Flame Characterisation and Nitrogen Oxides Assessment of Coal Replacement with Hydrogen in Rotary Kilns for Iron Ore Induration

Licentiate thesis, 2025

The iron and steel industry is a major contributor to global CO2 emissions, largely due to the reliance on fossil fuel combustion, as well as emissions of nitrogen oxides (NOx) due to high temperatures flames and fuel-nitrogen content. The induration of iron ore pellets is small but increasingly important step in the global iron and steel industry. The major rote for the European mining industry dominated by the Swedish mining company LKAB. In the electrification of the Swedish mining industry, LKAB seeks to substitute coal by hydrogen with hydrogen-coal co-firing as an intermediate solution. The replacement of coal with hydrogen presents both technical challenges, particularly in controlling flame behaviour and mitigating NOx formation, as well as economic challenges, in capital intensive investments and uncertain energy markets.

This thesis investigates flame characteristics (such as flame area, stability, ignition zone, and particle temperature) and NOx formation during hydrogen-coal co-firing under kiln conditions. The work focuses on a scenario in which 30% of the kiln heat demand is covered by hydrogen. A combination of experimental work and modelling is employed to assess flame behaviour and to identify optimal strategies for full-scale implementation of hydrogen-coal co-firing. Specific research objectives include defining the interactions between hydrogen and coal during combustion, and tailoring burner designs and hydrogen injection methods to enhance performance.

The results show that introducing a solid fuel into a gaseous flame environment rapidly shifts the combustion dynamics towards a predominantly solid fuel flame. Meanwhile, low amounts of hydrogen significantly improve the flame stability, intensity, and ignition of coal combustion. In a 30%-hydrogen combi-burner set-up, reducing the hydrogen velocity enhances flame stability. Lancing configurations exhibit reduced flame stability, delayed ignition, and decreased flame area and intensity as the distance between the hydrogen and coal injection sites increases.

Regarding NOx formation, the formation of NOx from the nitrogen bound in the fuel dominates the emissions profiles for both lignite and bituminous coal. Early release of volatile nitrogen compounds helps reduce the overall level of NOx formation.

For hydrogen-coal co-firing, combi-burners consistently achieve lower NOx emissions than coal-only firing. In contrast, NOx performance in hydrogen lancing varies with the design parameters. Positioning the hydrogen lance closer to the coal burner improves NOx performance. However, hydrogen inlet velocity is critical: a higher hydrogen velocity close to the coal burner increases NOx formation, while a higher velocity in distant lancing configurations reduces NOx emissions.

Future work will expand these findings to larger-scale systems and develop advanced measurement techniques for more-precise flame characterisation and NOx control strategies.