Prospects of Bioethanol from Agricultural Residues in Bangladesh

Abstract

Bangladesh is a middle-income country. With the development of the industrial and agricultural sectors, the demand for petroleum-based fuels in the transport sector has been steadily growing. Diesel, petrol, octane (C₈H₁₈), liquid petroleum gas (LPG), and compressed natural gas are mainly used as fuels in the transportation sectors of Bangladesh. The government imports LPG as well as refined, crude, and furnace oil from abroad to meet the country’s growing energy demand. Apart from that, Bangladesh has a shortage of natural gas reserves, which is a great concern. As a result, it is essential to find and use renewable fuel sources. Since Bangladesh is an agricultural country, bioethanol could be the best alternative fuel generated from agricultural residues and waste. Every year, a large amount of agricultural residue is generated in this country, from which a vast amount of bioethanol could be produced. Bioethanol derived from agricultural residue and waste can reduce dependency on fossil resources, reduce fossil fuel’s environmental impact, and improve engine performance. This article comprehensively reviews the bioethanol production potential from agricultural residues and investigates the opportunities and possibilities in Bangladesh. The research outcomes reveal that in the fiscal year 2019–2020, approximately 46.5 million tons of agricultural residue were generated from the available major crops, from which about 19.325 GL (gigalitres) of bioethanol could be generated. This current study also investigates the practical methods of bioethanol production from different agricultural feedstocks and identifies the challenges related to bioethanol production in Bangladesh.

1. Introduction

Energy is one of the most critical factors in global prosperity. Globally, the demand for fossil fuels to generate energy is the highest and grows at the rate of 1.3% per year. Currently, about 84% of the world’s energy is generated from fossil fuels [1,2]. As a result, worldwide fossil fuel reserves are declining rapidly, which is a cause for global concern. Increasing usage of fossil fuels leads to increased greenhouse gas (GHG) emissions in the environment as well as global warming [3]. Already, the expanded burning of fossil fuels has increased the concentration of CO₂ in the atmosphere by 43% and raised the global temperature by 0.85 °C [4]. Additionally, fossil fuels release nitrogen oxides (NO_x) and sulfur oxides (SO_x) into the atmosphere. These can cause acid rain, which leads to environmental damage [5]. As the supply of fossil fuels is declining because of global demand and damage to the environment, there is a necessity for alternative renewable and sustainable fuels. The worldwide transportation sector accounts for about 28% of the total energy consumption, and 95% of that transport energy is obtained from oil-based fuels [6]. Only gasoline and diesel fuels accounted for 75% of the total delivered transportation energy that was used in 2012 [7]. Producing bioethanol from renewable biomass is a way to reduce fossil fuel demand and GHG emissions [8]. Currently, bioethanol is a prevalent alternative fuel throughout the world because combining bioethanol with fossil fuels to use as a transportation fuel helps to reduce environmental pollution. The United States of America, Brazil, China, and many countries produce a vast amount of bioethanol as an alternative to fossil fuels. Biofuels (mainly bioethanol) represent 25% of transport fuels in Brazil and 5% in the USA, which is significantly high, and its production has increased gradually [9]. On the other hand, Bangladesh mainly relies on fossil fuels for energy production and transportation.

The production of bioethanol from biomass resources can be classified into several generations based on their feedstock sources. First-generation (1G) feedstocks refer to crops such as soybean oil, corn starch, and sugarcane. Second-generation (2G) feedstocks include lignocellulosic biomass such as rice straw, husk, wood, wheat straw, bagasse, and other similar materials. Third-generation (3G) feedstocks are derived from algae [10,11]. 1G bioethanol can be generated from sugar by direct fermentation or both hydrolysis and fermentation for starch-type raw materials [12]. Producing bioethanol from lignocellulosic biomass includes the following primary steps: pretreatment to remove structural and compositional barriers, cellulose and hemicellulose hydrolysis into fermentable sugars, and sugar fermentation into bioethanol [13]. The basic steps of producing bioethanol from 1G and 2G feedstocks are shown in Figure 1.

Currently, most bioethanol is produced from 1G feedstock, and global ethanol production is expected to exceed 132 billion liters by 2030 [14]. However, 1G feedstocks compete with food in terms of land occupancy and need to be improved to fulfill the growing demand for fuel. They negatively influence biodiversity and may cause deforestation to gain more farmland [15]. The retrograde impact of these circumstances has raised the necessity of designing second-generation bioethanol technology from lignocellulosic materials, which are plentiful renewable organic materials in the biosphere [16].

Since Bangladesh is an agricultural country, significant agricultural residues are generated yearly. Therefore, bioethanol could be an alternative fuel source to reduce Bangladesh’s demand for fossil fuels. Every year, a considerable amount of agricultural residue is disposed of without processing. Additionally, these residues (e.g., crop and plant residues) are regarded as waste and are not included in the national accounting system for calculating GDP [17]. However, proper biomass waste utilization through producing bioethanol can contribute to the country’s GDP growth and reduce dissipation. Bangladesh does not currently generate bioethanol commercially [18]. Thus, it is essential to understand the opportunities and limitations of bioethanol production in Bangladesh. This study aims to provide an overview of Bangladesh’s bioethanol potential using agricultural residues, focusing mainly on 2G bioethanol production. The properties of bioethanol, its worldwide status, and its potential in Bangladesh are sequentially described in the following sections.

