sustainability

Review
Role of Biofuels in Energy Transition, Green Economy and
Carbon Neutrality
Nida Khan 1 , Kumarasamy Sudhakar 1,2,3, * and Rizalman Mamat 4

1 Faculty of Mechanical and Automotive Engineering Technology, Universiti Malaysia Pahang,

Pekan 26600, Malaysia; nidak.881@gmail.com
2 Automotive Engineering Centre, Universiti Malaysia Pahang, Pekan 26600, Malaysia
3 Energy Centre, Maulana Azad National Institute of Technology, Bhopal 462003, India
4 School of Mechanical Engineering, Ningxia University, Yinchuan 750021, China; rizalman@ump.edu.my
* Correspondence: sudhakar@ump.edu.my

Abstract: Modern civilization is heavily reliant on petroleum-based fuels to meet the energy demand
of the transportation sector. However, burning fossil fuels in engines emits greenhouse gas emis-
sions that harm the environment. Biofuels are commonly regarded as an alternative for sustainable
transportation and economic development. Algal-based fuels, solar fuels, e-fuels, and CO2 -to-fuels
are marketed as next-generation sources that address the shortcomings of first-generation and
second-generation biofuels. This article investigates the benefits, limitations, and trends in different
generations of biofuels through a review of the literature. The study also addresses the newer genera-
tion of biofuels highlighting the social, economic, and environmental aspects, providing the reader
with information on long-term sustainability. The use of nanoparticles in the commercialization of
biofuel is also highlighted. Finally, the paper discusses the recent advancements that potentially



enable a sustainable energy transition, green economy, and carbon neutrality in the biofuel sector.



Citation: Khan, N.; Sudhakar, K.; Keywords: biofuels; sustainability; bioeconomy; solar fuels
Mamat, R. Role of Biofuels in Energy
Transition, Green Economy and
Carbon Neutrality. Sustainability 2021,
13, 12374. https://doi.org/10.3390/ 1. Introduction
su132212374
Our planet is experiencing more natural calamities that are severe in terms of intensity
and duration. The use of non-renewable fuels as primary energy sources for several
Academic Editor: Paris Fokaides
years resulted in increasing the speed of global warming and the emission of various air
pollutants that are detrimental to the environment and public health. According to a review
Received: 28 July 2021
of five leading international datasets by the World Meteorological Organization (WMO),
Accepted: 28 October 2021
2020 was one of the three warmest years on earth, tied with 2016 for first place [1]: another
Published: 9 November 2021
stark reminder of the accelerated pace of climate change, which is devastating health and
lives around our world. Based on current policies, global energy demand is expected to
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
rise by 1.3% per year until 2040 without dramatic energy production and recycling [2].
published maps and institutional affil-
Progress must be made even sooner to reduce greenhouse gas associated with industrial
iations.
development and energy usage. By 2070, the International Energy Agency (IEA) anticipates
global transportation (measured in passenger kilometers) to be increased fourfold and car
ownership rates to rise by 60%. According to the Energy Technology Perspective study, the
demand for passenger and freight aircraft will triple [3].
The idea of using biofuels appears to be feasible to bring our planet on the pathway to
Copyright: © 2021 by the authors.
meet energy-related sustainable development. Henry Ford (1896) pioneered bioethanol,
Licensee MDPI, Basel, Switzerland.
while Rudolf Diesel was an innovator in peanut oil. Biofuel is one of the sustainable energy
This article is an open access article
distributed under the terms and
sources obtained from processing various feedstocks such as plant, algae, or animal waste.
conditions of the Creative Commons
Biodiesel (fatty acid methyl ester, or FAME, fuels derived from vegetable oils and fats,
Attribution (CC BY) license (https:// including wastes such as used cooking oil) and bioethanol (produced from corn, sugar
creativecommons.org/licenses/by/ cane, and other crops) are the two most popular biofuels. Since liquid fossil fuels dominate
4.0/). the transportation sector, replacing these fuels with renewable energy will significantly

Sustainability 2021, 13, 12374. https://doi.org/10.3390/su132212374 https://www.mdpi.com/journal/sustainability

Sustainability 2021, 13, 12374 2 of 30

contribute to the achievement of the comprehensive energy and sustainability goals. The
most widely used biofuels are ethanol (from various sources), which is well suited to
Otto cycle engines, and biodiesel (from multiple sources), which is better suited to diesel
cycle engines. Biodiesel can be used as a fuel additive in compression ignition (diesel)
engines, primarily in 20% blends (B20) with petroleum diesel. Biodiesel blend levels
are often determined by the cost of the fuel and the projected advantages. Biomethane
fuels in CNG buses demonstrate the sustainability concept while also improving overall
environmental performance.
Several countries have now passed regulations approving biofuels to meet the po-
tential transportation requirements [4]. The integration of biofuels will reduce a nation’s
reliance on conventional petroleum imports from other countries, which will help mitigate
the impacts of the fluctuations in oil prices, boost the economy, and reduce carbon emis-
sions. Moreover, biofuels encourage new entrepreneurs while simultaneously increasing
economic activity globally. They also provide community-level growth alternatives for
small and medium-size power grids [5].
Overall, global ethanol production decreased about 15% in 2020 and biodiesel pro-
duction decreased by 5% in 2020. The International Energy Agency (IEA) estimates that
worldwide transportation biofuel output will return to approximately 162 billion liters
in 2021, similar to 2019 [6]. In 2025, biofuels are expected to provide roughly 5.4% of the
energy requirement for road transport [7].
Dependent on feedstocks and technique, biofuels are grouped into multiple categories
known as 1st, 2nd, 3rd, and 4th generation. Agricultural products or traditional biofuels are
used to generate first-generation biofuels. Fermentation, transesterification, and anaerobic
digestion are examples of comparatively well-established processes for producing these
fuels [8]. The primary concern with first-generation biofuel is that it is primarily made from
agricultural resources, which has a negative impact on financial, ecological, and political
considerations because mass biofuel production necessitates more fertile land, resulting
in far fewer lands available for human and animal food production [9]. Lignocellulosic
feedstocks, agro-residues, and non-edible plant biomass constitute the second-generation
feedstocks [10]. Biofuels of the second generation overcome the impact on the climate and
social aspects. However, it has a negative energy yield, feedstock transportation issues,
high downstream production costs, and modest greenhouse gas (GHG) reduction, limiting
their use [11].
On the other hand, biofuels of the 3rd generation have gained broad interest as a
substitute for biofuel production to address the problems associated with the first and
second generations [8]. The most promising feedstock for renewable fuel production is
macro and microalgae. Microalgae and macroalgae require sunlight [12], water, nutrients,
and carbon dioxide to create energy biofuels. Algae biomass has the distinct benefits of not
competing with soil, having low lignin content, requiring less energy, and competing less
with food crops [13]. Genetically modified (GM) algae are used in fourth-generation biofuel
(FGB), but there is still considerable concern about the negative environmental impacts.

Biofuels for Transportation

Figure 1 depicts non-renewable biofuels used in the different transportation sectors.
Biomethane is utilized to fuel CNG buses, demonstrating that the gas used to power
CNG buses may be produced sustainably while also increasing overall environmental
performance. The most promising bio-derived fuels for ship use are SVO and biodiesel.
The importance of marine transportation in global freight distribution cannot be overstated.
However, the maritime sector accounts for nearly 2.6% of worldwide GHG emissions.
ExxonMobil has completed a successful sea trial of its first marine biofuel oil with Stena
Bulk, which is a shipping firm bunkered in Rotterdam. The study showed that marine
biofuel oil, which may reduce CO2 emissions by up to 40% when compared to traditional
marine fuel, can be utilized in a relevant maritime application without modification,
allowing operators to make substantial progress toward their carbon reduction goals. The
 Sustainability 2021, 13, x FOR PEER REVIEW 3 of 32

Bulk, which is a shipping firm bunkered in Rotterdam. The study showed that marine
Sustainability 2021, 13, 12374 biofuel oil, which may reduce CO2 emissions by up to 40% when compared to traditional 3 of 30
marine fuel, can be utilized in a relevant maritime application without modification, al-
lowing operators to make substantial progress toward their carbon reduction goals. The
aviation industry is responsible for 12% of all transportation-related GHG emissions and
aviation
2–3% ofindustry is responsible
all anthropogenic GHGfor 12% of all[12].
emissions transportation-related GHG emissions
Hundreds of demonstration flights and
have
2–3% of all anthropogenic GHG emissions [12]. Hundreds of demonstration flights
been flown by more than 20 airlines using a combination of regular jet fuel and aviation- have
been
gradeflown by more
biofuel than 20
generated airlines
from using
various a combination
feedstocks, of regular
including wastejetcooking
fuel andoil aviation-
and oil
grade
crops such as rapeseed, jatropha, camelina, and palm oil, to produce an alternateoilaviation
biofuel generated from various feedstocks, including waste cooking oil and crops
such as
biofuel.rapeseed, jatropha, camelina, and palm oil, to produce an alternate aviation biofuel.

Light
Heavy Heavy Marine Aviation
Vehicles
Vehicles Machinery - Bioethanol - Sustainable
- Bioethanol
- Biodiesel - Biodiesel -Biodiesel Aviation Fuel
- Biodiesel

Figure1.1.Biofuels
Figure Biofuelsasasan
analternative
alternativefor
fornon-renewable
non-renewablefuels
fuelsininthe
thedifferent
differenttransportation
transportationsectors.
sectors.

The
Thepurpose
purposeofofthis
thisarticle
articleisistotoshed
shedlight
lighton
onthe
thefollowing
followingaspects
aspectsofofbiofuel:
biofuel:
(a)
(a) To
Toinvestigate
investigatethe
thebenefits,
benefits,limitations,
limitations,and
andtrends in in
trends different generations
different generationsof biofuels.
of biofu-
(b) Toels.assess the social, economic, and environmental effects for the long-term sustain-
(b) ability of biofuels.
To assess the social, economic, and environmental effects for the long-term sustaina-
(c) To highlight the recent advancements in the biofuel sector that potentially enable
bility of biofuels.
(c) carbon neutrality,
To highlight the sustainable energy transition,
recent advancements and a greener
in the biofuel economy.
sector that potentially enable
carbon neutrality, sustainable energy transition, and a greener economy.
2. Overview of 1G, 2G, 3G, and 4G Generations of Biofuels
Sustainability 2021, 13, x FOR PEER REVIEW
Figure
2 of
depicts different generations of biofuels based on the feedstock and4 of
the32
2. Overview 1G, 2G, 3G, and 4G Generations of Biofuels
development of the conversion process.
Figure 2 depicts different generations of biofuels based on the feedstock and the de-
velopment of the conversion process.

Figure2.2. Biofuels
Figure Biofuels generation.
generation.

2.1.
2.1. First
First Generation
Generation (1G)
(1G)
First-generation
First-generation biofuels
biofuels include
include biodiesel,
biodiesel, bioethanol,
bioethanol, and
and biogas,
biogas, which
which are
areused
used
commercially.
commercially. Biodiesel is a diesel substitute produced by the oil transesterification of
Biodiesel is a diesel substitute produced by the oil transesterification of
natural sources as well as leftover fats and oils. At the same time, bioethanol is a gasoline
substitute that is produced through the fermentation of sugar or starch as illustrated in
Figure 3. First-generation biofuels are being evaluated based on two main claims: For in-
stance, they explicitly attempt to compete with crops for feed. Second, their energy, eco-
 Figure 2. Biofuels generation.

