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Nano-zirconia has been widely applied due to its excellent physical and chemical properties (e.g., high strength, corrosion resistance, oxygen ion conductivity). Existing preparation methods of nano-zirconia tend to require long reaction time, and the sizes of final particles are large with uneven distributions. Sub-/supercritical hydrothermal synthesis of nanoparticles is favored by researchers owing to controllable reaction process, uniform particle size distribution, good reproducibility, short reaction time, high conversion rate and harmlessness to environment. In this paper, the characteristics and mechanisms of dissolution, crystallization and growth of nano-zirconia during sub-/supercritical hydrothermal synthesis are systematically reviewed. The influences of process and material parameters on the size and purity of particles are analyzed. Then, the reaction mechanism and product phase transition mechanism during hydrothermal synthesis of zirconia are summarized to provide a theoretical reference for the oriented preparation. Finally, the improvement and commercialization of sub-/supercritical hydrothermal synthesis technology are evaluated, and the future research topics are proposed.

Hydrothermal liquefaction of the third-generation biomass represented by microalgae can produce biocrude. However, the directly obtained biocrude has a high heteroatom content and a low higher heating value (HHV), which cannot meet the standards of biofuel. In this work, water-insoluble biocrude which was directly gained from microalgae hydrothermal liquefaction (HTL) was upgraded under four kinds of solvents (i.e., methanol, ethanol, acetone, and H₂O) and one type of catalyst (H₂O + Ru/C) at 240–400 °C for 1 h. The results show that the HHV and C and H contents of upgraded bio-oil increased and the O/C ratio decreased significantly after solvent upgrading. The highest upgraded bio-oil yield appeared in the case of ethanol upgrading and reached the maximum value of 82.8 wt % at 360 °C. The upgraded bio-oil yield of acetone upgrading increased from 45.8 to 68.2 wt % as the temperature increased within 240–400 °C. Also, esterification reactions between alcohol and acid in the supercritical system remarkably reduced the content of carboxyl-containing organic matter.

For strengthening the safety and reliability for supercritical water oxidation (SCWO) process, the essential causes and solutions of key challenges such as the high corrosion risk of the heat-exchanger and the entrance part of SCWO reactor, the blockage problems resulted from the solid deposition, and the safety and environmental issues caused by the emergency discharge of supercritical aqueous fluids, were investigated theoretically and experimentally. The thermodynamic state division between chemical- and electrochemical corrosion and the corrosion sensitive temperature range of 280−450 °C featuring the predominant electrochemical corrosion were proposed. A set of optimized parameters for the petrochemical sludge treatment such as a temperature of 520–580 ℃, a pressure of 23−25 MPa, and an oxidation coefficient of 1.1–1.3, which not only can ensure the high enough removal rate of organic pollutants but also prevent the corrosion of construction materials as much as possible; the critical velocity of conveying fluid for avoiding the deposition of solid particles were also obtained. On the bases of innovatively developing the “indirect heat transfer” unit, the mixing-preoxidation equipment, the oxygen removal process for SCWO effluents, and the emergency discharge protection device, a novel SCWO process and its corresponding control scheme were proposed and analyzed.

Supercritical gasification (SCWG) and supercritical partial oxidation (SCWPO) technologies have emerged as preferred means of converting wet biomass to hydrogen-rich gases. We experimentally investigated the effects of moisture content, pressure and oxidation coefficient (n) on mole fraction, yield, gasification efficiency and energy recovery of gaseous products from SCWG or SCWPO of municipal sewage sludge, as well as on the carbon and nitrogen contents in liquid products. Potential of sludge for producing gaseous products was thermodynamically analyzed by an Aspen Plus model. The results show that 87 wt%, 25 MPa and n = 0 were optimum conditions for sludge gasification. Sludge with 87 wt% moisture content was pumpable at 75 °C, and further increasing the moisture content decreased the heating value and energy recovery of gaseous products. Pressure played little role in both the experimental and equilibrium gas yields. Highest mole fractions and yields of H2 and CH4 were achieved at n = 0.

Lithium iron phosphate (LiFePO4) is one of the most important cathode materials for high-performance lithium-ion batteries in the future, due to its incomparable cheapness, stability and cycle life. However, low Li-ion diffusion and electronic conductivity, which are related to the charging rate and low-temperature performance, have become the bottleneck problem. This review begins with the introduction and comment of and phase transition mechanism in lithium iron phosphate particles, followed by the analysis the application potential of nanotechnology in high performance batteries. Nanoscale LiFePO4 has been prepared easily in the laboratory, but few were prepared on a macroscopic scale, and it is more difficult to enter the industrial production stage. Supercritical hydrothermal synthesis is a nano-preparation technology with great potential for industrial application. This article reviews the key parameters (temperature, pressure, concentration, etc.) and core equipment (mixer/reactor) of supercritical hydrothermal synthesis from the perspective of crystallization mechanism, and then point out the structural optimization design of mixer and development of micro-reactors may be the core work for industrialization of supercritical hydrothermal synthesis.

