BIOMASS CONVERSION TO VALUABLE CHEMICALS SUCH AS BIOFUEL

AND BIOPLASTICS: A COMPREHENSIVE REVIEW OF THE

THERMOCHEMICAL PROCESSES

BY

JAMES PERPETUAL DOTUCHOWO

PRESENTED TO

DR UMAR AHMED

AUGUST, 2025

Abstract: Fossil fuels have fueled the world economy for decades. However, given their limited
nature, fluctuating prices and the escalating environmental concerns, there is an urgent need to
develop and valorize cheaper, cleaner and sustainable alternative energy sources to curb these
challenges. Biomass represents a valid alternative to fossil fuels, especially for fuel and chemical
production as it represents the only natural organic renewable resource with vast abundance. A
vast array of conversion technologies is used to process biomass from one form to another, to
release energy, high-value products or chemical intermediates. This paper extensively reviews
the thermochemical processing of biomass to fuels and high-value chemicals, with an emphasis
on the process performance, conditions, and weaknesses. Technologies with great future
prospects as well as those with possible linkage to CO2 capture and sequestration are
highlighted. The important chemical compositions of biomass feedstock, their conversion
technologies and most importantly, the role of catalysis in their conversion to fuels, fuel
additives, based chemicals, and added-value chemicals are also discussed. Special attention is
given to biofuel production for transportation as this sector is responsible for the highest global
greenhouse gas emissions, and has an emerging market with promising future prospects for
sustainable large-scale biomass processing. The processes involved in the purification and
upgrading of biomass-derived products into higher-value products are equally discussed and
reviewed. The increasing demand for sustainable alternatives to fossil fuels and petrochemical-
derived plastics has driven research into biomass conversion technologies. Thermochemical
processes—including pyrolysis, gasification, hydrothermal liquefaction, and torrefaction—offer
efficient pathways to convert lignocellulosic biomass, agricultural residues, and organic waste
into biofuels (e.g., bio-oil, syngas) and bioplastics (e.g., polylactic acid, polyhydroxyalkanoates).
This review critically examines recent advancements in thermochemical conversion techniques,
their mechanisms, influencing parameters (temperature, catalysts, feedstock type), and the
economic and environmental challenges of scaling these processes. The study highlights the
potential of integrated biorefineries to optimize biomass valorization while addressing
sustainability concerns.

1.0 INTRODUCTION
Fossil fuels, the main source of global energy (81% of the energy supply)
satisfactorily meet the current energy and chemical needs, Biomass
composition plays a critical role in biorefining processes (Hayes, 2013; Ruiz &
Luque et al 2013). despite their environmental setbacks and other concerns.
They are used mainly as fuels for transportation or combusted in gas/thermal
engines or power plants for electricity production. The use of fossil fuels,
however, leads to the emissions of greenhouse gases (GHGs). These
emissions, together with their limited nature, rapid depletion, price
fluctuations, environmental concerns (global warming or climate change),
and the growing demand for energy has led to the current strong drive,
widespread interest and investments in renewable energies (Pande &
Bhaskarwar, 2013; Heidenreich & Ugo, 2015). Biomass energy is a
sustainable substitute for fossil fuels (oil, natural gas, and coal) in the
provision of energy, fuels, and chemicals (Hayes, 2013; Baruah &
Baruah,2014). It is the only renewable organic resource, relatively cleanest
and most abundant among the renewable energy resources (Pande &
Bhaskarwar, 2013). and is regarded as one of the most promising fossil fuel
alternatives for energy, chemicals and biofuels production (Caprariis et al
2017; Hu et al, 2017). The negligible amounts of nitrogen, Sulphur, and ash
in biomass make it a cleaner fuel compared to fossils due to its low SO 2, NO2
and soot emissions. Zero net CO2 emissions can be achieved with biomass
due to the quantitative recycling or sequestration of its CO2 released by
plants during photosynthesis (Balat, et al, 2009). Also, the interest in the
development and valorization of biomass as a source of fuel for the reduction
of the environmental concerns from energy generation and in the expansion
of domestic and renewable energy sources for both richer and poorer
countries stems right from the 1970s energy crisis (Sharma et al, 2015).
Apart from its direct energy production, there is currently a growing interest
and attention to develop processes capable of converting biomass to fuels
(bio-oil, bio syngas, etc.) and vital industrial bio-based chemicals (Caprariis
et al, 2017). Currently, biomass fuels (agricultural residues, wood, and
herbaceous materials) constitute the third largest primary, energy resource
worldwide. Their conventional conversion to energy is regarded as carbon
neutral. However, the harvesting of biomass for energy purposes seemingly
may be unsustainable as it depletes soil carbon and nutrients, leading to a
reduction in soil productivity (Hrncic et al, 2016). Recent studies (Muh &
Tabet, 2019) indicate that roughly 3 billion people globally rely on traditional
biomass for heating, lighting, and cooking, resulting in the annual premature
death of about 3.1 million due to indoor pollution, mainly in developing
countries. In addition, about 2.7 billion people lack clean cooking facilities in
sub-Saharan Africa and Asia-Pacific, necessitating improved cooking facilities
through biogas from anaerobic biomass processing (Muh et al., 2018). The
abundant and sustainable availability of biomass feedstocks is continuously
being dominated and challenged by biomass-derived wastes and residual
materials, with the inclusion of municipal solid waste, which when
unprocessed causes serious problems (Hrncic et al, 2016). The use of
biomass wastes (agricultural, domestic, etc.) as bioenergy feedstocks
compared to energy crops will, among others, reduce GHG emissions, stress
on land usage, as well as other associated impacts, conflicts, and ethical
issues arising from food crop usages. This stands as a strong booster for
bioenergy production and in the sustainability of biofuels and chemical
feedstocks (Adams et al, 2018). The optimal use and applications of
renewable energy resources and technologies can potentially minimize
waste generation, enhance energy access and security, promote sustainable
development, and above all, redress the current environmental concerns
associated with fossil fuel use and dependency (Panwar et al, 2012). Biomass
energy resources are utilized in two ways: directly through combustion and
indirectly by transforming them into solid, liquid, and gaseous fuels or
intermediate energy carriers (Sharma et al, 2015). A variety of techniques,
categorized into biochemical, thermochemical, biotechnological, and
physiochemical, are currently being employed to convert biomass into
various energy forms heat, power, fuels, their conventional conversion to
energy is regarded as carbon neutral. However, the harvesting of biomass
for energy purposes seemingly may be unsustainable as it depletes soil
carbon and nutrients, leading to a reduction in soil productivity (Hrncic et al,
2016). Recent studies (Muh & Tabet, 2019) indicate that roughly 3 billion
people globally rely on traditional biomass for heating, lighting, and cooking,
resulting in the annual premature death of about 3.1 million due to indoor
pollution mainly in developing countries. In addition, about 2.7 billion people
lack clean cooking facilities in sub-Saharan Africa and Asia-Pacific,
necessitating improved cooking facilities through biogas from anaerobic
biomass processing (Muh et al, 2018). The abundant and sustainable
availability of biomass feedstocks is continuously being dominated and
challenged by biomass-derived wastes and residual materials with the
inclusion of municipal solid waste, which when unprocessed causes serious
problems (Hrncic et al, 2016). The use of biomass wastes (agricultural,
domestic, etc.) as bioenergy feedstocks compared to energy crops, will
among others reduce GHG emissions, stress on land usage, as well as other
associated impacts, conflicts and ethical issues arising from food crops
usages. This stands as a strong booster for bioenergy production and in the
sustainability of biofuels and chemicals feedstocks (Adams et al, 2018). The
optimal use and applications of renewable energy resources and
technologies can potentially minimize waste generations, enhance energy
access and security, promote sustainable development, and above all
redress the current environmental concerns associated with fossil use and
dependency (Panwar et al, 2012). Biomass energy resource is utilized in two
ways: directly through combustion and indirectly by transforming it into
solid, liquid and gaseous fuels or intermediate energy carriers (Sharma et al,
2015). A variety of techniques, categorized into biochemical,
thermochemical, biotechnological and physiochemical, are currently being
employed to convert biomass into various energy forms (heat), chemicals
and other value-added products (Pande & Bhaskarwar, 2012).

The thermochemical processes make use of heat energy and chemical

catalysts to decompose biomass into valuable energy-rich products, while
the physical methods use densification techniques that include crushing,
heat and pressure applications, to convert biomass into biofuels. Also, the
biochemical processes utilize naturally occurring micro-organisms and
enzymes to process biomass into desirable energy products. Biomass direct
combustion primarily produces thermal energy, used for electricity
production, in combined heat and power (CHP) systems or remotely for
cooking and heating especially in developing countries. The indirect
processing (gasification and pyrolysis) gives rise to mainly liquid fuels
(biofuels), chemicals, charcoal, electricity and CHP generations with internal
combustion engines, turbines, and boilers. The liquefaction process
(hydrothermal) directly converts biomass into crude oil for use as premium
fuels after upgrading or for heat and power generation (Sharma et al., 2015).
Given that the required infrastructural developments for electric vehicles or
hydrogen may last longer, biomass energy provides a huge potential for oil
substitution in the transport sector (Hayes, 2013). The development of new
processes is very crucial for the assurance of a smooth transition to
renewable carbon sources in the future. As such, pathways for the
generation of bio-derived chemicals and alkanes required for the production
of various transport fuels, chemicals and materials are urgently needed.
Alkanes (linear, branched and cyclic), mainly obtained from fossils are vital
chemical intermediates and end products of the chemical industry, and
biomass is an important alternative for the production of these chemicals
and fuels (Deneyer et al, 2015).

Recent studies (Marshall & Alaimo, 2010; Serrano-Ruiz et al, 2011; Bond et
al, 2013) have shown that several value-added chemicals (halocarbons,
aromatics, alkanes, arenes, alkenes, ethers and epoxides, alcohols and
phenols, carbonyls, and many other bifunctional organic species), energy
and transport fuels can be derived directly from biomass or indirectly as
biomass-derived intermediates via both thermochemical and biochemical
pathways. However, more research is still required to optimize these
processes to make them commercially viable for large-scale applications.
These processes will in the nearest future become competitive with a
continuous decrease in fossil supply and their rising costs. The transition to
biomass-based economy is presently being demonstrated by the current
multidisciplinary drives towards the development of biorefineries that can
successfully transform biomass to chemicals, high-volume commodities, and
platform molecules. This is challenged by the lack of novel separation,
refinement and transformation techniques that will maximize feedstocks
conversion, thereby minimizing waste and promoting large-scale applications
(Bond et al, 2013).

This study presents a comprehensive review of the thermochemical

processes for biomass conversion to fuels, chemicals, and vital material
products, with special reference to the concept of the biorefinery as well as
recent developments in this area. Considering the rapid progress so far in
biomass conversions into energy, this review concentrates mainly on the
available thermochemical routes of biomass transformations into fuels and
value-added chemicals, with a special focus on the role of catalysis in these
conversions. Recent developments and future prospects of these conversion
processes are also discussed. Details about reactors technologies and
designs are not covered in this review. The depletion of fossil fuel reserves,
coupled with environmental concerns over greenhouse gas emissions and
plastic pollution, has intensified the search for renewable alternatives.
Biomass, as a carbon-neutral resource, presents a viable feedstock for
producing biofuels and bioplastics through thermochemical conversion.
Unlike biochemical methods (e.g., fermentation), thermochemical processes
offer faster reaction rates, higher scalability, and the ability to process
diverse feedstocks, including non-edible biomass.

