(735j) Kinetic Mechanism Investigation On NOx Removal With Hydrazine Hydrate At Moderate to High Temperatures

The moderate temperature selective non-catalytic reduction (SNCR) process is needed in many complicate practices. In this research kinetic mechanism of NO_x reduction using hydrazine hydrate in SNCR process was studied and verified by the experimental results, the dominant radicals and reactions were confirmed and key parameters such as residence time and normalized stoichiometric ratio (NSR) in the SNCR process have been obtained through sensitivity analysis. The results of kinetic calculations based on the proposed mechanism showed that the effective temperatures for SNCR De-NOx using hydrazine hydrate were bimodal distributed with the optimum temperatures being around 893K and 1248K respectively, and the lower temperature window fell in the range of 850-1000 K; while the experimental results proved the bimodal distribution of effective temperatures with the peak values being around 896 K and 1241 K respectively and the lower temperature window was within the range of 837-957 K for the hydrazine-based SNCR process. Chemkin/Fluent coupling simulation was adopted to eliminate the discrepancy between the results of experiments and perfectly stirred reactor (PSR) model, and the calculation results from Chemkin/Fluent coupling simulations and the experimental data matched well with respect to reaction behaviors versus temperature. Through sensitivity analysis it had been found that the most effective reaction contributing to NO reduction was the decomposition reaction of N₂H₄ to NH₂ and the most helpful radical was NH₂. At temperature below 900 K, increasing of residence time and NSR could raise the efficiency of NO reduction, and the increased residence time would expand the temperature window to lower side significantly; however, if the reaction temperature is higher than 900 K, increase of residence time or NSR has almost none effect on NO reduction. All of these results showed that hydrazine hydrate corresponds to a much lower temperature window than the traditional reagents and it will be widely applied to those SNCR processes for NO_x reduction where a high temperature reactor is not available.