Biogas Safety first!

General requirements
photo: Uwe Mühling
4.8. Explosion and fire hazards
An explosion is defined as the sudden chemical reaction
of a flammable substance with oxygen, releasing
large amounts of energy. There is a sudden
expansion in the volume of gases as the energy is
released. This can be brought about by an explosive
atmosphere, for example.
Flammable substances may be present in the form
of gases, vapours, mists or dusts. An explosion can
only occur if three factors apply simultaneously:
flammable substance (in distribution and
concentration conducive to explosion)
oxygen (from air)
source of ignition
Depending on the circumstances, two types of explosion
can take place in biogas plants: detonation
and deflagration. A deflagration is characterized by a
subsonic flame propagation. The overpressure level
resulting from a deflagration depends on the flame
speed, which depends on the turbulene and number
of objects in the combustion zone. In a completely
unconfined area the overpressure is limited while
a confined space can result in significant overpressures.
Lower levels of overpressrue can result in ear
damage and bruises and cuts from missiled objects.
Higher levels can damage a building and as a result
of that als injure people in and around that building.
A detonation is characterized by supersonic flame
propagration velocities and always creates high
overpressures. If many obstacles are present in the
combustion zone or if gas expansion is limited due to
partial confinement, the flame speed may accelarate
from subsonic to supersonic levels. This is called
the ‘deflagration to detonation transition’ (DDT).
The overpressure from a detonation is high enough
to destroy builings within the exploding cloud or to
seriously damage buildings away from the cloud.
People within the exploding cloud can suffer fatal
lung injury. Away from the cloud, ear damage and
damage from missiled objects is possible.
If the concentration of biogas in the atmosphere is
between 6 and 22 % v / v, there is a risk of explosion
in the presence of an ignition source (explosive range,
explosive atmosphere). In the case of pure methane
gas, the explosive range is between 4.4 and 16.5 %
v / v. The ignition temperature of biogas is 700°C
(methane 595°C). The composition of biogas may vary
with regard to the proportions of methane and carbon
dioxide, with the result that the explosive range of the
gas mixture in the presence of air also varies.
Figure 3 therefore shows by way of example the explosive
limits of a methane/carbon dioxide mixture
(70 % CH 4
– 30 % CO 2
) and their trend (upper and
lower limit). Gas-air mixtures above or below the explosive
range are not ignitable.
Figure 3: Explosion triangle for biogas
LEL
0
CH 4
in Vol.-%
20
UEL
40
60
80
100
0
Explosion range
Air/CO 2
/CH 4
in Vol.-%
Mixture Methane/CO 2
70/30
CO 2
in Vol.-%
100 80 60 40 20 0
20
40
60
80
100
15