2. Fuel Properties of Bioethanol

Bioethanol (C₂H₅OH) is a liquid fuel that can be produced from different types of biomass feedstocks through conversion processes. Bioethanol is the only transportation fuel that does not impact the GHG effect. After burning ethanol, carbon dioxide (CO₂) is reclaimed back into plant material because plants utilize it to produce cellulose during photosynthesis, resulting in a closed carbon dioxide cycle. The ethanol manufacturing process exclusively employs renewable energy sources, and no net CO₂ is released into the atmosphere [19]. Ethanol contains 35% oxygen and assists in the full burning of fuel, resulting in the minimization of particulate emission, which is harmful to human beings and animals. Ethanol’s exhaust emissions are less harmful than those from petroleum sources [20]. Additionally, ethanol has a substantially higher octane number than regular gasoline, which increases an engine’s performance [21]. As a result, it is preferred for ecologically friendly transportation plans and to fuel public vehicles. Incorporating bioethanol into other fuels is a frequently used method of producing sustainable energy. Adding bioethanol to other fuels alters the fuel blend’s physicochemical qualities and improves its quality. Currently, many countries use the blending technique, like Brazil, which uses 27%; the USA, 10%; and India, which uses a 6.4% blend of bioethanol in gasoline [22]. In 2017, Bangladesh’s government granted a decree allowing a 5% bioethanol blend in automobile fuels [23]. This fuel blend contains 5% ethanol and 95% gasoline, known as E5, which may be utilized in vehicles without major modification [18].

Table 1. The physiochemical characteristics of bioethanol and E5 [24].

Fuel Characteristic	Bioethanol	E5
Density at 15 °C (gm/cm3)	0.790	0.834
Calorific value (kJ/kg)	(Lower) 26,700	(Higher) 43,632
Kinematic viscosity at 40 °C (mm2/s)	1.130	2.53
Cetane number	5.8	-
Octane number	110	96
Flashpoint (°C)	13	24
Moisture level (mg/kg)	2024	100
Boiling point (°C)	78	-

displays the physiochemical characteristics of bioethanol and E5 [24].

3. Current Trend of Bioethanol Production in the World

Bioethanol has already been highlighted as one of the most promising biofuels globally because it considerably decreases crude oil use and environmental pollution and helps to prevent high fuel prices [25]. The global bioethanol market was about 110 billion liters in 2019, dropping to 98 billion liters in 2020 due to the pandemic but increasing again after the pandemic [26]. The market is predicted to develop at a compound annual growth rate of more than 4.5% over the forecasted years (2021–2026) [27]. Global bioethanol output grew by 456.2%, from 13.2 gigalitres (GL) in 2000 to 73.5 GL in 2013 [28]. Currently, corn dominates bioethanol production, accounting for 60% of total production; in second, sugarcane makes up 25% of the production, followed by wheat at 3% and molasses at 2%. The remaining percentage is comprised of other types of seeds [26]. The bioethanol production of top nations and their feedstocks in 2020 is shown in

Table 2. Top countries for bioethanol production and their feedstock in 2020 [29,30,31,32].

Countries	Production (107 Gallons)	Feedstock
United States	1390	Corn
Brazil	793	Sugarcane, Corn, and Soybeans
European Union	125	Sugar Beet and its Derivatives, Corn, and Wheat
China	88	Corn, Soybeans, Wheat, and Sugarcane
India	48	Sugarcane and Molasses
Canada	46	Wheat and Corn
Thailand	40	Sugarcane, Molasses, and Cassava
Argentina	23	Soybeans, Corn, Wheat, and Sugarcane

[29,30,31,32]. The USA is the world’s most significant ethanol producer, with over 13.9 billion gallons, which is 53% of the total bioethanol generated in 2020, followed by Brazil at 31%, the European Union at 5%, China at 3%, and Canada at 2%. The USA and Brazil jointly make up 84% of the world’s total bioethanol [32]. Figure 2 shows the different regions’ yearly bioethanol production and their contribution percentage to total production during 2017–2021 [32].

