Effects of oxidizer flexibility and bluff-body blockage ratio on flammability limits of diffusion flames

Concerns about global warming have encouraged the interest in hydrocarbon combustion techniques that allow easy capture of carbon dioxide. One technique for achieving this objective is through the use of pure oxygen instead of air for combustion or what is called oxy-combustion carbon capture technology. The main goal of the manuscript is to study flammability limits, visual flame appearance and exhaust emissions of diffusion flame stabilized over a bluff body over ranges of operating and design conditions. The operating conditions include flow Reynolds number, equivalence ratio and oxidizer composition. The design parameter considers the change of blockage ratio (BR) of the bluff body namely, BR = 0.36, 0.5, 0.67 and 0.82. Based on this, three sets of experiments were performed utilizing compressed natural gas (CNG) as a fuel to be burned with three different oxidizers including air, oxygen-enriched-air and oxy-fuel mixtures (O2 plus CO2 with a controlled oxygen fractions, OF). The three sets of experiments were performed to identify ranges for stable flame operation considering different oxidizers under different operating conditions. Stability limits, visual flame appearance and extinction limits of these flames are quantified and analyzed. Furthermore, three different regions were observed; precisely, jet flames, central-jet dominated flames and recirculation zone flames, depending on the ratio between oxidizer and fuel momentum. The flame color changed from yellow for air combustion, to bright white for oxygen-enriched-air combustion and finally to blue with yellow tips for oxy-combustion. The flame length was the highest for air combustion, then lower for oxy-combustion and the lowest for oxygen-enriched-air combustion. This was attributed to the effect of oxygen-enrichment which results in increase in the flame speed making flame length shorter. For the sake of comparison, the flammability limits of the three sets were reported and the results revealed that oxygen-enriched-air-flames have higher stability than air-flames and oxy-flames, respectively.