Experimental and simulation investigation on nitrogen transformation characteristics in ammonia/coal preheating combustion

The power plant boiler has the characteristics of large spatial scale. The flow field, temperature field and concentration field are disorderly, which is easy to lead to the surge of nitrogen oxide emission so as to limit the engineering application of coal co-firing with NH₃. To solve the problem, a de-NO_x strategy based on preheating combustion technology was proposed, whose core was to promote the advance conversion of NH₃ and volatile-N into N₂ in the preheating zone so as to reduce the amount of fuel-N in combustion zone. A two staged drop-tube furnace(DTF) was built to investigate the effects of excess air ratio in preheating zone(λ₁) and NH₃ proportion on NO emission and unburned carbon content in fly ash. And a detailed chemical reaction mechanism model was built to analyze the transformation path of ammonia nitrogen and coal nitrogen. The results from preheated combustion experiments and simulation calculations showed that, under low λ₁ conditions, the unburned carbon content in fly ash decreased as the NH₃ proportion increased. However, the maximum increase in NO emissions more than doubled. Increasing λ₁ at this stage gradually mitigated the rise in NO emissions. This effect was attributed to the progressive increase in the conversion rate of nitrogen from ammonia and coal volatiles into N₂, which could ultimately reach 100%. This indicated that NO emissions under these conditions were determined solely by the balance between the oxidation and reduction of nitrogen in the char. Furthermore, by implementing air staging, NO emissions under a 20% ammonia mixing condition could be comparable to those under pure coal conditions. Chemical reaction pathway analysis in the preheating zone revealed that coal pyrolysis products altered the conversion pathway of NH₃, primarily facilitating its transformation into HCN and HNCO through interactions with radicals such as CH₃. In the combustion zone, the pathway analysis further identified soot and HO₂ radicals as the critical species for NO reduction. Finally, the large-scale engineering application of coal co-firing with ammonia preheating combustion by using the developed new preheating burner and gasifier preheating system was forecasted.