Evaluation of NOx-Reduction Measures for Iron-Ore Rotary Kilns

The grate-kiln process is employed for sintering and oxidation of iron-ore pellets. In this process, a fuel (typically coal) is combusted with a large amount of excess air in a rotary kiln, and the high air-to-fuel ratio leads to significant NOx formation. The current Article is an assessment of NOx reduction measures that have been tested in pilot-scale and in full-scale by the Swedish iron-ore company Luossavaara-Kiirunavaara Aktiebolag (LKAB). The results show that the scaling between the full-scale kiln and the pilot-scale kiln is crucial, and several primary measures that reduce NOx significantly in pilot-scale achieve negligible reduction in full-scale. In the investigated full-scale kiln, thermal NOx formation is efficiently suppressed and low compared with the NO formation from the fuel-bound nitrogen (especially char-bound nitrogen). Suppressing the NO formation from the char-bound nitrogen is difficult due to the high amounts of excess air, and all measures tested to alter mixing patterns have shown limited effect. Switching to a fuel with a lower nitrogen content is efficient and probably necessary to achieve low NOx emissions without secondary measures. Simulations show that replacing the reference coal with a biomass that contains 0.1% nitrogen can reduce NOx emissions by 90%.