Search Results (620)

Waste lubricating oils (WLOs) represent a major stream of hazardous petroleum-based residues, with global generation exceeding 24 million tons annually. Improper disposal of WLOs poses risks to soil, water, and air quality, while their chemical composition makes them a potential secondary resource within circular economy frameworks. This review summarizes conventional, advanced, and emerging technologies reported for the recycling and valorization of WLOs into high-value petrochemicals and carbon-based materials. Established processes such as acid–clay treatment, solvent extraction, and vacuum distillation are discussed together with more recent approaches, including catalytic upgrading, hydrotreatment, membrane separation, and thermochemical conversion methods such as pyrolysis and catalytic cracking. Reported data on process performance, environmental considerations, techno-economic indicators, and life cycle assessment outcomes are comparatively analyzed to outline current trends, technical challenges, and future development directions in WLO recycling. Particular attention is given to thermochemical pathways capable of generating carbonaceous materials, including carbon black, porous carbons, and functional carbon nanostructures with potential applications in adsorption, catalysis, electrochemical systems, and tribological formulations. Hybrid and integrated process configurations described in the literature are highlighted for their potential to improve recovery efficiency, enhance product quality, and reduce environmental burdens. In addition, recent life cycle assessment (LCA) and techno-economic analysis (TEA) studies are reviewed to provide insight into the environmental and economic implications of advanced re-refining systems. Overall, the reviewed literature indicates that WLO recycling represents not only an important element of sustainable lubricant management but also a promising waste-to-carbon strategy for the production of value-added carbon-based materials and petrochemical products. Full article

Municipal sewage sludge was used to prepare sewage sludge pyrolysis char (SS-PC). The effects of pyrolysis temperature on the morphology and structure of SS-PC were investigated, and the performance of SS-PC-800, prepared at 800 °C, for promoting gas production from corn stover pyrolysis was evaluated in a fixed-bed reactor. The results suggested that adding SS-PC-800 promoted the pyrolysis of corn stover and reduced the activation energy required for thermal decomposition. A further comparison of five metal oxides indicated that CaO had the most pronounced effect on H₂ formation under the tested conditions. A synergistic effect was observed when reactive CaO was introduced into SS-PC. At a pyrolysis temperature of 800 °C, when the mass ratio of CaO to SS-PC-800 was 2:3 and the mass ratio of mixed catalyst to corn stover was 1:5, the H₂ yield was 26.5% higher than that obtained from corn stover pyrolysis alone. In this study, SS-PC was employed as a catalytic material, and the synergistic interaction between its catalytic components and CaO effectively enhanced H₂ production during biomass pyrolysis. These findings can provide a useful reference for the resource utilization of municipal sludge and the development of catalysts for biomass thermochemical conversion. Full article

Cellulose is the most promising abundant renewable polymer material with the highest potential for the future low-carbon biorefineries. However, its utilization in industry is limited by the structural recalcitrance as a result of organization of crystalline domains, fibrillar architecture hierarchy and intramolecular and intermolecular hydrogen bonding which is responsible for access restriction for the catalysts and consequent cleavage of the glycosidic bonds. Therefore, efficient depolymerization of cellulose is of paramount importance as a step in biomass conversion into the low molecular products. In this review, the recent advances in cellulose depolymerization are discussed. The chemical, enzymatic, thermal, thermochemical, mechanochemical, oxidative and hybrid catalytic method is thoroughly discussed. Attention is paid to the mechanism of the depolymerization reaction steps as glycosidic bond activation as hydrolytic, radical mediated, and energy assisted pathways. Selectivity and conversion efficiency based on substrate morphology, solvent system and catalyst design are also discussed. Further, there is a comparison of key performance metrics which are relevant for the industrial process as product yield, carbon efficiency, energy demand, stability of the catalyst, solvent recyclability and impact to the environmental lifecycle. The pros and cons of the various methods are also represented. Processes based on mineral acids enable rapid conversion. However, they suffer from corrosion, waste handling issues and degradation by-products. On the other hand, enzymatic depolymerization processes offer relatively high selectivity but they are limited in terms of feedstock sensitivity and slow reaction kinetics. The downstream valorization mechanisms are also described with the result being that no single available technology is capable of satisfying all industrial requirements. Thus, future progress expects integrated circular processes where advanced catalysis, process intensification and digital optimization strategies take place. Full article

