A modelling heat transfer study of a hydrogen flame in the iron ore pelletization process: An evaluation of the rotary kiln process

Examensarbete för masterexamen
Ehlmé, Elias
The supplied heat needed in the heat treatment of iron ore pellets commonly originates from fossil fuels that generate considerable emissions of greenhouse gases. Hence, there is a need for such mining industries to find possible solutions in order to reduce their emissions. One such company is Luossavaara-Kiirunavaara Aktiebolag, (LKAB), which is examining the suitability of a transition from combusting coal to hydrogen gas in the rotary kiln, used for a portion of their product. Their aim is by switching from coal to hydrogen, reach the climate targets set by LKAB and the Swedish government. Since such process changes may affect how the kiln should be operated it is of interest to examine the process changes with the use of modeling tools. This master thesis work focuses on examining the heat transfer conditions in a rotary kiln, using hydrogen gas by further developing a heat transfer model of a coal flame in a rotary kiln, developed at the division of Energy Technology at Chalmers University of Technology. By conducting sensitivity analyses of several parameters such as the gas temperatures, flame dimensions, and addition of coal or iron ore dust particles to the flame, suggestions on how the kiln should be operated to mimic a coal or oil flame may be evaluated. One of the main challenges with substituting coal for hydrogen is the reduced thermal radiation from char, ash, and soot particles as well as carbon dioxide in the kiln. These components dominate the heat transfer in the kiln as they absorb, emit, and scatter radiation, hence promoting heat transfer by radiation to the pellets, which lie like a bed within the kiln. As heat transfer is reduced, flue gas temperatures increase, which leads to increased heat losses and may also lead to an uneven heat transfer load to the pellet bed, which is undesired with respect to the quality of the pellets. The results show that by increasing the gas temperatures of the preheated air, or by adding particles, a hydrogen flame that assumes an adiabatic temperature profile, may deliver a heat transfer to the bed material similar to an oil or coal flame, respectively. However, an uneven heat load further remains an issue. The addition of particles in the extreme environment significantly influenced the emittance of radiation, resulting in elevated kiln wall temperatures, which may cause further complications in the kiln. This master thesis work concludes that while further research in the mentioned areas is required, the results may be used to evaluate the implementation of a hydrogen flame in a rotary kiln.
Iron ore pelletizing process, rotary kiln, hydrogen combustion, radiation, heat transfer
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