Exploring the Energy Potential of Residual Biomass: A Bibliometric Analysis

Abstract

The increasing challenge of waste disposal and the growing demand for reliable renewable energy sources are particularly critical in developing countries. Waste-to-Energy technologies have emerged as a promising approach to harness the energy potential of waste in an economically viable and environmentally sustainable manner. This study provides a global overview of scientific developments and technological trends in Waste-to-Energy through a bibliometric analysis of 1869 documents retrieved from the Web of Science database, covering the period 2017–2021 and focusing on the field of bioenergy. Here, the term bioenergy is used in a broad sense, encompassing energy recovery from both biogenic waste (e.g., food waste, agricultural residues) and non-biogenic waste (e.g., plastics, synthetic polymers) under the Waste-to-Energy framework. The analysis revealed that developing countries prioritize specific technologies for energy recovery: anaerobic digestion for organic waste, incineration for non-biodegradable mixed waste, and pyrolysis and gasification for carbon-rich waste streams such as biomass and plastics. Landfilling is mentioned solely as a final disposal route for inert materials, not as an energy recovery pathway. Additionally, research highlights the potential benefits of synergistic combinations of raw materials in improving product quality and reducing pollution in Waste-to-Energy processes. This bibliometric and content-based review supports future research efforts by identifying key trends, influential contributions, and critical implementation challenges. The findings underscore the role of Waste-to-Energy technologies as valuable tools in sustainable waste management strategies, especially in regions where improving energy access and reducing environmental impact are pressing concerns.

1. Introduction

In recent times, the extreme and incessant use of fossil fuels and the impact of greenhouse gases (GHGs) have steered research toward the production of renewable energy from organic resources and waste [1,2,3]. Despite remarkable socioeconomic and technological developments, a quarter of the world’s population still depends on conventional fuels for domestic use, such as firewood, charcoal, coal, kerosene, or other non-modern fuels traditionally used for cooking and heating [4]. Population growth, urbanization, and economic development have been key drivers of environmental degradation and the global increase in municipal solid waste (MSW) generation, such as paper, plastic, textiles, and food waste [5,6]. Within this context, the MSW generation rate per capita in developed countries is higher than that in developing countries, as this rate is commonly associated with the level of economic and social prosperity of a given country. However, in the next few decades, developing countries are expected to match the MSW generation rate of developed countries, mainly because of their eating habits, consumption patterns, and the standard of living of their urban population [7,8]. Consequently, Waste-to-Energy (WtE) technologies are now considered an essential option for the sustainable management of organic waste and economically viable and environmentally sustainable renewable energy sources [9]. Figure 1 describes the average recoverable energy contents (in terms of electrical power efficiency) for different MSW components using several Waste-to-Energy technologies: anaerobic digestion (AD), gasification, and incineration. In this figure, it is evident that AD is the most suitable option for food and organic waste, while gasification shows the highest theoretical energy yield for plastic waste. However, in industrial practice, plastic waste is more commonly valorized through pyrolysis or chemical recycling, whereas gasification is predominantly applied to lignocellulosic biomass. The designation of gasification as the “best alternative” for plastics in this context reflects the comparative results of the assessed energy recovery potential, rather than its current prevalence in industrial applications. Here, incineration also represents an interesting solution for all waste; however, other types of waste, such as inert waste, metals, and glass waste, etc., were not considered in this assessment.

Even though developed countries have already started implementing WtE technologies for effective waste management, significant implementation challenges remain. Identifying the best WtE option is one of those challenges, since there is social opposition to the development of WtE facilities due to their potentially polluting emissions, along with high infrastructure costs and difficulties in obtaining funds [11,12]. Therefore, to secure the successful implementation of any WtE technology, its acceptance by the local community is crucial [13,14]. This review offers an extensive analysis of scientific advances in current WtE technology at a global level.

Therefore, the main objective of this study is to provide a comprehensive bibliometric and content-based review of global research on Waste-to-Energy (WtE) technologies, with a particular emphasis on residual biomass. By integrating global and regional analyses, including a case study of Colombia, this work seeks to identify technological trends, highlight differences in adoption between developed and developing countries, and discuss implementation challenges. The findings aim to inform future research, policy-making, and technological development in the field of sustainable waste management and renewable energy generation.