Biomass valorization plays a vital role in achieving carbon neutrality and circular economy frameworks. Owing to its carbon-rich structure, biomass represents a promising feedstock to produce bio-based hydrocarbons via biological and thermochemical pathways. While biological conversion routes have been extensively studied, their deployment at commercial scale is constrained by high capital costs and low product yields. In contrast, thermochemical conversion technologies are increasingly being explored as viable large-scale biomass valorization routes. This review presents a comprehensive assessment of thermochemical pathways, with particular emphasis on hydrothermal liquefaction (HTL). The review identifies hydrothermal liquefaction (HTL) as a strategically advantageous route for wet and heterogeneous biomass valorization, due to simultaneous yields of liquid biocrude, and solid hydrochar. The review emphasizes the application of biocrude upgradation processes like hydrodeoxygenation under biphasic solvent systems using sulfided NiMo and CoMo catalysts. Further, the review also establishes hydrochar as a tunable functional material rather than a mere byproduct for applications in fields of energy production, soil amendment, and heterogeneous catalysis. The review article examines technology readiness levels of different biomass valorization techniques, and suggests that while combustion, anaerobic digestion, torrefaction, and transesterification are commercially mature, HTL and carbon capture utilization and storage (CCUS)-integrated fuel synthesis pathways remain at intermediate readiness. Additionally, the review carries out an in-depth study on artificial intelligence and machine learning (AI and ML) applications in biomass valorization, where it observes that Tree-based ensemble models, particularly Random Forest and XGBoost, show strong performance for several HTL prediction tasks, while Gaussian Process Regression and neural network–Bayesian optimization approaches provide additional advantages for uncertainty estimation and process-level optimization. Finally, the future research opportunities in biomass valorization and AI/ML application in HTL-process optimization have been identified for improving the bio-based fuel production techniques. Full article

Plastics pyrolysis is increasingly pursued as a pathway for producing circular hydrocarbon feedstocks for petrochemical integration. However, non-integrated reactor configurations often exhibit limited heat-transfer control, significant char-handling requirements, and variable product distributions. This work presents a system-level interpretation of the MLM-R™ process, an integrated pyrolysis–combustion loop in which a circulating solid heat carrier enables continuous thermal supply through internal oxidation of carbonaceous residues. Material Flow Analysis (MFA) was applied to reconcile mass, elemental carbon, and chemical energy distributions across the defined process boundary. For the representative case study (1000 kg polyolefin basis), ~81% of feed carbon and ~83% of feed chemical energy (HHV basis) were recovered in the condensed liquid product, while ~7% of feed carbon was internally combusted to sustain autothermal operation. Simulated distillation analysis indicates that removal—aimed at further reprocessing—of a ~15 wt% C34+ heavy fraction from the pyrolysis vapor stream enables compliance with refinery-relevant boiling range targets (≥95% below 480 °C). The MFA results, supported by the physicochemical interpretation, suggest that integrated control of solids circulation and heat transfer contributes to product selectivity and process scalability in circular feedstock production. Full article

Lignin is an abundant aromatic biopolymer generated as a major by-product in lignocellulosic biorefineries, and its efficient valorization is essential for improving process sustainability and economic viability. Among current upgrading strategies, the conversion of lignin into lignin-derived biochar (LDB) has emerged as a promising route because of its high carbon yield, scalable production, and tunable physicochemical properties. This review examines the relationships between lignin structure, thermochemical conversion pathways, and the resulting properties of LDB materials within biorefinery systems. The influence of different technical lignins and conversion routes, including pyrolysis and hydrothermal carbonization, is critically discussed together with post-functionalization strategies. Particular attention is devoted to emerging applications in contaminant adsorption and controlled release systems for agrochemicals. The adsorption mechanisms governing pharmaceuticals, pesticides, microplastics, and PFAS removal are analyzed, while the dual role of LDB as both adsorbent and delivery platform is highlighted. Current limitations include lignin heterogeneity, lack of standardized evaluation protocols, and insufficient validation under realistic environmental conditions. Overall, LDB represents a versatile and scalable platform for lignin valorization and sustainable material design within circular bioeconomy frameworks. Full article

