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POTENTIAL CONTRIBUTIONAND
RESEARCH CHALLENGES FORUTILISING
B IOFUELS IN GASOLINE ENGINES
TO 2030
Adam A Marsh
0404304
15th February 2009
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POTENTIAL CONTRIBUTIONAND
RESEARCH CHALLENGES FORUTILISING
B IOFUELS IN GASOLINE ENGINES
TO 2030
Adam A Marsh
0404304
15th February 2009
MSC SUSTAINABLE ENERGYAND ENVIRONMENT
CARDIFF SCHOOL OF ENGINEERING
CARDIFF UNIVERSITY
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ABSTRACT
Development of biofuels has seen an unprecedented increase in the last
decade, however, it is warned that too-rapid development will lead to
catastrophic mistakes being made, namely to local environments, habitat
destruction and concerns surrounding the fuel for food debate. This report
looks at some of the future potential contributions and research challenges
for the use of biofuels in gasoline, spark ignition engines.
It was found that gasoline fuel replacements to date are limited to alcohols,
and future research is concerned with the production of higher alcohols such
as butanol. The fermentation process is under constant development with the
genetic engineering of bacteria and fungi to increase yields, ferment more
complicated sugars and produce higher alcohols than ethanol.
Future research must aim to peruse the creation of a biofuel replicating
gasoline to ensure its success by enabling seamless integration with existing
infrastructure and spark ignition engines.
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CONTENTS PAGE
1.INTRODUCTION 2
2.FUELS 4
2.1GASOLINE 4
2.2ETHANOL 4
2.3BUTANOL 7
2.4BIOGASOLINE 8
3.M ICROORGANISMS 9
4.PROCESSES 12
4.1FERMENTATION 12
4.2ACETONE-BUTANOL-ETHANOL FERMENTATION 13
4.3SACCHARIFICATION 14
4.4EXPLOSIONS 14
4.5HYDROLYSIS 15
4.6FISCHER-TROPSCH SYNTHESIS 16
5.CONCLUSION 18
APPENDX I
APPENDIX II
REFERENCES
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1.INTRODUCTION
Transport is a key asset to the way we live in the modern world. With globalised
economies, international business and the desire to travel, the need for fuels to
provide the energy needed is ever increasing. With rising prices and depletingreserves of fossil fuels, national desire for security of fuel supply and
sustainability, coupled with the increased awareness of greenhouse gas effects
from fossil fuel combustion, the transport sector is in a critical state to meet
requirements set by governmental legislations and demands of their peers. Road
vehicles account for 93% of the UKs transport greenhouse gasses and are
heavily dependant on fossil fuels.1 Transport can reduce its consumption of fossil
fuels by utilising more energy efficient vehicles, improving public transportsystems and changing the fuels that are used. It would appear that the British
government are concentrating on the latter. In 2005, the Renewable Transport
Fuel Obligation (RTFO) was passed, requiring 5% of total fuel sales to be from
renewable sources by 2010/11, which was expected to be almost entirely from
biofuels.1 Alongside legislation, the government have included tax reductions for
biofuels to aid their market position.
Projections of fossil fuel use into 2030 indicate a gentle turning point around the
year 2020.2 The demand for fuels for transport is expected to continue to rise,
with some of the slack taken up with biofuels, figure 1.1. Bioethanol and
biodiesel are the primary transport biofuels of late and have received rapid
development of industrial scale production and quantity as the substitution of
biofuels for petroleum based fuels has become an important factor in energy
strategies worldwide. In 2007, 12.5 billion gallons of bioethanol was produced,62% of which was produced in the US and Brazil.3 The current interest in
biofuels stems from governments needing to secure energy supplies whilst the
cost of traditional fossil fuels is rising, making alternative fuels closer to being
economically viable, together with recent concerns about greenhouse gas
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emissions and global warming. Biofuels have the potential to be carbon neutral
as it is assumed that each mole of carbon dioxide released during combustion
has been sequestered from the atmosphere during growth of the biomass,
resulting in no net increase in levels of atmospheric carbon dioxide.
Figure 1.1 - Exx onMobil Projections of World petroleum Supply to 203 0 (D.L. Greene,
2007)
Development of biofuels has seen an unprecedented increase in the last decade,
however, it is warned that too-rapid development will lead to catastrophic
mistakes being made, namely to local environments, habitat destruction and
concerns surrounding the fuel for food debate.4 Most of these concerns spread
from the over eager use of first generation biofuels derived from food crops,
namely ethanol. Time to reflect on current technologies and their impacts will
give chance for the more advanced second and third generation biofuels to
reveal their potential and break into the biofuels market. This report looks at
some of the future potential contributions and research challenges for the use of
biofuels in gasoline, spark ignition engines.
