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Systematic Review

Valorization of Lignocellulosic Biomass to Biofuel: A Systematic Review

by
Mbuyu Germain Ntunka
1,*,
Siphesihle Mangena Khumalo
1,
Thobeka Pearl Makhathini
2,
Sphesihle Mtsweni
2,
Marc Mulamba Tshibangu
2 and
Joseph Kapuku Bwapwa
3
1
Department of Chemical Engineering, Faculty of Engineering & The Built Environment, Durban University of Technology, Durban 4000, South Africa
2
Department of Chemical Engineering, Mangosuthu University of Technology, Jacobs, Durban 4026, South Africa
3
Department of Civil Engineering, Mangosuthu University of Technology, Jacobs, Durban 4026, South Africa
*
Author to whom correspondence should be addressed.
ChemEngineering 2025, 9(3), 58; https://doi.org/10.3390/chemengineering9030058
Submission received: 22 March 2025 / Revised: 22 May 2025 / Accepted: 25 May 2025 / Published: 29 May 2025
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Abstract

Lignocellulosic biomass, derived from plant materials, represents a renewable alternative to fossil fuels and plays a crucial role in advancing environmental sustainability. This systematic review investigates recent developments in the conversion of lignocellulosic biomass into biofuels, with a focus on pre-treatment technologies that enhance enzymatic hydrolysis, a critical step in efficient biofuel production. This review addresses two primary questions: (1) What are the most effective pre-treatment methods for enhancing enzymatic hydrolysis in lignocellulosic biomass conversion? (2) How do these pre-treatment methods compare in terms of efficiency, environmental impact, and economic feasibility? Consequently, studies were selected based on inclusion criteria that focus on research investigating these pre-treatment methods and their comparative performance. A structured search of original studies was applied across databases such as Crossref, Google Scholar, Scopus, PubMed, and Semantic Scholar, resulting in the inclusion of 17 peer-reviewed articles published between 2019 and 2024. The findings highlight effective pre-treatment methods that significantly improve enzymatic accessibility and bioethanol yields. However, ongoing challenges such as feedstock variability, process efficiency, and cost-effectiveness remain. These results highlight the need for further research and development to optimize conversion technologies and identify new areas for exploration.

Graphical Abstract

1. Introduction

Lignocellulosic biomass (LCB), derived from plant materials such as agricultural residues and forestry waste, represents a promising renewable resource for sustainable energy production. Through biochemical and thermochemical processes, LCB can be converted into biofuels [1], providing a sustainable alternative to fossil fuels and contributing to effective agricultural waste management [2]. In addition to its environmental benefits, the utilization of LCB can boost rural economic development by creating employment opportunities, reducing dependence on imported fossil fuels, and improving energy security. For example, Brazil’s ethanol production from sugarcane biomass has successfully reduced its reliance on imported oil while generating thousands of rural jobs [3].
Unlike first-generation biofuels, LCB does not compete with food supply, as it is derived from non-edible plant parts. This characteristic makes LCB a more sustainable and environmentally friendly alternative [4]. Its use contributes to lower carbon emissions, supports efforts to mitigate climate change, and enhances energy independence by diversifying fuel sources [5]. As a carbon-neutral and abundant resource, LCB plays a key role in the transition to cleaner energy systems by replacing fossil fuels, which are major contributors to environmental degradation [6]. Table 1 summarizes common types of LCB and their regional availability.
The conversion of LCB into biofuels and other biomaterials presents a promising solution to growing global energy demands. This transition is facilitated by various technological methods designed to improve the efficiency of biomass to product conversion.

1.1. Conversion of LCB to Biofuel

The conversion of LCB into biofuels involves several key stages, each employing specific technologies designed to maximize efficiency and sustainability. A range of conversion technologies, including thermochemical, biochemical, and chemical methods, are used to transform LCB into energy and value-added products [13]. Among these key stages, pre-treatment plays a crucial role in reducing the inherent recalcitrance of biomass, thereby improving enzymatic hydrolysis and enhancing fermentation processes [14,15]. Pre-treatment techniques include physical, chemical, and biological methods, each with distinct advantages and limitations depending on the feedstock and processing goals. Following pre-treatment, enzymatic hydrolysis plays a crucial role in converting cellulose and hemicellulose into fermentable sugars through the action of specific enzymes, which are fermented into biofuels. The final step, distillation, purifies the biofuel to meet the required quality standard [8,16]. Overall, developing and optimizing these technologies is essential to fully harness the potential of LCB as a sustainable and renewable energy source.

1.2. Challenges and Opportunities

Despite its potential, the complex structure of LCB poses challenges in achieving high yields and selectivity for target products [17]. Continued research and optimization of biorefinery processes are necessary to exploit the potential of LCB fully [15]. Additionally, the scale-up of biorefinery processes presents logistical and financial challenges that must be addressed to make these technologies more commercially viable. However, with technological advancements and ongoing research efforts, LCB has excellent potential to play a significant role in the transition to a more sustainable and circular economy. By overcoming these challenges, biorefinery processes can help reduce the dependence on fossil fuels and create a more environmentally friendly and economically viable future.
LCB production is crucial for achieving sustainable energy goals, as it is a more environmentally friendly option and contributes to energy security, reducing dependence on fossil fuels and mitigating climate change by lowering carbon emissions [4,5]. However, significant challenges remain in converting LCB into biofuels. The complex structure of LCB hinders its efficient breakdown, while by-products generated during pre-treatment and hydrolysis can inhibit fermentation and enzymatic activity. High-energy demand and chemical-intensive pre-treatment methods such as steam explosion further complicate the process. In addition, these methods raise environmental concerns, including hazardous waste generation and the need for extensive wastewater treatment [18,19,20,21].
Table 2 summarizes recent review papers against this systematic review regarding their main objectives and contribution to the body of knowledge. The gap that exists between them motivates this study.
As illustrated in Table 2, recent studies on the conversion of LCB to biofuels reveal several critical research gaps. These gaps include the limited investigation of pre-treatment methods, a lack of comparative analysis of conversion process efficiencies, inadequate integration of by-products into biorefinery systems, and an insufficient understanding of the effects of feedstock variability on biofuel yields. In addition, many studies fall short in incorporating comprehensive environmental and sustainability assessments. Synthesizing these gaps offers valuable insights and forms the basis for practical recommendations to guide future research. Collectively, these shortcomings underscore the need for a more integrated and holistic research approach, thereby providing rationale and motivation for the present study.

