7 Biogas

Brought to you by Anant Kapoor and Alfie Wilson

Objectives • that biogas, mainly consisting of methane,

can be produced by the anaerobic fermentation of waste materials containing carbohydrates

• that when biogas is burnt it can provide energy for domestic and industrial use

• that different designs of biogas generators are suitable for different circumstances.

Outcomes • Most students should be able to:• describe the nature of biogas • explain how biogas is produced in a generator.• Some students should also be

able to:• evaluate the advantages and disadvantages of

given designs of biogas generator.

Specification • Biogas, mainly methane, can be produced by

anaerobic fermentation of a wide range of plant products or waste material containing carbohydrates.

• Evaluate the use of biogas generators.

Biology B3 4.7 Biogas

Answers to in-text questions

a Methane b One in which energy is released.

Summary answers

1 Biogas is a biofuel and is mainly methane with varying amounts of carbon dioxide, water and other gases. It is made by bacteria anaerobically breaking down plant and animal waste material that contains carbohydrates. 2 a More than one generator is needed. As one is being cleaned out, another is producing gas.

b A batch-type digester will use plant material rather than the waste needed for the other types shown. In some places animal waste may be in short supply, or there may be religious or hygiene objections to using waste, yet the batch digester can still be used to produce biogas.

What is biogas?

• Biogas is a flammable mixture of gases. It is formed when bacteria break down plant material or the waste products of animals in anaerobic respiration. Biogas is mainly methane but the composition of the mixture varies .

• that biogas, mainly consisting of methane, can be produced by the anaerobic fermentation of waste materials containing carbohydrates

• (explain p.276)

How is biogas produced

How is biogas used?• Domestic

-Any heating purpose e.g. cooking, gas lights etc.

• Industrial• vehicle fuel

biogas can be injected into existing gas networks so homes can readily use biogas in the home. Biogas power generation

Biogas energy can also be used to generate power. Both electricity and heat can be produced with the help of a gas powered generator. The proportions of heat and power generated depend of course on the design of the plant but are usually in the region of 35 per cent electricity and 65 per cent heat.

Linchun Village, a tourist resort at the southern tip of China's Hainan province is now lit at night by street lamps using methane for fuel, the first among villages in the island province.

BioBus, the uk’s first food and poo powered bus

How the design of a biogas fermenter is adapted for its circumstances

The bacteria involved in biogas production works best at 30͒.°C. So biogas generators tend to work best in hot countries. The process is also exothermic. This means that if you put some energy at the beginning to start things off, and have your generator well insulated to prevent energy loss, biogas generators will work anywhere.

Airtight so process kept anaerobic

What is biogas?Biogas is a gas produced by the anaerobic fermentation of organic waste containing carbohydrates by bacteria.

This gas contains:● 40͒%-70͒% methane ● carbon dioxide● traces of hydrogen, nitrogen

and hydrogen sulphide.

Biogas is currently used in several ways, including in the production of electricity, heating water for central heating systems and as fuel for buses.

Producing biogas

Biogas use in remote areas

In parts of rural India cow dung is used to produce biogas for cooking.

The production of biogas can be ideal in remote areas lacking mains electricity or sewage systems. It not only produces a useful fuel but can also be used to get rid of large quantities of sewage or other organic waste material.

On a small scale, biogas digesters can be used to provide energy for individual families or farms.

Biogas B3 3.6 How is biogas produced?

Cows in India have a poor diet. They produce large amounts of methane in the wind they pass from both ends of their bodies.The manure they produce also produces some methane if it decays without enough air.Collecting the methane directly from the cows is not very practical – but we can make use of the manure!

Scientists working at this village in India have developed a biogas digester that can make methane from a mixture of cow manure and other organic waste. It is providing useful gas for local people for heating water and cooking food.

The biogas digester is a huge upturned metal drum. Waste is pushed underneath through holes in the concrete base. As the manure decays in the absence of air, the methane bubbles to the top and passes to storage tanks through a metal pipe.

The biogas digester is sealed with a layer of oil around the base. You can just see the leaves from the tree overhead reflected in the surface. The oil keeps oxygen out so that the decay is anaerobic.The metal drum floats up and down as the gas is produced. The frame has guides with small rollers on the ends to let it move.

The system is so good that development workers and scientists from around the world come to visit and find out how it works. This picture shows visitors from several African countries, a few from South America and some

from Asia.

Link to methane powered streetlights in China

A Generalised Biogas Generator

Biogas generators take in waste material or plants, and biogas and useful fertilisers come out the other end.

Airtight so process kept anaerobic

Fixed Dome Biogas Plant

Advantages

• Low initial cost• Long useful life as no moving

or rusting parts involved• Well-insulated

Disadvantages

• Sealing of the gas holder not always gas-tight

• Gas pressure fluctuates

Types of biogas generator used by this company