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Systematic Review

Optimisation Strategies and Technological Advancements for Sustainable Direct Reduction Iron Production—A Systematic Review

by
Ratidzo Yvonne Nyakudya Ncube
* and
Michael Ayomoh
Industrial and Systems Engineering Department, University of Pretoria, Pretoria 0028, South Africa
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(5), 2266; https://doi.org/10.3390/su17052266
Submission received: 22 January 2025 / Revised: 21 February 2025 / Accepted: 25 February 2025 / Published: 5 March 2025
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Abstract

:
This systematic review examines optimisation strategies and technological advancements to foster sustainable direct reduction iron (DRI) production. The evaluation encompassed a meticulous review of journal articles, industrial reports, and conference papers published between 2002 and 2025, ultimately identifying 65 pertinent studies. A qualitative thematic analysis of the optimisation strategies enabled the identification of three primary themes: life cycle assessment strategies, modelling tools, and technological innovation strategies. This review highlights innovative approaches to using alternative reductants such as biomass and hydrogen, incorporating renewable energy sources in the process, and the economic feasibility of adopting these optimisation strategies. The research findings indicated that there is an urgent need to enhance waste management strategies, especially for coal-based reduction processes, as they are linked to environmental issues. Hydrogen-based reduction has been identified as an innovative methodology for waste control with the potential to reduce carbon dioxide emissions by up to 90%, though it has its limitations. The Circular Economy approach has been proposed as a viable strategy to reduce waste generation and extend the lifespan of materials used in the DRI process. This review provides essential insights on resource optimisation and utilisation and promotes technological innovation to improve the sustainability of DRI.

1. Introduction

Direct reduction iron (DRI) is a metallic substance produced through the direct reduction of iron oxide at temperatures below its fusion point, using either natural gas or coal as a reductant, yielding two primary categories: gas-based DRI and coal-based DRI. Sponge iron, also known as direct reduction iron (DRI) [1], has become a vital raw material for steel manufacturers, particularly in electric arc furnaces due to its superior iron content and increased cost of scrap metal [2]. DRI is produced through the direct reduction of iron oxide at temperatures below its fusion point, utilising either natural gas or coal as reductants, yielding two distinct types of direct reduction iron, namely, gas-based and coal-based. The reducing gas is a mixture of hydrogen (H2) and carbon monoxide (CO), which act as reducing agents [3,4,5,6]. DRI is preferred as a raw material for steelmaking compared to scrap owing to the residual elements (0.13–0.73%) available in scrap [7], which alter the consistency and composition of steel. Steel is a critical resource across various industries including construction, manufacturing, agriculture, and transport [8], and needs to be conserved for future generations. However, the iron and steel industry poses significant environmental concerns, releasing emissions, oil metals, sludge, fluo rides, sulphur compounds, and heavy metals that contaminate soils [9], causing environmental degradation and compromising the sustainability of the process.
Sustainable development is defined as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs” [10]. This necessitates sustainable DRI production to manage the rate of mineral resource depletion and to control the amount of waste and emissions. Production data from world DRI statistics [11] show that coal-based DRI has been gradually increasing over the past few years, especially in developing regions of Africa and India, despite the call to use alternative reductants that cause less pollution. Coal-based DRI processes are an environmental threat owing to the increased use of natural resources, land degradation, greenhouse gas emissions, and environmental pollution [12]. The need to address waste from DRI has prompted the development of innovative strategies which provide a road map for industries seeking to achieve sustainability in DRI. The sustainability of DRI is important to enable the continuity of the process for future generations, as this is a non-renewable resource material that is vital in the steelmaking process. Steel is the most vital raw material for infrastructural development for most manufacturing industries; thus, its main raw material needs to be preserved. Identifying these sustainable strategies for DRI helps industries to craft their policies with a clearer perspective, which enables the continuity of DRI production. Even though the systematic review methodology is presented to identify and compare strategies that can be adopted for sustainability, some geographic and resource constraints may present challenges to some researchers due to resource availability. From this review, the results of the analysis are presented together with the conclusions and recommendations.

2. Review Methodology

A systematic literature review methodology [13] has been used in this paper to aggregate research findings from various articles on strategies applied to improve the DRI process. This review was performed to establish the current strategies applied to enhance the sustainability of both coal-based and gas-based DRI processes. Several researchers have explored strategies that focus exclusively on either coal-based or gas-based processes rather than addressing both simultaneously [14,15,16]. This review seeks to identify strategies that can be adopted and modified to foster the sustainability of both processes, thus providing a comprehensive picture of sustainable DRI strategies. It also highlights the challenges facing the DRI industry such as the environmental impacts, energy efficiency, process optimisation, and economic feasibility of the process. A systematic search of articles was performed to identify studies related to improving the DRI process whilst minimising raw materials and waste. The search was conducted using the Scopus database and Google Scholar search engine for articles published from 2002 to 2025, and it was based on the steps proposed by Aromataris [13]. An initial search was performed using the title [TITL “Optimisation Strategies and Technological Advancements for Sustainable DRI Production”], and no article matched the exact title in the Scopus database or on Google Scholar. The search was refined to include the title, abstract, and keywords as follows, [TITL-ABS-KEY] (sustainable “direct reduction iron”) OR (“coal-based” “direct reduction iron”) OR (“gas-based” “direct reduction iron”) OR “optimisation AND direct reduction iron”. A total of 748 articles were retrieved from both databases; however, not all articles were relevant to the research. Journal papers, conference papers, and technical reports from industry were considered for the review. Book chapters, thesis reports, editorial notes, and magazine articles were not considered as articles for analysis in this review. A total of 65 articles finally matched the selection criteria after exclusions. Figure 1 shows the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA: Supplementary Materials) methodology flowchart with the elaborate steps taken to consider articles for review.
A thematic-based approach was further considered to categorise the articles based on the themes. The themes were further analysed to identify sustainability trends between coal-based and gas-based processes. The analysis was performed to identify the gaps in sustainability solutions applicable to coal-based and gas-based processes.
This review has been performed to address the following objectives:
  • Categorise the different optimisation strategies capable of enhancing the sustainability of the DRI process.
  • Establish new technologies used to enhance the sustainability of DRI.
  • Establish economic implications of sustaining the DRI process.
This systematic review considered articles on optimising different DRI processes as the population for the study. Various DRI processes were analysed and the PRISMA guideline was used to screen and identify relevant research articles. The selected articles were analysed using Atlas Ti-24 to categorise the approaches based on themes. This enabled a comparative analysis of the articles using assigned codes. The 1st-order coding was used to assign a code to the themes considered for classification. Three main themes were established based on the strategy used for optimisation; further comparative analysis of the sub-characteristics of the articles was performed using Excel and the comprehensive analysis is shown in the Qualitative Findings Section.

