Bioethanol is the world’s most abundant biofuel and is being considered an alternate substitute in gasoline and other transportation industries. Ethanol is also an important precursor and an excellent organic solvent to synthesize numerous valuable chemicals and other composites [75]. As a matter of fact, bioethanol is a historically significant product derived from lignocellulosic biomass, and therefore, it deserves a more detailed study. Bioethanol production can be achieved by hydrolyzing a wide range of carbohydrate-rich renewable materials into fermentable sugars, which are, in turn, converted into ethanol. Depending on the raw material from which it is derived and its manufacturing process, bioethanol can be classified into three different generations. First-generation (1-G) bioethanol is obtained via yeast fermentation of sucrose crops intended for use in human food or animal feed, such as sugarcane (juice, molasses), grains (maize, wheat), and tuber crops (potato, sugar beet) [76]. Second-generation (2-G) bioethanol is made from lignocellulosic raw materials, including no-food plants, such as switchgrass and trees, and residual materials (such as solid waste, municipal waste, wood processing residues, and agricultural waste) [77].

Lastly, algal biomass is used to produce third-generation (3-G) bioethanol [78].

2 g and 3 g bioethanol is also known as advanced bioethanol. Technologies and mechanisms for advanced bioethanol production are still in the research and development stage or in the pilot or adaptation stage for large-scale production [79].

Despite the controversy over food competition and negative impacts on the environment and land use, 1G-bioethanol still accounts for more than 95% of the global ethanol market [80].

However, the growing demand for bioethanol, combined with the increase in population, raises concerns about 1G bioethanol’s long-term sustainability. In fact, it competes with food supplies for human and animal consumption, aggravating problems regarding food security worldwide, land and water availability, as well as soil contamination from distillation residues [81]. Furthermore, because of the increasing production of food commodities, significant quantities of agro-industrial waste are generally untreated and disposed of as waste via burning, dumping, or unplanned landfilling, resulting in environmental pollution, public health problems, and decreased organic matter in the soil [82]. The valorization of agricultural residues for 2 g bioethanol production leads to environmental benefits.

Lignocellulose biomass mainly consists of cellulose fibers embedded in a matrix of hemicelluloses and lignin [83,84]. Cellulose, accounting for 40–50% of agro-industrial residues, is an insoluble homopolysaccharide composed of fermentable sugars and formed via β-D-pyranose units linked by glycosidic bonds. About 40% of agro-industrial residues are hemicellulose, which contains pentoses, hexoses, and uronic acids. Lignin is the most complex natural polymer, formed via the cross-linking of three major components: p-coumaric, coniferyl, and sinapyl alcohols. It ensures the mechanical strength of the cell wall as a whole and makes up between 20–30 wt% of agro-industrial waste [82,83,85]. In agro-industrial waste generated from different sources, biomass constituents can vary significantly (Table 1) [83].

The bioconversion of lignocellulose biomass into 2 g bioethanol requires three key steps: pretreatment, saccharification (or hydrolysis), and fermentation [84,86].

Pretreatment of lignocellulose biomass is essential to reduce the biomass size, solubilize, hydrolyze, and separate the cellulose, hemicellulose, and lignin components [16,87,88,89]. A variety of pretreatment methods are available (physical, chemical, physiochemical, biological, electrical, or a combination thereof) [15,83] (Table 2).

Following pretreatment, lignocellulosic biomass usually undergoes saccharification, which is usually carried out with lignocellulolytic enzymes that can break down lignocellulose biomass into its monomers [91,92].

Lignocellulolytic enzymes occur in several Fungi and Bacteria [93], and they are divided into two categories: hydrolases (cellulases, hemicellulases, xylanases, proteases, and amylases) that break down cellulose chains and ligninases that break down lignin chains [94]. Because of their high specificity and ability to work in mild conditions, lignocellulolytic enzymes deriving from microbes are more efficient than inorganic catalysts. However, several factors limit their use in industrial processes, including their low stability at high temperatures, the high cost of isolating and purifying them, and their difficulty in retrieving them from reaction mixtures [25,95].

Fermentation-based bioconversion of lignocellulose biomass has been investigated using a variety of microorganisms. One of the most used yeasts for CR fermentation is Saccharomyces cerevisiae [96].

However, several studies have shown that Fungi belonging to the genera Aspergillus, Fusarium, Rhizopus, Monilia, Neurospora, Trichoderma, and Paecilomyces, as well as Bacteria, especially Lactobacillus sp. (Lactic Acid Bacteria, LAB), Clostridium, and Bacillus sp., can ferment monomeric sugars from CRs into a variety of valuable compounds [25,96,97].

Lignocellulosic feedstock fermentation varies depending on the microorganisms and raw materials. Five types of microbial cultures are used in fermentation processes, as summarized in Table 3.

There are several strategies for fermentation-based bioconversion of lignocellulose biomass via microbes: separate enzymatic hydrolysis and fermentation (SHF), simultaneous saccharification and fermentation (SSF), simultaneous saccharification and co-fermentation (SSCF), consolidated bioprocessing (CBP) [25].

In SHF, saccharification (or enzymatic hydrolysis) and fermentation reactions take place in different bioreactors. In SSF, SSCF, and CBP technologies, enzyme hydrolysis and fermentation are combined into one reactor to reduce overall production time, operating costs, and inhibitors, as well as improve the hydrolysis rate [103].