1.1 BIOMASS RESOURCE, CONSTITUENTS, AND CHEMISTRY.

The Biomass Resources Biomass is generally referred to any mixture of

hydrocarbon material comprising carbon, hydrogen, and oxygen, with small
amounts of nitrogen, sulphur and some minerals (Demirbas, 2001; Demirbas,
2010; O’Connor, 2013). This very important and versatile renewable
resource is an indirect source of solar energy and stores its solar energy in
the form of chemical energy via photosynthesis (Demirbas, 2001; Pande &
Bhaskarwar, 2012). Biomass covers a broad range of plant and plant derived
materials, including biodegradable wastes. It comprises of all biological
materials, including biological waste or dead biomass. They include:
agricultural crops and residues, forestry crops and residues, wood and wood
wastes, sewage, municipal solid waste, industrial residues, animal residues,
food processing wastes, dedicated crops and residues, seaweeds, algae and
aquatic plants (Balat et al, 2009; Balat et al, 2009; Baruah & Baruah, 2014),
and these biomass resources are divided into the following categories:
wastes, standing forests, agricultural residues, and energy crops and aquatic
plants (algae, water weeds, water hyacinth, reeds and rushes) (Demirbas,
2001; Panwar et al, 2012). The energy crops include corn, sugar cane,
grains, sorghum, sugar beets, elephant grass, seaweeds (kelp) and many
others. The choice of an energy crop for energy production is dependent on
the crop’s dry materials yield per unit of land (hectare). Higher yield reduces
land requirements, thereby lowering the production cost of the biomass
energy (Demirbas, 2010). Virgin biomass has vast global energy potential.
The world’s terrestrial standing biomass carbon is estimated to around a
hundred times the total annual global energy consumption. Forest biomass
comprises between 80% to 90% of the total biomass carbon. Marine biomass
carbon (with least natural abundance) is highly concentrated in oceanic and
marshy land environments and is projected to precedes forest biomass
carbon with respect to the net annual energy production (Zhang et al, 2010).
Biomass is the only renewable carbon source capable of being converted into
convenient solid, liquid and gaseous fuels, as well as added-value chemicals.
Woody biomass is the oldest form of energy used by humans and is applied
traditionally in many parts of the world (especially in the poorer countries)
through direct combustion for heating and cooking or indirectly through
conversion into liquids or gaseous fuels. However, biomass combustion
produces pollutants such as CO2 (the major component of greenhouse
gases), SO2 and NO2 (both constituents of acid rain) and dust. The amount
of these pollutants released from biomass combustion is far less compared
to that emitted from fossil combustion (Panwar et al, 2012). The majority of
biomass energy is produced from wood and wood wastes (64%), municipal
solid wastes (24%), agricultural wastes (5%) and landfill gases (5%)
(Demirbas, 2010). Biomass energy can be classified as traditional or modern
biomass. Traditional biomass includes fuelwood and charcoal, animal wastes,
rice husk, and plant residues, used in small scales in developing countries.
Modern biomass consists of wood and agricultural wastes, urban wastes, and
biofuels (biogas, energy crops, etc.) and they are involved in large-scale uses
aimed at substituting for the conventional energy sources (Panwar et al,
2012). The biomass resource can be processed into three major end
products: transport fuels, chemical feedstock, and power/heat generation
through two principal conversion technologies: thermochemical and
biochemical pathways. Among the four thermochemical technologies,
biomass gasification has a great future prospect for renewable chemical
production and for power generation via internal combustion engines or
turbines (Damartzis & Zabaniotou, 2011). Good knowledge of the
physicochemical properties of biomass is instrumental for its sustainable use
for energy and chemical production. Parameters such as chemical
composition, moisture content, the content of ashes and inorganic
substances account for the diversity and disparities among biomass species.
The principal elements present in biomass in increasing order of abundance
are: manganese, sodium, chlorine, iron, Sulphur, aluminium, magnesium,
silicon, potassium, calcium, nitrogen, hydrogen, oxygen and carbon (Sharma
et al, 2014). Biomass utilization is mainly challenged by its low energy
density, inconvenient form, and bulkiness. As such, the handling, storage,
and transportation of raw biomass are costlier compared to fossils.
Therefore, to optimally and fruitfully valorise biomass, an improvement of
the biomass properties that enhances its handling, storage and transport is
very paramount. Biomass conversion technologies, therefore, seek to elevate
these properties and ameliorate these prominent setbacks of biomass
utilization (Adams et al, 2018). When compared to fossil fuels, biomass-
derived fuels have low heating values due to their high moisture and oxygen
contents, low energy density, high content of volatiles (about 80%), high
ignition stability, high density due to the presence of oxygenates, acidic,
corrosive and are very viscous (Sharma et al, 2015).

1.3 Biomass Chemistry and Conversion

Biomass (terrestrial) consists mainly of the following biological molecules:
carbohydrates (sugars, cellulose, and hemicellulose), proteins, lignin and
lipids (fatty acids, oils), extractives, starches, water, ash, hydrocarbons and
other compounds (Hayes, 2013; Marshall & Alaimo, 2010). Lignin, cellulose,
and hemicellulose made up the bulk of a biomass species whereas sugars
occupy less than 30% and lipids (oil) less than 10% of a bulk biomass
species. Lignin and carbohydrates are usually regarded together as
lignocellulose biomass (Marshall & Alaimo, 2010). Biomass feedstocks vary
greatly with respect to their chemical constituents and physicochemical
properties, and these have a strong influence on their appropriate choice of
conversion technologies (Adams et al, 2018). Agricultural wastes are known
to possess very high energy and chemical content as they have a higher
composition of organic constituents like cellulose, hemicellulose, lignin and
trace amount of other organic compounds or polymers (Demirbas, 2001).
The conversions of biomass vary and are dependent on the desired end
products (energy, fuels or chemicals) and biomass streams available (Brown,
2014). Whereas carbohydrates (starches and sugars) are usually converted
through biological fermentation into ethanol, lipids (fatty acids) are often
transformed into biodiesel through esterification (transesterification) with
methanol or ethanol. The sugars from photosynthesis are usually
metabolized into lipids, proteins, and lignin. Cyclic carbohydrate (glucose)
are often converted into fatty acids with long-chain hydrocarbons having
high energy contents and excellent liquid properties (Marshall & Alaimo,
2010). Alternatively, solid biomass is often converted into synthetic gas (CO
+ H2), which is later transformed into liquid fuels and other chemicals
through the Fischer-Tropsch (FT) process, requiring several complex steps,
energy, and capital investment. Direct liquefaction provides a simpler and
more robust solid biomass to liquid (BTL) conversion. Several BTL opment
(Marshall & Alaimo, 2010). The choice of biomass as a source of energy
depends on its moisture content, calorific value, fixed carbon and volatile
matter content, alkali metal content, ash, and residual matter content, and
the cellulose/lignin ratio (Demirbas, 2010). The conversion of lignocellulosic
biomass to fuels and chemicals is often difficult, expensive and low-yielding
due to the complexities and the exact chemistry of the lignocellulose
biomass polymers. Some of the processes available for the processing of
lignocellulosic biomass, together with their products and possible
pretreatments steps are illustrated in Fig. (1) (Adams et al, 2018). The
worldwide distribution, vast abundance and the renewable nature of biomass
have led to growing interest and the devoted efforts currently employed for
organic chemical production from various biomass streams, with particular
focus on sugar conversions to value-added chemicals. The most suitable of
the biomass sugar feedstocks used as chemical precursors or intermediates
are the hexoses, especially glucose and D-fructose (Tong et al, 2010).
1.4 Biomass Composition:
Biomass is generally composed of organic and inorganic constituents and a
fraction of water (Sharma et al, 2015). The development of processes for the
generation of fuels and chemicals from biomass is strictly dependent on its
chemical structure and the basic organic constituents (Demirbas, 2010).
Biomass is composed of three main groups of naturally occurring polymeric
materials on a dry mass basis. These are cellulose (about 50%),
hemicellulose (10-30% in woods and 10-40% in herbaceous biomass), and
lignin (20-40% in woods and 10-40% in herbaceous biomass). It also consists
of other constituents such as inorganic minerals compounds like the alkali
metals compounds (potassium, calcium, sodium, silicon, phosphorus,
magnesium and chlorine in herbaceous biomass) and extractives (usually
smaller organic molecules and polymers such as proteins, salts, and acids).
However, the quantities of these inorganic compounds vary from one
biomass species to another (wood (less than 1%), herbaceous biomass
(15%), and 25% in agricultural and forestry residues) (Demirbas, 2010).
Lipids, simple sugars, proteins, water, starches, hydrocarbons, ash, and
other compounds are equally present and these components are mainly
found in the trunks, foliage, and barks of plants (Demirbas, 2010). The
distributions of the three major biomass constituents are also species-
dependent, with very prominent disparities between hardwoods and
softwoods. The deciduous woods (hardwoods) have a relatively higher
amount of cellulose, hemicellulose, and extractives compared to the
softwoods which only have a higher lignin content. Overall, hardwood is
made up of around 43-47% cellulose, 25-35% hemicelluloses, 16-24% lignin,
and 2-8% extractives, whereas softwood is compose of about 40-44%
cellulose, 25-29% hemicellulose, 25-31% lignin and 1-5% extractives
(Demirbas, 2010; Marshall & Alaimo, 2010). As discussed by Demirbas
(2010) and Tong et al. (2010), biomass is composed of the following major
elemental composition on the dry mass basis: oxygen (30-40%), carbon (30-
60%), and hydrogen (5-6%) depending on the ash content. Sulphur, chlorine,
and nitrogen make up less than 1% of the biomass. These elemental
composition in increasing order of abundance are: Al, Mg, Si, K, Ca, N, H, O,
and C. The typical levels and description of the main biomass constituents
are summarized in Table 1 (Sharma et al, 2015). The major biomass organic
constituents (cellulose, hemicelluloses, and lignin) and biomass elemental
composition are characterized using the proximate and ultimate analysis
(Sikarwar et al., 2017). Carbohydrates are polyhydroxy organic compounds
generally represented elementally as (CH2O)n. They have a uniform carbon
of about 40%, far less than in hydrocarbons. However, their oxygenated
nature gives them superior physicochemical properties that enhance their
conversion and utilization. Carbohydrates are classified as monosaccharides,
disaccharides and oligosaccharides, and polysaccharides.

Table 1. Basic levels and description of the main biomass organic

constituents [25].
Componen Percent Dry Description
t Weight (%)
Cellulose 40-60 High molecular weight (≥106) linear glucose
chain linked by β-glycosidic bonds. The chain is
stable and resistant to chemical attack
Hemicellulo 20-40 Highly branched short sugar chains (pentoses (D-
se xylose, L-arabinose) and hexoses (D-galactose,
D-glucose, and D-mannose) and uronic acid.
They have lower molecular weight than cellulose
and are relatively easy to hydrolyzed into basic
sugars.
Lignin 10-25 A biopolymer rich in three-dimensional, highly
branched polyphenolic constituents that provide
plants structural integrity. Amorphous with no
exact structure. More difficult to be dehydrated
compared to cellulose and hemicellulose.

First-generation biofuels feedstocks mainly comprise starch and sugar,

whereas cellulose and hemicellulose carbohydrates constitute the second-
generation biofuels feedstocks (Adams et al, 2018). Cellulose and
hemicellulose make up the carbohydrate portion of biomass, whereas the
non-carbohydrate portion is composed of lignin (Demirbas, 2010; Tong et al,
2010).
1.5 Catalysis in Biomass Conversion
Organic catalysis (enzymatic catalysis) has traditionally been used in
biomass processing. However, inorganic catalysis used in the conversion of
raw biomass feedstocks to valuable products is to some extent more
economically performant (Marshall & Alaimo, 2010). Acid catalysis is widely
applicable in biomass valorisation owing to their superior ability towards
molecular deoxygenation through multiple chemical reactions (Bond et al,
2013).
Catalysts play a key role in biomass processing either in promoting the
conversion processes or in the upgrading of the conversion products (liquids
or gases) into high-value fuels or chemicals. However, their nature and
action vary, depending on the conversion technology being used.
Heterogeneous catalysts play a central role in fossil conversion to fuels,
power, and chemicals. However, their role in biomass processing is still
unclear, given the diametrically opposing chemical nature of both resources.
Zeolites over the past years have shown great potential for use in biomass
valorisation, especially in the conversion of lignocellulosic biomass to fuels
and chemicals. They play a key role in the conversion of oxygenates to
hydrocarbons, catalysing reactions like dehydration, esterification,
decarboxylation, and acylation. Thus, the use of zeolite catalysts in biomass
processing is found to be a promising alternative method for the production
of transport fuels and chemicals (Taarning et al, 2011). However, the design
of novel catalytic routes for the selective, efficient, and direct conversion of
biomass feedstocks for the production of targeted chemicals is a major
challenge in the field of biochemicals (Chen & Wang, 2017).
The transesterification process for biodiesel production is usually catalyzed
by both homogeneous and heterogeneous catalysts. Heterogeneous
catalysts (acid or base) like alkali metal oxides and derivatives, transition
metal oxides and derivatives, alkali earth metal oxides and derivatives,
mixed metal oxides and derivatives, sulfated oxides, ion exchange resins,
carbon-based catalysts, enzyme base catalysts, boron-based catalysts,
waste material-based catalysts, have all been reported in literature recently
and their applications in lab-scale biodiesel production. They are highly
active, selective, and water-tolerant depending on the amount and strength
of the active acid or basic sites (Chouhan & Sarma, 2011).
However, the role of heterogeneous catalysts in the production of biodiesel
and bioethanol is limited due to their production pathways (bacterial
fermentation and transesterification using homogeneous bases).
Heterogeneous catalysis is vastly applicable in the production of advanced
biofuels (high energy density and infrastructural compatibility). This involved
some relevant catalytic routes like pyrolysis/gasification accompanied by
catalytic upgrading, aqueous-phase processing of sugars and platform
molecules, and hydrotreating of vegetable oils and related feedstocks (Chen
& Wang, 2017).
Catalysts also play a fundamental role in the gasification products. They
improve the gas quality, conversion efficiency, and reduce the tar content.
Dolomite, alkaline metal oxides, and Ni-based oxides are widely used
gasification catalysts. Iron, cobalt, ruthenium, and potassium-based catalysts
are well known for their role in the Fischer-Tropsch (FT) synthesis of diesel,
hydrocarbons and other liquid fuels from bio-syngas. However, the
performance of these catalysts varies and their choice is determined by their
properties, reaction pathways, and the desired end product (Sikarwar et al,
2017).
Recently, catalysis has become one of the principal processes required for
the achievement of sustainable chemicals and energy production. This is
evident from the production of furan derivatives in the past decades from
biomass sugar, used as substitutes for oil-derived chemicals or as starting
materials for new product synthesis like vital polymeric materials,
pharmaceutical agents (fungicides), liquid fuels or solvents, macrocyclic
ligands (Chen & Wang, 2017; Tong et al, 2010). Despite the great and
intriguing progress made in the catalytic transformation of biomass sugars
into essential chemicals and chemical precursors, there is still an urgent
need for further research to enhance the selectivity of the catalysts and to
improve their conversion efficiencies in order to fully commercialize those
processes. Also, further catalytic progress for biomass processing should be
focused on the rapid, economical, and environmentally benign production of
organic chemicals with recent developments and the concepts of green
chemistry. In addition, priorities in catalytic chemistry should be given to
multi-purpose catalysts originating.

Table 2. Classification of biofuels [3, 19].

