C H A P T E R
9
Pyrolysis of Biomass
*
Vaibhav Dhyani*, Thallada Bhaskar*,†
Thermo-Catalytic Processes Area, Bio-Fuels Division, CSIR-Indian Institute of Petroleum,
Dehradun, India †Academy of Scientific and Innovative Research, New Delhi, India
9.1 INTRODUCTION
Because of population explosion and rapid urbanization, there has been a steady increase
in the demand for energy and petroleum-based commodity. Growth in energy consumption
is a parameter associated with the development of any nation and living standard of its
citizens. Currently, the energy demand is majorly fulfilled by conventional resources such
as coal, petroleum, and natural gas. However, the insufficiency of conventional resources
and environmental threats such as the greenhouse effect and global warming associated
with their usage are the severe demerits [1]. An emergent political commitment to address
this detrimental climate change came in the form of United Nations Convention, COP21,
which has been ratified by nearly 200 countries including India. Thus, there is an indispensable global requirement for clean and renewable sources of energy. Biomass, which is an
agglomeration of organic polymers is a plentiful and renewable source of organic carbon
which can be utilized for the production of transport fuel, petrochemical feedstocks, and
valuable chemicals, via the biochemical or thermochemical routes. Although biofuels have
a low energy content, they are considered environment friendly for having negligible
content of nitrogen, sulfur and ash in comparison to crude oil; thus their combustion causes
lesser emissions of SO2, NOx, and soot [2]. Additionally, the CO2 produced during the
combustion of biofuels is used up by plant life for photosynthesis, consequently balancing
the CO2 cycle [3].
While the biochemical processing of biomass uses enzymes and microorganisms to convert
biomass into lower molecular weight compounds, the thermochemical processing uses
heat and catalysts to do the same. The biochemical route for conversion of biomass is often
contemplated as a “mature” process with limited room for expansion [4]. In comparison to the
Biofuels: Alternative Feedstocks and Conversion Processes for
the Production of Liquid and Gaseous biofuels
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9. PYROLYSIS OF BIOMASS
biochemical processes, the thermochemical processes are fast and can complete within a few
seconds to hours depending on the process severity; on the other hand the former requires a
long duration of time (in the range of days) to complete. Another significant difference
between the two pathways comes from the fact that lignin is an unusable component of
the feedstock in the biochemical route and is usually produced as the undigested
by-product from the process. Fermentation of the C5 sugars is also difficult in these process.
But the thermochemical route is capable of utilizing the complete feedstock and is flexible
to different feedstock types [5].
Among the various thermochemical methods of conversion, pyrolysis has shown the
highest potential for commercialization. The word “pyrolysis” is derived from the Greek
words pyr “πύρ-fire” and lysis “λύσις-breakdown/separation,” stressing the breakdown of
the matter by heating. In a more coherent form, the term pyrolysis stands for thermal decomposition of an organic molecule in the absence of air. Biomass, when subjected to such operation yield an assortment of solid, liquid, and gaseous products. The chief benefit of pyrolysis
is that the liquid product (bio-oil) thus obtained can be not only be stored and transported
easily but can also be used as a fuel substitute. Although the research in the field of pyrolysis
has emerged strongly in the past few decades, the concept is prehistoric. The practice of conversion of wood into charcoal and production of wood-vinegar using pyrolysis has been in
existence since centuries. Pyrolysis is a flexible process in which the operating parameters can
be tuned for the desired combination of bio-oil, biochar, and noncondensable gases. It can be
used to selectively cleave the bonds of biomass constituents to produce high-value chemicals
which are conventionally produced from crude oil after several steps of functionalization.
The processes such as catalyst assisted pyrolysis and integrated hydropyrolysis and
hydroconversion can provide directly usable transportation fuels. Also, the highly functional
biomasses can be exploited for the production of high-value compounds of nutraceutical
significance. Pyrolysis technology may offer a “greener” solution to produce energy and
chemicals.
9.2 COMPONENTS OF LIGNOCELLULOSIC BIOMASS
Biomass refers to any organic matter originating from plants and animals. All kinds of biomass fundamentally contain energy derived from the sun: Plants absorb the energy from the
sun through photosynthesis and convert carbon dioxide and water into nutrients (carbohydrates); the energy of the plants is further taken up in the food chain by animals. Chemically,
biomass is a renewable source of hydrocarbons. Some biomass types also carry substantial
quantities of inorganic species. The lignocellulosic biomass refers to the dry plant biomass,
which is primarily made up of three constituent units: cellulose, hemicellulose, and lignin.
The content of these components in biomass varies depending on the biomass type. The abundance of lignocellulosic biomass has made it an attractive feedstock for pyrolysis. However,
other than the lignocellulosic feedstock, much research on pyrolysis has also been done on
algal biomass, poultry litter, and sewage sludge. Seaweed refers to a large number of species
of macroscopic, multicellular, and marine algae. They have been investigated as a potential
source of bioethanol since long. Algae remains exempt from the disadvantages associated
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9.2 COMPONENTS OF LIGNOCELLULOSIC BIOMASS
219
FIG. 9.1 The lignocellulosic composition of different biomass sources. (Adapted from V. Dhyani, T. Bhaskar,
A comprehensive review on the pyrolysis of lignocellulosic biomass, Renew. Energy 129 (2018) 695–716, doi:10.1016/j.
renene.2017.04.035.)
FIG. 9.2 Repeating D-glucose unit of cellulose.
with terrestrial biomass resources, which are said to be responsible for higher food prices and
which also impact water sources, biodiversity, and rainforests. Chief sources of lignocellulosic biomass include materials such as wood wastes, agricultural and industrial residues,
municipal solid waste, and dedicated energy crops having a high photosynthetic efficiency.
Fig. 9.1 presents a ternary plot of cellulose, hemicellulose, and lignin in different biomass
types. It can be interpreted from the plot that while leaves and grasses are rich in hemicellulose and wooden biomass is rich in cellulose; the shells are mostly rich in lignin. Knowledge of
lignocellulosic composition in biomass can be helpful in controlling the product chemistry.
The chemical behaviors of the components of the lignocellulosic biomass are described as
follows:
9.2.1 Cellulose
Cellulose is an essential structural component of the primary cell wall of green plants,
many forms of algae, and the oomycetes. It is the most abundant organic polymer on
earth and can be identified as a linear chain polysaccharide of reiterating D-glucose
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9. PYROLYSIS OF BIOMASS
FIG. 9.3 Cellulose fibers in the cell wall.
