Comparative Analysis of Combustion Characteristics and Pollutant Formation in Radiant Tubes with Different Structural Designs

This study aims to investigate the combustion characteristics, thermal distribution, and nitrogen oxide (NOx) formation of two radiant tube designs—conventional and staged combustion—under air–fuel ratios of 1:10 and 1:11. A three-dimensional numerical model was developed in ANSYS Fluent 2023 R1 to compare flame temperature, wall temperature gradients, and pollutant emissions. The results reveal that flame temperature is the dominant factor in NOx formation. The conventional tube, with flame temperatures around 1800 °C, shows decreasing NOx emissions as the air–fuel ratio increases (corresponding to lower flame temperatures). In contrast, the staged combustion tube exhibits flame temperatures exceeding 1900 °C, where the thermal mechanism dominates, leading to a sharp increase in NOx emissions far above the conventional design. These findings highlight that in staged combustion systems, inadequate consideration of flame temperature and mixing characteristics may cause NOx control to fail or even reverse.