Biofuel Description Examples
type
First Produced from raw Bioethanol from sugar cane, sugar beet and
generatio materials competing with starch crops (corn and wheat)
n biofuels the food and feed industry • Biodiesel from oil-based crops (rapeseed
sunflower, soyabean, palm oil, and waste
edible oils
• Starch-derived biogas
Second- Produced from non-food • Biogas from waste and residues
generatio crops (energy crops), or • Biofuels from lignocellulosic materials
n biofuels waste residues • Biofuels from energy crops
Third- Produced from aquatic • Algal biodiesel
generatio microorganisms like algae • Algal hydrogen
n biofuels
Fourth- Biofuels based on high solar • Carbon-negative technology
generatio efficiency cultivation • Technology of the future
n biofuels
from the incorporation of transition metals with acid/base solid catalysts.
This will permit several reaction steps to be conducted in a single reactor,
avoiding the costly intermediate separation processes. Moreover, catalysts
recycling and the efficient recovery and separation of the targeted products
are also very instrumental in biomass catalytic conversion and in catalytic
studies as a whole. However, these are challenged due to lack of perfect
understanding of the catalysts structure-property relationships and exact
biomass conversion reaction mechanisms, multi-purpose catalysts and their
suitable solvent systems, process compositions, catalyst optimization, and
development, especially for large-scale production systems (Tong et al,
2010).

2.0 BIOMASS CONVERSION TECHNOLOGIES

Overview of Biomass Conversion Processes
Biomass conversion processes mainly achieve two goals: energy production
and environmental clean-up. Two major pathways are used for biomass
processing: the biological (biochemical) and the thermochemical pathways.
The thermochemical pathways usually have higher efficiencies than the
biological processes due to their low reaction times and superior ability to
destroy the organic constituents of biomass (Demirbas, 2010; Sharma et al,
2015).
The feedstocks for biomass conversion can be highly variable in terms of
mass and energy density, moisture content, size, and intermittent supply. As
such, modern industrial technologies are often hybridized with fossil fuel
such that in case of biomass supply irregularities, the fossil fuel is used for
preheating, drying and in the maintenance of fuel supply (Zhang et al, 2010).
The pathways for biomass conversion into various energy forms vary. The
choice of an appropriate conversion process depends on the biomass
quantity, type, desired energy or chemical end product, the feedstock
characteristics, economic conditions, policy, and environmental standards
(Brown, 2014). Among these, the dominant factors are the desired energy
form and feedstock availability.
Three principal process technologies govern biomass conversion into energy,
fuels, and chemicals. These are the biochemical, thermochemical, and
physiochemical processes. The biochemical pathway consists of two main
processes (anaerobic digestion and fermentation), the thermochemical
pathway consists of four major processing options (combustion, gasification,
pyrolysis, and liquefaction), and the physiochemical route mainly consists of
extraction, followed by esterification, whereby oils are obtained from the
crushing of oilseeds (Brown, 2014). Through these conversion processes,
biomass can be used to produce different forms of energy (heat, electrical,
chemical energy or fuels). Its energy density is often upgraded through the
production of fuels such as liquid fuels (transport fuels), charcoal, gaseous
fuels (H2, biogas, producer gas). The bioconversion processes of biomass
mainly give rise to biofuels (bio-alcohols, bio-dimethyl ether, synthetic
natural gas (bio-methane), Fischer-Tropsch fuels, and hydrogen), broadly
classified into four groups (Table 2) depending on the biomass feedstock
used (Balat et al, 2009; Demirbas, 2010).
Biomass is mainly utilized through combustion for heat and power
generation, conversion into gas-like fuels (CH4, H2, and CO), or conversion
into liquid fuels (biofuels). Presently, biomass energy contributes about 14%
of the total global energy consumption, compared to 12% from coal, 15%
from gas, and 14% from electricity (Marshall & Alaimo, 2010).
The first-generation biofuels, with commercialized technologies, are primarily
produced with food or feed raw material feedstock like simple sugars, fats,
starch, and vegetable oils. The controversy of food versus energy lead to the
development of second-generation biofuels, produced mainly from
lignocellulose biomass such as non-feed crops, forest residues, domestic,
agricultural, and industrial wastes. Although the second-generation biofuels
overcame the controversial food versus fuel challenge of the first-generation
biofuels, they still require vast arable land to cultivate the feedstock crops.
Thus, depriving food cultivation and indirectly imposing the same
controversy with the first-generation biofuels. These issues are addressed
with the third-generation biofuels that use algae and seaweeds which are
grown on unproductive land, marshy land, and sea waters. These
technologies are still under development. The fourth-generation biofuel
technologies are still at the conceptual stage. They are intended to be
produced by technologies that will successfully transform biomass into fuel
in a way that the CO2 consumed in their generation is larger than that
produced during their use or combustion. As such, these biofuels would be
very vital in mitigating climate change through the reduction of atmospheric
GHGs. They result from genetically engineered algae, a carbon-negative
energy resource, with enhancing hydrocarbon yields, giving rise to an
artificial carbon sink (Balat et al, 2009; Demirbas, 2010).
The second-generation biofuels are mainly produced via biochemical and
thermochemical methods from lignocellulosic materials. Thermochemically,
the process consists of gasification and/or pyrolysis, followed by the
processes for gas cleaning and conditioning and finally the FT synthesis to
generate synthetic liquid fuels. Biochemically, the cellulose and
hemicellulose components of biomass are first broken down enzymatically
into their constituent sugars, which are then fermented to bioethanol. This
pathway is less prone to commercialization than the former but shows great
future cost reduction potentials (Patel et al, 2016). Whereas the
thermochemical processes occur at higher efficiencies with short reaction
times and have the ability to completely decompose vast organic compounds
in biomass, the biological processing shows low efficiencies with higher
reaction times (days, weeks, etc.) and is unable to completely breakdown
most organic compounds in biomass such as lignin (Brown, 2014). The next
sub-sections of this paper present an overview of the current biomass
thermochemical conversion routes for fuels and chemical production -
pyrolysis, gasification, and hydrothermal processing (precisely hydrothermal
liquefaction).
2.1 The Thermochemical Conversion Pathways
Thermochemical conversion pathways are commonly used to convert
biomass into higher heating value fuels, reducing the biomass oxygen
content to increase their energy density as well as increasing the weight of
the final hydrocarbon fuel via the creation of carbon-carbon bonds
(Demirbas, 2010). These processes do not solely produce useful energy from
biomass directly, but also convert biomass into more convenient, easily
transportable, and more energy-dense forms of energy carrier (producer gas,
oils, alcohols, etc.) under controlled temperatures and oxygen conditions
(Zhang et al, 2010). They are dependent on the biomass feedstock type,
physicochemical properties, and the process operating conditions, and these
affect process conversion time, product distribution, and quality (Demirbas,
2010).

The products of biomass thermochemical processing are usually classified as

a carbon-rich residual fraction and the volatile fraction (gases, vapor, and
tar) (Demirbas, 2010), and are generally in the form of solid, liquid, and
gaseous fuels with equal ecological and industrial importance (Demirbas,
2001). The thermochemical processing of biomass occurs via combustion,
gasification, pyrolysis, carbonization, and liquefaction processes, with
pyrolysis being a preliminary stage in all the processes. These processes
release the energy content of biomass by transforming it into solid
(charcoal), liquids (bio-oils) or gaseous fuels (synthetic gas) via pyrolysis,
gasification or liquefaction, and directly as heat through combustion and/or
co-firing. The most valuable of these technologies are those with liquid or
gaseous intermediate energy carriers with higher potentials of upgrading to
more energetic molecules or fuels (Brown, 2014; Patel et al., 2016). Among
these processes, gasification is the most efficient and cost-effective for
bioenergy generation from lignocellulose biomass (Demirbas, 2010).
These processes occur by varying their operating conditions, namely heating
rate, vapor residence time, reactor configuration, etc. The pyrolysis solid
product (biochar) is highly applicable in the sequestration of carbon through
soil management systems. The gasification process mainly generates flue
gas, together with syngas which is the feedstock for the production of
synthetic liquid fuels via the Fischer-Tropsch (FT) synthesis or burnt for heat
and power generation (Demirbas, 2010). The liquefaction process
(hydrothermal) focuses only on liquid fuels production under high pressures
in various solvents like water, acetone, methanol or their mixtures (Sikarwar
et al., 2017). The stages, processes, and products involved in biomass
thermochemical transformations are illustrated in Fig. (2) (Brown, 2014), and
their characteristics are summarized by Brown (2014) and Patel et al, (2016).
The thermal depolymerization and decomposition of the structural
constituents of biomass (cellulose, hemicellulose, and lignin) give rise to
chemicals in the form of liquids, gases, together with residual solid charcoal.
Pyrolysis oil comprises an array of chemicals such as cyclopentanone,
methanol (most valuable), methoxyphenol, acetone, acetic acid, phenol,
levoglucosan, furfural, guaiacol and their alkylated phenol derivatives, and
formic acid. The composition of these chemicals is mostly dependent on the
process heating rate and temperature. Lignin conversions give rise to
chemicals like syngas, methanol, dimethyl ether, ethanol, mixed alcohols,
C1-C4 gases, hydrocarbons, Fischer-Tropsch liquids, styrenes, oxygenates,
phenol, cyclohexane, biphenyls, substituted phenols, cresols, catechols,
eugenols, resorcinols, syrinols, guaiacols, vanillin, vanillic acid, aromatic
acids, aliphatic acids, quinones, syringaldehydes, aldehydes, cyclohexanal,
beta keto adipate, cyclohexanol/al, toluene, benzene, xylene and derivatives,
higher alkylates, substituted lignins, drugs, mixed aromatic polyols, carbon
fiber, fillers, and so on (Marshall & Alaimo, 2010).
2.2 Pyrolysis
Pyrolysis refers to the thermal breakdown of biomass (mainly lignocellulosic
biomass) to liquid (tar or bio-oil), carbon-rich solid (char or biochar) and a
mixture of non-condensable gases in the absence of oxygen at elevated
temperatures (Sharma et al, 2015). Gases result when the process
temperatures are high with longer residence times, and longer vapor (hot)
residence times. Lower process temperatures favor charcoal production
whereas short vapor residence times and lower temperatures are best for
liquid production. The proportion of these three key products produced
during a pyrolysis process is dependent on the process condition or how the
process conditions are varied (Brown, 2014). This process generates fuels
with high fuel-to-feed ratios, and this makes it the most efficient biomass
conversion route. In addition, the optimization of the high-value fuel products
via thermal and catalytic methods is crucial.

Table 3. Biomass pyrolysis modes, process conditions, and product

distribution (P. Adams, et al 2018)

Mode Conditions Product

Distributio
n
Liquid (wt. Solid (wt. Gas (wt.
 %) %) %)
Fast -500°C. Shot 75 12 13
hot vapor
residence
time <2s
Intermediate 50(2 phases) 25 25
-500°C.
Moderate
hot vapour
residence
time 5-30s
Carbonization(slow 30 (2 35 35
) -400°C. Long phases)
hot vapour
residence
time(hours
to days)
1 96
Gasification(allothe -750- 3
rmal) 900°C.Moder
ate hot
vapour 80 20
time>5s 0, unless
Torrefaction (slow) -280°C. Solid vapours are
residence condensed,
time, -10- then up to
60mins 15%
pathways is the main objective of this process (Demirbas, 2010). In a
nutshell, pyrolysis offers a broader perspective and opportunities for biomass
valorization than the other thermochemical conversion processes and is
broadly classified into two groups (fast and slow) depending on the operating
conditions (Demirbas, 2010). Pyrolysis plays a fundamental role in reactors
design, reaction kinetics, and in the determination of product distribution,
properties, and composition, in all the viable thermochemical biomass
conversion processes (Demirbas, 2010). Generally, four key pathways
generate transport fuels via pyrolysis: slow pyrolysis and syngas upgrading,
fast pyrolysis and hydroprocessing, catalytic pyrolysis and hydroprocessing,
and hydropyrolysis and hydroprocessing (Brown, 2014). Hydroprocessing
refers to the integrated hydrocracking and hydrotreating processes. The
feedstock chemical composition and the process temperature strictly
determine the quantity, types, and quality of the resulting products
(Demirbas, 2001). Table 3 highlights the various types of pyrolysis, process
conditions, and product distribution (Brown, 2014). The end products of
pyrolysis originate both from the primary breakdown of the solid biomass
species and the secondary reactions of the condensable volatile organic
products into char, lightweight gases, and secondary tar. Contrary to
combustion and gasification that occur at higher temperatures, pyrolysis is
conducted at lower temperatures, ranging from 400-700°C, and are found to
be much lower in biomass species containing metallic compounds. Also, it
occurs at lower pressures (0.1-0.5 MPa) compared to higher pressures
ranging from 10 MPa to 25 MPa with hydrothermal liquefaction (Demirbas,
2010). Temperature is a major determinant for the distribution of pyrolysis
products. The products are mainly produced between 352°C-452°C. At
higher temperatures, the gaseous fraction is enriched with lighter molecules
through the breakdown of heavier molecules found in the liquid and residual
solid. High liquid production arises at high temperatures, high heating rate,
and short residence time, while high residence time, low heating rate, and
low temperatures favor char production. Increasing temperatures reduces
charcoal yield. The fuel gases are produced under high temperatures, long
gas residence time, and high heating rate (Demirbas, 2010). Heat is usually
provided externally to thermally crack the biomass constituents into gases
and vapor, which through secondary reactions give rise to the broad product
spectrum (Brown, 2014). A number of factors affect biomass pyrolysis
performance, product types, distribution, and quality. These include:
feedstock type, temperature and heating rate, volatiles residence time and
pressure, particle size, shape and orientation, reactor configuration,
catalysts, additives, and physicochemical properties such as thermal
conductivity and emissivity, permeability and density, specific heat capacity
and heat of reaction, particle shrinkage and moisture content, and the
external heat transfer coefficient. The outcome of any pyrolysis process,
therefore, depends on how these parameters are varied (Demirbas, 2010).
The much interest in pyrolysis arises from its operational flexibility,
technology versatility, and its adaptability to a wide range of biomass
streams and products (Brown, 2014).

