Jc. Kramlich et Wp. Linak, NITROUS-OXIDE BEHAVIOR IN THE ATMOSPHERE, AND IN COMBUSTION AND INDUSTRIAL-SYSTEMS, Progress in energy and combustion science, 20(2), 1994, pp. 149-202
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.