The reduction of the nitrogen oxides NO and N2O to molecular nitrogen in the presence of iron, its oxides, and carbon monoxide in a hot fluidized bed

Chemical reactions between iron, or its oxides, and the oxides of nitrogen, NO and N2O, have been studied in detail in an electrically heated bed of sand, fluidized by nitrogen, containing controlled amounts of NO or N2O (1-100 ppm). Batches (1.0-5.0 g) of iron particles (diam. 106-250, 250-300, or 300-425 mu m) were injected into the bed (mass 2 kg), while the concentrations of NO and N2O were measured as a function of time in the freeboard. The oxides of iron were found to be relatively unreactive in the decomposition of NO and N2O, but metallic iron caused a sudden drop by up to 80% in the concentration of these nitrogen oxides released from the fluidized bed. The oxides of nitrogen and iron were reacting to form N-2 and an oxide of iron. The effects of temperature, the size of the iron particles, and the mass of iron added are discussed in detail. Plots of [N2O] vs time during the above experiments enabled the initial rate of reaction to be measured and the rate constant expressed as 0.4 exp(-2350/T) (m/s). This expression was confirmed in a thermogravimetric analyzer (TGA). Differences in behavior between NO and N2O reacting on Fe were found and investigated using an electron scanning microscope (ESM) to study the iron oxides produced. This revealed that, whereas the oxide layer produced by NO was very porous and almost sponge-like, that produced by N2O was relatively smooth, continuous, and impervious. This difference in the structure of the oxides explains the low conversions of iron to iron oxide in the presence of N2O. The very porous oxide formed by NO and Fe does not hinder further oxidation. The different structures of the oxides also explains the different products formed (Fe2O3 by NO and FeO by N2O). The presence of CO (a strong reducing agent and present in most combustion situations) in addition to NO or N2O and iron causes the decreased emissions of the oxides of nitrogen to continue ad infinitum. Carbon monoxide reacts with the oxides of iron to regenerate metallic iron, allowing the continuous reduction of NO or N2O to N-2. Here, then, is an excellent technique for reducing the emissions of NOx and N2O where by iron catalyzes the reduction of NO and N2O by CO in, e.g., N2O + CO --> N-2 + CO2. (C) 1997 by The Combustion Institute.