www.rsc.org/ees
Volume 1 | Number 5 | November 2008 | Pages 513–596
ISSN 1754-5692
COVER ARTICLE
Rafael Luque et al.
Biofuels: a technological perspective
OPINION
José Goldemberg
The challenge of biofuels
1754-5692(2008)1:5;1-2
PERSPECTIVE
www.rsc.org/ees | Energy & Environmental Science
Biofuels: a technological perspective
Rafael Luque,*ab Lorenzo Herrero-Davila,b Juan M. Campelo,a James H. Clark,b Jose M. Hidalgo,c
Diego Luna,a Jose M. Marinasa and Antonio A. Romeroa
Received 28th April 2008, Accepted 1st August 2008
First published as an Advance Article on the web 5th September 2008
DOI: 10.1039/b807094f
Environmental issues, the growing demand for energy, political concerns and the medium-term
depletion of petroleum has created the need for development of sustainable technologies based on
renewable raw materials. Biofuels might help to meet the future energy supply demands as well as
contributing to a reduction of green house gas emissions. Although this topic is highly controversial
and many investigations are currently ongoing, this review is intended to give a brief overview about
certain aspects of the complex biofuel issue, providing the latest update of the production and potential
of biofuels in the transport sector including types of biofuel, feedstocks and technologies and some of
the possible socio-economic, environmental and political implications of the widespread use of biofuels
in our society.
Introduction
There is now a general scientific consensus that observed trends
in global warming are been caused by fossil fuel combustion and
anthropogenic emissions of greenhouse gases (GHG) including
nitrous oxide (N2O) and especially carbon dioxide (CO2) and
methane (CH4).1 Initial concerns about the impact of GHG in
our society led to the development of the United Nations
Framework Convention on Climate Change (1992), which in
turn resulted in the 1997 Kyoto Protocol as a way to tackle the
problem. In 2002 the European Union ratified the Kyoto
Protocol and emphasised the potential for scientific innovation
as a means of countering GHG emissions. Neither these targets
nor the Kyoto targets have so far been met. The reality is that the
a
Departamento de Quı´mica Orgánica, Universidad de Córdoba, Campus
Universitario de Rabanales, Edificio Marie Curie (C3), E-14014 Córdoba,
Spain. E-mail: [email protected]; Web: www.uco.es/senecagreencat
b
Green Chemistry Centre of Excellence, Chemistry Department, The
University of York, Heslington, York, UK YO10 5DD
c
Seneca Green Catalyst S.L., E-14011 Córdoba, Spain
Rafael Luque
Rafael Luque has been a postdoctoral researcher working in
the Green Chemistry Centre of
Excellence over the last three
years. His PhD is from Universidad de Cordoba in Spain
under Prof. Campelo’s supervision. His interests are materials science, catalysis and
more recently biofuels. He is
(co-)author of over 35 manuscripts and 2 patents. He will
return to Prof. Campelo’s
group in late September.
542 | Energy Environ. Sci., 2008, 1, 542–564
situation is likely to become more complicated in the next few
years. Transport has shown the highest rates of growth in GHG
emissions in any sector over the last ten years (20% global CO2
emissions, 25% UK emissions), with a predictable 80% higher
energy use and carbon emissions than now by 2030.2
Oil is the world’s primary source of energy and chemicals with
a current demand of about 12 million tonnes per day (84 million
barrels a day)3 with a projection to increase to 16 million tonnes
per day (116 million barrels a day) by 2030. While 30% of the
global oil consumption accounts for transport, a striking 60% of
the rising demand expected for 2030 corresponds to transport.4
With the transport sector expanding in the US and Europe and
especially developing in the newly industrialised and emerging
economies of China and India, these values can easily be
underestimated. The availability of conventional oil is again
becoming geographically restricted and a general agreement now
is that the era of cheap and secure oil (cheap energy) is over.5
Several alternatives are currently being explored, including
a range of carbon free and renewable sources (photovoltaics,
wind and nuclear power, hydrogen), in an attempt to replace
Juan M. Campelo is the head
of the Departamento de Quimica Organica at the Universidad de Cordoba in Spain.
For three decades he has been
involved in acid–base heterogeneous catalysis research
applied to organic chemistry.
He has recently joined the
interest in biofuels, starting
a new research line in Cordoba
around 2nd generation biodiesel. He is (co-)author of
over 140 peer-reviewed publications and 6 patents.
James H. Clark holds a Chair
in Chemistry at the University
of York. James has been at the
forefront of green and
sustainable chemistry for over
10 years, being founding
scientific editor of the journal
Green Chemistry and founding
Director of the Green Chemistry Network. He currently
runs the Green Chemistry
Centre of Excellence at York
bringing together research and
industrial collaboration on
renewable resources and clean
synthesis.
This journal is ª The Royal Society of Chemistry 2008
Fig. 2 Greenhouse gas emissions from production and utilisation of
biofuels (source: Sustainable biofuels: prospects and challenges, The
Royal Society. Policy document 01/08, ISBN 978 0 85403 662 2.
Reproduced with permission of the Royal Society).
Fig. 1 Biofuels consumption (1991–2006) in the EU27. Source: REFUEL,
IEA, Eurovserv’ ER. Reproduced with permission of Marc Londo.
natural gas, coal and oil in the electricity generation sector.
However, there is no such equivalent in transportation yet, since
fuel cells, electric/hybrids and natural gas based cars are still
a long way from becoming mainstream vehicles.
A short and medium term alternative is needed. Crop-based
fuels denoted as biofuels including biodiesel and bioethanol,
emerged as a real alternative to the use of gasoline and conventional diesel in transportation. An exponential increase in the
consumption of such biofuels has taken place in the last few years
(Fig. 1).
Ideally, such oil alternatives should reduce (or even remove)
the dependence of oil as well as contributing as much as possible
to meet the GHG emissions target. However, it is also widely
accepted that joint efforts from politicians, regulators, scientists
and consumers will be needed to support an independent oil/
GHG controlled scenario in the future.
In view of the predictions, the need for a secure energy supply
for transportation make it essential to explore biofuels as alternatives to mineral oil based fuels addressing and evaluating
socio-economic and environmental consequences originated in
their implementation.
From a wider context, there are three main drivers for the
promotion, development and implementation of biofuels in our
society. In principle, these are energy independence, climate
change and rural development.3 The political motivation to
support biofuels arises from each individual driver or combinations of them. The main theoretical reasons for the promotion of
biofuels can be summarised as follows:
1.
Biofuels can improve independence and energy security
Local, national or global production of energy can avoid the
reliance on politically and/or socially unstable energy suppliers.6
In addition, the global oil demand is increasing exponentially
and there is a need to find alternatives to fossil fuels derived from
petroleum.
2. Biofuels may contribute to a reduction in carbon emissions
(aka climate change mitigation)
They have been often considered a solution to climate change. In
fact, net emissions from biofuels have been reported to be
This journal is ª The Royal Society of Chemistry 2008
remarkably lower than those generated from the combustion of
fossil fuels.7 Nevertheless, the GHG emissions from the
production of biofuels are a key issue that needs careful attention
as they arise from every single stage in the supply chain from
feedstock production and transport to conversion, biofuels
distribution and end use (Fig. 2). Recently, some studies point
out the CO2 reduction may be far less than originally thought due
to the inclusion of crop production costs including fertilisers,
machinery, etc. as well as harvesting, transformations and
distribution.3
3. Biofuels can help to increase farm income and contribute to
rural development
With a growing biofuels market, many countries will be able to
grow more types of crops to cover national or foreign demands
on energy crops. The increasing demand for agriculture due to
the labour-intensive plant derived technologies8 is expected to
improve farm income, which in countries with oversupply can
also help to reduce the need for subsidies. Traditionally deprived
rural areas could experience a renaissance through the implementation of biofuels and biorefineries. In addition, there is also
a lot of controversy over food vs. fuel and the growth of specific
crops to be transformed into biofuels. The resolution of this
process will necessarily take place on the basis of very different
variables over time.9
With a wide array of potentially renewable energy resources,
the concept and proposed benefits evolving from the use of
biofuels are inspiring; therefore they need to be taken into
account in order to contribute to a sustainable and energy secure
future.
In this review, we aim to provide a general (not exhaustive)
overview of the state of the art in the production and potential of
biofuels for transport as well as their socio-economic, environmental and technological implications of their widespread
implementation on our society.
What are biofuels?
Biofuels (also denoted sometimes as agrofuels to make reference
to biofuels from agriculture and forestry) can be broadly defined
as any sort of fuel that is made from organic matter (aka
biomass). The most common biofuels are biodiesel and
bioalcohols, including bioethanol and biobutanol (also called
biogasoline).
Energy Environ. Sci., 2008, 1, 542–564 | 543
The past of biofuels: an overview of the history of biofuels
Biofuels are not a recent discovery. The transesterification of
vegetable oils has been widely known and employed since the 19th
century. In fact, the process currently in use for making biofuels
from biomass is the same inherited from the past. The feedstocks
utilised for their preparation were also very similar. Peanut,
hemp and corn oil and animal tallow were conventionally used
and have been partially replaced by soybean, rapeseed, recycled
oil, forest wastes and trees and sugar cane. But the history of
biofuels is more political and economical than technological.
Liquid biofuels for transport have been used since the early
days of the automobile industry. Nikolaus August Otto, the
German inventor of the internal combustion engine (Fig. 3),
conceived his invention to run on ethanol. In 1898, Rudolph
Diesel (Fig. 4) used peanut oil (the ‘‘original’’ biodiesel) in the
first demonstration of his compression ignition engine at the
World’s Exhibition in Paris. Vegetable oils were used in diesel
engines until the 1920’s when an alteration was made to the
engine, enabling it to use a residue of petroleum (currently
known as No. 2 diesel fuel).
Nevertheless, Diesel was not the only person to believe that
biofuels could be the way forward of the transportation industry.
Henry Ford (Fig. 5) was so convinced that renewable resources
were the key to the success of his automobiles that he designed
them (from the 1908 Model T) to be run with ethanol. He also
Fig. 3 Nikolaus August Otto, inventor of the internal combustion
engine.
Fig. 4 Rudolf Diesel, inventor of the diesel engine.
544 | Energy Environ. Sci., 2008, 1, 542–564
Fig. 5 Henry Ford, founder of the Ford company.
built an ethanol plant in the Midwest and established a partnership with Standard Oil to sell it in their distributing stations.
At the beginning of World War II, both the Allies and
Germany utilized biomass fuels in their machines. Wartime
Germany experienced extreme oil shortages. The vehicles were
then powered with blends including Reichskraftsprit that was
a blend of gasoline with alcohol fermented from potatoes. In
Britain, grain alcohol was blended with petrol by the Distillers
Company Ltd. Under the name Discol and marketed through
Esso’s affiliate Cleveland. Despite their use during World War II,
biofuels remained in obscurity to which they had been forced
from fossil fuels (petroleum). Until the 70s, the solid petrol-based
industry establishment made us very dependent on foreign oil.
However, the supply of crude oil, as are all supplies of fossil fuels,
is limited and some issues were about to take place. In 1973 we
experienced the first of two oil crises. The Organisation of the
Petroleum Exporting Countries (OPEC) that controls the
majority of the global oil reduced supplies and increased prices.
The second one came five years later in 1978.
Then, the potential of biofuels reappeared in the public
consciousness, brought back by various bio-energy programs
including the Programa Nacional do Alcool (Proálcool) established in Brazil in 1975 (which was critical in the development of
ethanol fuel) and international organisations such as the International Energy Agency (IEA) Bioenergy, established in 1978 by
the Organisation for Economic Co-operation and Development
(OECD), that aimed to improve cooperation and information
exchange between countries that have national programs in
bioenergy research, development and implementation.
