Evaluation of Technological Alternatives for the Energy Transition of Coal-Fired Power Plants, with a Multi-Criteria Approach

Abstract

This paper investigates technological pathways for the conversion of coal-fired power plants toward sustainable energy sources, using an integrated multi-criteria decision-making approach that combines Proknow-C, AHP, and PROMETHEE. Eight alternatives were identified: full conversion to natural gas, full conversion to biomass, coal and natural gas hybridization, coal and biomass hybridization, electricity and hydrogen cogeneration, coal and solar energy hybridization, post-combustion carbon capture systems, and decommissioning with subsequent reuse. The analysis combined bibliographic data (26 scientific articles and 13 patents) with surveys from 14 energy experts, using Total Decision version 1.2.1041.0 and Visual PROMETHEE version 1.1.0.0 software tools. Based on six criteria (environmental, structural, technical, technological, economic, and social), the most viable option was full conversion to natural gas (ϕ = +0.0368), followed by coal and natural gas hybridization (ϕ = +0.0257), and coal and solar hybridization (ϕ = +0.0124). These alternatives emerged as the most balanced in terms of emissions reduction, infrastructure reuse, and cost efficiency. In contrast, decommissioning (ϕ = −0.0578) and carbon capture systems (ϕ = −0.0196) were less favorable. This study proposes a structured framework for strategic energy planning that supports a just energy transition and contributes to the United Nations Sustainable Development Goals (SDGs) 7 and 13, highlighting the need for public policies that enhance the competitiveness and scalability of sustainable alternatives.

1. Introduction

Sustainability and the energy transition are critical components in the current global energy landscape, where the need to mitigate the environmental impact of the electricity sector has become a priority. A key aspect of this transition is the reduction of greenhouse gas (GHG) emissions, particularly carbon dioxide.

The adoption of cleaner technologies and energy sources reflects the imperative to advance toward decarbonization and foster a more sustainable energy system. It is essential to gradually replace traditional fossil fuels and transform them into a more efficient and environmentally friendly model.

The energy transition can be defined as a structural change in energy systems at national, regional, or global scales. This process encompasses technical, political, and social dimensions: from technological progress to natural resource use and public policies [1]. However, the main challenge lies in ensuring a just transition, which integrates not only environmental criteria but also labor protection and the reduction of inequality [2,3]. Considering economic, environmental, and social sustainability, both developed and developing nations have come to understand that economic growth must go hand in hand with reduced environmental pressure [4]. For these reasons, it is crucial to integrate climate and development strategies across various sectors to move toward a sustainable future.

In this context, according to the University of Calgary, coal-fired power plants are infrastructures designed to generate electricity through coal combustion [5].

However, the International Energy Agency (IEA) states that these plants are responsible for around 40% of global carbon dioxide emissions. While they have historically been essential to energy security, converting these plants to incorporate cleaner energy sources is gaining significant relevance as part of the energy transition [6].

From this perspective, and according to the Brazilian Ministry of Mines and Energy, the growing search for and inclusion of alternative renewable energy sources to diversify the energy matrix is evident both nationally and globally. In addition to meeting rising electricity demand, this diversification also contributes to the decarbonization of the electricity sector and promotes energy sustainability [7]. This reflects the interdependence between energy security, decarbonization, and sustainable development.

The progressive elimination of coal-based electricity generation through policies and instruments has been emphasized by several countries. In this process, coal-fired thermal power plants must evolve into models that complement renewable energy sources and transform their production sustainably over time, ensuring energy supply stability and a gradual, competitive transition—not by eliminating thermoelectric systems, but by reconfiguring them [8].

Given this background, this study addresses the following research question: What are the most feasible alternatives for the conversion of coal-fired power plants that promote the energy transition through the integration of more sustainable energy sources?

The search for conversion alternatives in this study is supported by a structured evaluation using a multi-criteria approach, analyzing them according to the following dimensions: (a) Environmental, with special emphasis on CO₂ emissions reduction; (b) Structural, in terms of the reuse of existing infrastructure; (c) Technical, regarding performance and flexibility; (d) Technological, considering technology readiness levels; (e) Economic, focusing on investment cost; (f) Social, through an employment impact assessment.

In today’s energy landscape, it is increasingly necessary for coal-fired power plants to adapt their operations flexibly to enable a transition toward systems with lower carbon emissions. In this context, it becomes essential to understand, develop, and evaluate solutions that promote deeper decarbonization of the electricity system, without requiring a complete restructuring of thermal systems, but rather by adapting them to the current economic, climate, and technological scenario. Hence, this study addresses the evaluation of technological alternatives for the energy transition of coal-fired power plants using a multi-criteria approach.

This process faces the challenge of transforming existing facilities without compromising energy security or causing negative social impacts. This is where multi-criteria analysis emerges as an indispensable tool, enabling a comprehensive assessment of the multiple dimensions involved in such conversions. These approaches effectively handle the inherent multidimensionality of energy problems, transforming complex data into structured evaluations [9]. The use of multi-criteria decision-making (MCDM) methods is essential to classify and select alternatives through an efficient configuration that minimizes bias in complex decision-making. However, their application in thermal reconversion studies has been limited and lacks standardization [10].

This study seeks to address an identified gap: the lack of systematic studies that prioritize sustainable reconversion alternatives by offering an innovative approach that combines analytical rigor with applicability. By evaluating the alternatives through multiple interrelated criteria, it not only identifies technically viable options but also those that best balance environmental goals with economic and social realities. Going beyond predominant unidimensional analyses, as Vanatta (2022) warns, previous studies ignore the social impact of the transition, focusing only on technical and economic aspects [11]. While isolated research exists on specific technologies such as biomass, solar hybridization, and carbon capture, there is a lack of comparative evaluations that simultaneously weigh technical efficiency, environmental impact, and social equity.

This study offers a flexible assessment framework that can be adapted to different national and regional contexts, providing decision-makers with a structured tool. Beyond its academic contribution, this research holds practical impact potential: its findings can inform the design of more effective public policies, guide investments toward sustainable alternatives, and promote decarbonization of the power sector without sacrificing social equity or energy security. In a world facing climate urgency and growing energy demands, such integrative approaches are not only valuable but are necessary [1]. This approach ensures the consideration of the specificities of each energy context while aligning with the United Nations Sustainable Development Goals (SDGs 7 and 13), as established by the United Nations (2015) [12].

The general objective of this study is to evaluate and rank technological alternatives for the energy transition of coal-fired power plants using multi-criteria methods. Specifically, it aims to: (i) identify potential alternatives through literature and patent analysis using the Proknow-C method; (ii) evaluate these alternatives according to predefined criteria using the Analytic Hierarchy Process (AHP); and (iii) rank the alternatives to determine the most feasible pathways for conversion or infrastructure reuse using PROMETHEE.

The motivation for this study stems from the growing need to transform coal-based power generation systems toward more sustainable and efficient configurations, in line with international climate goals. As countries accelerate their decarbonization agendas, coal-fired power plants, many of which still play a strategic role in energy supply, must be retrofitted rather than abruptly decommissioned. This is especially true in regions where economic, technical, and infrastructure constraints hinder the immediate implementation of fully renewable systems. In this context, structured methodologies capable of comprehensively and comparatively evaluating retrofitting alternatives are essential.

Despite the growing number of studies on coal-fired power plant retrofits, there remains a marked lack of integrated assessments that systematically combine literature mapping, hierarchical evaluation, and rank-ordering methods into a unified analytical framework. Based on the review and study of the scientific and technological literature, no proposals similar to those presented in this study have been made. They jointly apply the Proknow-C, AHP, and PROMETHEE methodologies to analyze technological alternatives for thermal infrastructure retrofitting. This gap is accentuated in the Latin American context, where the energy transition must reconcile decarbonization objectives with regional structural and socioeconomic constraints.

In this sense, this study introduces an innovative methodological approach that integrates these three decision-support tools, providing a replicable framework for the identification, evaluation, and prioritization of retrofitting alternatives. The contribution of this work lies not only in its methodological innovation but also in its contextual relevance, offering strategic guidelines for energy planning tailored to coal-based thermal systems.

This study makes several noteworthy contributions to the field of sustainable energy planning and thermal infrastructure conversion, which can be summarized as follows:

Systematic identification of reconversion alternatives: A comprehensive mapping of technological options for coal-fired power plant transformation was conducted through a structured literature and patent review using the Proknow-C methodology.

Integration of advanced multi-criteria decision-making tools: The combined use of AHP and PROMETHEE enables a robust prioritization of alternatives, considering six strategic dimensions: environmental, structural, technical, technological, economic, and social.

Evidence-based decision support for policymakers and investors: The proposed framework provides a flexible and replicable model that can inform public policy design and investment strategies under different national contexts.

Ranking of feasible alternatives with quantified impact: The study delivers a comparative ranking that highlights the most viable technological pathways, favoring those that optimize emissions reduction, infrastructure reuse, and socioeconomic benefits.

Alignment with global sustainability goals: The outcomes directly contribute to the achievement of SDG 7 (Affordable and Clean Energy) and SDG 13 (Climate Action), supporting a fair and effective energy transition.

This paper is structured as follows: Section 2 presents the theoretical foundations and a literature review. Section 3 describes the methodological framework, detailing the integrated use of Proknow-C, AHP, and PROMETHEE techniques. Section 4 discusses the results and provides a comparative analysis of the identified technological alternatives. Section 5 addresses the discussions, integrating policy implications and future research directions. Finally, Section 6 presents the conclusions of the study.

2. Fundamentals and Literature Reviews

The decarbonization of the power generation sector involves phasing out conventional fossil-fueled systems and introducing renewable-based alternatives capable of meeting electricity demand. As Cuji and Galarza (2022) state, energy generation is a major source of greenhouse gas (GHG) emissions, making it a priority for both environmental and technical intervention [13].

However, as noted by Mac, Brouwer, and Samuelsen (2018) [14], electricity from renewable sources often has higher costs and lower energy density than fossil fuels. Renewable energy sources like wind and solar are inherently variable, requiring complementary technologies to stabilize the grid supply. Moreover, the best renewable resources are not always located near population centers, demanding major upgrades or expansions of transmission infrastructure [14].

