Emissions and Soot in Partially Premixed Combustion - Novel Combustion Concepts for Sustainable Energy Development - page 428

In the post- ame region, there might be a quenching of the chemical reactions,

due to the entrainment of the combustor liner cooling air, especially in the
primary or premixed zone in a gas turbine engine.

fl

￿

ciency and hence CO
emissions are the engine and the combustor inlet temperatures, combustion pres-
sure, primary-zone equivalence ratio; and for liquid fuels, the mean droplet size of
the spray is also signi

The main factors which in fl uence the combustion ef

cant.
The unburned hydrocarbon is a collective term coined for the hydrocarbon
species, which are not completely burnt or oxidized to CO 2 and are produced during
the course of combustion reaction, and includes unburnt fuel molecules. They
mainly occur in the combustor in the form of drops or vapour, as well as the
products of thermal degradation of the parent fuel into species of lower molecular
weight. They might be formed due to poor atomization (in case of liquid fuels),
inadequate burning rates, chilling effects of film-cooling air, etc. The reaction

kinetics of UHCs are much more complex than that for CO formation, though more
or less the same factors which affect the CO formation would affect the formation of
UHCs as well. Compounds like Poly Aromatic Hydrocarbons (PAHs), Benzene,
Toluene, Xylene (BTX) and Aldehydes like methanal (HCHO) can also be grouped
in this category. Control of these unburned hydrocarbons is vital, as these com-
pounds are extremely hazardous to health, in that they have both short-term as well
as long-term health effects. For example, HCHO can cause irritation in eyes over a
short term of exposure, and BTX is supposed to be a carcinogen.
'

, also known as particle pollution or PM, suspended partic-
ulate matter (SPM), and soot, is a complex mixture of extremely small particles and
liquid droplets. It basically implies extremely small solid particles suspended in a
gaseous or liquid medium. Particle pollution is made up of a number of compo-
nents, including acids (such as nitrates and sulphates), organic chemicals, metals
and soil or dust particles (USEPA 2013 ). The size of particles is directly linked to
their potential for causing health problems. Environmental Protection Agency
(EPA) is concerned about particles that are 10

Particulate matter '

m in diameter or smaller, because
those are the particles that generally pass through the throat and nose and enter the
lungs. Once inhaled, these particles can affect the heart and lungs and cause serious
health effects. EPA groups particle pollution into two categories (USEPA 2013 ):

μ

￿ '

Inhalable coarse particles or PM 10 ' , such as those found near roadways and

dusty industries, are larger than 2.5 μ m and smaller than 10 μ m in diameter.

￿ ' min

diameter and smaller. These particles are emitted directly from sources such as
forest

Fine particles or PM 2.5 ' , like those found in smoke and haze, are 2.5 μ

fires, or they can form when gases emitted from power plants, industries
and automobiles, react in the air.

PAHs are clusters of aromatic rings, up to 4 rings in size, which are formed under
fuel-rich combustion conditions. Soot is formed when under the appropriate con-
ditions; these PAHs grow in size up to 20 - 50 nm. Soot is formed from combustion
processes in locally fuel-rich zones at elevated temperatures (generally >1,300 K),