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

1.4 Motivation and Goals of this Thesis
these problems led to political decisions for stricter regulations on NO x emissions
since the 1980s. Thus, two goals have to be considered in the development
of new combustion applications: First, the decrease in operating costs
represented by an increase in efficiency. Second, the public opinion represented
by legislation for a decrease in NO x emissions. In light of the above
described correlation between combustion temperatures and resulting NO x
emissions, a conflict becomes inevitable. Both combustion modification and
post-combustion methods need to be considered to meet the most stringent
regulations enacted for NO x reduction [50, 51, 289, 443].
1.4 Motivation and Goals of this Thesis
Global emissions of nitrogen oxides increased at an averaged rate of 3.4 % per
year over the past 150 years [103]. 2 As a significant amount of the global NO x
emissions is attributed to combustion of biomass and fossil fuels, increasingly
stringent regulations have been implemented in a number of industrialized
countries. European aeronautics, for instance, has the environmental goal for
2020 to reduce NO x emissions by 80% in conjunction with a CO 2 reduction of
50% per passenger kilometer and a reduction of the perceivable aircraft noise
by 50%, using the year 2000 as a baseline [5, 7].
As a result of the continuing trend towards increased efficiency in liquid fuel
combustion and the detrimental effects of NO x , the development of enhanced
NO x control and reduction concepts becomes necessary. Since most liquid
fuels contain virtually no fuel nitrogen, NO formation via the thermal mechanism
is dominant in most of the related technical applications and, hence,
starting point for NO x control techniques that rely on combustion modification.
The notion of the dependence of the NO formation rate on temperature
and oxygen concentration is employed, which involves a reduction of
the combustion gas temperature or availability of oxygen, or both. Temperature
reduction may be achieved in various ways, including exhaust gas recirculation,
water injection, and reduction in fuel-air ratio. Staged combustion
is also effective in producing substantial NO x reduction, as it limits the
2 While Germany reports an absolute decrease of anthropogenic NO x emissions by 54% in the period of 1990 to
2009 [449], an increase by 275 % was projected for Asia for the same period with even higher dynamics for the
decades to come [1]. Generally, after World War II, the most rapid increases in emissions have been registered
in Asia, South America, and Africa with high dynamics in the power and transport sectors [103, 451].
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