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REVIEW ARTICLE
Pathways of lignocellulosic biomass conversion
to renewable fuels
Sonil Nanda &Javeed Mohammad &Sivamohan N. Reddy &
Janusz A. Kozinski &Ajay K. Dalai
Received: 15 June 2013 /Revised: 9 September 2013 /Accepted: 10 September 2013
#Springer-Verlag Berlin Heidelberg 2013
Abstract The increased worldwide demand for energy, par-
ticularly from petroleum-derived fuels has led to the search for
a long-term solution of a reliable source of clean energy.
Lignocellulosic biomasses appear to hold the key for a contin-
uous supply of renewable fuels without compromising with the
increasing energy needs. However, the major possible solutions
to the current energy crisis include ethanol, bio-oils and syn-
thesis gas (syngas) produced from lignocellulosic biomass.
Recently, a great deal of research has been made in the fields
of biomass conversion through biochemical, hydrothermal or
thermochemical pathways to biofuels. However, a broad-
spectrum assessment of the above pathways is rare in literature
in terms of technology used, biofuel yields, potential challenges
and possible outcomes. This review paper discusses different
routes for biofuel production, particularly ethanol, bio-oil and
syngas with the bio-oil upgrading techniques. This review
highlights ethanol fermentation and available biomass pretreat-
ment as the biochemical mode, not limiting to the pros and cons
of the pretreatments. Supercritical water gasification (hydro-
thermal pathway) of biomass for syngas production followed
by gas-to-liquid technologies (syngas fermentation and
Fischer–Tropsch catalysis) has been discussed. In addition,
thermochemical pathway dealing with biomass gasification
for syngas and pyrolysis for bio-oils has been presented with
compositional analysis of bio-oils and their upgrading technol-
ogies. The review focuses on various engineering limitations
encountered during biomass conversion and bioprocessing
with the potential solutions which do not restrict them to
different biofuel production pathways.
Keywords Lignocellulosic biomass .Bioethanol .Bio-oil .
Fermentation .Gasification .Pyrolysis
Abbreviations
AFEX Ammonia fibre explosion
CNT Carbon nanotubes
CHP Combined heat and power
CSTR Continuous stirred tank reactor
DME Dimethyl ether
FT Fischer–Tropsch
FFV Flexible fuel vehicle
GTL Gas-to-liquid
GHG Greenhouse gas
HMF Hydroxymethylfurfural
LHSV Liquid hourly space velocity
MPa Megapascal
PI Performance Index
PCR Polymerase chain reaction
SHF Separate hydrolysis and fermentation
SSF Simultaneous saccharification and fermentation
SCCO
2
Supercritical CO
2
SCW Supercritical water
SCWG Supercritical water gasification
WGS Water–gas shift
Symbols
M
n
Average molecular weight
P
c
Critical pressure
T
c
Critical temperature
ΔH
R
Heat of reaction
K
w
Ionic product of water
M
w
Molecular weight
M
w
/M
n
Polydispersity Index
wt% Weight percent
S. Nanda :J. Mohammad :S. N. Reddy :J. A. Kozinski
Lassonde School of Engineering, York University,
Toronto, ON, Canada
S. Nanda :A. K. Dalai (*)
Department of Chemical and Biological Engineering,
University of Saskatchewan, Saskatoon, SK S7N 5A9, Canada
e-mail: ajay.dalai@usask.ca
Biomass Conv. Bioref.
DOI 10.1007/s13399-013-0097-z
1 Introduction
The present energy crisis is considered as a serious issue in
terms of sustainability of human development and civilization.
In the transportation sector, the number of vehicles on roads
worldwide is projected to increase to 1.3 billion by 2030 and to
over two billion by 2050 from 806 million vehicles in 2007 [1].
Although at the global scale, about 27 % of primary energy
from fossil fuels is used for transportation, yet it is the fastest-
growing sector and currently contributes to nearly 80 % of
anthropogenic greenhouse gas (GHG) emissions [2]. Over
97 % of transportation fuels (e.g. petroleum, diesel, gasoline
etc.) are derived from crude oil whose demands are sky
rocketing, although the knowledge of their exhausting fossil
resources is well-known. The consumption of petroleum and
other liquid transportation fuels was 85.7 million barrels per
day in 2008 and is expected for increase to 112.2 million
barrels per day by 2035 [3]. Figure 1shows the trend of some
significant consumers of petroleum over the years (1980–
2011). This ever-increasing demand for fossil fuels has led to
the search for alternative energy supplies from renewable
resources that can fulfill future energy requirements.
Low-molecular-weight alcohols such as methanol (CH
3
OH),
ethanol (C
2
H
5
OH), propanol (C
3
H
7
OH) and butanol (C
4
H
9
OH)
have great potentials to replace fossil fuels in the transportation
sector. Alcohol fuels from renewable organic sources have the
tendency to curb GHG emissions, reduce fuels cost, enhance
overall energy efficiency and improve employment in agricul-
tural sector. In the recent times, biomass-derived ethanol has
emerged as a biofuel, generating a great deal of interest in its
production pathways. During the last decades, bioethanol was
produced from fermentation of variety of food-based materials
including grains such as corn, potato mashes, fruit juices, beet
and sugarcane. The ethanol production from corn became com-
mercially viable in USA during 1980s. With an annual yield of
13 billion gallons of ethanol from corn, USA utilized 23 % of its
corn harvest in 2009 for ethanol production which resulted in an
economic crisis with the rise in corn prices for human and
animal consumption [5]. China, being the world’s largest pro-
ducer of rice and wheat and ranking second in corn production,
derived 1.4 million metric tons of ethanol from grain-based
feedstock in 2007 [6]. In India, where the demand for gasoline
has grown at an average annual rate of 7 % during the last
decade, molasses are used as feedstock for ethanol production.
Every year, nearly 2.7 billion litres of ethanol is generated from
molasses in India [7].
The use of such starch-based feedstocks for ethanol produc-
tion is mostly surrounded by criticisms of “food versus fuel”
associating with the risk of diverting crop farmlands for fuel
production and thereby affecting the food supply on a global
scale. On the other hand, lignocellulosic biomasses are consid-
ered as prospective resources for biofuels not only because they
are available on a renewable basis but also they have no net
increase in CO
2
release into the atmosphere. Lignocellulose
comprises about half of the plant matter produced by photo-
synthesis and is the most abundant renewable organic resource
[8]. The utilization of waste biomass in the production of
sustainable energy signifies bioenergy. Bioenergy or biomass
energy refers to any source of renewable energy produced
from nonfossil biological materials. In fact, bioenergy is a
potential solution to the challenges faced by the world
economy for energy security due to dependency on deplet-
ing fossil fuel resources. Bioenergy has the ability to
decrease net emissions of carbon into the atmosphere per
unit of energy delivered making it environmental friendly.
In addition, biofuels have a tendency to offset GHG emis-
sions and global warming. Although CO
2
is released dur-
ing combustion of biofuels, it also reutilized to grow new
biomass which leads to no net CO
2
accumulation in the
atmosphere.
Ethanol is an oxygenated fuel containing 35 % O
2
which
reduces the emission of particulate matters and GHG emis-
sions from combustion. As a fuel, it can be used directly (95 %
ethanol and 5 % water) or as a gasoline blend. The nearly pure
ethanol fuel demonstrates a clean burning characteristic fea-
ture along benefits of low vapor pressure and reduced emis-
sion into the atmosphere. Ethanol when blended with gasoline
oxygenates, thus reducing the formation of CO and ozone.
Bioethanol has a potential to replace 32 % of global gasoline
consumption when used in E85 (85 % ethanol and 15 %
gasoline) for a midsize passenger vehicle [9]. Ethanol–gaso-
line blended fuels (e.g. gasohol) are a blend of E10. The
countries that have implemented ethanol–gasoline blending
programs include USA (E10 and E85 for flexible fuel vehi-
cles, FFV), Canada (E10 and E85 for FFV), Sweden (E5 and
E85 for FFV), Brazil (E20 and E25 for FFV), China (E10),
Fig. 1 World petroleum consumption, 1980–2011 (data source, [4])
Biomass Conv. Bioref.
Australia (E10), Thailand (E10), Columbia (E10), Peru (E10),
Paraguay (E7) and India (E5) [1].
The current five major bioethanol-producing countries are
USA, Brazil, China, Canada and France [4]. Figure 2high-
lights these major bioethanol producers along with their pro-
duction trend since 2007. At present, USA and Brazil produce
over 90 % of world’s bioethanol. In 2008, Canada produced
about 800 million litres of bioethanol representing a 400 %
increase over its production in 2005 [10]. Canada has a
potential to meet about 50 % of its gasoline demand from
the lignocellulosic biomass accessibility, of which 12–28.5 %
is contributed from the energy crop systems. In order to meet
the energy demands, several countries including USA and
European Union have implemented biofuel programs. USA
has a goal of 30 % reduction in gasoline consumption with
biofuels usage by 2030 [11], whereas the European Union has
a mandate of having 10 % of transportation fuel coming from
biofuels in 2020 [12]. It could be predicted that by 2020, the
total international utilization of bioethanol as fuel will reach
10 million metric tons.
Biomasses from agriculture, forestry, municipal, industrial
and urban residues are suitable raw material for biofuel produc-
tion. The worldwide production of biomass from terrestrial
plants is 170–200× 10
9
tons, with an estimated 70 % made of
plant cell walls [13]. However, only a small proportion of this
biomass is used for biofuel production as the major share of
plant part is used for sugar production, electricity generation and
as compost in crop fields. Exploring the potential of these plant
residues for biofuel production is essential to minimize the need
for other energy sources and to promote their ecofriendly utili-
zation. Substrates for ethanol production contain raw materials
that can be transformed into sugars. These raw materials are
classified as directly fermentable sugars, starch-based and lig-
nocellulosic materials. Among these substrates, starch- and
sugar-containing materials do not require extreme and costly
pretreatments. There is an ease of hydrolysis, and the sugars in
these feedstocks are relatively easy to extract, transform into
glucose and ferment to produce ethanol. Today, the majority of
ethanol is derived from starch- and sugar-based feedstocks to
make large-scale ethanol production affordable. Ethanol cannot
only be produced via bioconversion of biomass, but also
through thermochemical and hydrothermal pathways.
Synthesis gas or syngas is a product of hydrothermal and
thermochemical conversion of biomass. This syngas can be
directly used as a fuel or can be converted into liquid fuels such
as ethanol and other alcohols and hydrocarbons via gas-to-
liquid (GTL) routes.
Bio-oil is a synthetic fuel obtained from the pyrolysis of
biomass. Bio-oils are complex mixture of oxygenated com-
pounds namely alcohols, acids, aldehydes, esters, ketones
and many other aromatic compounds [14]. Bio-oils have
found wide range of applications as a sustainable fuel in-
cluding use in boilers for power generation and in the
synthesis of chemicals. The heating value (16–18 MJ/kg)
of bio-oils makes it as a possible substitute to the petroleum-
based fuels. Upon upgrading, these bio-oils can be used as
transportation fuels. However, high oxygen and water con-
tents in the crude bio-oil pose considerable challenges for
their upgrading.
In this review, the technical aspects of selected different
biomass conversion pathways namely biochemical, hydro-
thermal and thermochemical pathways to produce liquid fuels
have been emphasized. Bioconversion processes such as eth-
anol fermentation and available biomass pretreatment have
been discussed along with the benefits and drawbacks
pertaining to the pretreatments. Supercritical water gasifica-
tion process as hydrothermal conversion along with thermo-
chemical gasification, pyrolysis and liquefaction has been
presented. Although the GTL conversion of syngas to higher
alcohols through Fischer–Tropsch (FT) catalysis is well-
established, yet syngas fermentation to ethanol is new to the
existing literature. In addition, the commercial worldwide
status of bio-oil production from pyrolysis and gasification
technologies has been presented. In general, this review aims
to provide the different pathways to produce liquid fuels from
lignocellulosic biomass along with the potential limitations
and possible solutions during biomass and biofuel processing.
To the best of the knowledge, rare literature is available on the
comparative evaluation of biochemical, hydrothermal and
thermochemical conversion pathways of biomass to liquid
sustainable fuels.
2 Lignocellulosic biomass—composition and conversion
Lignocellulosic materials are economical resource that are
abundantly available and have the capability to support the
sustainable production of renewable fuels. Lignocellulosic
Fig. 2 Major five world producers of bioethanol, 2007–2011 (data
source, [4])
Biomass Conv. Bioref.
biomass is usually categorized into agricultural and forage
crops, dedicated energy crops, wood residues from soft- and
hardwood and municipal paper waste. A kind of nonrenewable
lignocellulosic feedstocks is also accessible which is referred as
“disturbance wood and crops”that are typically the forest and
crop resources damaged by insects, pests and disease.
Lignocellulose is a major component of plants that provides
them structure and is usually present in roots, stalks and leaves.
Plant cell walls are primarily made of cellulose (C
6
H
10
O
5
)
n
,
hemicellulose (C
5
H
8
O
4
)
m
, lignin [C
9
H
10
O
3
(OCH
3
)
0.9–1.7
]
x
,
pectin and glycosylated proteins. Pectins are cross-linked poly-
saccharides forming a hydrated gel that holds the cell wall
components together. The primary functions of glycosylated
proteins are plant growth and development, physical strength,
water and solute conduction and defence against pathogens.
Lignocellulose forms a complex crystalline structure held to-
gether by covalent bonding, intermolecular bridges and van der
Waals forces that makes it insoluble in water and robust to attack
by enzymes. On the dry matter basis, a typical lignocellulosic
biomass has 30–60 % cellulose, 20–40 % hemicellulose and
15–25 % lignin [15]. About 90 % of dry matter in lignocellu-
losics comprise of cellulose, hemicelluloses and lignin, whereas
the rest consists of extractives and ash. Extractives are regarded
as nonstructural biomass components that are soluble in neutral
organic solvents or water. Extractives comprise of structural
biopolymers such as terpenoids, steroids, resin acids, fats, lipids,
waxes and phenolic constituents in the form of stilbenes,
flavanoids, lignans and tannins. The composition of cellulose,
hemicellulose and lignin vary in different lignocellulosic feed-
stocks as shown in Table 1. The biomass is a heterogeneous
mixture of both organic and mineral composites [17–20,22].
The mineral matter in biomass includes both major elements
(e.g. Na, Mg, K, Ca and Si) and minor elements (e.g. Al, Fe,
Mn, P and S). These major and minor elements occur as less
than 1 wt% in wood and shells, whereas in straws and husks
they range up to 25 wt% [16]. Furthermore, the chemical
composition of biomass is influenced by the plant’s genetic
and environmental factors that vary considerably [15,21].
Cellulose is a glucose polymer consisting of β(1→4)
linked D-glucose subunits with an average molecular weight
of around 100,000 that are synthesized at the plant cell mem-
brane and aggregated by hydrogen bonding and van der Waals
forces. Cellulose is a straight chain polymer derived from the
dehydration of glucose (C
6
H
12
O
6
) molecules as shown in
Fig. 3a. Cellobiose is the repeat unit of cellulose and its
molecular weight is approximately 30,000. Cellulose contains
both amorphous and crystalline regions alternating with each
other in the form of microfibrils. Because of the fibrous nature
and strong hydrogen bonding, cellulose is found to be insol-
uble in majority of the solvents [23].
Hemicellulose is a mixture of polysaccharides composed of
pentose and hexose sugars such as glucose, mannose, xylose
and arabinose as well as sugar acids such as methylglucuronic
and galaturonic acids. Hemicellulose has its molecular weight
less than 30,000 and degree of polymerization near to 200. The
chemical structure of hemicellulose is shown in Fig. 3b.Unlike
cellulose that requires severe hydrolysis conditions for dena-
turation to simple glucose units due to its crystalline structure,
hemicellulose is relatively easy for denaturation using acids,
bases or enzymes. The monomeric sugar components such as
D-glucose, D-mannose, D-galactose, D-xylose, L-arabinose, L-
rhamnose, D-glucuronic acid, and D-galacturonic acid (e.g.
carboxylic acid) obtained from hemicellulose can be subjected
to bioconversion for ethanol and other useful byproducts.
Lignin is a phenyl propane polymer linked with ester bonds
that acts as glue and tightly binds with cellulose and hemicel-
lulose. The macromolecular polymeric structure of lignin is
shown in Fig. 3c. Lignin consists of macromolecules that
contain highly branched phenolic compounds. Some major
structural components of lignin are p-coumaryl alcohol,
coniferyl alcohol and sinapyl alcohol. The phenyl propane
units in lignin are joined by C–O–CandC–C linkages.
Lignin is also known to contain methoxyl, phenolic, hydroxyl
and terminal aldehyde groups in the side chain with limited
solubility in most solvents. The average molecular weight of
lignin is in the order of 20,000. The polydispersity index
(molecular weight (M
w
)/average molecular weight (M
n
)) or
the ratio of weight average M
w
to M
n
of lignin is reported to be
higher than that of cellulose [23]. Generally, softwoods have
higher lignin content than hardwoods. In contrast, hardwoods
have a greater amount of holocellulose (i.e. sum of cellulose
and hemicellulose) and extractives than softwoods [24]. The
presence of lignin in biomass makes it difficult to obtain
holocellulose to produce fermentable sugars. During the deg-
radation process, lignin can form furan compounds that could
inhibit fermentation.
The production of biofuels from lignocellulosic feedstocks
can be achieved through three different processing pathways.
They are: (1) biochemical, where enzymes and microorgan-
isms are used to convert the cellulosic and hemicellulosic
sugars to alcohols; (2) hydrothermal, where supercritical water
acts as a medium in the biomass conversion to fermentable
sugars and H
2
-rich syngas; and (3) thermochemical, where
pyrolysis and gasification technologies produce a synthesis
gas (CO+ H
2
) from which a wide range of long carbon chain
biofuels such as synthetic gasoline and bio-oils can be
reformed.
Figure 4illustrates the pathways and subpathways for
lignocellulosic biomass conversion to biofuels. It should be
noted that gasification in case of hydrothermal pathway uses
supercritical water (SCW) as a medium for biomass conver-
sion, whereas in thermochemical gasification inert atmosphere
by N
2
or other inert gases act as the medium for conversion.
The distinction between hydrothermal and thermochemical
gasification is important as the reaction medium for gasifica-
tion is a determining factor for syngas composition.
Biomass Conv. Bioref.
Tabl e 1 Percent dry weight
composition of some lignocellu-
losic feedstocks and organic
wastes
Feedstock Cellulose Hemicellulose Lignin Reference
Bagasse 41 23 18 [16]
Bamboo 26–43 15–26 21–31 [8]
Banana waste 13 15 14 [8]
Barley straw 32 26 23 [17]
Bast fibre jute 45–53 18–21 21–26 [8]
Bast fibre kenaf 31–39 22-23 15–19 [8]
Bast fibre seed flax 47 25 23 [8]
Blue gum 36 12 31 [18]
Coastal Bermuda grass 25 36 6 [19]
Coconut coir 48 26 18 [16]
Coconut shell 36 25 29 [16]
Coffee pulp 35 46 19 [8]
Coir pith 29 15 31 [16]
Corn cobs 45 35 15 [19]
Corn stalks 43 24 17 [16]
Corn stover 40 22 18 [20]
Cotton gin waste 78 16 –[16]
Cotton seed hair 80–90 5–20 –[19]
Elephant grass 22 24 24 [8]
Esparto grass 33–38 27–32 17–19 [8]
Ethiopian mustard 33 14 19 [18]
Flax straw 29 27 22 [17]
Grasses 25–40 25–50 10–30 [21]
Groundnut shell 36 19 30 [16]
Hardwood bark 22–40 20–38 30–55 [21]
Hardwood stem 40–50 24–40 18–25 [19]
Leaves 15–20 80–85 –[19]
Millet husk 33 27 14 [16]
Newspaper 40–55 25–40 18–30 [19]
Nut shells 25–30 25–30 30–40 [19]
Oat straw 31–37 27–38 16–19 [8]
Orchard grass 32 40 5 [8]
Pinewood 39 24 20 [15]
Poplar wood 35 17 26 [18]
Rice husk 31 24 14 [16]
Rice straw 37 23 14 [16]
Rye grass (early leaf) 21 16 3 [8]
Rye grass (seed setting) 27 26 7 [8]
Rye straw 33–35 27–30 16–19 [8]
Sabai grass –24 22 [8]
Silver birch 34 40 23 [22]
Softwood stem 45–50 25–35 25–35 [21]
Subabul wood 40 24 25 [16]
Sorted plant refuse 60 20 20 [1]
Sweet sorghum bagasse 45 25 18 [18]
Switchgrass 45 31 12 [8]
Timothy grass 34 30 18 [15]
Waste papers from chemical pulps 60–70 10–20 5–10 [19]
Wheat straw 39 24 16 [15]
Biomass Conv. Bioref.
Syngas is the major product of hydrothermal and thermo-
chemical gasification of biomass, although minor amounts of
tars and particulates (e.g. ash, soot or char) are obtained from
both pathways. The temperature, pressure and catalyst have
major impact on the syngas composition. Syngas contains
mostly CO and H
2
with varying amounts of CO
2
,CH
4
and
water vapor with traces of impurities such as NH
3
,H
2
S, HCl,
COS and HCN [25]. Syngas obtained can be converted into
ethanol through the available GTL technologies such as syn-
gas fermentation and Fischer–Tropsch catalysis. Syngas fer-
mentation can be classified as a hybrid pathway which in-
volves a series of conversions, i.e. hydrothermal and thermo-
chemical followed by biochemical processes.
3 Pretreatment technologies for lignocellulosic biomass
The bioconversion of lignocellulosic biomass requires: (1)
delignification to liberate cellulose and hemicellulose from the
complex with lignin, (2) depolymerization of the carbohydrate
polymers to produce free sugars and (3) fermentation of mixed
hexose and pentose sugars to ethanol. The initial step is separa-
tion of cellulose and hemicellulose from lignin followed by
hydrolysis. The presence of five sugars (e.g. glucose, galactose,
mannose, xylose and arabinose) found within cellulosic mate-
rials makes it relatively difficult for hydrolysis compared to
starch which is associated with the single sugar (glucose). A
suitable pretreatment of biomass is necessary to ensure good
yields of sugars from the polysaccharides. Pretreatment disrupts
the plant cell wall and improves enzymatic access to the poly-
saccharides as raw and untreated biomass is usually resistant to
enzymatic degradation [26]. The pretreatment step usually rep-
resents nearly 20 % of the total production costs of the fuel [27].
Due to the nonfermentable nature of lignin, biomass is
pretreated to separate cellulose, hemicellulose and lignin [26].
After biomass pretreatment (using acids and enzymes), the
cellulose and hemicelluloses containing fermentable pentose
and hexose sugars are recovered in the biomass hydrolysate
which is fermented to ethanol [27,28]. Lignin remains in the
pretreated or hydrolyzed biomass as it is recalcitrant to acidic
and enzymatic pretreatments [14]. However, this hydrolyzed
biomass containing lignin can be pyrolyzed and/or gasified to
produce bio-oils, chars and gases (see Fig. 4). A number of
biomass pretreatment technologies are available today that are
broadly described by various authors [19,26–28]. However, in
this review, a few of such pretreatments are discussed under the
categories of physicochemical, chemical, hydrothermal and
biological modes. In addition, Table 2summarizes the widely
used biomass pretreatment technologies along with their ad-
vantages and disadvantages.
3.1 Physicochemical pretreatment
The biomass is comminuted by chipping, grinding and milling
to reduce their particle size. This not only increases the surface
area of the biomass but also reduces the crystallinity of cellu-
lose for better hydrolysis. Ammonia fibre explosion (AFEX)
is a type of pretreatment in which lignocellulosic biomass is
Fig. 3 Chemical structures of acellulose, bhemicellulose, and clignin
Biomass Conv. Bioref.
exposed to liquid NH
3
at high temperature (60–200 °C) and
pressure (1.4–4.8 MPa) for 10–60 min, with a swift reduction
in pressure. With the reduction in pressure, NH
3
evaporates
explosively causing material breakdown and removal of lignin
and hemicellulose without producing inhibitory degradation
compounds [27]. AFEX can significantly improve the sac-
charification rates but is not efficient for biomass with high
lignin content.
