Environmental Engineering Reference
In-Depth Information
least 3:1, otherwise it requires too much work to produce energy (Hall et al., 2009). As a
comparison, gasoline has an EROI of 11-18:1 and corn-based ethanol varies between
0.78 and 1.29:1 (Randolph and Masters, 2008).
2. Land use. Plants are inefficient in terms of capturing energy from the sun. On average
only 0.7 percent of sunlight energy is transformed into carbohydrates (Gebhardt, 1986).
As a result, large extensions of land will be needed for the production of feedstocks,
which may come from crop substitution, deforestation of tropical forest, or conversion of
grasslands.
3. Need of fertilizers and water. With exception of soybeans, which needs little nitrogen
fertilization because of its symbiotic association with nitrogen-fixing bacteria, all crops
need fertilization with nitrogen and phosphorus. Current nitrogen production via the
Haber-Bosch process is highly energy intensive and short lived once applied to the ground.
Phosphorus is produced from concentrated deposits in the earth and it is a limited resource
that eventually will be depleted with devastating consequences for our agricultural system
(see Chapter 3). Water availability is becoming an issue for food production (see
Chapter 9). The addition of dedicated crops for fuels and chemicals is going to make
water stress even more significant, by accelerating the depletion of aquifers and reducing
surface water availability. For instance, production of ethanol from potatoes and sugar
beets needs between 60 and 100 m
3
of water per gigajoule (GJ) of energy, from sugar cane
110 m
3
/GJ and from sorghum 400 m
3
/GJ. Biodiesel from soybean and rapeseed requires
400 m
3
/GJ, whereas jatropha needs 600 m
3
/GJ (Gerbens-Leenesa et al., 2009).
4. Impact of monocultures. Large scale monocultures reduce biodiversity and eliminate
natural predators that control pests. Therefore, alternative pest management systems need
to be put in place. Also, many of the proposed crops for fuel and chemicals are exotic
species for most regions with the associated risk of becoming invasive species.
5. Threat to food security. In terms of resources, feedstocks for a biobased economy will
compete directly with food production. A result of this competition will be a change in the
availability and affordability of food with a high impact on those regions of the world that
are food importers.
6. Lack of mature technology. Many of the technologies mentioned in this chapter about
production of fuels and chemicals from biomass are at the conceptual stage. Even when
many of those concepts are theoretically feasible, such as the production of cellulosic
ethanol or chemicals from syngas, technologies are still not completely developed. A key
issue here is the lack of a cost-effective process that also has an EROI significantly higher
than the break-even point of 1.
7.
High demand of energy for separation. Water-soluble compounds produced by fermenta-
tion require energy for their separation from water. For instance, the final concentration
of ethanol in a fermentation broth is around 8 percent and the other 92 percent is essen-
tially water. Ethanol is separated from the water by distillation, which is an energy-
intensive operation. In the future, this could change if new separation methods that do not
require phase change (e.g., membranes) become available.
SUMMARY
Long term sustainability is compromised by a decline in fossil feedstocks that is supported by
persistent predictions of peak oil in a foreseeable future. The planned alternative to fossil-
based feedstocks for the production of liquid fuels, polymers, and chemicals is the use of plant
materials, which had been used for centuries before the advent of petroleum and gas to
