Page 244 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

220 7 Combustion Process and Air Emission Formation

Fig. 7.6 TEM image of soot
particles from a diesel engine
(used with permission from
ACS)

The composition of the ash particles depends strongly on the fuel, although
SiO 2 ,Al 2 O 3 ,Fe 2 O 3 and CaO are usually the primary components. Ash from oils
contains vanadium (V) and nickel (Ni), plus magnesium (Mg). These metals are
added to the fuel as a corrosion inhibitor.

7.8.2 Soot Particles

Carbonaceous particles (10–80 nm) in the combustion system can agglomerate and
form larger particles (could be larger than 10 µm) [4, 5, 18]. These clusters are
called soot. A soot “particle” could have a family of thousands of carbonaceous
particles in it. A transmission electron microscope (TEM) image of diesel engine
soot is shown in Fig. 7.6 [41].
Chemically, soot particles are mainly composed of carbon, sulphur, and nitrogen
compounds and trace elements. Hydrocarbons can also be adsorbed into soot. These
particles could be as small as less than 10 nm in diameter.
There are mainly two mechanisms for the soot formation, depending on the type
of fuel [25, 28]. Aliphatics in gaseous or light liquid fuels can be converted to
acetylene (C 2 H 2 ) at high temperature, followed by a “polymerization” of C 2 H 2 to
form soot. This mechanism is also possible for combustion at high temperatures, up
to 1,600 °C.
For heavy oil and coal that contain more aromatics than aliphatics, soot is
formed by condensation and other processes that have aromatics as the starting
point [4, 11, 15, 22, 28, 38, 52]. The overall reaction of soot formation during sub-
stoichiometric stages of combustion can be written as