(also known as pyranose) units connected by acetyl bonds (β(1 ! 4) glycoside linkage)
(Fig. 9.2). Because of its long and linear molecules, cellulose does not dissolve readily in water.
The cellulose chains (20–300) are grouped to form cellulose fibers, which are linked together
by hydrogen and van der Waals bonds forming microfibrils. These long microfibrils are arranged as a crisscross mesh in the cell wall, giving it strength and shape (Fig. 9.3). Hemicelluloses and lignin cover the cellulose microfibrils. Interaction of the three hydroxyl groups
present in the pyranose rings forms intra- and intermolecular hydrogen bonds. These hydrogen bonds provide a crystalline structure to the cellulose which leads to its mechanical
strength and chemical stability. The crystalline structure of cellulose provides strength and
toughness to a plant’s leaves, roots, and stems. Fermentable D-glucose can be obtained from
cellulose using either acids or enzymes for breaking the β(1 ! 4) glycoside linkage [6–8].
9.2.2 Hemicellulose
Hemicelluloses are heteropolysaccharides of different monomers such as arabinose, galactose, glucose, glucuronic acid, mannose, and xylose. These monomers are present in the form of
polysaccharides such as xylan, glucuronoxylan, arabinoxylan, glucomannan, and xyloglucan.
The composition of the hemicellulose varies considerably depending on the plant species. Angiosperm hemicelluloses are mostly composed of xylans, whereas gymnosperm hemicelluloses contain mostly glucomannans [9]. The structures of different monomers present in
hemicellulose are shown in Fig. 9.4. One macromolecule of hemicellulose may contain
50–200 monomer units, which is considerably lesser than the degree of polymerization of
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9.2 COMPONENTS OF LIGNOCELLULOSIC BIOMASS
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FIG. 9.4 Structures of different monomers present in hemicellulose.
cellulose. Hemicelluloses also rank second to cellulose in abundance in the lignocellulosic
biomass, making up one-fourth to one-third of the plant matter. In comparison to cellulose,
hemicellulose is amorphous and is readily hydrolyzed by dilute acids, bases, and hemicellulose enzymes [6].
9.2.3 Lignin
Lignin may constitute as much as 20%–35% of the organic matrix of wood and other
vascular plants [10]. It is a three-dimensional highly branched polymer, made up of
phenylpropane units attached by alkyl-aryl ether bonds (Fig. 9.5). The phenylpropane monomers can be classified into p-hydroxyphenyl, guaiacyl, and syringyl units based on the number of methoxyl groups (Fig. 9.6). These structural elements are linked to each other by
ether, carbon-carbon, and ester bonds. Ether bonds are predominantly present as linkages
between the phenylpropane side chains and a benzene ring (α-O-4, β-O-4, γ-O-4), between
benzene rings (4-O-5), and between phenylpropane side chains (α-O-β0 , α-O-γ0 ). They may
account for 60%–70% of the total linkages. Carbon-carbon bonds primarily consist of linkage
between 5-5, β-1, β-5, β-6, α-6, α-β0 , etc., and account for 30%–40% of the total linkages.
Ester bonds are very low in numbers and primarily exit in herbaceous plants [10]. Lignin
is relatively hydrophobic and aromatic. It is entangled in the empty spaces between cellulose
and hemicellulose and provides structural rigidity to the lignocellulose biomass [11].
Softwood lignin is principally made from coniferyl alcohol (>95%), with the remained
consisting of p-coumaryl alcohol type units, and trace amount amounts of sinapyl alcoholderived units. Hardwood lignins, also termed as guaiacyl-syringyl lignins are composed
of coniferyl alcohol and sinapyl alcohol-derived units in varying ratios. In these lignins,
the methoxyl content per phenylpropanoid unit is in the range of 1.2–1.5. Grass lignins also
fall into the category of guaiacyl-syringyl lignins; however, unlike hardwood lignins, they
also contain p-coumaric acid and ferulic acid residues attached to the core lignin through ester
linkages [12, 13].
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9. PYROLYSIS OF BIOMASS
FIG. 9.5 Structure of lignin.
Lignin content in the biomass usually varies from 24% to 33% in softwoods, from 19% to
28% in the temperate-zone hardwoods, from 26% to 35% in tropical hardwoods, and from
11% to 27% in the nonwoody fiber sources. In compression wood, the average lignin content
can be as high as 35%–40%, but tension wood, the reaction wood of the angiosperms, contains
only 15%–20% lignin [13].
9.2.4 Ash
A small amount of inorganic minerals are also present in the biomass in the form of compounds of calcium, chlorine, magnesium, phosphorus, potassium, silicon, sodium, sulfur, etc.
Biomass can also contain traces of aluminum, cobalt, copper, iron, manganese, molybdenum,
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9.3 PYROLYSIS PRODUCTS
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FIG. 9.6 Phenylpropane monomers present in lignin.
nickel, titanium, vanadium, and zinc [10]. Although the inorganic content varies in the
biomass, it is observed that agricultural wastes have high inorganic minerals in comparison
to the wood-based biomass [14–16].
9.2.5 Extractives
Other than hemicellulose, cellulose, lignin, and ash, the lignocellulosic biomass also
comprises a significant amount of extractives. Extractives are the natural products
peripheral to a lignocellulose cell wall, which can be readily removed using inert solvents such as acetone, benzene-alcohol solution, cold water, and ether. These compounds
are formed as a result of the plant’s metabolic processes and can be categorized as
primary and secondary metabolites. The primary metabolites are the intermediates in
the intermediary metabolic processes of the plant and include amino acids, carboxylic
acids, simple fats, simple sugar, etc. The secondary metabolites are more complex than
the primary and have a restricted taxonomic distribution. They form the base for chemotaxonomy as they characterize the enzyme systems producing them. Some common
examples of secondary extractives include the alkaloids, complex flavonoids, monoterpenes, sesquiterpenes, acetogenins, phenol glycosides, coumarins, cyanogenic glycosides, and other complex phenolics [17]. Although the extractive content in biomass
has been reported as <10%, being the volatile fractions of the biomass they are known
to increase the heating value of biomass, and enhance the decomposition of lignin to
form phenolic homologues without obvious thermal difference between the original
biomass and the extracted residues [18–20].
