New Advances in CO₂ Reduction and H₂ Promotion Techniques in Energy Systems

1. Introduction

The pursuit of sustainable energy solutions is increasingly centered on combating climate change by reducing CO₂ emissions [1,2] and promoting hydrogen as a cleaner fuel source [3]. Among the strategies proposed by researchers are carbon capture and storage (CCS) technologies [4], which aim to capture CO₂ emissions from industrial activity and power plants. By reducing greenhouse gases, these innovations support the adoption of hydrogen-based energy systems [5]. Recent advancements in simulation methods for green hydrogen production—using renewable energy sources—are paving the way for more efficient and environmentally friendly processes. These cutting-edge strategies include blue hydrogen production, enhanced carbon sequestration, and emerging technologies such as coal pyrolysis. Such innovations hold significant potential in reducing greenhouse gas emissions and propelling the hydrogen economy towards sustainability. To realize this potential, scientists and policymakers are working together to develop a comprehensive framework that integrates these approaches, with the aim of reducing carbon footprints and establishing hydrogen as a viable clean energy carrier for transportation, industrial processes, and electricity generation. It is hoped that this will, in turn, create a sustainable energy system and decarbonization pathways that enable global climate objectives to be met.

This article aims to provide an overview of the latest advancements in our Special Issue, “CO₂ Reduction and H₂ Promotion Techniques in Energies”, that are vital to achieving the above.

2. A Review of the Latest Advances

Panpan et al. [6] examine the influence of key mineral components (SiO₂, Al₂O₃, and CaO) on the supercritical water gasification (SCWG) of semi-coke, aiming to understand their roles in inhibiting or promoting the gasification process [7,8]. Given coal’s significance as an energy source, particularly in China, SCWG represents a cleaner alternative to the conventional coal process, which poses environmental concerns [9]. Using a batch autoclave system, the authors investigate the impact of these minerals with and without a K₂CO₃ catalyst [6]. The results indicate that while CaO enhances gasification, the effects of SiO₂ and Al₂O₃ vary depending on the catalyst’s presence. Notably, Al₂O₃ is shown to improve the gasification efficiency by preventing mineral agglomeration and facilitating near-complete carbon gasification, providing valuable insights for making coal conversion more sustainable.

Oleg et al. [10] explore the thermal–hydraulic characteristics of heat transfer in tubular pyrolysis reactor channels with hemispherical protrusions, aiming to enhance the efficiency of this process in organic-waste thermal decomposition. Pyrolysis is a promising method for converting hydrocarbon waste into valuable products such as hydrogen, making efficient heat transfer crucial for improving product quality and reducing secondary reactions [11,12,13,14,15]. The authors combine Computational Fluid Dynamics (CFD) modeling with experimental analysis to assess different channel configurations and coolant flow velocities [12]. The findings reveal that hemispherical protrusions significantly boost heat transfer, with an average increase of 2.23 times compared to smooth channels, and an optimal protrusion height of 2 mm is identified for maximizing efficiency [16].

Joo-Youn et al. [4] investigate the most suitable conditions for biohydrogen production by combining food waste and sewage sludge, analyzing the effects of alkali pretreatment. They also consider solid volatile levels and the influence of blending patterns. Achieving net-zero emissions by 2050 necessitates biological hydrogen production techniques that surpass fossil fuel-based ones [17]. One study found that alkali pretreatment enhances the biodegradability of waste material [18]. The researchers obtained a maximum specific hydrogen production rate of 163.8 mL H₂/g volatile suspended solid/h at a 5.0% volatile solid concentration, with food waste making up 62.5% of the mixture. Integrating waste materials with alkali pretreatment holds promise for renewable hydrogen production by alleviating pressure on waste management, supporting worldwide carbon reduction [19].

Tae-Hoon et al. [20] evaluate the use of crude hydrolytic extracellular enzymes (CHEEs) as an economical pretreatment method for waste-activated sludge (WAS) to improve methane production during anaerobic digestion. Efficient WAS management is vital in Korea, where 4 million tons of waste were produced in 2020 and anaerobic digestion is a vital energy conversion technology [21]. The study demonstrates that CHEEs achieve methane production levels similar to those of a four-enzyme commercial mix, thus proving their viability in sustainable solutions [22]. It also reveals that microbial community changes can lead to increased methane production, while indicating that CHEEs offer the potential to reduce operational expenses as well as boost waste management efficiency and renewable energy generation. Sunel et al. [13] investigated the performance of a drop-tube coal gasifier in terms of how O/C ratios and wall temperature parameters affect process outcomes. Another study uses a three-dimensional Computational Fluid Dynamics (CFD) model to evaluate syngas composition, temperature distribution, and carbon conversion under different operating conditions [23]. The optimal conditions for CO production and carbon conversion (higher than 97%) were achieved at an O/C ratio of 0.9 and a wall temperature of 1800 °C. The results demonstrate the critical need to optimize gasification parameters, which enhances the coal utilization process while minimizing pollutant emissions.

Lijian et al. [24] investigate the impact of adding ruthenium (Ru) to nickel (Ni) catalysts supported on silica (SiO₂) to enhance hydrogen iodide (HI) decomposition in the sulfur–iodine (SI) cycle, aiming to improve catalytic activity and sulfur tolerance. They highlight the challenges related to sulfur deactivation in hydrogen production catalysts, which is crucial for reducing CO₂ emissions. The methodology involved preparing and characterizing Ni/SiO₂ and Ni-Ru/SiO₂ catalysts, followed by HI decomposition tests under varying sulfuric acid concentrations, alongside density functional theory (DFT) modeling. The key findings indicate that adding Ru significantly boosts catalyst performance, increasing HI conversion and sulfur resistance. At the same time, theoretical modeling suggests that Ru enhances HI adsorption and reduces sulfur adsorption, leading to a more stable and efficient catalyst for hydrogen production.

Oleg et al. [25] focus on enhancing deep oil sludge processing using innovative pyrolysis methods, examining the impact of a stirrer in a stirred-tank reactor for hydrogen production. Oil sludge, a significant oil and gas industry waste product, poses environmental hazards and contains valuable hydrocarbons [26]. The study introduces a two-stage process that involves first pyrolyzing the oil sludge to generate non-condensable gases, followed by the conversion of these gases into hydrogen. The key findings reveal that using a stirrer significantly improves reaction product concentrations and hydrogen yields, demonstrating its effectiveness in enhancing heat transfer and catalyst interaction, and thus providing a promising solution for industrial waste management and supporting the transition to cleaner energy.

3. Conclusions

The Special Issue “CO₂ Reduction and H₂ Promotion Techniques in Energies” showcases the latest advancements in green energy production and waste management, for example, an investigation of supercritical water gasification (SCWG) using mineral additives of CaO and Al₂O₃. Adopting hemispherical protrusions for pyrolysis reactors was shown to enhance heat transfer performance significantly, and biological processes for hydrogen generation from food waste and sewage sludge reached impressive production rates. Cost-effective waste treatment solutions adopted crude hydrolytic extracellular enzymes, while new coal gasification parameters reached maximum carbon conversion potential. Readers of this Special Issue will discover new possibilities within supercritical water gasification, innovative waste processing, biological hydrogen production from food waste and sewage sludge, and the numerical simulation of coal gasification. The findings within demonstrate how combined strategies promise to improve energy efficiency while confronting environmental problems, accelerating the shift towards renewable systems with reduced greenhouse gas output.