4. Current Energy Status and Fuel Consumption Pattern in Bangladesh

Along with development, Bangladesh’s energy demand has also increased by an average of more than 6% per annum [27,33]. Currently, biomass supplies about 27% of primary energy and the remaining 73% is supplied by other commercial energy. Of the total biomass resources, agricultural residues accounted for 43%, livestock residues 34%, and municipal waste 7%. In Bangladesh, renewable energy sources include solar, wind, hydropower, and biomass resources. All of these sources have the potential to solve the country’s energy crisis by contributing to the total national energy production. Of the total national energy production, hydropower, solar photovoltaic, and wind energy contribute 230, 732.65, and 2.9 MW (megawatts), respectively [34,35]. However, solar energy is the fastest-growing renewable energy sector. The installation capacity of Bangladesh was 26,550 MW until 31 March 2023, of which more than 51% was generated from burning natural gas and 33% from a couple of furnace oils and high-speed diesel oils [36,37]. In December 2021, daily oil consumption was 178.99 barrels (per day) [38]. Natural gas reserves are diminishing since they meet around 72% of commercial energy demand [39]. Natural gas does not meet the need for fuel, and a considerable amount of fuel has to be imported from oil-supplying nations. Bangladesh imported about 8.5 million tons of crude and refined petroleum oil worth USD 775.31 million from oil-supplying nations in 2020, which was about 94% of the liquid fuel used in Bangladesh [39]. Figure 3a shows the current energy scenario of Bangladesh [39]. Bangladesh’s overall fuel consumption rises yearly as the country’s industry and transportation sectors are expanding. The demand for petroleum-based fuel is anticipated to grow at a rate of 2 to 4%, and if this growth continues, the need for fuel in Bangladesh will reach 15 million tons by 2030 [37]. From 1994 until the present, Bangladesh has produced an average of 3.95 BBL/D/1K (thousands of barrels per day) of crude oil.

The transportation sector alone consumes 3,451,580 metric tons (MT) of oil which is about 63% of the total petroleum-based oil used in the country. The total petroleum consumption by sector for 2019–2020 is shown in Figure 3b [39]. Diesel, petrol, octane (C₈H₁₈), and compressed natural gas are mainly used as transport fuels in the transport sector whereas diesel is the dominant fuel consumed in the country. The scenario of the consumption of petroleum products from the Bangladesh Petroleum Corporation (BPC) during the last eight years is shown in Figure 4 [39].

5. Biomass Potentiality in Bangladesh

5.1. Availability of Agricultural Residues

Bangladesh has massive biomass resources due to its extensive non-commercial usage. Additionally, the rainfed ecosystem produces vast amounts of biomass resources. Biomass is considered as a renewable energy source. The most common biomass sources are based on agricultural, forest, animal dung, and municipal solid waste, of which agricultural residue dominates in biomass generation. In 2018, about 70.69% of the total land area of Bangladesh was agricultural land [40]. The amount of residue production depends on the type of agricultural crop production. Thus, agricultural residues represent the key component of total recoverable biomass, because of the widespread area of cropland. Increasing crop production has led to an increasing biomass fuel supply in Bangladesh during the last few decades. The total supply of biomass fuel in 1980 was 236.08 PJ (petajoule) and increased to 1344.99 PJ over the next 30 years [41]. The GDP growth rates of the agriculture and forestry sectors at constant prices have been 2.65% in the fiscal year (FY) 2020–2021, compared with 4.10% in the previous FY (2019–2020) [33]. The major crops generated in Bangladesh are rice, wheat, jute, pulses, corn, sugarcane, and vegetables [42]. The production target of rice, wheat, and maize together in FY 2020–2021 was to be 46.635 MT, where rice was 39.643 MT, wheat was 1.299 MT and maize was 5.693 MT, which was a combined 45.344 MT in the last fiscal year [33]. Although agricultural residues are spread across the country, the same amount of crop is not produced in all regions due to land fertility, changeability in topography, insufficient infrastructural conveniences, etc. The 26.56% of districts in Bangladesh with high productivity are located in the three north-western divisions of the country, Khulna, Rajshahi, and Rangpur, of which the Kushtia, Naogaon, Dinajpur, and Jessore districts are notable [43]. Figure 5 depicts the agricultural productivity of different regions in Bangladesh [43].

The potential of agricultural residue relies on the amount of crops and agricultural fields. It is very hard to measure the availability of residues from field and grain processing areas because of the unavailability of real residue generation data. Grain residues also differ significantly in characteristics and rate of decomposition.

Instead of direct measurements, the production of crop residues is estimated based on different crop areas and yield data and research data on the straw/grain ratio (known as residue yield) [44].

$$\mathrm{Residue} \mathrm{production} = \mathrm{grain} \mathrm{production}\times \frac{\mathrm{straw}}{\mathrm{grain}}\mathrm{ratio}$$

(1)

Crop residues are classified into two types: field residue and process residue [45]. Field residues are usually utilized as fertilizer and are gathered from the land after harvesting. Process residues are produced during crop processing (e.g., milling) [46]. The field residues are not fully recoverable. It depends on the specific local climatic and soil conditions [47]. Agricultural residues and their recovery factors are shown in

Table 3. Agricultural residues and recovery factor [45,48].

Agricultural Residues		Recovery Factor (%)
Field Residues	Straw, Stalks, and Leaves	35
Process Residues	Husks, Bagasse, Seeds, Bran, and cob	100

[45,48].

Crops produce large amounts of residues which represents an important source of energy like bioethanol. The major crops produced in Bangladesh and residues produced from these crops during the years 2019–2020 are shown in

Table 4. Annual selected agricultural crop production and residue recovery in 2019–2020 [49,50].