This paper is organized into three sections. Section 2 discusses different options for converting waste into energy through WtE technologies. The methodology used and developed in this document, as well as the results and the bibliometric analysis for the contributions to bioenergy and WtE technologies, are presented in Section 3. Finally, Section 4 discusses the conclusions derived from the study.

2. Waste-to-Energy Conversion Options

WtE technologies offer an efficient solution to the problem of waste disposal and energy demands [15,16]. WtE technologies include various emerging technologies capable of turning waste into chemicals, such as methanol, biodiesel, and clean renewable energy. These technologies can have thermal, chemical, or biological conversion routes and have been successfully used in waste management in Europe and Asia [17]. Continuous development of specialized technologies for incineration and generation of other forms of energy from waste, including using waste as an alternative fuel for industrial production, is supported by an effort to minimize waste [18] following global guidelines for CO₂ reduction, especially when waste is considered an important GHG source. As illustrated in Figure 2, WtE conversion processes can be classified into two broad categories: biochemical and thermochemical processes. Biochemical processes refer to the biological decomposition of the organic portion of waste under microbial action, either in the presence (aerobic) or absence (anaerobic) of oxygen. Conversely, thermochemical processes refer to the thermal decomposition of matter.

3. Bibliometric Analysis of Methodological References

This section discusses the development of the bibliometric analysis, conducted to study the scientific advances in current WtE technology scenarios. The analysis included the methodological development, the definition of the databases checked, the keywords used, and the time horizon, among other elements required to complete the study.

3.1. Methodological Development

As part of this assessment, 1869 bioenergy-related documents (papers, reviews, books, among others) were selected from the Web of Science (Wos) database. For this scientific evaluation, search criteria were first defined covering the period 2017–2021 to address current and accurate information regarding our topic of interest. Subsequently, we used R-Studio 1.3 and HistCite 12.3.17 software to generate chronological reports categorizing the most influential works by author, journal, institution, country, cited documents, and keywords. From the initial publications, a sample was extracted according to predefined inclusion and exclusion criteria, to conduct a content analysis that can act as a source of scientific information on technological trends in using waste for generating energy.

Bibliometric and content assessments identified methodologies that estimate the energy potential of waste and provided a description of the significant contributions in the field of bioenergy. The information selection process included three phases: (i) literature review and data collection; (ii) extraction, loading, and conversion; and (iii) data synthesis. The information for this study was obtained from the WoS database, which covers a wide variety of areas related to bioenergy and hosts publications from Journal Citation Reports [20].

For extensive data collection, we selected the following keywords: “bioenergy”, “biomass”, “biofuel”, and “municipal solid waste”. These were combined using the Boolean operators AND and OR to refine the results. The search covered documents published between 2017 and 2021 (Information compiled on 13 December 2021, at 15:57:47). The selection of the 2017–2021 period was intentional to capture recent and consolidated trends in bioenergy research, while ensuring sufficient citation accumulation to support bibliometric significance. Although more recent data exist, limiting the range to this five-year period allowed for clearer identification of established patterns, avoided bias from incomplete citation curves, and maintained methodological consistency with prior bibliometric studies in environmental science. Once the information from the database had been collected, it was downloaded in “BibTex” format for execution in the Command Line Interface of R-Studio software, using the Bibliometrix library and the Biblioshiny web interface. Simultaneously, WoS files were downloaded in “Plain Text File” format, so that they could be processed in HistCite 12.3.17 software, which generated chronological historiographies that highlighted the most cited works of the collection obtained. It also included classifications by author, journal, institution, country, cited documents, and keywords [21]. Figure 3 presents the details of the search procedure, how it was applied in the database, and the results obtained.

These Figures were developed manually based on information generated by bibliometric analysis software and processed in OriginPro 8 and VOSviewer 1.6.17.

Table 1. Summary of the data, with the main information.