1.3. Objectives

This systematic review evaluates recent research on converting LCB into biofuels, with the aim of identifying the most effective pre-treatment and processing methods. It assesses how these techniques influence conversion efficiency and biofuel yield while highlighting the key challenges and limitations of current conversion technologies. In addition to technical performance, this review explores the environmental and economic impacts of LCB valorization, including reduced greenhouse gas emissions, potential land use changes, water consumption, and the cost competitiveness of lignocellulosic biofuels relative to fossil fuels.

2. Methodology

2.1. Protocol

A protocol was developed following the guidelines provided by Booth et al. [23] to enhance transparency and replicability, making this literature review systematic. The PRISMA framework was thereafter used to guide this review [24,25].

2.2. Integrative Search Approach

The search strategy involved pre-defined research questions and an integrated search through major electronic academic databases like Crossref, Google Scholar, Scopus, PubMed, and Semantic Scholar. English-language publications were included, and search keywords were derived from the research questions and synonyms. Considering the upward trend in biofuel research, the search string was defined for articles published between 2019 and 2024. Table 3 lists the identified keywords and their search terms.

2.3. Screening and Selection of Eligible Studies

The screening and selection of eligible studies began with a comprehensive search across various databases using a predefined search string, as outlined in Table 3 [26]. Duplicates were identified and removed through a manual review to ensure accuracy. The remaining studies were systematically screened based on the clearly defined inclusion and exclusion criteria shown in Figure 1.
The results of the screening process are summarized in Table 4 following the Preferred Reporting Items for Systematic Review and Meta-analyses [27] framework, with detailed records maintained for all excluded documents at each step to ensure transparency and reproducibility.

2.4. Quality Control

The initial search identified 4974 potential documents, of which 1794 were duplicates. After removing the duplicates, 3180 articles were systematically analyzed using an MS Excel spreadsheet, focusing on the pre-defined research questions. This review applied strict inclusion and exclusion criteria shown in Figure 2, excluding grey literature, government documents, industry manuals, and conference proceedings due to concerns about authenticity [28]. The research questions were central to the criteria to minimize bias, and authors were required to disclose any conflicts of interest as McDonagh et al. [29] recommended, ensuring this review’s reliability. Ultimately, 17 studies were selected for data extraction and analysis, which provides a comprehensive overview of the valorization of LCB to biofuel. Table 4 summarizes the number of articles retrieved from each database and the search terms used, while Figure 2 presents a PRISMA flow diagram detailing the study selection process.

2.5. Data Analysis

The analysis of 17 research articles focused on improving the enzymatic hydrolysis process for LCB conversion into biofuel. Different biochemical conversion pathways were compared for efficiency, environmental, and economic impact. Comparing ethanol and butanol production helped determine the best balance of efficiency, sustainability, and cost-effectiveness, comprehensively responding to the research question.