Sustainability Indicators for the DRI Process

This review enabled the identification of sustainability indicators for the DRI process, which are crucial for evaluating the environmental social and economic impacts. Sustainability indicators suggested by various authors include energy consumption per ton of DRI [17] produced, carbon dioxide emission per tonne of DRI produced [18], raw material and water usage per tonne of DRI [19], waste generated per tonne of DRI [20], and the economic feasibility of the DRI process compared to the traditional methods. Indicators vary depending on specific DRI production methods and technology and regional factors. Indicators help measure sustainability and identify areas of improvement for the DRI process; thus, it is important for organisations involved in DRI production to regularly assess various indicators for them to adopt indicators that help them make informed decisions and process improvements. One author suggested that the increased rate of iron and steel production imposes challenges to sustainability due to emissions associated with the process [12]. He suggested sustainability indicators assessed in the iron and steel industry include “economic parameters, greenhouse gas emission, freshwater consumption, and land use”. Fruehan [21] highlighted that the international community is facing challenges in developing processes that will make steel production more sustainable in the future. He suggested that sustainable steelmaking goals should include conserving natural resources such as coal and ore; reducing emissions such as carbon dioxide, sulphur dioxide, and nitrogen dioxide; reusing waste material and landfills; and reducing the disposal of hazardous waste. The demand for DRI as the main raw material for steel production has pushed organisations to look for strategies to enhance the sustainability of DRI to enable the continuity of the process [22].

3. Qualitative Findings

The qualitative approach proposed by Rambree [23] was also used to present the findings from the study. These qualitative results describe the themes and codes assigned to different articles under study to determine the grouped characteristics of the articles. Three main themes, namely, modelling techniques, the use of alternative technologies, and life cycle assessment strategies, were established from the various approaches for optimising the DRI process. Subthemes were further created from the main themes, to allow for an in-depth analysis of the articles. The modelling techniques’ theme was divided into simulation models, mathematical models, and experimental models, whilst alternative technology’s theme was further divided into biomass reductants, hydrogen reductants, and energy-efficient technologies. The initial coding from Atlas TI-24 shows the classification of the themes and subthemes, as shown in Figure 2. By visualising the distribution of themes and subthemes, the study ensures that the categorisation is both transparent and replicable.
This study established the current optimisation strategies for sustainable DRI production. The modelling techniques theme presents the highest number of articles on strategies to enhance the sustainability of DRI from the study This highlights the multifaceted ways researchers attempt to understand and improve the DRI process. Simulation and mathematical models provide a theoretical or computational approach to predicting outcomes and optimising parameters, while experimental models offer hands-on validation, and this layered approach can help identify which modelling strategies yield the most reliable or rational insights, potentially pointing to areas where further empirical validation might be needed.
The use of alternative technology presents the diverse options available for sustainable DRI other than the traditional coal-based and gas-based processes, which are associated with carbon emissions. Biomass and hydrogen reductants signal a shift towards reducing carbon emissions and the dependence on traditional fossil fuel-based imports; similarly, energy efficient technologies reflecting efforts towards lower energy consumption, are sustainably promoting the DRI process. This thematic breakdown not only demonstrates innovation but also maps out ways for sustainability to be targeted. LCA strategies emphasise that researchers not only focus on process optimisation from a technical perspective but also evaluate the environmental impact across the life cycle of the DRI process. A detailed summary of the articles under each subtheme is presented in Table 1. The data in the table elaborate on the problems addressed by the various strategies to improve the sustainability of the process. The summarised data from Table 1 show that many of the articles reviewed focused on enhancing the sustainability of the DRI process through variations in process parameters [24,25,26,27,28,29,30,31], mass and energy balances [31,32,33], energy efficiency [34,35,36,37], and emission reduction [25,26,38,39,40,41,42,43]. Addressing these issues fosters the sustainability of the DRI process.

3.1. Comparison of Various Reductants Used in Various Optimisation Strategies

A comparative analysis of the reductants used in the various optimisation strategies was conducted from the literature review. The data reveal that more research has been performed on natural gas and hydrogen-based reductants as shown in Figure 3, though these reductants have less impact on carbon emissions compared to coal-based processes. Natural gas and coal are the two main types of primary fuels used in the DRI process, and both are fossil fuels and non-renewable and compromise sustainability. Some researchers have considered the use of biomass reductants as a panacea to sustainable DRI; however, these reductants have their limitations that hinder their application on a commercial scale. Gas-based DRI has been mainly adopted in countries that have abundant reserves of natural gas [79]. However, in most developing countries, natural gas has a prohibitive cost, which inhibits its use in energy-intensive processes. Coal is used to power energy-intensive industrial processes in most developing countries because of its availability and economic price. The Middle East is the greatest producer of DRI followed by Asia [11]; this has been attributed to the huge iron deposits and gas reserves in the Middle East, and the abundant coal and iron reserves in India. Developing countries in Asia and Africa are still struggling with the use of coal-based reductants for the DRI process and this poses a threat to the environment due to the pollution from the process, hence the need for sustainability measures. However, given the substantial environmental impact of coal-based DRI, it is crucial to prioritise research efforts in coal-based DRI, to mitigate detrimental effects on the environment.

