The efficient conversion of biomass and the controlled fabrication of carbon materials with hierarchical pore structures are of great significance for energy transition and pollution control. However, conventional pyrolysis under inert atmospheres tends to generate tar deposits, which hinder pore development and limit material performance. In this work, biomass was subjected to thermochemical reduction under supercritical CO2 (scCO2) atmosphere, followed by KOH activation to produce activated biochar. The effects of reaction temperature (200–400 ℃) and residence time (20–60 min) on structural evolution and adsorption performance were systematically investigated, with a comparative analysis between scCO2 and N2 atmospheres. The results show that scCO2 regulates carbon structure evolution and alleviates pore blockage by promoting tar migration and cracking, as well as enhancing the carbon-CO2 reaction. The activated biochar exhibited a high specific surface area (759.58–1895.05 m2/g) with a micropore-dominated structure. Compared with N2, scCO2 pretreatment led to a more developed and uniform pore network. The maximum adsorption capacity of the activated biochar, prepared by pre-treatment followed by 20 min reaction at 200 ℃, reached 436.63 mg/g in the methylene blue adsorption experiment. Kinetic analysis indicated that chemisorption plays a dominant role in dye adsorption process (R2 > 0.99). The results demonstrate that adsorption performance is governed by the synergistic effects of pore structure and surface chemical properties. It elucidates the multifunctional role of scCO2 in biomass conversion and pore structure regulation, providing a theoretical basis for the targeted design of high-performance biochar.

Supercritical water gasification (SCWG) represents a promising technology for the treatment of high-salinity oily wastewater; however, its industrial application is hindered by reactor blockage due to inorganic salt deposition. Using wastewater from the LD27-2-A23H offshore oilfield as a reference, this study systematically investigated the effects of temperature, pressure, and operational parameters on the deposition behavior and morphological characteristics of inorganic salts in a self-designed continuous SCWG system. Experiments were conducted with both salt-only aqueous solutions (NaCl, KCl, and CaCl2, single and ternary mixtures) and glycerol-based simulated organic-inorganic matrices representing oily wastewater. Results indicated that increasing temperature from 673 K to 823 K decreased the bottom deposition fractions of NaCl, KCl, and CaCl2 from 84%, 90%, and 95% to 67%, 70%, and 79%, respectively, while wall adhesion fractions increased from 11%, 7%, and 2% to 35%, 29%, and 20%. In the mixed-salt system, bottom deposition decreased from 84% to 67%, with COD and TOC removal efficiencies exceeding 82% and 91%, respectively. CaCl2 exhibited the highest initial bottom deposition fraction and the weakest temperature sensitivity, whereas NaCl and KCl responded more significantly. Deposition evolved through abrupt solubility reduction, rapid nucleation, crystal growth and agglomeration, followed by directional deposition. Deposit locations varied with flow direction, indicating that nucleation and early growth are strongly influenced by local thermal conditions and hydrodynamics. These findings provide mechanistic insight into inorganic salt deposition in SCWG systems and offer guidance for reactor design, anti-clogging strategies, and stable gasification of high-salinity oily wastewater.

Supercritical water gasification (SCWG) enables the clean resource utilization of acrylonitrile-butadiene-styrene (ABS) plastic waste. The elucidation of the reaction mechanism of ABS is a prerequisite for obtaining the target gas and controlling N pollution. Combining experiments and ReaxFF molecular dynamics simulations, this study investigated the effects of reaction temperature, time, feedstock concentration and pressure on the SCWG of ABS. Results showed that at 700 °C, 23 MPa, 10 min and 6% concentration, the carbon gasification efficiency (CE) and hydrogen gasification efficiency (HE) reached 59.78% and 142.96%, respectively, with H2 yield increasing from 1.38 mol/kg at 500 °C to 15.87 mol/kg. Simulations revealed that SCW acted as both hydrogen donor and reaction medium, accelerating the cleavage of ABS molecular bonds by overcoming an energy barrier of 165 kJ/mol. Benzene rings underwent ring-opening via hydroxylation, hydrogenation and pyrolysis-induced pathways. H2 was derived from chain reactions of hydrogen radicals released by ABS cracking and dissociated from supercritical water. Nitrogen was converted to ammonia via hydrogenation and hydrolysis of nitrogen-containing compounds, or to nitrogen via direct denitrification to form hydrogen cyanide and cyano groups followed by subsequent transformation. This study may contribute to understanding the transformation mechanism of ABS in SCW from a multi-scale perspective, thereby facilitating the development of regulatory strategies.

