Effects of coal replacement by hydrogen on flame structure, heat release, and NOₓ emissions under rotary-kiln iron ore induration conditions

Journal article, 2026

We investigate the transition from coal to hydrogen in a downscaled industrial burner adapted for hydrogen injection under rotary-kiln conditions. Although replacing coal with hydrogen is an attractive decarbonization route, its effects on combustion behaviour, heat transfer, ignition, flame stability, and NOx emissions are complex and must be understood before full-scale implementation.The results show that the largest changes occur when moving from pure coal to 30% H2 co-firing, with earlier ignition, higher flame stability, and increased temperature. Further increases from 30% to 50% and 70% H2 cause comparatively smaller changes, suggesting that once co-firing is established, higher hydrogen shares up to 70% do not introduce major additional limitations. NOx emissions decrease by 38% at hydrogen substitution up to 30%, but then rise sharply with further hydrogen addition, reaching 219% above pure coal under full hydrogen firing. This indicates functional limitations of the tested burner above 50–60% hydrogen substitution.