Page 228 - Advances in bioenergy (2016)

2−
0 75,76
2−
involving oxidation of S to S and those based on the capture of S through
74
precipitation of its metallic salts owing to their very low Ksp. Semi-batch processes based
on selective adsorption of H S on solid adsorbents have been also developed. Materials based
2
on iron oxides and activated carbons, alkali doped or undoped have been explored for this
77
purpose. Biological methods are also employed based on biofilters, bioscrubbers, and
74
biotrickling filters. Thiopaq is one of the most known technologies for large-scale industrial
biogas desulfurization using microorganisms to oxidize the sulfide to elemental sulfur. 78

Another problem that concerns only biogas from landfill facilities or waste composting is the
74
presence of siloxanes in the landfill gas. Siloxanes, silicones containing SiO bonds with
organic groups, are widely employed in industry for the production of shampoos, detergents,
cosmetics, pharmaceuticals, and so on. The decomposition of these products generates
siloxanes that are found in landfill gas because of their high vapor pressure and low water
79
solubility. Upon landfill gas combustion siloxanes are converted to silicon dioxide
(crystalline silica), that adheres and is deposited inside the boiler. Similar problems have been
reported for other engines or catalytic processes. 79,80 There are various methods for removing

siloxanes through adsorption, absorption, cooling (cryogenic condensation), biological, and
catalytic methods, some of them being commercialized. 74,79 A detailed description of biogas
cleaning methods is beyond the scope of this review. The interested reader is referred to the
relevant literature.

Among reforming processes, CO reforming or dry reforming of methane (DRM) [Eq. (7),
2
Table 9.1] is more suitable for biogas, as both major constituents are incorporated in the final
product. However, the process faces difficulties due to the fact that it is energy demanding, as
it is thermodynamically favored at high temperatures (above 973 K), whereas catalysts may

suffer from sintering and from the formation of carbonaceous deposits. The latter constitutes
the most important challenge of the process.

Reaction Mechanism

The first step in the DRM reaction sequence is the adsorption of methane. At low temperatures,
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the adsorption is precursor mediated, whereas at higher temperatures, it is direct. It has been
suggested that CH adsorbs reversibly on the surface of transition metals, arriving at
4
equilibrium. This conclusion is derived from results of steady-state isotopic transient kinetic
analysis, which detected methane on the surface of various Ni- and Pd-based catalysts under
reaction conditions. 82,83 There is general agreement that one of the slow reaction steps in the
reaction sequence is the cracking of methane on the metal surface, as the dissociation energy of
the CH H(g) bond is high (439.3 kJ/mol). However, the total dissociation energy of the bond
3
CH H depends on the hosting surface and the entire catalytic system which may be
x
controlling the surface metal work function. Consequently, lower CH H bond dissociation
x
energies are required in catalyzed decomposition. Nevertheless, for many catalytic systems,

methane decomposition is considered as the rate-determining step (RDS). 81,84,85