With the growing demand for clean energy, biomass pyrolysis has attracted increasing attention for producing high-value carbon-neutral products, but pyrolysis gas treatment remains a key bottleneck. This study compares mixed-gas separation driven by pyrolysis gas pressure using a Laval nozzle and absorption refrigeration liquefaction. An accurate biomass pyrolysis reactor model was first developed to predict pyrolysis gas distribution. By incorporating Gibbs free energy minimization and the catalytic effect of biochar, the model reduced prediction deviation by 2.53% compared with the original thermodynamic model. Based on the separation model, Laval nozzle separation generated CO2 microdroplets with an average mass fraction of 0.175 and reduced the CO2 molar fraction in pyrolysis gas by 23.91%. The separated liquid phase reached a CO2 molar fraction above 0.925, while requiring a small footprint and showing favorable economic feasibility, making it suitable for small-scale separation. In terms of exergy destruction, the Laval nozzle showed clear advantages: when only evaporator heat exchange was considered in the absorption refrigeration cycle, its exergy destruction was only 44.28% – 55.07% of that of absorption refrigeration liquefaction. Absorption refrigeration liquefaction required additional low-quality heat. For the optimal working-fluid pair and a pyrolysis gas flow rate of 1.0 kg h−1, the maximum condensed liquid reached 0.5378 kg h−1, 3.07 times that of Laval nozzle separation, with a COP of 0.6654 and low-quality heat consumption of 0.0778 kW. However, due to phase equilibrium limitations, the CO2 purity was only 0.576 – 0.844, making it more suitable for large-scale separation.

The degradation of food waste via supercritical water (SCW) may offer an attractive H2-rich gas production method. Current research focuses on screening high-performance catalysts to enhance conversion efficiency while ignoring the introduction of O2 for in-situ heat supply and radical generation. This study explores the reaction mechanism, kinetic behavior, and predictive models for the supercritical water partial oxidation (SCWPO) of food waste. The results showed that as the oxidant equivalent ratio (ER) increased, the CO2 yield continued to rise, while the yields of combustible gases such as H2 and CH4 decreased simultaneously. The carbon gasification efficiency (CE) increased by 8.25%, while the hydrogen gasification efficiency (HE) decreased by 11.76%. Through optimization of process parameters (higher oxidation temperature, longer residence time, lower system pressure, and lower feed concentration), the CE of this system can reach up to 93.17%. The complete conversion pathway for food waste in an SCWPO system comprises four core processes: macromolecular hydrolysis, hydrocarbon cracking and rearrangement, interconversion of gaseous products, and multistage oxidation. The reaction kinetics follow a classic decelerating kinetic model. The activation energy for the reaction in the 550–650 °C range is only 50.56 ± 12.78 kJ/mol, demonstrating that partial oxidation can significantly lower the reaction energy barrier.Finally, combine machine learning (ML) to accurately predict product distribution. This paper proposes a generative adversarial network (GAN)-based data augmentation framework to expand the dataset and compares four machine learning models: decision tree (DT), random forest (RF), extreme gradient boosting (XGB), and support vector regression (SVR). Among these, RF and XGB demonstrated the best predictive performance. The R2 greater than 0.967 was attained for the test set, and the R2 of the real data prediction is as high as 0.997. This study may provide insights into the transformation behavior of food waste in SCW under the influence of O2.

Transpiration cooling is a promising thermal protection technology for hypersonic vehicles, yet its coupled two-phase heat transfer mechanisms in porous media require deeper understanding. This study develops a 1D numerical model using the local thermal non-equilibrium two-phase mixture model (LTNE-TPMM) to evaluate how porous-medium properties and operating conditions impact cooling performance. Key findings indicate that higher solid thermal conductivity lowers wall temperatures, while increased porosity and particle diameter reduce pressure drop but intensify thermal non-equilibrium. Additionally, an inlet thermal diffusion effect was identified, in which backward heat conduction through the porous solid skeleton causes thermal dissipation near the inlet, leading to lower-than-expected outlet fluid temperatures. To quantify this phenomenon, a modified porous-medium Peclet number Pep is proposed. Results show that a lower Pep strongly correlates with enhanced inlet thermal diffusion. Within the investigated parameter range, these results provide preliminary guidance for the analysis and early-stage design of transpiration cooling systems.

