Page 232 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

208 7 Combustion Process and Air Emission Formation

Table 7.4 Typical values for
Fuel Sulphur content (wt%, dry)
sulphur content of fuels (wt%,
Fossil fuels Coal 0.2–5
dry)
Oil 1–4
Natural gas 0–10
Light fuel oil <0.5
Heavy fuel oil <5
Peat <1
Petroleum coke *5
Biomass Wood <0.1
Straw *0.2
Bark <2

The more sulfur in the fuel, the higher level of SO 2 emission. SO 2 is mainly
formed when the sulfur elements are oxidized by O 2 ,

S þ O 2 ! SO 2 ð7:35Þ

and SO 2 can be oxidized to SO 3 .
Their concentrations at equilibrium can be determined by the overall reaction,
using the knowledge we learned in Chap. 3.

1
SO 2 þ = 2O 2 $ SO 3 ð7:36Þ
¼ 1:53 10 5 exp 11;760 atm 1
K P;SO 3 2
T
The van’t Hoff equation (7.36) of the chemical equilibrium constant indicates that
the concentration of SO 3 increases with the decreasing combustion temperature. The
kinetics of SO 2 oxidation without catalytic effect can be described as follows [51].

6
4

SO 2 þ O þ M ! SO 3 þ M k f1 ¼ 8:0 10 exp 1;400=TÞ m mol s ð7:37Þ
ð
4
6

SO 3 þ O þ M ! SO 2 þ O 2 þ M k f2 ¼ 7:04 10 exp 785=TÞ m mol s
ð
ð7:38Þ

7
3
SO 3 þ H ! SO 2 þ OH k f 3 ¼ 1:5 10 m mol s ð7:39Þ
Assuming constant-temperature and fuel-lean combustion, the net rate of SO 3
formation is thus described by Eq. (7.40)
dSO 3 Š
½
½
¼ ¼ k f1 SO 2 Š O½Š M½Š k f2 SO 3 Š O½Š M½Š k f3 SO 3 Š H½Š ð7:40Þ
½
½
r SO 3
dt