3.2. Thematic Analysis of the Strategies

The thematic analysis offers an overview of how various research articles approach the optimisation of DRI processes by categorising the strategies into three main themes, which are modelling techniques, the use of alternative technologies, and life cycle assessment strategies. Figure 4 shows the number of articles considered in the systematic reviews according to their themes. The modelling theme constitutes the bulk of the articles, with the mathematical model offering several strategies to enable efficient resource utilisation and waste reduction, thus enabling the sustainability of the process. Simulations and mathematical models provide theoretical and computational frameworks to predict outcomes and optimise parameters while experimental models offer hands-on validation, and this layered approach can help to identify which modelling strategies yield the most reliable actionable insights, potentially pointing to areas where further empirical validation might be needed. The bar chart in Figure 4 shows fewer articles on energy-efficient technologies and LCA strategies. However, these two strategies provide a comprehensive framework to quantify energy requirements and greenhouse gas emissions across the entire life cycle of the DRI process [78]. There is a need for more research on these themes as they enable a reduction in GHG emissions, which is a global challenge.
An in-depth thematic analysis was carried out to enable a comparison of findings across various articles and to identify the problems addressed and variables considered to optimise the DRI process, thus fostering sustainability. Strategies reviewed from different themes enable sustainability through various concepts such as emission reduction, energy optimisation, and the adoption of greener technologies like hydrogen and biomass reductants.
The data presented in Table 2 and Figure 5 detail the distribution of articles that tackle problems with the DRI process identified in the review. Among 65 articles analysed, emission reduction is the main issue addressed by various researchers (29%), highlighting it as a critical area of interest for numerous researchers. Each strategy examined had at least one article that presented a solution for decreasing emissions. The global significance of carbon emissions prompted researchers to explore innovative solutions aimed at mitigating these environmental challenges. LCA emerges as a favoured strategy for emission reduction due to its capacity to identify and implement emission reduction opportunities across various stages of the process’s life cycle. Furthermore, the review indicates that researchers have tried to look for solutions to challenges related to the optimisation of DRI processes (28%). Strategies considered for process optimisation include the use of mathematical, simulation, and experimental models. All these models play a significant role in enhancing the sustainability of DRI. In addition, the adoption of energy-efficient technologies and advancements in hydrogen technology collectively play a significant role in addressing problems associated with energy efficiency and the economic feasibility of the DRI process, thus enhancing the overall sustainability.

3.2.1. Modelling Techniques as a Strategy

The analysis of modelling techniques explored a range of mathematical, simulation, and experimental models designed to improve the sustainability of direct reduction iron (DRI) processes. The document coding applied in this study using Atlas-ti24 enabled a visual representation of the relationships between codes applied for the modelling techniques and documents, as shown in Figure 6. The mathematical models presented in this study enabled the identification of optimisation strategies that can be applied to both coal-based and gas-based processes to reduce waste and increase productivity. Several mathematical models analysed and focused on process optimisation and increasing the efficiency of systems [3,27,51,52]. The primary objectives of these researchers included refining operating conditions, boosting conversion rates, and minimising energy usage. Kumari [34] developed a mathematical model to estimate the optimum parameters for the rotary hearth furnace and established that maintaining optimum temperatures during reduction resulted in a reduced residence time, thus enabling significant energy savings. Moussa [27] also presented a mathematical model for gas-based shaft furnaces, which enabled improved thermal efficiency and reduced energy consumption. Ranzani [55] combined the mathematical and experimental models and enabled both process optimisation and CO2 reduction and established that CO2 emissions could be reduced by up to 80% with the use of hydrogen reductants.
The 2D model by Hamadeh [53] also enabled process optimisation and waste reduction. These models incorporate various reducing gases, such as hydrogen and syngas, enabling accurate predictions of essential process variables while offering deeper insights into process sustainability.
Simulation models contribute significantly to sustainability by optimising various aspects of the DRI process, including the mass and energy balance of the system [32,33,46], which helps in minimising energy losses within the reactors, as shown in the studies. The studies highlight that these sophisticated mathematical models and simulation methodologies hold significant promise for improving process efficiency, curbing greenhouse gas emissions, and enhancing product consistency within the iron and steel sector. The efficacy of these models has been validated across numerous plant environments, demonstrating their practical industrial relevance. Other authors used Fuzzy Logic [4,80] to develop simulation models to reduce waste generated from accretion build-up during the reduction process. Bechara [57] developed a simulation model for waste reduction using computer-aided optimisation, and this resulted in a 15% emission reduction. These models play a significant role in both process optimisation and waste reduction, thus offering a balance between productivity and sustainability.
The experimental models reviewed in the study play a crucial role in enhancing the sustainability of DRI by providing alternative process conditions, which enable the optimisation of the process and waste reduction. Several researchers [8,14,41,71] have suggested ways of using hydrogen as an alternative reductant to significantly lower CO2 emissions compared to the traditional carbon-based methods. Major highlights of the experimental models include the effect of using different parameters on process optimisation and emission reduction [26,81]. These experiments established optimum operational conditions, which enabled increased productivity with minimum waste generation. Hydrogen-based experiments are considered a greener alternative that reduces the overall carbon footprint of iron production. Experiments to optimise the DRI process [24,28,82] enabled productivity improvement by increasing the degree of metallization for the iron. This was achieved through improved resource efficiency and optimised operational parameters. Improved metallization leads to less waste generation, contributing to a more sustainable production process with less environmental impact. Experimental strategies are pivotal in making DRI production more sustainable by reducing emissions, utilising renewable resources, and improving process efficiency, and this aligns with the broader goals of sustainable development.