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2 FUELS
2.1 Gasoline
Gasoline is a mixture of four major hydrocarbons with 5-12 carbon atoms each;
olefins, aromatics, parafins and napthenes with some contaminants such as
sulphur, nitrogen and oxygen.5 The properties of gasoline such as octane
number, density, reactivity and composition, vary with the source of the crude oil
which results in blending of different gasoline to meet specifications. Key
parameters are volatility and octane number. Gasoline spark ignition engines are
designed to be most efficient at specific volatilities and burning fuels outside the
specified range will result in poor performance, especially during cold start,
engine warming and acceleration. The octane rating is an indicator of a fuels
resistance to detonation under compression. A higher octane rating fuel can
withstand higher compression ratios without detonating, a positive feature of a
spark ignition fuel. Typical gasoline ratings are 91-99.
Is important in this current youthful state of biofuel utilisation in internal
combustion engines that biofuels match gasoline properties to ensure their
uptake in the market and compatibility with existing combustion techniques,
distribution and storage methods. The European Normal for gasoline can be
found in appendix I.
2.2 Ethano l
Ethanol is the same alcohol found in drinks, has the chemical formula C2H5OH
and is the most produced biofuel worldwide.6 Bioethanol can be produced from
sugars or starch by fermentation and has the potential to be used in unmodified
gasoline engines with ethanol blends up to 10% (E10) but specific engines can
run higher blends up to E85.7,8
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Ethanol is perfectly miscible and hydroscopic with water and creates an
azeotrope due to hydrogen bridges formed with the OH branch. This can lead to
the absorption of water vapour into fuel blends from atmospheric humidity
creating storage issues. Petrol-ethanol blends have a higher vaporisation
pressure and low temperatures can cause water and fuel separation. In these
conditions the ethanol will be removed from the gasoline as it will remain in
solution with the water, essentially loosing the ethanol to the atmosphere and
changing the fuel characteristics.9
The use of ethanol fuels in internal combustion engines reduces overall
emissions of particulate matter and CO, however ethanol and acetaldehyde,
which can form dangerous secondary particulate matter, are both released and
resistant to existing catalytic converters. It is rather undetermined if alcohols
increase or decrease NOx emissions, with reports on both sides.10, It is suggested
that NOx formation is dependant on air-fuel ratios. Ethanol poses many threats to
surface soils and ground water, figure 2.2. The corrosive nature of ethanol and
its miscibility with water enable rapid soil and ground water infiltration, risking
damage to underground steel storage and pipe work and requiring new
distribution infrastructure. Clays are dehydrated by ethanol posing building
foundation threats.
Bioethanol has a higher octane rating, flame speed, heat of vaporization and
wider flammability range than gasoline.11 Ethanol does however have a lower
energy density than gasoline, containing about 70% of that in gasoline, due to
constituent water. This lower energy density results in a higher fuel demand for
the same work rate, increasing total CO2 emissions from exhaust gasses. Taking
into account the carbon life cycle of bioethanol results in a marginal CO 2 saving
of 1-5% against gasoline, making it very expensive per tonne of CO2 saved
(103/tonne).10 The net energy value (NEV) of ethanol is heavily debated and
there are voices arguing that ethanol has a NEV deficit whilst others argue
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otherwise.12 Positive ratios appear marginal however, with ratios such as 1.24,13
appendix II. Energies into the production of ethanol include the use of farm
machinery, the cultivation of yeasts, distillation and transport between different
stages. The NEV needs to be optimized for ethanol to be economically and
environmentally viable and this will stem from research into production
processes.
Figure 2.2 - Schemat ic representation of the environmental impacts of ethanol in
gasoline (2005-Niven)
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2.3 Butanol
Butanol is an alcohol of four carbon atoms, C4H9OH and has the potential to be
used in conventional gasoline engines in blends up to 85% butanol.14 Of the
higher order alcohols, 1-butanol has proven to be the easiest to produce using
microbial technology. Butanol shows a number of advantages over its shorter
chained counterpart ethanol. It easily mixes with gasoline, has a similar calorific
value, is less susceptible to water contamination and has a lower vapour
pressure than ethanol.15,16 These properties allow the high potential blends and
eliminate any alterations to conventional engines and fuel distribution and
storage infrastructure. Higher alcohols over ethanol have the ability to be
branched forming isomers, which have higher octane numbers to their straight
chain counterparts.17
Sources of biobutanol from fermentation are the same feedstock as used for
ethanol and bioethanol plants can be retro fitted simply and cost effectively.