Consequently, a sharp increase in renewable capacity may not be feasible in the short term. To address these limitations, various low-carbon technologies have been proposed to meet future energy needs while minimizing emissions. These include fossil fuel-based generation integrated with renewables or equipped with carbon capture and storage (CCS) systems. Additionally, reducing electricity demand through improvements in generation, transmission, and end-use efficiency can lower the need for primary production without compromising supply [14].

This section outlines key technological alternatives discussed in the literature, selected for their GHG mitigation potential, technological readiness, scalability, and representation in scientific studies.

Biomass Co-Firing: Kamble et al. (2019) describe biomass as a “carbon-neutral renewable resource,” where the CO₂ released during combustion is reabsorbed by plant regrowth [15]. Xu et al. (2020) classify biomass co-firing in coal plants as a “hybrid transitional solution,” utilizing existing infrastructure with technical modifications such as co-milling [16]. Miedema et al. (2017) support its role in “incremental decarbonization,” allowing 20–30% coal substitution without extensive boiler upgrades [17].

Natural Gas Conversion: Natural gas is viewed as a “bridge fuel” due to its high operational flexibility, aiding renewable integration (Mac et al. 2018) [14]. In González et al. (2018), it is described as a “techno-economic enabler” emitting up to 50% less CO₂ than coal and adaptable to existing turbines [18]. Yet, Safari et al. (2019) caution that its role is transitional and depends heavily on pipeline infrastructure availability, limiting its application in underdeveloped regions [4].

Solar-Coal Hybridization: Solar-assisted coal systems are described by Mills (2018) as “synergistic integrations,” using concentrated solar or PV to preheat steam, thereby reducing coal usage [19]. Jiang et al. (2022) extend this to hybrid combined cycle systems with thermal storage to enhance stability [20]. Khanmohammadi et al. (2022) propose “solar-thermoelectric multigeneration,” where solar energy powers both electricity and hydrogen production, maximizing infrastructure use [21].

Hydrogen Integration: Hydrogen is framed by Szima et al. (2021) [22] as a “zero-emission energy vector” when produced from renewable-powered electrolysis. However, much of today’s hydrogen comes from coal gasification with CCS, an interim pathway [22]. Wei et al. (2024) identify hydrogen as a “combustion modifier,” improving efficiency and reducing NOx emissions when co-fired with coal [23]. Despite its promise, Li et al. (2019) note that hydrogen adoption is limited by high production and cryogenic storage costs [24].

Carbon Capture and Storage (CCS): Lockwood (2017) and Wang et al. (2017) characterize CCS as an “end-stage mitigation technology” that removes CO₂ post-combustion using chemical solvents [25]. Rogieri et al. (2023) contextualize CCS within the “circular carbon economy,” where captured CO₂ is stored or reused [26]. Yet, successful implementation hinges on regulatory frameworks that internalize carbon costs. Most systems reduce plant efficiency by 15–20%, posing challenges for large-scale deployment [27].

The specialized literature emphasizes that selecting reconversion technologies requires a multi-factor analysis, not only technical and economic, but also political and social dimensions must be addressed. Several empirical studies provide comparative insights in

Table 1. Reconversion Technologies Evaluated.

Technology	Emissions Reduction	Technical Maturity	Cost Efficiency	Infrastructure Reuse	References
Biomass Co-Firing	Moderate (up to 48%)	High	Moderate	High	[15,16,17]
Natural Gas Conversion	High (up to 50%)	Very High	High	High (existing turbines)	[4,14,18]
Solar Hybridization	Moderate	Medium	Low to Moderate	Medium	[19,20,21,28]
Hydrogen Co-Firing	Emerging	Low to Medium	Low	Medium	[22,23,24]
CCS Integration	High (15–20% CO2 removal)	Medium	Low	Low	[25,26,27]

.

It is worth mentioning that nuclear energy was not included among the technological alternatives evaluated because its implementation does not represent a direct conversion option for existing thermoelectric infrastructure. Its incorporation would require completely new facilities and different regulatory conditions. Furthermore, in the Latin American context, nuclear energy is not part of priority energy transition strategies, presents limited social acceptance, and faces risks associated with radioactive waste management. For these reasons, it was considered outside the scope of this analysis.

Mac, Brouwer, and Samuelsen (2018) [14] analyzed the U.S. coal-to-gas transition (1995–2012), reporting reductions of 23% in CO₂, 40% in NOx, and 44% in SO₂ emissions. They highlighted efficiency gains from combined heat and power (CHP) systems [14].

Safari et al. (2019) [4] noted that natural gas emits 15.3 kg CO₂/GJ compared to 26.2 kg/GJ for coal and that combined-cycle gas plants achieve up to 65% efficiency. Capital costs are lower—only 25% of other new generation technologies [4].

Miedema et al. (2017) found that 60% biomass co-firing can cut GHG emissions by 48% and raise renewable shares by 35%, albeit with reduced energy efficiency (41.2%) [17].

Xu et al. (2020) showed advanced pulverized coal boilers achieve 92% thermal and 45% electrical efficiency when adapted for co-firing, offsetting earlier concerns [16].

Rogieri et al. (2023) assessed CCS feasibility at Brazil’s Jorge Lacerda complex, concluding its success depends on financing and policy support [26]. Lockwood (2017) warned that European CCS faces barriers: amine-based scrubbers reduce plant efficiency by ~10% and can increase electricity costs by up to 80% [25].

Serrano, Olmeda, and Petrakopoulou (2019) [28] compared solar-coal hybrid and conventional plants. The hybrid achieved 4.6% lower emissions and 1.6 percentage points higher exergetic efficiency, but had higher capital costs ($8050/kW vs. $5979/kW) and a levelized cost of $0.19/kWh (vs $0.12/kWh for coal) [28].

Szima et al. (2021) [22] explored Integrated Gasification Combined Cycle (IGCC) plants with hydrogen production. These offered flexible generation, 25 €/ton CO₂ avoidance costs (vs. 44 €/ton for pre-combustion), and improved annual return rates (+11%) by selling hydrogen during low-renewable periods [22].

Despite the wide range of technological alternatives explored in the literature, a significant research gap remains. There is a notable absence of integrated, systematic assessments that evaluate and compare these solutions using standardized multi-criteria frameworks. Most existing studies focus on isolated aspects, technical or economic, while frequently neglecting social and infrastructural dimensions. Moreover, few analyses account for contextual feasibility, scalability across different regions, or the compounded effect of combining technologies. This gap highlights the need for a unified, decision-support approach capable of holistically ranking reconversion options across environmental, economic, and social dimensions, an unmet need directly addressed by the methodology proposed in this study.

The literature reviewed presents a diverse panorama of emerging technologies for the energy transition, each with specific advantages but also with technical, economic, or structural limitations that condition their applicability. The use of biomass as a co-combustion alternative is valued for its renewable nature and carbon neutrality, as mentioned by Kamble et al. (2019) and Miedema et al. (2017), but it faces restrictions in energy efficiency and large-scale supply logistics [15,17]. Conversions to natural gas, although more efficient and less polluting according to Safari et al. (2019), depend on a gas pipeline infrastructure that limits their implementation in less developed regions [4]. Hybrid solar-coal solutions show potential in reducing emissions (Serrano et al. 2019) but still present high investment costs and operational complexity [28]. The integration of hydrogen, whether as an energy carrier or additive, is still affected by its high energy demand for production, cryogenic storage, and transportation, as mentioned by Li et al. (2019) and Szima et al. (2021) [22,24]. On the other hand, while carbon capture and storage (CCS) is proposed as an advanced mitigation tool (Lockwood, 2017; Rogieri et al. 2023), its massive implementation faces regulatory barriers, high operating costs, and loss of energy efficiency [25,26]. In general, studies tend to analyze each alternative in isolation, without integrating multiple relevant criteria such as technological maturity, economic viability, environmental impact, and suitability for existing infrastructure within a structured comparative framework. This fragmentation makes it difficult to identify optimal solutions in contexts with specific constraints, such as developing countries. Therefore, there is a need to apply integrative multi-criteria methods that allow for a hierarchical, weighted, and contextualized evaluation of the most viable energy conversion alternatives, thereby overcoming the methodological limitations observed in the existing literature. The current literature on the conversion of coal-fired power plants presents fragmented approaches, with many studies addressing specific technological solutions without articulating integrated comparative assessments.

Along these lines, Dash et al. (2024) [29] apply a combination of the VIKOR method with self-organizing maps (SOM), a technique based on unsupervised artificial neural networks, to evaluate the performance of sustainable energy systems. This approach represents a breakthrough in the classification of complex alternatives [29]. However, as the authors themselves point out, methods such as VIKOR depend on trade-off parameters and ideal solutions, the definition of which can introduce analytical rigidity. Therefore, this study proposes a framework that combines AHP and PROMETHEE, which allows not only weighing criteria based on validated pairwise comparisons (AHP), but also generating a robust and visually interpretable ordering of the alternatives (PROMETHEE), without requiring extreme assumptions or artificial trade-offs. This methodological combination directly responds to the limitations observed in the literature, offering a reproducible tool that is sensitive to dimensions and adaptable to contexts with structural and social constraints.

This gap highlights the need for a unified decision-support approach that can rank reconversion options across environmental, economic, and social dimensions—a gap directly addressed by the methodology proposed in this study.

Figure 1 provides a conceptual overview of the main technological pathways for reconverting coal-fired power plants, grouped into five categories: biomass co-firing, natural gas conversion, solar-coal hybridization, hydrogen integration, and carbon capture and storage (CCS). Each pathway is linked to key attributes that influence its feasibility and performance. For instance, biomass is noted for being a carbon-neutral resource with incremental implementation potential, though it requires adaptations in boiler systems. Natural gas is highlighted for its operational flexibility and existing infrastructure compatibility, yet its dependence on pipeline networks limits its universal applicability. Solar hybrid systems are characterized by their synergistic coupling with coal and potential to reduce emissions, but also face high capital costs. Hydrogen co-firing, while promising as a zero-emission vector, is constrained by technological readiness and high storage costs. CCS, finally, is identified as an end-stage mitigation solution whose adoption is hindered by efficiency penalties and regulatory complexity. This visual synthesis reinforces the need for a multi-criteria framework to evaluate trade-offs among environmental benefits, cost, infrastructure reuse, and scalability—aligning with the analytical approach proposed in this study.

