Microwave Pretreatment for Biomass Pyrolysis: A Systematic Review on Efficiency and Environmental Aspects

Abstract

Microwave pretreatment (MWP) has emerged as a promising strategy to enhance the pyrolysis of lignocellulosic biomass due to its rapid, volumetric, and selective heating. By disrupting the recalcitrant structure of cellulose, hemicellulose, and lignin, MWP improves biomass deconstruction, increases carbohydrate accessibility, and enhances yields of bio-oil, syngas, and biochar. When combined with complementary pretreatments—such as alkali, acid, hydrothermal, ultrasonic, or ionic-liquid methods—MWP further reduces activation energies, facilitating more efficient saccharification and thermal conversion. This review systematically evaluates scientific progress in this field through bibliometric analysis, mapping research trends, evolution, and collaborative networks. Key research questions are addressed regarding the technical advantages of MWP, the physicochemical transformations induced in biomass, and associated environmental benefits. Findings indicate that microwave irradiation promotes hemicellulose depolymerization, reduces cellulose crystallinity, and weakens lignin–carbohydrate linkages, which facilitates subsequent thermal decomposition and contributes to improved pyrolysis efficiency and product quality. From an environmental perspective, MWP contributes to energy savings, mitigates greenhouse gas emissions, and supports the integration of renewable electricity in biomass conversion.

1. Introduction

Global energy demands are continuously increasing, yet petroleum, historically the primary energy source, faces limited medium-term availability, making energy supply from fossil fuels uncertain [1]. Moreover, global climate change and environmental pollution, due to high carbon dioxide and particulate matter concentrations, have become major scientific concerns [2]. Consequently, attention has shifted toward renewable feedstocks as sustainable sources of fuels and industrially valuable chemicals [3,4].

Among renewable sources, biomass—defined as the biodegradable fraction of products, residues, and wastes from agriculture, forestry, and related industries (including fisheries and aquaculture), as well as the biodegradable portion of industrial and municipal waste [5,6]—represents a promising energy source capable of meeting current and future demands [7,8]. Various thermochemical and biological processes have been utilized to convert biomass into value-added products (Figure 1), with pyrolysis being particularly advantageous due to its benefits in storage, transportation, and flexibility for applications such as turbines, combustion systems, boilers, and engines [9,10]. However, processing solid biomass and waste for pyrolysis remains challenging, and the technology is still in the early stages of development, facing numerous practical barriers compared to conventional fossil fuel-based methods [11,12]. The production of liquid biofuels, alongside by-products like solid char and gas, from pyrolysis of lignocellulosic residues has been extensively studied, with biomass sources such as beechwood, bagasse, woody biomass, straws, seedcakes, and municipal solid waste (MSW) being explored [13]. Pyrolysis is defined as the thermal decomposition of lignocellulosic materials under an inert, oxygen-deficient environment [14,15,16]. The practice has evolved, and pyrolysis is now recognized as an effective method for transforming biomass into bio-oil, with the ultimate goal of replacing non-renewable fossil fuels with high-value energy products [17]. However, advancing technology to achieve this goal remains a significant challenge. Continued improvements in pyrolysis are necessary for producing solid fuels such as char, carbonaceous materials, and syngas, with systems typically consisting of pre-processing equipment, the pyrolysis reactor, and downstream processing units. These systems can be classified into those producing only heat and biochar (slow pyrolysis) or those generating both biochar and bio-oils (fast pyrolysis) [18].

Pyrolysis is an emerging technology capable of producing more than 300 organic chemical compounds, mainly condensable vapors, non-condensable gases, and a solid residue known as biochar [20,21,22]. In particular, pyrolysis is widely investigated for producing bio-oil, syngas, and biochar due to its adaptability to diverse biomass types and operational flexibility [23]. However, the efficiency of pyrolysis strongly depends on the physical and chemical properties of the feedstock, including moisture content, particle size, and lignocellulosic structure [24]. The decomposition of biomass during pyrolysis involves a wide range of reactions, including dehydration, depolymerization, isomerization, aromatization, carbonization, and decarboxylation [25,26,27,28]. Condensable vapors consist of hydrocarbons, alcohols, phenols, sugars, furans, nitrogenous compounds, carbonyls, among others [29]. The liquid fraction can serve as an alternative fuel or diesel energy source [30]. Through oxygen removal and waste reduction, the quality of the liquid product can be significantly improved; it does not need to be consumed at the production site, as it can be easily stored and is transported [31]. Non-condensable gases (H₂, CO, C₂H₂, CH₄, C₂H₄, etc.) generated during pyrolysis processes constitute another valuable by-product [32]. The solid fraction, or biochar, is a carbonaceous residue formed during primary and secondary reactions [33].

Pretreatment of lignocellulosic biomass is a crucial step designed to modify the inherent recalcitrance of its complex structure, thereby improving conversion efficiency [34]. Depending on the intended pathway, pretreatments may be categorized as mechanical (size reduction, milling), thermal (steam explosion, microwave heating), or chemical (acid/alkali treatments), and can be tailored, either to facilitate the conversion process or to optimize the properties of the final products—commonly referred to as the process—or product-oriented strategies [35]. In the context of thermochemical routes such as pyrolysis, pretreatments aim primarily at reducing cellulose crystallinity, depolymerizing hemicellulose, and improving porosity and heat transfer within the feedstock, while maintaining compatibility with downstream conversion technologies [36]. By contrast, chemical pretreatments focused on extensive lignin removal are mostly relevant to biochemical conversion pathways and are not directly applicable to pyrolysis [35]. Thus, the selection of an appropriate pretreatment strategy depends strongly on both the targeted process and the desired product performance [36].

An emerging heating technique is the use of microwaves, which have garnered significant interest due to several advantages over conventional thermal methods [37]. Notable benefits include non-contact volumetric heating, short reaction times, minimal solvent requirements, limited side reactions, and reduced parametric complexity [38,39]. Microwave radiation can achieve up to 50% higher heating efficiency than natural gas or steam [40,41]. It converts electromagnetic energy directly into heat at the molecular level, promoting uniform energy dissipation across the material—particularly effective in bulk systems [42,43].