Sustainability 2021, 13, 12374 2.1. First Generation (1G) 4 of 30

First-generation biofuels include biodiesel, bioethanol, and biogas, which are used
commercially. Biodiesel is a diesel substitute produced by the oil transesterification of
naturalsources
natural sourcesas aswell
wellasasleftover
leftoverfats
fatsand
andoils.
oils.At
Atthe
thesame
sametime,
time,bioethanol
bioethanolisisaagasoline
gasoline
substitute that
substitute that isisproduced
producedthrough
throughthe thefermentation
fermentationof ofsugar
sugaror orstarch
starchasasillustrated
illustratedinin
Figure3.3. First-generation biofuels
Figure biofuels arearebeing
beingevaluated
evaluatedbased
basedonontwotwomain
mainclaims:
claims:ForFor
in-
stance, they
instance, theyexplicitly
explicitly attempt
attempt to to
compete
compete with crops
with for for
crops feed. Second,
feed. theirtheir
Second, energy, eco-
energy,
nomic, andand
economic, environmental
environmental balance
balancewillwill
notnot
be as
beoptimal as previously
as optimal planned.
as previously planned.Accord-
Ac-
cording to many
ing to many researchers,
researchers, if food
if food pricesprices are influenced
are influenced by biofuel
by biofuel production
production to thetosame
the
same extent,
extent, the number
the number of food-insecure
of food-insecure peoplepeople in developed
in developed countries
countries will increase
will increase to
to nearly
nearly 1.2 billion
1.2 billion by 2025 by[13].
2025 [13].

Figure3.3.First-generation
Figure First-generationbiofuels.
biofuels.

Several
Severalstudies
studieshavehavefound
foundthat
thatswitching
switchingto tofirst-generation
first-generationbiofuels
biofuelsmay mayresult
resultin
in
an increase in GHG emissions. Senauer [13] stated that agricultural use
an increase in GHG emissions. Senauer [13] stated that agricultural use and fertilizer ap- and fertilizer
application
plication will willdouble
doubleemissions
emissionsover
overthe
thenext
next3030 years
years rather
rather than
than the
the anticipated
anticipated 20%20%
reduction
reduction in GHG emissions from biofuel. Furthermore, 1G biofuels such as as
in GHG emissions from biofuel. Furthermore, 1G biofuels such ethanol
ethanol re-
require
quire aalarge
largeamount
amountof ofmaize,
maize,which
which requires
requires aa large
large amount
amount of of water
water ranging
rangingfromfrom55toto
2138
2138liters
liters(L)
(L)per
per1 1L Lofof
ethanol,
ethanol,depending
dependingon on
how howandand
where ethanol
where maize
ethanol is grown.
maize [14].
is grown.
This
[14].appears to havetonegative
This appears environmental
have negative consequences,
environmental as its intake
consequences, as itsfrom water
intake fromsources
water
can put those areas at risk of drought. Pursuing biofuel production in water-scarce
sources can put those areas at risk of drought. Pursuing biofuel production in water-scarce locations
would further strain an already constrained resource, mainly if a crop requires irrigation.
Water resources and wetlands are expected to suffer as a result of increased water intake [15].
Chaudhary et al. [16] examined the ecological impacts of ethanol production in various
parts of the world. It was demonstrated that the cultivation of sugar cane in Brazil suffers a
greater loss of biodiversity than the production of sugar beet in France and maize (grain or
stover) in the United States [16]. The expansion of 1G biofuels has been a source of social
stress, particularly in developing countries where biofuel expansion has occurred in the
absence of advanced facilities to control it. Biofuel-based community conflicts are typically
related to land contract issues. Citizens in Tanzania, Mozambique, Ghana, Kenya, and
Zambia have been reported to have lost access to their shared land due to extensive jatropha
farming. Land leases are frequently at the core of biofuel-related community disputes [17].
The Indian government’s and the biofuel industry’s rapid adoption of jatropha threatens
to drive millions of underprivileged rural farmers out of areas where they get their food,
fuel, wood, fodder, and lumber [18]. Conventional agricultural production is already
facing extreme water constraints; therefore, the regional and local water supply burden
would be enormous with 1G biofuel. Policymakers would be hesitant to pursue biofuel
alternatives based on conventional food and oil crops [19].The pros and cons of the 1G
biofuels are highlighted in Figure 4. The biofuel yield parameters from 1G feedstock is
provided in Table 1.
 ers out of areas where they get their food, fuel, wood, fodder, and lumber [18]. Conven-
tional agricultural production is already facing extreme water constraints; therefore, the
regional and local water supply burden would be enormous with 1G biofuel. Policymak-
ers would be hesitant to pursue biofuel alternatives based on conventional food and oil
crops [19].The pros and cons of the 1G biofuels are highlighted in Figure 4. The biofuel
Sustainability 2021, 13, 12374 5 of 30
yield parameters from 1G feedstock is provided in Table 1.

+ -

Figure 4. Pros and cons of first-generation biofuels.

Figure 4. Pros and cons of first-generation biofuels.

Table 1. Parameters and yield of biodiesel, biomethane, bioethanol, and syngas for various feedstocks of first-generation biofuels.

Biofuels Biodiesel Bioethanol Biomethane Biobutanol Syngas

Feedstock Soybean Corn Corn Corn Rapeseed
Fermentation,
Primary liquefaction, Fast pyrolysis,
Transesterification, heat treatment Anaerobic digestion, Conversion to
Hydrotalcite as basic (105–110 ◦ C) pH-range—6.5–8.2. AB Fermentation, biochar
catalyst, for 5–7 min, mesophilic Strain: C. acetobutylicum, Steam gasification
Parameters
methanol/oil molar α-amylase Secondary (30–40 ◦ C), Temperature 34 to 39 ◦ C Steam flow:
ratio of 20:1, liquefaction: 95 ◦ C, or thermophilic for 40 to 60 h 172 g min−1 kg−1
reaction time of 10 h 1–2 h, enzymatic (50–60 ◦ C) biochar
liquefaction, Temperature = 750 ◦ C
pH 4.5
H2 = 58.7%
Yield 189.6 g/kg 449 g/kg 205–450 dm3 kg−1 12–20 g/L
CO = 10.6%
References [20] [21] [22] [23] [24]

2.2. Second Generation (2G)

Figure 5 provides the 2G biofuels feedstock. Second-generation biofuels address
many of the issues related to first-generation biofuels. The prospects for fostering regional
growth and improving the economic situation in developing regions is envisaged with 2G
biofuels. Around the world, various strategies for the production of second-generation
biofuels are being considered. Still, the focus is primarily on two distinct paths, either
the thermo or bio route generated by biomass of cellulose and lignin, tree surplus, and
 2.2. Second Generation (2G)
Figure 5 provides the 2G biofuels feedstock. Second-generation biofuels address
many of the issues related to first-generation biofuels. The prospects for fostering regional
growth and improving the economic situation in developing regions is envisaged with 2G
Sustainability 2021, 13, 12374 biofuels. Around the world, various strategies for the production of second-generation 6 of 30
biofuels are being considered. Still, the focus is primarily on two distinct paths, either the
thermo or bio route generated by biomass of cellulose and lignin, tree surplus, and sea-
sonal forage crop. Thermochemical manufacturing has the significant benefit of higher
seasonal forage
versatility crop. Thermochemical
of feedstock manufacturing
than biological production. The has the significant
“thermo” route is benefit
focusedofon
higher
the
versatility of feedstock than biological production. The “thermo” route
heat processing of biomass under decreased oxidizing agent concentrations. Under theis focused on
the heat processing of biomass under decreased oxidizing agent concentrations.
temperature range of 300 to 1000 °C, the ◦bottom range will primarily produce solid biofuel Under
the temperature range of 300 to 1000 C, the bottom range will primarily produce solid
called biochar. At elevated temperatures, pyrolytic oil and syngas are the most concen-
biofuel called biochar. At elevated temperatures, pyrolytic oil and syngas are the most
trated substances in the middle range. The “bio” approach includes the pretreatment of
concentrated substances in the middle range. The “bio” approach includes the pretreatment
lignocellulosic material, enzymatic hydrolysis, and the fermentation of sugars by specific
of lignocellulosic material, enzymatic hydrolysis, and the fermentation of sugars by specific
strains of microorganism. It is more challenging to convert lignocellulose to reducing sug-
strains of microorganism. It is more challenging to convert lignocellulose to reducing
ars than it is to convert starch. Biological, physical (thermal), or chemical catalysts are used
sugars than it is to convert starch. Biological, physical (thermal), or chemical catalysts
to pretreat biomass in the biochemical pathway as shown in Figure 6; hence, enhanced
are used to pretreat biomass in the biochemical pathway as shown in Figure 6; hence,
advances in the development of 2G biofuels are hampered due to the chemical and struc-
enhanced advances in the development of 2G biofuels are hampered due to the chemical
tural properties of the extracellular matrix. The biodiesel yield parameters from the 2G
and structural properties of the extracellular matrix. The biodiesel yield parameters from
biofuel are highlighted in Table 2.
the 2G biofuel are highlighted in Table 2.

Second generation

Bioethanol from crops Biodiesel from Fat

Lignocellulosic Biomass Waste Vegetable Oil

Figure 5. Feedstock of second-generation biofuels.

Figure 5. Feedstock of second-generation biofuels.

Table 2. Parameter and yield of biodiesel, bioethanol, biomethane, and syngas from various feedstocks of second-generation biofuels.

Biofuels Biodiesel Bioethanol Biomethane Biobutanol Syngas

Feedstock Palm oil Sugarcane bagasse Corn Stover Rice straw Corn Stover
Fermentation, Acid Anaerobic
Gasification,
(H2 SO4 ) digestion,
Transesterification, Fermentation, C. Fluidized bed gasifier
hydrolysis, Cellulase
Parameters H2 SO4 —5% v/w, sporogenes BE01, GA: steam;
Kluyveromyces sp. (Spezyme CP)
95 ◦ C/540 min (37◦ C and 6.7 pH) T: 600–710 ◦ C;
IIPE453, Fermentation T = 37 ± 1 ◦ C,
ER: N.A.
at 50 ◦ C t = 30 days
H2 : 26.9,
Yield 97 w/w 165 g/kg 135 dm3 kg−1 VS 5.52 g/L CO: 24.7
CO2 : 23.7 CH4 : 15.3
References [25] [26] [27] [28] [29]

New technologies have focused on genomes as well as structural and artificial ge-
netics that would offer demanding opportunities for enhancing the digestibility of cell
walls [30] and have the potential to raise PFCE from biomass radically. In species such
as Caldicellulosiruptor saccharolyticus and Acidothermus cellulolyticus [31], enzymes have
recently been used to degrade lignocellulose and speed up the process. The efficiency
of cellulosic biofuels can also be significantly strengthened by supplying engineered mi-
crobes with the potential to digest lignocellulosic biomass even without the application of
costly enzymes. Some see genetic modification (GM) techniques as key to achieving high
yield, thus boosting the total energy stability of crop residues, for example, developing
resistance to fertilizers and pesticides and flooding. However, GM discussions may result
from socioeconomic and political decisions debates [32]. Second-generation biofuels are
contemporary and innovative, but they do have a specific impact on sustainability. The
balance of the life cycle of GHG emissions remains a problem depending on the location
of the 2G biofuels produced, the conservation methods, the modes of transportation, and
the methods of processing. The second-generation biofuels include waste from operations,
 Sustainability 2021, 13, 12374 7 of 30

such as methane from garbage dumps or the conversion of waste from processes derived
from fossil fuels [33]. A detailed study by Havlík et al. [34] (reported that 2G biofuel pro-
duction powered by wood from clean sources would reduce overall emissions, considering
the deforestation, agriculture water consumption, and increased crop prices especially
Sustainability 2021, 13, x FOR PEER REVIEW 7 of 32
with the rise of biofuel land area. Still, some other environmental requirements, such as
ecosystem preservation, climate regulation, and fuelwood availability to the inhabitants,
could be influenced by biomass feedstocks and land use [34].