Petrochemical sludge is difficult to be degraded by traditional methods due to the complex organic pollutants, supercritical water oxidation (SCWO) is a promising technology for treating different industrial wastes. In this article, a comprehensive SCWO plant coupled with ammonia distillation and biochemical treatment with the processing capacity near 50 tons per day is designed. Experiments are conducted to determine the influence of reaction temperature, oxidation coefficient and residence time, meanwhile, the species of organics and the concentration of heavy metals in raw feedstock and products are also analyzed to determine the follow-up process. Furthermore, an indirect heat exchange unit between supercritical fluids, capillary device for pressure regulation, and gas–liquid–solid three-phase separator sealed by spiral are proposed to overcome the corrosion and plugging issues. Moreover, an economic evaluation including investment and running cost and the application prospect of the whole system are also analyzed in detail. These information will be significant for designing and constructing the commercial SCWO plant for other industrial wastes.

The optimization and mechanism for oxidation of the waste anion exchange resin from the nuclear industry in supercritical water were investigated. To achieve the maximum chemical oxygen demand (COD...

Corrosion continues to be a major challenge in developing supercritical water-cooled reactor and supercritical water oxidation system. Corrosion behaviors of candidate alloy materials used in these...

Supercritical hydrothermal synthesis of nanosized metal oxides is of great interest for scholars due to its one-pot and readily realizing commercial production process. Hydrothermal synthesis of nanosized TiO2 in subcritical and supercritical water was investigated with different reaction temperatures (250–450°C), time (3–10 min), precursor species (Ti(SO4)2, TiCl4), and the addition of a surfactant (CH3(CH2)5NH2). TiO2 synthesized under supercritical state has a higher degree of crystallinity when compared with TiO2 obtained from subcritical state, which is attributed to a higher reaction rate. Extending reaction time from 3 to 10 min leads to the growth of crystal and broader size distribution (36.66 ± 8.5 nm). More uniform products can be obtained when Ti(SO4)2 acting as the precursor compared with TiCl4 being the precursor. The application of surfactant CH3(CH2)5NH2 in the synthesis makes chemical bonding between the particle surface and organic ligand. It affects the growth direction of TiO2 crystal; as a result, rod-like TiO2 crystal is obtained.

In this study, the oxidation behaviour of alloy 617, alloy 740, alloy HR6W and Sanicro 25 was investigated in SCW and vapour environment at 650°C for 320 h. The weight gain and the thickness of the...

The corrosion behavior and mechanism of Inconel 600 exposed to the oxidizing high-salinity supercritical water (SCW) containing chlorides, sulfates, and phosphates at 450, 500, and 580 °C were investigated and proposed. The corrosion film had an outer layer dominated by NiO, Fe₂O₃, and eutectic phosphate deposit and an inner layer consisting of Cr-rich oxides. Some produced oxides may generate ionic reactions with the low-melting phosphates precipitated out from SCW, triggering the generation of the eutectic FePO₄–Ni₃(PO₄)₂–Na₃PO₄–Na₂HPO₄ salts in the outer layer. The formation and thickening of the outer layer results from the eutectic phosphate disposition and the common solid-state growth mechanism. Nonmolten chlorides and sulfates in SCW predominantly exist on the outmost surface and are easily washed off, being supported by the absence of S and Cl on sample surfaces. Based on this investigation, a comprehensive corrosion mechanism, involving the solid-state growth of oxide scales, salt precipitation, and phosphate melting processes, was proposed and discussed.

Supercritical water gasification (SCWG)having been developed for over 30 years is an efficient and clean hydrogen production technology. Biomass and coal are often used as good raw materials of SCWG, meanwhile achieving their clean conversion and utilization. However, salts such as sodium sulfate, generally exist in the raw materials and can poison the necessary catalyst during SCWG, so that they should be removed as much as possible before the raw materials are pumped into the catalytic reactor. Additionally, a large amount of inorganic salts will be generated simultaneously during the gasification reaction. As the solubility of various inorganic salts decreases sharply beyond the water critical temperature, salts in the supercritical water easily separate out and deposit on the walls of equipment and piping, resulting in a series of potential issues such as reduced heat-transfer efficiency, increased flow resistance and even the related-facilities blockage. In this paper, the experimental analysis and numerical simulation for the solubility and deposition characteristics of the sodium sulfate in supercritical water are systematically summarized; on this basis the corresponding deposition and plugging mechanisms are described and analysed. Additionally, this paper also introduces the potential technical methods being able of avoiding the equipment blockage caused by sodium sulphate deposition, and proposes the necessary subsequent research, which is of great significance for solving the disadvantageous effect of sodium sulfate deposition on the long-term stable operation of SCWG commercial plants.

