Advancing Circular Economy Principles: Green Fuels Production and Sustainable Solvents Utilization

This Ph.D. thesis explores the implementation of Circular Economy principles through distinct yet complementary studies, addressing the urgent need to reduce CO2 emissions and reycle waste materials. In recent years, CO2 capturing technologies are increasingly gaining attention, with Carbon Capture and Utilization (CCU) standing out for its dual approach of reducing emissions and converting CO2 into valuable products like methane (CH4), methanol (CH3OH), and ethanol (C2H6O). These products can be used as green fuels, renewable industrial reagents, or, in the case of ethanol, as a food-grade solvent. After an Introduction and General Overview (Chapter 1) to lay the foundations of the work, the first part of this thesis focuses on synthesizing green hydrogen (Chapters 2 – 4), which can be possibly combined with recovered CO2 to produce other renewable fuels, particularly emphasizing methane production (Chapter 5). The last part examines the use of ethanol, compared to other food-grade solvents, for extracting polyphenols from hazelnut skin wastes, highlighting biomass waste valorization (Chapter 6). Finally, conclusions are drawn and and future applications are proposed (Chapter 7). Currently, green fuels and green solvents are synthesized via various routes, both traditional (from fossil sources) and renewable. Among renewables, reliance on biomass and its derivatives is often proposed. However, while biofuels and biochemicals from biomass are valuable, they cannot alone meet our energy demands due to scalability issues, competition with food supply, and dependence on fossil fuel-derived fertilizers. Therefore, integrating bio-based technologies with green products synthesis from renewable H2 and recovered CO2 is crucial for sustainable energy transition. A key technology in this context is green hydrogen production, which can be achieved through renewable-powered processes such as water electrolysis (WE) and thermochemical water-splitting cycles (TWSCs). TWSCs are closed cycles in which water (H2O) decomposes through a series of chemical reactions where all the produced intermediates are recycled, except for H2 and O2, which represent the net products of the process. Among the TWSCs, the Iodine-Sulfur (IS) cycle is one of the most well-known and extensively studied. This cycle consists of three steps: the Bunsen reaction, where feed water reacts with iodine (I2) and sulfur dioxide (SO2) to form hydriodic acid (HI) and sulphuric acid (H2SO4); sulfuric acid splitting (SAS) reaction, which produces O2 and regenerates SO2; HI decomposition, yielding H2 and regenerating I2. Two notable variants of the traditional IS cycle are the Hybrid Sulfur (HyS) cycle, first proposed by the Westinghouse Electric Corp in 1975, and the Nickel-Iodine-Sulfur (NIS) cycle, recently introduced by ENEA. The HyS cycle includes two reactions, one of which is the highly endothermic 6 Sulfuric Acid Splitting (SAS) reaction, standing as the most energy-intensive step in both the HyS and IS cycles. In contrast, the NIS cycle comprises five reactions, one of which is the Bunsen reaction. Similar to the IS cycle, the Bunsen reaction is where water is introduced into the process. Due to its energy-intensive role in both IS and HyS cycles, the Sulfuric Acid Splitting reaction was extensively studied in Chapter 2 of this thesis. The endothermic SAS reaction occurs in two steps: a thermal Sulfuric Acid Dissociation (SAD) at 450–500 °C, transforming sulfuric acid (H2SO4) into water (H2O) and sulfur trioxide (SO3), followed by a Sulfur Trioxide Splitting (STS) where SO3, in the presence of catalysts, transforms into oxygen (O2) and sulfur dioxide (SO2). Collaborating with the German Aerospace Center (DLR), intensive experimental tests were carried out on Fe2O3-coated SiSiC foams for the STS reaction at atmospheric pressure and 850 °C. The experimental set up used was also described with an innovative model, which took into account both mole expansion and the reaction equilibrium. Literature data for Fe2O3 catalyst pellets and honeycombs were used to fit the kinetic and equilibrium parameters for the high-temperature STS reaction. The model, validated with the experiments performed at DLR and with additional literature data, incorporated the fitted parameters, outperforming existing simplified literature models in describing Fe2O3 catalyst behavior. The Bunsen reaction, central to both the IS and NIS processes, was also deeply investigated in Chapter 3 of this thesis using the same approach applied to the SAS reaction – comprising an extensive experimental campaign followed by detailed reaction modelling. The Bunsen reaction, exothermic and fast within the temperature range of 20 – 120 °C, involves a heterogeneous gas-liquid-liquid system between the three reagents SO2, I2 and H2O and the two products H2SO4 and HI. In the laboratories of ENEA Casaccia, a continuous counter-current flow Bunsen reactor fed with 5 NL h-1 of gasoues SO2 was constructed and tested. Solid I2 pellets were introduced from the top to enhance saturation at the two acids interface, promoting segregation. This innovative design achieved nearly complete SO2 conversion and optimal H2SO4 and HI concentrations in the respective product phases while avoiding undesirable byproduct formation. This study also advanced the modeling of the experimental reactor using innovative kinetic and liquid-liquid equilibrium approaches, achieving an exceptional validation accuracy of R2 = 0.9988. A microscopic model describing gas phase trends in bubble or packed Bunsen-type reactors was also developed. The dimensionless analysis revealed a characteristic times ratio (Bu) that comprehensively describes all gas phase phenomena within the Bunsen reaction section, identifying an optimal Bu value of 0.4 for pure SO2 gas inlet. The innovative experimental setup and modeling demonstrate the readiness for upscaling, paving the way for future Bunsen reactor engineering. 