Ozonolysis is another physicochemical pretreatment that
uses ozone to degrade lignocellulosic materials with effective
removal of lignin without producing inhibitors for down-
stream processing [28]. The process is carried out at room
temperature and requires considerable amount of ozone [19].
Some other physicochemical pretreatments include gamma
rays [29], pulsed electrical field [1], electron beam [30],
ultrasound [31] and microwave digestion [32].
3.2 Chemical pretreatment
Chemical pretreatments mostly involve the use of acids and
bases in biomass hydrolysis [26]. Acid hydrolysis employs
concentrated and diluted H
2
SO
4
or HCl to treat the lignocel-
lulosic biomass [19]. Dilute acid hydrolysis has replaced the
concentrated acid hydrolysis due to the fact that concentrated
acids are: (1) hazardous to handle and need reactors resistant to
corrosion, (2) required to be recovered after digestion to make
the process economically feasible and (3) less efficient in
achieving high reaction rates than dilute acids. Despite its
many advantages, dilute acid hydrolysis results in the forma-
tion of furfurals. Other undesirable components found in bio-
mass hydrolysates that are inhibitory to fermentation include
sugar degradation products (e.g. hydroxymethylfurfural or
HMF and levulinic acid), hemicellulose degradation products
(e.g. acetic acid, ferulic acid, glucuronic acid and p-coumaric
acid) and lignin breakdown products (e.g. syringaldehyde and
syringic acid). Irrespective of the pretreatment and hydrolysis
method used, it is often difficult to eliminate the chances for
generation of these inhibitory compounds.
Alkaline hydrolysis uses bases such asNH
4
OH and NaOH.
However, this pretreatment is expensive and the recovery and
recycling of bases from the system is often difficult [34,35].
An advantage of using NH
4
OH is that the volatile nature of
NH
3
makes it effective in improving cellulose digestion.
Alkaline hydrolysis of lignocellulosic biomass depends on
its lignin content and overrules acid hydrolysis in degrading
lesser sugars with easy recovery of caustic salts. It causes
saponification of intermolecular ester bonds, cross-linking
xylan hemicellulose and other components.
The organosolv process has the direct action of water and
dissolved organic solvents such as ethanol, methanol and
acetone usually in combination with an acid to solubilize the
lignin and hydrolyze hemicellulose [36]. The process temper-
atures may vary from room temperature up to 180–200 °C
depending on the nature of organic solvent [42]. The major
drawback of this pretreatment is the production of furfurals
that may inhibit subsequent processes and result in low recov-
ery of pentose sugars. Furfural is a significant inhibitor of
ethanol production from hemicellulose hydrolysate and even
its low concentrations of 3–15 mM can adversely affect the
ethanol production rates [33]. The production of inhibitors is
found in both steam explosion and organosolv pretreatments.
The final concentrations of inhibitors in steam explosion can
be reduced by washing the exploded biomass with water [26].
This is quite difficult in case of organosolv process because of
the presence of organic solvents (e.g. ethanol, methanol and
acetone) in the hydrolysing medium [27]. There are additional
costs involved in solvent draining, evaporation and condensa-
tion to separate the aqueous and organic phases. Moreover,
ethanol acts as an inhibitor for the hydrolysis process as
suggested by Chiaramonti et al. [27]. However, organosolv
pretreatment is beneficial in higher cellulose digestibility and
recovery of hemicelluloses present in soft- and hardwood
from the water-soluble stream.
Fischer-Tropsch catalysis Combined heat-power
Tar
Biochemical pathway
Biomass
Hydrothermal pathway Thermochemical pathway
Pretreatment SCW gasification Pyrolysis
Gasification,
co-gasification
LigninCellulose, hemicellulose
Fermentation
Syngas
Syngas fermentation
Bioethanol
Gas
Bio-oil Char
Liquefaction
Fig. 4 Pathways of bioethanol
and bio-oil production using
biochemical, hydrothermal, and
thermochemical routes
Biomass Conv. Bioref.
Tabl e 2 Advantages and disadvantages of various pretreatment methods
Pretreatment Advantages Disadvantages Reference
Physicochemical pretreatments
Mechanical comminution Increases surface area of biomass Power consumption usually
higher than inherent biomass energy
[19]
Reduces cellulose crystallinity
Ammonia fibre explosion Increases accessible surface area Not efficient for biomass with high
lignin content
[19]
Removes lignin and hemicellulose
Does not produce inhibitors for
downstream processes
Increases saccharification rate
Ozonolysis Reduces lignin content Large amount of ozone required [19]
Does not produce inhibitory residues Expensive process
Pulsed electrical field Operates in ambient conditions High cost related [1]
Disrupts plant cells efficiently Energy intensive process
Gamma rays Mutagenic effect in increasing
cellulase activity
High cost related [29]
Difficulties in large-scale applications
Electron beam Increases enzymatic digestibility High cost related [30]
Energy intensive process
Ultrasound Faster process High cost related [31]
Energy intensive process
Microwave digestion Better hydrolysis High cost related [32]
Faster process Energy intensive process
Chemical pretreatments
Acid hydrolysis Hydrolyzes hemicellulose to xylose
and other sugars
High cost related [30,33]
Alters lignin structure Equipment corrosion issues
Formation of inhibitory substances
Alkaline hydrolysis Removes hemicelluloses and lignin Long residence times required [34,35]
Increases accessible surface area Irrecoverable salts formed and incorporated
into biomass
Expensive process
Difficulty in recovering bases
Organosolv Hydrolyzes lignin and hemicelluloses Solvents need to be drained from the
reactor, evaporated, condensed and recycled
[36,37]
Production of inhibitors
Lower rec overy of p entoses
High cost related
Hydrothermal pretreatments
Liquid hot water Elevated recovery rates for pentoses Degradation of monosaccharide sugars [38]
Generates low amount of inhibitors
Steam explosion Causes hemicellulose degradation
and lignin transformation
Destruction of a portion of the xylan fraction [39]
Cost-effective process Incomplete disruption of the lignin–carbohydrate matrix
Generation of compounds inhibitory to
microorganisms
Supercritical CO
2
Increases accessible surface area Does not alter lignin or hemicelluloses [40]
Does not produce inhibitory compounds High cost related
Supercritical water Better mass transfer Production of inhibitors such as furfural,
hydroxymethylfurfural and acetic acid
[37]
Good solvation potential
Enhances enzymatic digestibility
Biological pretreatments
Enzymatic hydrolysis Simple equipment Degrades lignin to certain extent [41]
Efficiently degrades cellulose and
hemicelluloses
Rate of hydrolysis is low
Low energy requirements
Uses both high and low moisture
containing biomass
Biomass Conv. Bioref.
Recently, ionic liquids are gaining interest in biomass hy-
drolysis and being attractive alternatives to volatile and unsta-
ble organic solvents due to their high thermal stability and
nearly absolute nonvolatility. Ionic liquids are organic salts that
usually melt at temperatures less than 100 °C and are able to
dissolve polar and nonpolar organic, inorganic and polymeric
compounds. The ionic liquid, 1-ethyl-3-methylimidazolium
acetate, has found application in solubilizing cellulose and
reducing its crystallinity in switchgrass [43], and extracting
lignin from wood [44]. The reconstituted cellulose after ionic
liquid dissolution had lower degrees of crystallinity than native
cellulose which resulted in better accessibility of the polysac-
charide chains to cellulases.
3.3 Hydrothermal pretreatment
Steam explosion is a kind of hydrothermal pretreatment in
which chipped biomass is treated with high-pressure saturated
steam (160–260 °C) with a swift reduction in pressure. This
causes the organic material to undergo an explosive decom-
position including hemicellulose degradation and lignin trans-
formation, thus enhancing cellulose hydrolysis. The disrup-
tion of the lignin–carbohydrate matrix generates certain com-
pounds inhibitory to microorganisms in downstream process-
ing. To overcome this, pretreated biomass is usually washed
with water to remove the toxic materials along with water-
soluble hemicellulose [39]. The addition of H
2
SO
4
,SO
2
and
CO
2
to the system improves enzymatic hydrolysis with com-
plete removal of hemicelluloses and decreased accumulation
of inhibitory compounds. Autohydrolysis is a similar method
in which steam-saturated water (150–230 °C) is used for plant
cell wall denaturation to release hemicellulose and alter the
lignin structure [45]. Upon heating the biomass, acids are
released that hydrolyze the lignocellulosic material.
Boiling of lignocellulosic biomass in liquid hot water is a
traditional hydrothermal pretreatment practice, commonly
known as cooking of biomass. During this process, a phase
of high pressure with hot water (200–230 °C) enhances elevat-
ed recovery rates for pentose sugars and generates a low
amount of inhibitors [38]. About 40–60%ofthetotalbiomass
gets dissolved during boiling with a recovery rate of 4–22 %
cellulose, 35–60 % lignin and 100 % hemicellulose [45]. The
treatment lasts for about 15 min and maintaining a pH 4–7
minimizes the degradation of monosaccharide sugars [1]. A
study by Biermann et al. [46] has shown to depolymerize about
60 % lignin during hydrothermal treatments of hardwood with
degradation of about 20 % cellulose. Furthermore, Saska and
Ozer [47] demonstrated the removal of 39.4 % lignin from
sugarcane bagasse with less than 2 % cellulose degradation. As
hemicelluloses are easy to hydrolyze due to their lower degree
of polymerization than cellulose, various hydrothermal and
enzymatic pretreatments have shown their recovery of about
80 % from corn stalks, 65 % from cottonwood poplar, 60 %
from hardwoods and 55 % from bamboo grass [45].
Supercritical fluids are novel advancements in the available
hydrothermal pretreatments. A compound above its critical
temperature (T
c
) and critical pressure (P
c
) but below the pres-
sure required to condense it into a solid is called supercritical
fluid. Some commonly investigated supercritical fluids are CO
2
(T
c
=31 °C, P
c
=7.38 MPa), water (T
c
=374 °C, P
c
=22.1 MPa)
and propane (T
c
=96.7 °C, P
c
=4.25 MPa). With increased
temperature, the density of liquid reduces due to thermal expan-
sion, whereas with increased pressure, the gas gains density. The
phase distinction between liquid and gas disappears when the
densities are equal, further signifying the critical point.
Supercritical fluids exhibit specific properties such as partition
coefficients and solubility. A slight change in temperature or
pressure close to their critical points can result in up to 100-fold
changes in solubility offering better separation [37].
Water at a pressure and temperature higher than its critical
point (i.e. 22.1 MPa and 374 °C) is known as supercritical
water. With a pressure above P
c
and temperature below T
c
,a
subcritical condition occurs and when the pressure drops be-
low P
c
, subcritical steam is generated. SCW has gas-like
viscosity and liquid-like density, providing better mass transfer
and solvation properties. Near the critical point, the ionic
product of water (K
w
) increases considerably which makes it
behave as a weak polar solvent to hydrolyze many compounds
catalyzed by its ions (H
+
and OH
−
) and dissolve organic
substances that can provide a homogeneous phase for reac-
tions. K
w
increases up to a maximum of 6.34× 10
−12
at 250 °C,
resulting in a water pH of 5.5 at 220 °C [48]. Such conditions
facilitate complete separation of hemicellulose from the ligno-
cellulosic network and significantly enhance their enzymatic
digestibility.
The above features make SCW behave as a catalyst in the
decomposition of lignocellulosic biomass which are nonpolar in
nature and are easily hydrolyzed by the catalyst. At supercritical
conditions, biomass denatures to release cellulose which de-
grades to polysaccharides and oligosaccharides. The properties
of SCW make it an ideal solvent for lignocellulosic materials,
providing a reaction medium as well as the protons necessary
for the hydrolysis reaction. In the first step of degradation,
cellulose is hydrolyzed to polysaccharides which are subjected
to fragmentation and dehydration forming oligosaccharides and
finally monosaccharides. Hydrolysis of cellulose starts at tem-
peratures above 230 °C resulting in glucose monomers, whereas
hemicelluloses dissolve in hot water at temperatures around
100 °C to produce pentose monomers, chiefly xylose [49].
The monomer sugars produced from hemicellulose and cellu-
lose can further degrade depending on pressure, temperature
and residence time, forming either fragmentation or dehydration
products that continue breaking down to produce organic acids
such as formic acid, acetic acid, lactic acid, glycolic acid and/or
pyruvic acid [50]. These organic acids are useful substrates for
Biomass Conv. Bioref.
methane production in anaerobic fermentation but for the pur-
pose of ethanol production these products are undesirable and
the reaction should be stopped prior to fragmentation and de-
hydration of the monomers. In high-pressure systems, fragmen-
tation products are predominant, whereas dehydration products
are principal in low-pressure systems.
A comparative study on the decomposition of cellulose
with supercritical (400 °C and 40 MPa) and subcritical water
(280 °C and 40 MPa) revealed high yields of hydrolyzed
products (i.e. oligosaccharides, glucose and fructose) in su-
percritical conditions, whereas the fragmented products (i.e.
erthrose, methylglyoxal, glycolaldehyde and dihydroxy ace-
tone) were higher in subcritical conditions [48]. In subcritical
conditions, cellulose is found to produce higher amounts of
dehydration products which act as inhibitors to fermentation.
In a study by Ehara and Saka [48], a combined system of
subcritical and supercritical water was found to increase the
hydrolyzedproducts, thus decreasing the level and diversity of
dehydrated products. From similar studies, the amount of
dissolved hemicellulose sugars is known to increase as a
function of increased temperature and duration of treatment
[37]. Hydrolyzed hemicelluloses have a tendency to react with
furfurals and other toxic byproducts.
Supercritical CO
2
(SCCO
2
), being nonflammable, nontoxic
with ambient critical temperature (31.1 °C) and moderate pres-
sure (7.39 MPa) makes it a suitable biomass pretreatment
methods [32]. Liquid-like densities and gas-like diffusivities
with molecular size in par with water infers to combine CO
2
explosion in the presence of water for extraction of fermentable
sugars. Delignification of biomass can be enhanced by using
mixed co-solvents such as water–ethanol, water–acetate in
SCCO
2
process at high temperatures (190 °C) and pressures
(16 MPa) [51]. The penetration capabilities of SCCO
2
into
cellulosic structures of the biomass can be enhanced by in-
creasing the temperature and then disruption of the structure
can be achieved by the sudden reduction in the pressure [40].
Conversely, these hydrothermal pretreatments often result in
the generation of inhibitory byproducts such as furfural, HMF
and acetic acid. They have adverse effects on enzymatic hy-
drolysis and fermentation. Although the formation of inhibitors
during pretreatment is undesirable, yet several post-treatment
steps such as detoxification, neutralization and nutrient supple-
mentation to the hydrolysate medium could curb the inhibitory
effects as described by Lenihan et al. [52]. The detoxification is
performed by adsorption of inhibitors on active carbon, e.g.
charcoal. Neutralization is done by adding chemicals that neu-
tralise the acidic inhibitors (e.g. acetic and carboxylic acid) to
form salts that are removed by filtration. The hydrolysates
containing inhibitors could also be supplemented with several
essential nutrients to enhance microbial multiplication thus
masking the effects of inhibitors. Furthermore, pretreatments
at low pH have been found to produce lesser amount of
inhibitors [27].
On the other hand, it is essential to employ enzymes and
microorganisms that are resistant to the inhibitory compounds.
This not only avoids the additional steps in eliminating the
inhibitors, but also reduces the overall process cost. Most of
the physicochemical and hydrothermal treatments result in a
reduced yield of fermentable sugars due to their extreme
treatment conditions [53]. Additionally, these pretreatments
necessitate high energy, high pressure and corrosion-resistant
reactors and generate acidic or alkaline waste residues that
require certain pre-disposal procedures to ensure environmen-
tal safety which increases their overall operational costs. In
contrast, biological pretreatment is a benign substitute that is
accomplished by microorganisms for degrading lignin and
hemicellulose, followed by the production of cellulases and
other fermentative enzymes.
3.4 Biological pretreatment
Saccharification is the process of breaking down of a complex
carbohydrate (e.g. starch or cellulose) into its monosaccharide
components. During hydrolysis, cellulose is degraded by cel-
lulase enzyme-reducing sugars that are fermented to ethanol
by microorganisms. Compared to other pretreatments, the
operational cost of enzymatic hydrolysis is low. In addition,
the process involves mild reaction conditions (e.g. pH 4.8 and
temperature of 45–50 °C) and does not have a corrosion
problem with reactors [41].
The pretreatment of biomass removes the lignin, hydrolyzes
the hemicellulose and decrystallizes the cellulose. As a result
of the decrystallization of cellulose, cellulase enzymes have
increased access to the cellulose fibres [54]. Furthermore, the
pretreated cellulose is enzymatically hydrolyzed either by si-
multaneous saccharification and fermentation (SSF) or by
separate hydrolysis and fermentation (SHF). In SSF, the cellu-
lose enzyme and inoculum are added together in the medium,
whereas in SHF the enzymatic hydrolysis is carried out sepa-
rately from the alcoholic fermentation [18]. The SSF is similar
to SHF except that both hydrolysis and fermentation are
performed in the same reactor in SSF process. The accumula-
tion of sugars within the reactor is lower as the presence of
yeast together with the cellulolytic enzymes results in in-
creased saccharification and ethanol yield [55].
In SHF process, hydrolysis can be performed at higher tem-
peratures taking advantage of the enzyme’s stability at extreme
temperatures to increase productivity and minimize bacterial
contamination. Moreover, it is beneficial in terms of the ease of
separation of the sugar syrups from the hydrophobic lignin
which can further serve as solid fuel and yeast cells that can be
recycled. On the other hand, in SHF, the accumulation of glucose
which is derived from the hydrolysis of cellulose has a tendency
to inhibit the endoglucanases, exoglucanases and β-glucosidase,
thus affecting the ethanol yields. SSF is profitable in being a
faster process and requiring lower amounts of enzyme because
Biomass Conv. Bioref.
the end-product inhibition from the cellobiose and glucose
formed during enzymatic hydrolysis is relieved by yeast fermen-
tation [56]. However, the drawbacks of SSF are heat transfer,
culture homogeneity and necessity of the enzyme and culture
conditions to be compatible with respect to pH and temperature.
Nevertheless, a suitable microbial culture (e.g. filamentous fungi
and thermophilic bacteria) able to grow in high temperatures,
low moisture and on solid substrates can be employed to over-
come these drawbacks.
Microorganisms involved in ethanol production from lig-
nocellulosic feedstocks include delignifiers (e.g. fungi) and
cellulase producers (e.g. fungi, yeast and bacteria). Plant cell
wall-degrading enzymes occur in cellulosomes that are found
in anaerobic bacteria and fungi. In contrast, the aerobic mi-
croorganisms produce discrete enzymes secreted into their
growth media as secondary metabolites [57]. The production
and regulation of cellulases and hemicellulases in microor-
ganisms has been extensively investigated. The widely used
industrial cellulases are obtained from Trichoderma reesei
and Saccharomyces spp. with optimal catalytic conditions at
pH 4.5 and temperature 55 and 37 °C, respectively.
Cellulases comprise three types of hydrolytic enzymes: (1)
endoglucanases which attack regions in cellulose fibre with low
crystallinity creating free chain ends, (2) exoglucanases (i.e.
cellobiohydrolases) which further degrade the molecule by re-
moving cellobiose units from the free chain ends and (3)
β-glucosidases which hydrolyze soluble cellobiose to produce
glucose. Hemicellulases include glucuronidase, acetylesterase,
xylanase, β-xylosidase, galactomannanase and glucomannanase.
Hemicellulases aid in cellulose hydrolysis by exposing the cel-
lulose fibres making them more accessible for saccharification.
The application of hemicellulases depends on the pretreatment
method. In case of dilute acid pretreatment, most of the hemicel-
luloses are removed before saccharification [57]. However, with
non-acid pretreatment methods, the hemicellulose fraction re-
mains intact requiring hemicellulases [34]. In order to reduce
the overall operational costs, it is industrially desirable to hydro-
lyze and harvest hemicellulose before subjecting the recalcitrant
biomass to severe pretreatments for cellulose recovery. In enzy-
matic hydrolysis, substrate inhibition may be a factor affecting
saccharification. Unlike conditions of low substrate levels, where
an increase in substrate concentration results in an increased yield
and hydrolysis; high substrate concentration often cause substrate
inhibition which substantially lowers the hydrolysis rate.
White rot fungi are the most effective basidiomycetes used
for bioconversion of lignocellulogic biomass. Phanerochaete
chrysosporium, a white rot fungus, produces lignin-degrading
enzymes, such as lignin peroxidases and manganese-dependent
peroxidises as a response to carbon or nitrogen limitation during
their secondary metabolism [58]. The peroxidase enzyme can
catalyze lignin biodegradation in presence of H
2
O
2
[59]. The
lignin-degrading enzymes have found many applications in the
degradation of wood cell walls.
4 Bioconversion pathways
Much of the research for bioconversion of biomass have
focused on two types of fermentation systems, namely sub-
merged fermentation and solid-state fermentation. In sub-
merged fermentation, microorganisms are cultivated in a liq-
uid medium containing biomass and nutrients, whereas in
solid-state fermentation both microbial growth and product
formation occurs on solid biomass with optimal available
water. Solid-state fermentation is advantageous over sub-
merged fermentation in having higher ethanol yields with
better product characteristics, low capital and operating costs,
smaller fermenter size, higher enzymatic activity, improved
process control and monitoring, reduced downstream process-
ing, reduced stirring and lower sterilization costs [60].
Another major benefit of solid-state fermentation is that the
transformed biomass after fermentation acts as a value-added
product that can be used as biopulp, compost, biofertilizer,
biopesticide and/or biopromoter [61]. This makes the biolog-
ical treatment an ecofriendly technology for ethanol produc-
tion. Solid-state fermentation has found wide industrial appli-
cations in the commercial production offood flavors, aromatic
compounds, enzymes (e.g. alpha-amylase, fructosyl transfer-
ase, lipase and pectinases) and organic acids (e.g. lactic acid
and citric acid) [60].
Microorganisms for ethanol fermentation can be assessed in
terms of the process parameters, nutritional requirements, com-
patibility with existing products, type of fermentation and
equipment. The chief fermentation parameters that govern
microbial metabolism are temperature, pH, growth rate, spec-
ificity, productivity, yield, alcohol tolerance, osmotic tolerance,
inhibitor tolerance and genetic stability [1,56]. Diverse groups
of microorganisms have been employed in the fermentation of
lignocellulosic materials to bioethanol. An ideal microorgan-
ism for ethanol production should have: (1) ethanol yield of
more than 90 % of theoretical estimation; (2) ethanol produc-
tivity of more than 1 g/L/h; (3) ethanol tolerance of more than
40 g/L; (4) robust metabolic characteristics with simple growth
requirements preferably through inexpensive media formula-
tion; (5) ability to grow in undiluted hydrolyzates; (6) growth
conditions to retard contaminants; and (7) resistance to inhib-
itors, acidic pH and higher temperatures [56].
Performing the fermentation as a batch, fed-batch or con-
tinuous process depends on the microbial growth kinetics, the
type of lignocellulosic hydrolysate and the economics associ-
ated with the operation [55,56]. Batch fermentation is
performed in a closed culture system with a stipulated amount
of nutrient inoculated with the fermentative microbial culture.
It is a multivessel method that has flexibility in operation but is
characterized by low productivity, labour intensive and elab-
orate preparatory procedures [62]. In continuous fermentation,
the substrate, culture medium and nutrients are pumped con-
tinuously into a bioreactor where the microorganisms are
Biomass Conv. Bioref.
active. In addition, it has an ease of control and relatively less
labour intensive; although contamination is considered a seri-
ous problem that needs the entire process interruption and
clean-up [62]. Fed-batch reactors combine the advantages of
both batch and continuous processes and have found exten-
sive industrial applications. Through fed-batch fermentation,
it is possible to regulate process parameters at specific levels
through feedback control, maximized viable cell count,
prolonged culture lifetime and increased productivity.
