Numerical investigation of combustion characteristics under oxygen-enriched combustion combined with flue gas recirculation in a cement rotary kiln

Abstract

A new technology combining oxygen-enriched combustion with flue gas recirculation (OEC-FGR) in the cement rotary kiln is put forward, based on the technical complementarity of these strategies. The effects of the flue gas recirculation rate on the combustion characteristics and NOx formation with a primary air oxygen concentration of 27% in a cement rotary kiln of 5000 t/d capacity are numerically investigated. The introduction of recirculating flue gas has significant influences the flame size, temperature distribution, and NOx formation. As the FGR rate increases, the flame lengthens and the high-temperature region shrinks. At the same time, the position of the temperature peak is delayed and the length of the calcination section is shortened. Furthermore, NOx formation is remarkably reduced, owing to the lengthened low oxygen reduction zone and lower temperature. As the FGR rate increased from 0% to 27%, the maximum NOx concentration on the center line in the kiln and the NOx concentration at the outlet decreased by 392 ppm and 343 ppm, respectively, and both decreased the most when the FGR rate increased from 0% to 10%. Moreover, good burnout is obtained with the new combustion technology, although considerable CO is generated near the burner. Since the length of the clinker calcination zone may be adversely affected by an excessively high FGR rate, the FGR rate should be carefully determined. In this study, a better compromise between calcination performance and NOx reduction could be achieved when the FGR rate was 20% .