On a worldwide scale, the United Nations (UN) International
Biofuels Forum is formed by Brazil, China, India, South Africa,
the United States and the European Commission, the United
States, Brazil, France, Sweden and Germany being the world
leaders in biofuel development and use.
As examples, some interesting available data are given. France
was the second largest biofuel consumer among the EU States in
2006 (increasing by 62.7%). Biodiesel represents the largest share
of this (78%, far ahead of bioethanol with 22%).10 Germany
itself remained the largest European biofuel consumer, with a
consumption estimate of 2.8 million tons of biodiesel (equivalent
to 2 408 000 toe), 0.71 million ton of vegetable oil (628 492 toe)
and 0.48 million ton of bioethanol (307 200 toe), being ADM
Ölmühle Hamburg AG (a subsidiary of the American group
This journal is ª The Royal Society of Chemistry 2008
Fig. 6 The Veggie Van project. Reproduced with permission of Barry J.
Holmes.
Archer Daniels Midland Company) the largest biodiesel
producer.10 In Brazil, the government hopes to build on the
success of the Proálcool ethanol program by expanding the
production of biodiesel which must contain 2% biodiesel by
2008, increasing to 5% by 2013.
More interesting and entertaining examples that illustrate the
effects of biofuels in our current society can be found in journals,
press and the internet. Biodiesel produced today can be used in
unmodified diesel engines. Over 200 major fleets in the United
States now run on biodiesel with entities including the United
States Post Office, the US Military, metropolitan transit systems,
agricultural concerns, and school districts being major users. In
Europe, biodiesel is readily available to purchase in many gas
stations (well over 1000 stations in Europe). Journey to Forever,
a non-government organization, traveled from Hong Kong to
Southern Africa producing their own biodiesel along the way and
teaching people from small villages how to make their own
biofuels to employ in heaters, tractors, buses, automobiles, and
other machines they might have. Josh Tickell traveled on The
Veggie Van (Fig. 6) 25 000 miles around the United States on
new and recycled fryer oil. Nowadays, The Veggie Van Organization is a non profit educational service that promotes biofuels
through hands-on demonstrations, school presentations and
road tours.11 David Ramey also drove his unmodified 1992 Buick
across America in the summer of 2005 using 100% butanol as
fuel.12
For a more detailed study about various industrial uses and
biofuels programmes worldwide, we refer the readers to recently
reported documents3,10,13 and the discussion around biofuels.
Present of biofuels: the biofuels ladder
In this changing environment, many opportunities are arising for
bioenergy. There has been a relatively high acceptance from
general public, governments, producers and part of the agricultural sectors in promoting the expansion of biofuels in our
society in an attempt to switch from the petrol-based industry we
have been relying on for the last 50+ years to a biobased industry
and society that can guarantee a more secure energy supply. In
this regard, much of the work carried out in the last few years has
been focused on the development of various methodologies for
the production of biofuels for transport. There are several
This journal is ª The Royal Society of Chemistry 2008
Scheme 1 The biofuels ladder. Road map of biofuels production from
feedstocks and technologies.
important factors that we have to take into account for
a successful implementation of biofuels in our society and we will
deal with them in the discussion around biofuels. However, since
we started to climb the biofuels ladder, we have already achieved
a certain level of development (2nd generation of biofuels with
a potential 3rd generation in sight) that is already contributing to
prove the real potential of biofuels in our lives. We will briefly go
through the different types of biofuels that have been developed
until now. It is generally accepted that there are two different
generations of biofuels that have been classified starting from the
conventional technologies and feedstocks (1st generation) to the
latest advances in the field (2nd and potential 3rd generation).
Scheme 1 summarises a very simplified classification of
biofuels we propose for a better understanding of the subject.
The latest results reported by Howard et al.14 right before the
submission of this manuscript might lead to a third generation of
biofuels as we will highlight in future perspectives.
First generation biofuels
The first generation biofuels referred to biofuels manufactured
from readily available energy crops including sugar, starch and
oil crops (edible feedstocks) using conventional technologies.
The most common first generation biofuels are biodiesel and
bioethanol. Some other biofuels in this category including
biofuels integrating glycerol, biofuels from catalytic cracking,
biobutanol and 2,5-dimethylfuran (DMF) will also be briefly
discussed. The various biofuels will be grouped according to the
technology employed for their preparation. These are chemical,
thermo-chemical and biological conversion.
Biofuels produced by chemical conversion
Biodiesel
First generation biodiesel is currently the most common biofuel
in Europe. It remains in the political and economic arena and is
playing a part in the biofuels expanding process as the awareness
alternative fuel spreads through the consciousness of the general
public. Only in 2007, 19 biodiesel plants in the new EU member
states were starting operations, or were under construction/
Energy Environ. Sci., 2008, 1, 542–564 | 545
drawbacks make impractical its successful implementation with
hardly any examples of commercial processes available.26
Scheme 2 Conventional transesterification of TG for the production of
biodiesel.
planning. Relatively large plants can be found in Lithuania,
Poland and Romania, with capacities of 100 000 tonnes y1. The
conventional methodology for the production of biodiesel
involves the transesterification of triglycerides (TG) from vegetable oils (palm, corn, soybean, rapeseed, sunflower, etc.) with
short chain alcohols including methanol and ethanol to yield
fatty acid (m)ethyl esters (FAM/EE) and glycerol as a by product
(Scheme 2).
Several reviews on the preparation of biodiesel from different
feedstocks can be found in the literature.15,16 A very good
overview of such technologies has been recently published by
Al-Zuhair.17
The methods of preparation of biodiesel can be classified in:
chemical catalytic (base- or acid catalysis), biocatalytic (enzyme
catalysis) and non-catalytic processes.
Biodiesel produced by chemical catalytic methods
Homogeneous catalysis. The conventional and traditional
methodology for the production of biodiesel primarily involved
the transesterification of the vegetable oils using NaOH and
KOH18–20 or mineral Brønsted acids (sulfuric, phosphoric or
hydrochloric acids)21–23 as homogeneous catalysts and vegetable
oils or waste oils and fats as feedstock at relatively mild
temperatures (50–80 C).
Few reports on the production of biodiesel using a variety of
homogeneous catalysts including guanidines24 and different
amines as catalysts (yielding conversions higher than 98% in
a one-step reaction, minimizing the production of waste water)25
can also be found.
Regardless of the limitations of the methodology, the process
is also far from being environmentally friendly. The final mixture
needs to be separated, neutralised and thoroughly washed,
generating a great amount of salt, soaps, and waste water which
need to be further purified or treated. The catalyst cannot also be
recycled. These several additional steps certainly put the total
overall biodiesel production costs up, at the same time reducing
the quality of its main by-product (glycerol). This phase needs to
be separated from the biodiesel for further washing/drying to
remove the traces of glycerol and from the fuel to comply with
EU quality standard regulations (EN 14214). The standard
prescribes 0.02% or lower glycerol content must be present in the
biodiesel.
The acid catalysed homogeneous transesterification has not
been widely investigated compared to the alkali-catalysed
process due to its slower reaction rates, the need of harsher
conditions (higher temperatures, methanol to oil molar ratios
and quantities of catalysts) and the formation of unwanted
secondary products such as dialkyl or glycerol ethers.16b These
546 | Energy Environ. Sci., 2008, 1, 542–564
Heterogeneous catalysis. Several reports can be recently found
on the production of biodiesel involving other chemically catalysed protocols as greener alternatives using vegetable oils using
solid bases27–30 and solid acids.23b, 27,31–33 Di Serio et al. have
recently reviewed the use of heterogeneous catalysts for biodiesel
production.27
The advantages of the heterogeneously catalysed protocols
from the green chemistry standpoint are that the catalyst may
be recycled and subsequently employed in the reaction. The
biodiesel prepared has improved properties compared to
the homogeneously catalysed process. The elimination of the
pre-treatment steps and the minimisation of waste, avoiding the
production of waste salts, also improves the green credentials of
the reaction. Excellent yields of FAME/FAEE can be obtained
under relatively mild conditions with many of these heterogeneous catalysts. However, the separation, disposal or use of the
glycerol generated in the process as well as the washing of the
crude biodiesel obtained to remove the traces of glycerol to meet
the EEC regulations are often a problem associated to the
chemical production of biodiesel.
Biodiesel produced by biochemical catalytic methods. The
increasing environmental concerns have led to a growing interest
in the use of enzyme catalysis as it usually produces a cleaner
biodiesel under milder conditions. It also generates less waste
than the conventional chemical process. Many authors have
reported a wide range of efficient and low energy intensive
protocols obtaining very promising results with lipases (in both
free and immobilised form)34,35 and combining lipases with alkali
catalysts.36
The limitations of the industrial use of enzymatic methodology
is mainly due to their high production costs, which may be
overcome by molecular technologies to enable the production of
the enzymes in higher quantities as well as in a virtually purified
form.37
Biodiesel produced by non-catalysed processes. The most
common and simple non-catalysed biodiesel production process
has been performed using supercritical methanol via simultaneous transesterification of triglycerides and esterification of
fatty acids.38 The alcohol supercritical conditions are essential
because a very low reaction rate is obtained under subcritical
conditions.
The procedure has been claimed to be very effective yielding
high FAME contents in a very short time of reaction (typically
less than 30 min). The presence of water also facilitated the
formation of the methyl esters. Nevertheless, the supercritical
methodology is still very expensive and the implementation of
such costly technology in industry is currently a challenge.
Properties and advantages/disadvantages of first generation
biodiesel. Biodiesel is non toxic, sulfur-free and biodegradable
being used in unmodified diesel engines pure or as a blend. It has
a significant added value compared to petrol-diesel because of its
higher lubricity, flash point and lower aromatic content which
extend engine life and reduces maintenance costs. Biodiesel has
This journal is ª The Royal Society of Chemistry 2008
also lower combustion emissions [except for nitrogen oxides
(NOx)], particulate matter and unburned hydrocarbons
compared to petrol-based diesel.15
However, biodiesel suffers from low mileage (higher fuel
consumption compared to No. 2 diesel fuel) and oxidative
instability as well as being corrosive.17 It also produces slightly
lower power and torque.
Biofuels integrating glycerol
Novel methodologies for the preparation of esters from TG
which directly afford alternative biofuels are currently under
development.39
The transesterification reaction of triglycerides with dimethyl
carbonate (DMC),40 methyl acetate41 or ethyl acetate42 produced
a mixture of three molecules of FAME or FAEE and one of
glycerol carbonate (GC) or glycerol triacetate (triacetin). Such
a mixture (FAME + GC) has suitable physical properties to
be employed as fuel, constituting a novel biofuel denoted as
DMC-BioD.20b,20c
Gliperol is another patented novel biofuel.41a Gliperol is
composed of a mixture of three molecules of FAMEs and one
molecule of triacetin. It can be obtained after the transesterification of one mole of TG with three moles of methyl
acetate employing a lipase as a catalyst.41,42
Similarly, we have patented a novel biofuel denoted as
Ecodiesel-100 obtained via 1,3-selective partial ethanolysis of the
triglycerides with pig pancreatic lipase (PPL).43 The biofuel is
a mixture of two parts of FAEE and one part of MG, with minor
quantities of DG.
