Supercritical Water Research Articles

Corrosion characteristics of Fe, Ni and Ti based alloys near the critical point of water and during supercritical water gasification and oxidation of municipal sludge

Supercritical water gasification (SCWG) and oxidation (SCWO) were promising technologies for treating municipal sludge; however, corrosion hindered their development. To obtain the candidate material for preheaters, corrosion characteristics of 316, 316 L, Incoloy 800, 825, and TA-10 were investigated at 350 °C and 400 °C. For SCWG and SCWO reactors, corrosion behaviors of 316, Incoloy 800, 825, Inconel 600, 625, and Hastelloy C276 were explored at 450 °C and 520 °C, with and without oxidants. Near the critical point of water, corrosion rates of all materials were below 0.4 mm·a−1, with 316 L being the candidate material. According to the pH variation, materials underwent activation (350 °C) and passivation (400 °C) pathways. During SCWG and SCWO, Incoloy 825 and Inconel 625 were prioritized at 450 ℃ and 520 ℃, exhibiting corrosion rates below 0.40 mm·a−1 and 0.50 mm·a−1, respectively. Chemical corrosion dominated and double-layer oxide films (Fe3O4-FeCr2O4 and NiO-Cr2O3) formed.

Upcycling of waste disposable medical masks to high value-added gasoline fuel and surfactants products by sub/supercritical water degradation and partial oxidation

The amount of waste disposable medical masks (DMMs) and the potential environmental risk increased significantly due to the huge demand of disposable medical surgical masks. In this study, two effective and environmentally friendly processes, supercritical water degradation (SCWD) and subcritical water partial oxidation (SubCWPO), were proposed for the upcycling of DMMs. The optimal conditions for the SCWD process (conversion ratio>98 %) were 410 ℃, 15 min, and 1:5 g/mL. The oil products obtained from the SCWD process were mainly small molecule hydrocarbons (C7-C12) with a content of 86 % and could be recycled as fuel feedstock for gasoline. Alkyl radicals in the SCWD reaction formed double bonds and ring structures through hydrogen capture reactions, β-scission, and dehydrogenation reactions, and aromatic hydrocarbons were formed by olefin cyclization and cycloalkane dehydrogenation. The introduction of an oxidant (H2O2) to the reaction system could significantly reduce the reaction temperature and shorten the reaction time. At 350 ℃, 15 min, 1:20 g/mL, V(H2O2): V (H2O) of 1:1, the conversion ratio of the SubCWPO process was 88 %, which was higher than that of the SCWD process at 400 ℃ (71.49 %). Oil products produced from the SubCWPO process were rich in alcohols and esters, which could be used as raw materials for nonionic surfactant of polyol and fatty acid ester. The abundant hydroxyl radical in the SubCWPO system trapped hydrogen atoms on PP and reacted with the resulting alkyl radical to form alkanols, which was oxidized to form acids. The esterification of acids and alkanols formed high level of esters. The SCWD and SubCWPO processes proposed in this study are believed to be promising strategies for DMMs degradation and the recovery of high value-added hydrocarbons.

Development of nickel-based bimetallic catalysts for production of hydrogen-rich syngas via supercritical water gasification of liquid residue from hydrothermal pretreatment of food waste

Hydrothermal carbonization has become increasingly prevalent in the pretreatment of food waste, leading to the generation of substantial volumes of liquid effluent. This hydrothermal liquid effluent from food waste (HLF) contains high content of organic compounds, making it challenging to treat using conventional methods. In this regard, the HLF was considered a potential source for energy production via supercritical water gasification (SCWG). This study aimed to develop effective bimetallic catalysts based on Ni/Al2O3 with four different promoters (Cu, Co, La, and Ce), and investigated their performance in the catalytic SCWG for the treatment of HLF and hydrogen production. Among the tested promoters, the Ce-Ni catalyst was considered the most suitable in terms of its highest H2 yield and H2 selectivity. The optimal Ce content was found to be 5 wt%, with a catalyst dosage of 5 % by weight of HLF. These conditions resulted in the highest H2 selectivity of 1.61 and a relatively high H2 yield of 749.60 mmol/L under SCWG operating conditions of 480 °C and 30 min. Excessive catalyst dosages increased total syngas yield but reduced H2 selectivity. The stability test demonstrated that the Ce promoter contributed to enhancing the catalyst resistance to deactivation by coke deposition. The hydrogen production of 5Ce-Ni decreased by 13.8 % after three reuses, whereas a larger decrease of 26.0 % was observed with Ni/Al2O3. These findings indicate that Ce-Ni catalyst show promise for enhancing H2-rich syngas production in the SCWG of hydrothermal liquid effluent from food waste.