Continuous fermentation often provides a higher productivity
than batch fermentation but all the substrate is not consumed
in the process. In contrast, fed-batch fermentation works at
low substrate concentrations with an increasing ethanol con-
centration during the course of fermentation. However,
irrespective of the fermentation type, the addition of buffers
for pH control, antifoaming agents, supplement of vitamins,
amino acids and antibiotics and aeration for aerobic fermen-
tations are indispensable.
Fungi, principally many yeast species, are well-known for
ethanol production. For bacterial strains to be more efficient
than yeast in ethanol fermentation, the former should: (1) be
capable of producing ethanol reliably in larger bioreactors, (2)
greatly reduce needs for saccharification enzymes and (3) be
able to ferment the media in bulk even though not fully aseptic.
Some notable fungus belonging to genus Neurospora,Monilia ,
Paecilomyces ,Fusarium ,Sclerotium ,Phanerochaete ,
Trichoderma,Aspergillus ,Schizophyllum and Penicillium are
reported to have the ability to ferment cellulose directly to
ethanol [41]. The yeast Schizosachharomyces pombe has a
characteristic feature of resisting a high osmotic pressure within
the fermenter [63]. Saccharomyces cerevisiae is referred to as
one of the model organisms in industrial biotechnology for
exhibiting a high ethanol tolerance and producing ethanol at
an elevated rate even in limiting oxygenation conditions. A
major drawback in using S.cerevisiae for hemicellulose con-
version is that it is unable to ferment xylose naturally. However,
it can ferment xylulose and in the presence of xylose isomerase,
xylose is converted to xylulose which is then fermented to
ethanol. Attempts have been made to improve sugar uptake by
S.cerevisiae for enhancing its ethanol production from ligno-
cellulosics. In an investigation by Katahira et al. [64], a Pichia
stipitis gene encoding a sugar transporter, SUT1 was expressed
in S.cerevisiae strain that expresses xylose reductase,
xylosedehydrogenase and xylulokinase. The yield of ethanol
during xylose and glucose co-fermentationbytheSut1-
expressing yeast strain was 0.44 g/g consumed sugar, whereas
the parental strain produced only 0.39 g/g consumed sugar.
Yeasts such as P.stipitis ,Pachysolen tannophilus and
Candida shehatae can convert xylose to xylulose through
sequential reduction reactions. The oxidation steps and the
phosphorylation of xylulose allow entry of the sugar phos-
phate into the pentose-phosphate pathway [65]. P.stipitis,C.
shehatae and C.parapsilosis can metabolize xylose through
the action of xylose reductase to convert xylose to xylitol and
through the action of xylitol dehydrogenase, xylitol is
converted to xylulose. This has made it possible for the
recombinant S.cerevisiae to perform ethanol fermentation
from xylose by carrying the gene for heterologous xylose
reductase and xylitol dehydrogenase from P.stipitis and
xylulokinase from the wild-type S.cerevisiae [66].
Over the last five decades, aerobic fungus T.reesei has gained
attention in yielding cellulases for ethanol biorefineries. T.reesei
secretes three types of extracellular cellulolytic enzymes includ-
ing five endoglucanases, two cellobiohydrolases and two
β-glucosidases. Cellobiohydrolases are of special interest as they
tend to constitute 60–80 % of natural cellulase systems. T.reesei
is also known for its xylanolytic activities to degrade hemicellu-
lose by secreting two endo-β-xylanases- xylanase I and xylanase
II. Besides T.rees ei ,Aspergillus oryzae is an efficient xylan-
degrading fungi for its ability of β-xylosidase production.
A few bacteria and actinomycetes that have been explored
for cellulase production include Clostridium ,Cellulomonas,
Bacillus,Rumino coccus ,Bacteriodes ,Erwinia,Acetovibrio,
Microbispora and Streptomyces [67]. Thermomonospora
fusca is an aerobic filamentous bacteria having the ability to
produce β-1,4-endoglucanases, endo-cellulase, exo-cellulase
and xylanases. Ruminococcus albus is an anaerobic cellulo-
lytic rumen bacterium that produces highly active cellulolytic
enzymes and in which β-glucosidase catalyzes the hydrolysis
of cellobiose and cello-oligosaccharides during the final deg-
radation of cellulosic materials [19]. The potential bacteria for
industrial ethanol production are Zymononas mobilis,
Zymobacter palmae,Escherichia coli,Erwinia chrysanthemi
and Klebsiella oxytoca [68]. Z.mobilis produces ethanol up
to 97 % of the theoretical yield [63], but has narrow substrate
specificity for glucose, fructose and sucrose. The bacterium
is found to produce undesirable compounds such as sorbitol
which reduces the efficiency of bioconversion [69]. However,
the recombinant Z.mobilis has the benefits of requiring
minimal nutrients and grows at low pH and high temperatures
[1].
Fungi are able to degrade cellulose, hemicelluloses and
lignin through a series of enzymatic reactions involving hydro-
lytic and oxidative enzymes such as cellulases, hemicellulases
and ligninases, whereas actinomycetes (i.e. Streptomyces spp.)
are able to degrade lignocellulose found in soil and compost as
plant debris via the same enzyme series. In addition, certain
bacteria and yeast are found helpful in reducing the levels of
furfural in the medium by partially transforming it to furfuryl
alcohol and/or furoic acid [70]. Furfural reductase, an enzyme
significant in the detoxification of furfural during ethanol pro-
duction, has been successfully purified and characterized from
E.coli strain LYO1 by Gutierrez et al. [69]. Thermophilic
anaerobic bacteria are advantageous over the conventional
yeasts in ethanol production for their capability to withstand
extreme temperatures and utilizing a variety of inexpensive
Biomass Conv. Bioref.
feedstock but their low ethanol tolerance (<2 %, v/v)isa
major impediment in fermentation [71]. Some thermophilic
anaerobic bacteria have been examined for their ethanol
productivity including Clostridium thermohydrosulfuricum
[72], Clostridium thermosaccharolyticum [73],
Thermoanaerobacter brockii [74], Thermoanaerobacter
mathranii [75], Thermoanaerobacter ethanolicus and
Thermoanaerobacter thermohydrosulfuricus [76].
Biofuel yields from lignocellulosics vary significantly
among feedstocks. Some studies have shown that bioconver-
sion is dependent upon the chemical nature of the feedstock
and that the easiest bioconversions are achieved with herba-
ceous residues. Compared to woody biomass, agricultural
residues have a higher surface area and small pore size which
reduces their susceptibility to enzymatic hydrolysis [77]. In
contrast to softwood residues, hardwood biomass has more
cellulose and less hemicellulose which produce more glucose
and xylose for easier bioconversion [78]. In addition, the
hemicellulose of hardwood contains more xylose which is
difficult to hydrolyse than other pentose sugars.
The use of recombinant microorganisms in ethanol
biorefineries to improve product yield is gaining attention.
Recombinant strains rely on plasmids for gene expression
which is often lost from cultures growing in nonselective media
[56]. Therefore, a vigorous microbial physiology for the prop-
agation of robust strains with stable gene expressions should be
maintained for high ethanol productivity. There is a growing
interest in understanding and exploiting the industrial impor-
tance of yeasts in displaying various enzymes for digesting the
waste biomass. Yeast cell-surface engineering is one of the
emerging new strategies for making the yeast express several
bioprocessing enzymes on its cell surface in a larger density
even in inexpensive media [63]. Yeast cell surfaces bear many
glucoamylase-extractable proteins, such as agglutinin and
flocculin having glucosylphosphotidylinosital anchors that
play a vital role in the expression of cell surface proteins [79].
5 Hydrothermal conversion pathways
5.1 Supercritical water gasification
As discussed earlier in the hydrothermal pretreatments, water
above its critical temperature (374 °C) and pressure (22.1 MPa)
is called as supercritical water. The thermophysical properties
of SCW make it suitable for oxidation of waste streams. The
nonpolar characteristic of SCW enhances its ability to dissolve
nonpolar compounds forming a homogeneous phase suitable
for the processing of organic wastes [80]. Water acts as a
reactant and also as a medium to convert biomass into gaseous
fuels. The unique solvating and transport properties of SCW
make it attractive for various hydrothermal conversion of
waste biomass such as gasification.
Biomass when subjected to pyrolysis at ambient pressure
results in hydrocarbon-rich syngas, some refractory tar compo-
nents and biochars [81]. However, the biochar formation can be
avoided by using SCW for the gasification of biomass. The
breakthrough works on conversion of wet biomass into
hydrocarbon-rich gas in the presence of catalysts at high tem-
peratures (400–450 °C) and pressures (≤35 MPa) drives to
consider lignocellulosic biomass as a potential feedstock for
alternative fuels. The major issues of conventional gasification
such as biochar formation and requirement of low moisture
content biomass are ruled out with this supercritical water
gasification (SCWG) technology. Hence, this technology has
been found to be promising for the production of H
2
from
biomass over last few years.
During gasification, cellulose and hemicellulose break into
simple sugars such as glucose and fructose [82], whereas lignin
converts to phenolics [83]. These intermediates further convert to
gases in SCW medium. The knowledge of behaviour of biomass
constituents and their reaction mechanisms are essential for the
effective production of fuel products. The pathways for the
intermediates and their degradation help in designing suitable
reactor with optimum operating conditions for maximum H
2
production. Glucose is a model compound to understand the
gasification of the biomass constituents, especially holocellulose.
Glucose, on gasification, results in H
2
and CO as major compo-
nents along with CH
4
,CO
2
and little amounts of biochar. The
knowledge on the gasification of model compounds such as
catechol and guaiacol helps to understand the SCWG of lignin.
The gasification products of lignin mainly comprise of H
2
,CH
4
and CO
2
with small amount of CO. The feed concentration,
temperature and pressure influence the product gas compositions
to a greater extent. Glycerol serves as a substitute model com-
pound in the cases where glucose undergoes dehydration to form
ring structure compounds such as HMFs which usually occurs
below the critical temperature of water. Depending on the oper-
ating temperature, the gasification process can be classified as
low-temperature gasification (300–500 °C) and high-
temperature gasification (500–800 °C).
Low-temperature gasification of biomass leads to low con-
centrations of H
2
in the gaseous products. Along with the
gaseous product, oil-based liquids are also formed. The liquid
contains a wide range of products such as acids, phenols,
aldehydes and furfurals. Ions (H
+
and OH
−
) from SCW ioni-
zation at high density are favoured that further supports the
cleavage of ring compounds to form simple molecules. Polar
and ionic reaction pathways are dominant for gasification of
biomass at low temperatures in SCW [80]. In contrast to the
ionic reaction pathways, free radical mechanisms dominate in
high temperature SCWG of biomass [84]. The concentration
of free radical depends on the gasification temperature and the
reactants. The low density at high temperatures makes the free
radical mechanisms as major pathways for the conversion of
organic compounds in SCW.
Biomass Conv. Bioref.
Hydrogenation is a key step for the chain termination of
free radical mechanisms to breakdown polymeric molecules
into lighter components. Water at a very high temperature and
pressure has weak hydrogen bonding (inter- and intramolec-
ular) properties, which makes it a good source of H
2
.
Gasification of glucose below 600 °C results in H
2
, CO,
CO
2
,CH
4
and low concentrations of other gaseous products
[85]. At temperatures above 600 °C, H
2
and CO
2
are the major
products. Water–gas shift (WGS) reaction is another route to
increase H
2
production during the gasification of biomass at
high temperatures. WGS reaction is weakly exothermic and
becomes prominent at high temperatures in the SCWG. The
concentration of CO decreases significantly while that of H
2
and CO
2
increases. The concentration of glucose has a signif-
icant impact on syngas production. With higher concentra-
tions of glucose, the levels of H
2
and CO
2
decrease, while the
levels of CO, CH
4
and C
2
H
6
increase.
Lignin in the biomass primarily dissociates into phenolics in
SCW with the help of H
+
and OH
−
ions; subsequently, these
phenols decompose to gases. Lignin initially decomposes via
hydrolysis and dealkylation forming phenols, formaldehyde and
low molecular weight compounds with reactive functional
groups. These reactive functional group compounds with form-
aldehyde undergo cross-linking to form higher molecular
weight components. Gasification of lignin results in four phases,
especially oil, aqueous, gas and solid phase [83]. The ether
linkage components of lignin which are dissolved in SCW form
oils. The components such as guaiacols, syringols undergo
hydrolysis and dealkylation to produce water-soluble methanol
and catechols. Dealkylation of the components form gases and
hydrocarbons along with aqueous compounds such as acids,
alcohols and aldehydes. Repolymerization of degraded compo-
nents is favoured at high temperatures to form phenolic char.
Phenolic char is formed on the reaction of the degradation
products of lignin (e.g. phenolics) with aldehydes. The
nondissolved components of lignin lead to formation of gas,
hydrocarbons, phenolic mixtures and water-soluble compounds.
The aqueous phase phenolics are hydrolyzed and dealkylated to
form phenolic oils, gases and other soluble compounds.
The addition of phenol to the lignin for SCWG enhances the
degradation of lignin and significantly inhibits the repo-
lymerization of the reactive components [83]. All the higher
and substituted phenols (e.g. cresols, guaiacols and catechols)
deforms to stable phenolic compounds. CO
2
and CH
4
are the
major products of lignin gasification at high temperatures in
SCW. The gasification of lignin near critical temperature results
in maximum production of CO
2
.However,H
2
production can
be enhanced with the increase in temperature (600–700 °C),
while reducing the concentration of CO. Increase in the lignin
concentration for SCWG decreases H
2
and CO, while substan-
tially increasing CH
4
concentrations.
Studies on both catalytic and noncatalytic routes for SCWG
of biomass and its constituents such as cellulose, hemicellulose
and lignin have been reported in literature [80,82,86]. The
different operating conditions and product yields for catalytic
and noncatalytic SCWG have been presented in Table 3.The
noncatalytic studies on SCWG of biomass infer that the prod-
uct gas results in higher amounts of CO which can be subse-
quently upgraded to H
2
and CO
2
via WGS reaction. Higher
conversions can be obtained by heterogeneous catalysis of
SCW at low temperatures while similar conversions can be
attained at the expense of high temperature and pressures
without catalyst. The gasification products of biomass are
CO, H
2
,CO
2
and CH
4
. The syngas obtained from the
SCWG can be further processed to produce either H
2
-rich
gas product (via WGS reaction) or other liquid fuels (via
syngas fermentation or FT catalysis). The production of etha-
nol from the gasification products of biomass can be done by
two routes: (1) syngas fermentation (biosynthesis route) and
(2) FT catalysis (thermochemical route).
5.1.1 Noncatalytic SCW gasification
The lignocellulosic biopolymers (e.g. cellulose, hemicellulose
and lignin) can be hydrolyzed to sugars and phenolics.
Subsequently, these intermediates undergo gasification to pro-
duce gaseous products (e.g. CO, H
2
,CO
2
and CH
4
). The basic
reactions that occur during noncatalytic and catalytic SCWG
canbesummarizedas:
Cellulose=hemicellulose→sugars mainly glucoseðÞ→syngas
Lignin→phenolic compounds→gaseous products
C6H10O5
ðÞ
nþnH2O→nC6H12O6ð1Þ
C6H12O6→6CO þ6H2ð2Þ
C10H10 O3
ðÞ
nþnH2O→nC10H12 O4ð3Þ
C10H12 O4→Phenolics ð4Þ
Phenolics þH2O→CO þCO2þH2ð5Þ
CO þH2O→CO2þH2ð6Þ
CO þ3H2→CH4þH2Oð7Þ
CO þ2H2→CH4þ0:5O2ð8Þ
Resende et al. [91] reported that CH
4
and CO
2
are produced
in higher quantities compared to that of H
2
andCObySCWG
of lignin in quartz reactors. In addition, the gas yields from
cellulose and lignin have been compared in the same study at
the same operating conditions. The gas product from cellulose
hadhighH
2
compared to that of lignin products. The yield of H
2
(1.8 mmol/g) was found to be high for cellulose compared to
that of lignin (0.7 mmol/g). Susanti et al. [84] studied the effects
of temperature, feed concentration and reactions times on
SCWG of glucose in updraft gasification equipment and found
that the maximum H
2
yield (10–11.5mol/molofglucose)was
Biomass Conv. Bioref.
obtained at the lowest investigated concentration (1.8 wt%) in
60 s.
5.1.2 Catalytic SCW gasification
Research on the catalytic SCWG of biomass for H
2
produc-
tion has been extensively growing over the last few decades.
To understand SCWG of biomass, the model compounds (e.g.
cellulose, glucose and lignin) have been studied and these
insights can be extended to the real biomass. For the catalytic
SCWG, various parameters have been widely examined
which include type of reactors, catalysts, activity, selectivity,
reaction pathways, stability, operating conditions, feed con-
centration and feed constituents [80,86,92]. The design of
reactors, corrosion and salt precipitation problems of SCWG
have been technically reviewed by Bermejo and Cocero [93].
To screen the catalysts for the production of H
2
from biomass,
the following needs are to be considered: (1) high catalytic
activity for C–C bond cleavage (breaking down of sugar
monomers), (2) high activity for WGS reaction, (3) stability
and durability of the catalyst/support at the operating condi-
tions and (4) low activity for C–O cleavage (to reduce the
formation of hydrocarbons).
ThecatalystswhichareusuallyemployedinSCWGare
activated carbon, transition metals with or without supports
[e.g. Ni, Ru, Pt, Pd, Ru/C, Ni/silica-alumina, Pt/γ-Al
2
O
3
,Ru/
Al
2
O
3
, Ni/MgO, Ni/α-Al
2
O
3
, Ni/carbon nanotubes (CNT), Ru/
CNT and Ru/TiO
2
] and oxides (e.g. ZrO
2
,CeO
2
and RuO
2
).
The catalysts such as Ni and Ru are found to have high catalytic
activity for the SCWG of biomass [80,86]. The catalytic
stability and lifetime of Ni catalysts can be increased when
they are doped with other metals such as Ru, Cu and Ag [92].
Addition of Ru to Ni/γ-Al
2
O
3
improved the catalytic activity
and stability for SCWG of glucose along with an increase in the
Tabl e 3 Operating conditions
and product yields from various
hydrothermal and thermochemi-
cal conversion processes
Data source for pyrolysis, [87,
88]; liquefaction, [89]; and gasi-
fication, [90]
Conversion pathways Operating conditions Product yields
Hydrothermal Noncatalytic SCWG Temperature: 400–900 °C CO
2
:29–38 mol%
Pressure: 23–35 MPa CO: 1–13 mol%
H
2
:25–68 mol%
CH
4
:2–15 mol%
Catalytic SCWG Temperature: 300–600 °C CO
2
:25–52 mol%
Pressure: 23–35 MPa CO: 1–8 mol%
H
2
:12–55 mol%
CH
4
:5–21 mol%
Themochemical Slow pyrolysis Temperature: 300–700 °C Bio-oil: ∼30 wt%
Vapor residence time: 10–100 min Biochar: ∼35 wt%
Heating rate: 0.1-1 °C/s Gases: ∼35 wt%
Fast pyrolysis Temperature: 400–800 °C Bio-oil: ∼50 wt%
Vapor residence time: 0.5–5s Biochar:∼20 wt%
Heating rate: 10–200 °C/s Gases: ∼30 wt%
Flash pyrolysis Temperature: 800–1000 °C Bio-oil: ∼75 wt%
Vapor residence time: <0.5 s Biochar: ∼12 wt%
Heating rate: >1,000 °C/s Gases: ∼13 wt%
Liquefaction Temperature: 200–330 °C Bio-oil: 2.5–22.5 wt%
Retention time: 30 min Gases: 1–30 wt%
Volatile organics: 1–45 wt%
Water solubles: 1–17 wt%
Temperature: 320 °C Bio-oil: 17–27 wt%
Retention time: up to 120 min Gases: 12–32 wt%
Volatile organics: 30–45 wt%
Water solubles: 5–17 wt%
Gasification Temperature: 1,000 °C CO
2
:∼8 vol%
Pressure: 0.1 MPa CO: ∼17 vol%
H
2
:∼65 vol%
CH
4
:∼9 vol%
C
2
H
4
:∼0.6 vol%
C
2
H
6
:∼0.3 vol%
Biomass Conv. Bioref.
H
2
yield (50 mol/kg of glucose) [94]. A better performance of
Ni/CeO
2
-γ-Al
2
O
3
has been reported over Ni/γ-Al
2
O
3
for
SCWG of glucose in an autoclave reactor. H
2
selectivity and
yield are reported to be higher for Ni/CeO
2
-γ-Al
2
O
3
since Ce
showed inhibition to coke forming [95]. Azadi et al. [96]
investigated the activity and selectivity for H
2
and addition of
promoters (e.g. K, Na, Cs and Sn) to Ni catalyst for SCWG of
glucose with different supports. They reported that Ni/α-Al
2
O
3
with the addition of alkali promoters (e.g. K and Na) improved
the H
2
selectivity and carbon conversion. Different catalysts
such as Ni/α-Al
2
O
3
, Ni/hydrotalcite, Raney Ni, Ru/C and Ru/
γ-Al
2
O
3
have been tested for SCWG of biomass model com-
pounds (e.g. glucose, cellulose, fructose, xylan, lignin, bark and
pulp); however, it was found that Ni-based catalysts show high
activity and H
2
selectivity compared to the other catalysts [97].
Stability of supports for Ru (e.g. Ru/TiO
2
,Ru/γ-Al
2
O
3
and
Ru/C) has been tested for SCWG of lignin. The results show
that Ru/TiO
2
has high gasification efficiency and stability com-
pared to the other two Ru-supported catalysts [98]. Ru-trivalent
salts have also been used for lignin gasification in SCW [99].
Ni/MgO catalyst has also been found to be a promising catalyst
forSCWGoflignin[100]. SCWG of biomass in the presence of
transition metals (Pt and Pd), homogenous base oxide catalysts
(e.g. KOH, NaOH and CaO) and heterogeneous oxide catalysts
(e.g. ZrO
2
, CeO
2
and RuO
2
) have been studied [80,86].
Madenoglu et al. [101] investigated SCWG for different bio-
mass with high lignin content in presence of two catalysts (i.e.
K
2
CO
3
and Trona) and observed that both catalysts are effective
to produce high H
2
/CH
4
-rich gaseous products. Bimetallic cat-
alysts are capable for effective gasification of biomass with high
catalytic activity, stability and selectivity [102]. The knowledge
about the catalyst structure, synthesis to its activity helps in
designing novel catalysts for SCWG. Furthermore, the addition
of alkalis to Ru/α-Al
2
O
3
has been found to enhance both H
2
and
CH
4
selectivity (14.7 and 12.8 mol/kg of glucose, respectively)
forSCWGofglucose[103].
Another technology to produce H
2
from biomass is appli-
cation of solar energy for SCWG. The energy required for
SCWG can be harvested from solar energy instead of fuel and
thereby reducing its high energy requirements. The investiga-
tions by Lu et al. [104] on SCWG of biomass with solar energy
showed promising results to produce H
2
from the decomposi-
tion of water and biomass with higher efficiencies. Further
details about the technology have been presented in the recent
review by Nzihou et al. [105]. The effect of a catalyst, its
activity, selectivity and stability are the major concerns of
catalytic gasification of biomass in SCW.
5.2 Syngas fermentation
The raw syngas obtained from gasification of the biomass can be
directly used to produce ethanol by syngas fermentation.
Fermentation of syngas for ethanol production requires
acetogens which are capable of producing ethanol (solventogenic
phase) over acetic acid (acidogenic phase). An appropriate pH is
essential during the fermentation to enhance the mass transfer of
gaseous substrates and remove inhibitory substances from the
medium. Acetogenic bacteria are able to generate acetate as a
product of anaerobic respiration through acetogenesis.