3. Materials and Methods

This study adopts a multi-criteria methodological framework that integrates both qualitative and quantitative analyses to evaluate technological alternatives for the reconversion of coal-fired power plants. The approach combines three complementary methods: Proknow-C (Knowledge Development Process—Constructivist), the Analytic Hierarchy Process (AHP), and the Preference Ranking Organization Method for Enrichment Evaluations (PROMETHEE).

These methods were selected due to their proven applicability in complex decision-making contexts, where multiple criteria, including environmental, economic, technical, and social, must be considered simultaneously. Proknow-C was applied to identify and structure the relevant literature and patent base; AHP was employed to assign weights to the evaluation criteria; and PROMETHEE enabled the prioritization and ranking of reconversion alternatives based on performance under each dimension.

To ensure a robust and reproducible application of these methods, the analysis was supported by dedicated software tools: Total Decision version 1.2.1041.0 for AHP and Visual PROMETHEE version 1.1.0.0 for PROMETHEE. These tools enabled efficient quantitative evaluation and clear graphical comparisons across alternatives.

Compared with other common multi-criteria decision-making methods such as TOPSIS and ELECTRE, the combination of AHP and PROMETHEE offers key methodological advantages that justify its selection in this study. AHP allows for hierarchical weighting of criteria through pairwise comparisons, ensuring a logical and validatable structure using the consistency index (CR). PROMETHEE, on the other hand, facilitates a complete and visually interpretable ordering of alternatives through preference flows, which is useful for complex contexts with multiple dimensions, such as the one addressed in this work. Although TOPSIS is widely used due to its computational simplicity, it has limitations in the treatment of non-compensatory criteria and its sensitivity to the choice of ideal and anti-ideal alternatives. ELECTRE, while useful for preselection in contexts with strong conflicts between criteria, can generate partial rankings that are difficult to interpret and rely on subjective preference thresholds. In contrast, AHP + PROMETHEE combines theoretical robustness in the weighting of criteria with the ability to discriminate finely between alternatives, without requiring rigid assumptions or extreme reference values. This methodological synergy improves the transparency, traceability, and usefulness of the results for strategic decision-making in energy transition policies.

Figure 2 illustrates the methodological workflow, which aligns with the general objective of this research and is structured around three specific objectives and their corresponding analytical stages.

Figure 2 presents the methodological flowchart adopted in this study, structured around the general objective and three specific objectives that guide each stage of the process. Objective 1 involves the identification of alternatives and criteria using the Proknow-C method, based on a systematic review of scientific literature (Step 1) and intellectual property documents (Step 2). Objective 2 corresponds to the multicriteria evaluation of the alternatives, integrating two steps: the literature-based evaluation (Step 3) and the evaluation through an expert survey (Step 4), applying the AHP method and Total Decision software version 1.2.1041.0. Finally, Objective 3 focuses on the ranking of preferences through the PROMETHEE method, which allows the ordering of alternatives (Step 5) and their graphical representation (Step 6) using Visual PROMETHEE software version 1.1.0.0. This sequence allows for a structured integration of decision-making methods for the conversion of coal-fired power plants. The following subsections describe each methodological phase in detail.

3.1. ProKnow-C Method

The Proknow-C (Knowledge Development Process–Constructivist) method was applied as a systematic and constructivist approach to scientific literature review, specifically designed to structure and organize bibliographic research aligned with the study’s objectives [30].

The purpose of this method was to search for and select high-relevance scientific articles related to the reconversion of coal-fired power plants. Through this process, the literature review remains aligned with the research objectives, offering a structured framework that supports both the organization and evaluation of academic contributions.

This stage consisted of identifying conversion alternatives for coal-fired thermal power plants through a systematic review of the most relevant scientific literature and intellectual property records. The first step involved selecting keywords (KWs) derived from the study’s thematic axes. To ensure broad coverage and include the most relevant and recent research, keywords were applied in English, Portuguese, and Spanish.

Table 2. KeyWords (KW).

Axis (1)	(KW)
Specification	“generation unit” “power plant” “thermoelectric” “coal” “conversion” “modernization” “energy transition” “decarbonization”
Axis (2)	(KW)
Energy Sources	“alternatives” “integration” “hybridization” “renewable energy” “clean sources” “alternative energy” “sustainable energy”
Axis (3)	(KW)
Analysis Methods	“feasibility” “viability” “impact” “method” “criterion” “multicriteria” “AHP” “PROMETHEE”

shows the keywords grouped by thematic axis, enabling clear visualization of how each keyword set relates to a specific research dimension. This structure ensures comprehensive thematic coverage, making the search more targeted and efficient.

For the scientific literature search, three databases were used: (i) Scopus: due to its comprehensive coverage of scientific and technological publications; (ii) CAPES Journal Portal: a major Brazilian gateway to indexed research; and (iii) Google Scholar: useful for broader searches via algorithms similar to general Google search.

Boolean operators “OR” and “AND” were applied to combine keywords across thematic axes, as shown in Table 2. These operators were fundamental for expanding and refining search results. To ensure topicality, a 10-year time filter (2015–2024) was applied. As a result, a bibliographic portfolio of 26 peer-reviewed scientific articles was compiled as the analytical basis for this research.

Table 3. Selected Scientific Articles—Part I.

Articles	Ref.	Specific Contribution to the Analysis
A Review of Post-combustion CO2 Capture Technologies from Coal-fired Power Plants	[27]	Evaluates post-combustion capture technologies, their efficiencies, and operational limitations, providing a technical basis for the alternative installation of carbon capture systems (CCS).
Review of the operational flexibility and emissions of gas- and coal-fired power plants in the future with growing renewables	[18]	Presents emissions and flexibility comparisons between gas and coal plants, essential for evaluating full conversion to natural gas and coal-natural gas hybridization.
The role of natural gas and its infrastructure in mitigating greenhouse gas emissions, improving regional air quality, and renewable resource integration	[14]	Highlights the role of gas as a transition fuel, its compatibility with renewable energy, and its infrastructure. Supports the alternative of full conversion to natural gas and coal-natural gas hybridization, as well as structural and environmental criteria.
Integrating sustainability into strategic decision-making: A fuzzy AHP method for the selection of relevant sustainability issues	[9]	Justifies the use of the AHP method in prioritizing complex sustainability criteria, strengthening the methodological approach of this study.
Natural gas: A transition fuel for sustainable energy system transformation?	[4]	Supports gas as a cleaner and more efficient transitional solution. Provides evidence for evaluating full conversion to natural gas and coal-natural gas hybridization.
Co-gasification of coal and biomass an emerging clean energy technology: Status and prospects of development in Indian context	[15]	Reviews co-gasification technologies, their efficiency, and technical challenges relevant to the total conversion to biomass alternatives and coal-biomass co-firing.
Coal-Biomass Co-Firing Power Generation Technology: Current Status, Challenges and Policy Implications	[16]	Details the technical requirements, benefits, and regulatory challenges of co-firing coal with biomass, providing evidence for the coal-biomass co-firing alternative.
A Comparative Review of Next-generation Carbon Capture Technologies for Coal-fired Power Plant	[25]	Analyzes emerging advanced solvent capture technologies, energy efficiency, and cost reduction. Supports the technical feasibility of installing carbon capture systems.
Renew, reduce or become more efficient? The climate contribution of biomass co-combustion in a coal-fired power plant	[17]	Provides quantitative evidence of GHG emission reductions and energy efficiency associated with coal-biomass co-firing.
A systematic approach for assessment of renewable energy using analytic hierarchy process	[31]	Supports the use of AHP in multi-criteria energy contexts, serving as a methodological reference for prioritizing criteria and alternatives.
Finding synergy between renewables and coal: Flexible power and hydrogen production from advanced IGCC plants with integrated CO2 capture	[22]	Provides a technical basis for the coal-hydrogen cogeneration alternative, highlighting the use of IGCC plants with flexible energy and hydrogen production.
Combining solar power with coal-fired power plants, or cofiring natural gas	[19]	Evaluates the operational benefits of integrating solar energy with coal-fired plants, relevant to coal-solar hybridization and coal-natural gas hybridization alternatives.
Comparative analyses of a novel solar tower assisted multi-generation system with re-compression CO2 power cycle, thermoelectric generator, and hydrogen production unit	[21]	Describes multigeneration systems based on solar energy, including electricity and hydrogen generation. Provides a technical justification for the coal-solar hybridization alternative.

presents the first half of the selected articles (13 papers), and

Table 4. Selected Scientific Articles—Part II.

