Environmental Chemistry Letters (2021) 19:4075–4118

https://doi.org/10.1007/s10311-021-01273-0

REVIEW

Conversion of biomass to biofuels and life cycle assessment: a review

Ahmed I. Osman1 · Neha Mehta1,2 · Ahmed M. Elgarahy3,4 · Amer Al‑Hinai5 · Ala’a H. Al‑Muhtaseb6 ·
David W. Rooney1

Received: 6 May 2021 / Accepted: 9 July 2021 / Published online: 23 July 2021
© The Author(s) 2021

Abstract
The global energy demand is projected to rise by almost 28% by 2040 compared to current levels. Biomass is a promising
energy source for producing either solid or liquid fuels. Biofuels are alternatives to fossil fuels to reduce anthropogenic
greenhouse gas emissions. Nonetheless, policy decisions for biofuels should be based on evidence that biofuels are produced
in a sustainable manner. To this end, life cycle assessment (LCA) provides information on environmental impacts associated
with biofuel production chains. Here, we review advances in biomass conversion to biofuels and their environmental impact
by life cycle assessment. Processes are gasification, combustion, pyrolysis, enzymatic hydrolysis routes and fermentation.
Thermochemical processes are classified into low temperature, below 300 °C, and high temperature, higher than 300 °C,
i.e. gasification, combustion and pyrolysis. Pyrolysis is promising because it operates at a relatively lower temperature of
up to 500 °C, compared to gasification, which operates at 800–1300 °C. We focus on 1) the drawbacks and advantages
of the thermochemical and biochemical conversion routes of biomass into various fuels and the possibility of integrating
these routes for better process efficiency; 2) methodological approaches and key findings from 40 LCA studies on biomass
to biofuel conversion pathways published from 2019 to 2021; and 3) bibliometric trends and knowledge gaps in biomass
conversion into biofuels using thermochemical and biochemical routes. The integration of hydrothermal and biochemical
routes is promising for the circular economy.

Keywords Biomass · Biofuel · Thermochemical · Biochemical · Life cycle assessment

Introduction

* Ahmed I. Osman In recent decades, urbanisation, modernisation and indus-

aosmanahmed01@qub.ac.uk trialisation linked to energy production and utilisation have
* Amer Al‑Hinai been a fundamental loop in various economic, scientific and
hinai@squ.edu.om social sectors (Ahmad Ansari et al. 2020; Shrivastava et al.
* Ala’a H. Al‑Muhtaseb 2019). The depletion of non-renewable fuel sources, accom-
muhtaseb@squ.edu.om panied with greenhouse gas emissions, has become a critical
1
School of Chemistry and Chemical Engineering, Queen’s issue (Fawzy et al. 2020; Osman et al. 2021). Therefore, the
University Belfast, Belfast, Northern Ireland BT9 5AG, UK necessary shift for exploring alternative options to overcome
2
The Centre for Advanced Sustainable Energy, David Keir the world-scale looming energy crisis, considering the envi-
Building, Queen’s University Belfast, Stranmillis Road, ronmental concerns and its mitigation, while confronting
Belfast, Northern Ireland BT9 5AG, UK the spiralling energy demand has become an urgent need
3
Environmental Science Department, Faculty of Science, Port of the hour.
Said University, Port Said, Egypt Biomass, unlike other sustainable energy sources such
4
Egyptian Propylene and Polypropylene Company (EPPC), as wind, solar, geothermal, marine and hydropower, can
Port‑Said, Egypt directly produce fuel along with chemicals (Quereshi et al.
5
Department of Electrical and Computer Engineering, College 2021; Farrell et al. 2019; Farrell et al. 2020). Thus, it is
of Engineering, Sultan Qaboos University, Muscat, Oman not feasible to substitute fossil-based fuels with the afore-
6
Department of Petroleum and Chemical Engineering, College mentioned sustainable energy sources; hence, biomass
of Engineering, Sultan Qaboos University, Muscat, Oman

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4076 Environmental Chemistry Letters (2021) 19:4075–4118

utilisation to produce fuel and chemicals is required (Bharti as shown in Fig. 1. There are unprecedented challenges with
et al. 2021). Biomass is classified as non-lignocellulosic or the integration of thermochemical and biochemical routes.
lignocellulosic in nature and exists in various forms such as For instance, the catalysts or solvent utilisation of the ther-
woody, herbaceous, aquatic debris, farming manure and by- mal routes may result in poisoning or kill the microorganism
products and other forms (Osman et al. 2019; Kaloudas et al. or generate various inhibitors that can affect the biological
2021). Various technologies are used to convert biomass into progress routes. Furthermore, this integration may lead to
fuel or chemicals, such as gasification, combustion, pyroly- additional costs.
sis, enzymatic hydrolysis routes and the fermentation pro- Identifying sources of biofuels such as biodiesel and
cesses (Abou Rjeily et al. 2021; Peng et al. 2020). biochar can potentially reduce the environmental impacts
A recent review discussed integrating hydrothermal and of fossil fuels (Balajii and Niju 2019; Gunarathne et al.
biochemical routes in biomass utilisation from a circular bio- 2019). Biofuels can also counter the increasing use of fossil
economy approach (Song et al. 2021). The thermochemical resources and prevent pressure on non-renewable sources
methods usually involve a high energy intake along with sol- (Peng et al. 2020; Hassan et al. 2020). However, it is impor-
vent or catalyst addition. Meanwhile, the biochemical route tant to use practical, scientific and robust tools to evaluate
has a lengthy cycle period and is less efficient in breaking the real benefits of using biofuels over conventional energy
down recalcitrant biomass materials. Thus, combining those sources (Chamkalani et al. 2020; Kargbo et al. 2021). Life
two routes can be promising by incorporating the benefits of cycle assessment (LCA) has been identified as a compre-
both methods in biofuel processing. They proposed a sche- hensive evaluation approach (Astrup et al. 2015) to measure
matic route where hydrothermal routes are being used in the environmental impacts over the entire production chain of
pretreatment stage to prepare the appropriate biomass feed- biofuels (Collotta et al. 2019).
stock for the following biological routes to improve the over- Therefore, this review aims to critically evaluate exist-
all process efficiency and final product yields and vice versa, ing biomass to biofuel pathways and associated studies

Fig. 1  Integration of hydrothermal and biochemical routes in biomass ical route or vice versa by thermochemical pretreatment for the fol-
utilisation from a circular economy approach. Firstly, the biomass is lowing biochemical route and eventually producing biofuel or chemi-
pretreated using a biochemical process for the following thermochem- cals

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Environmental Chemistry Letters (2021) 19:4075–4118 4077

which evaluated environmental impacts for the entire life sectors comprising heat, power and transport fuel (Kargbo
cycle. The main objectives were to: (1) critically review et al. 2021; Wang et al. 2020a). For this purpose, two
recent advances in biofuel production processes, (2) ana- grouped distinct routes, namely thermochemical and bio-
lyse existing LCA studies and highlight key methodological chemical, are currently used. Thermochemical methods use
approaches and present critical findings. the whole biomass in the presence of a heat source and con-
trollable oxygen atmosphere to modify it to different energy
forms.
Bibliometric analysis In contrast, biochemical methods employ enzymes, bac-
teria or other engineered organisms to transform it to liquid
Bibliometric research methodology: TOPIC: ‘biomass fuels such as drop-in-biofuels (Shen and Yoshikawa 2013;
pyrolysis’ OR TOPIC: ‘biomass gasification’ OR TOPIC: Singh et al. 2016). During the past decades, the biomass-
‘biomass combustion’ OR TOPIC: ‘biomass hydrothermal derived fuel synthesis process has upgraded from the first-
liquefaction’ OR TOPIC: ‘biomass torrefaction’ OR TOPIC: generation biofuel to fourth-generation biofuel, passing by
‘biomass fermentation’ OR TOPIC: ‘anaerobic digestion’ second and third generations. Food crops, inedible biomass,
AND TOPIC: ‘biomass into fuels’ AND TOPIC: (‘thermo- macro/microalgal biomass and genetically bioengineered
chemical’ OR ‘thermo-chemical’) AND TOPIC: ‘biochemi- algal and microbial systems-based biofuels are examples for
cal’. The document type selected in the bibliographic search the first, second, third and fourth generations, respectively
was articles, the timespan: All years. (Martin 2010; Adelabu et al. 2019; Aro, 2016; Ben-Iwo
It is a complex process to assess the sustainability of bio- et al. 2016). Innumerable biomass-based fuels, chemicals
fuels. This is because the use of energy crops can cause and organic compounds such as methane, ethane, propane,
the transformation of natural and agricultural land for the butane, ethylene, methanol, ethanol, butanol, dimethyl
cultivation of these crops. Moreover, various technical path- ether, ammonia, acetic acid, formaldehyde, gasoline, diesel,
ways range from biological to thermochemical conversion wax, paraffin, bio-jet fuels and others have been produced
processes, all involving range of products and co-products. throughout different biomass to liquid routes and presently
Therefore, it is imperative to conduct the LCA of the biofuel available in the markets throughout the world (Demirbas and
production chain. This study provides an overview of LCA Demirbas, 2010).
approaches by recent studies. Keywords: ‘biomass’, ‘bio-
fuel’, ‘life cycle assessment’, ‘environmental impact assess- Thermochemical conversion methods
ment’, were used for literature search in the Web of Sci-
ence database. Forty most complete studies published from Thermochemically, diverse technologies including direct
2019–2021 were selected for analysis in the present study. combustion, torrefaction, hydrothermal liquefaction, pyroly-
sis and gasification have been implemented to produce liquid
fuels from biomass, as shown in Fig. 2. Practically, the bio-
Biomass conversion technologies mass is decomposed in controllable operational conditions to
produce solid, liquid and gas (syngas), which need a supple-
Harnessing various renewable energy resources is consid- mentary catalytic promotion process to produce liquid fuels
ered affordable, reliable and sustainable solutions for their called drop-in biofuel. One of its most important features is
excessive availability, such as agriculture wastes, domestic its capabilities to utilise any biomass type as a biomaterial
wastes, forest residues, industrial wastes and human excreta. feedstock, unlike biochemical conversion methods (Raheem
Among them, biomass is the most significant contributor et al. 2015).
with 9% (~ 51 EJ) of the global overall primary energy sup- The thermal-based processes are also classified into low-
ply, out of which about 55% is traditionally used in daily temperature, which typically operate at < 300 °C, such as
living activities such as heating and cooking, especially in torrefaction, hydrothermal carbonisation and high-temper-
developing countries (Chan et al. 2019). Slade et al. (2014) ature that operate at > 300 °C, such as gasification, combus-
revealed the possibility of biomass, wastes and energy crops tion and pyrolysis biomass conversion methods (Quereshi
for sharing up to ~ 100 EJ in the world energy supply (Slade et al. 2021). The direct combustion (flaming or smoulder-
et al. 2014). ing) of biomass to produce energy usually operates in the
Economically, biomass combustion is not the best strat- temperature range of 1000–2000 °C in the presence of air.
egy to utilise biomass because of causing severe environ- However, there are emissions associated with such processes
mental damage as well as not recovery of the total energy as NOx and pollutants (Osman 2020). Hence, gasification
stored in the substrates (Ullah et al. 2015). In this context, which typically operates at 800–1300 °C is seen as a poten-
biomass conversion into solid, liquid and gaseous forms is tial substitute to produce energy and chemicals as well. The
deemed an efficient and green energy supplier for various synthesis gas produced from the gasification process can

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4078 Environmental Chemistry Letters (2021) 19:4075–4118

Fig. 2  Thermochemical conversion of biomass, including hydro- cessing plants into liquid biofuels for energy purposes or for the pro-
thermal liquefaction, pyrolysis, torrefaction, gasification and direct duction of value-added chemicals
combustion processes. Different types of biomass are treated in pro-

be used for electricity production, as well as the conversion In waste-to-energy facilities, biomass can be separately
into liquid fuel via the Fischer–Tropsch route. Interestingly, burned or combined in co-firing with coal to produce steam,
pyrolysis is considered a promising route requiring a lower used later in electricity generation. The net electricity effi-
temperature of up to 500 °C compared to the gasification cacy generated from coal/biomass in co-firing power plant
process. system varied from 36 up to 44% based on as-used strategy
and biomass specifications (quantity and quality). Despite
Direct combustion the present feasibility of 20% of co-firing as energy basis
in addition to a theoretical achievability of 50%, only less
Biomass utilisation as fuel was closely linked with the than 5% and sometimes surpasses to 10% of biomass con-
beginning of human civilisation. Moreover, it is the high- tribution continuously. It has been estimated that only 10%
est contributor source of clean energy globally (Mladenović of biomass usage in co-firing systems in power plants can
et al. 2018). Biomass combustion is described as a group decline the release of C
­ O2 to the atmosphere from 45 to 450
of chemical reactions involving carbon dioxide and water million tonnes/year by 2035 (Sahu et al. 2014). Considering
formation resulting from the transformation of carbon and the physicochemical properties of fuel and its required vol-
hydrogen, respectively, by oxidation reactions. Improper ume to air for avoiding any troubles in the fuel-to-air ratio,
oxygen quality can result in incomplete combustion associ- excessive air can be forwarded to the reaction to control
ated with release of atmospheric polluters (i.e. CO, ­NOX, the temperature of the burning system and ensure complete
­SO2 and particulate matter) (Yang et al. 2020). Nowadays, combustion (Vicente and Alves, 2018). Majorly, combus-
the usage of effective fabricated combustion systems such tion physicochemical features of biomass can be catego-
as combustion control systems that simultaneously use con- rised into macroscopic and microscopic. Comprehensively,
ventional and alternative biomass resources has become a macroscopic features are provided by macroscopic analyses
predominant feature industrially. such as proximate analysis such as moisture tenor, sulphur,

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Environmental Chemistry Letters (2021) 19:4075–4118 4079

particle magnitude, calorific value, bulk density, fixed car- Table 2  Physicochemical characteristics of biomass and coal-based
bon and ash fusion. This is coupled with ultimate analysis fuels (Demirbas, 2004)
such as C, H, N, O and S %, whereas microscopic analyses Property Biomass Coal
include chemical, thermal and mineral data (Khodaei et al.
Particle size (mm) 3 100
2015).
Fuel density 500 1300
Main combustion reactions are as follows: (1) drying
Carbon content* 43–54 65–85
out of biomass, (2) pyrolysis, (3) pre-combustion reaction,
Oxygen content* 35–45 2–15
(4) primary combustion, (5) secondary combustion and (6)
Sulphur content* Max 0.5 0.5–7.5
effluent stack gas. Parameters that influence the combus-
AL2O3 content* 2.4–9.5 15–25
tion process include biomass magnitude, specific gravity,
SiO2 content* 23–49 40–60
moisture tenor, ash percentage, elemental composition and
K2O content* 4–48 2–6
anatomical structure as lignin, cellulose and hemicellulose.
Fe2O3 content* 1.5–8.5 8–18
Different researchers highlighted that about 95–97% of the
Ignition temperature 418–426 490–595
global bioenergy production is based on direct biomass com-
Heating value 14–21 23–28
bustion (Fouilland et al. 2010; Zhang et al. 2010). Given the
massive quantity of ash produced from coal burning (∼780 *(wet % of dry fuel)
million tonnes), a less quantity of biomass ash (∼ 480 mil-
lion tonnes/year) was assumed to be generated coincided
with the burning of 7 billion tonnes of biomass (Vassilev As a solid fuel, fossil-based fuels are still dominating this
et al. 2013; Izquierdo and Querol, 2012). Retrofitting invest- sector for power generation and heat. However, biomass uti-
ment cost characterised to various power plants was USD lisation in the co-combustion along with fossil-based fuel
430–500/kW, USD 760–900/kW and USD 3000–4000/kW is seen as a cost-effective and interesting option (Variny
for co-feed plants and separate feed plants indirect co-fir- et al. 2021). Co-combustion of those two feedstocks offers
ing, respectively. These investigated costs were totally more higher power generation than biomass alone, and biomass
minuscule than the specified outlays of 100% biomass power ash is acting as a sulphur capture and mitigates the sulphur
generations facilities (Sahu et al. 2014). Table 1 presents oxide emissions. On the other hand, some challenges arise
the energy content in MJ/kg for several kinds of biomass, when mixing fossil-based coal with biomass, leading to
whereas Table 2 displays the differences in the physicochem- higher corrosion, slagging and fouling due to the high alkali
ical characteristics of biomass and coal-based fuels. contents within the biomass. In fact, the projected gradual

