Environmental Engineering Reference
In-Depth Information
sulfur sorbents are their sulfur sorption capacity and temperature at which good absorp-
tion takes place. Mixed oxides comprising a combination of the properties of various
metals and supported metal oxides are the most promising for effective and stable sul-
fur capture, but depending on the optimal absorption temperature, other metal oxide
sorbents may be used.
Among such materials, ZnO has the most favorable thermodynamic properties for
H
2
S capture in high-temperature gas cleaning systems. Vaporization, though, is an
issue (Westmoreland and Harrison, 1976), and zinc migration and agglomeration limit
the use of ZnO as sulfur sorbent to approximately 600
C. Therefore, zinc sorbents
with higher stability comprising the inclusion of other metal elements have been
developed. They include zinc copper ferrite that can ensure H
2
S reduction to lower
levels than zinc ferrite. Moreover, zinc ferrite doped with titanium shows higher
stability under certain preparation conditions.
Copper oxide is also able to ensure H
2
S concentration reduction from >> 1000
ppm
v
to sub-ppm
v
levels. An issue regarding CuO is that metallic copper is easily
formed by the reducing action of the product gas compounds H
2
and CO. This of
course decreases the desulfurization efficiency.
Manganese oxides have demonstrated an attractive combination of high sulfur cap-
ture capacity and high reactivity even in a moderate temperature range, without the
need for sorbent preconditioning or activation. The required process conditions better
match biomass gasification and tar reforming temperatures. Manganese oxide sor-
bents, though, are prone to the formation of sulfates and also need a regeneration
at very high temperature.
Also iron oxide, an abundant and relatively cheap material class, can desulfurize
product gas, but its potential is slightly lower than the aforementioned compounds,
mainly as a result of severe reduction as well as iron carbide formation at temperatures
in excess of 550
C. The sulfidation or absorption step forms iron sulfide, which can be
regenerated effectively by oxidation using air or nitrogen-diluted air at considerably
lower temperatures than other metal sulfides. Furthermore, the sulfidation product,
FeS
x
, can react with SO
2
to form Fe
3
O
4
and elemental sulfur, which is the most pref-
erable route for SO
2
capture from the regeneration product gas.
As yet another material type, cerium oxide may be utilized for sulfur capture, and it
can be regenerated well with reasonable elemental sulfur recovery. The extent of sul-
fur absorption increases when the temperature and CO/CO
2
ratio are increased.
Cerium oxide, though, shows lower sulfur removal efficiency than ZnO. This is in
particular the case at comparatively low temperatures (<700
C) in the range of hot
gas cleaning. Compared to other sulfur capture materials, cerium oxide is far more
expensive.
Additive elements, such as Ti, Al, Si, Zr, Co, Ni, and Fe, and promoters (Co, Ni,
and Fe) are included in different metal oxide-based sorbents to enhance their sulfur
capture capacity as well as their regeneration properties. Moreover, mesoporous
materials and zeolites, composed of Al
2
O
3
,TiO
2
,Fe
2
O
3
,andSiO
2
, are used as sup-
port. Zinc oxide-based sorbents (ZnFe
2
O
4
,ZnTiO
3
,Zn
2
TiO
4
,andZn
3
TiO
8
), mixed
and dispersed copper oxide-based sorbents (CuO/Al
2
O
3
,CuO/Fe
2
O
3
/Al
2
O
3
,
CuO/Fe
2
O
3
,andCuO/MnO
2
), manganese oxide-based sorbents (MnO/
γ
−
Al
2
O
3
,
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