Acetogens are anaerobic in nature and mostly use CO
2
as carbon
source and H
2
as energy source. A few mesophillic bacteria such
as Acetobacterium woodii,Clostridium carboxidivorans,
Peptostreptococcus spp., Clostridium aceticum ,Clostridium
ljungdahlii,Clostridium carboxydivorans ,Clostridium
ragsdalei and Clostridium autoethanogenum have been found
useful in fermenting syngas to liquid fuels more effectively than
the conventional catalytic process [106–108]. Majority of studies
on syngas fermentation to ethanol have been performed using
Clostridium spp., especially C.ljungdahlii [109].
Tab le 4presents a few bacteria that have been used for
syngas fermentation to produce liquid fuels, mainly ethanol.
Cotter et al. [113] have investigated the process parameters on
autotrophic bacteria C.ljungdahlii and C.autoethanogenum
and reported their abilities to produce significant amounts of
ethanol (2.2–4.7 mM) and acetic acid (35–37 mM) from the
biomass gasification products (e.g. H
2
,COandCO
2
). The
microorganisms (acetogens) which are used for syngas fer-
mentation use acetyl–CoA pathway to produce liquid prod-
ucts such as ethanol, acetic acid, butanol and butyrate [122].
Acetic acid concentrations are high during the growth phase of
the bacteria, whereas high ethanol concentrations are attained
during the nongrowth phase (solventogenic phase) [123].
Acetogens converts the gaseous products of biomass gasifica-
tion in the presence of various enzymes (reduction reactions)
to an intermediate product such as acetyl CoA. Acetyl CoA
with the help of phospotransacetylase and acetate kinase
transforms to acetic acid, while ethanol can be produced by
aldehyde dehydrogenase and alcohol dehydrogenase en-
zymes. The reactions involving the synthesis of ethanol and
acetic acid from syngas fermentation are as follows:
2CO2þ6H2→C2H5OH þ3H2Oð9Þ
6CO þ3H2O→C2H5OH þ4CO2ð10Þ
2CO2þ4H2→CH3COOH þ2H2Oð11Þ
4CO þ2H2O→CH3COOH þ2CO2ð12Þ
The enzymes hydrogenase and CO dehydrogenase are
essential for syngas fermentation [108]. The electrons required
for reduction reactions are primarily supplied by CO followed
by H
2
which implies that the composition of syngas has
significant impact on the ethanol production. The mechanisms
for production of acetic acid and ethanol from syngas fermen-
tation can be found elsewhere [106,109,124].
Biomass Conv. Bioref.
The metabolic activity of microorganisms can be enhanced
by adding necessary supplements such as nutrients, reducing
agents and other medium components. The solventogenic
phase is influenced by a nutrient limited medium, pH, H
2
composition, addition of reducing agents and presence of trace
metals in the medium [115]. Recent studies have shown that
corn seed extract [125] and corn steep liquor [119] can be used
for ethanol production via syngas fermentation. The optimum
pH for ethanol production is around four to six depending on
the bacteria used for fermentation. The solventogenic phase is
found to be prominent at a pH 4–4.5 for C.ljungdahlii with
maximum ethanol production of 48 g/L [126]. The acetic acid
generated in the growth phase decreases the acidity of the
medium, thereby favouring ethanol production. Temperature
is another optimal parameter which needs to be considered for
metabolic activity of the microorganisms to maximize ethanol
yields. The optimum temperature for syngas fermentation has
been reported to be around 37–40 °C for mesophilic bacteria to
produce ethanol [109].
Another significant criterion for effective fermentation of
syngas is the mass transfer of gaseous substrates into liquid
phase. Bredwell et al. [127] have discussed various reactor
configurations for better mass transfer of the gaseous substrates
(i.e. syngas) into the liquid medium. Different reactor configu-
rations for syngas fermentation have been implemented to
enhance the mass transfer of gaseous substrate in the liquid
phase [124]. Most of the investigations of syngas fermentation
have been carried out in continuous stirred tank reactors
(CSTR). Mohammadi et al. [117] conducted fermentation in
CSTR and reported a maximum of 6.5 g/L of ethanol produc-
tion from syngas (55 % CO, 20 % H
2
,10%CO
2
and 15 % Ar).
Munasinghe and Khanal [128] reported that CO mass transfer
can be enhanced by using composite hollow fibre membrane
reactor. Immobilized hollow fibre membrane reactors are also
promising for effective mass transfer rates of gaseous substrates
during syngas fermentation [129,130].
The effects of impurities present in gas feeds on syngas
fermentation have been thoroughly discussed [131,132].
Nitrogen oxide has been reported to be a potential inhibitor
for hydrogenase enzyme in ethanol production from syngas
[133]. In addition to NO, NH
3
has also been found to inhibit
the acetogenic process [134]. Xu and Lewis[131]investigated
the effect of NH
3
on the activity of hydogenase enzyme which
is crucial for ethanol production. Nickel has been found to
promote activities of CO dehydrogenase and acetyl CoA
synthase which are important in the production of ethanol
[122]. A recent study on the effect of trace metals on syngas
fermentation for liquid fuels (ethanol) in the presence of C.
ragsdalei revealed that Cu
2+
inhibits the ethanol production,
while Ni
2+
and Zn
2+
enhance it [135]. Moreover, addi-
tion of reducing agent (e.g. methyl viologen) improved
the ethanol production by C.ragsdalei during the fer-
mentation of syngas [136].
Tab l e 4 Microorganisms used in fermentation of syngas
Microorganism Temperature (°C) pH Reactor Products Reference
Alkalibaculum bacchi CP11 37 8–8.5 –Acetate, ethanol [110]
37 8 Batch Acetate, ethanol [111]
Bacterium P7 37 5-6 Bubble column reactor Acetic acid (0.03 g), ethanol (0.2 g), [112 ]
butanol (0.08 g) per mole of CO
Clostridium autoethanogenum 37 6 –Acetate (8.6–23.3 mM), ethanol (0.83–1.5 mM) [113]
37 4.7 CSTR Acetate (0.3 g/L), ethanol (0.3 g/L) [114]
30 4.8–5.8 Batch Acetate (1.7 g/L), ethanol (0.7 g/L) [115]
Clostridium carboxidivorans 38 6.2 Batch Acetate (0.03 mmol), ethanol (0.2 mmol),
butanol (0.04 mmol) per mmol of CO
[116]
Clostridium ljungdahlii 37 4.5–6.8 CSTR Acetate (5.4 g/L), ethanol (6.5 g/L) [11 7]
37 4 CSTR Acetate (4.1 g/L), ethanol (4.5 g/L) [118]
37 5.5, 6.8 –Acetate (35–37 mM), ethanol (2.2–4.7 mM) [113]
Clostridium ragsdalei 37 4–6 Batch Acetate (50 mM), ethanol (120 mM) [119]
32 5–7 - Acetate (7.9 g/L), ethanol (1.9 g/L) [120]
37 4.7–6 CSTR Acetate (4.8 g/L), ethanol (25.3 g/L), [121]
2-propanol (9.3 g/L), butanol (0.5 g/L)
Biomass Conv. Bioref.
5.3 Fischer–Tropsch catalysis
Conversion of gaseous products (mostly syngas) into liquid
fuels by using catalysts is known as FT catalysis. The biomass
gasification products after purification are required to be ad-
justed for their H
2
/CO ratio (∼1–4) which is usually done via
WGS reaction. The syngas with specified ratio in presence of
various catalysts (e.g. Fe, Cu, Co, Rh, Ru and Ni) produces
liquid fuels [137]. Depending on the catalysts, the syngas
products undergo various reactions to form ethanol, higher
alcohols and hydrocarbons. The chemical reactions involved
in the process are as follows:
2CO þ4H2→C2H5OH þH2Oð13Þ
2CO2þ6H2→C2H5OH þ3H2O the same with 9ðÞðÞ
CO þ3H2→CH4þH2O the same with 7ðÞðÞ
CO2þ4H2→CH4þ2H2Oð14Þ
CO þ2H2→CH3OH ð15Þ
CH3OH þCO þH2→C2H5OH ð16Þ
nCO þ2nþ1ðÞH2→CnH2nþ1ðÞ
þnH2Oð17Þ
nCO þ2nH2→CnH2nþnH2Oð18Þ
C2H2n−1OH þCO þ2H2→CH3CH2
ðÞ
nOH þH2Oð19Þ
Noble metal-based catalysts (e.g. Rh, Ru and Re) show
higher activity and selectivity for ethanol, whereas non-noble
metal-based catalysts (e.g. Zn, Mo, Fe, Mn, Co and Cr)
produce a mixture of alcohols [138]. Different mechanisms
have been proposed for ethanol and higher alcohols produc-
tion from syngas [139–141]. Rh-based catalysts have been
investigated extensively for ethanol production because of
their high selectivity towards ethanol. Rh/CNT is found to
improve the catalytic activity for producing ethanol [142]. CO
hydrogenation and selectivity is high for Rh/CNT compared to
other carbon supports [143]. Haider et al. [144]havestudied
the performance of the catalysts Rh/SiO
2
and Rh/TiO
2
by
adding Fe to synthesize ethanol from syngas. Mei et al.
[145] conducted both theoretical and experimental studies
over Rh/SiO
2
with different promoters for CO hydrogenation
to produce ethanol. Rh promoted with Mn and Li on the
supports SiO
2
,TiO
2
and SiO
2
-TiO
2
showed selectivity for
ethanol and C
2
+ oxygenates [146]. Rh-Mn supported on
molecular sieves was able to convert syngas to ethanol with
selectivity near to 13 % [147]. The addition of alkali metals
improved the selectivity for alcohols over to the hydrocarbons
for CO hydrogenation [148,149]. Cu/Co/Cr catalysts modi-
fied with Zn, Mn, Li, Na, K, Rb and Cs have been investigated
for CO hydrogenation to synthesize higher alcohols [148].
These studies suggest that ethanol productivity and selectivity
depend on temperature and the catalyst composition.
Gong et al. [150] proposed a new route for ethanol produc-
tion with Cu/SiO
2
where CO gets coupled with methanol to
form dimethyl oxalate followed by hydrogenation. The per-
formance of the Cu-based catalysts with different Cu loadings
has been evaluated for maximum ethanol production, al-
though 20 % Cu/SiO
2
demonstrated a maximum selectivity
(83 %) for ethanol. Another new method has been proposed
for ethanol synthesis by using dimethyl ether (DME) and
syngas [151]. DME undergoes carbonylation to form methyl
acetate which is further hydrogenated to form ethanol and
methanol. Liu et al. [152] presented the catalysts that can be
applied for this process along with the reaction conditions.
DME can be synthesized from syngas via methanol synthe-
sisfollowedbydehydration[153]. CO and CO
2
combine with
H
2
to form methanol, which undergoes dehydration to DME.
WGSreactionisalsoinvolvedinDMEsynthesis[154]. The
synthesis of methanol from syngas has been found to be
effective in the presence of metal catalyst while the acid
catalyst activates dehydration of methanol [155]. The effects
of various catalysts [(hybrid/bifunctional: CuO-ZnO-Al
2
O
3
/γ-
Al
2
O
3
, CuO-ZnO-Al
2
O
3
/NAHZSM-5, Cu-ZnO-Al
2
O
3
/
(ferrierite or ZSM-5, NaY, HY)] on the direct synthesis of
DME from syngas have been discussed in literature [153–157].
Ethanol and methanol are produced in considerable
amounts (30 and 40 %, respectively) along with other alcohols
when Cu-Co supported on a composite MWCNT/silica [158].
Higher alcohols along with ethanol can be produced by using
Rh as a promoter on Mo-K/MWCNT with significant selec-
tivities of 25 and 16 % [159]. Ethanol was found to be a major
product for Cu-promoted Fe/MnO
2
in a slurry reactor over
260–300 °C [160]. About 60 % ethanol of total oxygenates
has been produced from CO hydrogenation with Cu addition
to Fe-K/activated carbon [161]. Rh-La/V/SiO
2
showed a max-
imum selectivity (52 %) for ethanol with 8 % CO conversion
[162]. The selectivity and productivity of alcohols (ethanol) is
higher compared to that of hydrocarbons with MoS
2
/γ-Al
2
O
3
[163].
CO
2
hydrogenation has also been investigated for ethanol
synthesis from syngas. The high stability of CO
2
implies the
need for a high temperature and pressure with maximum
catalytic selectivity for ethanol. The direct hydrogenation of
CO
2
is considered as a combination of reverse WGS reaction
followed by conversion into alcohols. This implies that the
catalysts for the conversion of syngas to alcohols need to
catalyze both the reactions for higher yields. Various catalysts
have been studied for the ethanol selectivity through CO
2
hydrogenation [164]. A combination of Fe- and Cu-based
catalysts with suitable promoters favours alcohol formation
by suppressing CH
4
formation [165]. It has been reported that
K-Cu/Fe/Zno catalyst enhanced the selectivity of ethanol to
20 % at 300 °C and 7 MPa pressure [166].
Rh-based catalysts with SiO
2
support are also found to be
effective for ethanol production. The effects of promoters on
Biomass Conv. Bioref.
5 % Rh/SiO
2
for ethanol selectivity have been tested and a
selectivity of 15.5 % and conversion of 7 % towards ethanol is
found to be significant with Li promoter [167]. A maximum
conversion of 26.7 % for CO
2
has been achieved with 16 %
selectivity for ethanol using 5 % Rh-Fe/SiO
2
at 260 °C [168].
Ethanol can be produced significantly by CO
2
hydrogenation
in the presence of multifunctional (mixture of Rh, Fe and Cu)
catalysts [169]. Ru-based catalysts with CNT showed better
performance than other carbon supports [143]. Rh-Mn-Li-Fe/
CNT showed maximum production of C
2
oxygenates with
76 % ethanol [142].
The process evaluation of chemical route for ethanol pro-
duction involves gasification of biomass followed by syngas
cleaning, conditioning (adjusting H
2
/CO/CO
2
ratio), FT catal-
ysis and separation of products. The exergy analysis for etha-
nol production by chemical route from wood biomass in the
presence of Rh and Mo-based catalysts has been evaluated by
Heijden and Ptasinski [170]. The studies confirm that both the
catalysts have same efficiency for ethanol production. Wei
et al. [171] conducted the process evaluation for ethanol pro-
duction from wood using different pathways. Performance
Index (PI) is the process parameter which is suitable to com-
pare different pathways to produce ethanol and is represented
as follows:
PI ¼weight of ethanol
electricity þsteamðÞinput time ð20Þ
Wei e t a l . [ 171] reported that maximum PI has been
obtained for [gasification+chemical] route over to [gasifica-
tion+ biosynthesis] route. Although the energy inputs for bio-
synthesis route are less but their high processing times
(>20 days) result in their lower PI. The high processing rates,
less water consumption and minimal energy requirement
make the chemical route an alternative pathway for ethanol
production.
Tab le 5compares both biosynthesis (syngas fermentation)
and chemical route (FT catalysis) for synthesis of ethanol from
biomass derived syngas. Griffin and Schultz [25]compared
gas fermentation and FT catalysis for ethanol production from
biomass. Biosynthesis route operates at ambient temperature
and pressure with less processing rates. The biosynthesis route
does not require any specific ratio of H
2
/CO/CO
2
and also has
high selectivity for ethanol production. On the other hand, FT
catalysis operates at high temperature and pressure with a
specified H
2
/CO ratio. The catalysts have low selectivity for
ethanol and are also prone to poisoning. High carbon to fuel
conversion efficiency, higher selectivity with lesser GHG
emissions and flexible H
2
/CO ratio makes biosynthesis route
compete with chemical route for ethanol production.
6 Thermochemical conversion pathways
6.1 Pyrolysis
Thermochemical conversion has been defined as a chemical
reforming process that converts long-chain organic com-
pounds from the biomass into short-chain oxygenated hydro-
carbons. The thermochemical conversion of lignocellulosic
feedstocks is achieved through pyrolysis, gasification and
co-gasification. These are considered as the major thermo-
chemical technologies for syngas and bio-oil production.
Pyrolysis is defined as the degradation of macromolecular
organic materials at elevated temperatures in the absence of
oxygen. Pyrolysis can be divided into three basic types, name-
ly slow (or conventional) pyrolysis, fast pyrolysis and flash
pyrolysis. These three types of pyrolysis are performed by
selectively varying the reactor conditions, especially operating
temperature, heating rate, feedstock particle size and solid/
vapor residence time. The operating conditions for slow, fast
and flash pyrolysis have been given in Table 3.Bothfastand
flash pyrolysis results in higher amount of bio-oils, whereas
slow pyrolysis results in considerable amount of biochars
[172]. The ideal particle sizes of biomass for flash and fast
pyrolysis are less than 0.2 and 1 mm, respectively [88]. For
slow pyrolysis, biomass particles of 5–50 mm are preferred. A
significant fraction of noncondensable gases (e.g. H
2
,CO,
CO
2
,CH
4
,C
2
H
4
and C
2
H
6
) are found in both slow and fast
pyrolysis processes. This is due to the secondary reactions that
Tabl e 5 Comparison between
syngas fermentation and Fischer-
Tropsch catalysis
a
Productivity referred from Grif-
fin and Schultz [25]
Property Syngas fermentation Fischer-Tropsch catalysis
Catalysts Microbial enzymes Solid catalysts
Selectivity Highly specific to ethanol Active for other reactions
Residence time Hours to days Hours
H
2
/CO ratio Not specific 1–4
Productivity (million gallons/year)
a
82.1 64.7
Catalyst poisoning No Yes
Temperature Ambient (∼37 °C) High (200–350 °C)
Pressure 1 atm 10–200 atm
Products Ethanol, butanol, acetic acid Alcohols, hydrocarbons
Biomass Conv. Bioref.
occur during the mass transfer process catalyzed by char fines
and other forms of particulate matter. The noncondensable
gases could be recycled for heat recovery during the pyrolysis
process. The organic vapor resulting from pyrolysis is a com-
plex mixture of aerosols, mist, particulate matter and
noncondensable gases.
Pyrolysis of biomass leads to both primary and secondary
reactions during the vapor release process. Primary reactions
result in gas evolution from the biomass which is quickly
quenched during the condensation process. High vapor con-
densation efficiency at a fast rate is essential for enhanced
quality and quantity of bio-oil. A low rate of vapor condensa-
tion results in secondary reactions that cause lower yields of
bio-oil due to the release of noncondensable gases and water
vapor. Secondary reactions usually lead to the formation of
higher molecular weight compounds such as tar. Tars tend to
plug the condenser lines and hence increase the shutdown
frequency of the reactor which is a major issue in the refineries.
In general, slow pyrolysis with low to medium heating rates
has been used for production of biochar targeted for adsorbent
and solid fuel applications. Biochar also acts as a soil fertilizer
as it contains alkali (e.g. Li, Na and K) and alkaline (e.g. Ca,
Mg and Ba) earth metals. Unlike slow pyrolysis biochar, ash
content of the fast pyrolysis biochar tends to be high because of
its higher reactor temperatures and smaller biomass particle
size requirements. This makes fast pyrolysis less valuable for
biochar production. Furthermore, biomass char can be physi-
cally or chemically activated to emulate the properties of
activated carbon. Currently, fast pyrolysis is the major pyroly-
sis process geared towards producing bio-oils as an alternative
to the dwindling fossil fuel reserves.
The bio-oil as an oxygenated hydrocarbon fuel is known to
recover 80 % of the energy content (maximum yield) from the
feedstocks on a dry matter basis assuming that the biochars
and gases are utilized in the pyrolysis process for heat gener-
ation. Moreover, bio-oil has almost half the high heating value
of hydrocarbon fuels (petroleum, 42–44 MJ/kg) because of its
high oxygen and water content. Removal of moisture and
oxygen from bio-oils through catalytic hydrodeoxygenation
is found to increase the heating value of the fuel [81].
6.2 Bio-oil composition
Bio-oil is a complex mixture of oxygenated aliphatic and aro-
matic compounds. Bio-oil compared to petroleum derived or
conventional oil is largely composed of oxygenated compounds
with negligible quantities of hydrocarbons. Conventional oil on
the contrary is predominantly a hydrocarbon-based liquid fuel.
The pyrolysis liquid consists of both organic- and aqueous-rich
fractions. These fractions are usually produced from the con-
densation process during biomass pyrolysis. Aqueous-rich frac-
tion is termed as an acid phase, whereas the organic-rich fraction
is termed as an oil phase. Acid phase is reported to contain
mainly acetic acid, methanol and acetone. Oil phase consists
mostly of phenolic and carbonyl compounds. Aqueous phase of
the pyrolysis liquid consists mostly of water, acids and a small
concentration of low molecular weight compounds such as
aldehydes, ketones, alcohols and ethers [173]. A number of
acids, aldehydes, ketones, esters, alcohols, furans, phenols,
ethers and saccharides in the organic and aqueous-rich phases
of pyrolysis liquids are identified qualitatively and quantitative-
ly through gas chromatography-mass spectroscopic analysis as
listed in Table 6. It should be noted that except for methanol and
acetic acid most of the chemical compounds are less than 1 wt%
in the aqueous-rich fraction.
Certain amount of chemical compounds originate in the bio-
oils from high-pressure liquefaction, which include volatile or-
ganic acids, alcohols, aldehydes, ethers, esters, ketones, furans,
phenols, hydrocarbons and nonvolatile components.
Liquefaction of biomass is another attractive thermochemical
process (discussed further) that results in the biomass conversion
to liquid fuels without gasification or pyrolysis. Fast pyrolysis of
biomass results in considerable amount of components such as
cyclopentanone, methoxyphenol, acetic acid, methanol, acetone,
furfural, phenol, formic acid, levoglucosan, guaiocol and
alkylated phenol derivatives in the bio-oils [24]. The C
1
com-
pounds in bio-oils consist of formic acid, methanol, formalde-
hyde and ketones. The C
2
–C
4
compounds in bio-oils consist of
linear hydroxyl and oxo-substituted aldehydes and ketones. The
C
5
–C
6
compounds consist of hydroxyl, hydroxymethyl, oxo-
substituted furans, furanones and pyranones. The C
6
compounds
consist of anhydro-sugars and anhydro-oligosaccharides. The
other major compounds in bio-oils are hydroxyacetaldehydes,
hydroxyketones, sugars, carboxylic acids and phenolics [174].
The chemical composition of pyrolysis oil is expected to vary
depending on the type of feedstock utilized, type of pyrolysis
reactor and process characteristics. Pyrolysis oils from softwood
bark contain greater levels of lignin-derived components com-
pared to those from hardwood bark [175].
During pyrolysis, the polymeric components of biomass
such as cellulose, hemicellulose and lignin undergo a series of
complex thermal degradation reactions. Cellulose and hemicel-
lulose undergo cycloreversion and dehydration reactions
followed by transglycosylation. Both low molecular and high
molecular weight compounds are formed during this
holocellulosic decomposition. Bio-oil from lignocellulosic bio-
mass is a heterogeneous mixture of thermochemical derivatives
from cellulose, hemicellulose and lignin. The typical degrada-
tion products from cellulose and hemicellulose in bio-oils in-
clude acids, esters, alcohols, ketones, aldehydes, sugars, furans
and miscellaneous oxygenates, whereas lignin derivatives in-
clude phenols, guaiacols and syringols [176]. Some high mo-
lecular weight compounds in bio-oils include 2-methoxy-3-(2-
propenyl)-phenol, 4-ethyl-2-methoxy-phenol, 2-methoxy-4-
vinylphenol, 2,4-dimethoxyphenol, diethoxymethyl acetate, 3-
hexenoic acid ethyl ester, methylmaleic acid, glycolaldehyde
Biomass Conv. Bioref.
dimer, pentopyranose and levoglucosan [174]. The low molec-
ular weight compounds include acetic acid, propanoic acid,
butanoic acid, hydroxy-acetaldehyde, acetone, 1-hydroxy-2-
propanone, formic acid ethyl ester, ethanol, 1,2-ethanediol,
glycidol, 1,3-propanediol, furfural and phenol. Lignin un-
dergoes dehydration reaction resulting in many side-chain un-
saturated compounds such as styrene derivatives, eugenol, iso-
eugenol and p-hydroxy-cinnamic alcohols [177].
