4.4 The Technical Parameters of Biogas/Methane

Methane and gases having a considerable methane content have long been researched on to establish their physical properties and technical behavior.

Some of the properties, which have an effect on the combustion process in an engine, shall be explained hereunder:

- Ignitability of CH₄ in a mixture with air
CH₄: 5 . . . 15 Vol %
air: 95 . . .85 Vol %

Mixtures which are leaner, i.e. CH₄ content less than 5 Vol % or richer, i.e. CH₄ content more than 15 Vol %, will not properly ignite with spark ignition.

-Combustion velocity c_c in a mixture with air at a pressure of p = 1 bar

c_c = 0.20 m/s at 7% CH₄
c_c = 0.38 m/s at 10% CH₄
c_c = 0.20 m/s at 13% CH₄

The combustion velocity is a function of the volume percentage of the burnable component, here CH₄. The highest value is near the stoichiometric air/fuel ratio, mostly at an excess air ratio of 0.8 . . . 0.9. It increases drastically at higher temperatures and pressures.

-Temperature at which CH₄ ignites in a mixture with air

T₁ = 918 K. . .1023K(=645°C...750°C)

- Compression ratio of an engine, e, at which temperatures reach values high enough for self-ignition in a mixture with air (CO₂ content decreases ignitability, i.e. increases possible compression ratio)

e = 15...20

- Methane number, which is a standard value to specify a fuel's tendency to "knocking", i.e. uneven combustion and pressure development between TDC and BDC

Fig.4.4: "Knocking" in a p, alfa-diagram of an engine

CH4, 100%:	100
biogas (CH4 70%):	130
for comparison:
butane:	10
propane:	33.5

Methane and biogas are very stable against "knocking" and can therefore be used in engines of higher compression ratios than petrol engines. Fig. 4.4 illustrates the cause of the pressure and hence the force on the piston when the engine "knocks". Operation under such conditions will gradually destroy the engine.

- Stoichiometric air/fuel ratio on a mass basis at which the combustion of CH₄ with air is complete but without unutilized excess air