3. Results

3.1. Overview of Included Studies

This study reviews various pre-treatment techniques for LCB, focusing on bioethanol yield and delignification rate. Bioethanol yield is a crucial metric for assessing the efficiency of the entire bioethanol production process, including pre-treatment [30,31].
The results in Table 5 show varying degrees of effectiveness and delignification rates in breaking down biomass for further processing. Some methods, like ethanol-assisted deep eutectic solvent, are more efficient in ethanol production per gram of glucose. A study that compared the pre-treatment methods of steam explosion and organosolv found that steam explosion was more efficient and delignified than organosolv. This analysis suggests that steam explosion may be a better way to break down LCB for biofuel production.
Although Table 5 summarizes the findings of various biomass pre-treatment studies, it also shows several critical data gaps that limit its holistic utility for direct comparative analysis. Some studies focus exclusively on pre-treatment efficiency or lignin removal without proceeding to fermentation steps [32,33,34,35,36]. As a result, they do not report bioethanol yield, making it difficult to assess the overall process efficiency from biomass to biofuel. Furthermore, in some studies, the degree of delignification is either qualitatively described or not reported at all [37,38]. This shortcoming may be due to a focus on other performance parameters such as enzymatic digestibility or sugar release, which are often used as indirect indicators of lignin removal. Moreover, some data are presented using different units (e.g., percentage vs. mg/g) or under different experimental conditions (e.g., varying biomass types, temperatures, or timeframes), making standardization difficult. In some cases, the absence of experimental details prevents meaningful interpretation. Furthermore, many studies lack key information such as solid recovery, sugar conversion efficiency, or inhibitor formation, which are crucial for a full techno-economic or environmental assessment.
Life cycle assessment (LCA) was conducted for each biomass pre-treatment method, evaluating both cost-effectiveness and sustainability. The summarized results are presented in Table 6. Notably, the LCA served as a critical tool for assessing the environmental impact and economic feasibility of each pre-treatment approach. From the results presented in Table 6, it is apparent that the deep eutectic solvent method has been demonstrated to be promising due to its cost-effectiveness and sustainability. This method uses ethanol as a renewable solvent and significantly reduces the use of harsh chemicals, aligning with green chemistry principles. The results suggest that deep eutectic solvents offer a sustainable pathway for biomass valorization, enhancing the environmental friendliness of the conversion process and making them a promising option for future applications.
Table 5. Summary of key findings on pre-treatment effectiveness and delignification rates.
Table 5. Summary of key findings on pre-treatment effectiveness and delignification rates.
Pre-Treatment Method TypeDelignification Rate (%)Bioethanol YieldReference
Deep eutectic solvent90.458.8 g per 100 g[39]
Organosolv49.741.4 g/L[40]
Sequential alkaline extraction62N/A[41]
Hydrothermal94.60.48 g/g[42]
Steam explosion63.9N/A[43]
Dilute-acid hydrolysisN/A0.47 g of ethanol per gram of glucose[44]
Deep eutectic solvent78.88N/A[45]
Ternary deep eutectic solvent55.81N/A[46]
OrganosolvN/A28.7 g/L[47]
Ethanol (20 wt%)-assisted deep eutectic solvent64–69N/A[48]
Table 6. Cost-effectiveness and sustainability of the selected pre-treatment processes.
Table 6. Cost-effectiveness and sustainability of the selected pre-treatment processes.
Pre-Treatment ProcessCost-EffectivenessSustainabilityReference
Ball millingLow-cost availability and efficient conversion processes.Remarkable renewable nature, reduced greenhouse gas emissions, and promotes circular economy practices.[35]
Gluconic acidUse of low-cost Cu-biochar catalysts lowering production costs significantly.Offers a green alternative to traditional catalysts.[36]
Xylonic acidCan be cost-effective when optimized.Reduced reliance on harmful chemicals, supporting a more circular economy.[37]
Dilute-acid hydrolysisEconomically viable for large-scale operations.Minimizes environmental impacts by lowering the overall carbon footprint when compared to conventional processes.[44]
Deep eutectic solventOffer a cost-effective solution for biomass pre-treatment by being derived from renewables.Eco-friendly and biocompatible, making them an attractive alternative to traditional toxic solvents.[49]
OrganosolvCost-effective due to the use of relatively low-cost solvents.More sustainable pre-treatment method because it uses environmentally friendly solvents.[38]
Ternary deep eutectic solventCost-effective because they can be synthesized from inexpensive, biodegradable components.Ternary DESs are sustainable due to their environmentally friendly nature.[46]
OrganosolvProvides cost-effectiveness by using readily available solvents and offering efficient delignification.Pre-treatment is favorable as it generates fewer toxic byproducts compared to other methods, making it an environmentally friendly option for biofuel production.[47]
Ethanol (20wt%)-assisted deep eutectic solventThe use of this method demonstrates cost-effectiveness due to the low-cost, renewable nature of ethanol and the simple equipment required for processing.Sustainability is achieved by utilizing a green solvent system that reduces the need for harsh chemicals.[48]
Supramolecular deep eutectic solvent
(SUPRA-DES)		Potentially reduces the associated costs of waste disposal with its green approach.It aligns with the principles of sustainability and minimizes dependence on fossil fuels.[39]
Alkaline—16 wt% KOHOptimizes the enzymolysis process and maximizes the utilization of agricultural waste, providing an economically viable pathway for biomass valorization.Contributes to the circular economy and reduces reliance on fossil resources.[34]
FeCl3 catalyzation which increased the release of cellulaseIt can be cost-effective due to its low cost and efficiency in promoting enzymatic activity, but its economic viability depends on the scale of application and downstream process integration.The enzyme-undigestible residues were effectively used as bio-sorbents for cadmium adsorption, demonstrating a circular approach to biomass utilization.[33]
Steam explosion pre-treatment enhances the enzymatic digestibilityThe method is a clean and practical choice for producing biofuel on a big scale without requiring any chemical additions.The work promotes production of biofuel from commercial biofuel production LCB systems by demonstrating how steam explosion pre-treatment can improve the valorization of tiger nut biomass.[43]
Fermenting, hydrothermally pre-treating, and using enzyme hydrolysisIncrease in the value of agricultural waste by turning pineapple leaves known as waste and consequently lowering disposal expenses.Sustainability promoted by utilizing abundant pineapple leaf waste, and agricultural waste disposal is addressed while producing renewable energy.[50]
Organosolv-pretreated outer-tunic biomass in biofuel productionThe entire biomass is used, whilst biogas and fertilizer are produced from the inner portion of the tunic.Tunicates are a marine resource that does not compete with land utilized for food production, minimizes dependency on fossil fuels, and is a sustainable strategy.[40]
Hydrolysis of cellulose by alkaline extractionReduction in the cost of producing bioethanol by increasing the yields of fermentable sugars from ryegrass through the optimization of the extraction and hydrolysis process.Process viewed as a sustainable alternative to fossil fuels. By improving the pre-treatment processes and enzymatic conversion efficiencies, a more sustainable biofuel production system is promoted.[51]
Larvae fed hydrolyzed digestateAn alternative microbial pre-treatment, which uses naturally occurring organisms to boost nutrient availability without incurring the high expenses associated with enzymes, may be a more economical option.From a sustainability standpoint, employing BSF larvae as a technique to valorize anaerobic digestate has a dual benefit: lowering waste and potentially transforming it into high-value biomass for animal feed or other purposes.[32]

3.2. Pre-Treatment Technologies

Pre-treatment methods for LCB are crucial for efficient bioethanol production, as they break down the complex structure of biomass and enhance enzymatic digestibility. This process increases fuel yield by improving the accessibility of cellulose and hemicellulose in biomass. Investing in research and developing efficient pre-treatment methods is essential for advancing bioethanol production technology. The primary objectives of pre-treatment are the extraction of hemicellulose and/or lignin and the reduction of biomass particle sizes.
According to Rezania et al. [52], pre-treatment methods can be broadly categorized into physical, chemical, and biological methods, each with distinct mechanisms and outcomes. The selection of an appropriate pre-treatment method depends on the type of biomass and the specific requirements of subsequent processing steps. In addition, it is crucial for optimizing bioethanol production efficiency while considering the specific characteristics of the feedstock and the overall process design.

3.2.1. Physical Pre-Treatment

Pineapple leaf waste, which is rich in lignocellulose content, holds considerable potential for bioethanol production and other value-added products. With over 91% fermentation efficiency, pre-treatment using hydrothermal heat followed by enzymatic hydrolysis and fermentation has shown promising results [50]. In Costa Rica, a simple mechanical extraction and fermentation was estimated to produce 2.1 tonnes of bioethanol, 1.55 tonnes of spent yeast biomass, and 11.65 tonnes of dry fibrous material annually [53].
Steam explosion is another effective physical treatment method capable of processing various LCB. It enhances cellulose accessibility, making the biomass 2.5–4.2 times more amenable to enzymatic hydrolysis, particularly in tuber-based feedstock [43]. In tiger nuts, steam explosion increased cellulose accessibility and oil yield by 8.72–18.85%. It has also been shown to improve the quality of other LCBs like aspen wood and sugarcane trash [54]. The most vigorous treatment, 214 °C for 10 min, produced 83.9 L CH4/kg and a glucan conversion of 45.2%, highlighting the method’s effectiveness across various biomass types [55].
Milling techniques are widely used to reduce the size and disrupt the crystalline structure of lignocellulosic materials, enhancing enzymatic digestibility. Among these, ball milling is recognized as an effective pre-treatment method. It facilitates enzymatic breakdown and supports acid-catalyzed alcoholysis by reducing structural resistance [35]. Intermittent ball milling has shown promise for achieving high glucose yields in shorter processing times and with reduced solvent usage [56]. Understanding the connections between structure, property, and degradability is crucial for improving the cost-effectiveness and the practicality of biorefinery processes [35,57].

