NOx Formation in Rotary Kilns for Iron Ore Pelletization

Licentiatavhandling, 2017

The production of iron ore pellets is often performed in the so-called Grate-Kiln process. The aim of the process is to oxidize the magnetite (Fe3O4) to hematite (Fe2O3) and to sinter the pellets so they can be used in steel manufacturing. The heat required for this is produced by combusting a pulverized fuel in a rotary kiln, forming a suspension flame. Due to the need to oxidize the pellets, large amounts of air are introduced to the kiln. Relating the amount of air to the fuel, an air-to-fuel equivalence ratio of 4-6 is obtained. Furthermore, the air is pre-heated to above 1000°C. High temperatures and large amounts of excess air are known to promote NOx formation and NOx emissions from iron ore processing plants are in general high. The aim of this work is to describe the NO formation in the rotary kiln and to identify governing parameters that may be altered to reduce the emissions. The thesis contains results from experiments in a pilot-scale kiln and from modeling work based on the same experiments. In the experiments, four coals were tested as well as co-firing coal with biomass. In-flame measurements of temperature and gas concentrations were performed with the use of a suction pyrometer and FTIR spectroscopy (+paramagnetism). Different primary measures for NOx reduction were also tested. Overall, reducing the primary air flow in the burner and co-firing coal with biomass were the most effective measures for reducing NOx emissions, compared to the reference case. Using natural gas and oil resulted in three times the amount of NOx. Reducing the total amount of excess air only resulted in a small NOx reduction, and increasing the secondary air temperature resulted in slightly decreased NOx formation. The general assumption in rotary kilns is that NOx is mostly formed by the thermal NO mechanism due to the high temperatures involved. Although this is certainly true for the cases with gas and oil, the experimental results indicate that NOx formed from the fuel-bound nitrogen is dominating the total NOx formation when solid fuels are used. The results from the detailed reaction modeling show that the thermal NO formation is of minor importance. Instead, the reduction of NO by char appears to be remarkably low in the kiln and responsible for the high net conversion of fuel-bound nitrogen to NO.