The cement industry is a major contributor to global carbon emissions. Therefore, reducing emissions while utilizing industrial wastes is critical for its sustainable development. Red mud, a solid waste byproduct of alumina smelting with main components like SiO₂, Al₂O₃, and CaO, can partially replace limestone as a raw material in cement production. TG-DSC thermal analysis clarified red mud’s three-stage weight loss characteristic during calcination (total weight loss rate of 22.11%), and orthogonal experiments identified calcination temperature as the core factor for its CaO content, with the optimal calcination pretreatment process confirmed (0.075–0.09 mm particle size, 1373 K, 1 h residence time, CaO content up to 21.1%). Based on the results, this study uses ANSYS Fluent 2021 R1 to simulate a TTF-type precalciner, establishing a validated multi-physical field model (all relative errors < 5%) to explore red mud blending ratios of 0%, 2.5%, 5%, 7.5% and 10%. Unlike previous experimental studies, this work uses a CFD model to quantify how red mud blending ratios affect the coupled thermo-chemical environment in a TTF precalciner, revealing a mechanism-driven trade-off among decomposition rate, CO₂, and NO_x that experiments alone cannot capture. Results show red mud slightly alters the internal temperature field and reduces the raw meal decomposition rate. The decomposition rate remains within the industrial acceptable range of 85–95% when the red mud blending ratio is no more than 5%, while further increasing the blending ratio to 7.5% and 10% causes the decomposition rate to drop below 85%. Therefore, a blending ratio of 5% is recommended, which balances waste utilization, decomposition rate, and emission reduction, providing solid technical support for red mud’s large-scale use in cement production. Full article

The urgent demand for sustainable carbon management and environmental remediation has accelerated research on biochar as a multifunctional material. This review critically evaluated over 250 peer-reviewed studies to elucidate the relationships between feedstock composition, thermochemical conversion processes, and the resulting physicochemical properties of biochar. The analysis revealed that pyrolysis temperature is the dominant parameter governing biochar yield and structure, contributing up to ~50% of the variability, while feedstock composition strongly influences surface functionality and pore architecture. Low-temperature biochar (300–400 °C) exhibits higher cation exchange capacity and functional group density, whereas high-temperature biochar (>600 °C) demonstrates enhanced aromaticity, stability, and carbon sequestration potential. Advanced modification strategies significantly improve the adsorption capacity, catalytic activity, and energy applications. Despite these advances, major challenges remain, including lack of process standardization, limited long-term field validation, and uncertainties in carbon stability. This review identifies key research gaps and proposes future directions focusing on scalable production, life-cycle assessment, and integration into circular economy systems, thereby providing a comprehensive framework for the development of high-performance biochar technologies. Full article

Industrial processes consume large amounts of thermal energy, while many recoverable heat streams remain unused because heat sources and sinks differ in time, temperature level, power demand, and operating schedule. Thermal energy storage (TES) can decouple heat supply from heat demand and support waste heat recovery, peak-load reduction, process heat electrification, and flexible operation of continuous, batch, and intermittent processes. This narrative review assesses industrial TES from a process integration perspective rather than from a storage-material perspective alone. Sensible, latent, thermochemical, sorption-based, hybrid, and steam-based storage systems are compared with respect to delivery temperature, storage duration, charging and discharging power, response time, heat losses, reliability, integration complexity, and techno-economic feasibility. Sector-specific opportunities are discussed for the iron and steel, cement, ceramics, chemical and petrochemical, pulp and paper, and food and beverage industries. The review shows that deployment is constrained less by the availability of storage concepts than by heat exchanger limitations, inconsistent Key Performance Indicator (KPI) definitions, unclear system boundaries, scarce long-term operating data, and insufficient coupling with pinch analysis, heat exchanger network design, control, and safety requirements. A practical technology-selection workflow and a research roadmap are proposed for scalable, reliable, and economically viable industrial TES deployment. Full article