Safety Management and Accident-Control Strategy for a Commercial-Scale Plant for Supercritical Water Oxidation of Sludge

Supercritical water oxidation is a promising technology for decomposition of industrial wastewater and sludge. However, the system is operated under high temperature and pressure (usually higher than 500 °C and 25 MPa). Corrosion of component materials and salt deposition may lead to leakage or even the burst of the pressure vessel in the SCWO system, resulting in a high level of worry about the safe operation of the system. In this paper, the safety management and accident- control strategies are introduced according to a commercial-scale SCWO plant in China. The safety management strategy refers to the special design and operation strategies of some facilities. Different types of potential accidents are analyzed and the coping strategies for accidents of different levels of severity are described in detail. The strategies are valuable for the safe operation of commercial SCWO plants and other plants operated in high temperature and high pressure.

Experimental investigation and thermomechanical performance evaluation of supercritical water oxidation of n‐dodecane/tributyl phosphate

AbstractBACKGROUNDNuclear energy has brought immense benefits while also generating a large amount of wastewater that urgently needs to be processed. Supercritical water oxidation (SCWO) technology serves as an efficient method for wastewater treatment. In this study, a SCWO experiment was carried out with organic solvent of 30% tributyl phosphate (TBP) and 70% n‐dodecane. This study investigated the operating parameters and the effects of enhanced oxidation techniques on chemical oxygen demand (COD) removal efficiency. An organic solvent SCWO process was developed, and its energy and exergy efficiencies were assessed using Aspen Plus.RESULTSThe COD removal was about 96.18% at the experimental parameters of 500 °C, 10 min, 25 MPa, oxidation coefficient of 1.5 and material concentration of 4 wt%. COD removal increased by 3.09% with the addition of CeO2. Finally, the overall energy and exergy efficiencies of the SCWO system were found to be 58.8% and 9.1% respectively by Aspen Plus.CONCLUSIONIt was found that the reaction temperature had the greatest effect on the COD removal rate compared with other reaction parameters. The decomposition of organic solvents can be divided into two stages, fast and slow, and the addition of alkali and catalyst can effectively promote the decomposition of waste organic solvents. The experiments showed that SCWO is feasible for the treatment of n‐dodecane/TBP. © 2024 Society of Chemical Industry (SCI).

Combining experiment and theory to study the mechanism of lignin supercritical water gasification

Supercritical water gasification (SCWG) presents notable advantages in biomass energy utilization. The exploration of the reaction mechanism of lignin in supercritical water holds significance in advancing and refining the biomass SCWG process. In this work, syringol was selected as a lignin model compound to study the SCWG mechanism of lignin through a combination of molecular simulation and experiments. The results show that the ether bond is rapidly cleaved in the reaction initial stages. While the phenolics dearomatization has a slower rate and temperature exerts a considerable influence on the dearomatization process. Four degradation pathways are proposed based on the molecular simulations. In the context of the dearomatization reaction, DFT calculations are performed to determine the energy barriers associated with each of the four pathways. Notably, the energy barrier for the most favorable pathway is found to be 291.96 kJ/mol. A lumped kinetic model is constructed based on the reaction pathway, and the reliability of the model is verified by both experiments and DFT calculations. This paper offers valuable assistance in optimizing reaction pathways.