Description	Results
Data Overview
Sampling Period	2017–2021
Sources (Magazines, Books, etc.)	388
Documents	1869
Average years since publication	1.74
Average number of citations per document	13.7
Average number of citations per year per document	4.885
Document Types
Papers	1475 (78.92%)
Review Papers	299 (16.00%)
Proceedings Paper	87 (4.66%)
Editorial Material	6 (0.32%)
Letters	1 (0.05%)
Meeting Abstract	1 (0.05%)
Authors
Authors	5818
Author Participation	8668
Authors of Single-Author Documents	41
Authors of Multiple-Author Documents	5777
Author Collaboration
Single-Author Documents	46
Documents per Author	0.321
Authors per Document	3.11
Co-authors per Document	4.64
Collaboration Index	3.17

summarizes the resulting dataset.

This combined methodological framework goes beyond conventional bibliometric reviews by integrating quantitative trend mapping with an in-depth content analysis of purposively selected high-impact and context-relevant publications. This dual approach enables a richer interpretation of scientific output, capturing both macro-level patterns and micro-level technological insights. Additionally, the inclusion of a comparative perspective between developed and developing countries, together with a focused case study on Colombia, enhances the practical relevance of the findings by linking global research trends to specific policy, socio-economic, and technological contexts. Such an integrated design has been rarely applied in the Waste-to-Energy (WtE) literature and represents a methodological innovation that broadens the applicability and interpretive depth of bibliometric studies in this field.

3.2. Classification Method

For the content analysis, a sample from all publications was taken, and its size was calculated based on Equation (1).

$$n={\displaystyle \frac{N\times Z^2\times p\times q}{e^2\times \left(N-1\right)+Z^2\times p\times q}}$$

(1)

where N is the population size, Z represents the confidence level, e is the maximum accepted estimation error, p is the probability of the event occurring, and q is the probability that the event does NOT occur. With a 90% confidence level and 10% margin of error, the sample size was determined as 66 documents from the total set of 1869. Although this sample size was calculated assuming a random sampling approach, in practice the selection followed purposive criteria, prioritizing the most cited and most recent publications, as well as those most relevant to the study’s scope. This approach aimed to maximize the relevance and depth of the content analysis, rather than achieve strict statistical representativeness. Therefore, the results should be interpreted as illustrative of key technological and thematic trends, rather than as a probabilistic generalization of the entire dataset. The review of these documents was conducted based on the evaluation criteria listed in

Table 2. Evaluation criteria for content analysis.

Sample Size	Assessment Criteria
66	25 publications with the highest number of citations
	16 publications from Colombia
	25 most recent publications

.

It is important to note that the high proportion of publications from Colombia (approximately 24% of the total among developing countries) reflects the actual distribution of indexed scientific production in the Web of Science database for the 2017–2021 period. This proportion does not aim to suggest that Colombia is representative of all developing countries; rather, it illustrates a specific national research dynamic in the Waste-to-Energy (WtE) and bioenergy fields. Colombia’s prominence in the dataset is linked to its significant scientific activity within the Latin American context and the alignment of national policies with renewable energy development.

3.3. Results Analysis

In addition to the global analysis, a regional focus was applied using Colombia as a case study. This selection was motivated by the authors’ interest in highlighting the country’s scientific output and its alignment with global Waste-to-Energy (WtE) research trends, as well as by the availability of sufficient bibliographic records to enable a meaningful analysis.

3.3.1. Top Publications and Authors

This bibliometric analysis presents the authors, institutions, and countries that have contributed research in the field of bioenergy. Figure 4 illustrates the annual scientific production, and the corresponding number of citations recorded during the 2017–2021 period. This visualization highlights the evolution of research activity and its academic impact, offering insights into trends in publication frequency and the growing relevance of the topic within the scientific community.

The number of publications reveals a growth rate of 13.57%, indicating an increase in the volume of knowledge, associated with the growing need to find science-based solutions for renewable energy. However, this increase is also the result of social awareness about seeking ways to reduce the environmental impacts of poor waste management.

