Emissions and Soot in Partially Premixed Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 429

and is primarily composed of carbon (can be even up to 96 %), up to 10 % hydrogen
on a molar basis, and even more in young soot, oxygen and other elements. For-
mation of soot proceeds in three main chemical and physical steps:

1. Formation and growth of large aromatic hydrocarbons and their transition to
particles (inception of soot particles)
2. Growth of these solid particles by the addition of components from the gas
phase (surface growth)
3. Coagulation of the primary particles to large aggregates (particle agglomeration).

The net emission of soot depends on the competition between soot formation
and oxidation (Hadef et al. 2010 ). The basic mechanism for the formation of PAHs
and soot is that, first a precursor species, like acetylene (ethyne) is formed under

fuel-rich conditions. This ethyne and other hydrocarbon fragments combine through
pyrolytic reactions (in the absence of oxygen), to form the larger ringed structures,
which are known as PAHs. These clusters grow in size, in the absence of oxidation
reactions, which break the ring structures. Small particles of a certain critical size
are formed by the growth of such aromatic clusters by both physical and chemical
means, i.e. coagulation. At this point, these particles are termed as primary soot
particles. Many of these soot particles, now grow by particle agglomeration
(physical growth) and surface growth (chemical reactions), while remaining sus-
pended with the precursor species, clusters and adolescent soot particles.
Thus, while low temperatures generally inhibit combustion and lead to the
formation of more unburnt hydrocarbons, even very high temperatures would
hinder combustion. At extremely high temperatures, the hydrocarbons in the fuel
are pyrolysed and stripped of the hydrogen atoms. The remaining carbon atoms
form soot at these temperatures.
As such particulate matter is known to cause irritation in eyes, coughing and the
blackening of face. Particulate matter also increases the risk of coronary heart
disease. Soot particles smaller than 300 nm in size are known to penetrate deep into
the lungs and its alveoli, and hence can have a negative impact on the health. Soot
also contributes to visible air pollution and haze.
Particulate matter and soot formed in gas turbine engines are present at high
altitudes, which affect cloud formation and weather patterns. Since these particles
are present above the cloud formation zones, they do not come down with rains and
hence remain suspended in the stratosphere. Also, PAHs are oxidized more rapidly
in the urban atmosphere and may get stabilized if they get adsorbed on the surface
of particulates like soot. Then they become quite long-lived and capable of pene-
trating into the lungs as well. PAHs are also known mutagens, co-carcinogens and
carcinogens.
Exhaust smoke is caused by the production of finely divided soot particles in

fuel-rich regions of the flame, which in conventional combustors are always close to

the fuel spray. But most of the soot produced near to the fuel spray, is consumed in
the high-temperature regions downstream. Therefore, in a gas turbine engine, the
primary-zone dictates the rate of soot formation, while the intermediate zone

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