Sustainable globalization demands expanded fuel feedstocks. In the twentieth
century main studies were concentrated on the development of cheaply use of
fossil fuels for heat, electricity (industrial, commercial and residential),
transportation fuels and chemicals including pharmaceuticals, detergents, synthetic
fiber, plastics, pesticides, fertilizers, lubricants, solvent, waxes, coke,
asphalt to meet the growing demand of the population (Bender,
2000; Demirbas, 2006; Scott et
al., 2010). The transportation sector, moreover, requires considerable
supplies of liquid transportation fuel which accounts for a major part of oil
reserves. Currently, the fossil resources are not considered as sustainable
resources so that it is predicted that oil reserves are relatively depleted
by 2025 (Greene et al., 2006). The vast combustion
of fossil fuels over the past century has brought serious concerns for most
nations in the world due to environmental impact of CO2 emission
on the atmosphere contributing global warming. It has been realized that the
green house emission from widespread utilization of fossil fuels has a deleterious
effect on climate temperature. Consequently, there is a growing trend towards
exploiting alternative, renewable and environmentally friendly fuels which are
cost-wise competitive with fossil fuels (Vasudevan et
Plant biomass, agricultural residues and forest wastes have received much attention
as feedstock for the production of fuels which are known as biofuels. Biofuels
could help decrease global demands for fossil fuels resulting reduction of greenhouse
gas emission and environmental warming (Goh et al.,
2010). This review study is aimed at highlighting biomass resources, different
type of biofuel (first and second generation of biofuels) and biotechnological
process used for biofuels production.
Biomass: Biomass is known as an organic, non-fossil material with a
biological origin including plants, agricultural residues, forest wastes, microbial
cells and municipal wastes derived from biological sources all which represent
a potential sustainable energy source (Wu et al.,
2010). Agro-industrial residues obtained in food bioprocess constitute the
large sources of biomass such as palm kernel cake produced in palm oil industry
(Abdeshahian et al., 2010a). The total amount
of biomass produced on the Earth is approximated 100 billion tones organic dry
matter of land biomass annually and 50 billion tones of aquatic biomass (Naik
et al., 2010).
Lignocellulose is the most plentiful renewable biomass which forms approximately half of the plant matter produced by photosynthesis indicating a vast sustainable organic resource in soil.
Lignocellulose is composed of three types of biopolymers namely, cellulose,
hemicellulose and lignin that are strongly linked by non-covalent forces and
by covalent crosslinkages with a relatively low content of monosaccharides,
starch, protein, or oils. Only a small part of the lignocellulosic substances
obtained in agriculture or forestry is used as food, feed, industrial raw materials
and energy resources, the rest being considered waste recycled to the Earth
system (Sanchez, 2009).
Lignocellulose is a potential feedstock for the production of biofuels, industrial
enzymes, animal feed, biofertilizers, biopesticide and biopromoter products.
Lignocellulosic raw materials are also used in the paper industry (Tangerdy
and Szakacs, 2003). Lignocellulosic raw materials are also used in the production
of industrial enzymes (Abdeshahian et al., 2010b).
Plant biomass has been realized as a major non-fossil resource of energy due
to sustainable development of plant biomass for biotechnological applications,
positive effect on global warming with the mitigation of atmospheric CO2
and making independency to fossil fuel energy (Demirbas,
2007). In the sector of bioenergy production plant biomass has been known
for decades as one of the most potentially renewable energy sources that could
be utilized for the production of biofuels. Moreover, plant biomass can contribute
to provide approximate 14% the total worlds energy demand representing
a key role for global economy (Zhao et al., 2009).
Biofuel: The term of biofuel refers to liquid fuels and blending components
produced from renewable biological products (biomass) for using preliminary
in the transport sector (Wu et al., 2010). Biofuels
are considered as a favorite, green alternative to fossil sources of energy
offering several advantages including compatibility used in transportation infrastructure,
contribution to mitigating carbon dioxide emissions, sustainability in production,
plant biomass-based origin, security of supply and development of rural economy
(Reijnders, 2006; Yan and Lin, 2009).