Properties and advantages/disadvantages of biofuels integrating
glycerol. DMC-BioD, Gliperol and Ecodiesel-100 incorporate
glycerol in their monophasic homogeneous mixtures, avoiding
the generation of residues or by-products in their preparation
processes. The main difference with respect to the conventional
biodiesel (FAME) production is that no additional separation
steps are needed. GC and/or triacetin can be perfectly burnt
together with the FAME in the blend. In terms of green chemistry, the incorporation of glycerol into the biofuel improves the
efficiency of the process (from current 90% to 100%), without
substantial modifications of its physico-chemical properties. The
atom efficiency is also improved as the total number of atoms
involved in the reaction is part of the final mixture. Recent
studies have also demonstrated that the presence of MG adds
value to the biofuel by improving the lubricity on the engine.44
However, the implementation of such interesting biofuels in the
transport sector can be greatly conditioned by the performance
of the biofuels in actual diesel engines due to their high content in
‘‘additives’’ including MG, triacetin and GC. These compounds
may also have higher melting points (e.g. MG) and potentially
form carbonaceous deposits when burnt. Furthermore, the
current legislation on biodiesel does not restrict the presence of
MG and glycerol so that a special legislation promoting these
biofuels might be necessary.
2,5-dimethylfuran (DMF)
DMF has attracted a great deal of attention as a potential
biofuel. Advances in the production of DMF from fructose and
This journal is ª The Royal Society of Chemistry 2008
glucose using catalytic biomass-to-liquid (BTL) processing have
been recently reported.45 The process involves, in the first step, an
acid-catalysed dehydration of fructose to produce 5-hydroxymethyl furfural (HMF). HMF is then extracted from the organic
phase and subsequently converted to DMF by the hydrogenolysis of C–O bonds using a carbon supported copperruthenium catalyst.45 The potential of DMF is currently under
investigation. A DMF joint venture is currently under development involving the University of Wisconsin (Madison, US) and
Virent Energy Systems.
Properties and advantages/disadvantages of DMF. DMF has
interesting physico-chemical properties. Its energy density is 40%
greater than ethanol, making it comparable to gasoline. It is also
chemically stable, insoluble in water (does not absorb moisture
from the atmosphere) and has a higher boiling point (92 C) than
ethanol. However, the toxicological properties of DMF have not
been thoroughly tested. The limited information available
suggests that DMF is no more toxic than current fuel components.45 The scaling up of the technology, in terms of costs and
production, might be another issue to take into account.
Biofuels from catalytic cracking
Catalytic cracking of vegetable oils has also been reported as
a promising alternative to the preparation of biofuels. Several
vegetable oils (e.g. palm, canola, soybean) have been employed in
the process that involves conversion of the oils into biofuels
suitable for gasoline engines using acid catalysts such as transition metal catalysts (that yield biofuels enriched in diesel fraction—over 50% by weight)46 and zeolites and mesoporous
materials that give biofuels with higher gasoline fractions—over
40%—with higher aromatic content.47 The reaction is normally
performed at moderate to high temperatures (300–500 C) using
different oils to catalyst ratios depending on the oil and the
catalyst.47
Properties and advantages/disadvantages of biofuels from
catalytic cracking. The technology is relatively simple and efficient and it has the advantage that industrially similar processes
using micro- and mesoporous materials to produce fuels and
chemicals are well developed. The main shortcoming of the
successful implementation of such biofuels lies on the economics
of the process with the use of expensive vegetable oils as raw
materials
Biofuels produced by biological conversion
Bioalcohols
Bioethanol. Bioethanol is the other common first generation
biofuel that is generally used as a blend that can go up to 85%
content (E85).48 It is the most employed biofuel on a world level
with the US currently being the world’s largest producer and
Brazil the largest exporter, accounting together for 70% of the
world’s production and 90% of ethanol used for fuel.48 In Sweden
and the US, a high-proportion bioethanol blend E85 (85%
ethanol and 15% petrol) is being used in Flexible Fuel Vehicles
(FFVs) with modified engines that are able to run on either E85
or petrol, or any mixture of the two. The E85 can nowadays also
Energy Environ. Sci., 2008, 1, 542–564 | 547
Fig. 7 E85 bioethanol blend can be found at cost-competitive prices in
many petrol stations all over the UK (April 2008).
be purchased in several petrol stations in the UK (Fig. 7). Neat
ethanol (E100) has also been employed in large scale in Brazil in
specially modified engines.
The common feedstocks employed for the production of first
generation bioethanol are energy crops including sugar cane,
corn, wheat, maize and sugar beet (aka ‘food’ crops) although
a great potential of grain or sweet sorghums in replacing maize
and sugar cane, respectively, has been reported.49
First generation bioethanol is generally obtained by biological
conversion involving two key steps: hydrolysis and fermentation,
followed by a distillation and dehydration of the bioethanol
produced to obtain a higher concentration of alcohol to make it
suitable for its use as automotive fuel.
Hydrolysis (saccharification). The digestion of the feedstock is
normally performed via enzymatic hydrolysis using mixtures of
amylases enzymes to convert the starch into sugars. Sugar cane
and beet directly produce sugars that can be directly fed into the
bioreactor.
Fermentation. The released sugars are subsequently fermentated to ethanol using yeast (e.g. Saccharomyces cerevisiae) using
a similar process to that used in beer and wine-making.50,51 The
invertase enzymes present in the yeast catalyse the conversion of
sucrose into glucose and fructose that are subsequently transformed into ethanol and carbon dioxide by the zymases enzymes
(Scheme 3).
Bacteria strains such as Zymmomonas mobilis have been
demonstrated as an alternative to yeasts offering several
advantages in the fermentation including higher specific
productivity, ethanol yield and alcohol tolerance.3
Properties and advantages/disadvantages of first generation
bioethanol. First generation bioethanol offers a very convenient
Scheme 3 Production of bioethanol via fermentation of hydrolysed
sugars from energy crops.
548 | Energy Environ. Sci., 2008, 1, 542–564
blend with gasolines for engines due to its higher octane number
(over 100), lower cetane number (less than 10) and a higher heat
of vaporisation compared to gasoline. Such properties allow
a higher compression ratio, shorter burn time and leaner burn
engine, which lead to theoretical efficiency advantages over
gasoline in an internal combustion engine.52 Bioethanol is also
believed to give a 70% carbon dioxide reduction compared to
petrol, reducing also the particulate and NOx emissions.50
However, bioethanol has various disadvantages. Firstly, it is
hygroscopic and this is possibly the main disadvantage of the use
of such biofuel as transport fuel. The blend of bioethanol with
gasoline has to take place right before its use. Therefore, storage
and distribution issues of the biofuels will arise, putting up the
costs of the final product. Secondly, bioethanol has several
problems, in terms of physical properties, entailing the automobile engines needing to be modified to be run on a blend
containing more than 5 (E5, Europe) to 10% (E10, North
America) bioethanol according to the respective EN228 and
ASTM D5798 regulations. Bioethanol also has low energy
density (over 30% less energy than gasoline, although it can be
partly compensated by its better efficiency, reducing it to about
10–12%),50 low flame luminosity and lower vapor pressure than
gasoline (making cold starts difficult). It is also corrosive,
miscible with water and relatively toxic to ecosystems.50
Biobutanol. Biobutanol (also denoted as biogasoline) is
another interesting candidate that recently entered the battle of
the alcohols and has the potential to become one of the key
biofuels of the future due to its interesting properties.53–55
The biobutanol is produced via fermentation in which the
sugars from the source (so far from edible feedstocks) are firstly
converted to butyrate and hydrogen, then turned into butanol via
fermentation using various bacteria strains.53 The process has
been reported to work with a wide range of bacteria and
biomass.53–56 Four main species have been in use, namely C.
acetobutylicum, C. beijerinckii, C. saccharoperbutylacetonicum
and C. saccharobutylicum. Most data are available from C.
acetobutylicum that has been widely employed in the fermentation of starchy raw materials.53
Dupont and BP announced a partnership in 2006 to develop
the next generation of biofuels, with biobutanol as first
product.55,57 A biobutanol demonstration plant has recently
started to be built at an existing BP site in the UK that is expected
to start test production of biobutanol by 2009 using sugar beet as
feedstock.57 Similar biobutanol pilot plant projects are also
ongoing in the US.58
Properties and advantages/disadvantages of biobutanol.
Biobutanol offers a number of potentially significant advantages.
It is believed to have around 85 percent of the energy content of
gasoline (compared a less than 70% of bioethanol) and the
existing engines can run on it without modification (compared to
the maximum 85% for bioethanol), one of the reasons why
biobutanol is also known as biogasoline. Biobutanol has low
vapor pressure therefore it is safe to handle and it would not
affect RVP.55 It is also less corrosive and hygroscopic compared
to ethanol and therefore less prone to water contamination
allowing the possibility of blending within the biorefinery,
allowing its implementation in the current infrastructure (tanks,
This journal is ª The Royal Society of Chemistry 2008
pipes, filling stations). Another key feature of biobutanol is that
one of its derivatives, dibutyl ether, may have the potential to be
employed as biofuel.53 However, butanol also has some disadvantages. It has low RVP and a low flash point, therefore it is not
very safe.54
Biofuel produced by thermo-chemical conversion
Biofuels from fast pyrolysis
Vegetable oils can be employed as fuels for diesel engines, but
their viscosities need to be reduced in order to allow their use in
such engines without substantial modifications. Different methodologies have been developed to reduce such viscosities
including dilution, microemulsification and pyrolysis as well as
the already discussed transesterification and catalytic cracking.
Fast pyrolysis involves the breaking down of high molecular
weight compounds to smaller molecules by heating at moderate
to high temperatures (300–500 C).59 The vapors generated in the
pyrolysis of the oil are condensed and normally two liquid
fractions (aqueous and organic) are obtained. The fractions are
separated and the organic fraction, containing a wide range of
hydrocarbons, is usually analysed and sometimes catalytically
upgraded59,60 prior to its use as biofuel. A variety of vegetable oils
(e.g. sunflower, castor, soybean, palm) have been employed in
the process.59,60
Properties and advantages/disadvantages of biofuels from fast
pyrolysis. Fast pyrolysis of vegetable oils for the production of
biofuels is a simple, efficient and inexpensive technology to
produce biofuels. Compared to transesterification, pyrolysis has
many advantages as the liquid fuel produced in this process has
similar chemical components to conventional petroleum diesel
fuel and can be used without major modifications as alternative
engine fuels. The production costs (due to the expensive raw
materials employed, the biofuels obtained are 50 to 100% more
expensive than conventional fuel!61), the corrosive nature of the
fuels obtained and the variety of product compositions that is
highly dependent on temperature and/or catalyst content (for the
particular case of biofuel upgrading) are important issues related
to the potential commercialisation of such biofuels.
Prospects for 1st generation biofuels
Various interesting conclusions can be drawn from the use of
first generation biofuels. Biodiesel, bioethanol and specially
biobutanol have many advantages as petrol-fuel replacements
but also important disadvantages.
Despite the discussed drawbacks associated to each methodology, the main concern related to the production of first
generation biofuels comes from the fact that the conventional
biofuel production process generally involves the use of ‘food’
crops. This issue has generated much controversy in a world
where the limited area of arable land and grain reserves will
contribute to skyrocket the food prices if we carry on using such
food crops extensively for biofuel production. That and other
issues that arise related to deforestation, global warming and
threats to biodiversity, in particular in developing countries (e.g.
Malaysia as a consequence of the demand of edible oils [e.g. palm
This journal is ª The Royal Society of Chemistry 2008
Fig. 8 Road map of potential development pathways for first and
second generation biofuels, including implications for different markets.
Source: REFUEL. http://www.refuel.eu/fileadmin/refuel/user/docs/
REFUEL_final_road_map.pdf. Reproduced with permission of Marc
Londo.
oil]) encouraged the search for alternative technologies and
feedstocks for biofuels production.