9.3 PYROLYSIS PRODUCTS
The products obtained from pyrolysis can be classified into condensable vapors,
noncondensable (permanent) gases, and solid biochar. The condensable vapors are those
products of pyrolysis which can be easily converted into liquid form or bio-oil at room
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9. PYROLYSIS OF BIOMASS
FIG. 9.7
Yield of liquid, gaseous, and solid products for individual biomass components. (Based on S. Wang, X.
Guo, K. Wang, Z. Luo, Influence of the interaction of components on the pyrolysis behavior of biomass, J. Anal. Appl. Pyrolysis
91 (2011) 183–189.)
temperature. Bio-oil is addressed in detail in the later sections of the chapter. The yield of
these liquid, gaseous, and solid depending on the lignocellulosic composition of the biomass
(Fig. 9.7).
The main components of the noncondensable gases (NCG) are permanent gases such
as CO, CO2, and CH4 [21]. The chemical composition of NCG highly depends on the
differences in the chemical structure of biomass being pyrolyzed. Hemicellulose pyrolysis
produces a high yield of CO2 because of the presence of high carboxyl content. On the other
hand, cellulose yields more CO because of the presence of the carbonyl groups. Since lignin
has an aromatic structure and methoxyl, functional groups, its pyrolysis produces more H 2
and CH4 [22]. The NCG can be either combusted for heat or can be utilized to sustain
fluidization. Recycling of NCG has been demonstrated in the work of Mante et al. [23].
They reported an increase in liquid yield and HHV of bio-oil with a simultaneous reduction in the yield of char when recycling of NCG was performed. Increase in the aromatic
fractions and decrease in the carboxylic, methoxy, and sugar fractions was also reported in
their work.
Biochar comprises varying fractions of unconverted solids, inorganic materials, and carbonaceous residues from the pyrolysis operation, depending on the severity of the pyrolysis
process. Being rich in carbon, biochar has higher calorific value in comparison with the feedstock biomass. Similar to NCG, biochar can also be combusted to provide heat of operation
[24]. Addition of biochar to soil is also a common practice which can aid in increasing its
fertility and water holding capacity [6, 25]. Because of their high specific surface area and presence of large number of charged inorganic species biochar exhibits excellent chemisorption.
Thus, the removal of metal ions from wastewater by their adsorption on biochar is another
promising application [26].
Other than the lignocellulosic composition of the biomass, pyrolysis severity is another parameter that affects the product distribution. Evans and Milne [27] performed
a thorough study of different pyrolysis pathways depending on the reaction severity
(Fig. 9.8).
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9.4 TYPES OF PYROLYSIS
FIG. 9.8 Pyrolysis pathways depending on the reaction severity (Adapted from R.J. Evans, T.A. Milne, Molecular
characterization of pyrolysis of biomass. 1. Fundamentals, Energy Fuel 1 (1987) 123–138, doi:0887-0624/87/2501-0123.)
9.4 TYPES OF PYROLYSIS
Based on the operating condition, pyrolysis can be broadly categorized into three
types: slow, intermediate, and fast. These types differ in terms of process temperature, solid
residence time, biomass particle size, and heating rate (Table 9.1).
The differences in these process parameters lead to different product distribution in each
case. Different pyrolysis types are further described as follows:
9.4.1 Slow Pyrolysis
Slow pyrolysis is a batch operation, performed at low temperatures, and slow heating rates.
The solid residence time is usually long and can range from hours to days, depending on the
product desired. Although slow pyrolysis has the lowest liquid yield, it has the advantage of
TABLE 9.1 Characteristic Operating Conditions and Product Distribution for Different Pyrolysis Types [28, 29]
Product Yield (%)
Category
Temperature (°C)
Residence Time
Particle
Size (mm)
Heating
Rate (°C/s)
Liquid
Gas
Solid
Slow
400
Hours-days
5–50
0.1–1
25–30
25–35
30–40
Intermediate
500
5–30 s
1–50
1–10
40–50
25
25–30
Fast
500
1–2 s
<1
10–200
60–75
13–20
12–20
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9. PYROLYSIS OF BIOMASS
being more tolerant of moisture in the feedstock [30]. Biomass valorization techniques such as
carbonization and torrefaction come under the heading of slow pyrolysis. While carbonization
is used to produce coal [31], torrefaction partially removes the organic volatiles yielding
“torrefied biomass” which is light, hydrophobic, high in energy density, and can be stored
for a long time [32, 33]. Such biomass also needs less energy to crush, grind, or pulverize [33, 34].
9.4.2 Intermediate Pyrolysis
Intermediate pyrolysis is performed in the range of 300–500°C. The bio-oil obtained from
intermediate pyrolysis has a lower tar yield and viscosity in comparison to fast pyrolysis.
Although the lower yield of condensables (up to 55%) is obtained in this process, it provides
an advantage of being more flexible. It offers higher control of the decomposition reactions
(thus the scope for process optimization) and accepts larger feed sizes (coarse, shredded,
chopped, or ground) [14, 35].
9.4.3 Fast Pyrolysis
Fast pyrolysis of biomass is conducted when bio-oil is the desired product. In this operation, the biomass decomposes rapidly to generate vapors which are quickly condensed to
avoid any secondary decomposition. Fast pyrolysis of wood can give liquid yields up to
75% [35, 36]. The essential conditions for fast pyrolysis conditions are:
• The high rate of heat transfer to the biomass particles. Since biomass has poor thermal
conductivity, the small particle size is a necessary requisite to achieve rapid heating of the
biomass.
• Control over pyrolysis reaction conditions.
• Small vapor residence times (<2 s) to minimize secondary cracking of the pyrolysis
products. This can be achieved by rapid removal of pyrolysis products from the reaction
environment, followed by quick condensation to liquid products.
9.5 MECHANISM OF PYROLYSIS
The mechanism of pyrolysis of biomass is more complicated than other solid state reactions
because of the complex behavior of the biomass. Different components of the biomass
decompose at different temperature ranges, following different reaction mechanism, and
decomposition rates. A large number of reaction types have been identified as taking place
in series and parallel, such as dehydration, depolymerization, isomerization, aromatization,
decarboxylation, and charring [37].
