Energy from Biogas

Table of Contents

Biogas is generated mainly from organic wastes such as cattle manure and other bio-degradable organic wastes through anaerobic digestion. The estimated potential of household biogas based plants based on animal wastes is 12 million.

Applications: Biogas can be used for:

Benefits: 

  • Clean and renewable source of energy.
  • Efficient way of energy conversion to clean gaseous fuel and saves forests and fossil fuels.
  • Saves women and children from drudgery of collecting firewood, exposure to smoke in kitchen, and time saving in cooking and cleaning.
  • Produces enriched organic manure, rich in nitrogen, which can supplement chemical fertilizer.
  • Leads to improvement in environment and sanitation and hygeene.
  • Provides a source for decentralized electrical / mechanical power generation and also for refrigeration and cooling.
  • Leads to employment generation for rural areas and augments fuel supply.

Technology

Anaerobic digestion comprises three steps:

  • Decomposition (hydrolysis) of plant, animal or other biomass matter to breakdown 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 - 40 °C with optimum of 35 °C. It is also possible to operate in the thermophilic range 55 - 65 °C under controlled conditions. Apart from temperature, the rate of biogas production depends on factors such as the carbon-nitrogen ration, hydraulic reaction time, solid concentration and type of feedstock.

Biogas mainly consists of methane, carbon dioxide and traces of hydrogen sulphide, hydrogen and nitrogen. The relative percentage of these gases depends on the quality of feed and the process condition. The percentage of methane in the gas determines its calorific value, as other constituents do not contribute to energy content. The methane content of biogas is appreciably high, 60 %, which is high enough for use in many energy applications including power generation. The content of methane can be enhanced to 97 % by enrichment and purification.

Components of biogas plant: 

  • Mixing tank: The feed in collected in the mixing tank. Water is added till ratio of feed and water is 50:50 and then it is throughly mixed till a homogeneous slurry is formed.
  • Inlet pipe: To discharged the slurry into the digester.
  • Digester: For fermentation of slury and production of biogas through bacterial action.
  • Gas holder / storage dome: For collection of biogas.
  • Outlet pipe: For discharging the digested slurry into the outlet tank.
  • Gas pipeline: To carry the gas to the point of utilization, such as stove or biogas engine, after removal of moisture and hydrogen-sulphide.

Types of Biogas Plants

Fixed-dome type: It consists of one lower segment (for the digester) and a hemisphere over it (for both the digester and gas holder). The mixing tank is connected to the digster by a 15 cm asbestos cement pipe. Throgh the outlet opening provided in the digester, the slurry is pushed into the outlet tank and automatically overflows through another slurry discharge hole provided on the upper edge of sidewall of outlet tank.

Floating-drum type: It consists of a deep well-shaped underground digester connected by inlet and outlet pipes. A MS, FPR, or HDPE material gas storage drum is put inverted over the digester, which moves up or down along a guide pipe corresponding to the accumulation and withdrawl of gas from plant.

Prefabricated type: It is made up of HDPE / rubberized nylon fabric and components are portable, which can be conveniently placed at any location. The model of choice can be selected on the basis of gas requirements and distance between kitchen and cattle shed.

Standard capacity: For family type biogas, 1 - 6 m³ and 1 - 10 m³ capacities are available in Fixed and Floating type respectively. Most commonly used are 1 - 4 m³ capacity. Medium and large size biogas plants of capacity 25 - 1000 m³ have been designed for community, institutional and industrial applications.

Cost: The average cost of Fixed Dome (Deenbandhu) and Floating Dome (KVIC) models is:

Plant capacity (m³)Fixed DomeFloating Dome
112 00015 000
217 00024 000
320 00028 000
424 00035 000
632 00045 000

The cost of plant increases in hilly regions by about 30 % and in NE region by 50 - 60 %.

Cost Effectiveness: The lifespan of biogas plant is above 20 years. However, taking into account the utilization pattern, socio-economic aspects and climatic factors, the average life of familty type plant is 15 years. A comparison of costs of diesel and kerosene, shows that biogas plant is cost effective. Morever, a biogas plant genetated about 30 man-days of employment and 1.2 tonnes of organic manure.

Biogas Based Distributed / Grid Power Generation

Biogas based power plants are a reliable decentralized power generation option in the country, especially in the capacity range of 3 - 250 kW, to provide electricity to individual / community / grid.

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