KINETICS OF THE CONVERSION OF NO TO N-2 DURING THE OXIDATION OF IRON PARTICLES BY NO IN A HOT FLUIDIZED-BED

The kinetics of the conversion of poisonous NO gas to N-2 in Fe + qNO --> FeOq + q/2 N-2 have been measured in hot fluidised beds of sand fr om 973 to 1173 K at 1 bar. The reaction involves NO (in N-2) reacting with tiny (diam. 139-400 mu m) iron particles and, because NO is there by converted to harmless N-2, the reaction is of interest for removing NO from, e.g. fluidised bed or other combustors. Whether the oxide pr oduced is FeO, Fe3O4 or Fe2O3 is one subject of discussion and it is c oncluded that the product is mainly FeO. Two stages were found for the reaction. Initially, the rate of oxidation of iron is first order in NO and, in fact, is kinetically controlled. The outer layer of oxide p roduced on each iron particle is found to be very porous at the highes t temperature (1173 K), but much less porous at the lowest temperature (973 K). The associated rate constant has a low activation energy (41 +/- 4 kJ/mol) and a pre-exponential factor close to the theoretical u pper limit of the collision frequency. During the second stage of reac tion, the observed rate is independent of the concentration of NO. Thi s is because the rate is controlled by diffusion of Fe2+ ions and elec trons from a particle's metallic core, through vacancies in the layer of FeO (covering the unreacted Fe), to the outer surface of the partic le. Interestingly, the second stage has a larger activation energy (15 5 +/- 20 kJ/mol) than the first stage. At the highest temperature (900 degrees C) these iron particles can be oxidised completely, but at 70 0 degrees C the reaction stops after only a conversion of similar to 2 0% of Fe to FeO. (C) 1998 Elsevier Science Ltd. All rights reserved.