Page 96 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

70 3 Basics of Gas Combustion

include, but are not limited to, those listed in Table 3.1, where M can be any species
present in the combustion system that acts as a collision partner. Species such as
H, O, OH and HO 2 are called radicals. Radicals play important roles in combustion.
They are highly reactive and short-lived.
Example 3.5: Reaction rate
Determine the reaction rate at low pressure for the mechanism reaction
H þ H þ M ! H 2 þ M

at temperature of 3,000 k, where M is H 2 as a function of [H].

Solution
According to Eq. (3.22), the chemical reaction rate for the reaction

H þ H þ M ! M þ H 2

is described in terms of the formation rate of H 2

d½H 2 Š 2
¼ k½HŠ
dt
where the reaction rate constant k can be described using Eq. (3.25)

B E A
k ¼ AT exp ð3:25Þ
RT
Therefore, the chemical reaction rate is described as

d½H 2 Š B E A 2
¼ AT exp ½HŠ
dt RT
There is no negative sign because H 2 is produced rather than consumed in this
one-way reaction.
From Table 3.1, we can get
16
A ¼ 0:7 10 ; B ¼ 0:6; and E A ¼ 0

Thus the rate of reaction at T = 3,000 K is

d½H 2 Š 16 0:6 2 19 2
¼ 0:7 10 3,000 exp 0Þ½HŠ ¼ 5:74 10 ½HŠ
ð
dt
The solution indicates that molecular hydrogen is produced extremely fast at
high temperature.