NOx formation in iron ore rotary kilns

Doctoral thesis, 2019

Iron ore pellets are often produced using the so-called Grate-Kiln process, which is designed 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 process comes from the combustion of a pulverized fuel in a rotary kiln, involving the formation of a jet flame. To oxidize the pellets, large amounts of air are introduced into the kiln, and an air-to-fuel equivalence ratio of 4–6 is obtained. Furthermore, the air is pre-heated to >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.
This thesis describes the NOx formation in the rotary kiln and identifies the governing parameters, with the aim of reducing the emissions. The work involves experiments in a pilot-scale kiln, as well as modeling work based on the same experiments. Data from a full-scale iron ore pelletization plant are also provided.
From the experiments and the modeling work in this thesis, thermal NO is deemed to be of low importance in iron ore rotary kilns when solid fuels are combusted. Instead, the conditions during char combustion contribute significantly to the overall NOx formation. These results explain why many of the primary measures used to date have failed to achieve reductions in NOx emissions. Suggested additional primary measures include: raising the pyrolysis temperature (e.g., through oxygen addition) to deplete the char of nitrogen; or switching to a fuel with a lower nitrogen content (e.g., wood pellets). These are interesting alternatives for the future, and the latter may be tested in the coming years.