OPTIMAL COST OF PRODUCTION OF BIOETHANOL: A REVIEW

1
Obiora Chiemerie Martins

Department of Agricultural and Bioresources Engineering, Faculty of Engineering, University of Nigeria,

Nsukka, Nigeria.

Abstract

With the environmental impact of burning fossil fuels, biofuels have gained a lot of attention with
bioethanol having a positive significant effect on the environment and ensuring a promising future.
Bioethanol cost of production have roughly been estimated to 0.5usd/liter for first generation
bioethanol and 1usd/liter for lignocellulosic bioethanol with USA and Brazil lead in global production
chat. These prices must have doubled with the the current economic inflation the world is facing.
Optimal production of bioethanol from either generations is the current focus while bioethanol from
genetic modification of enzymes and algae is been explored. The ethanol yield from sugar and starch
feedstocks (first generation) is lucrative and easier than Lignocellulosic biomass but the first generation
competes with food availability which leads to hunger thereby creating more problems. Processes that
are cost effective and Environmentally friendly in the production of bioethanol are been discussed.

Keywords: Bioethanol, cost of production, optimal, first generation, lignocellulosic, ethanol yield.

1
Corresponding author's email address: chiemerie.obiora.241539@unn.edu.ng

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1. Introduction

Cost of production has been one of the major limitations faced in the development of recent
technologies. The optimization of production cost ranging from consumable supply to labour used in
production of these technologies are usually revisited after the desired outcome has been developed.
Fossil fuels have been the primary source of energy but are none renewable and are faced the
challenges of emitting greenhouse gases which deplet the ozone layer. For these reasons renewable
fuels (like biodiesel and bioethanol) that are environmentally friendly are been developed. Bioethanol is
one of the most important biofuels and also a potential substitute for the conventional gasoline and can
be used directly in vehicles (E100) or blended with the gasoline (E10, E15, E85), thereby reducing
greenhouse gas emissions [1]. Bioethanol is a liquid oxygenated fuel containing 35% oxygen produced
from the microbial fermentation of monomeric sugar obtained from carbohydrate sources such as corn,
soybeans and sugar cane (first generation) and other plant materials [2].

About 40% of the global bioethanol production is from sugar cane and sugar beet and nearly 60% is
from starch-containing feedstocks [3]. Ethanol can be made from a variety of plant-based feedstocks
which are used to classify bioethanol production into generations. Bioethanol production can be
classified into first generation, second generation, and third generation, which are produced from
starch- and sugar-based feed stocks, cellulose biomass, and algal biomass, respectively [4]. In Europe,
the most convenient renewable raw materials for bioethanol production are grains (mostly wheat) and
sugar beet [5]. In France they also made bioethanol from wine surplus [6]. The prices of raw materials
have a considerable impact on the bioethanol production costs and they can represent 40–75% of the
total costs depending on the type of feedstock [7]. The costs in China (wheat, sweet sorghum or cassava)
are 0.28–0.46 USD/L depending on the feedstock costs [8]. The cost of bioethanol production from the
sugar-containing raw materials is around 0.44 USD/L in India, while from lignocellulose-containing raw
materials it is 0.80–1.20 USD/L depending on the type of feedstock [9]. The costs of bioethanol
production from sugar cane in Brazil are in the range of 0.20–0.30 USD/L [10]. In the USA and European
Union bioethanol produced from sugar beet and corn reached the lowest production costs of 0.30 and
0.53 USD/L [8].

Bioethanol blendend with gasoline is widely used due to its reduced cost whilst reducing the
emotion of greenhouse gases. The blending of bioethanol with gasoline might not require modifying the
engine, rather it will help to enhance ignition or engine performance. The most commonly used blends
are E85 and E10 [2]. Bioethanol requires higher heat of vaporization than gasoline making it more
difficult to ignite the engine at low temperature or cold weather. In attempt to reduce the cost of
production, lignocellulosic biomass (second generation) is being considered as feedstocks because of
availability and low cost of acquisition but the processing cost is still high [11].

