To produce bioethanol, any feedstock with a considerable level of sugar or sugar-producing materials, such as starch or cellulose, can be utilized ^[1][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) ^[2][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 ^[3][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 ^[4][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 ^[5][58]. Table 1 represents the cellulose, hemicellulose, and lignin compositions of lignocellulosic feedstocks available in Bangladesh ^[6][7][8][19,59,60].

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 ^[9][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 ^[10][62]. Through the biological process of hydrolysis, cellulose releases glucose, which is then converted into a variety of compounds, such as bioethanol.

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 ^[11][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 ^[12][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 ^[11][63]. Yeasts that can ferment pentose may convert xylose into single-cell proteins (SCP) and a range of solvents and fuels like bioethanol ^[3][56].

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 ^[13][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 ^[14][66]. 

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  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 ^[15][68]. A report by Ahuja and Deb of Global Market Research mentions that the lignin market would exceed USD 960 million by 2024 ^[16][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 ^[17][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. 

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 ^[18][72]. Each stage must be combined properly to obtain a larger bioethanol output in a cost-effective and long-term manner.

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 ^[19][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 ^{[5][6][20][21][22]}[19,58,74,75,76]. Table 2 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 ^[23][13].

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 ^[22][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 ^[37][38][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 ^[39][40][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 ^{[41][42][43][44]}[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% ^[45][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 ^[46][47][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 ^[48][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 ^[49][102]. Overall, in order to convert cellulose to bioethanol efficiently, the influencing parameters must be optimized.

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 ^[50][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) →→ 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) ^[51][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 3 shows the main advantages and disadvantages of the different bioethanol production processes ^[3][52][24,56].

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. ^[52][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. ^[59][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, researchers 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. ^[60][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 5 ^[60][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.