Implications of Fuel Choice and Burner Settings for Combustion Efficiency and NOx Formation in PF-Fired Iron Ore Rotary Kilns

The combustion process applied in the grate-kiln process for iron ore pellet production employs air-to-fuel equivalence ratios in the range of 4-6, typically with coal as fuel and high-temperature air (>1000 degrees C) as oxidant. The NOx emissions from these units are in general significantly higher than those in other combustion systems, and the large flows of flue gases make the implementation of secondary measures for NOx control costly. Therefore, it is of importance to investigate NOx. formation under combustion conditions relevant for iron ore production, in order to control the emissions from these units. The present work examines NO formation during the combustion of four pulverized coals, as well as during cofiring with biomass in a pilot-scale kiln (580 kWfuel) based on a two-week experimental campaign. The influence of burner settings was also included in the investigation. Based on the presented experimental results and the results of previous modeling and experimental studies, we suggest that the NOx emissions are mainly the result of a high conversion of fuel-bound nitrogen (fuel-N) to NO. In particular, char-bound nitrogen (char-N) conversion appears to be higher than in conventional pulverized fuel flames, presumably due to the high levels of oxygen present in the char-burnout region. The temperatures in the kiln varied between the test cases, but thermal NO formation is estimated to be of low importance.