Abstract: Nitrogen oxides (NOx) are pollutants that can be produced by natural or anthropogenic processes. NOx emissions of anthropogenic origin are mainly the result of the burning of fossil fuels at high temperatures. In pelletizing kilns, mineral coals are used and natural gas is burned, these processes contribute to the nitrogen oxides emissions. The pelletizing plants located at the Tubarão Unit – Vitoria, Espirito Santo/Brazil, account for a significant fraction of Vale’s NOx emissions, corresponding to around 982 g per tonne of pellets produced, in the base year of 2015. Vale has a target to reduce 10% of all NOx emissions by 2030. Given this scenario and aiming to reduce NOx emissions at the Tubarão Unit, a case study is proposed for the development of a burner for pelletizing kilns with low NOx emission. To better understand the NOx formation in pellet kilns, as well as what can be done to reduce NOx emissions in existing plants, a series of CFD (Computation Fluid Dynamics) modelling scenarios were studied. The working scenarios were: Scenario number 1 – a 3 MW burner being powered by natural gas and atmospheric air mixture, defined as Base Case; Scenarios 2, 3, 4 and 5, respectively: i) a 3 MW burner being powered by natural gas and atmospheric air mixture, with addition of water at a flow rate of 60 l/h (75 μm droplet size and 36 degrees open cone); ii) 120 l/h (75 μm droplet size and 36 degrees open cone); iii) 120 l/h (120 μm droplet size and 88 degrees open cone); 160 l/h (120 μm droplet size and 88 degrees open cone). To make the CFD modelling , the integral geometric model by one combustion chamber was considered. The computational grid created was composed by 1 milion of control volumes, refined during calculations in the regions with high gradients. The models used were: k-ε for the turbulent flow, EDC (Eddy Dissipation Concept) for the chemical turbuluent interactions, the reduce mechanism DRM19 (19 species and 84 reactions) and DPM (Discrete Phase Model) for the water drops. Regarding NOx simulation, just the thermical NOx generation was considered because this is the aim producing mechanism present in pelletizing kilns. With the modeled scenarios, showed over the isosurfaces of carbon monoxide (CO) at 2000 ppm, it was not observed significant changing in the flame shape due to the water addition. It was observed reductions of temperature peaks until 50°C on the flame center to the scenarios with water injection at the 120 l/h and 160 l/h flow rates. Considering the evaluated scenarios, a reduction in the range of 40-50% in NOx emission was observed, being the scenarios applying 120 l/h with droplets of 75 μm and 160 l/h with droplets of 120 μm, the ones with the greatest NOx reduction (50%). Given the results obtained, an injection nozzle was developed for spraying water into burners to be tested in an industrial environment to validate the NOx reduction observed in the simulations.

Keywords: NOx emissions, emissions control, low NOx burner, pelletizing plant.