Biogas is a fuel gas made mainly of methane and carbon dioxide. Microorganisms produce biogas when they break down organic matter without oxygen.

What is the typical composition of biogas?

Biogas is usually about 50–75% methane, with most of the rest carbon dioxide. Small amounts of hydrogen sulfide and water vapour are also present.

Which component of biogas provides its fuel value?

Methane is the combustible component that releases energy when burned. The carbon dioxide adds bulk but no energy.

Why must hydrogen sulfide be removed from biogas?

Hydrogen sulfide is toxic and corrosive, and damages engines and boilers. The gas also forms sulfur dioxide when burned.

Biomethane is biogas upgraded by removing the carbon dioxide and impurities. The purified gas matches natural gas and can be injected into the gas grid or used as vehicle fuel.

How is biogas from a commercial digester usually used?

Most biogas is burned in an engine that drives a generator, with its heat also put to use. Some plants upgrade the gas to biomethane instead.

CCEA A-Level Environmental Technology 3930

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Energy From Biomass

Topic overview Defining biomass and its main categories Energy crops grown for biomass Advantages and disadvantages of biomass fuel Combustion, woodchip and fuel quality Gasification and syngas Biogas and commercial anaerobic digesters

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Energy From Biomass

Biogas and commercial anaerobic digesters

3 things to know
AS 1

Overview

The composition of biogas, removing hydrogen sulfide, biomethane, feedstocks used in Northern Ireland, digestate, the advantages and disadvantages of digesters, methane leaks, water pollution and the case for a dairy farm.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Evaluation

Give three advantages of commercial anaerobic digesters.

Evaluation

Give three disadvantages of commercial anaerobic digesters.

Key term

Discuss whether a dairy farmer should install an anaerobic digester.

Questions from this subtopic

Each one with the model answer underneath.

1
What is biogas?
Model answer
Biogas is a fuel gas made mainly of methane and carbon dioxide. Microorganisms produce biogas when they break down organic matter without oxygen.
2
What is the typical composition of biogas?
Model answer
Biogas is usually about 50–75% methane, with most of the rest carbon dioxide. Small amounts of hydrogen sulfide and water vapour are also present.
3
Which component of biogas provides its fuel value?
Model answer
Methane is the combustible component that releases energy when burned. The carbon dioxide adds bulk but no energy.
4
Why must hydrogen sulfide be removed from biogas?
Model answer
Hydrogen sulfide is toxic and corrosive, and damages engines and boilers. The gas also forms sulfur dioxide when burned.
5
What is biomethane?
Model answer
Biomethane is biogas upgraded by removing the carbon dioxide and impurities. The purified gas matches natural gas and can be injected into the gas grid or used as vehicle fuel.
6
How is biogas from a commercial digester usually used?
Model answer
Most biogas is burned in an engine that drives a generator, with its heat also put to use. Some plants upgrade the gas to biomethane instead.

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Fossil Fuels

Global Action on Energy Conservation and Climate Change

Macrogeneration

Microgeneration

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