As may be observed in Figure 5, Meisam Tabatabaei, Professor of Environmental Biotechnology at University Malaysia Terengganu and Editor-in-Chief of the Biofuel Research Journal (BRJ), is the most prolific author in this research area. Meisam has collaborated with the United Nations Development Program to encourage the development of biofuels around the world. Likewise, since 2016, he has served as the main contributor to the “Lancet Countdown on Health and Climate Change”, an international collaboration of more than 120 scientists from 43 universities, research institutes, and UN organizations, including the WHO and the World Bank [22]. The author with the second highest number of publications is Mortaza Aghbashlo, Associate Professor in Biosystems Engineering at the University of Tehran, Iran. Mortaza has published more than 150 papers in journals, such as “Progress in Energy and Combustion Sciences” (Impact Factor: 26.467), and “Renewable and Sustainable Energy Reviews” (Impact Factor: 14.982), etc. Dr. Aghbashlo has been on the WoS’s list of Highly Cited Researchers (top 1% of world scientists) in the engineering category since 2017. He is the editor of book volumes, such as “Biodiesel: from Production to Combustion” and “Fungi in Fuel Biotechnology”, which were published by Springer Nature in 2018. Moreover, he is on the BRJ Editorial Board [23].

The number of citations was used to identify the most cited articles, which indicates the impact and degree of development that these publications have had in the field [21]. The scientific impact was studied by assessing the ten most cited publications for papers published from 2017 to 2021.

Table 3. Top 10 publications with the highest number of citations from 2017 to 2021.

No.	Document Title	Number of Citations	Reference
1	A review of the hydrothermal carbonization of biomass waste for hydrochar formation: Process conditions, fundamentals, and physicochemical properties	349	[25]
2	A Technological Overview of Biogas Production from Biowaste	209	[26]
3	Recent advances in the gasification of waste plastics. A critical overview	205	[27]
4	Fates of Chemical Elements in Biomass during Its Pyrolysis	203	[28]
5	A review of technological options of waste-to-energy for effective management of municipal solid waste	202	[4]
6	Co-pyrolysis of biomass and waste plastics as a thermochemical conversion technology for high-grade biofuel production: Recent progress and future directions elsewhere worldwide	186	[29]
7	Anaerobic co-digestion of animal manures and lignocellulosic residues as a potent approach for sustainable biogas production	181	[30]
8	Biomass pyrolysis: A review of the process development and challenges from initial research up to the commercialization stage	163	[31]
9	Hydrogen production from biomass using dark fermentation	153	[32]
10	Biorefineries in circular bioeconomy: A comprehensive review	145	[33]

provides a list of the most cited publications. Here, the ten publications are review papers, which have a high level of importance to the researchers in this field. These papers are valuable as they project theoretical or referential frameworks, which are essential for future studies [24].

3.3.2. Top Journals

Journals with a high impact factor contribute to the publication of works related to the field of bioenergy; therefore,

Table 4. Top 5 journals with the greatest contributions from 2017 to 2021.

No.	Journal	Impact Factor	Number of Publications	Global Percentage	Number of Citations	Global Percentage
1	Waste and Biomass Valorization	3.703	112	6.0	853	3.3
2	Waste Management	7.145	102	5.5	2127	8.3
3	Journal of Cleaner Production	9.297	95	5.1	1488	5.8
4	Renewable & Sustainable Energy Reviews	14.982	86	4.6	3308	12.9
5	Bioresource Technology	9.642	81	4.3	2096	8.2

presents the Top 5 journals that have contributed the most to the subject matter. For example, the Waste and Biomass Valorization journal reported the largest number of publications. This journal addresses technologies related to environmental engineering, renewable energy, sustainability and the environment, and waste management and disposal [34]. Furthermore, the Renewable and Sustainable Energy Reviews journal had the largest number of citations, positioning itself as one of the scientific journals with the highest level of impact in the field of bioenergy.

3.3.3. Contributions by Institutions

The institutional and country-level analysis revealed that 1998 research centers from 103 countries contributed to bioenergy research between 2017 and 2021. Among them, five institutions published at least thirty articles on the topic during this period. The University of Tehran emerged as the most prolific institution, accounting for 2.73% of the total publications (51 out of 1869 articles), followed by the Indian Institute of Technology with 2.30%. These were closely followed by the Chinese Academy of Sciences (2.25%), the Council of Scientific and Industrial Research (1.77%), and Tsinghua University (1.66%). Figure 6 illustrates the chronological distribution of publications from these leading institutions.