Second generation biofuels
Alternative feedstocks, generally non-edible feedstocks including
waste vegetable oils and fats, non-food crops and biomass
sources, and/or technologies are starting to be developed in an
attempt to overcome the major shortcomings of the production
of first generation biofuels. The biofuels obtained from such
technologies have been denoted as second generation biofuels.62
In theory, these can solve these problems and can supply a larger
proportion of fuel supply in a more sustainable and reasonably
priced way with greater environmental benefits (Fig. 8).
Several advances have been made in the last few years/months.
The majority of the second generation biofuels processing
technologies are not yet available on a fully commercial scale so
the biofuels are expected to enter the market within a few years.
Moreover, the development of many other approaches are
currently ongoing and many more are to be reported, so the list
included below, far from being exhaustive, provides the most
interesting technologies reported until very recently. Second
generation biofuels will be classified in various groups depending
on the technologies employed for their preparation. In a similar
way to those of the first generation biofuels, these are prepared
by chemical, biological, and thermo-chemical conversion.
Biofuels prepared by chemical conversion
Biodiesel from non-edible feedstocks (via transesterification)
Biodiesel from non-food crops. A second type of feedstock
becoming relevant for the production of biodiesel is the so-called
non-edible raw materials including non-food crops and waste
oils and fats. Non-food crops, generally not suitable for
human consumption or animal feed, have comparable or even
higher oil yields (27–40% w/w) and lower resource consumption
(i.e. cultivation inputs) than conventional ‘food’ crops,63 making
then specially suitable for a more sustainable biodiesel production, in terms of a more efficient use of resources, minimal
Energy Environ. Sci., 2008, 1, 542–564 | 549
interaction with food crops and expected lower environmental
impact.64
Examples of these crops including Brassica carinata65,66 and
Jatropha curcas64,66,67 for the preparation of biodiesel have
recently been reported. Jatropha is a particularly good example
of a non-food crop for biofuel production since it thrives on poor
soil and land unsuitable for food crops, actually creating topsoil,
and gives a high oil yield.
The preparation of biodiesel from non-food crops is very
similar to the chemical transformations (transesterifications)
previously described for the use of traditional vegetable oils from
food crops.
In the summer of 2007, Brazil opened its first commercial
Jatropha biodiesel facility (Compahnhia Productora de Biodiesel
de Tocantins) with an estimated production of 40 000 tonnes
biodiesel y1 by 2008.67c Some other Brassica and Jatropha
projects including pilot plants in India, Africa and South
America are also ongoing.
Biodiesel from used vegetable oils (UVO) and fats. UVO and
animal fats are also being considered as very attractive feedstocks
for the production of biodiesel due to their lower market value
compared to virgin oils and their fact of being recycled materials
from other industrial sectors.68 The processing of the oil often
requires a reduction of the high FFA content via acid catalysed
esterification before the actual raw material can be transesterified
to biodiesel.67b
Kulkarni et al. have recently reported the use of a heterogeneous solid acid catalyst that is able to carry out a simultaneous
esterification of the free fatty acids and transesterification of the
triglycerides, giving high FAME yields.69 Efficient and low
energy intentive protocols of the production of biodiesel from
waste oils and animal fats combining lipases with alkali catalysts
have also been reported.70
Biodiesel from microbial oil (via transesterification)
Biodiesel from algae oil. Research is currently ongoing into the
production of biodiesel from microalgae, which are believe to
afford greater oil yields than any known feedstock as has been
recently reported.71
Microalgae are sunlight-driven cell organisms that convert
atmospheric CO2 (via photosynthesis) into a plethora of chemicals including methane, hydrogen, polysaccharides and oil.71,72
The production of microalgal oil is remarkably more efficient
compared to conventional oil crops, providing higher oil yields
(up to a 75% dry weight) and lower land area utilisation (Tables 1
and 2).
Table 2 Comparison of oil yield vs. required land for different biodiesel
feedstocks in the US71
Crop
Oil yield/L ha1
Required
land/M ha1a
Microalgaeb
Microalgaec
Oil palm
Jatropha
Canola
Soybean
Corn
136 900
58 700
5950
1892
1190
446
172
2
4.5
45
140
223
594
1540
a
To meet 50% of all US current transport consumption. b 70% (w/w) oil
yield in biomass. c 30% (w/w) oil yield in biomass.
The process involves the extraction of the oil from microalgae
and subsequent transesterification with alcohols using homogeneous or heterogeneous catalysts (in a similar way to that of
biodiesel obtained from (non) edible feedstocks) to give
biodiesel.
Significant advances in the field have recently been reported
with biodiesel from microbial oil. Cellana, a joint venture of Shell
and HR Biopetroleum started the construction of a pilot facility
in the Kona coast of Hawaii Island to grow algae as biofuel
feedstock.73
Biodiesel from other microbial oils. Many reports can be found
on the subject using different microbes including various yeasts
and bacteria.74 A summary of the main reported microorganisms
and their respective oil yields has been included in Table 3.
In general, the cultivation of such microorganisms is not
dependent on seasons or climate. They can also be easily grown
on a variety of inexpensive substrates including waste residues
from agriculture and industry,74b providing they have the nutrients needed for the microorganisms.
Properties and advantages/disadvantages of biodiesel from nonedible feedstocks. Second generation biodiesel has very similar
properties to those of first generation biodiesel, without significant differences between edible and non-edible feedstocks. A
reduction in the NOx emissions of the biodiesel obtained from
used oil has also been reported,75 although some authors highlight the larger fumes production with the use of UVO as feedstock.76 Most importantly, the use of cheap feedstocks (e.g. waste
oils and fats) and the potential commercialisation of glycerol
Table 3 Oil production (oil content and yield) of different microorganisms grown on various carbon sources74
Table 1 Microbial oil content (% dry weight) of various algae species71
Microalgae
Oil content
(% dry wt)
Botryococcus braunii
Chlorella sp.
Cylindrotheca sp.
Nannochloropsis
Nitzschia sp.
Schizochytrium sp.
25–75
28–32
16–37
31–68
45–47
50–77
550 | Energy Environ. Sci., 2008, 1, 542–564
Carbon
Microorganism source
Trichosporon
fermentans
Lipomyces
starkeyi
Mortierella
isabellina
Cunningamella
echinilata
Oil content Oil yield/
Biomass/g L1 (%)
Ref.
g L1
Molasses 36.4
35.3
12.8
74a
Sewage
sludge
Starch
Pectin
Starch
Pectin
9.4
68.0
6.4
74b
10.4
8.4
13.5
4.1
36.0
24.0
28.0
10.0
3.7
2.0
3.8
0.4
74c
74c
74c
74c
This journal is ª The Royal Society of Chemistry 2008
(and glycerol derived products) considerably reduces the
biodiesel production costs, especially taking into account that
70–90% of the biodiesel cost arises from the cost of the oil.77
However, the use of high temperatures in the transesterification,
incomplete conversion and variability of the incoming feedstock
(with marked differences in water content and FFA depending
on the source, location and usage) are problems related to such
feedstocks for biodiesel production.
The production of methyl esters from algal oil has recently
attracted a great deal of attention. The enormous diversity of
species of algae with high oil content that require tiny land
utilisation compared to oil crops offers an interesting possibility
of industrial exploitation of such organisms in the production of
biodiesel.
However, there are major limitations in their successful
implementation, the economic feasibility of the technology being
the most important. Firstly, the recovery of such bio-oil from
algae is a very challenging task. The algal broth produced in the
biomass production generally needs to be further processed to
recover the biomass78 and then the concentrated biomass paste is
extracted with an organic solvent (e.g. hexane) to recover the
algal oil that can be transesterified into biodiesel.
Secondly, microalgal oil is rich in long-chain polyunsaturated
acids including eicosapentaenoic (20:5 n-3, EPA) and docosahexaenoic acids (22:6 u-3, DHA) which are generally undesirable
in conventional biodiesel due to the negative impact of the
polyunsaturations on oxidation stability. The presence of EPA
and DHA is not contemplated in the EU (EN 14214 and EN
14213, biodiesel for transport and heating) and US (ASTM
D6751) quality biodiesel standards that specify a limit of 130 g
(EN 14213) and 120 g (EN 14214) iodiene per 100 g biodiesel
(iodine value). The storage issues arising from the oxidation
instability may either be overcome through chemical transformations (e.g. hydrogenations) of the polyunsaturated
compounds.79 It is yet unclear how the presence of much more
saturated FAME will affect cold performance (CFPP) of the
biodiesel
These main drawbacks will undoubtedly put up the costs of an
already costly process in which problems related to capital
infrastructure costs, contamination through open pond systems
and costs associated with harvesting, drying and valorisation of
the rest of the algae may also have a major contribution. A full
and precise estimation of the economics of the process, that have
been argued to be far too good from what Chisti et al.71 originally
reported, is needed in order to demonstrate its feasibility.72,78
and fats, cellulosic feedstocks, urban and municipal waste have
been reported.46,80,84
Properties and advantages/disadvantages of biofuels for catalytic cracking. As mentioned for the first generation biofuels
from catalytic cracking, the technologies are relatively simple
and inexpensive. An important advantage is that the raw materials used are also inexpensive (e.g. waste oils, cellulosic biomass)
as compared to first generation feedstocks. However, the efficiency of the process using crude feedstocks (with variable
compositions) and biomass needs to be addressed.
Biofuels produced by biological conversion
Bioalcohols
Bioethanol. Second generation bioethanol is usually produced
from a range of alternative readily abundant and inexpensive,
cellulosic biomass feedstocks including woody biomass, grasses,
forestry and agricultural waste.85 Very interesting reviews about
the progress in bioethanol and lignocellulosic processing have
been recently reported.50,86
An overview of the production routes of second generation
bioethanol is included in Fig. 9.
The process (Fig. 9, right hand side) is identical to that
described in the production of first generation bioethanol:
decomposition of the material into fermentable sugars (hydrolysis) and transformation of the sugars into bioethanol
(fermentation).
The main changes are the processing technologies and the
feedstocks that usually account for the majority of the plant cost.
Cellulosic biomass comprises of two main components; cellulose
and hemicellulose (complex carbohydrate polymers), accounting
roughly for about a 70–75 wt% of the lignocellulose. A mixture
of enzymes (cellulases and hemicellases) different from those of
the first generation bioethanol production are employed in the
hydrolysis step. In the particular case of lignocellulosic (woody)
materials, lignin is obtained as a by-product of the process.
Lignin can be burned to produce heat and power for the
processing plant and potentially for surrounding homes and
businesses and it is hoped that it can become a future source of
aromatic chemicals and materials. Alternative organisms also
need to be employed due to the impossibility of the traditional
yeast and bacteria to process the pentose (C5) sugars derived
from hemicellulose.87
Biofuels from catalytic cracking
Several feedstocks and catalysts have been employed for the
preparation of biofuels46,80–83 in the so-called biomass-to-liquid
(BTL) process. Corma et al. and Dumesic et al. have recently
reviewed the technologies and feedstocks for the catalytic
cracking of biomass to produce fuels and chemicals.80
Transition metal catalysts, common materials in the hydroprocessing industry, under high hydrogen partial pressures yield
diesel-type biofuels46 while molecular sieves (e.g. zeolites and
mesoporous materials from the M41S family including MCM-41
and MCM-48 patented by Mobil) afford gasoline-like fuel with
higher aromatic content.80,83 Feedstocks including TG, waste oils
This journal is ª The Royal Society of Chemistry 2008
Fig. 9 Thermochemical and biological routes to second generation
bioethanol (source: Sustainable biofuels: prospects and challenges, RS
Policy document 01/08, ISBN 978 0 85403 662 2. Reproduced with
permission of The Royal Society).