Articles	Ref.	Specific Contribution to the Analysis
Potential of biomass and coal co-firing power plants in Indonesia: a PESTEL analysis	[32]	Analyzes strategic and regulatory factors affecting the viability of coal and biomass hybridization, informing structural, and social criteria.
Performance analysis of tower solar aided coal-fired power plant with thermal energy storage	[20]	Evaluates the technical and economic impact of solar-coal hybrid systems, providing data relevant to coal and solar hybridization.
Innovative hybridization of the two-archive and PROMETHEE-II triple-objective and multi-criterion decision making for optimum configuration of the renewable hybrid energy system	[10]	Demonstrates the usefulness of PROMETHEE II for optimizing energy configurations, supporting its methodological application in this study.
Do homeowners benefit when coal-fired power plants switch to natural gas? Evidence from Beijing, China	[33]	Evaluates positive externalities of switching to gas, such as increased values of nearby properties, contributing to the social criteria and full conversion to natural gas.
The costs of replacing coal plant jobs with local instead of distant wind and solar jobs across the United States	[11]	Provides empirical evidence on just transition and job substitution, informing the social criteria for all alternatives with labor impact.
Decarbonizing coal-fired power plants: Carbon capture and storage applied to a thermoelectric complex in Brazil	[26]	Evaluates the technical, regulatory, and economic feasibility of installing carbon capture systems (CCS) in the Latin American context.
Experimental Study and Thermodynamic Analysis of Hydrogen Production through a Two-Step Chemical Regenerative Coal Gasification	[24]	Supports the feasibility of advanced hydrogen technologies, such as regenerative gasification, relevant to coal and hydrogen cogeneration.
Technical and economic analysis of the conversion on an existing coal-fired thermal power plant to solar-aided hybrid power plant	[34]	Provides technical and economic support to justify solar integration in coal-fired plants, evaluating efficiency and return on investment.
Coal to Biomass Conversion as a Path to Sustainability: A Hypothetical Scenario at Pego Power Plant (Abrantes, Portugal)	[35]	A real-life case study on full conversion to biomass in Europe, useful for estimating operational and economic feasibility.
Exergy and Economic Evaluation of a Hybrid Power Plant Coupling Coal with Solar Energy	[28]	Quantitatively supports the benefits of coal-solar hybridization in terms of exergy efficiency and emissions reduction.
The Return of Coal-Fired Combined Heat and Power Plants: Feasibility and Environmental Assessment in the Case of Conversion to Another Fuel or Modernizing an Exhaust System	[36]	Analyzes strategies for maintaining existing infrastructure through modernization or fuel conversion, linked to the structural reuse criterion.
Assessment on Energy Conversion Efficiency and GHG Emissions Rate for Coal, Natural Gas and Biomass Power Plant in Malaysia	[37]	Provides key comparative values on energy efficiency and CO2 emissions for full conversion to natural gas, full conversion to biomass, and coal-biomass hybridization.
Numerical simulation of hydrogen co-firing distribution on combustion characteristics and NOx release in a 660 MW power plant boiler	[23]	Reports on the technical and operational challenges of coal-hydrogen cogeneration in large boilers, including effects on combustion and NOx emissions.

presents the second half (13 papers). These publications provide a comprehensive and diverse overview of technological reconversion possibilities, including renewable integration, carbon capture, and hybrid energy systems.

To expand the scope of technological insights, a parallel search was performed for international intellectual property records, using: PATENTSCOPE (WIPO) and Google Patents.

Given the generalized terminology used in patent filings, Boolean operators were again used to combine keywords across thematic axes. This process resulted in the identification of 13 relevant patents, summarized in

Table 5. Selected Patents.

Patents	Ref.	Specific Contribution to the Analysis
istema de integración sinérgica de fuentes de electricidad de origen renovable no gestionable y bombas de calor de dióxido de carbono en centrales termoeléctricas. ES2893976	[38]	Provides innovation for integrating renewable energy into thermal power plants using supercritical heat pumps and thermal storage, relevant to the coal and solar hybridization alternative.
Combustion method for high-concentration hydrogen-rich liquid. WO2019131765	[39]	Provides a technical solution for improving combustion with safe and efficient liquid hydrogen, applicable to coal and hydrogen cogeneration.
Large-scale coal-fired power plant CO based on artificial intelligence2Optimal scheduling method for trapping system. CN113341716	[40]	Uses artificial intelligence to optimize the operation of carbon capture systems in large plants, informing the design and feasibility of carbon capture systems.
A kind of method of coal-burning power plant’s coupled biological matter direct combustion power generation. CN110388639	[41]	Describes a direct technical integration of biomass with coal, improving the technical justification for the coal and biomass hybridization alternative.
Method and calculation system for reducing CO2 emissions from co-fired biomass at a coal-fired power plant. CN116502393	[42]	Provides a quantitative method for estimating emissions reductions in the coal and biomass hybridization alternative.
Sistema y procedimiento de captura y almacenamiento de dióxido de carbono de los gases de combustión. ES2618290	[43]	Technically supports the operational feasibility of carbon capture systems in thermal plants with multiple types of fossil fuels.
Planta termoeléctrica alimentada con calor ambiental y enfriada mediante regasificación de gas natural licuado. ES2580879	[44]	Presents an efficient thermal configuration for the use of natural gas in electricity generation, relevant for full conversion to natural gas.
A kind of double reheat formula solar energy and coal fired power plant complementary power generation system and operation method. CN109185085	[45]	Provides a solar-coal hybrid design for steam reheating, expanding the technical justification for coal and solar hybridization.
Source-end coupled solar energy gain type coal-fired power plant boiler. CN113091041	[46]	Describes a system for direct solar integration in coal-fired boilers, contributing to coal and solar hybridization.
A kind of coal-saving high temperature tower-type solar thermal complementation coal fired power plant integrated system. CN106918030	[47]	It improves the thermal efficiency and stability of hybrid systems, strengthening the technical foundation of coal and solar hybridization.
System for realizing low-carbon emission of coal-fired power plant by taking green hydrogen as carrier. CN118300174	[48]	Describes a comprehensive electrolysis, storage, and CO2 conversion system, key to the alternative coal and hydrogen cogeneration.
High calorific biomass molded fuel for replacing coal for thermoelectric power plant by using vegetable oil by-product and high efficiency compression molding technique, and method for manufacturing same. KR1020200069865	[49]	Proposes a molded biomass fuel optimized for thermal use, technically supporting full conversion to biomass.
System for the circular production of hydrogen and oxygen with feedback of thermal energy waste recovered in the Stirling engine step and in the electrolysis step. WO2023035089	[50]	Introduces an innovative electrolysis system powered by waste heat, relevant to coal and hydrogen cogeneration and future renewable hydrogen integrations.

. Each entry includes the title, patent, and number.

This comprehensive literature and patent mapping stage plays a fundamental role in identifying high-potential technologies backed by scientific rigor and technological maturity. It provides a solid foundation for the next stages of the study, ensuring that decision-making is supported by robust and up-to-date evidence from both academia and industry. The criteria considered for the inclusion of articles and patents are presented in Appendix A.

3.2. AHP Method

The methodological procedure in this section aimed to conduct a quantitative evaluation of the identified alternatives for the reconversion of coal-fired power plants toward a sustainable energy transition, integrating cleaner energy sources. This evaluation was performed using the Analytic Hierarchy Process (AHP).

The AHP is a multi-criteria decision-making technique that decomposes a complex problem into a hierarchy of criteria and subcriteria. It simplifies comparisons among various factors and enables the assignment of relative weights to criteria and alternatives in line with the study’s overall objective. This method was developed by Saaty, T.L. [51]. The hierarchical structure consists of the following levels: Level I (objective), Level II (criteria), Level III (subcriteria), and Level IV (alternatives), as illustrated in Figure 3.

A key feature of this process is the pairwise comparison of elements, which enables a clearer determination of the relative importance of each component.

Table 6. Saaty’s Importance Scale. Adapted from [51].

Intensity	Definition	Explanation
1	Equal importance	Both activities contribute equally to the objective.
3	Moderate importance	Experience and judgment slightly favor one activity over the other.
5	Essential importance	Experience and judgment strongly favor one activity over the other.
7	Very strong importance	One activity is strongly favored over the other; its dominance of importance is demonstrated in practice.
9	Extreme importance	The evidence favors one activity over the other with the highest degree of certainty.
2, 4, 6, 8	Intermediate values	When a compromise condition is sought between two definitions.
Reciprocals	If an activity receives a value compared to j, as reciprocity, j will receive the reciprocal value when compared to i.

presents the scale of importance defined by Saaty, which is essential for pairwise comparisons of criteria and alternatives. This procedure is used to assign a relative weight to each element involved in the analysis.

Using this approach, a comparison matrix is developed to calculate the relative weights of each criterion and alternative through normalization and eigenvector calculations, ensuring that all weights fall within a valid range and are comparable to each other.

The validation process is also crucial, and it is supported by the Consistency Ratio (CR), which verifies whether the pairwise comparisons are logically consistent. If the CR is below 10%, the comparisons are considered consistent, thus validating the reliability of the model and reinforcing the quality of the analysis.

The study collects, normalizes, and analyzes two datasets that include the weights for both evaluation criteria and reconversion alternatives. The first dataset is derived from an in-depth literature review, while the second dataset is based on expert surveys in the energy sector.

Initially, the comparison of alternatives is based on scientific literature findings and prior studies, whose details are found in Appendix B. Subsequently, an additional comparison is performed using the results of surveys distributed to domain experts, specifically within the National Electricity Administration (ANDE), whose information is included in Appendix C.

ANDE (1964) [52] is the public entity responsible for operating Paraguay’s electric system, encompassing generation, transmission, distribution, and commercialization. The institution employs highly qualified professionals with expertise in various areas of the energy sector [52]. The questionnaire, designed in accordance with AHP standards, was sent to 27 experts, of whom 14 provided complete responses. The criteria considered for the inclusion of experts are presented in Appendix A. Each expert offered performance scores for each alternative, contributing to the analytical robustness of this phase.

3.3. PROMETHEE Method

This stage consisted of producing a ranking of the obtained results to propose the most feasible alternatives for the reconversion of coal-fired generation units and/or the reuse of existing infrastructure. This was achieved through the application of the PROMETHEE method, using the Visual PROMETHEE software version 1.1.0.0, which enabled both a robust quantitative evaluation and a representative graphical comparison.

PROMETHEE (Preference Ranking Organization Method for Enrichment Evaluations) is an outranking-based multi-criteria decision-making model developed by Jean-Pierre Brans [53].

Based on the comparison matrix and the weightings derived from the AHP method, a final decision matrix is generated. This matrix organizes the performance of each alternative (rows) under each evaluation criterion (columns), as shown in

Table 7. Decision Matrix [53,54].

Alternatives (A)	Criteria (C)
	C1	C2	C3	Cm
A1	a11	a12	a13	a1m
A2	a21	a22	a23	a2m
A3	a31	a31	a33	a3m
…	…	…	…	…
An	an1	an2	an3	anm

.

Next, PROMETHEE I is applied to generate a partial ranking, based on the positive preference flow (ϕ⁺) and the negative preference flow (ϕ⁻) for each alternative. These flows are expressed in Equations (1) and (2), respectively. The positive flow indicates how much an alternative outranks the others, while the negative flow indicates how much it is outranked by the others [53,54].

$${\varphi }^{+}\left(a\right)={\sum }_{xϵA}\pi (a,x)$$

(1)

This represents the sum of the preference of alternative (a) over all other alternatives (x) in the set A.

$${\varphi }^{-}\left(a\right)={\sum }_{xϵA}\pi (x,a)$$

(2)

This represents the sum of the preferences of all other alternatives (x) over alternative (a) in the set A.