Sludge is a by-product of wastewater treatment process with high organic and nitrogen contents. Supercritical water oxidation (SCWO) rates of refractory species in sludge might be accelerated by reactive alcohols, which is referred to as a co-oxidation phenomenon. In this work, influences of three typical reactive alcohols (methanol, ethanol and isopropanol) on TOC and of NH3–N removal rates of sludge by SCWO were analyzed and their co-oxidation products were characterized by GC–MS, FT–IR, EDS and TG–DTG methods. Additionally, to capture the nature of co-oxidation, HO2 and OH radicals released by alcohols were theoretically compared with detailed chemical kinetics models using the Chemkin Software. The results show that methanol, ethanol and isopropanol all exhibited co-oxidation accelerated effects on TOC and NH3–N removal rates of sludge. One reason was that alcohols provided reactive HO2 and OH radicals. The other reason was that adding alcohols not only prevented forming recalcitrant products (non-nitrogen and nitrogen aromatic compounds) but also encouraged producing reactive products (non-nitrogen open chain compounds). Ethanol offered the best co-oxidation promotion effects on the removal of TOC and NH3–N in the liquid products and also gave the lowest organic contents in the solid products, followed by isopropanol and methanol. This order was consistent with the difficulty of how the alcohols themselves could be oxidized in supercritical water since ethanol could support the highest amount of HO2 radicals for sludge in the shortest time.

For 9–12Cr ferritic–martensitic steels in supercritical water, the dependencies of the thicknesses of three oxide layers (diffusion, inner, and outer layers) on each of seven principal independent variables were separately investigated using three supervised artificial neural networks (ANN) and fuzzy curve analyses. The latter were employed to evaluate the relative significances of independent variables, indicating that on the whole, temperature and exposure time are the most important variables, while the oxide dispersion strengthening (ODS) comes in the first place for the thickness of the diffusion layer. A thicker diffusion layer occurs easily at intermediate temperature approximately 600 °C and/or on ODS steels, due to higher growth rate of the barrier layer than that of the outer layer. The periodic growth of the diffusion layer, including the “shrinking”/“thickening” stages, was revealed by ANN prediction, which may be the basic cause of periodic pore-assembled layers within the inner layer. Finally, the physicochemical basis of classical point defect model was extended to describe the growth of oxide scales for explaining the ANN predictions at the atomic level. The growth of the inner layer into the metal is attributed to the inward transport of oxide ions (actually via outward transport of oxygen vacancies), while the outward transport of cations through the inner layer via a cation vacancy mechanism or as interstitials results in the thickening of the outer layer. The periodic variation in oxygen potentials at the diffusion/inner layer interface is responsible for the periodic growth of the diffusion layer. The iron caves at the inner–diffusion interface left behind by the generation reaction of cation interstitials may be the intrinsic sources of pores within the inner layer.

Nano/micro crystalline copper is widely used in catalysts and it has potential for being used as conductive additive to ink for inkjet printed electronics. Copper is attractive, because it has excellent electrical conductivity and low cost compared to noble metals. The nucleation and phase transitions from the precursor to the final micrometer sized Cu in supercritical methanol have been studied for the first time using in-house in situ powder X-ray diffraction (PXRD). Temperatures have a significant impact on the reduction process of Cu2+, at low synthesis temperature (250 °C), it was observed how the Cu2+ precursor initially formed copper hydroxy nitrate (Cu2(OH)3NO3) and transformed to copper(II)oxide (CuO) i.e no reduction took place. At 300 °C multiple phase transformation could be observed from initial copper hydroxy nitrate to zero-valent copper, the in situ investigations reveal the following reaction scheme; CuII2(OH)3NO3 → CuIIO → CuI2O → Cu0. Increasing the synthesis temperature causes the pure Cu0 to form much faster; at 350 °C, it takes 8.7 min to produce phase pure Cu0, while at 450 °C, the formation takes ∼0.7 min. Increasing the initial concentration of Cu2+ in the precursor causes formation of larger Cu0 crystallites in the final product. Finally, the in situ observations were used as guidance for making Cu0 using a supercritical flow setup.

There are many advantages to using supercritical carbon dioxide (ScCO2) fracturing technology to exploit shale gas reservoirs in China, including minimal damage to the environment or formation, and displacing methane (CH4) in the adsorbed state. When ScCO2 enters fractures in the formation, ScCO2-water-shale interactions may affect the physicochemical properties of shale. In this study, a high-pressure reaction system was adopted to simulate ScCO2-water-shale interactions under ScCO2 stimulation conditions. The element mobilization and pore structure before and after the reaction were measured using ICP-MS, XRF. The results show that the major elements, including Ca, Mg, Na, K, and Al, exhibit varying degrees of mobilization after the interactions because of dissolution of carbonate and silicate minerals in shale samples. Compared with the major elements, trace elements have a lower mobility, quantified as

Supercritical water gasification (SCWG) is a promising technology for converting organic wastes to hydrogen. Less amount of oxygen is beneficial for increasing hydrogen generation rate. However, the corrosion rate of reactor material would be accelerated. TiO2 coating with a thickness of 0.1 mm was prepared on the surface of 316L stainless steel (SS316L) to improve its corrosion resistance in supercritical water (SCW). The corrosion performances of TiO2/SS316L were tested in a bath SCW reactor at 400 °C, 25 MPa. The influences of oxygen concentration (0–1000 mg/L) on surface morphologies and corrosion depths were studied thoroughly. Results indicated that the surface of TiO2/SS316L exhibited cracks and pores after exposed in SCW. And the average corrosion rates accelerated at higher oxygen concentrations. The interface between the coating and medium was relatively smooth and there was no obvious change in the thickness of the coating with oxygen concentration of 0 and 500 mg/L. While for that with 1000 mg/L oxygen, the surface of TiO2/SS316L exhibited reticulate crack. The cross section showed a serrate structure, and only 0.08 mm thick of the coating was remained. In addition, the corrosion mechanism of coating was discussed.

