Analysis of unburned hydrocarbon species for air and oxy-fuel flames in a semi-industrial combustion chamber using Fourier Transform Infrared Spectroscopy

Abstract

The combustion of biomass in an oxy-fuel atmosphere shows significant differences compared to air combustion, primarily due to the changes in gas properties caused by CO₂. While multiple studies have analyzed aspects such as flow field, temperature distribution, and CO occurrence in the flame, this study provides a comprehensive analysis of the occurrence of short unburned hydrocarbons (

) and aromatic compounds like benzene in both oxy-fuel and air flames. Short chained hydrocarbons are known as a significant group of molecules for cracking longer hydrocarbons and thus influencing the reaction rate. Simultaneously those species have also a big influence on the formation of aromatic compounds, whereas aromatic structures favor the formation of soot and tar. For this purpose, three different oxygen contents in the oxidizer, ranging from 27% to 33%, are compared with an air flame in a semi-industrial combustion chamber equipped with flue gas recirculation and a thermal power of up to 670 k$W\text{th}$. The gas species are analyzed using a Fourier Transform Infrared Spectroscopy analyzer (FTIR) and a suction probe to extract the measurement gas from different radial positions in the flame. The results show that the upper part of the flame along the center line in the oxy-fuel case reveals an extended volatile release and pyrolysis due to the availability of CO₂ and partially higher temperatures, leading to an increase in CO and shorter hydrocarbons. As the radial distance from the center line extends, the occurrence of hydrocarbons is mainly dominated by lean combustion reactions. This trend is consistent across all oxygen concentrations in the oxidizer stream during oxy-fuel combustion. At greater distances from the burner plane, the air case and the oxy-fuel condition with 33% oxygen concentration show the highest similarities in the concentrations of unburned hydrocarbons. The analysis of aromatic compounds reveals significant changes in the upper part of the flame, which can be attributed to the influence of CO₂ on the formation of shorter hydrocarbons and the enhanced tar cracking mechanism in the oxy-fuel case.