Subsequently, PROMETHEE II is used to generate a complete ranking of the alternatives through the calculation of the net flow (ϕ), which is the difference between the positive and negative flows, as shown in Equation (3):

$$\varphi \left(a\right)={\varphi }^{+}\left(a\right)- {\varphi }^{-}\left(a\right)$$

(3)

This net flow allows for the ordering of alternatives in a single ranking, where higher values indicate more favorable alternatives. Additionally, the preference flows serve as the basis for graphical representations that illustrate the performance and dominance of each alternative.

4. Analysis and Results

This section presents the main findings derived from the application of the multi-criteria decision-making approach, combining the Proknow-C, AHP, and PROMETHEE methods. The analysis integrates data obtained from both a structured literature review and patent review, as well as expert surveys, enabling a comprehensive evaluation of eight technological alternatives for the reconversion of coal-fired power plants. The results are organized into three main parts: (i) the relative weights assigned to evaluation criteria, (ii) the ranking of alternatives based on literature data, and (iii) the ranking based on expert assessments. This structure allows for a comparative and robust interpretation of the most feasible technological pathways, reflecting the multidimensional nature of the decision-making process.

4.1. Results of the ProKnow-C Method

Using the Proknow-C method, the most relevant studies from scientific literature and intellectual property databases related to reconversion alternatives for coal-fired power plants were identified. This process resulted in a curated selection of 26 scientific articles and 13 patents. A detailed analysis was conducted to categorize and evaluate the collected sources, forming the theoretical and conceptual foundation that supports the development of the proposed alternatives in this study.

Given that the main objective of this research is to present feasible alternatives for the potential repurposing of existing coal-fired power plants, the documents were classified into the following technological categories: solar energy, natural gas, biomass, hydrogen, and carbon capture. Subsequently, the percentage of documents pertaining to each category was determined. This analysis integrated both scientific and patent sources, providing a comprehensive overview of how the literature is distributed across key technology areas. The results of this categorization are presented in Figure 4.

Following this and based on a thorough review and quantification of the literature and patents, eight reconversion alternatives were identified for coal-based generation units. These alternatives, defined using the Proknow-C framework, are summarized in

Table 8. Reconversion Alternatives.

Alternative	Description	Advantages	Challenges
Alternative 1- Full conversion to natural gas	Complete replacement of coal with natural gas in boilers or turbines. Requires retrofitting burners, controls, and connections to gas pipelines. Can include combined cycles for greater efficiency [14,18].	It reduces CO2 emissions by up to 50%, lowers SO2 and NOx, has high efficiency (~65%), and has relatively low capital costs [4,14].	Dependence on gas infrastructure, impact if supply is limited, transition not completely carbon-free [4].
Alternative 2—Full conversion to biomass	Complete replacement of coal with solid biomass (pellets, waste). This requires adaptations to feed, combustion, and ash management systems [15,16,17].	Renewable source, net-zero CO2 balance, reuse of agricultural residues. Forty-eight percent GHG reduction in high mixes [17].	Lower energy density, greater storage and transportation volumes, risk of thermal instability [15].
Alternative 3—Coal and natural gas hybridization	Combined use of coal and natural gas in dual combustion systems. Maintains current infrastructure with partial gas integration [14].	Partial emissions reduction, improved efficiency, and facilitated renewable integration, and it is lower cost than full conversión [14].	Operational complexity, need for advanced controls, and dependence on multiple fuels [4].
Alternative 4—Coal and biomass hybridization	Co-firing of coal with biomass in the same boiler. Minor modifications: mixing, grinding, and control systems [16,17].	Reduces coal use by up to 20% to 30%, gradual transition, low investment costs [15,17].	Biomass variability, potential reduction in energy efficiency, adjustment of combustion parameters [16].
Alternative 5—Coal and hydrogen cogeneration	Hydrogen injection, as a complementary fuel alongside coal, generates electricity and steam [22,23].	Reduction in NOx and CO2 emissions, increased efficiency, and shift toward carbon-free systems [23].	High production and storage costs, safety requirements, and infrastructure development [24].
Alternative 6—Coal and solar hybridization	Integration of solar thermal or photovoltaic systems to preheat water or generate auxiliary energy in coal-fired plants [19,20,21].	Reduction in coal use, improved thermal efficiency, and modular option for progressive retrofitting [19].	High initial investment, solar intermittency, space requirements, and thermal redesign [28].
Alternative 7—Carbon capture systems	Installation of post-combustion capture systems with solvents (such as amines) to remove CO2 before emission [25,26,27].	Direct emissions reduction, usable in other industries, promotes the circular carbon economy [26].	Efficiency reduction (10% to 20%), high operating costs, dependence on regulation and storage infrastructure [25,27].
Alternative 8—Decommissioning and subsequent reuse	Permanent closure of the plant and use of the land for renewable, industrial or logistics projects [14].	Total emissions elimination, possible productive restructuring, environmental regeneration.	Loss of installed capacity, social impact due to job losses, high investment in retrofitting [14].

, the technical description of the eight reconversion alternatives.

Table 8 provides a detailed technical and operational description of each of the eight reconversion alternatives summarized in the same table. These narratives clarify the specific scope, technological configurations, performance parameters, and implementation challenges associated with each strategy. The aim is to enhance the depth and applicability of the findings by elucidating how each alternative can be realistically adopted in coal-fired power plant retrofitting scenarios.

Alternative 1 (Full Conversion to Natural Gas) presents the complete replacement of coal with natural gas as the primary fuel in existing thermal power plants. The conversion process requires retrofitting of burners, combustion chambers, control systems, and the establishment of reliable connections to a gas supply network. In some configurations, the implementation of combined-cycle gas turbines (CCGT) may be adopted to optimize efficiency. Natural gas-fired systems typically achieve thermal efficiencies of up to 65%, significantly higher than conventional coal-based systems (35–42%). The environmental benefits are substantial: carbon dioxide emissions are reduced by approximately 50% (0.45 kg CO₂/kWh compared to 0.94 kg CO₂/kWh for coal), while sulfur dioxide (SO₂) and nitrogen oxides (NOx) emissions can also decrease by over 40% [4,14,18]. However, this alternative is highly dependent on the availability and proximity of natural gas infrastructure, and, while cleaner than coal, it remains a fossil fuel with associated lifecycle emissions.

Alternative 2 (Full Conversion to Biomass) involves the total replacement of coal with solid biomass fuels such as wood pellets, agricultural residues (e.g., sugarcane bagasse, rice husk, soybean cake), and forest by-products. The transition requires significant adaptations to the fuel handling systems, including feeding mechanisms, combustion chambers, and ash management infrastructure. For efficient and stable operation, the biomass should have a moisture content below 20% and a calorific value compatible with the existing thermal configuration. According to Miedema et al. (2017), high-percentage biomass substitution (up to 60%) can lead to greenhouse gas emission reductions of up to 48% [17]. However, biomass has a lower energy density compared to coal, leading to higher requirements for storage space and transport logistics. These limitations, combined with thermal instability risks during combustion, necessitate careful operational adjustments [15,16].

Alternative 3 (Coal and natural gas hybridization) consists of operating the power plant using a dual-fuel system in which coal and natural gas are combusted simultaneously or in alternating phases. This hybridization allows for partial retention of existing coal infrastructure while improving combustion control and thermal efficiency. It enables flexible operations during peak demand periods and facilitates gradual decarbonization. In technical terms, the integration involves the addition of natural gas burners and control systems capable of regulating the proportion of fuel used. Studies indicate this configuration may reduce CO₂ emissions by approximately 30–40% compared to full coal operation [4,14]. The approach is also considered cost-effective, as it avoids full replacement of core components. Nonetheless, it introduces operational complexity due to the need for precise combustion control and dual-fuel management systems [4].

Alternative 4 (Coal and biomass hybridization) involves co-firing biomass and coal in the same boiler, usually with minimal retrofits to existing systems. Technical modifications include co-milling, adjusted feeding mechanisms, and combustion control systems. This strategy allows up to 30% substitution of coal with biomass, significantly reducing greenhouse gas emissions while minimizing capital investment [15,16,17]. Infrastructure reuse is high, and operational flexibility is preserved. However, biomass variability in moisture content and calorific value may affect combustion efficiency and stability. Despite these challenges, this approach is often regarded as a transitional decarbonization pathway with strong potential in rural regions where biomass is abundant and affordable [15,16].

Alternative 5 (Coal and Hydrogen Cogeneration) integrates hydrogen into coal-fired power generation either by co-firing it with coal or through parallel cogeneration systems that produce both electricity and hydrogen. Hydrogen can be injected directly into the boiler or used in separate turbines, depending on the plant’s configuration. The main advantages include reduced CO₂ and NOx emissions and the introduction of a zero-carbon energy vector [22,23]. The technical setup requires advanced combustion control, storage facilities, and hydrogen-compatible components. While promising, this alternative faces significant economic and infrastructural barrier, including high production and storage costs, low energy density, and safety concerns [24]. Currently, it is considered a medium-to-long-term solution contingent on future advances in hydrogen technologies and cost reductions.

Alternative 6 (Coal and Solar Hybridization) involves the integration of solar thermal or photovoltaic (PV) systems into existing coal plants to preheat water or provide auxiliary power. Solar-assisted systems can reduce fossil fuel consumption and improve overall thermal efficiency. Solar energy is introduced via heat exchangers, often supported by thermal energy storage units to mitigate intermittency. This hybrid model achieves moderate emission reductions and allows for modular, phased retrofitting [19,20,21]. Despite its environmental and flexibility benefits, this alternative is constrained by high initial capital costs (ranging from USD 5000 to 8000/kW), space requirements for solar fields, and design complexities in thermal coupling [28]. It is especially attractive in regions with high solar irradiation and policy support.

