Climate change is driven mainly by the burning of fossil fuels, which results in greenhouse gases (GHG) emissions into the atmosphere; it has been recorded that 89% of total carbon dioxide, a greenhouse gas, that was emitted into the atmosphere in 2018 originates from fossil fuel sources ^[1][2][1,2]. However, following the menace of the COVID-19 pandemic, these GHG emissions have dropped by 4.6% in 2020 due to the hindered economic activity. Nevertheless, the emissions rose again significantly by 6.4% in 2021, with the manufacturing and energy sectors contributing the more substantially [3]. British Petroleum reported that fossil energy demand has declined in 2020 (Figure 1), but also highlighted that the demand for renewable energy has noticeably increased [4].

Another important contributor to the emission of GHGs is the agricultural sector, as it has sought to achieve two goals: increase crop yields and maximize economic profit. To reach those outcomes, more modern practices were developed, including monoculture, rigorous tillage, irrigation, and chemical pest control. However, all these practices require fossil fuels for their application, in addition to the fossil fuels utilized for transportation and machinery ^{[5][6][7][8][9]}[5,6,7,8,9], implying that GHG is released into the atmosphere at every stage, from land to final product. For instance, farming releases considerable amounts of methane and nitrous oxide, which respectively originate from livestock during the digestion of foods in the animals’ digestive tract and mineral nitrogen fertilizers [10].

Furthermore, this sector is also responsible for significant waste generation, which is consequently hazardous to food safety, the environment, and global health ^[11][16]. One of the current priority of world leaders is to provide sufficient food and energy for the increasing population, all while reducing fossil fuel and solid wastes ^[12][17]. Thus, as agriculture contributes to climate change, the latter harms the agricultural sector through changes in precipitation patterns and negative effects on crop yield, for example ^[13][14][15][18,19,20]. In addition, many agricultural practices excessively exploit the earth’s natural resources, further negatively altering the environment ^[16][21] (Figure 2).

Therefore, in light of the multiple environmental issues that the planet is facing today, there has been a rising awareness about the repercussions of these issues and an increasing demand for a shift toward more sustainable strategies and practices, as well as for more sustainable energy alternatives. The latter include solar energy, hydropower, geothermal power, wind energy, and biomass ^[18][23]. The alternatives are presented as more natural solutions, and they all allow for the reduction of GHG emissions. However, the additional advantage of biomass is that it can stem from wastes and can be transformed into various forms of products, such as heat when burned or biofuel when processed. Planting crops specifically for biofuel production, in itself, induces environmental stress, as there will be no variation in planting ^[19][20][24,25]. However, since biomass can also originate from wastes, this valorization becomes an asset in reducing wastes left in the environment. There exist different types of biomasses for the production of biofuels; these biomasses can be starch-based, sugar-based, or lignocellulosic, meaning they are composed of cellulose, hemicellulose, and lignin. As for lignocellulosic biomass, they are of three types, shown in Figure 3 along with some examples of each ^[21][26].

The literature describes distinct types of promising biomasses tested for bioethanol production, and whose chemical composition is described in Table 1. Overall, lignocellulosic biomass (LCB) are promising raw materials because they are abundant and economic. For instance, wheat is considered one of the most ancient and most important crops worldwide, with a world production of 779,243 (1000 MT) in 2021 ^[22][23][27,28]. Moreover, the hemp market has risen in popularity, even if about 50% of the world’s hemp fiber supply is produced in China, which planted 667 km² of hemp in 2019 ^[24][29]. Additionally, the United States' (U.S.) grew approximately 220 km² of industrial hemp in the open in 2021, and the E.U. cultivated approximately 350 km² in 2019 ^[25][26][30,31]. Furthermore, corn is one of the most distributed crops worldwide, in addition to being the U.S.’s leading crop, having planted 97,000 km² of land with corn in 2019 ^[27][28][32,33]. Rice, on the other hand, is a primary necessity for over half of the world’s population. The U.S. Department of Agriculture (USDA) estimates World Rice Production to reach 514.76 million metric tons in 2022–2023, which represents a 0.23% increase in worldwide rice production compared to 2021–2022 ^[29][30][34,35]. Finally, sugarcane is also classified as one of the world’s most important crops, especially that 267,743 km² were harvested worldwide in 2016. Moreover, Brazil ranked first worldwide in sugarcane production volume, with a volume equivalent to 41% of world production in 2017 ^[31][36]. The abundance and mass production of these crops implies a significant generation of agricultural wastes. The latter is also inexpensive and is characterized with its naturally complex structure, so its valorization for energy production can be presented as promising and as an attractive alternative.

It is worth noting that the production of biofuels from biomasses has already been taken to an industrial level, with several facilities built around the world: 86 million tons of bioethanol were produced in 2018, with the main producers divided as shown in Figure 4. Additionally, the U.S. Energy Information Administration (EIA) reported that 4835 trillion British thermal units (TBTu) were provided by biomass in 2021, and that is equivalent to 1.416 × 10¹⁶ kWh ^[43][48].

However, some of these facilities are operational, while others are either ceased, planned, still under construction, or empty. The main reasons for the lack of operational facilities are due to financial difficulties or lack of adequate technology ^[44][49]. In fact, it is important to know that the structure and organization of the three polymers constituting lignocellulosic biomass depend on the types of biomasses, which also have a varying composition of each polymer. Furthermore, the multi-scale structure of those fibers, as well as the presence of lignin, is recalcitrant to enzymatic hydrolysis, which is applied to allow the release of fermentable sugars that will, in their turn, yield bioethanol following fermentation ^[45][46][47][50,51,52]. Therefore, this biomass requires a pretreatment to enhance the accessibility of the enzymes to the fibers, hence increasing the bioconversion and fermentation yields. The various other kinds and varieties of lignocellulosic biomass, as well as the different pretreatment parameters to be considered, led to the suggestion of numerous pretreatment techniques for different types of available lignocellulosic biomasses, and were presented with the overall procedure in Figure 5. Choosing the appropriate pretreatment technique and conditions plays a role in the outcomes of subsequent processes. Therefore, several factors need to be considered, such as energy cost, eco-friendliness, duration of pretreatment, and conservation of the fermentable sugars. In general, the efficiency of pretreatment for biofuel production is often defined by the lignin, cellulose, and hemicellulose contents after pretreatment, reducing sugar yield/concentration obtained following enzymatic hydrolysis, and biofuel yield/concentration obtained following fermentation.