Table 1. Conventional heating vs. microwave heating [43,44,45].

Conventional Heating	Microwave Heating
Energy transfer	Energy conversion
Surface heating by conduction, convection, and radiation	Volumetric and uniform core heating at the molecular level
Absence of hot spots	Presence of hot spots
Slow, inefficient, and limited	Fast and efficient
Lower electricity-to-heat conversion efficiency	Higher electricity-to-heat conversion efficiency
Non-selective	Selective
Limited dependence on material properties	Strong dependence on material properties
Limited heating controllability	Precise and controllable heating
Limited process flexibility	High process flexibility
Bulky and less portable equipment	Portable equipment
Polluting process	Cleaner and less polluting process
Higher thermal inertia	Lower thermal inertia

illustrates these advantages in comparison with conventional heating.

Microwaves can rapidly and selectively heat biomass, reducing moisture content, disrupting the lignocellulosic matrix, and improving mass and heat transfer during subsequent pyrolysis. In conventional heating, heat is transferred from the surface to the core of the material through conduction driven by temperature gradients, and mass flux consistently proceeds from the interior toward the exterior. In contrast, microwaves induce heating at the molecular level by directly converting electromagnetic energy into thermal energy, leading to heat and mass flows that are opposite to those observed under conventional heating [46].

Microwave radiation lies between infrared and radio waves in the electromagnetic spectrum, with the most commonly used frequencies being 915 MHz and 2.45 GHz [47,48]. Unlike conventional heating, microwaves penetrate materials and generate heat internally, enabling rapid and volumetric energy transfer [49,50,51]. The efficiency of this process is determined by the dielectric properties of the material, particularly the loss tangent, which governs the conversion of microwave energy into heat. According to their interaction with microwaves, materials can be classified as insulators, conductors, or absorbers [52]. Insulators, such as glass and ceramics, are largely transparent, while conductors, such as metals, reflect microwaves. Absorbers, or dielectrics, are of primary interest in biomass processing because they efficiently transform microwave energy into heat [53]. These distinctive heating features allow faster heating rates, selective absorption, and reduced formation of undesired by-products [54]. Such advantages make microwave-assisted processes highly attractive for applications in energy, environment, and chemical engineering [55].

In recent years, the interdisciplinary field of microwave biomass processing has drawn significant attention due to its potential to integrate and benefit from advances in energy, environmental sciences, and chemical engineering. From the perspective of energy, microwave-assisted methods offer more efficient heating, shorter reaction times, and often higher energy yields than conventional thermal treatments, enabling improved conversion of biomass into biofuels, bio-oils, syngas, and biochar. From an environmental standpoint, these technologies can reduce greenhouse gas emissions, lower pollutant formation, enable resource recovery (e.g., biochar as soil amendment, or valorization of waste biomass), and support circular bioeconomy principles. Meanwhile, chemical engineering contributes the necessary tools for optimizing process design—such as reactor configuration, catalyst development, dielectric properties of biomass, scale-up, techno-economic analysis, and life-cycle assessment—that ensure those energy and environmental benefits can be realized in practice.

For example, the review by Liu et al. [56] discusses recent progress in microwave pretreatment, pyrolysis, and biomass conversion, highlighting how chemical engineering parameters (power input, reactor design) influence conversion efficiency and product distribution. Another relevant study [57] emphasizes how catalytic materials and absorbents play a role not only in improving energy efficiency but also in reducing environmental impact of byproducts. Recent reviews emphasize the role of microwave absorbents and catalysts in achieving efficient heating and high product selectivity [58]. Experimental studies using biochar as a low-cost microwave absorber have demonstrated significant improvements in heating rate and control, corroborating the advantages of microwave systems over conventional thermal methods [58]. Further investigations using lignin model compounds highlight that microwave pretreatment can reduce the formation of undesired pyrolysis byproducts, such as tars or chars [59]. The combination of microwave pretreatment and pyrolysis represents a viable strategy to optimize biofuel production from diverse biomass feedstocks, aligning with global efforts to develop sustainable and low-carbon energy technologies. Building on these findings, recent studies confirm that microwave pretreatment offers distinct advantages over conventional heating methods by enabling selective depolymerization, reducing activation energy requirements, and improving the homogeneity of heating across lignocellulosic substrates [60,61,62]. Moreover, microwave-assisted strategies have been shown to lower overall energy inputs while enhancing the quality of bio-oil and biochar fractions, thus contributing to the environmental sustainability of biomass valorization processes [63,64].

In this context, the present work does not intend to provide a comprehensive critical analysis of all microwave-assisted processes reported in the literature, supported by a bibliometric analysis conducted using the PRISMA methodology with clearly defined search criteria. Rather, it presents a systematic overview supported by a bibliometric analysis and guided by specific research questions, with the aim of elucidating the benefits of applying microwave pretreatment to biomass prior to pyrolysis. The bibliometric analysis covers a broad period of the last two decades to identify long-term trends in scientific output, while a more targeted review of the most relevant studies over the last five years is performed to provide an updated and detailed understanding of current advances. While the discussion addresses biomass pyrolysis in general, particular emphasis is placed on efficiency indicators such as bio-oil yield, given its central role in biofuel production, alongside related applications such as biochar generation for soil amendment. By systematically summarizing current knowledge, identifying trends, and highlighting critical factors influencing process performance, this review contributes to the optimization of biomass pyrolysis and the advancement of sustainable and efficient biofuel production strategies.

2. Methods

This study employed a qualitative methodology grounded in a systematic review of the scientific literature, structured into two main phases.

2.1. Phase One: Focused Literature Review

An extensive search was conducted across internationally recognized academic database (Scopus). This database was selected due to their strong relevance and impact in disseminating science, technology, and engineering advances. Search queries combined keywords such as microwave pretreatment, biomass and pyrolysis. The search was carried out across the full text of the documents, not limited only to Title, Abstract, and Keywords. This broader approach ensured a more comprehensive retrieval of relevant publications. The search was conducted on 19 August 2025. The inclusion criteria targeted peer-reviewed publications that examined microwave-assisted biomass pretreatment and its effects on physicochemical properties, pyrolysis performance, or product distribution. The selected studies were critically evaluated regarding relevance, methodological soundness, and overall contribution to the field.