9.5.1 Mechanism of Cellulose Pyrolysis
The thermal decomposition of cellulose takes place by cleavage of β(1 !4) glycoside
linkage sharply in the temperature range of 315–400°C. As per the decomposition
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FIG. 9.9 Cellulose decomposition mechanism suggested by Lin et al. [38].
mechanism suggested by Lin et al. [38], cellulose initially depolymerizes to oligosaccharides, which continue to decompose until anhydro-monosaccharides are formed
(Fig. 9.9). Levoglucosan (LGA) formed as the first product of this breakdown sequence
can further undergo dehydration and isomerization reactions to form other anhydromonosaccharides such as 1,4:3,6-dianhydro-α-D-glucopyranose (DGP), levoglucosenone
(LGO), and 1,6-anhydro-β-D-glucofuranose (AGF). These anhydrosugars may further
react to form furans by dehydration reactions, or hydroxyacetone, glycolaldehyde, or glyceraldehyde by fragmentations and retro-aldol condensation reactions. CO and CO2 are
formed as a result of decarbonylation and decarboxylation reactions, while polymerization
of pyrolysis products produces char.
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9. PYROLYSIS OF BIOMASS
9.5.2 Mechanism of Hemicellulose Pyrolysis
Irrespective of the variance in the polysaccharide constituents of hemicellulose, the breakdown of hemicellulose mainly occurs in the temperature range of 200–350°C. For the sake of
simplicity, most of the studies on hemicellulose pyrolysis, reported in the literature are based
on “xylan” as the model compound. Even with the structural similarities between cellulose and
hemicellulose, the mechanism of hemicellulose decomposition varies considerably from that of
cellulose. During the pyrolysis of cellulose, the breakdown of the glycosidic bond between
pyranose rings yields a glucosyl cation, which is subsequently stabilized by the formation of
1,6-anhydride. Further decomposition of anhydride leads to the formation of LGA. On the other
hand, xylose cation is unable to form any stable anhydride because of the absence of a sixth
carbon and substituted oxygen in the fourth position. Instead, it undergoes further glycosidic
bond cleavage and dehydration reactions to form dianhydro-xylopyranose. The main decomposition products of xylan are acetic acid, furfural, hydroxyacetone, CO2, CO, and H2O [39].
Acetic acid is formed at low temperatures as a result of dissociation of O-acetyl groups that
are attached to the main xylan chain [40]. Hexoses and pentoses in hemicellulose have different
pyrolytic behaviors. Di Blasi et al. (2010) reported that hydroxymethylfurfural is the main product in softwood hemicellulose (primarily composed of hexoses) pyrolysis, but is not produced
from hardwood hemicellulose (consisting primarily of pentoses). A similar trend in the production of hydroxymethylfurfural has also been reported by R€ais€anen et al. [41]. Patwardhan et al.
[42] postulated a decomposition scheme of xylan based on three competing pathways:
(a) depolymerization to sugars and anhydrosugars, (b) dehydration to furan and pyran ring
derivatives, and (c) furanose and pyranose ring breakage to give light oxygenated species.
Transglycosylation of xylosyl cation with other xylosyl anions into longer polymers and
subsequent dehydration was explained as the mechanism of char formation. Fig. 9.10 shows
the decomposition of Xylan as scheme proposed by Patwardhan et al. [42].
9.5.3 Mechanism of Lignin Pyrolysis
The pyrolysis mechanism of lignin is more complex compared to cellulose and hemicellulose. While the pyrolysis of cellulose demonstrates a sharp decomposition of glycoside bond
breakdown near 350°C, lignin does not show any sharp decomposition curve, and the overall
reaction sequence can be classified into primary decomposition reactions at 200–400°C, secondary reactions at temperatures higher than 400°C. Aromatic methoxy groups are stable
during the primary stage of pyrolysis but become highly reactive in the temperature range
of 400–450°C. Therefore, the aromatic compounds produced in the course of the primary pyrolysis stage are principally 4-substituted guaiacols from G-lignins and 4-substituted
syringols from S-lignins. Most of the side-chains are unsaturated alkyl groups along with
a smaller amount of saturated alkyls groups. The main volatile products from G-lignins in
primary stage include coniferyl alcohol, coniferyl aldehyde, isoeugenol, 4-vinyl guaiacol,
vanillin, acetovanillone, and dihydroconiferyl alcohol [43, 44].
When the pyrolysis temperature is raised from 400°C to 450°C, secondary pyrolysis reactions
occur, and guaiacols/syringols rapidly transform to catechols/pyrogallols and o-cresols/
xylenols along with phenols. At temperatures >700°C the formation of polycyclic aromatic
hydrocarbons (PAH) is increased. Compounds like phenols and o-cresols which are stable at
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9.5 MECHANISM OF PYROLYSIS
229
FIG. 9.10 Xylan pyrolysis mechanism proposed by Patwardhan et al. [42].
high temperatures are also observed along with PAHs. It has also been pointed out that at
400–450°C the methoxyl group-related reactions occur while in the temperature range of
550–600°C decomposition involves the gasification of catechols [43].
A detailed study on the pyrolysis of lignin has been reported by Patwardhan et al. [45].
They reported the formation of monomeric phenolic compounds with phenol, 2,6-dimethoxy
phenol, 2-methoxy-4-vinyl phenol, and 4-vinyl phenol, as the major primary products.
The liquid bio-oil produced by subsequent condensation was rich in dimeric and oligomeric
compounds. It was postulated that the oligomeric species were formed by re-oligomerization
of the monomeric products, formed through condensation. It was also observed that the yield
of alkylated phenols increased with temperature.
9.5.4 The Interaction Between the Biomass Components
The interactions between the components of the biomass, hemicellulose, cellulose, and
lignin are also significant, which makes the overall pyrolysis mechanism prediction based
on the decomposition characteristics of the individual components difficult. Several studies
have been reported in the literature to explicate the interaction of the biomass components.