Fast pyrolysis (thermolysis) operates under moderate temperatures of about

500°C and is characterized by high heating rate, short hot vapor residence
time (<2s), and overall short reaction time. Here, the biomass is broken
down swiftly to mainly vapor and aerosols, with some charcoal and gas.
Thereafter, the vapor is cooled and condensed into a dark brown
homogeneous liquid (Brown, 2014). It favors the production of liquid
products but inhibits solid char formation (Sharma et al, 2015). Its feedstocks
generally have small particle size, with a provision in the system for the
rapid removal of the vapor to prevent further contact with the hot solid
particles. The reactor systems for fast pyrolysis include: fluidized bed,
ablative systems, vacuum pyrolysis systems, and the stirred or moving beds
reactors (Demirbas, 2010). Fast pyrolysis is currently of great commercial
interest as it gives rise to liquids which can be stored, transported, used as
energy, efficient energy carriers, chemicals, fuel precursors, and transport
fuels. It produces about 60-70wt.% of liquid bio-oil, 15-25wt.% of solid char,
and about 10-20wt.% of non-condensable gases. To maximize liquid
production, the process is kept under low temperature, high heating rate,
and short gas residence time, and to maximize gas production, high
temperature, long residence time, and the low heating rate is required
(Demirbas, 2010). However, with rapid heating and quenching rates,
intermediate pyrolysis liquid results which condense prior to the further
decomposition of its higher weight constituents into gaseous products
(Demirbas, 2010; Demirbas, 2001). The key features of fast pyrolysis for
liquid productions (high yields) are (Demirbas, 2010; Brown, 2014; Demirbas,
2001; Sharma et al, 2015):
 Particle size of less than 5mm for fast devolatilization and high heating
rates.
 Short vapor residence time (<2s) to minimize secondary reactions.
 A controlled reaction temperature of around 500°C to maximize liquid
yields.
 Feedstock moisture content of less than 10 wt.% as all the feed in
water settle in the liquid phase together with water from the pyrolysis
reactions.
 Very high heating and heat transfer rates at the biomass particle
reaction interface. This requires a finely ground biomass fed of less
than 3mm due to its low thermal conductivity. The particle heating rate
is the rate-limiting step in this process.
 Rapid char removal to minimize vapor cracking.
 Rapid cooling of pyrolysis vapor and aerosols to produce bio-oil.
As fast pyrolysis for liquids occurs in few seconds, mass and heat transfer
processes, reaction kinetics, and phase transition phenomena play a major
role. Thus, bringing the reacting biomass particles quickly to the optimum
process temperature and minimizing their exposure to lower temperatures
(charcoal production) and higher temperatures (thermal cracking) is very
important in this process (Brown, 2014). This is mainly achieved in fluidized
bed reactors. The reactor commonly used for fast pyrolysis includes bubbling
fluidized-bed, circulating fluidized-bed, ablative flow, entrained flow, rotating
cone, and vacuum reactors (Sharma et al, 2015). Fast pyrolysis is also
reported to produce hydrogen gas at elevated temperatures (700-1000°C),
through steam reforming and water-gas shift reactions. These reactions
mainly convert methane, simple aromatics, other hydrocarbon vapors (C2-
C5), and others, into hydrogen. Overall, steam reforming converts the
hydrocarbons into CO and H2, and the CO then combines with H2O to
produce H2 and CO2 via the water-gas shift reactions (Sharma et al, 2015). It
is a commercialized technology for chemical production but is still under
development for the production of liquid fuels. Compared to the other
thermochemical conversion processes, small scale fast pyrolysis processes
have higher energy efficiencies and relatively low investment costs
(Demirbas, 2010). Slow and intermediate pyrolysis focus on the production of
solid char with liquids and gases as by-products. Slow pyrolysis occurs under
gentle heating at low temperatures with longer vapor residence times and
larger particle sizes. It is a well-known process that occurs between 277-
677°C and often appears in the traditional charcoal kiln (Demirbas, 2001).
The agitated drum kilns, large retorts (continuous or batch), rotary kilns, and
screw pyrolyzers are commonly used slow pyrolysis systems (Demirbas,
2010). Slow pyrolysis has often been used to reduce the harmful effects of
waste on the environment as well as reduces waste quantities for disposal.
Also, pyrolysis has been used for centuries for the production of charcoal,
until recently (last 35 years) that fast pyrolysis for liquid production was
developed (Brown, 2014). The flash pyrolysis process operates at
temperatures within 777-1027°C to produce the petroleum equivalent of bio-
oil of up to 70% yield efficiency. This process also gives rise to pyrolytic
water, posing major setbacks to this process. This bio-crude oil can be used
directly as fuels in boilers, engines, and turbines or as refined fuels for heat
and power generation (Demirbas, 2001).

2.3 Pyrolysis Products and Uses

Biomass pyrolysis is a very prominent pathway for the generation of solid
(char), liquid (tar), and gaseous products which are potential alternative
energy sources or substitutes for petroleum fuels. The liquids may be used
directly as liquid fuels for boilers, diesel engines, and gas turbines for heat
and power production, added to the feedstock of the petroleum refinery, or
catalytically upgraded into liquid transport fuels. Other applications may
require its alkalis content to be removed and oxygen content lowered via
catalytic cracking and hydrogenation (Demirbas, 2010; Demirbas, 2001). It
mainly gives rise to a carbon-rich solid and volatile matter or flue gas. The
flue gas is generally used as solvents (acetone, methanol), hydrocarbons.

Table 4. Chemical constituents of fast pyrolysis liquid (M. Balat, et

al 2009).
Major Components Mass %
Water 20-30
Lignin fragments: insoluble pyrolysis 15-30
lignin 10-20
Aldehydes: formaldehyde,
acetaldehyde, 10-15
hydroxyacetaldehyde, glyoxal,
methylglyoxal
Carboxylic acids: formic acid, 5-10
propionic acid, butyric
pentanoic,hexanoic,glycolic(hydroxy 2-5
acetic) 1-4
Carbohydrates: cellobiosan, ɑ-ß- 2-5
levoglucosan, oligosaccharides, 1,6- 1-5
 anhydroglucofuranose
Phenols: phenol, cresols, guaiacols,
syringols
Furfurals
Alcohols: methanol, ethanol
Ketones: acetol (1-hydroxy-2-
propanone), cyclopentanone
and electricity productions. The bio-oil component is a major source of
chemicals like acetic acid and levoglucosan, which can be upgraded into
motor fuel or combusted for electricity generation. The produced char could
either be used as slurry fuel, soil enrichment agent, or used as activated
carbon in industrial applications (Demirbas, 2010). Pyrolysis plays a
fundamental role in reaction kinetics, reactor design, and in the
determination of product distribution, composition, and properties in all the
thermochemical conversion processes (Demirbas, 2001).
The pyrolysis liquids are dark brown viscous oils with well-known chemistry
and no exact precision on their exact quantitative constituents. They are
mainly composed of ketones, aldehydes, organic acids, phenols,
anhydrosugars like levoglucosan, pyrolytic lignin (guaiacyl- and a syringyl-
based fragment of the original polymeric lignin), and a significant amount of
water (about 25%). The pyrolysis liquid consists of two distinct phases: the
aqueous phase (higher methanol, acetic acid, and acetone ratios) comprising
a broad range of low molecular weight organo-oxygen compounds, and a
non-aqueous phase (tar or bio-oil) of high molecular weight insoluble
organics (aromatics) (Demirbas, 2010). Table 4 shows the chemical
composition of fast pyrolysis liquids (Demirbas, 2010).
The bio-oil fraction of the pyrolysis products is a liquid mixture of oxygenated
compounds that contains the carboxyl, carbonyl, and phenolic groups. Its
composition varies greatly from the petroleum-derived fuels and chemicals,
but are very similar to that of the original biomass, thereby posing a major
challenge to these pyrolytic oils or in its possibilities in substituting fossil-
derived fuels. Bio-oil is made up of about 20-25% water, 25-30% water-
insoluble pyrolytic lignin, 5-12% organic acids, 5-10% anhydrosugars, 5-10%
nonpolar hydrocarbons, and 10-25% of other oxygenated compounds. The
low energy density of bio-oils is attributed to their water content, which
reduces the oils flame temperatures, causes ignition difficulties, possible
injection difficulties, and premature evaporation of the oil when preheating.
They are very polar and readily absorb over 35% water, contrary to the non-
polar and insoluble petroleum oils. They also have a low heating value of 16
MJ/kg compared to 43 MJ/kg for conventional diesel fuel due to their
oxygenated nature and water content. Their moderately acidic nature (pH
range 2.5-3.0) contributes to the corrosive nature of the oils. Also, the kinetic
viscosities and densities of oxygenated bio-oils are much higher than their
petroleum counterparts (Demirbas, 2010).
Bio-oils are generally used as fuels in engines, boilers, turbines, and CHP
plants for heat and power generation, upgraded through thermal or catalytic
cracking into transport fuels, or can be used for the synthesis of valuable
chemicals and organic solvents. However, bio-oils' applications are so far
limited by their poor volatility, high viscosity, corrosiveness, and coking. The
range of chemicals that can be derived from the fast pyrolysis oils, their
minimum, and maximum weights are found in Demirbas's (2010) study and
it covers the following chemical groups: organic acids, ketones, alcohols,
aldehydes, esters, sugars, oxygenates, phenolics, hydrocarbons, and
steroids. These consist of a range of compounds like cyclopentanone, acetic
acid, methoxyphenol, methanol, acetone, phenol, levoglucosan, furfural,
formic acid, guaiocol, and their alkylated phenol derivatives.