Activity and stability of the supported Ni-based catalysts for the gasification performances of phenol solution and coal-gasification wastewater in supercritical water were studied in a continuous reactor at 480 °C, 25 MPa and oxygen ratio of 0.2 for 50 h operation. The influences of the supports (γ-Al2O3, active carbon (AC) and carbon nanotube (CNT)) on gas yields, gasification efficiencies for phenol solution were investigated, and the loading amount of Ni were optimized. Results showed that the catalytic activity and the stability of the catalysts followed the order of Ni/CNT > Ni/AC > Ni/γ-Al2O3. The activity of Ni/AC and Ni/γ-Al2O3 decreased after 30 h continuous operation, and there occurred significant leaching of Ni2+. For Ni/CNT catalyst, H2 yield increased obviously when the loading amount of Ni lower than 15 wt%, while increased little at higher loading amount. Then, 15 wt% Ni/CNT with a thickness of 1.5 mm was coated on 316 L stainless steel (SS316L, an economic material usually used as the reactor material), which can act as a "catalytic tube wall" in reactor. The catalytic activity and corrosion resistance of Ni/CNT/SS316L for the gasification of real coal-gasification wastewater were studied. Results showed that Ni/CNT/SS316L gave a great positive effect on H2 production. H2 yield increased from 25.36 mmol/g (total organic carbon) without catalyst to 75.12 mmol/g (total organic carbon) with Ni/CNT/SS316L after operated for 20 h, respectively. However, obvious pealing of the coating was found after 50 h operation. Further study is necessary for the improvement of the coating preparation method.

Supercritical water oxidation is a promising alternative for treatment of wastewater and sludge. The addition of auxiliary fuel can achieve the optimization of the system. Owing to the discrepancy ...

Organic materials, such as ion exchange resins, plastic, oils, and solvents, are widely used in the operation and decommission of nuclear facilities. The generated radioactive organic wastes are both radioactive and organic; therefore, the degradation of such wastes becomes more difficult. Due to delays in the disposal of radioactive organic wastes, potential safety risks are increasing. With the advantages of degrading refractory organics rapidly and thoroughly, supercritical water oxidation (SCWO) has become a potential alternative way to degrade radioactive organic wastes. This review focused on the degradation characteristics of different radioactive wastes from the perspective of potential practical applications. Some improved methods for facilitating the degradation of radioactive wastes by SCWO are considered and analyzed. Moreover, the kinetics and intermediate pathways of radioactive organic wastes are further analyzed. The distribution, migration and transformation of radionuclides during the SCWO reaction, as well as the further processing of radionuclides in gas-, liquid- and solid-phase products, were summarized and discussed. Furthermore, some fruitful areas for further work were reviewed for the highly efficient degradation of radioactive organic wastes. This review can provide useful information and guidance for the industrial applications of SCWO treatment for radioactive wastes.

Supercritical hydrothermal synthesis is a promising technology to produce nano-ZrO2 due to its simple craft, short reaction time and low cost. In this paper, nano-ZrO2 particles with an average particle size of 5.66 – 35.43 nm were successfully prepared by supercritical hydrothermal method. Meanwhile, the influences of reaction temperature, pressure, time, precursor concentration and pH value on the particle size distributions and surface topographies of nano-ZrO2 were investigated under different conditions. Reaction time and precursor concentration were strong influencing factors of particle sizes and distributions. The reaction mechanisms of different crystal forms were controlled by pH value of the precursor. Under acidic conditions, soluble [Zr4(OH)8(H2O)16] 8+ was produced due to the hydrolysis of precursors, and the appearance of abundant monoclinic nucleus was triggered subsequent condensation of [Zr4(OH)8(H2O)16] 8+. In-situ crystallization dominated the crystallization process and the tetragonal phase was formed by the rearrangement of the precursor structures. The intermediate product Zr(OH)3NO3 was detected under high pH conditions.

Supercritical hydrothermal combustion, a new and promising homogeneous combustion technology with a wide range of application scenarios and broad development prospects, provides creative ideas and means for the enhanced degradation of organic wastes, hydrothermal spallation drilling, thermal recovery of heavy oil, etc. This technology is elaborated upon in five parts: (1) introducing the main devices including semi-batch reactor and continuous reactor to study the hydrothermal flame in accordance with research institutions, (2) presenting the research status of related numerical simulation from the angles of reaction kinetics and flow-reaction, (3) summarizing the characteristics of hydrothermal flame and combustion by five key parameters, (4) dividing up ignition process and explaining ignition mechanism from the perspectives of critical physical properties of water and heat transfer and mixing conditions, (5) discussing and forecasting its industrial applications including hydrothermal spallation drilling, the thermal recovery of heavy oil, the clean conversion and utilization of coal-based fuel, and the harmless treatment of pollutants. By and large, this paper analyzed in detail everything from experimental equipment to industrial applications, from combustion characteristics to ignition mechanisms, and from summary conclusions to prospect prediction. In the end, herein is summarized a couple of existing paramount scientific and technical obstacles in hydrothermal combustion. Further significant studies in the future should include excellent reactors, advanced monitoring techniques, and powerful computational fluid dynamics.