Crops (Field + Process)	Crop Production 2019–2020 (105) ton	Residue Yield × Recovery Factor	Residue Recovery (105) ton
Rice Straw	366.03	1.695 × 0.35	217.15
Rice husk		0.267 × 1	97.73
Rice Bran		0.083 × 1	28.57
Wheat Straw	10.29	1.75 × 0.35	6.30
Corn Stalks	40.16	2 × 0.35	28.11
Corn Cob		0.3 × 1	6.13
Corn Husk		0.3 × 1	12.05
Jute Stalks	80.45	2 × 0.35	56.32
Pulses	3.98	1.9 × 0.35	2.65
Sugarcane	36.83	0.3 × 0.35	3.87
Tobacco	0.86	2 × 0.35	0.60
Vegetables and Others	45.75	0.4 × 0.35	6.41
Total	584.34		465.87

[49,50].

5.2. Utilization of Agricultural Residues

Biomass energy can be used to produce heat, electricity, or biofuel. In Bangladesh, most biomass resources are primarily used as rural cooking fuel. Rice straw, husk, bran, sugarcane bagasse, and jute stalks account for 46% of the total biomass energy [45]. Rice is the leading agricultural crop, occupying 76% of all farming land. In the fiscal year 2019–2020, this sector’s total recoverable residue was 34,345,000 tons (a detailed breakdown is included in Table 4). Rice straw and rice bran are commonly used as feed for cattle, poultry, and fish in Bangladesh. Rice husk, a by-product of paddy processing, is used as fuel to generate electricity by steam turbines and gasification processes and is also used for rice parboiling systems [51]. The country has approximately 100,000 rice mills, which consume approximately 70% of the husk energy for rice parboiling. Two rice husk gasification power plants with capacities of 250 kW and 400 kW are already in operation and funded by “Infrastructure Development Company Limited (IDCOL)” [52]. Rice husk is also used to make biomass briquettes; the yearly production is about 19,881 tons, which are used as cooking fuel in rural areas of the country [47]. Bangladesh also has many sugar mills, making sugarcane bagasse a promising source of power generation. Fourteen sugar mills have already installed cogeneration power plants to meet their energy demand and can generate 38.1 MW of electricity [53]. Biofuel production, on the other hand, is still in its early stages in Bangladesh. Nitol Motors in Bangladesh is developing bioethanol from molasses [54].

Table 5. Utilization pattern of agricultural residues in Bangladesh [50].

Primary Source	Residues	Utilization
Rice	Straw	(i) Fuel; (ii) Animal feed; (iii) Animal bedding; (iv) Housing material
	Husk	(i) Fuel; (ii) Cattle feed; (iii) Poultry feed; (iv) Fish feed
	Bran	(i) Fuel; (ii) Animal feed
Wheat	Straw	(i) Fuel; (ii) Housing materials
Jute	Stalk	(i) Fuel; (ii) Housing materials
Sugarcane	Leaf	(i) Fuel; (ii) Animal feed
	Bagasse	(i) Fuel
Corn	Stalk	(i) Fuel; (ii) Animal feed
	Cob	(i) Fuel
	Husk	(i) Fuel
Pulse	Straw	(i) Fuel; (ii) Animal feed
Tobacco	Plants	(i) Fuel
Vegetables	Plants	(i) Fuel; (ii) Animal feed

depicts the agricultural residue utilization practices in Bangladesh.

6. Feedstock for Bioethanol

To produce bioethanol, any feedstock with a considerable level of sugar or sugar-producing materials, such as starch or cellulose, can be utilized [55]. Bioethanol feedstocks are categorized into three major kinds: sugar-containing feedstocks (e.g., sugar beets and sugar cane), starchy materials (e.g., cassava, potatoes, and root crops), and lignocellulosic biomass (LCB) (e.g., agricultural residues) [3]. LCB, such as agricultural residues, is a sustainable alternative feedstock for bioethanol production because of its availability, low cost, higher ethanol yields, and efficiency. LCB from agricultural residues provides a plentiful, renewable supply of carbohydrates for microbial processing into fuels and chemicals [56]. Every year, more than 442 billion gallons of bioethanol can be generated from lignocellulosic biomass, which is about 16 times the current world bioethanol production [57]. Rice straw is one of the most plentiful lignocellulosic wastes in the world as well as in Bangladesh. LCB is mainly composed of three significant elements, i.e., cellulose, hemicellulose, and lignin, and each changes according to the source of the lignocellulosic material [58].

Table 6. The percent of biochemical mix of lignocellulosic biomass feedstocks that are the most accessible in Bangladesh [19,59,60].

Residues	%wt. on a Dry Matter Basis
	Cellulose	Hemicellulose	Lignin
Rice straw	28–36	23–28	12–14
Wheat straw	33–38	26–32	17–19
Maize stover	35–40	21–25	19–21
Jute	37–48	12.18–12.3	27.9–35.3
Sugarcane	25–45	28–32	15–25
Tobacco	30.22	40.28	21.06

represents the cellulose, hemicellulose, and lignin compositions of lignocellulosic feedstocks available in Bangladesh [19,59,60].

6.1. Cellulose

Cellulose (C₆H₁₀O₅) is a hexose sugar that is an important part of plant cell walls. It can be easily produced from biomass by using a pretreatment process. It is a linear, unbranched, homopolysaccharide-type organic polymer of glucose monomers (D-glucose anhydrous) coupled to β-(1,4)-glycosidic linkages. It consists of a long chain of small repetitive glucose units collected in microfibril bundles [61]. Cellulose is extremely crystalline, insoluble in water, and permits hydrolysis processes, known as saccharification, to dissolve the polysaccharide to release sugar molecules by increasing the water concentration [62]. Through the biological process of hydrolysis, cellulose releases glucose, which is then converted into a variety of compounds, such as bioethanol.