Energy Environ. Sci., 2008, 1, 542–564 | 551
Branched alcohol mixtures. The preparation of a branchedlonger chain alcohol mixture with a potential use as biofuel has
been recently reported by Liao et al..88 Such an alcohol mixture
with high isobutanol content is produced via synthetic nonfermentative pathways employing metabolic engineered bacteria
(e.g. E. coli) and glucose as carbon source. This strategy diverts
the 2-ketoacid intermediates in the aminoacid biosynthetic
pathway of E. coli for alcohol synthesis, converting them into
aldehydes (by 2-ketoacid decarboxylases) and then to alcohols
(by alcohol dehydrogenases).88 The process has already been
licensed to Gevo (spin-off company from Pasadena, US) that
hopes to begin commercial scale production within a few years.89
Properties and advantages/disadvantages of bioalcohols. Second
generation bioethanol offers several improvements with respect
to first generation bioethanol. It can reduce GHG emissions in
around 90% compared with fossil fuels,50 providing greater
savings than those obtained with first generation bioethanol
(over 60% reduction). However, the development of an efficient
pre-treatment process in order to break up the fibre structure of
the biomass is needed because the methodologies investigated
(mechanical, thermal, chemical, enzymatic-cellulase and various
combinations) have been proven to be unsuitable due their high
costs, low yields, produced waste or undesired by-products.
Secondly, an efficient microorganism for the fermentation of
pentoses, present in hemicellulose, needs to be developed.
Therefore, there is a need for a joint effort from chemists,
microbiologists and chemical and biochemical engineers in
order to demonstrate the potential of the second generation
bioethanol.
Branched-mixed alcohols have interesting properties as biofuel
due to their higher energy density and lower hygroscopicity as
well as higher octane numbers compared to their linear chain
counterparts (e.g. bioethanol). The reported strategy also opens
up many interesting possibilities for biofuels production as more
user-friendly microorganisms (e.g. Saccharomyces cerevisiae) are
readily applicable.88 Furthermore, it enables the exploration of
biofuels beyond those naturally produced in microbial fermentation (e.g. in the production of bioethanol).
Biogas
Biogas is an environmentally friendly, clean, cheap and versatile
fuel, composed of a mixture of CH4 and CO2 that is usually
generated from bacterial digestion of biomass in the absence of
air between 10–72 C.90,91 Almost any type of organic matter (e.g.
sewage sludge, animal wastes, industrial effluents) is suitable for
the production of biogas, which can be directly utilised in
cooking and heating systems.92 The process is carried out in
anaerobic digesters that can vary in size from 1 m3 (small
household unit) to as large as 2000 m3,93 involving a step-wise
series of reactions that require the cooperative action of several
microorganisms. Initially, a group of microorganisms (acidogens) break down the organic matter into a digestible form
(usually simpler fatty acids) that can be utilised by methanegenerating anaerobic bacteria (methanogens) that produce
biogas as a metabolic by-product.94
The use of biogas as transport fuel has been explored in its
application in explosion engines. Biogas has shown a great
552 | Energy Environ. Sci., 2008, 1, 542–564
potential for its uses in Brazil,95 and in places such as Sweden it
has been used in urban buses since 2004.96 Also in Sweden, some
studies have evaluated the economic and environmental feasibility of biogas as a renewable source of energy in large scale
showing positive results in its applicability CHP (centralised heat
and power).97
Properties and advantages/disadvantages of biogas. Biogas
might be of relevance in renewable energy markets both for
transport and for generation of electricity. It is also a realistic
alternative to the accumulation of waste in landfills as new sites
can be specially configured to optimise gas output (as high as
1000 m3 h1 biogas). However, LCA studies have identified an
impact to GHG in its production, associated to the generation
and emission of CO2 and N2O in the process.97
Biohydrogen
Various authors have recently reviewed the prospects and
potential in the production of biohydrogen.98–100 Biohydrogen
can be produced by three different biological pathways:
fermentation and direct or indirect (bio)photolysis.
Fermentation. Dark and photo fermentation are technologies
under development (currently at lab scale) to produce biohydrogen from wet biomass (e.g. molasses, organic wastes,
sewage sludge) using (an)aerobic hydrogen fermenting
bacteria.100,101 The advantage of dark fermentation is that the
biohydrogen is produced directly without formation of methane.98c,100 During dark fermentation, various organic acids are
also produced. These compounds can subsequently be converted
to hydrogen by a process denoted as photo fermentation.
Direct photolysis. In this approach, the process takes advantage of the photosynthetic capability of algae and cyanobacteria
to split water into O2 and H2 via direct absorption of light
and transfer of electrons to two groups of enzymes that participate in biological hydrogen metabolism: hydrogenases and
nitrogenases.99
Indirect photolysis. Alternatively, biohydrogen can be
prepared through the use of some microorganisms (algae) that
can directly produce hydrogen under certain conditions.98,99
Most specifically, sealed cultures of green algae become anaerobic in the light under deprivation of sulfur nutrients and
spontaneously induce the ‘‘hydrogenase pathway’’ to photosynthetically produce hydrogen.98c Substantial rates of hydrogen
production were obtained over 60 h in the light although the
hydrogen production leveled off reaching a point (after 100 h) in
which the algae go back to the normal photosynthetic pathway in
order to restore the consumption of internal starch and proteins
that takes place in the course of the hydrogen production.102
Properties and advantages/disadvantages of biohydrogen.
Biohydrogen is believed to be one of the biofuels for the future,
combining its ability to potentially reduce the dependence of
foreign oil and contribute to lower the GHG emissions from the
transportation sector. However, storage (biohydrogen has to
be compressed, liquefied, or stored in metal hydrides),
This journal is ª The Royal Society of Chemistry 2008
transportation and use (fuel cell vehicles are not commercially
available yet and a distribution infrastructure for hydrogen
cannot be realised in the short term) as well as the technological
advances needed for its successful implementation limit
biohydrogen only as a longer-term option for the transport
sector.
Biofuels produced by thermo-(chemical) conversion
Biofuels included under this headline are also prepared from
various non-edible biomass feedstocks. Thermo-chemical
conversion pathways include processes such as gasification and
pyrolysis (Fig. 10).80,90
Biofuels from gasification
The process involves the partial combustion of the feedstock to
produce syngas (a mixture of carbon monoxide (CO) and
hydrogen (H2) denoted as bio-synthetic natural gas, bio-SNG)
via conventional or alternative gasification processes. Then,
bio-SNG is subsequently transformed into liquid hydrocarbons
(mostly diesel and kerosene-type fuels) and/or gases via different
processes, leading to a variety of biofuels that will be outlined.
Such prospective liquid/gas biofuels for transport (Fig. 10)
include bioalcohols (methanol, ethanol and linear higher chain
alcohol mixtures) and synthetic biofuels.
At this point, it is worth mentioning that although bio-SNG
could be classified as synthetic biofuel, it comes first since all the
reported biofuels from gasification are obtained from it and thus
the technologies (up to the preparation of the syngas) are very
similar.
Bio-SNG. Bio-SNG can be produced by a conventional gasification process (methanation) at high temperatures (800–
1000 C) aiming at producing large quantities of methane. The
current technology employed allows the use of a wide range of
biomass feedstocks including wood chips and waste wood.103,104
The conventional gasification process involves various steps
(Scheme 4).104b Firstly, the biomass undergoes endothermal
steam gasification (reaction 1) to give a mixture of CO and H2,
which is subsequently converted into methane, CO2 and
hydrogen (reactions 2 and 3). The net overall reaction from
biomass to methane and CO2 (reaction 4) is slightly exothermic.
Fig. 10 Biomass gasification and pyrolysis routes to synthetic biofuels
(source: Sustainable biofuels: prospects and challenges, RS Policy
document 01/08, ISBN 978 0 85403 662 2. Reproduced with permission of
The Royal Society).
This journal is ª The Royal Society of Chemistry 2008
Scheme 4 Reactions involved in the conventional gasification of
biomass.
However, the main drawback of the conventional gasification
technology is the formation of tars and char.61
Interestingly, the gasification of biomass can be performed at
lower temperatures (250–400 C) in supercritical water. It has
currently been reported at lab scale, employing different Ni and
Co based catalysts.104 In this process, the biomass disintegrates in
supercritical water forming a mixture of carbon dioxide, carbon
monoxide and methane (SNG). The technology is expected to be
especially suitable for wet (polluted) biomass and has higher
efficiency than the conventional gasification process at a lower
temperature.104b Bio-SNG can also be produced from biogas.
Various projects in the Netherlands, including the largest
existing bio-SNG plant located in Buggenum, currently produce
bio-SNG from the co-gasification of biomass with coal at
different proportions.105
Properties and advantages/disadvantages of bio-SNG.
Bio-SNG has various advantages and disadvantages. Its octane
number is very high, but the cetane number is very low, which
means that bio-SNG has to be used in spark ignition engines,
which have to be adapted for the use of bio-SNG. Storage is also
an issue for bio-SNG as it is also a gas at room temperature so it
needs to be compressed or liquefied to be used as an automotive
fuel. Furthermore, larger storage and fuel tanks are needed due
to the lower volumetric energy content of the fuel.
The supercritical water low-temperature gasification technology may overcome the main technological barriers in the
process. Still, gas cleaning (especially tar removal) and catalyst
development are important technological issues, although if
active and selective catalysts are used (e.g. Ru/C), no significant
quantities of tars or char are formed.104b However, the cost of the
supercritical water production of bio-SNG is several times higher
than that of the conventional gasification process.106
Bioalcohols
Biomethanol. Biomethanol can be produced from synthesis
gas106 via conventional gasification of biomass (partial oxidation)
at high temperatures (800–1000 C) and subsequent catalytic
synthesis of the CO + H2—in a 1 : 2 ratio—under high pressures
(4–10 MPa).106,107
The biofuel can be blended with petrol up to 10–20% without
the need of any engine modifications.107 Several feedstocks
including bark, woodchips, bamboo, waste wood and pulp106,107
and even glycerol108 have been reported to be used in the process.
There are a few biomethanol pilot plants under development,
mainly in the US (e.g. North Shore Energy Technologies, 40
MMgy plant) and Japan (e.g. Norin Green no1, MAFF and
Mitsubishi heavy industries).107a
Energy Environ. Sci., 2008, 1, 542–564 | 553
Bioethanol. Bioethanol can also be obtained via conventional
thermal gasification of biomass to syngas combined with catalytic processes in a similar way to those for the production of
biomethanol (Fig. 9, left side).3
Alternatively, following biomass gasification, the syngas can
be directly fermented to ethanol using anaerobic bacteria
(Fig. 9).109 This eliminates the need of the hydrolysis step to
break up the cellulose and hemicellulose fractions of the biomass.
The lignin fraction can also be converted into ethanol. The
process has been reported at lab scale and is still under development.3,109 However, the efficient delivery of the syngas to the
microorganisms still remains a challenge.3
There are some examples of ongoing industrial processes. An
operating lignocellulosic bioethanol production plant is located
in Ottawa (Canada), run by the IOGEN Corporation.110 The
demonstration plant produces up to 3 million litres of bioethanol
per year. The feedstocks employed are wheat, oat and barley
straw. A bioethanol plant in Ulmea (Sweden) is running using
waste stream of cellulose-based materials and another pilot
plant production for the preparation of bioethanol from lignocellulosic materials (e.g. Norway Spruce) has recently started
production.111
Linear bioalcohol mixtures. Mixed linear alcohols (i.e.,
mixtures of mostly ethanol, propanol and butanol, with some
pentanol, hexanol, heptanol and octanol) can also be produced
from syngas in a similar way to that described for methanol
and ethanol.112 One of these linear alcohol mixtures denoted as
Ecalene is currently registered with the US Environmental
Protection Agency per 40 CFR 79.23 as a fuel blending
additive.112
Properties and advantages/disadvantages of bioalcohols.