3.2.2. Chemical Pre-Treatment

Chemical pre-treatment methods, such as acid hydrolysis, alkaline treatment, and the use of deep eutectic solvents (DESs), are widely used to break down lignin and hemicellulose structures, enhancing cellulose accessibility for enzymatic hydrolysis. Out of the 17 selected studies, 7 employed chemical methods as a core pre-treatment strategy [58].
Recent studies have increasingly focused on DES as a sustainable alternative to conventional chemical treatments. For example, Tan et al. [39] found that supramolecular DES mixed with β-cyclodextrin effectively removes lignin while preserving its structure. Similarly, a ternary DES containing ethylene glycol enables efficient lignin separation and retention, which supports high yields of fermentable sugars and microbial lipids [59]. Another DES formulation, composed of choline chloride, boric acid, and polyethylene glycol-200, which is neutral, successfully removes both lignin and hemicellulose while keeping the structure of cellulose [60].
Alkaline pre-treatment using sodium hydroxide (NaOH) or potassium (KOH) has proven effective in enhancing biomass biodegradability. NaOH pre-treatment (0.5–10%) removes lignin and releases cellulose, while KOH pre-treatment improves kinetics [61,62]. Compared to acid pre-treatment, alkaline pre-treatment can yield up to 240% more biogas [63], with concentration alkaline pre-treatment achieving biogas yield increases of 21.5% [64]. However, increased phenol concentrations released during alkaline treatment may inhibit anaerobic digestion.
Sequential alkaline extraction techniques have shown promise for cocksfoot grass, and ryegrass by effectively separating hemicellulose and enhancing cellulose conversion. These extractions break down the lignin structure and can release up to 89.4% of the original lignin from the cell walls of ryegrass [65,66]. Combined with ultrasonic and hydrothermal pre-treatments, sequential alkali post-extractions have been able to recover more than 90% of the original hemicelluloses, which significantly improves biorefinery efficiency [66].
Innovative chemical strategies are also being explored to increase the overall utility of LCB. FeCl3-based treatments have been shown to enhance cellulase release, boost ethanol yield, and transform corn stalks into valuable materials like biosorbents and supercapacitors [33,67]. Additionally, studies by Wu et al. [68] proposed a corncob biorefinery using metal chlorides as catalysts to co-produce platform chemicals and lignin.
Eco-friendly chemical treatment methods are also being developed to produce xylooligosaccharides (XOSs) and fermentable sugars from LCB. Methods using xylonic acid [41], glutamic acid [53], and acetic acid with poly (ethylene glycol) ether-assisted alkali treatment [68], as well as mechanical refining [69], have enhanced XOS yields and improved enzymatic digestibility. These methods support the conversion of by-products like xylose into valuable compounds, making LCB more useful in biorefineries producing multiple products.
In addition, DES has shown promise in delignifying oil palm empty fruit bunches and producing valuable chemicals. A ternary DES consisting of choline chloride, oxalic acid, and ethylene glycol delignified 55.81% of fibers and produced furfural [46], outperforming other choline chloride-based DES formulations [69]. Delignification efficiency is temperature-dependent with aqueous DES yielding up to 38.53% furfural, and their recyclability further enhances economic feasibility.
Novel methods such as alkaline-assisted ohmic heating (AA-OH) are emerging. AA-OH simultaneously reduces lignin and hemicellulose content while increasing cellulose accessibility, which represents a promising direction for future pre-treatment development.

3.2.3. Biological Pre-Treatment

Biological pre-treatment methods use microorganisms or microbial enzymes to degrade lignin and hemicellulose, thereby improving the digestibility of LCB. These eco-friendly processes are slower than chemical methods [58]. In this study, seven peer-reviewed articles focus on biological treatment.
Microbial strains such as hyper-cellulase and hyper-xylanase-producing cultures have demonstrated significant potential for enhancing lignocellulosic breakdown. One promising biological system is the black soldier fly (BSF) larvae, which efficiently process a range of organic waste streams, including biogas digestate, food waste, and chicken manure [32,70]. They can significantly reduce waste volume, with reported reductions ranging from 37 to 80% [32,70]. The larval biomass, rich in proteins and lipids, can be used as a substitute for fishmeal in animal feed and as a soil amendment or fertilizer [71]. However, some substrates, like untreated digestate, may require pre-treatment to improve larval growth and biomass production. BSF larvae offer a sustainable solution for organic waste management and resource recovery [32].
Saccharomyces cerevisiae has received considerable attention for its role in metabolic engineered biofuel production. Through techniques such as pathway blocking, enzyme overexpression, and carbon flux optimization, this yeast can produce more bioethanol and biobutanol [72]. Fatty acid-derived biofuels, which offer high-energy density are also being explored [73]. Although evolutionary engineering and combined bioprocessing show promise for cellulosic ethanol production [74], challenges remain, including incomplete substrate catabolism, low heterologous protein expression, and inhibitor accumulation [75]. Current research aims to address these limitations and improve production efficiency.
Recent research focuses on transforming corncob residues into high-value nanomaterials [34]. Enzymatic hydrolysis combined with DESs and high-pressure homogenization has been used to produce lignin-containing cellulose nanofibrils (LCNFs) and lignin nanoparticles [76]. Microwave- and ultrasound-assisted techniques can further improve the extraction of cellulose and hemicellulose from corncob. At the same time, a three-step fractionation method can separate xylooligosaccharides, nanolignin, and nanocellulose, demonstrating the potential of biomass utilization [77].
Microbial valorization presents another sustainable approach to waste management, converting several types of waste into valuable products through fermentation [78]. For instance, waste cooking oils serve as feedstock for the microbial synthesis of biopolymers, biosurfactants, and lipases [79]. Pseudomonas putida has shown promise in valorizing synthetic wastes like plastics, oils, and agricultural waste into platform chemicals and biopolymers [80]. This outcome could lead to multi-product biorefineries using lignocellulosic and plastic wastes [81]. However, key challenges remain, such as waste pre-treatment efficiency, microbial cell factory capabilities, and the practicality of synthetic biology tools [80].
Research has explored innovative corncob residue (CCR) methods for high-concentration fermentable sugars and valuable products. Higher enzyme-based methods, such as corncob residue (PFI) homogenization, have yielded glucose levels up to 187.1 g/L [36]. Pre-treatment with tetrahydrofuran with water or potassium hydroxide has produced high glucose yields of 498.2 mg/g and 91%, respectively [82,83]. These processes also enable the production of lignin-based bio-adhesives and fertilizers, promoting sustainable biorefinery concepts [35,83]. Moreover, metal chloride pre-hydrolysis in two-phase systems can produce key platform chemicals like glucose, xylose, and furfural [84].
Fungal biodegradation has also been proposed as a long-term, sustainable solution for LCB valorization. White-rot fungi species like Trametes versicolor produce enzymes that can break down lignin, cellulose, and hemicellulose [85]. These enzymes have various industrial applications, including biofuel production, wastewater treatment, and biotransformation [86]. In addition, mushroom cultivation on agro-industrial waste offers a low-energy, cost-effective, eco-friendly approach to biomass reuse [86,87]. While fungal biodegradation shows potential to create bio-based products and contribute to a circular bio-economy, further research is needed to optimize and scale up these processes [88,89].