6.2. Hemicellulose

Hemicellulose (C₅H₈O₄)n is a short, branched heteropolymer composed primarily of 5-carbon (pentose) (e.g., D-xylose) and 6-carbon (hexoses) (e.g., D-glucose) sugars that are typically located in primary and secondary cell walls of biomass [63]. It is present in almost all soil-plant cell walls along with cellulose. Although cellulose is crystalline, robust, and immune to hydrolysis, hemicellulose has little strength and is in a random, amorphous arrangement. It is readily hydrolyzed by dilute acids or bases and countless hemicellulose enzymes [64]. Because of its low degree of polymerization, hemicellulose has a lower molecular weight than cellulose. This distinctive polymer forms a complex network with cellulose via hydrogen bonds and lignin via covalent interactions [63]. Yeasts that can ferment pentose may convert xylose into single-cell proteins (SCP) and a range of solvents and fuels like bioethanol [56].

6.3. Lignin

Lignin [C₉H₁₀O₃(OCH₃)]n is a three-dimensional, heterogeneous, and crosslinked aromatic polymer of propyl phenol. The three main aromatic phenols are coniferyl alcohol, sinapyl alcohol, and a minor quantity of p-coumaryl alcohol [65]. Since lignin is covalently linked to distinct hemicellulose side groups, strong carbon-carbon (C–C) and ether (C–O–C) connections in lignin offer strength and protection to the plant tissue against assault by cellulolytic microorganisms and functions like glue. Lignin is one of the barriers to LCB fermentation since it is unaffected by chemical and biological degradation yet has an impact on the quality of the bioethanol output [66]. Figure 6 depicts the molecular networks of cellulose, hemicellulose, and lignin [67].

Utilization of Lignin

Due to the heterogeneity and recalcitrant tendency of lignin, which is the second most abundant natural polymer (when considering only forest biomass otherwise chitin is second when considering all types of biomass), it is not affected by the fermentation process reviewed in this article as a bioethanol production method (described in Section 7) resulting in fermentation residue (FR) waste after producing bioethanol from agricultural residues. Therefore, the appropriate use of FR will make the biorefinery process more viable, thereby reducing waste, maximizing resource use, and achieving circular economy goals. The amount of lignin found in nature is estimated to be 0.5–3.6 billion tons per year, with the cellulosic ethanol industry producing 1–2 lack tons and pulp and paper manufacturing producing 40–50 million tons [68]. A report by Ahuja and Deb of Global Market Research mentions that the lignin market would exceed USD 960 million by 2024 [69]. Furthermore, the aromatic lignin marketplace is predicted to proliferate by more than 4.5% by 2024 due to strong manufacturing demand for phenol derivatives, which are used in several industries including the cosmetic industry. Therefore, it is crucial to utilize lignin more effectively to make industrial-scale bio-refinery plants cost-competitive. Lignin is an aromatic feedstock that is present in almost every plant cell, typically found in the range of 15–30% by dry mass and 40% by energy [70]. Considering an ethanol yield of 355 L per dry ton of biomass, 46.5 million tons of biomass could generate 12.9 million tons of lignin (i.e., FR) in the fiscal year 2019–2020 in Bangladesh. The lignin can be utilized to produce fuels, heat, value-added materials, and chemicals. Utilizing these materials, several industries like the chemical industry, plastic industry, dyeing industry, cosmetic industry, automobile factories, resin production, power production, etc., could meet their demand which can lead to a circular economy in Bangladesh. Figure 7 represents the processes of lignin conversion, and their valuable products and their utilization sectors.

7. Bioethanol Conversion from Lignocellulosic Biomass

Bioethanol generated from LCB is usually identified as 2G bioethanol. Different steps are used to generate bioethanol from LCB such as (1) pretreatment, (2) enzymatic hydrolysis, and (3) fermentation of sugar. After these main steps, distillation and purification are used to fulfill fuel standards. These steps can be performed separately as well as together with several advancements. Different methods are available for biomass bioconversion to prepare bioethanol, as shown in Figure 8 [72]. Each stage must be combined properly to obtain a larger bioethanol output in a cost-effective and long-term manner.

7.1. Pretreatment

Pretreatment is the first, most expensive, and most important step of the bioconversion process to produce ethanol from LCB, and it differentiates LCBs as a 2G feedstock from the 1G feedstock. The objective of the various pretreatment processes is to change or eliminate the structural and compositional obstacles from cellulose and hemicellulose that prevent hydrolysis to maximize the rate of enzyme hydrolysis as well as fermentable sugar yields [73]. Pretreatment is commonly used to separate lignin and hemicellulose from cellulose, which allows the cellulose to be hydrolyzed and converted into bioethanol. Pretreatment can be performed in various ways, such as (1) physical (chipping, milling, and grinding), (2) physio-chemical (steam pretreatment, hydro thermolysis, and wet oxidation), (3) chemical (organic solvents, oxidizing agents, dilute acids, and alkali), and (4) biological pretreatments. The different types of pretreatment methods available for LCB processing to produce accessible cellulose for effective hydrolysis have been reviewed comprehensively in several review articles [19,58,74,75,76].

