Page 254 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

230 8 Pre-combustion Air Emission Control

concentration the H 2 S gas stream. The concentrated H 2 S streams usually contain
>50–60 %ofH 2 S. H 2 S smells like rotten eggs; it is corrosive and toxic. H 2 S loaded
gas streams must be further treated [5].

8.1.2.1 Claus Process

One of the common practices is to convert H 2 S to nontoxic and elemental sulfur by
the Claus process, which was first developed by London chemist Carl Friedrich
Claus in 1883. A Claus process converts H 2 S into elemental sulfur by two steps,
thermal step and catalytic step [1, 8, 33, 43]. In the thermal step 1/3 of the H 2 Sis
oxidized into SO 2 and water by the oxygen in the air.

H 2 S þ 1:5O 2 ! SO 2 þ H 2 O ð8:1Þ

The resultant gas steam contains H 2 S and SO 2 in a 2:1 mol ratio. This step
requires a high-reaction temperature of 1,000–1,400 °C.
In the following catalytic step, SO 2 reacts with the remaining H 2 S in presence of
a catalyst to form elemental sulfur. With the catalysts, the reactions in the catalytic
step proceed at a much lower temperature of 200–350 °C.
3
2H 2 S þ SO 2 $ S n sðÞ þ 2H 2 O ð DH ¼ 108 kJ=moleÞ ð8:2Þ
n
where typical value of n is 8. Following Eq. (3.27), the equilibrium constant of Eq.
(8.2) can be described as

y 2 y 3=8
H 2 O S 8 5
K P ¼ 2 P 8 ð8:3Þ
y y
H 2 S SO 2
The reaction described in Eq. (8.2) is exothermic and equilibrium-limited
reaction that calls for low temperatures. However, low temperatures will result in
low elemental sulfur yield rate. That is why a catalyst is necessary. Typical catalysts
for Claus processes are activated alumina, activated bauxite, or cobalt molybdenum
hydrogenation catalyst [33, 43]. In addition, a multistage process with interstage
timely removal of elemental sulfur by cooling and sulfur condensation further
improves the conversion rate.
By two- or three-stage processes the H 2 S conversion efficiencies could reach
about 95 or 97 %, respectively.
Alternatively, with the super-Claus process and special catalysts, efficiencies of
>99 % can be achieved because it prevents the formation of SO 2. However, a
separate hydrogenation reactor has to be employed between the second and third
stage. Oxygen enrichment of the air to the burner in the final Claus stage also
reduces soot formation and poisoning of the catalysts.