Biomethanol is considerably easier to recover than bioethanol as
it does not form an azeotrope with water like ethanol. The
technology to prepare biomethanol from syngas has been
reported to be in place, requiring efficiency improvements and
scale up to make methanol an economically viable transportation
fuel.113 However, biomethanol suffers from severe shortcomings
including a low vapour pressure, low volumetric energy density
(about half of that of petrol), toxicity (methanol is highly
poisonous), high flammability and the incompatibility with
engines except at relatively low biomethanol content.
In terms of properties, bioethanol obtained from gasification
of biomass does not have significant differences compared to that
obtained by biological conversion. However, the processes are
remarkably dissimilar. The conventional gasification step is
a costly process compared to the relatively inexpensive biological
conversion. Nevertheless, the biomass feedstock can effectively
be turned into syngas (without the need of any microorganisms)
and subsequently into bioethanol in a simple catalytic reaction
similar to that of biomethanol.3 Compared to bioethanol
produced by biological conversion, this protocol avoids issues
such as the inefficient degradation of biomass to fermentable
sugars as well as dealing with the processing of the pentoses (C5)
generated in the hydrolysis of biomass.
The mixture of linear alcohols has superior energy content
compared to biomethanol or bioethanol, lower heat of vaporisation (essential for cold starts) and increasing compatibility of
554 | Energy Environ. Sci., 2008, 1, 542–564
the ethanol-gasoline in a blend due to an improved water tolerance and a decrease in gas emissions. The main problems are the
costly gasification technology as well as the lack of standards for
producers and users.
Synthetic biofuels. Synthetic biofuels can be defined as fuels
prepared from syngas via different processes. Bridgwater and
Demirbas have recently reported comprehensive overviews of
the development of these technologies for the preparation of
biofuels.61,106
Under this headline we can include a selection of some interesting options such as bio-dimethylether (bio-DME), biofuels
obtained by steam reforming, biofuels obtained via HydroThermal Upgrading (HTU) and biofuels produced via Fischer–
Tropsch Synthesis (FTS).
Bio-DME. DME derived from fossil fuels was mainly
employed in the past as a substitute propellant for chlorofluorocarbons (CFCs) in spray cans.114 DME is a liquefied gas that
can be produced from a variety of feedstocks including residual
oils, organic waste and biomass.114
Bio-DME can be directly produced from syngas in a process
that is very similar to that of biomethanol.3 Alternatively, it can
be produced from biomethanol via catalytic dehydration using
slightly acidic catalysts including g-Al2O3115 and ion exchange
resins116 at temperatures ranging from 100–400 C depending on
the methodology employed.3,115,116
Biofuels obtained by steam reforming. Steam reforming can be
applied to various solid waste materials including organic waste,
sewage sludge, waste oils, black liquor and agricultural waste to
produce biofuels.106 Steam reforming of natural gas (often
referred as steam CH4 reforming) is the most common method to
produce commercial H2.106
Biohydrogen can therefore be produced from a biomass
feedstock via conventional gasification at high temperatures to
syngas to obtain methane (reaction 4, Scheme 4) and subsequent
steam CH4 reforming at high temperatures (700–1100 C) using
Ni supported catalysts (e.g. Ni/Al2O3, Ni/MgO) at 3–25 bar
pressure (Scheme 5, reaction 1).106,117 For the production of high
purity H2, the reforming of the biofuel that includes multiple
catalytic steps is followed by two water gas-shift (WGS) reaction
steps (Scheme 5, reaction 2), a final CO purification and removal
of the remaining CO2 by pressure swing adsorption or ceramic
membrane separation.106,117
Alternatively, the gasification step of biohydrogen can also be
performed in supercritical water (in a similar way to that of the
bio-SNG) with the advantages of the direct use of wet biomass
without drying and a high gasification efficiency at lower
temperature.106,118 However, the cost of H2 production using this
technology is several times higher than the current price of H2
obtained from steam reforming.106
Scheme 5 Steam CH4 reforming (1) and WGS (2) reactions for the
preparation of biohydrogen.
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Biofuels obtained via HydroThermal Upgrading (HTU).
HTU-diesel can be produced from various feedstocks including
dry (wood and lignocellulose)119 and wet (beet pulp, sludge or
bagasse) biomass.120,121 This methodology involves the hydrothermal treatment of biomass that is converted into a mixture
of hydrocarbons at relatively low temperatures (250–350 C)
and moderate (autogenous pressure)119 to high (120–180 bar)
pressures.120
The biocrude obtained is a heavy organic liquid immiscible
with water that contains a wide range of hydrocarbons including
acids (e.g. acetic acid), alcohols (e.g. isopropyl alcohol) and
phenolic derivatives (in the particular case of lignocellulosic
materials).119–121 Often, the obtained hydrocarbon mixture needs
further processing (via catalytic hydro-de-oxygenation [HDO])
to yield a liquid biofuel similar to fossil diesel that can be blended
with fossil diesel in any proportion without the necessity of
engine or infrastructure modifications.121 HTU research has been
mainly performed in The Netherlands, with an HTU demonstration plan in Amsterdam that is able to generate over 12 000
tonnes of biocrude (including ash) per year.120
Biofuels obtained via Fischer–Tropsch Synthesis (FTS). The
Fischer–Tropsch (FT) process is one of the advanced biofuels
conversion technologies. It has been known since 1923 when
German scientists Franz Fisher and Hans Tropsch aimed to
synthesize long-chain hydrocarbons from a CO and H2 gas
mixture, but it was mainly used in the past for the production of
liquid fuels from coal or natural gas.122
Prior to the FTS, the gasification of biomass feedstocks takes
place in a similar way as that described for the production of bioSNG, to obtain syngas (Scheme 4). Then, a cleaning and
conditioning step of the produced syngas is normally performed
to remove all the impurities present prior to the catalytic reaction
to minimise the poisoning of the catalyst.123
The FTS process is then carried out. It comprises of various
steps described by the set of equations in Scheme 6, where x is the
average length of the hydrocarbon chain and y is the number of
H2 atoms per carbon.
The first step involves the reaction of CO with H2 in the
presence of a Co or Fe catalyst (Scheme 5, top reaction) to afford
a hydrocarbon chain extension (–CH2–) that is a building block
for the formation of longer hydrocarbons. Typical operation
conditions are temperatures between 200–400 C and 15–40 bar
pressures, depending on the process.106,122
All reactions are exothermic and the product is a sulfur free
mixture of different predominantly linear hydrocarbons
(primarily alkanes and alkenes) that frequently undergoes
upgrade and refining steps to be turned into automotive fuels,
namely FT-diesel (main product) and gasoline-like biofuels
(by-products).106 The FT catalysts are mainly based on iron and
cobalt.106,122,124 Cobalt catalysts have the advantage of a higher
conversion rate and they are also more reactive in hydrogenation, producing less unsaturated hydrocarbons and alcohols
compared to iron catalysts that produce higher alkenes and
oxygenates content.106,122
The process using biomass as feedstock is currently under
development. In theory, there are no restrictions in the type of
biomass that can be used as feedstock. Woody and grassy
materials and agricultural and forestry residues have been
investigated in the process.123,125
Pilot production facilities for Fischer–Tropsch liquids from
biomass are currently in operation in Germany and the Netherlands.105
Properties and advantages/disadvantages of synthetic biofuels.
Synthetic biofuels have several advantages as they can be used in
unmodified diesel engines (with the exception of bio-DME)105,106
and they are cleaner that traditional fuels due to the removal of
all contaminants to avoid the poisoning of the catalysts used in
the processing steps.3,123
Synthetic biofuels can have excellent autoignition characteristics (except bio-DME that has a low-self-ignition temperature
and lower volumetric energy content) as they have similar energy
content, density and viscosity than of fossil diesel as well as
a higher cetane number and lower aromatic content (which
results in lower particle emissions). They are also S-, N-free and
less corrosive than other biofuels (e.g. bioethanol and biodiesel)
therefore being more environmentally friendly than fuels
produced from crude oil. Some of them (e.g. FT-diesel) have
been proved to reduce the CO, NOx and particulate matter
compared to diesel fuel.105,124
However, the production of synthetic biofuels faces a similar
technological barrier to that of the listed gasification-derived
biofuels (i.e. bioalcohols and other synthetic biofuels): the
production of the synthesis gas has to be adapted to the higher
reactivity and different properties of biomass with respect to
coal. This includes two key steps in the process that need thorough improvements: biomass pre-treatment (via torrefaction
and/or pyrolysis) to avoid the aggregation of the biomass particles that can plug the feeding lines and economically viable
inferior temperatures of gasification (e.g. via supercritical
water gasification) that have been reported to provide higher
efficiencies.
For instance, the FTS biofuel production can be more cost
effective reducing both the capital and the operating costs of the
plant,126 being the purification of the syngas the most expensive
section to take into account for costs in an FT plant. The
development of active and selective catalysts and the utilisation
of by-products including electricity, heat and steam are some
other inputs that need to be addressed.
Biofuels from fast pyrolysis
Scheme 6 FTS reactions for the production of linear long-chain
hydrocarbons.
This journal is ª The Royal Society of Chemistry 2008
Fast pyrolysis is an attractive alternative to gasification to
convert biomass into more valuable chemicals and fuels (Fig. 10).
This process involves the thermal decomposition of biomass in
extremely short periods of time to obtain a solid residue (char)
and hydrocarbon-like mixtures (bio-oils) of higher value-added
chemicals (e.g. carboxylic acids, ketones, aldehydes and
Energy Environ. Sci., 2008, 1, 542–564 | 555
aromatics). Fast pyrolysis usually is a pre-treatment step to
increase the energy density of biomass in further combustion or
gasification processing as the dark viscous liquid obtained after
cooling and condensation has a heating value about half of that
other conventional fuels.61,94a
A fast pyrolysis process involves several steps including feedstock drying (ideally leaving less than 10% water content to
minimise the water in the bio-oil) and grinding (to provide
sufficiently small particles to ensure a rapid reaction), thermal
decomposition, separation of char and collection of the final
product (bio-oil).
Almost any form of biomass can be considered for fast
pyrolysis. Wood forestry and agricultural waste have been
largely explored.61,94a Lignocelluloses, hemicellulose and other
types of biomass break down through thermal treatment in
absence of air. The conversion takes place at temperatures
between 450–550 C at high heating rates, (103–104 K s1),
followed by rapid condensation of the vapours, being solid char
and bio-oil (up to 80% yield) the final products of the process,
with a considerable water content.46,61 The heating rate, much
higher than in conventional pyrolysis, is essential in order to
optimise the process to the desired products, where lower values
of heating rate will generate a larger amount of tar than volatiles,
as it does in conventional pyrolysis.127
Alternatively, microwaves might be a realistic option for
optimised compositions of bio-oil, attempting to minimise
downstream upgrading processes. Other feedstocks have also
been studied, vegetable (waste) oils and algal biomass being the
most significant. The use of TG yields a bio-oil with relatively
similar properties to that of the feedstock at temperatures
between 300–500 C with portions of such liquids having similar
properties to that of diesel fuel.46,128 The bio-oil obtained is either
normally gasified or refined in conventional petrol refineries.
