Emissions and Soot in Partially Premixed Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 431

On the other hand, nitrous oxide liberates NO, which reacts with ozone. It is the
main, naturally occurring regulator of stratospheric ozone. It is also a major
greenhouse gas, and has almost 300 times greater Global Warming Potential than
CO 2 . Also, its long lifetime allows its transportation into the stratosphere, where it
can participate in ozone depletion. It is often associated with asphyxiation hazards.
Exposure to N 2 O might also cause short-term decrease in mental performance,
audiovisual ability and manual dexterity. Long-term exposures can cause Vitamin
B 12 de

ciency, numbness, reproductive side effects, neurological problems, etc.
In general, NO x reacts with ammonia, moisture and other compounds to form
small particles, which penetrate deeply into the sensitive parts of the lungs like the
alveoli. This can cause or aggravate respiratory diseases like emphysema and
bronchitis. It is also dangerous for heart patients, and can even cause premature
deaths. Also, Ozone is formed when NO x and VOC react in the presence of heat and
sunlight. Ozone might prove hazardous, especially for children and for elderly
people, with lung diseases such as asthma.
There are basically four different mechanisms of NO x formation (Lefebvre
1999 ): Thermal NO, Nitrous Oxide Mechanism, Prompt NO and Fuel NO. Thermal
NO Mechanism is also popularly known as the Zeldovich Mechanism. According
to this mechanism, NO is formed by the oxidation of atmospheric nitrogen in high
temperature regions of the flame gases. The thermal NO

production declines rapidly as the temperature is reduced. All in all, we can say that
the high

fl flame and in the post- fl

flame temperatures (>1,800 K), large residence time of the burnt products
in the combustor and availability of an excess of oxygen, all would promote the
formation of thermal NO x .
Prompt NO mechanism is also known as the Fenimore NO x mechanism. The
NO x formed here is in the

fl

flame region, very close to the burner and hence is termed
as prompt NO x . Prompt NO x can be signi

fl

cant in quantity; under low temperature,
fuel-rich conditions, where the residence times are short. Such conditions can be
created in surface burners, staged combustion systems and gas turbines. This
mechanism is generally not prevalent in extremely lean conditions.
Nitrous oxide (N 2 O) mechanism is not a major source of NO production and
occurs basically under fuel-lean, low-temperature conditions. Based on the origin of
the fuel, nitrogen can be present in a bound form in the fuel itself. Plants contain
nitrogen in the form of proteins, amino acids, alkaloids, chlorophyll and porphyrins,
etc. These were transformed during the coali cation process into polycyclic aromatic

compounds with pyridinic, pyrrolic or other functional groups, and hence the
nitrogen is

fixed in the coal, which can get converted to NO x during combustion.
Such a path of NO x formation is called fuel-bound NO x . Light distillate fuels contain
less than 0.06 % of organically bonded nitrogen or fuel-bound nitrogen, but the
heavy distillates may contain as much as 1.8 %. This nitrogen from the fuel might
react during combustion to form the fuel NO. Fuel-bound NO is the main source of
NO in rocket engines, as no atmospheric nitrogen is available there. The fraction of
nitrogen undergoing oxidation to NO, changes slowly with the increasing ame

temperature. Gaseous fuels like LPG, which contain little or no fuel-bound nitrogen
would have an insigni

fl

cant amount of NO production through this mechanism.