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 m ore significant anthropogenic sources, a fact that has generated consi derable research interest over the last several years. The debate has principally focused on (1) the identity of the sources, and (2) the co nsequences of increased N2O concentrations. Both questions remain open , to at least some degree. The environmental concerns stem from the su ggestion that diffusion of additional N2O into the stratosphere can re sult in increased ozone (O3) depletion. Within the stratosphere, N2O u ndergoes 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 re search on N2O is measurement. Both electron capture gas chromatography and continuous infrared methods have seen considerable development, a nd both can be used reliably if their limitations are understood and a ppropriate precautions are taken. In particular, the ease with which N 2O is formed from NO in stored combustion products must be recognized; this can occur even in the lines of continuous sampling systems. In c ombustion, the homogeneous reactions leading to N2O are principally NC O + NO --> N2O + CO and NH + NO --> N2O + H, with the first reaction b eing the most important in practical combustion systems. Recent measur ements have resulted in a revised rate for this reaction, and the sugg estion that only a portion of the products may branch into N2O + CO. A lternatively, recent measurements also suggest a reduced rate for the N2O + OH destruction reaction. Most modeling has been based on the ear lier kinetic information, and the conclusions derived from these studi es need to be revisited. In high-temperature combustion, N2O forms ear ly in the flame if fuel-nitrogen is available. The high temperatures, however, ensure that little of this escapes, and emissions from most c onventional combustion systems are quite low. The exception is combust ion under moderate temperature conditions, where the N2O is formed fro m fuel-nitrogen, but fails to be destroyed. The two principal examples are combustion fluidized beds, and the downstream injection of nitrog en-containing agents for nitrogen oxide (NO(x)) control (e.g., selecti ve noncatalytic reduction with urea). There remains considerable debat e on the degree to which homogeneous vs heterogeneous reactions contri bute to N2O formation in fluidized bed combustion. What is clear is th at the N2O yield is inversely proportional to bed temperature, and con version of fuel-nitrogen to N2O is favored for higher-rank fuels. Fixe d-bed studies on highly devolatilized coal char do not indicate a sign ificant role for heterogeneous reactions involving N2O destruction. Th e 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 de gree of char devolatilization has a profound influence on both the yie ld 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 contribu tions in practical systems. Formation of N2O during NO(x) control proc esses has primarily been confined to selective noncatalytic reduction. Specifically, when the nitrogen-containing agents urea and cyanuric a cid are injected, a significant portion (typically > 10%) of the NO th at is reduced is converted into N2O. The use of promoters to reduce th e optimum injection temperature appears to increase the fraction of NO converted into N2O. Other operations, such as air staging and reburni ng, do not appear to be significant N2O producers. In selective cataly tic reduction the yield of N2O depends on both catalyst type and opera ting condition, although most systems are not large emitters. Other sy stems considered include mobile sources, waste incineration, and indus trial sources. In waste incineration, the combustion of sewage sludge yields very high N2O emissions. This appears to be due to the very hig h nitrogen content of the fuel and the low combustion temperatures. Ma ny industrial systems are largely uncharacterized with respect to N2O emissions. Adipic acid manufacture is known to produce large amounts o f N2O as a by-product, and abatement procedures are under development within the industry.