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9. PYROLYSIS OF BIOMASS
Wang et al. [46] have reported that the interaction between cellulose and hemicellulose
fractions promote the production of 2,5-diethoxy-tetrahydrofuran, while simultaneously
hindering the production of altrose and LGA. It was further reported that while cellulose
increases the production of hemicellulose-derived acetic acid and 2-furfural, lignin inhibits
their formation. A decrease in the yield of LGA along with a simultaneous rise in the yield
of furans and other low molecular weight compounds was reported by Zhang et al. [47]
for the pyrolysis of the cellulose-lignin mixture. A similar decrease in LGA has also been
reported by Wu et al. [48] for cellulose-lignin co-pyrolysis.
9.6 BIO-OIL
Part of the pyrolysis vapors can be condensed to give a dark brown liquid product termed
as bio-oil. This liquid product mainly contains oxygenated hydrocarbons, moisture, and
fragments of biomass components which were able to escape the reactor environment [49].
The water content in the bio-oil is inherited from the moist raw material and also from the
secondary reactions taking place between the bio-oil components during the storage. Because
of this water, the crude bio-oil forms two phases: high-density aqueous phase, and lowdensity organic phase. Out of these two, the organic phase can be either blended with
petroleum-based fuel or can be upgraded to high calorific value fuel. The main components
of the aqueous phase are the water-soluble compounds such as acetic acid, hydroxyl acetone,
and phenol [50, 51]. This phase cannot be used as fuel; however, it can be catalytically reformed
to produce hydrogen [51].
9.6.1 Production
9.6.1.1 Feedstock Requirements
Drying of feedstock before its pyrolysis is essential as the moisture embedded in the
feed not only consumes process heat but also lowers the yield of organics in the bio-oil
subsequently produced. The moisture present in the biomass vaporizes during pyrolysis
and recondenses with the bio-oil product, thus lowing the calorific value of the liquid
product. Moisture content <7% has been recommended for reasonable performance [52].
Feedstock drying usually requires 50% additional energy than the theoretical minimum of
2442 kJ/kg of moisture evaporated [53].
9.6.1.2 Heat Transfer Requirements
For the efficient conversion of biomass, adequate heat transfer into the biomass is necessary. Since biomass has poor thermal conductivity, reduction of biomass particle size can enhance the heat transfer rates. The insulating char layer developed on the surface of biomass
with pyrolysis progression also adds to the heat transfer resistance. The size reduction of the
biomass particle can also decrease the incremental effect of char formation on the heat transfer
resistance. However, size reduction or comminution is an energy-intensive process that adds
to the feedstock preparation cost. Fast heating rates favor quick break down of the biomass
and yield more gases and produce less char. Bio-oil production is also high at rapid heating
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9.6 BIO-OIL
231
FIG. 9.11 Modes of heat transfer in the pyrolysis process.
rates due to the decrease in mass and heat transfer limitations and lesser time available for
secondary reactions.
Several heating methods are used in different pyrolysis reactors to ensure the efficient conversion of biomass into liquid fuel. These can be heat transfer via convection by hot gas, or
direct heat transfer by solid-solid conduction. Some of these heat transfer methods are shown
in Fig. 9.11.
9.6.1.3 Reactor Configurations
9.6.1.3.1 FLUIDIZED-BED REACTOR
Fluidized-bed reactors (FBR) are the most popular reactor configurations employed for
reactions involving solid reactants. In the FBR, a fluidization medium (gas or liquid) is passed
through the bed of solid reactants at high enough velocities to suspend the solid and
cause it to behave like a fluid. The FBR configuration is preferred because of the inherent advantage of high heating rate, uniform heat, mass transfer, and good control over the reaction
parameters as the engineering of FBR is well understood. The FBR is ideally suited for fast
pyrolysis operations and allow continuous productivity. A bubbling fluidized bed (BFB) is
a well-established approach where the gas at low velocities is used and fluidization of the
solids is relatively stationary. The BFB bed can be a blend of biomass particles with inert sand
or with powdered catalysts [54]. Careful selection of feed particle size is important for efficient pyrolysis operation. If large biomass particles are fed to the reactor, the char produced
as the pyrolysis product will be challenging to entrain out of the reactor; and if the fine particles are fed to the reactor, much of the biomass may get entrained out of the reactor without
undergoing full pyrolysis.
9.6.1.3.2 CIRCULATING FLUIDIZED-BED REACTOR
In 1978 Choi et al. [6] were granted a patent for a loop pyrolysis reaction system, which is
now known as circulating fluidized-bed (CFB) reactors. They are similar to FBR/BFB, but in
this configuration the solids are not contained within the reactor, and char leaves the reactor
with the vapor products. Higher fluidization gas velocity is required in CFB in comparison to
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9. PYROLYSIS OF BIOMASS
FBR/BFB. A CFB plant for the production of food additives and nutraceuticals has been
successfully commercialized by Ensyn [55].
9.6.1.3.3 ABLATIVE PLATE REACTOR
In ablative plate reactors, the thermal “erosion” of the biomass particles takes place by
pressing against the hot reactor wall. The rate of reaction is a function of reactor surface temperature, ablative pressure, and the relative velocity of the biomass on the heating surface.
Since particle reduction takes place within the reactor, the ablative plate reactors accept large
feedstock sizes and allow an excellent mechanical abrasion of char [6].
9.6.1.3.4 AUGER/SCREW REACTOR
A central rotating screw is the key component of the auger/screw reactors. The rotating
screw blends the feed mixture, moves the biomass inside the reactor, and controls the biomass
residence time. The heat of reaction can be provided through the tubular reactor wall. Auger
reactors are compact and can be made portable, allowing on-site biomass conversion [56].
9.6.1.3.5 ROTATING CONE REACTOR
In a rotating cone reactor, the biomass along with sand is fed near the bottom of a rotating
cone and is carried up the wall of the rotating cone in a spiral motion due to the centrifugal
force. The main advantage of this configuration is that carrier gas is required to transport the
vapors, thus reducing the cost of operation [6]. The University of Twente, Netherlands, developed this technology and it has been commercialized by BTG-BTL (Biomass Technology
Group-Biomass to Liquid, Netherlands) [57]. A rotating cone pyrolyzer of 50 ton/day capacity has been successfully employed in Malaysia for the conversion of empty fruit bunches
from palm oil trees. Another unit of 5 ton/h capacity, for converting wood residues into pyrolysis oil has been installed by Empyro in Hengelo, the Netherlands. In both of these units,
the un-condensed gas and biochar are burned to heat the sand, which is recycled back to the
pyrolysis reactor [58].
