Mechanism of mineral Fe on fuel-N oxidation during ammonia-coal co-combustion: Experimental and quantum chemistry study

Ammonia-coal co-combustion can significantly reduce CO2 emissions from pulverized coal boiler. However, since ammonia is a blended high-nitrogen fuel, an inevitably increases the risk of high NOx emissions. Therefore, in-depth research on the transformation path of fuel-N during ammonia-coal co-combustion is the key to achieving low-nitrogen combustion. Since naturally occurring minerals in coal impact the migration and transformation of fuel-N, we report an experimental study coupled with quantum chemistry calculations to study the generation of nitrogen oxides during ammonia-coal co-combustion, in the presence of the inherent mineral Fe. The experimental results showed that under all the temperatures and ammonia co-firing ratios studied in this work, ammonia-coupled Fe-impregnated pulverized coal inhibited NO generation compared to coal without Fe impregnation. Theoretical calculations provided the possible existing forms of Fe in this system, and revealed the molecular pathways for the oxidation of ammonia-N to the nitrogen-containing intermediates of HNO and NCO, as influenced by the presence of mineral Fe. It was found that with impregnated Fe, the activation energy of the rate-determining step for the oxidation of fuel-N was about 30-40 kJ/mol higher than that without Fe impregnation; thus, Fe reduced the oxidation rate of fuel-N. The theoretical calculations elaborated the mechanism of the inhibited generation of nitrogen oxides with mineral Fe. The results indicated the enhancement of binding energy between nitrogen products and the surface of coal char.