Energy-Efficient Chemistry

1. Introduction

The growing environmental concerns related to climate change and resource depletion urgently call for a global and profound innovation of industrial production models and energy systems to foster the adoption of environmentally efficient technologies, circular economy principles, and long-term sustainable value chains [1].

According to the latest IEA data, global energy-related CO₂ emissions increased by 900 million tonnes (Mt) between 2019 and 2023 [2]. In 2023 alone, emissions rose by an additional 410 Mt, setting a new record of 37.4 gigatonnes (Gt). Industry remains the largest energy-consuming sector, which includes chemicals, petrochemicals, iron, and steel, accounting for more than one-third (34%) of industrial energy demand worldwide [3]. The chemical industry alone is responsible for around 5% of global GHG emissions annually, including both direct and indirect emissions [4]. With energy consumption currently representing one of the most pressing challenges, the chemical sector must urgently transform to reduce energy requirements and minimize environmental impacts. Importantly, energy demand in this sector not only includes the manufacturing processes that convert feedstocks into products but also the production of petrochemical feedstocks themselves, which accounts for roughly 60% of the total energy input. Consequently, rethinking conventional chemical production models and promoting alternatives based on renewable feedstocks, energy-saving technologies, and sustainable product design have become imperative [5]. For instance, the use of biomass for chemical production offers a potential breakthrough opportunity to reduce dependence on fossil resources, promote sustainability, and foster the development of new green industrial value chains [6].

Within the circular economy, energy-efficient chemistry can be achieved only when material and energy issues are addressed in an integrated manner. This requires not only the rational use of resources but also the achievement of energy savings through technological advances that reduce electricity and fuel consumption or through the substitution of fossil fuels with renewable energy sources.

This transition towards a more energy-efficient chemistry appears slower than expected despite significant global efforts to ensure technical and economic feasibility. While technological progress continues and remaining challenges are gradually addressed, technology itself does not seem to constitute the main barrier to large-scale implementation. Rather, the environmental benefits of such innovations, particularly regarding their impacts across the entire production chain, require careful and comprehensive assessment [7].

This Reprint, titled “Energy-Efficient Chemistry”, gathers select research contributions that explore innovative approaches aimed at achieving these goals across theoretical, experimental, and applied perspectives. The goal is to offer a multidisciplinary overview of recent advances in energy-efficient chemical processes, emphasizing both environmental benefits and technological feasibility.

2. An Overview of Published Articles

This Reprint covers a broad spectrum of research topics related to resource and energy efficiency in chemical processes, reflecting the urgent need to decarbonize energy systems and industrial production while advancing circular economy principles. The presented contributions illustrate the diversity of technological, methodological, and conceptual approaches being explored to reconcile economic growth and industrial development with environmental protection and climate change mitigation.

A central theme in the collected papers is the role of green hydrogen as a key enabler of the low-carbon transition. Hydrogen produced from renewable energy sources has increasingly been recognized as a cornerstone in the shift away from fossil fuel-based energy systems. Ghisellini et al. (contribution 13) provide a comprehensive assessment of the environmental and social implications of deploying green hydrogen across multiple sectors. Their study highlights not only the promise of green hydrogen in decarbonizing industry, transport, and power generation but also the necessity of considering the full life cycle impacts associated with its production and use. Such a holistic view is essential to avoid burden-shifting, whereby reductions in greenhouse gas emissions are offset by negative impacts in other environmental or social categories.

Complementing this broad sustainability analysis, Scheepers and Lehnert (contribution 9) investigate the technical dimension of hydrogen production, specifically the modeling of polymer electrolyte membrane (PEM) electrolyzers. By introducing alternative statistical methods to analyze polarization curves, they enhance the accuracy of kinetic descriptions and provide tools for improving electrolyzer efficiency. These advances are critical because PEM electrolyzers represent one of the most mature technologies for large-scale hydrogen production, and their optimization is a prerequisite for cost-competitive and sustainable green hydrogen deployment.

The exploration of hydrogen as a clean energy vector is expanded by Żukowski et al. (contribution 8), who investigate polyolefin pyrolysis in multilayer fluidized beds. Their study identifies gaseous mixtures rich in hydrogen precursors, offering potential feedstocks for hydrogen generation. By linking plastic waste valorization with hydrogen production, this research exemplifies how circular resource flows and decarbonization strategies can be synergistically combined. Together, these studies on green hydrogen illustrate a multi-layered research landscape in which systemic sustainability assessments, advanced electrochemical modeling, and innovative feedstock pathways converge to support the development of efficient, low-carbon hydrogen technologies.

Beyond hydrogen, many contributions in this Reprint focus on the enhanced use of biomass as a renewable resource for producing valuable chemicals, energy carriers, and fertilizers. Biomass is increasingly regarded as a game changer in the transition toward sustainable manufacturing paradigms given its potential to replace fossil resources while contributing to carbon sequestration and soil health. Corinto Cavalloni et al. (contribution 12) present oxidative pyrolysis of wood pellets as a carbon-negative heating system capable of simultaneously producing useful heat and storing carbon in the form of biochar. Their work highlights oxidative pyrolysis as a simplification of conventional reactor design, demonstrating how negative emissions technologies can be implemented in practice.

Adamski et al. (contribution 11) examine the valorization of biomass for energy and fertilizer applications by investigating the influence of thermal conditions on compost transformation processes. Their study underscores the importance of optimizing composting parameters not only to ensure biological stabilization of biomass but also to support waste-to-energy strategies. In this way, biological processes become integral to circular bioeconomy models that aim to close material loops and reduce dependence on synthetic fertilizers.