9.6.1.3.6 CYCLONE/VORTEX REACTOR
In a vortex pyrolysis reactor, the biomass particles are entrained in a hot inert gas (steam or
nitrogen) flow and then tangentially enter into the reactor tube. Inside the reactor, the biomass
particles are then forced to slide on the reactor wall at high velocity using high centrifugal
forces. The particles are melted on the hot reactor wall which is maintained at a high temperature and leave a liquid film of bio-oil. Unconverted particles are recycled with a special solid
recycle loop. Vapors generated on the reactor wall are quickly swept out by carrier gases. This
design can meet the requirements of fast pyrolysis and has been demonstrated for a bio-oil
yield of 65% [29]. Different pyrolysis reactor configurations are shown in Fig. 9.12.
9.6.1.4 Char Separation
Char is customarily removed from pyrolytic vapors by centrifugal gas-solid separation
equipment like cyclones. For char particle size below 2–3 μm, the separation efficiency of
the cyclone drops considerably, and fine char particles are inevitably carried over and
suspended in the bio-oil subsequently produced. The downstream separation via liquid
filtration is challenging because of the accelerated blockage of filters [59]. Char acts as a
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9.6 BIO-OIL
233
FIG. 9.12 Different pyrolysis reactor configurations.
catalyst and leads to secondary cracking of the pyrolytic products leading to a reduction in
bio-oil yield by up to 20%. Reduction in the temperature of maximum organic yield because of
the presence of char has also been reported [57]. Char retains nearly all of the alkali metals of
the biomass which affect the yield and composition of obtained bio-oil [60]. Nowakowski
et al. [61] reported the catalytic effect of potassium in favoring the char formation reactions
during biomass pyrolysis. Higher char yields have also been attributed to the presence of
phosphorus, indicating the different pyrolytic mechanisms during biomass decomposition
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9. PYROLYSIS OF BIOMASS
in the presence of inorganic components [62]. Agricultural wastes usually contain a higher
amount of ash (and alkali metals) in comparison to the wood-based biomass, and henceforth
char separation is of extreme significance for their bio-oils. Solid particles are furthermore
undesirable as they may block the injection nozzles during the combustion. Since suspended
char in bio-oil essentials comprises of alkali metals and compounds containing sulfur and
nitrogen, it increases the risk of catalyst poisoning during the upgradation process. Also,
the metal ions in char promote polymerization reaction leading to poor stability of the
bio-oil. Separation methods such as hot vapor filtration and microfiltration are often used
to remove the char particles to micron levels, but these methods also suffer from disadvantages such as filter clogging, catalytic decomposition by filter cake, and pressure drop across
the filter with cake buildup [63].
9.6.1.5 Liquid Recovery
Rapid quenching of pyrolysis vapors generated in the reactor is of utmost importance in
the pyrolysis process to prevent the bio-oil components from further cracking into permanent
gases, or polymerize to char. Although rapid cooling leads to the higher yields of condensable
products, cooling at temperatures below 0°C may lead to the formation of agglomerated solid
lumps composed of micro char particles. Chen et al. [64] advocated the use of differential
condensation as useful to separate the water and other chemical constituents of the bio-oil.
9.6.2 Properties
9.6.2.1 Physical Properties
Different physical properties of bio-oil are tabulated in Table 9.2.
TABLE 9.2 Physical Properties of Bio-oil and Mineral Oils [65, 66]
Property
Typical Bio-Oil
Light Fuel Oil
Heavy Fuel Oil
0.01–0.1
0.01 (max)
0.08 (max)
Density (15°C) (kg/dm )
1.10–1.30
0.845 (max)
0.99–0.995
Flash point (°C)
40–110
60 (min)
65 (min)
LHV (MJ/kg)
13–18
42.6
40.6
Nitrogen (%)
<0.4
0.02
0.4
pH
2–3
–
–
Pour point (°C)
9 to 36
5 (min)
15 (max)
Sulfur (%)
<0.005
0.001 (max)
1.0 (max)
Suspended solids (%)
Below 0.5
–
–
Viscosity (40°C) (cSt)
15–35
2.0–4.5
180–420
Water (%)
20–30
0
0
Ash (%)
3
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9.6 BIO-OIL
235
9.6.2.1.1 MOISTURE
The moisture in the bio-oil is derived from the dampness of the feedstock and also from the
thermochemical reactions taking place during pyrolysis. Even when bone-dry biomass is
subjected to pyrolysis, a water content of 12%–15% can be predicted in the biomass. Moisture
not only lowers the ignition delay but also lowers the heating value, combustion rates, and
flame temperature of the bio-oil [67]. Inhomogeneity, layering, and phase separation in the
liquid because of the presence of high moisture content has been reported in the literature.
9.6.2.1.2 OXYGEN CONTENT
Presence of high oxygen content (35%–40%) in the bio-oil is what distinguished the properties of bio-oil from the conventional hydrocarbon fuel. This oxygen content is distributed
over 300 compounds which can be categorized as carboxylic acids, hydroxy aldehydes,
hydroxy ketones, sugars, and phenolics [68]. The existence of oxygen in bio-oil not only
reduces the energy density (by up to 50%) but also makes bio-oil immiscible with conventional fuels [69].
9.6.2.1.3 VISCOSITY
The high viscosity [35–1000 cP (at 40°C)] of the bio-oil is mainly contributed by the high
molecular weight pyrolytic lignin [9]. Length of the fatty acid chain and number of saturated
bonds make the bio-oil more viscous and thus undermine its flow and combustion properties
[70]. Viscosity of the bio-oil may also increase during the storage because of the reactivity of
the components [71].
9.6.2.1.4 STABILITY
Thermodynamically, bio-oil is a nonequilibrium product from biomass pyrolysis produced by rapid quenching of pyrolysis vapors. Because of the reactivity of the oxygenated
functional groups, the bio-oil has thermal instability and storage difficulties [72]. The oxygenated compounds present in the bio-oil tend to achieve equilibrium during storage by undergoing reactions which lead to an increase in the average molecular weight and consequently
the viscosity of the bio-oil with the storage time and temperature. Condensation reactions
such as esterification and etherification take place during the storage period, leading to the
formation of heavier compounds and unwanted moisture [73]. At high temperatures
(100°C) deterioration of the bio-oil by polymerization can take place, which results in the
formation of solid coke like products during distillation of the oil.
