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7.4 Formation of VOCs and PAHs 205

7.4 Formation of VOCs and PAHs

Volatile organic compounds are a family of organic compounds that are volatile in
nature. They are mainly lower (C 1 –C 4 ) paraffin, olefins, aldehydes (e.g., formal-
dehyde), ketones (e.g., acetone) and aromatics (e.g., benzene, toluene, benzalde-
hyde, phenol). Sometimes, polycyclic aromatic hydrocarbon (PAH) (boiling point
218 °C) is also referred to as a VOC. The organic compounds that are not involved
in the formation of smog such as methane, CO, and halogenated organics like 1,1,1-
trichloroethane and CFCs are not considered as VOCs.
PAHs are produced during combustion when temperature is about 500–800 °C,
and they are oxidized further above 800 °C. Therefore, PAHs mainly present in the
low temperature zone of the flame or a combustion facility due to the poor fuel/
oxygen mixing. Heavy duty diesel engines (mainly trucks) emit *1,300 µg/km of
lighter PAHs such as pyrene, fluoranthene, etc. while gasoline-fueled cars emit
*100 µg/km of more hazardous heavier PAHs such as benzo(a)pyrene and dibenz
(a,h)anthracene [36].

7.5 Formation of CO and CO 2

7.5.1 Volatile Oxidation

The volatiles from liquid fuel vaporization or solid fuel devolatilization contain
various combustible gas molecules. It is challenging to decide their exact formula if
we were to use the combustion principles introduced in Chap. 3. Alternatively, we
can use a simple step reaction model [48] for volatile combustion. Assuming
oxygen is the oxidant,

k f1
Volatiles þ O 2 ! CO þ H 2 O ð7:19Þ
1 k f2
CO þ O 2 ! CO 2 : ð7:20Þ
2

The rate of combustion really depends on the rate of mixing.
Another key factor is the heat of combustion of the volatiles, which can be
calculated or experimentally determined. With this information known, the reaction
rate can be estimated using the combustion kinetics introduced in Chemical
Kinetics and Chemical Equilibrium in Chap. 3.