SHF is the predominant fermentation strategy, even if it has numerous disadvantages, including the high production cost due to long processing times and expensive equipment [104]. In addition, because of SHF’s long duration, it is susceptible to microbial contamination [105]. The released sugars, primarily cellobiose and glucose, inhibit the hydrolytic enzyme activity. Approximately 6 g/L of cellobiose reduces enzyme activity by 60%. Contamination could also be caused by enzymes [106].

SSF combines enzymatic hydrolysis and fermentation in one reactor [107], and it has several advantages compared to SHF. In the first place, the use of a single vessel for fermentation and saccharification reduces residence times and capital costs. In addition, the inhibitory compounds from enzymatic hydrolysis are reduced, improving the overall efficiency of the process [108,109,110].

A significant drawback of SSF limiting its use on an industrial level compared to the SHF is the different optimal temperatures and pH for hydrolysis and fermentation. Indeed, the optimal temperature of enzymatic hydrolysis is typically greater than the fermentation temperature. Consequently, a proper equilibrium point must be found for the process to work [111]. Currently, several thermotolerant bacteria and yeasts (i.e., Candida acidothermophilum and Kluyveromyces marxianu) have been investigated for increasing fermentation temperatures, approaching optimal hydrolysis temperatures [112]. Another obstacle to SSF is the difficulty of implementing continuous fermentation by recirculating and reusing the fermenting microbes [83]. As a result, yield losses in SSF processes constitute an inherent weakness [113].

SSCF allows the fermentation of both hexoses and pentoses in a single bioreactor [114], reducing energy consumption and process costs compared to using SSF, resulting in higher efficiency [115]. A major drawback of the SSCF process is the difference in temperature, pH, and other conditions between hydrolytic enzymes and fermentative microorganisms, as well as between microorganisms used in co-fermentation [116]. Thermophilic microorganisms can be engineered for this purpose [115].

In CBP, enzymes are produced in a single bioreactor via a single microorganism community. In this process, also known as direct microbial conversion (DMC), fermentation, saccharification, and hydrolytic enzyme production are performed in a single step, reducing operational costs and capital investments. For this purpose, several thermophilic cellulolytic anaerobic bacteria are investigated, including Thermoanaerobacter ethanolicus, Clostridium thermohydrosulfuricum, Thermoanaerobacter mathranii, Thermoanaerobium brockii, Clostridium thermosaccharolyticum strain. [117]. Currently, numerous studies focus on identifying and exploiting mixed cultures able to hydrolyze lignocellulosic biomass simulta neously with fermentation [118].

Biomethane production from lignocellulosic biomass offers a promising avenue for sustainable and renewable energy generation. Lignocellulosic biomass, composed of cellulose, hemicellulose, and lignin, is an abundant and widely available feedstock that can be effectively utilized for biomethane production via anaerobic digestion [119]. Anaerobic digestion is a biological process in which microorganisms break down complex organic matter in the absence of oxygen, resulting in the production of biogas, primarily composed of methane (CH₄) and carbon dioxide (CO₂) [120]. Lignocellulosic biomass presents unique challenges due to its complex structure and resistance to degradation. However, efficient pre-treatment methods have been developed to enhance the accessibility of the biomass components, such as steam explosion, alkali treatments, and enzymatic hydrolysis [71]. These pre-treatment techniques facilitate the breakdown of complex polymers and increase the bioavailability of substrates for microbial conversion, leading to improved biomethane yields.

The crystallinity of cellulose is reduced, and the porosity of cellulose is increased via various methods such as acid, base, and enzymatic hydrolysis, biological treatment, and steam explosion, which remove lignin and hemicellulose. [15,121,122].

The anaerobic digestion process involves a diverse consortium of microorganisms, including bacteria, archaea, and fungi, working synergistically to convert lignocellulosic biomass into biomethane [123]. The microorganisms hydrolyze the complex carbohydrates into simple sugars, ferment them into organic acids, and subsequently convert them into methane and carbon dioxide via methanogenesis. The process conditions, such as temperature, pH, and substrate concentration, need to be carefully controlled to optimize microbial activity and ensure efficient biomethane production [124].

Buitron et al. [125] studied the waste generated in the hydrothermal pretreatment (HPT) process. This waste is rich in organic matter, which can be used to produce methane via anaerobic digestion processes. The generation of methane from HPT waste via anaerobic digestion has economic and environmental benefits, but the efficiency of this process is low due to inhibitory compounds generated during biomass pretreatment, such as furan and lignin derivatives (phenolic compounds). The authors of this study evaluated the biodegradability rate and the theoretical biochemical methane potential (BMP) of the waste material obtained from HPT lignocellulose pretreatment, its composition and structure, and hydrolysis rates. Their results revealed that sugarcane has the highest organic matter content per mass of biomass and, therefore, the highest BMP and degradability. They also demonstrated that the liquid fraction produced higher BMP values than the solid fraction and that the relatively low biodegradability of HPT waste compared to natural carbohydrates is due to an inhibitory effect of furfural and lignin. However, at lower concentrations, furfural has an inhibitory effect, which becomes stimulating at high concentrations; therefore, the addition of furfural optimizes biomethane production [11,125]. As a matter of fact, Differences in furfural content in steam-exploded hydrolyzates are responsible for different degrees of inhibition of anaerobic digestion. Furfural concentrations ranging from 100 to 500 mg/L inhibited methane production, but a concentration of 1000 g/L had a stimulating effect on anaerobic digestion [125,126]. Mwene–Mbeja et al. [127] investigated the detailed mechanisms of the enzymatic reactions that transform residual proteins, carbohydrates, and lipids into biomethane and fertilizers as a strategy to improve the efficiency of biomethanation in industrial applications, thereby maximizing biomethane production or biofertilizer quality. These authors investigated the function of various types of enzymes in organic reactions that occur during anaerobic digestion, such as hydrolysis, acidification, acetate synthesis, and methane synthesis. Each type of substrate (proteins, carbohydrates, or lipids) is degraded under anaerobic conditions via specific enzymes, and the intermediates are substrates for the production of methane and fertilizers [127].