2.2. Phase Two: Thematic Analysis Guided by Research Questions

Based on the selected studies, a thematic analysis was conducted to address the following research questions, aiming to synthesize the key findings and identify trends, gaps, and opportunities for future research in microwave-assisted biomass pretreatment for pyrolysis applications:

• RQ1: How does microwave-assisted pretreatment alter the physicochemical structure of lignocellulosic biomass (e.g., cellulose crystallinity, lignin removal, and porosity enhancement)?
• RQ2: How do combined microwave-assisted pretreatments influence the physicochemical properties of lignocellulosic biomass?
• RQ3: What are the current challenges, limitations, and future perspectives in applying microwave pretreatment at pilot and industrial scales for sustainable bioenergy production?
• RQ4: What were the most important advances and discoveries in the field of microwave-assisted biomass pretreatment prior to pyrolysis in the last five years?

To address the research question of the bibliometric analysis presented in Section 3.1.1, the PRISMA methodology was applied to refine the search and highlight the most relevant studies from the last five years on microwave pretreatment of biomass prior to pyrolysis. In the Scopus search, several restrictions were applied. Only research articles were considered, excluding reviews and conference proceedings. Furthermore, the selection was limited to texts written in English, as it is the predominant language of scientific publications. To capture the most recent research trends, the search was also restricted to studies published within the last five years. The description of the selection and exclusion of articles was carried out according to the following search line:

TITLE-ABS-KEY [microwave pretreatment, biomass, pyrolysis] AND [LIMIT-TO [DOCTYPE, “ar”]] AND [LIMIT-TO [LANGUAGE, “English”]] AND [LIMIT-TO [PUBYEAR, 2021] OR LIMIT-TO [PUBYEAR, 2022] OR LIMIT-TO [PUBYEAR, 2023] OR LIMIT-TO [PUBYEAR, 2024] OR LIMIT-TO [PUBYEAR, 2025]]

This approach allowed for a structured synthesis of the literature, highlighting both experimental outcomes and theoretical insights. It ultimately provided a comprehensive understanding of the current state of the art in microwave-assisted pretreatment of biomass prior to pyrolysis.

3. Results

3.1. Phase One: Focused Literature Review

3.1.1. Publications per Year

Figure 2 shows the number of publications per year on microwave-assisted biomass pretreatment, as retrieved from Scopus on 19 August 2025. The data indicate a gradual but notable increase in scientific output over the past two decades. Between 2008 and 2013, the field shows very low activity, with only 1–2 publications per year, reflecting the early stage of research in microwave-assisted biomass pretreatment. From 2014 to 2016, a steady increase was observed (3–4 publications per year), suggesting growing interest and initial experimental studies. A temporary dip occurs in 2017 (1 publication), likely due to the small sample size of niche studies or database indexing effects. Starting in 2018, the number of publications rose significantly, peaking at 12 in 2021, indicating intensified research activity, likely driven by advances in microwave reactor technology and increasing interest in sustainable bioenergy solutions. The fluctuation between 2022 and 2025 (10, 8, 11, and 7 publications) can be attributed to variations in research funding cycles, the impact of global events (e.g., COVID-19 recovery), and the natural delay between project completion and publication. Such year-to-year differences are common in emerging research areas and reflect the dynamic nature of scientific output. Overall, the trend demonstrates that microwave-assisted biomass pretreatment has evolved from a niche topic to a recognized study area, highlighting its importance for improving pyrolysis efficiency and promoting sustainable bioenergy production.

3.1.2. Distribution of Publications by Subject Area

Figure 3 presents the distribution of publications on microwave-assisted biomass pretreatment across various subject areas according to Scopus classification. The Energy category dominates with 46 publications, representing the largest share of research output. This reflects the field’s strong focus on bioenergy production, pyrolysis optimization, and renewable energy applications. Environmental Science follows with 36 publications, indicating the relevance of microwave-assisted biomass pretreatment for sustainability, emission reduction, and environmental impact assessments. Chemical Engineering contributes 31 publications, highlighting the importance of process design, reactor optimization, and scale-up considerations. Engineering (20 publications) and Chemistry (18 publications) show active interdisciplinary engagement, covering materials characterization, reaction kinetics, and thermal processes. Agricultural and Biological Sciences (10 publications) emphasizes the role of feedstock selection, biomass composition, and pretreatment strategies derived from biological studies. Lower representation is observed in Materials Science and Biochemistry, Genetics and Molecular Biology (6 publications each), reflecting more specialized investigations into biomass structure, enzymatic interactions, and material properties. Although Physics and Astronomy (5 publications) and Medicine (4 publications) show lower representation in the dataset, their potential applications remain significant. This limited presence may be explained by the current research focus on energy, environmental sciences, and engineering, which dominate the field, reflecting its relevance for sustainable energy production and process innovation. Nevertheless, in medicine, possible uses include advances in medical imaging and diagnostic tools, while in physics, modeling and simulation approaches may provide valuable insights. These underrepresented areas, therefore, represent opportunities for future research expansion.

3.1.3. Keywords

Figure 4 summarizes the most frequently used keywords in publications on microwave-assisted biomass pretreatment retrieved from Scopus. The frequency of each keyword provides insights into the main research themes and priorities in the field. Biomass (60 occurrences) and pyrolysis (59 occurrences) are the most prominent keywords, reflecting the core focus on biomass conversion and thermochemical processes. Keywords related to microwave technology, including microwaves (28), microwave radiation (22), microwave heating (14), and microwave pyrolysis (14), highlight the central role of microwave-assisted methods as an emerging and widely investigated pretreatment strategy. Terms such as pretreatment (21), pre-treatment (13), heating (13), and microwave (15) emphasize the methodological aspects and process optimization efforts within the field. Keywords associated with feedstock components, notably lignin (16) and cellulose (21), indicate that structural and compositional analysis of lignocellulosic biomass is a critical research focus. Bioenergy-related terms such as biofuel (14) and biofuels (16), and biofuel production (12), underscore the ultimate application goal: sustainable energy production from biomass. The distribution of keywords demonstrates that research in this field is methodologically driven (microwave techniques, heating, pretreatment) and application-oriented (biofuel generation, biomass composition), reflecting the interdisciplinary nature of microwave-assisted pyrolysis research. Overall, the keyword analysis confirms that microwave-assisted pretreatment is a highly active study area, bridging materials characterization, process engineering, and sustainable energy applications.

