Renewable Energy xxx (2016) 1e12

Contents lists available at ScienceDirect

Renewable Energy
journal homepage: www.elsevier.com/locate/renene

Strategies for selection of thermo-chemical processes for the
valorisation of biomass
Rawel Singh a, b, Bhavya B. Krishna a, b, Garima Mishra a, b, Jitendra Kumar a,
Thallada Bhaskar a, b, *
a
b

Thermo-catalytic Processes Area(TPA), Bio-Fuels Division (BFD), CSIR-Indian Institute of Petroleum (IIP), Dehradun 248005, India
Academy of Scientific and Innovative Research (AcSIR), New Delhi, India

a r t i c l e i n f o

a b s t r a c t

Article history:
Received 10 January 2016
Received in revised form
3 March 2016
Accepted 4 March 2016
Available online xxx

Research on biomass conversion has been gaining a lot of interest as biomass is renewable and sustainable in nature. Products from biomass can be obtained by different methods amongst which thermochemical route has a very high potential. Biomass is generally available in a localised manner in varying
quantities and qualities throughout the year and hence, region specific technologies have to be developed considering the end user requirement. Pyrolysis is a very versatile technique with the above
considerations. The process parameters can be tweaked to necessity to produce more bio-oil or bio-char.

Thermogravimetric analysis is essential for understanding the decomposition behaviour of the feedstock
before the lab scale pyrolysis is carried out. Pyrolysis using several regional feedstocks has been carried
out under nitrogen and hydrogen atmosphere and different biomass feedstocks were also liquefied using
sub/supercritical solvents. This review aims to provide a comparison of the results obtained using various
processes. This helps in the decentralised processing of biomass (dry biomass using pyrolysis and wet
biomass by hydrothermal liquefaction) to produce bio-crude which can be upgraded to produce fuels/
chemicals/petrochemical feedstocks in an environmental friendly manner.
© 2016 Elsevier Ltd. All rights reserved.

Keywords:
Pyrolysis
Hydrothermal liquefaction
Kinetic analysis
Lignocellulosic biomass
Aquatic biomass
Algae

1. Introduction
Fossil resources derived fuels have played the most important
role in the rapid technological progresses over the past few centuries. It is estimated that more than 85% of the world's energy
requirements are obtained from conventional fuels [1]. Energy
scenarios project that world's annual energy consumption will increase steeply from current value of 500 to 1000e1500 Exa Joules
per annum by 2050 [2e4]. Use of fuels derived from fossil resources
leads to global warming due to high levels of CO2 emission in atmosphere. Renewable, sustainable and environment friendly
alternate resources are required to address these issues. Solar radiation, winds, tides and biomass are renewable resources and
while first three resources can be used to obtain energy, biomass
can be used to produce energy, chemicals and materials [5]. Need
for a secure source of transportation fuels and chemicals make it
essential to explore bio-fuels/bio-based hydrocarbons as


* Corresponding author. Thermo-catalytic Processes Area(TPA), Bio-Fuels Division
(BFD), CSIR-Indian Institute of Petroleum (IIP), Dehradun 248005, India.
E-mail addresses: , (T. Bhaskar).

alternatives to hydrocarbons derived from fossil resources [6]. The
transition from the current fossil-based to bio-based carbon economy is expected to evolve continuously in the coming decades and
a continuous changeover to more complex bio-renewable feedstocks like agricultural residues, industrial wastes, green plants,
wood, or algae will occur [7].
2. Types of biomass feedstocks
Biomass is a plant matter of recent (no geologic) origin or material derived there from and can be used to produce various useful
chemicals and fuels [8,9]. Biomass contains variety of plant species
with varying morphology and chemical composition. Low
hydrogen to carbon ratio and high oxygen to carbon ratio in
biomass suggests that biomass can be utilised for the production of
fuels as well as functional chemicals [7]. Depending on the nature
of biomass used different biomass generation are shown in Fig. 1.
First-generation bio-fuels are derived from edible feedstock
from the agricultural sector such as corn, wheat, sugarcane, and
oilseeds. First generation biofuels have limitation of food versus
fuel issue. Second-generation bio-fuels are non-edible and

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Second
generation
biofuels

First generation
biofuels

Edible parts of
agricultural crops
and forest trees

Competes with
human and animal
food

Third
generation
biofuels

Fourth
generation
biofuels

Non edible parts of
crops, forest
residues, energy
crops

Macro algae, micro
algae


Modified organisms
for better yield

No food vs fuel issue

Does not compete
with agricultural land

Yet to be used in
large scale

Food production for
human population is
essential-hence
availability of
biomass is plenty

Utilises CO2 from
atmosphere or from
industrial emissions
for growth

Food vs fuel issue

Expected higher
yield per hectare

Fig. 1. Different biofuel generations depending on biomass type.


comprise of raw materials derived from lignocellulosic biomass and
crop waste residues from various agricultural and forestry processes [10,11]. Lignocellulosic biomass has three major components: cellulose, hemicellulose and lignin. The agricultural residues
can be classified as field and seed crop, fruit and nut crop, vegetable
crop and nursery crop [12]. The residues generated from the forest
products industry can be divided into two categories: (1) logging
residues-generated from logging operations, e.g., from final fellings
and (2) industrial by-products- generated by the forest industries
during processing of timber, plywood, particleboard, pulpwood,
etc. [13,14]. Energy crops are specifically grown to produce some
form of energy. Energy crops are generally divided into two types:
herbaceous and woody. Herbaceous energy crops are mostly types
of grasses, which are harvested like hay. Perennial grasses, such as
switchgrass, miscanthus, bluestem, elephant grass, and wheatgrass
could all potentially be grown as energy crops [15]. Third generation bio-fuels are based on algal matter (micro- and macro algae)
and cyanobacteria, which yield carbohydrates, proteins, vegetable
oils (lipids), and, subsequently, biodiesel and hydrogen gas, are
gaining considerable interest. The term algae can refer to microalgae, cyanobacteria (the so called “blue-green algae”), and macro
algae (or seaweed). The differences between microalgae and macro
algae are shown in Fig. 2.
3. Thermochemical conversion of biomass
There are several methods of conversion of biomass viz: mechanical, chemical, biochemical and thermochemical. Mechanical
processes only perform a size reduction of feedstock. Chemical
processes carry out a change in the chemical structure of the
molecule by reacting with other substances. These processes
include the wide class of chemical reactions where a change in the
molecular formula occurs [16]. Bio-chemical processes occur at
lower temperatures and most common types of biochemical
processes are fermentation and anaerobic digestion. The
fermentation uses microorganisms and/or enzymes to convert a
fermentable substrate into recoverable products (usually alcohols


Fig. 2. Comparison of microalgae and macro algae.

or organic acids) [17]. Anaerobic digestion involves the bacterial
breakdown of biodegradable organic material in the absence of
oxygen over a temperature range from about 30 to 65  C. The main
end product of these processes is biogas (a gas mixture made of
methane, CO2 and other impurities) [16,18]. An overview of
thermochemical and biochemical processes during biorefinery is
shown in Fig. 3.
Thermochemicals processes are carried out in the presence of
heat and can also use catalyst. Thermo-chemical methods utilize
the entire biomass without any pre-treatment steps to produce
value added hydrocarbons. In comparison to the biochemical
processes, thermochemical processes occur faster in the range of
few seconds, minutes or hours when the former takes time in the
range of days to complete. The other advantages of thermochemical methods of conversion are that they are not feedstock specific and can also process moisture-rich/aquatic feedstocks. The micro-organisms are feed specific and even the
slightest of change could lead to its non-functionality. This poses
a major risk in the commercialisation of the process at an

