Nitrous oxide behavior in the atmosphere, and in combustion and industrial systems

Tropospheric measurements show that nitrous oxide (N2O) concentrations are increasing over time. This demonstrates the existence of one or more significant anthropogenic sources, a fact that has generated considerable research interest over the last several years. The debate has principally focused on (1) the identity of the sources, and (2) the consequences of increased N2O concentrations. Both questions remain open, to at least some degree. The environmental concerns stem from the suggestion that diffusion of additional N2O into the stratosphere can result in increased ozone (O3) depletion. Within the stratosphere, N2O undergoes photolysis and reacts with oxygen atoms to yield some nitric oxide (NO). This enters into the well known O3 destruction cycle. N2O is also a potent absorber of infrared radiation and can contribute to global warming through the greenhouse effect. A major difficulty in research on N2O is measurement. Both electron capture gas chromatography and continuous infrared methods have seen considerable development, and both can be used reliably if their limitations are understood and appropriate precautions are taken. In particular, the ease with which N2O is formed from NO in stored combustion products must be recognized; this can occur even in the lines of continuous sampling systems. In combustion, the homogeneous reactions leading to N2O are principally NCO + NO → N2O + CO and NH + NO → N2O + H, with the first reaction being the most important in practical combustion systems. Recent measurements have resulted in a revised rate for this reaction, and the suggestion that only a portion of the products may branch into N2O + CO. Alternatively, recent measurements also suggest a reduced rate for the N2O + OH destruction reaction. Most modeling has been based on the earlier kinetic information, and the conclusions derived from these studies need to be revisited. In high-temperature combustion, N2O forms early in the flame if fuel-nitrogen is available. The high temperatures, however, ensure that little of this escapes, and emissions from most conventional combustion systems are quite low. The exception is combustion under moderate temperature conditions, where the N2O is formed from fuel-nitrogen, but fails to be destroyed. The two principal examples are combustion fluidized beds, and the downstream injection of nitrogen-containing agents for nitrogen oxide (NOx) control (e.g., selective noncatalytic reduction with urea). There remains considerable debate on the degree to which homogeneous vs heterogeneous reactions contribute to N2O formation in fluidized bed combustion. What is clear is that the N2O yield is inversely proportional to bed temperature, and conversion of fuel-nitrogen to N2O is favored for higher-rank fuels. Fixed-bed studies on highly devolatilized coal char do not indicate a significant role for heterogeneous reactions involving N2O destruction. The reduction of NO at a coal char surface appears to yield significant N2O only if oxygen (O2) is also present. Some studies show that the degree of char devolatilization has a profound influence on both the yield of N2O during char oxidation, and on the apparent mechanism. Since the char present in combustion fluidized beds will likely span a range of degrees of devolatilization, it becomes difficult to conclusively sort purely homogeneous behavior from potential heterogeneous contributions in practical systems. Formation of N2O during NOx control processes has primarily been confined to selective noncatalytic reduction. Specifically, when the nitrogen-containing agents urea and cyanuric acid are injected, a significant portion (typically > 10%) of the NO that is reduced is converted into N2O. The use of promoters to reduce the optimum injection temperature appears to increase the fraction of NO converted into N2O. Other operations, such as air staging and reburning, do not appear to be significant N2O producers. In selective catalytic reduction the yield of N2O depends on both catalyst type and operating condition, although most systems are not large emitters. Other systems considered include mobile sources, waste incineration, and industrial sources. In waste incineration, the combustion of sewage sludge yields very high N2O emissions. This appears to be due to the very high nitrogen content of the fuel and the low combustion temperatures. Many industrial systems are largely uncharacterized with respect to N2O emissions. Adipic acid manufacture is known to produce large amounts of N2O as a by-product, and abatement procedures are under development within the industry.