The solutions of phenol and the mixture of phenol and ammonia were gasified in a continuous supercritical water reactor to evaluate the influences of ammonia on the hydrogen production and the degradation pathways of phenol. The effects of temperature (560–640 °C), reaction time (2–20 s) and concentration of ammonia (500–2000 mg/L) on gaseous distributions, gasification efficiencies and reactants removal efficiencies were investigated. In addition, the effects of ammonia on the kinds of intermediate products were analyzed. Results showed that the increasing temperature greatly promoted the hydrogen production and degradation of phenol. Longer reaction time gave a positive effect on gasification efficiencies in 10 s, while the effect was little when the reaction time longer than 10 s. Ammonia prevented the production of hydrogen and the degradation of phenol, which was mainly due to the generation of more stable nitrogen polycyclic compounds. The possible degradation pathways for the mixture of phenol and ammonia were proposed.

As one of the main unconventional gas reservoirs, the exploration and exploitation of shale gas reservoirs has been a hot spot of global resources development. The traditional water-based fracturing fluid with the disadvantages of large water consumption and serious formation damage does not apply to shale gas reservoirs stimulation. Supercritical CO 2 fracturing technology, using harmlessness CO 2 for sand-carrying fluid, was brought forward to apply to development of shale gas reservoirs, with the advantages of no water consumption, no damage to formation, contributing to CH 4 desorption, etc. When applying supercritical CO 2 as the fracturing fluid, in fractures supercritical CO 2 interacts with shale in the presence of water. The study on the interactions of shales and supercritical CO 2 in the presence of water is a basic issues in stimulation of supercritical carbon dioxide within shale reservoirs. The objective of this study is to solve the key problems which the interactions of shale, supercritical CO 2 and water involves, such as mineral elements migration, microscopic pore structure and so on.

Early oxidation characteristics of ferritic-martensitic steel T91 in supercritical water (SCW) from 1 h to 120 h were investigated. Relative to austenitic steels, occurrence of each oxidation stage for T91 was delayed. Both inner Cr-rich layer and outer magnetite layer formed even after a 1h-exposure at 540 °C, implying their almost simultaneous formations. Several assembled-pore lines were observed in the inner layer. Abundant pores accumulated at the boundary between the inner/outer oxide layers in steam, while mainly along the inner/diffusion layer interface in SCW, due to lower difference between iron diffusion through the inner and outer layers for the later.

In this study, the oxidation behavior of Ni-based alloys 625 and 825 in supercritical water containing 5000 ppm of dissolved oxygen at 500 °C was investigated by scanning electron microscopy coupled with energy-dispersive spectroscopy, X-ray diffraction, and laser Raman spectroscopy. Alloy 625 and 825 showed a pitting-like phenomenon after 500 h of exposure. The structure and composition of the oxides formed on alloy 625 and 825 were similar, namely, an outer layer of Fe oxides, spinel structures, and Ni(OH) 2 ; and an inner NiO layer. The mechanism for the formation of the oxide films was proposed, and the factors affecting corrosion of Ni-based alloys were discussed in detail. In addition, the corrosion resistance performance of alloy 625 and 825 was compared.

A novel supercritical water system was proposed to treat sewage sludge harmlessly in this work, with its core to produce syngas by supercritical water gasification (SCWG) of sewage sludge and dispose of the residual liquid organics by supercritical water oxidation (SCWO) for heat release and harmless disposal. Moreover, a cool wall reactor was employed to solve corrosion and salt deposition, while the reaction heat was recovered for power generation, and the heating exchange net was optimized. The effects of SCWG temperature (400–550 °C), moisture content of sewage sludge (87–95 wt%), pressure (23–29 MPa), and oxidation coefficient (0–0.5) on the characteristics of the SCWG-SCWO combined system, and the influence of heating method for supercritical water on the exergy destruction and system efficiency, were investigated and discussed. The results showed that an increase in SCWG temperature, the decrease in moisture content and oxidation coefficient, except for the effect of pressure, could significantly improve the net exergy efficiency of the system. Under the condition of SCWG temperature being 450 °C, moisture content of 87 wt%, 25 MPa and oxidation coefficient being 0, the electricity self-sufficient rate, cold gas efficiency, exergy efficiency and net exergy efficiency were 18.08%, 13.09%, 15.94%, 15.41% and 10.06%, respectively. Inputting external energy could tremendously impact the whole system. When the temperature of heat source for heating water was higher than the reaction temperature of SCWO, the net exergy efficiency of the system could reach 27.73%. The thermodynamic efficiency of the SCWG-SCWO combined system outperforms the single SCWO case. This study provided an industrial basis for the hierarchical treatment of sewage sludge, and offered a new direction for pollutant treatment by the combined supercritical water technologies.