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3

Biomass

Pre-treatment


Holocellulose

Fermented to
ethanol, butanol,
xylitol etc. by
biochemical routes

Conversion to high
value chemicals by
thermochemical
routes

Lignin

Only burnt in
biochemical
processes

Thermo-chemical
route can be used
for the production
of aromatics

Fig. 3. Overview of thermochemical and biochemical processes for biomass conversion.

industrial level since the biomass availability in terms of quality
and quantity keeps varying all through the year. Various thermochemical processes for biomass conversion are shown in
Fig. 4.
This review article focuses on the strategies for selection of

thermochemical processes for valorisation of diverse biomass
feedstocks and need to have decentralised units that may be the
most immediate solutions for introduction of bio-based energy
systems. For the implementation of these units most important
thing is to do the fundamental research on diversified feedstocks

Combustion

to know the thermal decomposition behaviour, kinetics of
biomass and effective heat management through thermodynamic
data. Based on the fundamental studies from TG/DTG studies
pyrolysis approaches for dry biomass have been proposed for
centralised and decentralised biorefineries. For effective utilisation of wet biomass hydrothermal liquefaction has been proposed and effect of various parameters has been discussed. The
utilisation of biomass components viz. cellulose and lignin to
valuable chemicals has also been discussed using appropriate
catalytic and thermal methods.

Heat/ electricity

Centralised heating,
electricity by IGCC

Syn gas

FT to form fuels/
chemicals

Gasification
Steam reforming to
hydrogen


Biomass

Upgraded to
fuels/fuel blends
Bio-oil
Pyrolysis/
hydrothermal
liquefaction
Bio-char

Upgraded to produce
chemicals or
petrochemical
feedstocks
Adsorbents,
catalysts, electrodes,
soil management and
C-sequestration etc.

Fig. 4. Thermochemical processes for biomass conversion.

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4. TG/DTG studies of different feedstocks

“Thermogravimetric analysis” is a technique in which the mass
of a substance is measured as a function of temperature, while the
substance is subjected to a controlled temperature programme. The
thermal behaviour of biomass is determined using TGA. The TG and
derivative thermogravimetric (DTG) curves observed from TGA can
be used to determine the apparent weight loss of samples. Various
thermodynamic parameters and kinetic studies of different feedstocks have been carried out and discussed below.
4.1. Thermodynamic parameters
Pyrolysis behaviour and kinetic study of different lignocellulosic
biomass feedstocks (Cerus, Cheed, Cokad, Sagwan and Shimbal)
have been done. The thermodynamic properties were calculated
using the kinetic triplet values determined using the model free
approaches. The results obtained from thermal decomposition
processes indicate that there are three main stages in the pyrolysis,
i.e., dehydration, active and passive pyrolysis. The value of apparent
activation energy calculated using isoconversional methods are
used to evaluate the thermodynamic properties such as enthalpy,
Gibbs free energy and entropy of biomass pyrolysis. A shift in the
DTG curve, i.e. increase in the DTG peak temperature is observed as
the heating rate increases, but this shift is not uniform for all the
biomass studied. The estimated thermodynamic parameter values
are found to be different for biomasses all of which have forest as
their origin; but they are similar for a particular biomass at different
heating rates.
The model-free approach (Friedman method, Kissinger Akahira
Sunose method and Flynn-Wall-Ozawa method) does not require
assumption of specific reaction models, and yields unique kinetic
parameters as a function of either conversion (isoconversional
analysis) or temperature (non parametric kinetics). Model free kinetic methods are conversional in evaluation of pre-exponential
factor and reaction mechanism and the constraints involved do

not permit a straight forward evaluation of the remaining kinetic
parameters A and f(a). Thus a combination of pyrolysis data from
TGA and model free (isoconversional) methods can be a potent tool
for predicting the reaction kinetics as well as the thermodynamic
parameters of the biomass pyrolysis process.
The results obtained from the TGA studies of the feedstocks
showed that the apparent activation energy values calculated from
the isoconversional methods (150e170 kJ/mol) are found to be
similar for the all studied forest biomass except for Cokad which is
showing a relative high value of Ea (~200 kJ/mol). The values of the
pre-exponential factor are found to lie in the range of 108e1014 sÀ1.
The value of A for Cokad is of the order of 1014 from which it can be
attributed that the rotations of the active complex and the reagent
do not change during the reaction. The reaction order values are
found to be high which can be attributed to the multiscale and
multiphase nature of biomass feedstock. The estimated thermodynamic parameter values are found to be different for studied
biomasses all of which have forest as their origin; but they are
similar for a particular biomass at different heating rates.
4.2. Kinetic studies and reaction mechanism during pyrolysis
The thermal decomposition of biomass proceeds via a very
complex set of competitive and concurrent reactions with formation of over a hundred intermediate products and thus the exact
mechanism for biomass pyrolysis remains a mystery till date.
Modelling pyrolysis reactions with its unrevealed reaction mechanism presents a great challenge. The specific temperatures at
which various heterogeneous reactions occur, their reaction rates

and the energies involved in these reactions are valuable information useful for pyrolysis system design.
The development of thermochemical processes for biomass
conversion and proper equipment design requires the knowledge
of several process features which include a good understanding of
the governing pyrolysis mechanisms, the determination of the

most significant pyrolysis parameters and of their effect on the
process and knowledge of the kinetics [19]. Understanding both
multiscale and multiphase complexities represents a vital step
forward in optimizing pyrolysis and developing next-generation
biofuel technologies [20]. A precise conception of solid state pyrolysis kinetics is very crucial in designing and operating industrial
biomass conversion systems. The kinetic modelling studies of
biomass pyrolysis assists in analysis and optimization of reaction
conditions, process parameters and adaption of pyrolysis systems
to regionally differing surrounding conditions and diverse nature of
biomass feedstock. Kinetic studies form basis for development of a
prototype model for energy provision to remote rural areas.
Fundamental research will lead to a ‘building-up’ approach
whereby chemical mechanisms are integrated into particle models
(accounting for transport phenomena) which are capable of predicting global performance (i.e., bio-oil yield and composition).
Kinetics is the study of the dependence of the extent or rate of a
chemical reaction on time and temperature. Study of kinetics involves using mathematical models that quantify the relationship
between the rate of reaction, time, and temperature [21]. Kinetic
analysis is expected to be capable of.
 Revealing complexities in the reaction kinetics and prompting
some mechanistic clues
 Adequately describing the temperature dependence of the
overall reaction rate
 Producing reasonably consistent kinetic characteristics from
isothermal and non isothermal data [22].
A comprehensive kinetic analysis of a solid state reaction has
four main stages: Stage 1: Experimental collection of data; Stage 2:
Computation of kinetic characteristics for the data from stage 1;
Stage 3: Validation of kinetic parameters estimated; Stage 4:
Interpretation of the significance of any parameters evaluated in
stage 2. The different stages are shown in Fig. 5.