Some widely used pyrolysis reactors with their advantages
and disadvantages are listed in Table 7. During the pyrolysis
process, heat transfer to the biomass particles can be achieved in
three ways, especially through: (1) external and/or indirect
heating, (2) internal or direct heating using a heat transfer medi-
um and (3) energy supplied by partial combustion [88]. Pyrolysis
units with direct heating systems are more common worldwide.
Pyrolysis systems, in general, have been known to operate in
batch, continuous and semicontinuous modes. Most large-scale
pyrolysis systems are operated in a continuous mode to reduce
the operating costs as the capacity of the plants is relatively
optimal and the process parameters are easy to control. In order
to deliver high bio-oil yields from pyrolysis, small particle size
biomass is preferred for enhanced heat and mass transfer rates.
However, size reduction significantly increases the overall cost
of feedstock preparation. High holocellulosic content in the
feedstocks is beneficial in increasing the yields of oils during
fast pyrolysis.
Many reactor technologies such as circulating fluidized bed,
ablative reactor, rotating cone, transported bed and vacuum
moving bed have been studied for bio-oil production from
biomass. The bio-oil yields from most reactor systems seem to
be in the range of 65–75 wt%. A few major pyrolysis reactor
units with industrial-scale operating capacities located world-
wide are shown in Table 8. However, fluidized bed and circulat-
ing bed reactor systems are more commonly used and these are
reported to produce significantly higher bio-oil yields than most
other reactor systems currently available. Bio-oil yields as high
Tabl e 6 Major chemical com-
pounds present in the pyrolysis
liquids
Organic-rich fraction Aqueous-rich fraction
Components Wt% Components Wt%
Methanol 0.9–1.2 Methanol 1.8–2.1
Acetic acid 4–5Aceticacid 9.4–11.3
Furfural 3–4Furfural 0.9-1
Methyl furfural 1–2 Methyl furfural 0.2-0.3
Guaiacol 4.5–5 Guaiacol 0.2-0.3
4-Methyl guaiacol 4–5 4-Methyl guaiacol 0.2-0.3
4-Ethyl guaiacol 3–4 4-Ethyl guaiacol 0.1-0.15
m-Cresol and p-Cresol 5–5.5 Acetone 0.5-0.75
2,4-Xylenol 1.5–2.5 2,4-Xylenol 0.1-0.15
Va n i ll i c a l c o h o l 9 –10 Vanillic alcohol 0.7-1.1
Vanillic acid 9.5–10.5 Vanillic acid 0.9-1.5
Eugenol 2.5–3 Propionic acid 0.6-0.8
3-Methoxy, 4-hydroxyphenyl ethylcarbinol 6–8 Phenol 0.3-0.4
Phenol 3–4 Acetaldehyde 0.1-0.2
4-Propyl guaiacol 4–4.5 Methyl acetate 0.3-0.4
Guaiacol propionate 2–2.5 Ethyl acetate 0.1-0.2
o-Cresol 3.5–4o-Cresol 0.1-0.15
Coniferyl alcohol 1–2 Cyclopentanone 0.3-0.4
3-Methoxy-4,5-dihydroxyphenyl ketone 3–4 3-Methoxy-4,5-dihydroxyphenyl
ketone
<0.1
2,6-Methoxy-4-propenylphenol 1–1.5 2,6-Methoxy-4-propenylphenol <0.1
Methyl formate <0.1 Methyl formate <0.1
Acetone <0.1 2,5-Methyl furan <0.1
Acetaldehyde <0.1 Guaiacol propionate <0.1
Methyl acetate <0.1 Other organic compounds (<0.05 %) 4–5
2,5-Methyl furan <0.1 Water 74–78
Propionic acid <0.1
2-Methyl-5-ethylfurfural <0.1
2-Hydroxy-3-methyl cyclopentanone <0.1
Other organic compounds (<0.05 %) 6–7
Biomass Conv. Bioref.
as 75 % can be obtained using bubbling fluidized bed, circulating
fluidized bed, ablative, rotating cone, vortex, vacuum and few
others reactors. The most common and widely used reactors
have been bubbling fluidized and circulating fluidized beds
because they provide a higher tolerance for the feedstock’s
particle size (∼2–6 mm). In each of the above reactor systems,
an electrostatic separator is preferably installed after the conden-
sation unit to separate condensable and noncondensable compo-
nents from a complex stream of aerosol, mist, particulates,
organics and water vapor [177].
6.3 Quality improvement for bio-oils
Bio-oil upgrading to transport fuels can be conducted by a
variety of different techniques. Such techniques include catalytic
hydrodeoxygenation, hot vapor filtration and stabilization by
adding suitable solvents. Most widely used upgrading technolo-
gies for bio-oils include catalytic hydrodeoxygenation or zeolite
cracking. Catalytic hydrodeoxygenation involves hydrotreating
and catalytic vapor cracking [81]. Hydrodeoxygenation is a
catalytic process through which the O
2
can be removed in the
form of simple molecules such as H
2
O, CO and CO
2
. The high
amount of O
2
in bio-oils is a contributing factor in its low
stability and low heating value. Based on the catalyst selectivity,
the carbon loss can be minimized. However, some undesirable
byproducts are also formed in coke and acids. These compounds
tend to poison the catalyst and decrease its operating lifetime
[180].
During upgrading, bio-oil is subjected to moderate tempera-
tures (200–450 °C) and high pressures (13.8–20 MPa) during
contact with H
2
[181]. However, catalytic upgrading of bio-oils
offers some technical challenges in the form of heavy tars that
can foul the catalyst and render it inactive. Hence, performing
bio-oil upgrading is desired in two stages. The first stage in-
volves mild hydrotreating followed by the severe hydrotreating
stage to minimize the heavy tar formation. Although,
hydrodeoxygenation is one of the most promising bio-oil
upgrading routes, it is often associated with higher processing
cost [182].
Zeolite cracking is another upgrading technology that exists
for bio-oil which removes O
2
merely through the cracking
reactions at atmospheric pressure in the absence of H
2
.
However, due to the formation of low-grade fuel with a high
carbon content (20–40 wt%) and catalyst deactivation issues
this process is commercially unattractive. Since there are many
condensation and polycondensation reactions that occur in bio-
oil, the chances of catalyst poisoning due to the heavy aromatic
hydrocarbons is generally high. Hence, the catalyst life is
greatly affected by the process type, operating time, concentra-
tion of impurities such as sulphur and nitrogenous compounds,
and affinity for the carbon formation to the chemical groups
present in catalyst. High yields of C
5
rich upgraded oil is
reported for low liquid hourly space velocity (LHSV), whereas
aromatic rich crude oil is reported for high LHSV [183]. LHSV
can be defined as the ratio of reactant liquid flow rate to the total
reactor or catalyst volume. Furthermore, hydrogenation and
cracking are reported to be the rate-limiting steps in biomass
upgrading and hence the catalysts chosen for a particular pro-
cess type must be able to overcome these challenges.
By using an appropriate temperature, H
2
pressure and suit-
able catalysts, the oxygen present in the bio-oil can be removed
as water. This process provides additional benefit of lowering
bio-oil viscosity by cracking the large sized polyaromatic
molecules. However, the above steps can significantly increase
the overall production costs for bio-oils [184]. Viscosity and
ash content of the bio-oils are generally high than the crude
Tabl e 7 Most common reactor types used in pyrolysis
Reactor type Heat transfer mode (predominant) Advantages Disadvantages
Ablative Conduction Compact design Inconsistency with heat supply
Accepts bigger particle size of biomass High char abrasion from biomass
Gas for heat transfer is not required
Circulating fluid bed Conduction High heat transfer rate Solids recycle required
Maximum biomass particle size of 6 mm High char abrasion from biomass
Possible liquids cracking by hot solids
Wearing out of reactor possible
Complex system
Fluidized bed Conduction High heat transfer rate Requires biomass with particle size <2 mm
Limited char abrasion
Better solids mixing
Simple reactor configuration
Entrained flow Convection High heat transfer rate Low heat transfer rates
Higher throughput Requires biomass with particle size <2 mm
Limited tar formation Limited gas/solid mixing
Biomass Conv. Bioref.
pyrolysis oils. By successfully upgrading the bio-oils, most of
the previously stated challenges are addressed. Removal of
high molecular weight compounds is essential to increase the
fuel atomization efficiencies, which otherwise adversely plugs
the fuel injectors, spray nozzles and other engine equipment.
During the pyrolysis process, certain amounts of alkali
metals are trapped in the submicron biochar particles which
appear in the pyrolysis liquids. This significantly lowers the
combustion efficiencies of the bio-oil. Hot gas filtration dur-
ing pyrolysis is a useful technique to upgrade the pyrolysis oil
quality for its successful utilization as a fuel in turbines, diesel
engines and boilers. At the same time, it lowers the ash content
and alkali metals (<10 ppm) in the bio-oils [185]. Hot gas
filtration has a great potential to reduce the biochar and alkali
Tabl e 8 Large-scale pyrolysis and gasification reactor systems in operation worldwide
Company/entity Location Reactor type Feed rate
(tons/day)
Pyrolysis reactor units
Pyrovac Canada Vacuum 93
Red Arrow USA Circulating fluidized bed 45
Renewable Oil International USA Auger reactor 24
Pyne UK Fluidized or circulating bed 20
Dynamotive Canada Bubbling fluidized bed 11
Fortum Finland –9.3
Wellman UK Bubbling fluidized bed 6.6
University of Waterloo Canada Bubbling fluidized bed 6
Biomass Technology Group Netherlands Rotating cone 5.3
Red Arrow Canada Circulating transported bed 3
Ensyn Canada Circulating transported bed 1
RTI International Canada Bubbling fluidized Bed 0.5
National Renewable Energy Laboratory USA Ablative Vortex 0.5
VTT Technical Research Centre Finland Circulating fluidized bed 0.5
Gasification reactor units
Energy Products of Idaho USA Bubbling bed 1040
Uhde Finland Circulating bed 576
SilvaGas USA Dual bed 350
Taylor Biomass Energy USA Dual bed 300–400
Solena Group Italy Plasma 250
Wood Plastic Composite Japan Plasma 150–210
Carbona Finland Bubbling bed 100–150
Foster Wheeler Energy Finland, Sweden Circulating bed 70–170
CHRISGAS Sweden Circulating bed 86
Plasco Energy Group Spain, Canada Plasma 70
ThermoChem Recovery International USA Bubbling bed 69
VTT Technical Research Centre Finland Circulating bed 60
Energy research Centre of the Netherlands Netherlands Dual bed 48
REPOTEC/TUV Austria Dual bed 40
Pearson Technology USA Entrained flow 26
InEnTec USA Plasma 15
Karlsruhe Institute of Technology Germany Entrained flow 12
Iowa State University USA Bubbling Bed 5
Range Fuels Inc. USA Entrained Flow 5
Enerkem Canada Bubbling Bed 4
CUTEC Institute Germany Circulating bed 2.7
Fraunhofer Institute Germany Circulating bed 2.4
Mitsubishi Heavy Industries Japan Entrained Flow 2
Data source for pyrolysis reactors, [87,178] and gasification reactors, [179]
Biomass Conv. Bioref.
metal content at the expense of lower bio-oil yields. Although
leaching of metals is not observed during the storage span of
bio-oils, the agglomeration of submicron biochar particles
could significantly affect their storage stability. Proper control
of pyrolysis reaction and vapor cracking conditions increase
the bio-oil yields with less ash and alkali content.
Addition of solvents has been postulated to positively affect
the stability of bio-oils in three ways which are: (1) physical
dilution, (2) reaction rate control and (3) inhibition of network
polymerization and repolymerization. Phase separation in bio-
oils can be minimized by the addition of solvents like methanol,
ethanol, ethylene glycol and acetone. These solvents help
maintain the homogeneity of bio-oil by dispersing the
aqueous-rich and organic-rich phases. Moreover, they are
known to regulate the viscosity increase in bio-oils during their
storage [186]. The addition of solvents has been predicted to
terminate or even reverse the higher-order polymerization re-
actions, physically dilute the high molecular weight com-
pounds and produce a change in the oil microstructure. The
chain termination reactions could prevent the monomers from
polymerizing. On the other hand, the monomers in the bio-oils
could be chain terminated as dimers and oligomers. There is a
reduction in the viscosity of the bio-oil by the addition of low
molecular weight solvents, especially methanol (5–10 %).
Among the additives used (e.g. 10 % ethanol, 10 % acetone,
10 % methanol, 10 % ethyl acetate, 5 % methanol+5 % ace-
tone, and 5 % methanol+5 % methyl isobutyl ketone), 10 %
methanol is reported to provide the least increase in viscosity as
a function of bio-oil aging time [187]. The addition of solvents
such as methanol, ethanol and butanol has been reported to
decrease the flash point, thereby increasing the stability and
reducing the odor of bio-oils.
Apart from the techniques discussed previously, few re-
searchers have found other ways of improving overall quality
of the bio-oils. Removal of low molecular weight compounds
that cause unpleasant odor and a flash point decrease is
predicted to improve the stability of bio-oils [188]. Low
molecular weight or light compounds causing unpleasant odor
are mainly acids, aldehydes and ketones. These light com-
pounds can be removed using a rotary evaporator operated
under vacuum at lower temperatures. Concentration methods
have been found helpful in improving the overall bio-oil
quality without its unpleasant odor [188]. The heating value
of bio-oils can be enhanced by lowering its water content and
viscosity. Bio-oils could be upgraded to straight chain gaso-
line using catalytic hydrodeoxygenation, although lower
yields of gasoline are expected via this route due to the loss
of H
2
OandCO
2
.
6.4 Gasification
Gasification could be performed either directly or in combi-
nation with coal via co-gasification. The primary advantages
of co-gasification are that the GHG and other pollutant gas
emissions could be significantly reduced along with abridged
tar formation. The tar and other heavy compounds produced
from gasification could be recycled through pyrolysis for
further cracking into useful chemical compounds (Fig. 4).
The syngas produced from the pyrolysis process could be
either used for combined heat and power (CHP) or recycled
for process heat.
Similar to SCWG, syngas produced from thermochemical
gasification can be further utilized for chemical or fuel synthesis
via FT catalysis. The syngas can be recycled for the process heat
recovery whereas bio-oil can be utilized directly or catalytically
upgraded to meet the fuel standards. The mixture of these gases,
referred as producer gas, is utilized in internal combustion
engines as a substitute for furnace oil, and for synthesis of C
1
–
C
6
alcohols via FT catalysis. While syngas mostly comprises of
CO and H
2
, producer gas contains CO and N
2
(or any other inert
gas used in the process) as its major components. However,
prior to be used as a fuel gas, the producer gas needs to be
cleaned of tar and dust removal and cooled. The entire gasifi-
cation process takes place in four steps that involve drying of
fuel, pyrolysis and combustion followed by reduction [189].
During the combustion step, the pyrolysis gases produced from
the preceding pyrolysis step react with biochar in the absence of
O
2
at higher temperatures (800–900 °C) as seen in the following
reactions.
CþO2→CO2ð21Þ
4HþO2→2H2Oð22Þ
CmHnþ0:25mþ0:5n
ðÞO2→nCO2þ0:25mH2Oð23Þ
On the other hand, hot gases released from the combustion
step are converted to producer gas in the reduction phase by
the following endothermic reactions.
CþCO2→2CO ΔHR¼160:9KJ=molðÞð24Þ
CþH2O→H2þCO ΔHR¼118:4KJ =molðÞð25Þ
Each of the above steps is considered to have a separate
zone of reaction where different chemical and thermal reac-
tions take place. The fuel undergoes the above series of
reactions for its complete conversion.
The gasification reactions for biomass take place both at
high (>1,200 °C) and low (<1,000 °C) temperatures under
inert atmosphere to support complete conversion. During low-
temperature gasification, the syngas typically retains half of
the energy in the gas stream. However, the remaining energy
is contained in CH
4
and higher chain hydrocarbons [190].
During high-temperature gasification, there is limited CH
4
and tar formation, which makes the gas clean-up and recovery
relatively easy. Gasification is conducted typically below the
ash softening point or above the slagging temperature to avoid
agglomeration or liquid formation.
Biomass Conv. Bioref.
Three major types of gasification processes that currently
exist in the market are in the form of fixed bed, fluidized bed
and entrained flow. The main difference among these process-
es exists in the ways biomass is fed to the gasifier whether
from the top or side way, under gravity or with gas flow
operation, and above or below the melting point of ash and
biochar. The operating pressure and the choice of oxidant
involved, i.e. oxygen, air or steam also affect the biomass
gasification reactions [191]. The syngas produced from gasi-
fication with air results in low H
2
level (8–14 vol%) and lower
heating value of 4–6MJ/m
3
[192]. This shifts interests to-
wards other gasification agents such as steam or steam com-
bined with air/oxygen. The use of oxygen in the gasification
involves huge production costs and makes it less vulnerable.
Air can replace oxygen during the steam gasification to pro-
duce syngas (30–60 vol%) with heating value of 10–16 MJ/
m
3
[193]. The introduction of steam as gasifying agent im-
proves the H
2
content and reduces formation of tar and char.
Steam gasification has been found to be promising in
producing high-quality syngas. The temperature and steam-
to-biomass ratio plays a crucial role in the syngas composi-
tion. With the increase in temperature, steam reforming and
WGS reactions become prominent, which further increases
the H
2
content of the syngas [194]. Depending on the operat-
ing temperature (600–900 °C) and steam-to-biomass ratio
(0.4–0.9w/w)andH
2
composition vary from 20–60 vol%
[195–197]. Awide range of gasifier configurations have been
developed globally, each tailored to feedstock type and the
quality of syngas desired, as summarized in Table 8.
Gasification reactors are broadly classified into three major
types based on the mode of air flow namely downdraft,
updraft and entrained flow [198]. Downdraft gasifier is ad-
vantageous in offering consistent quality of syngas production
with low sensitivity to char dust and ash. Its disadvantages are
tall design and inefficiency towards small particle size bio-
mass. The gas leaves the downdraft gasifier at high tempera-
tures (900–1,000 °C), which lowers the overall energy effi-
ciency [199]. Moreover, the tar content of syngas is lower,
although the particulates content is high. In contrast, the
updraft gasifier tends to have smaller pressure drop, good
thermal efficiency, lower tendency towards slag formation
and ability to use moderately moisturized biomass. Due to
the low temperature (200−300 °C) of syngas leaving the
gasifier, the overall energy efficiency of updraft gasification
is high [199]. In entrained flow gasification, the biomass is fed
into the gasifier with pressurized inert gas or steam creating a
turbulent flame at high temperatures (1,200–1,500 °C). The
syngas exits the gasifier at temperatures of 800–900 °C, which
lowers the energy efficiency of the process. The benefit of
using entrained flow gasification is fast conversion of bio-
mass; however, the process suffers from pressure drop, high
tar content of syngas and high sensitivity towards slag forma-
tion [179,189,191]. Conversely, biomass treatments such as
flash pyrolysis, slow pyrolysis and torrefaction prior to
entrained flow gasification are found to be promising.
The low-temperature gasification is usually performed
using a catalyst medium [200]. Depending upon the presence
of catalytically active components such as K and Na in the
biomass, the gasification temperature can be as low as 500 °C.
During low temperature biomass gasification, a fluidized bed
gasifier and a downstream catalytic reformer in series are
operated at a temperature of 900 °C. Apart from the typical
composition (i.e. H
2
,CO,CO
2
and water vapors), the gas
produced through low temperature gasification contains con-
siderable amount of hydrocarbons such as CH
4
,C
2
H
4
,C
6
H
6
and tars. The product gas is suitable for CHP; however, it does
not meet the requirements of syngas to be used as a fuel or
precursor for liquid fuels and value-added chemicals.
Therefore, the product gas needs further upgrading using a
catalytic reformer where hydrocarbons can be converted
mainly to H
2
and CO mixture with limited quantities of CO
2
and water vapor. Typically, most syngas conversions to liquid
fuels using gasification and catalytic reforming require raw
product gas with almost no inert gases. Hence, it is advanta-
geous to use pure O
2
instead of air with steam as a moderator
[200].
6.5 Liquefaction
The range of thermochemical processes for biomass conver-
sion also extends to direct liquefaction and hydrothermal
liquefaction. Direct liquefaction involves the conversion of
biomass into liquid fuels without the gasification step [201].
This has been linked to hydrogenation and high pressure
thermal decomposition processes that involve the use of CO
and H
2
for fuel production from carbonaceous materials [202].
Hydrothermal liquefaction or hydrous pyrolysis involves the
use of water and catalysts to convert the solid biomass into
liquid oil [203]. During this process, the complex organic
materials inthe form of biomass and biogenic waste is cracked
and reduced into heavy oil and useful chemicals.
Some of the complex chemical reactions that occur during
liquefaction are [202,204]: (1) cracking and reduction of
cellulose, hemicellulose, lignin and lipids; (2) hydrolysis of
cellulose and hemicelluloses to glucose and other simple
sugars; (3) hydrogenolysis in the presence of H
2
; (4) reduction
of amino acids; (5) reformation reactions via dehydration and
decarboxylation; (6) degradation of C–OandC–C bonds and
(7) hydrogenation of functional groups.
During liquefaction, biomass undergoes thermal decompo-
sition into its monomer units. These monomer units are further
repolymerized to produce liquid oils or condensed into unde-
sirable solid chars. The addition of solvent slows down the
higher order solid-state reactions and reduces the undesirable
condensation reactions for char formation [205]. The addition
of catalysts also reduces the reaction temperature, improves
Biomass Conv. Bioref.
reaction kinetics and enhances the bio-oil yields [198]. While
alkali catalysts enhance the oil yields by reducing char forma-
tion [206], acid catalysts decrease the reaction temperature
and time [207]. Apart from bio-oil generation, liquefaction of
lignocelluloses is also found helpful in the production of
polyurethane foams, epoxy resins and adhesives for plywood
[198].
The temperatures and pressures used in liquefaction are
usually in the range of 250–350 °C and 5–20 MPa, respec-
tively [198]. Compared to pyrolysis, drying of biomass is not
necessary for direct liquefaction. However, the requirement of
catalyst is essential for liquefaction. Alkali metal catalysts
such as Na
2
CO
3
,K
2
CO
3
along with CO and H
2
as supple-
mental reactants are used to facilitate the overall liquefaction
process [22,206]. In addition, use of different catalysts such as
NaOH, H
3
PO
4
,H
2
SO
4
and toluenesulfonic acid on sawdust
have resulted in about 70.6–99.4 wt% of liquefied product
yields [208].
The liquid yields from chestnut wood liquefaction varied
from 15.6–56.3 wt% [209]. However, the increase in reaction
temperature (190–240 °C) and the use of acid catalysts
(H
2
SO
4
and H
3
PO
4
) affected the liquid yields positively.
Another similar study by Liang et al. [210] reported high
liquid yields (67–80 wt%) obtained from liquefaction of
wheat straw, rice straw and corn stover. High H
2
pressures
(2–10 MPa) and catalysts are necessary to enhance the rate of
liquefaction reactions and improve the process selectivity.
Hydrogen stabilizes the free radicals formed during decom-
position of cellulose and lignin at high temperatures and
prevents any condensation, cyclization and repolymerization
reactions resulting from these radicals [203].
Liquefaction of lignin-rich wood resulted in a higher abun-
dance of low molecular weight phenolic compounds com-
pared to cellulose confirming the phenolic nature of lignin
[202]. The amount of solid residue formed in the same study
was also proportional to the lignin content owing to its mac-
romolecular structure. For facilitating the recovery phenolic
fractions, a mixture composed of small amount of phenol and
some lower alcohols is usually employed. This is due to the
fact that phenol prevents the condensation reaction by lignin
or polyphenol in the biomass that produces an insoluble
polymerized material during acid-catalyzed liquefaction
[207].
7 Challenges in biomass conversions
Despite a great deal of promise, fuels from biomass still
remain a controversial subject. The production of biofuels
has a tendency to alleviate poverty, boost the rural economy
through employment opportunities and increase national en-
ergy security due to investments in domestic biomass.