Table 7. Different methods of biomass pretreatment for bioethanol yield and their main pros and cons.

Pretreatment	Condition	Main Advantage	Main Disadvantage	Pretreated Residue Examples	Sugar Yield/ Cost [77]	Ref.
(a) Physical
Milling and grinding	Ball mill: 0.2–2 mm final particle size	No chemical used Reduces cellulose crystallinity	Consumes more power	Hardwood, corn straw, corn stover, sugarcane, bagasse	L/H	[19,78,79]
Extrusion	Screw speed: 75 rpm, barrel temperature: 125 °C	No degradation products formed	Considerable aberration of metal face	Corn cobs, switchgrass, wheat bran,	H/H	[80,81]
Microwave	1% NaOH, 600 W, 4 min	Quick heat transfer	High reactor cost	Sugarcane bagasse	L/H	[82]
(b) Physicochemical
Acid-catalyzed steam explosion (ACSE)	T = 160–200 °C, dilute H3PO4 or H2SO4 (1–3% w/v), t = 5–30 min	Increased enzymatic accessibility	Higher acquisition and handling costs	Barley straw, Arundo donax, green wood	H/H	[83,84,85,86]
Ammonia fiber explosion (AFEX)	T: 90–140 °C, P: 1.12–1.36 MPa, t: 30–60 min; ammonia: dry biomass = 1:1–1:2	Volatile ammonia is recoverable and reusable	Inefficient for lignin-rich biomasses	Wheat straw, barley straw, rice husk, corn stover	H/H	[19,87]
(c) Chemical
Ozonolysis	Ozone	No inhibitors formed	Requires a significant amount of ozone	Wheat straw, cotton straw	H/H	[19,79]
(d) Biological	Fungus or bacteria	No chemicals required	Slow process	Corn stover, wheat straw	L/L	[88,89]

Note: T = temperature, P = pressure, t = time.

depicts various pretreatment methods for LCB that have been previously experimented with to improve ethanol production. A successful pretreatment procedure aims to (i) create the maximum amount of available sugars directly or indirectly through hydrolysis, (ii) limit inhibitory product creation, and (iii) reduce expenses [13].

7.2. Hydrolysis

The cellulose is prepared for hydrolysis after the pretreatment process. Cellulose hydrolysis is the process of converting glucose from cellulose, known as saccharification. Polysaccharides are broken down into sugars by the hydrolysis process. The strategy of hydrolysis of biomass is broadly categorized into two major divisions: chemical (concentrated acid) and enzymatic [76]. Because of some disadvantages of chemical methods (costly, toxic, corrosive, inhibitor formation, and dangerous), enzymatic hydrolysis is more promising. Enzymatic hydrolysis is more interesting because it builds a higher yield than acid-catalyzed hydrolysis without inhibitor formation and because the use of advanced biotechnology reduces enzyme prices [90,91]. The productivity of enzymatic hydrolysis is affected by several factors, including molecular structure, fiber surface area, hydrolysis duration, and enzyme loading. Cellulases, which are extracted from fungi (e.g., Trichoderma reesei) or bacteria (e.g., Bacteroides), are in high demand as industrial enzymes because they are widely used in a variety of sectors, such as the pulp and paper industry, the textile industry, food factories, and lignocellulosic processing for ethanol production [92,93]. To increase the yield and hydrolysis rate, researchers have concentrated on optimizing the hydrolysis procedure and boosting cellulase activity. Currently, several surfactants such as polyethylene glycol (PEG), bovine serum albumin (BSA), Triton X-100, Tween, sodium dodecyl sulfate (SDS), and lignosulfonate are widely used as lignin blockers to reduce the inhibition of unproductive enzyme binding on enzymatic saccharification and improve enzyme efficiency and stability [94,95,96,97]. For instance, adding Tween 80 improves hydrolysis efficiency by increasing enzyme accessibility to the substrate and enhancing mass transfer rate, which can enhance glucose output by 26.6–99.6% [98]. Similarly, polymers containing polyethylene glycol (PEG) have been used to improve hydrolysis efficiency because they can change the surface properties of cellulose, resulting in lower enzyme loading [99,100]. Improving the operability of enzymatic hydrolysis by using higher substrate concentrations, such as xylanase, is promising for bioethanol production because it affects the rate of hydrolysis to maximize glucose yields [101]. Ostadjoo et al. carried out a study with xylanase, which allows hemicellulose hydrolysis, and different feedstocks of varying concentrations, such as xylans from wheat straw biomass and sugarcane bagasse [102]. Overall, in order to convert cellulose to bioethanol efficiently, the influencing parameters must be optimized.

7.3. Fermentation

The hydrolysate formed after hydrolysis is utilized by microorganisms (like yeast and bacteria) for bioethanol fermentation. Under various fermentation conditions, the most popular hexose- and pentose-fermenting yeasts utilized in bioethanol production are Saccharomyces cerevisiae (S. cerevisiae) and Pichia [103].