3.2.2. Technological Innovations for Sustainability Enhancement of DRI

Technological innovations in DRI processes have enabled the identification of technologies that can be adopted and modified for coal-based processes to have less environmental impact. Findings from the innovative technologies theme involve the use of energy-efficient technologies and alternative reductants such as biomass and hydrogen. Figure 7 shows the document codes assigned to each theme.
The utilisation of biomass reductants as a reducing agent was explored by several researchers as an effective way to reduce emissions and waste, thereby enhancing the sustainability of the DRI process. This approach was investigated in countries like China [25,35], India [65], and Germany, which shows the growing interest in sustainable alternatives for coal-based reduction. A study by Zuo [35] established that biomass char has a higher reactivity than coke and coal; however, the production of biomass char on a large scale remains a challenge due to the cost of production and the carbon footprint. Syngas derived from biomass has the potential to increase productivity whilst at the same time reduce emissions. A study by Guo [83] on syngas derived from biomass showed that the reducibility of iron oxide can increase from 88% at a temperature of about 900 °C to 99.5% at a temperature of 1100 °C, which is almost an optimum reduction according to standards of DRI reduction. Heat losses and pollutants emitted from coal-based processes have been the major drivers for the development of strategies to optimise the DRI process.
Hydrogen technology has been considered the most promising iron technology with the potential to reduce emissions by 80–90% [84]. Green hydrogen has the potential to reduce CO2 emissions to nearly zero in the iron and steel industry [85]. Transitioning from carbon-based reduction processes to hydrogen-based reduction is a significant step towards sustainability. Hydrogen can be produced using renewable energy sources, making the process more environmentally friendly. Additionally, hydrogen-based reduction produces water as a byproduct rather than carbon dioxide [86]. DRI produced from hydrogen has less carbon footprint than DRI produced using fossil fuels such as natural gas and coal. Direct reduced iron is produced from iron ore, which exists as iron oxide (hematite Fe2O3). The chemical reduction of iron oxide (Fe2O3) to iron (2Fe) by carbon (C) from carbon monoxide (CO) produces carbon dioxide (CO2) (Equations (1) and (2)), which heavily pollutes the environment. The use of hydrogen substitutes the carbon and carbon monoxide reductants, and the following reaction (Equation (3)) occurs [87], which results in the production of water, which does not pollute the environment:
F e 2 O 3 + 3 2 C = 2 F e + 3 2 C O 2
F e 2 O 3 + 3 C O = 2 F e + 3 C O 2
F e O 3 + 3 H 2 = 2 F + 3 H 2 O
Hydrogen used as a reductant instead of carbon needs to be fossil-free; thus, green electricity must be used for water electrolysis to reduce the carbon footprint. Wang [70] investigated the feasibility of producing hydrogen for direct reduction technology, which is 100% green hydrogen, rather than the natural gas conventional process. He concluded that the economic feasibility of hydrogen direct reduction is dependent on low-cost green hydrogen in the long term and 80 to 90% of carbon emissions could be avoided by the application of hydrogen direct reduction (H-DR) technology. Hydrogen is mainly produced through carbon-intensive processes by processing fossil fuels, among which natural gas reforming is the main source of hydrogen in the direct reduction process [88]. For areas that lack natural gas and have abundant coal resources, coal gasification technology can be employed for producing hydrogen. Hydrogen can also be produced through water electrolysis; however, production can be very expensive [89]. If the power source used for electrolysis originates from fossil fuels, then the carbon footprint and carbon emission are greater than those of the reforming process. Only hydrogen produced from the electrolysis of water using renewable energy sources such as wind and nuclear is considered green hydrogen.

3.2.3. Life Cycle Assessment

The life cycle cost analysis strategy is used to evaluate the economic performance of the DRI process over its entire life cycle, considering all the relevant costs. The perspective of this cycle provides a holistic approach that helps in understanding the immediate economic cost and long-term economic implications. A life cycle cost analysis was performed by Nduagu [43] for the various methods of DRI production. A hybrid life cycle inventory approach was used for the three distinct technological pathways for producing DRI. A comparison of life cycle assessments was performed on the coal-based rotary kiln, coal gasifier shaft furnace, and reformed natural gas shaft furnace. Variations in the input parameters, operating conditions, and certain assumptions were considered for the three distinct technologies. It was observed that greenhouse gas (GHG) emissions were significantly reduced from (1391.1–1880) kg of CO2/t of DRI when using coal-based processes to (815.1–1160) kg of CO2/t of DRI for the reformed natural gas DRI process, which shows a huge reduction in the green GHG emissions. Another study conducted in America [42] on cost and life cycle analyses for CO2 reduction in DRI technologies showed that the use of natural gas as a reductant could reduce the CO2 emissions by 33%, the use of hydrogen as a reductant has the potential to reduce CO2 emissions by 67%, and the use of renewable hydrogen as a reductant has the potential to reduce CO2 emissions by up to 90%; however, the prohibitive cost of renewable hydrogen was a major drawback. A hybrid study involving the life cycle assessment strategy and biosyngas [15] analysed the compatibility and environmental impacts of different reducing gases, and it was established that the GHG effect of biosyngas is 75% lower than natural gas-based reduction and 85% lower than the blast furnace route. Life cycle cost analysis for DRI is important as it provides a comprehensive view of the economic implications of the process. This information is valuable to policymakers, businesses, and researchers aiming to promote the sustainability of DRI. The literature on the economic sustainability of DRI production underscores the growing recognition that sustainability and economic goals are interconnected. It highlights the essential components of economic decision-making in the DRI sector, ultimately aiming to achieve a more sustainable and economically viable industry.