To improve biomethane yields, co-digestion strategies have been explored by co-feeding lignocellulosic biomass with other organic substrates, such as animal manure, food waste, or energy crops [128]. Co-digestion enhances the nutrient balance, improves the carbon-to-nitrogen ratio, and increases the diversity of microbial communities, leading to more stable and efficient biomethane production. Furthermore, process optimization via reactor design, operational parameters, and control strategies has been investigated to maximize biomethane production from lignocellulosic biomass [129].

In addition to biomethane production, the anaerobic digestion process offers the additional benefits of waste treatment and nutrient recycling. The digestion residues, known as digestate, can be used as a nutrient-rich fertilizer, thereby closing the loop in a sustainable and circular bioeconomy [120]. Moreover, the utilization of lignocellulosic biomass for biomethane production contributes to reducing greenhouse gas emissions and dependence on fossil fuels.

In summary, biomethane production from lignocellulosic biomass holds great potential as a renewable energy pathway. Pre-treatment methods, process optimization, and co-digestion strategies are being continuously researched and developed to enhance the efficiency and economic viability of biomethane production. The utilization of lignocellulosic biomass not only offers a sustainable energy source but also provides waste management solutions and contributes to environmental sustainability.

The production of biodiesel from lignocellulosic biomass holds great promise as a renewable and sustainable alternative to fossil fuels. In fact, many nations have limited biodiesel production due to the vast land area required for cultivating oilseed crops and the competition of energy crops with traditional food crops [130]. As a result, research has focused on biodiesel production from waste elements such as used oils, food residues, and residual biomass [131,132,133,134,135].

Lignocellulosic biomass, composed of cellulose, hemicellulose, and lignin, is a widely available and abundant feedstock that can be effectively utilized for biodiesel production via various conversion pathways [136]. The complex structure and recalcitrant nature of lignocellulosic biomass require appropriate pretreatment methods to enhance the accessibility of biomass components and facilitate subsequent conversion processes [137]. Physical, chemical, and biological pretreatment methods have been explored to break down the complex structure, remove impurities, and improve enzymatic hydrolysis efficiency [138]. Physical pretreatment methods increase the surface area and enhance the accessibility of cellulose and hemicellulose to enzymes [38]. Chemical pretreatment methods help to disrupt the lignin matrix and solubilize hemicellulose, facilitating enzymatic hydrolysis and subsequent biodiesel production [139]. Biological pretreatment methods offer the potential for selective lignin degradation and improved enzymatic hydrolysis [140,141,142,143,144,145].

Enzymatic hydrolysis is a key step in biodiesel production from lignocellulosic biomass, where cellulose and hemicellulose are enzymatically converted into fermentable sugars. Cellulases and hemicellulases are commonly employed enzymes in this process, breaking down polysaccharides into monomeric sugars suitable for fermentation [146,147]. Selection and optimization of enzyme cocktails, including cellulases, hemicellulases, and accessory enzymes, are essential to achieve efficient hydrolysis and maximize sugar yields [148,149,150,151]. Enzyme loading, hydrolysis conditions (temperature, pH), and substrate composition significantly influence hydrolysis efficiency and subsequent biodiesel production [152,153,154].

The fermentation of liberated sugars into biodiesel can be realized via microbiological processes, such as yeast fermentation or microbial consortia. Yeasts, such as Saccharomyces cerevisiae, are commonly used for ethanol production from lignocellulosic biomass, which can be subsequently converted into biodiesel via transesterification reactions [155,156,157]. Alternatively, microbial consortia, including bacteria and archaea, have been investigated for the direct production of biodiesel from lignocellulosic sugars via the fermentation pathway [158,159,160]. The choice of microorganisms, fermentation conditions, and downstream processing stages significantly affect the yield and quality of the produced biodiesel [161].

Recently, emerging biofuels such as renewable diesel have gained attention, which are obtained by hydro-deoxygenating renewable resources such as biodiesel, vegetable oils, and single-cell oils. In addition to green diesel, renewable diesel is also known as second-generation biodiesel, which exhibits superior cleanliness, oxidative stability, and cold compatibility, giving renewable diesel a competitive advantage over conventional biodiesel created via transesterification [162]. Renewable biodiesel can reduce nitrogen oxide and hydrocarbon emissions, while biodiesel reduces carbon dioxide emissions and other particulate matter. The existing petroleum-based refining infrastructure can be utilized for the production and distribution of renewable diesel. In addition to traditional methods, hydrodeoxygenation is an efficient method for reducing the viscosity of triacylglycerols [163].