Experimental study on hydrogen–oxygen spontaneous ignition limits in supercritical water using a wall-cooled reactor

Hydrogen is a promising fuel for many industrial applications because of its carbon-free properties. The characteristics of hydrogen–oxygen reaction in supercritical water were thoroughly investigated using a newly constructed experimental facility. Three typical temperature profiles were obtained, representing different reaction states: mild oxidation, transition state, and intense combustion. The spontaneous ignition limit was mapped based on 42 experimental data points. The minimum ignition temperature decreases with increasing fuel concentration, reaching as low as 517 °C at a hydrogen concentration of 23.2 mol%. The reliability of the spontaneous ignition limit was confirmed by the first hydrogen hydrothermal flame images obtained. The initial reactor temperature and fuel concentration increase allows the hydrogen–oxygen reaction to transition from mild oxidation to intense combustion. The oxygen excess ratio does not affect the reaction states, but it affects the temperature profile and hydrogen conversion efficiency. The introduction of cooling water changes the concentration and temperature of the reactant mixture in the reactor, thus affecting the stability of the hydrothermal flame.

Structural Alterations of Oils and Domanic Shales During Supercritical Water Injection

Structural Alterations of Oils and Domanic Shales During Supercritical Water Injection

Hydrochar Production by Hydrothermal Carbonization: Microwave versus Supercritical Water Treatment

Hydrochar Production by Hydrothermal Carbonization: Microwave versus Supercritical Water Treatment

Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy

AbstractSupercritical water provides distinctly different solvation properties compared to what is known from liquid water. Despite its prevalence deep in the Earth's crust and its role in chemosynthetic ecosystems in the vicinity of hydrothermal vents, molecular insights into its solvation mechanisms are still very scarce compared to what is known for liquid water. Recently, neutral metal particles have been detected in hydrothermal fluids and proposed to explain the transport of gold species to ore deposits on Earth. Using ab initio molecular dynamics, we elucidate the solvation properties of small gold species at supercritical conditions. The neutral metal clusters themselves contribute enormous THz intensity not because of their intramolecular vibrations, but due to their pronounced electronic polarization coupling to the dynamical supercritical solvent, leading to a continuum absorption up to about 1000 cm−1. On top, long‐lived interactions between the gold clusters and solvation water leads at these supercritical conditions to a sharp THz resonance that happens to be close to the one due to H‐bonding in liquid water at ambient conditions. The resulting distinct resonances can be used to analyse the solvation properties of neutral metal particles in supercritical aqueous solutions.

Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy.

Supercritical water provides distinctly different solvation properties compared to what is known from liquid water. Despite its prevalence deep in the Earth's crust and its role in chemosynthetic ecosystems in the vicinity of hydrothermal vents, molecular insights into its solvation mechanisms are still very scarce compared to what is known for liquid water. Recently, neutral metal particles have been detected in hydrothermal fluids and proposed to explain the transport of gold species to ore deposits on Earth. Using ab initio molecular dynamics, we elucidate the solvation properties of small gold species at supercritical conditions. The neutral metal clusters themselves contribute enormous THz intensity not because of their intramolecular vibrations, but due to their pronounced electronic polarization coupling to the dynamical supercritical solvent, leading to a continuum absorption up to about 1000 cm-1. On top, long-lived interactions between the gold clusters and solvation water leads at these supercritical conditions to a sharp THz resonance that happens to be close to the one due to H-bonding in liquid water at ambient conditions. The resulting distinct resonances can be used to analyse the solvation properties of neutral metal particles in supercritical aqueous solutions.

Effect of CO2-H2O Interaction with High-Rank Coal on Mechanical Properties and Acoustic Emission Characteristics under Confining Pressure.