Biotechnological efforts in 21nd century have been aimed at finding industrial
feedstock and green processes for the production of biofuels and bioproducts
using renewable biomass sources (Stevens and Verhe, 2004).
FIRST GENERATION OF BIOFUELS
The first generation of biofuels refers to the fuels produced from edible feedstock
including carbohydrate derived from corn, wheat, sugarcane and sugar beet, as
well as oil-seed component produced from plants such as palm oil and soybean
oil (Moore, 2008). There are three main types of biofuels
in this category which are commercially used in transportation infrastructure
with the large production around the world. These fuels include biodiesel (bio-esters),
ethanol and biogas.
Biodiesel: Biodiesel is an alternative diesel which is produced by transesterification of vegetable oil, residual oil and fats. Transesterification is a reversible reaction in which vegetable oil and animal fat are mixed with methanol in the presence of a liquid catalyst to produce methyl esters known as biodiesel and glycerol which is a valuable co-product. As shown in Eq. 1, in transesterification process triglyceride and methanol are reactants. This process is catalyzed by a liquid acid or a liquid base:
However, transesterification is hampered in the case of a high content of free
faty acids in oil which leads to a decrease in methyl esters quantity due to
saponification reaction. The use of solid catalyst can catalyze the simultaneous
tranesterification of triglyceride and esterification of high free fatty acid
existing in oil. The tranesterification process occurs between triglyceride
and methanol while esterification process occurs between high free fatty acid
and methanol in each which the final product is methyl esters (Kulkarni
et al., 2006; Meher et al., 2006).
Carbohydrate sources used in ethanol production: A large number of carbohydrate based feedstocks are used for the production of ethanol through fermentation process. Three main groups of carbohydrate sources used in this process are:
||Agricultural crops containing sugar: Sugar cane, wheat,
beet root, fruits, palm juice
||Agricultural crops containing starch: Grain (wheat,
barley, rice, corn and sweet sorghum) and root plants like potato and cassava
||Cellulosic biomass: Wood and wood waste, cedar, pine,
agricultural residues and fibers
Agricultural food sources like corn, wheat, barley, sweet sorghum form grain
alcohol whereas the alcohol obtained from lignocellulolosic biomass is called
biomass ethanol or bioethanol (Minteer, 2006).
The conversion of starch feedstocks to ethanol: Starch components consist
of a long polymeric chain of glucose. It is not possible to directly convert
starch feedstocks to ethanol by conventional fermentation technology. The polymeric
structure of starch is first broken down into simple molecules of glucose and
other oligosaccharides. In this process, a mash contained 15-20% starch concentration
is produced by grinding starch substances and mixing with distilled water. The
mash is then heated at or more than its boiling point and subsequently processed
with two enzyme preparation. The first enzyme is amylase which releases maltodextrin
oligosaccharides by liquefaction process. The second enzymes are pullulanase
and glucoamylase which hydrolyze the dextrin and oligosaccharide to produce
glucose, maltose and isomatose. The process of second enzyme reaction is known
saccharification. The mash is then cooled to 30°C to be treated by yeast
in fermentation process (Lee et al., 2007).
Fermentation process is a next stage of ethanol production. Fermentation process
refers to biochemical processes in which organic substances are metabolized
by microorganisms producing enzymes (Abdeshahian et al.,
2009). Fermentation process can be performed in aerobic or anaerobic conditions.
In the production of ethanol simple sugar (hexoses and pentoses) are fermented
in anaerobic conditions by microorganisms including yeast (Sacchromyces
species), bacteria (Zymomonas species) and mold (mycelium). These microorganisms
live in natural environment and very specific for fermentation of hexoses or
pentoses or mixture of them. Different yeasts are used in ethanol production
under anaerobic conditions such as Saccharomyces cerevial, Saccharomyces
uvarum, Shiozosaccharomyces pombe and Kluyveromyces species.
In anaerobic conditions yeasts hydrolyze glucose to ethanol with an efficiency
conversion of 51% on the weight basis. In practice, 40-48% of glucose is converted
to ethanol with 46% fermentation efficiency because of partly consumption of
glucose for the production of cell components and metabolic products other than
ethanol production (Lee et al., 2007). The Eq.