In parallel, Fernández et al. (contribution 10) analyze microwave-assisted pyrolysis (MAP) of forest biomass, identifying this technology as an efficient alternative to conventional pyrolysis. MAP allows for rapid and uniform heating, improved yields of valuable bio-products, and reduced energy consumption. Such advances are particularly relevant as global energy demand rises and the need to exploit forestry residues sustainably becomes ever more pressing. Similarly, Bielecki et al. (contribution 1) focus on the densification of agricultural residues and its effects on pyrolysis behavior. Their results show how pre-treatment methods influence the composition and structure of pyrolytic products, opening pathways for tailoring biomass-derived products to specific applications. Finally, Şen et al. (contribution 5) critically review bark-based biorefinery systems, emphasizing their potential to generate both biochars and high-value extractives such as antioxidants. By mapping existing knowledge gaps and future directions, their work points to the emerging relevance of bark valorization in integrated biorefinery concepts.

Together, these contributions underscore the centrality of biomass in sustainable transitions. Whether through pyrolysis, composting, or integrated biorefineries, biomass-based strategies provide multiple benefits: renewable energy production, carbon sequestration, waste reduction, and the creation of value-added products. This multifunctionality makes biomass a cornerstone of energy-efficient and circular chemical systems.

A further thematic block in this Reprint involves the development of alternative fuels, which play an increasingly important role in reducing dependence on fossil energy sources. Sirviö et al. (contribution 6) examine the compatibility of B20 biodiesel blends with steel components, addressing a critical issue for the large-scale adoption of biodiesel in transportation. Their findings indicate that while some corrosion concerns remain, biodiesel blends made from used cooking oil methyl ester represent a viable renewable fuel option. This aligns with the broader strategy of valorizing waste oils within circular resource frameworks.

Glushkov et al. (contribution 3) provide an extensive review of gel fuels, a relatively less explored class of alternative fuels with promising safety and environmental benefits. Compared to conventional liquid and solid fuels, gel fuels offer improved combustion control, reduced emissions, and enhanced handling safety. By systematically reviewing rheological properties, atomization, ignition, and combustion behavior, their work paves the way for more widespread exploration of gel fuels in both civilian and defense applications. Both studies stress the importance of innovation in fuel formulation and performance optimization, demonstrating that the diversification of fuel options is an essential component of sustainable energy systems.

The Reprint also features contributions that address advances in materials, modeling, and systemic approaches, which are equally critical to achieving sustainable innovation. Qiu and Liu (contribution 2) review stannous tungstate semiconductors, exploring their potential applications in photocatalysis and photoelectrochemical water splitting. Their analysis highlights both the opportunities and challenges associated with this material, such as excessive electron–hole recombination and limited stability. Overcoming these issues could significantly enhance solar energy utilization, thereby contributing to clean hydrogen production and pollutant degradation.

Kačur et al. (contribution 7) shift the focus toward underground coal gasification (UCG), presenting a review of laboratory-scale investigations. While coal is a controversial resource in the context of decarbonization, UCG technologies have the potential to convert coal into syngas more efficiently and with lower environmental impacts compared to conventional mining. The reviewed studies explore process optimization, the role of gasification agents, and environmental risks, providing insights that may inform future deployment of UCG under stricter sustainability conditions.

From a systemic perspective, Ncube et al. (contribution 4) discuss the integration of circular economy and green chemistry, highlighting their combined potential to drive radical innovation in product design. By advocating renewable feedstock, waste minimization, and the extension of product lifespans, they demonstrate how chemistry can be a central driver of circular production models. Importantly, their work also emphasizes that achieving this integration requires supportive policies, adequate investment, and widespread environmental education. In this way, systemic approaches connect technical innovation with broader societal and institutional changes, ensuring that the transition to sustainability is not only technologically feasible but also socially inclusive and economically viable.

Collectively, the contributions included in this Reprint reflect the multi-dimensional nature of sustainability challenges. They cover a wide spectrum of approaches, ranging from technological innovations in hydrogen, biomass conversion, and alternative fuels, with a focus on optimizing processes and materials, to systemic strategies and policy dimensions. Complementary pathways emerge; only through such an integrated and multidisciplinary approach can energy-efficient and environmentally sound chemical processes be developed and implemented on a larger scale.

3. Conclusions

The goal of this Reprint is to offer a comprehensive overview of recent advances in resource and energy-efficient chemical processes, emphasizing not only their technical feasibility but also their environmental and social implications. By showcasing both conceptual frameworks and practical case studies, it highlights the synergies between green chemistry, circular economy, and clean energy transitions. Simultaneously, it acknowledges the complexities and trade-offs that must be managed to ensure that solutions intended to mitigate climate change do not create unintended environmental burdens elsewhere.

The global community continues to strive toward net-zero targets and sustainable development goals, and the contributions gathered here provide valuable insights into how chemistry, materials science, and engineering can converge to support this transformation. By aligning innovation with sustainability imperatives, they chart pathways toward an energy-efficient and circular future, where chemical processes contribute not only to economic development but also to environmental regeneration and societal well-being.

This Reprint is addressed to researchers, engineers, industry professionals, and policymakers interested in green chemistry, sustainable engineering, and clean energy technologies. It serves as a resource for those seeking to understand and apply innovative solutions that reduce the energy and carbon intensity of chemical manufacturing while maintaining high performance and economic viability.

We thank all the authors and reviewers whose contributions made this Reprint possible, and we hope that this collection will stimulate further research and collaboration toward a more sustainable chemical future.