Environmental Engineering Reference
In-Depth Information
palm oil, or 58-93% of life-cycle GHG emissions, respectively. Processing palm oil into biodiesel
was estimated to produce an additional 0.7 t CO
2
/t biodiesel.
Reijnders and Huijbregts (2008) assumed 75% fossil fuel use during palm production, but if the
processing facility were 98% powered by palm residue, as de Vries (2008) suggests to be more accu-
rate, life-cycle GHG emissions would be 3-21% lower than those reported above. However, other
recent findings (Science Daily 2007) suggest that CO
2
emissions from peat land conversion may
be more than 4 times higher than the values used in Reijnders and Huijbregts (2008), significantly
increasing life-cycle GHGs.
Jatropha is a tropical plant that can be converted to biodiesel. Except for promising select variet-
ies that are not currently used, most
Jatropha curcas
varieties are toxic. This makes it more difficult
to process the seeds and not possible to generate co-product animal feeds from the meal (King et al.
2009). However, several Asian and African countries, most notably China and India, have shown
interest in developing jatropha as a biodiesel feedstock (Fairless 2007).
11.3
BIomass electrIcIty
11.3.1 i
introduction
In 1978, the Bioenergy Feedstock Development Program began at Oak Ridge National Laboratory
and soon recognized the potential value of herbaceous bioenergy crops such as switchgrass.
Research in soil science, management techniques, biotechnology, and other areas led to yield
increases of 50%, projected production cost reductions of 25%, and nitrogen fertilizer reductions of
40% (McLaughlin and Kszos 2005). Different species of switchgrass were identified and optimized
regionally. Switchgrass is prized for its stress tolerance and ability to grow on marginal lands. As
will be discussed in Section 11.4, bioenergy feedstocks that are noncompetitors for cropland can
avert significant life-cycle GHG emissions.
Short rotation woody crops (SRWC) are another biomass source with large potential. In northern
temperate areas, SRWC development has focused on willow shrubs and hybrid poplar, whereas
eucalyptus has been studied for warmer climate applications. Willow has several advantageous
characteristics: high biomass production in short time periods, a broad genetic base and ease of
breeding, and the ability to re-sprout after multiple harvests (Keoleian and Volk 2005). Like switch-
grass, willow is able to reach its peak height and mass after only a few years.
The biomass electricity systems considered in this section utilize perennial feedstocks—crops
are not replanted each year, only harvested. This is beneficial from life-cycle energy and GHG
perspectives because it reduces farm operations and retains the soil carbon stored by plant root sys-
tems. Next-generation biofuel feedstocks (e.g., switchgrass and giant miscanthus) are also perennial,
but they result in an end product that is not necessarily comparable because electricity and liquid
fuel are not functionally equivalent (pending the coming electric vehicle market). However, as will
be shown below, biomass electricity systems are better at leveraging fossil fuel inputs than biofuel
systems.
11.3.2 l
ifE
-c
yclE
E
nErgy
11.3.2.1 Willow
Willow biomass can be burned to directly generate heat or produce electricity and could possibly
serve as an ethanol feedstock in the future. It is typically harvested on a 3- or 4-year cycle, with
seven to ten harvests per system before replanting. As an example, see Table 11.7 (Heller et al. 2003).
The biomass is not available for harvest until the third year and is subsequently cut and collected
every 3 years to allow the plants to reach sufficient density. Biomass density factors include genetic
variety, length of growing season, soil conditions, and climate conditions. Root systems remain in
the ground until the 23rd year, with only the upper portion of the plant harvested during this period.