Table 1  Energy contents of biomass sources, in MJ/kg

Biomass type Energy content Biomass type Energy content Biomass type Energy content
(MJ/kg) (MJ/kg) (MJ/kg)

Greenwood 8 Sour cherry stalk 17.59 Coconut shell 20.00

Manure 8.650 Tobacco leaf 17.97 Oven dry plant matter 20
Sewage 10.510 Corncob 17.99 Wood bark 20.3
Mustard stalk 10.73 Soybean cake 18.30 Hazelnut shell 20.47
Corn stover 10.730 Peanut shell 18.46 Spruce wood 20.5
Rice husk 13.524 Cereals 18.61 Wood bark 20.57
Peat 15.30 Ailanthus wood 18.93 Peach stones 20.657
Cotton gin 15.500 Tobacco stalk 19.02 Olive cake 21.57
Olive refuse 15.77 Hazelnut seed coat 19.2 Olive husk 21.8
Fuelwood 16.10 Eucalyptus (Grandis) 19.35 Oak bark 22
Peach bagasse 16.24 Poplar 19.38 Olive pits 22
Potatoes 17.00 Colza seed 19.38 Apricot stones 22.082
Potato peel 17.18 Spruce wood 19.45 Tuncbilek lignite 23.212
Corncob 17.3 Coir pith 19.50 Rape seed 27.80
Sugar beet 17.40 Black locust 19.71 Tyres 36.800
Cotton cake 17.50 Groundnut shell 19.80 Methane gas 55

Data sourced from: Sami et al. 2001; Demirbas 2001; Demirbas 2005; Atimtay 2010; Haykiri-Acma 2003; Borjesson 1996; Raveendran and
Ganesh 1996; Erol et al. 2010; Friedl et al. 2005

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4080 Environmental Chemistry Letters (2021) 19:4075–4118

shutdown of fossil-based power plants will limit this co- Commercially, the torrefaction process is a potential
combustion approach. Technologies such as gasification and scenario to be applied in various applications. Salapa et al.
pyrolysis should be the main focus, to be competitive and revealed the high adsorption capacity of torrefied barley
fully available in the near future. straw of 11.65 mg/g at operating parameters of 220 °C and
20 min (Salapa et al. 2018). Other researchers evaluated the
torrefaction process’s impacts at different contact periods
Torrefaction (i.e. 20, 40 and 60 min) and temperature (160–260 °C) on
ethanol generation based on rice straw. They found out that
Torrefaction is an endothermic pretreatment pathway that the best yield of 351 mg/g was obtained at operating param-
mainly proceeded at temperatures ranging from 200 to eters of 220 °C and 40 min.
300 °C and a non-accelerated heating rate of less than 50 °C/ Additionally, they confirmed that the torrefied biomass
min in an oxygen-free atmosphere. This process is used for enhanced the yield of ethanol production at a value of
upgrading the solid biomass to produce a torrefied prod- 50.67% compared with the untreated one (Chiaramonti et al.
uct used later as a suitable alternate to coal (Cahyanti et al. 2011; Sheikh et al. 2013). An enhancement in the ethanol
2020). Three transformational reactions, including: vola- production yield is based on torrefied sugarcane bagasse and
tilisation, polymerisation and carbonisation, occur during waste jute caddies by 19.34 and 20.28%, respectively, com-
the torrefaction process. The process efficacy is influenced pared with the untreated biomass (Chaluvadi et al. 2019).
by temperature, reaction time, particle magnitude, carrier Igalavithana et al. demonstrated that torrefied product
gas type and flow, catalyst and performance index (Chen could be positively utilised in soil improvement because it
et al. 2021). This strategy significantly improves the phys- increases air space, water retention efficiency, plant pros-
icochemical properties of utilised biomass such as hydro- perity, microbial community and enzymes activity (Igala-
phobicity, grindability, mass/energy density, ignitability, vithana et al. 2017). Ogura et al. observed an increase in
moisture expelling and homogeneity (Chen and Kuo 2011). the growth of the J. curcas when it was exposed to varied
Commonly, the torrefied product is termed bio-coal or ratios of 250 °C torrefied biomass of 1, 3 and 5% (Ogura
green coal and biochar when used as fuel and soil amend- et al. 2016). The feasibility of using torrefaction condensate
ment. Based on the mode of operation, the torrefaction pro- in plant safeguarding pathways (pest repellent, insecticide
cess can be practically classified into dry, wet and steam and herbicide) was confirmed considering its minimum
modes (Barskov et al. 2019). In dry torrefaction, biomaterial amounts of polycyclic aromatic hydrocarbons and phenolic
feedstock can be torrefied in non-oxidative; for example, compounds. Comparing the as-generated condensate result-
nitrogen and carbon dioxide are carrier gases or oxidative ing from different biomass feedstocks such as pine bark,
mediums such as air, flue gases and other streams with vari- forest residue, wheat straw and willow biomass, the con-
ous oxygen concentrations at working temperatures ranging densate based on willow had the best pesticide performance
from 200 to 300 °C. Attributing to oxygen presence, the (Hagner et al. 2020). Table 3 presents more details about the
oxidative scenario has a better reaction rate than the other physicochemical properties of numerous biomass after the
non-oxidative scenario and minimises the reaction time torrefaction process.
(Thanapal et al. 2014; Lynam et al. 2011).
Contrarily, in the wet torrefaction, the biomass is torre- Hydrothermal liquefaction
fied in a wet environment, typically water and dilute acids
at 180–260 °C and 5–240 min for surrounding temperature Hydrothermal liquefaction is defined as a thermochemical
and holding time, respectively, and the produced solid is pathway at which the lignocellulosic feedstock, whether wet
termed as hydrochar. Under these subcritical conditions, or dry, is effectively decomposed into renewable liquid fuel
physicochemical properties of water such as density, dif- (Guo et al. 2015). Based on the mode of operation, it can
fusivity, dielectric constant and viscosity alter and improve be divided into two main subclasses: (1) indirect liquefac-
the biomass degradation process, which further upgrades tion and (2) solvent liquefaction (Mika et al. 2018). In the
the torrefaction process (Bach and Skreiberg 2016; Balat first scenario, biomass or its liquefied products are first con-
et al. 2008). verted into syngas followed by a subsequent fuel synthesis
Besides the two mentioned routes, steam torrefaction (i.e. alcohols and alkanes). In contrast, in the second, direct
by introducing steam with elevated temperature and pres- conversion of biomass into liquid fuel occurs by the action of
sure is conducted to torrefy the biomass at 200–260 °C and proton solvents such as water, alcohols, phenols, sulpholane
5–10 min for environmental temperature and contact period, and ionic liquid. Solvent liquefaction has the priority to be
respectively. The subsequently accelerated venting of the implemented over the other scenario because of its remark-
pressure will allow steam to bulge the biomass and split it able merits, moderate operational conditions and higher
with minor loss in the feedstock. yield of products (Gollakota et al. 2018).

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Environmental Chemistry Letters (2021) 19:4075–4118 4081

Table 3  Physicochemical properties of biomass after torrefaction

Biomass Fixed carbon (wt.%) Volatile matter Ash content O/C ratio H/C ratio Higher heating References
(wt.%) value (MJ/kg)

Cotton stalk 16.48–34.01 53.62–75.44 4.79–9.88 0.25–0.83 0.06–0.14 18.68–25.43 Chen et al. (2014a),
Corn stalk 18.41–41.19 35.30–69.32 9.18–20.52 0.18–0.85 0.06–0.15 18.26–23.61 Chen et al.
(2014b), Chen
et al. (2014c)
Rice straw 17–48 44.5–79.6 6–8 0.24–0.81 0.09–0.16 19.0–28.6 Nam and Capareda
(2015)
Oil palm fibre 5–25 3–48 6–7 0.22–0.62 0.02–0.10 11.13–23.98 Chen et al. (2013)
Cryptomeria 12–25 5–54 0 0.23–0.63 0.02–0.09 21.94–28.81
japonica
Coconut fibre 2–27 2–40 0.5–3 0.22–0.55 0.02–0.09 11.86–26.46
Eucalyptus 15–28 10–61 0 0.28–0.64 0.02–0.09 20.59–26.28
Bamboo 25.05–47.03 48.05–70.20 1.43–1.95 0.34–0.63 0.07–0.10 21.02–27.26 Chen et al. (2012a),
Rice husk sugar cane 22.27–41.17 33.33–61.88 13.18–23.11 0.32–0.70 0.06–0.10 17.68–21.48 Chen et al.
bagasse Madagas- (2012b)
car almond
Oil palm 32.58–43.67 39.60–53.76 9.97–13.52 0.29–0.54 0.06–0.08 20.61–23.54
Reed canary grass 13.3–21.3 70.5–80.3 6.4–8.3 0.67–0.74 0.11–0.13 20.0–21.8 Bridgeman et al.
Wheat straw 15.6–38 51.8–77 7.4–10.2 0.48–0.72 0.10–0.12 19.8–22.6 (2008)
Olive tree pruning 20.4 74.5 2.4 0.89 0.16 N/A Martin-Lara et al.
(2017)
Olive trimmings 16.54–32.57 55.96–77.65 3.87–7.87 0.18–0.70 0.08–0.12 19.6–28.4 Martin-Lara et al.
Olive pulp 19.98–50.40 41.99–77.22 1.50–6.67 0.24–0.72 0.08–0.12 20–26.8 (2017), Volpe
et al. (2015)
Orange peel waste 38.9–62.2 37.8–61.1 3.3–5.6 0.24–0.65 0.06–0.10 21.45–28.74
Lemon peel waste 35.7–61.8 38.2–64.3 3.5–6.3 0.22–0.68 0.06–0.10 21.09–28.75
Jatropha seed 13.5–35.5 55.5–79.8 6.25–7.60 0.29–0.49 1.14–1.50 21.8–23.8 Hsu et al. (2018)
residue

The main process parameters that directly influence the viscosity as well as dielectric constant and weakening of
hydrothermal liquefaction process include biomass composi- water hydrogen bonds consequently enhance the solubil-
tion, particle size, pressure, temperature, heating rate, resi- ity of hydrophobic organic compounds associated with an
dence time, feed/solvent ratio and presence of the catalyst improvement in the catalytic activity of acid–base reac-
and reducing gas. Generally, the hydrothermal liquefaction tions. This results in biomass conversion into four main
process operates at mild operational parameters of tempera- fractions: bio-crude fuel (liquid), water-soluble products,
ture in the temperature range of 250–500 °C and pressure of solid residue and gases.
5–35 MPa and contact periods of 5–60 min. These processes Moreover, hydrothermal liquefaction is regarded as lesser
are conducted in the presence of solvents such as sub/super- energy consumption and a higher efficiency strategy than
critical water, organic solvents and mixed solvents such as pyrolysis because of the better physicochemical properties.
the combination of water + organic solvent (Yang et al. 2019; The produced bio-crude from the hydrothermal liquefaction
Akalın et al. 2017). process has an oxygen content of 10–20 wt.% and a heating
The utilisation of water as a solvent has numerous value of 30–35 MJ/kg, which is typically higher than those
advantages over conventional organic solvents due to its obtained from the conventional pyrolysis process (Guo et al.
natural occurrence in biomass and eco-friendliness. On 2019). During the hydrothermal liquefaction process, the
the contrary of water state at ambient conditions, the oxygen contained in the biomass is partially eliminated by
compressed water in a liquid state at the following criti- dehydration, decarboxylation and decarbonylation reactions
cal conditions of temperature and pressure of 374 °C and associated with producing CO, ­CO2 and ­H2O. Despite its
22.064 MPa, respectively, generates higher ionic products higher quality, the higher oxygen content produces a highly
(i.e. ­H3O+ and ­OH−) ions. Proximity to the mentioned crit- sticky and acidic bio-crude product with a low heating value.
ical conditions, the physicochemical properties of water Distinctly, its quality enormously varies depending on the
such as density, viscosity, dielectric constant, polarity and operational parameters and biomaterial feedstock composi-
permittivity change (Arun et al. 2021). Decrease in the tion (Scarsella et al. 2020).

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4082 Environmental Chemistry Letters (2021) 19:4075–4118

Hence, the importance of downstream upgrading treat- coffee grounds of approximately 15 and 17.4% for lipids and
ment such as catalytic cracking and hydrotreating for the proteins, respectively, were liquefied to output crude bio-oil
obtained bio-crude cannot be ignored for further utilisation under ­N2 atmosphere at (200–300 °C) and (5–25 min). The
as a transportation drop-in fuel. In situ upgrading using vari- highest yield of acetone-recovered bio-crude oil (47.3 wt%)
ous acidic or alkaline homogenous catalysts such as HCl, was obtained at 275 °C and 10 min with an estimated higher
­H3PO4, ­K2CO3, ­Na2CO3, NaOH, KOH and Ca(OH)2 and heating value of 31.0 MJ/kg−1, better than that of spent cof-
heterogeneous catalysts has been studied in detail (Perkins fee grounds of 20.2 MJ/kg (Yang et al. 2016). Xiu et al. per-
et al. 2019). Despite its cost and energy-saving nature, it has formed a study on swine manure composed of < 1%, 17.1%,
some disadvantages, such as operating at high pressure that 22.3% and ∼35% for lignin, crude protein, ash content and
results in the necessity of (1) solid/water slurries reliable saccharide, respectively. They successfully liquefied into
pumping, (2) suitable unit metallurgy to avoid the potentially bio-crude oil under ­N2 atmosphere at a temperature range
corrosive nature of slurries at the operational parameters of 260–340 °C and a contact period of 15 min with a yield
of elevated pressures and mild temperatures and (3) usage of 24.2 wt.% and higher heating value of 36.05 MJ/kg (Xiu
of heat exchangers with high surface areas to overcome et al. 2010). Table 4 presents the proximate as well as ulti-
the problem of low heat transfer coefficients (Beims et al. mate investigations of crude bio-oil prepared from several
2020). Li et al. (2017) liquefied wheat straw biomass at a biomaterials through the hydrothermal liquefaction process.
pre-adjusted temperature of 270 °C for 120 min, and the
resulting oil was forwarded for manufacturing of bio-polyols Pyrolysis
and polyurethane foams (Li et al. 2017).
Crude oil originated from liquefaction of bark biomass Pyrolysis is counted as one of the most as-used thermo-
was directed to produce bio-based phenol–formaldehyde chemical scenarios to degrade the carbonaceous biomass,
formable resole as reported by Li et al. (Li et al. 2016). Spent such as cellulose, hemicellulose and lignin (Aravind et al.