3.3.4. Contributions per Country

Asian countries, such as China and India, have made significant contributions to bioenergy research. It is evident that out of the first five universities with the highest number of publications, two are located in the People’s Republic of China (Chinese Academy of Sciences and Tsinghua University), with two in India (Indian Institutes of Technology and Council of Scientific & Industrial Research). During 2017–2021, a large part of the bioenergy research came from the People’s Republic of China, with 467 publications (25% of all publications), followed by India with 186 documents (9.957%), the United States with 175 documents (9.368%), Italy with 144 documents (7.709%), and England with 110 documents (5.889%). Figure 7 presents more information on the geographical distribution of the number of publications.

The bibliometric analysis revealed a marked concentration of contributions from Asian countries, reflecting their significant role in advancing Waste-to-Energy (WtE) technologies. This strong regional presence may be linked to aggressive governmental policies, rapid urbanization, and pressing energy and environmental needs that stimulate intensified research and technological development in the field.

Figure 8 presents an analysis of the average annual citations for bioenergy research publications from countries with over 100 documents. Publications originating from the People’s Republic of China received approximately 1463 citations per year on average, underscoring the significant impact of Chinese research in this field. They were followed by publications from the United States (~598 citations/year), Italy (~480), England (~380), Iran (~378), India (~461), and Spain (~216). This ordering highlights not only the volume, but also the citation influence of research outputs by country during the studied period.

3.3.5. Keywords

Author keywords (keywords assigned by the authors) and KeyWords Plus (terms generated by Web of Science based on the titles of cited references) are useful tools for determining research focus and future trends in the field [35]. This study identified a total of 6604 KeyWords Plus terms, with “municipal solid waste” appearing in 830 documents (approximately 44.4% of the total documents). Similarly, author keywords totaled 4316, with “municipal solid waste” assigned in 269 documents (about 14.4%). Other frequently occurring KeyWords Plus terms included “biomass” (455 documents, 24.3%), “sewage sludge” (217 documents, 11.7%), “food waste” (199 documents, 10.7%), and “energy” (190 documents, 10.2%). Regarding author keywords, other common terms included “anaerobic digestion” (209 documents, 9.7%), “biomass” (165 documents, 7.6%), “gasification” (152 documents, 7.0%), and “biogas” (131 documents, 6.1%). The relationship between the twenty most used keywords in bioenergy research is indicated by lines and connectors in Figure 9.

While the bibliometric indicators provide a quantitative understanding of publication trends, future research should also integrate qualitative assessments, to better contextualize the drivers behind national or institutional productivity. For instance, the prominence of China and India may be linked to national policies promoting circular economy models and renewable energy transitions. Additionally, differences in keyword usage or citation impact suggest not only research volume but also variations in strategic focus and scientific influence across regions. These aspects deserve further exploration through mixed-method approaches that combine bibliometric data with policy and funding analyses.

3.3.6. Discussion on Selected Publications

Recent studies published after 2021 further reinforce the importance of the trends identified in this work. For instance, ref. [36] offered a systematic review of hybrid WtE systems combining thermochemical conversion (gasification, pyrolysis, or hydrothermal routes) with other renewable energy sources such as solar thermal and anaerobic digestion. Similarly, ref. [37] discussed the technological convergence between anaerobic digestion and gasification as a hybrid approach for sustainable Waste-to-Energy generation. More recently, ref. [38] presented an integrated municipal solid waste gasification system coupled with absorption heat-pump drying, highlighting advances in energy efficiency for practical applications. The alignment between these findings and our 2017–2021 bibliometric and content analysis supports the continued relevance of the technological priorities and challenges we have identified.

Developing countries commonly adopt the following technologies for MSW management [4]: anaerobic digestion (AD) for organic waste, incineration for mixed MSW (not biodegradable), pyrolysis and gasification for specific types of waste (plastic, tires, electronic equipment, electronic waste, wood waste, etc.), and landfills for inert waste. In contrast, developed countries place more emphasis on improving process efficiency, recycling/recovery, and pollution control strategies [4].

Table 5. WtE technology cost comparisons. Source: [5].

WtE Technology	Capital Costs (US$/Ton MSW/Year)	Operation Costs (US$/Ton MSW/Year)
Incineration	400–700	40–70
Pyrolysis	400–700	50–80
Gasification	250–850	45–85
Anaerobic Digestion	50–350	5–35
Gas Recovery Landfill	10–30	1–3

lists the estimated costs of WtE technologies; however, the actual cost depends on several factors, such as government incentives, raw material prices, and the availability of skilled labor [16].