The adsorption of CO2 by coal leads to changes in its mechanical properties, particularly when considering supercritical CO2 and water with supercritical CO2 adsorption. This is strongly linked to the efficiency of CO2-enhanced coalbed methane (CO2-ECBM) extraction and the safety of CO2 geological storage. This study focuses on 3# coal from the Datong Mine in Gaoping City, Shanxi Province. The high-rank coal's mechanical properties, including the triaxial compressive strength and elastic modulus, were examined under the combined effects of CO2 injection pressure, CO2 injection time, and moisture content. The triaxial compressive strength and elastic modulus of the coal showed a decrease following CO2 injection. Increasing the CO2 injection pressure, prolonging the CO2 injection time, and increasing the moisture content were favorable for coal softening. In particular, the triaxial compressive strength and elastic modulus of the coal sample after 144 h of water and supercritical CO2 softening decreased by 67.67 and 64.15%, respectively. Injecting CO2 into coal changed its failure mode. The dry raw coal sample exhibited a brittle shear failure mode, while the coal samples showed transitional shear failure after injecting 6 MPa CO2 and 8 MPa CO2 and ductile nondilatant barreling failure after injecting water and 8 MPa CO2 (with a moisture content of 3.02%). Moreover, the cumulative acoustic emission energy of the coal samples followed a similar trend to the decrease in mechanical properties under different conditions. The physical and chemical interactions among coal, CO2, and water caused the softening of coal; these included the generation of the swelling stress, the dissolution of minerals by carbonate solutions, the reduction in surface energy of coal owing to CO2 adsorption, and the extraction and plasticization reactions of organic matter in coal. The research results are of great significance for further understanding CO2-ECBM and CO2 geological sequestration.

Macro-characteristics and micro-mechanisms of Na2SO4 precipitation dissolved by KNO3 molten salt in a continuous supercritical water system

This work aims to explore the ability of molten salt to solve salt deposition in supercritical water (SCW) related technologies including supercritical water oxidation and supercritical water gasification, with KNO3 and Na2SO4 as examples. In the pure KNO3 solution, the two-phase layering of high-density KNO3 molten salt (settling at the reactor bottom) and low-density saturated KNO3-SCW salt solution (flowing out at the top outlet of the reactor) was formed in a kettle-reactor with about 6.5 ratio of depth to inner diameter, thereby improving the accuracy of measured solubilities. The precipitation macro-characteristics of mixed KNO3 and Na2SO4 in SCW were investigated under different feed concentration conditions. The results showed that Na2SO4 deposition on the reactor sidewall could be reduced by more than 90 % when the mass ratio of KNO3 to Na2SO4 in the feed was only 0.167. No visible salt deposition was observed on the sidewall when the ratio was 0.374. All solid deposited salts were converted into the liquid molten salt as the ratio reached 3.341, and thus could easily flow out of the reactor, without plugging. ‘Molten salt dissolution’ mechanism may provide a more plausible explanation for mixed KNO3 and Na2SO4 in SCW. In addition, the precipitation micro-mechanisms of mixed KNO3 and Na2SO4, and the critical conditions of avoiding sidewall deposition and reactor plugging were proposed. This work is valuable for overcoming the salt deposition problem in SCW-related technologies.

Catalytic supercritical water gasification of oily sludge with the FeOOH/AC for hydrogen production

Along with the continuous increase of crude oil consumption in China, the production of oily sludge is increasing. The traditional oily sludge treatment technology cannot realize the purpose of green and efficient resource utilization of oily sludge. Supercritical water gasification hydrogen technology can efficiently convert oily sludge into hydrogen and realize the resource utilization of oily sludge. However, the traditional supercritical water gasification process requires high energy consumption. It is necessary to study the catalysts in the gasification process. Loaded catalysts have a large specific surface area and can effectively reduce the reaction activation energy. This paper investigated the effects of different catalyst types and concentrations on gasification. The results showed that adding FeOOH/AC effectively increased hydrogen production, and the best catalytic effect was achieved when the FeOOH loading was 10 wt%. The catalyst concentration was 5%, with a hydrogen production rate of 4.638 mol/kg. It was found that the activation energy of the reaction with the addition of the catalyst was reduced by 8.61 kJ/mol compared to that without the catalyst through kinetic analysis. 8.61 kJ/mol.

