Mixing biogas with other fuels or
increasing the oxygen fraction
in the oxidizer can improve the combustion performance of biogas.
In this study, we investigated the effects of propane addition and
oxygen enrichment on the flame characteristics of biogas with a constant
heating value in a half-open duct. The evolution of the flame structure
revealed that the tulip structure took on two types of patterns at
the oxygen fraction θ = 0.21. However, there was no salient
tulip flame formation when θ > 0.21. Oxygen enrichment significantly
increased the flame propagation velocity (V) and
explosion pressure (P). Propane addition significantly
reduced V
max at θ = 0.25 and 0.29.
The maximum rate of pressure rise [(dP/dt)max] exhibited a nonlinear dependence on the maximum
flame velocity (V
max). A similar trend
was observed for the dependence of (dP/dt)max on the laminar burning velocity (LBV). Therefore,
the LBV is an important parameter to predict V
max and (dP/dt)max values of mixtures. Furthermore, by using the PREMIX code and the
UC San Diego mechanism, we simulated laminar flame burning properties
to reveal the macroscopic flame propagation characteristics. The rate
of propane consumption was higher than the rate of methane consumption.
Additionally, oxygen enrichment played an important role in the entire
reaction process, leading to an increase in the molar concentrations
of microscopic free radicals and a promotion of the rates of production
(ROP) of free radicals. In contrast, the addition of propane decreased
the molar concentrations of microscopic free radicals and inhibited
the ROP of free radicals.
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