Alternative 7 (Carbon Capture Systems) involves the implementation of post-combustion carbon capture technologies, typically using chemical solvents like monoethanolamine (MEA), to remove CO₂ from flue gases before atmospheric release. Capture units are added downstream of the combustion process, requiring additional infrastructure for chemical handling, compression, and CO₂ transport or storage. Studies report capture efficiencies of up to 90% under ideal conditions [25,26,27]. However, these systems impose a parasitic energy load, reducing overall plant efficiency by 10–20%, and increasing the levelized costs of electricity by up to 80% [25]. Widespread deployment also depends on the development of regulatory frameworks and storage infrastructure. Despite these challenges, CCS is considered a crucial end-stage mitigation tool within the circular carbon economy [26].

Alternative 8 (Decommissioning and Subsequent Reuse) involves the permanent shutdown of coal-fired power units and the repurposing of the site for renewable energy generation, industrial parks, or logistics centers. Decommissioning includes the dismantling or retirement of boilers, turbines, and auxiliary systems based on factors such as age, efficiency, and retrofitting infeasibility. The approach eliminates direct emissions and opens pathways for urban regeneration and economic diversification. However, it also results in significant social impacts due to direct job losses and potential local economic decline [14]. Although reuse opportunities exist, such as conversion into solar farms or hydrogen hubs, the transition requires substantial investment and coordinated socio-economic planning.

The eight reconversion alternatives proposed in this study—ranging from full conversion to natural gas and biomass, to hybridization with hydrogen and solar energy, carbon capture systems, and eventual decommissioning—represent a diverse set of technological pathways for the transition of coal-fired power plants. These detailed technical descriptions highlight the heterogeneity of each option in terms of environmental impact, technical feasibility, infrastructure compatibility, and socioeconomic implications. While alternatives such as full conversion to natural gas or coal-solar hybridization offer immediate emission reductions with mature technologies, others—like hydrogen cogeneration and carbon capture systems—are more dependent on future advances in cost, regulation, and operational efficiency. The feasibility of each solution is closely tied to local and regional factors, including energy infrastructure, resource availability, and public policy frameworks. This section complements the multi-criteria evaluation by contextualizing each option within its real-world implementation conditions, thus reinforcing the practical relevance and decision support.

Upon completing the qualitative analysis of these alternatives—emphasizing their alignment with energy transition goals toward more sustainable sources—the evaluation criteria were defined. These criteria were derived from the environmental, structural, technical, technological, economic, and social dimensions that most influence the feasibility of reconversion options. The structure of the criteria and sub-criteria is detailed in

Table 9. Evaluation Criteria and Subcriteria.

Criteria	Subcriteria	Technical Description
Environmental	Carbon dioxide emissions	Level of CO2 emissions generated by the alternative. Measured in kg CO2/kWh. Source: Comparative emission values by fuel type (coal: 0.94; gas: 0.45; biomass: ≈0) [4,14,17,28].
	Waste generation	Includes solid waste derived from the process: ash, slag, non-combustible organic waste, and biomass by-products. It also considers the volume, composition, and management of the waste generated [15,16,17].
Structural	Infrastructure reuse	Extent to which the alternative allows the reuse of existing equipment (boilers, turbines, internal transmission systems), reducing investment and conversion times [14,15,16,17].
Technical	Flexibility	Ability of the alternative to operate under different loads, integrate with other (renewable) sources, and respond to variations in demand or supply [14,20,22].
	Performance	Considers net thermal efficiency (%, electrical efficiency in combined cycles (%), and the useful energy/primary energy conversion ratio (natural gas 48–65%, coal 35–42%, biomass 30–40%) [4,16,17,23,28].
Technological	Technological maturity	Level of technology development and validation, from prototypes to commercial solutions. Based on technical literature and actual plant adoption [19,21,25,26].
Economic	Implementation cost	Estimated investment cost (CAPEX), expressed in USD/installed kW. Reference values: natural gas (~1000–1200 USD/kW), biomass (~2000–3000 USD/kW), CCS (>3500 USD/kW), and solar hybrids (>5000 USD/kW) [4,14,25,27,28].
Social	Job creation	Potential of the alternative to generate or maintain direct and indirect employment. It includes local job retraining, maintaining existing jobs, and creating new technical profiles [14,15,26,28].

.

4.2. Results of the AHP Method

By applying the Analytic Hierarchy Process (AHP) method, a hierarchical decision structure was established to calculate the relative weights of each criterion and subcriterion. Based on the previously defined Figure 2 and the reviewed studies, the objectives, criteria, subcriteria, and alternatives were organized according to the corresponding levels shown in

Table 10. Definition of Hierarchical Structure.

Level	Component	Description
Level I	Objective	Evaluate technological conversion alternatives for the energy transition of coal-fired power plants
Level II	Criteria	C1: Environmental
		C2: Structural
		C3: Technical
		C4: Technological
		C5: Economic
		C6: Social
Level III	Criteria	C1: CO2 emissions, Waste generation
		C2: Infrastructure reuse
		C3: Flexibility, Performance
		C4: Technological maturity
		C5: Implementation cost
		C6: Job creation
Level IV	Alternatives	A1: Full conversion to natural gas
		A2: Full conversion to biomass
		A3: Coal and natural gas hybridization
		A4: Coal and biomass hybridization
		A5: Coal and hydrogen cogeneration
		A6: Coal and solar hybridization
		A7: Carbon capture systems
		A8: Decommissioning and subsequent reuse

.

The coal power plant reconversion alternatives were quantitatively evaluated according to the defined criteria. Carbon dioxide emissions reduction and infrastructure reuse were identified as essential components of this analysis. All proposed alternatives consider the partial or full repurposing of existing facilities, with the exception of Alternative 8 (decommissioning and subsequent reuse). Alternatives that do not involve infrastructure reuse or contribute to the decarbonization process were excluded. Therefore, environmental and structural criteria were prioritized in the analysis.

Next in importance is the technical feasibility, which reflects the practical viability of implementing each alternative effectively and efficiently. This includes factors such as operational flexibility and performance. Based on this rationale, the weighting of each criterion was defined using the Total Decision software version 1.2.1041.0, enabling a systematic and rigorous evaluation. Figure 5a presents the hierarchy of criteria and their relative importance within the context of the analysis.

The results indicate that the environmental criterion (C1) has the highest weight at 33.05%, followed by structural (C2) at 28.28%, and technical (C3) at 14.68%. The technological (C4), economic (C5), and social (C6) criteria were assigned equal weights of 8.00% each. The consistency ratio was found to be 0.05%, well below the acceptable threshold of 10%, which indicates a high degree of coherence in the pairwise comparisons.

Subsequently, the alternatives were compared based on their performance against each criterion, allowing for the identification of the most promising options and the establishment of priorities. The first comparison was based on an in-depth review of the literature and previously analyzed data, as illustrated in Figure 5b.

This assessment revealed the alternatives most strongly supported in scientific literature for their feasibility and energy transition potential. The results show that Alternative 1 (Full conversion to natural gas) obtained the highest weight at 14.10%, followed by Alternative 3 (Coal and natural gas hybridization) at 13.51%, Alternative 6 (Coal and solar hybridization) at 13.39%, and Alternative 2 (Full conversion to biomass) at 12.89%. The remaining alternatives showed balanced values between 11.19% and 11.82%. The model’s consistency ratio of 0.63% further reinforces the reliability and robustness of these results.

Following this, the alternatives were evaluated based on expert input. The survey was administered to specialists from the Administración Nacional de Electricidad (ANDE), Paraguay’s national electricity utility. Of the 27 experts contacted, 14 completed the questionnaire. Figure 5c presents the weights assigned to each alternative, combining the previously established criterion weights with expert evaluations.

In this expert assessment, Alternative 3 (Coal and natural gas hybridization) ranked highest at 13.60%, followed by Alternative 1 (Full conversion to natural gas) at 13.52%, Alternative 6 (Coal and solar hybridization) at 13.35%, Alternative 2 (Full conversion to biomass) at 13.10%, and Alternative 4 (Coal and biomass hybridization) at 13.09%. Lower-ranked alternatives include Alternative 5 (Hydrogen cogeneration) at 11.48%, Alternative 7 (Carbon capture) at 11.26%, and Alternative 8 (Decommissioning and reuse) at 10.59%. The consistency ratio of 0.05% confirms the reliability and accuracy of these comparative judgments, ensuring that the weights reflect the experts’ evaluations with high validity in the multi-criteria decision-making framework.

4.3. Results of the PROMETHEE Method

Through the application of the PROMETHEE method, a ranking of alternatives for the reconversion of coal-fired power generation units and/or the reuse of existing infrastructure was obtained. Based on the weights of alternatives for each criterion established by the AHP method—drawn from the literature review and expert survey—an arithmetic mean was calculated to represent optimal solutions. These values were then processed in the Visual PROMETHEE software to obtain the positive (ϕ⁺) and negative (ϕ⁻) preference flows for each alternative, as shown in

Table 11. Positive and Negative Flows of Each Alternative.

Alternatives	(ϕ+)	(ϕ−)
Alternative 1	0.0791	0.0422
Alternative 2	0.0472	0.0431
Alternative 3	0.0627	0.0370
Alternative 4	0.0497	0.0455
Alternative 5	0.0459	0.0517
Alternative 6	0.0473	0.0349
Alternative 7	0.0512	0.0708
Alternative 8	0.2741	0.3319

.

Using these preference flows, graphical representations were generated for each alternative. These visuals help to interpret each option’s relative position in the final ranking, thus aiding the analysis of feasibility and competitiveness in the coal-fired power plant reconversion process.

Figure 6a illustrates the results of Alternative 1 (Full conversion to natural gas), where the technological criterion performs notably well (C4 = 0.2374), followed by structural (C2 = 0.1429) and technical (C3 = 0.900). Environmental (C1 = −0.0451), economic (C5 = 0.0219), and social (C6 = −0.0473) aspects are less significant.

Figure 6b shows Alternative 2 (Full conversion to biomass), with a negative impact on environmental (C1 = −0.1003) and strong positive performance in structural reuse (C2 = 0.1429). Other criteria are near-neutral or modestly positive.

Figure 6c presents Alternative 3 (Coal and natural gas hybridization), with environmental downsides (C1 = −0.0769), but strong structural reuse (C2 = 0.1429), moderate technical (C3 = 0.0846), and economic advantages (C5 = 0.0929).