3.1.4. Correlation Keywords

Figure 5 demonstrates that microwave-assisted pretreatment of biomass is at the intersection of fundamental biomass chemistry (green cluster), thermochemical conversion processes (red cluster), and energy valorization (blue and yellow clusters). The visualization of correlation keywords was generated using VOSviewer 1.6.20 software, which allows mapping and clustering of bibliometric networks. The strong centrality of biomass, pyrolysis, and microwaves/microwave radiation reflects the field’s consolidation around microwave-assisted pyrolysis (MAP) as a process-intensification route that reduces energy use and can improve carbon efficiency relative to conventional heating, according to recent comprehensive reviews [65]. Links between the green cluster (biomass composition and pretreatment) and the red cluster (pyrolysis and product distribution) are consistent with evidence that microwave pretreatment alters lignocellulosic structure and dielectric behavior, lowering apparent activation barriers and shifting devolatilization kinetics, which in turn affects product yields and selectivity [60]. Connections among lignin, cellulose, hemicellulose, temperature, and pretreatment methods align with reports that microwave pretreatment adjusts moisture, porosity, and functional groups, facilitating subsequent pyrolysis and enabling shorter residence times and higher quality of value-added products [61]. The dense links between catalysis, reaction kinetics, bio-oil, and biochar match the literature showing that microwave-responsive absorbents and catalysts (e.g., carbons, zeolites, ferrites) localize heating (“hot spots”) and steer pathways toward deoxygenation and aromatic formation, thereby upgrading bio-oil and tailoring biochar properties [58]. Edges connecting the yellow/blue clusters [process intensification, biofuel production] to the core terms reflect MAP’s scalability considerations—reactor design, absorber selection, and parameter optimization—as key levers for improving efficiency and lowering global-warming potential in life-cycle terms [65]. At the study level, microwave pretreatment before catalytic fast pyrolysis has been shown to modify surface morphology and increase ketone/aromatic formation (e.g., with CaO), exemplifying the product-distribution links in the map [62]. Overall, the network’s high inter-cluster connectivity quantitatively mirrors the literature’s interdisciplinary integration of materials science (dielectric/structure), chemical engineering (reactors/catalysts), and energy valorization (bio-oil/biochar/syngas) in microwave-enabled biomass upgrading [63].

3.1.5. Correlation Keyword Clusters

The keyword co-occurrence network highlights the main research domains associated with biomass pretreatment using microwave technology before pyrolysis. Four main clusters can be identified:

Green Cluster—Biomass Composition and Pretreatment: This cluster revolves around biomass, cellulose, lignocellulose, pretreatment methods, and hydrolysis. It reflects studies focused on the structural characteristics of biomass and the role of pretreatment in enhancing thermal conversion efficiency. The presence of temperature, lignin, and hemicellulose indicates strong research interest in how microwave-assisted pretreatment modifies the physicochemical properties of biomass components, particularly lignocellulosic feedstocks.

Red Cluster—Pyrolysis and Product Distribution: Centered around pyrolysis, bio-oil, biochar, and carbon, this cluster emphasizes the thermochemical conversion process and the resulting products. The association with microwave pretreatment, reaction kinetics, and catalysis suggests that research is strongly directed toward understanding how microwaves influence reaction pathways, heating mechanisms, and yield distribution, particularly in improving bio-oil quality and biochar functionality.

Yellow Cluster—Energy Applications and Process Intensification: Keywords such as microwave heating, activation energy, fast pyrolysis, gasification, and biomass pretreatments form a cluster focused on process optimization and scaling-up perspectives. This indicates efforts to enhance energy efficiency, reduce activation barriers, and integrate microwave systems with other thermochemical routes for renewable energy generation.

Blue Cluster—Biofuel Production and By-products: This cluster is represented by terms such as biofuel production, phenols, decomposition, microwave cooking, and biotechnology. It highlights the valorization of pyrolysis-derived compounds into high-value chemicals and biofuels. The co-occurrence of phenolic derivatives and polyphenols reflects growing attention to the selective recovery of functional chemicals beyond conventional bioenergy applications.

3.1.6. Publications per Country

Figure 6 presents the number of publications on microwave-assisted biomass pretreatment by country, based on Scopus data. China mainland leads the field with 52 publications, demonstrating its dominant role in developing microwave-assisted biomass pretreatment and bioenergy technologies. India (13) and the United States (10) follow, highlighting significant contributions from other major players in renewable energy research. Canada (5), and Australia (4) represent countries with growing interest and active research groups in this area. Countries such as Egypt (3), Denmark (3), and several others with two publications (e.g., United Kingdom, Thailand, Spain, South Korea, Poland, Philippines, Malaysia, Japan, Ireland, Iran, Greece, Colombia, Belgium) show emerging contributions, often reflecting collaborative or niche studies. A long tail of countries with one publication each, including Vietnam, Sweden, Pakistan, Norway, New Zealand, Netherlands, Latvia, Indonesia, Hungary, France, Ecuador, Czech Republic, Croatia, Chile, Brazil, and Bangladesh, illustrates the global reach and interdisciplinary interest of microwave-assisted biomass research, even in countries with nascent research activities. Overall, the distribution highlights that microwave-assisted biomass pretreatment research is highly concentrated in China mainland, followed by India and the United States, while a diverse set of countries are contributing to the global expansion of knowledge in sustainable bioenergy and pyrolysis technologies. This geographic pattern reflects the availability of research infrastructure and national priorities in renewable energy and biomass utilization.

