferences of
the beneficiaries, and so on.
Standard Capacity
For family-type biogas plants, approved models are available
for 1-6 m3 and 1-10 m3 capacities for fixed-dome and floating-drum
plants, respectively. The commonly used capacities of these
models are 1-4 m3.
 Area
of Applications
Cooking Biogas can be used for cooking in a specially designed
burner. A biogas plant of 2 m3 capacity is sufficient for
providing cooking fuel to a family of four to five.
Lighting Gas lamps can be fuelled by biogas. To power a 100
candle lamp (60 W), the biogas required is 0.13 m3 per hour.
Power generation: Biogas
can be used to operate a dual-fuel engine and can replace
up to 75% of the diesel.
Biogas digested slurry
The use of biogas digested slurry as organic manure can supplement
the usage of chemical fertilizers. The effluent manure does
not produce any odour and hence does not create any pollution.
The biogas slurry is rich in nitrogen, the essential nutrient
for plant growth. Moreover, this nitrogen is in water-soluble
form and can be easily absorbed by the plants.
The biogas slurry can also be mixed with biomass and composted.
The slurry being full of bacteria (which can break down vegetable
matter) makes excellent composting material. When a large
mass of biomass waste with high carbon: nitrogen ratio is
available, the slurry helps stimulate decomposition by acting
as a nitrogen primer. Earthworms could either be cultured
in dry sludge or in a compost pit.
Benefits
- Provides a non-polluting and renewable source of energy
- Efficient way of energy conversion (saves fuelwood)
- Saves women and children from drudgery of collection and
carrying of firewood, exposure to smoke in the kitchen,
and time consumed for cooking and cleaning of utensils
- Produces enriched organic manure, which can supplement
chemical fertilizers
- Leads to improvement in the environment, and sanitation
and hygiene
- Provides a source for decentralized power generation
- Leads to employment generation in the rural areas
Cost
The cost of installation varies according to the model and
size of the plant. The average estimated cost of the most
popular Deenbandhu model is given in Table.
The cost of a biogas plant increases by about 30% in hilly
areas and by 50%-60% in the north-eastern region of the country.
The average estimated cost of the most
popular Deenbandhu model
| Plant Capacity |
Cost per Plant |
| 1 M3 |
Rs 5500 |
| 2 M3 |
Rs 9000 |
| 3 M3 |
Rs 10 500 |
| 4 M3 |
Rs 13 500 |
Strategy for promoting the Technology
The MNES provides central financial assistance for construction
and maintenance of biogas plants, development of skilled manpower,
training for use and maintenance, awareness creation, and
support to implementing agencies and technical centres for
implementation of the programme.
Financial Incentives
Under the National Biogas Programme, central subsidy is available
to users of difference categories and areas for setting up
biogas plants
National Biogas programme: central subsidy for different
categories and areas for setting up biogas plants
| Category/Area |
Central subsidy per plant |
| North-eastern states
and Sikkim (except plain areas of Assam) |
Rs 11 700 |
| Plain areas of Assam
|
Rs 9000 |
| Jammu and Kashmir, Himachal
Pradesh, Uttaranchal ( excluding Terai region), Nilgiris
of Tamil Nadu, Sadar Kurseong and Kalimpong sub-divisions
of Darjeeling, Sunderbans, Andaman and Nicobar Islands |
Rs 4500 (restricted to
Rs 3500 for 1 m3 fixed dome type) |
| Sucheduled Caste, Scheduled
Tribe, desert districts, small and marginal farmers, landless
labourers, Terai region of Uttaranchal, Western Ghats
and other notified hilly areas |
Rs 3500 (restricted to
Rs 2800 for 1 m3 fixed dome type) |
| All others |
Rs 2700 (restricted to
Rs 2100 for 1 m3 fixed dome type) |
Main Application: Energy
Contact:
Ministry of Non-Conventonal Energy Sources,
Govt. of India
Block-14, CGO Complex, Lodi Road
New Delhi – 110 003
Tel: 91 11 2436 0707/ 2436 0404
Fax: 91 11 2436 1298
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