The decarbonisation of industries, such as the iron and steel industry, is a significant step towards phasing out fossil fuels. Hydrogen combustion offers a zero-carbon alternative to produce the necessary heat in the furnaces used in these industries, but is known to produce harmful N O x emissions. The simulated configuration in this study replicates a hydrogen-fuelled, straight-grate furnace for iron ore pelletisation. Two flame regions exist in this high-temperature furnace: typically, the primary flame features a rich fuel/air mixture followed by a secondary reaction zone where the burnt products of the primary flame interact with hot secondary air. The use of hydrogen as fuel in such a rich primary flame results in unburnt hydrogen in the primary flame, which further reacts with hot secondary air, leading to an increase in the N O x emissions. This study is novel in that it systematically investigates high-temperature operating conditions specific to hydrogen combustion using a detailed kinetic framework, enabling a wide parametric exploration that is computationally impractical in multidimensional CFD models. This paper conducts a detailed parametric study to understand the effects of various operating conditions, such as the equivalence ratio of the primary flame, inlet temperatures of the primary and secondary streams and the flame strain rate, on the rate of production of N O ( R O P N O ) and suggests potential N O mitigation strategies in such high-temperature furnaces. Here, one-dimensional ideal reactors are used to explore the underlying chemical kinetics in detail. A 39-species, 269-reactions chemical mechanism, which also includes N O x submechanisms, models hydrogen combustion in this study. It is observed that richer primary flames lead to higher R O P N O , compared to lean or stoichiometric alternatives. In these cases, the primary source of N O x is the thermal N O x formation pathway. Moreover, the interaction of the unburnt H 2 in the primary flames with the hot secondary air seems to play a significant role in N O production. Also, the N 2 in the secondary air stream is a more significant contributor to R O P N O compared to that in the primary flame in the rich cases. Furthermore, while R O P N O increases with an increase in the inlet temperatures, it decreases with an increase in the flame strain rate. Importantly, this work illustrates that R O P N O can be reduced by strategically changing these operating conditions. For instance, an increase in R O P N O due to a richer primary flame and/or hotter primary/secondary inlets can be compensated, at least partly, by increasing the flame strain rate. Such an optimisation of the operating conditions, which leads to N O mitigation, paves the way for cleaner designs of high-temperature furnaces. • Iron ore pelletising: fuelling with H 2 decarbonises the process, but increases NO x . • Effects of salient operating conditions on NO x emissions are systematically studied. • Rich primary flames and hot primary/secondary inlets increase NO production rate. • Increased flame straining: decreases the rate of production of NO (ROP NO ). • For clean furnace design: ROP NO can be controlled by optimising these conditions.
Palulli et al. (Fri,) studied this question.