Reactive particle-laden flow in industrial-scale supercritical water gasification reactor: Modeling and study on flow patterns

Coal gasification in supercritical water (SCW) is a highly complicated reactive particle-laden flow process, which couples multiphase flow hydrodynamics, conjugate heat transfers, and complex chemical reactions. A quantitative understanding is the key issue for reactor design and optimization. In this study, the Eulerian-Eulerian model is used to describe the particles-SCW flow in a novel industrial-scale thermally integrated supercritical water gasification (TISCWG) reactor with conjugate heat transfer boundary conditions and porous media model. The sophisticated chemical reactions are using a species transport model with a seven-step lumped kinetics. Four zones of the TISCWG reactor and four typical flow patterns in the gasification chamber are firstly revealed as 1) the bottom accumulation region with the nonhomogeneous solids distribution and back-mixing of particles, 2) the upper developed fluidization region with the SCW and coal particles moving in a plug-flow regime in the center and a downward annular gravity-driven flow near the wall, 3) the jet-affected region strongly affected by the vortexes near the nozzle and presented as a trans-critical jet regime and 4) the dilute region where dilute solids slow down and fall back into the gasification zone. Flow in the gasification chamber is strongly affected by the heat transfer with the preheat zone, as reflected by an annular gravity-driven flow film flowing downwards near the gasification wall. A funnel-shaped structure promotes a more uniform distribution of solids in the accumulation region rather than a cylindrical structure. The effect of flow patterns on coal gasification processes is also studied. Volatile pyrohydrolysis reactions depend on the time-varying solids distribution and the gaseous products at the system outlet with a typical H2 mole fraction of 61.42% and CO2 of 34.98%.

Hydrothermal recycling of polyolefins as potential alternative method for fuel production

The hydrothermal degradation of different polyolefins (virgin and recycled HDPE, recycled LLDPE, metallocene LLDPE, LDPE waste, virgin and recycled PP, PP waste) and their blends in different forms (granulate, foil) was investigated with supercritical water at 450 °C. The degradation of HDPE was investigated in a time range from 15 min to 240 min. The maximum yield of the oil phase was obtained at a degradation time of 60 min and was over 90 %, therefore all further experiments were carried out at a reaction time of 60 min. It was found that the composition of the oil obtained, and its calorific value depend on the type of material degraded. The oils obtained from PE materials contained between 65.3 % and 69.5 % saturated hydrocarbons, 11.4 %–17 % olefins and 8.9 %–20 % aromatics, while the oils from PP materials contained 40.8 %–45.9 % aromatics, 28.4 %–33.4 % saturated hydrocarbons and 10.6 % to 19.2% olefins. The main compounds in the oils from PP materials were the C9 compounds and from PE materials the C16 compounds. An exception were the oils from rLLDPE-1-g and LDPE-b, in which C8 compounds were most abundant and which contained the highest proportion of C17 – C31 hydrocarbons (approx. 17 %) compared to oils from other PE and PP materials (1.3 %–7.1 %). The HHV of the oils was between 29.4 and 45.7 MJ/kg and was highest for the oils where the gasoline/heavy oil ratio was less than 1, while the HHV of the gas phase was between 48.4 MJ/kg and 50.7 MJ/kg.

Phytochemical Screening, Antimicrobial and In-vitro Cytotoxic Activities of Several Extracts of Six Medicinal Plants (Crossopteryx febrifuga Benth, Khaya anthotheca, Gardenia ternifolia, and Terminalia glaucescens) from Central Africa Region