Although increased production of biomass for energy can
offset substantial use of fossil fuels, there is the possibility to
threaten conservation areas to manage bioenergy crops, con-
taminate water resources with agricultural pollutants and de-
crease food security due to competition for land. There is
always a risk associated with international food security when
the food crops are considered as feedstocks for biofuel pro-
duction rather than for human consumption or as animal feed.
For instance, in 2007 USA produced nearly 26.5 billion litres
ethanol from a major proportion of its corn harvest from the
same year [211]. This greatly affected the corn export to the
developing countries, resulting in an unparalleled rise in corn
prices that rose to 73 % towards the end of 2010.
Furthermore, biofuels are regarded as “carbon neutral”and
basically free from detrimental compounds such as sulphur
and aromatics.A few requisites should be considered for mak-
ing bioethanol as a clean fuel with reference to GHG manage-
ment, such as: (1) bioethanol producing industries should use
biomass and not fossil fuels as energy sources, (2) cultivation
of annual feedstock crops on land rich in carbon (above- and
below-ground) should be avoided, (3) byproducts from the
process should be utilized efficiently to maximize their energy
and GHG benefits and (4) N
2
O emissions should be mini-
mized through efficient organic fertilization approaches
[212–214].
Another constraint in biomass conversion occurs with the
diverse variety of forest residues. There are various challenges
related to the conversion of woody biomass such as under-
standing the recalcitrance of the wood matrix and level of
enzymes required to degrade the hemicelluloses and elucidat-
ing the inhibitory effect of lignin and its derivatives on the
fermentation process. Hardwood has more cellulose and less
hemicellulose than softwood which implies better bioconver-
sion due to greater amounts of glucose in the former. It is more
challenging to delignify softwood because of its stable lignin
chemistry which is condensed when exposed to acidic condi-
tions [215]. The proportions of the wood constituents vary
between species and there are distinct differences between
soft- and hardwood. On an average, the cellulose and hemi-
cellulose content is 70.3 and 78.8 % in soft- and hardwood,
respectively. Similarly, lignin composition in softwood is
29.2 % and in hardwood is 21.7 % [216].
Certain natural barriers that contribute to the recalcitrance
of lignocellulosic biomass to chemicals or enzymes are: (1)
epidermal plant tissues, mainly the cuticle and epicuticular
waxes, (2) density of sclerenchymatous tissue, (3) configura-
tion of vascular bundles, (4) lignin composition, (5) structural
heterogeneity and complexity of cell wall and (6) other inhib-
itors occurring naturally in cell walls and/or generated during
bioconversion [217]. Moreover, a pretreatment is very critical
for ensuring better sugar yields from the biomass prior to
bioconversion. For an ideal lignocellulosic pretreatment,
Taherzadeh and Karimi [40]havesummarizedafewsalient
features such as: (1) production of reactive cellulosic fibre for
Biomass Conv. Bioref.
enzymatic attack, (2) avoiding destruction of cellulose and
hemicelluloses, (3) avoiding formation of inhibitors for hy-
drolytic enzymes and fermentation, (4) minimizing energy
consumption, (5) reducing expenses for feedstock prepara-
tion, reactors and chemicals and (6) producing fewer residues.
On the other hand, a major problem encountered in the
ethanol refineries is contamination during bioconversion,
irrespective of first or second generation feedstock utilized.
Considerable economic losses occur during processing be-
cause the contaminating species competes for sugars and
nutrients which lead to inhibition of fermenting microflora
and a subsequent reduction in ethanol productivity with
undesired products. This makes it very crucial to detect and
control any possible chance of contamination at its initial
stage. Unavoidable cases can lead to shutdown of the fermen-
ter, requiring cleansing of the contaminants, proper system
sterilization and re-inoculation of the fermenting species into
fresh substrate. As per Muthaiyan and Ricke [218], the
sources of contamination during bioconversion can be either
direct or indirect. Direct sources of contaminants originate
from materials added to the fermenter such as biomass, inoc-
ulum, enzymes, nutrients and aerosols. Indirect contaminant
sources are dirty transfer lines, connecting pipelines and water
for pumping and agitation. A few predominant bacterial con-
taminants include Lactobacillus,Leuconostoc ,Pediococcus,
Weisella and Acetobacter [219]. Dekkera bruxellensis,
Candida tropicalis and Pichia galeiformis are a few fungal
contaminants commonly found in a fermenter [220].
Various detection means for contamination include plating
methods, fermentation metabolite detection, high-performance
liquid chromatography, particle size distribution analysis, fluo-
rescence spectroscopy and molecular methods, such as poly-
merase chain reaction (PCR), real-time PCR and real-time
immuno-PCR [218]. Electromagnetic engineering and nano-
technology are recently being implemented as detection
methods. One such example is a superconducting quantum
interference device which is a highly sensitive detector of
magnetic flux [221]. Bacterial contaminants in the bioethanol
production facilities are often controlled with antibiotics such
as penicillin G, streptomycin, tetracycline, virginiamycin and
monensin.
Although the SCWG of lignocellulosic biomass is one of
the promising pathways to produce H
2
-rich gas, it has a few
limitations for industrial applications. The plugging of the
reactor has been found to be a major problem with supercrit-
ical processes. Furthermore, the biomass should be free of salt
forming components as the salts are less soluble in SCW
[222]. Moreover, the salts along with biochar make the reactor
lines more prone to plugging. Although Ni- and Ru-based
catalyst for SCWG of biomass are active to produce H
2
and
CO, but they also show activity for methanation reactions
leading to high concentrations of CH
4
which is a major
GHG [80,86]. Selectivity of H
2
is another major issue of
SCWG. A suitable catalyst/support active to H
2
by suppress-
ing the side reactions is essential to synthesize H
2
-rich gaseous
products from biomass. As the gasification of biomass is done
at high temperature and pressure, the catalyst/support should
be stable without sintering, oxidation and phase transforma-
tion for longer times. Metals such as Ni and Rh with alumina
supports and CNTs are found to be stable and active for H
2
production from biomass with SCW [80]. The economics of
the overall process also needs to be considered for the com-
mercial applications.
There are many challenges which need to be addressed to
make the syngas fermentation commercially viable in produc-
ing biofuels and other value-added products. The yields of the
products from syngas fermentation are usually low; hence
new recombinant microorganisms with high yields of ethanol
are essential for industrial scale fermentation of syngas [124].
Genetic manipulation of microorganisms to amplify solvent
production over acetic acid can be considered as a possible
option. The syngas obtained from the gasification processes
should be free from the impurities such as NO
x
and SO
x
as
they might inhibit the activity of enzymes during fermentation
and result in undesirable products [131]. The major issue of
the syngas fermentation is lower mass transfer rate of gases
into the liquid fermentation medium [128]. Advance reactor
configurations to enhance the mass transfer rates of the sub-
strates need to be developed for effective production of etha-
nol during fermentation process. The end products with mar-
ket specificity imply essential separation processes for ethanol
from the other products.
The need of sterile environment, low processing rate and
mass transfer limit the syngas fermentation to scale-up the
process for commercialization. Companies which currently
produce ethanol by fermentation of syngas are INEOS Bio
and Coskata Inc. in USA and LanzaTech in New Zealand.
INEOS Bio has employed C.ljungdahlii to produce 100
gallons of ethanol per dry ton of biomass at the pilot scale
[223]. The commercial plant is in progress and targets to
produce eight million gallons of ethanol per year from the
vegetative, yard and household waste. The unused syngas
during fermentation is utilized to generate power. Coskata
Inc. employs C.ragsdalei,C.carboxidivorans and C.
coskatii in syngas fermentation to produce ethanol [224].
The plant aims to produce 16 million gallons of ethanol per
year and further scale-up to 78 million gallons from wood
chips and solid waste. LanzaTech uses the CO-rich industrial
off-gas to synthesize ethanol and other valuable byproducts
using C.autoethanogenum [225]. The pilot plant has been
scaled-up to produce 100,000 gallon of ethanol fromsteel mill
off-gases. Further details about the current production and
their collaboration details have been presented in the recent
review by Kopke et al. [226].
As syngas could be used in synthesizing alcohols and
DME, the limitations however are the requirement of gas
Biomass Conv. Bioref.
cleaning and conditioning. Conventional gas technologies for
the cleaning and conditioning of syngas typically comprise of
a filter, rectisol unit and a CO
2
gas polishing unit [200]. The
synthesis of ethanol from syngas through FT catalysis in-
volves many other side reactions with various byproducts.
The activity, stability and life of catalyst along with product
recovery are the major technical challenges for ethanol pro-
duction from CO
2
of syngas. The catalysts for FT process
synthesize mainly hydrocarbons and mixture of alcohols. Rh-
based catalysts are not only found to be active and selective to
ethanol but also for CH
4
. Catalysts which are active for
oxygenates also catalyze other reactions to form CH
4
and
other byproducts. The selectivity and activity of a catalyst to
catalyze the reactions for direct synthesis of ethanol from
syngas are usually low. The low catalytic activity for ethanol
from syngas limits the process for commercialization. The
effectiveness of a catalyst for alcohol production can be im-
proved to a greater extent by understanding the structure,
formulation and physicochemical properties of the feedstock
(syngas) and catalyst. The rational design of a catalyst for
maximum ethanol production by suppressing other reactions
need to be developed by correlating catalyst activity to the
material properties.
A major technical challenge in gasification is removal of
tar. Huber et al. [81] have summarized a few solid catalysts
(e.g. dolomite, Rh/CeO
2
/SiO
2
,aswellasPd,Pt,RuandNi
supported on CeO
2
/SiO
2
) and alkali metal catalysts (e.g.
NaCl, KCl, AlCl
3
6H
2
O, K
2
CO
3
,Na
2
CO
3
,Na
3
H(CO
3
)
2
,
CsCO
3
and Na
2
B
4
O
7
10H
2
O) for biomass gasification. Using
alkali metal catalyst with the biomass feedstock by dry mixing
or wet impregnation not only decreases tar formation but also
elevates biochar yields. One of the major drawbacks encoun-
tered during the thermochemical conversions is the corrosive
nature of bio-oils due to their acidic properties. Bio-oils are
reported to have a corrosive effect on ordinary steel and
aluminum but they are noncorrosive for stainless steel and
polymers. In a study by Lu et al. [227], the corrosiveness of
rice husk bio-oils and their emulsions with biodiesel were
examined on four metals of aluminum, brass, mild steel and
stainless steel. Maximum corrosion was observed in case of
mild steel followed by aluminum, whereas least corrosion was
found in brass, with no sign of corrosion in stainless steel.
Hence, bio-oils could be stored in tanks made of stainless steel
(304/316) or plastic either at room temperature or refrigerated
conditions [228]. Studies also suggest that the influence of
copper and stainless steel on the phase separation of softwood
and hardwood derived bio-oils is not significant [229].
Accurate testing of pH is another difficulty for bio-oils as the
electrodes are prone to fouling. Alcohols are characteristically
amphoteric, i.e. they are either weakly acidic or weakly basic.
Alcohols can be protonated by a strong acid to form oxonium
ions and in aqueous solutions they dissociate to form alkoxides
[230]. On the other hand, phenols which are a major component
of bio-oils are more acidic than alcohols and form phenoxide
ions on reaction with OH
–
ion. Moreover, pyrolysis of lignin is
known to be difficult to achieve due to the unstable polymeric
reactions that take place during the pyrolysis process. Lignin
also plugs the reactor lines during the feeding process as well as
causes foaming issues inside the reactor. In spite of these
challenges, there is a great potential for converting lignin into
valuable aromatic chemicals as its structure is known to be rich
in syringol and guiacol units. The use of a H
2
-donating catalyst
is useful in breaking down the lignin effectively through
hydrodeoxygenation and thus preventing the polymerization
reactions [231].
The bio-oil recovery efficiencies are subject to variation
with the process conditions and feedstock types utilized.
Minimal time lag during the heat transfer seems to be
primarily responsible for higher yields of bio-oil as pro-
duced from fluidized bed and circulating bed pyrolysis.
Typically, high pyrolysis temperature and low residence
time in combination seem to produce the high yields of
bio-oil with low biochar formation [87]. In spite of the
advancements in pyrolysis technologies at present, the
scaling-up of the pyrolysis reactors is critically considered
before achieving the commercial status of biofuel produc-
tion. Furthermore, designing an effective filtration system
for the particulates present in the pyrolysis gases is also
necessary to enhance the stability of bio-oils.
There are some challenges associated with liquefaction for
large-scale commercialization due to the low oil yields (20–
55 wt%), formation of tars, operational difficulties and cata-
lyst requirements [232]. Among all lignocellulosic compo-
nents, cellulose is resistant to chemical transformation during
liquefaction due to its highly ordered macro and microfibrils.
The variability in structural and chemical composition poses a
technical challenge for liquefaction of woody feedstocks
[208]. Moreover, the liquefaction reactor systems tend to be
expensive due to the complexity of their fuel feeding facilities
[198]. Despite some of the limitations that exist with lique-
faction, the liquid yields could be optimized by the use of a
proper solvent, catalyst, temperature and pressure.
At present large-scale commercialization and utilization of
bio-oil as a fuel alternative is under evaluation. A promising
sign for their mass utilization is that they have been tested in
motor engines, turbines and boilers successively with consid-
erably lower emissions [184]. To achieve success on wide-
scale, bio-oils should be homogeneous, low in solid content
(e.g. char fines and ash), viscosity and acidity, and have a
high heating value. Furthermore, a lower flash point is helpful
in increasing the stability of bio-oils and reducing their un-
pleasant smell. Unlike biodiesel that has an ease of benign
handling and storage due to its higher flash point, bio-oils
with lower flash point necessitate adaption of safer handing,
transportation and storage facilities similar to gasoline [81,
188].
Biomass Conv. Bioref.
8 Conclusions and perspectives
Lignocellulosic biomasses are the inexpensive and most
abundant form of biofuel feedstock available today.
Their conversion to biofuels requires application of bio-
chemical, hydrothermal and thermochemical processes
along with some hybrid technologies. The bioconversion
of lignocellulosic feedstocks to alcohol-based fuels ne-
cessitates an appropriate pretreatment technology to dis-
rupt the plant cell wall and release monosaccharides
sugars for subsequent fermentation. Since, lignin is a
large fraction of biomass, in an ideal biorefinery it
should be used for biofuels and biochemicals production
through thermochemical routes. The hydrothermal and
thermochemical routes involve gasification, pyrolysis,
liquefaction and hybrid gas-to-liquid fuel technologies
such as syngas fermentation and Fischer–Tropsch
catalysis.
Although the production of bio-oils by fast pyrolysis is
commercially viable, their applications have been found in
chemical manufacture and heat/power generation rather
than in transportation sector. This is because bio-oils de-
grade with time and thus cannot be used directly as a
transportation fuel without upgrading or blending.
However, another limitation is that fuels derived from
bio-oils have not been extensively investigated and the
process of bio-oils upgrading is under steady development
worldwide for enhanced efficiency and improved fuel
properties. In addition, low-cost processing technologies
that could efficiently convert a large fraction of the ligno-
cellulosic biomass into liquid or gaseous fuels do not yet
exist. Nevertheless, the introduction of new hybrid tech-
nologies that link gasification with fermentation and cata-
lytic engineering seem to make the ethanol production
from lignocellulosic biomass economically feasible.
In order to achieve a sustainable and economical supply
of bioenergy, a few objectives should be focused on such
as: (1) development of newer cost-effective conversion
technologies, (2) engineering the existing technologies for
increased productivity with low energy/power consump-
tion, (3) studying the biomass chemistry that would deter-
mine the process complexities, and (4) generation of lesser
amount of byproducts and process waste for a lower
carbon footprint. Regardless of all the conversion path-
ways, the future of biofuels as an alternative energy in the
fuel market appears very bright because of the shrinking
supplies of fossil fuels and booming demand for sustain-
able energy sources.
Acknowledgments The authors express their acknowledgments towards
the Natural Sciences and Engineering Research Council of Canada
(NSERC), Canada Research Chair (CRC) program and BioFuelNet Canada
for the financial support in this research.
References
1. Balat M (2011) Production of bioethanol from lignocellulosic ma-
terials via the biochemical pathway: a review. Energ Convers Man-
age 52:858–875. doi:10.1016/j.enconman.2010.08.013
2. International Energy Agency (IEA) (2012) CO
2
emissions from fuel
combustion highlights. 2012 ed. Luxembourg
3. International Energy Outlook (IEO) (2011) U.S. Energy Informa-
tion Administration, Washington, DC. www.eia.gov/ieo/pdf/
0484(2011).pdf.Accessed1March2011
4. U.S. Energy Information Administration (USEIA) (2013) U.S. Depart-
ment of Energy, Washington. http://www.eia.gov. Accessed 14 April
2013
5. Demain AL (2009) Biosolutions to the energy problem. J Ind
Microbiol Biotechnol 36:319–332. doi:10.1007/s10295-008-0521-
8
6. Fang X, Shen Y, Zhao J, Bao X, Qu Y (2010) Status and prospect of
lignocellulosic bioethanol production in China. Bioresour Technol
101:4814–4819. doi:10.1016/j.biortech.2009.11.050
7. Sukumaran RK, Surender VJ, Sindhu R, Binod P, Janu KU, Sajna
JV, Rajasree KP, Pandey A (2010) Lignocellulosic ethanol in India:
prospects, challenges and feedstock availability. Bioresour Technol
101:4826–4833. doi:10.1016/j.biortech.2009.11.049
8. Sanchez C (2009) Lignocellulosic residues: biodegradation and
bioconversion by fungi. Biotech Adv 27:185–194. doi:10.1016/j.
biotechadv.2008.11.001
9. Kim S, Dale BE (2004) Global potential bioethanol production from
wasted crops and crop residues. Biomass Bioenerg 26:361–375.
doi:10.1016/j.biombioe.2003.08.002
10. Mabee WE, Saddler JN (2010) Bioethanol from lignocellulosics:
status and perspectives in Canada. Bioresour Technol 101:4806–
4813. doi:10.1016/j.biortech.2009.10.098
11. Foust TD, Aden A, Dutta A, Phillips S (2009) An economic and
environmental comparison of a biochemical and a thermochemical
lignocellulosic ethanol conversion processes. Cellulose 16:547–
565. doi:10.1007/s10570-009-9317-x
12. Trostle R (2011) Global agricultural supply and demand: factors
contributing to the recent increase in food commodity prices.
USDA, a report from the economic research service; 2008. www.
ers.usda.gov/Publications/WRS0801/WRS0801.pdf. Accessed 23
April 2011
13. Pauly M, Keegstra K (2008) Cell-wall carbohydrates and their
modification as a resource for biofuels. Plant J 54:559–568. doi:
10.1111/j.1365-313X.2008.03463.x
14. Nanda S, Azargohar R, Kozinski JA, Dalai AK (2013) Character-
istic studies on the pyrolysis products from hydrolyzed Canadian
lignocellulosic feedstocks. Bioenerg Res. doi:10.1007/s12155-013-
9359-7
15. Nanda S, Mohanty P, Pant KK, Naik S, Kozinski JA, Dalai AK
(2013) Characterization of North American lignocellulosic bio-
mass and biochars in terms of their candidacy for alternate
renewable fuels. Bioenerg Res 6:663–677. doi:10.1007/
s12155-012-9281-4
16. Raveendran K, Ganesh A, Khilar KC (1995) Influence of mineral
matter on biomass pyrolysis characteristics. Fuel 74:1812–1822.
doi:10.1016/0016-2361(95)80013-8
17. Naik S, Goud VV, Rout PK, Jacobson K, Dalai AK (2010)
Characterization of Canadian biomass for alternative renewable
biofuel. Renew Energ 35:1624–1631. doi:10.1016/j.renene.
2009.08.033
18. Ballesteros M, Oliva JM, Negro MJ, Manzanares P, Ballesteros I
(2004) Ethanol from lignocellulosic materials by a simultaneous
saccharification and fermentation process (SFS) with Kluyveromyces
marxianus CECT 10875. Process Biochem 39:1843–1848. doi:10.
1016/j.procbio.2003.09.011
Biomass Conv. Bioref.
19. Prasad S, Singh A, Joshi HC (2007) Ethanol as an alternative fuel
from agricultural, industrial and urban residues. Res Conserv Recycl
50:1–39. doi:10.1016/j.resconrec.2006.05.007
20. Kim TH, Kim JS, Sunwoo C, Lee YY (2003) Pretreatment of corn
stover by aqueous ammonia. Bioresour Technol 90:39–47. doi:10.
1016/S0960-8524(03)00097-X
21. Malherbe S, Cloete TE (2002) Lignocellulose biodegradation: fun-
damentals and applications. Rev Environ Sci Biotechnol 1:105–
114. doi:10.1023/A:1020858910646
22. Qian Y, Zuo C, Tan J, He J (2007) Structural analysis of bio-oils
from sub-and supercritical water liquefaction of woody biomass.
Energy 32:196–202. doi:10.1016/j.energy.2006.03.027
23. Sjostrom E (1993) Wood chemistry fundamentals and applications,
2nd edn. Academic Press, San Diego
24. Demirbas MF (2006) Current technologies for biomass conversion
into chemicals and fuels. Energ Source Part A 28:1181–1188. doi:
10.1080/00908310500434556
25. Griffin DW, Schultz MA (2012) Fuel and chemical products from
biomass syngas: a conversion of gas fermentation to thermochem-
ical conversion routes. Environ Prog Sustain Energ 31:219–224.
doi:10.1002/ep.11613
26. Kumar P, Barrett DM, Delwiche MJ, Stroeve P (2009) Methods for
pretreatment of lignocellulosic biomass for efficient hydrolysis and
biofuel production. Ind Eng Chem Res 48:3713–3729. doi:10.1021/
ie801542g
27. Chiaramonti D, Prussi M, Ferrero S, Oriani L, Ottonello P, Torre P,
Cherchi F (2012) Review of pretreatment processes for lignocellu-
losic ethanol production, and development of an innovative method.
Biomass Bioenerg 46:25–35. doi:10.1016/j.biombioe.2012.04.020
28. Sun Y, Cheng J (2002) Hydrolysis of lignocellulosic materials for
ethanol production: a review. Bioresour Technol 83:1–11. doi:10.
1016/S0960-8524(01)00212-7
29. Youssef BM, Aziz NH (1999) Influence of gamma-irradiation on
the bioconversion of rice straw by Trichoderma viride into single
cell protein. Cytobios 97:171–183
30. Bak JS, Ko JK, Han YH, Lee BC, Choi IG, Kim KH (2009)
Improved enzymatic hydrolysis yield of rice straw using electron
beam irradiation pretreatment. Bioresour Technol 100:1285–1290.
doi:10.1016/j.biortech.2008.09.010
31. Imai M, Ikari K, Suzuki I (2004) High-performance hydrolysis of
cellulose using mixed cellulase species and ultrasonication pretreat-
ment. Biochem Eng J 17:79–83. doi:10.1016/S1369-703X(03)
00141-4
32. Zhu S, Wu Y, Yu Z, Wang C, Yu F, Jin S, Ding Y, Chi R, Liao J,
Zhang Y (2006) Comparison of three microwave/chemical pretreat-
ment processes for enzymatic hydrolysis of rice straw. Biosyst Eng
93:279–283. doi:10.1016/j.biosystemseng.2005.11.013
33. Zaldivar J, Martinez A, Ingram LO (1999) Effect of selected alde-
hydes on the growth and fermentation of ethanologenic Escherichia
coli. Biotechnol Bioeng 65:24–33. doi:10.1002/(SICI)1097-
0290(19991005)65:1<24::AID-BIT4>3.0.CO;2-2
34. Mosier NS, Wyman C, Dale B, Elander R, Lee YY, Holtzapple M,
Ladisch R (2005) Features of promising technologies for pretreat-
ment of lignocellulosic biomass. Bioresour Technol 96:673–686.
doi:10.1016/j.biortech.2004.06.025
35. Wyman CE, Dale BE, Elander RT, Holtzapple M, Ladisch MR, Lee
YY (2005) Coordinated development of leading biomass pretreatment
technologies. Bioresour Technol 96:1959–1966. doi:10.1016/j.
biortech.2005.01.010
36. Zhao XB, Cheng KK, Liu DH (2009) Organosolv pretreatment
of lignocellulosic biomass for enzymatic hydrolysis. Appl
Microbiol Biotechnol 82:815–827. doi:10.1007/s00253-009-
1883-1
37. Girio FM, Fonseca C, Carvalheiro F, Duarte LC, Marques S, Bogel-
Lukasik R (2010) Hemicelluloses for fuel ethanol: a review.