The chart below shows global ethanol production by country or region, from 2007 to 2020. Overall,
global production continues to increase, but production fell worldwide in 2020 due to the COVID-19
pandemic. The United States is the world's largest producer of ethanol, having produced over 13.9
billion gallons in 2020. Together, the United States and Brazil produce 84% of the world's ethanol. The
vast majority of U.S. ethanol is produced from corn, while Brazil primarily uses sugarcane [12].

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Fig.1 Global Bioethanol production by country or region

Fig.2 Annual World Fuel Ethanol Production (Mil.

Gal.)

Source: Renewable Fuels Association [12].

2. 0ptimised approach for bioethanol production process

The Production process involves pretreatment of feedstock, Hydrolysis, Fermentation and

Distillation and dehydration [13]. The handling of biomass feedstock is the preliminary step to prepare
the feedstock, involving cleaning and preliminary cutting to reduce the physical size beforehand the

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pretreatment process [14]. During feedstock handling, storage of biomass feedstock is equally essential
to maintain the quality of the supply for bioethanol production [13]. Pretreatment help increase the rate
of reaction and Ethanol yield from biomass thereby increasing productivity. However, some
pretreatment process are very expensive especially when using lignocellulosic biomass as feedstock.

2.1 Pretreatment of feedstock

Pretreatment is essential in processing of biomass into bioethanol because it provides a larger

surface area for chemical reaction and maximize the productivity of the succeeding processes. As said
earlier, pretreatment is one of the costliest steps in the production of bioethanol from lignocellulose
biomass accounting to approximately $0.30/gallon of ethanol produce [2]. There are various types of
pretreatment process of which some are less expensive than the other.

2.1.1 Physical pretreatment

Physical pretreatment operates on biomass by raising accessible areas and the volume of the pores,
lowering cellulose level of polymerization and its crystallinity, hemicellulose hydrolysis, and incomplete
lignin depolymerization [15]. It involves grinding, chipping, and milling of fine particle size of the
material. Chipping converts into 10–30 mm particles, while milling and grinding convert into 0.2–0.4 mm.
It is easy to handle and results in decrystallization of cellulose, depolymerization, and an increased
surface area [16]. These physical pretreatment does not change the chemical structure of the feedstock
and they are cheap.

2.1.2 Physicochemical Pretreatment

The extrusion process has been expanded as one of the physical continuous pretreatment methods
towards bioethanol production due to its significant improvements of sugar recovery from different
biomass feedstocks [17]. This process practices heating, mixing, shearing and screw speed to shatter the
lignocellulose structure and finally increasing accessibility of carbohydrates to enzymatic attack [21].
Extrusion process can be improved in removing hemicelluloses and lignin by micro/nano fibrillation [22].
This process yields five times higher than hot compressed water treatment, which resulting 77%
delignification, and conversion of 69% and 38% of cellulose and hemicelluloses respectively into glucose,
xylose and arabinose [23]. Zheng et al. stated that twin-screw extruder can remove 80% of xylose with
soluble lignin [22]. Extrusion pretreatment has some advantages over other pretreatments: (1) low cost
and provides better process monitoring and control of all variables [18]; (2) no sugar degradation
products [19]; (3) good adaptability to different process modifications [20]; and (4) high continuous
throughput [20]. It seems therefore that extrusion pretreatment is more feasible for the pretreatment
of lignocellulosic biomass towards bioethanol production [17].

Steam-Explosion pretreatment is one of the most commonly used pretreatment options, as it uses
both chemical and physical techniques in order to break the structure of the lignocellulosic material.
This hydrothermal pretreatment method subjects the material to high pressures and temperatures for a

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short duration of time after which it rapidly depressurizes the system, disrupting the structure of the
fibrils. The disruption of the fibrils increases the accessibility of the cellulose to the enzymes during
hydrolysis. Particle size is a major contributing factor on the effectiveness of the process, and it has been
seen that relatively large particle sizes have been able to yield maximum sugar concentrations [24].
Steam explosion technique is usually done at 180⁰C and 210⁰C not exceeding 240⁰C. The steam-
explosion pretreatment process has been a proven technique for the pretreatment of different biomass
feedstocks. It is able to generate complete sugar recovery while utilizing a low capital investment and
low environmental impacts concerning the chemicals and conditions being implemented and has a
higher potential for optimization and efficiency [25].