The qualitative study of secondary metabolites of the root and stem bark of Crossopteryx febrifuga Benth, Khaya anthotheca, Gardenia ternifolia, and Terminalia glaucescens obtained by maceration were estimated by tub reaction. Antimicrobial and cytotoxic activities of the root and stem bark of the same plants obtained by maceration and subcritical water extraction were respectively tested whit antibiogram disc test on Pseudomonas Aeruginosa (ATCC 27853), Enterobacter Hormaechei (ATCC700323), Bacillus Spizizenii (ATCC6633), Enterococcus Casseliflavus (ATCC700327), Staphylococcus aureus subsp. Rosenbach (ATCC 6538), and Salmonella Typhus (ATCC 6539). The Tetrazolium test (MTT test) was realized by using the mouse fibroblast cells (NIH3T3). Extracts based on the bark of Terminalia glaucescens and Crossopteryx febrifuga Benth induce sensitivity to 4/6 bacterial strains, in particular Enterobacter Hormaechei (ATCC700323): 3±2cm; Bacillus Spizizenii (ATCC6633): 5±2 cm; Staphylococcus aureus subsp. Rosenbach (ATCC 6538): 5±2; and Salmonella Typhus (ATCC 6539): 3±2cm. Extracts based on the bark of Khaya anthotheca and those of the bark of Gardenia ternifolia induce only the sensitivity on 2/6 bacterial strains specially Enterobacter Hormaechei (ATCC700323) and Bacillus Spizizenii (ATCC6633) respectively 4±2cm and 5±2cm. Extracts based on the barks and roots of Khaya anthotheca and those of the roots and barks of Gardenia ternifolia only induce sensitivity on 2/6 bacterial strains tested, in particular the strain of Enterobacter Hormaechei (ATCC700323) and Bacillus Spizizenii (ATCC6633). Only extracts based on the bark of Crossopteryx febrifuga Benth and Terminalia glaucescens show sensitivity against Salmonella Typhus (ATCC 6539). All the plant extracts tested show very low cytotoxic power. Plant extracts obtained by the maceration method (Hydro Methanol 90%) induce 19% inhibition of cell growth compared to extracts obtained by supercritical water 125°C 60ATM and 125°C80 ATM. The extracts obtained using the supercritical water 125°C 60ATM method are the least offensive on cell proliferation. The most concentrated extracts show the greatest inhibitions. The extracts of plant E show the greatest inhibition at a concentration of 1 g/ml.

Partial upgrading of bitumen with supercritical water— liquid products characteristics, gas composition and coke morphology

The study aimed to investigate the partial upgrading of bitumen in the absence of water (non-water) and the presence of supercritical water (SCW). Partial upgrading experiments were conducted at the target temperature of 420 °C, with the reaction time varying from zero to 60 min in a batch reactor. The liquid products were analyzed for density, viscosity, composition, total acid number (TAN), olefin content, and elemental analysis. The gas composition and morphology of solid products were also studied. The experimental results revealed that while partial upgrading of bitumen with SCW could accelerate improvement in oil quality, particularly in terms of API and viscosity, it concurrently led to elevated gas and coke formation levels. Moreover, the difference in the distribution of oil fractions between the two environments (absence and presence of SCW) tends to diminish with an extended residence time. Additionally, as the residence time increases, the efficiency of TAN reduction under SCW conditions is less pronounced compared to the non-water conditions. Liquid products obtained in an SCW environment exhibited higher olefin content (between 3 and 4 wt %) compared to non-water conditions (between 1.8 and 2.7 wt %). In both environments, the olefin content was initially increased up to a residence time of 20 min and then decreased. Regarding heteroatoms, the sulphur (S) and nitrogen (N) contents decreased up to 36 % and 72 %, respectively, while a greater reduction was observed in the presence of SCW. More non-hydrocarbon gases (H2S, CO2 and CO) were produced in the presence of SCW. SEM images showed that an extended residence time led to a shift in coke morphology towards a more uniform and compact structure, regardless of the upgrading environment. However, the porous structures of coke samples obtained under SCW are distinguished from those formed without water, which is attributed to the phase inversion of precursors. The findings of this study confirm that SCW plays a beneficial role as a solvent in the partial upgrading of bitumen, expediting the process. However, the increased level of coke formation and olefin contents suggest that it may not function effectively as a hydrogen donor in the process.

Back Cover: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. Int. Ed. 28/2024)

Back Cover: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. Int. Ed. 28/2024)

Rücktitelbild: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. 28/2024)

Rücktitelbild: Solvation Properties of Neutral Gold Species in Supercritical Water Studied By THz Spectroscopy (Angew. Chem. 28/2024)