Butanol production from sugar is however, three times less efficient than the
production of ethanol.18 Engine combustion characteristics of butanol are not
well known and more research is needed to determine and control the multitude
of potential reaction pathways. Recent studies suggest relatively high emission
levels of carbon monoxide, methane and propene.19
British Petroleum (BP) and DuPont have recently joined their market forces and
biotechnology capabilities to develop the biobutanol process methods.15 This will
be produced at an existing British Sugar mill that currently produces bioethanol
with aims to have a commercially suitable method during 2010.
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2.4 Biogasoline
It has been claimed recently that gasoline can be made directly from poplar and
switch grass biomass, although results are yet to be published. The chemical
changes of cellulosic biomass during a rapid heating and cooling process over
catalysts has been investigated at the University of Massachusetts, USA.20
Cellulose is heated rapidly and briefly and immediately cooled in one step into
gasoline components. At a heating rate of 1000oK/s, half the cellulose energy is
transformed into naphthalene (two fused benzene rings, C10H8) and toluene (a
benzene ring with a single methyl group, C6H5CH3), liquid state high octane
aromatic hydrocarbons which are found in petrol. The catalyst is readily available
and used in the petroleum industry,21 although specifics have been withheld. The
rates of heating and cooling determine the products; slow heating produces coke
and carbon whilst too rapid heating produces vapours. The chemistry of this
process are not yet fully understood, but current yields lie at 50% and estimates
cost 100% yields at $1 a gallon. It may be possible to synthesize further
products from the naphthalene and toluene that are closer to the gasoline blend,
or alternatively add these to existing blends to increase the octane rating.
Alternative methods include the generation of specific fatty acids with carbon
chains of length and composition to those found in gasoline, a similar concept to
biodiesel from waste cooking oils.22 Generic engineering ofE. colihas lead to the
creation of the fatty acids and the enzymes to refine them into equivalent
mineral fuels.23
The creation of a biofuel with the same chemical structures as those of gasoline
would allow seamless integration of the new fuel, using existing infrastructure
and internal combustion engines. If this can be done cost effectively, the future
use of biofuels in the transport sector will be secured for energy security and
independence from fossil fuel reasons alone.
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3.M ICROORGANISMS
There are many microorganisms from a vast range of environments that have
the ability to produce a variety of chemicals that are of use to industry, including
natural fuels. The most common of these natural processes is fermentation bythe yeast Saccharomyces cerevisiaeproducing ethanol. Propanediol and butanol
have also been found to be generated by fermentation of glucose by
microorganisms.24 Species of bacteria, fungi, algae and yeasts have been found
to produce oils, table 3.1. An algal species, Botryococcus brauniihas regularly
been cultivated to produce 50wt% hydrocarbons between C27H52 and C34H58
which individually have high heating values of around 33.8 MJ.kg-1.25,26 The
simplicity of
algae enable very high photosynthetic efficiencies, resulting inincredibly rapid growth rates, far exceeding those of food crops. Biomass can be
doubled in under a day, and can be grown on any land condition in closed cycle
systems.27 Further products can be synthesised from those produced by
microbial activity with additional synthetic processes, such as the cracking of
natural oils into conventional diesel or gasoline.
Table 3.1 Oil content of some m icro-organisms (Meng, 2009)
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Key fields of development in microbial fuel production surround understanding of
the complicated product synthesis pathways, figure 3.1, with the aim to increase
target product yield, predictability and reliability whilst reducing environmental
stresses to the microorganism, production of less desirable products and
economic cost.28 Natural yields of fuel products are often low as increasing yields
tend to create environments in which the organism can no longer function. For
example, butanol is toxic to the bacteria Clostridium acetobutylicum that can
produce it through acetone-butanol-ethanol (ABE) fermentation.29 This makes it
necessary for continuous removal of the product to keep the microbial process
functional, but results in a dilute harvest requiring further processing and
increasing production costs.
Figure 3.1 Schematic representation of 1-butanol production in engineered E. coli.
(2008, Atsumi)
There exists a vast array of sequenced genomes which aids the genetic
engineering of microorganisms to increase the variety of chemicals that can be
produced. Genetic engineering of a single trait often requires manipulation of
multiple genes, which has knock on effects to other processes of the cell.