In this work, Fe-Ni-based alloy HR6W, Ni-based alloys Haynes 282 and Inconel 740 were investigated in supercritical water at 450 °C and 25 MPa for 30–200 h. HR6W, Haynes 282 and Inconel 740 all showed corrosion weight gain in the whole exposure duration and the oxidation kinetics follow the parabolic law. HR6W and Inconel 740 samples displayed lower corrosion weight gain and better corrosion resistance under experimental conditions. A layer composed of network-shaped oxides was formed on the surface of Inconel 740 sample. The height of the oxide particles on Haynes 282 sample surface is not consistent enough after exposure in supercritical water for 200 h, and there was large amount of vacancies among oxide clusters, which increases the risk of substrate destruction. The dominant phases on the surface of HR6W, Haynes 282 and Inconel 740 samples were [FeCr2O4, NiFe2O4], [Fe3O4, NiFe2O4 and Cr2O3], and [Fe2O3, Fe3O4], respectively.

The initial corrosion behaviour of austenitic TP347H exposed to supercritical water was investigated. The early oxidation can be categorized into three stages (rapid oxidation, transitory, and diffusion-controlled stages). Higher temperature resulted in a transformation towards the subsequent stage earlier and less oxidation resistance. Cr6+ species (likely CrO2(OH)2) were only generated on the surface in the very early phase, and then they gradually disappeared as the exposure time increased. Compared with a complete outer Fe-rich oxide layer, the formation priority of a continuous inner Cr-rich layer was observed at 540 °C within 35 h. A corresponding early oxidation process was determined.

Al2O3 and TiO2 coatings were fabricated on 316L stainless steel by atmospheric plasma spraying to improve the corrosion resistance of 316L stainless steel in supercritical water. The corrosion characteristics of the samples were evaluated in a batch reactor at 500 °C and 25 MPa with an oxygen concentration of 1000 mg/L for 80 h. The adhesive strengths of the coated samples were tested, and the weight changes, morphologies and elements distributions of the fresh and corroded samples were analyzed. Results showed that the bond strength of TiO2/316L was 1.5 times than that of Al2O3/316L (26.639 N/mm2). The surface morphology of Al2O3/316L showed gully erosion with much pores and cracks after exposed in SCW, which provided channels for oxygen and SCW to get into the substrate and also the elements in substrate to diffuse to the surface of the coating. The corroded Al2O3/316L suffered significant weight loss, and most of the coatings were peeled off. However, the surface morphology of TiO2/316L was relatively dense and the thickness of the coating was not found to decrease obviously.

A new -Mn-Cu- mixture catalyst with components of 50.9 wt% CuMn 2 O 4 , 46.1 wt% CuO and 3.0 wt% Mn 2 O 3 was made by a co-precipitation method firstly. We adopted the multi-component catalyst together with activated carbon (AC) and Na 2 CO 3 (N) for the first time to conduct supercritical water partial oxidative gasification (SWPO) of sewage sludge at 450 °C, 25 MPa, 0.3 of oxidation coefficient and 5 h of residence time, so as to improve H 2 production and organics removal. The results show that MnO 2 -AC-N, CuO-AC-N and -Mn-Cu-AC-N could obviously improve H 2 yield and removal efficiency of organics in sewage sludge SWPO. At -Mn-Cu-AC-N conditions, H 2 yield and X COD (removal efficiency of chemical oxygen demand) were 202.4 mL/L and 95.6 wt%, much higher than 48.7 mL/L and 86.2 wt% obtained without catalyst, respectively. The -Mn-Cu-AC-N catalyst seemingly had the best catalytic effect on gas formation as well as on organics removal in sewage sludge SWPO among MnO 2 -AC-N, CuO-AC-N and -Mn-Cu-AC-N. Potential catalytic reaction pathways were proposed as well.

The oxidation behaviors of alloy C-276 in two harsh environments: high-temperature air and supercritical water (SCW), respectively representing the working conditions of the external and internal surfaces of reactors for SCW gasification biomass to produce H2, were investigated. In two environments, all oxidation kinetics followed parabolic laws, while the corrosion rate of alloy C-276 exposed to supercritical water gasification (SCWG) environments was 2.5–3 times higher than that in high-temperature air. The oxide scale formed in air at 500 °C consisted of an outer Fe-rich layer (Fe2O3 and NiCr2O4) and an inner layer of Cr2O3 and NiCr2O4, while the outer Fe-rich layer disappeared as the temperature increased to 550 °C. Compared to the scales formed on nickel-base alloys in near-pure SCW, the absence of NiO and Ni(OH)2 phases within the scales formed on the C-276 samples in present SCWG environment may be due to higher molar proportion of hydrogen.

