Laminar Burning Characteristics Of Biogas-Air Mixtures In Spark Ignited Premix Combustion

Laminar burning velocities of biogas-air mixtures in premixed combustion have been studied to
elucidate the fundamental flame propagation characteristic of biogas as a new alternative and renewable fuel.
The results are compared with those from a methane-air flame. Biogas is a sustainable and renewable fuel that is
produced in digestion facilities. The composition of biogas discussed in this paper consists of 66.4% methane,
30.6% carbon dioxide and 3% nitrogen. Burning velocity was measured using a photographic technique in a
high pressure fan-stirred bomb, the initial condition being at room temperature and atmospheric pressure. Based
on this experimental investigation, the laminar burning velocities of biogas-air mixtures were 0.2086 m/s for lean
(�=0.8), 0.2638 m/s for stoichiometric (�=1.0) and 0.1864 m/s for rich (�=1.2) conditions. Compared to a
methane-air mixture, the presence of carbon dioxide and nitrogen causes a reduction in the laminar burning
velocity for two reasons. The dilution effect leads to a lower concentration of reactive species in the fuel-air
mixture for a given equivalence ratio, which leads to a lower overall chemical reaction rate of bimolecular
reactions in the fuel oxidation reaction mechanism. Also, the presence of this additional inert gas will absorb
some of the heat generated, thus lowering the flame temperature which in turn will tend to reduce the overall rate
of many of the chemical reactions within the fuel oxidation mechanism. These effects lead to a different
behaviour in burning velocity of biogas as a function of equivalence ratio. Whereas a rich (�=1.2) methane-air
mixture has a higher burning velocity than a lean (�=0.8) mixture, the reverse is the case for the equivalent
biogas-air mixtures where the lean mixture has a higher burning velocity than the rich mixture. This is a
consequence of the rich biogas-air mixture having a higher fraction of the carbon dioxide and nitrogen
components from the fuel compared to the lean biogas-air mixture, and shifts the optimum equivalence ratio for
operation of a biogas-air mixture to a leaner mixture than would be the case for methane-air mixtures.