Inserting genes to yeast for ethanol fermentation of xylose weakens the microbe
and reduces ethanol tolerance.12 Genetic engineering of Escherichia coli is
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showing great potential for the biofuels industry and has proved to be successful
in producing butanol.30 Currently, potential microorganisms producing molecules
of interest are metabolically engineered to enhance the creation of these
products which are then assessed alongside growth rates. Bottlenecks to the
process are determined and attempts to remove them are performed by cellular
profiling, adjustments and repeating of the cycle, figure 3.2. The creation of
systematic models describing microbial pathways are needed to aid the
understanding of cellular physiology enabling rapid prototyping, testing,
optimisation, diagnosis of problems and suitable solutions to further the potential
use of microorganisms to generate fuel products.31
The formation of higher alcohols has required glucose as feedstock. This will
need to be expanded to lower cost feedstock to be economically viable. Future
research will include changing the fermentation process from anaerobic to
aerobic, interspecies transfer of genes for further products, to ensure continued
growth of the organism and formation of the product in higher alcohol
concentrations.24,30
Figure 3.2 Flowchart of systems biology applied to achieving a production target
(2008, Mukhopadhyay)
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fermentation.35 Ethanol derived from sugars or starch crops behave with greater
similarity to gasoline than those from oils or lignocellulosic fibres.36
Removing ethanol from water is performed by distillation which proves to be
energy intensive, most notably when achieving purities over 95%. It has been
estimated that 60% of the energy provided by the combustion of produced
ethanol is needed in its distillation.12 The future of fermentation may be limited
economically by the relatively long, unpredictable nature and low yields of the
process.37 Improvements in separation techniques are evidently required to
improve the process of fermentation and provide a continuous system.
Possibilities aside from distillation include pervaporation (the partial vaporisation
of the batch through a selective membrane), online solvent extraction and
selective adsorption of ethanol from a small stream of fermentation broth. The
latter requires highly selective ethanol adsorbents, alternating adsorption -
desorption columns but has the potential of producing pure ethanol. Powdered
adsorbents are most rapid due to increased surface area.
4.2 Acetone-butanol-ethanol (ABE) fermentation
Solventogenic closteria bacteria can ferment sugars into acetone, ethanol and
most uniquely, butanol.38 The aim of ABE fermentation is to increase the yield of
butanol production and reduce the yield of acetone and ethanol. Yields of
15.8g/L.h have been achieved, however butanol is toxic to the bacteria and
hence needs to be continuously removed.29 This can be done by gas stripping,
using the by bubbling the by-products, hydrogen and carbon dioxide, through
the medium to capture the butanol which is collected after condensing and the
gasses recycled. Alternatively, liquid-liquid extraction is possible by adding oleyl
alcohol (C18H35OH) which is insoluble in the aqueous fermentation broth but
miscible with butanol and floats atop the broth.
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ABE fermentation is not yet commercially viable however. Higher productivity
rates are required via microbial development. Cheaper collection techniques and
developing the ability to use lignocellulosic biomass will reduce the cost of
butanol production by this method. Clostridium acetobutylicumhas a particularly
complicated genetic make-up and E. coli is receiving interest to develop this
field.39
4.3 Saccharification
Saccharification is the degeneration of cell walls into sugars and is required as an
intermediate step to ferment lignocellulosic feedstocks which have carbohydrate
structures such as lignin, cellulose, xylan and starch.40 Cellulose is a linear
crystalline chain of glucose (C6H12O6) containing up to 10,000 units per chain.
Hemi cellulose is a branched polymer with a mix of glucose and xylose (C5H10O5),
with their respective isomers, and is found predominantly in woody biomass.
The cells cellulose crystalline structure wall first needs to be physically broken.
Mechanical methods include chipping, milling and grinding resulting in
approximate sizes of 10-30mm, 2mm ad 0.2mm respectively. Low temperature
pyrolysis (
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than is needed in mechanical processing to the same size, and is most effective
in hardwoods and agricultural residue. Steam explosion may generate inhibitory
compounds to later processes requiring a wash which will also remove any
soluble sugars, however the addition of small amounts of H2SO4, SO2 or CO2 has
been found to decrease the production of such compounds.
Ammonia fibre explosions (AFEX) do not produce inhibitory components, works
at lower temperatures and id good for herbatious crops and grasses. The
ammonia must be recycled however to protect environment, is not very good for
the break down of hemicellulose and longer residence times (30min) are
required. Carbon dioxide explosions are more cost effective than AFEX and dont
form inhibitory compounds but result in lower yields. It is envisaged that the CO2
will form carbonic acid which will increase the rate of hydrolysis.
4.5 Hydrolysis
Hydrolysis is the chemical break down of carbohydrates and starch into simple
sugars.41The equation for starch into glucose is given as equation (4.2).