3.2.4. Combined Pre-Treatment Methods

While single pre-treatment methods have been widely studied, combining various pre-treatment methods often yields superior results. Combined pre-treatment techniques can significantly enhance the breakdown of biomass components, thereby improving efficiency.
Combining physical and chemical methods is one of the most widely synergistic approaches. Physical methods such as milling or extrusion increase the surface area and reduce particle size, which facilitates chemical reaction. When combined with chemicals like alkalis or acids, these techniques allow for more efficient cellulose exposure, thereby improving enzyme cellulose accessibility [90,91].
Studies have shown that this combination may improve biomass digestibility compared to using either method alone. Recent research has highlighted promising techniques like microwave-assisted pre-treatment combined with alkali for improved enzymatic hydrolysis [92], which is advantageous for industrial-scale applications. However, energy demands and the potential formation of inhibitor by-products pose challenges to the sustainability of this approach [93].
The combination of physical and biological pre-treatments offers an eco-friendly solution by leveraging the natural enzymatic activity of microorganisms to degrade lignin and hemicellulose. Physical methods like milling or steam explosion break the biomass structure, while biological treatments such as fungal or bacterial action target lignin and hemicellulose degradation. In addition, this approach reduces the need for harsh chemicals and is considered more sustainable. Recent studies have shown the potential of combining steam explosion with fungal pre-treatment, which improves the degradation of lignin and increases the availability of cellulose. Therefore, biological treatment reduces the need for chemicals and lowers the environmental impact of pre-treatment processes.
Additionally, microbial inoculation post-physical pre-treatment has shown promising results in enhancing biomass digestibility [94]. One major limitation of physical–biological pre-treatment is the slower processing time, as biological treatments can be time-consuming. However, the variability in microbial efficacy and the need for controlled conditions make scaling up challenging. Research is therefore needed to improve the consistency and efficiency of biological agents in industrial settings [93].
Chemical–biological pre-treatment combines the immediate effects of chemical treatments with the long-term advantages of biological degradation. Chemicals like alkalis, acids or oxidants break down lignin and hemicellulose, followed by biological agents that further degrade the remaining biomass. This two-step approach enhances both delignification and the enzymatic digestibility of cellulose. Chemical treatments prepare the biomass for the biological agents, thus allowing for more efficient enzymatic hydrolysis. Recent developments in using weak alkaline and acidic deep eutectic solvents (DESs) followed by fungal treatments have shown significant improvements in lignin removal and enzymatic hydrolysis efficiency. DES-based pre-treatments, when combined with biological degradation, have achieved high levels of delignification and improved sugar yields while offering a sustainable and efficient solution for LCB pre-treatment [35,45,95]. Chemical–biological combinations can face challenges such as the excessive cost of chemicals and the longer time required for biological treatment. Furthermore, the toxic by-products generated by chemical treatments may inhibit microbial growth during the biological stage. Therefore, more research is needed to optimize the interaction between chemical and biological agents to mitigate these issues [92].
Acid–oxidant pre-treatment uses acid and oxidizing agents to break down lignin and hemicellulose. Acidic conditions facilitate hemicellulose hydrolysis, while oxidants help in lignin degradation. This then increases the enzymatic digestibility of cellulose. Studies have shown that using 4% NaOH and acid oxidants results in high lignin removal and improved glucose yields. However, this method may yield toxic by-products and requires careful control of oxidative conditions. The alkali–oxidant combination has been shown to significantly reduce lignin content and enhance the efficiency of subsequent enzymatic hydrolysis. The alkali treatment disrupts lignin and hemicellulose structures, while the oxidants further degrade these components, improving cellulose accessibility. This method has demonstrated satisfactory performance in increasing glucose yields but faces similar issues with oxidants’ excessive cost and potential toxicity [96].
AA-OH is a novel method that uses electric currents to generate heat, significantly reducing lignin and hemicellulose content while increasing cellulose availability [96]. Ab Aziz et al. [96] say that using 4% NaOH at 120 °C for 25 min is the best way to make AA-OH work because it reduces lignin by 86.9%. Researchers are exploring new methods to separate oil palm fronds for efficient biomass fractionation. One approach involves adding 20% ethanol to DES, which yields 85.8% glucose while maintaining lignin β-O-4 content [48]. With the addition of ultrasound and alkaline pre-treatment, delignification increases to 47% and glucose yield to 90.12% [97]. Alkaline hydrogen peroxide and Type II DES can remove 55.14% of lignin and hemicellulose [82]. These methods offer promising alternatives for efficient biomass fractionation, potentially improving biofuel production and lignin valorization.
More research remains necessary to develop pre-treatment strategies that are more environmentally friendly and cost-effective for biorefinery applications, for example. This can also impact bio-based circular economic growth [90,98]. LCB pre-treatment is also crucial for biorefinery applications, including addressing the growing demand for sustainable energy [94]. Emerging trends in using deep eutectic solvents (DESs) offer promising options for sustainable biomass conversion, particularly with the ability to dissolve lignocellulosic components and increase cellulose accessibility. When combined with acids or alkalis, DESs provide an efficient method for biomass pre-treatment, particularly for corn and wheat straw [34]. Acidic DESs remove significant amounts of lignin, while alkali–DES combinations enhance cellulose hydrolysis. These methods are seen as cost-effective and environmentally friendly and with promising results in both lignin removal and enzymatic hydrolysis efficiency [45]. The use of DESs, especially in combination with acids or alkalis, is an emerging trend in biomass pre-treatment. These methods are green, cost-effective, and may offer high efficiency in biomass fractionation. However, DESs can be expensive to produce, whereas the recyclability of specific DES formulations still requires further investigation. Additionally, forming by-products during pre-treatment remains challenging as these may inhibit downstream processes [90].

