A review of nitrous oxide behavior in the atmosphere, and in combustion and industrial systems

Abstract

Tropospheric measurements show that nitrous oxide (N 2 O) concentrations are increasing over time. This demonstrates the existence of one or more significant anthropogenic sources, a fact that has generated considerable research interest for several years. The debate has principally focused on (1) the identity of the sources, and (2) the consequences of increased N 2 O concentrations. Both questions remain open, to at least some degree. The environmental concerns stem from the suggestion that diffusion of additional N 2 O into the stratosphere can result in increased ozone (O 3 ) depletion. Within the stratosphere, N 2 O undergoes photolysis and reacts with oxygen atoms to yield some nitric oxide (NO). This enters into the well known O 3 destruction cycle. N 2 O is also a potent absorber of infrared radiation and can contribute to global warming through the greenhouse effect. In combustion, the homogeneous reactions leading to N 2 O are principally NCO + NO → N 2 O + CO and NH + NO → N 2 O + H, with the first reaction being the more important in practical combustion systems. During high-temperature combustion, N 2 O 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 N 2 O is formed from fuel nitrogen, but fails to be destroyed. The two principal examples are combustion in fluidized beds, and in applications of nitrogen oxide (NO x ) control by the downstream injection of nitrogen-containing agents ( e.g. , selective non-catalytic reduction with urea). There remains considerable debate on the degree to which homogeneous vs. heterogeneous reactions contribute to N 2 O formation in fluidized bed combustion. What is clear is that the N 2 O yield is inversely correlated with bed temperature, and conversion of fuel nitrogen to N 2 O is favored for higher-rank fuels. Formation of N 2 O during NO x control processes has been confined primarily to selective non-catalytic 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 N 2 O. The use of promoters to reduce the optimum injection temperature appears to increase the fraction of NO converted into N 2 O. Other operations, such as air staging and reburning, do not appear to be significant N 2 O producers. In selective catalytic reduction, the yield of N 2 O 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 N 2 O 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 N 2 O emissions. Adipic acid manufacture is known to produce large amounts of N 2 O as a byproduct, and abatement procedures are under development within the industry.