2.4 Catalytic Pyrolysis

This occurs in the absence of air under moderate temperatures (about
500°C), high heating rate, and short residence time, to produce mainly
liquid, used as a source of fuel or valuable chemicals. This is the most
attractive approach to minimize the polymerization, corrosivity, low thermal
stability, and high viscosity challenges associated with the liquid pyrolysis
products, as well as facilitates the handling and treatments of these liquid
products (Demirbas, 2010).
Cracking reactions and the upgrading of biomass products are influenced by
the catalyst during a pyrolysis process, with strict dependence on the
catalyst type and reactor configuration. Various liquids and gaseous fuels
arise from the catalytic cracking of pyrolysis vapor over different catalysts.
Zeolites catalysts are particularly useful in the reduction of oxygenates
(which decreases the energy content) of the specific pyrolysis oil (Demirbas,
2010).
Catalysts are mainly used to facilitate the cracking of heavier molecules in
bio-oil to lighter ones, giving rise to a less viscous oil, less corrosive bio-oil
(by reducing the synthesis of carboxylic acids), and lastly to promote the
synthesis of high-value products (hydrocarbons) capable of raising the
heating value of bio-oil (Demirbas, 2010). Catalysts modify the pyrolysis
process as follows (Demirbas, 2010):
 May cause a remarkable decrease in the breakdown temperatures of
biomass constituents.
 Affects reaction networks (deoxygenation), as well as reduces
polymerization precursors (multifunctional phenols) for stabilizing bio-
oil.
 May lead to the release of CO, CO2, and H2O during decarboxylation,
decarbonization, and dehydration reactions.
 Promote coke formation via dehydration reactions (arises mainly from
catalysts high acidity).
Table 5 Properties comparison between diesel fuel and pyrolysis oil
(40°C and 25% water) (E. Taarning,et al 2011).
Physical properties Pyrolysis Oil Diesel Fuel
Moisture content 20-30wt% 0.1wt%
pH 2.0-2.5 -
Density 1.2kgL 4
0.94kgL-1
Elemental analysis (wt
%)
C 55-58 85
H 5-7 11
O 35-40 1
N 0-0.2 0.3
Ash 0-0.2 0.1
HHV as 16-19 MJKg -1
40MJKg4
produced
Viscosity 40-100 cp 180cp
Solids (char) 0.1-0.5 1.0
(wt %)
Vacuum distillation Up to 50wt% 1wt%
residue
2.5 Bio-oil Upgrading to Fuels and Chemicals
Biomass-derived bio-oil in its unprocessed form has very high moisture
content, low energy density, and no free flowing physical form, making it
unsuitable for direct use in engines. Therefore, these necessitated the need
for bio-oil cleaning and upgrading, to enhance its energy density, fuel
properties and remove any impurities present (Brown, 2014). Table 5
highlights the distinct characteristics of pyrolysis oil compared to diesel fuel
(Taarning et al, 2011).
Bio-oil is generally known for its viscosity, acidity, thermal instability, and its
high constituent oxygenated compounds. The production of a high-quality
bio-oil capable of substituting fossil fuels perfectly is often very challenging.
To overcome this challenge, the bio-oil quality needs to be enhanced either
at source before full production or via the product upgrading (Demirbas,
2010). The upgrading of bio-oil to quality transport fuels is done through the
following three principal routes: hydrodeoxygenation with a hydrotreating
catalyst (alumina supported sulphided CoMo or NiMo), zeolite upgrading, and
emulsions formation with diesel fuel. Also, steam reforming can be used to
convert char and bio-oil into syngas or H2 (Demirbas, 2010).
Bio-oil is often produced with impurities and therefore needs to be upgraded
to enhance its quality and purity. This can be done either physically,
chemically, or catalytically (Brown, 2014). The catalytic bio-oil upgrading
consists of the full deoxygenation of bio-oil and conventional refining,
achieved either by integrated catalytic pyrolysis or decoupled liquid phase
hydrodeoxygenation. The partial upgrading into intermediate products
compatible with refinery streams is also envisaged to exploit the benefit of
the economy of scale and the broad experience in the conventional refinery.
The main upgrading methods through refineries integration are
hydrodeoxygenation, catalytic vapor cracking (in-situ or ex-situ), and
gasification to syngas followed by the synthesis of alcohols or hydrocarbons
(Brown, 2014).
Hydrodeoxygenation involves the treatment of bio-oil with high-pressure
hydrogen at moderate temperatures (302°C-602°C) in the presence of
heterogeneous catalysts to form saturated C-C bonds via oxygen removal,
raising the fuel energy content and stability. Sulphided CoMo/Al2O3 and
NiMo/ Al2O3 are often used to hydrotreat industrial feedstocks (Demirbas,
2010). This requires a source of hydrogen, moderate temperature of about
400°C, and high pressure of about 20 MPa. Complete deoxygenation is often
challenging due to the presence of phenols in bio-oil. Full
hydrodeoxygenation produces a naphtha-like product, requiring conventional
refining to derived conventional transport fuels such as gasoline, diesel,
methane, LPG, and kerosene. The naphtha equivalent yield from biomass
without hydrogen provision is projected to about 55% in terms of energy or
25wt.%. With the inclusion of hydrogen from biomass gasification, this
reduces the yields to about 33% in terms of energy and 15wt.% (Brown,
2014).
The hydrogen supply should be renewable and sustainable. This can be done
through biomass gasification, CO shifting to H2 followed by CO2 scrubbing,
or steam-reforming to H2 of bio-oil or the aqueous phase from a phase-
separated product or H2 locally generated through the electrolysis of water.
External H2 supply is unlikely due to high transport and storage costs
(Brown, 2014).
Zeolite cracking/upgrading takes place under atmospheric pressure at
temperatures within 352°C-502°C in a closed process coupled to pyrolysis
and removes oxygen mainly as CO2 with trace amount of CO due to catalyst
coking. This improves the thermal stability of the oil. The process yield is
projected to around 18wt.% aromatics. The production of aromatics, a very
important base chemical through this process, is of great significance to the
chemical industry (Brown, 2014). Due to the undesirable effects of pyrolysis
oil that disfavors its direct use, the oil is often upgraded via conversion into
gasoline using zeolites (crystalline microporous aluminosilicate materials). In
this process, the oil vapors are passed through the catalyst between 300°C-
500°C to produce hydrocarbons and by-products like H2O, CO2, and coke.
Coke formation (on the zeolites) eliminates the coking effects of the
unprocessed bio-oil. The catalyst removes the oxygen content of the oil in
the form of water, CO2, and CO, depending on the class of organic
compound involved (Taarning et al, 2011). Zeolite cracking of bio-oil is highly
beneficial in that there is no requirement for H2, it takes place under
atmospheric pressure, which greatly reduces the operating cost, and the
process occurs under similar temperatures to those for the production of bio-
oil. The ZSM-5 zeolite catalysts with strong acidity, high activities, and shape
selectivities convert oxygenated oil to hydrocarbon mixture ranging from C1
to C10. The major drawbacks to this process include easy coking, low yield
(14-22.5%), and short catalyst lifespan (Demirbas, 2010).
Catalytic steam reforming occurs over a Ni-based catalyst at temperatures
between 752°C-852°C. It is a two-step process that includes the shift
reaction, and occurs as follows (Demirbas, 2010): (1) Bio-oil + H2O → CO
+H2 and
(2) CO + H2O → CO2 +H2
3.0 Gasification
The gasification process converts carbonaceous materials (coal, petroleum
coke, biomass, etc.) into combustible gases (CO, H2, CO2, CH4, etc.) and
small amounts of light hydrocarbons (Brown, 2014), in the presence of
gasifying agents, such as oxygen, air, steam, CO 2 or their mixtures (Sharma
et al, 2015; Patel et al, 2016), with impurities like nitrogen, sulphur, tars and
alkali compounds (Sikarwar et al, 2017). Bio-oil is produced as an
intermediate that gives rise to the final product, syngas. A catalytic or
chemical upgrading unit is usually coupled to the conversion unit to
transform syngas and/or bio-oil to potential biofuels and chemicals
(Demirbas, 2010).
Biomass gasification is a well-known technology that produces mainly
gaseous products at high temperatures through the partial oxidation of
biomass, alongside a small amount of tar and ash. The gas production is
optimized at higher temperatures and the process is often classified based
on the gasifying agent: steam, air, air-stream, steam-oxygen, oxygen-
enriched air, etc. (Sikarwar et al, 2017). Advanced gasification processes,
such as plasma gasification and the supercritical water gasification have
recently been employed for the decomposition of biomass into mainly H2, as
well as CO and CO2 for the plasma gasification process (Adams et al, 2018;
Brown, 2014; Sharma et al, 2015).
Biomass gasification occurs in gasifiers which vary based on their
hydrodynamics (precisely the manner of contact between the gasifying
agent and solid fuel), operating conditions (temperature, pressure, etc.), and
the gasifying agents (air, oxygen, or steam). The most common types of
gasifiers are fixed-bed (updraft, downdraft or cross draft) gasifiers, fluidized-
bed gasifiers, and the entrained flow gasifier. Among these gasifiers, the
most suitable are the fixed-bed gasifiers (Adams et al, 2018; Sharma et al,
2015; Sikarwar et al, 2017; Patel et al, 2016). The designs of these gasifiers,
however, depend on the energy requirement, fuel type and characteristics
(moisture, particle size, density, ash content, and toxicity), nature of oxygen
injection, and the combustion bed type (Demirbas, 2001; Demirbas, 2010).
The supercritical water gasification (SCWG) is a highly efficient gasification
pathway for H2 (with low tar) production from high moisture biomass species
like algae, manure, olive mill water, sludge, etc. (Demirbas, 2010). In this
subsection, design details, operation principles, performance characteristics,
and comparisons of biomass gasifiers are not included or discussed.
However, the gasification process, product transformations, purification, and
upgrading are highlighted and deeply discussed.

3.1 Principles of Biomass Gasification

This process converts organic feedstocks (solid fuel or liquid) into gases
depending on the process temperature. Syngas is primarily produced at
extremely higher temperatures (>1200°C) and at lower temperatures, a
gaseous mixture comprising CO, H2, CH4, and CO2, with small amounts of
tar and ash. Tar and ash are potential by-products of gasification and have
adverse effects on the process performance and downstream end-uses
(Brown, 2014). Char (the unconverted organic fraction and the inert material
found in the treated biomass) production arises from the partial oxidation of
carbon in the feedstock material in the presence of gasifying carriers (air,
oxygen, steam, or CO2), and is largely a mixture of ash and the unconverted
carbon (Sharma et al, 2014). The main product of gasification (producer gas
or syngas) is a gaseous mixture of carbon monoxide (CO), methane (CH4),
carbon dioxide (CO2), hydrogen (H2), and nitrogen, together with light
hydrocarbons (ethane, propane) and heavier hydrocarbons (tar), which
condense at temperatures within 250°C to 300°C. Impurities such as tar,
particulate matter, alkalis, halides, Sulphur, hydrogen sulfide (H2S),
chloridric acid (HCl), and inert gases such as nitrogen (N2) are also present.
However, the relative presence of each component, as well as ash and
unconverted carbon depends on the biomass feedstock species, the
gasifying agent, and the operating conditions of the gasification process
(Demirbas, 2001; Demirbas, 2010; Sikarwar et al., 2017; Sharma et al.,
2014). Overall, biomass gasification is generally represented as: Biomass +
O2 (or H2O) gives rise to CO, CO2, H2, CH4 + other CHs + tar + char + ash
+ H2S + NH3 + C + trace species (Demirbas, 2010; Sikarwar et al, 2017).
The individual chemical reactions that take place during the biomass
gasification process are shown in Table 6 (Demirbas, 2001; Patel et al, 2016).
Table 6 Typical biomass gasification reactions (N.L. Panwar, et al
2012;T.Damartzis and A. Zabaniotou 2011).
Reaction Heating Value (KJ/Mol)
2C + O2 ↔ 2CO +246.4
C + O2 ↔ CO2 +408.8
CH4 + H2O ↔ CO + 3H2 -206
CH4 + 2H2O ↔ CO2 + 4H2 -165
C + CO2 ↔ 2CO -172
C + H2O ↔ CO + H2O -131
The overall gasification process is grouped into primary, secondary, and
tertiary reaction stages based on the temperature ranges and reaction
chemistry. Below 500°C, biomass is converted to oxygenated vapor and
liquid species, together with water and CO2 during the primary reactions.
The secondary reactions take place between 700°C-850°C to produce CO,
H2, CO2, water vapor, phenols, gaseous olefins, and aromatics, from primary
vapor and liquid species. The produced tar consists of mixed oxygenates,
phenolic esters, alkylphenols, heterocyclic esters, and PAHs. Methanation,
steam reforming, cracking, and water gas shift reaction occurs with the
remaining gases and tars. At temperatures between 850°C-1000°C, CO, H2,
and CO2 together with water vapor, PAHs, and liquid tar results during the
tertiary reactions (Demirbas, 2010). This process is limited by the biomass
feedstock requirements, homogeneity, bulk density, moisture and ash
content, particle size and energy content, which all require effective control
for efficient gasification (Brown, 2014).
During the gasification process, the woody biomass is first dehydrated before
the process temperatures exceed 200°C. Then pyrolysis is initiated,
producing vapor and char. The presence of oxygen here oxidizes the char
and vapor into a gaseous mixture of CO, H2, CO, and water, as well as
sulphur, tar, ammonia, and other impurities (Brown, 2014). The overall
gasification process is endothermic and the biomass oxidation stage
normally provides the required energy through either an autothermal
(internal heating of gasifier via partial combustion) or an allo-thermal
process (energy supplied from an external source) (Adams et al, 2018).
The major stages of biomass gasification process include (Adams et al.,
2018; Sharma et al, 2014; Sharma et al, 2015; Patel et al, 2016):
 Oxidation (exothermic): This is required to generate the thermal
energy needed by the endothermic processes (whole process) and to
maintain the required operating temperature of the system. It occurs
in the absence of oxygen and respecting the stoichiometric ratio so as
to only partially oxidize the fuel.
 Drying (endothermic): Here, the moisture contained in the feedstock
is evaporated and the heat required is proportional to the moisture
content of the feedstock. This process is complete at temperatures
around 150°C.
 Pyrolysis (endothermic): This process occurs at temperatures
between 250°C to 700°C to produce lightweight molecules such as
solid (char), liquid (tars), and gases (CO, H2, CO2, and light
hydrocarbons) via the thermochemical breakdown of the biomass
species (the decomposition of cellulose, hemicelluloses, and lignin)
(Patel et al, 2016; Sikarwar et al, 2017). Process phenomena such as
heat transfer, series reactions, and product diffusions are involved. The
kinetics of the reactions acts as the rate-limiting step at low
temperatures whereas heat transfer or product diffusion controls the
process at high temperatures.
  Reduction (endothermic): Here the products of the previous stages
together with char are reduced through a series of reactions to
produce the final syngas (Patel et al, 2016).
The reduction temperatures determine the final properties or constituents of
syngas and the solid residue. The normal operating temperatures of full-
scale biomass gasification vary between 800°C to 1100°C, whereas for
processes requiring oxygen for the gasification stage, the process
temperature ranges from 500°C to 1600°C (Sharma et al, 2014). The
gasification pathway offers ideal possibilities of combination with cheap CO2
capture and storage technologies, giving rise to both primary fuel input
flexibility and product mix concepts with possibilities achieving zero or even
negative carbon emissions (Zhang et al, 2010). The process enhances the
utilization of biomass and is currently being used to increase the efficiency
and reduce the investment costs of biomass electricity generation through
integration with gas turbine technology. A combined-cycle gas turbine
system is capable of achieving higher efficiencies up to 50%, as the gas
turbine’s waste gases are recovered to produce steam for the steam turbine
(Panwar et al, 2012).
Apart from its use for heat and power generation, nearly all hydrocarbon
compounds, as well as premium fuels (ethanol, methanol, transport fuels,
dimethyl ether (DME), and methane) and high-value chemicals, can be
synthesized from syngas after purification. Syngas can also be upgraded into
biomethane for injection into the gas grid or converted to synthetic diesel via
the Fischer-Tropsch process, a process catalyzed by transition metal-based
catalysts (Cobalt or iron) at a higher temperature. Hydrogen can equally be
isolated for used in fuel cells for electricity generation and to power electric
vehicles. However, the final end-use of the syngas is largely dependent on
the end demand and plant scale (Balat et al., 2009; Brown, 2014). Also,
reacting the clean syngas over a cobalt, platinum or iron catalyst via Fischer-
Tropsch pathway gives rise to hydrocarbon waxes and long-chain alkanes
(Brown, 2014). The purification and upgrading of these products are energy-
intensive, costly and require more R & D to improve their efficiencies for
large scale use (Brown, 2014).
Catalysts play a fundamental role in the gasification products. They improve
the gas quality, conversion efficiency, and reduce the tar content. Dolomite,
alkaline metal oxides, olivine, and Ni-based oxides are widely used
gasification catalysts. Iron, cobalt, ruthenium, and potassium-based catalysts
are well known for their role in the Fischer-Tropsch synthesis of diesel,
hydrocarbons, and other liquid fuels from bio-syngas. However, the
performance of these catalysts varies and their choice is determined by their
properties, reaction pathways, and the desired end product (Sikarwar et al,
2017). Dolomite or Ni-based catalysts are used in most gasifiers for the
partial tar oxidations, catalytic steam reforming as well as to enable the
catalytic upgrading of the gasification products into liquid biofuels. This is
usually done after effective gas cleaning and composition adjustments are
made (Brown, 2014).
Table 7 Biomass gasification feedstocks (A. Molino, et al 2016).
Supply sector Type Example
Forestry Dedicated forestry, Short rotation
forestry by-products plantations (e.g willow,
popular, eucalyptus),
wood blocks, wood
chips from thinning
Agriculture Dry lignocellulosic Herbaceous crops(e.g.
energy crops, oil, miscanthus, reed
sugar, and starch canary grass, giant
energy crops reed), oilseeds for
methyl esters(e.g
rapeseed, sunflower),
sugar crops for
ethanol(e.g sugar cane,
sweet sorghum), starch
crops for ethanol (e.g.
maize, wheat)
Industry Agricultural residues, Straw, pruning of
livestock waste, vineyards and fruit
industrial residues trees, wet and dry
manure, industrial
waste wood, sawdust
from sawmills, fibrous
vegetable waste from
paper industires

waste Dry lignocellulosic Residues from parks

contaminated waste and gardens(e.g
pruning, grass),
gemolition wood,
organic fraction of
municipal solid waste,
biodegradable
landfilled waste, landfill
gas, sewage sludge

