Page 154 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN006K-933 June 29, 2001 12:14

264 Fuel Chemistry

No definite rules exist to determine which nitrogen ox- to show that prompt NO is formed only when hydrocarbon
ide formation mechanism dominates for a given stationary radicals are available. It has been shown that the amount
combustor configuration because of the complex interac- of NO formed in the fuel-rich systems is proportional to
tions between burner aerodynamics and both fuel oxida- the concentration of N 2 and also to the number of car-
tion and nitrogen species chemistry. But in general, fuel bons present in the gas phase. Hence, this mechanism is
nitrogen has been shown to dominate pulverized coal fired much more significant in fuel-rich hydrocarbon flames or
boilers, although thermal NO is also important in the post- in the reburning flames. Two reactions are believed to be
flame regions where over-fire air is used. Thermal NO con- the most significant to this mechanism:
tributions only become significant at temperatures above
◦
2500 F in coal flames. Prompt NO formation is not typ- CH + N 2 HCN + N (3)
ically an important mechanism during coal combustion. C + N 2 CN + N. (4)
In the absence of fuel nitrogen, fuel NO is not a problem
Reaction (4) was originally suggested by Fenimore in
for natural gas flames. Prompt NO, however, is impor-
1971. Reaction (5) is considered to be only a minor, but
tant in the vicinity of the inlet burners where reacting fuel
nonnegligible, contributor to prompt NO with its impor-
fragments mix with the oxidizing air. Thus, in natural gas
tance increasing with increasing temperature.
burners, both prompt and thermal NO contribute to the
formation of nitrogen oxides.
E. Fuel NO
C. Thermal NO Fuel NO is by far the most significant source of nitric ox-

Theprincipalreactionsgoverningtheformationofthermal ide formed during the combustion of nitrogen-containing
NO are fossil fuels. Fuel NO accounts for 75 to 95% of the total
NO x accumulation in coal flames and greater than 50% in
N 2 + O NO + N fuel oil combustors. The reason for fuel NO dominance
in coal systems is because of the moderate temperatures
N + O 2 NO + O.
(1500–2000 K) and the locally fuel-rich nature of most
These two reactions are usually referred to as the coal flames. Fuel NO is formed more readily than ther-
thermal-NOformationmechanismortheZeldovichmech- mal NO because the N H and N C bonds common in
anism. In fuel-rich environments, it has been suggested fuel-bound nitrogen are weaker than the triple bond in
that at least one additional step should be included in this molecular nitrogen which must be dissociated to produce
mechanism: thermal NO.
The main step, at typical combustion temperatures, con-
N + OH NO + H
sists of conversion of fuel nitrogen into HCN, step which
The reactions (1), (2), and (3) are usually referred to as the according to some investigators (Fenimore, 1976; Rees
extended Zeldovich mechanism. Experiments have shown et al., 1981) is independent of the chemical nature of the
that as the complexity of the reactors and fuels increase, it initial fuel nitrogen. Once the fuel nitrogen has been con-
is difficult to evaluate the rate forms. The general conclu- verted to HCN, it rapidly decays to NH i (i = 1, 2, 3) which
sion is that although the nonequilibrium effects are impor- reacts to form NO and N 2 .
tant in describing the initial rate of thermal-NO formation,
the accelerated rates are still sufficiently low that very little
F. NO x Control Technologies
thermal NO is formed in the combustion zone and that the
majority is formed in the postflame region where the res- In order to comply with the regulations for nitrogen ox-
idence time is longer. ides emissions, various abatement strategies have been de-
veloped. The most common methods used for NO x con-
trol are air staging, fuel staging, flue gas recirculation,
D. Prompt NO
selective noncatalytic reduction (SNCR), or selective cat-
Prompt NO occurs by the fast reaction of hydrocarbons alytic reduction (SCR). A determination of the most ef-
with molecular nitrogen in fuel-rich flames. This Fenni- fective and least expensive abatement technique depends
more mechanism accounts for rates of NO formation in the on specific boiler firing conditions and the emission stan-
primary zone of the reactor, which are much greater than dards. The principle of air staging is mainly reduction the
the expected rates of formation predicted by the thermal level of available oxygen in zones where it is critical for
NO mechanism alone. NO measurements in both hydro- NO x formation. By doing so, the amount of fuel burnt
carbon and nonhydrocarbon flames have been interpreted at the peak temperature is also reduced. Air staging is