Direct reduction of iron ore pellets by N2/H2 mixture: In-situ investigation and modelling of the surface temperature during reduction progression

Commercially available iron ore pellets were analyzed thermogravimetrically under an N2/H2 atmosphere in the temperature range of 800-1100 °C using coupled optical measurement technology in the special HITECOM reactor system. The pellet surface temperature was determined non-invasively in-situ with a thermal camera. The N2/H2 ratio was synthetically adjusted according to the decomposition of ammonia (NH3) with 25 vol% N2 and 75 vol% H2 to simulate the use of ammonia as hydrogen source. The heat and mass transfer between pellet, gas phase and reactor system were analyzed by a Computational Fluid Dynamics (CFD) model. The CFD model was validated using the decrease in surface temperatures measured by a thermal camera. The thermogravimetric analysis showed an increase in reduction rate with increasing temperature. On this basis, an activation energy (EA) of 43 kJ/mol and a pre-exponential factor (k0) of 7.60E-02 g/s were determined as kinetic parameters. The pellet surface temperature is lower than the reactor temperature due to the endothermic reaction. The temperature difference between pellet surface temperature and reactor temperature increases with rising temperature and is directly proportional to the reactivity of the pellets with respect to H2. Depending on the reduction temperature, the H2 concentration on the pellet surface and in the reaction chamber is lower than the gas inlet concentration and decreases more sharply with increasing temperature. The characterized iron ore pellets are subject to raw material-related fluctuations, which are reflected in different reactivities with regard to H2. This "raw material influence" also affects the change in pellet surface temperature. The results, combined with the CFD simulation, provide a clearer insight into the heterogeneous reduction of iron ore pellets under N2/H2 atmosphere.