-[C6H10O6]-n + n(H2O) => n(C6H12O6) + nO2 (4.2)
Hydrolysis can be carried out by use of an acid or by enzymes and is most
effective at the breaking down of cellulose.42 Acid hydrolysis typically uses
Sulphuric acid which is also successful at breaking down xylon, which can
account for 30% of lignocellulosic carbohydrates. Continuous processes require
high temperatures (>160oC) and low loading whilst batch processes can use
lower temperatures and allow higher loading (
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Enzymatic hydrolysis makes use of microorganisms that produce a collection of
cellulose enzymes. The process can be performed in aerobic or anaerobic
conditions by bacteria or fungi although the aerobic conditions of fungi are
preferred. Low substrate concentrations are needed as high concentrations
inhibit enzymatic hydrolysis. Lignin must be removed before enzymatic hydrolysis
as it forms a barrier against access to cellulose. Mild conditions are required and
the process is not corrosive, so is cheaper than acid hydrolysis, but the results
are less predictable and longer times are required.
The development of pre-treatment to fermentation in the production of ethanol
continues to search for a solution that not only produces the highest yield of
simple sugars, namely glucose, but also has a low energy demand and doesnt
produce fermentation or hydrolysis inhibitors. These include eliminating the need
of acids with a movement to enzymatic methods, novel grinding techniques,43
production of many biofuels from one feedstock and process44 and specific yeast
cell surface engineering which may provide direct ethanol production from
lignocellulosic feedstock, bypassing saccharification and hydrolysis all together.45
4.6 Fischer-Tropsch Synthesis
Carbon monoxide and hydrogen (syngas) from gasification (the partial burning of
solid biomass or wastes in reduced oxygen levels) can be converted into liquid
hydrocarbons, or directly to methanol, by use of a catalyst in the Fischer-Tropsch
process. Different temperatures, catalysts and syngas H/C/O ratios will yield
products in varying ratios.46 The two most common reactions are given as
equation (4.3) and (4.4). The most suitable catalysts are iron, ruthenium or
cobalt based. Cobalt is almost 1000 times more expensive than iron, but
produces a higher activity and a longer life. Products contain no nitrogen or
sulphur compounds because syngas is used, creating fuels that are more
environmentally friendly than those derived from fossil oil.47
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5.CONCLUSION
Gasoline fuel replacements to date are limited to alcohols. Ethanol is the most
widely used biofuel for gasoline replacement. The benefits of ethanol have been
debated in recent years and a call to slow down the utilisation of biofuels is
primarily down to the destructive nature in which ethanol has been produced
from food crops. Ethanol is miscible with water which causes storage, transport
and combustion problems when mixed with gasoline. New fuels are emerging to
the market with properties closer to those of gasoline, most notable butanol
which is hydrophilic and highly miscible with gasoline. Fuel research focuses on
creating products from biomass with chemical compositions and properties
replicating those of gasoline. Progress has been made in the lab, creating some
part of the chemicals found in gasoline, but results are yet to be published.
Research in biofuel synthesis aims to improve the cost effectiveness of products
by reducing the creation of un desirable products whilst increasing the yields of
fuels. As alcohols dominate the gasoline biofuel market, most of this research
surrounds fermentation, and the microorganisms that carry out this process.
Genetic engineering of bacteria and fungi to increase yields, ferment morecomplicated sugars and produce higher alcohols to ethanol proves to be the
cutting edge, but modelling of changing microbe genetics would prove to speed
up this field dramatically. Aside from genetic engineering, improvements of
feedstock preparation by breaking down complex carbohydrates by non-
biological means continues to develop whilst catalyst research to convert syngas
into suitable hydrocarbons unearths new methods, but is yet to be economically
viable.
It must be the aim of future research to peruse the creation of a biofuel
replicating gasoline to ensure its success by enabling seamless integration with
existing infrastructure and spark ignition engines.
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37 N. Kosaric, J. Velikonja, 1995. Liquid and gaseous fuels from biotechnology: challenge and
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39 Jay D Keasling, Howard Chou, 2008.Metabolic engineering delivers next-generation biofuels.
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40 Ye Sun, Jiayang Cheng, 2002. Hydrolysis of lignocellulosic materials foe ethanol production: a
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42 Marija B. Tasic et al., 2009. The acid hydrolysis of potato tuber mash in bioethanol production.
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44 Prasad Kaparaju et al., 2009. Bioethanol, Biohydrogen and Biogas production from what straw
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46 Mark E. Dry, 2002. The Fischer-Tropsch process: 1950-2000. Catalysis Today; Volume 71, pg.
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47 Mark E. Dry, 2001. High quality diesel via the Fischer-Tropsch process a review. Journal of
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APPENDIX I
EN228 European Normal for Petrol 98 (biofuels platform)
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APPENDIX I I
Energy inputs and outputs for the life cycle analysis of mature ethanol
manufacture. (2007 Grands)
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