1.5. Biogas
Biogas is lighter than air and has an ignition temperature of approximately 700˚C (diesel oil 350˚C; petrol and
propane about 500˚C). The temperature of the flame is 870˚C. Biogas consists of about 60% methane (CH4) and
40% carbon dioxide (CO2) [7]. It also contains small proportions of other substances, including up to 1% hydrogen
sulphide (H2S). The methane content and hence the calorific value is higher the longer the digestion
process. The methane content falls to as little as 50% if retention time is short [10]. If the methane content is
considerably below 50%, biogas is no longer combustible [10] [11]. The first gas from a newly filled biogas
plant contains too little methane. The gas formed in the first three to five days must therefore be discharged unused.
The methane content depends on the digestion temperature. Low digestion temperatures give high methane
content, but less gas is then produced [11].