Figure 6d displays Alternative 4 (Coal and biomass hybridization), showing a negative environmental impact (C1 = −0.1159) and good structural reuse (C2 = 0.1429). Technical (C3 = −0.0161) and technological (C4 = 0.0110) scores are marginal.

Figure 6e corresponds to Alternative 5 (Coal and hydrogen cogeneration), with a substantial environmental drawback (C1 = −0.0984), positive structural reuse (C2 = 0.1257), and high social value (C6 = 0.1143), despite technical and economic challenges.

Figure 6f depicts Alternative 6 (Coal and solar hybridization), featuring slight environmental disadvantage (C1 = −0.0624), strong structural reuse (C2 = 0.1071), and a positive social impact (C6 = 0.1205), offsetting economic drawbacks (C5 = −0.0929).

Figure 6g presents Alternative 7 (Carbon capture systems), with notable environmental penalties (C1 = −0.0986), fair structural reuse (C2 = 0.1386), but technological and economic downsides (C4 = −0.0637, C5 = −0.2586).

Figure 6h visualizes Alternative 8 (Decommissioning and reuse), which has strong environmental benefits (C1 = 0.5976) but severe structural and social drawbacks (C2 = −0.9424, C6 = −0.3786).

Figure 6i presents a unified visual comparison of all alternatives. As shown, A1 (Full conversion to natural gas) ranks highest (ϕ = 0.0368), followed by A3 (Coal + gas, ϕ = 0.0257) and A6 (Coal + solar, ϕ = 0.0124). Alternatives A4 and A2 are in the middle tier, while A5, A7, and A8 rank lower, with A8 performing worst (ϕ = −0.0578), indicating it is the least favorable option in this assessment.

Finally, the net flow (ϕ), calculated as the difference between positive and negative flows, is shown in

Table 12. Alternative ranking based on net flow.

Ranking	Alternatives	(ϕ)
1	Alternative 1	0.0368
2	Alternative 3	0.0257
3	Alternative 6	0.0124
4	Alternative 4	0.0041
5	Alternative 2	0.0041
6	Alternative 5	−0.0058
7	Alternative 7	−0.0196
8	Alternative 8	−0.0578

. A higher net flow indicates a more favorable alternative.

When comparing the results obtained with the consulted bibliography, significant coincidences are observed that support the results of the study. Alternative 1—total conversion to natural gas, which occupies the first place in the ranking—shows a remarkable consistency with the findings of authors such as Mac, Brouwer, and Samuelsen (2018) and Safari et al. (2019) [4,14]. These studies highlight the high efficiency of natural gas, its contribution to the reduction of carbon dioxide emissions, and its profitability, especially in contexts where infrastructure for its distribution already exists. These advantages position natural gas as one of the most viable options and as a bridge fuel for the energy transition.

Second, Alternative 3—Coal and natural gas hybridization—also demonstrates outstanding performance, aligning with research such as that of Mills (2018) [19]. Both alternatives emerge as the most viable overall, with minimal differences between them, reflecting considerable competitiveness and suggesting that both could be complementary options in the transition to more sustainable energy systems.

Third, Alternative 6—Coal and solar hybridization highlights the growing importance of renewable energy in the energy transition. The results are consistent with studies that point to the great potential of solar energy, although they caution that its intermittency may limit its effectiveness when combined with traditional sources such as coal. This alternative represents a balance between innovation and leveraging existing.

Alternative 4—Coal and Biomass Hybridization and Alternative 2—Total Conversion to Biomass occupy fourth and fifth place, showing less efficient performance compared to natural gas but remaining a viable option due to their ability to reduce carbon emissions and promote sustainability. However, as Miedema et al. (2017) point out, their competitiveness may be limited by logistical costs and resource availability, suggesting the need for supportive policies for their large-scale implementation [17]. In contrast, Alternative 5—electricity and hydrogen cogeneration—presents a lower performance, indicating greater challenges in terms of technical and economic feasibility. This option, although promising in certain contexts, faces limitations that hinder its widespread adoption. Alternative 7—Carbon Capture Systems also shows a low score, reinforcing the arguments of previous studies that question its viability due to high costs and reduced energy efficiency.

Finally, Alternative 8—Closure and Subsequent Reuse—performed the worst, a result consistent with existing literature. This option is often considered the least favorable due to the loss of infrastructure and the negative social impact in terms of employment.

Comparing these results with previous studies confirms the trend observed in works such as those by González, Kirsten, and Prchlik (2018), who highlight the efficiency of natural gas in reducing emissions and its economic viability [18]. Likewise, the co-firing of coal and biomass, although slightly less efficient, remains a viable alternative for the energy transition, as described by Kamble et al. (2019) [15]. However, as Xu et al. (2020) point out, these alternatives require strong regulatory frameworks and supportive policies to be competitive in global markets [16].

These results are not only supported by the literature but also provide a solid basis and tool for decision-making in the design of energy transition strategies.

5. Discussions and Futures Perspectives

This section provides a critical discussion of the evaluated reconversion alternatives for coal-fired power plants, integrating the multi-criteria analysis results with theoretical foundations and empirical evidence. The aim is to consolidate the reasoning behind the most viable options, validate them through literature and expert insights, and reflect on their implications for public policy and future technological development. The discussion is structured into three parts: validation of alternatives, implications for policymaking, and study limitations with suggestions for future research.

5.1. Validation of the Alternatives

The choice of the approach based on the combination of AHP and PROMETHEE is based on its proven ability to generate robust and justified solutions in complex and multidimensional decision-making environments. The internal consistency of the model was verified using the Consistency Index (CR) in the AHP method, which yielded values below the 10% threshold in all comparative matrices, guaranteeing the logical validity of the assigned weights. Furthermore, the use of PROMETHEE allowed for a complete ordering of the alternatives based on net preference flows (ϕ), offering not only a ranking but also a visual representation of the relative dominance of each alternative. Although this approach does not seek global mathematical optimization in the traditional sense, it is a multi-criteria optimization based on preferences, reflecting the values assigned by experts and empirical evidence. Thus, the solutions obtained are justifiable from a methodological point of view, strengthening their applicability in real-life strategic decision-making processes.

To assess the model’s robustness to changes in preference judgments, the Walking Weight tool in Visual PROMETHEE was used. Two scenarios were simulated: (i) the environmental criterion drops to 23% and the technical criterion increases to 24%, and (ii) the structural criterion increases to 36% while the environmental criterion decreases to 26%. The results showed that, although slight variations were observed in the absolute values of the flows, the overall ranking of the alternatives remained stable, especially in the top positions (A1, A2, and A3). This indicates a high robustness of the model to reasonable changes in preference judgments. This consistency validates the reliability of the AHP–PROMETHEE methodological combination to support strategic decisions in complex contexts. The simulation of the scenarios can be seen in Figure 7.

The results of this study demonstrate that full conversion to natural gas emerges as the most viable alternative (ϕ = +0.0368), validating the findings of Mac et al. (2018), who observed significant reductions in CO₂ emissions (up to 23%) [14]. This alternative stands out due to its high thermal efficiency (up to 65% in combined cycle plants according to (Safari et al. 2019) and relatively low capital costs (approximately 25%), as well as its operational flexibility [4].

In contrast, the limited viability of carbon capture systems (ϕ = −0.0196) aligns with Lockwood’s analysis, which highlights substantial costs (up to +80% in power generation) and efficiency losses (around −10%) [25]. Practical case studies, such as the one by Rogieri et al. (2023) in the Jorge Lacerda Complex (Brazil), reinforce these conclusions, indicating that successful transitions require not only technological innovation but also accessible financing and robust regulatory frameworks [26].

Coal-solar hybridization (ϕ = +0.0124) confirms (Serrano et al. 2019) results, who reported efficiency improvements of 4.6%, albeit at higher capital costs ($8050/kW compared to $5979/kW for traditional coal) [28]. The PROMETHEE evaluation also confirmed the strategic potential of these alternatives, particularly when aligned with policy instruments that support energy diversification.

The integration of Proknow-C, AHP, and PROMETHEE methods offered complementary advantages. The Proknow-C approach provided a rigorous foundation through the selection of 26 scientific articles and 13 patents, establishing a robust theoretical and empirical basis. The AHP method enabled the structured quantification of evaluation criteria with high consistency (CR = 0.05% and 0.63%), and the convergence of results from bibliographic analysis and expert surveys—where the top three alternatives matched—reinforced the coherence and reliability of the findings.

As emphasized by (Calabrese et al. 2019), this methodological consistency minimizes bias in complex decision-making [9]. PROMETHEE confirmed these priorities mathematically via net flow analysis, highlighting the viability of hybrid solutions like coal-solar integration (Figure 6f), which not only reduces emissions (up to −35% compared to coal, as shown by Mills (2018) [19]. But also promote job creation (positive social impact), critical for a just transition [2].

The net flows (ϕ) revealed critical insights that can guide public policy:

Alternatives with ϕ > 0 (natural gas, hybridizations): Require incentives to offset weaknesses. For instance, natural gas transitions would benefit from carbon pricing mechanisms as suggested by (Mac et al. 2018) [14].

Alternatives with ϕ < 0 (hydrogen, carbon capture): Require R&D to enhance competitiveness. Findings by Wei et al. on hydrogen co-firing highlight the emerging potential of these technologies [23].

5.2. Implications for Public Policies

Full conversion to natural gas emerges as the most viable short-term solution due to its strong technical performance and economic feasibility. As such, it should be prioritized in national energy planning. To enable this transition, policy instruments should include the implementation of a minimum carbon tax aligned with the social cost of carbon, particularly for emerging economies. Additionally, tax credit schemes covering 30% to 50% of the capital costs could be established, with environmental performance requirements as conditional criteria. From a strategic planning perspective, retrofitting efforts should be concentrated in regions where natural gas distribution networks are already in place. Moreover, coal-fired plants older than 15 years should be prioritized, as they present greater potential for modernization benefits. An exemplary initiative could be the launch of a “Gas for Transition” program in Brazil, combining preferential credit lines with strict, continuous emissions monitoring requirements.