Figure 6. Pretreatment of lignocellulosic material.

Figure 6. Pretreatment of lignocellulosic material.
The bacteriological and physical properties of soils are also adversely affected by the
Table 2. Parameter and yield removal
of biodiesel, bioethanol,
of crop biomethane,
and forestry and syngas
residues. from various
For instance, feedstocks
according of second-genera-
to Powlson et al. [35] the
tion biofuels. energy savings related to burying wheat bran in croplands are greater to those produced
Biofuels due to Bioethanol
Biodiesel their removal for biodiesel production [35]. Various
Biomethane published data indicate
Biobutanol Syngasthat
when forest deadwood is removed, the occurrence and variability of bird species decrease
Feedstock Palm oil Sugarcane bagasse Corn Stover Rice straw Corn Stover
significantly. For cavity nesters (e.g., woodpeckers), the critical impact was recorded
Fermentation, Acid
(e.g., woodpeckers). The eggs and offspring of insects are captured and expelled from
Gasification,
the (Has
forest 2SO 4)
waste is removed Anaerobic
and shipped to energy plants, not only reducing insect
Transesterification, Fermentation, C. Fluidized bed gas-
hydrolysis,
proliferation digestion,
but also removing Cellulase
an important source of bird food [36,37]. Evidently, over
Parameters H2SO4—5%v/w, sporogenes BE01, ifier GA: steam; T:
the past decade(s), the growth of
Kluyveromycessp. cellulosicCP)
(Spezyme biofuel plants on an industrial level has become
95 °C/540 min weaker than anticipated. The latest biofuel output cost (37°statistics
C and 6.7 pH) 600–710
indicate °C; ER:
that biofuels of
IIPE453, Fermentation T = 37 ± 1 °C, t = 30 days
the second generation on an energy-average basis are two to three times more N.A.
expensive
at 50°C
than fossil fuels [10].
H2: 26.9,
Currently, producing 2G biofuel is cost-efficient, but there seem to be various tech-
nological challenges that need CO: 24.7
Yield 97 w/w 165 g/kg 135todm
be 3overcome
kg–1 VS before realizing
5.52 g/Ltheir potential. During
the pretreatment and extraction phase, the need for thermal energy and enzymes CO2: 23.7 CH4:
during
cellulose-based hydrolysis increases the production value of bioethanol [38]. The treatment 15.3
References [25] [26] such as wheat bran raises
of co-products [27] significant sustainability
[28] [29] pre-
concerns because
treatment for enzymatic saccharification is required to overcome the issues associated with
lignocellulosic biomasshave
New technologies [39]. focused
In additionon to the production
genomes as well of
assustainable, low artificial
structural and pretreatment
genet-
methods and highly productive fermentation processes, the incorporation of
ics that would offer demanding opportunities for enhancing the digestibility of cell walls hemicellulose
integration
[30] and haveisthea technical
potentialfactor thatPFCE
to raise should be taken
from intoradically.
biomass account when developing
In species such assus-
Cald-
tainable biorefineries [40]. Worldwide, several efforts are underway to commercialize the
icellulosiruptor saccharolyticus and Acidothermus cellulolyticus [31], enzymes have recently
second-generation biofuels provided from both routes’ extraction. The second-generation
been used to degrade lignocellulose and speed up the process. The efficiency of cellulosic
mainly operates on a pilot or prototype scale, as shown in Table 3.
biofuels can also be significantly strengthened by supplying engineered microbes with the
potential to digest lignocellulosic biomass even without the application of costly enzymes.
Some see genetic modification (GM) techniques as key to achieving high yield, thus boost-
ing the total energy stability of crop residues, for example, developing resistance to ferti-
lizers and pesticides and flooding. However, GM discussions may result from socioeco-
nomic and political decisions debates [32]. Second-generation biofuels are contemporary
and innovative, but they do have a specific impact on sustainability. The balance of the
life cycle of GHG emissions remains a problem depending on the location of the 2G bio-
fuels produced, the conservation methods, the modes of transportation, and the methods
of processing. The second-generation biofuels include waste from operations, such as me-
Sustainability 2021, 13, 12374 8 of 30

Table 3. Commercialization status of advanced biofuel production route [41].

Pilot/ Small
Conversion Process Demonstration Commercial
Demonstration Commercial
HEFA X
Gasification—FT X
Pyrolysis and upgrading X
HTL and upgrading X
Advanced sugar fermentation to hydrocarbons X
Ethanol production from agricultural residues
X
(pretreatment, enzymatic hydrolysis, and fermentation)

In 2007 and 2012, the Canadian company Iogen corporation operated a demonstration
plant and then planned to develop a production plant in Brazil that could manufacture
ethanol in 40 million liters of production by sugar cane bagasse [42]. Even so, since their
technology was not advanced enough or failed in their start-up process, several other firms
had to close down [43,44]. When compared to the fossil energy, they could replace the basis
cost of production. However, they are still just too expensive to manufacture. Changes
in policy could expedite the transition from first-generation biofuels to the commercial
deployment and adoption of second-generation biofuels. However, regulations must be
designed to encourage the development of the most favorable biofuels while discouraging
the production of “poor” biofuels [8]. Table 4 highlights the difference between first-
generation and second-generation biofuels.

Table 4. First-generation vs. second-generation biofuels.

First Second
Conversion Process
Generation Generation
Possibilities for greenhouse gases mitigation - X
Ability to reduced consumption of fossil fuels - X
The viability of using marginal land to produce feedstock - X
High manufacturing value X -
Relatively simple conversion procedure X -
High prospects for a net decrease in the use of non-renewable resources X -
More output of land use - X

2.3. Third-Generation Biofuel (3G)

First-generation and second-generation biofuels are not exclusively biological nor
reliant on environmentally sustainable feedstocks. Furthermore, high-energy inputs are
required in both feedstock processing and biofuel synthesis. Despite the popularity of
1G biofuels, they suffer from limitations due to disrupting the chain of food and feed,
while second-generation feedstocks are losing reputation owing to the increased cost of
the synthesis and chemical processing of biodiesel. The production of greener and more
efficient biofuels is essential to meet the challenge of entirely replacing conventional fossil
fuels with third-generation biofuels. The biofuel yield from 3G feedstock is highlighted in
Table 5. Figure 7 provides the third generation feedstock.
 biofuels, they suffer from limitations due to disrupting the chain of food and feed, while
second-generation feedstocks are losing reputation owing to the increased cost of the syn-
thesis and chemical processing of biodiesel. The production of greener and more efficient
biofuels is essential to meet the challenge of entirely replacing conventional fossil fuels
Sustainability 2021, 13, 12374 with third-generation biofuels. The biofuel yield from 3G feedstock is highlighted in9Table
of 30
5. Figure 7 provides the third generation feedstock.

Third generation

Microalgae Macroalgae

Figure 7. Third-generation biofuels.

Figure 7. Third-generation biofuels.
Table 5. Parameters and yield of biodiesel, biomethane, bioethanol, and syngas synthesis from various feedstocks of
Table 5. Parameters
third-generation and yield of biodiesel, biomethane, bioethanol, and syngas synthesis from various feedstocks of third-
biofuels.
generation biofuels.
Biofuels Biodiesel Bioethanol Biomethane Biobutanol Syngas
Biofuels Biodiesel Bioethanol
Chlorococcum Biomethane Biobutanol Syngas
Feedstock Spirulina platensis S. latissima (macroalgae) Macroalgae Spirulina
Chlorococcum
infusionum infu- S. latissima (macroal-
Feedstock Spirulina platensis Macroalgae Spirulina
Transesterification, reaction sionum gae)
Anaerobic digestion,
◦
temperature 55 C, 60% Fermentation, ◦
53 C for a period of Fermentation,
Transesterification, reac-alkaline pretreatment,
catalyst concentration, 1:4
Anaerobic digestion, C. beijerinckii
34 days, flushed with Pyrolsis,
Parameters
tionbiomass
algae temperature 55 °C, Fermentation,
to methanol temp. 120 ◦ C, alka-N53 °C2 (80/20%),
2 /CO for a period
to of 34 ATCC Fermentation,
35702, C.
Temp = 550 ◦ C.
ratio,catalyst
450 rpm stirring S. cerevisiae ◦ and 6.0 pH). Pyrolsis,
60% concentra- line pretreatment, obtain days,anaerobic
flushed with(37 Cbeijerinckii ATCC
Parameters intensity conditions. Temp =
tion, 1:4 algae biomass to temp. 120 °C, N2/CO2 (80/20%), to 35702,
H2 550
= 29,°C.
methanol ratio, 450 rpm S. cerevisiae obtain anaerobic − 1
condi- (37 °C and 6.0 pH). CO = 24,
Yield 60 g/kg 260 g /kg 340 ± 48.0 mL g VS 4 g/L
stirring intensity tions. CO2 = 24,
CH4 = 18
H2 = 29,
References [45] [26] [46] [47] [48]
CO = 24,
Yield 60 g/kg 260 g /kg 340 ± 48.0 mL g VS −1 4 g/L
CO2 = 24,
Algae are at the forefront of the production of third-generation biofuel. When CH4 algae
= 18
References are
[45] used to produce biofuels,
[26] CO 2 emissions will
[46] be reduced in comparison
[47] to sources
[48] of
fossil fuels, reducing rising temperatures. The production of 1 kg of microalgae, according
to Chisti [49],are
Algae involves the fixation
at the forefront of theof up to 1.8 kg CO
production 2 . This unique necessity
of third-generation biofuel.for biomass
When algae
production has evolved microalgae to focus on intensive bio-mitigation studies, which
are used to produce biofuels, CO2 emissions will be reduced in comparison to sources of
may vastly enhance life-cycle savings. The manufacture of biofuels through algae highly
depends upon lipid value. Fast-growing algae are thought to have low oil content, while
slow-growing algae have high lipid content [50]. Therefore, microalgal strains selection
with greater efficiencies and a rapid growth rate of metabolites is essential [51]. Green
microalgae (Chlorophyta) collect oil at a higher rate than other algal taxa such as cyanobac-
teria, brown algae, and red algae [52]. Owing to high lipids, approximately 60–70% [53]
and high efficiency, 7.4 g/L/day for Chlorella protothecoides [54], species such as Chlorella
are evidently targeted. Biofuel production through algal biomass could be commercially
viable when algal products and waste are flexibly used. A variety of methods can be used
to transform microalgae biomass into energy sources (Figure 8).
LCA analysis was conducted on the use of macroalgae for increased CO2 fixation and
biofuel generation [55]. It was showed that increased CO2 fixation by macroalgae could
provide an energy advantage linked with carbon recycling [55]. In the best-case scenario
thus far studied, macroalgae can yield a net energy of 11,000 MJ/t dry algae compared to
9500 MJ/t for microalgae gasification. Lam and Lee [56] evaluated the energy-efficiency
ratio (EER) of agricultural and microalgae-based biofuel manufacturing techniques. The
EER is defined as the energy output divided by the energy intake. EERs for crop-based
biofuel ranged from 1.44 to 5, whereas EERs for microalgae-based biofuel ranged from
0.35 to 434 [56]. Overall, the EER for microalgae-based biofuel generation was lower,
but this ratio may rise if the process continues to develop. Microalgae-based biodiesel
fuels have density, viscosity, flash point, heating value, cold filter clogging point, and
solidifying point in common with petroleum-based biofuels. As a result, they meet both
the American Society for Testing and Materials (ASTM) and the International Biodiesel
Standard for Vehicles (IBSV) requirements [57]. Table 6 compares the properties of macro
and micro-algae based biofuels.
 microalgae (Chlorophyta) collect oil at a higher rate than other algal taxa such as cyano-
bacteria, brown algae, and red algae [52]. Owing to high lipids, approximately 60–70%
[53] and high efficiency, 7.4 g/L/day for Chlorella protothecoides [54], species such as Chlo-
rella are evidently targeted. Biofuel production through algal biomass could be commer-
Sustainability 2021, 13, 12374 cially viable when algal products and waste are flexibly used. A variety of methods 10 ofcan
30