The iron and steel industry emits large amounts of CO2. It relies heavily on fossil fuels. Therefore, energy saving and carbon reduction are urgent. Biochar offers great potential. It can replace fossil fuels and sequester carbon. However, steelmaking processes are complex. Different routes require different biochar properties. This paper reviews sustainable biochar production strategies. These include torrefaction, pyrolysis, supercritical CO2 treatment, flash carbonization, gasification, hydrothermal carbonization, and microwave-assisted pyrolysis. We also review biochar modification methods. We focus on ball-milling and steam activation. Multidimensional analysis reveals key parameters: surface area, pore structure, and density. We then classify structure-activity relationships. Finally, we compare biochar with traditional fossil fuels in steelmaking and evaluate feasible blending ratios for each application: 20-30% for blast furnace pulverized coal injection, 10-20% for structural coke replacement in large blast furnaces, 2-10% for biocoke in coking blends, 20-40% for iron ore sintering, and variable partial replacement in electric arc furnaces depending on the function (slag foaming, carburization, or reduction).

With the depletion of fossil fuels and the advancement of carbon neutrality goals, biomass, as a renewable energy source, demonstrates significant potential in clean fuel production. To enhance the efficiency of gasification-reforming under different atmospheres while reducing energy consumption, the behavior and mechanisms of gasification-reforming were systematically investigated. The focus of this study was to produce H2/CO of 2 and 3 from eucalyptus sawdust under N2 and supercritical CO2 (scCO2) atmospheres for downstream synthesis of methanol and methane, respectively. The results indicated that during gasification at 500-700 degrees C, the scCO2 atmosphere favored the formation of liquid-phase products and inhibited the secondary cracking of light compounds. During reforming, CO production increased markedly as the temperature rose from 550 to 850 degrees C, while H2O addition notably promoted H2 production. After gasification-reforming in the scCO2 atmosphere, the total volume of H2 and CO was 1.60 times higher than that in the N2 atmosphere, indicating that the supercritical fluid environment effectively enhances syngas yield. Preliminary energy analysis under the screening conditions revealed that the production of methanol syngas and methane syngas feedstocks in scCO2 atmosphere required additional energy inputs of 7160.36 kJ/kg and 9813.82 kJ/kg, respectively. It provides a basis for process optimization and offers theoretical support for the efficient and low-carbon conversion of biomass into gaseous fuels.

To address the critical need for efficient utilization of agricultural residues, this study proposes a cascading valorization strategy for walnut shells via a two-stage supercritical CO2 process that integrates low-temperature phenolic extraction with high-temperature gasification. Experimental results demonstrated that at 200 degrees C, supercritical CO2 leveraged its superior transport and solvation properties to efficiently extract phenolic oligomers. A 40-min reaction duration was found to maximize the enrichment of high-value methoxyphenols, particularly 2,6-dimethoxyphenol, compared with inert atmospheres. Subsequent gasification at 700 degrees C demonstrated that, under supercritical conditions, both CO2 and H2O act as highly reactive gasification media. The simultaneous involvement of these two reactive media altered gasification pathways and product distributions. As a result, the lower heating value of the produced syngas increased by up to 50% compared with pure H2O gasification. Furthermore, characterization of the solid residues using XRD, Raman spectroscopy, SEM, and BET analysis confirmed that the synergy-competition mechanism between supercritical CO2 and H2O exerted a significant regulatory effect on biochar evolution. These analyses confirmed that higher water content selectively consumed amorphous carbon and edge defects, promoting microcrystallite densification and lateral aromatic growth to produce highly ordered biochar. This study concludes that cascading supercritical CO2 processing enables fullcomponent biomass utilization, offering a viable route for converting agricultural residues into valuable phenolics, energy-rich syngas, and functional carbon materials.