A quantitative kinetic model for the supercritical water partial oxidation of the mixture of acetic acid, naphthalene and phenol at 560 °C, 25 MPa was established. The model consisted a group of pathways that included two kind of compounds, which could be classified as stable cyclic compound (Int.1) and unstable ring-opening products (Int.2). The model was validated with the experimental data at oxygen ratio (OR) of 0 and 0.2, respectively. Results showed that the model could accurately predict the influences of reactants time on gas yields and intermediates concentrations. Reaction rate analysis indicated that the general trends of reaction rate at OR = 0 and OR = 0.2 were similar. The decomposition and steam reforming reactions of Int.2 were the main pathways for gas production. The oxygen during the gasification played a positive role in promoting the ring-opening reactions, which induced higher Int.2 production and increasing of steam reforming and decomposition rates, and finally increased the H2 production.

Supercritical hydrothermal synthesis is a green synthesis method for metal and metal oxide ultra-fine particles. Ultra-fine copper particles are of great interests for the researchers because of the excellent performance in recent years. In this paper, supercritical hydrothermal synthesis of copper ultra-fine particles with three different precursors (CuSO4, Cu(NO3)2, Cu(HCOO)2) are reported. This thesis reports that different products are produced with different precursors. Also, three kinds of reaction mechanisms with different precursors in supercritical water were explained. The conversion of copper ions in the reaction of Cu(HCOO)2 in supercritical water is the highest, the value reaches 100.0%. In the process of synthesizing ultra-fine copper particles, different additional HCOOH concentrations (0, 0.1 mol/L, 0.2 mol/L) and different reaction times (5 mins, 10 mins) were applied. Zero-valent ultra-fine copper particles without impurity were synthesized. The synthesized copper ultra-fine particles were cubic aggregations with micro-meter size

Copper(II) oxide ultrafine particles are of great interest as a new material for multiple applications. This paper expounds the synthesis and characterization of copper(II) oxide submicrometer particles formed by a facile and simple supercritical hydrothermal synthesis method. The copper(II) nitrate, copper salt, was used as a precursor aqueous solution heated by a preheated sand bath to reach supercritical conditions. The effects of process operating parameters, such as temperature, pressure, the addition of sodium hydroxide, and precursor concentrations on the morphology and the size of copper(II) oxide submicrometer particles have been investigated. The copper(II) oxide particles formed, with particle sizes of ca. 100 nm, were hexagon-flake-like and spindle-like and free of impurities. The average particle size decreased with the increase of temperature under subcritical conditions and decrease of pressure. It decreased with the increase of precursor concentration at lower concentration conditions and ...

Supercritical water treatment is a promising technology for sewage sludge disposal. The influences of oxidation coefficient (OC) as well as temperature on products of sewage sludge treatment in supercritical water were studied systematically. The results show that CO2 yield obviously increased with raising OC or temperature. Oxidant helped to convert N-containing compounds into N2. A small amount of oxidant increased H2 yield but excess oxidant decreased it. OC had significant effect on the COD (chemical oxygen demand) and NH3-N (ammonia nitrogen) concentration of liquid products. Most of organic compounds disappeared at OC=1.0 as temperature changed from 450 to 500, and diethyl phthalate was main stubborn substance in the liquid phase. The contents of some elements (such as Si, Al and Ca) in solid residue was almost double compared with those in dry basis SS.

In this study, the corrosion behavior of nickel-based alloys 625, 825, and 800 with a salt deposit layer was investigated in supercritical water containing a dissolved oxygen of 5,000 ppm a...

Energy, especially electricity, plays a vital role in our modern industrial society and in our current civilization. In order to increase the power generation efficiency of fossil fuels, supercritical and ultra-supercritical power plants, using supercritical water (SCW) as the working medium, have been developed over the past century [1].

Sewage sludge (SS) with a moisture content of about 80 wt% is a byproduct of wastewater treatment. With the development of the urbanization process, this amount has been increasing with the construction and expansion of wastewater treatment plants. Containing complex organic matters as well as heavy metal and viruses, SS is not only a pollutant but also a wet resource.

Above the critical point (T = 374.15 °C, P = 22.1 MPa), water exists as a single phase and has analogous transportation properties of gas and solvent properties like liquid, which is known as supercritical water (SCW) (Kutney in thermodynamic and transport property modeling in super critical water. Massachusetts Institute of Technology, 2005 [1]; Shaw et al. in Chem. Eng. News 69:26–39, 1991 [2]). SCW is a non-polar solvent that can be completely miscible with organic compounds and gases such as hydrogen, nitrogen, and oxygen.

Today’s commercial operation of power systems, such as Supercritical Generating Unit and IGCC, presents some congenital problems [1]. First, both the investment and operation cost of the desulfurization and denitration equipment are very expensive. Second, the carbon capture is not only inefficient but also leads to the power generation efficiency reduce exceeding 8%.

