Environmental Engineering Reference
In-Depth Information
Mn
V), iron oxide-based sorbents (Fe
2
O
3
/SiO
2
), and cerium
oxide-based sorbents (Ce/Mn) have been extensively studied. Some intrinsic pro-
blems using sulfur sorbents still need to be solved, even though many research
works have been carried out to find suitable sorbents for different desulfurization
processes.
Regarding the progress toward a low-cost, environmentally friendly, highly effi-
cient, rapidly reacting, and regenerable sorbent with high sulfur capture capacity
and durability, still, additional development work must be done, applying the material
under real conditions. Optimization of supported or mixed metal sorbents from zinc,
copper, and manganese is one of the options. Binary oxides and
-
Cu, and Mn
-
Cu
-
binary
oxides may have better attrition resistance and higher sulfidation equilibrium con-
stants. Experimental and modeling studies focusing on the molecular structure of dif-
ferent sulfur capture materials could be useful for the design and development of new
sulfur sorbents. Last but not least, mechanistic information for rational design and
modification of such materials can be conventionally achieved with the aid of a variety
of microscopic and spectroscopic characterization methods.
“
promoted
”
10.5.5 Chlorine Compound Removal
The chlorine content of biomass can vary significantly (Leibold et al., 2008)
(see also Chapter 2). Chlorine can act as an alkali vaporization shuttle, in such a
way that alkali-induced issues are occurring in the gasifier as well as in downstream
equipment. Moreover, ash softening can take place in high-temperature gas cleaning
processes due to the formation of low-melting compounds including in particular
chlorides.
Chloride poisoning (HCl) may harm relevant (Cu- and Zn-containing) catalysts in
the BTL chain via several parallel mechanisms (Twigg and Spencer, 2001):
i. Adsorbed chlorine atoms, formed by reaction, can block or modify cata-
lytic sites.
ii. The low melting point and high surface mobility of Cu(I) chloride mean that
even extremely small amounts of copper halide are sufficient to provide mobile
species that accelerate the sintering of Cu catalysts.
iii. Poisoning of Cu catalysts by reduced sulfur compounds (e.g., H
2
S) is wor-
sened by traces of mobile Cu(I) chloride.
iv. ZnO, often present in Cu catalysts, reacts to form Zn halides, which also have
low melting points, and causes further poisoning and sintering problems.
Therefore, chlorine-containing species need to be removed by sorbents. Natural
rock materials, such as dolomite and limestone, already substantially reduce the HCl
content. This component is absorbed and CaCl
2
is formed, which might contribute to
stickiness of the ashes as it has a relatively low melting point.
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