To improve the overall efficiency and economics of biodiesel production from lignocellulosic biomass, integrated biorefinery approaches have been proposed. These approaches aim to maximize the utilization of various biomass components, such as lignin and hemicellulose, to produce high-value products alongside biodiesel. Lignin, a byproduct of lignocellulosic biomass pretreatment, can be valorized into valuable chemicals, biofuels, or high-quality materials, reducing waste and improving the process’s overall economy [164]. Furthermore, the utilization of hemicellulose, a byproduct of cellulose hydrolysis, to produce basic chemicals or biopolymers further enhances the sustainability and value proposition of the biorefinery concept [165].

In summary, biodiesel production from lignocellulosic biomass offers a promising opportunity for renewable and sustainable energy. Studies are ongoing on pretreatment methods, enzymatic hydrolysis, fermentation pathways, and integrated biorefinery approaches to improve the efficiency, yield, and sustainability of the process. Further research and optimization are needed to address technical and economic challenges and facilitate the commercialization of biodiesel production from lignocellulosic biomass.

The production of hydrogen and jet fuel from lignocellulosic biomass holds significant potential as a sustainable and renewable energy solution for industry in general and the aviation sector in particular. Lignocellulosic biomass, derived from agricultural residues, energy crops, and forestry by-products, offers a plentiful and diverse feedstock for the production of both hydrogen and jet fuel [166,167]. Recent advancements in conversion technologies, such as thermochemical and biochemical processes, have shown promise in efficiently extracting hydrogen and synthesizing renewable jet fuel from lignocellulosic biomass.

Thermochemical processes, including gasification and pyrolysis, have gained attention for hydrogen and syngas production. Gasification, a process in which biomass is heated in the presence of a controlled amount of oxygen or steam, converts the lignocellulosic biomass into a mixture of hydrogen, carbon monoxide, and other gases [168,169,170,171]. Syngas obtained from gasification can be further processed to extract hydrogen via a water–gas shift reaction or utilized for the synthesis of liquid hydrocarbon fuels, including jet fuel, via Fischer–Tropsch synthesis [172,173,174,175]. Pyrolysis, on the other hand, involves the thermal decomposition of biomass in the absence of oxygen, producing a bio-oil that can be subsequently upgraded to obtain hydrogen-rich gases and liquid hydrocarbon fuels [176].

Biochemical routes, such as biological and enzymatic processes, offer an alternative approach to hydrogen and jet fuel production from lignocellulosic biomass. Biological pathways, including dark fermentation and photo-fermentation, utilize microorganisms to produce hydrogen by fermenting biomass sugars or volatile fatty acids derived from lignocellulose hydrolysis [177,178]. These processes can be optimized by selecting appropriate microorganisms, optimizing process conditions, and integrating co-cultures to enhance hydrogen yields and productivity. Enzymatic processes, employing cellulases and other hydrolytic enzymes, can effectively hydrolyze cellulose and hemicellulose fractions of lignocellulosic biomass into fermentable sugars, which can then be converted into hydrogen via microbial fermentation or further processed into jet fuel precursors [75,179,180].

Dark fermentation of lignocellulosic biomass has been shown to produce biohydrogen, a key component of renewable natural gas, along with other by-products such as methane, butyrate, acetate, and ethanol, depending on the specific microbes and operational conditions [181,182,183]. To optimize biohydrogen productivity and yield, various strategies have been explored, including sequential saccharification and fermentation, consolidated bioprocessing, separate hydrolysis and fermentation, and cell-free biocatalytic synthesis [184,185]. Several factors influence biohydrogen production, including nutrient availability, raw materials, temperature, and pH [186,187]. Typically, a combination of mixed substrates and microbial cultures is employed, often in conjunction with nanotechnology and carbon-biomaterials, to promote microbial growth and enhance the activity of enzymes involved in hydrogen production [188,189,190]. These approaches offer potential avenues for improving the efficiency and effectiveness of biohydrogen production from lignocellulosic biomass.

The production of biokerosene for aviation fuel (commonly referred to as jet fuel) involves a combination of biochemical and thermochemical processes, with vegetable oil being a primary feedstock [191,192]. However, due to concerns regarding the use of food crops for fuel production, there has been a growing focus on identifying alternative feedstock materials and exploring other biorefining pathways [193]. Lignocellulosic biomass can serve as a viable feedstock for the production of biokerosene via thermochemical conversion processes. In this approach, the feedstock is directly converted into sugars, which are then synthesized into biokerosene. The process typically involves a pretreatment step to obtain intermediate products, which are subsequently upgraded into biokerosene [194,195]. However, the high selling price of biokerosene remains a challenge that hinders its widespread commercialization [192]. Efforts are ongoing to address the cost-effectiveness and scalability of biokerosene production from lignocellulosic biomass, aiming to overcome these barriers and facilitate its broader adoption in the aviation industry.