Modeling of pyrolysis implies the representation of the chemical
and physical phenomena constituting pyrolysis in a mathematical
form. The inherent complexity of the pyrolysis process has posed
formidable challenges to modelling attempts. The pyrolytic
decomposition involves a complex series of interlinked reactions,
and consequently, changes in the experimental heating conditions
or sample composition and preparation may affect not only the rate
of reaction, but also the actual course of reactions [21]. Besides the
sheer extent and range of pyrolysis reactions, several other issues
complicate the modeling of pyrolysis. More often these issues are
inter-linked making it extremely difficult to separate the influence
of one from another.
Modeling biomass pyrolysis is a challenge because of the variety
of the raw materials involved and also because of the wide operating conditions. Shin et al. [23] observed that the accurate predictions of gas species and aromatics from the pyrolysis of
biomasses, and mainly the effect of different operating conditions,
not only require the description of the released components, but
also the definition of their successive gas phase reactions. The pyrolysis of various lignocellulosic materials is differentiated by the
various reaction rates and the final product distribution achieved.
The quantitative formulation of the pyrolysis of a single biomass
particle involving all the above-mentioned effects is a task
requiring considerable effort [21]. In view of the importance of

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5

Fig. 5. Different stages of kinetic analysis.


kinetics in pyrolysis of a biomass it is necessary to know the values
of kinetic parameters of the biomass under a particular set of
conditions. However, difficulty arises in studying the thermal
behaviour of biomass due to lack of exact knowledge of the course
of reactions and their degree of completion. Moreover, the vast
number of products resulting from the thermal degradation of
biomass hinders a thorough understanding of the process [21].
Solid state reactions ordinarily demonstrate a tangled interplay of
various chemical and physical processes such as solid-state
decomposition, reaction of gaseous products with the solid, sublimation, polymorphous transitions, diffusion, melting, evaporation,
adsorption, desorption, etc. Therefore, the effective activation energy of a solid state reaction is generally a composite value determined by the activation energies of various processes and by their
influence on the overall reaction rate. Even if the temperature is
kept constant (single isothermal experiment), the relative contributions of the elementary steps into the overall reaction rate vary
with the extent of conversion ultimately resulting in a dependence
of the effective activation energy on the extent of conversion.
Additionally, the kinetics of solid state reactions are known to be
sensitive to pressure, size of crystals, gaseous atmosphere and
many other factors which are likely to change during the process
[24].
The relative importance of the internal heat transfer to the
external heat transfer is defined by the ratio of their respective
characteristic times:

tinternal
hL
¼ Bi
¼
k
texternal


This is the definition of the Biot number, a dimensionless
number commonly used in thermal analysis. Biot numbers larger
than 10 characterise a heat transfer limited by the internal conduction. The internal pyrolysis number gives a measure of the
relative importance of the internal conduction and the chemical
reaction.

Py ¼

k
AeÀE=RT rCP L2

The external pyrolysis number is the product of biot number and
the internal pyrolysis number. It gives the ratio between heat
convection rate and chemical reaction rate and can be stated as
follows,

Py0 ¼

h
AeÀE=RT rCP L

Different solid state kinetic methods used for biomass pyrolysis
are shown in Fig. 6.
Studies of the kinetics of cellulose, hemicellulose and lignin
separately revealed that the interactions between fractions are
important, and the pyrolysis behaviour of biomass components is
not completely additive like that in consecutive reaction model. In
the TG-DTG analysis of lignocellulosic material two or three peaks
usually appear, that can be assigned to cellulose, lignin and hemicellulose, indicating that, although there are interactions between

fractions, their identity is maintained. Specific conclusions with
respect to each feedstock are as follows [25,26]:
Microcrystalline Cellulose: The master plots method predicts

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Fig. 6. Different solid state kinetic methods.

the pyrolysis process of microcrystalline cellulose by an autocatalytic reaction mechanism. Friedman method gave more stable
values than in comparison with Vyazovkin AIC method. Distributed
activation energy model gave a good fit to the kinetic data.
Lignin: The present work shows that lignin pyrolysis is a complex process and series of reactions occur rather than a simple
single step reaction. The lignin pyrolysis takes place over a wide
range of temperature where three or more peaks can be seen in the
DTG curve indicating presence of more than one pseudo components (sinapyl alcohol, coniferyl alcohol and guaiacyl alcohol). A
wide distribution of activation energy in DAE Model shows presence of complex process with multiple reactions.
Rice Straw: The kinetic results for rice straw showed that the
reaction mechanism for rice straw pyrolysis can be kinetically
characterized by two successive reactions. At conversion values less
than 0.3 the decomposition of rice straw is governed by diffusion
and it tends to third order rate equation at high conversion (predicted by compensation effect method).
Groundnut husk: The kinetic results for groundnut husk present a complicated analysis, as at conversion values less than 0.1 the
decomposition process is governed by diffusion but no clarity in
mechanism is seen at conversion values greater than 0.1.
Pine: In the case of pyrolysis process of pine wood at conversion

values less than 0.7 the pyrolysis process is governed by two and
three dimensional diffusion whereas at higher conversion values
(a > 0.7) the mechanism is controlled by reaction order mechanism
with order of reaction as 1.5.
Deodar: Kinetic predictions similar to pine wood were observed
in case of deodar pyrolysis showing that at conversion values less

than 0.8 the pyrolysis process is governed by two dimensional
diffusion whereas at higher conversion values (a > 0.8) the mechanism is controlled by a third order reaction mechanism. The entire
process for both pine and deodar wood pyrolysis is closer to
diffusion controlled mechanism.
Water hyacinth: Water hyacinth presented a complex pyrolysis
kinetics which could not be easily modelled. The model free kinetic
methods gave a bad fit for the case of water hyacinth showing its
complicated pyrolysis behaviour. DAE Model for three pseudocomponents used for this case gave a reasonable fit but the model
could not be validated at higher heating rates. Hence, more complex DAE Models with more number of pseudocomponents can be
used further for study of kinetics of water hyacinth.
In addition to these feedstocks, kinetics of tamarind seed husk,
another Indian biomass feedstock was studied. Tamarind seed husk
exhibited an abnormal behaviour giving very high activation energy values ranging from 144.53 to 639.57 kJ/mol with the model
free kinetic analysis methods.
Model fitting and model free methods both suffer from certain
drawbacks and have certain advantages over the other. A combination of model free and model fitting kinetic methods can help in
providing much more informative and meaningful results rather
than empirical values with no mechanistic meaning. Model free
kinetic methods can be used efficiently to give initial parameter
estimates for modelling biomass pyrolysis using distributed activation energy models. It can be used to give preliminary insights
into the kinetic triplet and to provide initial estimates for kinetic
parameters which will help to avoid the extra computation time
needed in optimization of objective function for prediction of these