2 shows the general anaerobic conversion of glucose to ethanol:
The ethanol produced from starch grain like corn is generally categorized into
two processes, namely dry mill and wet mill. Dry mills have small size processed
for manufacturing ethanol and animal feed. Wet mills are utilized for the production
of ethanol and a variety of high value co-products such as nutraceuticals, pharmaceuticals,
organic acids and solvent (Shapouri et al., 1995;
Biogas production: Anaerobic biodegradation of solid biomass leads to the production of biogas. In this process anaerobic bacteria digest organic matter of biomass to produce a mixture of methane and carbon dioxide gases with a typical ratio of 60-70% methane and 30% carbon dioxide. The gas has a heating value of 650-750 British thermal units (Btu) per cubic feet of gas (1 Btu = 1055 Joule). Anaerobic biodegradation of municipal solid waste (mainly biomass and biological waste) in landfills produce methane and carbon dioxide in approximate equal volume. These two basic gas together other gas of atmosphere (nitrogen and oxygen) and a slight amount of organic compounds form landfill gas (LFG).
However, LFG contains unfavorable contaminants including toxic vinyl chloride
and hydrogen sulphide. Thus, LFG requires the efficient technology for removal
of contaminants and carbon dioxide to be used as a substitute for natural gas
(Lee et al., 2007). The main use of LFG is the
production of energy as electricity using internal combustion engine, turbine,
micro turbine as well as direct use in boiler, dryer, kiln and green house.
The high cost of methane refinery has led to new trend of landfill gas utilization
toward the production of methanol (Lee et al., 2007).
The advantages of methanol as liquid fuel are low production of sulphur and
ash in commercial use and much simpler handle, transportation and store of methanol
than gas products.
SECOND GENERATION OF BIOFUELS
Second generations of biofules are produced from no-edible sources specifically
lignocellulosic biomass. The combustion of these biofuels releases low volume
of carbon dioxide in to the atmosphere mitigating green house gas (Goh
et al., 2010). In a commercial aspect, the cost of these biofuels
potentially comparable with standard petrol and diesel, hence they are cost-effective
for road transport.
Plant biomass is a promising feedstock for the production of 2nd generation
biofuels. In this time, the production of 2nd generation biofuels is not cost-effective
because of technological bottlenecks which need to be overcome before they can
potentially be used (Pauly and Keegstra, 2008; Wu
et al., 2010).
CONVERSION PROCESSES FOR SECOND GENERATION BIOFUELS
Mechanical extraction: In this method crude vegetable oil is obtained from
oil seeds by screw press (expeller) using mechanical pressure. Screw press works
in two ways of pre-pressing and full pressing. In pre-pressing, only a part
of the oil is separated from seeds and the remained cake with 18 to 20% oil
is further processed by solvent extraction. Oilseeds with high oil content (30-40%)
are processed in combined pre-pressing and solvent extraction methods. Full
pressing includes the imposition of 95000 kPa to extract most of oil content.
Full-pressing can also be carried out by an initial pre-pressing and a final
press (Stevens and Verhe, 2004).
Briquetting of biomass: Agricultural residues, forestry waste and other
plant biomass materials have a rough bulky shape. This drawback can be alleviated
by densification of residuals into compact regular shapes. In densification
biomass is placed in a press chamber. Densification is performed in two methods
of pressing and maceration. In pressing method an increase in density with an
increase of used pressure in the early stage of compression is applied, but
the rate of increase in density falls rapidly as the density of pressed materials
reaches close to the density of water, however, there is no close correlation
between density change and degree of maceration of materials which may be chopping,
grinding and pulverizing. A coarse chopping of some materials may be as effective
as ultrafine grinding. Thus, in chopping process the volume of tree branches
chipped is extensively reduced, while fin grinding provides a reduction in volume
that is not considerable higher than chopping method (Kitani
and Hall, 1989; Stevens and Verhe, 2004).
