Environmental Engineering Reference
In-Depth Information
4.4.5
Intermediate Oxidation Phases in the Combustion
of Quaternary Carbon Compounds
R
[O]
OCO
0
+4
ð
4
:
28
Þ
R
C
R
R
In any fuel combustion reaction, in the final oxidation phase, carbon changes to
dioxide (maximum oxidation state of the carbon atom) regardless of its chain
position, and the hydrogen contained in the organic molecule changes to water
[
21
-
23
].
Reactions (4.24-4.28) show that the intermediate oxidation phases for the
primary and/or secondary carbon compounds are the following:
Organic compound
!
!
!
!
alcohol
aldehyde (ketone)
carboxylic acid
carbon
dioxide + water
Hydrocarbons containing secondary, tertiary or quaternary carbon atoms do not
form all of the intermediate compounds; they skip some phases and change, through
combustion, directly from ketone, alcohol and hydrocarbon to carbon dioxide and
water.
Consequently, smoke produced by fuel combustion consists of a heterogeneous
hot gas suspension containing traces of the initial (or reformed) hydrocarbon under
the form of VOC (volatile organic compound), partial oxidation intermediates like
alcohols, aldehydes and ketones, carboxylic acids, carbon dioxide and water
vapour. As shown above in this chapter, smoke also contains other forms of
incomplete combustion: carbon monoxide (CO) and carbon particulates.
Depending on chain length, these intermediates exist in solid, nanoparticle, liquid,
very fine aerosols or gaseous state [
22
].
4.5 Dioxins and Furans
In the past decades, owing to the toxicity and long effect of these compounds in the
environment, extensive research has been carried out into the most important
precursors of dioxins and furans (polyhalogenated cyclic derivatives, chiefly biphe-
nyls and terphenyls, are mentioned frequently), and the mechanisms of formation,
decomposition and reformation of dioxins and furans in combustion processes.
Studies have identified 16 compounds whose emissions in the environment should
be limited on the whole planet (Table
4.2
)[
24
]:
TCDD is considered the most toxic of these chlorinated compounds. For this
reason it was assigned a value of 1.000. The toxicity of all the other compounds in
the two mentioned classes is expressed as an I-TEQ fraction to TCDD.