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values. Thus a combination of both these kinetic analysis methods
can be a powerful tool for predicting the reaction kinetics of
biomass pyrolysis process. For effective valorisation of lignocellulosic biomass into energy products/chemicals, the study of role of
solid catalyst is essential. In this direction, the kinetic studies for
selected feedstocks (rice straw, groundnut, pine, deodar) using the
conventional microporous catalytic materials (such as Mordenite,
y-zeolite, ZSM-5) has been initiated.
5. Pyrolysis of different feedstocks
Pyrolysis is said to be the basis of all thermochemical methods of
conversion and it is defined as the heating of any material in the
absence of oxygen. Pyrolysis processes produces bio-oil, bio-char
and non-condensable gases as products and the amount produced
varies on several factors. The operating parameters for pyrolysis
depend on the process type and the end product requirements.
Researchers have established the effect of pyrolysis temperatures,
particle size, gas flow rate and many other parameters through
their studies for the selected feedstock. The process parameters
have been identified in each case and the products have been
characterised. The end product utilisation is different in each case
and this dictates the development of indigenous technologies
depending on availability and costs involved with logistics. Bio-oil
obtained from slow pyrolysis of biomass does not have any direct
high commercial value. Hydropyrolysis experiments have been
carried out by various groups and some of the parameters that have

been tested are the effect of variations in final temperature, gas
used, pressure, heating rate, and particle size. Various reactor
configurations have been used, which give us information on the
heat and mass transfer effects. There are conceptual articles which
give us an insight into the possibilities of using hydropyrolysis in
the transition stage of shifting from fossil-based economy to
renewable feedstock-based economy. Basic fundamental data has
to be generated for both the processes using several feedstocks
available in the presence and absence of catalysts thereby generating a huge database of information which will be helpful to understand the process. No single process can produce solutions to all
fossil resource utilisation problems and hence, different processes
have to be used on different feedstocks to get different products.
5.1. Types of pyrolysis
There are various kinds of pyrolysis depending on the reactor
employed, gas atmosphere used and residence time inside the
reactor. The classification of pyrolysis based on the residence time
generally is slow, intermediate, fast and flash pyrolysis. Pyrolysis is
generally carried out in inert atmosphere like helium or nitrogen
etc and in cases where hydrogen atmosphere is used the process is
termed as hydropyrolysis. It can also be carried out under vacuum.
Based on the reactor used, the pyrolysis processes are ablative,
rotating cone, screw, auger, bubbling fluidised bed or circulating
fluidised bed and microwave pyrolysis. The products and ratios in
which they are formed vary depending upon the reaction parameters such as environment, reactor used, pyrolysis temperature,
rate of heating and source of heat. Longer vapour residence time
favours the production of bio-char. Moderate temperatures and
short vapour residence time are optimum for producing liquids.
One of the most important parameters in pyrolysis is the residence
time of the solid phase which can vary from seconds to days. Fast
pyrolysis is characterised by high heating rates and short residence
times. Fast pyrolysis generally requires the feedstock to be supplied

as fine particles; and the reactor design must facilitate rapid
removal of the hot vapours from the presence of the hot solids. In
fast pyrolysis, liquid fuel called bio-oil condenses from the vapours

7

and aerosols; the process also yields non-condensable gases of
medium calorific value. Other pyrolysis techniques include intermediate pyrolysis and flash pyrolysis. In intermediate pyrolysis,
reaction occurs at controlled heating rates thus avoiding tar formation. Interestingly, the size and shape of the feed particles are
less critical than in fast pyrolysis, which allows a wider variety of
biomass feedstock. Flash pyrolysis occurs with very fast heating
rates of !1000  C/s and uses even shorter solid residence times
(<0.5 s) than fast pyrolysis. The typical operating temperature for
flash pyrolysis is 800e1000  C and the biomass is supplied in the
form of dust [27]. This process gives a similar product distribution
as fast pyrolysis. A higher amount of liquid products can be produced through fast, intermediate and flash pyrolysis; whereas slow
pyrolysis produces a higher amount of the solid fuel (char) [28]. A
comparison of pyrolysis under nitrogen and hydrogen environment
is shown in Fig. 7.

5.2. Mechanistic differences of pyrolysis under nitrogen and
hydrogen environment
In the process of hydropyrolysis, as the reaction takes place
under the pressures of hydrogen, the formation of free radicals is
hindered. The amount of unsaturated hydrocarbons reduces
thereby increasing the quality of the bio-oil formed. The probable
reaction mechanism under nitrogen environment could be the
random thermal cracking of the large biomass molecule which
leads to an increased variation in the products formed. However, in
the presence of hydrogen it could be the systematic bond cleavage

(initiation with hydrogenolysis/hydrogenation, bond breaking in
the order of increasing bond strength and stability of formed
components etc.) in the macromolecular structure which leads to
the formation of few products with higher selectivity.
In case of rice straw, pyrolysis under hydrogen and nitrogen
environments showed that the reaction mechanism is different in
both cases as the product portfolio was different in two atmospheres. The bio-oils obtained under hydrogen atmosphere showed
more selectivity towards phenolic compounds. More amount of
bio-oil was obtained under nitrogen atmosphere but the selectivity
towards phenolic compounds was less [29].
Mechanistic differences between slow and hydropyrolysis are
presented as Fig. 8.

Pyrolysis
Presence of nitrogen

Presence of hydrogen

Endothermic

Exothermic

Possibility of free radicals and olefins
which can repolymerise leading to
less stable bio-oil

Presence of hydrogen produces
more saturated comounds in bio-oil
increasing shelf life of bio-oil


More yields of bio-oil and bio-char

When combined with in situ
hydroconversion, process is self
sustainable

Suitable in decentralised units

Centralised unit is more energy
efficient

Fig. 7. Comparison of pyrolysis under nitrogen and hydrogen environment.

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Hydrogen
• Selective bond
cleavage is higher
• Tendency to form
more vapours which
repolymerise into char

Nitrogen
• Random cleavage
• More gaseous products

when secondary reactions
occur
Fig. 8. Mechanistic differences between slow and hydropyrolysis.

5.3. Differences in behaviour of different biomass decomposition in
slow pyrolysis
5.3.1. Agricultural residues
Slow pyrolysis of rice straw under nitrogen atmosphere has
been carried out at 300, 350, 400 and 450  C [29]. The yield of biooil increased with increase in temperature up to 400  C beyond
which it reduced. It was concluded that 400  C is the optimum
temperature for slow pyrolysis of rice straw giving high yields of
desirable products namely the bio-oil and bio-char. In case of wheat
straw, the conversion was a maximum at 400  C. In case of wheat
husk, the bio-oil yield was almost similar at around 23 wt.% at 300,
350 and 400  C but increased to 28.5 wt.% at 450  C. The biochar
yield in case of thermal pyrolysis of wheat husk was 35.1 wt.% [30].
Cotton residue is an interesting feedstock for thermo-chemical
methods of conversion to produce value added hydrocarbons in a
sustainable manner and for slow pyrolysis under nitrogen environment, 400  C was found to be the optimum pyrolysis temperature to obtain maximum bio-oil yield. The bio-oil yield observed
was 38 wt.% and biochar yield was 30 wt.% at 400  C [31].
5.3.2. Forest residues
Slow pyrolysis of deodar under nitrogen atmosphere was carried out at 300, 350, 400 and 450  C. Maximum bio-oil yield of
46.5 wt.% was obtained at 350  C and with further increase in
temperature, the bio-oil yield decreased. Biochar yield observed at
350  C was 46.5 wt.%. Conversion was seen to increase with increase in temperature due to the primary decomposition of
biomass. The increasing temperature also had an effect on the noncondensable gases yield which increased at higher temperatures
due to secondary cracking [32]. Bio-oil under nitrogen atmosphere
was also majorly composed of phenolic compounds and aromatic
ethers. The complete loss of functionality and the aromatic nature
of bio-char indicated the complete conversion of biomass.