Algae have also a great potential. The bio-oil yields obtained
from algae are lower than those of terrestrial crops and a higher
tar yield is produced. The proportion of aromatic compounds is
also lower to that of wood and straw due to the absence of lignin
in algae.129 The studies made in microalgae showed a lower
oxygen content and a higher heating value than wood.130
Properties and advantages/disadvantages of biofuels from fast
pyrolysis. Fast pyrolysis for the production of biofuels has some
advantages. In general, it is a simple and inexpensive technology.
It also has the potential to build up an infrastructure of multiple
biomass pre-treatment plants where the bio-oil could supply
feedstock for gasification and combustion processes.131 In this
way, transport costs could be reduced up to 87%.3,131
Some of its limitations are the mentioned corrosive nature of
bio-oil due to its acidity, the presence of impurities such as salts
and solids (especially in marine biomass) plus the generation of
aromatic toxins. The high acidity, water content and variable
viscosity of the mixture is a limitation for its stability, complicating its processing and direct use.81 The high oxygen content of
the mixtures considerably decreases density energy, which is 40%
less compared to fuel oil.46 In this regard, further downstream
processing is usually required to lower the oxygen content to
optimise and stabilise the composition of the bio-oil for higher
aptitudes as fuel. Processes such as hydro-treatment (to reduce
water and oxygen content)132 hydro-cracking133 (analogous to
556 | Energy Environ. Sci., 2008, 1, 542–564
the conventional refinery), emulsification with mineral diesel for
direct use134 and steam reforming135 (to produce hydrogen as in
natural gas) are some of the key technologies employed.
Economically, the cost of bio-oil is 10 to 100% more expensive
than fossil fuel61 and therefore the scaling-up of the technology
needs to reduce costs to turn the process economically viable.
An improvement of the product quality ensuring norms and
standards for producers and users (due to its current lack of
standards for use and distribution) is also required.
Prospects for 2nd generation biofuels
Production of biofuels from second generation biofuels from
non-edible feedstocks have interesting features. Non-food crops
can be cultivated in alternative lands including the so-called
‘‘wastelands’’, tropical zones or even arid regions as they are
more likely to proliferate at relatively extreme conditions (e.g.
plagues and dry environments) with a low fertiliser input. Many
of the biomass feedstocks are also self-seeding crops (they do not
need to be planted and re-seeded after being harvested) and
require little or virtually no fertilizer input. These approaches can
therefore be applied to ‘‘marginal lands’’ where the soil cannot/
should not support food crops.136 In this way, they will not
interfere with the land dedicated to food crops.
They can also provide a solution for the production of quality
biofuels in developing countries (e.g. India) where, for example,
a blend of biodiesel obtained from jatropha and palm has been
reported to have the right balance of physical properties
conferring the product with an adequate cold low performance
and oxidation stability,63 also falling within the acceptable limits
by the American and European biodiesel standards.67b
However, the switch to these non-edible feedstocks poses
various concerns. The cultivation patterns of the crops are still
under investigation and early studies have shown relative
differences depending on the approach taken to crop cultivation
and oil production management.137 The crops have only been
employed by local communities for different uses (e.g. soap
production and natural crop protection for the inedible nature of
the oil and toxicity of the seeds, respectively).138 Therefore, the
evaluation of the sustainability index needs more data to estimate
the real global impact of these crops. Another important issue in
the production of biofuels from non-food crops deals with their
cultivation in poor soils which imply lower productivities and
hence higher costs of production.
Furthermore, the technologies available for the majority of the
second generation biofuels are still in their infancy and need
major developments to be able to sustain a scaled-up production
of biofuels for transport. The economics of the processes may
play a key role in the successful implementation of many of these
technologies as we will discuss in the next chapter.
Discussion around biofuels
The biofuels field is expanding and evolving on a daily basis. We
are aware that some of the trends and areas of this perspective
may have changed by the time the readers are able to go through
it due to changing government policies, financial incentives,
technological breakthroughs, market prices as well as socioeconomic and environmental issues. However, we can say that
This journal is ª The Royal Society of Chemistry 2008
the future of biofuels is far from clear and involves complex and
dynamic social, economic and environmental issues. Rather than
giving a full technical study about biofuels issues, we aim to focus
on ongoing discussions around biofuels and their impact in all
aspects of our society.
Use of fossil fuel and raw materials to produce biofuels
The whole life-cycle of the production of biofuels involves the use
of fossil fuel and raw materials to some extent. Whether the net
gain balance out of the fossil input obtained in terms of low
emissions is positive or not remains under discussion. Few
comparative studies can be found for the production of ethanol
(from various crops including corn, sugarcane and cassava139,140)
and biodiesel (from oilseeds141) in different locations. For
instance, the production of bioethanol from corn was reported to
have half of the net value of energy recovered from the fossil
input compared to that of the ethanol obtained from sugarcane
and cassava. Sheehan et al. also found that the replacement of
petroleum diesel for neat biodiesel (B100) in buses reduced the
consumption of oil in the whole life cycle by 95%.141 Janulis
found a reduction of around 80–86% fossil usage and net GHG
emissions using biodiesel from rapeseed with respect to petroldiesel.142 However, these promising figures have been taken from
a limited number of studies in very specific locations and they
cannot be generalised.
Availability of land for fuel. The food vs. fuel issue. The available land to produce virgin crops is a major limitation for first
generation biofuels. For instance, based on the DEFRA estimations,140,143 the 1.73 Mha easily available land in the UK for
biofuel production (around 10% from total agriculture land) will
struggle to meet the 5.75% bioethanol/biodiesel blend EU target
(estimated in 1.75 Mha) set for 2010.144 This is also the case for
many countries worldwide.
Current biofuel producers do not always have a secure access
to raw materials due to limited grain reserves and the fact that the
current costs of crude vegetable oil from ‘food crops’ are variable. This has a major impact on the production of first generation biofuel since the price of the feedstock is the major input
cost (70–90% of the total operating cost).77 Biobased energy
industries are also currently in competition with food producers
and we perceive them as being a primary cause of the increase in
food prices.
In order to make biofuel production profitable and more
sustainable, avoiding as much as possible competition with the
food market, companies have to focus on second generation
biofuels made from alternative cheap feedstocks (e.g. (waste)biomass, waste oils and fats, residues, etc.). (Ligno)cellulosic
ethanol and biodiesel from either waste oils, non-food crops or
algae emerge as real alternatives to tackle this problem (Fig. 11).
The implementation of such alternatives might considerably
improve the figures. Extensive second generation biofuels
production can also intensify and change land use and consequently have environmental, economic and social impacts which
are not easy to predict.
However, the food vs. fuel issue is far more complex than has
generally been presented since agricultural and export policies
and the politics of food availability are factors of far greater
importance and biofuels are only ‘‘part of the problem’’ but not
‘‘the problem’’ itself. The argument should be analysed against
the background of the world’s (or an individual country’s or
region’s) real food situation of food supply and demand (everincreasing food surpluses in most industrialized and a number of
developing countries), the use of food as animal feed, the underutilized agricultural production potential, the increased potential
for agricultural productivity, and the advantages and disadvantages of producing biofuels.145
Environmental impact. Despite the fact that some studies
carried out to date show first generation biofuels may offer a low
carbon balance, fossil fuel usage and GHG balance, further
outputs and environmental indicators must be addressed. Water
usage (in the growth of the crops), eutrophication (run off of
lawn fertilisers into natural waters) and soil erosion are some of
them. Moreover, intensive monoculture farming may have an
impact on ecosystems and biodiversity,146 to a higher extent if
genetically modified crops are widely spread.136 Deforestation as
a consequence of the replacement of the tropical rainforest for
fuel-crops (e.g. the Malaysian case) will also have a major impact
Fig. 11 (A) First vs. second generation energy crop production cost/supply. Wood and grass refer to the potential on arable land. Wood’ and grass’
refer to potential of both arable and pasture land. (B) Distribution of biofuel production costs from lignocellulosic materials (V/GJ). Source: REFUEL.
http://www.refuel.eu/fileadmin/refuel/user/docs/REFUEL_final_road_map.pdf. Reproduced with permission of Marc Londo.
This journal is ª The Royal Society of Chemistry 2008
Energy Environ. Sci., 2008, 1, 542–564 | 557
Fig. 12 Aerial photographs of oil palm plantations (left photo, bottom)
encroaching on natural rainforest (left, right photos, top) and deforestation in Malaysia. Source: Rhett Butler, http://www.mongabay.com/.
in biodiversity and climate change (Fig. 12).147 Nevertheless, it is
difficult to predict the overall environmental impact of biofuels
consumption in the future at this stage of development of the
various biofuels technologies.
Second and potential third generation biofuels are more
attractive in terms of crop economy. The Green Chemistry Centre
of Excellence at the University of York (UK) has recently started
to coordinate a new project (SUSTOIL) funded by the European
Commission through Framework 7 aiming to develop advanced
biorefinery schemes to convert whole EU oil-rich crops into
energy (biofuels, power and heat), food and bio-products including
speciality and commodity chemicals (www.sustoil.org).
However, the need to control the extensive usage of organisms
to hydrolyse cellulose might be essential because the potential
release of such organisms to the environment could have catastrophic consequences.
Socio-economic impact. Some sectors of the industry estimated
that a robust global biofuel market will be fully established
around 2012. Despite the great emphasis on the environmental
aspects for the promotion of biofuels, many authors agree on the
key role of socio-economic motivations in the rise of biofuel
markets,148 mainly from the OECD countries. However, the
impacts that biofuels could have on economies, especially in
those of developing countries, have not been properly evaluated.
Undoubtedly, their implementation in developing nations will
promote their development to some extent. Many issues might
arise from an inadequate implementation of the global market. A
generalised conception is that developing countries are likely to
be the main producers while higher industrialised countries (e.g.
OECD) will be the major consumers. This conception is based on
studies based on land availability, production costs, environmental conditions and levels of consumption in developing
countries.
The developing/developed countries approach has many
particularities. Asia is generally linked to the palm oil issue, with
major deforestation and threats to biodiversity. However,
jatropha cultivation shows a great potential for the whole region,
as well as cassava (production of bioethanol) in China and
Thailand.139,149 Several studies for the EU identify the potential
of Eastern European countries (the Czech Republic being one of
the best examples) for biofuels production based on the extensive
agricultural land available and the acquisition of a system of
incentives (Fig. 13).150 In contrast, Africa, despite its potential for
the production of biofuels, still has serious difficulties in developing a robust market due to a lack of enough political, legal and
economical infrastructure nationally promoted biofuels
programs to succeed as well as unskilled labour and cultural
constraints.151
Some authors welcome in this situation the opportunity for big
bioenergy players to use large available cultivable land.152 The
situation seems to be promising in places like Colombia, where in
2001 the government approved the E10 program based on better
economics of sugarcane in this country.153
The implementation of biofuels will also be highly dependent
on the feasibility of the technologies employed for their
production and the economics of the processes play a fundamental role in this regard. The price of crude oil is presently
increasing to about $140 per barrel (June 2008). At this price,
microalgal biomass with an oil content of 55% will need to be
produced at approximately $400 ton1 to make it competitive
with petroleum diesel. However, literature estimated data
pointed out that microalgal biodiesel can currently be produced
at $3000 ton1.71
The implementation of biofuels into economies131,154 will be
conditioned by several factors including local, global and
national issues, types of feedstocks or level of development of the
market.
