Life cycle analysis of alfalfa stem-based bioethanol production system
Date
2011-10-31
Authors
Journal Title
Journal ISSN
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Degree Level
Masters
Abstract
Alfalfa stem can be a potential feedstock for producing bioethanol. Numerous
studies have been carried out to assess the conversion of different feedstocks into
bioethanol, although studies related to life cycle assessment (LCA) of feedstocks such as
alfalfa are limited. However, LCA does serve to highlight areas where positive and
negative impacts can be expected in the overall biomass to ethanol process. This
research therefore focuses on investigating and evaluating an alfalfa bioethanol
production system in terms of five key life cycle impact categories: abiotic depletion
(AD), acidification (A), eutrophication (E), global warming (GW), human toxicity (HT),
and energy demand. The study concerns three subsystems: the cultivation subsystem
(S1), the baling and pre-processing subsystem (S2), and the ethanol conversion
subsystem (S3). Each subsystem could have different scenarios depending on specified
input combinations, and SimaPro 7.2 with CML 2 baseline 1990 V2.05 version was used
to assess environmental impacts.
The results of energy assessment correspond to LCA results, showing that the
environmental impact associated with alfalfa-ethanol production increases with increased
energy demand. Energy analysis of S1 showed that the energy requirements for
producing 1 kg of alfalfa under non-irrigated and irrigated cultivation were between
0.63 MJ to 1.30 MJ and 0.51 MJ to 0.94 MJ, respectively. The best input combination in
S1 was inorganic fertilizer with irrigation, for it consumed 0.51 MJ/kg energy and
resulted in the least environmental impact. The energy requirements for the postharvest
pre-processing of 1 kg of alfalfa biomass were 0.82 MJ to 1.62 MJ under different
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scenarios, with the drum drying requiring the highest energy in S2: 0.197 MJ of
electricity and heat per hectare.
Considering the three systems (namely S1, S2, and S3) demonstrates that irrigated
alfalfa production scenarios revealed lower energy demands in comparison to nonirrigated
scenarios; inorganic scenarios showed lower energy demand over organic
scenarios. Compared to the use of organic fertilizers, application of inorganic fertilizers
has decreased the impact with respect to AD, A, GW, and HT while slightly increasing E
in S1, S2, and S3. Therefore, the most favourable scenario was the inorganic irrigated
scenario in all subsystems. The LCA results concluded that GW was the most influential
impact category for all three subsystems, whereas AD, A, E, and HT had a comparatively
lower impact on each system.
To produce 1 L of ethanol, 32.78 MJ (minimum) to 38.43 MJ (maximum) of
energy input was required for S3 in all production scenarios at 50% water recycling. In
S1, S2, and S3, the inorganic irrigated scenarios had lower energy demands than the
organic irrigated scenarios. The highest energy consuming process in S3 was ethanol
plant heat energy. Overall, the inorganic, irrigated, and 50% water recycling represented
the best case scenario for all subsystems (S1, S2, and S3) in terms of energy demand with
an average of 7.82 MJ for 1 kg of alfalfa biomass input. Comparing the three subsystems
shows that the alfalfa cultivation subsystem (S1) consumed 6.2% to 15.1% of the total
energy. The ethanol conversion subsystem (S3) is the highest energy consuming
subsystem in this study, falling into the 77.5% - 94.8% range of the total for different
scenarios. The baling and pre-processing subsystem (S2) required between 3.5% and
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4.0% of the total energy. Future studies could assess different allocation methods and coproduct
credits for the establishment of a sustainable cellulosic biorefinery.
Description
Keywords
Life Cycle Analysis (LCA), Alfalfa, Bioethanol, Abiotic depletion, Acidification, Eutrophication, Global warming, Human toxicity, SimaPro 7.2, sustainable cellulosic biorefinery
Citation
Degree
Master of Science (M.Sc.)
Department
Agricultural and Bioresource Engineering
Program
Agricultural and Bioresource Engineering