Partnerships, Future, and Emerging Technologies - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 408

but tars are especially problematic since they have the additional detrimental effect
of fouling downstream fuel synthesis catalysts and system.
Proven technology approaches for removing the light hydrocarbons and tars
from the syngas is a two-step process: water quenching to remove the tars and other
particulates from the syngas stream followed by steam methane reforming to
change the methane and other light hydrocarbons to additional syngas. Although
water quenching is effective for removing tars and heteroatoms from the syngas, it
represents a carbon and efficiency loss since the carbon and energy contained in the
tars would be lost to the system. However, even more problematic is the large toxic
wastewater stream that water quenching would create. Steam methane reforming
does not represent a carbon or energy loss or create a problematic waste stream, but
it does add additional cost and complexity to the process.
A preferred approach to this two-step process would be to perform integrated tar
and light hydrocarbon reforming in a catalytic single-step process. This would have
the benefits of increasing the carbon and energy efficiency of the process, reducing
process steps and hence the cost and complexity of the conversion process, and
finally eliminating a large volume toxic wastewater stream. Although several
researchers have published results showing that a number of catalysts look prom-
ising for tar and light hydrocarbon reforming [ 38 ], the challenge is to maintain the
activity in the presence of sulfur. This area has been the focus of considerable effort
over that past several years, and a number of researchers and organization are
reporting some encouraging results [ 39 ].
The next major step in the process after the syngas has been cleaned and
conditioned is to perform the catalytic fuel synthesis. Several organizations have
developed mixed alcohol synthesis catalysts over the past decade or so [ 40 , 41 ]. In
order to be commercially viable, mixed alcohol synthesis must have good selectiv-
ity to the desired product, in this case ethanol, as well as good CO conversion.
Several researchers have reported significant improvements in mixed alcohol
synthesis catalyst performance [ 42 ]. Improvements in catalyst performance that
increase single-pass conversion and ethanol productivity are particularly beneficial
since these improvements have the added benefit of simplifying the process by
requiring fewer recycle loops, hence improving both costs and efficiencies.
Feedstock Considerations
Biomass composition affects thermochemical processing differently than biochem-
ical processing. Unlike biochemical conversion, which only converts the carbohy-
drate portion, gasification converts the entire organic component of biomass, both
carbohydrate and lignin fractions, into syngas, light hydrocarbons and some tars.
Trace inorganics predominantly sulfur, salts, and alkaline earth metals can be
problematic in a variety of ways. Potassium can be problematic in the gasifier for
fluidized bed gasifiers. The potassium interacts with the silica in the system to form
K 2 SiO 4 which has a low melting point of ~ 500 C, and its formation will lead to the