3.3. Economic and Environmental Assessment

The choice of pre-treatment techniques significantly impacts the economic viability and environmental sustainability of LCB conversion. These three methods—physical, chemical, and biological—are crucial for breaking down the complicated structure of LCB, making the following conversion processes more effective. Each method has distinct impacts on both economic and environmental aspects, which are essential for optimizing biofuel production and ensuring sustainable practices.

3.3.1. Physical Pre-Treatment Methods

Physical methods, such as milling and ultrasonication are energy-intensive, which can increase operational costs. However, they are often combined with other methods to improve efficiency. For instance, integrating physical methods with chemical pre-treatments can enhance biomass breakdown, leading to higher yields of fermentable sugars [90].
Physical pre-treatments have a lower environmental impact than chemical methods, as they do not involve hazardous chemicals. However, the high energy requirement can offset some of these benefits if not managed sustainably [90].

3.3.2. Chemical Pre-Treatment Methods

Chemical pre-treatments, such as alkaline and acid treatments, are effective in delignification and increasing cellulose accessibility. For example, alkaline peroxide pre-treatment has been shown to have the lowest energy consumption and waste generation, making it economically favorable [91]. However, the cost of chemicals and the need for corrosion-resistant equipment can increase capital and operational expenses [99].
Chemical methods can pose environmental risks due to the generation of toxic by-products and the need for waste management systems. However, new developments in green chemistry, like AA-OH, are being investigated to lessen these effects by using fewer chemicals and making the process more efficient [96].

3.3.3. Biological Pre-Treatment Methods

Biological pre-treatments, which use microorganisms to degrade lignin, are less costly regarding energy and equipment. They can be integrated with other methods to improve overall process efficiency and yield [100]. However, the longer processing times required for biological methods can be a drawback regarding throughput [47].
These methods are considered more environmentally friendly as they produce fewer harmful by-products and utilize renewable biological agents. For instance, fungi use has been shown to effectively reduce lignin content while maintaining a high yield of glucose, which is crucial for bioethanol production [100].

3.3.4. Combined Pre-Treatment Approaches

Combining different pre-treatment methods can offer a balanced pathway to optimize economic and environmental outcomes. For example, integrating physical and chemical methods can enhance biomass breakdown while reducing the required energy and chemical inputs [90]. Such synergies are essential for developing cost-effective and sustainable biorefinery processes [15]. While each pre-treatment method has advantages and limitations, the choice often depends on the specific type of biomass and the desired end products. Developing novel and combined pre-treatment strategies continues to be a research focus, aiming to balance economic feasibility with environmental sustainability. As the demand for biofuels and bioproducts grows, optimizing these processes will be crucial for advancing the bio-economy and reducing the reliance on fossil fuels.

4. Discussion

This review has comprehensively examined pre-treatment methods and sustainable strategies to enhance bioethanol production from LCB. As LCB is inherently resistant to enzymatic degradation due to its complex matrix of cellulose, hemicellulose, and lignin, effective pre-treatment methods remain a critical step in improving biomass digestibility and ensuring efficient bioethanol yield. The efficacy of pre-treatment significantly influences the accessibility of fermentable sugars, and consequently, the overall productivity and sustainability of bioethanol conversion.
Physical pre-treatment methods, such as milling and grinding, play a foundational role by reducing particle size and increasing surface area. These physical modifications enhance porosity and reduce cellulose crystallinity, which promotes efficient enzymatic hydrolysis. Although physical methods are less chemically invasive, they often require substantial energy input, which can impact process economics [46].
Chemical pre-treatment methods, including acid hydrolysis and alkaline treatments, are particularly effective in lignin and hemicellulose removal, thereby exposing cellulose for enzymatic digestion. Emerging solvents, such as DES, offer a promising alternative to conventional chemical treatments. Ternary DESs comprising choline chloride, oxalic acid, and ethylene glycol have demonstrated significant delignification capabilities, with studies reporting up to 55.81% lignin removal [41,45]. This enhanced lignin dissolution not only facilitates enzymatic hydrolysis but also contributes to the recovery of value-added compounds.
Biological pre-treatment methods employ lignin-degrading microorganisms or their enzymes to modify biomass in a milder and more environmentally benign manner. This method supports circular bio-economy principles by utilizing biological waste streams and minimizing harmful by-products. However, the scalability of biological methods remains a concern due to longer treatment times and sensitivity to environmental variables [101].
Among the strategies discussed, DES-based pre-treatment has emerged as a particularly versatile and sustainable option. Its efficiency in lignin removal, tunability, and low toxicity make it a valuable alternative to harsher chemical treatments. Moreover, the integration of DES with elevated temperatures has been shown to enhance delignification outcomes, thereby increasing cellulose accessibility and improving subsequent fermentation processes [15,45]. Despite these advantages, challenges such as the excessive cost of certain DES components and limited recyclability must be addressed through further research and processes.
In addition to bioethanol, DES pre-treatment has shown potential in valorizing lignocellulosic residues by enabling the production of chemicals like furfural. These advancements broaden the scope of biomass use beyond energy production, aligning with the goals of integrated biorefineries. Future investigations should explore the application of DESs across various biomass types, such as wheat straw, sugarcane bagasse, and oil palm fronds, to evaluate their effectiveness and economic viability in diverse feedstocks. Incorporating alkaline agents such as NaOH or KOH in conjunction with DES may further enhance biodegradability and improve yields of both bioethanol and biogas [38].
Despite these promising advancements, several key challenges remain. The environmental impact of chemical usage, the energy intensity of physical methods, and the slow kinetics of biological treatments all pose limitations to large-scale implementation. Moreover, selecting pre-treatment methods that balance efficiency, cost, and sustainability is critical for commercial viability. Continued research must focus on optimizing process conditions, improving DES recyclability, and developing hybrid systems that synergistically combine physical, chemical, and biological approaches.

