Effect of flue gas recirculation on the formation of unburned hydrocarbons in oxy-fuel and air flames in a semi-industrial scale burner

Oxy-fuel combustion is a promising technology for reducing CO 2 emissions in the energy sector. In this process, nitrogen from air is replaced with CO 2 , to ensure the purity of CO 2 in the flue gas, altering the properties of the oxidizer. For industrial applications, different methods of supplying CO 2 for the combustion are possible, typically categorized into different recirculation pathways: These include recirculation pathways with higher moisture and minority species content as well as a recirculation of cleaned flue gas after the scrubbing stages of the plant. These approaches differ primarily in oxidizer temperature, the amount of H 2 O present in the oxidizer stream and the occurrence of minority species. However, the presence of H 2 O in the combustion environment influences the gasification mechanisms within the flame, similar to the effects of CO 2 . This paper compares the formation and presence of unburned hydrocarbons between a conventional air flame and two oxy-fuel combustion cases, one using a mixture of synthetic CO 2 and O 2 and the other incorporating flue gas recirculation. Analyzing these intermediate species in the different flames, features a significant insight into the combustion chemistry and can be used as validation data for a detailed Computational Fluid Dynamics (CFD) and kinetic modeling of this process. This comparison for the same flame leads to increased knowledge about the differences between synthetic oxy-fuel combustion and flue gas recirculation and is a novelty in the field of semi-industrial oxy-fuel combustion. For the experiments, milled walnut shells are used as fuel. Comparing air and oxy-fuel combustion, the oxy-fuel flames shows significantly higher unburned hydrocarbons in the flame. However, the results indicate that differences in unburned hydrocarbon formation between the two oxy-fuel cases become apparent only at greater distances from the burner inlet, which is attributed to higher oxidizer inlet temperatures, adiabatic flame temperature, and increased H 2 O content in the oxy-fuel case with flue gas recirculation. This leads to increased hydrocarbons present in the end of the synthetic oxy-fuel case. • Analysis of unburned hydrocarbon species in oxy-fuel and air flames. • Investigation of aromatic hydrocarbon formation in oxy-fuel flames. • Analysis of the influence of flue gas recirculation.