Page 268 - Synthetic Fuels Handbook

254 CHAPTER EIGHT

A large number of options on the various aspects of bioconversion are available (Mabee
et al., 2004). The environmental performance of bioethanol, including air quality (NO ,
x
PM, SO , etc.) is also well documented as are the mass-balance and energy-balance of the
x
bioconversion process and economic analyses.
However, the most fundamental issues for the bioconversion option include improving
the effectiveness of the pretreatment stage, decreasing the cost of the enzymatic hydrolysis
stage, and improving overall process efficiencies by capitalizing on synergies between vari-
ous process stages. In fact, overall process economics needs improvement and this can be
achieved by creating added-value coproducts.
In fact, one of the major advantages of the bioconversion option is the implication that
the process can produce added-value products.
In the past, chemical products were a major part of the forest industry. A number of
chemical forest products such as pitch (partially dried resins), pine tar (liquefied resins),
turpentine (terpenes from distilled resins), rosin (the nonvolatile residues from resin distil-
lation), and tall oil (obtained from alkaline pulping liquors) were the basis of the industry.
These products were widely used in wooden shipbuilding and in the manufacture of soap,
paper, paint, and varnishes.
The resurgence of interest in the production of chemicals (including fuels) from bio-
sources is a means of searching to reduce reliance on petroleum-based products. Moreover,
the products that advanced manufacturing processes may be generated from biological
sources. There is the hope that the chemical products that can be derived from the biorefin-
ery have the potential to become a significant part of a country’s future economy.
To date, and in many countries, biochemical development has been based largely on
sugars. In fact, sugars are one of the main intermediate products of the bioconversion
option for a biorefinery. The issue, only in the context of the current text, then becomes
the potential for the conversion of sugar products into fuels. Whether or not this is feasible
technically and economically remains to be seen.

8.6.2 Thermal Conversion
The bioconversion option for a biorefinery uses biologic agents to carry out a structured decon-
struction of lignocellulose components. This process combines elements of pretreatment with
enzymatic hydrolysis to release carbohydrates and lignin from the wood (Fig. 8.10).
The thermal conversion option uses thermochemical processes to gasify wood, produc-
ing synthesis gases (sometimes called producer gases). This platform combines process
elements of pretreatment, pyrolysis, gasification, cleanup, and conditioning to generate a
mixture of hydrogen, carbon monoxide, carbon dioxide, and other gases. The products of
this platform may be viewed as intermediate products, which can then be assembled into
chemical building blocks and eventually end products.
In this process option, the only pretreatment required involves drying, grinding, and
screening the material in order to create a feedstock suitable for the reaction chamber. The
technology required for this stage is already available on a commercial basis, and is often
associated with primary or secondary wood processing, or agricultural residue collection
and distribution.
In the primary processing stage, the volatile components of biomass are subjected to
pyrolysis, or combustion in the absence of oxygen, at temperatures ranging from 450 to
600°C and, depending on the residence time in the chamber, a variety of products can be
achieved. If pyrolysis is rapid (short residence time), gaseous products, condensable liquids,
and char are produced and overall yield of bio-oil can, under ideal conditions make up 60
to 75 percent of the original fuel mass. The oil produced can be used as a biofuel or as a
feedstock for value-added chemical products. If the pyrolysis is slow (long residence time), the