Page 233 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

7.6 Formation of SO 2 and SO 3 209

Sulfur mass conservation gives SO 2 Š þ SO 3 Š ¼ S½Š ¼ constant, or SO 2 Š ¼
½
½
½
ð S ½Š SO 3 ŠÞ and Eq. (7.40) becomes
½
¼ k f1 S½Š SO 3 ŠÞ O½Š M½Š k f2 SO 3 Š O½Š M½Š k f3 SO 3 Š H½Š ð7:41Þ
ð
½
½
½
r SO 3
¼ 0, the
When the SO 3 concentration reaches steady state, corresponding to r SO 3
equilibrium SO 3 concentration is described in terms of [O] and [H] as
k f1 O½Š M½Š S½Š
½ SO 3 Š ¼ ð7:42Þ
s
ð k f1 þ k f2 Þ O½Š M½Š þ k f3 H½Š
This equation can be used to estimate the steady-state SO 3 concentration as a
fraction of the total SO x concentration S½Š ¼ SO 2 Š þ SO 3 Š. Mostly [O] and [H] are
½
½
assumed constant.
The formation of sulphuric pollutants during solid fuel combustion can also be
described by the following step reactions [51].
Fuel sulfur is first heated and devolatilized through,
Fuel SsðÞ ! H 2 S þ COS þ þ Char SsðÞ ð7:43Þ
Both solid and gases (vapors) are produced through this reaction. The solid
phase char sulfur can be oxidized through the following reactions, where H 2 S, SO 2
and COS are produced.

Char S þ O 2 ! SO 2 ð7:44Þ
Char S þ CO 2 ! COS ð7:45Þ

Char S þ H 2 O ! H 2 S: ð7:46Þ

The gas phases produced during the above three reactions can further react
through

1
H 2 S þ 1 = 2O 2 ! SO 2 þ H 2 O ð7:47Þ
H 2 S þ CO 2 ! COS þ H 2 O ð7:48Þ
H 2 S þ CO ! H 2 þ COS ð7:49Þ

H 2 S þ COS ! CS 2 þ H 2 O ð7:50Þ

CS 2 ! C þ 2=x S x : ð7:51Þ

In the products, the concentrations of SO 2 and SO 3 at equilibrium can still be
determined by the overall reaction in Eq. (7.36)