Furthermore, we also estimated the Global Warming Potential of these WtE technologies (See

Table 6. Global warming potential from WtE technologies. Source: [5].

WtE Technology	Global Warming Potential ($\mathbf{k}\mathbf{g} \mathbf{C}{\mathbf{O}}_{2_{\mathbf{e}\mathbf{q}}} \mathbf{P}\mathbf{e}\mathbf{r} \mathbf{M}\mathbf{W}\mathbf{h}$)
Incineration	424
Pyrolysis and Gasification	412
Anaerobic Digestion	222
Gas Recovery Landfill	746

below).

Table 7. Contributions by technology.

Technology	Contribution	Author(s)
Anaerobic Digestion (AD)	The energy potential of animal manure for biogas production is suboptimal due to its high nitrogen content, which lowers its C/N ratio. To address this deficiency, co-digestion with lignocellulosic waste (agricultural, forestry, municipal solids, and agro-industrial waste) compensates for the carbon deficit of the manure and improves the methane yield of the process.	[30]
	To improve the biodegradability of agricultural biomass, pretreatments are used according to the physicochemical characteristics of each raw material. The following treatments can be used: Chemicals (acid, alkali or oxidant) Thermal (hydrothermal or steam) Mechanical (crushing, grinding, chipping, etc.) Biological (industrial enzymes, lignolytic enzymes, and cellulose)	[39]
	Based on the technological surveillance methodology conducted for the 2009–2019 period in the Scopus and SciELO databases on AD studies of Biological and Food Waste, the main factors that affect the AD process were determined. The most frequently evaluated factor is related to experimental configuration, since it has the largest number of associated variables. Nevertheless, most factors have not yet been widely studied. The organic load rate and the number of stages involved in the process are the most commonly studied variables. Substrate and environmental factors follow, with pretreatment being the main studied variable. Finally, there is the inoculum, which is mainly associated with variables such as the inoculum mixture and the substrate to inoculum ratio (S/I). On the other hand, among the least studied aspects are headspace (2.16%) and retention time (3.03%). In addition, after reviewing the studies and the variables assessed, only 18% simultaneously assessed two or more variables. This analysis is useful to establish whether simultaneous interactions exist among these variables.	[40]
	Manure was highlighted as a significant source of ammonia, methane, and nitrogen oxide emissions, which were estimated to be 40%, 22.5%, and 28%, respectively, of total United Kingdom (UK) anthropogenic emissions. AD is a technology that can drastically reduce the potential for manure contamination compared to process absence, thus reducing approximately 90% of methane and 50% of nitrogen oxide emissions. More than 1615 TWh of electricity could be generated, since most of the 90 million tons of manure produced by the UK would be processed through biogas capture.	[41]
Gasification	Three gasification agents were used: air, steam, and oxygen, which produce different gas compositions, calorific values, and yields. Based on this, the authors deduced that steam produces a greater amount of ${ \mathrm{H}}_2$, with a higher calorific value compared to gasification by air and oxygen, thus also reducing tar content from 17% to 24% more than the other agents. Although air is the most common agent for gasification, since it is an abundant natural compound and it is easy to use, it has a major drawback: it contains approximately 79% nitrogen, which largely dilutes the resulting gas and increases its separation cost. Moreover, the re-sulting gas has a low calorific value $(3.5-7.8 \mathrm{MJ}/{\mathrm{m}}^3)$, being constrained to the on-site generation of heat and power. Oxygen has two drawbacks: the high cost of pure oxygen and the large cost needed to separate oxygen from syngas; hence, it is often combined with steam as a gasification agent.	[42]
	Plastic waste steam gasification produces a synthesis gas rich in ${ \mathrm{H}}_2$, of interest both for synthesis and energy applications. Furthermore, the absence of ${\mathrm{N}}_2$ increases the calorific value of synthesis gas in values exceeding $15 \mathrm{MJ}/{\mathrm{m}}^3$. Despite obtaining a higher quality syngas through steam gasification of plastics, this process must overcome two main limitations: process endothermicity, and the high tar content of the final product.	[27]
	The combined use of biomass/waste results in higher carbon conversion, production of a large amount of gas, and reduced hazardous toxic elements. The authors also deduced that fluidized bed gasifiers are more efficient in conversion, with less tar formation compared to fixed bed gasification techniques, which is one of the main issues of using this technique.	[43]
	The production of electricity and heat from biowaste resources in a Fluidyne Micro-Lab Class air-blown gasifier, which operates at atmospheric pressure, was also as-sessed. Poultry litter and digestate (obtained from an AD plant working with a mix-ture of animal manure and green waste) were selected for analysis. All materials were pelletized to increase their energy density. Consequently, the authors found that drying is essential for biowaste with high moisture content (50–60%), such as poultry waste, digestate, and MSW. The PCI of the Synthesis Gas produced oscillated between $3.1 \mathrm{and} 5.4 \mathrm{MJ}/{\mathrm{m}}^3$. In addition, agricultural and forestry waste do not require any pretreatment drying processes and can produce a useful amount of electricity.	[44]
