Reaction behavior and mechanism analysis of ammonia reduction of pellets

Ammonia, as a hydrogen carrier, has well-established trade routes and relatively low liquefaction costs, and can be directly used as a clean reducing agent for iron ore. Existing studies have provided thermodynamic parameters and reaction activation energy of ammonia reduction through thermodynamic calculations, basic experiments, and kinetic fitting, but the mechanism of ammonia reduction remains unclear. In this study, the reaction behavior, mineral composition and micro-structure of ammonia reduction of pellets at different temperatures and times was investigated, and further combined the changes in reduction expansion rate, thermodynamics analysis, and kinetic analysis, the mechanism of ammonia reduction of pellets were obtained. The results showed that the time for the reduction degree to reach 0.95 decreased from 93.5 min to 43.2 min as the temperature increased from 700 ℃ to 900 ℃. In addition, the mass fraction of nitrogen in the reduced pellet decreased from 0.029% to 0.019%. The results of kinetic fitting indicate that the mechanisms of ammonia reduction of pellets at various temperatures are diverse. The reduction is controlled by diffusion, chemical reaction, and nucleation respectively as the reduction proceeded at 700 ℃. However, the whole reduction duration is controlled by diffusion at 900 ℃. Overall, The reduction between ammonia and hematite mainly occurred on the outside of pellet, and the reduction of hematite to magnetite led to a first swelling of the pellets. The decomposition of ammonia is promoted by the catalysis of metallic iron as the reaction progresses, which led to the second swelling of pellets and the formation of a large number of cracks, and the interior of pellets are further reduced by hydrogen produced from ammonia decomposition. The research results have certain significance for understanding the ammonia reduction mechanism of pellets, promoting the application of ammonia in the iron-making process and the development of low-carbon technologies.