Table 4  Proximate and ultimate analyses of bio-crude oil prepared from several biomass through hydrothermal liquefaction process. Where,
the higher heating value is abbreviated as HHV
Feedstock Elemental composition of product spe- Physicochemical properties of the prod- References
cies after Hydrothermal liquefaction ucts
C (%) H (%) N (%) O (%) S (%) Ash % Moisture % HHV Yield %

Spirulina algae 68.9 8.9 6.5 14.9 0.86 – – 33.2 32.6 Vardon et al. (2011
Anaerobic sludge 66.6 9.2 4.3 18.9 0.97 – – 32.0 9.4
Cladophora glomerata 26.8 3.53 2.14 20.48 0.22 36.5 9.1 10.29 – Plis et al. (2015)
Nannochloropsis gaditana 40.3 5.97 6.30 14.49 0.37 28.3 4.1 18.53 –
Almeriansis 73.2 9.3 5.1 0.8 11.7 35.8 20.0 – 42.6 López Barreiro et al. (2015)
Gaditana 74.2 9.3 4.0 0.6 11.8 36.2 12.4 – 50.8
Swine manure 71 8.9 4.1 0.21 14.2 35 – – 61 Toor et al. (2011)
Algae Dunaliella tertiolecta 63.55 7.66 3.71 – 25.8 30.7 – – 25.8
Porphyridium 66–83 5–11 0–12 0–1 8–27 22.8–36.9 – – 5–25
A.esculenta 73.8 8 3.8 14 0.8 – – 33.8 17.8 Anastasakis and Ross (2015)
L.digitata 70.5 7.8 4.0 17 0.7 – – 32 17.6
L.Saccharina 31.3 3.7 2.4 26.3 0.7 24.2 9.2 12 –
Fucus vesiculosus 32.88 4.77 2.53 35.63 2.44 11.8 – 15.0 – Ross et al. (2008)
Laminaria hyperborea 34.97 5.31 1.12 35.09 2.06 11.2 – 16.54 –
Miscanthus 46.32 5.58 0.56 41.79 0.2 2.1 – 19.08 –
A.azuera 40.82 5.56 0.63 52.99 – 10.61 6.03 52.99 – Aysu et al. (2016)
Nannochloropsis Oceana 77.6 4.9 3.4 – 0.3 – – 37.70 54.2 Caporgno et al. (2016)
Aspen wood 75.2 8.2 0.5 15.8 0.3 0.48 3.8 34.3 – Pedersen et al. (2016)
Arthrospira platensis 74.5 10.2 6.8 7.5 1.0 – – 38.65 30 Lavanya et al. (2016)
Tetraselmis 71.4 9.5 5.7 12.3 1.1 – – 35.58 29
Nannochloropsis Salina 55.16 6.87 2.73 33.97 1.27 2.48 4.95 25.40 – Toor et al. (2013)
Seaweed meal 43.99 5.95 5.21 36.13 1.02 7.7 7.92 18.35 – Ferrera–Lorenzo et al. (2014)
Laurel algae 48.97 6.38 3.02 41.63 – 10.53 9.95 19.77 – Ertas and Alma (2010)

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Environmental Chemistry Letters (2021) 19:4075–4118 4083

2020). This results in the generation of solid, liquid and gas The primary product is liquid oil (Bridgwater, 2012a). Ultra-
biofuels in an oxygen-free atmosphere via endothermic reac- fast or flash pyrolysis is a highly accelerated pyrolysis at a
tion (Perkins et al. 2018). The yield of the pyrolytic products high heating rate with major gas and oil products. The oper-
is influenced by factors, including feedstock composition ating conditions are as follows temperature (medium–high
such as structure and complexity. This is also coupled with (700 – 1000 °C), shortest residence time and fastest heating
pyrolysis impacting factors such as particle size, tempera- rate. Yields of outputs are: (1) liquid condensate (10 – 20%),
ture, heating rate, residence time, inert gas type, inert gas (2) gases (60 – 80%) and (3) char (10 – 15%) (Priharto et al.
flow, catalyst type and others (Azizi et al. 2018). 2020).
Complex reactions such as dehydration, decarboxylation, Catalytic pyrolysis is a process in which catalysts such as
decarbonylation, hydrogenation, isomerisation, aromatisa- natural zeolite, Cu/Al2O3, Co/Mo/Z, Zeolite-ß, ­Fe2O3 and
tion, depolymerisation and charring are involved in the ther- Ni-CaO-C are used to decrease the reaction operating tem-
mal decomposition process of biomass. Typically, pyrolysis perature and increase the selectivity towards desired prod-
of biomass undergoes the following steps: (1) transfer of ucts. This process is used to optimise the biomass conversion
heat from its source to biomass to initiate the reaction, (2) into liquid fuels with improved physicochemical properties
elevated pyrolysis temperature of primary vapours con- (Cai et al. 2019; Chai et al. 2020). Catalytic co-pyrolysis
tributes to volatiles and char formation, (3) because of the of biomass and plastic waste showed promising results in
influx of hot vapours to the biomass, heat migration contin- upgrading the oil quality by removing oxygen from the
ues between unpyrolysed fuel and hot volatiles, (4) volatiles biomass and producing more aromatics and olefins (Wang
condensation associated with secondary reactions leads to et al. 2020b). This is due to the high hydrogen and carbon
tar formation and (5) autocatalytic secondary pyrolysis reac- contents within the plastic waste and consequently acting as
tions take place in conjugation with primary pyrolytic reac- a hydrogen donor in the catalytic co-pyrolysis process and
tion (Dabros et al. 2018). thus eliminates the oxygenated compounds. This approach
The impact of different pyrolysis operational parameters is seen as a sustainable, efficient and economical approach
occurs at different stages such as dehydration, decomposi- to upgrading the bio-oil quality, along with extending the
tion and reforming. With elevated heating rate, minimum catalyst’s lifetime.
vapour contact periods and a surrounding temperature of Microwave-assisted pyrolysis is a new thermochemi-
500 °C, liquid yields can be maximised (Chintala, 2018). cal process that transforms biomaterial feedstock into liq-
These conditions directly prohibit (1) thermal or catalytic uid oil using microwave input heat energy. In contrast to
cracking of the primary decomposition products due to char the conventional process, microwave-assisted pyrolysis is
presence to lesser non-condensable gas molecules as well a more effective and controllable technique. ­CO2-assisted
as (2) their polymerisation to char (Kasmuri et al. 2017). pyrolysis is a process by which C ­ O2 is delivered as a reactive
Table 5 presents different working modes of the pyrolysis medium instead of inert ­N2 in utilising the pyrolysis process
process. Other pyrolysis types such as catalytic and assisted and enhancing the syngas yield and declines the produced
microwave, carbon dioxide, additives, solar and hydro-pyrol- oil but also decreases the greenhouse gas emissions (Kwon
ysis can be performed to upgrade the product’s yields. et al. 2019).
Slow pyrolysis is a process in which organic materi- Additive–assisted pyrolysis is a type of pyrolysis at which
als are slowly heated at a low heating rate between 5 and metal salts such as sodium, potassium and calcium salts and
50 °C min-1 and the longest residence time above 10 s in the inorganic additives (zeolites, biochar) are added and thus
absence of oxygen, typically producing about 80% of car- having some advantages over conventional pyrolysis. It has
bon as the main product (Antoniou and Zabaniotou, 2013). a great potential to decrease the required operating tempera-
Fast pyrolysis is a strategy by which organic materials are ture, cracking time and solid residue yield, in addition, to
quickly heated at faster heating rates of > 103 °C ­s−1 and increase the cracking efficacy of wastes and improves the
shorter contact periods of up to 3 s without air existence. quality of pyrolysis products (Wang et al. 2019a).

Table 5  Different working Mode of action Working tem- Residence time (seconds) Yields (wt. %)
modes of the pyrolysis process perature (°C)
(Bulushev and Ross, 2011) Char Liquid Gas

Slow 450 Very long (> 30) 35 30 (70% water) 35

Medium 500 Moderate (10–30) 25 50 (50% water) 25
Fast 500 Short (< 2) 13 70 (30% water) 12
Flash 500 Very short (< 0.5) < 13 75 < 12
Gasification > 800 Long 10 5 85

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4084 Environmental Chemistry Letters (2021) 19:4075–4118

Solar pyrolysis is a process in which solar renewable decomposition stage and secondary cracking of vapours.
energy can be introduced as thermal input sources rather Bio-oil is formed by subjecting the heavier molecules con-
than electrical energy that maximise biofuel production and densable gases to subsequent cooling and condensation pro-
reduce ­CO2 emissions. Hydropyrolysis is a particular type of cesses, while the lower molecules non-condensable gases
pyrolysis at which biomaterial feedstock is decomposed with (i.e. CO and ­CO2) are not condensed during the cooling
the assistance of high pressurised hydrogen. Using the men- stage.
tioned technique above, a higher yield of hydrocarbons with
improved structures can be attained (Marcilla et al. 2013).
Pyrolysis products can be categorised as solid, liquid and Gasification
gases that can be exploited to generate chemicals, energy,
electricity and transportation fuels. Proximate and ulti- Gasification is a process by which carbonaceous materials
mate analyses are beneficial to characterise the obtained are thermochemically converted into valuable gases, com-
products. Char, pyrolytic char or biochar, is the produced monly referred to as synthesis gases in the presence of a
solid, chemically not pure carbon, and contains carbon as gasifying agent such as air, oxygen, steam, C ­ O2 or a combi-
the main constituent, hydrogen, nitrogen, ash and some nation of them at a temperature above 700 °C. Primarily, the
volatiles. The highly porous char is used in several appli- produced gas consists of CO, H ­ 2, ­CO2 and C
­ H4 (Shahabud-
cations as adsorbent and soil amendment for wastewater din et al. 2020). Generally, the gasification process com-
treatment and enhancement of crop yields. Bio-oil (tar) prises four main steps: (1) heating or drying (100–200 °C)
is a dark brown, sticky liquid produced from the thermal to decrease its moisture content, (2) pyrolysis, (3) oxida-
degradation of biomass. Typically, it consists of more than tion or partial combustion and (4) gasification. Firstly, the
400 chemical compounds (i.e. aldehydes, alcohols, amines, moisture content (30–60%) of the biomass is vaporised at
acids, esters, ethers, ketones, phenol derivatives, ketones, about 200 °C. Then, pyrolysis includes the decomposition
guaiacols, furans, oligomers, syringols and sugars) (Hen- of different biomass, including cellulose, hemicellulose and
rich et al. 2016). Considering its low carbon, nitrogen and lignin, into solid residues and volatiles occurs (Thomson
sulphur content, CO, S­ OX and NOx emissions are low, hence et al. 2020). Oxidation or partial combustion is the third
preparing bio-oil and conventional fuels. stage in which the resultant volatiles and char residues are
Table 6 displays a comparison between the properties oxidised to CO, ­CO2 and H ­ 2O with gasifying agent assis-
of prepared bio-oil (after water removal) and conventional tance beyond 700 °C (exothermic reaction).
liquid fuels. Concurrently, Table 7 shows physiochemical By the action of ­CO2 or steam as gasifying agents, car-
properties of as-formed bio-oil resulting from the pyroly- bon and volatile compounds react with them in terms of
sis of several biomaterials feedstocks without applying reduction reaction to produce syngas at a temperature over
any upgrading strategy as well as comparing it with other 800 °C in an endothermic reaction (Situmorang et al. 2020;
conventional fuels, respectively. Both condensable and Hanchate et al. 2021). Briefly, simultaneous exothermic
non-condensable gases are generated throughout the first and endothermic reactions are included in the gasification
process, and the first previously mentioned reactions are
considered as heat suppliers for the endothermic one. Main
Table 6  Comparison of bio-oil properties with conventional fuels reactions involved in the gasification include carbon reaction
(Bridgwater, 2012b) such as primary or secondary steam reforming, hydrogasi-
Property Bio-oil Diesel Heating oil Gasoline fication, oxidation, shift reaction, mechanisation and steam
reaction.
pH 2.0–3.0 – – –
The efficiency of the gasification process is impacted by
Viscosity at 40 °C (cP) 40–100 2.4 1.8–3.4 0.37–0.44
different operational parameters such as feedstock composi-
Density at 15 °C (kg/m3) 1200 820–950 865 737
tion, moisture content, ash content, granulometry, pressure,
Heating value (MJ/kg) 18–20 42 45.5 44
temperature, gasifier’s type, gasifying agents, equivalence
Pour point (°C) − 15 − 29 −6 − 60
ratio and steam to biomass ratio (Díaz González and Pacheco
Flash point (°C) 48–55 42 38 40
Sandoval 2020). Basically, the gasifier’s selection: fixed or
Solids (wt%) 0.2–1.0 – – –
moving bed (dry ash/slagging), fluidised bed (circulating,
Ash < 0.02 < 0.01
bubbling) and entrained flow (upflow, downflow), is con-
Carbon 42–47 87.4 86.4 84.9
trolled by several factors, for instance, feedstock composi-
Hydrogen 6–8 12.1 12.7 14.76
tion, gasifying agent and product requirements (Mehrpooya
Oxygen 46–51 – 0.04 –
et al. 2018). Syngas chemical composition and its heating
Nitrogen < 0.1 392 ppm 0.006 0.08
values vary based on the as-used gasification method, as
Sulphur < 0.02 1.39 0.2–0.7 –
presented in Table 8.

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 Table 7  Physiochemical properties of bio-oil produced from different biomasses without using any upgrading technique. Where, the higher heating value is abbreviated as HHV
Biomass Reactor type Operating Bio-oil elemental analysis Bio-oil fuel properties References
temperature (wt.%)
(°C)
C% N% O% H% pH Density (g/ml) Viscosity Water con- HHV Acid value
tent (wt.%) (MJ/kg) (mg of KOH/
g)

Saccharina Fluidised bed 300 60.15 5.77 16.48 7.74 5.9 – – 1.76 28.63 – Choi et al. (2017)
Japonica
Corn stover Fixed bed 400 – – – – 2.67 1.25 2.6b – 15.3 – Sundaram et al. (2016)
Environmental Chemistry Letters (2021) 19:4075–4118

Switchgrass Fixed bed 400 – – – – 2.77 1.25 2.1b – 14.9 –

Prairie cordgrass Fixed bed 400 – – – – 2.59 1.25 2.5b – 15.2 –
Pinewood Auger reactor 450 – – – – 3.08 1.18 55.2c 16.9 22.49 69.5 Hassan et al. (2009)
Sweetgum Auger reactor 450 – – – – 2.65 1.16 8.26c 38.3 2.65 119.2 Wang et al. (2012)
Corn stover Auger reactor 450 – – – – 2.66 1.08 1.60c 54.7 2.66 85.8
Saccharina Fixed bed batch 470 69.2 3.7 15.4 8.3 – 1.13 – 34.7 35.0 43 Choi et al. (2014)
Japonica
Pine wood Pilot-scale reactor 500 42.64 0.22 49.59 7.55 – 1.21 178.2c 23.5 18.9 – Zhang and Wu, (2014)
Oak Fluidised bed 500 54.9 0.07 38.7 6.28 – 1.24 57d 20.3 – 110 Elliott et al. (2014)
Loblolly pine Fluidised bed 500 50.1 0.53 42.7 6.65 – – 16.4b 19.8 – – Meng et al. (2014)
Chips
Hardwood Fluidised bed 500 61.35 0.24 32.07 6.34 – – 715b 8.93 23.5 Martin et al. (2014)
Eucalyptus Free-fall 550 – – – – 2.78 1.13 – 26.07 12.45 – Pidtasang et al. (2013)
Bark pyrolysis unit
Pine nuts Continuous 550 58 0.3 33.5 8.2 4.84 1.09 1244d 9.36 19.31 – Xu et al. (2018)
fixed bed
Walnut shell Spouted bed 550 37.91 1.19 25.02 8.78 4.28 0.95 3.29d 23.29 – – Zhu et al. (2017)
Napier grass Fixed bed 600 45.32 0.81 46.60 7.17 2.95 1.05 2.71b 26.01 20.97 – Mohammed et al. (2017)
Rice husk Fluidised bed 600 – – – – 3.59 1.15 58.16d 15.82 22.99 – Zhang et al. (2014)
Oakwood – 600 59.99 0.92 31.91 7.18 – – 173.35a 15.15 24.87 – Farooq et al. (2019)
Softwood – 600 39.96 0.11 52.19 7.74 – 1.20 67.39a 28.05 15.27 79.23 Jiang and Ellis (2010)