Bioresour Technol 101:4775–4800. doi:10.1016/j.biortech.2010.01.
088
38. Tomas-Pejo E, Olive JM, Ballesteros M (2008) Realistic approach
for full-scale bioethanol production from lignocellulose: a review. J
Sci Ind Res 67:874–884
39. McMillan JD (1994) Pretreatment of lignocelluloses biomass. In:
Himmel ME, Baker JO, Overend RP (eds) Conversion of hemicellu-
lose hydrolyzates to ethanol. Am Chem Soc Symp, Washington, pp
292–324
40. Taherzadeh MJ, Karimi K (2008) Pretreatment of lignocellulosic
wastes to improve ethanol and biogas production: a review. Int J
Mol Sci 9:1621–1651. doi:10.3390/ijms9091621
41. Duff SJB, Murray WD (1996) Bioconversion of forest products
industry waste cellulosics to fuel ethanol: a review. Bioresour
Technol 55:1–33. doi:10.1016/0960-8524(95)00122-0
42. Pan XJ, Gilkes N, Kadla J, Pye K, Saka S, Gregg D, Ehara K, Xie D,
Lam D, Saddler J (2006) Bioconversion of hybrid poplar to ethanol
and co-products using an organosolv fractionation process: optimi-
zation of process yields. Biotechnol Bioeng 94:851–861. doi:10.
1002/bit.20905
43. Li C, Knierim B, Manisseri C, Arora R, Scheller HV, Auer M, Vogel
KP, Simmons BA, Singh S (2010) Comparison of dilute acid and
ionic liquid pretreatment of switchgrass: biomass recalcitrance,
delignification and enzymatic saccharification. Bioresour Technol
101:4900–4906. doi:10.1016/j.biortech.2009.10.066
44. Lee SH, Doherty TV, Linhardt RJ, Dordick JS (2009) Ionic liquid-
mediated selective extraction of lignin from wood leading to en-
hanced enzymatic cellulose hydrolysis. Biotechnol Bioeng 102:
1368–1376. doi:10.1002/bit.22179
45. Garrote G, Dominguez H, Parajo JC (1999) Hydrothermal process-
ing of lignocellulosic materials. Eur J Wood Wood Prod 57:191–
202. doi:10.1007/s001070050039
46. Biermann CJ, Schultz TP, McGinnis GD (1984) Rapid steam
hydrolysis/extraction of mixed hardwoods as a biomass pretreatment.
J Wood Chem Technol 4:111–128. doi:10.1080/02773818408062286
47. Saska M, Ozer E (1995) Aqueous extraction of sugarcane bagasse
hemicellulose and production of xylose syrup. Biotechnol Bioeng
45:517–523. doi:10.1002/bit.260450609
48. Ehara K, Saka S (2005) Decomposition behavior of cellulose in
supercritical water, subcritical water, and their combined treatments.
J Wood Sci 51:148–153. doi:10.1007/s10086-004-0626-2
49. Peterson AA, Vogel F, Lachance RP, Forling M, Antal J, Micheal
WT (2008) Thermochemical biofuel production in hydrothermal
media: a review of sub- and supercritical water technologies. Energy
Environ Sci 1:32–65. doi:10.1039/B810100K
50. Kumar S (2013) Sub- and supercritical water technology for
biofuels. In: Lee JW (ed) Advanced biofuels and bioproducts.
Springer, New York, pp 147–183
51. Pasquini D, Pimenta MTB, Ferreira LH, da Silva Curvelo AA
(2005) Extraction of lignin from sugar cane bagasse and Pinus
taeda wood chips using ethanol-water mixtures and carbon dioxide
at high pressures. J Supercrit Fluid 36:31–39. doi:10.1016/j.supflu.
2005.03.004
52. Lenihan P, Orozco A, O’Neill E, Ahmad MNM, Rooney DW,
Walker GM (2010) Dilute acid hydrolysis of lignocellulosic
biomass. Chem Eng J 156:395–403. doi:10.1016/j.cej.2009.10.
061
53. Shi J, Sharma-Shivappa RR, Chinn M, Howell N (2009) Effect of
microbial pretreatment on enzymatic hydrolysis and fermentation of
cotton stalks for ethanol production. Biomass Bioenerg 33:88–96.
doi:10.1016/j.biombioe.2008.04.016
54. Hu F, Ragauskas A (2012) Pretreatment and lignocellulosic chem-
istry. Bioenerg Res 5:1043–1066. doi:10.1007/s12155-012-9208-0
55. Wyman CE, Hinman ND (1990) Ethanol: fundamentals of produc-
tion from renewable feedstocks and use as a transportation fuel.
Appl Biochem Biotechnol 24–25:735–753
Biomass Conv. Bioref.
56. Dien BS, Cotta MA, Jeffries TW (2003) Bacteria engineered for
fuel ethanol production: current status. Appl Microbiol Biotechnol
63:258–266. doi:10.1007/s00253-003-1444-y
57. Gray KA, Zhao L, Emptage M (2006) Bioethanol. Curr Opin Chem
Biol 10:141–146. doi:10.1016/j.cbpa.2006.02.035
58. Boominathan K, Reddy CA (1992) cAMP-mediated differential
regulation of lignin peroxidase and manganese-dependent peroxi-
dase production in the white-rot basidiomycete Phanerochaete
chrysosporium . PNAS 89:5586–5590
59. Azzam AM (1989) Pretreatment of cane bagasse with alkaline
hydrogen peroxide for enzymatic hydrolysis of cellulose and etha-
nol fermentation. J Environ Sci Health B 24:421–433. doi:10.1080/
03601238909372658
60. Couto SR, Sanroman MA (2006) Application of solid-state fermen-
tation to food industry—a review. J Food Eng 76:291–302. doi:10.
1016/j.jfoodeng.2005.05.022
61. Tengerdy RP, Szakacs G (2003) Bioconversion of lignocellulose in
solid substrate fermentation. Biochem Eng J 13:169–179. doi:10.
1016/S1369-703X(02)00129-8
62. Olsson L, Hahn-Hagerdal B (1996) Fermentation of lignocellulosic
hydrolysates for ethanol production. Enzym Microb Tech 18:312–
331. doi:10.1016/0141-0229(95)00157-3
63. Fukuda H, Kondo A, Tamalampudi S (2009) Bioenergy: sustainable
fuels from biomass by yeast and fungal whole-cell biocatalysts.
Biochem Eng J 44:2–12. doi:10.1016/j.bej.2008.11.016
64. Katahira S, Ito M, Takema H, Fujita Y, Tanino T, Tanaka T, Fukuda
H, Kondo A (2008) Improvement of ethanol productivity during
xylose and glucose co-fermentation by xylose-assimilating S.
cerevisiae via expression of glucose transporter Sut1. Enzym
Microb Tech 43:115–119. doi:10.1016/j.enzmictec.2008.03.001
65. Hahn-Hagerdal B, Jeppsson H, Skoog K, Prior BA (1994) Bio-
chemistry and physiology of xylose fermentation by yeasts. Enzym
Microb Tech 16:933–943. doi:10.1016/0141-0229(94)90002-7
66. Katahira S, Mizuike A, Fukuda H, Kondo A (2006) Ethanol fermen-
tation from lignocellulosic hydrolysate by a recombinant xylose- and
cellooligosaccharide-assimilating yeast strain. Appl Microbiol
Biotechnol 72:1136–1143. doi:10.1007/s00253-006-0402-x
67. Bisaria VS (1991) Bioprocessing of agro-residue to glucose and
chemicals. In: Martin AM (ed) Bioconversion of waste materials to
industrial products. Elsevier, London, pp 187–223
68. Jarboea LR, Shanmugama KT, Ingram LO (2007) Ethanol. In:
Majumder-Russell D (ed) Encyclopedia of microbiology. Elsevier,
New York
69. Gutierrez T, Ingram LO, Preston JF (2006) Purification and character-
ization of a furfural reductase (FFR) from Escherichia coli strain LYO1:
an enzyme important in the detoxification of furfural during ethanol
production. J Biotechnol 121:154–164. doi:10.1016/j.jbiotec.2005.07.
003
70. Gutierrez T, Buszko ML, Ingram LO, Preston JF (2002) Reduction
of furfural to furfuryl alcohol by ethanologenic strains of bacteria
and its effect on ethanol production from xylose. Appl Biochem
Biotechnol 98–100:327–340. doi:10.1385/ABAB:98-100:1-9:327
71. Georgieva TI, Skiadas IV, Ahring BK (2007) Effect of temperature
on ethanol tolerance of a thermophilic anaerobic ethanol producer
Thermoanaerobacter A10: modeling and simulation. Biotechnol
Bioeng 98:1161–1170. doi:10.1002/bit.21536
72. Cook GM, Morgan HW (1994) Hyperbolic growth of
Thermoanaerobacter thermohydrosulfuricus (Clostridium
thermohydrosulfuricum) increases ethanol production in pH-
controlled batch culture. Appl Microbiol Biotechnol 41:84–89.
doi:10.1007/BF00166086
73. Baskaran S, Ahn HJ, Lynd LR (1995) Investigation of the ethanol
tolerance of Clostridium thermosaccharolyticum in continuous cul-
ture. Biotechnol Prog 11:276–281. doi:10.1021/bp00033a006
74. Lamed R, Zeikus JG (1980) Glucose fermentation pathway of
Thermoanaerobium brockii. J Bacteriol 141:1251–1257
75. Larsen L, Nielsen P, Ahring BK (1997) Thermoanaerobacter
mathranii sp. nov., an ethanol-producing, extremely thermophilic
anaerobic bacterium from a hot spring in Iceland. Arch Microbiol
168:114–119
76. Avci A, Donmez S (2006) Effect of zinc on ethanol production by
two Thermoanaerobacter strains. Process Biochem 41:984–989.
doi:10.1016/j.procbio.2005.11.007
77. Houghton TP, Thompson DN, Hess JR, Lacey JA, Wolcot MP,
Schirp A, Englund K, Dostal D, Loge F (2004) Fungal upgrading
of wheat straw for straw-thermoplastics production. Appl Biochem
Biotechnol 113:71–93
78. Biely P, Kremnicky L (1998) Yeasts and their enzyme systems
degrading cellulose, hemicelluloses and pectin. Food Technol
Biotechnol 36:305–312
79. Watari J, Takata Y, Ogawa M, Sahara H, Koshino S, Onnela ML,
Airaksinen U, Jaatinen R, Penttila M, Keranen S (1994) Molecular
cloning and analysis of the yeast flocculation gene FLO1. Yeast 10:
211–225
80. Guo Y, Wang SZ, Xu DH, Gong YM, Ma HH, Tang XY (2010)
Review of catalytic supercritical water gasification for hydrogen
production from biomass. Renew Sust Energ Rev 14:334–343. doi:
10.1016/j.rser.2009.08.012
81. Huber GW, Iborra S, Corma A (2006) Synthesis of transportation
fuels from biomass: chemistry, catalysts, and engineering. Chem
Rev 106:4044–4098. doi:10.1021/cr068360d
82. Kruse A (2008) Review: supercritical water gasification. Biofuels
Bioprod Bioref 2:415–437. doi:10.1002/bbb.93
83. Fang Z, Sato T, Smith RL Jr, Inomata H, Arai K, Kozinski JA
(2008) Reaction chemistry and phase behaviour of lignin in high-
temperature supercritical water. Bioresour Technol 99:3424–3430.
doi:10.1016/j.biortech.2007.08.008
84. Susanti RF, Dianningrum LW, Yum T, Kim Y, Lee BG, Kim J
(2012) High-yield hydrogen production from glucose by supercrit-
ical water gasification without added catalyst. Int J Hydrogen Energ
37:11677–11690. doi:10.1016/j.ijhydene.2012.05.087
85. Fang Z, Minowa T, Fang C, Smith RL Jr, Inomata H, Kozinski JA
(2008) Catalytic hydrothermal gasification of cellulose and glucose.
Int J Hydrogen Energ 33:981–990. doi:10.1016/j.ijhydene.2007.11.
023
86. Azadi P, Farnood R (2011) Review of heterogeneous catalysts for
sub- and supercritical watergasification of biomass and wastes. Int J
Hydrogen Energ 36:9529–9541. doi:10.1016/j.ijhydene.2011.05.
081
87. Bridgwater AV, Peacocke GVC (2000) Fast pyrolysis processes for
biomass. Renew Sust Energ Rev 4:1–73. doi:10.1016/S1364-
0321(99)00007-6
88. Maschio G, Koufopanos C, Lucchesi A (1992) Pyrolysis, a prom-
ising route for biomass utilization. Bioresour Technol 42:219–231.
doi:10.1016/0960-8524(92)90025-S
89. Zheng CY, Tao HX, Xie XA (2013) Distribution and characteriza-
tions of liquefaction of celluloses in sub- and super-critical ethanol.
Bioresources 8:648–662
90. Portofino S, Donatelli A, Iovane P, Innella C, Civita R, Martino M,
Matera DA, Russo A, Cornacchia G, Galvagno S (2013) Steam
gasification of waste tyre: influence of process temperature on yield
and product composition. Waste Manage 33:672–678. doi:10.1016/
j.wasman.2012.05.041
91. Resende FLP, Fraley SA, Berger MJ, Savage PE (2008)
Noncatalytic gasification of lignin in supercritical water. Energ Fuel
22:1328–1324. doi:10.1021/ef700574k
92. Elliott DC (2008) Review: catalytic hydrothermal gasification of
biomass. Biofuels, Bioprod Bioref 2:254–265. doi:10.1002/bbb.74
93. Bermejo MD, Cocero MJ (2006) Supercritical water oxidation: a
technical review. AIChE J 52:3933–3951. doi:10.1002/aic.10993
94. Zhang L, Xu C, Champagne P (2012) Activity and stability of a
novel Ru modified Ni catalyst for hydrogen generation by
Biomass Conv. Bioref.
supercritical water gasification of glucose. Fuel 96:541–545. doi:10.
1016/j.fuel.2012.01.066
95. Lu Y, Li S, Guo L, Zhang X (2010) Hydrogen production by
biomass gasification in supercritical water over Ni/γAl
2
O
3
and Ni/
CeO
2
-γAl
2
O
3
catalysts. Int J Hydrogen Energ 35:7161–7168. doi:
10.1016/j.ijhydene.2009.12.047
96. Azadi P, Afif E, Azadi F, Farnood R (2012) Screening of nickel
catalysts for selective hydrogen production using supercritical water
gasification of glucose. Green Chem 14:1766–1777. doi:10.1039/
C2GC16378K
97. Azadi P, Khan S, Strobel F, Azadi F, Farnood R (2012) Hydrogen
production from cellulose, lignin, bark and model carbohydrates in
supercritical water using nickel and ruthenium catalysts. Appl Catal
B Environ 117–118: 330–338. doi:10.1016/j.apcatb.2012.01.035
98. Osada M, Sato M, Arai K, Shirai M (2006) Stability of supported
ruthenium catalysts for lignin gasification in supercritical water.
Energ Fuel 20:2337–2343. doi:10.1021/ef060356h
99. Yamaguchi A, Hiyoshi N, Sato O, Osada M, Shirai M (2008)Lignin
gasification over supported ruthenium trivalent salts in supercritical
water. Energ Fuel 22:1485–1492. doi:10.1021/ef8001263
100. Furusawa T, Sato T, Sugito H, Miura Y, IshiyamaY, Sato M, Itoh N,
Suzuki N (2007) Hydrogen production from the gasification of
lignin with nickel catalysts in supercritical water. Int J Hydrogen
Energ 32:699–704. doi:10.1016/j.ijhydene.2006.08.001
101. Madenoglu TG, Boukis N, Saglam M, Yuksel M (2011) Supercrit-
ical water gasification of real biomass feedstocks in continuous flow
system. Int J Hydrogen Energ 36:14408–14415. doi:10.1016/j.
ijhydene.2011.08.047
102. Alonso DM, Wettstein SG, Dumesic JA (2012) Bimetallic catalysts
for upgrading of biomass to fuels and chemicals. Chem Soc Rev 41:
8075–8098. doi:10.1039/C2CS35188A
103. Onwudili JA, Williams PT (2013) Hydrogen and methane selectiv-
ity during alkaline supercritical water gasification of biomass with
ruthenium-alumina catalyst. Appl Catal B Environ 132–133:70–79.
doi:10.1016/j.apcatb.2012.11.033
104. Lu Y, Zhao L, Guo L (2011) Technical and economic evaluation of
solar hydrogen production by supercritical water gasification of
biomass in China. Int J Hydrogen Energ 36:14349–14359. doi:10.
1016/j.ijhydene.2011.07.138
105. Nzihou A, Flamant G, Stanmore B (2012) Synthetic fuels from
biomass using concentrated solar energy: a review. Energy 42:
121–131. doi:10.1016/j.energy.2012.03.077
106. Mohammadi M, Najafpour GD, Younesi H, Lahijani P, Uzir MH,
Mohamed AR (2011) Bioconversion of synthesis gas to second
generation biofuels: a review. Renew Sust Energ Rev 15:4255–
4273. doi:10.1016/j.rser.2011.07.124
107. Heiskanen H, Virkajarvi I, Viikari L (2007) The effects of syngas
composition on the growth and product formation of Butyribacterium
methylotrophicum. Enzyme Microb Tech 41:362–367. doi:10.1016/j.
enzmictec.2007.03.004
108. Henstra AM, Sipma J, Rinzema A, Stams AJM (2007) Microbiol-
ogy of synthesis gas fermentation for biofuel production. Curr Opin
Biotechnol 18:200–206. doi:10.1016/j.copbio.2007.03.008
109. Daniell J, Kopke M, Simpson SD (2012) Commercial biomass syngas
fermentation. Energies 5:5372–5417. doi:10.3390/en5125372
110. Allen TD, Caldwell ME, Lawson PA, Huhnke RL, Tanner RS (2010)
Alkalibaculum bacchii gen. nov., sp. nov., a CO-oxidizing, ethanol
producing acetogen isolated from livestock-impacted soil. Int J Syst
Evol Microbiol 60:2483–2489. doi:10.1099/ijs.0.018507-0
111. Liu K, Atiyeh HK, Tanner RS, Wilkins MR, Huhnke RL (2012)
Fermentative production of ethanol from syngas using novel mod-
erately alkaliphilic strains of Alkalibaculum bacchii . Bioresour
Technol 104:336–341. doi:10.1016/j.biortech.2011.10.054
112. Rajagopalan S, Datar RP, Lewis RS (2002) Formation of ethanol
from carbon monoxide via a new microbial catalyst. Biomass
Bioenerg 23:487–493. doi:10.1016/S0961-9534(02)00071-5
113. Cotter JL, Chinn MS, Grunden AM (2009) Influence of process
parameters on growth of Clostridium ljungdahlii and Clostridium
autoethanogenum on synthesis gas. Enzym Microb Tech 44:281–
288. doi:10.1016/j.enzmictec.2008.11.002
114. Guo Y, Xu J, Zhang Y, Xu H, Yuan Z, Li D (2010) Medium optimi-
zation for ethanol production with Clostridium autoethanogenum with
carbon monoxide as sole carbon source. Bioresour Technol 101:8784–
8789. doi:10.1016/j.biortech.2010.06.072
115. Abubackar HN, Veiga MC, Kennes C (2012) Biological conversion
of carbon monoxide to ethanol: effect of pH, gas pressure, reducing
agent and yeast extract. Bioresour Technol 114:518–522. doi:10.
1016/j.biortech.2012.03.027
116. Liou JSC, Balkwill DL, Drake GR, Tanner RS (2005) Clostridium
carboxidivorans sp. nov., a solvent-producing clostridium isolated
from agricultural settling lagoon, and reclassification of the
acetogen Clostridium scatologenes strain SL1 as Clostridium
drakei sp. nov. Int J Syst Evol Microbiol 55:2085–2091. doi:10.
1099/ijs.0.63482-0
117. Mohammadi M, Younesi H, Najafpour G, Mohamed AR (2012)
Sustainable ethanol fermentation from synthesis gas by Clostridium
ljungdahlii in a continuous stirred tank bioreactor. J Chem Technol
Biotechnol 87:837–843. doi:10.1002/jctb.3712
118. Younesi H, Najafpour G, Mohamed AR (2006) Liquid fuel produc-
tion from synthesis gas via fermentation process in a continuous
tank bioreactor (CSTBR) using Clostridium ljungdahlii. Iran J
Biotechnol 4:45–53
119. Saxena J, Tanner RS (2012) Optimization of a cornsteep medium for
production of ethanol from synthesis gas fermentation by Clostridium
ragsdalei. World J Microbiol Biotechnol 28:1553–1561. doi:10.
1007/s11274-011-0959-0
120. Kundiyana DK, Wilkins MR, Maddipati P, Huhnke RL (2011)
Effect of temperature, pH and buffer presence on ethanol production
from synthesis gas by Clostridium ragsdalei. Bioresour Technol
102:5794–5799. doi:10.1016/j.biortech.2011.02.032
121. Kundiyana DK, Huhnke RL, Wilkins MR (2010) Syngas fermentation
in a 100-L pilot scale fermentor: design and process considerations. J
Biosci Bioeng 109:492–498. doi:10.1016/j.jbiosc.2009.10.022
122. Ragsdale SW (2009) Nickel-based enzyme systems. J Biol Chem
284:18571–18575. doi:10.1074/jbc.R900020200
123. Abubackar HN, Veiga MC, Kennes C (2011) Biological conversion
of carbon monoxide: rich syngas or waste gases to bioethanol.
Biofuels Bioprod Bioref 5:93–114. doi:10.1002/bbb.256
124. Munasinghe PC, Khanal SK (2010) Biomass-derived syngas fer-
mentation into biofuels: opportunities and challenges. Bioresour
Technol 101:5013–5022. doi:10.1016/j.biortech.2009.12.098
125. Kundiyana DK, Huhnke RL, Maddipati P, Atiyeh HK, Wilkins MR
(2010) Feasibility of incorporating cotton seed extract in Clostrid-
ium strain P11 fermentation medium during synthesis gas fermen-
tation. Bioresour Technol 101:9673–9680. doi:10.1016/j.biortech.
2010.07.054
126. Klasson KT, Ackerson CMD, Clausen EC, Gaddy JL (1993) Bio-
logical conversion of coal and coal-derived synthesis gas. Fuel 72:
1673–1678. doi:10.1016/0016-2361(93)90354-5
127. Bredwell MD, Srivastava P, Worden RM (1999) Reactor design
issues for synthesis-gas fermentations. Biotechnol Prog 15:834–
844. doi:10.1021/bp990108m
128. Munasinghe PC, Khanal SK (2012) Syngas fermentation to biofuel:
evaluation of carbon monoxide mass transfer and analytical model-
ling using a composite hollow fiber (CHF) membrane bioreactor.
Bioresour Technol 122:130–136. doi:10.1016/j.biortech.2012.03.053
129. Hickey R, Datta R, Tsai SP, Basu R (2011) Membrane supported
bioreactor for conversion of syngas components to liquid products.
U.S. Patent 2011/0256597 A1, 20 October 2011
130. Tsai SP, Datta R, Basu R, Yoon SH (2009) Syngas conversion
system using asymmetric membrane and anaerobic microorganism.
U.S. Patent 2009/0215163 A1, 29 August 2009
Biomass Conv. Bioref.