3.2 Gasification
Gasification Feedstocks and Pretreatments
The gasifiable biomass feedstocks (Table 7) are composed of cellulose,
hemicellulose, lignin, and proteins, with their percentage composition in
softwood, hardwood, and straw indicated by (Sharma et al, 2014). The fibers
of biomass (cellulose and hemicellulose) consist of saccharides that
polymerize into long chains. Lignins are phenolic polymers which play a
pivotal role in ensuring the structural stiffness of proteins and also functions
as the fibers' glue. Mainly the herbaceous species produce proteins (Sharma
et al, 2014). These feedstocks are broadly classified as wood and residues,
agricultural and herbaceous, marine biomass, human and animal waste,
contaminated and industrial biomass waste, and biomass mixtures
(Demirbas, 2010).
Biomass pretreatment is aimed to provide homogeneous feedstocks with
respect to composition, size, and moisture content (25-30wt.%) required for
the smooth and efficient operation of the gasification process. Apart from
drying, the other gasification pretreatment methods are torrefaction and
hydrothermal upgrading (HTU) (Sharma et al, 2014). Torrefaction occurs in
the absence of oxygen at temperatures between 200°C to 300°C, under an
inert atmosphere to enhance biomass potentials for gasification through
moisture, hydrogen, and oxygenated species loss. As a form of mild
pyrolysis, it potentially enhances biomass resistance to moisture and its
heating value. During this process, the biomass feed loses both its moisture
content and the rigid fibrous structure, resulting in an increase in the energy
density of the material. A combination of torrefaction and palletization is also
possible (Demirbas, 2010; Sharma et al, 2014).
Hydrothermal upgrading (HTU) breaks down biomass using water as a
solvent to produce bio-crude. It is mainly conducted in two stages: treating
the biomass feed in the water at pressures of around 30 bars and
temperatures between 200°C to 300°C, and the biomass (bio-crude)
conversion at temperatures of 300°C-350°C and pressures of 120-180 bar
and in a variable time period from 5-10 minutes. The produced bio-crude is
made up of a variety of hydrocarbons that can be used as a co-fuel in coal
plant, for chemicals or synthetic diesel-like fuel production in the chemical
industry (Sharma et al, 2014).
3.3 Biomass Gasification Products
The exact constituents of biomass gasification products depend on the fuel
type and composition, the gasifying medium or gasifier design, the
temperature, operating pressure, fuel moisture content, and the way the
reactants (biomass fuel and gasifying agent) are brought into contact within
the gasifier. As such, it is impossible to predict the exact composition of
biomass producer gas (Sikarwar et al, 2017). Straw produces a high
hydrogen content, highest char content in softwood biomass, lowest char
content in straw, higher dust content in straw, and higher tar content in
hardwood (Sharma et al, 2014). The feedstock properties with a strong
influence on the gasification process are moisture content, ash content,
volatile matter, char, organic constituents, thermal conductivity, and
inorganic constituents (Adams et al, 2018).
The gasification product can be in a solid phase and a gas/vapor phase -
often split into a gas and a condensable phase. The solid phase (ash) is
made up of the unreacted char and the inert materials found in the
feedstock, with char having <1wt.% in the quantity of ash. Char has a carbon
content greater than 76%, making it possible for its direct industrial use. In
the gas/vapor phase (syngas), the gas phase comprises an incondensable
gaseous mixture of CO, H2, CO2, CH4, light hydrocarbons, and some C2-C4
hydrocarbons, at room temperature as well as minor quantities of NH3, H2S,
and HCl depending on the feedstock composition. They can be burnt for
electricity or heat generation, or used to synthesize.
Table 8 Classes of tar (A. Molino, et al 2016).
Tar Type Type Name Peculiarity Characteristic
Compounds
1 GC-undetected Very heavy tars, -Determined by
cannot be subtracting the
detected by GC GC-detectable
tar fraction from
total gravimetric
tar
2 Heterocyclic Tars containing -Pyridine, phenol,
aromatics heteroatoms, eresols,
highly water- quinoline,
soluble isoquinoline,
compounds dibenzo phenol
3 Light aromatics Usually light -Toluene,
(1 ring) hydrocarbons ethylbenzene,
with a single xylenes, styrene
ring; do not pose
a problem
regarding
condensability
and solubility
4 Light PAH 2 and 3 ring -Indene,
compounds (2-3 compounds; naphthalene,
rings) condense at low methylnaphthale
temperature ne, biphenyl,
even at very low acenaphthalene,
concentration fluorene,
phenanthrene,
anthracene
5 Heavy PAH Larger than 3 -Fluoranthene,
compounds (4- rings, these pyrene,
7rings) compounds chrysene,
 condense at high perylene,
temperature at coronene
low
concentration
liquid transport fuels, hydrogen, or chemicals. The quantity of syngas
produced on a dry mass basis may range from 1-3 Nm³/kg, with a low
heating value (LHV) range between 4-15 MJ/Nm³. In the production of
gaseous fuels from biomass, char gasification determines the rate of the
process (rate-limiting step) (Sharma et al, 2014; Sikarwar et al, 2017).
The condensable phase (tar) consists of various organic compounds,
considered as bituminous oil in condensed form. Tar (a complex mixture of
condensable hydrocarbons), according to the European board of
standardization, refers to all organic compounds present in syngas with
defined analysis procedures, except the gaseous hydrocarbons C1-C6. The
composition of tar is strictly dependent on the gasification technology used,
the operating conditions, and the biomass feedstock. They are generally
divided into five major groups (Table 8) according to their molecular weights
(Sharma et al, 2014).
In the gasification process, tar originates from the pyrolysis stage and is
subject to decomposition and recombination. They are classified as primary,
secondary, and tertiary tars according to their respective formation
mechanisms. The primary tars are dependent on the biomass feed and are
produced directly during the pyrolysis step. Primary tars containing
oxygenated compounds (carbon acids, aldehydes, alcohols, ketones, etc.)
arise from cellulose and hemicellulose feedstocks due to their high oxygen
contents. Lignin pyrolysis principally gives rise to aromatic compounds,
mainly bi- and tri-functional substituted phenols (xylenol, cresol, etc.)
(Sharma et al, 2014). The secondary tars are produced during the oxidation
stage. They are alkylated mono- and di-aromatics, as well as hetero-
aromatics such as furan, pyridine, thiophene, and dioxin. Their formation
arises from the transformations of primary tars through rearrangement
reactions (dehydration, decarbonylation, and decarboxylation), at
temperatures above 500°C and in the presence of an oxidant such as air,
oxygen or steam (Sharma et al, 2014). At temperatures above 800°C,
tertiary tars (also known as recombination or high-temperature tars) are
formed. They comprise mainly of aromatics and polynuclear aromatic
hydrocarbons (PAH) such as naphthalene, benzene, pyrene, benzopyrene,
and phenanthrene. Biomass gasification generally does not produce tertiary
tars but they arise from the recombination and decomposition of primary
tars in the syngas reducing environment. They also do not co-exist with
primary tars, but only appear when all the primary tars are converted into
the secondary tars (Sharma et al, 2014). Fig. (3) depicts a schematic
representation of the different classes of tars as a function of temperature.
3.4 Syngas Processing
Synthetic gas, a raw material for biofuels, chemicals, and power generation,
usually contains impurities such as tar, particulate matter, nitrogen (NH3,
HCN), alkali, halides, sulphur (H2S, COS), and trace elements mainly
responsible for gasifier clogging, corrosion, and catalyst deactivation. Thus,
rendering syngas unfit for Fischer-Tropsch synthesis, biomethanol
production, fuel cell uses, and other applications. The majority of syngas is
used for ammonia synthesis (50%), bio-hydrogen production (25%), and the
remaining used for bio-methanol production, FT synthesis, and other
processes. The processes for syngas cleaning are broadly classified as cold
or hot gas clean-up based on the condensation temperatures of its various
constituents. Some of these impurities and their cleaning processes are
discussed by Demirbas (2010).
Biomass gasification for power generation produces mainly flue gases and
unwanted by-products such as ashes and tar (gasification bottleneck) which
are often left unutilized, especially tar, despite its rich fuel and chemical
potentials. Due to the complex nature of gasification tar with hundreds of
chemical constituents, there has been a recent interest in the valorization of
this biomass gasification waste to liquid fuels and chemicals, given its high
potential for soil contamination when discharged to the environment
untreated (Knoef, 2011). However, the complete treatment, removal or
conversion of tar is a huge technical challenge (low efficiency, fouling, plug
pipes and tubes, operational difficulty, etc.) in the development and in the
successful application of biomass-derived gas. Some commonly used
methods are: end pipe tar syngas clean-up, in-bed catalytic tar reforming,
and in-bed thermal tar cracking processes (Sharma et al, 2014). Catalysts
play a key role in enhancing reaction rates at low temperatures and also
promote the conversion of tar into valuable combustible gases through
steam reforming, thermal cracking, dry reforming, hydrocracking, water-gas
shift reactions, and hydroreforming. They are used either as bed materials or
as feedstock additives (Sharma et al, 2015).

The production of low- or medium heating value fuel gas for use in internal
combustion engines for power generation is well-documented. However, less
research has been published on gasification for liquid transport fuels or
chemical generation. Gasification is a very promising technology for biomass
valorization with high flexibility and efficiency. Promoting this technology in
the future requires advanced, cost-effective, and highly efficient gasification
systems and processes. This, therefore, necessitates further research and
development in biomass gasification to improve the future performance of
this technology, enhance process efficiencies, improve gas quality and
purity, and lower investment costs. To achieve these, the development of
innovative catalysts, sorbents, and high-temperature filtration media are
fundamental requirements, coupled with high-temperature gas cleaning and
catalytic conditioning.
3.5 Hydrothermal Processing of Biomass
Biomass hydrothermal conversion has drawn much attention since the
adoption of hydrothermal energy as a high potential energy resource.
Initially, much focus was on selecting the right solvent and mainly on high
temperature and pressure processing materials, given the omission of
compound solubility knowledge and its effect on the overall process
(Demirbas, 2010). It is an excellent approach for converting high moisture
energy-rich biomass into various products, such as solid (biochar), a liquid
(bio-oil), or a gas (hydrogen, methane, etc.) (Tong et al., 2010). This involves
heating organic wastes or aqueous biomass slurries at higher pressures and
low or moderate temperatures to generate a high-density energy carrier or
liquid product in the presence of a catalyst (Panwar et al., 2012), using
subcritical and supercritical water as the processing media (Demirbas, 2010).
The basic processing conditions are high pressures (4-22 MPa) and
temperatures between 250-374°C. The key factors responsible for the recent
widespread interest in this technology are its low temperatures, low tar yield,
and high energy efficiency compared to other thermochemical processes
(Demirbas, 2010; Brown, 2014). Suitable feedstocks are those with high ash
and moisture contents such as manures, food wastes, anaerobic digestion
digestate, sewage sludge, aquatic biomass (micro- and macroalgae), and
municipal wastes. The slurries can be fed up to 30wt.% solids (Brown, 2014).
Four main processes make up hydrothermal processing: carbonization,
aqueous phase reforming (APR), liquefaction, and gasification (Table 9), with
water acting as a reactant, solvent, and a catalyst (Demirbas, 2010; Brown,
2014).
 Hydrothermal carbonization (HTC) is the mildest process, occurring
at pressures between 20 bar to 40 bar and temperatures between
180°C to 250°C to produce hydro-char (solid), which is similar in
properties to low-rank coal. This process is governed by temperature
and residence time and allows for the reduction of both the oxygen
and hydrogen content of the biomass feedstock via dehydration and
decarboxylation reactions (Reza et al, 2013).
 Hydrothermal liquefaction (HTL) occurs at pressures up to 180 bar
and temperatures between 250°C and 375°C to produce liquid bio-
crude, which can be upgraded via catalytic hydrotreatment to a wide
range of distillate petroleum-derived products.
 Hydrothermal gasification (HTG) or supercritical water
gasification (SCWG) occurs at pressures beyond 200 bar and
temperatures above 375°C in the absence of oxidants to produce
 syngas, CO2, CH4, and C1-C4 hydrocarbon gases like C2H4 and C3H 6
(Brown, 2014; Tong et al, 2010). At low temperatures, byproducts like
small bio-oil, tar, and char are formed.
 The aqueous phase reforming (APR) (a subsection of HTG) takes
place within a temperature range of 220°C-250°C and pressures 1.5-5
MPa to generate H2 syngas, alkanes, and a range of bio-based chemical
products including fibers and plastic chemicals (Demirbas, 2010).
These processes produce a broad range of chemicals and fuels in the
gaseous, liquid, or solid state with opportunities for carbon capture, storage,
and sequestration, have high conversion rates, and most importantly use
wet biomass without any prior dewatering (Demirbas, 2010).
Despite these benefits of hydrothermal biomass processing, it uses more
complex and expensive reactors, requires large water handling equipment
and capabilities, is challenging to manage large-scale separation and
extraction processes, and is also difficult to compute gas yield due to a
complex mass balance resulting from the variabilities in the hydrothermal
media (Demirbas, 2010; Tong et al, 2010).
The hydrothermal biomass processing takes place under two main process
conditions: the subcritical and supercritical water conditions, which are each
determined by the critical point of water (374°C and 22.1 MPa). Table 10
shows the various properties of water at these process conditions. Biomass
constituents like cellulose and lignin are insoluble in water at ambient
temperature but are soluble in supercritical water (high-temperature water).
The hydrothermal breakdown of biomass takes place as follows: the water-
soluble biomass portion disperses into the water at about 100°C, and above
150°C, the water acts as a reactive medium.
Table 9 Main hydrothermal process and operating conditions (M.K.
Hrncic, et al 2016).
Hydrother Temperatur Pressu Reacti Catalyst Main
mal e(K) re on Produc
Reaction (MPa) Time t
HT
carbonizati
on
Low 523 2 Several Not essential Hydroch
temperatur hours ar
e
High 573-1073 2 Several Optional Hydroch
temperatur hours ar
e
HT
liquefactio
n
High 553-643 10-25 Few Optional Bio-oil
temperatur second
e s
Low 573-873 10-25 Few Alkaline salts: Increasi
temperatur second Na2CO3, ng bio-
e s KCL,KOH, oil yield,
heterogenous improve
catalysts under d to
high pressure H2 transpor
t fuel by
increasi
ng C/H
ratio
HT
gasification
Near- 573-773 Various Few Metal catalysts CH4
critical second and alkaline salts
s
Supercritic 773-1073 Various Few Metal catalysts Syngas
al second and alkaline salts H2 with
s minor,
CO2, C1-
C4 gases
Aqueous 493-523 1.5-5 Few Pt\Al2O3,Pt/ H2 and
phase- second ZrO2,Rh,Ni and CO2 with
reforming s on SiO2 etc. minor
C1-C6
alkanes