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Table 8  Composition of syngas from several biomass types and operating parameters, where N.A is abbreviated for not available
Biomass type Operating parameters Syngas Composition (% v/v) Lower heating
value
Temperature Steam: biomass Gasification Gasifying agent Equivalence CO CO2 CH4 H2 (MJ/Nm3)
(°C) reactor ratio

Bamboo 400–600 – Fluidised bed Air 0.4 23.5–30.6% 59–63% m/m 4–5% m/m 6.6–8.1% m/m 1.6–1.9
m/m
Bamboo 400–600 0:1, 1:1 Fluidised bed Air & air–steam 0.4 23.5–30.6% N.A N.A 6.6–8.16% m/m N.A
m/m (air); (air);10.9–
36.1–40.3% 16.5% m/m
m/m (air–steam)
(air–steam)
Empty Fruit 700–1000 – Fluidised bed Air 0.15–0.35 21–36 10–65 5–14 10–38 7.5–15.5
Bunch
Empty Fruit 850 – Fluidised bed Air 0.15–0.35 32–45 16.6–36 12–15 18.3–27.4 12.3–15.3
Bunch
α–cellulose 800 0–1.5 Fluidised bed Air–steam 0.27 6.5–11.2 26.3–27.7 2.2–3.7 13.5–18.5 6.5–7.6
Palm oil wastes 800 0.67–2.67 Fixed bed Steam – 14–33 14–26 3–6 47–58 8.7–12
Pine sawdust 700–900 2.7 Fluidised bed Air–steam 0.22 35–43 18–20 6–10 21–39 7.4–8.6
Palm oil wastes 750–900 1.3 Fixed bed Steam – 15–25 20–25 4–5 48–60 9.1–11.2
Olive kernel 950 – Fixed bed Air 0.14–0.42 15–20% w/w 40–55% w/w 10–12% w/w 20–30% w/w 8.8–10.4

Data sourced from Ahmad et al. (2016), Lv et al. 2004), Luo et al. (2009), Mohammed et al. 2011), Skoulou et al. (2008)
Environmental Chemistry Letters (2021) 19:4075–4118
Environmental Chemistry Letters (2021) 19:4075–4118 4087

Other gasification types such as plasma, supercritical and organic solvents, and (4) biological or combination of
and microwave have been operated to improve gasification them (Haghighi Mood et al. 2013). Secondly, pretreated
yields. In plasma gasification, an intense plasma thermal biomass is decomposed into monomers by the action of
process is used to catalyse and ionise the organic compounds acid/enzymatic hydrolysis. Lastly, the intermediate mono-
in biomass and gas, respectively, into syngas with slag using mers are converted into alcohols using yeast/bacteria (Liu
a plasma torch powered by an electric arc (over 2500 °C). et al. 2015).
Supercritical water gasification is a type of gasification typi- Based on the consolidation degree of the mentioned
cally performed in the presence of a vast amount of water stages, ethanol production can be configured into four
for the generation of H ­ 2 and C ­ H4 (Rodriguez Correa and routes: (1) separate hydrolysis and fermentation, (2)
Kruse, 2018). The process yield is very high, mainly affected simultaneous saccharification and fermentation, (3)
by different parameters such as temperature, catalyst and simultaneous saccharification and co-fermentation and
biomass/water ratio. (4) consolidated bioprocessing. In separate hydrolysis and
Microwave gasification is a compelling scenario for trans- fermentation scenarios, enzyme generation, hydrolysis,
forming biomass. This technique’s benefits over the tradi- hexose and pentose fermentation are employed in separate
tional methods include uniform distribution of temperature, OR individual reactors. Despite execution of hydrolysis
efficacy for large particle handling and higher heating values in addition to fermentation at their optimised conditions,
(Chen et al. 2015). Different technologies such as scrubbers accumulation of cellobiose and glucose enzymes during
(i.e. spray, dynamic wet, cyclonic, impactor, venture and hydrolysis process negatively prohibits the efficiencies of
electrostatic) and filters (fabric bag, fibrous ceramic, metallic cellulases (Margeot et al. 2009).
foam and granular bed) have been used to clean up the syn- In the simultaneous saccharification and fermentation
gas from different contaminants and hence improve its qual- scenario, cellulose hydrolysis and hexose fermentation
ity for numerous applications. A list of global biofuel-based simultaneously run at the same reactor that overcomes
facilities comprising its manufacturer, country, starting-up cellulase inhibition because of instant consumption of
year, feedstock composition, downstream products and as- sugars by fermenting microorganisms (Hahn-Hägerdal
used technology is shown in Table 9. et al. 2007). In the simultaneous saccharification and co-
fermentation scenario, two genetically modified strains
Biochemical conversion methods of Saccharomyces cerevisiae and Zymomonas mobilis are
used to co-ferment glucose and xylose in the same reac-
Biochemical conversion pathways such as anaerobic diges- tor. In the consolidated bioprocessing scenario, only one
tion and fermentation can be employed to generate various microorganism is simultaneously utilised for hydrolysis
biofuels from waste biomass, as shown in Fig. 3. Biochemi- and fermentation, which decreases the operation cost and
cal conversion methods have numerous merits, including enhances the process efficacy (Lin and Tanaka, 2006).
high product selectivity, high product yield and flexibility Different modes of fermentation can be briefly viewed
to be operated at ambient temperature and pressure condi- as follows: photo-fermentation is a fermentative trans-
tions (Singh et al. 2016). Ethanol and bio-hydrogen can be formation of organic substrate to produce bio-hydrogen
produced from the fermentable biomass via alcoholic fer- driven by miscellaneous groups of photosynthetic bacteria.
mentation and dark fermentation/photo-biological routes, This is occurring throughout a set of biochemical reac-
respectively, whereas biogas can be produced anaerobic tions in three steps like anaerobic conversion. The dif-
digestion as follows (Osman et al. 2020). ference between photo-fermentation and dark fermenta-
tion is its proceeding in the presence and absence of light,
Fermentation respectively.
Alcoholic fermentation is another type of fermentation
Fermentation is a process by which biological activities driven by yeast by which sugars are transformed into cel-
are utilised conjugated with air existence known as aerobic lular energy associated with the generation of ethanol and
fermentation or without air called anaerobic fermentation carbon dioxide. Considering its occurrence in the absence
(Karimi et al. 2021). Bioconversion of biomass to biofuel of oxygen, it can be categorised as an anaerobic integration
comprises of sequential stages: pretreatment, hydrolysis process. Heterotrophic algae or yeast can transform sugars
(acid/enzymatic) and fermentation (Alvira et al. 2010). into lipids inwards their cells associated with using suitable
The pretreatment step aims to damage the cell wall as solvents to break down the cells (Łukajtis et al. 2018). The
well as exhibit cellulose and hemicellulose for subsequent resultant lipids can be further purified and improved to liq-
hydrolysis. It can be classified into four main categories, uid forms of transport fuels by hydro-treated vegetable oil
including: (1) physical, e.g. grinding, (2) physicochemi- diesel scenario, whereas genetically modified bacteria con-
cal, e.g. wet oxidation, (3) chemical, e.g. oxidising agents sume sugars and consequently produce short-chain gaseous

13
 Table 9  Second-generation biofuel plants, including manufacturer, country, starting up a year, feedstock composition, downstream products and as-used technology
4088

Company/Institute/University Starting-Up Year Feedstock composition Downstream products As-used Technology TRL-scale Country
name

13
Lahti Energia Oy 1998 Wood waste Renewable diesel Circulating Fluidised Bed TRL 9 commercial Finland
(HVO) (70 MWth) gasifier
CHP Agnion Biomasse 2001 Wood waste (80,000 t/year) Synthetic natural gas (32.5 Agnion Heatpipe-Reformer TRL 4–5 pilot Germany
Heizkraftwerk Pfaffenhofen MWth)
CHOREN Industries GmbH 2003 Dry wood chips from recy- Fischer–Tropsch liquids (53 Not available TRL 4–5 pilot Germany
cled wood and t/year)
residual forestry wood
Vienna University of 2005 Syngas from FICFB gasifier Fischer–Tropsch liquids Not available TRL 4–5 pilot Austria
Technology/BIOENERGY (5 ­m3/h) (5 kg/day)
2020 +
West Biofuels 2007 Clean wood, waste wood (5 Fischer–Tropsch liquids Dual fluidised bed thermal TRL 6–7 demonstration United States
t/day) reforming
Bio SNG Guessin 2008 Syngas from gasifier (350 Synthetic natural gas (576 t/ Not available TRL 6–7 demonstration Austria
­m3/year) year)
TUBITAK MRC—ENERGY 2009 biomass Synthetic natural gas Downdraft fixed bed gasifier TRL 4–5 pilot Turkey
INSTITUTE—TURKEY (0.2 MW)
Greasoline GmbH 2011 Bio-based oils and fats, resi- Diesel type hydrocarbons (2 Catalytic cracking of bio- TRL 4–5 pilot Germany
dues of plant oil process- t/year) based oils + fats primarily
ing, free fatty acids used produces diesel fuel-range
bio-based oils and fats (3 hydrocarbons
t/year)
BioTfueL-consortium 2012 Forest waste, straw, green Fischer–Tropsch liquids (60 Not available TRL 4–5 pilot France
waste, dedicated crops t/year),
jet fuel component
TUBITAK 2013 Combination of hazelnut Fischer–Tropsch liquids (250 Pressurised fluidised bed TRL 4–5 pilot Turkey
shell, olive cake, wood chip t/year) gasifier
and lignite blends (0.2 t/h)
Goteborg Energi AB 2014 Forest residues, wood pellets, Synthetic natural gas (11,200 Repotec indirect gasification Demonstration Sweden
branches and treetops t/year) technology and Haldor
Topsoe fixed bed methana-
tion
BioMCN 2017 Wood chips Methanol (413,000 t/year) Not available TRL 8 first-of-a-kind Netherlands
Go Green Fuels Ltd 2018 Refuse derived fuel and Synthetic natural gas (1500 Not available TRL 8 first-of-a-kind com- United Kingdom
waste wood (7500 t/year) t/year) mercial demo
ECN 2019 Not available Synthetic natural gas Not available TRL 6–7 Netherlands
(300 MW) demonstration
Red Rock Biofuels 2019 Not available Diesel type hydrocarbons Not available TRL 8 first-of-a-kind com- United States
(1 t/year) mercial demo
Fulcrum BioEnergy Sierra 2019 Waste (20,000 t/year) Fischer–Tropsch liquids Not available TRL 9 commercial United States
Biofuels Plant (314,913 t/year)
Environmental Chemistry Letters (2021) 19:4075–4118
 Environmental Chemistry Letters (2021) 19:4075–4118 4089

alkenes that can be transformed by oligomerisation and

hydro-treatment into jet/gasoline.
Electro-fermentation is a novel fermentation pathway by
Country

Canada which microorganisms can be induced by using an electric

field that can positively: (1) stabilise and optimise metabo-
lisms of fermentation integration process by regulating
redox and pH imbalances, (2) stimulate whether breakdown
or elongation of carbon chain via different oxidative/reduc-
tion conditions, (3) enhance the synthesis of adenosine
triphosphate and upgrade the yield of microbial biomass, (4)
demonstration

extract purposed products via selective membranes and (5)

TRL-scale

TRL 6–7

the possibility of directing the fermentation reaction towards

the manufacture of a single and specific product (Schievano
et al. 2016).

Anaerobic digestion
As-used Technology

Anaerobic digestion is a biochemical, cost-effective and

environmentally sustainable approach for upcycling biomass
Not available

(Al-Wahaibi et al. 2020). It is a recovery process by which

biodegradable organic substrates’ bioconversion into renew-
able biogas occurs by several anaerobic organisms in an
oxygen-free environment (Kainthola et al. 2019). Typically,
the produced biogas comprises 50–75% of C ­ H4, 30–50% of
­CO2, (0–3% of ­N2, ~ 6% of ­H2O and 0–1% of ­O2. The biogas
Downstream products

could also contain other minor impurities such as ammonia,

hydrogen, hydrogen sulphide, nitrogen and water vapours
Ethanol (30,000

(Wainaina et al. 2020).

The growth rate of biogas production was 11.2%, with
t/year)

approximately 58.7 billion ­Nm3 in 2017. It has been inves-

tigated that the production outlay of biogas resulting from
anaerobic digestion plants will be declined by about 38% in
2050 compared with 2015. Collectively, more than 17,240
Company/Institute/University Starting-Up Year Feedstock composition

operating anaerobic digestion facilities in Europe generated

63.3 TWh of electricity based on biogas, which represented
about 14.6 million European households of the global con-
sumption rate per year in 2014. The American Biogas Coun-
cil announced that about 2,000 anaerobic digestion plants
N.A

were operated to handle the residues from municipal waste-

water treatment facilities, food waste and animal manure
digestion in 2015 (Shrestha et al. 2017).
In addition to biogas energy generation, the anaerobic
Data were taken from (Molino et al. 2018)

digestion process contributes to nutrient recovery, mitiga-

2019

tion of greenhouse gas emissions as well as depletion of

dissolved oxygen (Bharti et al. 2021). Several parameters,
including alkalinity, organic loading rate, temperature, pH,
feedstock composition, hydraulic retention time and con-
centration of volatile fatty acids, directly affect the anaero-
Vanerco (Enerkem&
Table 9  (continued)

Greenfield Ethanol)

bic digestion process and the physiochemical properties of

biogas (i.e. composition and heating value). Considering the
organic substrates, the anaerobic digestion process can be
categorised into wet, semi-dry and dry due to varying % of
name

total solids (Feng and Lin 2017).