3.3. The Economic Impact of Adopting Sustainable Approaches for DRI Production

Adopting economic strategies for the sustainability of DRI ensures the long-term viability of the process through optimum uses of resources, thus increasing profitability. DRI is a critical component of steel production and making it more sustainable is essential for reducing the overall carbon emissions in the steel industry. Organisations must invest in energy-efficient technologies and equipment to reduce energy consumption [90]. The economic sustainability of the DRI process can be enhanced through cost-effective strategies such as process optimisation, efficient resource utilisation, and minimising energy consumption and waste. A viable option for waste heat recovery for the MIDREX process [91] has shown that heat recovered from the exhaust flue gas could be recycled back into the system and has the potential to lower energy demand. Waste heat recovery for coal-based DRI can be used for preheating raw materials before they are reduced in the kiln, thus contributing to energy savings and cost reduction [11,73]. Companies can not only reduce production costs but also ensure long-term economic viability. The economic sustainability of DRI relies on the factor of cost efficiency, which can be achieved by reducing raw materials and energy consumption as they are the main contributors to production costs. For economic sustainability, there is a need for organisations to outweigh the various approaches for sustainability enhancement available, depending on the region’s location and availability of raw materials within a particular region. Regions with abundant gas reserves tend to use natural gas for the reduction process because of its economic viability, while regions like India and South Africa tend to use coal for the direct reduction process because it is readily available. DRI is an energy-intensive process; thus, recycling and reusing waste energy recovered from the system can go a long way in saving energy.

Challenges with Adopting Hydrogen DRI

Hydrogen technology is considered the latest technology for DRI and is proving to be very expensive compared to conventional methods. The economic feasibility of using hydrogen technology for the DRI process was investigated by Rosner [69], who concluded that renewable hydrogen for heating and reduction could reduce CO2 emissions by 85% at USD 1.63 per kg of hydrogen, (H2). Another author investigated a hydrogen technology called Flash Iron Technology (FIT) [92]. In this research, it was observed that renewable hydrogen used in the reduction process causes a 97% reduction in CO2 with a hydrogen cost of USD 2.50/kg. The cost of using hydrogen for the reduction of DRI differs depending on the main energy sources used for hydrogen production and the cost of carbon dioxide emissions from the process. Bhaskar [8] investigated the feasibility of decarbonizing the iron and steel industry using hydrogen from the electrolysis of water; he concluded that the energy consumption for hydrogen direct reduction was 3.72 MWh, which is high compared to the 3.48 MWh required when using the conventional method. However, the carbon emissions were reduced by 35%. The techno-economic analysis for the different hydrogen production technologies [14] shows the cost comparison for different hydrogen production methods and the % hydrogen yield.
The data from Table 3 show that biomass and coal gasification have a high production cost and a low yield of hydrogen; thus, this can pose a challenge for the adoption of these methods on a large scale. The GHG emissions from the coal gasification method compromise the emission reduction goal. Methane steam reforming has a lower production cost and high yield of H2; however, its carbon emission is quite high. Methanol steam reforming has an average production cost, low-to-average CO2 emission, and a high yield of H2 production. Organisations need to consider the cost associated with emissions and production before they adopt a particular hydrogen production method for them to strike a balance between the economic viability and sustainability of DRI.

4. Proposed Approach to Enhance the Sustainability of DRI

From all the articles reviewed, the concept of Circular Economy (CE) has not been explored as an approach or strategy to enhance the sustainability of DRI. This concept can be applied to all DRI processes to reduce waste and enable an elongated life span for primary non-renewable raw materials. CE implies the shift from the classical linear process, (take, produce, and dispose of), to a ‘circular’ economy approach that involves reusing, recycling, and regenerating material flow [93]. Recycling waste material or energy back into the system has an economic benefit on the process as it cuts the cost of raw material inputs. The reuse of waste heat in the MIDREX DRI process was proposed by Sina [91]; a techno-economic analysis for energy harvesting using the Kalina cycle was carried out, and it was noted that 70% of the energy that was lost from the heat exchangers and condensers could be recycled back into the system. Calzadilla [94] showed the trade-offs and synergies between climate change policies, Circular Economy, and technological developments. A generic CE framework that can be used for all DRI processes has been proposed as a strategy to reduce waste and emissions to enable sustainability. This framework highlights potential areas of improvement focusing on material recycling, waste utilisation, energy efficiency, emission reduction, circular design principles, and life cycle assessment. Figure 8 shows the generic circular framework for DRI processes.
The proposed CE framework uses the 5Rs: Reduce, Reuse, Recycle, Recover, and Reclaim. It starts with incorporating circular design principles in equipment design for durability, modularity, and maintainability to prolong the lifespan of equipment use. Adopting CE principles enables efficient resource utilisation and waste reduction, thus enhancing the sustainability of the process. Circular Economy principles can be adopted for each stage of the DRI process throughout the value chain to reduce waste and enhance resource efficiency. This enables sustainability by moving away from the linear production process of ‘take, make, and dispose’, to a more systemic and circular approach that encourages the reuse and recovery of waste material back into the system to reduce raw material and energy consumption. The continued improvement of the DRI production will aid in reducing the detrimental effects of this process on the environment.
  • Reduce
The energy consumption of the DRI process can be greatly reduced by incorporating waste heat back into the system. The waste heat generated during the DRI process comes from the reduction and cooling processes. The virgin raw material usage can also be reduced by using waste material such as accretion build-up and char as blends for the input feed material.
  • Reuse
Reuse involves repurposing waste materials to enable sustainability [95]. Waste elements from the coal-based DRI process such as waste heat, accretion build-up, and char can be directly reused at various stages of the process without further processing. Char, a solid waste derived from unburnt coal and iron, can be directly fed in the rotary kiln as a valuable fuel substitute, as it reduces the rate of coal consumption. Char can also be sold to other industries as a valuable aggregate material for construction products such as bricks [96]. The reuse of waste products by other industries contributes to additional revenue streams, which improves the economic viability of the process.
  • Recycling
Wastewater from the cooling process can be recycled by removing excess heat with a heat recovery system and reintroducing it into the system [72]. This water can be replenished to account for evaporation losses and reused in the cooling system. The wastewater from this process can be easily recycled because it contains fewer contaminants from the indirect cooling process, eliminating the need for further processing to remove contaminants.
  • Recover
Valuable elements from fly ash can be recovered by using electrostatic precipitators, which impart charges to exhaust gas streams, allowing the trapping of fly ash by metal screens with opposite charges [97]. The recovered fly ash can be used in other industries such as concrete production, brick manufacturing, and fertiliser production [98]. Waste heat can also be recovered and utilised for preheating and electricity generation, thereby promoting energy efficiency and sustainability. Innovative heat recovery systems enable the capture and processing of waste heat and exhaust gases [99].
  • Reclaim
Landfills from the disposal of char from the coal-based DRI process can be reclaimed to recover useful material, though this may require specialised and expensive equipment. Wastewater reclamation can be achieved by constructing wetlands using natural filtration and purification methods.