Distillation: The evaporation of volatile components of a mixture to
separate them from non-volatile parts is called distillation. This process is
a main method for recovery of plant oil. Plants are crushed before distillation
to extract the oil easier. Plants are then steamed and the important oils vaporize,
move up with steam and condensed to be come liquid. An advanced technology is
molecular distillation. This method is utilized for production of fragrances
which are difficult to be distilled in conventional method (Stevens
and Verhe, 2004).
Direct combustion: Combustion (burning) is the chemical reaction between
a fuel and oxygen. As a consequence, carbon dioxide and water are released with
the production of heat. In direct combustion plant biomass are used as a substitute
of fossil fuels in combustion reaction. Advantages of direct combustions are
lower sulfur emission (0.05-0.2 w.t. %) and the better control of particulate
formation in the source (Lee et al., 2007).
Gasification: Gasification is a technology for the conversion of biomass
to sustainable fuels. Gasification step includes reacting biomass with oxygen
and/or steam to produce a gaseous blend of CO and H2 with various
amounts of CO2. CH4, N2 and other gases under
elevated temperature which depending on the proportion of gaseous constituents
are called producer gas or synthesis gas (Syngas) (Rowlands
et al., 2008; Wu et al., 2010).
Gasification can be performed by two routes, namly catalytic and non-catalytic
method. In non-catalytic method a very high temperature as high as 1300 °C
is required, while catalytic process can be operated at lower temperature (900
°C) and even lower than that with using high technology (Lee
et al., 2007). Producer gas is mainly used for stationary power generation,
whereas syngas is currently applied for the production of transportation fuels
and chemical intermediates. The chemical reaction routes for the fuel production
using syngas are hydrogen production by Water-Gas-Shift reaction (WGS), hydrocarbon
production by Fischer-Tropsch (F-T) synthesis and methanol synthesis (Balat,
2006; Van Steen and Claeys, 2008). In WGS reaction,
the chemical reaction of CO and H2O results in H2 and
CO2. This process can be used to enhance producer gas to syngas by
enriching the H2 component or production of H2 as an end
product. F-T synthesis is utilized for the production of hydrocarbon from syngas.
The production of methanol from syngas has been carried out since 1920s (Rowlands
et al., 2008).
Hydrogen as a biogas can also be produced in biological processes (Alshiyab
et al., 2008a). In the process of biohydrogen production biomass
is fermented in anaerobic conditions to produce hydrogen by microorganisms such
as Rhodobacter sphaeroides and Clostridium acetobutylicum (Alshiyab
et al., 2008b; Jaapar et al., 2009).
Liquefaction: Liquefaction of biomass is usually performed in the presence
of a solution of alkalis, glycerin, propanol, butanol or direct liquefaction
(Demirbas, 2004). In liquefaction water insoluble oil
with high viscosity is produced using solvents for decreasing gases such as
CO and H2. In this method catalyst is added to biomass (Rowlands
et al., 2008). Lignocellulosic biomass can also be converted to a
liquid like heavy fuel oils by reacting lignocellolosic substances with prepared
gases using suitable catalyst.
Aqueous liquefaction includes desegregation of wood ultrastructure and subsequent
partial depolymerization of structural components. Alkali liquefaction involves
deoxygenation through decarboxylation from ester produced by hydroxyl group
and formate ion derived from carbonate. Alkali salts (e.g., sodium carbonate
and potassium carbonate), can work as catalyst for hydrolysis of cellulose and
hemicelluloses to release smaller components.
The heavy oil produced from liquefaction has a high viscosity and tarry property
which may not be handled easily; therefore, organic solvents such as propanol,
butanol, aceton, methyl ethyl keton and ethyl acetate are added to reaction
system. These solvent except ethyl acetate, can be recovered during wood liquefaction.
Catalytic aqueous liquefaction produces higher oil yield (63%) than that in
non-catalytic (31%) (Demirbas, 2004).
High Pressure Liquefaction (HPL) process can be used to produce bio-oil containing
a complex blend of volatile organic acids, alcohols, aldehydes, ethers, esters,
ketones and non volatile compounds. These oils can be enhanced by using catalyst
to produce an organic distilled product which enriched with hydrocarbons and
valuable chemicals (Demirbas, 2004).
