Assessment of Conversion Technologies for Bioalcohol Fuel ...

successfully demonstrated their technologies by building and systematically testing full
scale operating systems.
Syngas Cleanup and Conditioning
Without sufficient cleanup and conditioning, syngas produced from biomass may not
be useful for alcohol synthesis. Synthesis catalysts work optimally with a certain ratio
of H 2 to CO, and the effectiveness is reduced when a large concentration of inert
compounds (like N 2 ) are introduced to the catalyst system. Catalysts used for
synthesis can also be extremely sensitive to gaseous contaminants like sulfur,
chlorine, metal poisons and particulate contaminants such as tars. These compounds
occupy active sites of the catalyst, reducing catalyst activity and catalyst life. Syngas
cleanup and conditioning strategies must address the major and minor constituents in
the syngas to meet the requirements of the catalyst being utilized.
The requirements for syngas purity have not been well established for ethanol and
mixed alcohol catalysts. However, years of industrial experience with methanol
production catalysts has established some basic guidelines for syngas quality to
maintain a catalyst life of several years (Table 3) (Spath and Daton, 2003). Note the
very low levels required for constituents that reduce catalyst life that will typically
require specialized syngas cleanup. Particulate matter and tars also have to be
controlled to very low levels.
Table 3–Syngas Quality and Conditioning Requirements for
Catalytic Conversion to Methanol
Stoichiometric Ratio
(H2–CO2) / (CO + CO2)
~2
CO 2 4-8%
Sulfur
Halides
Fe and Ni
< 0.1 ppmv
< 0.005 ppmv
< 0.001 ppmv
Syngas cleanup can include various scrubbers, precipitators and adsorbents to
remove undesired compounds. Many of these approaches have been used
commercially in natural gas systems, coal gasification systems and other industrial gas
applications. While gas cleanup and conditioning present complexities and cost
challenges for system developers, many existing technologies can be applied to
biomass-derived syngas.
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