To overcome the challenges associated with lignocellulosic biomass conversion, various research efforts have focused on improving process efficiency, feedstock availability, and cost-effectiveness. Advancements in catalyst development, process integration, and reactor design have shown promise in enhancing the conversion efficiency of lignocellulosic biomass into hydrogen and jet fuel [196,197,198]. Additionally, advancements in feedstock preprocessing, such as fractionation and pretreatment methods, have facilitated the extraction of key components from lignocellulosic biomass and improved the overall process economics [15,69,199,200,201].

Furthermore, the development of sustainable supply chains and the utilization of advanced feedstock sourcing methods have been explored to ensure the availability of lignocellulosic biomass for hydrogen and jet fuel production. Strategies such as agricultural residue management, dedicated energy crop cultivation, and utilization of forest residues contribute to the sustainable sourcing of biomass feedstocks and minimize the potential environmental impacts [202,203,204].

In summary, the production of hydrogen and jet fuel from lignocellulosic biomass presents an environmentally friendly and renewable pathway for the aviation industry. Thermochemical and biochemical conversion technologies, along with advancements in catalysts, reactors, and feedstock sourcing, are driving progress in the efficient and cost-effective production of hydrogen and jet fuel from lignocellulosic biomass. Continued research and development efforts are crucial to further optimize these processes and enable their commercial implementation.

Utilizing lignocellulosic biomass for the production of biobutanol and biogas presents a sustainable and renewable alternative to conventional fuels. Lignocellulosic biomass, including agricultural residues, forestry waste, and energy crops, serves as a promising feedstock for biofuel production, such as biobutanol [205]. The biobutanol industry yields a diverse range of value-added by-products, encompassing fibers, solvents, coatings, and plastics, and acts as a precursor for various allied chemicals, such as butyl acetate, acrylic acid, and adhesives, fostering economic growth via a wide array of products [206,207]. Biobutanol can be generated via a two-step fermentation process known as acetone–butanol–ethanol (ABE) fermentation. By subjecting a variety of biomass feedstocks to solventogenic Clostridium species fermentation, the industrial acetone–butanol–ethanol fermentation process enables biobutanol production. Initially, lignocellulosic biomass undergoes pretreatment and enzymatic hydrolysis, releasing sugars that are subsequently fermented via solvent-producing microorganisms to yield biobutanol [208,209,210,211]. Recent advancements in the ABE fermentation process involve the development of genetically modified microorganisms with enhanced capabilities for biobutanol production [75,211,212]. However, the acetone–butanol–ethanol process encounters challenges such as low yield, increased toxicity of butanol to microbes, and difficulties in downstream recovery of butanol. Compared to petrochemical-based butanol production ($1.50/kg), biobutanol produced via acetone–butanol–ethanol fermentation yields a fuel price of $1.80/kg. Hence, future research strategies should focus on reducing the cost of biobutanol processing via cutting-edge genetic manipulation techniques [212,213,214].

Researchers have explored various approaches to biobutanol production from lignocellulosic biomass. For instance, Moradi et al. achieved a biobutanol yield of 112 g per kilogram of alkali-/acid-pretreated rice straw [215]. Another study utilized Clostridium sporogenes in the acetone–butanol–ethanol fermentation of detoxified, enzyme-hydrolyzed, and acid-pretreated rice straw, achieving optimal biobutanol production and a productivity rate of 0.05 g/L per hour [216]. A two-stage fermentation process involving acidogenic fermentation followed by acetone–butanol–ethanol fermentation resulted in a biobutanol production rate of 0.5 g/L per hour using pretreated rice straw [217]. Mild alkali pretreatment and enzymatic hydrolysis of rice straw prior to acetone–butanol–ethanol fermentation demonstrated an efficacy range of biobutanol production from 0.53 to 2.93 g/L [218].

Anaerobic digestion of lignocellulosic biomass provides a pathway for the generation of biogas, predominantly composed of methane and carbon dioxide. This biological process involves microbial consortia transforming biomass via anaerobic conditions, resulting in the production of methane-rich biogas [219,220]. Several factors impact the efficiency of biogas production, including biomass composition, operating conditions, and microbial activity. Strategies such as co-digestion, which involves blending different biomass types, and thermal pretreatment methods have been investigated to enhance biogas yields [221,222,223]. Advances in anaerobic digestion technology, including high-rate digesters and microbial enrichment techniques, have contributed to improved process performance and methane production [224]. Various factors, such as pH, temperature, organic loading rate, retention time, and carbon-to-nitrogen ratio, exert cumulative effects on the efficiency of biogas production techniques. The utilization of psychrophilic, mesophilic, and thermophilic microorganisms in the bioreactor, depending on their temperature sensitivity, is crucial for anaerobic digestion [225]. However, the recalcitrant structure of lignocellulosic biomass poses challenges for effective anaerobic digestion. Further investigation into co-digestion processes and different pretreatment methods is necessary to enhance microbial growth and improve the biogas production rate [226].

The integration of biobutanol and biogas production from lignocellulosic biomass offers synergistic benefits. By utilizing the by-products of biobutanol fermentation, such as residual sugars and lignin, as substrates for biogas production via anaerobic digestion, the overall energy efficiency and valorization of lignocellulosic biomass can be enhanced. This integrated approach contributes to a more sustainable and economically viable bioenergy production system [227,228,229].