9.6.2.1.5 ACIDITY
The carboxylic acids, such as acetic acid and formic acid present in the bio-oil, increase its
acidity (pH 2–3) and make it corrosive for the handling equipment and storage vessels, primarily at high temperatures and high water content.
9.6.2.1.6 INORGANIC CONTENT
The inorganic content of the bio-oil is mainly composed of alkali metal compounds derived
from the parent biomass. These include the compounds of metals such as sodium, potassium,
and vanadium.
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9. PYROLYSIS OF BIOMASS
9.6.2.1.7 HEATING VALUE
The heating value of the bio-oil is about half the heating value of the petroleum fuels and is
affected by the oxygen content present in the bio-oil.
9.6.2.2 Chemical Composition
The oxygenated bio-oil compounds can be categorized into phenolics, esters, furans,
hydroxy aldehydes, carboxylic acids, hydroxy ketones, and sugars [74]. The phenolics and
aromatic compounds are derived from the lignin and can be present as oligomers (molecular
weight 900–2500). The sugars are primarily derived from cellulose and hemicellulose [75].
The chemical composition of the bio-oil varies depending on the source and process conditions, providing an opportunity to manipulate the properties of bio-oil [14, 76]. “Fractional
pyrolysis” of biomass has also been reported in literature for segregation of bio-oil
compounds [77].
9.6.3 Upgradation
Bio-oil has a high potential as a liquid fuel since it constitutes approximately 70% of the
initial energy of the feedstock biomass and contains less nitrogen and sulfur than fossil fuels
[78]. However, the excessively high content of water and oxygen hinder its direct application
as a high-quality fuel for transportation. Other deficiencies of the untreated bio-oil such as
low calorific value, high corrosiveness due to its low pH, high viscosity, and limited chemical
stability in comparison with conventional fuels also inhibit its use as fuel. Therefore, to use the
pyrolysis bio-oil as a liquid fuel in conventional engines, it is necessary to carry out additional
processing or upgradation. Bio-oil can be upgraded in several ways which can be categorized
as physical or chemical methods.
9.6.3.1 Physical Upgradation
As mentioned in the previous section, suspended solid particles and char in the bio-oil are
undesirable and undermine its use as fuel. The traditional method of removal of solids from
pyrolysis vapors is to employ a cyclone separator, where the solid char particles are separated
from the low-density vapors at the cost of the centrifugal force generated during the swirling
motion inside the cyclone. The amount of centrifugal force generated is proportional to
the mass of the char particle and square of the gas velocity, and inversely proportional to
the diameter of the cyclone. Thus the efficiency of the cyclone separation process decreases
for small particle size and large cyclone sizes which are needed during scale-up. Also, using
high gas velocities are associated with the problems of high-pressure drop and erosion of
the cyclone surface. “Hot vapor filtration (HVF)” is an ideal method for production of better
bio-oils by filtration of vapors obtained from pyrolysis, before its condensation. In comparison to the conventional practice of cyclone separation, ash removal via HVF is considerably
higher (up to 10 ppm) [79]. Major work in HVF has been carried out at NREL [80], VTT and
Aston University [81].
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9.6 BIO-OIL
9.6.3.2 Chemical Upgradation
The popular techniques for chemical upgradation of bio-oil are hydrodeoxygenation
(HDO), catalytic cracking, and steam reforming. In HDO, the catalytic reduction of bio-oil
with hydrogen gas is carried out at high pressures. During this reaction the oxygen present
in the bio-oil is removed as water, simultaneously increasing the degree of saturation in
the compounds present in the bio-oil. Removal of oxygen from bio-oil not only increases
its energy content but also enhances its stability. Table 9.3 shows the comparison of bio-oil
properties before and after HDO upgradation.
Hydrogenation on aromatics present in the bio-oil is avoided as it leads to a lower octane
number and higher hydrogen consumption. Although most studies conducted on HDO are
based on industrial hydrotreating catalysts such as sulfided CoMo and NiMo-based catalysts
[78], nonsulfided catalysts such as zeolite supported platinum [82], vanadium nitride [83],
and ruthenium [84] have also been used.
Similar to HDO, catalytic cracking of bio-oil compounds can be used to increase the quality
of bio-oil via integrated in situ process or ex situ downstream processing. Although both
routes decrease the yield of condensable bio-oil components, they considerably enhance
the product quality by reducing the oxygen content and moisture present in the bio-oil.
Product selectivity is an additional feature of upgradation by catalytic cracking [85]. Use
of several mesoporous silica-based catalysts and metal-doped catalysts has been reported
in the literature [85–92].
Steam reforming refers to the reaction of hydrocarbons with steam at high temperatures,
producing the syngas. The process can be used for upgradation of oxygenated hydrocarbons
present in the bio-oil, as per the following empirical reaction:
Cm Hn Ok + ðm kÞH2 O ! mCO + ðm k + n=2ÞH2
The primary steam reforming reaction is also accompanied by water gas shift reaction,
providing the complete empirical reaction as
Cm Hn Ok + ð2m kÞH2 O ! mCO2 + ð2m k + n=2ÞH2
TABLE 9.3
Properties of Bio-Oil Before and After HDO Upgradation
Untreated Bio-Oil
Bio-Oil After HDO Upgradation
Carbon (wt%)
43.5
85.3–89.2
Hydrogen (wt%)
7.3
10.5–14.1
Oxygen (wt%)
49.2
0.0–0.7
Density (g/mL)
24.8
0.796–0.926
Moisture (wt%)
24.8
0.001–0.008
HHV (MJ/kg)
22.6
42.3–45.3
Viscosity (cP)
59 (40°C)
1.0–4.6 (23°C)
Adapted from D.C. Elliott, G.F. Schiefelbein, Liquid hydrocarbon fuels from biomass, in: American Chemical Society,
Division of Fuel Chemistry Annual Meeting Preprints, American Chemical Society, 1989, pp. 1160–1166.
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238
9. PYROLYSIS OF BIOMASS
As per the Le Ch^
ateliers principle, high temperatures and low pressures favor the
reforming reaction. It has been reported that high temperatures promote CO/H2 ratio in
the product stream while lower steam to carbon ration in the feed has the reverse effect
[93, 94]. The attractive feature of stream reforming is that H2 is a clean fuel and can be utilized
in a wide range of processes in the chemical industry.