7 Bunsen-characteristic design charts, provided for the first time, offer solid guidance for optimizing reactor performance, enhancing the industrial applicability of this innovative process. In Chapter 4, the results of the study on the Bunsen reaction were incorporated in a comprehensive Aspen Plus V10 simulation of the closed NIS cycle, using the IS cycle as a benchmark. A sensitivity analysis optimized the NIS process parameters, revealing that the NIS cycle offers 17% higher thermal efficiency compared to the IS cycle while maintaining stoichiometric hydrogen yield. Additionally, operating the NIS cycle at atmospheric pressure enhances economic viability and safety, crucial for scalable H2 production. H2 produced through renewable processes like IS, HyS and NIS thermochemical water splitting cycles, when combined with CO2, can create green fuels such as methane, methanol, and ethanol. In the context of Power-to-Gas (PtG), Synthetic Natural Gas (SNG) emerges as central, with catalytic methanation at its core. Among various catalysts studied for direct methane production from H2 and CO2, ruthenium (Ru)-based catalysts are particularly significant. As new Ru-based catalyst configurations emerge, there is a growing need for a standard methodology to determine their activity. To address this, a study was conducted in collaboration with BASF to develop a methodology for analysing Ru-based catalyst activity in fixed-bed methane synthesis applications, and the results of this study are reported in Chapter 5 of this thesis. This new method needed to be user-friendly, quick, and safe at the high temperatures required for the methanation reaction (250-410°C). Inspired by the established Engelhard approach for Pt- and Pd-based catalysts, the proposed method simplifies the complex kinetic model of Lunde and Kester by assuming first-order kinetics at lower temperatures (200–330 °C). This adaptation enables rapid calculation of an observed first-order pre-exponential factor, which serves as a quantitative metric for catalyst activity, specific to each structural configuration. The methodology was validated using 0.5 wt.% Ru/Al2O3 egg-shell cylindrical pellets. Experimental tests conducted with intact and pulverized pellets demonstrated consistent results, confirming the suitability of intact pellets for routine quality control without requiring labor-intensive pretreatments. The results were further validated by accurately predicting experimental CO2 conversion data at higher temperatures (up to 410 °C) with a root mean square error below 10%. Complementary to Power-to-Gas, Power-to-Liquid (PtL) offers another option for green fuel production, with ethanol being particularly promising. If produced from green H2 and recovered CO2 and used as a solvent (thus, not burned at its end-of-life), the entire process becomes Carbon Negative, permanently capturing CO2 within its chemical bonds. Due to its potential, in this thesis ethanol was investigated as a green solvent for polyphenol extraction from hazelnut skin waste. 8 According to the Food and Agriculture Organization of the United Nations (FAO), global hazelnut production reached nearly 1.1 million tons in 2020, with Italy being the second world leading producer. Hazelnut Skin (HS), often considered a waste by the industry, concentrates polyphenols up to 22 times more than the fruit itself. If extracted with a liquid solvent, polyphenols can be used as antioxidants in functional foods and beverages, tropical drugs and cosmetics, resulting in a valorization of HS waste. Among the solvents conventionally used for extracting polyphenols from vegetable matrices, only water, ethanol, or water–ethanol mixtures are acknowledged as being fully green, safe, and non-toxic. For this reason, extensive experimental testing was conducted in Chapter 6 of this thesis to evaluate the polyphenols extracting potential of ethanol compared to other solvents, i.e. water, the simplest and most cost effective option, and natural deep eutectic solvents (NADES), an innovative green alternative emerging in recent years. Experimental tests were performed both in a batch setup and a in fixed-bed column by varying the solid-liquid ratio at a constant temperature of 70 °C, investigating both equilibrium and kinetic aspects of polyphenol recovery. The extraction potential of all three solvents was evaluated, yielding 173.2 mg/g, 254.7 mg/g, and 470.2 mg/g of free polyphenols per unit mass of solid for water, a 50% v/v water-ethanol mixture, and NADES, respectively. While NADES showed superior extraction efficiencies, challenges such as NADES’ recoverability and high viscosity make it less suitable for large-scale use. Thus, ethanol emerged as the most effective solvent, balancing extraction efficiency with solvent recyclability and manageability. Moreover, if the ethanol used is produced from green H2 and recovered CO2, the entire process results doubly virtuous, since it simultaneously uses a carbon-negative product as solvent, ensuring the captured CO2 is not released, and valorizes a biomass waste, providing a sustainable solution for polyphenol recovery. In summary, the production of green fuels from CO2 and renewable hydrogen, combined with the use of green solvents for polyphenol extraction, showcases the potential for sustainable and efficient systems. By integrating these diverse yet interconnected studies, the thesis offers a holistic view of how Circular Economy principles can drive innovation and sustainability in both energy production and agro-food waste management sectors, advancing scientific knowledge and providing practical solutions for contemporary environmental challenges, aligning with global efforts towards a sustainable future.