5.3.3. Defatted biomass
After the production of bio-diesel or jet fuel from non-edible
oils, the de-oiled cakes are generally thrown away or burnt. These
cakes can be an effective and renewable source of valuable hydrocarbons as they do not compete for fodder also in many cases.
Slow pyrolysis of jatropha seed deoiled cake (JSDC) was carried out
under nitrogen at various pyrolysis temperatures of 300, 350, 400,
450 and 500  C. The yield of bio-oil was a maximum of 38.4 wt.% at
450  C. The biochar yield observed was 37 wt.% at 450  C. Phenolic
compounds and aromatic ethers were the major fraction of

compounds present in the organic fraction of the bio-oil. The first
order rate equation was used to calculate the frequency factor and
activation energy of the process and the lower values as compared
to thermo-gravimetric analysis indicated the efficient heat and
mass transfer in the reactor system [33].
The slow pyrolysis of de-oiled microalgae has been carried out
at 350, 400 and 450  C. Low amount of bio-oil yield also could be
attributed to the very high ash content of the de-oiled microalgae. It
can thus be concluded that the conditions required for pyrolysis
increase in terms of severity in the following order: forest
residue < agricultural residue < defatted biomass. The bio-oils were
majorly composed of phenolic compounds and aromatic ethers in
most cases. The next in line were hydrocarbons, furans and
carbonyl compounds. Alcohols, acids and esters were found in
small fractions. In case of de-oiled algae, the bio-oil had considerable quantities of nitrogen compounds due to the protein content of
algae. The bio-oil obtained by slow pyrolysis can only be tried to
blend with transformer/bunker/furnace oil in best case scenario in
terms of fuels. Since it has been identified that it is rich in phenolic
compounds, the bio-oil can be used for the production of very high
value chemicals after the development of efficient separation

processes.
5.4. Behaviour of different biomass feedstocks under hydropyrolysis
5.4.1. Agricultural residues
Rice straw pyrolysis has been carried out in hydrogen environments to understand the effect of reaction atmosphere on the pyrolysis products. Optimum temperature was found to be 400  C and
higher pressures of 30 bar were required to produce maximum
yield of phenolic derivatives in the presence of hydrogen [29].
Hydropyrolysis of cotton residue has been carried out and it has
been observed that the optimum conditions in the used experimental set up are 400  C and 20 bar pressure of hydrogen [34].
5.4.2. Forest residues
Hydropyrolysis of deodar was carried out at 350 and 400  C at
different pressures of hydrogen of 1, 10, 20, 30 bar. At both the
temperatures, the trend followed by bio-oil, bio-char and gas yields
were similar. The bio-oil yield at every pressure condition was
higher at 400  C than at 350  C. The bio-oil yield was a maximum at
1 bar pressure and with increase in pressure from 10 to 30 bar, the
bio-oil yield reduces though the difference in yields at 20 and
30 bar is negligible. It has been well established that the bio-oil
quality produced under hydrogen pressure is better than the case
with nitrogen [35]. With increase in pressure, the biochar yield
increases since the vapours are more in the vicinity of bio-char
leading to more bio-char formation [36]. At 30 bar pressure, secondary cracking is seen to take place as the yield of noncondensable gases increases at the expense of bio-oil and biochar. Thus, it can be concluded that 400  C and 20 bar pressure
are optimum conditions for the hydropyrolysis of deodar.
5.4.3. Defatted biomass
The hydropyrolysis of jatropha seed de-oiled cake has been
carried out at various hydropyrolysis temperatures (300, 350, 400
and 450  C) and pressures of hydrogen (1, 20, 40 and 52 bar). It has
been observed that the liquid bio-oil yields have increased with
pyrolysis temperature and pressure. It was also found that 40 bar is
the optimum value of hydrogen pressure for the hydropyrolysis
reactions at 450  C to obtain maximum liquid yield (17 wt.%) in the

given experimental setup. The frequency factor of 98 sÀ1 and
activation energy of 29 kJ/mol are the lowest at 40 bar reaffirming
the fact that heat and mass transfer limitations are the least at
40 bar pressure [37,38].

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5.5. Catalytic pyrolysis
5.5.1. Nitrogen atmosphere
Pyrolysis can be carried out in the absence (thermal decomposition) or presence of catalyst (catalytic decomposition). The basic
criterion to use the catalysts in any conversion process is to carry
out the reactions at lower temperatures compared to thermal runs.
The decomposition behaviour and product profile of the thermal
and catalytic pyrolysis of wheat straw and wheat husk and the effect of position of catalyst for wheat straw and wheat husk has been
studied. Solid phase contact (SPC) and vapour phase contact (VPC)
where catalyst is mixed with the feed and kept separately in a
holder respectively have been studied. The degradation mechanism
in the SPC of feed is very much different than in the case of VPC of
feed and hence, the products formed are different in both the cases.
From the above experiments, it can be seen that VPC has produced
higher and effective conversion of biomass to bio-oil. This can be
useful in a way that it reduces the cost of catalyst regeneration as it
is easy to remove char from the catalyst used in VPC than in SPC. In
the case where the catalyst is cheap e.g., as waste of some other
process industry such as fly ash etc. it can be thrown without being
regenerated. The catalyst used and its contact method has to be
modified according to the requirement of the end products. Hence,

a detailed understanding of these mechanisms is very much
required to develop an optimised process for the pyrolysis of
biomass to produce value added hydrocarbons in a sustainable
manner [30].
5.5.2. Hydrogen atmosphere
Catalytic hydropyrolysis of jatropha seed de-oiled cake has been
carried out at optimum conditions of 450  C and 40 bar pressure of
hydrogen. It can be observed that the feed quantity plays a major
role in the product yield. When all the other conditions are maintained constant other than the feed weight, it has been observed
that the bio-oil yield has reduced by half with a corresponding
increase in the biochar and gas yield. This indicates that the feed
weight is also an important parameter in the hydropyrolysis of
jatropha seed de-oiled cake. Some of the other observations from
the experimental results are that the catalytic experiments have
produced more oil than the thermal runs. In case of n-ZSM-5 and
feed of particle size 0.5e2 mm, plain support has given maximum
liquid yields and maximum conversion when 0.5 g catalyst was
mixed with 20 g feed. When Mo was incorporated it is seen to have
triggered secondary reactions thereby producing more char. Particle size is also a very important reaction parameter for the process
of hydropyrolysis as differences in product yields have been
observed with changes in the particle size of the feed used. With
increase in the amount of n-ZSM-5 added to feed with particle size
of <0.149 mm, the conversion was seen to reduce from 54% to 52.5%
and then to 44%. The incorporation of metal did not make any
significant difference in the yields of bio-oil in case of n-ZSM-5
though the conversion decreased. Highest conversion of 54% was
found in case of n-ZSM-5 added in the order of 0.25 g. In case of
hierarchical zeolites, the increase in the amount of FAPTMS
increased the bio-oil content. When Mo was impregnated it did not
produce any significant increase in the product yields. But when Ni

was impregnated, it is observed to have produced maximum
amount of bio-oil among all experiments in this feed. This might be
attributed to the hydrogenation activity of Ni increasing the bio-oil
yields. The yields of h-zeolite Beta (8%) and h-ZSM-5 with higher
FAPTMS are similar. It can be concluded that the presence of catalyst plays a key role in the hydropyrolysis of biomass. The reaction
mechanism in the absence and presence of catalysts is different as
evident from the differences in the product profile and distribution.
Thermal and catalytic hydropyrolysis has been carried out using

