Page 228 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

204 7 Combustion Process and Air Emission Formation

Two-step models take into consideration the heating rate and the competition
between different volatiles.

k f1
ð
Fuel ! x 1 V 1 þ 1 x 1 ÞCh 1
ð7:16Þ
k f2
ð
Fuel ! x 2 V 2 þ 1 x 2 ÞCh 2
This simplified reaction formula indicates that the particle after devolatilization
is decomposed into volatiles and chars, and there is nothing else. The corresponding
reaction rate can be described using (modified) Arrhenius expressions

E i
k i ¼ A i exp ; i ¼ 1; 2 ð7:17Þ
RT
With the 2-step model, the conversion rate of the fuel can be described as

dx F
¼ k f1 x F þ k f2 x F ð7:18Þ
dt
where x F is the remaining mass fraction of the solid fuel. The 2-Step model
parameters are summarized in Table 7.3.
The 1-step model is easy to use but it could not produce a good agreement over a
broad temperature range. The 2-step model is more practical for its ease to use and
reasonable kinetic agreement over a wide range of temperature; however, it is not
based on physical mechanisms [14]. Other models for devolatilization can be found
in literature, e.g., the report by Fletcher [14].

Table 7.3 Parameters for calculation of reaction rates of 2-step devolatilization

E 1 E 2 References
k f1 ¼ A 1 exp k f2 ¼ A 2 exp
RT RT
A 1 ¼ 2:0 10 5 A 2 ¼ 1:3 10 7 [30]
E 1 = 25,000 E 2 = 40,000
x 1 ¼ 0:3 x 2 ¼ 1:0
A 1 ¼ 3:7 10 5 A 2 ¼ 1:46 10 10 [46]
E 1 = 17,600 E 2 = 60,000
x 1 ¼ 0:39 x 2 ¼ 0:8
5 10 [43]
A 1 ¼ 3:7 10 1=sð Þ A 2 ¼ 1:46 10 ð 1=sÞ
E 1 =R ¼ 8;857 K E 2 =R ¼ 30;200 K
x 1 = proximate analysis volatile matter x 2 = 0.8