Page 242 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

218 7 Combustion Process and Air Emission Formation

Hydrogen peroxide radical is formed when a hydrogen atom reacts with oxygen
in the presence of a third component (M).
H þ O 2 þ M $ HO 2 þ M ð7:89Þ

This reaction is significant at low temperatures. As a result, considerable HO 2
concentration may be present in the low temperature zones of the flame, and
consequently a significant part of the NO present in the cooler zones may react into
NO 2 through Eq. (7.88).
In the high temperature zone of the flame, hydrogen and oxygen tends to react
directly to form hydroxyl radicals and oxygen atoms:

H þ O 2 ! OH þ O ð7:90Þ
The NO 2 formed at lower temperature decomposes rapidly back to NO when it
drifts into the high temperature zone of the flame.

NO 2 þ H ! NO þ OH ð7:91Þ

NO 2 þ O $ NO þ O 2 ð7:92Þ
Partially due to this decomposition process, NO 2 remains less than 5 % with
95 % or more NO in a typical flue gas. However, these reactions slow down at low
concentrations of O and H. This situation arises when hot and cold streams are
mixed rapidly.

7.8 Formation of Particulate Matter

In combustion inorganic minerals in fuel are converted into solid, liquid, and
vapors. The solid and liquid contribute directly to the particulate matter formation,
while the vapors could condense, solidify, and form secondary particulate matter
before and after the emission. The final phase of the ash-forming materials depends
on many factors including temperature, pressure, residence time, fuel particle size
and size distribution, and the compounds in the combustion system. For solid fuels
the ash-forming minerals are mainly converted to oxides of silicon, aluminum, and
iron (SiO 2 ,Al 2 O 3 and Fe 2 O 3 ). For liquid and gaseous petroleum fuels, the partic-
ulate matter is mainly due to the incomplete combustion and secondary reaction of
VOCs and SO x /NO x . However, this general knowledge may not apply to alternative
and renewable fuels.