Contribution of the Research

This systematic review makes a significant contribution to the sustainability of the direct reduction iron process by providing a comprehensive synthesis of the existing literature and identifying evidence-based best practices for optimising the process and reducing waste, thus enhancing the sustainability of the process. This review integrates the current state of knowledge on DRI sustainability using various reductants to reduce the carbon footprint, thus addressing the key research gap. The findings from the literature have significant implications for emission reduction, waste minimisation, and natural resource conservation, ultimately enhancing the process and supporting sustainable development. The review yields practical insights for industrial stakeholders, policymakers, and industrial practitioners, thus informing strategic decisions and policy formulations that promote the sustainability of DRI. Ultimately, this systematic review has far-reaching implications for supporting sustainable development and mitigating environmental impacts associated with the DRI process, thus supporting the United Nations Sustainable Development Goals SDG 9 (Industry, Innovation, and Infrastructure) and SDG 13 (Climate change).

5. Conclusions

This systematic review has provided a comprehensive analysis of optimisation strategies and technological advancement for fostering sustainability through the evaluation of 65 studies published between 2002 and 2025. Three primary themes were identified: life cycle assessment strategies, modelling tools, and technological innovation strategies. The findings highlight significant progress in utilising alternative reductants such as biomass and hydrogen, integrating renewable energy sources, and assessing the economic feasibility of various optimisation strategies.
A critical insight from this review is the pressing need for waste management, particularly in coal-based reduction processes, which remain a significant source of environmental pollution. In response, hydrogen has emerged as a transformative approach demonstrating the potential to reduce carbon dioxide emissions by up to 90%. However, limitations such as high energy demand, infrastructural constraints, and cost must be addressed for the widespread industrial adoption of hydrogen DRI. Furthermore, the study underscores the importance of a Circular Economy approach in the DRI process to enhance its sustainability. Promoting material use, waste minimisation, and resource efficiency presents the Circular Economy strategy as a viable solution for reducing environmental impact while extending the lifespan of raw materials. Additionally, modelling tools such as LCA play a crucial role in optimising resource utilisation and ensuring economic and environmentally feasible DRI technologies. The review demonstrates that the adoption of these sustainable DRI practices can lead to reduced greenhouse gas emissions, minimised waste generation, and conserved natural resources, thus enhancing the sustainability of the DRI process
In conclusion, this review provides valuable insights into the evolving landscape of sustainable DRI production, emphasising the need for continued research, policy support, and industrial collaboration. Future research should focus on overcoming the limitations of hydrogen reduction, improving waste management strategies, and integrating renewable energy sources into the DRI process. By advancing these optimisation strategies, the iron and steel industry can transition towards a more sustainable, low-carbon future while maintaining economic viability and operational efficiency.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/su17052266/s1. Ref. [100] is cited in the Supplementary Materials.

Author Contributions

R.Y.N.N.—original draft, writing, and editing; M.A.—reviewing and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support was provided by OWSD as a PhD fellowship to assist research from women in developing countries. Fund No: 3240378607.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article..