This study focuses on a 155 mm 32CrNi3MoV steel barrel and presents a thermochemically coupled phase transformation and diffusion dynamics model. The model leverages the significant disparity between radial and axial temperature gradients to simplify the heat conduction problem to a one-dimensional transient formulation. The temperature field distribution during firing sequences is solved analytically, accounting for the dynamic shift in critical phase transformation temperatures under high heating rates. The evolution of the martensitic layer thickness under repeated thermal shock is subsequently calculated. A numerical model for the pulsed diffusion of C and N is established based on Fick’s second law, incorporating the competitive diffusion–phase transformation mechanisms that govern martensite/austenite interface migration. To quantitatively evaluate the synergistic contribution of C and N to austenite stabilization, a carbon equivalent (Ceq) model is introduced, with the weight coefficient of N relative to C determined to be 0.68 and the critical Ceq required to lower the martensite start temperature below 25 °C calculated as 1.15 wt%. Concurrently, the microstructure and elemental distribution within the austenite layer of the retired barrel are systematically characterized using multi-scale techniques. The results indicate that the austenite layer on the inner bore surface arises from the synergistic effects of cyclic thermal-shock-induced phase transformation and elemental diffusion. Based on the Ceq criterion, the austenite layer thickness increases rapidly during the initial ~100 firing cycles, after which the growth rate slows significantly: it reaches approximately 1.27 μm after the first cycle and 2.94 μm after 1000 cycles, with only 0.2 μm of additional thickening between 100 and 1000 cycles—consistent with the experimentally observed range of 1.52–4.16 μm. The martensitic layer formed during the first firing cycle exhibits low thermal conductivity, which impedes subsequent heat transfer and leads to stabilization of its thickness at a characteristic depth. Grain refinement induced by repeated thermal shock provide short-circuit diffusion paths for elemental diffusion, accelerating compositional homogenization within the austenite layer and resulting in a stepped concentration profile at the interface. This study provides a representative example of non-equilibrium coupled phase transformation–diffusion phenomena under extreme transient loading. The established thickness prediction model can provide guidance for service life assessment of large-caliber barrels, offering both theoretical foundations and practical engineering guidance for their material design and performance optimization. Full article

With the continuous advancement of thrust-to-weight ratios in modern aero-engines, turbine inlet temperatures have reached levels that far exceed the thermal endurance limits of conventional superalloys and emerging ceramic matrix composites (CMCs). Consequently, thermal barrier coatings (TBCs) and environmental barrier coatings (EBCs) have become indispensable multifunctional systems for hot-section component protection. This review systematically delineates the evolutionary trajectory of TBC/EBC systems, transitioning from traditional yttria-stabilized zirconia (YSZ) and simple silicates to advanced multi-rare-earth-doped oxides, A₂B₂O₇ pyrochlore structures, and high-entropy ceramic systems. A critical comparative assessment is provided regarding their phase stability, thermal-physical properties, and durability challenges above 1200 °C. Furthermore, this paper provides an in-depth analysis of high-temperature degradation mechanisms, focusing on the thermochemical and thermomechanical interactions under calcium-magnesium-alumino-silicate (CMAS) attack, water-oxygen corrosion, and molten salt infiltration. By synthesizing current research gaps, we highlight the trade-offs between low thermal conductivity, high toughness, and environmental resistance. Finally, a strategic roadmap for next-generation coatings is proposed, emphasizing the integration of high-entropy material design, multi-scale structural optimization, and AI-driven life prediction models to meet the stringent reliability requirements of future propulsion systems. Full article

The growing demand for sustainable packaging materials has accelerated interest in biomass-derived carbon fillers as functional reinforcements for biodegradable polymer composites. This review critically evaluates the use of carbon materials produced from agricultural residues, particularly palm kernel shell (PKS) and coconut shell (CS), in biopolymer composite coating films for food packaging applications. Recent thermochemical conversion routes, including carbonization, activation, and catalytic graphitization, are discussed in relation to their influence on filler morphology, porosity, surface chemistry, and graphitic ordering. Particular emphasis is placed on structure–property relationships in composite systems containing matrices such as polylactic acid (PLA), starch, chitosan, gelatin, and polyvinyl alcohol (PVA). Published studies indicate that properly dispersed carbon fillers can improve tensile strength, thermal stability, ultraviolet shielding, and oxygen/water vapor barrier performance through stress-transfer mechanisms and tortuous diffusion pathways. However, excessive filler loading or poor interfacial compatibility frequently causes agglomeration, brittleness, and loss of transparency. Surface modification strategies including oxidation, silanization, and surfactant-assisted dispersion, are therefore reviewed as key approaches to optimize composite performance. Finally, current limitations involving migration safety, process scalability, and the lack of standardized testing protocols are discussed. Overall, PKS- and CS-derived carbon fillers represent promising sustainable additives for next-generation biopolymer composite packaging systems. Full article