9

hierarchical and nanocrystalline zeolites for the first time. A
maximum bio-oil yield of 19.1 wt.% has been obtained using
nanocrystalline zeolite. In addition to pressure and temperature,
particle size and feed weight are also important operating parameters for the process of hydropyrolysis. In the presence of catalysts,
selective cracking is observed to have happened and majorly
aliphatic type compounds have been observed in the process.
6. Hydrothermal liquefaction
Hydrothermal liquefaction is a process for obtaining fuels/
chemicals from biomass in the presence of sub/supercritical solvent
at moderate to high temperature (250e350  C) and pressure
(5e25 MPa). The hydrothermal liquefaction can be applied to a
variety of biomass feedstocks without any drying. Superior to pyrolysis hydrothermal liquefaction produces better quality bio-oil
containing various chemicals including vanillin, phenols, aldehydes, and organic acids, etc. Subcritical water has unique properties different from water at ambient conditions that include a
high ion product (Kw) and a low dielectric constant, which are
favourable for promoting reactions without catalysts. The changes
and optimization of reaction parameters and catalysts can produce
the functional hydrocarbons/specialty chemicals in a single step
[39].
6.1. Hydrothermal liquefaction of different feedstocks

Hydrothermal liquefaction of high or low moisture containing
lignocellulosic and algal biomass for effective and complete conversion of organic content (lignin, cellulose and hemicellulose) into
value added hydrocarbons has been carried out to understand the
structure activity relationship with respect to the different feedstocks. The various works carried out include:
 Hydrothermal liquefaction of lignocellulosic biomass components (Cellulose and lignin)
 Utilization of cellulose using hydrothermal approach to functional chemicals
 Lignin valorisation to substituted phenols and aromatic ethers
using organic solvents
 Hydrothermal liquefaction of agricultural residues (Wheat
Husk, Wheat Straw, Sugarcane Bagasse, Rice Straw and Tamarind Seed Husk)
 Effect of reaction environment on hydrothermal liquefaction of
rice straw for production of functional chemicals
 Hydrothermal liquefaction of forest biomass residue (Pine
wood, Deodar)
 Hydrothermal liquefaction of aquatic biomass (water hyacinth,
microalgae, macro algae)
 Effect of macro algae composition on product distribution and
nature of products
 Effect of organic solvents on hydrothermal liquefaction of
various second and third generation biomass

6.2. Behaviour of different biomass feedstocks under hydrothermal
liquefaction
The work focused on the utilization of sub-critical water as a
reaction medium as well as acid/base catalyst to depolymerise
various biomass feedstocks to value added hydrocarbons. A single
step method for the liquefaction of biomass has been employed to
get maximum liquid products yield. The fundamental studies to
understand the role of individual biomass components (cellulose
and lignin) on the production of valuable hydrocarbons and its


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10

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comparison with real biomass has been carried out. The reaction
conditions have been optimized in order to get maximum conversion to liquid products as well as maximum bio-oil yield. The
experimental studies of lignin and cellulose were carried out using
water and alkaline catalysts and characterization of the various
products was carried out. Conversion of cellulose into high yields of
methyl glucosides ca. !90% over sulfonated carbon based catalyst
was carried out. The carbon catalyst was demonstrated to be stable
for five cycles with slight loss in catalytic activity [40]. Hydrothermal conversion of lignin to substituted phenols and aromatic ethers
was performed using methanol and ethanol at various temperatures (200, 250 and 280  C) and residence times of 15, 30 and
45 min. FTIR and 1H NMR showed the presence of substituted
phenols and aromatic ethers in liquid products [41]. Hydrothermal
liquefaction of rice straw and tamarind seed husk was also carried
out using organic solvents. Thermal and catalytic hydrothermal
liquefaction of water hyacinth was performed at temperatures from
250 to 300  C under various water hyacinth:H2O ratio of 1:3, 1:6
and 1:12 under various residence times (15e60 min. The use of
alkaline catalysts significantly increased the bio-oil yield. Bio-oil
with high aliphatic carbon content was observed [42]. De-oiled
microalgae cake obtained after the lipid extraction from microalgae was utilized through hydrothermal liquefaction to produce
some value added hydrocarbons. A comparative study on hydrothermal liquefaction of various macro algae feedstocks viz. Ulva
fasciata (MA’UF), Enteromorpha sp. (MA’E) and Sargassum tenerrimum (MA'ST) was carried out to understand the effect of the
compositional changes of macro algae samples on product distribution and nature of products. Maximum conversion of 81% was

observed with MA’UF. FTIR and NMR spectra showed high percentage of aliphatic functional groups for all bio-oils [43]. Hydrothermal liquefaction of U. fasciata (MA’UF) was also carried out
using organic solvents (CH3OH and C2H5OH). The use of alcoholic
solvents significantly increased the bio-oil yield and the acids obtained in bio-oil to the corresponding methyl and ethyl esters. The
results showed that liquefaction with supercritical alcohols is an
effective way to produce functional hydrocarbons for chemical
feedstock [44]. The preliminary studies on hydrothermal liquefaction of rice straw using solid catalysts have been performed. There
was no significant change in the product distribution using solid
catalysts. However, the use of solid catalysts increased the TOC
content in aqueous products compared to thermal case indicating
the maximum carbon presence in aqueous products.
It was observed that addition of alkali enhances the inherent
function of high temperature water, leading to high yields of
desired products (bio-oil). Appropriate use of bases/acids and oxidants based on an understanding of the reaction mechanism can
lead to an excellent yield of the desired product. Properties of sub/
supercritical solvent abolish the effect of biomass particle size or
heating rates on bio-oil yield since it can act both as a heat transfer
medium and as an extractant during liquefaction. Solvent under
sub/supercritical conditions help to overcome the heat transfer
limitations that makes hydrothermal liquefaction relatively independent of the size of biomass particle or heating rates. The hydrothermal liquefaction of carbohydrates (cellulose and
hemicelluloses) generally leads to a mixture of water soluble hydrocarbons. Conversion products include acetic acid, formic acid,
lactic acid, levullinic acid, 5-hydroxymethyl-2-furaldehyde, and 2furaldehyde etc. Hydrothermal liquefaction of lignin is a promising way of recovering phenolics rich bio-oils. Both aromatic
aldehyde and phenolic compound are important chemical intermediates. It was also observed during this work that hydrothermal liquefaction using organic solvents is beneficial for high
liquid products. Temperature, type of biomass, solvent used (water
or organic) are major parameters that influence yield and

