A kinetic study of an advanced reburning process

The possibility of a positive synergism among flue gas recirculation, staged combustion and selective non-catalytic reduction technologies to achieve N-containing emission reductions has been investigated by simulating a 320 MW industrial-scale furnace boiler. A detailed chemical kinetic scheme coupled with a simplified fluid dynamic of the system has allowed us to compare the effectiveness of ammonia and isocyanic acid in removing N-containing pollutant emitted (NPE) with hot exhausts. By keeping the same flue gas composition for both cases, the effect of temperature and the impact of different amounts of NH3 and HNCO injected in the final combustion stage has been analysed. Ammonia was the most effective reducing agent for the particular conditions examined. Due to its high reactivity, the maximum effectiveness of the NH3-promoted process occurs at 940 K and an NH3/NOx, molar ratio of about 1.25. NH2 dominates the process mainly following two paths: N-2 can be produced from direct interaction between NH2 and NO or via intermediate formation of NNH. On the other hand, HNCO is less effective than ammonia, owing to the influence of the particular radical environment within the boiler. HNCO oxidation is influenced by the amine radical pool which partly contributes to N-2 formation but is, at the same time, an important source of NOx in the conditions analysed.