Kinetics of the conversion of NO to N 2 during the oxidation of iron particles by NO in a hot fluidised bed

Abstract

The kinetics of the conversion of poisonous NO gas to N 2 in Fe+q NO→ FeO q+ q 2 N 2 have been measured in hot fluidised beds of sand from 973 to 1173 K at 1 bar. The reaction involves NO (in N 2) reacting with tiny (diam. 139–400 μm) iron particles and, because NO is thereby converted to harmless N 2, the reaction is of interest for removing NO from, e.g. fluidised bed or other combustors. Whether the oxide produced is FeO, Fe 3O 4 or Fe 2O 3 is one subject of discussion and it is concluded 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 produced on each iron particle is found to be very porous at the highest 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 upper limit of the collision frequency. During the second stage of reaction, the observed rate is independent of the concentration of NO. This is because the rate is controlled by diffusion of Fe 2; ions and electrons from a particle’s metallic core, through vacancies in the layer of FeO (covering the unreacted Fe), to the outer surface of the particle. Interestingly, the second stage has a larger activation energy (155±20 kJ/mol) than the first stage. At the highest temperature (900°C) these iron particles can be oxidised completely, but at 700°C the reaction stops after only a conversion of ∼20% of Fe to FeO.