Environmental Engineering Reference
In-Depth Information
89 MJ as ethanol
Corn production
81 MJ
Corn transport
Energy Input
Ethanol conversion
15 MJ as coproducts
“energy credits”
Ethanol distribution
Energy captured as ethanol:
89
“Energy credits” captured:
15
81
×
100%
=
109%
81
×
100%
=
18%
Fig. 1.11 Energy balance for producing 1 gallon of ethanol [52]
combusted to produce electricity with an efficiency of
35% VE, its ECE is
30%. However, its low PEE (9%) drives results in an OE of only
3%.
Although ethanol production (at least from corn) appears to have a minimal
energy gain, the impacts of intense agriculture on the environment are of
concern. Modern agricultural practice is associated with negative environmen-
tal impacts, such as soil erosion, loss of biodiversity, use of pesticides, and
release of non-point pollution, such as nitrogen. The release of nitrogen is a
particular concern, because agricultural sources currently account for 58% of
the nitrogen load to the Mississippi basin [53, 54], and approximately 31% of
the agricultural nitrogen comes from fertilizers [55], which is a main input that
goes into producing corn [56]. An expert committee appointed by the National
Researcher Council (NRC) indicated recently that significant harm to water
quality and availability can result from intense farming of corn for ethanol if
new practices and techniques are not adopted [57]. Environmental concerns are
not a concern exclusive to bioethanol, but for any biomass-based technologies
that may result in an increased agricultural activity in the region. In that respect,
anaerobic digestion and MFCs that typically target wastes, not an agricultural
resource like corn, as fuel sources have an inherent advantage.
1.4 Conclusion
Biomass-based renewable energy is a carbon neutral technology that can con-
tribute to the world's energy supply. Oxygenic photosynthesis produces bio-
mass with a high energy value: the goal of biomass-based technology is to
cleanly and efficiently harness this energy value by driving the photosynthetic
reaction in reverse. Since raw biomass often is not convenient for direct use as a
fuel, microbiological processes are currently being used to convert biomass to
convenient biofuels. Two major microbiological processes today for biofuel
generation are methanogenic anaerobic digestion and ethanol production, each
