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

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 NO x 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% H 2 co-firing, with earlier ignition, higher flame stability, and increased temperature. Further increases from 30% to 50% and 70% H 2 cause comparatively smaller changes, suggesting that once co-firing is established, higher hydrogen shares up to 70% do not introduce major additional limitations. NO x 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.