Page 274 - Synthetic Fuels Handbook

260 CHAPTER EIGHT

Forest biomass or agricultural residues are almost completely comprised of lignocel-
lulosic molecules (wood), a structural matrix that gives the tree or plant strength and
form. This type of biomass is a prime feedstock for combustion, and indeed remains a
major source of energy for the world today (FAO, 2005). The thermal conversion method
utilizes pyrolysis and gasification processes to recover heat energy as well as the gaseous
components of wood, known as synthesis which can then be refined into synthetic fuels
(Chap. 7).
Lignocellulose is a complex matrix combining cellulose, hemicellulose, and lignin,
along with a variable level of extractives. Cellulose is comprised of glucose, a six-carbon
sugar, while hemicellulose contains both five- and six-carbon sugars, including glucose,
galactose, mannose, arabinose, and xylose. The presence of cellulose and hemicellulose
therefore makes lignocellulose a potential candidate for bioconversion. The ability of the
bioconversion platform to isolate these components was initially limited, as the wood
matrix is naturally resistant to decomposition. Recent advances, however, have made this
process more commercially viable and there is added potential for value-added products
that can utilize the lignin component of the wood.
The most fundamental issues for the bioconversion platform 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. There is also a need to improve process economics by creating coprod-
ucts that can add revenue to the process.
This type of application is a logical step on the path toward greater process efficiencies
and increased energy self-generation. These types of systems could also provide surplus
bioenergy, becoming an additional revenue stream.
Greenhouse gas production associated with lignocellulosic-based feedstocks is antic-
ipated to be much lower than with conventional fuels. The environmental performance
depends very much on the specific life cycle of the fuel, including the feedstock on which
the fuel is based and the technology employed (VIEWLS, 2005).
The recent proliferation of global biofuel programs is due to several factors, not the least
of which is high oil prices. Other factors, such as concern about (a) political instability in
oil-exporting countries, (b) various countries seeking to bolster their agricultural industries,
(c) climate-altering greenhouse gas emissions, and (d) urban air pollution are of equal
importance depending upon the country under study. Continuing developments in biorefin-
ing technology have also brought greater attention to biofuels as a potentially large-scale
and environmentally sustainable fuel.
However, the potential benefits of biofuels will only be realized if environmentally
sustainable technologies are employed. Under the correct stewardship, the technologies
described above will make it possible to produce biofuels from agricultural and forestry
wastes, as well as from nonfood crops such as switchgrass that can be grown on degraded
lands (Bourne, 2007).
Another potential benefit of biofuels is the role they could play in reducing the threat
of global climate change. The transportation sector is responsible for about one-quarter
of global energy-related greenhouse gas emissions, and that share is rising. Biofuels
offer an option for reducing the demand for oil and associated transport-related warming
emissions. However, the overall climate impacts of biofuels will depend upon several
factors, the most important being changes in land use, choice of feedstock, and the vari-
ous management practices.
Nevertheless, the greatest potential for reducing greenhouse gas emissions lies in the
development of next-generation biofuel feedstocks and the associated technologies from
conversion of these feedstocks to energy (Worldwatch Institute, 2006; Bourne, 2007).