13
4090 Environmental Chemistry Letters (2021) 19:4075–4118

Fig. 3  Biochemical conversion route of biomass utilisation into bio- turn, along with hydrogen, could be converted into liquid fuel. The
fuel, including fermentation and anaerobic digestion processes. Two other route is the fermentation process, where the biomass is firstly
types of digestion: aerobic digestion, which produces carbon dioxide pretreated and then followed by enzymatic hydrolysis and fermenta-
and fertiliser, while anaerobic digestion produces biogas which in tion, and finally, the production of liquid biofuel

Commonly, an anaerobic digestion scenario comprises produce methane (biogas) by two sets of methanogens: ace-
four consecutive steps, including hydrolysis, acidogenesis, toclastic and hydrogen utilising organisms (Matheri et al.
acetogenesis and methanogenesis, catalysed by different 2018; Ganesh Saratale et al. 2018). The first group (ace-
microorganisms, whereas hydrolysis is deemed as the rate- toclastic methanogens) convert acetate into methane and
determining step. carbon dioxide, while the other ones (hydrogen-utilising
Hydrolysis is the first stage of anaerobic digestion at methanogens) generate methane by applying hydrogen and
which complex biopolymers (i.e. carbohydrates, lipids, carbon dioxide as electrons donor and acceptor, respectively.
proteins, polysaccharide and nucleic acid) are converted Several enhancement techniques as pretreatment steps
into simple soluble compounds (i.e. amino acid, fatty acids, such as: (1) physical, e.g. milling, (2) chemical, e.g. ionic
monomers, sugar, purines and pyrimidine) by the action of liquid and surfactant, thermophysical, e.g. microwave irra-
enzymes (i.e. amylases, lipases and proteases) produced diation, (3) thermochemical, e.g. supercritical ­CO2, ammo-
from hydrolytic bacteria (Sawatdeenarunat et al. 2015). nia fibre explosion and ammonia fibre percolation, and
Secondly, acidogenesis is the second stage of anaerobic (4) biological (microbial and enzymatic) can precede to
digestion at which the simplified amino acids, sugar, fatty enhance anaerobic digestion process (Gautam et al. 2020).
acids and monomers are converted into intermediate bio- To upgrade the quality of biogas (impurities removing),
molecules (i.e. alcohols, volatile fatty acids, propionic and an additional cleaning step can be added to capture C ­ O 2,
butyric acids) by fermentative bacteria. ­H2S and water vapours and avoid mechanical and chemical
Acetogenesis is the third step at which the mentioned appliances throughout its utilisation. Different materials (i.e.
acidogenesis products serve as a substrate to produce acetate silica gel) can be used to tackle ­H2S and water vapours.
by homoacetogens bacteria. Methanogenesis is the last step In contrast, other techniques (i.e. water scrubbing, organic
at which both acetate and carbon dioxide are directed to scrubbing, membrane separation, cryogenic technology and

13
Environmental Chemistry Letters (2021) 19:4075–4118 4091

pressure swing adsorption) can be delivered to sequester residence time (Li et al. 2021). Where steam and oxygen
­CO2 from the product and subsequently elevate its calorific (95% vol.) are commonly the gasification agents, the heat
value (Nag et al. 2019). The raw biogas can be used for required for the gasification is supplied by biochar com-
producing electrical energy, whereas the improved biogas bustion. The gasification gas usually has a low content of
can be directly inserted into the natural gas grid or utilised light hydrocarbon and high-water content; thus, an in situ
as fuels for vehicles. From the economic point of view, reformer with steam is used to convert them into carbon
two substrates can be simultaneously mixed (anaerobic co- monoxide and hydrogen, followed by cooling of the high-
digestion) to overcome the disadvantages of mono-digestion temperature reformer effluent gas before subjecting it to gas
and enhances its feasibility. Numerous types of reactors, composition adjustment (compression and sulphur removal
including submerged packed beds, fluidised beds and other steps). This is then followed by the water gas shift reaction
types, have been employed for the anaerobic digestion treat- process, where steam is introduced into the unit to increase
ment process of wastewater with high biochemical oxygen the hydrogen to carbon monoxide ratio to 2, and then, car-
demand (Paudel et al. 2017). bon dioxide is removed before the methanol synthesis stage.
Finally, the compressed synthesis gas is pre-heated before
entering the methanol reactor, where carbon monoxide
Production of liquid biofuel such hydrogenation produces bio-methanol.
as methanol and bio‑oil from biomass Interestingly, bio-methanol derived from biomass feed-
stocks can be used to produce light olefins of 230 million
Historically, more than a hundred years ago, Giacomo Cia- tonnes demand worldwide (Li et al. 2021). Ethylene, pro-
mician mentioned in his manuscript entitled ‘Photochem- pylene and butylene as light olefins are commercially pro-
istry of the Future’ about the urgent need for the sustain- duced from petroleum-based hydrocarbon via steam crack-
able transfer from non-renewable to renewable sources ing, where currently, biomass into olefins route gains interest
(Sharma et al. 2020; Qasim et al. 2020). In 2017, the global through bio-methanol, dimethyl ether or Fischer–Tropsch
energy consumption was rated at 13.5 billion tons of oil process.
(∼656 exajoules) by a yearly growth rate of 1.7%. Relat- Bio-methanol can produce biodiesel via the transesteri-
edly, an increase in the uncontrolled population has directly fication process, where triglycerides/ lipids are transformed
deepened the negative effects of the ascending pressure on into fatty acid methyl ester using a catalyst and alcohol,
non-renewable resources globally (Pradhan et al. 2018). mainly methanol (Al-Mawali et al. 2021; Al-Muhtaseb
Considering the new United Nations reports, it has been et al. 2021; Hazrat et al. 2021). There is also a non-catalytic
stated that with an introduction of approximately 83 mil- route for microalgal biodiesel production via subcritical and
lion people to our globe per year, the current global popula- supercritical methanol (Karpagam et al. 2021).
tion of 7.6 billion is anticipated to increase to 8.6, 9.8, and As a type of biomass, algae recently showed some merits
11.2 billion by 2030, 2050 and 2100, respectively. Based in producing biofuels, such as high lipid productivity, carbon
on the United States Energy Administration (EIA) estima- dioxide capture, high growth rate, limited land requirement
tions, the global energy requirement is increasing annually and high production yields (Sekar et al. 2021; Peter et al.
and projected to rise by almost 28% in 2040 (∼739 quadril- 2021). Then again, there are still challenges, such as the
lions Btus) (Sharma et al. 2020). Majorly, high pressure on post-processing of algae and the cultivation process. Besides
energy consumption originates from countries with robust the biofuels mentioned earlier, microalgae can produce bio-
economic growth. A total enhancement in energy consump- oil via different processes, most commonly pyrolysis and
tion has been investigated by non-OECD (Organisation for others such as gasification and liquefaction as thermochemi-
Economic Co-operation and Development) countries (∼473 cal routes. Pyrolysis is preferred herein due to its simplicity,
quadrillions Btus) by 2040, compared with its counterpart of speed, better yields, along with operating conditions.
Organisation of the Petroleum Exporting Countries (∼266 Bio-oil is the main pyrolytic product in fast and flash
quadrillions Btus) (Kumar et al. 2020). pyrolysis, along with biochar and gaseous products (Xiao
The main route of biomass into liquid fuel ‘drop-in’ is et al. 2021). However, when applying bio-oil directly in
through the gasification process. On a small scale, woody petroleum (gasoline and diesel), engines will not produce
biomass gasification outperforms combustion and pyroly- sufficient heat due to its low calorific value, a high number
sis in terms of technological and economic impacts, while of oxygenated compounds (> 300) and high water content
pyrolysis has been identified as the best large-scale method (20–40 wt.%), which is negligible in hydrocarbon fuels
for upgrading woody biomass (Solarte-Toro et al. 2021). (gasoline and diesel) (Gupta et al. 2021). Furthermore, due
The biomass into liquid fuel such as bio-methanol starts to its high viscosity and the presence of acidic compounds,
with biomass gasification under low pressure using down- it will provide a flow barrier when it passes into injectors
draft gasifier owing to its low tar formation along with long and engines, resulting in engine corrosion. Besides, crude

13
4092 Environmental Chemistry Letters (2021) 19:4075–4118

bio-oil will generate coking complications in the combus- range of biomass feedstocks, geographical span, biofuels
tion stage due to the presence of a high number of solid produced, life cycle tools and inventories used. Even if the
particles. Therefore, upgrading the bio-oil via the integrated geographical span or biomass feedstock considered was sim-
refinery is crucial for its commercialisation and producing ilar, no two studies were identified as identical to each other.
value-added chemicals, char utilisation and gasoline grade This demonstrates that LCA practitioners and decision-mak-
fuel. The bio-oil upgrading process starts with moisture ers would need to identify the routes towards environmental
separation either by distillation (fractional or azeotropic), sustainability and energy efficiency while paying heed to the
using catalysts, additional pyrolysis or biomass pretreatment specific processes modelled in the studies.
techniques (demineralisation and torrefaction). This is fol-
lowed by value-added chemicals extraction from the aqueous
phase (acids, ketones, alcohols, ethers and esters) to improve Goal and scope definition
the overall economics of the process. Some chemicals could
also be extracted from the organic phase. The final organic Goal and scope definition includes defining specific pur-
residue of the bio-oil is then upgraded into a transportation pose, aim and objectives for conducting LCA. This stage is
fuel via various techniques such as deoxygenation, emulsifi- imperative to understand overall results and LCA findings. It
cation, hydrocracking, esterification, catalytic cracking and incorporates defining functional unit and respective system
heavy fuel blending. boundaries. Functional units are quantified description of
However, there are challenges associated with the upgrad- the performance requirements that the product system fulfils
ing routes for bio-oil, mentioned above, as they are still not and are linked with functions of the product rather than with
commercialised due to the high cost of the catalyst, short physical products. It was observed that about 32% of the
catalyst life and complex operating conditions (high-pres- reviewed studies used ‘units of bioenergy in J or kWh’ as
sure, special reactor requirements). Furthermore, extracting the functional unit, while about 22% recorded LCA results
the chemicals in their low concentration is expensive and for ‘amount of biofuels produced’ such as in kg (Fig. 4).
will require more investigation on the low-cost solvent, cata- The system boundaries included in the LCA studies
lyst and process optimisation, primarily as the physicochem- control what processes will be considered for computing
ical characteristics of the bio-oil rely on the catalyst used. environmental impacts. Figure 5 shows the generalised three
The catalyst minimises the heteroatom content of the bio- crucial phases for biofuel production: (1) Phase 1 includes
oil and increases the hydrogen-to-carbon ratio (H/C). This biomass cultivation, fertiliser application, impacts of ferti-
consequently lowers the harmful emissions of N ­ OX, ­SOX liser on soil, carbon emissions from land use, use of mar-
and increases the calorific value of the bio-oil (Nagappan ginal and/or forest land, transportation of produced biomass
et al. 2021). Selling the biochar produced during pyrolysis to the production system, (2) Phase 2 incorporates chemical,
can also increase the overall economics, which can be used thermal, biochemical, thermochemical processes for con-
in the carbon sequestration, adsorption of the contaminants, version of biomass to biofuels and related environmental
soil amendments and catalytic supports in bio-oil upgrading impacts due to chemicals, electricity and energy procure-
that enhances the circular bioeconomy of the process (Fawzy ment, upgradation of biofuels for final purpose, and (3)
et al. 2021). Phase 3 involves environmental impacts due to co-products
management and emissions due to biofuel use.
The system boundaries included in the LCA studies var-
Life cycle assessment of biomass to biofuel ied for processes and systems considered. It was observed
conversion processes that about 90% of the reviewed studies considered Phase
1, Phase 2, while only 25% of the studies included Phase 3
Life cycle assessment is recognised as an effective frame- (Fig. 6). This highlights a paucity of research in the biofuel
work for assessing impacts on natural environment, humans LCA field containing a holistic approach and includes all the
and natural resources for processes, products and systems. phases of the biofuel production chain. Even where the stud-
It provides evidence-based data to policymakers to make ies focused on Phase 3, they primarily dealt with anaerobic
long-term strategic decisions and improve environmental digestion. Most of the studies focused on specific processes
sustainability. The four main stages defined by ISO 14,040 of the biofuel production chain, focusing on critical areas
and IS0 14,044 for conducting LCA are: (1) goal and scope of concern rather than evaluating the overall impacts of the
definition, (2) life cycle inventory analysis, (3) environ- entire production chain. Interestingly, use of biofuels has
mental impacts assessment and (4) life cycle interpretation increasingly been recognised as a measure to reduce green-
(Lewandowski et al. 2000). house gas emissions; however, very few studies addressed
Herein, we analysed 40 LCA studies published from 2019 the use phase of biofuels and compared them to conventional
to 2021 on biofuels (Table 10). These studies covered a wide sources.

13
 Table 10  Characteristics of life cycle assessment research analysed in the present study
Reference Region Functional unit Feedstock Process Product LCA tools Database

Aberilla et al. (2019) Southeast Asia 1 kWh of electricity Rice and coconut Comparison of: Com- Electricity GaBi 7.3 Ecoinvent 3.1
produced residues bustion
Anaerobic digestion
Gasification
Amezcua-Allieri et al. Southern Mexico 1 MJ of energy pro- Sugarcane bagasse Boiler Bioenergy n/i Ecoinvent
(2019) duced
Ardolino and Arena EU-27 1 MW of biomethane Biowaste Comparison of: Biomethane SimaPro 8.2 n/i
(2019) Anaerobic digestion
with
Gasification
Bacenetti (2019) North Italy 1 ha of vineyard Pruning residues from Combustion for use in Heat and Cold n/i Ecoinvent 3.5
grapevine cultiva- boiler followed by
tion Chiller
Environmental Chemistry Letters (2021) 19:4075–4118

Banerjee et al. (2019) India Mass of ­CO2 equiva- Microalgal biomass Comparison of Trans- Biodiesel GaBi 7 n/i
lent released per MJ esterification ­(Fe2O3
of fuel produced catalyst with HCl
catalyst)
Chung et al. (2019) n/i 1000 kg of biodiesel Waste cooking oil Transesterification Biodiesel openLCA 1.8 Agribalyse and NEEDS
produced per day (waste chicken egg-
shell derived CaO
catalyst)
Dasan et al. (2019) n/i 100,000 kg of dry Microalgal biomass Lipid extraction Biodiesel n/i n/i
algae biomass for from followed by lipid
340 days of yearly Chlorella vulgaris recovery and trans-
operation esterification
Derose et al. (2019) n/i Mass of ­CO2 equiva- algae harvested from Comparison of: Biodiesel n/i GREET
lent released per MJ an algal turf scrub- Biochemical process-
of fuel produced ber ing with
Thermochemical
processing
Di Fulvio et al. (2019) EU-27 + United Biodiversity loss per Biomass from land n/i Biofuel GISa GLOBIOM
Kingdom ha of land use for model
spatially explicit
LCA
Dupuis et al. (2019) East England, United Mass of ­CO2 equiva- Lignocellulosic n/i High octane biodiesel n/i DNDC and STAMINA
Kingdom lent released per Biomass from Mis- models
unit of delivered canthus and Wheat
feedstock
Zhu et al. (2019) Hubei province, 1 MJ of electricity Cotton straw Combustion Electricity n/i Water footprint
China output ­databaseb
Wang et al. (2019b) China 1 kg hydrogen Biomass and water Gasification Hydrogen n/i n/i

13
4093
 Table 10  (continued)
4094

Reference Region Functional unit Feedstock Process Product LCA tools Database

13
Wagner et al. (2019) Germany 1 GJ of electricity Miscanthus Anaerobic diges- Electricity n/i Ecoinvent 3.4 and
tion followed by KTBL
combined heat and
power generation
Timonen et al. (2019) Southern Finland g ­CO2 eq per MJ of Pig slurry Anaerobic digestion Heat and SimaPro LIPASTO
energy produced electricity
and kg C
­ O2 eq per
kg of
nitrogen (N) in
digestate
Tanzer et al. (2019) Brazil and Sweden 1 GJ of fuel Agroforestry residues Comparison of: Gasi- Marine fuels CMLCA 5.2 Ecoinvent 2.2
fication
Fast pyrolysis hydro-
treatment
Hydrothermal lique-
faction
Quispe et al. (2019) Peruvian north coast 1 MJ of energy from Rice husk Combustion Electricity n/i Ecoinvent 3.1
rice husk
Krzyżaniak et al. Poland 1 Mg of dry biomass Poplar n/i Biofuel SimaPro Ecoinvent
(2019)
Im-Orb and Arporn- n/i kg ­CO2 eq per kg of Rice straw Integrated biomass Methanol LCSoft version 6.1 ASPEN Plus
wichanop, (2020) biofuel produced pyrolysis, Aspen Engineering
gasification, and 8.4
methanol synthesis
Jeswani et al. (2020) United Kingdom 1 MWh of heat Woodchips from Boiler Heat GaBi 4.4 Ecoinvent
forestry and sawmill
waste
Lan et al. (2020) Southeast United 1 MJ of biofuel pro- Blended feedstock: Fast pyrolysis fol- Gasoline and diesel n/i GREET 1.8
States duced switchgrass and lowed by combus- Ecoinvent
pine residues tion
Lask et al. (2020) Southwest Germany 1 kWh of electricity Perennial wild plant Anaerobic digestion Electricity openLCA Ecoinvent
produced mixtures and 1.8.0
maize silage
Mendecka et al. Italy Treatment and Olive pomace Hydrothermal car- Bioenergy n/i European Reference
(2020) upgrade of 1 kg of bonisation Life Cycle Database
biomass and Ecoinvent 3.2
Reaño, (2020) Southeast Asia 1 kg of biofuel pro- Rice husk Comparison of: Bio-hydrogen openLCA 1.10 Ecoinvent 3.5
duced Alkali water elec-
trolysis
Gasification
Dark fermentation
Environmental Chemistry Letters (2021) 19:4075–4118
 Table 10  (continued)
Reference Region Functional unit Feedstock Process Product LCA tools Database