5. Conclusions and Way Forward

This review highlights the critical role of pre-treatment technologies in enhancing the efficiency and sustainability of LCB conversion to bioethanol. Various physical, chemical, biological, and combined methods have been examined, each offering unique advantages and limitations. Among the emerging strategies, DESs stand out as a promising green alternative due to their substantial effectiveness in lignin removal, while supporting environmentally friendly processing.
While remarkable progress has been made in developing efficient and scalable pre-treatment approaches, challenges related to energy consumption, chemical use, microbial performance, and overall process economics remain. Integrating multiple pre-treatment strategies, such as physical–chemical or chemical–biological combinations, can potentially overcome these barriers and improve process outcomes. However, further research is needed to improve such integrated systems for industrial applications.
Moving forward, the development of cost-effective, eco-friendly, and scalable pre-treatment technologies is vital to advancing lignocellulosic biorefineries. Such progress will not only improve bioethanol yields but also contribute to a circular bio-economy by enabling the co-production of bio-based chemicals and materials. Ultimately, innovative pre-treatment strategies are foundational to realizing the full potential of LCB in meeting future energy and sustainability goals.

Author Contributions

M.G.N. conceived and presented the idea. M.G.N., M.M.T., J.K.B., T.P.M., S.M.K. and S.M. developed the search strategy and screening protocol and searched different databases. All authors read and analyzed the data on the remaining papers and produced results. M.G.N., T.P.M. and S.M.K. drafted the paper with input from all authors. M.G.N. coordinated and supervised the manuscript writing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is available on request.

Acknowledgments

The authors acknowledge Mangosuthu University of Technology and Durban University of Technology for their valuable support.

Conflicts of Interest

The authors declare no competing financial interests or personal relationships that could influence the work presented in this review paper.

Abbreviations

The following abbreviations are used in this manuscript:
ASTMAmerican Society for Testing and Materials
LCBLignocellulosic biomass
LCALife cycle assessment
PRISMAPreferred Reporting Items for Systematic reviews and Meta-Analyses
DESDeep eutectic solvent
SUPRA-DESSupramolecular deep eutectic solvent
XOSXylooligosaccharide
AA-OHAlkaline-assisted ohmic heating
BSFBlack soldier fly
LCNFLignin nanoparticle
CCRCorncob residue
PFIPermeabilized fiber