Pyrolysis: Heating biomass in anaerobic conditions results in the production of solid charcoal, liquid bio-oil and fuel gases. This process is called pyrolysis. Prolysis can be categorized into three groups depending on environment conditions, namely conventional pyrolysis, fast pyrolysis and flash pyrolysis.
Conventional pyrolysis involves heating wood biomass in a slow rate (0.1-1
Kelvin/second) for a range time of 45-550 sec. This process is divided to three
stages. In the first stage biomass degradation occurs at a temperature range
of 550-950 K which is known as pre-pyrolysis. The second stage is the main stage
of pyrolysis in which a high rate of temperature is used. In this stage the
pyrolysis products are produced. In the third stage char formed is heated at
a very slow rate and it results in carbon rich solid residues (Shafizadeh,
Fast pyrolysis consists of heating the biomass at a high temperature range
of 850-1250 K with a heating rate 10-200 K sec-1, short time of heating
(0.5-10 sec) and a particles size less than 1 mm. The fast pyrolysis is used
for the production of liquid or gaseous products. In this process biomass decomposes
to produce vapors, aerosol and some charcoal similar to chare. Cooling and condensation
of vapors and aerosol results in a dark brown liquid which its heating value
is half of that in conventional fuel oil. Depending on raw material used in
fast pyrolysis, 60-75% bio-oil, 15-25% solid char and 10-20% non condensed gaseous
products can be obtained (Shafizadeh, 1982).
Flash pyrolysis represents the pyrolysis process in which operating conditions
are a temperature range of 1050-1300 K, a heating rate higher than 1000 K sec-1,
residence time less than 0.5 sec and a particle size less than 0.2 mm. Flash
pyrolysis is used for the production of bio-oil which can be mixed with the
char to produce bioslurry. Bioslurry can be easily used in gasifier conditions
(pressure of 26 bars and temperature range of 927-1227 K) to produce syngas.
Biomass can be converted to crude oil in flash pyrolysis with efficiency up
to 70%. There is a type of fuel called bio-crude which can be used in engine
and turbine. Furthermore, bio-crude is capable of using as feedstock in refineries
(Demirbas, 2004; Mohan et al.,
Production of green diesel fuel using vegetable oils: As previously
discussed, oil derived from plants such as rapeseed, soybean, canola and palm
tree are widely used in the production of fatty acid methyl ester (biodiesel).
Energy biotechnology has made huge efforts to present biofuels which are compatible
with fossil fuels with an economical feasibility. Isoparaffin-rich diesel called
green diesel is a new products of biodiesel that is produced from renewable
plant oils comprising triglycerides and fatty acids processed under catalytic
saturation, hydrodeoxygenation, decarboxylation and hydroisomerization. Isoparaffin-rich
diesel is an aromatic and sulfur free diesel fuel with a very high cetan blending
value. Green diesel fuel is capable of adjusting to any environmental climate;
it used as neat or blended fuel. Any type of feedstock oil can be converted
to this product. Compared to petroleum based diesel fuel and fatty acid methyl
ester (biodiesel), green diesel represents a high cetan value, good cold flow
property and high storage stability (Kalnes et al.,
2007). Green diesel also is satisfactorily compatible for blending with
existing diesel fuels derived fossil feedstocks.
Bio-oil production using pyrolysis process: Bio-oil is a fuel oil that
is produced in fast pyrolysis process by heating biomass feedstock in anaerobic
conditions. During this process the lignocellulosic substances are thermally
degraded to produce liquid bio-oil (60-70%), char (30-35%) and gas such as CO,
H2 and volatile hydrocarbons (13-25%) under operating conditions
of 2-5 mm particle size and heating time of 0.1-2 sec. The amount of bio-oil
produced and the chemical content of bio-oil is varied depending upon the feedstock
composition, operating conditions and bioreactor type. Physically bio-oil is
a dark brown, viscous, corrosive and acidic liquid with a specific smoky odor.
Bio-oil is usually used as fuel for boiler, gas turbine, diesel engines, furnaces
and stationary engines. Chemical content of bio-oil is generally includes chemicals
derived from lignocellulose compounds such as aliphatic alcohols/aldehydes,
furanoids, pyranoids, benzenoids, fatty acids and high molecular mass hydrocarbons.