3.1.7. Publications per Affiliation

Figure 7 presents a treemap visualization of the analysis of publications by institutional affiliation. Figure was processed and visually represented using Microsoft Excel, which enabled the creation of the chart from the extracted dataset.

The treemap reveals that the Ministry of Education of the People’s Republic of China and the Chinese Academy of Sciences are the leading contributors, each with 10 publications. They are followed by the Guangzhou Institute of Energy Conversion of the Chinese Academy of Sciences, with 8 publications, and Huazhong University of Science and Technology, with 6 publications. Other notable contributors include Nanchang University (5 publications), Shenzhen University, the University of Chinese Academy of Sciences, and the Department of Bioproducts and Biosystems Engineering, each with 4 publications. In addition, international collaborations are evident through the contributions from institutions such as Washington State University Tri-Cities (3 publications), Tunghai University (3 publications), and Shanghai Jiao Tong University (3 publications). These results highlight a strong research concentration in Chinese institutions, particularly within the Chinese Academy of Sciences and its affiliated institutes, while also demonstrating an emerging network of global collaboration in this research field.

3.1.8. Publications per Funding Sponsors

The analysis of funding sponsors (Figure 8) indicates that the National Natural Science Foundation of China (NSFC) is the primary supporter of research on microwave-assisted biomass pretreatment, with 25 publications acknowledging its funding. This underscores the pivotal role of NSFC in promoting fundamental and applied research in biomass conversion and renewable energy technologies in China mainland.

Other significant Chinese funding agencies include the Fundamental Research Funds for the Central Universities (5 publications), the Natural Science Foundation of Guangdong Province (4), and the National Key Research and Development Program of China (4). Additionally, organizations such as the Youth Innovation Promotion Association of the Chinese Academy of Sciences (3) and the Science and Technology Planning Project of Guangdong Province (3) contribute to supporting emerging research initiatives and regional projects.

International and other institutional funding is also evident. For example, the European Commission is acknowledged in 3 publications, highlighting cross-border collaboration, while the Ministry of Science and Technology (3) and Ministry of Education (3) show that regional governments outside mainland China also contribute to this research area. Other contributors include the Key Research and Development Program of Jiangxi Province (3), National Cheng Kung University (2), and the Ministry of Science and Technology of the People’s Republic of China (2).

Overall, the funding landscape reveals a strong dominance of Chinese national and provincial agencies, emphasizing the strategic importance of biomass and bioenergy research in China mainland, while international and regional sponsors also play a complementary role in supporting research diversity and collaboration.

3.1.9. Publications per Source

Figure 9 presents the distribution of publications on microwave-assisted biomass pretreatment by journal source. Bioresource Technology is the leading journal, with 13 publications, indicating its central role in disseminating research on biomass conversion, bioenergy, and thermochemical pretreatment processes. Biomass Conversion and Biorefinery follows with 4 publications, highlighting its focus on biorefinery strategies and sustainable biomass utilization. Journals such as the Journal of the Energy Institute, Journal of Analytical and Applied Pyrolysis, Energy Conversion and Management, Chemosphere, and Biomass and Bioenergy each contribute 3 publications, reflecting interest in energy engineering, pyrolysis analytics, environmental impact, and bioenergy applications. Several other journals, including Science of the Total Environment, Fuel, Frontiers in Chemistry, Environmental Research, Energy and Fuels, Energy, and Advanced Materials Research, have published 2 papers each, demonstrating the interdisciplinary nature of the field, encompassing energy, chemistry, materials science, and environmental studies. Overall, the distribution of sources shows that research on microwave-assisted biomass pretreatment is published predominantly in journals focused on energy, biomass, and environmental science, underscoring the intersection of chemical engineering, sustainable energy production, and environmental applications.

3.2. Phase Two: Thematic Analysis Guided by Research Questions

Pretreatment of lignocellulosic biomass is a critical preparatory stage in thermochemical conversion processes, such as pyrolysis, aiming to improve the accessibility and reactivity of biomass components [64,66,67]. This process involves the modification of the complex lignocellulosic structure, including cellulose, hemicellulose, and lignin, to enhance heat and mass transfer, reduce cellulose crystallinity, increase porosity, and optimize the distribution of products during pyrolysis [68,69]. Microwave-assisted pretreatment, in particular, has emerged as a promising approach due to its ability to induce rapid and uniform internal heating at the molecular level, leading to structural rearrangements that facilitate biomass conversion [37,70,71,72]. This targeted pretreatment strategy is relevant to improving process efficiency, product yield, and the quality of biofuels and biochar, while aligning with sustainable and environmentally friendly conversion objectives [73,74,75].

Microwave irradiation, a form of non-ionizing electromagnetic energy, induces rapid internal heating of biomass particles [76]. This internal heating generates localized pressure and structural disruptions that weaken the rigid plant matrix [77]. Such effects contribute to the rearrangement of crystalline regions within cellulose and enhance molecular mobility [78]. In lignocellulosic matrices, the coexistence of amorphous and crystalline domains facilitates dipole reorientation, leading to the disruption of hydrogen bonds and partial breakdown of the cell wall architecture, thereby improving the accessibility of cellulose to enzymatic attack [34,79,80]. Hardwoods, in particular, show high susceptibility to microwave treatment, which results in the formation of micropores and a notable increase in available surface area. Overall, microwave irradiation emerges as an alternative to conventional heating methods by altering the compact structure of cellulose and contributing to the partial solubilization of hemicelluloses and lignin [81].

To better understand the influence of microwave pretreatment on biomass prior to pyrolysis, four research questions were formulated and are addressed below.