Table 10 Properties of water under ambient, subcritical and

supercritical conditions (K. Tekin et al, 2014).
- Ambient Subcritical Supercritical Supercritical
Water Water Water
Temperature, 25 250 400 400
T(°C)
Pressure, 1.0 5 50 50
P(MPa)
Density, 0.997 0.80 0.17 0.58
P(gcm-3)
Dielectric 78.5 27.1 5.9 10.5
constant, (ε)
pK 14.0 11.2 19.4 11.9
Heat 4.22 4.86 13 6.8
capacity, Cp
(KJKg-1K-1)
Viscosity, 0.89 0.11 0.03 0.07
µ(mPa s)
 Thermal 608 620 160 438
conductivity
λ (mWm4K4)
hydrolysis occurs. The solid biomass is transformed into a slurry at about
200°C and 1 MPa, and finally, at about 300°C and 10 MPa, liquefaction takes
place, giving rise to oily products. Here, biomass is converted into a solid
(biochar), a liquid (bio-oil), and gaseous products via the variation of process
conditions like temperature, reactor pressure, reaction time, and catalyst
presence (Demirbas, 2010). Supercritical water oxidation is used for the
removal of toxic compounds (pollutants) from biological and organic waste,
and in power generation cycles. To better understand biomass hydrothermal
processing and the various degradation pathways, a thorough knowledge of
water properties under hydrothermal conditions (subcritical and
supercritical) is required. At higher temperatures, water has a low dielectric
constant. As such, under supercritical conditions, water is a very good
solvent for non-polar substances, but these substances are insoluble in water
at room temperatures (Demirbas, 2010).
4.1 Hydrothermal Liquefaction (HTL)
Hydrothermal Liquefaction (HTL) is one of the most investigated
hydrothermal processes for the conversion of waste biomass into chemicals
and bio-based fuels at moderate temperatures and high pressures, enough
to keep the water in the liquid phase (Demirbas, 2010). HTL is a promising
technology for the production of high-quality bio-oil from biomass. Here,
mainly wet feedstocks or those with high moisture content are used. This
eliminates the need for feedstock drying (energy consuming), as performed
with traditional thermochemical processes such as gasification and pyrolysis,
which require only dry feedstocks (Elliott et al, 2015). This process is
performed in water under moderate temperatures of about 280-370°C and at
high pressures (10-25 MPa), to generate liquid bio-crude with high energy
content, together with aqueous, gaseous, and solid-phase by-products (Tong
et al, 2010; Brown, 2014).
HTL involves several complex reactions, giving rise to high energy density
products and an enhanced heat recovery process. Two major feedstock
classes characterize the process mechanism: the dry feedstocks
(lignocellulose biomass) comprising cellulose, hemicellulose, and lignin
constituents, and the wet feedstocks (algal biomass) made up of vital
components such as proteins, carbohydrates, lipids, and algaenans (Brown,
2014). A broad range of chemicals can be obtained through HTL alongside
bio-crude. These mainly consist of monoaromatic compounds, fatty acids,
alkenes, alkanes, polyaromatic compounds, nitrogenous, and other
oxygenated compounds, as detailed by Brown (2014).
In this section, key concepts of HTL such as the process principles, bio-crude
elemental composition, feedstock types, process energy efficiency, as well as
future scope and prospects, are highlighted. HTL gives rise to four distinct
product phases comprising bio-oil, light gases (CO 2 and trace amounts of
CH4, CO, and H2), a solid residue (char), and a carbon-rich water phase
(Elliott et al, 2015). As discussed in a study by Anastas and Warner (2000),
process parameters, such as temperature, pressure, feedstock composition,
residence times, heating rate, water ratio, catalyst, particle size, solvent
density and type, and reaction medium, greatly affect bio-oil yield. The bio-
oil generated can be improved into high-quality liquid fuels via chemical,
catalytic, and physical (extraction, solvent addition, or separation) methods.
This process is advantageous in that it uses wet biomass which does not
require drying, uses water which is a unique and environmentally friendly
solvent, occurs at lower temperatures, is highly energy-efficient, and does
not require drying (Demirbas, 2010; Sikarwar et al, 2017). It produces high
quality bio-oil, higher yields, lower water and oxygen content, which gives it
a higher heating value (HHV) compared to pyrolysis or gasification oil, but
still higher than fossil fuels. However, the high-pressure process leads to high
equipment cost for large-scale or industrial processing (Elliott et al, 2015).
The low oxygen content and HHV of the bio-oil main product originate from
the high contribution of dehydration and decarboxylation reactions in the
HTL process, removing oxygen in the form of water and CO 2 respectively
(Anastas & Warner, 2000), which reduces biomass oxygen from 40wt.% to
about 10wt.% (Demirbas, 2010).
According to (Elliott et al,2015). Increasing the HTL process temperature
correspondingly raises bio-oil yield prior to the occurrence of bio-oil cracking
and repolymerization reactions that lead to an opposite trend. In addition,
increasing the residence time (normal range of 5-30 minutes) equally
increases bio-oil yield. However, a further increase in the reaction time
rather leads to a decrease in bio-oil yield, originating from cracking and
repolymerization reactions to form lighter gases and char respectively. A
detailed review of the techniques for the separation and extraction of HTL
products, chemical composition, and products analysis using mass
spectroscopy, Fourier transform infrared (FTIR) spectroscopy, elemental
analyzer, nuclear magnetic resonance (NMR) spectroscopy, high
performance liquid chromatography (HPLC), etc., are discussed by Demirbas
(2010).
The HTL process mainly converts biomass to bio-oil, which can substitute
fossil fuels. This process is however challenged by its high-pressure
conditions, making its economics very questionable. The HTL of algal
biomass gives rise to by-products such as CO 2 and nutrients, which are
reused for algal cultivation in the algal culture for growth and
photosynthesis, and wastewater which can be reused to reduce the process
water need, as well as organic fertilizer for agricultural use. The process
generates about 85-90% of energy, consuming only 10-15% of the feedstock
energy, thus a very energy-efficient process. Moreover, the resultant
biocrude is very similar to their petroleum counterparts, and thus requires
very little upgrading for commercial use (Brown, 2014). All the above-listed
by-products and applications, if properly valorized, can potentially overcome
the economic challenges of HTL. However, further research to optimize the
process mechanisms and parameters is still needed to overcome the
setbacks of HTL and to effectively commercialize the process.
During the process, water is kept in the liquid phase by operating at or above
its saturation point, which greatly minimizes the enthalpy change linked with
the latent heat of vaporization of water (Brown, 2014). In the past decades,
compressed hot water has attracted much attention as a green reaction
medium. Presently, it is widely applicable in resource recovery from wastes,
polymers recycling, and biomass energy recovery, waste treatments,
homogeneous and heterogeneous catalysis, inorganic and organic materials
synthesis, and others (Demirbas, 2010). When the process occurs below the
critical point of water, it is known as subcritical. The supercritical process
takes place above water's critical point (274°C and 22.1 MPa), with the water
properties within the liquid-like and gas-like phase. Under these conditions,
water functions both as a solvent and a catalyst or reactant. Its dielectric
constant decreases, raising the solubility of insoluble organic compounds.
Also, the reaction rates are higher due to the high diffusion coefficient as the
water viscosity decreases (Elliott et al, 2015).
In the past few decades, there has been a strong drive and interest in the
use of biomass and biomass derivatives for the production of liquid fuels.
Research studies within this scope range from biomass gasification,
pyrolysis, HTL of lignocellulose material, and the upgrading processes of the
derived bio-crude. The choice of biomass generates high prospects and
expectations for fuels, fine chemicals, and raw material synthesis for the
petrochemical industry. Among the biomass to fuels and chemicals
processing methods, HTL has drawn huge attention recently as one of the
promising routes for both dry and wet biomass (Brown, 2014). Presently,
fuels (biofuels) and chemicals production rely mainly on dry biomass
feedstocks (dry wood) through various pyrolysis and gasification processes.
Only very little interest and attempts are reported (Brown, 2014) which have
so far been made towards the development of the HTL process for wet
biomass valorization, despite the huge prospects in wet biomass feedstocks.
Microalgae, due to their high photosynthetic efficiency, fast growth rate,
maximum biomass production, and the lack of arable land requirements, is
regarded as an excellent fuel and chemical source through wet biomass
processing. Thus, the selection of the best process feedstocks and
operational conditions, as well as the optimization of the existing parameters
and process mechanisms to enhance HTL process efficiency and yields are
very important for enhancing future performance, prospects, and
applicability of this technology, especially for wet biomass, as well as in
assuring the economic viability of HTL.
4.1 Dry biomass HTL
This process produces bio-crude either in a continuous or batch reactor, with
detailed pathways as shown in Fig. (4). The feedstock is pretreated to reduce
the particle size, remove impurities, and to make a stable slurry for easy
pumping through alkaline treatment. The HTL process then takes place at
around 350°C and 150 bar for approximately 15 minutes, followed by phase
separation to produce a gaseous mixture of CO 2, bio-crude, solid residue, and
traces of the aqueous phase (water) which can be reused in the HTL unit to
enhance the yield of the bio-oil or reduce the process water requirement.
Anaerobic or catalytic hydrothermal gasification treatment of the other water
stream results in a hydrogen-rich or methane-rich syngas. However, the
presence of phenols and furfurals limits the anaerobic treatment process.
The resulting bio-crude has less moisture and oxygen content, thus it can be
used directly or upgraded by further hydrotreatment for commercial use.
This process is highly energy efficient as the overall heat generated can be
used efficiently and effectively to operate the hydrothermal gasifier (Brown,
2014).
Wet biomass HTL
The HTL of wet/algae biomass differs only slightly (no pretreatment) from
that of dry biomass and offers great potential for nutrient recycling. Fig. (5)
depicts a schematic representation of wet biomass hydrothermal processing.
Here, the algae biomass feed from an algal culture is first dewatered to
obtain a feed slurry with about 20% solid, which is then pumped into the HTL
unit to produce biocrude which is then further hydrotreated to generate fine
hydrocarbon fuels. The CO2 and nutrients that result from the aqueous phase
are reused for algal cultivation, making the process more economically
viable and sustainable (Brown, 2014).
HTL Process Mechanism
Prior to the degradation of the biomass constituents into smaller molecules
via dehydrogenation, dehydration, decarboxylation, and deoxygenation, the
biomass feedstock is initially fragmented by hydrolysis. Repolymerization of
the lighter molecules results in the synthesis of some complex chemicals,
containing aldehydes, phenols, acids, ketones, alcohols, esters, and other
aromatics (Brown, 2014). HTL is initiated by solvolysis of biomass in micellar
forms, followed by the disintegration of biomass's major polymeric
constituents (cellulose, hemicellulose, and lignin), and then concludes with
its thermal depolymerization into smaller fractions (Tong et al, 2010).
The actual mechanism of HTL is still unclear. However, the process
comprises depolymerization, preceded by decomposition and recombination
reactions. The biomass is first decomposed and depolymerized into highly
reactive lighter molecules (water-soluble oligomers), preceded by the
cleavage of the inter- and intramolecular hydrogen bonds to produce simpler
monomers (glucose and other products like acetic acid, furfural, and
aldehyde compounds) (Tong et al, 2010), which subsequently repolymerizes
to form bio-crude, gas, and solids. Due to the complex nature of biomass, the
process mechanism and reaction chemistry are also complex, and the major
process parameters like residence time, temperature, process
decomposition, condensation, and repolymerization of the constituents also
vary within the phases (Brown, 2014). Temperature, pressure, particle size,
and reaction times affect bio-oil production via HTL (Tong et al, 2010).
Biomass depolymerization sequentially dissolves the polymeric components
of biomass, aided by their physicochemical properties. The process
temperature and pressure modify the polymer long-chain structure, while the
organic materials’ energy contents are recycled in the presence of water
(Brown, 2014). The decomposition of the biomass monomers occurs via
dehydration, cleavage, deamination, and decarboxylation. Here, water
molecules are lost via dehydration, CO2 lost by decarboxylation, and
deamination removes the amino acid content. The decarboxylation and
dehydration processes remove oxygen from biomass as CO 2 and H2O
respectively, and this hydrolysis converts the polymers to polar monomers
and oligomers. The applications of water at high temperature and pressure
dissociates the hydrogen-bonded cellulose structure into its glucose
monomers. The isomerization, hydrolysis, reverse-aldol breakdown,
dehydration, recombination, and rearrangement reactions of fructose
compared to glucose results in a broad stream of products like furfural, polar
organic molecules, phenols, organic acids, and glycoaldehydes, which are
highly water-soluble (Demirbas, 2010; Brown, 2014). Among these reactions,
the decomposition and deoxygenation reactions are the key reactions that
give rise to the resulting biocrude, consisting of acids, aldehydes, and
aromatics (Anastas & Warner, 2000). The repolymerization and
recombination stage arises from the non-availability of hydrogen compounds
in the system. As such, with high concentrations of free radicals (reactive
fragments from previous reactions), the fragments tend to repolymerize or
recombine into heavier char molecules, noted for their coke formation.
However, with sufficient presence of hydrogen in the organic mixture during
the liquefaction process, stable weight species will be formed from the
capping of the free radicals with the hydrogen molecules (Brown, 2014).
4.2 Catalytic Hydrothermal Liquefaction
Homogeneous and heterogeneous catalysts are generally employed in HTL
to enhance the process efficiency through the reduction in tar and char
formation while improving oil (product) yields. The homogeneous catalysts
(alkali salts, with the K salts more active than Na salts) like KHCO 3, KOH,
K2CO3, Na2CO3, and NaOH, improve the product yield of the HTL process by
facilitating water-gas shift reactions through ester formation via
decarboxylation reaction between biomass hydroxyl groups and the formate
ions in the alkali carbonates, followed by dehydration, deoxygenation,
decarboxylation, and dehydration of micellar-like fragments into lighter ones.
Then, the cycle of rearrangement reactions via cyclization, polymerization,
and condensation gives rise to the final products. This group of catalysts
preferentially decreased solid yields, increased bio-crude yield and
properties, and decreased dehydration reactions (with elevated pH) that
produce unstable and unsaturated molecules. However, the recovery of
these catalysts is costly, owing to the energy and cost-intensive separation
process (Tong et al, 2010).
Heterogeneous catalysts are most commonly used in HTG, with only a few
applications so far with HTL, especially with the improvement of the bio-
crude quality from lignocellulose biomass. Platinum, nickel, and palladium
are common heterogeneous catalysts, but due to the rareness of these
metals, attention has been shifted to metallic oxides like ZrO2, MnO, MgO,
NiO, ZnO, CeO2, La2O3, and so on (Tong et al, 2010). The very first catalytic
hydrothermal liquefaction of wood biomass with CsOH and RbOH (very
potent bases) has been performed by Akiya and Savage (2002) and noted
that these strong bases (CsOH and RbOH) like other base catalysts greatly
improve oil yields by hindering char formation, producing mainly benzenediol
derivatives and phenolic compounds. Also (Karagöz et al,2006) investigated
the effect of K and Na carbonates and hydroxides on the product yields and
boiling point distribution of oxygenated hydrocarbons from low-temperature
HTL of wood biomass (pine). They found that the treatment of wood biomass
with catalytic alkaline solutions (NaOH, Na2CO3, KOH, and K2CO3) produces
mainly phenolic compounds whose distribution depends solely on the exact
base solution used, and thermal treatments produce mainly furan
derivatives. They reported the catalytic activity of these bases as follows:
K2CO3 > KOH > Na2CO3 > NaOH. The effects of various catalysts and process
parameters on the product yields, distribution, and chemical constitution
from various biomass streams like bagasse (Mohan et al., 2006), waste
biomass (Yu et al, 2011), food waste using CeZrOx (Zhang et al., 2016),
aquatic biomass (Zhang et al., 2013; Zhang et al., 2017), and barley straw
(Yu et al, 2013) are also reported in literature. The use of bifunctional
catalysts like Ni/HZSM-5 (Wang et al, 2012) has also been investigated and
seen to exhibit better catalytic activity on bio-oil production as it integrates
the cracking and hydrodeoxy biomass conversion technologies like
gasification is also feasible with catalytic HTL. This has been demonstrated
by (Zhang et al,2013) by using K2CO3 catalyst to produce aqueous
gasification liquids and gases from pine sawdust. This in a nutshell integrates
a biomass to liquid gasifier to a catalytic HTL process and this improves the
overall gasification efficiency. Biocrude quality in algae HTL is best improved
using Ni/SiO2-Al2O3, Ni-Mo/Al2O3, Pt/C, Pt/Al2O3, and Ce/HZSM-5 catalysts, with
Ce/HZSM-5 catalyst presently seen as the first-choice option. The highest
catalytic denitrogenation, deoxygenation, and desulfurization activities are
exhibited respectively by Ce/HZSM-5, Co/Mo/Al2O3, and Ni/SiO2-Al2O3. Also, Ru
supported catalysts like Ru/C, Ru/C+alumina, show higher deoxygenation,
denitrogenation, and hydrogenation performances in biocrude upgrading,
whereas Pt supported catalysts, such as Pt/C and Pt/γ- Al 2O3, showing optimal
catalytic activities in heteroatom content reduction and in improving algal
HTL biocrude quality and upgrading, especially with respect to catalytic
hydrogenation, deoxygenation, and denitrogenation (Zhang et al, 2017).