Spatari et al. (2020) United States 1 kg of bio-oil Forest residues Comparison of: Biodiesel ASPEN Plus GaBi 9 LCA
Fast pyrolysis GaBi 9
Catalytic fast pyroly-
sis
Thengane et al. (2020) California, United 1 MJ of torrefied rice Rice husk Torrefaction Torrefied rice husk openLCA 1.8.0 Ecoinvent 3.4
States husk pellets pellets
Ubando et al. (2020) Taiwan 1 kg of torrefied Microalgal biomass Torrefaction Torrefied microalgal Simapro 8.5.2 Simapro 8.5.2 LCA
microalgal biomass from Chlorella biomass
vulgaris
Nilsson et al. (2020) Sweden 1 ha of land under Perennial grass n/i Bioenergy GISa DNDC agricultural
cultivation model
Aristizábal-Marulanda Colombia 1 MJ of energy Coffee cut stems Comparison of: Ethanol and electric- SimaPro 8.3 Ecoinvent 3
et al. (2021) Enzymatic hydrolysis ity + steam
Environmental Chemistry Letters (2021) 19:4075–4118

followed by gasifi-
cation system
Gasification only
system
Brassard et al. (2021) Metropolitan France 1000 kg of dry bio- Primary forestry Pyrolysis Bio-oil, Simapro 9.0 Ecoinvent 3
mass residues biochar) and gaseous
(non-condensable
gases)
Cusenza et al. (2021) Sicily, Italy 1 kWh of electricity Residual biomasses: Anaerobic digestion Electricity and heat n/i Ecoinvent 3
feed into the grid olive pomace,
chicken and bovine
manure, whey, citrus
processing waste,
and Hedysarum
coronarium
silage
Da Silva et al. (2021) Region state of Rio 1 kg of methanol Rice straw Gasification Methanol Aspen Plus Aspen Plus
Grande do Sul,
Brazil
Fu et al. (2021) Dongying, Shandong 1 kg of ethanol Sweet sorghum Combustion Ethanol GISa Harmonised world soil
province, China
Martillo Aseffe et al. Ecuador (Los Rios 1 ton of seed-corn Corncob Comparison of: Electricity SimaPro 8 SimaPro 8 LCA
(2021) and Guayas) produced Combustion
Gasification
Schonhoff et al. Germany 100 GJ thermal Sida hermaphrodita Comparison of: chips, pellets and bri- GaBi 9.2 Ecoinvent 3.5
(2021) energy Chopping quettes as carriers
Pelletisation of Thermal energy
Splitting

13
4095
 Table 10  (continued)
4096

Reference Region Functional unit Feedstock Process Product LCA tools Database

13
Schmidt Rivera et al. United Kingdom 1 tonne of spent cof- Spent coffee grounds Comparison of: Biodiesel, GaBi 8.7 Ecoinvent 3.3
(2020) fee grounds treated Incineration compost
Landfilling,
Anaerobic digestion,
Composting fol-
lowed by
direct application to
land
Al-Mawali et al. Gulf cooperation 1000 kg of biodiesel Waste date seeds Esterification using a Biodiesel SimaPro SimaPro LCA
(2021) council countries magnetic catalyst
Yang et al. (2021) China 1 ton of crop residues Crop residues Slow pyrolysis Biofuel and biochar GaBi 8.7 PRC ­2019c
Saranya and Karnataka, India Biomass achievable in Microalgal biomass Comparison of: Biodiesel OpenLCA 1.10.3 Ecoinvent 3.6
Ramachandra, 1 ha area Acid catalysis
(2020) Biocatalysis
Bora et al. (2020) United States 1000 kg of wet poul- Poultry litter Comparison of: Biofuel and biochar n/i Ecoinvent 3
try litter Direct land use
Pyrolysis
Gasification
Hydrothermal pro-
cessing
Al-Muhtaseb et al. Pakistan 1000 kg of biodiesel Waste loquat seeds Transesterification Biodiesel SimaPro SimaPro LCA
(2021) using a CaO/CeO2
catalyst

n/i: Not included, EU-27: The European Union; GREET: Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation
a
GIS: Some life cycle assessment (LCA) studies analysed environmental impacts of land use for biofuel production and thus used geographic information systems software
b
Water footprint database: Data sourced from the water footprint assessment manual
c
PRC 2019: Data sourced from the national development and reform commission of the People’s Republic of China (PRC)
Environmental Chemistry Letters (2021) 19:4075–4118
Environmental Chemistry Letters (2021) 19:4075–4118 4097

Fig. 4  Types of functional units Types of functional units considered in the reviewed
used in the life cycle assessment studies
35
studies reviewed in the present
work (N = 40)
30

% of studies reviewed
25

20

15

10

0
Bioenergy Biofuel Biomass Biomass Land use Other
produced produced produced used

Fig. 5  General system boundary for conducting life cycle assessment of conversion of biomass feedstocks to biofuels

Fig. 6  Phases of system bound- Phases of system boundary considered in the reviewed
ary considered for conduct-
100
studies
ing life cycle assessment of
converting biomass feedstocks 90
% of studies reviewed

into biofuels in the reviewed 80

literature (N = 40) 70
60
50
40
30
20
10
0
Phase 1 Phase 2 Phase 3 Phase 1+2 Phase 1+3 Phase 2+3 Phase
1+2+3

13
4098 Environmental Chemistry Letters (2021) 19:4075–4118

Life cycle inventory analysis of reactive substances injurious to human health and ecosys-
tems. Acidification measured in kg ­SO2 equivalent is caused
Inventory analysis involves quantifying all the inputs and by the emission of acidifying substances. Land use calcu-
outputs for the processes considered in the system bound- lated in ­m2 is categorised as the transformation of urban
ary of the LCA. This includes raw material requirements, land, agricultural land and natural land such as forests. Water
energy input, emissions to air, wastewater production, solid depletion ­(m3) is the use of water for the entire production
waste generation, emissions to land and others. It should be chain of biofuels. Particulate matter formation expressed
noted that more the systems involved in the system bound- as PM2.5 equivalent and/or PM10 equivalent relates to the
ary, greater would be the need for data for inventory analy- emission of PM 2.5 (particulate matter with ≤ 2.5 µm in
sis, which also is explained in Fig. 6 with only some studies diameter) and/or PM10 (particulate matter with ≤ 10 µm in
considering all the phases of biofuel production. diameter). Eutrophication consists of the effect of releas-
Table 10 shows databases for conducting inventory analy- ing an excessive amount of nutrients reported as kg P ­ O4
sis, used in the reviewed studies such as SimaPro LCA data- equivalent. Ionising radiation (kg U235 equivalent) transfers
bases, Ecoinvent, GREET (Greenhouse Gases, Regulated energy into the body tissue and may thereby interfere with
Emissions, and Energy Use in Transportation). Some LCA the structure of molecules (Table 11).
studies also used agricultural models for considering the Finally, human toxicity is recorded as kg 1,4 dichloroben-
impacts of land use on overall environmental sustainability, zene equivalent or cumulative toxic units. Human toxicity
such as GLOBIOM, DNDC and STAMINA (Di Fulvio et al. (carcinogens) is an index that corresponds to potential harm
2019; Dupuis et al. 2019; Nilsson et al. 2020). of a unit of cancer-causing chemical released into the envi-
ronment and is based on both the inherent toxicity of a com-
Environmental impacts assessment pound and its potential dose. Human toxicity (non-carcino-
gens) index is associated with non-carcinogenic chemicals
Mid‑point indicators release, doses and exposure.

In this stage of LCA, key environmental impacts are quan- Endpoint indicators
tified and distributed in various environmental categories
depending upon the functional unit, system boundary, mod- The mid-point categories are aggregated to present results
elled systems and need of the decision-makers. Some studies as endpoint categories. It is argued that the environmental
computed net energy ratio to evaluate the usability of biofu- impacts should be presented as mid-point categories to pre-
els as energy sources (Al-Mawali et al. 2021; Al-Muhtaseb vent oversimplification or misinterpretation of environmen-
et al. 2021; Dasan et al. 2019; Im-Orb and Arpornwichanop tal impacts (Kalbar et al. 2017). Nevertheless, some studies
2020; Reaño 2020; Saranya and Ramachandra 2020), which did not present detailed mid-point indicator impacts but only
is defined as the ratio of output energy to input energy for the endpoint indicators (Amezcua-Allieri et al. 2019; Martillo
overall process (Pleanjai and Gheewala 2009). Aseffe et al. 2021; Bora et al. 2020).
Mid-point categories used for expressing life cycle The endpoint categories used in reviewed studies were:
environmental impacts were: global warming potential (1) human health (disability-adjusted life year) is related
(100 years), which includes greenhouse emissions is gen- to the impacts of environmental degradation that results in
erally expressed as kg ­CO2 equivalent for a time horizon an increase in and duration of loss of life years due to ill
of 100 years. Some studies also considered greenhouse gas health, disability or early death, and (2) ecosystem quality
emissions for a temporal scale of 20 years, in accordance (species × year) is linked to the impact of global warming
with the life span of infrastructure (Aberilla et al. 2019; potential, ozone layer depletion, acidification, ecotoxicity,
Cusenza et al. 2021). Abiotic depletion reported as kg Sb eutrophication and indicates biodiversity loss. It is recorded
equivalent corresponds to the depletion of fossil fuels, min- as local species loss integrated over time, and (3) resources
erals, clay and peat. Abiotic depletion (fossil fuels, recorded are related to the depletion of raw materials and energy
as MJ) is linked to the depletion of fossil deposits. Ozone sources expressed generally in US dollars ($), representing
layer depletion (kg trichlorofluoromethane equivalent) is the extra costs involved for future mineral and fossil resource
typically accounted for a time scale of 40 years. extraction (Al-Muhtaseb et al. 2021).
Ecotoxicity potential evaluated in kg 1,4 dichlorobenzene
equivalent or cumulative toxic units is calculated in three Uncertainty, scenario and sensitivity analysis
separate categories, which examine damage to terrestrial,
freshwater and marine sources for the entire production pro- Life cycle assessment (LCA) studies are models which
cess. Photochemical oxidation recorded in kg non-methane are simplified versions of the real-world system and thus
volatile organic compounds equivalent refers to emissions are inherently uncertain (Wang and Shen 2013). These

13
 Table 11  Environmental impacts and main findings recorded in the life cycle assessment case studies analysed
Reference Environmental impacts considered Findings

Aberilla et al. (2019) Global warming (20 years), ozone layer depletion, particulate matter forma- The study compared anaerobic digestion, combustion and gasification for
tion, photochemical oxidant formation, freshwater eutrophication, marine rice and coconut residues in South East Asia to produce 1 kWh of electric-
eutrophication, terrestrial acidification, freshwater ecotoxicity, marine ity as the end product. It was observed that anaerobic digestion was the
ecotoxicity, terrestrial ecotoxicity, fossil depletion, mineral depletion, best option for 14 out of 18 impacts (the remaining four impacts categories
water depletion, agricultural land occupation, natural land transformation, were: global warming, photochemical oxidant formation, particulate matter
urban land occupation, human toxicity, ionising radiation formation and terrestrial ecotoxicity potentials)
potentials Global warming potential for anaerobic digestion = 100 kg ­CO2 eq
Global warming potential for gasification and combustion = -30 to 30 kg C ­ O2
eq
Ozone layer depletion for anaerobic digestion = − 36 µg CFC-11 eq
Ozone layer depletion for gasification and combustion = − 1 µg CFC-11 eq
Human toxicity (carcinogens) for anaerobic digestion = − 74 g 1,4 DB eq
Environmental Chemistry Letters (2021) 19:4075–4118

Human toxicity (carcinogens) for gasification and combustion = 72 to 182 g

1,4 DB eq
Amezcua-Allieri et al. (2019) Abiotic resource depletion, acidification, eutrophication, ozone layer deple- Although all these environmental categories were analysed, the results were
tion, freshwater aquatic ecotoxicity, marine ecotoxicity and terrestrial only presented as normalised or % change in the value of environmental
ecotoxicity potentials impacts compared to fossil fuel. Potential environmental impact index
(PEI) recorded using a summation of all environmental categories was
2528 (for the use of sugarcane bagasse) vs 20,200 PEI/GJ for using fossil
fuel in a sugar mill
Ardolino and Arena (2019) Global warming, non-renewable energy and respiratory inorganics poten- Both the biomethane scenarios consisting of using biomethane produced
tials from anaerobic digestion and gasification were better than the diesel sce-
nario, improving 56% and 65% for global warming potential, and 31% and
52% for non-renewable energy potential, respectively
Bacenetti (2019) Global warming (100 years), ozone layer depletion, human toxicity (car- When compared with the business-as-usual scenario where the pruning resi-
cinogens), human toxicity (non-carcinogens), particulate matter forma- dues are wasted, pruning residues for bioenergy production showed better
tion, photochemical oxidant formation, terrestrial acidification, freshwater environmental impacts in most categories. Below are some of the results
eutrophication, terrestrial eutrophication, marine eutrophication, freshwa- recorded for business-as-usual and reuse of residues for 1 ha of vineyard
ter ecotoxicity, mineral fossil and renewable resource depletion potentials land as a functional unit (in respective order)
Global warming potential = 89 and − 807 kg C ­ O2 eq
Ozone layer depletion = 5 and − 43 mg CFC-11 eq
Human toxicity (carcinogens) = 9.6 × ­10–7 and -1.1 × ­10–6 comparative toxic
units
Human toxicity (non-carcinogens) = 3.7 × ­10–5 and -4.3 × ­10–5 comparative
toxic units
Particulate matter formation = 0.06 and 2.44 kg PM2.5 eq
Banerjee et al. (2019) Global warming, eutrophication, human toxicity, ozone layer depletion Biodiesel production from wet biomass and using ­Fe2O3 nanocatalyst in the
potentials transesterification process reported less environmental impacts as com-
pared to the conventional biodiesel production process in all four environ-
mental categories

13
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 Table 11  (continued)
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Reference Environmental impacts considered Findings