Acknowledgments

The authors would like to acknowledge fellowship funds from OWSD to the corresponding author’s PhD study research. Ratidzo Y. Nyakudya Ncube acknowledges the financial support provided by the Organization for Women in Science for the Developing World.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the study design; in the collection, analyses, or interpretation of the data; in the writing of the manuscript; or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
CCarbon
CECircular Economy
COCarbon monoxide
CO2Carbon dioxide
DRIDirect reduction iron
FeIron
FITFlash iron technology
GHGGreenhouse gas
H2Hydrogen
H-DRHydrogen direct reduction
KgKilogram
LCALife cycle assessment
MWHMegawatts hour
PRISMAPreferred Reporting Items for Systematic Reviews
SDGSustainable Development Goals
USAUnited States of America
UKUnited Kingdom

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Figure 1. PRISMA methodology for the systematic literature review of strategies for sustainable DRI production.
Figure 1. PRISMA methodology for the systematic literature review of strategies for sustainable DRI production.
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Figure 2. Themes and subthemes for the various strategies used to optimise the DRI process.
Figure 2. Themes and subthemes for the various strategies used to optimise the DRI process.
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Figure 3. Comparison of coal-based and gas-based strategies to optimise the DRI process.
Figure 3. Comparison of coal-based and gas-based strategies to optimise the DRI process.
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Figure 4. Overview of articles reviewed for each subtheme.
Figure 4. Overview of articles reviewed for each subtheme.
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Figure 5. Problems addressed by various articles in the review.
Figure 5. Problems addressed by various articles in the review.
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Figure 6. Relationship between themes for modelling techniques and the coded documents.
Figure 6. Relationship between themes for modelling techniques and the coded documents.
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Figure 7. Relationship between themes for technological innovations.
Figure 7. Relationship between themes for technological innovations.
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Figure 8. Proposed Circular Economy framework.
Figure 8. Proposed Circular Economy framework.
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Table 1. Tabulated summary of articles in the subthemes.
Table 1. Tabulated summary of articles in the subthemes.
ApproachProblem AddressedReductant TypeCountry/RegionYear Published
Simulation models
Simulation of direct reduction moving-bed reactor [33]Mass and energy balance within the reactorNatural gasIran2018
Computer-aided optimisation to reduce carbon footprint [44]Emission reductionNatural gasFrance2020
Fuzzy Logic System for accretion prevention [45]Waste reductionCoal-basedSouth Africa2013
CFD simulation of two-phase gas-particle flow [46]Optimisation of process parametersNatural gasIran2017
Numerical simulation and parameter optimisation [47]Mass and energy balance on the reduction rateCoke oven gasChina2015
Accretion control in sponge iron production using Fuzzy Logic [5]Process optimisationCoal-basedKenya2014
Neural network to optimise sponge iron production [48]Optimisation of process parametersCoal-basedIndia2016
Simulation of impacts of mechanical pellets on iron reduction [36]Process optimisationNatural gasBrazil2023
Artificial Neural Network to predict solid conversion process of DRI [25]Process optimisationNatural gasIran 2022
Productivity increment of coal-based sponge iron using simulation [49]Waste reductionCoal-basedIndia2016
Modelling and simulation of the Midrex shaft furnace [33]Mass and energy balance within the shaft furnaceNatural gasCanada2015
Simulation of direct reduction reactor [50]Process optimisationNatural gasIran2011
Numerical simulation to optimise reduction temperature of HR [51]Mass and energy balances in the shaft furnaceHydrogen gasChina2023
Mathematical models
Mathematical model and expert system to optimise reduction [52]Control and optimisation of process parametersCoal-basedChina2012
Computational fluid dynamic analysis of sponge iron rotary kiln [53]Optimisation of process parametersCoal-basedIndia2017
Modelling 2D model for direct reduction shaft furnace [3]Emission reductionNatural gasFrance2018
Modelling and optimisation of rotary kiln DRI using the FORTRAN model [54]Process optimisationCoal-based South Africa2015
Analysis of temperature profile and % metallization in rotary kin [55]Optimisation of process parametersCoal-basedIndia2017
Mathematical model to estimate parameters and efficiency of RHF [35]Thermal efficiencyCoal-based India2014
Mathematical analysis of parameters affecting the reduction of iron ore [28]Efficiency of the reduction processNatural gasEgypt2015
Modelling of counter-current moving-bed reactor for DRI [32]Mass energy balance within the shaft furnaceNatural gasArgentina2004
Modelling kinetics of iron oxide reduction using CO [29]Effect of residence time on the reduction rateNatural gasIndia2022
Modelling a new low-emission hydrogen DRI process using a 2D model [38]CO2 emission reductionHydrogen gasFrance2012
Mathematical modelling of sponge iron DRI [30]Energy efficiency, CO2 emission reductionCoal-basedIndia2010
Modelling and environmental economic analysis of DRI with different gases [56]Emission reduction by using different reducing gasesCoke oven gas and
hydrogen		Italy2023
Online modelling of the Energiron Direct Reduction Shaft furnace [31]Process optimisationNatural gasItaly2013
Modelling the complex iterations of the Midrex shaft furnace [57]Mass and energy interactions.Natural gasSaudi Arabia2012
Multiscale process modelling of iron ore DRI using Aspen [58]Process optimisation
Emission reduction		Natural gasFrance2018
Experimental models
Effect of iron ore coal pellets during reduction with hydrogen and CO [59]Optimisation of process parametersHydrogen gasChina2016
Statistically designed experiments for hydrogen-based direct reduction iron [27]CO2 emission reductionHydrogen gasBrazil2022
The effect of water gas shift reaction and other parameters on DRI [40]Reducing the production costNatural gasUSA2015