composition of bio-oil. It was observed that compositional variations in biomass types cause changes in product distribution and
nature of products obtained from hydrothermal liquefaction since
lignin, hemicelluloses, and cellulose behave differently during hydrothermal liquefaction and the interaction between these components and products derived from the can greatly influence the
product distribution and nature of products.
Catalytic hydrothermal liquefaction of wheat husk using alkali

catalysts gave high yield of light hydrocarbons (ether fraction).
Alkaline catalysts promote the decomposition of lignin fraction to
low molecular weight phenolic compounds [45]. From hydrothermal liquefaction of rice straw carried out using different reaction
environments viz. N2, O2 and CO2 it was observed that inert atmosphere (N2) gave highest bio-oil yield (17 wt%) as well as conversion (78%) compared to O2 and CO2 on basis of gravimetric yields
as well as on organic carbon basis. GC/MS showed that bio-oil obtained under different reaction conditions was composed of
phenol, guaiacol, catechol, syringol etc. and their derivatives [46].
To see the effect of organic solvents on whole biomass, hydrothermal liquefaction of rice straw was also carried out using
methanol and ethanol at different conditions of temperature and
residence time. Similar to the biomass components, high liquid
product yield (47 wt%) was obtained using organic solvents. Liquid
products were mainly composed of alkyl derivatives of phenol and
guaiacol [47].
To understand the effect of different biomass with varying
composition, hydrothermal liquefaction study of agricultural and
forest biomass residues was carried out. The agricultural biomass
(wheat straw and sugarcane bagasse) exhibited higher conversion
under thermal and catalytic conditions compared to forest biomass
(pine wood and deodar). Alkaline catalysts (KOH and K2CO3) have
found an important effect on the decomposition of both agricultural and forest biomass residue in terms of both bio-oil yield and
conversion.
K2CO3 showed higher catalytic activity in terms of both bio-oil
yield as well as conversion for agricultural (wheat straw and sugarcane bagasse) biomass compared to forest (pine wood and
deodar) biomass. Varying biomass composition had a significant
effect on bio-oil yield as well as conversion. Alkali catalysts also
showed different activity for agricultural and forest biomass residue [48].
7. Centralised and decentralised biorefinery systems in Indian
scenario
Bio-refineries are seen as the best future option for the production of energy in a sustainable and environment friendly
manner. Biomass is a much dispersed resource and its availability is
also too varied depending on several factors. Several region specific

processes have to be developed and deployment of the same has to
be carried out based on supply and demand scenario. With this
background, slow pyrolysis can be carried out in decentralised locations using resources from a group of villages or so. The products
can be used at that location or be transported to a centralised unit
for further upgradation. In case where centralised facilities can be
built, hydropyrolysis can be a better option for the production of
renewable hydrocarbons.
8. Future prospects
It has been well established that there is a requirement to move
towards sustainable sources of energy and hydrocarbons. Fuels and
chemicals can be effectively produced from biomass which is a
renewable source of energy. The thermo-chemical methods have
the maximum potential to produce value added hydrocarbons in an

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economic way in the future. Among the various pyrolysis techniques, slow and hydropyrolysis approaches are seen to be most
suitable for decentralised and centralised conversion units
respectively.
Study of kinetics for biomass conversion is a vast and emerging
field and it requires research at every level to develop realistic
models for real world reactors. In this context study for kinetics of
biomass pyrolysis can be extended further to a batch scale reactor
or a fluidized bed reactor to examine if the predictions made by this
study can be validated for a scaled up process. The use of new
experimental techniques capable of providing molecular-level
insight is needed to improve upon existing global kinetic

schemes. Further efforts have to be made to deduce the reaction
mechanism for the process of biomass pyrolysis and establish the
reaction chemistry of pyrolysis process.
Knowledge of synergistic interactions between the main
biomass components and the effect of their interactions on quantitative and qualitative yield of products needs to be given importance in the future works. The significance of TGA to quantitatively
resolve complex mixtures because of the characteristic thermal
decomposition temperature of each component can be utilized to
improve the further works on kinetic study at batch or continuous
scales. Further improvements in kinetic models and the modeling
strategies need to be made to incorporate study of some complex
biomass feedstocks and analyse their pyrolytic behaviour. Kinetics
for catalytic pyrolysis of lignocellulosic biomass is crucial in getting
an insight into the pyrolysis behaviour of lignocellulosic biomass
and hence kinetic studies for catalytic pyrolysis process need to be
implemented.
The works carried out on slow and hydro pyrolysis are preliminary results and are a basis to understand the two processes.
The effect of operating parameters on the processes has been understood and certain process parameters have been optimised.
Detailed design for the fabrication of continuous reactors for both
the processes has to be carried out. The detailed reaction mechanism for both the processes is still not known and yet to be understood. As the reaction mechanism is different at different
conditions, the reaction conditions can be modified to obtain the
required product slate with increased yield and selectivity. The
identification of set generation of this data is essential for various
feedstocks available. The differences in the set of reaction parameters for different feedstocks and processes have to be understood
using the first principles. Catalysts play a major role in the biomass
conversion process and hence, their design in terms of surface area,
hydrothermal stability, functionality to be impregnated and acidity
etc. has to be optimised for the process. Biomass is a natural
polymer which contains a wide range of functionalities (CeO, C]O,
CeH, C]C, OeH, CeC) and can be exploited effectively for the
production of high value chemicals. When the biomass is cleaved

effectively at specific locations in the presence of hydrogen various
useful specialty chemicals can be obtained in an economically
viable manner. Depending on the end product required the catalyst
has to be modified to maximize the production of fuels or
chemicals.
More studies have to be carried out to use cheap/waste materials as catalyst and non-noble metals for impregnation to provide
hydrogenation/dehydrogenation activity. The basic understanding
of the reaction mechanism will help in estimating the order of the
reaction and various other kinetic parameters associated with the
process like enthalpy change, entropy change etc. The process has
to be optimised in terms of operating conditions, type of reactor
used, heat and mass transfer limitations and downstream separation steps. After the process has been established, detailed technoeconomic analysis has to be carried out to understand the profitability of the process. Supply chain mechanisms have to be fit in