Gasification and pyrolysis of lignocellulosic biomass are promising thermochemical conversion technologies that offer sustainable and efficient routes for the production of valuable biofuels and bioenergy. Lignocellulosic biomass, including agricultural residues, forest waste, and energy crops, represents a vast and renewable resource for bioenergy production [230,231]. Gasification is a process that converts solid biomass into a mixture of combustible gases, primarily carbon monoxide (CO), hydrogen (H2), and methane (CH4), known as syngas, via high-temperature reactions in an oxygen-limited environment [232,233]. Pyrolysis, on the other hand, involves the thermal decomposition of biomass in the absence of oxygen, leading to the formation of liquid bio-oil, solid char, and non-condensable gases [234].

Brownstein [60] explored the production of synthesis gas from lignocellulosic feedstock as a means of utilizing basic fuels. Initially, industries favored treatment and fermentation as the primary processes for valorizing lignocellulose waste, but some have since shifted towards converting it into synthesis gas. While gasification of fossil fuels is a well-established technique, utilizing lignocellulose waste as a raw material for synthesis gas production enables the use of carbon sources to generate liquid fuels.

Several prominent companies, including Lanzatech, Ineos, Coskata, and Syntec Biofuels, are engaged in gas synthesis applications such as the Fischer–Tropsch method, the ExxonMobil methanol-to-gasoline system, and acetogens for gas-to-liquid fuel fermentation. Similarly, companies like Velocys, Maverick, Fulcrum, and Enerkem have compared the cost of biogas to that of natural gas derived from fossil fuels and have opted to focus on the former. Enerkem, a Canadian company, has replaced fossil sources with waste to produce sustainable transportation fuels and chemicals used in everyday products [235]. Commercial-scale production of renewable methanol and ethanol is achieved by Enerkem from non-recyclable, non-compostable municipal solid waste. This innovative and environmentally friendly approach to waste management and energy diversification is based on eco-fuel feedstocks and aligns with the principles of the circular economy.

The production of ethanol involves the gasification of waste cellulose in multiple steps. First, methanol is synthesized from the produced synthesis gas, which is then converted to methyl acetate and acetic acid using rhodium-based catalysis. The acetic acid is esterified to obtain additional methyl acetate, and finally, all the methyl acetate is hydrogenated to produce ethanol. This multi-step technology can utilize both in situ-produced methanol and commercial methanol [235].

Asadullah et al. [236] emphasized the importance of efficient supply chain management, appropriate biomass pretreatment, and efficient fuel conversion in the development of biomass power generation. The authors investigated critical parameters for the generation of fuel gas with an optimal composition for turbines or internal combustion engines. These parameters include the type of gasifier (updraft, downdraft, fixed bed, fluidized bed), gasifying agent (air, steam), temperature, pressure, and air/fuel ratio. For example, a fluidized bed gasifier allows for homogeneous heat distribution and fast heat transfer to the particles, resulting in improved reaction rates. However, it requires small biomass particles, leading to higher energy and economic costs. Similarly, a fixed bed gasifier (either updraft or downdraft) operates with high carbon conversion, longer residence time, and low gas velocity but is suitable only for small-scale power generation. Additionally, the product gas often contains impurities such as tar, particles, sulfur and nitrogen oxides, and ammonia, the quantities of which vary depending on the gas composition. Consequently, gas composition is crucial as internal combustion engines can only tolerate a limited concentration of contaminants, necessitating a purification process to minimize their presence. Physical filtration and catalytic hot-gas cleaning are the primary methods employed for purification. The primary approaches for electricity generation from the fuel gas obtained via biomass gasification include combined heat and power generation, fuel cells, and synthetic diesel production.

Gasification of lignocellulosic biomass offers several advantages, including high energy efficiency, flexibility in feedstock selection, and low greenhouse gas emissions [237,238]. The produced syngas can be utilized for various applications, such as electricity generation, heat production, and the synthesis of liquid fuels and chemicals [239]. Several factors influence the gasification process, including biomass composition, particle size, gasification temperature, residence time, and gasification agent [240,241]. To optimize gasification performance, research efforts have focused on improving reactor design, developing efficient catalysts, and exploring novel biomass pretreatment techniques [242,243,244]. Integration of gasification with other processes, such as gas cleaning and syngas upgrading, further enhances the overall efficiency and environmental sustainability of the biomass conversion process [245,246].

Pyrolysis of lignocellulosic biomass is a thermochemical process that offers an attractive route for the production of bio-oil, biochar, and syngas [247,248]. Bio-oil, also known as pyrolysis oil, is a complex mixture of oxygenated organic compounds that can be further refined into transportation fuels and chemicals [249,250,251]. Biochar, a solid residue obtained from pyrolysis, has applications in soil amendment and carbon sequestration [252]. Pyrolysis conditions, such as heating rate, temperature, and residence time, strongly influence product distribution and quality [253,254,255]. Various pyrolysis technologies, including fast pyrolysis, slow pyrolysis, and intermediate pyrolysis, have been developed to optimize bio-oil yields and properties [256,257,258]. Catalysts and additives are often employed to enhance the selectivity and quality of the bio-oil [259,260,261]. However, challenges such as the instability of bio-oil, high oxygen content, and the need for upgrading processes for bio-oil utilization remain areas of active research [262,263].