Pyrolysis	A critical review of the fate of the main chemical elements (C, H, O, N, P, Cl, S and metals) and advances in research on the emission, transformation, and distribution of pyrolysis elements was undertaken. First, the authors concluded that future work must be geared toward on-site or online detection and monitoring technologies, such as TG-FTIR-MS and Py-GC/MS, or advanced characterization techniques, such as solid carbon-13 spectroscopy nuclear magnetic resonance and synchrotron-based X-ray absorption spectroscopy, which could provide a feasible strategy to elucidate more deeply and precisely the mechanism and behaviors of lignocellulosic biomass in the pyrolysis process. Second, efficient sustainable catalysts and processes must be de-veloped to convert C, H, and O into desired products (i.e., bio-oil) and to avoid un-wanted products (i.e., tar and persistent organic pollutants). Third, any future works must ultimately predict and prevent the emission of ${\mathrm{NO}}_{\mathrm{x}}$ through the development of new reducing agents or catalysts. Fourth, the development of technologies based on the calcium loop within the pyrolysis process could effectively retain N, P, S, and Cl in the biocarbon phase, and thus control their emissions in the bio-oil and gas phases. Likewise, some heavy metals, such as Cu and Ni denote catalytic effects in biomass pyrolysis to improve the final product. Finally, biomass co-pyrolysis with carbon or synthetic polymers (i.e., plastics, and rubbers) contributes to improving both the quality and the quantity of the pyrolysis oil, without having to improve the system process.	[28]
	The study indicated that co-pyrolysis has been recognized for its ability to significantly improve the quantity and quality of pyrolysis oil, without the addition of catalysts, solvents, or hydrogen pressure to the process. On the other hand, co-pyrolysis has been considered an efficient and economical method for the production of high-grade biofuels from waste co-processing.	[29]
	Several biomass pretreatments have been developed, such as physical, thermal, chemical, and biological methods to improve the performance and quality of bio-oil; however, the structural configuration of raw materials and how they relate to bio-oil composition and properties must be studied further. Likewise, many researchers have developed different reactor configurations to maximize the yield of bio-oil or to improve the properties or certain specific properties thereof. Despite this, a large number of reactors still cannot fully meet the requirements of commercial pyrolysis reactors on an industrial scale, since a pyrolizer must have the ability to process versatile raw materials and accommodate low raw and reactor material requirements, as well as high heat transfer efficiency, among other aspects to reduce the operation costs and meet commercial use requirements. In this context, zeolite-based catalysts have emerged as promising agents for converting biomass into a low-oxygen fuel. Even so, they still suffer from coke fuel formation and its consequent rapid deactivation. Moreover, low bio-oil yields and carbon deficiency are the other parameters that must be considered when working with these catalysts.	[31]
	A pyrolysis study of different refuse derived fuel (RDF) samples was conducted in a semi-batch rotary kiln. These RDF samples were obtained from two different waste treatment plants operated by Bharuch Enviro Infrastructure Limited in the city of Ahmedabad, India, at an average feed particle size of 50 mm with different shapes. The thermal behavior of the RDF samples was studied using a thermal gravimetric analysis by heating in the presence of pure nitrogen (100 mL/min) at 20 °C/min up to 900 °C. Pure nitrogen was used to remove gaseous condensable products and air from the pyrolysis zone. The results showed that the quality of the pyrolysis product varied according to the feed composition and the operating temperature, wherein the highest oil yield (43%) was obtained with the RDF sample containing 77% of plastics at 500 °C.	[45]
Other Technologies	A review of the critical Hydrothermal Carbonization (HTC) parameters was conducted, including temperature, residence time, heating speed, reagent concentration, and water quality. This review provided new perspectives that can be used to enhance the potential of this technology. The authors concluded that future work efforts must address the influence of a wide variety of organic substances contained in the biomass and identify their conversion mechanisms during the HTC process. Temperature was identified as the critical parameter governing hydrocarbon characteristics; however, more variables must be considered, such as heating rate, particle size, substrate concentration, packing ratio, catalyst addition, and liquid quality.	[25]
	HTC from organic MSW fractions could reduce the problem of landfills and provide a sustainable source of solid fuel. The research indicated that upon increasing carbonization severity (higher temperatures and longer residence times), the solid yield and volatile hydrocarbon matter content generally decreased, while elemental carbon content increased, leading to higher heating values. However, at carbonization temperatures exceeding 260 °C, the amount of extractable “secondary” carbon decreased, after an initial increase to 220–240 °C.	[46]
	In Ecuador, a study assessed the energy potential of ${ \mathrm{H}}_2$ from MSW considering thermochemical (gasification combined with steam reform-ing) and electrochemical (gasification combined with electrolysis) paths for their conversion. This study also proved the feasibility of satisfying the energy demand of urban public transportation in 91% of the country with ${ \mathrm{H}}_2$ derived from MSW. Here, the authors offered very promising benefits, including a solution to the problem of final MSW disposal, with an improvement in urban transport infrastructure and its corresponding environmental indicators.	[47]