be used to transform microalgae biomass into energy sources (Figure 8).

Figure 8. Biofuel
Figure 8. Biofuel generation
generation from
from microalgae.
microalgae.

Table 6. Properties of micro and macro-algal biodiesel as compared to conventional diesel [58,59].
LCA analysis was conducted on the use of macroalgae for increased CO2 fixation and
biofuel generation [55].Microalgae
It was showed Macroalgae
that increased Biodiesel
CO2 fixation by macroalgae could
Standard
Fuel Property Unit Diesel
provide an energy advantage Biodiesellinked withBiodiesel
carbon recyclingEN 14214
[55]. In the best-case scenario
Cetane Number thus far studied,
- macroalgae46.5can yield a net energy of 11,000 51
58.23 MJ/t dry algae compared
53.3 to
Kinematic Viscosity @40 ◦ C 9500 MJ/ mmt for2/s
microalgae 5.06
gasification. Lam4.3and Lee[56] evaluated
3.5–5.0 the energy-efficiency
2.64
Density @15 ◦ C ratio (EER)kg/L
of agricultural0.912
and microalgae-based
0.868 biofuel manufacturing
0.86–0.90 techniques.
0.84 The
Acid Value EER is defined
mg KOH/g as the energy
0.14 output divided
0.13 by the energy intake.
0.5 max EERs for crop-based
0
biofuel ranged from 1.44 to 5, whereas EERs for microalgae-based biofuel ranged 100 1-D
from
Flashpoint ◦C
0.35 to 434. [56]. Overall, the- EER for microalgae-based
155 101 min
biofuel generation was126 lower,
2-D but
Cloud Point this ratio may◦ C rise if the process
16.1 continues−to4 develop. Microalgae-based
- biodiesel
4 fuels
Sulfur Content
have density,
mg/kg
viscosity, flash
7.5
point, heating value,
8.9
cold filter clogging
10 max
point, and solidify-
5.9
ing point in common with petroleum-based biofuels. As a result, they meet both the
Copper Strip Corrosion
-
American Society for Testing1 and Materials (ASTM)
1 1
and the International Biodiesel1 Stand-
(3 h at 50 ◦ C)
ard for Vehicles (IBSV) requirements [57]. Table 6 compares the properties of macro and
micro-algae based biofuels .
Approximately 30% is the algal biomass’s oil portion, and the leftover 70% is the algae
by-product. This by-product can be used for medical chemicals, cosmetics, toiletries, and
fragrance products.
High energy and cost-intensive downstream processes, such as enzymatic hydrolysis
and metabolic pathway extraction, remain primary techno-economic obstacles to the full
commercialization of microalgal biodiesel production [60]. Efroymson et al. [61] suggested
that by reducing the number of phases in the manufacturing and co-production of a
more energetic fraction, the value of algal biofuels could be dramatically lowered. Many
algae specimens are not appropriate for industrial cultures, as the structure of microalgal
lipids in fatty acids may not be ideal when used as biofuel. In challenging situations, the
accumulation of lipids leads to cell development and division being stopped, resulting in a
clear limitation of the productivity of biomass [62]. Genetic manipulation engineering can
deliver innovative routes to lipid and algal biomass production [63]. Through calculation,
it was demonstrated that replacing 1% of US road fuel source with macroalgal biofuel only
involves 0.09% area of the Exclusive Economic Zone (EEZ) [64]. Such a prospect is most
likely to remain on a document before promoting strategies are carried out. Microalgae-
derived jet fuels have also been extensively tested in commercial and military aircraft.
Solazyme Inc. produced the world’s first jet fuel made entirely of algae using the UOP
HEFA process technology and fermentation. The US Navy has tested Solazyme’s jet
fuel [64]. To analyze the environmental effect of an algal-based BAF supply chain in the
Sustainability 2021, 13, 12374 11 of 30

United States, Agusdinata and Laurentis [65] combined LCA and multi-actors (stakeholder
decisions) found that algal biofuels have the potential to reduce the country’s aircraft
industry’s life-cycle CO2 emissions by up to 85% by 2050 [65]. Massive algae processing
also faces technical and logistical challenges, but respondents believe that algae-based
biofuels can play an important role in the advancement.

Emerging Trend of Nanotechnology

Nanotechnology, as a creative and ground-breaking technique, has a broad array
of applications and prominent roles. NPs are reliable, cost-effective, and environmen-
tally sustainable, with high stability, a faster synthesis rate, and a simple procedure. A
nanoparticle is described as a structure with a diameter of 0.1 to 100 nm. As a result of
their extraordinary physicochemical properties, nanoparticles are now being used strategi-
cally in biofuel development. Many nanomaterials with unique properties such as TiO2 ,
Fe3 O4 , SnO2 , ZnO, sulfur, graphene, and fullerene have been used in biofuel processing.
Nanoparticle-aided microalgal harvesting has become the new trend to enhance energy
usage, total microalgal concentration, quality, and process cost [66]. In a large-scale sam-
ple, the use of nanoparticles on microalgae harvesting claimed a 20–30% reduction in
microalgae production cost [67]. The use of nanoparticles in bio-diesel blended fuels has
improved efficiency and lowered emissions. More emphasis should be put on this in
the future. Karthikeyan et al. have concentrated on preparing various biodiesel blends
using CeO2 additives in the hope of long-term applications in single-cylinder compression
ignition engines that would benefit the whole population [68]. In the presence of ion–silica
nanocomposites, algal oils have a high yield of production [69]. The Ames Laboratory
has created a new technique dubbed “nano-farming” that extracts oil from algae using
sponge-like mesoporous nanoparticles. The process does not damage the algae in the same
way that other methods are being produced, lowering production costs and shortening the
production cycle [70]. Table 7 highlights the use of nanoparticles in algal biofuels.

Table 7. Role of various nanoparticles in algal biofuels.

Nanoparticles Functions Ref.

Improved the Chlorella sp. microalgae growth by 18.9% after 4 days of exposure to a
Al2 O3. [71]
concentration of 1000 mg L−1 Al2 O3
Improved total yield in microalgae processing (e.g., cell
TiO2 [67]
suspension, cell division, and cell harvesting)
Increased harvesting productivity by 95% of Scenedesmus
Fe3 O4 [72]
ovalternus and Chlorella vulgaris when grown with iron oxide NP
During scaled-up catalytic transesterification, the conversion
CaO [73]
efficiency of biodiesel was 91%
TiO2 , CeO2 Enhance 10–11% of biogas yield from wastewater treatment [74]

Although nano-additive applications were essential to microalgae growth, harvesting,

biofuel conversion, and biofuel applications to enhance efficiency, there were still certain
obstacles prior to the deployment of nano-additives for the commercialization scale. Nano-
additives in micro-algal biofuels are limited to the laboratory and pilot size, which is a sig-
nificant constraint. Despite its benefits, one of the primary difficulties with NPs is the high
cost of manufacture, which has hampered the commercialization of nanofluids (Figure 9).
 ing, biofuel conversion, and biofuel applications to enhance efficiency, there were still cer-
tain obstacles prior to the deployment of nano-additives for the commercialization scale
Nano-additives in micro-algal biofuels are limited to the laboratory and pilot size, which
Sustainability 2021, 13, 12374 is a significant constraint. Despite its benefits, one of the primary difficulties
12 with
of 30 NPs is
the high cost of manufacture, which has hampered the commercialization of nanofluids
(Figure 9).

9. Application
Figure 9.
Figure Applicationofof
nanoparticles.
nanoparticles.
2.4. Fourth-Generation Biofuel (4G)
The most promising advanced biofuels are those from the fourth generation of biofuels.
The feedstocks of the fourth-generation biofuels are genetically engineered microalgae,
microbes, yeast, and cyanobacteria; these microorganisms are genetically engineered. The
best way to cut the price, nutrients consumption, and ecological footprint is to boost produc-
tivity and lipid accumulation. Ketzer et al. found that from a biological standpoint, a better
energy return on investment (EROI) might be obtained by improving photo-conversion
efficiency, which would result in higher biomass and energy yields. Increasing and al-
tering the buildup or release of energy products (e.g., lipids, alcohol) is currently being
researched [75]. Genome editing strategies are frequently used to improve the efficiency
and lipid composition of algae. Currently, three types of genetic modification tools are
commonly used for genomic editing of microalgae strains: zinc-finger nuclease (ZFN), tran-
scription activator-like effector nucleases (TALEN), and clustered frequently interspaced
palindromic sequences (CRISPR/Cas9) [76]. As a result of the complexity and difficulty
of the experimental design of ZFN and TALEN, the CRISPR-Cas9 method is the most ac-
tively developed in microalga [77,78]. Wang et al. performed precise CRISPR/Cas9-based
genome editing of commercial algal strains such as Nannochloropsis, which accumulates oil
as a source of plant-like fats for biofuel generation under nitrogen shortage [79]. Engineered
ZFNs were utilized by Sizova et al. [80]) and Greiner et al. [81] to target the COP3 and COP4
genes in C. reinhardtii. The effectiveness of the ZFNs was only observed in the tailored
model strain of C. reinhardtii. The most difficult challenge is to generate unique ZFNs
with high specificity and affinity for the target sites [80,81]. Before executing the actual
experiment, ZFNs must be validated using a gene-targeting selection method [80]. Using
genetic and metabolic engineering, it is possible to connect the third and fourth generations
compared to 3G biofuels where the main focus is on the production of biomass of algae
to generate biodiesel. On the other side, the most attractive feature of fourth-generation
biofuel is introducing the incorporation of modified photosynthetic microorganisms [82].
Figure 10 shows the process of biofuel production from genetically modified algae.
 fore executing the actual experiment, ZFNs must be validated using a gene-targeting se-
lection method [80]. Using genetic and metabolic engineering, it is possible to connect the
third and fourth generations compared to 3G biofuels where the main focus is on the pro-
duction of biomass of algae to generate biodiesel. On the other side, the most attractive
feature of fourth-generation biofuel is introducing the incorporation of modified photo-
Sustainability 2021, 13, 12374
synthetic microorganisms [82]. Figure 10 shows the process of biofuel production13from of 30

genetically modified algae.