The thermochemical conversion of single cylindrical biomass particles in supercritical water crossflow is numerically investigated using a multi-region CFD model that couples heat transfer, mass transfer, and chemical reactions. This study conducts a comparative analysis between a global single-step mechanism (Lumped biomass model) and a multi-step component-based mechanism (Three components biomass model) to quantify their impact on conversion characteristics under crossflow conditions. The results reveal that while both models predict gasification as the rate-limiting step, the Three components biomass model predicts significantly longer conversion times and a higher gasification time proportion (similar to 80%) compared to the Lumped biomass model (> 60%), attributed to the broad decomposition temperature range and slow kinetics of lignin. Particle size is identified as the dominant factor controlling conversion, with the Three components biomass model exhibiting a stronger sensitivity to diameter (t(c) sigma d(p)(1.6971)) than the Lumped biomass model (t(c) epsilon d(p)(1.38)). This magnified sensitivity stems from a "production delay" in the Three components biomass model, where thermal lag in larger particles retards the activation of refractory lignin, thereby delaying char formation and coupling the entire reaction sequence to internal heat transfer. Conversely, convection intensity has a limited impact, with the Three components biomass model showing reduced sensitivity (tc a Uin a Uin -0.118). These findings underscore that accurate modeling of supercritical water gasification requires explicit consideration of component-specific kinetics and their interplay with intra-particle transport phenomena.

The dye and textile industry commonly employs activated carbon adsorption technology due to its costeffectiveness and high efficiency. However, disposing of waste-activated carbon has a significant environmental and human health impact, and it's a huge economic waste. This study investigates the kinetic and isothermal adsorption characteristics of Acid Red G dye adsorption by Powdered Activated Carbon derived from coconut shells. To effectively reuse activated carbon and maximise resource conservation, regeneration experiments were carried out using Supercritical Water at 24 MPa and 400 degrees C for 30 min. The experimental results demonstrated that, in comparison with thermal regeneration, supercritical water possesses the benefits of environmental protection, economic efficiency and extensive applicability. This is of considerable importance to the field of research concerning the regeneration of activated carbon.

By employing the Eulerian-Eulerian Two Fluid Model, the effect of different particle size, supercritical CO2 (scCO2) velocity at slit jet (Ujet) and initial bed height on the macroscopic characteristics (i.e., fountain morphology, profiles of particle velocity, momentum transfer characteristics among particles, transient temperature evolutions of particles, interphase heat transfer coefficient and wall to bed heat transfer characteristics) in the pseudo 2D rectangular spouted bed using scCO2 as fluidizing agent is numerically studied in detail herein. Considering there are currently no relevant visualized experiments reported using scCO2 as a fluidized agent due to the extreme operating pressure of CO2 (25 MPa in this paper) under supercritical conditions, present numerical model was validated with experimental data by using air as the fluidizing agent, confirming simulated instantaneous volume fraction distribution of air and transient temperature evolutions of particles basically consistent with the experiments. Numerical results reveal some of the internal relations among hydrodynamics characteristics in bed, momentum transfer characteristics among particles and relevant heat transfer behaviours. Results show larger Ujet and smaller particle size will accelerate the particles' translational motion in spout, spout core and fountain core zone. Larger particle concentration will promote inter-particle collisions while suppress the kinetic motion of particles in above zones. Decrease the particle size will enhance interphase convective heat transfer coefficient, while increasing Ujet results insignificant impacts. Finally, we also observe the transition zone between annular and periphery zone has a certain enhancing effect on the wall to bed heat transfer coefficient. (c) 2024 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights are reserved, including those for text and data mining, AI training, and similar technologies.