3.2.1. How Does Microwave-Assisted Pretreatment Alter the Physicochemical Structure of Lignocellulosic Biomass?

In a microwave pretreatment of lignocellulosic biomass, information about its dielectric properties is necessary to understand its interaction with electromagnetic energy, primarily to find an optimal condition during heating [82]. Thus, the hydroxyl groups of lignocellulosic biomass are polar, and lignocellulosic fibers give rise to the formation of dipoles. Furthermore, electric current flows through the crystalline region of the biomass, and higher moisture content aids flow in the amorphous regions [83]. A more dipolar material exhibits greater dielectric properties and subsequently generates heat within its interior [82]. The crystalline region present in biomass facilitates the flow of electric current within it, and the presence of moisture aids flow in the amorphous regions [83]. Furthermore, the addition of acids or alkalis can significantly affect the polar orientation within the material during the pretreatment process [82]. Under the influence of an electromagnetic field such as microwaves, the polar structure of cellulose molecular chains orients [84], and due to microwave vibration, these polar molecules collide [85], and these collisions form hot spots that are randomly located within the biomass [86]. Due to the increase in heat, explosions occur that accelerate the relocation of the crystalline structure [77]. On the other hand, the OH groups present in the biomass absorb microwaves, accelerating heating and increasing pressure, resulting in lignocellulosic destruction [87]. During the process, water loss by evaporation and the microwave power level must also be analyzed as necessary mechanisms to control the reaction [82]. Other advantages of applying microwaves as a pretreatment are that a minimal amount of solvent is required, and the formation of secondary reactions is limited [88]. During the hydrocracking of α-cellulose from merbau wood waste (Intsia spp.), microwaves (399 W) were used to pretreat the feedstock [43]. Wood charred at 800 °C and microwave-irradiated for 5 min was compared to wood flakes irradiated with microwaves for 30 min. A Ni catalyst (1.0, 1.5, and 2.0 wt%) was also used. The charred and microwave-treated wood showed the best performance and had a lower specific surface area (364.12 m²/g), total pore volume (0.28 cm³/g), mean pore diameter (3.03 nm), and acidity (2.18 mmol/g). The conversions to liquid were 58.76 wt% for the Ni_1.5 catalyst, 57.51 wt% for the Ni₁ catalyst, and 34.18 wt% for the Ni₂ catalyst [89].

• Physical principle and selectivity.

Microwave (MW) fields couple mainly through dipole rotation and ionic conduction, delivering rapid, volumetric heating to polar phases (bound water, acids/alkalis, hydrotropes, DES/ILs). This accelerates autohydrolysis, deacetylation, and cleavage of lignin–carbohydrate complexes (LCC), while moisture acts as an in situ susceptor that generates internal steam pressure and micro-fractures [90]. The net effect is fast softening/swelling of the cell wall and greater enzyme accessibility compared with conventional convective heating [91].

• Lignin removal [delignification].

Under MW with suitable media (e.g., hydrotropes, alkali, organosolv, or ionic liquids), solubilization of biomass components is markedly enhanced [91]. High-pressure MW hydrotropic pretreatment of softwood, hardwood, and non-wood feedstocks using sodium cumene sulfonate (NaCS) led to the partial removal of hemicellulose and extractives, with up to 55% overall mass loss depending on the substrate and conditions, while weakening lignin bonds and making the biomass more amenable to subsequent treatments [92]. FTIR confirmed the attenuation of lignin aromatic bands (1512–1605 cm⁻¹) after MW pretreatment, indicating structural modifications rather than complete lignin removal [92].

• Cellulose crystallinity.

X-ray diffraction generally shows an apparent increase in crystallinity index (CrI) because MW-assisted pretreatment preferentially removes amorphous hemicellulose and lignin, enriching crystalline cellulose. For example, MW pretreatment (water, NaOH, NaCS, ethanol/H₂SO₄) increased CrI across pine, beech and straw; reported increases include bamboo (from 49.5 to 56.9% with MW hot water), catalpa sawdust (from 32.3 to 39.4% with MW + NaOH), and eucalyptus (from 55.0 to 59.5% with MW + IL). Note that CrI can also decrease when severe conditions induce cellulose chain scission or partial amorphization; however, recent high-pressure MW studies predominantly report CrI increases associated with amorphous-phase removal rather than lattice disorder [92]. In another work, Biomass samples were subjected to microwave pretreatment under varying powers (259, 462, 595, and 700 W) and exposure times (1, 2, 3, and 5 min). According to Venegas et al. [93], the maximum temperatures reached under the 700 W–5 min condition were 147.69 °C for Pinus radiata (PR) and 130.71 °C for Eucalyptus globulus (EG), which induced rearrangement of cellulose crystalline chains through vibrational excitation, increased the internal energy of the biomass, and promoted lignin decomposition by reaching its glass transition temperature.

• Porosity and surface morphology.

MW pretreatment produces pronounced microstructural changes—fiber fibrillation, fissures, and pore formation—observed by SEM. After MW under alkaline/hydrotropic media, surfaces transition from smooth/compact to porous/fibrous with visible openings; this correlates with higher enzymatic digestibility due to increased accessible surface area and pore volume (Several studies report clear SEM evidence; when BET is reported, increases are consistent with the SEM trends) [92].

• Hemicellulose deconstruction and acetyl removal.

MW-assisted liquid hot water in bamboo removed hemicellulose and other non-crystalline fractions, exposing crystalline cellulose; MW-hydrothermal pretreatment of Acacia at 170 °C for 60 min enhanced hemicellulose dissolution, further contributing to porosity and reduced recalcitrance [94].

• Mechanistic implications for pretreatment design.

Because MW fields directly heat polar phases and reaction media, combining MW with alkali, hydrotropes, DES/ILs, or organosolv accelerates β-O-4 ether cleavage in lignin, breaks LCCs, deacetylates xylan, and promotes selective solubilization of non-cellulosic fractions. The resulting biomass exhibits (i) lower lignin content, (ii) increased CrI (by enrichment), and (iii) higher porosity—conditions that jointly improve enzymatic hydrolysis kinetics and overall saccharification yields [91].

3.2.2. How Do Combined Microwave-Assisted Pretreatments Influence the Physicochemical Properties of Lignocellulosic Biomass and What Are Their Implications for Enhancing Energy Efficiency, Product Selectivity, and Environmental Sustainability in Subsequent Pyrolysis Processes?