Figure 10. Fourth-generation biofuel production.

2.4.1. Microorganisms Use in Fourth-Generation Biofuel

In biofuel processing, several microbes, yeast, cyanobacteria, and microalgae are used,
with cyanobacteria and microalgae being the best choices for this reason. The selection
of suitable strains, as all microbial species, cannot be genetically modified due to the
complexity of structure, high nutrient demand, or environmental intolerance. Tables 8–10
provides the lipid yield, risk-mitigation and yield parameters of the GM algae

Green Algae
The Chlamydomonas reinhardtii has been genetically engineered to express many es-
sential biofuel characteristics [83]. However, the production rate of biomass is low. Some
examples of green algal such as Chlorella, Parachlorella, Nannochloropsis, Scenedesmus, Botry-
ococcus, and Neo-chloris are rich in lipid content and hence mostly use for biofuel instead of
having low biomass.

Blue-Green Algae
Cyanobacteria are among the first microorganisms to have lasted for a few billion
years. They are a crucial source of atmospheric oxygen and play a vital role in the daily
lives of ordinary people [84]. There are many possible uses for cyanobacteria, such as
feed sources, agricultural biofertilizers, and wastewater treatment [85]. Compared to other
photoautotrophs, biofuel production from cyanobacteria has a lot of potential as a biofuel
platform, since they do not need fermentable sugar or arable land to grow. They will
have far less competitiveness with farmland capacity to fix carbon dioxide gas. Genetic
tractability, horizontal gene transfer, and competitiveness among genetically modified
cyanobacteria and other microorganisms may impact natural ecosystems [61,86]. Syne-
chocystis sp. PCC 6803, Synechococcus elongatus sp. PCC 7492, Synechococcus sp. PCC 7002,
and Anabaena sp. PCC 7120 all have been used as model organisms for genetic engineering.
The optimal production host, on the other hand, is challenging to forecast [87]. Fourth-
generation processes include pyrolysis (at temperatures ranging from 400 to 600 ◦ C [88]),
gasification, and solar-to-fuel pathways in addition to genetic modification [89].
Sustainability 2021, 13, 12374 14 of 30

Table 8. Genetically engineered microalgae generate lipids for biofuel production.

Species Attainments Reference(s)

Increased lipid content by 1.5 times
Chlamydomonas reinhardtii [83,90]
20% increase in triglyceride content
35% increase in lipid
Phaeodactylum tricornutum [91,92]
1.1 times increase in triglyceride content

According to Snow and Smith [93], genetically engineered microalgae-induced compli-

cations include future environmental challenges such as ecological changes, toxicity, lateral
transference of genes, and competition with native organisms [94]. The disposal of GMOs
is one of the critical issues with their use. Since the intentional or unintentional release of
chromosomal or plasmid DNA at specific concentrations might result in horizontal gene
transfer through transformation, there are stringent rules for its disposal [95].

Table 9. Risk mitigation for GM algae.

Biological Method Physical Method

To assess possible or unknown hazards, controlled field experiment
Inactivation of a gene that regulates sexual behavior
should be conducted.
UV, chemical, and heat deactivation during the harvesting of
Upon escape, terminator gene expressed
genetically modified algae.

To reduce the danger of large-scale GM algae being released into the environment, two
main containment strategies are being considered: first, physically stopping the algae from
escaping into the atmosphere, and second, genetically preventing the algae from replicating
and competing in nature [96]. GE algae outperform native strains in the context of ecological
compatibility and cost-effectiveness, making algal biofuels more viable. There is proof of
their superiority and the absence of significant drawbacks on a lab and prototype size, but
this must be demonstrated commercially, since it is necessary for the genetic stability of
GE algae. Although CRISPR technology eliminates the fear of GMOs, it is not universally
embraced. For example, gene-edited organisms must be subjected to the same onerous
restrictions as traditional GMOs, according to a judgment by the European Union’s Court
of Justice ECJ [97].

Table 10. Parameters and yield of biodiesel, biomethane, bioethanol, and syngas synthesis from various feedstocks of
fourth-generation biofuels.

Biofuels Biodiesel Bioethanol Biomethane Biobutanol Syngas

Feedstock E. coli Synechocystis sp. PCC6803 - E. coli -
Deletion of Aas gene in strain SS3B to produce
strain SS34. Integration of pyruvate
Introduced DmJHAMT decarboxylase from Z.
Deletion of adh, ldh, frd,
(Drosophila melanogaster mobilis and endogenous
fnr and pta and insertion
Juvenile Hormone Acid alcohol dehydrogenase
of bcd-etfAB from C.
O-Methyltransferase) slr1192 under control of
Parameters - acetobutylicum. -
inlet temperature 250 ◦ C with split ratio 1:1; different promoters.
Cultures grown
carrier gas: helium; flow: 5 mL/min; oven Temperature: 37 ◦ C,
semi-aerobically in shake
temperature: initial temperature of 160 ◦ C, shaking at 80 rpm,
flasks at 37 ◦ C for 24 h
hold 3 min; gradient to 255 ◦ C at 5 ◦ C/min; induced by 0.4 mM
hold 3 min; inlet temp: 270 ◦ C, detector isopropylthiogalactoside
temp: 330 ◦ C
Yield 0.56 g/L 5.50 g/L 0.37 g/L
References [98] [99] [100]
Sustainability 2021, 13, 12374 15 of 30
Sustainability 2021, 13, x FOR PEER REVIEW 16 of 32

Industries Involved in Third and Fourth Generation Biofuels

Chemical reaction, Direct
The Scottish Association
combustion, for Marine Science (SAMS) is working on MacroFuels, a
Thermochemical
Horizon 2020 project to develop improved
conversion biofuels from seaweed or macroalgae. The
project’s goal is to make a breakthrough in biofuel production. It also aims to develop
Methane, Bioethanol, Biobutanol, Methane, Bioethanol, Biobutanol,
Generated fuel
technologies for fuels that can be utilized in heavy transportation and aircraft.
Syngas, Biodiesel Biodiesel, Syngas
Craig Ventor’s Synthetic Genomics is currently developing microbes capable of pro-
Biomass and production yield both
ducing fuel directly from Easy
carbonto dioxide
cultivate(CO2 ). By 2050, it is predicted that the fourth
Advantages
generation will be wholly developedfor andfood
playcrops are
a significant role in the high.power industry.
global
No competition
A brief comparison of 3G and 4G biofuels are presented Increase CO11.
in Table 2 absorption capacity

3. Sustainable
Table Assessment
11. Comparisons between of Third-Generation
third Biofuels
and fourth-generation biofuels.
The sustainability concept is multidimensional. It acknowledges that there are inher-
Biofuel Generation Third Generation Fourth Generation
ent relations between economic, social, and environmental well-being as shown in Figure
Biomass used Algae
11. If any one of and microorganism
the dimensions changes, it will haveEngineered
an impactcrops
uponand thesolar fuels
other two di-
mensions. Sustainable biofuel production
Biochemical conversion, should include preserving biodiversity,
Genetically modified algae, sustain-
Processing methodology able water utilization, healthyDirect
Chemical reaction, air quality, soil conservation, social issues
Biochemical (such as storage,
conversion,
combustion, Thermochemical conversion Thermochemical conversion
transportation, health effects, etc.), and most importantly, fair labor practices. On the one
hand, biofuel contributes
Methane, Bioethanol,to the prospects of CO2 reduction,
Biobutanol, Methane,improves air quality,
Bioethanol, and pro-
Biobutanol,
Generated fuel
vides net energy gain. Biodiesel
Syngas, Biodiesel, Syngas
On the other hand,
Easy the continuous production
to cultivate of biofuels
Biomass harms biodiversity,
and production yield both are causes
high.
Advantages soil degradation, and affects food security. Since biodiesel production has risen steadily
No competition for food crops Increase CO2 absorption capacity
globally, food prices for vegetable oils have increased significantly [101]. Studies on ma-
rine algal biofuels have received interest in the last few decades. A potential solution to
3. Sustainable Assessment of Third-Generation Biofuels
energy and environmental problems is a commercially feasible algal cultivation. It is cost-
The sustainability
effective, concept land,
requires no additional is multidimensional.
uses less water, and It acknowledges
reduces atmosphericthat there
CO2are. Thein-
herent
global efficiency, net productivity per hectare, avoided CO2 emissions, net present valuein
relations between economic, social, and environmental well-being as shown
Figure
(NPV), 11.
andIflevelized
any onecost
of the dimensions
of energy (LCOE) changes, it will
are the key haveused
metrics an impact
for theupon the other
sustainability
two dimensions. Sustainable biofuel production should include preserving
evaluation of biofuels. Third-generation biofuels, with higher pollution reductions, aim to biodiversity,
sustainable water utilization,
be more sustainable. healthy
These biofuels areair quality,
focused onsoil conservation,
biomass social
sources that areissues (such
not used foras
storage, transportation,
other primary purposes,health
such aseffects, etc.), and most
food processing importantly,
and cultivation. fair demonstrate
Algae labor practices. On
great
the one hand, biofuel contributes to the prospects of CO reduction, improves
promise as a possible future green energy source because of their environmental friendli-
2 air quality,
and
nessprovides
and highnet energy gain.
oil-yielding ability per given field.

Sustainability

Social Environmental Economic

Employability GHG Emission Energy Sufficiency

reduction and self sufficiency
Land Issues
Soil Quality Balance of payments
Small holder
integration Water use and quality Financing
Food Security Biodiversity Fuel Cost

Figure Sustainability aspects

Figure 11. Sustainability aspects of
of biofuel
biofuelproduction.
production.

On the other hand, the continuous production of biofuels harms biodiversity, causes
soil degradation, and affects food security. Since biodiesel production has risen steadily
globally, food prices for vegetable oils have increased significantly [101]. Studies on
marine algal biofuels have received interest in the last few decades. A potential solution to
energy and environmental problems is a commercially feasible algal cultivation. It is cost-
Sustainability 2021, 13, 12374 16 of 30

effective, requires no additional land, uses less water, and reduces atmospheric CO2 . The
global efficiency, net productivity per hectare, avoided CO2 emissions, net present value
(NPV), and levelized cost of energy (LCOE) are the key metrics used for the sustainability
evaluation of biofuels. Third-generation biofuels, with higher pollution reductions, aim
to be more sustainable. These biofuels are focused on biomass sources that are not used
for other primary purposes, such as food processing and cultivation. Algae demonstrate
Sustainability 2021, 13, x FOR PEER REVIEW 17 of 32
great promise as a possible future green energy source because of their environmental
friendliness and high oil-yielding ability per given field.