Hybrid technological solutions, which combine renewable and conventional energy sources, represent a promising mid-term pathway by balancing sustainability and operational adaptability. In this context, public policy must support the scale-up of these systems through targeted economic measures. For instance, thermal storage systems could be partially financed—between 15% and 20% of total investment—via green climate funds. Local biomass integration should also be encouraged by offering financial incentives to farmers for the use of agricultural residues, such as soybean waste in Paraguay. To guide infrastructure deployment, national energy planning could establish “Hybrid Priority Zones” in areas with high solar irradiation (greater than 5 kWh/m²/day) or within a 50-km radius of abundant biomass supply. These policies are particularly relevant for rural or semi-rural regions with strong agricultural sectors or favorable solar profiles.

For emerging technologies such as carbon capture and hydrogen integration, policies should focus on future readiness. This includes the creation of public-private consortia aimed at scaling up innovative solutions, with pilot programs in collaboration with universities and research institutes. Special Economic Zones (SEZs) located near retrofitted plants could provide shared R&D infrastructure, enhancing innovation capacity. Human capital development will also be essential. Technical training programs such as “Clean Energy Schools” should be established, offering specialized curricula for plant operators and technicians. To facilitate workforce transition, active employment policies should be adopted—for example, “Labor Transition Subsidies” offering a 20% wage supplement for up to two years to workers affected by reconversion.

A phased and adaptive policy strategy is recommended. In the short term, efforts should focus on accelerating gas conversion and deploying key financial instruments. In the medium term, hybrid systems should be scaled, alongside the development of local supply chains for renewable energy components. In the long term, emerging technologies should be integrated into the energy matrix, while regulatory frameworks and clean energy markets are consolidated. This gradual, multi-stage approach—when combined with complementary social policies—can offer a pragmatic and equitable pathway toward power sector decarbonization. It ensures both environmental sustainability and regional socioeconomic development.

Ultimately, the success of such a strategy will depend on regulatory flexibility to adjust incentives based on technological maturity, enhanced regional cooperation in supply chains (such as a common biomass market in Mercosur), and continuous performance monitoring using standardized indicators, such as emissions and jobs per megawatt-hour generated. If effectively implemented, this framework offers a realistic roadmap to achieve SDGs 7 (Affordable and Clean Energy) and 13 (Climate Action), bridging the gap between climate commitments and national development agendas.

Regarding regional applications and social effects, the applicability of each retrofit alternative is influenced by regional energy infrastructure, resource availability, and socioeconomic conditions. In Latin America, many coal-fired plants are located in economically vulnerable areas, where they not only serve energy functions but also represent key sources of employment. Therefore, strategies such as full conversion or closure can have significant social impacts, including job losses and local economic disruption. Hybrid options such as the integration of biomass or solar power allow for more gradual transitions that maintain part of the operation while introducing low-carbon technologies. Furthermore, the typology of each plant affects technical feasibility and investment requirements. These contextual variables reinforce the need for regionally tailored policies that integrate just transition principles, ensuring that environmental benefits are not achieved at the expense of social equity.

5.3. Limitations and Future Research

While this study provides a robust and methodologically sound assessment of reconversion alternatives for coal-fired power plants, some limitations remain that suggest fruitful directions for future research. One of the primary constraints lies in the study’s predominantly quantitative nature. Although this approach ensures objectivity and replicability, it does not fully capture the complex social and human dimensions inherent in energy transition projects. A complementary integration of qualitative methods—such as interviews with local communities affected by reconversion efforts or analyses of socio-environmental conflicts—would allow for a more nuanced understanding of the distributional impacts and governance mechanisms involved. Such mixed-methods approaches are essential to evaluate aspects of energy justice, including how benefits and risks are spatially allocated and how inclusive the decision-making processes are.

Although this study proposes a structured framework for the evaluation of retrofit alternatives, it is acknowledged that some considered technologies, such as carbon capture and storage (CCS) and hydrogen integration, still present significant challenges for large-scale implementation. However, recent research reports significant advances that could modify their future viability. In the case of CCS, developments in adsorbent materials such as membranes and MOF-8 modified with deep eutectic solvents stand out, which have improved CO₂ capture at low pressures, showing high capacity even under post-combustion conditions (Zhang et al. Separation and Purification Technology, 2025) [55]. In the hydrogen field, recent studies have demonstrated the development of highly selective and high-permeability membranes for H₂/CO₂ separation, including inorganic membranes, MMMs, and advanced polymers. These technologies allow hydrogen purities exceeding 99.9% to be achieved, with improvements in thermal stability, resistance to contaminants, and reduction of operating costs (Sun et al. 2024) [56,57]. These advances, although still in the expansion phase, reinforce the relevance of including these technologies in the evaluated retrofit scenarios, anticipating their increasing maturity and applicability.

Although this study presents a structured and replicable framework for evaluating alternatives to coal-fired power plant conversion, it has not yet been applied to a specific case study. This limitation is acknowledged, and future research will focus on implementing the proposed methodology in a real-world context—ideally at a coal-fired power plant in Latin America currently engaged in decarbonization planning. Such a pilot application would enable validation of the decision-making model, refinement of criteria based on field conditions, and integration of stakeholder perspectives. This process would enhance the model’s practical relevance, support its usefulness for policy formulation, and facilitate the exploration of complementary mixed solutions [58,59,60,61,62,63,64,65,66,67,68,69].

An additional avenue for future exploration is the incorporation of full Life Cycle Assessments (LCA) for each reconversion alternative. By analyzing the environmental, economic, and social impacts across the entire value chain—from resource extraction to end-of-life—LCA studies would offer a more comprehensive perspective on the long-term sustainability of each option. These methodological enhancements, particularly when combined with the multi-criteria framework already applied, would provide a holistic evaluation tool to guide energy transition strategies. Ultimately, integrating such dimensions is critical for designing energy policies that not only reduce emissions effectively but also promote social inclusion, regional equity, and environmental responsibility throughout the entire lifecycle of power sector transformation projects.

6. Conclusions

This study provided a clear and objective answer to the research question: What are the most feasible alternatives for the conversion of coal-fired power plants that support the energy transition through the integration of more sustainable energy sources? The findings revealed that the most viable options are: (i) full conversion to natural gas, which ranked highest due to its high thermal efficiency and cost-effectiveness—particularly in regions where gas distribution infrastructure is already in place; (ii) hybridization of coal and solar energy, reflecting the increasing relevance and maturity of renewable sources; and (iii) hybridization of coal and natural gas, which stands out as a technically competitive and complementary solution. These alternatives demonstrate the potential to integrate cleaner, technologically mature energy sources, making them the most practical paths for repurposing existing coal-fired infrastructure.

In contrast, alternatives such as full conversion to biomass or hybridization with biomass exhibited lower feasibility, mainly due to technical, logistical, and economic limitations. Similarly, the cogeneration of electricity and hydrogen, and carbon capture systems, ranked among the least viable, aligning with literature that highlights their high costs and reduced operational efficiency. Lastly, the decommissioning and subsequent reuse of plants ranked lowest in the final PROMETHEE results, primarily due to the total loss of infrastructure and the negative social impact caused by significant job reduction.

These conclusions are firmly supported by the reviewed literature and offer a strong, decision-oriented framework for policymakers and energy planners. Furthermore, the study aligns with key SDGs, notably SDG 7, which promotes universal access to modern, affordable, and sustainable energy, and SDG 13, which focuses on climate action and greenhouse gas mitigation [12]. A just and sustainable energy transition—such as the one modeled in this study—can foster new investment opportunities, enhance innovation, and generate employment, laying the foundation for a more competitive and resilient economy.

This research contributes to the SDGs in four critical areas: (i) Technically, by identifying viable technological alternatives for coal plant conversion and infrastructure reuse; (ii) Environmentally, by offering low-emission pathways that support decarbonization; (iii) Economically, by presenting feasible options that maintain the operational and market competitiveness of coal-fired facilities; and (iv) Scientifically, by consolidating technical and intellectual property evidence, fostering knowledge sharing and collaboration across Latin America.

In practical terms, the findings guide energy policy design across temporal horizons. In the short term, full conversion to natural gas should be prioritized, supported by carbon pricing mechanisms and tax credits covering 30–50% of capital expenditures—especially in regions with existing gas networks. In the medium term, scaling up renewable hybrid solutions will require investment in local supply chains, such as thermal storage funds and incentives for agricultural biomass (e.g., soybean residue). Priority zones should be established based on solar potential (>5 kWh/m²/day) or biomass availability (<50 km). In the long term, emerging technologies like hydrogen and carbon capture should be promoted through public–private consortia and complemented with inclusive labor transition programs, such as targeted training for affected communities. Successful implementation will depend on regulatory flexibility, regional cooperation (e.g., a Mercosur biomass market), and performance monitoring via indicators such as emissions and employment per MWh generated.

For future research, this study recommends integrating qualitative methods—such as community interviews or participatory workshops—to better capture local perceptions and justice-related dimensions of the energy transition. The multi-criteria framework could be adapted for national contexts with rigid coal-based energy systems or limited access to natural gas. Conducting full life cycle assessments (LCAs) using regionalized databases would enhance sustainability evaluations, while studies on governance models for Priority Zones could improve long-term planning and institutional coordination. The robustness of such future work will depend on both scientific rigor and practical policy relevance, as illustrated in this study.

In conclusion, the results of this study directly support its objective by identifying and ranking the most feasible alternatives for coal plant conversion, using a robust multi-criteria approach. The integration of Proknow-C, AHP, and PROMETHEE methods provided a consistent and transparent evaluation framework. By addressing the lack of integrated and standardized assessments in the literature, the study makes both academic and practical contributions to energy planning and policy design.

Thus, the findings reinforce the effectiveness of the proposed methodology in bridging the identified research gap: the absence of integrated, standardized multi-criteria assessments for coal plant reconversion. By combining a rigorous bibliographic foundation, expert-informed evaluation, and mathematical decision ranking, the proposed framework proves to be a powerful tool for informed, context-sensitive energy planning. It strengthens the scientific contribution and provides strategic value for guiding coal phase-out strategies in regions where infrastructure remains relevant, and transformation is urgent.