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Biogas: A renewable energy source

Introduction

Biogas is a combustible mixture of gases. It consists mainly of methane (CH4) and carbon dioxide (CO2) and is formed from the anaerobic bacterial decomposition of organic compounds, i.e. without oxygen. The gases formed are the waste products of the respiration of these decomposer microorganisms and the composition of the gases depends on the substance that is being decomposed. The substrate, the production technology and the collection of the gas, all affect the composition of the gas (Table 1). If the material consists of mainly carbohydrates, such as glucose and other simple sugars and high-molecular compounds (polymers) such as cellulose and hemicellulose, the methane production is low. However, if the fat content is high, the methane production is likewise high.

Table 1 — Composition of biogas, landfill gas and natural gas

Biogas Landfill gas Natural gas
Methane (vol-%) 60 –70 35 – 65 89
Other hydro carbons (% v) 0 0 9.4
Hydrogen (vol-%) 0 0-3 0
Carbon dioxide (% v) 30 – 40 15 – 50 0.67
Nitrogen (vol-%) ~0.2 5 – 40 0.28
Oxygen (vol-%) 0 0-5 0
Hydrogen sulphide (ppm) 0 – 4000 0 –100 2.9
Ammonia (ppm) ~100 ~ 5 0
Lower heating value (kWh/nm3) 6.5 4.4 11.0

Methane – and whatever additional hydrogen there may be – makes up the combustible part of biogas. Methane is a colourless and odourless gas with a boiling point of -162 C and it burns with a blue flame. Methane is also the main constituent (77-90%) of natural gas. Chemically, methane belongs to the alkanes and is the simplest possible form of these. At normal temperature and pressure, methane has a density of approximately 0.75 kg/m3. Due to carbon dioxide being somewhat heavier, biogas has a slightly higher density of 1.15 kg/m3. Pure methane has an upper calorific value of 39.8 MJ/m3, which corresponds to 11.06 kWh/m3. If biogas is mixed with 10-20% air, you get explosive air, which – as the name indicates – is explosive!

To increase the quality of the raw biogas, the gas is usually cleaned of unwanted substances such as hydrogen sulphide, oxygen, nitrogen, water and particulates. The main reason for doing this is to prevent corrosion and mechanical wear of the equipment in which the biogas is used. The methods used in cleaning and upgrading biogas will be dealt with in a different section.

The basic texts listed on the right pane contain further details of the biogas process.


Biogas resource documents

Downloadable reference documents

Basic texts
Biogas - what it is, how it is made, how to use it

Biogas – Green energy process, design, energy supply, environment

Biogas from AD - Training manual

Biogas from AD of biomass

Biogas methane explained & other articles

Biogas Digest Volume I - Biogas basics
Biogas – An introduction
A system approach to biogas technology
About dry fermentation in agriculture

Alternative energy sources - Biogas production

Anaerobic digestion-02
Biomethane from biomass, biowaste and biofuels
Dry fermentation – From biowaste to biogas

Biogas brochure

Microbiological handbook for biogas plants