In the field of nano energy,investigating the specific heat capacity and coordination number of nano-confined water is highly significant for gaining a better understanding of the energy and microstructure of confined water.In this work,we employed the method of molecular dynamics（MD） simulation to calculate the specific heat capacity at constant volume and coordination number of water molecules confined in carbon nanotubes（CNTs） under different conditions（T=600-700 K,P=21.776 and 25 MPa,CNT diameter=0.949-5.017nm）.The results showed that near the critical point,the specific heat capacity at constant volume of confined water was lower than that of bulk water,and the energy fluctuation showed a trend of first increasing and then remaining unchanged with the increase of temperature and CNT diameter.Among them,the saturation point of temperature is 650 K（reduced pressure P r =1） and 660 K（P r =1.15）,and the saturation point of CNT diameter is 2.034 nm.Additionally,the pseudo-critical temperature of confined water was the same as bulk water,and it increased with the increase of critical pressure.Moreover,with the increase of CNT diameter,the coordination number of confined water increased rapidly,and reaches the saturation state when the CNT diameter is 2.034 nm.This investigation revealed the mass and energy characteristics of nano-confined water near the critical point,which could provide guidance for the critical phase transition of nano-confined water.

Compared to macroscopic bubbles, nanobubbles have unique physicochemical properties that make them highly promising for scientific research. Hydrogen nanobubbles (HNBs), in particular, combine hydrogen's properties with nanobubble-specific effects, offering significant applications in energy, water treatment, and medicine. This study systematically investigated the stability and dynamics of HNBs using molecular dynamics simulations. The results show that the sustained formation of HNBs requires hydrogen in water to reach supersaturation. Bubbles with diameters between 1.1 and 2.8 nm exhibit pressures ranging from 969.5 to 406.6 atm. Theoretical solubility, calculated using Henry's law based on pressure, remains valid at the nanoscale. An electrical double layer was observed at the gas–liquid interface, resulting from the reorientation of water molecules. The electrostatic force generated by the double layer counteracts the ultra-high Laplace pressure, aiding in the stability of nanobubbles. The dynamic properties, such as the diffusion coefficients of hydrogen and water, decrease as HNB size increases. The viscosity of solution containing nanobubbles of different sizes decreases by 6.64%, 8.14%, 14.16%, 19.29%, and 27.08%, respectively. These findings provide valuable insights for advancing the research and application of HNBs.

scCO2 (supercritical CO2) is now an efficient, clean, and pollution-free solvent widely used in industrial productions, such as extraction, printing and dyeing, as well as pharmaceutical productions. In general, a radial mass flux will exist on the surface of active component particles or drug particles during the process of particle dissolution or re-forms due to variations in operating condition of scCO2, which is called Stefan flow. Comparing with the conventional fluids (0.744 < Pr(Prandtl number) < 1.5), typical high-Pr characteristic (Pr > 10) of scCO2 near critical point will present a unique heat transfer performance, while the relative variations of density and viscosity near critical point affect the mechanics characteristic of particle and distribution details of flow field near particle surface, especially considering the effect of different intensities and directions of Stefan flow. To this, based on the PR-DNS (Particle Resolve-Direct Numerical Simulation) method, this work investigates the Stefan flow-affected drag of the sphere, the flow and temperature field near sphere surface, as well as the interphase heat transfer on the sphere surface. We pay more attention to the high-Pr characteristic and relative variations of density and viscosity of scCO2 near critical point. Results point out that the correlation of Nu (Nusselt number) vs. Resf (Stefan Reynolds number) is no longer linear compared with the cases of conventional fluids, and variable physical properties lead larger drag coefficient while worse heat transfer performance.

The fluid and non-spherical particle interaction exists widely in multiphase systems and is more complex due to the shape factor and the Stefan flow that emits from the particle surface will further affect the mass, momentum, and energy transfer between the particle and the fluid. Works about the drag coefficient and Nusselt number relations for Stefan flow-affected non-spherical particles are rare. Motivated by this fact, a particle-resolved direct numerical simulation study of Stefan flow-affected non-spherical particles in supercritical water is carried out to investigate the flow and heat transfer process. The sphere, ellipsoid, cylinder, cube, and cuboid particles are considered in steady-state regimes corresponding to Reynolds numbers from 10 up to 200. This work analyzes the pressure and friction drag coefficients of the particles and illustrates the flow, velocity, and temperature distribution around the particles. The drag coefficient and Nusselt number show significant differences between different shape particles, and Stefan flow further reduces the drag force and Nusselt number. The Reynolds number, particle shape, and Stefan flow both influence particle-fluid interaction: introducing two particle shape descriptors, the drag coefficient depends primarily on the average sphericity (Phi(Av)), while the crosswise sphericity (Phi(perpendicular to)) influences the Nusselt number. The new fitting formulas are developed for the drag coefficient and the Nusselt number of the Stefan flow-affected non-spherical particles.

