Page 270 - Synthetic Fuels Handbook

256 CHAPTER EIGHT

synthesis gas, so-called because of the presence of carbon monoxide and hydrogen in the
product stream.
After the production of syngas, a number of pathways may be followed to create bio-
fuels. Proven catalytic processes for syngas conversion to fuels and chemicals exist, using
syngas produced commercially from natural gas and coal. These proven technologies can
be applied to biomass-derived syngas.
Methanol is one potential biofuel that can be generated through catalysis. The majority
of methanol produced today is being derived from natural gas, however. Methanol has a
high octane number (129) but relatively low energy (about 14.6 MJ/L) compared to gaso-
line (91–98 octane, 35 MJ/L). Because methanol has a favorable hydrogen/carbon ratio
(4:1), it is often touted as a potential hydrogen source for future transportation systems.
Another potential biofuel that can be produced through the thermochemical platform
is Fischer-Tropsch diesel (or biodiesel). This fuel was first discovered in 1923 and is com-
mercially based on syngas made from coal, although the process could be applied to biomass-
derived syngas. The process of converting carbon monoxide (CO) and hydrogen (H )
2
mixtures to liquid hydrocarbons over a transition metal catalyst has become well estab-
lished although process efficiency leaves much to be desired.
It is also possible to convert synthesis gas to higher-molecular-weigh products, includ-
ing ethanol. Ethanol and other higher alcohols form as by-products of both Fischer-Tropsch
and methanol synthesis, and modified catalysts have been shown to provide better yields.
The thermal conversion option provides the opportunity for a number of additional coprod-
ucts, as well as energy in the form of heat or electricity and biofuels. Each component (i.e.,
carbon monoxide, carbon dioxide, methane, and hydrogen) of the gaseous products may be
recovered, separated, and utilized.
Pyrolysis/gasification systems have been reported to be much more efficient for energy
recovery, in terms of electricity generation, than traditional combustion. It has been estimated
that typical biomass steam-generation plants have efficiencies in the low 20 percent range,
compared to gasification systems with efficiencies that reach 60 percent (DOE, 2006). High
efficiencies have been noted for both co-firing systems (where biomass is gasified together
with a fossil fuel such as coal or natural gas) and in dedicated biomass gasification processes
(Gielen et al., 2001). Because the potential for energy recovery is so much higher, gasification
systems without any downstream catalysis may be able to increase bioenergy production with
minimal impact on existing product streams in sawmilling or pulping operations.
Gasification technologies for the production of fuels from biomass are available but are
often bypassed in favor of fossil fuels although this may change with rising fuel costs and
uncertainty about the security of fossil reserves (Faaij, 2006).
Another issue is the quality of bio-based synthesis gas which are often more heteroge-
neous than natural gas-based synthesis gas. While technical approaches are well documented
for the production of hydrogen, methanol, and Fischer-Tropsch liquids from synthesis gas,
the input gases must be relatively clean in order for these processes to function in a com-
mercially viable sense. Therefore, before catalysis, raw synthesis gas must be cleaned up
in order to remove inhibitory substances that would deactivate the catalyst. These include
sulfur, nitrogen, and chlorine compounds, as well as any remaining volatile tar.
The ratio of hydrogen to carbon monoxide may need to be adjusted and the carbon diox-
ide by-product may also need to be removed. One major problem with methanol synthesis
is that biomass-based syngas tends to be hydrogen-poor compared to natural gas syngas.
Methanol synthesis requires a ratio of 2:1 hydrogen/carbon monoxide to be cost-effective.
Common problems associated particularly with Fisher-Tropsch synthesis (Chap. 7) are
low product selectivity (the unavoidable production of perhaps unwanted coproducts,
including olefins, paraffins, and oxygenated products), and the sensitivity of the catalyst to
contamination in the syngas that inhibit the catalytic reaction. With biomass-based syngas,
this problem is amplified due to the heterogeneous nature of the syngas.