3.1.
3.1.Land
Land
The
Theprimary
primarygoalgoalof ofbiofuel
biofuelconservation
conservationisistotoconserve
conserveland.
land. Field
Field use
use can
can be
be ex-
ex-
panded
panded from food to social growth and biofuels as the world population increases.As
from food to social growth and biofuels as the world population increases. Asaa
resourceful
resourcefulevolution,
evolution,third-generation
third-generation(algal)
(algal)biofuels
biofuelsmay
mayavoid
avoidfood
foodcompetition
competitionand and
land
land use. Thefast
use. The fastgrowth
growthrate rateofof algae
algae enables
enables thethe massive
massive cultivation
cultivation in non-arable
in non-arable land-
landmasses, thereby
masses, thereby eliminating
eliminating competition
competition withwith
landland in for
in use usecrop
for crop production.
production. Com-
Compared
pared
to conventional forests, agroecosystems, and other aquatic plants, microscopic algaealgae
to conventional forests, agroecosystems, and other aquatic plants, microscopic have
have
higherhigher
growthgrowth
ratesrates and efficiency.
and efficiency. Unlike
Unlike otherother agricultural
agricultural biodiesel
biodiesel feedstocks,
feedstocks, it re-
itquires
requires
muchmuchlessless surface
surface areaarea [49].
[49]. ForFor example,
example, brown
brown seaweeds
seaweeds produce
produce 13.113.1 kg
kg dry
dry weight m −2 yr−1 compared to 10 kg dry weight m−2 yr−1 from sugarcane [102,103].
weight m−2 yr−1 compared to 10 kg dry weight m−2 yr−1 from sugarcane [102,103]. Figure 12
Figure 12 highlights
highlights the land requirements
the land requirements of microalgae
of microalgae compared compared to other feedstock.
to other feedstock.

Area to produce global oil demand

(hectares × 106)

Rapeseed 5121

Soybean 10932

Sunflower 4097

Oil palm 819

Algae (50% TAG) 2.5

0 2000 4000 6000 8000 10000 12000

Figure12.
Figure 12.Land
Landdemand
demandof
ofmicroalgae
microalgaeoil
oilcompared
comparedtotodifferent
differentbiomass
biomass[104].
[104].

Inaddition,
In addition,due
duetotothe
thelimited
limiteddependency
dependencyon onfarmland
farmland(compared
(comparedtotocrop-based
crop-based
biofuels), algae
biofuels), algae lead to less
less habitat
habitat destruction.
destruction.Consequently,
Consequently,bybyusing
using microalgae
microalgae as as
bi-
biodiesel feedstock,competition
odiesel feedstock, competitionforfor agricultural
agricultural land, particularly for human
human consumption,
consumption,
isissignificantly
significantlyreduced
reduced[105].
[105].Oil
Oilyields
yieldsfrom
frommicroalgae
microalgaecancansurpass
surpassthose
thosefrom
fromoil
oilplants
plants
such
suchas asrapeseed,
rapeseed,palm,
palm,ororsunflower
sunflowerper perhectare
hectareasasshown
shownininFigure
Figure13.
13.
Sustainability
Sustainability2021,
2021,13,
13,x12374
FOR PEER REVIEW 1817
ofof3230

Figure 13. Comparison of microalgae oil yield (L oil/ha/yr) with other biodiesel feedstock [105].
Figure 13. Comparison of microalgae oil yield (L oil/ha/yr) with other biodiesel feedstock [105].
In Malaysia, coastal areas and underutilized rice land are promising sites for massive
In Malaysia,
microalgae coastal[106].
cultivation areas Dueand underutilized rice land arethese
to saltwater penetration, promising
lands aresitesunproductive
for massive
microalgae cultivation [106]. Due to saltwater penetration, these
and therefore can be used to produce marine microalgae appropriate for saltwater [107,108]. lands are unproductive
and
The therefore
‘Submariner’ can research
be used team to produce
has explored marine themicroalgae
possibilitiesappropriate
of connecting forboth
saltwater
macro-
[107,108].
and micro-algaeThe ‘Submariner’
development research
facilitiesteam has an
to use explored the possibilities
operational offshore wind of connecting
farm in the
both
Baltic macro-and
Sea to reducemicro-algae
the burden development
on land facilities
availabilityto use
[109].an operational offshore that
The DOE estimates windif
farm in the Baltic Sea to reduce the burden on land availability
algae fuel replaced all the petroleum fuel in the United States, it would require only [109]. The DOE estimates
that
15,000if algae
squarefuelmiles,
replacedwhich all the
is apetroleum
few thousand fuel in the United
square miles States,
larger itthan
would require only
Maryland. This
15,000 square miles, which is a few thousand square miles
is less than one-seventh of the area devoted to corn production in the United States larger than Maryland. This isin
less
2000 than
[70].one-seventh
Wigmosta et of al.
the[110]
areaexamined
devoted tothe corn
land,production
water, and inresource
the United States in in
availability 2000
the
[70].
United Wigmosta
States and et al. [110] examined
determined that about the 43,107
land, water,
hectares andof resource
land wereavailability
suitable forinalgaethe
United
cultureStatesin open and determined
ponds. that about to
This corresponds 43,107 hectares
a possible of land
yearly output were suitable
of 2.20 1011forL ofalgae
algal
culture in open ponds. This corresponds to a possible
oil, which is equivalent to 48% of the United States’ annual petroleum imports.yearly output of 2.20 1011 L of algal
oil, which is equivalent to 48% of the United States’ annual petroleum imports.
3.2. Water
3.2. WaterWater use concern is the main drawback associated with first-generation and second-
generation
Water use biofuels.
concernWater is theismain
a limited
drawback resource, and awith
associated lack first-generation
of it can severely andaffect well-
second-
being. In addition, current water problems are predicted to be intensified
generation biofuels. Water is a limited resource, and a lack of it can severely affect well- by climate change.
As a result
being. of the lack
In addition, of freshwater
current water problems sources worldwide
are predicted andtothebeinefficient
intensified usage of fresh-
by climate
water aquifers,
change. As a resultonly ofbrackish
the lack water or seawater
of freshwater sourcescan worldwide
be considered andinthe
broader application.
inefficient usage
ofThe water footprint
freshwater aquifers,ofonly a biofuel
brackish refers to the
water total volume
or seawater can beofconsidered
surface water neededap-
in broader for
its production. Three types of green, blue, and gray algae
plication. The water footprint of a biofuel refers to the total volume of surface water are commonly considered for
the water
needed for footprint of biofuel
its production. Three production
types of green,[111].blue,
Footprints
and gray in green
algae and blue watercon-
are commonly refer
sidered for the water footprint of biofuel production [111]. Footprints in green and blueto
to evaporation during the period of processing. The footprint of graywater applies
the water
water referultimately
to evaporationreleased duringas waste. The water
the period footprint of
of processing. The microalgae
footprint and terrestrial
of graywater
plants was examined by Zhang et al. [112]. It was found that
applies to the water ultimately released as waste. The water footprint of microalgae and the green water footprint for
microalgae
terrestrial processing
plants was aboutby
was examined one-quarter
Zhang et al. of the average
[112]. It wasgreen
found water
thatfootprint
the greenfor three
water
plant species. Microalgae biodiesel has a WF of about
footprint for microalgae processing was about one-quarter of the average green water 3726 kg water/kg biodiesel. Still,
it is possible to recycle about 84% of this water, taking the
footprint for three plant species. Microalgae biodiesel has a WF of about 3726 kg water/kgWF down to 591 kg water/kg
biodieselStill,
biodiesel. [113]. Bypossible
it is using nutrients
to recycleinabout wastewater
84% of thisandwater,
seawater, using
taking the algae
WF down reduces the
to 591
Sustainability 2021, 13, 12374 18 of 30

need for fresh water. Furthermore, by recycling and reusing the discharged water from the
harvest process, up to 90.2% of the usage of topically discharged water can be restored to the
manufacturing process [114]. Table 12 highlights the water footprint of biofuel feedstocks.

Table 12. A comparison of the blue–green water footprints of microalgae biofuel and other feedstocks.

Type of Water Water

Biofuel’s Feedstock Biofuel’s Feedstock
Footprint Footprint
Sugar cane (Bioethanol) Blue + Green 139 [115]
Rapeseed (Biodiesel) Blue + Green 165 [116]
Microalgae (Biodiesel) Blue + Green 14–87 [117]

3.3. Energy
The net energy ratio is the ratio of the energy of algal biofuel to the energy invested in
algal production. Micro-algae have an energy content of 5–8 kWh/kg (18,000–28,800 kJ/kg)
of dry weight [118]. The development of algal biodiesel could be feasible if the energy
needed to generate the microscopic algae and the energy necessary to turn the microscopic
algae into operational fuel is lower than that sum. As a result, the Net Energy Ratio can be
written as
E Energy in Algal Bio f uel
ENER = Out = .
E In Energy Invested
Microalgae are solar-powered cell factories that turn carbon dioxide into potential bio-
fuels [119]. Microalgae are a quickly evolving photosynthetic species capable of converting
9–10% of solar energy (average sunlight irradiance) into biomass, with a potential yield of
around 77 g/biomass/m/day, which is about 280 tons per hectare per year [120,121]. In a
highly efficient seaweed processing method, prices for energy return on investment from
seaweed (0.44 to 1.37) for fermentation and ethanol distillation could be equivalent to corn
(1.07) [122].

3.4. Socio-Economic Aspects

Jobs and profits, food security, economic progress, sustainable energy, economic via-
bility, health and security, public acceptance, and equality of opportunity were defined as
socio-economic measures. Both jobs and local revenue are vital factors of development in
algal energy generation. To meet the challenge of long-term viability, the development of
safer and more sustainable biofuels must be planned with third-generation biofuels. The
growing of macroalgae is relatively easy; the crop can be harvested in around 6 weeks with
modest initial capital expenditure. These features provide women with a significant oppor-
tunity to generate capital for themselves and their families [123]. About 116,000 households,
representing over one million people, planted over 58,000 hectares of seaweed in the Philip-
pines. It is worth noting that more than half of the seaweed-harvesting population is
unskilled or semi-skilled [124,125]. Wild seaweed harvesting is a vital part of the history
and practice of many nations. Women form a large proportion of the harvester workers in
Brazil (assumed to be about 80%), and nori (Pyropia spp.) collection is typically performed
by women in Japan.
Similarly, the bulk of seaweed harvesters in South Africa are women [126] whose
annual average income was quoted as US $5000; therefore, microalgae biofuel production,
as a long-term sustainable sector, can also create opportunities for job development at
all skill levels, close to traditional biofuels [127], and it also can be a safer choice when
paired with existing complementary industries. The correlation regarding waste effluent
bio-fixation and the production of usable co-products (e.g., feed, fertilizer) [128] may be
economically advantageous to native communities in parallel, supplementing seasonal
industry profits. Algae (seaweeds and microalgae) is recognized by the European Commis-
sion (2012, 2016) as such a good food safety choice that by 2054, the combined cultivation
of algae could achieve 56 million metric tons of protein production, accounting for 18%
Sustainability 2021, 13, 12374 19 of 30

of the worldwide alternative protein industry [129,130]. Many algal biofuel companies
have pilot plant job figures that can be registered. Wholesalers of algal biofuels products
and technology, such as nutrients, CO2 , polyethylene liners, PBRs, pumps, and workers
from plants that have mutual storage services (e.g., CO2 , nutrients) to biofuel facilities,
are examples of indirect jobs [131]. Gallagher claims that [132] the economic viability of
microalgae biofuel development seems reasonable and relies on government support and
potential oil price. The list of sustainable indicators are presented in Table 13.

