Combustion characteristics of methane–oxygen enhanced air turbulent non-premixed swirling flames

• Turbulent non-premixed swirling flames were studied upon the oxygen enrichment.
• Flame stability is enhanced with oxygen enrichment even for low rates.
• Reverse flow velocities near the burner exit lead to improve flame stabilization.
• Oxygen enrichment decreases CO emissions but increases NOx emissions.
• Increasing swirl intensity has noticeably effects on NOx emission reduction.

Application of oxygen-enhanced combustion to existing fossil fuel energy systems to facilitate CO2 capture presents several challenges. This work investigates the combustion characteristics of methane oxygen enriched air turbulent non-premixed swirling flames. It focuses on the stability of flames, NOx, CO2 and CO emissions and the flow field dynamics. The burner configuration consists of two concentric tubes with a swirler placed in the annular part for the oxydant. The experiments are conducted using a 25 kW water cooled combustion chamber. The exhaust gas compositions are measured using gas analyzers. OH chemiluminescence experiments are conducted to investigate the structure and the stability of the flames without and with oxygen enrichment. Flame liftoff heights, fluctuations of the flame base and flame lengths are determined. Particle Image Velocimetry is used to analyze the dynamics of swirling flows. The measurements are performed for oxygen concentrations ranging from 21% to 30% by volume, with swirl numbers from 0.8 to 1.4 and global equivalence ratios from 0.8 to 1. The results show that the addition of oxygen to air, while keeping the oxidant flow rate constant, enhances the combustion efficiency and flame stability. It is observed that increasing oxygen concentration leads to lower lift-off heights and reduces flame height fluctuations. Increasing the swirl number significantly improves the flame stability. The results demonstrate that the CO2 emissions in the exhaust gases linearly increase with increasing O2 content in the oxidant. The CO emissions are shown to decay exponentially, whereas the NOx emissions, mainly produced through the thermal pathway, increase strongly with oxygen enrichment. The PIV results illustrate that increasing the swirl intensity increases the reverse flow velocities close to the burner exit. The decay of axial velocity presents favorable flow patterns for the stabilization of the flame.