Kinetics of iron oxide reduction using CO, experiments, and modelling [29]Optimisation of process parametersNatural gasIndia2022
Prediction of solid conversion process in DRI using machine learning [25]Optimisation of process parametersNatural gasIran2022
Reaction reactivity of low-grade iron ore biomass for sustainable process [60]CO2 emission reductionBiomass gasIndonesia2022
Effects of preheating temperature and pellet size on reduction in DRI with biomass [61]Improving the degree of metallization Biomass gasChina2013
Process modelling of hydrogen DRI, varying process parameters [62]Variability in energy consumptionHydrogen gasAustralia2024
Effects of metallization degree on CO2 emission yield [63]Productivity and emission reductionCoke oven gasChina2024
Solar-aided DRI using hydrogen as a reductant [64]Low-CO2 alternativesHydrogen gasFrance2024
Using molten carbonate fuel cells to reduce energy consumption [65]Energy efficiency and emission reductionNatural gasItaly2024
Biomass Reductant
Utilisation potential of biomass volatiles and biochar as reducing agents for DRI [66]Emission reduction Biomass gasIndia2024
Biomass reducing agent utilisation in the RHF process in DRI [41]Emission reductionBiomass gasChina2015
Direct reduction of iron ore by biomass char [36]Productivity and process optimisationBiomass gasChina2013
Direct reduction of oxidized iron ore pellets using biomass syngas as a reducer [26]CO2 emission reductionBiomass gasChina2016
Reduction behaviour of iron ore pellets using hardwood biomasses as reductants [67]Process optimisation with alternative reductantsBiomass gasGermany2022
Using biomass as a reductant of DRI in the RHF [68]Emission reduction, optimisation of process parametersBiomass gasChina2017
Synergetic conversion laws of biomass and iron ore for DRI and syngas coproduction [69]Productivity, emission reductionBiomass gasChina2022
Hydrogen Reductants
Decarbonisation of the DRI process using green hydrogen [8]Emission reduction Hydrogen gas Norway2020
Hybrid hydrogen-based reduction of iron ore processes [42] Emission reduction Hydrogen gasGermany2022
Design and cost analysis of hydrogen-based DRI [70]Optimising the operational cost of hydrogen reductionHydrogen gasUSA2023
Hydrogen direct reduction; an overview of challenges and opportunities [14]Feasibility of the hydrogen DRI processHydrogen gasGermany2021
Direct reduction of iron ore with hydrogen [14]Emission reduction, reducing energy consumptionHydrogen gasChina2021
The perspective of hydrogen direct reduction of iron [71]Emission reduction, process optimisationHydrogen gasAustralia2023
Transitioning to hydrogen-based reduction technologiesEmission reduction
Challenges associated with hydrogen DRI		Hydrogen gasCanada2023
Energy-Efficient Technologies
Energy conservation in sponge iron process through proper utilisation of waste heat [72]Waste heat recoveryCoal-basedIndia2013
Energy survey for coal-based sponge iron industry [36]Energy losses, increase in energy efficiencyCoal-basedIndia2015
Most efficient technologies for greenhouse emission Abatement [73]Emission reduction, energy efficiencyCoke oven gas and natural gasSwitzerland2019
Techno-economic analysis of DRI through the integration of carbon capture and Storage technology [74]Emission reductionNatural gasKorea2024
Economic analysis of the pressurised chemical looping system
integrated with the Midrex process [75]		Energy efficiency and emission reductionNatural gasCanada2024
Decarbonization using chemical looping technology and biomass [76]The need for low-cost syngasBiomass gasJapan2024
Life Cycle Assessments
Comparative life cycle assessment of natural gas and coal-based DRI [43] Emission reduction throughout the life cycleCoke oven gas and natural gasIndia2022
Cost and life cycle analysis for CO2 reduction in DRI technologies [42] Emission reduction throughout the life cycleNatural gas and hydrogenUSA2023
Life cycle assessment of biosyngas-based DRI production process [15]Emission reduction Biomass gasSweden2023
Life cycle assessment for sponge iron production process [37]Reducing energy consumptionNatural gasIran2023
Life cycle assessment of hydrogen DRI to reduce CO2 emissions [77]Emission reductionHydrogen gasChina2024
Prospective LCA approach for decarbonization options in the UK [78]Emission reductionNatural gasUK2025
Table 2. Comparative analysis of problems addressed by various strategies.
Table 2. Comparative analysis of problems addressed by various strategies.
Problems Addressed Total
Emission reduction133331519
Process optimisation6741 18
Emission reduction and optimisation 1131 6
Energy efficiency211 217
Economic feasibility 1 21 4
Mass and energy balances42 6
Energy efficiency and emission reduction 11 12 5
Total131511776665
StrategySimulation modelsMathematical modelsExperimental modelsBiomass technologyHydrogen technologyEnergy-efficient techLife cycle assessment
Table 3. Cost and CO2 emission per yield of H2 from various hydrogen production methods [14].
Table 3. Cost and CO2 emission per yield of H2 from various hydrogen production methods [14].
forMethane Steam
Reforming		Coal GasificationMembrane ElectrolysisBiomass GasificationMethanol Steam
Reforming
Cost (USD/kg H2)1.251.57.731.8–2.2
GHG emission
(kg CO2/kg H2)		8.1–1113–170+/−0<7
H2 yield %70–8550–607020–40<90
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Nyakudya Ncube, R.Y.; Ayomoh, M. Optimisation Strategies and Technological Advancements for Sustainable Direct Reduction Iron Production—A Systematic Review. Sustainability 2025, 17, 2266. https://doi.org/10.3390/su17052266

AMA Style

Nyakudya Ncube RY, Ayomoh M. Optimisation Strategies and Technological Advancements for Sustainable Direct Reduction Iron Production—A Systematic Review. Sustainability. 2025; 17(5):2266. https://doi.org/10.3390/su17052266

Chicago/Turabian Style

Nyakudya Ncube, Ratidzo Yvonne, and Michael Ayomoh. 2025. "Optimisation Strategies and Technological Advancements for Sustainable Direct Reduction Iron Production—A Systematic Review" Sustainability 17, no. 5: 2266. https://doi.org/10.3390/su17052266

APA Style

Nyakudya Ncube, R. Y., & Ayomoh, M. (2025). Optimisation Strategies and Technological Advancements for Sustainable Direct Reduction Iron Production—A Systematic Review. Sustainability, 17(5), 2266. https://doi.org/10.3390/su17052266

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