2.1.3 Chemical pretreatment

Ionic liquids (ILs) are salt solutions consisting of significant quantity of organic cations and
small/inorganic anions that exists as liquid at relatively low temperatures like room temperature. They
are used to fractionate lignocellulose to obtain specific, purified and polymeric raw materials which are
intact and are easily separated and used as value-added co-products [2]. While most ILs are created in
laborious, multi-step syntheses, protic ILs can be synthesized in a one-step process from commodity
chemicals, which makes them less expensive; it is estimated that some ILs could be produced at bulk
scale for a price of $1.24 per kg which is comparable to common organic solvents such as acetone and
toluene [26]. The ability to recycle and reuse these customizable ILs in a process that operates at
comparatively lower temperatures and pressures makes this a more benign alternative and an
economically attractive candidate for biorefining [27].

Acid pretreatment utilizes acidic substances (typically H2SO4 or HCl) for lignocellulosic biomass due
to its powerful ability to rupture the components into simpler form [28, 29]. The low temperature for
this process makes it as a low cost pretreatment, and it can loosen the cell wall matrix through
hemicellulose degradation [30]. The process does not affect lignin, but cellulose microfibrils is sufficient
to produce high yield of monomeric sugars for fermentation [31, 32]. However, the utilization of acid
pretreatment will increase the tendency of corrosion to the equipment, hence it is important to
overcome this issue especially in a large scale production with high acid reagent [28, 33]. Therefore,
many acid processes have been modified to improve all downstream processes as well the bioethanol
yield and prices [31].

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Fig.3 Cost of different pretreatment methods

[35].

Fig.4 Advantages and disadvantages of other pretreatment methods showing relative cost

[36].

*LHW: Liquid hot water

*AFEX: Ammonia fiber explosion

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 *ARP: Ammonia recycle p

2.1.4 Biological pretreatment

In this method of pretreatment, microorganisms are employed to degrade the structure of biomass.
The typical microorganisms employed for this technique are white, brown and soft-rot fungi [33].
Several of white-rot fungi, namely Ceriporia lacerata, Phanerochaete chrysosporium, Pleurotus ostreaus,
Cyathus stercolerus, etc., have been evaluated and resulted in high delignification efficiency of various
lignocellulosic biomasses [30]. Biological pretreatment has several advantages over conventional or
alternative strat-egies of pretreatment. The main drawback of this strategy is the slow process as well as
the fact that the sugars may be used by the microbes for their growth. Development of an integrated
strategy such as a combined process as well as fine-tuning of each process variable can improve the
overall process economics [34].

There are other effective methods for pretreating biomass during the production of bioethanol but
are not cost effective. Either the process involves low cost but cost of purchasing equipment is relatively
expensive or the process is expensive but the equipment is cheap or both. In the case of optimal cost
the above methods were found cost efficient.

2.2 Hydrolysis of feedstock

Hydrolysis is used for converting cellulosic feedstocks into lower sugar polymer in the production of
bioethanol. In hydrolysis, the polysaccharides (cellulose and hemicellulose) present in a feedstock are
broken down to free sugar molecules (glucose, mannose, galactose, xylose, and arabinose) before
Fermentation takes place [37]. Hydrolysis are sometimes combined with fermentation process.
Hydrolysis are not particularly expensive but some factors affect this production process and makes it
looks expensive. Factors may include difficulty in acid recovery, disposal, concentration control and
recycling [38]. Also, the feedstock used in the production of bioethanol affects the hydrolysis method
used. Acid-catalysed Hydrolysis method is one of the commonly used methods. The acid can either be
concentrated or diluted. Example of such acids are H2SO4 and HCl [2]. The concentrated acid-catalyzed
hydrolysis is used at lower temperature and high acid concentration, resulting to 90% sugar recovery at
a short period of time [39]. The disadvantage of this method is the high cost of production due to
difficulty in acid recovery, recycling and concentration control [38].

