(735c) Reaction Characteristics of Sncr De-NOx in Moderate Temperature Range Based Urea Spiked Hydrazine Reductant

Large amounts of NOx are discharged from waste incinerator and sintering furnace in iron and steel industry. Some suitable reagent such as N₂H₄.H₂O can reduce NOx efficiently through selective non-catalytic reduction (SNCR) process within temperature range of 500~600°C, however adding urea into hydrazine hydrate would reduce the cost of the reagents.

Thermal decomposition of urea and hydrazine hydrate in N₂ and flue gas, the SNCR de-NOx characteristics of the urea/hydrazine hydrate mixture, the influence of factors such as flue gas temperature, oxygen concentration, normalized stoichiometric ratio (NSR) and proportion of urea in hydrazine hydrate were investigated.

It has been found that urea starts decomposing around 200°C and completes the decomposition around 500°C with NH₃ emission as an index. Under N₂ atmosphere, hydrazine hydrate decomposes around 200°C and NH₃ emission increases with the temperature rising. While in flue gas, very little NH₃ emission can be detected when hydrazine hydrate decomposes. The appearance of NOx from the mixture oxidation increases with the rising of temperature.

The effective temperature window of urea / hydrazine hydrate mixture is 450~550°C, much lower than that of urea and even lower than that of N₂H₄.H₂O. By adding 16.7% of urea into hydrazine hydrate the effective temperature window slightly moved to lower temperature side.

Through the sensitive analysis it was found that by adding urea into hydrazine hydrate, these following reactions can promote De-NOx process:

NH₂CONH₂=>NH₃+HNCO

H+O₂+M<=>HO₂+M,

H₂O₂+M<=>2OH+M,

NH₂+NO<=>N₂+H₂O

2N₂H₃<=>N₂H₄+N₂H₂

N₂H4(+M)<=>2NH₂(+M).