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

2 Combustion Theory
termed “natural sources” [391, 443]. These include among others geological
processes, microbial processes, and animal excrements. Microbial processes,
for instance, form carbon dioxide (CO 2 ), carbon monoxide (CO), methane
(CH 4 ), ammonia (NH 3 ), nitrous oxide (N 2 O), nitrogen oxides (NO x ), hydrogen
sulfide (H 2 S), and organic sulfur compounds. Lightning and forest fires
are thermal processes producing NO x , ozone (O 3 ), CO, volatile organic compounds
(VOCs), sulfur monoxide (SO), and particulate matter (PM). Anthropogenic
emission sources are mostly related to the burning of different kinds
of fuel, and they may be classified into process-related and user-related emissions.
Differentiation is also possible according to chemical composition,
physical state, and the manner in which pollutants reach the atmosphere (primary
or secondary pollutants) [391, 443].
The term “nitrogen oxides” (NO x ) in the first instance refers to any binary
compounds of nitrogen and oxygen. However, in the particular field of atmospheric
chemistry and air pollution, it is typically assigned to the sum of nitric
oxide (NO) and nitrogen dioxide (NO 2 ), as is the case in this thesis. Besides,
the term “oxides of nitrogen” is a synonym for nitrogen oxides in its general
meaning, and thus is sometimes used to prevent ambiguousness. Nitrous oxide
(N 2 O) is commonly also known as laughing gas. Here, it is not included in
the entity of NO x [214, 391, 443, 451].
2.2.1 Carbon Monoxide
Apart from the major combustion products CO 2 and H 2 O, carbon monoxide
(CO) is the most copious air pollutant in the lower atmosphere. With respect
to anthropogenic sources, emissions of CO exceed in quantity the mass of all
other air pollutants combined. According to Annamalai and Puri [19] and
Seinfeld [391], carbon monoxide pollution in urban areas correlates closely
with motor traffic volume, with 84% of the overall CO fraction stemming from
vehicle exhaust. Increased levels of CO production are typically encountered
in rich combustion systems, in nonpremixed combustion systems, in systems
with partial oxidation of unburned fuel (e.g. in over-lean regions or during operation
with insufficient liquid fuel vaporization), and in areas with quenching
by cold surfaces or aerodynamic quenching. Carbon monoxide is a product of
the incomplete oxidation of carbon-containing compounds before the final
product CO 2 is attained. Thus, the CO content within the exhaust is a mea-
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