In Enzymatic Hydrolysis Method, Enzymes are used to catalyse the breakdown cellulose material
into fermentable starch. These enzymes are usually very expensive due to high demand from various
industries such as paper, textile and food processing industries [40]. The high cost of these enzymes also
impacts on the overall cost of production especially as large quantities of enzymes are required. Based
on the cost, microorganisms with the potential of secreting cellulolytic enzymes are broadly used in the
recent times [2]. The advantage of enzymatic hydrolysis is able to be operated at only a mild hydrolysis
environment (pH of around 4.8 and temperature of around 318-323 K). Enzymatic hydrolysis produce
high yield and better end-product result compared to acid hydrolysis which does not promote any

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corrosion tendency to the processing equipment [41]. Although some difficulties like product inhibition
and catalytic efficiency are faced in this method, genetic modification can be used to enhance these
enzymes [13].

2.3 Fermentation of feedstock

Fermentation is a biomass conversion process into bioethanol by microorganisms(yeast and fungi or

bacteria), which by digesting fermentable sugars and producing ethyl alcohol and other byproducts
[31,33]. Saccharomyces and Pichia are the most common yeast type employed in bioethanol production
as well asbacteria Zymomonas and Escherichia and Aspergillus [42].

Simultaneous saccharification and fermentation (SSF) technique is a single fermentation reactor in

order to minimalize the inhibitors production, combining both processes (saccharification and
fermentation) at one time, and to reduce additional equipment cost [29, 43]. The SSF technique can
decrease production period and enhances the efficiency of the production. Additionally, ethanol
accumulation in the reactor does not inhibit the hydrolysis activity, making SSF as one favorable method
for bioethanol production [44,45].

Separate hydrolysis and fermentation (SHF) technique is a fermentation process can perform under
different vessels, where each carries specific task [46]. SHF allows each both processes, hydrolysis and
fermentation at their most optimum conditions and yielding the desired products to the maximum
potential [44]. SHF technique is considerably cost effective when it focuses on the substrates,
substances employed and the high quality ethanol yield, although it may not be cost effective in the
equipment installment [46].

Simultaneous saccharification and co-fermentation (SSCF) is a combination of microbial

incorporation of sugars released previously at the time of pre-treatment and hydrolytic routes of ligno-
cellulosic biomass. Mixed cultures of the yeasts could be employed, which can incorporate both hexoses
and pentoses [47]. This method combines both hydrolysis and fermentation process together reducing
cost for each individual processes.

Consolidated bioprocessing (CBP) of biomass to bioethanol refers to the combining of the three
biological events required for this conversion process (production of saccharolytic enzymes, hydrolysis
of the polysaccharides present in pretreated biomass and fermentation of hexose and pentose sugar
into bioethanol) [48].

Other fermentation process like continuous fermentation and batch fermentation are also
considered. While batch fermentation is the simplest form of fermentation, bioethanol yield is also of
maximum necessity.

3. Recovery of ethanol

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 This process is not expensive and easily carried out with the exception of large scale production
where more sophisticated procedure can be used. The bioethanol produced after fermentation usually
contain some percentage of water as ethanol is water soluble. Distillation mechanism can be used to
separate the ethanol-water mixture. The conventional techniques used for separating the ethanol-water
mixture include azeotropic distillation, liquid-liquid extraction and extractive distillation [49]. Extractive
distillation is the most predominantly used for large scale operations [2].

4. Conclusion

The second generation bioethanol need more optimism in cost of production ranging from 0.5-
0.9usd$/L. The first generation bioethanol which produces bioethanol from edible Feedstocks (sugar
and starch) competes with the world food production thereby, limiting available food. From recent
analysis, USA and Brazil are the world leading producers of bioethanol producing about 110-140 Liters of
bioethanol from corn and sugarcane in a year. To optimism the hunger impact of bioethanol production,
there should be a mixture of both first and second generation bioethanol, reducing the amount of
feedstock necessary for bioethanol production. On the other hand, pretreatment of lignocellulosic
biomass is expensive which in turn will increase the price per liter of bioethanol. The third generation
bioethanol where algae are used as feedstock is still in advancement and not yet attractive economically.
The production processes list above are considered to be more optimal in the yield of bioethanol while
being environmentally friendly and cost efficient.

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