Environmental Engineering Reference
In-Depth Information
• In developing economies, development projects should include provisions or outcomes that
improve socioeconomic conditions for small-scale farmers who rely on agriculture to feed
themselves and their families and that do not require the involuntary displacement of local
populations.
• High conservation value areas and native ecosystems should not be cleared and converted
for fuel plant source development.
Current and future producers are targeting sustainable production scenarios that, in addition
to minimizing impact on LUC and food and water resources, provide an energy alternative that
is economically competitive with current petroleum-based fuels. Market growth will require a
coordinated effort between feedstock producers, refiners, and industry regulators to ensure that
environmental impacts are minimized.
If done responsibly, increasing HRD and HRJ usage in the transportation sector can significantly
reduce GHG emissions as well as diversify energy sources, enhance energy security, and stimulate
the rural agricultural economy.
8.3 thermochemIcal conversIon oF Plant Woody BIomass
Archeological findings show the first evidence of thermochemical processing of woody biomass
by humans to have occurred approximately 1.9 million years ago, the first controlled use of fire by
humans approximately 400,000 years ago, and charcoal production and controlled burns some tens
of thousands of years ago (Bowman 2009). Although the above examples are thermochemical reac-
tions, this review will focus on more advanced thermochemical processing routes to upgrade woody
biomass into liquid transportation and gaseous energy products.
The second major platform covered in this review is for conversion of woody biomass into
biofuels and high-value chemicals through the use of thermochemical processing steps conducted
at high temperature and pressure, often in the presence of chemical catalysts. Gasification and
pyrolysis of wood are the two most important thermochemical processing routes. In both cases,
wood is thermally decomposed into intermediate compounds of small molecular weight relative
to the starting polymeric carbohydrate and lignin wood fractions. The predominant products of
gasification constitute a synthesis gas, whereas in pyrolysis, depending on reaction conditions, the
major products could be a crude bio-oil, a synthesis gas, or a solid carbonaceous char. The following
sections will introduce these major thermochemical processing platforms, present the main features
of reaction chemistries, and estimate energy efficiencies from conversion.
8.3.1 g
aSification
-B
aSEd
c
onvErSion
of
p
lant
B
iomaSS
Gasification of woody biomass provides a means for production of renewable energy products,
notably synthesis gas for liquid fuels production, fuel gas for heat or power applications, and hydrogen
for fuel cells. Figure 8.4 shows a process flow diagram for gasification of woody feedstocks for
production of these three main energy products. Wood chips are transferred to a storage operation,
and upon entering the process pass through a size reduction step before gasification.
Gasification is a partial oxidation/thermal decomposition reaction carried out at high temperature
(600-900°C) in the presence of a gasification medium (air, oxygen, steam) to yield a synthesis
gas containing major products carbon monoxide (CO), H
2
, CO
2
, and H
2
O but with significant
amounts of minor products, mainly ash, ammonia, hydrogen sulfide (H
2
S), tars, and particulate
char (Torres et al. 2007) as well as trace contaminants such as hydrogen cyanide (HCN), halogens
[e.g., hydrogen chloride HCl], alkali metals, and other metals (Pb, As, Hg) (NSF 2008). These
minor products, or imp¡urities, must be removed (gas cleanup) before using the synthesis gas for
electricity generation, heat production, or catalytic conversion to the high-value fuels and chemicals,
as shown in Figure 8.4. For example, (1) tars can coat surfaces within downstream processes and
