18 April 2019
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. 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
In the case of pure methane gas, the explosive range is between 4.4 and 16.5% v / v. 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 below shows by way of example the explosive limits of a methane/carbon dioxide mixture (70% CH4 – 30% CO2) and their trend (upper and lower limit). Gas-air mixtures above or below the explosive range are not ignitable.The ignition temperature of biogas is 700°C (methane 595°C).
There are various potential sources of ignition in biogas plants.
Hot surfaces >500 °C (turbochargers)
Naked flames Fire, flames, embers
Mechanically generated sparks Friction, beating, grinding
Electrically generated sparks: Switching operations, loose connection, equalising currents
Exothermic reaction: Spontaneous combustion of dusts
Electrostatic discharge Caused by missing potential equalization