These chemicals are mixed with water formed in pyrolysis to produce an emulsion.
Some value added chemicals can be obtained from biofuels which are used for
the production of natural resin, food flavors, wood preservatives, slow release
fertilizer and pharmaceuticals (Ates and Isikday, 2008;
Elliott, 2007; Ozbay et al.,
2006; Rout et al., 2009; Scott
et al., 1993).
Production of FT oil (green motor fuel): Syngas (CO and H2
mixture) obtained in gasification of biomass can be processed for the production
of liquid fuels and hydrocarbonic compounds using transition metal catalyst.
Theses process refers as Fisher-Tropsch synthesis (FTS). The liquid fuels obtained
in FTS process are called FT oil or green motor fuel. The feedstocks used for
FTS are coal, biomass and natural gas. The gasification of lignocellulosic biomass
produces biosyngas which consists of carbon monoxide (28-36%), carbon dioxide
(22-32%), benzene/toluene/xylene (0.84-0.96%), ethan (0.16-0.22%), hydrigen
(21-30%), methane (8-11%) and tar (0.15-0.24%) (Balat, 2006).
The general chemical reaction of FTS is shown in Eq. 3:
where n is the average length of hydrocarbon chain and m is the number of hydrogen atom per carbon. As shown in Eq. 4, one mole CO reacts with two mole H2 in the presence of catalyst to form a hydrocarbon chain:
The CH2 is a unit for building longer chains. Most of
the products formed in FTS are aliphatic straight chain with a low amount of
branched hydrocarbon. Main hydrocarbon products in FTS are olefin and paraffin,
other hydrocarbons include methane (CH4), ethylene (C2H4),
ethane (C2H5), propane (C3), butane (C4),
gasoline (C5-C12), diesel fuel (C13-C22)
and wax (C23-C33). The proportion of products depends
on operating condition like the temperature, pressure and the type of catalyst
used. The range of temperature and pressure used for FTS are 202-352°C and
15-40 bar, respectively. Cobalt and iron are the main catalyst for FTS process
(Chew and Bhatia, 2008; Rout et
Production of bioethanol from renewable lignocellulosic feedstock: Biotechnological
process for bioethanol production is based on microbial and enzymatic reaction
for sugar synthesis from biomass feedstock like lignocellulosic substances and
starch. Sugar obtained is then converted to alcohol and other fuels as well
as chemicals by fermentation process. The pathway for conversion of lignocellulosic
biomass to ethanol requires different operating units including pretreatment,
enzyme production, hydrolysis, fermentation and ethanol recovery. Biotechnological
efforts have focused on reducing cost of conversion process of lignocellulosic
biomass to liquid fuels and chemicals by means of integrated xylose and glucose
fermentation, reduced energy needed for pretreatment, bioprocess of lignin to
valuable products and well-established process for recovery of alcohol (Lee
et al., 2007; Minteer, 2006). Main lignocellulosic
resources for the production of liquid fuel are agricultural residues, agro-industrial
wastes, forest residues and post harvest materials of food crops which generate
a reach source of carbohydrate (Huber and Corma, 2007).
As discussed previously, the lignocellulosic biomass is constructed from three main parts including cellulose, hemicellulose and lignin. Cellulose is a crystalline polymer of glucose. Hemicellulose is an amorphous polymer of xylose, mannose and arabinose while lignin is a large polyaromatic compounds. The conversion of linocellulosic biomass to alcohol consists of pretreatment of biomass, hydrolysis by acid or enzyme, fermentation process and distillation. The pre-treatment process is used for separation of xylose and lignin from crystalin cellulose.
One of the important pre-treatment methods is steam explosion process. In this
method lignocellulosic biomass is exposed to a high pressure of saturated stem
inside a vessel for a short residence time of 20 sec to 20 min at a temperature
range of 473-543 K and pressure 14-16 bar. The pressure in vessel is then quickly
dropped to atmospheric pressure to make explosion. Steam explosion causes disintegration
of hemicellulose and lignin in biomass structure which is subsequently converted
to low molecular weight components. A large part of water soluble components
is removed by water extraction. At the same time steam explosion reduce crystallinity
of cellulose to lessen the polymerization of cellulose and hemicelluloses-lignin
compound which in turn increases the accessibility area for enzyme reaction.