4.3 CURRENT DEVELOPMENTS AND FUTURE OF BIOMASS

CONVERSION PROCESS
4.3.1 Current Developments
Current research and development in renewable energy technologies have
led to significant engineering advances and discoveries for all renewable
energy resources. Some of these advances in biomass conversion
technologies are (Zhang et al, 2010):
 Advanced biorefinery system designs for the sustainable production of
multiple products.
 Advanced gasification processes for the efficient production of medium
energy content fuel gas and power.
 Advanced plantation designs for multicultivation of virgin biomass
species for integrated biomass production and conversion systems.
  Closed-coupled biomass gasification-combustion systems for the
production of steam and hot water for schools and commercial
buildings.
 Advanced catalysts for thermochemical gasification for high yield
chemicals, gases, and fuels production.
 Zero-emission biomass waste combustion systems for energy
recovery, waste disposal, and recycling.
 Fast pyrolysis processes for liquid fuels and chemicals production from
biomass.
 Genetically engineered microorganisms for the simultaneous
conversion of pentose and hexose sugars to ethanol for cellulosic
biomass.
4.3.2 Future of Biomass Processing
Biomass has the full potential to provide a cost-effective and sustainable
supply of energy in the future, while simultaneously helping countries to
meet their targeted GHG emission reduction. Biomass integrated
gasification-gas turbine technology with high energy conversion efficiencies
holds the future of biomass electricity generation. The renewable, flexible,
and adaptable biomass resource is expected to favorably compete for niches
with fossils in the chemical industry. However, the success of the biomass-
derived chemicals depends on the demand and supply of feedstock, primary
chemicals, and key intermediates such as lactic acid, cellulose, levulinic acid,
etc., that the petrochemical industries shall be unable to make (Panwar et
al., 2012).
Biomass resource, with its great natural abundance, is a perfect substitute
for fossil fuels for energy generation if fully harnessed. Third and fourth
generation biofuels processing technologies shall have great future
prospects and impact on climate change mitigations, thereby creating a
more carbon-neutral environment free from greenhouse gases. The present
biorefineries generate and attempt to generate costlier fuels and value-
added products (chemicals) with an almost negligible cost of the initial raw
materials, making biomass processing a great attraction to investors in the
chemical and energy industries owing to its potentially huge benefits and
sustainability.
Efficient feedstock upgrading and refining processes would be employed in
future biorefineries. This would greatly improve biorefinery efficiency, given
the added uniformity of the process with the fractionating of the biomass
feedstock into its core constituents before usage. As such, a complete
biomass usage will be assured as only the residual biomass shall be used for
heat and power generation. Nanoparticles-based catalysts would more likely
be used to improve the catalytic upgrading/cracking processes of the
products of biomass thermochemical conversion. Also, greener processes
with higher efficiencies shall be applicable for biomass conversions under
mild conditions through biocatalysts development. Biodiesel and bioethanol
in the near future will completely replace gasoline, given the unlimited scope
and rapid advances of biomass biochemical processing and algal biomass
conversion processes. In addition, the production efficiencies of cellulases
and hemicellulases shall be improved through genetic engineering (genetic
manipulation), and this would greatly reduce the conversion times for the
biochemical processing of lignocellulosic biomass. Furthermore, transgenic
plants with less lignin content and an upregulated biosynthesis of cellulose
shall be grown. These plants would have the capacity to capture and store
more carbon so as to enhance the energy density of biomass. Conclusively,
the bright future of biomass to energy conversions is evident with the vast
number of integrated biorefineries already planted globally (Balat et al.,
2009).

4.4 CONCLUSION
Biomass can be converted to solid, liquid, and gaseous fuels and high-value
chemicals or high-density energy and chemical precursors. Biomass energy
is released directly through combustion or co-firing with fossils (natural gas
or coal) for heat and power generation, or indirectly through thermochemical
processes (gasification, pyrolysis, liquefaction, etc.) to produce fuels (syngas,
bio-oils, charcoal, etc.) and value-added chemicals or intermediates. The
vast array of biomass feedstocks for chemicals and biofuels production vary
substantially with respect to their physicochemical constituents, which
greatly influence their choice of conversion technology.
A number of processes transform biomass into fuels and chemicals.
However, all these processes show some major shortcomings. This ushers
the need for a modern biorefinery to maximize environmental and economic
benefits through the combination of the various processes. The use of small,
local or mobile pyrolyzers could make the transformation of agricultural
waste and forestry materials into liquid attractive and this can easily be
shipped and further processed in a central biorefinery. Biomass conversion
technologies comprise both biochemical and thermochemical processes,
suitable for both direct conversion or the formation of high energy or
chemical intermediates. The sufficient variability of these processes enabled
the gaseous or liquid fuels produced to be identical or non-identical to fossil
fuels. Virtually, all fossil-derived fuels and chemicals can be produced from
biomass. The modern biorefinery uses almost all of the processes involved in
the refining of petrochemicals and fuels in the petroleum refinery. Also,
many chemical species, natural polymers, pharmaceuticals, and other value-
added products are derived from specialized biomass feedstocks.
Gasification and pyrolysis processes play a key role in the production of fuels
and chemicals from biomass. They both show great future prospects for use
in biomass valorization, especially in small scale applications. However,
further research and development are still needed to enhance their
conversion efficiencies, reactor configurations, process parameters, and
product upgrading to transportation fuels or valuable chemicals, as well as
in-process integrations with diesel engines, turbines, boilers, and CHP plants
for heat and power generation. The knowledge and productivity of energy
crops have reasonably advanced recently. Thus, modern biotechnological
research provides an opportunity for more advances in this trend, thereby
encouraging and promoting the development of cost-competitive biofuels
worldwide. In addition, high-pressure studies will play a central role in future
bioenergy research and in the deployment of suitable economic viable
technologies for biomass processing into biofuels (liquid or gaseous) and
useful biochemicals (organic acids, ketones, furfurals, aldehydes) via the
application of supercritical fluids. This will significantly contribute and
enhances both upstream and downstream processing in the energy industry
and most importantly in the future biorefinery.

4.5 Recommendations
Based on the comprehensive review of thermochemical processes for
biomass conversion into Valuable chemicals, the following recommendations
are proposed to advance research and development in this field:
 Focus on Hybrid and Integrated Processes: The future of biomass
conversion lies in moving beyond single-process technologies.
Research should be directed toward developing and optimizing hybrid
systems that combine multiple thermochemical processes (e.g.,
pyrolysis followed by gasification of char) to maximize the yield of
valuable products and improve overall energy efficiency. The concept
of an integrated biorefinery, which converts biomass into a spectrum of
bioproducts and bioenergy, should be a primary focus.
 Investigate Novel Catalysts for Enhanced Selectivity: A critical
bottleneck is the development of catalysts that can selectively steer
reactions toward desired products, such as specific biofuels or
bioplastics precursors. Further research is needed to design and
synthesize advanced heterogeneous catalysts that are robust, cost-
effective, and resistant to deactivation from tar and other impurities
present in biomass-derived streams. Exploring the use of in-situ
catalysts and understanding their interaction with biomass
components is crucial.
 Address Process Optimization for Specific Biomass Feedstocks: The
review highlights the diverse nature of biomass feedstocks.
Recommendations include a more focused effort on optimizing
thermochemical processes for specific types of feedstocks (e.g.,
agricultural waste, algae, forestry residue). This requires detailed
studies on how feedstock composition (lignin, cellulose, hemicellulose
content) and moisture content influence process efficiency, product
distribution, and the need for specific pre-treatment methods.
  Explore Sustainable Waste Management Integration: The economic
viability of biomass conversion can be significantly improved by
integrating it with waste management systems. Further research is
recommended on co-processing various wastes, such as municipal
solid waste and industrial by-products, with biomass. This approach not
only provides a renewable energy source but also offers a sustainable
solution for waste disposal.
 Conduct In-Depth Techno-Economic and Life-Cycle Assessments: While
the technical feasibility of many thermochemical processes has been
demonstrated, more detailed techno-economic analyses (TEA) and life-
cycle assessments (LCA) are needed. These studies should provide a
holistic view of the economic viability and environmental impact of
different conversion pathways, from feedstock sourcing to product
end-use. This will help in identifying the most promising pathways for
large-scale commercialization.

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