13
Chung et al. (2019) Global warming, ozone layer depletion, eutrophication, terrestrial acidifica- The highest environmental impacts were observed during transesterification
tion, ecotoxicity, fossil depletion, minerals depletion, land use, human of the entire biodiesel production process. The transesterification pro-
toxicity(carcinogens), ionising radiation, respiratory inorganics and cess contributed a 1010 MJ surplus on fuel consumption due to the high
respiratory organics potentials demand of electrical energy for 1000 kg of biodiesel produced
Dasan et al. (2019) Net energy ratio and global warming potential (10,000 years) This study assessed the impact of different microalgae cultivation systems on
biomass productivity. The net energy ratio was below 1 for all microalgae
cultivation systems (open pond, tubular, bubble column photo-bioreactors)
Derose et al. (2019) Global warming potential Biochemical and thermochemical pathways were compared for bioenergy
production (1 MJ of fuel produced)
Global warming potential for biochemical pathway = 111.2 g ­CO2 eq
Global warming potential for thermochemical pathway = − 2 g ­CO2 eq
Di Fulvio et al. (2019) Biodiversity loss Increased cultivation of perennial crops for bioenergy production could be an
option for climate change mitigation. However, it could negatively impact
biodiversity through loss of species habitats in EU-27 and the UK
Dupuis et al. (2019) Global warming potential per unit of delivered feedstock (100 years) Perennial energy crops rather than wheat husk should be considered biofuel
in East England based on the life cycle assessment of the use of wheat husk
and energy crops
Zhu et al. (2019) Water footprint in L per MJ of bioenergy The total life cycle water use intensity for using cotton straw as a bioenergy
source via combustion was 11.708 L/MJ, lower than bio-oil power genera-
tion, however, much greater than other renewable energy sources (such as
geothermal, solar photovoltaic and wind power). The study also noted that
biomass agricultural production accounted for 84.61% of the total water
use
Wang et al. (2019b) Global warming potential An increase in gasification temperature and decrease in biomass slurry con-
centration and pressure could decrease global warming potential
Wagner et al. (2019) Global warming, agricultural land occupation, freshwater ecotoxicity, This study noted that marginal German electricity mix, when substituted by
human toxicity, marine ecotoxicity potential, freshwater eutrophication miscanthus-based biogas, can lead to net benefits in the impact categories
and marine eutrophication potentials of global warming potential, human toxicity, marine ecotoxicity, freshwater
ecotoxicity and freshwater eutrophication. This was observed even when
the biomass is transported over longer distances, biomass yields are lower,
or a lower heat utilisation rate is applied
Timonen et al. (2019) Global warming potential The study compared the use of pig slurry with grass and pig slurry with food
waste as feedstocks for anaerobic digestion
The highest global warming potential was observed when pig slurry and
grass from uncultivated fields were used for anaerobic digestion. This
could be due to due to prominent N ­ 2O emissions from grasslands, while
the lowest level of global warming potential was in the scenario, which
involved the digestion of pig slurry and side streams from the food industry
due to avoided emissions from co-feedstock procurement (covered storage
and zero emissions to produce food industry sludge as waste)
Environmental Chemistry Letters (2021) 19:4075–4118
 Table 11  (continued)
Reference Environmental impacts considered Findings

Tanzer et al. (2019) Global warming potential (100 years), ­NOx and ­SO2 emissions All biofuel systems of agroforestry residues have substantially lower life
cycle environmental impacts compared to heavy fuel oil (HFO, marine fuel
reference)
Global warming potential for biofuels = − 30 to 40 kg ­CO2 eq/GJ
Global warming potential for HFO reference = 90 kg C
­ O2 eq/GJ
SO2 emissions all biofuels < 0.3 kg ­SO2/GJ
SO2 emissions for HFO reference = 2.5 kg ­SO2/GJ
NOX emissions from biofuels ≈ HFO reference = 1.6 kg ­NOX/GJ
Quispe et al. (2019) Global warming (100 years), acidification potential, eutrophication and For generating 457 MJ of bioenergy from fuels considered, the results
water depletion potentials obtained are summarised below
Environmental Chemistry Letters (2021) 19:4075–4118

For rice husk, with yield = 9.5 t/ha and dryer efficiency as 0.7:
Global warming potential = 1.96 kg ­CO2 eq
Water depletion potential = 2.3 ­m3
Eutrophication potential = 12.5 g ­PO4 eq
Acidification potential = 50.3 g S
­ O2 eq
For coal system:
Global warming potential = 61 kg ­CO2 eq,
Water depletion = 23 ­m3
Eutrophication potential = 63 g ­PO4 eq
Acidification potential = 462 g S
­ O2 eq
The study concluded that if all rice husk available in Peru were used for dry-
ing processes instead of coal, it could mitigate around 708,540 metric tons
of ­CO2 eq per year

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Reference Environmental impacts considered Findings

13
Krzyżaniak et al. (2019) Global warming, particulate matter formation, terrestrial acidification, This study compared poplar production (1 tonne of dry matter) as lignocel-
freshwater eutrophication, human toxicity, freshwater ecotoxicity, fossil lulosic feedstock for use in bioenergy systems considering four fertilisation
depletion potentials systems: unfertilised, mineral, lignin and lignin + mineral, and observed the
following results
Global warming potential for lignin + mineral fertilisation system = −20 kg
­CO2 eq
Global warming potential for unfertilised system = 20 kg C
­ O2 eq
Particulate matter formation potential for lignin + mineral fertilisation sys-
tem = 0.3 kg PM10 eq
Particulate matter formation potential for unfertilised system = 0.3 kg PM10
eq
Terrestrial acidification potential for lignin + mineral fertilisation sys-
tem = 0.7 kg ­SO2 eq
Terrestrial acidification potential for unfertilised system = 0.3 kg ­SO2 eq
Freshwater eutrophication potential for lignin + mineral fertilisation sys-
tem = 0.01 kg P eq
Freshwater eutrophication potential for unfertilised system = 0.3 kg P eq
Human toxicity potential for lignin + mineral fertilisation system = 9 kg 1,4
DB eq
Human toxicity potential for unfertilised system = 19 kg 1,4 DB eq
The study concluded that fertilised systems could cause negative environ-
mental impacts for poplar production in Poland
Im-Orb and Arpornwichanop (2020) Net energy ratio and The study analysed the life cycle impacts of methanol production. The
potential environmental impact impact categories considered global warming, ozone layer depletion,
acidification, eutrophication, photochemical oxidant formation, ecotoxicity,
respiration effects, human toxicity (non-carcinogens) and human toxic-
ity (carcinogens) were all merged as potential environmental impact. The
highest net energy ratio achieved was 0.612. The lowest PEI/kg methanol
was observed as 0.18
Environmental Chemistry Letters (2021) 19:4075–4118
 Table 11  (continued)
Reference Environmental impacts considered Findings

Jeswani et al. (2020) Global warming, abiotic depletion, abiotic depletion for fossil fuels, acidifi- The study compared the use of biomass for boiler compared to fossil
cation, eutrophication, freshwater ecotoxicity, human resources (i.e. natural gas, oil and electricity). The findings demonstrated
that heat from biomass boilers had lower impacts than from the coal and oil
equivalents in all categories (except terrestrial ecotoxicity potential). Some
of the environmental impacts observed for the production of 1MWh of heat
are summarised below
toxicity, marine ecotoxicity, ozone layer depletion, photochemical oxidant Global warming potential during biomass use = 14.7 kg ­CO2 eq
formation and terrestrial ecotoxicity potentials
Global warming potential during use of fossil resources = 233.9–468.4 kg
­CO2 eq
Terrestrial ecotoxicity potential during biomass use = 0.6 kg DB eq
Environmental Chemistry Letters (2021) 19:4075–4118

Terrestrial ecotoxicity potential during use of fossil resources = 0.03–0.11 kg

DB eq
Human toxicity potential during biomass use = 29 kg 1,4 DB eq
Human toxicity potential during use of fossil resources = 4–192 kg 1,4 DB eq
Lan et al. (2020) Primary energy consumption and global warming potential (100 years) The life cycle energy consumption and global warming potential of 1 MJ of
gasoline and diesel production utilising switchgrass and pine residues were
observed as 0.7 to 1.1 MJ and 43.2 to 76.6 g ­CO2 eq, respectively
Lask et al. (2020) Global warming potential When considering land-use scenarios due to maize silage and wild energy
crop production, blended feedstock consisting of both showed the lowest
global warming potential as 198 kg C ­ O2 eq/kWh of electricity produced
Mendecka et al. (2020) Global warming, acidification, eutrophication and For treatment and upgrade of 1 kg of olive pomace, following impacts were
observed
freshwater ecotoxicity potentials Global warming potential = 0.468 to 1.024 kg ­CO2 eq
Acidification potential = 0.0007 to 0.0015 mol H + eq
Eutrophication potential = 0.036 to 36.290 g P eq
Freshwater ecotoxicity potential = 165.8 to 254.5 comparative toxic units
Reaño (2020) Net energy ratio and The study compared alkali water electrolysis, gasification and dark fermenta-
tion for hydrogen production. Dark fermentation pathway for the produc-
tion of 1 kg of ­H2 was recorded as the most efficient process based on:
Global warming (100 years), acidification, eutrophication, and terrestrial Net energy ratio = 1.25,
ecotoxicity potentials
Global warming potential = 46 kg ­CO2 eq
Acidification potential = 75 g ­SO2 eq
Eutrophication potential = 350 g ­PO4 eq
Terrestrial ecotoxicity potential = 2.5 mg 1,4 DB eq

13
4103
 Table 11  (continued)
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Reference Environmental impacts considered Findings

13
Spatari et al. (2020) Global warming potential (100 years) For 1 MJ of biodiesel produced:
Global warming potential for catalytic pyrolysis = − 80 g ­CO2 eq
Global warming potential for fast pyrolysis = 20 g ­CO2 eq
Thengane et al. (2020) Global warming potential Global warming potential = 6.04 ton C­ O2 eq/ton torrefied rice husk for severe
torrefaction
Ubando et al. (2020) Global warming, water depletion, fossil resource scarcity, mineral resource Global warming potential was observed as 12.72 C ­ O2 eq for the entire
scarcity, land use, human toxicity(carcinogens), human toxicity (non-car- process from cultivation to torrefaction. For all other categories, only
cinogens), marine ecotoxicity, freshwater toxicity, terrestrial ecotoxicity, normalised results as % were recorded
marine eutrophication, freshwater eutrophication, terrestrial acidification,
ozone layer depletion, particulate matter formation, ionising radiation
potentials
Nilsson et al. (2020) Global warming (100 years) and eutrophication potentials The mean global warming potential was 1170 ± 460 and 1200 ± 460 kg ­CO2
eq/ha year for a 140 and 200 kg N/ha fertilisation rate. The eutrophication
potential was observed as 11 ± 6.1 kg N-eq/ha to use 1 ha of land to pro-
duce perennial grass as a biofuel source in five different sites in Sweden.
The variation between sites highlights the use of temporal and spatial data
for assessing whole life cycle impacts
Aristizábal-Marulanda et al. (2021) Global warming, ozone layer depletion, terrestrial acidification, freshwater The study compared the use of coffee-cut stems for ethanol production
eutrophication, human toxicity, photochemical oxidant formation, par- concerning the production of electricity and steam. Lesser environmental
ticulate matter formation, freshwater ecotoxicity, water depletion, fossil impacts were observed for the latter. Some of the findings for both 1 MJ of
depletion and agricultural land occupation potentials ethanol and 1 MJ of electricity and steam are:
Global warming potential for 1 MJ of ethanol = 1.37 kg ­CO2 eq
Global warming potential for 1 MJ of electricity and steam = 0.27 kg ­CO2 eq
Ozone layer depletion potential for 1 MJ of ethanol = 2.5 × ­10–8 kg CFC-
11 eq
Ozone layer depletion potential for 1 MJ of electricity and
steam = 0.7 × ­10–8 kg CFC-11 eq
Fossil depletion potential for 1 MJ of ethanol = 0.05 kg oil eq
Fossil depletion potential for 1 MJ of electricity and steam = 0.01 kg oil eq
Environmental Chemistry Letters (2021) 19:4075–4118
 Table 11  (continued)
Reference Environmental impacts considered Findings

Brassard et al. (2021) Global warming (100 years), ozone layer depletion, human toxicity (car- This study compared the use of primary forestry residues for pyrolysis to the
cinogens), freshwater eutrophication, mineral resource use, terrestrial, business-as-usual of no use and concluded that pyrolysis could result in
freshwater acidification, water scarcity, photochemical ozone forma- better environmental impacts in nine out of a total of 16 categories. Some
tion, human toxicity (non-carcinogens), respiratory inorganics, marine results recorded for the use of 1000 kg of feedstock are given below
eutrophication, terrestrial eutrophication, freshwater ecotoxicity, ionising
radiation, resource use (energy carriers), land use potentials Global warming potential = 906 kg ­CO2 eq during pyrolysis
Global warming potential = 1750 kg ­CO2 eq during no use
Ozone layer depletion potential = 3 × ­10–5 kg CFC-11 eq during pyrolysis
Ozone layer depletion potential = 2 × ­10–5 kg CFC-11 eq during no use
Mineral resource use potential = 1.5 × ­10–3 kg Sb eq during pyrolysis
Mineral resource use potential = 2.5 × ­10–4 kg Sb eq during no use
Environmental Chemistry Letters (2021) 19:4075–4118

Human toxicity(non-carcinogens) = 5 × ­10–5 cumulative toxicity units, during

pyrolysis
Human toxicity(non-carcinogens) = 2.25 × ­10–4 cumulative toxicity units,
during no use
Cusenza et al. (2021) Cumulative energy demand, The study analysed environmental impacts for residual biomass feedstocks to
produce 1 kWh of electricity to feed in the grid:
and Cumulative energy demand = 3.8 MJ
Global warming (20 years), ozone layer depletion, human toxicity (non- Global warming potential = 1.1 kg ­CO2 eq
carcinogens), human toxicity (carcinogens), particulate matter formation,
ionising radiation, photochemical oxidant formation, acidification, ter-
restrial eutrophication, freshwater eutrophication, marine eutrophication,
ecotoxicity and abiotic deletion potentials
Ozone layer depletion potential = 3.3 × ­10–8 kg CFC-11 eq
Abiotic depletion potential = 2.9 × ­10–8 kg Sb eq
Human toxicity(non-carcinogens) = 3.6 × ­10–8 cumulative toxicity units
Human toxicity(carcinogens) = 9.6 × ­10–9 cumulative toxicity units
Particulate matter formation potential = 1.1 × ­10–4 kg PM2.5 eq
The study conducted an environmental performance of bio-waste valorisa-
tion practices considering the double perspective of producing renewable
energy and treating bio-wastes
Da Silva et al. (2021) Global warming potential (100 years) Global warming potential = 0.8 kg ­CO2 eq for 1 kg of methanol produced
using rice straw through gasification
Fu et al. (2021) Net energy gain and global warming potential For 1 kg of ethanol produced using rice straw through combustion in the
boiler
Net energy gain potential = 4.33 MJ
Global warming potential = 0.005 kg C ­ O2 eq

13
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 Table 11  (continued)
4106

Reference Environmental impacts considered Findings

13
Martillo Aseffe et al. (2021) Global warming potential The study analysed the use of corncob for electricity production through
gasification and combustion and its impacts on the seed-corn supply chain.
It concluded that if waste-to-energy scenarios are included with the agri-
business sector, it could reduce environmental impacts
and Global warming potential = 913 kg CO2 eq/t seed-corn through combustion
Endpoint indicators: human health, ecosystem quality, resource depletion Global warming potential = 797 kg CO2 eq/t seed-corn through gasification
Schonhoff et al. (2021) Global warming (including biogenic carbon), global warming (exclud- The study concluded that different production and processing pathways for
ing biogenic carbon), acidification, freshwater eutrophication, terrestrial Sida hermaphrodita biomass did not significantly differ in the overall envi-
eutrophication, marine eutrophication, human toxicity (carcinogens), ronmental impacts. Moreover, when comparing Sida hermaphrodita bio-
human toxicity (non-carcinogens), land use, particulate matter formation, mass was performed with alternative biomasses like wood or Miscanthus;
water depletion, freshwater ecotoxicity, ionising radiation, photochemical in most cases (except Miscanthus and mixed logs), beneficial properties of
zone formation, abiotic depletion potentials Sida hermaphrodita biomass were observed
For 100 GJ of thermal energy from Sida hermaphrodita biomass, some of
the environmental impacts recorded were:
Global warming potential (including biogenic carbon) ≈ 12,000 kg C ­ O2 eq
Global warming potential (excluding biogenic carbon) ≈ 1500 kg C ­ O2 eq
Ozone layer depletion potential ≈ 10 × ­10–5 kg CFC-11 eq
Abiotic depletion potential ≈ 0.15 kg Sb eq during pyrolysis
Human toxicity(carcinogens) = 14 to 19 × ­10–5 cumulative toxicity units
Human toxicity(non-carcinogens) ≈ 10.5 × ­10–4 cumulative toxicity units
Schmidt Rivera et al. (2020) Global warming, primary The study compared waste treatment cases for spent coffee grounds. It
concluded that overall, incineration (with electricity and heat recovery)
showed the least environmental impacts compared to landfilling, biodiesel
production and composting, followed by the spreading of compost on
land. It showed that use for waste as an energy source should follow not
only waste hierarchy but also life cycle assessment. Some of the impacts
recorded for incineration (with electricity and heat recovery—best case)
and biodiesel production using 1 tonne of spent coffee grounds are below
energy demand, fossil depletion, metal depletion, particulate matter forma- Global warming potential for biodiesel production = 31 kg ­CO2 eq
tion, stratospheric ozone depletion, photochemical oxidant
(ecosystems), photochemical oxidant (humans) formation, freshwater Global warming potential for incineration = 525 kg ­CO2 eq
eutrophication, marine eutrophication, terrestrial acidification, human
toxicity (carcinogens), human toxicity (non-carcinogens), freshwater
ecotoxicity, marine ecotoxicity and terrestrial ecotoxicity potentials
Ozone layer depletion for biodiesel production = 0.25 g CFC-11 eq
Ozone layer depletion for incineration = 0.18 g CFC-11 eq
Human toxicity (non-carcinogens) for biodiesel production = 357 kg 1,4 DB
eq
Human toxicity (non-carcinogens) for incineration = 5 kg 1,4 DB eq
Environmental Chemistry Letters (2021) 19:4075–4118
 Table 11  (continued)
Reference Environmental impacts considered Findings

