Effect of the flue gas recirculation supply location on the heavy oil combustion and NOx emission characteristics within a pilot furnace fired by a swirl burner

• Heavy oil combustion suffering from high NOx emissions.
• Performing low-NOx heavy oil swirl combustion experiments in a pilot-scale furnace.
• Evaluating the effect of the FGR supply location in the burner.
• The centrally-located FGR in the burner attaining the best performance.

To reduce NOx emissions within industrial furnaces fuelled with heavy oil, a combustion configuration consisting of a low-NOx swirl burner, overfire air (OFA) and flue gas recirculation (FGR) was developed and then demonstrated in a pilot-scale furnace. In considering that FGR supply location always affects greatly combustion and NOx reduction performance, four combustion experiments with varying FGR were performed in turn: (i) under the circumstances without FGR and (ii) at three different FGR supply locations, i.e., FGR supplied through the burner periphery (BP-FGR), the main air pipe of the burner (MAP-FGR), and the duct between primary air and inner secondary air (DPS-FGR). Comparisons of gas temperatures and species concentrations downstream of the oil/air flame as well as NOx and CO emission levels were performed among the four settings to evaluate the effect of varying the FGR supply location. In comparison with the circumstances without FGR, supplying FGR in the furnace generally resulted in the change trends consisting of (i) decreasing the oxygen content and peak gas temperature in the primary combustion zone, (ii) prolonging the entire combustion process, and (iii) restraining the NOx generation while slightly increasing the CO emission. However, among the three settings with FGR, BP-FGR exhibited the least contribution in the aforementioned change trends, whereas DPS-FGR attained the optimal FGR performance. Consequently, the swirl burner plus an OFA ratio of approximately 25% and a DPS-FGR pattern with a 10% FGR ratio achieved low NOx emissions of approximately 200 mg/m3 at 3% O2 and an acceptable CO emission of approximately 50 ppm within the furnace.