9.6.4 Standards, Norms, and Legislation
Because of the differences in the chemical composition of the biomass, the properties of the
bio-oils vary considerably. Not only biomass properties but processing and operating conditions also affect the bio-oil quality. Hence, it is necessary to implement certain norms and
standards for the successful introduction and sell of bio-oil and bio-oil blends in the market.
Standardization is even more important for bio-oil as its properties are entirely dissimilar
from conventional liquid fuels running the world economy. Although work on standardization of bio-oil properties has been in progress since 1985, the major challenge in this work is
the fact that standard testing methods for mineral oils cannot be directly implemented for the
bio-oil without any suitable validation. The ASTM developed the very first standards for fast
pyrolysis oil. To date, two different grades have been defined. The difference between Grades
D and G is only the maximum allowable ash and solids content [95]. These grades are
described in Table 9.4.
In Europe, the European Committee for Standardization (CEN) received a mandate from
the European Commission in 2013 to develop standards for the use of fast pyrolysis oil in
different applications [95]. In early 2014, a dedicated working group (WG41) was established
under TC19. In the mandate different qualities of fast pyrolysis bio-oils (FPBO) are foreseen
for five different applications, being.
1. FPBO replacing heavy fuel (HFO).
2. FPBO replacing light fuel oil (LFO).
3. FPBO to be used as a fuel in stationary engines and turbines.
TABLE 9.4 ASTM D7544 for Fast Pyrolysis Bio-Oil [95]
Property
Grade G
Grade D
Ash (wt%)
>0.25
>0.15
Density at 20°C (kg/L)
1.1–1.3
1.1–1.3
Flash point (°C)
>45
>45
Higher heating value (HHV) (MJ/kg)
<15
<15
Kinematic viscosity (40°C) (cSt)
>125
>125
Pour point (°C)
<9
<9
S sulfur (wt%)
>0.05
>0.05
Solids (wt%)
>2.5
>0.25
Water content (wt%)
>30
>30
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9.7 ECONOMIC ANALYSIS OF THE PYROLYSIS PROCESS
239
4. FPBO as feedstock for gasification to syngas.
5. FPBO suitable for coprocessing with mineral oil in a conventional refinery.
A European Standard will be developed for (1) and (2). For the third application, a
Technical Specification will be prepared, whereas the work on the fourth and fifth application
has been postponed. A Standard is mandatory for each EU member state to instrument in the
national law; this is not the case for a Technical Specification.
Furthermore, to produce or import Fast Pyrolysis Liquid in the European Union a so-called
REACH registration is required. REACH means Registration, Evaluation, Authorisation &
restriction of Chemicals [96].
9.6.5 Applications
The bio-oil can be used for heating, without considerably altering the machine designs [6].
Other than their application as fuel blends, they can also be used for the extraction of valuable
chemicals such as phenols, furans, and hydroxyl aldehydes. Phenolic compounds present in
the bio-oil can be extracted using separation technologies such as supercritical fluid extraction
[97]. These compounds are a valuable commodity for food, paint, and pharmaceutical industries [98, 99]. Also, the extraction of these compounds from bio-oil can reduce the dependency
on petroleum based productions. Companies like Red Arrow Products Company, USA and
Chemviron, Germany have already patented and commercialized food flavoring products
obtained from bio-oil [100]. More devotion for developing dependable, low-cost separation
and refining systems can aid in the commercialization of valuable chemicals present in the
bio-oil.
9.7 ECONOMIC ANALYSIS OF THE PYROLYSIS PROCESS
Ji et al. [101] studied the determined the production cost of a pyrolysis unit working with
the waste agricultural biomass feeding rate of 4000 kg/h, as 262.43 USD/ton, which was less
than the expected selling cost of 315.28 USD/ton. It was reported that the production cost is
highly sensitive to the liquid fuel yield and feedstock cost.
Jaroenkhasemmeesuk and Tippayawong [102] reported the production cost of the bio-oil
as a function of plant capacity. In their work bio-oil was produced at the cost of 0.91–1.06
USD/L for the feed capacity of 20–30 kg/day. However, when the capacity of the plant
was increased to 100 kg/h, production cost dropped to 0.45–0.61 USD/L.
A study on bio-oil production along with its catalytic upgradation by performed by Islam
and Ani [103]. Their study was based on FBR based catalytic treatment of bio-oil using the biooil produced from rice-husk. The data which was produced in a plant with a capacity of
0.3 kg/h working on a feed cost of 20 USD/ton was scaled up for 100 and 1000 kg/h for further analysis. The scale-up study showed the bio-oil costs of 0.38 and 0.18 USD/kg for the 100
and 1000 kg/h scales, respectively. Cost of feedstock and equipment and labor costs were
acknowledged as the major variables determining the cost of bio-oil production in this study.
Other noteworthy studies on the economic analysis of bio-oil production have been carried
out by Mullaney et al. [104], Cottam and Bridgwater [105], Gregoire and Bain [106], and
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9. PYROLYSIS OF BIOMASS
Solantausta et al. [107]. Studies based on online optimization of the pyrolysis process,
accounting for diverse feedstocks and including more process parameters can help in resolving the economic challenges in a pyrolysis plant. Cost cutting by optimization of process
heat and simultaneous enhancement of the quality of the end product via upgradation is
another field that needs thorough research. Study of bio-oil applications in power generation,
combustion in vehicles, and associated environmental issues can aid in the long-term
development of the pyrolysis processes.
9.8 CONCLUSIONS AND PERSPECTIVES
Pyrolysis is a promising technology that can be used for the conversion of biomass into fuel
and valuable hydrocarbons that can be used as industrial chemicals. To exploit the process on
the substantial scale, it is necessary to study the biomass composition, decomposition behavior, decomposition mechanism, and product chemistry. Different reactor types have been
configured to exploit the pyrolysis process in large scale. Efficient char separation and liquid
recovery are important factors for process design. Because of high oxygen content, poor
calorific value, and lack of stability, the bio-oil produced via pyrolysis is unsuitable to be used
as a transport fuel, however via physical upgradation methods like blending and emulsification
a stable burning oil can be prepared. Catalyst based upgradation process can obtain better
calorific value.
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