On the Formation of Nitrogen Oxides During the Combustion of ...

1 Introduction
1.3 Oxides of Nitrogen Formation
The ongoing public discussion of environmental problems caused by the
combustion of fossil fuels is mainly focused on the production of carbon dioxide
(CO 2 ). Accordingly, there are a large number of research activities in the
field of CO 2 reduction and separation. This includes work on an increase in
the efficiency of combustion related processes, as the amount of CO 2 is linearly
dependent on the amount of fuel burned. However, a gain in the efficiency
of heat engine processes is often also linked to an increase in combustion
temperature. This is due to the fact that the achievable efficiency of a
thermodynamic process rises with an increase of the temperature differences
in the process. Moreover, a higher temperature level may bring with it the
added advantage of a lower level of carbon monoxide (CO) production, due
to higher fuel burnout. The downside of this increase in combustion temperature
is a higher production rate of thermal NO, described by a mechanism
postulated by Zeldovich in 1946 [471, 472]. Thermal NO is generated in combustion
processes at temperatures above 1800 K, and its generated amount increases
exponentially with even higher temperatures. This amount is a function
primarily of temperature and of the residence time at the said temperature.
Hence, not only the flame zone but its associated post-flame zone is
usually of major importance.
Nitric oxide (NO) must not be confused with nitrogen dioxide (NO 2 ) or nitrous
oxide (N 2 O). In principle, NO and NO 2 are collectively referred to as “nitrogen
oxides” (NO x ) [51, 443, 458]. While the NO portion used to contribute up to
90% of the NO x emission in the flue gas of most combustion devices burning
fossil fuels, it decreased over the last decades, mostly due to combustion modification
techniques or post-combustion methods, both aiming at a general
NO x reduction [51, 69, 201, 380]. Still, NO is considered to be rapidly oxidized
in air, forming NO 2 . Nitrous oxide is accounted for separately in the majority
of combustion literature.
Oxides of nitrogen are, in part, responsible for acidic precipitation, cause photochemical
smog in cities, act as a greenhouse gas, participate in the ozone
(O 3 ) chemistry by building harmful ozone in the troposphere and destroying
beneficial ozone in the stratosphere, and are directly noxious to humans as
they affect the respiratory system [51, 103, 391, 451]. Public debates about
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