Hybrid pretreatment strategies that integrate microwaves with chemical, physical, or solvent-based approaches have recently attracted attention as a means of improving the efficiency and selectivity of biomass pyrolysis. By leveraging the volumetric and rapid heating of microwaves together with mechanisms such as alkali- or acid-driven delignification, hydrothermal solubilization, ultrasonic disruption, or ionic liquid dissolution, researchers have reported significant improvements in lignin removal, cellulose accessibility, and sugar or bio-oil yields. Such synergies not only reduce energy barriers and process times but also contribute to higher overall conversion efficiency. Table S1 compiles representative studies of combined microwave-assisted pretreatments, summarizing their key findings and impacts on subsequent biomass pyrolysis.

3.2.3. What Are the Current Challenges, Limitations, and Future Perspectives in Applying Microwave Pretreatment at Pilot and Industrial Scales for Sustainable Bioenergy Production?

From an environmental perspective, microwave pretreatment (MWP) can help decarbonize thermochemical biomass conversion by reducing process energy demand through rapid, volumetric, and selective heating; shortening residence times and avoiding energy-intensive auxiliaries such as deep drying or extensive size reduction; and enabling direct electrification so that low-carbon renewable power can displace fossil process heat [95,96]. Recent assessments of microwave-assisted (catalytic) pyrolysis show that product selectivity improvements and the ability to heat moisture-containing feedstocks can offset some electricity penalties, especially when efficient microwave sources and strong microwave absorbers are used, and when grid mixes are low-carbon or coupled to onsite renewables [58]. In addition, pretreatment-driven structural changes—partial delignification, increased cellulose accessibility, and higher porosity—translate into faster conversion and higher yields, which reduce the feedstock and logistics footprints per unit of product. Overall, while scale-up and device efficiency remain critical bottlenecks, the literature points to a credible pathway where MWP lowers energy use and mitigates greenhouse gas (GHG) emissions, particularly in distributed systems and when powered by renewable electricity. MWP has emerged as a promising technology due to several advantages over conventional thermal conversion methods [45,60,95,97]. One of its key benefits is the ability to achieve rapid and uniform heating, which shortens processing times and reduces overall energy consumption as a result of direct energy conversion within the biomass, thereby improving energy efficiency [98,99]. In addition, microwave pretreatment alters the structural characteristics of biomass by disrupting cell wall integrity, increasing porosity, and reducing crystallinity. These changes enhance the efficiency of subsequent pyrolysis and contribute to higher product quality; for example, formic acid microwave-assisted pretreatment has been shown to increase the yield of aromatic compounds during catalytic fast pyrolysis [100,101,102]. Another significant advantage of MWP is its contribution to environmental sustainability. By improving biomass conversion efficiency and reducing dependency on fossil fuels, MWP lowers greenhouse gas emissions while promoting the circular economy through the valorization of biomass waste into valuable products such as biochar, bio-oil, and syngas [103]. Moreover, its versatility allows the conversion of diverse waste streams, including both biomass and plastics, into high-value products, thereby addressing challenges related to waste accumulation [96,97]. Finally, the integration of microwave pretreatment with other physicochemical methods enhances the sustainability of biofuel production, improving conversion efficiency while minimizing environmental impacts [60].

The most significant advances reported in this field are summarized in

Table 2. Key environmental contributions of microwave pretreatment [before pyrolysis].

Environmental/Sustainability Advantage	Why It Helps (Mechanism)	Ref.
Lower process energy via rapid, volumetric heating	Microwaves couple directly with dipoles/ions, cutting heat-up times and thermal losses vs. convective heating.	[60]
Less pre-drying/size-reduction energy	MAP tolerates higher moisture and larger particle sizes, avoiding energy-intensive drying and fine milling.	[58]
Integration of renewables	Microwaves are inherently electric—easy coupling to PV/wind or hybrid systems	[104]
Improved product selectivity → lower downstream upgrading burden	Selective, in-core heating and catalyst/absorber synergy yield higher-quality, lower-oxygen bio-oil, reducing hydrotreating severity and associated emissions.	[105]
Potential life-cycle GHG reduction (with biochar co-product)	Biochar/activated carbon from MAP can act as carbon sequestration; MAP systems can be designed for distributed conversion to cut transport emissions.	[106]
Reduced reagent intensity when paired with tunable pretreatments	MAP enhances physicochemical pretreatments (acid/alkali/organosolv/hydrothermal), enabling milder conditions or shorter times for delignification/demineralization.	[60,107]
Lower emissions from process intensification	Compact reactors, rapid start/stop, and targeted heating minimize off-gas/cooling loads relative to large, thermally massive units.	[58]
Valorization of wet/heterogeneous wastes	MAP handles moist, variable feedstocks (sludge, residues), enabling diversion from landfilling and fossil displacement.	[108]
Scalable routes to higher-surface-area biochar (adsorbents/soil)	Faster heating and localized hotspots can yield chars with higher surface area, supporting soil health, pollutant capture, and circular uses.	[105]

, which compiles key environmental contributions of microwave pretreatment prior to biomass pyrolysis.

3.2.4. What Were the Most Important Advances and Discoveries in the Field of Microwave-Assisted Biomass Pretreatment Prior to Pyrolysis in the Last Five Years?

Figure 10 presents the article selection scheme following the PRISMA methodology, which was applied to refine the search and identify the most relevant studies from the last five years on microwave pretreatment of biomass prior to pyrolysis. And Table S2 presents the most relevant results reported in the literature over the last five years in the field of microwave-assisted biomass pretreatment prior to pyrolysis.

4. Future Works

Future research on microwave pretreatment [MWP] of lignocellulosic biomass prior to pyrolysis should focus on six fronts:

• Reactor engineering, scale-up, and electrification efficiency.

Bench-scale benefits must be translated into continuous, kilo- to ton-scale systems with rigorous energy balances. Priorities include optimizing applicator geometry for heterogeneous, moist feedstocks; transitioning from magnetron to high-efficiency solid-state sources; and mapping spatiotemporal temperature and dielectric fields with in situ diagnostics for model-based control. Community-accepted reporting of wall-to-core energy efficiency and heat losses is needed to make results comparable across labs and scales [58].

