Energy Techonologies - 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 (30^oC-40^oC) with an optimum of 35^oC. It is also possible to operate plants in the thermophilic range (55^oC-65^oC) 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

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, rises 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

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

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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