Carbon dioxide (CO2) is one of the main factors contributing to the greenhouse effect. The dependence on fossil fuels has led to increasing levels of carbon dioxide in the atmosphere every year. And it is far from enough to solve the climate problem by reducing the consumption of fossil fuels to cut down carbon dioxide emissions. In recent years, a series of researches on Carbon Capture, Utilization and Storage (CCUS) have been carried out in various countries around the world. CO2 is a non-toxic, tasteless and stable gas at normal temperature. However, when it reaches supercritical state after rising temperature and pressure, it has the characteristics of low viscosity, high diffusivity and high density, and is widely used in green, pollution-free and efficient development technology. Because of these unique properties, supercritical carbon dioxide (sCO2) has attracted more and more attention from researchers. sCO2 has been widely used in many aspects by virtue of its high solubility and easy compression. Different from previous reviews which only introduced the application of sCO2 property, this paper introduces the current research status of the application of the thermodynamic property of carbon dioxide in extraction, dyeing, pharmaceutical, power generation, heat transfer and exploitation of unconventional oil and gas, and mainly analyzes each application in detail from the aspects of working mechanism and improving working efficiency. Finally, the research direction and problems needed to be solved for the application of CO2 thermal physics are proposed, which pave the way for other new applications.

Based on PR-DNS (Particle Resolve-Direct Numerical Simulation) method, the gasification process of a single spherical char particle moving in high-temperature supercritical CO2/gaseous H2O atmosphere at 9 MPa is conducted, and related mechanism on the reaction-affected drag of the sphere, flow field and interphase thermal as well as mass transfer near sphere surface is presented. The reaction temperature and Re (Reynolds number) of the fluid flowing over the sphere are variable within the range of 1273.15-1673.15 K and 10-200, respectively. The mass decomposition source on the sphere surface (Stefan flow), reaction heat, heat radiation between fluid species and char, porous inert ash layer and variations on physical properties of fluid mixture near the sphere surface are taken into account, while the Pseudo-Steady-State approach is employed for handling the particle shrinking. The results indicate that the higher reaction temperature leads the smaller drag coefficient, while enhances the performance of heat transfer compared with that of non-reactive cases. Due to the relative strength between convective heat transfer and endothermic effect of heterogeneous reaction, variation laws of temperature on the sphere surface vs. Re differs from each other under different reaction temperatures. Higher temperature and lower Re leads the higher surface Damko & BULL;hler numbers, which indicates that the heterogeneous reactions in gasification process have normal characteristic of reactions in the kinetically-controlled regime. Porous inert ash layer increases resistance to flow as well as diffusion of species, and the variations in relative concentration of fluid reactants can also result in varied influences on above variables.

In industrial applications, the phenomenon of scCO2 (supercritical carbon dioxide) flowing over particles is quite common. Considering that the scCO2 is chemically inactive but has high solubility, the pure Stefan flow will present without the related diffusion of a chemical reaction component and reaction heat, during the process of a spherical particle in the solid phase dissolved in a system of scCO2. To this, particle resolve-direct numerical simulation without considering the role of gravity and buoyancy is employed in this paper to investigate the high-temperature scCO2 flowing over a low-temperature stationary sphere with the uniformly, normally, and outward distributed Stefan flow on its surface, with the above cases conducted in the process of small variations on physical properties of scCO2. We present a series of variables in the flow field and temperature field near the sphere surface to study the effects of Stefan flow on them compared with cases without Stefan flow. Related distribution details of the velocity boundary layer and the temperature boundary layer near the sphere surface under conditions with or without the Stefan flow are also presented and analyzed. Different from other similar studies, our study also pays more attention to variables of the local fluid field as well as temperature field near the surface of the spherical particle. The results show that the presence of Stefan flow will reduce flow resistance of the freestream but inhibits heat transfer performance. Simpler correlations in form compared with previous well-established correlations are presented and are used to describe the operating conditions proposed herein.