131. Xu D, Lewis RS (2012) Syngas fermentation to biofuels: effects of
ammonia impurity in raw syngas on hydrogenase activity. Biomass
Bioenerg 45:303–310. doi:10.1016/j.biombioe.2012.06.022
132. Xu D, Tree DR, Lewis RS (2011) The effects of syngas impurities
on syngas fermentation to liquid fuels. Biomass Bioenerg 35:2690–
2696. doi:10.1016/j.biombioe.2011.03.005
133. Ahmed A, Cateni BG, Huhnke RL, Lewis SR (2006) Effects of
biomass-generated producer gas constituents on cell growth, prod-
uct distribution and hydrogenase activity of Clostridium
carboxidivorans P7T. Biomass Bioenerg 30:665–672. doi:10.
1016/j.biombioe.2006.01.007
134. Calli B, Mertoglu B, Inanc B, Yenigun O (2005) Effects of high free
ammonia concentrations on the performances of anaerobic bioreactors.
Process Biochem 40:1285–1292. doi:10.1016/j.procbio.2004.05.008
135. Saxena J, Tanner RS (2011) Effect of trace metals on ethanol
production from synthesis gas by the ethanologenic acetogen, Clos-
tridium ragsdalei. J Ind Microbiol Biotechnol 38:513–521. doi:10.
1007/s10295-010-0794-6
136. Panneerselvam A, Wilkins MR, Delorme MJM, Atiyeh HK,
Huhnke RL (2009) Effects of various reducing agents on syngas
fermentation by “Clostridium ragsdalei”. Biol Eng 2:135–144
137. Dry ME (2004) Present and future applications of the Fischer–
Tropsch process. Appl Catal A 276:1–3. doi:10.1016/j.apcata.
2004.08.014
138. Subramani V, Gangwal SK (2008) A review of recent literature to
search for an efficient catalytic process for the conversion of syngas
to ethanol. Energ Fuel 22:814–839. doi:10.1021/ef700411x
139. Li F, Jiang D, Zeng XC, Chen Z (2012) Mn monolayer modified Rh
for syngas-to-ethanol conversion: a first-principles study. Nanoscale
4:1123–1129. doi:10.1039/c1nr11121c
140. Choi Y, Liu P (2009) Mechanism of ethanol synthesis from syngas on
Rh (111). J Am Chem Soc 131:13054–13061. doi:10.1021/ja903013x
141. Spivey JJ, Egbebi A (2007) Heterogeneous catalytic synthesis of
ethanol from biomass-derived syngas. Chem Soc Rev 36:1514–
1528. doi:10.1039/B414039G
142. Pan X, Fan Z, Chen W, Ding Y, Luo H, Bao X (2007) Enhanced
ethanol production inside carbon-nanotube reactors containing cat-
alytic particles. Nat Mater 6:507–511. doi:10.1038/nmat1916
143. Fan Z, Chen W, Pan X, Bao X (2009) Catalytic conversion of syngas
into C
2
-oxygenates over Rh-based catalysts—effects of carbon sup-
ports. Catal Today 147:86–93. doi:10.1016/j.cattod.2009.03.004
144. Haider MA, Gogate MR, Davis RJ (2009) Fe-promotion of sup-
ported Rh catalysts for different conversion of syngas to ethanol. J
Catal 261:9–16. doi:10.1016/j.jcat.2008.10.013
145. Mei D, Rousseau R, Kathmann SM, Glezakou VA, Engelhard MH,
Jiang W, Wang C, Gerber MA, White JF, Stevens DJ (2010) Ethanol
synthesis from syngas over Rh-based/SiO
2
catalysts: a combined
experimental and theoretical modeling study. J Catal 271:325–342.
doi:10.1016/j.jcat.2010.02.020
146. Han L, Mao D, Yu J, Guo Q, Lu G (2012) Synthesis of C
2
-oxygenates
from syngas over Rh-based catalyst supported on SiO
2
,TiO
2
and SiO
2
-
TiO
2
mixed mode. Catal Commun 23:20–24. doi:10.1016/j.catcom.
2012.02.032
147. Chen G, Guo CY, Zhang X, Huang Z, Yuan G (2011) Direct
conversion of syngas to ethanol over Rh/Mn supported on modified
SBA-15 molecular sieves: effect of supports. Fuel Process Technol
92:456–461. doi:10.1016/j.fuproc.2010.10.012
148. Cosultchi A, Parez-Luna M, Morales-Serna JA, Salmon M (2012)
Charaterization of modified Fischer–Tropsch catalysts promoted with
alkaline metals for higher alcohol synthesis. Catal Lett 142:368–377
149. Surisetty VR, Dalai AK, Kozinski J (2010) Synthesis of higher
alcohols from synthesis gas over Co-promoted alkali-modified
MoS
2
catalysts supported on MWCNTs. Appl Catal A Gen 385:
153–162. doi:10.1016/j.apcata.2010.07.009
150. Gong J, Yue H, Zhao Y, Zhao S, Zhao L, Lv J, Wang S, Ma X
(2012) Synthesis of ethanol via syngas on Cu/SiO
2
catalysts with
balanced Cu
0
-Cu
+
sites. J Am Chem Soc 134:13922–13925. doi:10.
1021/ja3034153
151. Yang G, San X, Jiang N, Tanaka Y, Li X, Jin Q, Tao K, Meng F,
Tsubaki N (2011) A new method of ethanol synthesis fromdimethyl
ether and syngas in a sequential dual bed reactor with modified
zeolite and Cu/ZnO catalysts. Catal Today 164:425–428. doi:10.
1016/j.cattod.2010.10.027
152. Liu Y, Murata K, Inaba M, Takahara I (2013) Synthesis of ethanol
from methanol and syngas through an indirect route containing
methanol dehydrogenation, DME carbonylation and methyl acetate
hydrogenolysis. Fuel Process Technol 110:206–213. doi:10.1016/j.
fuproc.2012.12.016
153. Sai Prasad PS, Bae JW, Kang SH, Lee YJ, Jun KW (2008) Single-step
synthesis of DME from syngas on Cu-ZnO-Al
2
O
3
/zeolite bifunctional
catalysts: the superiority of ferrierite over other zeolites. Fuel Process
Technol 89:1281–1286. doi:10.1016/j.fuproc.2008.07.014
154. Erena J,Garona R, Arandes JM, Aguayo AT, Bilbao J (2005) Direct
synthesis of dimethyl ether from (H
2
+CO) and (H
2
+CO
2
)feeds.
Effect of feed composition. Int J Chem React Eng 3:1–15. doi:10.
2202/1542-6580.1295
155. Aguayo AT, Erena J, Sierra I, Olazar M, Bilbao J (2005) Deactiva-
tion and regeneration of hybrid catalysts in the single-step synthesis
of dimethyl ether from syngas and CO
2
. Catal Today 106:265–270.
doi:10.1016/j.cattod.2005.07.144
156. Zeng C, Sun J, Yang G, Ooki I, Hayashi K, Yoneyama Y, Taguchi
A, Abe T, Tsubaki N (2013) Highly selective and multifunctional
Cu/ZnO/Zeolite catalyst for one-step dimethyl ether synthesis: pre-
paring catalyst by bimetallic physical sputtering. Fuel 112:140–144.
doi:10.1016/j.fuel.2013.05.026
157. Moradi GR, Nosrati S, Yaripor F (2007) Effects of hybrid catalysts
preparation method upon direct synthesis of dimethyl ether from
synthesis gas. Catal Commun 8:598–606. doi:10.1016/j.catcom.
2006.08.023
158. Feng W, Yao J, Wu H, Ji P (2012) Synthesis of bioethanol from
biomass-derived syngas over carbon nanotube/silica supported catalyst.
Biotechnol Adv 30:874–878. doi:10.1016/j.biotechadv.2012.01.017
159. Surisetty VR, Dalai AK, Kozinski J (2010) Effect of Rh promoter
on MWCNT-supported alkali modified MoS
2
catalysts for higher
alcohols synthesis from CO hydrogenation. Appl Catal A Gen 381:
282–288. doi:10.1016/j.apcata.2010.04.036
160. Zhang X, Liu Y, Liu G, Tao K, Jin Q, Meng F, Wang D, Tsubaki N
(2012) Product distributions including hydrocarbon and oxygenates
of Fischer–Tropsch synthesis over mesoporous MnO
2
-supported Fe
catalyst. Fuel 92:122–129. doi:10.1016/j.fuel.2011.07.041
161. Ma W, Kugler EL, Dadyburjor DB (2011) Promotional effect of
copper on activity and selectivity to hydrocarbons and oxygenates
for Fischer–Tropsch synthesis over potassium-promoted iron cata-
lysts supported on activated carbon. Energ Fuel 25:1931–1938. doi:
10.1021/ef101720c
162. SubramanianND,GaoJ,MoX,GoodwinJGJr,TorresW,SpiveyJJ
(2010) La and/or V oxide promoted Rh/SiO
2
catalysts: effect of tem-
perature, H
2
/CO ratio, space velocity and pressure on ethanol selectivity
from syngas. J Catal 272:204–209. doi:10.1016/j.jcat.2010.03.019
163. Chiang SW, Chang CC, Shie JL, Chang CY, Ji DR, Tseng JY,
Chang CF, Chen YH (2012) Synthesis of alcohols and alkanes from
CO and H
2
over MoS
2
/γ-Al
2
O
3
catalyst in a packed bed with
continuous flow. Energies 5:4147–4164. doi:10.3390/en5104147
164. Wang W, Wang S, Ma X, Gong J (2011) Recent advances in
catalytic hydrogenation of carbon dioxide. Chem Soc Rev 40:
3703–3727. doi:10.1039/C1CS15008A
165. Centi G, Perathoner S (2009) Opportunities and prospects in the
chemical recycling of carbon dioxide to fuels. Catal Today 148:
191–205. doi:10.1016/j.cattod.2009.07.075
166. Takagawa M, Okamoto A, Fujimura H, Izawa Y, Arakawa H (1998)
Ethanol synthesis from carbon dioxide and hydrogen. Stud Surf Sci
Catal 114:525–528. doi:10.1016/S0167-2991(98)80812-4
Biomass Conv. Bioref.
167. Arakawa H (1998) Research and development on new synthetic
routes for basic chemicals by catalytic hydrogenation of CO
2
.Stud
Surf Sci Catal 114:19–30. doi:10.1016/S0167-2991(98)80723-4
168. Kusama H, Okabe K, Sayama K, Arakawa H (1997) Ethanol
synthesis by catalytic hydrogenation of CO
2
over Rh-Fe-SiO
2
cat-
alysts. Catal Today 22:343–348. doi:10.1016/S0360-5442(96)
00095-3
169. Inui T, Yamamoto T (1998) Effective synthesis of ethanol fromCO
2
on polyfunctional composite catalysts. Catal Today 45:209–214.
doi:10.1016/S0920-5861(98)00217-X
170. Heijden HV, Ptasinski KJ (2012) Exergy analysis of thermochem-
ical ethanol production via biomass gasification and catalytic syn-
thesis. Energy 46:200–210. doi:10.1016/j.energy.2012.08.036
171. Wei L, Pordesimo LO, Igathinathane C, Batchelor WD (2009)
Process engineering evaluation of ethanol production from wood
through bioprocessing and chemical catalysis. Biomass Bioenerg
33:255–266. doi:10.1016/j.biombioe.2008.05.017
172. Mohanty P, Nanda S, Pant KK, Naik S, Kozinski JA, Dalai AK
(2013) Evaluation of the physiochemical development of biochars
obtained from pyrolysis of wheat straw, timothy grass and pine-
wood: effects of heating rate. J Anal Appl Pyrol. doi:10.1016/j.jaap.
2013.05.022
173. Boucher ME, Chaala A, Roy C (2000) Bio-oils obtained by vacuum
pyrolysis of softwood bark as a liquid fuel for gas turbines. Part I:
properties of bio-oil and its blends with methanol and a pyrolytic
aqueous phase. Biomass Bioenerg 19:337–350. doi:10.1016/
S0961-9534(00)00043-X
174. He R, Ye XP, English BC, Satrio JA (2009) Influence of pyrolysis
condition on switchgrass bio-oil yield and physicochemical proper-
ties. Bioresour Technol 100:5305–5311. doi:10.1016/j.biortech.
2009.02.069
175. Ba TA, Chaala M, Garcia-Perez D, Rodrigue RC (2004) Colloidal
properties of bio-oils obtained by vacuum pyrolysis of softwood
bark. Characterization of water-soluble and water-insoluble frac-
tions. Energ Fuel 18:704–712. doi:10.1021/ef030118b
176. Joshi J, Lawal A (2012) Hydrodeoxygenation of pyrolysis oil in a
microreactor. Chem Eng Sci 74:1–8. doi:10.1016/j.ces.2012.01.052
177. Meier D, Faix O (1999) State of the art of applied fast pyrolysis of
lignocellulosic materials—a review. Bioresour Technol 68:71–77.
doi:10.1016/S0960-8524(98)00086-8
178. Ringer M, Putsche V, Scahill J (2013) Large-scale pyrolysis oil
production: a technology assessment and economic analysis.
NREL/TP-510-37779; 2006. http://www.nrel.gov/docs/fy07osti/
37779.pdf. Accessed 26 March 2013
179. E4tech (2009) Review of technologies for gasification of biomass
and wastes. NNFCC Project 09/008
180. Huber GW, Corma A (2007) Synergies between bio- and oil refin-
eries for the production of fuels from biomass. Angew Chem Int Ed
46:7184–7201. doi:10.1002/anie.200604504
181. Sharma RK, Bakhshi NN (1991) Catalytic upgrading of biomass-
derived oils to transportation fuels and chemicals. Can J Chem Eng
69:1071–1081. doi:10.1002/cjce.5450690505
182. Mortensen PM, Grunwaldt JD, Jensen PA, Knudsen KG, Jensen AD
(2011) A review of catalytic upgrading of bio-oil to engine fuels.
Appl Catal A Gen 407:1–19. doi:10.1016/j.apcata.2011.08.046
183. Couper JR, Penney WR, Fair JR, Walas SM (2010) Chemical
process equipment-selection and design, 2nd edn. Elsevier, USA
184. Czernik S, Bridgwater AV (2004) Overview of applications of
biomass fast pyrolysis oil. Energ Fuel 18:590–598. doi:10.1021/
ef034067u
185. Agblevor FA, Besler S (1996) Inorganic compounds in biomass
feedstocks. 1. Effect on the quality of fast pyrolysis oils. Energ Fuel
10:293–298. doi:10.1021/ef950202u
186. Oasmaa A, Czernik S (1999) Fuel oil quality of biomass pyrolysis
oils—state of the art for the end users. Energ Fuel 13:914–921. doi:
10.1021/ef980272b
187. Diebold JP, Czernik S (1997) Additives to lower and stabilize the
viscosity of pyrolysis oils during storage. Energ Fuel 11:1081–
1091. doi:10.1021/ef9700339
188. Oasmaa A, Sipila K, Solantausta Y, Kuoppala E (2005) Quality
improvement of pyrolysis liquids: effect of light volatiles on the
stability of pyrolysis liquids. Energ Fuel 19:2556–2561. doi:10.
1021/ef0400924
189. Rajvanshi AK (1986) Biomass gasification. In: Goswami DY (ed)
Alternative energy in agriculture. CRC Press, New York, pp 83–102
190. Higman C, van der Burgt M (2008) Gasification. Elsevier, London
191. Roddy DJ, Whitton CM (2012) Comprehensive renewable energy.
Biomass gasification and pyrolysis. Elsevier, New York, pp 133–137
192. Delgado J, Aznar MP, Corella J (1997) Biomass gasification with
steam in fluidized bed: effectiveness of CaO, MgO, and CaO-MgO
for hot raw gas cleaning. Ind Eng Chem Res 36:1535–1543. doi:10.
1021/ie960273w
193. Lv PM, Xiong ZH, Chang J, Wu CZ, Chen Y, Zhu JX (2004) An
experimental study on biomass air–steam gasification in a fluidized bed.
Bioresour Technol 95:95–101. doi:10.1016/j.biortech.2004.02.003
194. Franco C, Pinto F, Gulyurtlu I, Cabrita I (2003) The study of
reactions influencing the biomass steam gasification process. Fuel
82:835–842. doi:10.1016/S0016-2361(02)00313-7
195. Moon J, Lee J, Lee U, Hwang J (2013) Transient behavior of
devolatilization and char reaction during steam gasification of bio-
mass. Bioresour Technol 133:429–436. doi:10.1016/j.biortech.
2013.01.148
196. Weerachanchai P, Horio M, Tangsathitkulchai C (2009) Effects of
gasifying conditions and bed materials on fluidized bed steam
gasification of wood biomass. Bioresour Technol 100:1419–1427.
doi:10.1016/j.biortech.2008.08.002
197. Fushimi C, Araki K, Yamaguchi Y, Tsutsumi A (2003) Effect of
heating rate on steam gasification of biomass. 2. Thermogravimetric-
mass spectrometric (TG-MS) analysis of gas evolution. Ind Eng
Chem Res 42:3929–3936. doi:10.1021/ie0300575
198. Zhang L, Xu CC, Champagne P (2010) Overview of recent ad-
vances in thermo-chemical conversion of biomass. Energ Convers
Manag 51:969–982. doi:10.1016/j.enconman.2009.11.038
199. McKendry P (2002) Energy production from biomass (part 3):
gasification technologies. Bioresour Technol 83:55–63. doi:10.
1016/S0960-8524(01)00120-1
200. van der Drift A, Boerrigter H (2013) Synthesis gas from biomass for
fuels and chemicals. ECN-C-06-001; 2006. ftp://www.nrg-nl.com/
pub/www/library/report/2006/c06001.pdf. Accessed 26 March 2013
201. Behrendt F, Neubauer Y, Oevermann M, Wilmes B, Zobel N (2008)
Direct liquefaction of biomass. Chem Eng Technol 31:667–677.
doi:10.1002/ceat.200800077
202. Balat M (2008) Mechanisms of thermochemical biomass conver-
sion processes. Part 3: reactions of liquefaction. Energ Source A 30:
649–659. doi:10.1080/10407780600817592
203. Xu C, Etcheverry T (2008) Hydro-liquefaction of woody biomass in
sub- and super-critical ethanol with iron-based catalysts. Fuel 87:
335–345. doi:10.1016/j.fuel.2007.05.013
204. Blaschek HP, Ezeji TC, Scheffran J (2010) Biofuels from agricul-
tural wastes and byproducts. Blackwell, New York. doi:10.1002/
9780813822716. ISBN 978-0-813-80252-7
205. Miller IJ, Fellows SK (1981) Liquefaction of biomass as a sourceof
fuels or chemicals. Nature 289:398–399. doi:10.1038/289398a0
206. Zhong C, Wei X (2004) A comparative experimental study on the
liquefaction of wood. Energy 29:1731–1741. doi:10.1016/j.energy.
2004.03.096
207. Mun SP, Hassan EM (2004) Liquefaction of lignocellulosic biomass
with mixtures of ethanol and small amounts of phenol in the presence
of methanesulfonic acid catalyst. J Ind Eng Chem 10:722–727
208. Xu J, Jiang J, Dai W, Xu Y (2012) Liquefaction of sawdust in hot
compressed ethanol for the production of bio-oils. Process Saf
Environ 90:333–338. doi:10.1016/j.psep.2012.01.001
Biomass Conv. Bioref.
209. Krzan A, Kunaver M, Tisler V (2005) Wood liquefaction using
dibasic organic acids and glycols. Acta Chim Slov 52:253–258
210. Liang L, Mao Z, Li Y, Wan C, Wang T, Zhang L, Zhang L (2006)
Liquefaction of crop residues for polyol production. Bioresources 1:
248–256
211. Food and Agriculture Organization of the United Nations, FAO
(2008) The state of food and agriculture. Biofuels: prospects, risks,
and opportunities. FAO, Rome
212. Borjesson P (2009) Good or bad bioethanol from a greenhouse gas
perspective—what determines this? Appl Energ 86:589–594. doi:
10.1016/j.apenergy.2008.11.025
213. Fargione J, Hill J, Tilman D, Polasky S, Hawthorne P (2008) Land
clearing and the biofuel carbon debt. Science 319:1235–1238. doi:
10.1126/science.1152747
214. Searchinger T, Heimlich R, Houghton RA, Dong F, Elobeid A,
Fabiosa J, Tokgoz S, Hayes D, Yu TH (2008) Use of U.S. croplands
for biofuels increases greenhouse gases through emissions from land-
use change. Science 319:1238–1240. doi:10.1126/science.1151861
215. Shimada K, Hosoya S, Ikeda T (1997) Condensation reactions of
softwood and hardwood lignin model compounds under organic
acid cooking conditions. J Wood Chem Technol 17:57–72. doi:10.
1080/02773819708003118
216. Balat M (2009) Gasification of biomass to produce gaseous products.
Energ Source A 31:516–526. doi:10.1080/15567030802466847
217. Himmel ME, Ding SY, Johnson DK, Adney WS, Nimlos MR,
Brady JW, Foust TD (2007) Biomass recalcitrance: engineering
plants and enzymes for biofuels production. Science 315:804–807.
doi:10.1126/science.1137016
218. Muthaiyan A, Ricke SC (2010) Current perspectives on detection of
microbial contamination in bioethanol fermentors. Bioresour
Technol 101:5033–5042. doi:10.1016/j.biortech.2009.11.005
219. Heist P (2011) Identifying, controlling the most common microbial
contaminants. Ethanol Producer Magazine; 2009. www.
ethanolproducer.com/article.jsp?article_id=5464. Accessed 23
April 2011
220. Basilio ACM, de Araujo PRL, de Morais JOF, da Silva FilhoEA, de
Morais Jr MA, Simoes DA (2008) Detection and identification of
wild yeast contaminants of the industrial fuel ethanol fermentation
process. Curr Microbiol 56:322–326. doi:10.1007/s00284-007-
9085-5
221. Grossman HL, Myers WR, Vreeland VJ, Bruehl R, Alper MD,
Bertozzi CR, Clarke J (2004) Detection of bacteria in suspension
by using a superconducting quantum interference device. PNAS
101:129–134. doi:10.1073/pnas.0307128101
222. Gloyna EF, Li L (1995) Supercritical water oxidation research and
development update. Environ Prog 14:182–192. doi:10.1002/ep.
670140318
223. Gaddy JL (1998) Biological production of acetic acid from waste
gases with Clostridium ljungdahlii. U.S. Patent 5807722, 15 Sep-
tember 1998
224. Zahn JA, Saxena J (2011) Novel ethanologenic species Clostridium
coskatii. U.S. Patent 2011/0229947 A1, 22 September 2011
225. Heijstra B, Kern E, Koepke M, Segovia S, Liew F (2012) Novel
bacteria and methods of use thereof. Patent WO/2012/015317, 2
February 2012
226. Kopke M, Mihalcea C, Bromley JC, Simpson SD (2011) Fermen-
tative production of ethanol from carbon monoxide. Curr Opin
Biotechnol 22:320–325. doi:10.1016/j.copbio.2011.01.005
227. Lu Q, Zhang J, Zhu XF (2008) Corrosion properties of bio-oil and
its emulsions with diesel. Chin Sci Bull 53:3726–3734. doi:10.
1007/s11434-008-0499-7
228. Czernik S, Johnson DK, Black S (1994) Stability of wood fast
pyrolysis oil. Biomass Bioenerg 7:187–192. doi:10.1016/0961-
9534(94)00058-2
229. Perez MG, Chaala A, Pakdel H, Kretschmer D, Rodrigue D, Roy C
(2006) Multiphase structure of bio-oils. Energ Fuel 20:364–375.
doi:10.1021/ef050248f
230. Oasmaa A, Elliott DC, Korhonen J (2010) Acidity of biomass fast
pyrolysis bio-oils. Energ Fuel 24:6548–6554. doi:10.1021/
ef100935r
231. Wild PD (2011) Biomass pyrolysis for chemicals. Dissertation,
University of Groningen. ISBN: 978-90-367-4994-7
232. Verma M, Godbout S, Brar SK, Solomatnikova O, Lemay SP,
Larouce JP (2012) Biofuels production from biomass by thermo-
chemical conversion technologies. Int J Chem Eng. doi:10.1155/
2012/542426
Biomass Conv. Bioref.