presents a summary of the most relevant scientific contributions from some of the publications selected for content analysis and sorted by WtE technology.

Recently published studies (e.g., ref. [36,37,38,48,49]) reinforce the continued importance of the trends identified in this work, particularly regarding the role of anaerobic digestion, pyrolysis, and gasification in sustainable waste management and energy recovery. These newer contributions also highlight emerging research directions, such as hybrid WtE systems combining thermochemical and biochemical processes, digital optimization tools for process efficiency, and expanded life cycle assessment approaches to address environmental trade-offs. The consistency between these recent findings and the results presented in our 2017–2021 analysis supports the enduring relevance of the identified technological priorities and implementation challenges, confirming that the observed patterns are not limited to the studied period but remain critical in current WtE research and practice.

4. Conclusions

This review provides a comprehensive global overview of bioenergy research from 2017 to 2021, focusing on technologies aimed at harnessing energy from waste and minimizing its environmental impact. The bibliometric analysis highlighted the main scientific contributions and production trends, identifying key Waste-to-Energy (WtE) technologies and their evolution over time. Complementarily, the content analysis offered detailed insights into the most relevant studies, including technological preferences in both developed and developing countries. The findings confirm a growing scientific interest in waste management, evidenced by a 13.57% increase in related publications. The analysis also revealed that most high-impact contributions were review articles, which play a crucial role in shaping future theoretical frameworks. Notably, a significant share of the research originated from Asian countries, particularly China and India. The study identified preferred WtE technologies in developing regions, such as anaerobic digestion for organic waste and pyrolysis and gasification for specific types of waste. It is important to note that pyrolysis and gasification, while both thermochemical conversion processes, differ in technological maturity and application scope; for instance, pyrolysis (and chemical recycling) tends to dominate plastic waste valorization over gasification. Additionally, landfilling was mentioned as a common practice for inert materials; however, it does not constitute an energy recovery technology, as it lacks energy valorization. Although incineration was highlighted as a key solution for non-biodegradable mixed waste in developing countries, this review’s content analysis did not include focused studies on incineration among the selected publications. Future work should integrate more detailed assessments of incineration to provide a balanced perspective on its role within WtE strategies. While scientific advancements in this field have been substantial, successful implementation of WtE technologies in developing countries will require stronger public policies, regulatory frameworks, and financial support to facilitate sustainable and scalable energy recovery from waste. Future research should explore integrated WtE systems combining multiple technologies, assess socio-economic barriers to implementation, and analyze environmental trade-offs through life cycle assessment approaches.