In pre-treatment process low molecular weight fraction of hemicelluloses form
xylose and solid cellulose is converted to glucose through enzymatic process.
Glucose and xylose are processed in fermentation process to produce ethanol.
Ethanol can be mixed with gasoline for the production of an oxygenated fuel
with the lower hydrocarbon and green house emission (Minteer,
There are a number of important factors affecting chemical conversion of biomass
to biofuel. These factors include the ratio of surface to volume, the concentration
of acid used, temperature and time. Among these factors, the surface to volume
ratio is of great importance because of its determinant effect on the yield
of glucose. The smaller particle size, therefore, has a better hydrolysis process.
With consideration of the ratio of liquid to solid, a higher ratio causes chemical
reaction rate more quickly (Jensen et al., 2008).
Solvent extraction process: Extraction is defined as the process of
separation of a desired substance from the raw feedstock by dissolving the desired
materials into a suitable solvent and subsequent recovery of substance from
solvent. Solvent extraction needs to different operating parts including the
extraction of oil from oil seeds, evaporation of solvent, distillation of the
mixture of oil and solvent and heating of meal residue. Different types of solvent
are used in separation process such as hexane (for extraction of seed oil),
dichloromethane, acetone, ethanol and isopropanol. The extracted lignocellulos
biomass can be used in hydrolysis and fermentation process for the production
of biofuels (Stevens and Verhe, 2004).
Supercritical fluid processing: Supercritical fluid refers to a substance
in conditions which its temperature and pressure are above the critical point
of these variables in vapor liquid status. At supercritical conditions a substance
is not liquid or gas and reduced pressure at constant temperature doesnt
cause it to boil as it cannot be condensed by cooling at constant pressure (Saka
et al., 2006). Supercritical fluid process can be used for the extraction
of aromatic woods (cedar wood, sadal wood, pine wood) to separate extractive
Supercritical fluid processing can be used as a substitute for acid hydrolysis
or enzymatic hydrolysis since acid recovery in acid hydrolysis is expensive
and is a polluting methods and enzymatic hydrolysis needs to the pretreatment
of biomass. In supercritical conditions (temperature 300-644 K and pressure
200-250 bar) acid and base components (H+ and OH¯) of water
are released and dissolved in biomass. The dissolved acidic and basic components
of water break the bonds of cellulose and hemicelluloses to simple molecules
of sugar i.e. glucose, xylose and oligosaccharides (Sasaki
et al., 1998). Supercritical fluid gasification technology has been
used for the hydrolyzing cellulose to glucose in residence time 10-20 sec and
above 45 sec pyrolysis start. When temperature increases to 873 K, the complete
disintegration of biomass structure occurs by transfer of oxygen from water
to carbon atoms of the biomass. The hydrogen atom of water forms hydrogen. The
general reaction for this process is shown in Eq. 5:
where C6H12O6 represents the molecule of biomass
(Loppinet-Serani et al., 2008).
The concerns to the depletion of fossil fuels sources followed by global warming due to increasing CO2 emission (green house gas) as well as soared price of petroleum and subsequent rapid drop over past decades has kindled worldwide interest for the development of renewable energy resources as an alternative for unsustainable fossil fuels resources. Global efforts have employed biomass as potentially sustainable fuel feedstocks for the generation of energy with reducing green house gas and dependency to fossil resources. In this regard, the first generation of biofuel source provided a reliable substitute for fossil fuel sources. However, the controversy of food demand versus biofuel consumption caused that energy biotechnology introduces the second generation of biofuel sources which are mainly plant biomass feedstocks. The conversion of lignocellulosic biomass to liquid fuels is a costly process which demands more scientific studies for finding biotechnological processing of plant biomass to form liquid biofuels with a cost-effective green process.
The authors wish to express their gratitude to Science Alert for providing the opportunity of publishing this review article.