11

place to ensure easy procurement of feedstocks and proper sales of
products. To calculate the greenhouse gas emissions from the
process life cycle assessment studies have to be carried out taking
into consideration the direct and indirect land use changes. In
addition to the above mentioned studies, research has to be carried
out to design effective reactors that can reduce the heat and mass
transfer limitations. The new age catalysts have to be developed
that are hydrothermally stable over a wide range of temperatures,
have reduced coking tendencies, cheap and easily available.
Detailed catalyst characterisation techniques have to be developed
in addition to in-situ reaction monitoring studies. For the development of efficient biorefineries, a detailed study on the available
quantity and quality of feedstocks and processes/combination of
processes suitable for the same have to be identified in Indian
scenario. Detailed understanding of the chemistry and chemical
engineering of the processes is required to make the commercialisation of the developed process a success story in this competitive

global scenario.
Hydrothermal liquefaction is a promising process for the valorisation of biomass, especially high moisture biomass. Such
biomass with high water content may be directly utilized without
energy-intensive pretreatment and converted into bio-oil and
platform chemicals. The hydrothermal liquefaction of carbohydrates (cellulose and hemicelluloses) generally leads to a mixture of
water soluble hydrocarbons. Conversion products include acetic
acid, formic acid, lactic acid, levullinic acid, 5-hydroxymethyl-2furaldehyde and 2-furaldehyde etc. While hydrothermal technologies have many advantages over other thermochemical conversion routes of processing biomass, the fact remains that these
technologies are not being widely commercialized today. Part of
this is due to the high pressures needed for processing which requires special reactor and separator designs for development of
continuous processes and the capital investments needed for fullscale plants. The current size of continuous systems available is
not adequate for demonstration scale of operation. It is also
essential to understand the detailed chemistry and mechanism of
HTL during continuous process. There are a number of other critical
issues hindering commercialization that need to be addressed so
that the technologies can be ultimately scaled up. The challenges
with respect to robust (strength and stable) catalyst have to be
addressed. The process has to handle solids loading in excess of
15e20 wt% and feedstocks with impurities and different compositions. Feeding at high pressure and temperatures and heat recovery systems have to be designed. Feeding at high pressures into
the reactor has always been a challenge and this is a problem
majorly in small scale plants. In case of heterogeneous catalysts, the
catalyst should not deactivate easily due to the formation of coke.
With respect to the conversion methods, effective heat and mass
transfer is required for the proper conversion of feedstock into the
desired products. The hydrothermal upgradation technology has
immense potential for effective utilization of biomass functionality
for the production of valuable hydrocarbons.
Acknowledgement
The authors thank the Director, CSIR-Indian Institute of Petroleum, Dehradun, for his constant encouragement and support. The
authors thank CSIR in the form of XII Five Year Plan project
(CSC0116/BioEn) and Ministry of New and Renewable Energy for

providing financial support.
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straw, Bioresour. Technol. 169 (2014) 614e621.
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[28] A.K. Hossain, P.A. Davies, Pyrolysis liquids and gases as alternative fuels in
internal combustion engines e a review, Renew. Sust. Energy Rev. 21 (2013)
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[29] B. Balagurumurthy, V. Srivastava, J. Kumar, B. Biswas, R. Singh, P. Gupta,
K.L.N.S. Kumar, R. Singh, T. Bhaskar, Value addition to rice straw through
pyrolysis in hydrogen and nitrogen environments, Bioresour. Technol. 188
(2015) 273e279.
[30] B.B. Krishna, R. Singh, T. Bhaskar, Effect of catalyst contact on the pyrolysis of
wheat straw and wheat husk, Fuel 160 (2015) 64e70.
[31] B.B. Krishna, B. Biswas, J. Kumar, R. Singh, T. Bhaskar, Role of reaction temperature on pyrolysis of cotton residue, Waste Biomass Valor. 7 (2016) 71e78.
[32] A.E. Putun, N. Ozbay, E.P. Onal, E. Putun, Fixed-bed pyrolysis of cotton stalk for
liquid and solid products, Fuel Process. Technol. 86 (2005) 1207e1219.
[33] B.B. Krishna, B. Biswas, J. Kumar, R. Singh, T. Bhaskar, Slow pyrolysis of
jatropha seed de-oiled cake and estimation of kinetic parameters, J. Energy
Environ. Sust. (2015). Accepted manuscript.
[34] B. Balagurumurthy, R. Singh, T.S. Oza, K.L.N.S. Kumar, S. Saran, G.M. Bahuguna,
R.K. Chauhan, T. Bhaskar, Effect of pressure and temperature on the hydropyrolysis of cotton residue, J. Mater. Cycles Waste Manag. 16 (2014) 442e448.
[35] T.L. Marker, L.G. Felix, M.B. Linck, M.J. Roberts, P. Ortiz-Toral, J. Wangerow,
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[36] L. Wang, Ø. Skreiberg, M. Gronli, G.P. Specht, M.J. Antal, Is elevated pressure
required to achieve a high fixed-carbon yield of charcoal from biomass? Part
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[37] B. Balagurumurthy, T. Bhaskar, K.L.N.S. Kumar, H.B. Goyal, D.K. Adhikari, Effect
of pressure on the hydropyrolysis of Jatropha seed deoiled cake, J. Mater.
Cycles Waste Manag. 15 (2013) 328e334.
[38] B. Balagurumurthy, T. Bhaskar, H.B. Goyal, D.K. Adhikari, Hydropyrolysis of
Jatropha seed de-oiled cake: estimation of kinetic parameters, Waste Biomass
Valor 4 (2013) 503e507.
[39] R. Singh, A. Prakash, B. Balagurumurthy, T. Bhaskar, Hydrothermal liquefac€ cker, R. Sukumaran (Eds.),
tion of biomass, in: A. Pandey, T. Bhaskar, M. Sto
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269e291 (Chapter 10).
[40] S. Dora, T. Bhaskar, R. Singh, D.V. Naik, D.K. Adhikari, Effective catalytic conversion of cellulose into high yields of methyl glucosides over sulfonated
carbon based catalyst, Bioresour. Technol. 120 (2012) 318e321.
[41] R. Singh, A. Prakash, S.K. Dhiman, B. Balagurumurthy, A.K. Arora, S.K. Puri,
T. Bhaskar, Hydrothermal conversion of lignin to substituted phenols and
aromatic ethers, Bioresour. Technol. 165 (2014) 319e322.
[42] R. Singh, A. Prakash, B. Balagurumurthy, T. Bhaskar, Catalytic hydrothermal
liquefaction of water hyacinth, Bioresour. Technol. 178 (2015) 157e165.
[43] R. Singh, B. Balagurumurthy, T. Bhaskar, Hydrothermal liquefaction of macro
algae: effect of feedstock composition, Fuel 146 (2015) 69e74.
[44] R. Singh, T. Bhaskar, B. Balagurumurthy, Effect of solvent on the hydrothermal
liquefaction of macro algae ulva fasciata, Process Saf. Environ. Prot. 93 (2015)
154e160.
[45] R. Singh, T. Bhaskar, S. Dora, B. Balagurumurthy, Catalytic hydrothermal
upgradation of wheat husk, Bioresour. Technol. 149 (2013) 446e451.
[46] R. Singh, K. Chaudhary, B. Biswas, B. Balagurumurthy, T. Bhaskar, Hydrothermal liquefaction of rice straw: effect of reaction environment, J. Supercrit.
Fluids 104 (2015) 70e75.
[47] R. Singh, V. Srivastava, K. Chaudhary, P. Gupta, A. Prakash, B. Balagurumurthy,
T. Bhaskar, Conversion of rice straw to monomeric phenols under supercritical
methanol and ethanol, Bioresour. Technol. 188 (2015) 280e286.
[48] R. Singh, A. Prakash, B. Balagurumurthy, R. Singh, S. Saran, T. Bhaskar, Hydrothermal liquefaction of agricultural and forest biomass residue: comparative study, J. Mater. Cycles Waste Manag. 17 (2015) 442e452.


Please cite this article in press as: R. Singh, et al., Strategies for selection of thermo-chemical processes for the valorisation of biomass,
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