Agapi Vasileiadou

The energy and environmental crisis, growing energy demand, and the degradation of natural resources

Global demand for sustainability drives interest in bioenergy from sustainable feedstock. Agro-industrial waste such as brewer’s spent grains (BSG) is an important by-product of brewing. This study provides a comprehensive review of the current technologies of BSG for energy recovery and BSG-based materials for energy storage applications. The latest scientific progress, not only from conventional processes on anaerobic digestion, combustion, gasification, pyrolysis, torrefaction, and hydrothermal liquefaction but also from several integrated technologies, pretreatment methods, and additives/catalysts regarding the improvement of energy efficiency and process sustainability, was reviewed. In addition, the co-feedstock practices (co-combustion, anaerobic co-digestion, hydrothermal co-liquefaction, anaerobic co-fermentation) and co-production were examined. AD of BSG yields about 302 NL CH₄/kg COD, generating roughly 0.39 kWh of electricity/kg BSG and 1.71 MJ of thermal energy/kg BSG. Ultrasonic pretreatment enhances methane production up to four times (107 L CH₄/kg TVS) and reduces CO₂ emissions by 0.083 t CO₂eq/t BSG. Anaerobic co-digestion of BSG with other brewery waste increased the yield up to 88 mL CH₄/g TVS, generated approx. 0.348 kWh/kg TVS electricity, and reduced emissions by 0.114 kg CO₂eq/kg TVS. Bioethanol yields can reach 72%, while biohydrogen generation was up to 5154 mL H₂/g glucose. BSG pyrolysis provides up to 71.8% bio-oil, and its calorific value is 18–25 MJ/kg. BSG-derived activated biocarbon has a notable surface area (1792 m²/g) for lithium–sulfur batteries. The assessment showed that BSG’s transformation into bioenergy and energy storage materials aligns with waste reduction and sustainable development goals. However, future research on combined alternative wastes, integrated technologies, green nanotechnology, and artificial intelligence technology could lead to optimal performance and facilitate their industrial application.

Energy is a crucial factor for urban development. Cities have a crucial role in climate change, as they use 2/3 of the world’s energy, producing 70% of greenhouse gas (GHG) emissions. In order to reduce the large ecological footprint of the utilization of conversional energy sources (coal, gas, and oil) and enhance a nation’s energy independence (security), it is crucial to find alternative fuels. Biomass residues are characterized as a sustainable and carbon-neutral energy source. Hence, this review describes a critical assessment of not only the quality characteristics of several waste and biomass residues for bioenergy production and biofuels but also the value-added products that could be produced from wastes to enhance industry (e.g., pharmaceutical, cosmetics, packaging industry, etc.). Furthermore, the challenges and potential solutions of waste utilization for bioenergy production and the transformation of value-added products for urban sustainability are also explored. Despite the high-quality characteristics and the availability of these wastes, several critical factors should be taken into account. Biomass residues could contribute to sustainable development goals (SDG), such as sustainable cities and communities, clean energy, responsible consumption and production, the economic growth of a country, and, as a result, urban development.

Coal bottom ash is used globally in various applications in the construction industry to reduce its negative environmental impacts. In this study, the potential utilization of lignite bottom ash from four power plants in Greece in concrete manufacturing was evaluated through granulometric, chemical, and mineralogical analyses. The particle-size distribution of bottom ash obtained from dry sieving resembles that of sand, making bottom ash suitable for replacing fine aggregates in the production of concrete. Its chemical composition, determined with selective point analyses energy dispersive spectroscopy (EDS), reveals high amounts of silica and alumina indicating pozzolanic properties, and high calcium contents suggesting hydraulic/cementitious character. Mineralogical characterization, obtained from powder X-Ray diffraction analyses (XRD), displays the prevalence of amorphous matter, calcite, quartz, aluminosilicate minerals, and portlandite, implying a beneficial pozzolanic and hydraulic activity in concrete manufacturing.

The management of plastic waste is considered to be among the major environmental problems that must be urgently addressed. For various reasons, recycling of plastic waste is not always feasible. In this study, a comprehensive evaluation of a mixture of plastic wastes (of the municipal solid wastes, MSW) as potential fuel is performed. Precisely, the combustion of plastic waste and the co-combustion of plastic waste-lignite blends are studied. Thermochemical characteristics, chemical composition, and kinetic parameters are measured/estimated. The environmental impact of these samples is also evaluated in terms of CO₂ maximum potential emissions and ash production. In addition, the ash quality and its risk for slagging problems are explored. The random mixture of plastic waste revealed extremely high energy content (34 MJ/kg), which is higher than some well-established liquid fuels, e.g., ethanol and lower ash content (~5 wt.%), with lower activation energy and a higher maximum rate of mass loss (~9%/min) than lignite. Besides the much lower amount of produced ash, plastic waste, despite its higher carbon content, exhibits lower CO₂ maximum potential emissions (~75 g CO₂/MJ). The composition of the ash produced by plastic waste and lignite is different quantitatively but qualitatively is of the same type (similar medium risk ash). The superior characteristics of plastic waste are also evident in the blends. Provided that toxic emissions are captured, the utilization of plastic waste through combustion seems to be an attractive approach for simultaneous waste management and energy production, especially for plastic waste of limited recycling potential.

Recently, coal power plants across Europe have been reopened. Alternative fuels are needed for energy autonomy purposes, for a smoother transition to the post-lignite era and for sustainable development. In this work, different categories of municipal solid wastes (MSW) and their blends with lignite were studied for their potential use as alternative fuels. Seventeen samples were studied using several techniques: gross calorific value (GCV), proximate analysis, ultimate analysis, ion chromatography, ash elemental analysis, thermogravimetric analysis, kinetic modeling and thermodynamic analysis. A determination of empirical chemical formulas was performed. Slagging/fouling potential was evaluated with various indices including modified indices that take into account ash production and GCV. Maximum emission factors were calculated and defined per produced MJ. Also, an environmental footprint index was developed regarding the environmental impact of solid wastes. The GCV experimental results were compared with those of twenty different empirical models. Moreover, several case studies were performed to evaluate the potential of covering the energy demands, with combustion of MSW, in Greece and Europe. The results showed that MSW as a primary/secondary fuel is an attractive solution considering the fact that it boasts better characteristics in comparison with lignite. Moreover, the environmental footprint index (EFI_sw) of the MSW revealed a much smaller environmental impact. The high N content is not always translated to high emissions if NO is expressed per produced MJ (gNO/MJ). In addition, MSW can also be used as a significant contributor in covering energy demands regarding the energy recovery potential. Graphical