Recent advancements in gasification and pyrolysis technologies have focused on improving process efficiency, product quality, and environmental performance. Integrated gasification combined cycle (IGCC) systems have been developed to maximize energy conversion efficiency and minimize emissions by utilizing the syngas for power generation [264,265]. Co-gasification of biomass with coal or other carbonaceous materials has shown promise in improving the gasification process and diversifying feedstock options [266,267]. Moreover, the use of novel catalysts and catalytic gasification processes has demonstrated potential for enhancing gasification performance and syngas quality [268,269]. In the pyrolysis domain, the development of advanced reactors and integrated systems has aimed to increase bio-oil yields and reduce undesired by-products [270]. Upgrading techniques such as hydrodeoxygenation and catalytic cracking are being explored to improve the stability and quality of bio-oil for its utilization in transportation fuels [271,272,273,274].

In conclusion, gasification and pyrolysis of lignocellulosic biomass offer promising pathways to produce biofuels and bioenergy. These thermochemical conversion technologies provide opportunities for the efficient utilization of abundant biomass resources while reducing dependence on fossil fuels. Advancements in reactor design, biomass pretreatment, catalyst development, and process integration have contributed to improving the efficiency, sustainability, and economic viability of these technologies. Continued research and development efforts are essential to overcome challenges and further optimize gasification and pyrolysis processes for large-scale implementation in the bioenergy sector.

The energetic valorization of lignocellulosic biomass presents a promising approach for sustainable energy production, with both economic and environmental considerations being crucial aspects to be addressed. Lignocellulosic biomass, including agricultural residues, forest waste, and energy crops, is a widely available and renewable resource that can be utilized for bioenergy production [62,275]. The economic viability of utilizing lignocellulosic biomass for energy generation relies on several factors, including feedstock availability, processing costs, energy conversion efficiency, and market demand for bioenergy products [157,276]. Additionally, the environmental impact associated with the energetic valorization of biomass needs to be carefully assessed to ensure a sustainable and low-carbon energy pathway.

Economically, the utilization of lignocellulosic biomass offers opportunities for rural development, job creation, and reduced dependence on fossil fuels [180,277]. Biomass is found worldwide but is not evenly distributed, tending to be concentrated in forests and rural areas. Furthermore, raw biomass, especially agricultural biomass, is humid and irregular in size, cannot be stored in its place of origin, and is very expensive to transport. For these reasons, challenges related to the logistics and supply chain management of biomass feedstock collection, transportation, and storage need to be addressed to ensure a reliable and cost-effective biomass supply [202,278]. Technological advancements in biomass pretreatment, enzymatic hydrolysis, fermentation, and thermochemical conversion processes have been crucial in improving the overall efficiency and cost-effectiveness of lignocellulosic biomass conversion [75,279,280]. Integration of bioenergy production with other industries, such as pulp and paper or bio-refineries, can lead to synergies and value chain optimization, enhancing the economic viability of biomass valorization [281,282].

Environmental considerations play a crucial role in the evaluation of the energetic valorization of lignocellulosic biomass. Biomass-derived energy has the potential to reduce greenhouse gas emissions compared to fossil fuels, contributing to climate change mitigation and improved air quality [8,283]. However, the overall environmental performance depends on factors such as biomass sourcing, production processes, and waste management strategies [180,283,284]. Sustainable sourcing of biomass feedstock, including responsible land use practices and biodiversity preservation, is essential to ensure that biomass utilization does not have adverse impacts on ecosystems [285,286]. The selection of conversion technologies that minimize emissions and waste generation, as well as the proper management of by-products and residues, are key considerations for environmental sustainability [283,287,288].

In their studies, Liu et al. developed a new framework to accurately assess the climate change impacts of biomass utilization. They found that second-generation biofuels, including biofuels derived from logging residues, wood, and wood waste, resulted in significantly reduced total greenhouse gas (GHG) emissions compared to fossil fuels. This reduction can amount to approximately 50% of CO₂ emissions compared to fossil fuels [289].

Hsu [290] investigated GHG emissions from biomass-based pyrolysis oil and demonstrated that greenhouse gas emissions could be reduced by approximately 50% when using pyrolyzed biofuels instead of fossil fuels.

According to Wang et al. (2020) [276] and Steele et al. [291], the global warming potential for pyrolysis bio-oil production is reported to be 30.85 kg CO₂ eq and 32 kg CO₂ eq, respectively. These findings confirm the promising potential for commercially converting biomass into fuels.

Life cycle assessment (LCA) is a valuable tool for assessing the environmental impact of lignocellulosic biomass utilization. LCA studies enable a comprehensive evaluation of the entire life cycle, from biomass production and harvesting to energy conversion and end-use applications. They provide insights into environmental hotspots, resource consumption, emissions, and potential environmental trade-offs, allowing for informed decision-making and process optimization [292,293]. By considering the entire value chain and identifying opportunities for improvement, LCA studies contribute to the development of sustainable bioenergy systems that minimize environmental burdens [292,293,294,295].