Fig. 13 Potential energy yields for 1st and 2nd generation feedstocks (best feedstock for both generation production chains). Source: REFUEL http://
www.refuel.eu/fileadmin/refuel/user/docs/REFUEL_final_road_map.pdf. Reproduced with permission from Marc Londo.
558 | Energy Environ. Sci., 2008, 1, 542–564
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As examples, a number of studies have evaluated economic
implications of the implementation of large scale biofuel
production in developing countries.154,155 In the case of Tanzania,
the assumption estimated an initial production cost of biofuels
(e.g. biodiesel) of 2.23–2.29 $US L1 at a small scale plant(local)
compared to the 0.47 $US L1 price for conventional fuel.154
These figures could potentially drop to 0.7–0.8 $US L1 at a large
scale batch production in a very optimistic scenario.154 However,
a national biodiesel plan would be constrained by the economical
burden generated in the African country in terms of taxes not
collected from substitution of conventional diesel plus need for
subsidies to the emerging market in order to make it fully
competitive. These facts summarise some of the main issues for
Africa in the production of biofuels.
Furthermore, each biofuel has several particularities to
consider. Some useful examples are given.
Biodiesel economics in large continuous processes are suitable
for high volume productions where high capital plan cost is offset
by lower operational costs and large competitive volumes of fuel
produced.156 On the other hand, proportionally higher running
costs from smaller scale batch production would suit locally
produced and distributed fuel.151,156 However, full costs evaluation will be conditioned by the type of feedstock employed and
the possibility of economic premium through adding value to its
main by-product (glycerol). Low value feedstocks such as soapstocks, UVO, and free fatty acids from oil refining are generally
more accessible and economical than even high yield oil content
oilseeds (approx. 600–800 $US t1 to 1700–2000 $US t1, for
a product sold at 1200–1400 $US t1).157 Haas reported a 25%
saving in the use of biodiesel from waste soapstock with respect
to the use of pure virgin soyabean oil.158 Nonetheless, low value
feedstocks might have a negative cost implication for more
industrious downstream processing,159 sometimes compensated
by glycerine sales (6.5% saving in operational cost).157,160
Ethanol economics are largely dependant on the type of
feedstock employed.161 The use of sugar cane and cellulose
ethanol involves generally higher capital cost than corn
production; however there are larger returns in production cost
in terms of a more effective use of by-products (such as bagasses
and molasses for sugar cane) or the possibility to use virtually
free feestocks (wood chips, forestry). For corn, it has been
reported that a larger number of inputs might be needed for the
same ethanol volumes produced than for cane under the same
agricultural conditions.161 Here is where local factors (e.g.
availability of land and water, climate, etc.) would play
a fundamental role.153
Biobutanol preparation requires improvements at three levels
of production: preteatment (preferable from cellulosic materials
where appropriate), optimisation of the bacterial strain for
fermentation and purification of the biofuel from the broth.53
Synthetic biofuels production also finds a number of technical
and non technical limitations that might make them less
economic than fossil equivalents. Bridgwater et al. have largely
studied pyrolysis and gasification processes finding that a modest
10 MW facility operating at 35% would require approximately
40 000 t y1 (40 km2).127,131,134 Despite the high capital investment
and cost needed, a good deal of the operational cost (attributed
to collection of biomass and distribution) should be taken into
account when assessing the plant size.131
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Apart from the impact of biofuels on the economies of
developing/developed countries or the practicalities and
economics of the processes, there is also an increasing concern in
society regarding the subsidies in the promotion of biofuels.
Supporting a future scenario without subsidies is under
debate. A good example of that is the closure of certain biodiesel
companies in Germany conditioned by the removal of incentives
in taxation of B100 fuel.162 However, we must not forget or
ignore the huge subsidies (mostly hidden) given to the oil
industry that will have to be taken into consideration for a fair
comparison.
In summary, a careful consideration of the many factors
described above (together with the policies that promote the
development and implementation of such petrol-fuel replacements) will determine the implementation of one or another
technology, based in certain types of feedstocks and orientated to
certain markets.
Sustainability. A set of procedures, standards, metrics and
evaluations of management systems need to be implemented, at
technological, social, environmental and policy making levels,
adapting to the whole diversity of levels and scenarios in the
complex and politically sensitive energy/bioenergy markets.
One of these powerful tools is life cycle assessment (LCA).
This technique has its origin in engineering or process design
aiming to indicate factors of concern in a system (i.e. an
industrial process, and ecosystem or the potential effect of an
energy crop in a certain set of land).163 LCA also establishes
a balance of inputs/outputs plus precise boundaries for their
assessment, and most importantly a system of reference.163 In
the particular case of biofuels, the relevant factors will mainly
be emissions, erosion and the run off of fertilisers. The
boundaries might vary from specific steps in the production of
the fuel (gate to gate) to a full life cycle (cradle to grave), from
cultivation to end-of-use. The system of reference will usually be
fossil fuel.
Some of the useful metrics to evaluate the impacts of biofuels
(from a carbon/energy efficiency point of view) have been
detailed by Rajagopal and Zilberman90 and Farrell et al.164
These authors attempted to unify and give meaningful information to previously reported LCA studies of ethanol from
corn,9,165 developing a set of comparative methodologies where
the metrics were net energy values [MJ L1-biofuel (e.g. ethanol)]
and GHG emissions (gCO2 per MJ-ethanol) as follows:
Net energy value (NEV): Its simple definition is ‘‘energy
contained in a litre of ethanol minus fossil energy used to
produce it’’ (MJ L1). This metric relates to the used fossil energy
only (fossil energy to produce ethanol in; agriculture, conversion
technology, transportation of crops and eventually final fuel).
Net petroleum offset: Relates to the net petroleum saving
made by the use of biofuels (e.g. the number of gallons of
gasoline displaced by one gallon of bioethanol). This metric
might be useful to evaluate the impact of the use of biofuels in oil
depletion, taxation and imports.
Net carbon reduction: Net carbon emissions reduction
resulting from the consumption of a unit of biofuel. This value is
expected to result in lower net addition to the atmosphere than
fossil fuels, because the carbon emitted through combustion is
sequestered during recultivation.
Energy Environ. Sci., 2008, 1, 542–564 | 559
However, LCA has intrinsic limitations as a simplified LCAGHG/energy approach may lead to misleading conclusions.
These limitations are mainly difficulties in delimiting the
boundaries of the system and the need to consider a broader
set of indicators for a much more complete analysis of metrics
(e.g. water consumption of the crop, carbon sequestration in
soil, erosion, acidification, eutrophication and biodiversity
impact).90,164
A potential complementary tool with a primarily dynamic
approach is the so-called Strategic Niche Management
(SNM).166 This useful dynamic methodology is based on the
continuous and interacting process of learning from simulated
experiments that are created in order to evaluate the success of
a new and complex technology (e.g. biofuels).167
SNM has already been applied to different aspects of biofuels.
Various case-studies have been reported (i.e. introduction of
Jatropha cultivation in Tanzania, and the creation of experimental markets for biofuels in the UK and Germany-B100
diesel.168
The main limitation of this methodology is similar to that of
the LCA, that is, the delimitation of boundaries to be set. It is
also relatively new and it needs some improvements. However,
we believe SNM could have a positive influence in the expansion
of the use of more sustainable technologies.
Future perspectives and prospects
Several potential scenarios for biofuels can be foreseen in the
future. The big hopes for the transport sector are second and
future third generation biofuels, including biodiesel from
microbial oil, the production of biobutanol (from non-edible
feedstocks) as a more petrol-like fuel and the preparation of
biofuels from cellulosic and biomass non-edible feedstocks. The
production of 60 billion gallons of biofuels by 2030 by the US,
assuming commercially available cellulosic ethanol by 2012, has
been estimated by de la Torre Ugarte et al.169 Some other sources
reflect a similar increase in the volume of biofuels produced in the
next few decades (Fig. 14). According to the predictions from
REFUEL, the 1st generation bioethanol would disappear by
2015, while biodiesel might peak by 2025, the same time at which
FT-diesel may start to take off.
Fig. 14 Expected production of various biofuels in the EU27. Source:
REFUEL
http://www.refuel.eu/fileadmin/refuel/user/docs/REFUEL_
final_road_map.pdf. Reproduced with permission from Marc Londo.
560 | Energy Environ. Sci., 2008, 1, 542–564
The reality is that biofuels have started to have a negative
public perception in some countries at the time of writing this
review; this is supported by members of the scientific community
claiming adverse social5a and environmental effects9,170 in first
generation biofuels.
One of the critical factors that will influence the future prospects of biofuels is diversification. The future of biofuels as
a sustainable (economic, social and environmental definitions)
technology is directly linked to the maximum use of by-products
that will make its production more cost effective (e.g. glycerol
obtained as a by-product from biodiesel production can be
converted into a wide range of high-added value chemicals;171
by-products from forestry could generate commercially relevant
bioplastics from biological transformation;172 CO2 from ethanol
production might find uses after sequestration for highly efficient
carbon absorbent plants such as algae71).
Biology and synthetic biology would have the opportunity to
design plants with special properties through genetic engineering.
In this regard, a variety of genetically modified corn (Spartan
Corn III) has recently been developed by scientists at Michigan
State University to be specifically used as a biofuel feedstock.172
The crop has a specific enzyme that allows the leaves and stalk to
be processed into biofuels. However, little is known about
potential environmental concerns and effects on soil and land use
this might create.
A few weeks before the submission of this manuscript,
Howard et al. have reported the fixation of CO2 from plants can
be modulated depending on changes in the availability of light.14
Such a discovery can significantly change the future of biofuels,
as a major contribution to the GHG reduction can be done
through the regulation of the CO2 fixed by plants in crops utilised
for the production of biofuels. Many interesting advances in the
field are currently ongoing.
Conclusions
The potential for biofuels has been recognised throughout the
20th century but the new century has brought with it a widespread realisation that the petroleum age is coming to an end.
Rather than starting to introduce sustainable fuels in a steady,
planned way over the last 30 years (since the oil crisis of the 70s),
there is now a degree of panic in the need for alternatives to
conventional fuels with a not very well studied plan, poorly
judged policies and badly controlled practices. The use of petrolfuel replacements has generated a lot of controversy; ideally they
should contribute to global sustainability, ensuring the energy
supply and meeting the GHG targets (as well as being profitable
and cost competitive as much as possible) without compromising the economies, culture, societies and the environment of
our future. More thoughtful analysis is now showing that many
of these so-called first generation biofuels are little better and,
possibly much worse, than traditional fuels, at least in terms of
overall carbon footprint and environmental damage. These
important issues should however, not let us be distracted from
the potential benefits of biofuels and more widely, biomass
exploitation. It should rather encourage us to redouble our
efforts to produce later generation biofuels based on low value
and waste biomass, using the greenest and efficient technologies
and with properly measured and reported environmental
This journal is ª The Royal Society of Chemistry 2008
impacts. A joint effort from politicians, economists, environmentalists and scientists is needed now, more than ever, to
address the issues of the progressive incorporation of biofuels in
our society and to come up with alternatives, policies and
choices to advance the key technologies for a more sustainable
future. The debate of biofuels has just begun. Hail to the
winners!
Acknowledgements
LHD would like to acknowledge the Knowledge Transfer
Partnership (KTP) and Double Green Ltd. for their support and
fruitful discussion. The authors would like to thank colleagues at
the Green Chemistry Centre of Excellence and Departamento de
Quimica Organica from Universidad de Cordoba for their help
and support. The authors wish to thank Barry J. Holmes,
Marc Londo, Ron Cascone and Rhett Butler (http://www.
mongabay.com/) for their kind help and permission to partially
reproduce diverse material throughout the manuscript.
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