• Specific reaction rates of the formation of pyrolysis products

One important limitation identified in the present review is the scarcity of studies that explicitly analyze the specific reaction rates for the formation of pyrolysis products (biochar, bio-oil, and gas) after microwave pretreatment. While several works have investigated kinetic parameters and product distributions, none have directly quantified the individual reaction rates of product formation. Addressing this gap is essential, since reaction kinetics provide fundamental insights into the mechanisms of biomass conversion and are key to optimizing process efficiency, selectivity, and scalability. For instance, Chen et al. [109] analyzed the thermal decomposition of Dunaliella salina during microwave pyrolysis using composite additives and applied kinetic modeling to calculate activation energies, although their focus remained on overall mass loss rather than on product-specific rates. Similarly, Amer et al. [110] investigated Egyptian agricultural and woody biomasses subjected to microwave drying pretreatment and calculated kinetic parameters via Arrhenius and Coats-Redfern methods, but without separating the kinetics of biochar, bio-oil, and gas formation. Liang et al. [111] examined microwave pretreatment effects on the catalytic pyrolysis of water hyacinth, reporting kinetic data using Flynn-Wall-Ozawa analysis and correlating pretreatment power/duration with product composition, though again without addressing specific reaction rates. Finally, Qiu et al. [112] provided a comprehensive review of microwave-assisted pyrolysis, highlighting the influence of microwave pretreatment on reaction pathways and product distribution, and underlining the need for more detailed kinetic studies. Based on these findings, we emphasize that future research should focus on elucidating the product-specific reaction kinetics under microwave pretreatment, as this remains an underexplored but highly relevant area for advancing both fundamental understanding and industrial applications of biomass pyrolysis.

• Microwave absorbers and catalytic co-pretreatments.

Systematic screening and durability testing of absorbers (e.g., biochar, SiC, carbons) and supported catalysts under MWP conditions are needed to understand long-term stability, fouling, and regeneration, as well as their consequences for downstream pyrolysis selectivity (oxygenate suppression, aromatic enrichment). Hybrid pretreatments—alkali/acid, organosolv, and hydrothermal—should be rationally designed to couple selective dielectric heating with targeted delignification/hemicellulose removal [113].

• Microwave-assisted hydrothermal and organosolv routes.

MW-hydrothermal pretreatment has shown faster hemicellulose solubilization and improved carbohydrate recovery; future studies should quantify how such structural changes (porosity, crystallinity, lignin condensation) shift pyrolysis kinetics and product distributions in continuous reactors. Likewise, MW-assisted organosolv systems warrant solvent/acid selection maps (including recyclability and corrosion) tied to techno-economic and safety outcomes [114,115].

• Integrated TEA/LCA under renewable electricity supply.

Because MWP is inherently electrical, future work should couple reactor data with grid-specific emission factors and on-site renewable scenarios, explicitly propagating absorber dosage, electrical efficiency, and throughput into cost and GHG metrics. Solar-powered or hybrid renewable cases suggest favorable windows, but sensitivity analyses and uncertainty quantification are needed for policy-relevant comparisons to conventional heating [104].

• Feedstock breadth, moisture tolerance, and quality control.

MWP’s ability to process wet and ash-rich residues should be benchmarked across diverse feedstocks (agricultural residues, forestry by-products, sewage-derived biosolids), with standardized protocols for moisture/ash normalization, particle-size distributions, and scale-relevant handling. Kinetic/thermogravimetric studies indicate MWP can reduce activation energies and shift decomposition temperatures; connecting these shifts to continuous reactor performance and product upgrading remains an open task [69].

• Data standards and reproducibility.

Field-wide datasets that report dielectric properties vs. temperature and moisture, absolute power balances (delivered/absorbed), and absorber/catalyst inventories will enable predictive models and fair cross-study comparisons. Reviews call for harmonized metrics linking pretreatment conditions to structural descriptors (e.g., accessible cellulose, lignin S/G ratio) and to downstream pyrolysis yields/quality [113].

5. Conclusions

This review highlights microwave pretreatment as a transformative strategy for enhancing biomass deconstruction. When combined with chemical, hydrothermal, ultrasonic, or ionic-liquid methods, MWP promotes delignification, reduces cellulose crystallinity, increases porosity, and improves enzymatic hydrolysis and pyrolysis efficiency. It also enables shorter residence times, facilitates the use of wet or ash-rich feedstocks, and can be directly powered by renewable electricity, aligning with low-carbon energy transition goals.

Key insights from the literature indicate that MWP plays a pivotal role in advancing biomass valorization. It reduces activation energy and enhances selectivity, generating higher-value products such as bio-oil, functional biochar, and sugar-rich hydrolysates. However, progress toward pilot- and industrial-scale implementation requires engineering advances, including optimized applicator designs, solid-state microwave generators, and the real-time monitoring of dielectric properties. The integration of MWP with complementary pretreatments such as alkali, acid, hydrothermal, or ionic liquids demonstrates synergistic potential to reduce energy demand and improve fractionation efficiency. Moreover, techno-economic assessments (TEA) and life cycle analyses (LCA) suggest that MWP powered by renewable energy can significantly reduce greenhouse gas emissions and logistic footprints, reinforcing its role in sustainable biorefineries. At the same time, valorization of lignin-derived compounds, hemicellulosic sugars, and bioactive molecules generated during pretreatment could enhance profitability and resource efficiency. Finally, the adoption of standardized metrics for dielectric properties, energy balances, and pretreatment efficiency, together with open data sharing, will be critical to ensure the reproducibility, predictive modeling, and broader adoption of MWP technologies.

In conclusion, microwave pretreatment stands as a transformative enabler for sustainable bioenergy production. Its continued development, informed by mechanistic understanding, engineering innovations, and integrated TEA/LCA, offers a credible pathway to scale-up. As such, MWP has the potential not only to improve the efficiency of pyrolysis and related processes but also to accelerate the broader transition towards renewable, low-carbon energy systems.