Microorganisms consume fermentable carbohydrates as substrate and create ethyl alcohol and other byproducts in the process. The 6-carbon sugars are the most plentiful and are frequently used by these microbes (C₆H₁₂O₆ (glucose) $\to$ 2C₂H₅OH + 2CO₂) to produce bioethanol. As a result, cellulosic biomass materials that contain a high amount of glucose or glucose precursors are the simplest to transform into bioethanol. After the pretreatment process, the hydrolysis and fermentation processes can be performed individually or simultaneously. Currently, the following advanced methods are frequently used in bioethanol production: simultaneous saccharification and fermentation (SSF), separate hydrolysis and fermentation (SHF), simultaneous saccharification and co-fermentation (SSCF), and consolidated bioprocessing (CPB) [25]. In SSF, the cellulose is broken down and fermented simultaneously in the presence of the microbes. Fermenting more than one form of sugar, such as pentoses and hexoses, the SSCF fermentation process employs the integration concept of mixed microorganisms. LCB hydrolysis is performed separately from the fermentation stage in SHF.

Table 8. The main advantages and disadvantages of the SHF, SSF, SSCF, and CPB processes [24,56].

Process	Main Advantage	Main Disadvantage
SHF	Ability to complete each step under the best possible conditions	Cellulase and glucosidase enzymes are inhibited by glucose produced during hydrolysis
SSF	Lower enzyme requirements; higher product yields	SSF conditions are more difficult to optimize
SSCF	Reduced capital costs; higher ethanol productivity	Diverse assimilation rates of pentose and hexose, and expensive cellulase enzymes are required
CPB	One microbe produces all of the necessary enzymes, as well as sugars and ethanol	Conversion time is longer than for other processes

shows the main advantages and disadvantages of the different bioethanol production processes [24,56].

Table 9. Various operating methods for producing bioethanol from different lignocellulosic feedstocks.

Feedstock	Pretreatment/Hydrolysis	Microorganism	Modes	Ethanol Yield	Ref.
Rice straw	Alkali (NaOH)/ Accellerase® 1500 enzyme	S. cerevisiae, Candida tropicalis	Batch SSCF	28.6 g/L	[104]
Rice husk	0.1 M of FeCl3, HCl, and NaOH in triplicates at 121 °C for 15 min/Trichoderma reesei ATCC 26,921 enzyme	S. cerevisiae	SSF	3.8%	[105]
Sugar cane	Acid (H2SO4) followed by alkaline delignification (NaOH)/Trichoderma reesei	Recombinant S. cerevisiae containing the β-glucosidase gene	Batch SSF	51.7 g/L	[106]
Corn stover	AFEX/mix enzyme (Ctec 2, Htec 2, and Multifect pectinase)	S. cerevisiae Y35	SHF	45.5 g/L	[107]
White straw	2.15% (v/v) H2O2, 35 °C/T. longibrachiatum, A. niger, T. reesei	E. coli strain FBR5	SSF	66 g/L	[108]
Jute stalks	Alkali (2% NaOH)/commercial cellulase and β-glucosidase enzymes	S. cerevisiae JRC6	SHF	7.55 g/L	[59]
Tobacco	Alkali 2% (CaO)/liquid hydrolysates and β-glucosidase	S. cerevisiae	SHF	75.74 g/L	[109]

summarizes the recent studies on bioethanol production that have focused on using agricultural residue and waste that are readily available in Bangladesh, where SSF, SHF, and SSCF techniques are used to produce bioethanol. S. cerevisiae was used as a biocatalyst for fermentation in nearly 84 percent of the studies cited. These processes for generating bioethanol from several LCBs are presently being designed to fulfill sustainability and fuel standards, and the demands of transport.

8. Bioethanol Potential in Bangladesh

Bangladesh has vast potential for commercial bioethanol production. About 70.69% of the land is considered an agricultural area where many crops are produced. An enormous amount of residue is produced from the major crops that are readily available, such as rice, wheat, corn, sugarcane bagasse, pulses, and jute, from which a significant amount of bioethanol could be produced. Few theoretical studies on the feasibility of biofuel in Bangladesh have been conducted with a focus on bioethanol. Miskat et al. [24] conducted theoretical research on the accessibility of bioethanol production from agricultural residues. According to their estimates, the seven major crops (rice, jute, corn, wheat, sugarcane, cotton, and tobacco) produced approximately 65.36 million tons of crop residue, which could be converted into 32 million tons of bioethanol, with rice residues alone accounting for 27 million tons of that total. Mahmud et al. [37] researched the theoretical assessment of agricultural feedstock for biodiesel and bioethanol production. According to the findings, Bangladesh can produce approximately 44.4 million metric tons of bioethanol from five major crops (rice, jute, corn, wheat, and sugarcane) in the fiscal year 2019–2020, with rice accounting for 71% of the total. Here, we will examine how much organic ethanol can be produced from Bangladesh’s most available crop residues. These raw materials are rice, wheat, maize, jute, sugarcane, tobacco, pulses, and vegetable wastes. Many researchers report ethanol yields under various conditions, in different units and in different amounts. The ethanol yield (liters/ton) has been adapted from Tse et al. [110] in this paper for convenient calculation of the bioethanol potential from major agricultural residues. According to the findings, Bangladesh can produce 19.325 GL of bioethanol from crops in the fiscal year 2019–2020. Rice wastes are the most crucial contributor, from which about 14 GL of bioethanol can be produced; after rice, the most prominent contributors are jute and maize wastes. The potential for bioethanol production in gigalitres (GL) from significant agricultural residues in Bangladesh is shown in

Table 10. Bioethanol potential from agricultural residues in 2019–2020.

