Effect of Propane Addition and Oxygen Enrichment on the Flame Characteristics of Biogas

Abstract

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 Vmax 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 (Vmax). 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 Vmax 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.