Al-Mawali et al. (2021) Net energy ratio and The study conducted LCA to analyse the transformation of waste date seed
oil to biodiesel via esterification. It concluded that it is environmentally
feasible to produce biodiesel from waste-derived feedstocks
Global warming (100 years), ozone layer depletion, abiotic depletion (fossil Net energy ratio = 2.17
fuels)
abiotic depletion, human toxicity, freshwater ecotoxicity, marine ecotoxic- Global warming potential = 1.11 kg C
­ O2 eq/kg of biodiesel produced
ity
terrestrial ecotoxicity, photochemical oxidant formation, acidification, and Abiotic depletion potential = 2 × ­10–5 kg Sb eq/kg of biodiesel produced
eutrophication potentials
Abiotic depletion (fossil fuels) potential = 19 MJ/kg of biodiesel produced
Human toxicity potential = 0.6 kg 1,4 DB eq/kg of biodiesel produced
Environmental Chemistry Letters (2021) 19:4075–4118

Yang et al. (2021) Global warming (100 years), ozone layer depletion, abiotic depletion, The study used LCA to evaluate the potential of biochar for carbon seques-
human toxicity, freshwater ecotoxicity, marine ecotoxicity tration produced using crop residue in China. The carbon sequestration
potential of a country-level biochar system was estimated at 500 × ­1012 kg
­CO2 eq for a year. This accounts for 4.50% of the total greenhouse gas
emissions from China, which could be mitigated by use of biochar
terrestrial ecotoxicity, photochemical oxidant formation, acidification, and Global warming potential = -5 × ­1014 kg ­CO2 eq/kg of biochar
eutrophication potentials
Abiotic depletion potential = 4.5 × ­107 kg Sb eq/kg of biochar
Saranya and Ramachandra, (2020) Net energy ratio and The study compared acid catalysis and biocatalysis for the use of microalgal
biomass feedstocks (produced in 1 ha of land—FU) for biodiesel produc-
tion. The study concluded that biocatalysis was the best route to use with
the following impacts
Global warming (100 years), abiotic depletion, abiotic depletion (fossil Net energy ratio = 18.8
fuels)
photochemical oxidant formation, acidification, and eutrophication poten- Global warming potential = 2000 – 3000 kg ­CO2 eq/FU
tials
Abiotic depletion potential = 0.004 kg Sb eq/FU
Bora et al. (2020) Endpoint indicators: global warming potential, human health, ecosystem This study computed 15 mid-point indicators for thermochemical conversion
quality, resource depletion of poultry litter to fuel and biochar in the USA. However, the results were
aggregated to present endpoint indicators for 1000 kg of poultry litter (FU)
Global warming potential of thermochemical conversion = 659 to 1192 kg
­CO2 eq/FU
Global warming potential for spreading on fields = 1410 kg ­CO2 eq/ FU

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 4108 Environmental Chemistry Letters (2021) 19:4075–4118

uncertainties arise due to statistical variation, subjective

CFC-11: Trichlorofluoromethane; DB: dichlorobenzene; EU-27: The European Union; eq: Equivalent; FU: Functional unit; HFO: Heavy fuel oil; LCA: Life cycle assessment; NER: Net energy
The study conducted LCA to analyse the transformation of waste date seed
judgement, linguistic imprecision, variability in space

Abiotic depletion (fossil fuels) potential = 26 MJ/kg of biodiesel produced

oil to biodiesel via esterification. It concluded that it is environmentally

Abiotic depletion potential = 3 × ­10–6 kg Sb eq/kg of biodiesel produced

and time, inherent randomness, expert disagreement and

­ O2 eq/kg of biodiesel produced

model approximations. These uncertainties can be propa-
feasible to produce biodiesel from waste-derived feedstocks gated through the model using Monte Carlo simulations for
parameter uncertainties or by considering different scenarios
for biofuel production. Though it was noted that about 60%
of the reviewed studies considered scenario analysis, only
10% of the studies did uncertainty analysis by accounting
for parameter uncertainties (Fig. 7).
Sensitivity analysis identifies which process of the bio-
diesel production life cycle contributes directly to the bur-
Global warming potential = 1.13 kg C

densome environmental footprints. Relatedly, if impacts in

ratio; PEI: Potential environmental impact; PM2.5: Particulate matter with ≤ 2.5 µm in diameter; PM10: Particulate matter with ≤ 10 µm in diameter
environmental categories are to be minimised, these will
be the processes where future research and development
should focus on (Al-Muhtaseb et al. 2021). 50% of the stud-
Global warming (100 years), ozone layer depletion, abiotic depletion (fossil Net energy ratio = 2.23

ies reviewed here conducted sensitivity analysis.

Therefore, for reliable and robust decision-making, it is
necessary to analyse sensitivity and uncertainty for various
Findings

scenarios. There were about ten studies identified with no

analysis on sensitivity or uncertainty (due to parameters and
scenarios). Moreover, the presence of only four studies with
scenario, sensitivity and uncertainty analyses poses ques-
terrestrial ecotoxicity, photochemical oxidant formation, acidification, and
fuels), abiotic depletion, human toxicity, freshwater ecotoxicity, marine

tions on the applicability of findings presented in the LCA

studies. There was no study identified that included all three
phases of the life cycle of biofuel production system bound-
ary (described in Fig. 5) and uncertainty, scenario and sen-
sitivity analyses in the 40 reviewed studies. This presents
a considerable knowledge gap when it comes to the use of
LCA studies for strategic decision-making.

Interpretation of results
Environmental impacts considered

This stage of the assessment includes making interpreta-

tions, drawing conclusions, identifying the phases or pro-
cesses that can be improved in the life cycle of a biofuel
eutrophication potentials

production chain to improve the environmental feasibility

Net energy ratio and

of the environmental system. This stage could also involve

presenting and communicating results to stakeholders.
ecotoxicity

Key points observed in life cycle assessment studies

We conducted an intensive critical review of 40 LCA stud-

ies, including methods and findings. In this section, we pre-
sent key points observed in these studies.
There was no study identified that included all the three
Al-Muhtaseb et al. (2021)

phases of biofuel production along with uncertainty, sensi-

tivity and scenario analyses.
Table 11  (continued)

Moreover, most of the studies that computed net energy

ratio (Al-Mawali et al. 2021; Al-Muhtaseb et al. 2021; Dasan
et al. 2019; Im-Orb and Arpornwichanop 2020; Reaño 2020;
Reference

Saranya and Ramachandra 2020), which is defined as the

ratio of output energy/input energy for the overall process

13
Environmental Chemistry Letters (2021) 19:4075–4118 4109

Fig. 7  Details of the scenario,

sensitivity and uncertainty
analyses conducted in the
reviewed studies (N = 40)

(Pleanjai and Gheewala 2009), recorded net energy ratio > 1, only one study identified that highlighted that for spent cof-
showing the importance of biofuels as energy sources. fee grounds, incineration is a better route compared to bio-
Generally recognised contentious issue to produce bio- fuel production (Schmidt Rivera et al. 2020).
fuels was land use which could occur due to natural land, Finally, focusing on the comparison of biological and
agricultural and urban land transformation. This was high- thermochemical pathways, in general, it was observed in the
lighted by the fact that 7% of the studies used land use as comparative studies that thermochemical processes showed
a functional unit (Fig. 5); furthermore, about 15% of the lesser environmental impacts compared to biological pro-
reviewed studies analysed impacts on land use (Aberilla cesses for the same biomass and geographical and temporal
et al. 2019; Chung et al. 2019; Ubando et al. 2020; Aris- span (Ardolino and Arena 2019; Derose et al. 2019). Even in
tizábal-Marulanda et al. 2021; Brassard et al. 2021; Schon- Aberilla et al. (2019), which showed higher environmental
hoff et al. 2021). It was also noted that the use of perennial impacts in the thermochemical process, the greenhouse gas
energy crops is an interesting approach towards mitigation of emissions for gasification were lower than that of anaerobic
greenhouse gas emissions; however, it could result in loss of digestion for rice and coconut residues.
biodiversity for the European Union and UK (EU-27 + UK)
(Di Fulvio et al. 2019) (Table 11).
Other studies focused on water depletion and concluded Bibliometric analysis
that while producing biofuels can mitigate greenhouse gas
emissions, it is also necessary to compute water depletion Figure 8a, b depicts the bibliometric analysis mapping origi-
during crop production and biomass processing (Aberilla nated from the Web of Science core collection for the net-
et al. 2019; Quispe et al. 2019; Ubando et al. 2020; Aris- work visualisation and density visualisation, respectively.
tizábal-Marulanda et al. 2021; Schonhoff et al. 2021). In Firstly, the data were exported 500 entries at a time of 9947
fact, Zhu et al. (2019) concluded that water depletion for results and then were fed into the VOSviewer software that
biofuel production from cotton straws was lower than bio- plotted the data. The type of analysis used herein was co-
oil power generation, however, much greater than observed occurrence, and all keywords were included, as well as
for other renewable sources of energy (such as geothermal, the fractional counting method. We observed direct clus-
solar photovoltaic and wind power). Most of the water use ters connecting identifiable keywords to broad topics such
occurred due to biomass agricultural production, accounting as thermochemical, biochemical and processes associated
for 84.6% of the total water use. with those two routes (gasification, pyrolysis, hydrothermal
The impacts on land use and water depletion due to energy liquefaction, combustion, torrefaction, anaerobic digestion
crop production show that waste-derived feedstocks could and fermentation). This enabled the visualisation of most
provide more sustainable energy sources. Waste-to-energy of the significant keywords in publications in the period of
applications for biomass could mitigate the use of land, fer- 1970–2021 that were associated with the thermochemical
tilisers and water for agriculture of energy crops. This is also and biochemical conversion routes of biomass.
in accordance with zero-waste hierarchy for management of It is evident from Fig. 8a, b that keywords that have seen
waste biomass (Refuse/ redesign > Reduce > Reuse > Recy- a significant increase in popularity and, as a result, progress
cle > Material and chemicals recovery > Residuals man- in keyword research such as biomass gasification, pyrolysis
agement > Unacceptable, e.g. landfilling of non-stabilised and combustion as they part of the thermochemical route in
waste/energy recovery) (Simon, 2019). In fact, there was biomass conversion. In addition, other correlated keywords

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4110 Environmental Chemistry Letters (2021) 19:4075–4118

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 Environmental Chemistry Letters (2021) 19:4075–4118 4111

◂Fig. 8  Bibliometric mapping of biomass conversion processes into microorganism or generate various inhibitors that can affect
biofuels a network visualisation and b density visualisation maps. the biological progress routes. Furthermore, this integration
The bibliometric mapping was performed between 1970 and 2021
may lead to additional costs.
Moreover, to understand the recent advances in evaluat-
have shown in the bibliometric mapping, such as hydrogen ing environmental impacts due to biofuel production, we
production, catalysts and fuel performance. This signifies conducted an intensive critical review of 40 life cycle assess-
that the process of thermochemical conversion is at a very ment (LCA) studies published from the years 2019–2021,
mature stage in terms of research and development, as dem- including methods and findings. The important methods and
onstrated by the prominence of publication keywords over key findings observed were:
the last 51 years. Because of its higher productivity, eco-
nomic viability and existing infrastructure compatibility 1. Only eight studies included all three phases of biofuel
resources, it is ultimately readily available and easily scaled production (which includes biomass cultivation, biofuel
up for the industrial sector. production process and biofuel use and end-of-life man-
Although gasification appears to be most researched in agement phase).
the thermochemical route, this could be down to the fact 2. Waste-derived feedstocks could provide more sustain-
that gasification technology has existed longer than coun- able energy sources by mitigating impacts on land use
terpart technologies, apart from combustion. This also does and water depletion incurred during the production of
not indicate that it is the most efficient thermochemical energy crops.
technology in process efficiency and product quality. For 3. Focusing on thermochemical and biological processes
example, when using gasification, there is a need to remove for the same biomass feedstock and geographical and
the hydrogen sulphide and clean the synthesis gas produced temporal span, thermochemical processes caused lesser
and other requirements. greenhouse gas emissions compared to biological path-
On the other hand, the biochemical conversion route is ways.
less favourable since it suffers from certain limitations, such
as its time-consuming process and low product yield and This review has suggested interesting new avenues for
product inhabitation. Biochemical conversion keywords are evaluation of environmental impacts of the biofuel produc-
shown in Fig. 8a, b, such as ethanol production, bio-hydro- tion chain and key outcomes from a range of biofuel produc-
gen and others. tion processes. Based on the bibliometric mapping (network
and density visualisation maps) from the Web of Science
core collection, we have identified that the thermochemical
Conclusion conversion route of biomass is more researched and far out-
weighed and understood than the biochemical counterpart
Biomass as an affordable, reliable and sustainable energy route of research outputs. This indicates that the biochemical
source contributes 9% (~ 51 EJ) of the global overall primary route suffers from specific gaps in the research, as shown
energy supply. Thermochemical and biochemical technolo- from the lack of impact in the bibliometric mapping analy-
gies are the two main routes employed to convert biomass sis, thus opening doors for a scope for further research in
into biofuels. The former route includes hydrothermal lique- this area.
faction, pyrolysis, torrefaction, gasification and combustion
processes, while the latter route consists of fermentation and
anaerobic digestion processes. Funding The authors would like to thank OQ Oman for their generous
financial support (project code: CR/DVC/SERC/19/01). The authors
Herein, we critically reviewed each individual route along
would also like to acknowledge the support of the Sustainable Energy
with the integration between hydrothermal and biochemi- Research Centre at Sultan Qaboos University. Ahmed Osman and
cal routes of biomass utilisation from a bioeconomy per- David Rooney wish to acknowledge the support of The Bryden Centre
spective. Both routes have drawbacks: the former method project (Project ID VA5048). The Bryden Centre project is supported
by the European Union’s INTERREG VA Programme, managed by the
usually involves a high energy intake along with solvent or
Special EU Programmes Body (SEUPB). Neha Mehta acknowledges
catalyst addition. In contrast, the latter route has a lengthy funding from the Centre for Advanced Sustainable Energy, Belfast,
cycle period and is less efficient in breaking down recalci- UK.
trant biomass materials. Thus, combining those two routes
can be promising by incorporating the benefits of both meth- Declarations
ods in biofuel processing. However, there are outstanding
challenges associated with integration between those two Conflict of interest The authors declare no conflict of interest.
routes. For instance, the catalysts or solvent utilisation of the
thermochemical route can result in poisoning or killing the

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4112 Environmental Chemistry Letters (2021) 19:4075–4118

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