Crop Residue	Residue Recovery (105 tons)	Bioethanol Potential (Liters/ton) [110]	Bioethanol Production (GL)
Rice straw	217.15		9.03
Rice husk	97.73	416.00	4.07
Rice bran	28.57		1.19
Wheat straw	6.30	406.00	0.26
Corn stalks	28.11		1.32
Corn cob	6.13	470.00	0.29
Corn husk	12.05		0.57
Jute stalks	56.32	418.47 a	2.36
Pulses	2.65	108.21	0.03
Sugarcane bagasse	3.87	351.00	0.135
Tobacco	0.60	372.47	0.02
Vegetables and others	6.41	110.00 b	0.07
Total	465.87		19.325

^a,b bioethanol potential for jute stalks and vegetables were considered to be 418.47 and 110 L/ton, respectively.

[110]. In FY 2019–2020, BPC sold about 7 GL of petroleum-based oil equivalent to 5,488,668 MT, of which 62.69% was to the transport sector.

Based on the result and favorable scenario, Bangladesh can easily reduce the fossil fuel crisis and cost and contribute on a large scale to the renewable energy mix. Additionally, it can quickly meet the target of 5% blending for transportation fuel.

9. Challenges of Bioethanol Production from Agricultural Residues

Agricultural wastes/residues are plentiful and renewable materials for second-generation bioethanol production in Bangladesh. Research on using agricultural residues for second-generation bioethanol production has yielded promising results. However, there is still a significant disparity between predicted and actual bioethanol output at the industrial level. As a result, numerous challenges and barriers must be addressed to fully utilize these inexpensive, plentiful, and renewable resources for commercially viable bioethanol production. These challenges and barriers include:

• The biggest impediment to the development of bioethanol power plants is a lack of efficient technology for the pretreatment, hydrolysis, and fermentation processes, as well as adequate infrastructure.
• Generation of 2G bioethanol is two or three times more costly than fossil fuel because of high pretreatment and enzyme costs.
• The production of 2G bioethanol requires a significant amount of energy during growth, harvesting, transportation, and feedstock processing. As a result, developing new bioethanol production technologies with a positive energy balance remains a challenge.
• As crops are season-dependent, different crops are produced in different places at different times in Bangladesh. Additionally, farmers frequently change the crops cultivated on the same land. Therefore, it is always difficult to determine the best place to make bioethanol.
• As Bangladesh currently does not produce bioethanol commercially, there is a lack of clear, long-term, compatible policies and enough economic incentive policies.

However, research is needed to develop energy-efficient technologies to reduce production costs and address the challenges of converting lignocellulosic biomass to bioethanol. Additionally, sufficient ethanol production in Bangladesh is possible through adequate research and development in the agricultural sector and the formulation of effective and simple policies for bioethanol production.

10. Conclusions

This paper investigated the significant potential for the use of agricultural residues in Bangladesh. Currently, non-fossil energy sources have a very low share of the energy supply in Bangladesh. If the agricultural residues are efficiently utilized as biobased products such as bioethanol, imported oil demand will be reduced sufficiently and will also contribute to a large share of the renewable energy mix. Bangladesh is now attempting to enhance the use of biomass resources due to environmental concerns, future economic concerns, and a desire to have a positive impact on the environment. As a result, numerous researchers are attempting to promote the technology for producing biofuel and biopower. The government of Bangladesh has made a special effort to develop a biofuel plant for bioethanol. To successfully produce bioethanol, semi-government and private enterprises should also be involved in biofuel production to decrease costs and risks. This paper showed that rice residues alone contribute 73.72% of the total residues in Bangladesh, followed by jute stalks at 12.09% and corn residues at 9.94% in FY 2019–2020. Combined, rice, corn, and jute residues account for 95.75% of the total residues generated in Bangladesh in FY 2019–2020, from which 18.8 GL of bioethanol could be produced. In addition, utilizing residual lignin from bioethanol production could enable a circular economy that will help achieve the economic growth and sustainable development goals of Bangladesh. Additionally, the western zone of the country is more suitable for generating bioethanol because of its higher productivity index. An E5 can be an optimum alternative fuel for SI engines which can be managed by producing bioethanol from this agricultural waste. Based on current research headway, it is clear that bioethanol production from lignocellulosic agricultural residues will undoubtedly become a viable technology to fulfill fuel security very soon. Therefore, the government can start producing bioethanol commercially from major agricultural residues to follow the global trend. Finally, because of the abundance of biomass in Bangladesh, it may play a vital role as a sustainable energy source through public and private cooperation.