Lignocellulosic biomass from hardwood and softwood species represents a highly abundant and renewable resource for biofuel and bio-based material production. This review provides a comprehensive analysis of the chemical composition and structural organization of the three major polymers—hemicellulose, cellulose, and lignin—across different wood fractions, including bark, sapwood, and heartwood. Typically, wood consists of a significant number of these components, approximately 20–35% hemicellulose, 40–50% cellulose, and 20–30% lignin. Significant variations exist between hardwood and softwood species, particularly in lignin composition and hemicellulose structure, which strongly influence biomass recalcitrance and conversion efficiency. Bark is rich in lignin (often 20–40%) and extractives, making it suitable for thermochemical processes, while sapwood exhibits higher carbohydrate accessibility, favoring biochemical conversion. Heartwood, enriched with extractives and condensed lignin, shows reduced reactivity but high potential for value-added chemicals. The review also evaluates extraction techniques and conversion pathways, highlighting the importance of fraction-specific processing strategies. Understanding these variations is essential for optimizing biorefinery performance and advancing sustainable biomass utilization. Full article

Hakea sericea is one of the most aggressive invasive shrubs in Mediterranean ecosystems, producing large quantities of lignocellulosic residues during control operations. This study evaluates thermochemical liquefaction as a valorisation route for this biomass, linking biomass conversion with invasive species management. Whole-plant material was liquefied through acid-catalysed reactions using 2-ethylhexanol as the solvent and p-toluenesulfonic acid as the catalyst. A response surface methodology design was used to assess the effects of temperature, reaction time, and catalyst loading on conversion efficiency. The biomass contained 35.92% cellulose, 32.29% hemicellulose, and 17.36% lignin. Liquefaction yields ranged from 15.59% at 120 °C for 30 min to 82.7% at 160 °C for 90 min, with conversions above 70% achieved within 30 min at higher catalyst concentrations. The regression model explained 87.5% of the variability in liquefaction performance. Spectroscopic and thermal analyses confirmed extensive depolymerisation of lignocellulosic polymers and the formation of an aliphatic-rich bio-oil, with 57.5% of proton signals located in the alkane region of the ¹H NMR spectrum. The bio-oil exhibited a higher heating value of 31.91 MJ kg⁻¹, corresponding to an energy recovery of about 85%. Microscopic observations showed strong structural disruption of plant fibres. Overall, the results demonstrate efficient conversion of H. sericea biomass into energy-dense liquid products, supporting its use in invasive species control strategies. Full article

The increasing share of renewable energy, such as solar and wind energy, in the energy mix implies a demand for sustainable energy storage systems for the mitigation of the intermittency of these energy sources. One option, therefore, is stationary batteries based on abundant sodium, stored in hard carbon (HC) anodes. In this work, following the sustainable by design principle, HCs were synthesized from cotton-based textile waste using three different thermochemical routes: hydrothermal carbonization (HTC) followed by pyrolysis under nitrogen atmosphere (HC-250-N), HTC followed by pyrolysis under a water vapor stream (HC-250-W), and direct pyrolysis (HC-direct-N). The impact of the synthesis method on the physicochemical properties and electrochemical performance of the HCs was thoroughly investigated. X-ray diffraction, Raman spectroscopy, electron microscopy, and gas adsorption analyses revealed that the HTC pre-treatment significantly enhanced the carbon content, microporosity, and degree of structural graphitic order. HC-250-N exhibited the highest graphitic character and more uniform microstructure, while HC-250-W showed the largest specific surface area and broader micropore distribution. Electrochemical evaluation in sodium-ion half-cells indicated that HC-250-N delivered the most balanced performance, with a reversible capacity of 335 mAh g⁻¹ and good cycling stability. These findings confirm the potential of textile waste-derived HCs as promising and sustainable anode materials for sodium-ion batteries and highlight the importance of tailoring synthesis parameters—such as HTC treatment and pyrolysis conditions—to optimize their structural and electrochemical properties. Full article