Table 13. List of sustainable development metrics for bioenergy with a focus on terrestrial feedstocks [133].

Category Indicator Sustainable System Design Goals

Occupation High wages and more job opportunities
Social welfare Wealth of households High-paying jobs and reducing fuel prices, raising household income
Security of foods Algal biofuel using non-arable land and opportunities for food co-products
Maximize the advantages of substituting algal biofuel for fossil fuel for oil
Premium for energy security
Energy security dollars.
Volatility in the price of petrol
Reduce uncertainty of fuel prices.
Terms of commerce Build situations such that fewer capital leaves a government agency to buy crude.
External trade
Volume of commerce Minimizing net fuel imports
Investment return (ROI) Build a constructive ROI
Profitability
Net present value (NPV) Build a positive NPV
Non-renewable energy resources depletion
Resource Reduce the dependency on fossil fuels
Fossil fuel energy return on investment
conservation Raise fossil fuel EROI above 1 and finally above 3
(fossil EROI)
Public opinion
Show a promising estimation of a high percentage
Social Transparency
Display a steadily rising or high value
acceptability Efficient engagement from stakeholders
Maintain catastrophe level at current occurrence or based on comparable technologies
Risk of catastrophe

3.5. Environmental Aspects

The sustainable development focuses on eliminating GHG pollution from the atmo-
sphere, reducing human health by using renewable energy sources such as biofuels and
extracting toxic pollutants from the environment. Water contamination and freshwater
retention shortages, combined with global warming, are now overwhelming global fears
and threatening biodiversity. The third-generation biofuels offer valuable insight into the
clean energy approach and long-term sustainability. Algae is one of the most efficient
biological mechanisms for transforming sunlight into energy to absorb and convert carbon
dioxide to biomass. Around half of the global carbon fixation is carried out by algae [134].
Ecological science has shown that the combination of the production of macroalgae with
e.g., shrimp [103] and salmon [104], will remedy coastal eutrophication. Algal-based
wastewater treatment offers an efficient and cost-effective tool to eliminate organic and
inorganic contaminant wastes from wastewater. The wastewater from these sources can be
divided into organic and inorganic compounds. The critical portion of organic waste is
carbon-containing biodegradable substances. In contrast, inorganic waste includes nitrates,
phosphates, and heavy metals [135]; hence, it provides a reliable and cost-effective tool
to extract wastewater from organic and inorganic toxins. The author of [122] has recently
shown how the rare metal indium could be extracted from e-waste (old electronic materials)
that is frequently transported to developing nations, where workers (many of whom are
minors) burn circuit boards to remove valuable and rare metals. This has grave health and
pollution consequences. The brown seaweed Ascophyllum nodosum biomass was discovered
to aid in the ‘extraction’ of the metal indium, which is essential as an input to modern elec-
tronics, particularly screens. Biofuels derived from microscopic algae are one of the most
promising green energy options not just because of their reduced greenhouse gas emissions
but also because of their CO2 sequestration [136]. The CO2 sequestration in microalgae
is 10–50 times more than that of many terrestrial plants, while a higher concentration of
CO2 results in a higher yield of lipids. In contrast, fourth-generation biofuel is pushed to
 one of the most promising green energy options not just because of their reduced green-
house gas emissions but also because of their CO2 sequestration [136]. The CO2 sequestra-
tion in microalgae is 10–50 times more than that of many terrestrial plants, while a higher
concentration of CO2 results in a higher yield of lipids. In contrast, fourth-generation bio-
Sustainability 2021, 13, 12374 fuel is pushed to be carbon-negative with significant environmental benefits. Table 14
20 of 30
highlights the GHG reduction of genetically engineered microalgae

Table 14. Fixation of greenhouse gases by genetically engineered microalgae.

be carbon-negative with significant environmental benefits. Table 14 highlights the GHG
Species reduction of genetically engineeredMethod microalgae. Attainment References
Engineered small PSII antenna size Chlamydomonas
Fixation ofby
Table 14.reinhardtii greenhouse gasesaby genetically engineered Photosynthe-
microalgae.
transforming permanently active variant
sis efficiency
Chlamydomonas
Species reinhardtii NAB1* (mutagenized MethodNAB1) of the LHC translation re- Attainment [137]
has increased References
pressor
Engineered NAB1
small to decrease
PSII antenna antenna size
size Chlamydomonas by translation
reinhardtii by re-
by 50%.
transforming a permanentlypression.
active variant NAB1* Photosynthesis efficiency
Chlamydomonas reinhardtii [137]
(mutagenized NAB1) of the LHC translation repressor NAB1 to has increased by 50%.
Genetically modified cyanobacteria generate
decrease antenna size by translation repression.
and secrete
Synechococcus elongatus carbonic anhydrase (Cas) in the medium. The secreted CAs Carbon intake
Genetically modified cyanobacteria generate and secrete carbonic
Synechococcus
(Strain of elongatus
freshwater converted
anhydrase (Cas) indissolved
the medium.CO 2 to
The HCOCAs
secreted 3. The cyanobacteriaCarbon
converted has increased
intake has [138]
(Strain of freshwater [138]
cyanobacteria). dissolved CO to HCO . The cyanobacteria absorbed
absorbed HCO3 and converted it into biomass through
2 3 HCO 3 and increased by 41%.
by 41%.
cyanobacteria).
converted it into biomass through photosynthesis.
photosynthesis.

3.6.
3.6. Current
Currentand
andFuture
FutureProspects
ProspectsofofBiofuels
Biofuels
In
In2019,
2019,global
globalproduction
productionofofbiofuels
biofuelsincreased
increased5%,
5%,which
whichwas
wasled
ledmainly
mainlyby
byaa13%
13%
biodiesel
biodiesel expansion (with Indonesia overtaking the US and Brazil to develop into the the
expansion (with Indonesia overtaking the US and Brazil to develop into sig-
significant nationalproducer(Figure
nificant national producer (Figure14).
14).Meanwhile,
Meanwhile,bioethanol
bioethanol production
production increased
increased by
by
2%.
2%. In
In 2019,
2019, global
globalbiofuel
biofueljobs
jobswere
wereprojected
projectedat
at2.5
2.5million
million[139].
[139].

2020 2019

1.00
JOBS (millions)

0.80
0.60
0.40
0.20
0.00

Figure 14. Liquid biofuel employment in top 10 countries [139,140].

According to IEA, 3% annual production growth is projected for the next five years, but
the decline in oil prices in 2020 (USD 30 per barrel) due to lower global demand stemming
primarily from the COVID-19 pandemic decreased demand for biofuel crops [141]. Liquid
biofuels are believed to be one of the most cost-competitive suppliers of high efficiency and
a potential substitute for marine and aircraft fuels. The biofuel market was heavily affected
by the COVID-19 pandemic in 2020. Global biofuel transport production is expected to be
144 billion liters in 2020, which is equivalent to 2,480,000 barrels per day (kb/d): an 11.6%
decrease from peak production in 2019 and the first decline in annual production in several
decades [139]. Figure 15 depicts the increase in yearly biofuel demand in various countries
to meet the 2030 sustainable development scenario. Biofuel production in the United States
and EU member states will fall short of SDS demand in 2030. While biofuel production
in Brazil and India is estimated to rise, the SDS volume for 2030 must involve even faster
growth. China and ASEAN countries are also experiencing production growth, which, if
maintained, would meet the SDS’s 2030 biofuel volume requirements [142].
 production in several decades [139]. Figure 15 depicts the increase in yearly biofuel de-
mand in various countries to meet the 2030 sustainable development scenario. Biofuel
production in the United States and EU member states will fall short of SDS demand in
2030. While biofuel production in Brazil and India is estimated to rise, the SDS volume for
Sustainability 2021, 13, 12374 2030 must involve even faster growth. China and ASEAN countries are also experiencing 21 of 30
production growth, which, if maintained, would meet the SDS’s 2030 biofuel volume re-
quirements [142].

25.0%
22%
20.0%
19%
15.0% 15.3%
13.3%
11.8%
10.0% 10%
9%
7%
5.0% 5%
1.9% 1.7%
0.0% 0.5%
United European Brazil India China ASEAN
States Union

Forecast annual production growth (2019-25)

Annual production growth needed to meet SDS (2019-30)

Figure 15.
Figure 15. Biofuels annual production
production growth
growth to
tomeet
meetsustainable
sustainabledevelopment
developmentscenario,
scenario,2030
2030[142].
[142].

4.
4. Transition
Transition to
to aa Circular
Circular Economy,
Economy, Green
Green Economy,
Economy, and Bioeconomy
and Bioeconomy
4.1. Circular Economy
4.1. Circular Economy
The
The fundamental
fundamentalprinciple
principlethat
thatconnects
connectsthe
theideas
ideasofofcircular
circulareconomy,
economy, green economy,
green econ-
and bioeconomy is balancing economic, environmental, and social objectives.
omy, and bioeconomy is balancing economic, environmental, and social objectives. The circular
The
economy is a possible
circular economy solution solution
is a possible to the optimal
to the utilization of investments
optimal utilization and ensures
of investments and their
en-
long-term use. The processing units must demonstrate economic feasibility
sures their long-term use. The processing units must demonstrate economic feasibility while mini-
mizing waste and environmental
while minimizing effects to achieve
waste and environmental effectsatofully integrated
achieve a fullycircular bioeconomy.
integrated circular
The new green deal from the European Commission focuses on priority
bioeconomy. The new green deal from the European Commission focuses on priority areas where algae
ar-
production may make
eas where algae a significant
production contribution:
may make for example,
a significant the goals
contribution: of the EU the
for example, becoming
goals
climate neutral
of the EU by 2050,
becoming theneutral
climate protection of biodiversity
by 2050, the protection[143], and the development
of biodiversity of a
[143], and the
circular economy [144]. The circular economy is based on three basic principles:
development of a circular economy [144]. The circular economy is based on three basic
a. No waste, since products are renewable and biodegradable.
principles:
b. Consumed resources
a. No waste, since are recovered
products without
are renewable posing
and any security threats to the ecosystem.
biodegradable.
c.b. Consumed
Energy forresources
all processes is provided from renewable
are recovered without posing any and sustainable
security sources.
threats to the eco-
Vitamins,
system. proteins, amino acids, polysaccharides, fatty acids, sterols, pigments, fibers,
and enzymes
c. Energy for with
all unique properties
processes can be
is provided synthesized
from renewablefrom
and microalgae. In a microalgae-
sustainable sources.
based circular bioeconomy, production wastes are recycled and reintroduced as secondary
raw materials, i.e., to convert waste materials into new products in microalgae-based pro-
duction systems (microalgae biorefineries). Microalgae are helpful in a circular economy
as they can be used for the bio-remediation of nutrient waste and provide biomass for
various commercial uses. Microalgal farming on nonarable land or coastal ecosystems
reduces water demands, recycles nutrients, and converts atmospheric CO2 into nutrient-
rich sustainable feedstocks. This lays the groundwork for a circular aquaculture-based
industry as part of a larger circular bioeconomy [145] contributing to several UN Sustain-
able Development Goals. The circular bioeconomy principle is currently gaining attention
as a critical component of green technology. Recently, combined activated sludge (AS)
microalgae wastewater treatment systems have been suggested as a more energy and com-
mercially efficient alternative to traditional solutions for removing carbon and nutrients
from liquid streams.
Furthermore, microalgae cultivation in wastewater leads to faster nitrogen and phos-
phorus removal, with up to 1 kg of dry biomass generated per m3 of wastewater [146]. The
EU goals for creating circular economy from waste sources [147] align with current urban
water management paradigms [148]. Bioplastics are critical in transitioning the plastics
sector from a wasteful linear economy to a circular economy. Algae-based bioplastic is
considered to be a long-term solution for ensuring the circular economy practice. Bioplastic
could produce natural materials via composting as the end of life cycle management [149].
 Sustainability 2021, 13, 12374 22 of 30