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Chemengineering 09 00058 g001
Figure 1. The inclusion and exclusion criteria.
Figure 1. The inclusion and exclusion criteria.
Chemengineering 09 00058 g001
Chemengineering 09 00058 g002
Figure 2. PRISMA flow diagram describing the selection of studies for systematic review of valorization of LCB to biofuel.
Figure 2. PRISMA flow diagram describing the selection of studies for systematic review of valorization of LCB to biofuel.
Chemengineering 09 00058 g002
Table 1. Common types of LCB and their regional availability.
Table 1. Common types of LCB and their regional availability.
Type of BiomassExampleRegionRegion AvailabilityReference
Agricultural wastesCorn stoverUSAMidwestern states (corn belt)[7]
Sugarcane bagasseBrazilSão Paulo, Minas Gerais, and Paraná
Rice strawAsiaChang Jiang basin and Southern China (China), northern states (India), Java and Sumatra (Indonesia)[8]
Energy cropsMiscanthusEurope, Asia and USAUnited Kingdom, Germany, and Poland (Europe), parts of East Asia, and the Midwest/Southeast (USA)[9]
SwitchgrassEurope, USA and CanadaParts of Western Europe, the Great Plains, the Midwest and Southeast (USA), and southern provinces (Canada)
Forest residuesWood chips and sawdustSouth AfricaKwaZulu-Natal, Mpumalanga, and Eastern Cape provinces[10]
Wood chips and forest thinningRussiaSiberia, Northwest, and Far East
Sawmill residues and wood chipsNorth AmericaPacific Northwest, Southeast (USA), and British Columbia (Canada)
Forest thinning, sawdustEuropeScandinavia and Eastern and Western Europe
Industrial wastesBagasse and paper mill sludgeSouth AfricaKwaZulu-Natal and Mpumalanga provinces[11]
BrazilSão Paulo, Minas Gerais, Paraná (bagasse), and São Paulo, Paraná, and Santa Catarina (paper mill sludge)
IndiaUttar Pradesh, Maharashtra, Tamil Nadu (bagasse) and Maharashtra, Tamil Nadu, and West Bengal (paper mill sludge)
Municipal solid wastesDomestic waste, food, scraps, sewageGlobalGlobal[12]
Table 2. Recent studies on the conversion of LCB to biofuels.
Table 2. Recent studies on the conversion of LCB to biofuels.
Paper TitleObjective and ContributionReference
A Review on Renewable Energy: Conversion and Utilization of BiomassThe article reviews how LCB can be transformed into high-quality chemicals and biofuels, promoting sustainable production and reducing reliance on fossil fuels. It examines the pros and cons of different pre-treatment methods for converting cellulose, hemicellulose, and lignin, offering guidance for optimizing their use in the future.[17]
Lignocellulosic Biomass Decomposition and BioconversionLCB is highlighted as a sustainable energy resource due to its abundance and degradability, making it a viable option for energy production. The paper emphasizes the environmental compatibility and non-toxic nature of LCB, which contributes to its cost-effectiveness as an energy source.[6]
The Significance of Biomass in Achieving a Global Bio-economyThe paper explores different types of biomass, including lignocellulosic materials, organic waste, and algae, and their roles in the bio-economy. It also examines various conversion technologies, such as pyrolysis and gasification, and their contributions to energy and biofuel production.[13]
Exploitation of Lignocellulosic-based Biomass Biorefinery: A Critical Review of Renewable Bioresource, Sustainability and Economic ViewsThe paper evaluates different pre-treatment methods for LCB, highlighting their pros and cons in improving biorefinery efficiency and enzymatic digestion. It also analyses biorefining’s economic and environmental impacts, promoting green chemistry and offering sustainable options to guide policy decisions on biorefinery technologies.[15]
Lignin Used as a Green and Sustainable Agriculture Biomass for Renewable Applications: A Comprehensive ReviewThe paper highlights the significant production of lignin in the pulp and paper sector, which generates 50 to 70 million tonnes annually, primarily using it for energy recovery. This emphasizes lignin’s potential as a renewable energy source and its role in sustainable agriculture biomass applications. It also discusses the diverse applications of lignin, particularly in the production of polyurethanes (PUs) and other polymeric products, as well as its use in various industries, such as adhesives, furniture, building structures, and biomaterials. This showcases lignin’s versatility and importance in green and sustainable practices.[19]
Review of Biomass as a Renewable Energy for Sustainable EnvironmentThe paper emphasizes biomass’s potential as a renewable energy source, addressing pollution and waste management by using feedstocks like agricultural residues and municipal waste. It discusses how converting biomass into biofuels can reduce greenhouse gas emissions and promote sustainable energy while also highlighting the importance of responsible feedstock management.[20]
Microbial Lignocellulolytic Enzymes for the Effective Valorization of Lignocellulosic Biomass: A ReviewThe paper discusses various types of lignocellulolytic enzymes and their modes of action. It also highlights the importance of solid-state fermentation and enzyme immobilization.[22]
Bioenergy Generation from Thermochemical Conversion of Lignocellulosic Biomass-based Integrated Renewable Energy SystemsThe paper systematically reviews the thermochemical conversion technologies for LCB, such as gasification and pyrolysis, and their integration with renewable energy systems like solar thermal and fuel cells. It emphasizes the importance of selecting suitable configurations to achieve sustainable power generation and enhance environmental benefits.[21]
Table 3. Identified keywords and search terms.
Table 3. Identified keywords and search terms.
KeywordsSearch Terms
Pre-treatment techniques“pre-treatment techniques”
Enzymatic hydrolysis“hydrolysis”
“conversion”
Lignocellulosic“lignocellulose”
“lignocellulosic biomass”
Biomass“agricultural waste”
Valorization“valorization”
“waste conversion”
“bioconversion”
Biofuel“bio-oil”
Table 4. Total number of articles retrieved from each database and corresponding search terms.
Table 4. Total number of articles retrieved from each database and corresponding search terms.
DatabaseSearch TermsSearch String No. of ArticlesSearch Date
ScopusPrimary search terms—title, abstract, and keywords“pre-treatment techniques” AND “enzymatic hydrolysis” OR “hydrolysis” OR “conversion” AND “lignocellulosic” OR “lignocellulose” AND “lignocellulosic biomass” AND “biomass” OR “agricultural waste” AND “valorization” OR “waste conversion” OR “bioconversion” AND “biofuel” OR “bio-oil”119213 September 2024
Google ScholarPrimary search items—title, abstract, and keywords“pre-treatment techniques” “enzymatic hydrolysis”| “hydrolysis ”|“conversion” lignocellulosic “lignocellulose” “lignocellulosic biomass” “biomass ”|“agricultural waste” “valorization ”|“waste conversion ”|“bioconversion” biofuel “bio-oil”98113 September 2024
CrossrefPrimary search terms—title, abstract, and keywords“pre-treatment techniques” “enzymatic hydrolysis”| “hydrolysis ”|“conversion” lignocellulosic “lignocellulose” “lignocellulosic biomass” “biomass ”|“agricultural waste” “valorization ”|“waste conversion ”|“bioconversion” biofuel “bio-oil”100014 September 2024
PubMedPrimary search terms—title, abstract, and keywords“pre-treatment techniques” “enzymatic hydrolysis”| “hydrolysis ”|“conversion” lignocellulosic “lignocellulose” “lignocellulosic biomass” “biomass ”|“agricultural waste” “valorization ”|“waste conversion ”|“bioconversion” biofuel “bio-oil”157013 September 2024
Semantic ScholarPrimary search terms—title, abstract, and keywords“pre-treatment techniques” “enzymatic hydrolysis”| “hydrolysis ”|“conversion” lignocellulosic “lignocellulose” “lignocellulosic biomass” “biomass ”|“agricultural waste” “valorization ”|“waste conversion ”|“bioconversion” biofuel “bio-oil”23114 September 2024
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Ntunka, M.G.; Khumalo, S.M.; Makhathini, T.P.; Mtsweni, S.; Tshibangu, M.M.; Bwapwa, J.K. Valorization of Lignocellulosic Biomass to Biofuel: A Systematic Review. ChemEngineering 2025, 9, 58. https://doi.org/10.3390/chemengineering9030058

AMA Style

Ntunka MG, Khumalo SM, Makhathini TP, Mtsweni S, Tshibangu MM, Bwapwa JK. Valorization of Lignocellulosic Biomass to Biofuel: A Systematic Review. ChemEngineering. 2025; 9(3):58. https://doi.org/10.3390/chemengineering9030058

Chicago/Turabian Style

Ntunka, Mbuyu Germain, Siphesihle Mangena Khumalo, Thobeka Pearl Makhathini, Sphesihle Mtsweni, Marc Mulamba Tshibangu, and Joseph Kapuku Bwapwa. 2025. "Valorization of Lignocellulosic Biomass to Biofuel: A Systematic Review" ChemEngineering 9, no. 3: 58. https://doi.org/10.3390/chemengineering9030058

APA Style

Ntunka, M. G., Khumalo, S. M., Makhathini, T. P., Mtsweni, S., Tshibangu, M. M., & Bwapwa, J. K. (2025). Valorization of Lignocellulosic Biomass to Biofuel: A Systematic Review. ChemEngineering, 9(3), 58. https://doi.org/10.3390/chemengineering9030058

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