Energy & Fuels, Vol.25, No.4, 1510-1520, 2011
Latter Stages of the Reduction of NO to N-2 on Particles of Fe while Simultaneously Oxidizing Fe to Its Oxides
The gaseous pollutant, nitric oxide; is produced in almost every combustor but can be converted back to molecular N-2 by reacting it with metallic iron in NO + Fe -> FeO + 1/2N(2) (I) In fuel-rich regions of a combustor, the product, iron oxide, can be chemically reduced to metallic Fe (for subsequent reuse) by, e.g., carbon monoxide in FeO + CO -> Fe + CO2 (II) The net reaction of reactions I and II is NO + CO -> 1/2N(2) + CO2 (III) This paper investigates the latter stages of reaction I in a thermogravimetric balance, where the sample of iron was held in a specially designed bucket to ensure good contact of the gas with iron. In this case, the nearly spherical particles of Fe (initial diameter of 104-1.26 mu m) became covered with successive layers of FeO, Fe3O4, and even Fe2O3. Measurements of the rate of reaction I were made for different concentrations of NO in N-2 contacting the particles of Fe at fixed temperatures from 500 to 900 degrees C. At 900 degrees C, the reaction between NO and Fe proceeded rapidly and completely to Fe2O3 under kinetic control. At lower temperatures, the reaction became controlled by diffusion through the oxide product at progressively lower extents of the reaction. When [NO] in the gas was increased at a fixed temperature, the onset of diffusion control occurred at lower conversions of Fe. In contrast to its early stages Of reaction control, the rate of reaction I under diffusion control was found to be independent of the concentration of NO in the gas phase. It is concluded that, in these latter stages, the rate of reaction I is controlled by the diffusion of iron ions through an oxide layer around each particle. It is likely that this involves Fe2+ ions diffusing through FeO, but after Fe has been oxidized to FeO, the rate become: determined by both Fe2+ and Fe3+ diffusing through Fe3O4. In these cases, the diffusion coefficient varies with the temperature from similar to 0.6 x 10(-13) to similar to 3 x 10(-11) m(2)/s from 500 to 900 degrees C; the associated activation energy exceeds 100 kJ/mol. This study indicates that particles of either Fe or its oxides can be usefully included in a fluidized-bed combustor to remove NOx.