Microalgae are a viable alternative source for making bioplastics. Several recent stud-
ies have looked at the production of bioplastics from microalgae biomass. According to
Karan et al. (2019), the average requirement for microalgae cultivation to meet global
plastic manufacturing is about 145 000 km2 , which is only 0.028% of the Earth’s surface area
of 510,000,000 km2 [150]. Polysaccharides agar, carrageenan, and alginate are used to make
bioplastics from seaweeds, and seaweed waste from agar extraction has been suggested as
a material filler [151].The methods of producing PHA from genetically engineering algae is
presented in Table 15.

Table 15. Methods of producing PHAs from genetically engineered algae.

Polymer
Algae Type of Product Culture Mode Reference
(Percentage of Dry Cell Weight)
Production of P(3HB) using
Spirulina plantesis PHB 10 [152]
CO2 /acetate as a carbon source.
P(3HB) production under
phosphate-starved medium + 1%
Nostoc muscorum PHB 21.5 [153]
(w/w) glucose + 1% (w/w) acetate
with aeration and CO2 addition.

A circular economy-based business model for obtaining several products from mi-
croalgae biomass for agricultural, nutrition, cosmetics, and aquaculture use is proposed in
a study [154]. AlgaePro is developing technologies for growing microalgae in a circular
economy approach, using biodegradables from urban waste, CO2 , and waste heat from
industrial sites [155]. Researchers in Italy and Slovenia are cultivating microalgae that
absorb nutrients from agricultural wastewater as part of a European initiative called Salt-
gae. Once the water has been cleaned, the algae are dried and sold in cosmetics, animal
feed, and fertilizers. Aquaculture of algae on industrial sites would enable a24circular
Sustainability 2021, 13, x FOR PEER REVIEW of 32
economy, turning wastewater into a viable resource [156].A circular based economy using
Micro/Macroalgae is presented in Figure 16.

Figure 16. Systematic diagram of a circular economy.

Figure 16. Systematic diagram of a circular economy.
4.2. Green Economy
4.2. Green
TheEconomy
green economy is a viable alternative to today’s economic framework, which ag-
gravates inequality,
The green economy stimulates pollution,
is a viable induces
alternative resource
to today’s scarcity,
economic and poseswhich
framework, numerous
ag-
environmental health risks. According to the UNEP [157] (a green economy is
gravates inequality, stimulates pollution, induces resource scarcity, and poses numerous“low-carbon,
resource-effective
environmental andrisks.
health socially equitable”,
According with
to the the ultimate
UNEP [157] (, agoal of reducing
green economyenvironmental
is “low-car-
bon, resource-effective and socially equitable”, with the ultimate goal of reducing envi-
ronmental impact and biodiversity loss as well as improving human well-being and social
justice. The numerous benefits associated with algal energy, such as eco-friendliness and
high productivity, lead to a green economy and sustainable growth by improving human
Sustainability 2021, 13, 12374 23 of 30

impact and biodiversity loss as well as improving human well-being and social justice. The
numerous benefits associated with algal energy, such as eco-friendliness and high productivity,
Sustainability 2021, 13, x FOR PEER REVIEW
lead to a green economy and sustainable growth by improving human health and quality 25 of of
32
life [157].The benefits of green economy using algae is presented in Figure 17.

Figure17.
Figure 17. Algal-based
Algal-based circular
circulareconomy,
economy,green
greeneconomy,
economy,and
andbioeconomy.
bioeconomy.

4.3. Bioeconomy
5. Conclusions
Rapid
Many urbanization,
developed and improved
developingquality of life,
nations areand longer
steadily lifespans biofuel
supporting place demands
production on
all manufacturing sectors producing food, chemicals, and fuels. As a
due to its potential benefits. This study examined the prospects of third-generation andresult of the increased
strain, land usage, drinkable
fourth-generation biofuels in water, fossil fuels,
the context and other sustainability.
of long-term natural resources Theare anticipated
following are
to increase, resulting in unexpected
some of the key conclusions from the study. climate change, biodiversity loss, and a decline in the
capacity to manage ecosystems sustainably. The bioeconomy may offer a potential solution
• Greenhouse gas emissions, environmental impact, loss of habitat, community con-
to this rising demand by substituting biomass-based commodities for depletable resources,
flicts, and substantial production costs are all associated with first-generation and
reducing environmental impact. A bioeconomy is defined as “the development of long-
second-generation biofuel. The use of edible biomass in first-generation biofuels has
term biological resources and the conversion of waste biological resources into value-added
been of significant concern. It competes with the world’s food requirements that limit
products such as food, feed, bio-based products, and bioenergy” [158]. The European Union
its production
introduced a plan fortoimproving
a few countries. The otherin
the bioeconomy limitation
2014, whichincludes the high
was based investment
on microalgae.
costs and
Microalgae canpoor efficiencies
significantly of feedstock
contribute conversion
to the economy,toproviding
biofuel. Second-generation
required biomass bio- for
human applications such as new drugs, cosmetics, food, and feed. The plan alsohave
fuel has production limitations. Both the first and second generations their
included
optionsstrengths and weaknesses
for wastewater treatment inand
terms of environmental
atmospheric and social
CO2 mitigation. impact. Hence,
Increasing both
the market
generations will shortly be unable to meet the growing biofuel
development for microalgae-based products as long-term substitutes for currently available demand and energy
optionstransition
will be targets.
critical to the success of a microalgae-based bioeconomy. The industrial
•
units Developing third-generation
of the bioeconomy and fourth-generation
are biorefineries. The enormous potential biofuelsofhas
tinybroad implications
microalgae favors
on global socio-economic growth and sustainable development
a microalgae-based biorefinery and bioeconomy, generating huge opportunities in the goals. It contributes
globaltoalgae
carbon balance,
industry. biodiversity
Seaweeds conservation,
can also be used assustainable
feedstock inwater utilization,
biorefineries healthy
to produce
air quality, soil conservation, and sustainable social enterprise.
fuels, pesticides, food additives, medicines, and other products, making them an essential
• of
part A the
large number
future of companies
bioeconomy [159].are investing heavily in biofuels to accelerate the global
energy transition. Creating and applying sustainable biofuels standards will be more
5. Conclusions
critical, with more entrepreneurs or companies committed to thinking that benefits
will ultimately
Many developed outweigh the risks.
and developing nations are steadily supporting biofuel production
•
due to its potential benefits. This study to
Nanotechnology has the potential make next-generation
examined the prospects biofuels feasible. The
of third-generation ef-
and
ficacy of biofuel can be significantly enhanced by incorporating nanomaterials into
the process development. Magnetic nanoparticles, carbon nanotubes, metal oxide na-
noparticles, and other Nano catalysts have the potential to become an essential part
of long-term bioenergy production. However, most of the performance data are
based on small-scale biofuel generation. Further research is needed to study the effi-
cacy of nanotechnology in pilot-scale biofuel production.
Sustainability 2021, 13, 12374 24 of 30

fourth-generation biofuels in the context of long-term sustainability. The following are

some of the key conclusions from the study.
• Greenhouse gas emissions, environmental impact, loss of habitat, community conflicts,
and substantial production costs are all associated with first-generation and second-
generation biofuel. The use of edible biomass in first-generation biofuels has been
of significant concern. It competes with the world’s food requirements that limit its
production to a few countries. The other limitation includes the high investment costs
and poor efficiencies of feedstock conversion to biofuel. Second-generation biofuel has
production limitations. Both the first and second generations have their strengths and
weaknesses in terms of environmental and social impact. Hence, both generations will
shortly be unable to meet the growing biofuel demand and energy transition targets.
• Developing third-generation and fourth-generation biofuels has broad implications
on global socio-economic growth and sustainable development goals. It contributes to
carbon balance, biodiversity conservation, sustainable water utilization, healthy air
quality, soil conservation, and sustainable social enterprise.
• A large number of companies are investing heavily in biofuels to accelerate the global
energy transition. Creating and applying sustainable biofuels standards will be more
critical, with more entrepreneurs or companies committed to thinking that benefits
will ultimately outweigh the risks.
• Nanotechnology has the potential to make next-generation biofuels feasible. The
efficacy of biofuel can be significantly enhanced by incorporating nanomaterials into
the process development. Magnetic nanoparticles, carbon nanotubes, metal oxide
nanoparticles, and other Nano catalysts have the potential to become an essential part
of long-term bioenergy production. However, most of the performance data are based
on small-scale biofuel generation. Further research is needed to study the efficacy of
nanotechnology in pilot-scale biofuel production.
• Biofuels from macroalgae and microalgae contribute to a circular economy by gen-
erating natural bio-products, such as proteins, pigments, fatty acids, and bioplastics.
Therefore, algae-based green and bioeconomy opportunities include a new supply
chain in manufacturing, cleaner fuel, food security, and GHG mitigation benefits.
• With the advancement of technology, extensive research, and development, it is
reasonable to assume that third-generation and fourth-generation biofuel will become
more appealing for commercial usage globally.
• If specific considerations related to sustainability requirements are met, third-generation
and fourth-generations biofuels can be used realistically as a transitional approach.
In the future, nature-inspired solutions hold more excellent prospects as sustainable
energy sources for the planet.

Author Contributions: Conceptualization, K.S. and R.M.; investigation, K.S.; resources, R.M.; data
curation, N.K.; writing—original draft preparation, N.K.; writing—review and editing, K.S.; visu-
alization, N.K.; supervision, K.S., R.M.; project administration, K.S.; funding acquisition, K.S. All
authors have read and agreed to the published version of the manuscript.
Funding: The authors are grateful for the financial assistance granted by the Universiti Malaysia
Pahang (www.ump.edu.my accessed on 25 October 2021) under the Postgraduate Research Grants
Scheme (PGRS) PGRS210348.
Conflicts of Interest: The authors declare no conflict of interest.
Sustainability 2021, 13, 12374 25 of 30

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