To ensure the economic and environmental viability of the energetic valorization of lignocellulosic biomass, a holistic approach is required, integrating technological advancements, policy support, and stakeholder engagement. Research and development efforts should focus on improving biomass conversion technologies, optimizing process integration, and reducing costs via innovation and economies of scale [208,285,296,297]. Government policies and incentives that promote the utilization of biomass for energy purposes, such as feed-in tariffs and renewable energy targets, can stimulate investment and market development [203,298]. Collaboration between industry, academia, and policymakers is crucial to address technical, economic, and environmental challenges and foster the transition toward a sustainable bioenergy sector.

In conclusion, the energetic valorization of lignocellulosic biomass offers significant potential for sustainable energy production. Economic considerations, including feedstock availability and processing costs, must be carefully evaluated to ensure the viability of biomass conversion technologies. Environmental considerations, such as greenhouse gas emissions and resource utilization, should be addressed via responsible biomass sourcing, efficient conversion processes, and proper waste management. The integration of economic and environmental aspects, supported by technological advancements and policy frameworks, will pave the way for a sustainable and low-carbon bioenergy sector.

Life cycle assessment (LCA) studies focusing on bioenergy production from lignocellulosic biomass have provided valuable insights into the environmental impacts associated with different stages of the production process. These studies have identified specific impact categories that are significantly affected by the utilization of lignocellulosic biomass for bioenergy production. Among the impact categories commonly assessed, greenhouse gas emissions and fossil fuel depletion are often found to be substantially influenced by bioenergy production from lignocellulosic biomass [295,299]. This is primarily due to the displacement of fossil fuels with renewable biomass feedstock. For example, a study by Cherubini and Strømman (2011) evaluated the LCA of bioenergy production from lignocellulosic biomass and found that the substitution of fossil fuels with biomass feedstock led to a significant reduction in greenhouse gas emissions, contributing to climate change mitigation. Additionally, the study highlighted that the use of lignocellulosic biomass can reduce the depletion of fossil fuel resources, as the biomass feedstock is renewable and can be sustainably managed [300].

Other impact categories, such as eutrophication, acidification, and land use, also exhibit varying degrees of influence depending on the specific biomass utilization strategies and management practices employed. Guo, Song, and Buhain (2018) conducted an LCA of bioethanol production from corn stover and lignocellulosic biomass and found that the eutrophication potential was influenced by the agricultural practices associated with the production of the biomass feedstock. They noted that proper nutrient management and the adoption of sustainable agricultural practices can minimize eutrophication impacts [301].

Furthermore, the choice of land use for biomass cultivation and the potential impacts on biodiversity and ecosystem services have been highlighted in several studies [302,303]. Evaluating the environmental impacts of land use change and the preservation of natural habitats are crucial aspects in assessing the sustainability of lignocellulosic biomass utilization for bioenergy production [303,304].

Scientific studies have indicated that certain strategies for lignocellulosic biomass utilization are more viable and sustainable than others. Integrated biorefinery concepts, where various products are derived from different components of the biomass, have been shown to maximize resource efficiency and reduce overall environmental impacts. Cherubini and Strømman (2011) highlighted the potential of integrated biorefineries in their study, which demonstrated the simultaneous production of biofuels, bio-based chemicals, and bio-based materials from lignocellulosic biomass. This approach maximizes the value extracted from the biomass feedstock and contributes to the development of a more sustainable bioeconomy [300].

Zhang et al. report the improvement in greenhouse gas emissions in the production of bioethanol from sweet potatoes. They also report that the next step on the road towards a totally sustainable bioethanol production is to improve energy efficiency and environmental benefits during the cultivation unit [305].

Furthermore, the utilization of agricultural residues and dedicated energy crops as feedstock for bioenergy production has demonstrated favorable sustainability characteristics. Song, Guo, and Zhang (2019) [306] conducted an LCA and techno-economic analysis of lignocellulosic ethanol production from corn stover and emphasized the advantages of utilizing agricultural residues as feedstock. They highlighted that agricultural residues, such as corn stover, leverage existing agricultural practices and avoid potential competition with food crops, making them a more sustainable feedstock option. Additionally, the study highlighted the importance of advanced conversion technologies, such as biochemical and thermochemical processes, in enhancing the efficiency and environmental performance of lignocellulosic biomass utilization for bioenergy production [306]. Similar results have been obtained by other scientists. Roy P.’s investigation showed that GHG emissions and the production cost of ethanol are dependent on feedstock, conversion technologies, system boundaries, allocation methods, and the utilization of byproducts. The LCA study also confirmed that both technological pathways are environmentally and economically viable. Although the results of this study indicate that similar benefits can be gained, they seem to be inclined toward the gasification-biosynthesis pathway. Biotechnological advances, especially in enzyme production, would improve the viability of the enzymatic hydrolysis process [307].

Gerrior et al., in their life cycle analysis, highlighted the importance of energy efficiency in bioethanol production and the recovery and exploitation of byproducts to make corn ethanol refineries economically sustainable and commercially competitive [308].

In conclusion, LCA studies provide valuable insights into the environmental impacts of bioenergy production from lignocellulosic biomass. Greenhouse gas emissions and fossil fuel depletion are major impact categories affected by the utilization of lignocellulosic biomass. Strategies such as integrated biorefineries, utilization of agricultural residues, and advanced conversion technologies have been shown to be more viable and sustainable. However, it is crucial to consider the specific context and local conditions when assessing the sustainability of lignocellulosic biomass utilization for bioenergy production.