Numerical simulation of co-combustion of pulverized coal and hydrogen-rich gas and pollutant formation in oxidized pellet rotary kiln

To meet the requirements of energy conservation, carbon reduction, and ultra-low emissions in the iron and steel industry, ϕ6.858 m×45.72 m pellet rotary kiln was investigated. A co-combustion model of pulverized coal and natural gas/hydrogen was established based on computational fluid dynamics （CFD） and the finite-rate/eddy-dissipation （FR/ED） mechanism. Flow field, temperature field, and pollutant formation characteristics in the kiln were systematically investigated under pure pulverized coal combustion and under different substitution ratios of natural gas and hydrogen.Results show that, after hydrogen-rich gaseous fuel is introduced, typical recirculation zone is formed near the burner nozzle, which is favorable for more complete fuel combustion. With the addition of natural gas or hydrogen, the flame gradually changes from the slender cylindrical shape under the pure pulverized coal condition to compact spindle-like shape at high natural gas substitution ratios, effectively alleviating the ignition delay of pulverized coal and making the temperature distribution more uniform. In particular, the front-end combustion enhancement effect is more pronounced under hydrogen-blending conditions than under natural gas-blending conditions. However, at high substitution ratios, a new local high-temperature zone appears near the kiln head, especially under hydrogen-blending conditions, which is more likely to increase the thermal load on the burner nozzle and the front-end refractory materials. Fuel substitution can significantly suppress NO_x formation. At a substitution ratio of 60%, the outlet NO_x mass fraction decreases from 0.034 2% under the pure coal condition to 0.013 5% for natural gas and 0.013 2% for hydrogen, respectively, corresponding to reductions of more than 60%. Natural gas and hydrogen exhibit significant differences in the mechanism of CO₂ reduction. At a substitution ratio of 60%, the peak CO₂ mass fraction only decreases from 0.17 to about 0.13 under natural gas substitution, whereas it drops to below 0.08 under hydrogen substitution, with a reduction of more than 50%, indicating a much stronger decarbonization effect. Considering the thermal stability of the rotary kiln, equipment safety, and emission reduction requirements, 40% is recommended as the optimal substitution ratio for both natural gas and hydrogen. This study provides an important theoretical basis and engineering guidance for fuel structure optimization and low-carbon clean combustion in rotary kilns.