Supercritical water (SCW: T > 374.15 °C, P > 22.1 MPa) possesses specially physical and chemical properties such as high diffusivity, low viscosity, low dielectric constant, and a small amount of hydrogen bonds. Organic matter and oxygen can be dissolved in SCW with any ratio under high enough pressure condition while the solubility of inorganic salt in SCW is extremely low. Therefore, these properties are beneficial to reduce resistance of mass and heat transfer and to separate salts from SCW. As an attractive media for regulating chemical reaction process and performing salt separation process, SCW has attracted more attentions since the early 1980s. The definition of supercritical water oxidation (SCWO) is that the organic matter is oxidized into harmlessly small molecular compounds such as carbon dioxide, nitrogen, water, and inorganic salt by excess oxidant in SCW. Supercritical water gasification (SCWG) means that organic matter undergoes hydrolysis, pyrolysis reaction and so on in SCW to form a hydrogen-rich mixing gas.

Nanomaterials are always at the leading emerging area of material and nanotechnology. Especially in this age, the ever-increasing population in the world makes energy, chemicals and other resource efficiency particularly important in sustaining concept [1]. The significant reduction in size (1–100 nm) often improves their electronic and other properties due to quantum confinement effects.

Two major points in supercritical hydrothermal combustion were reviewed:(1) The structure of semi-batch reactors or continuous reactors used in different institutes and colleges. These investigations can be used to guide the design of reactors for later scholars and lay the foundation for the industrialization of supercritical hydrothermal combustion. (2) The research status of characterization of hydrothermal flame processes by various scholars. These investigations can be used to guide the process parameters of industrialization of supercritical hydrothermal combustion. The continuous reactor designed in each organization is very sophisticated, which can avoid the two major problems of reaction in the supercritical state: salt precipitation and corrosion. The ignition temperature, extinction temperature, and other characteristics of supercritical hydrothermal combustion studied by scholars are summarized and the laws are basically similar. The removal rate of different organic matters was also summarized under supercritical hydrothermal combustion, and the removal rate of more than 99% was basically achieved.

The harsh work environments caused by high-temperature, high-pressure water, especially the presence of aggressive species such as oxygen and salts, makes equipment corrosion to be a key problem restricting large-scale industrial application of the supercritical water technologies. In this work, the corrosion characteristics of Ni-Fe-Cr and Ni-Fe-Cr-Mo-Cu corrosion-resistant alloys, such as the corrosion rate, the structure and composition of oxide film, and the effects of various surface treatment processes, are discussed in detail. The corrosion mechanisms of typical Ni-based alloys in supercritical water are analyzed. Inconel 625 and Hastelloy C-276 possess good corrosion resistance over a wide range of test environments, in terms of current investigation data regarding Ni-based alloys. The outer layer is formed by a metal passive dissolution-oxide precipitation mechanism, and the precipitated metal ions are released from the barrier layer. The looseness and discontinuity of the outer oxide layer can be attributed to the decomposition of oxides in the layer, the scouring of external fluids, or growth via electro-crystallization. This information is valuable for theoretically guiding material selection and design and operating parameter optimization of key equipment in the supercritical water technologies.

Conventional physical, biochemical, and incineration treatments of hypersaline organic wastewater have some major drawbacks, such as the incomplete removal of organic matter, severe secondary pollution, increased salinity, or massive salt-out problems. Supercritical water oxidation (SCWO) is an advanced oxidation technology for treating organic wastewater, which has been in development for decades; however, a new technology that combines SCWO with supercritical water desalination (SCWD) has been proposed, which is uniquely advantageous for treating hypersaline and refractory organic wastewater because it causes no secondary pollution and exhibits a high removal rate, fast reaction speed, and effective desalination. To further improve this new combination method, the behavior characteristics and mechanisms of inorganic salts in sub-/supercritical water require significant attention. In this review, the authors evaluate the conventional technologies for treating wastewater, introduce the research and development of SCWO and SCWD processes, and then emphatically analyze the behavior and mechanisms of inorganic salts in sub-/supercritical water, such as their phase equilibrium, dissolution, crystallization, and deposition. Finally, the contents, methods, conclusions, and shortcomings of previous studies are systematically and objectively summarized, and a preliminary theory of salt behavior and future research topics are proposed.

The early oxidation of Super304H stainless steel exposed to supercritical water at 450 °C, 540 °C, and 580 °C was characterized. After a 40 h exposure, distribution density of oxide grains produced on samples increased with experimental pressures, while initial aqueous pH had a negligible influence on short-term corrosion morphologies, implying that the outer oxide layer of scales may form via solid growth mechanism rather than by metals dissociation/oxides precipitation process. After a repeated 24 h cycles of exposure at condition of 540 °C/25 MPa followed by that of 500 °C/23 MPa up to 5 cycles, partial spallation of the scales appeared along inner and outer layer interface, not substrate/scales interface due to the enhanced adhesion of scales on the substrate by pinning effects and production and convergence of cavities at the inner/outer layer interface.

With increasing construction of wastewater treatment plants and stricter policies, municipal sewage sludge (MSS) disposal has become a serious problem. Treatment of MSS in supercritical water (SCW) can avoid the pre-drying procedure and secondary pollution of conventional methods. SCW treatment methods can be divided into supercritical water gasification (SCWG), supercritical water partial oxidation (SCWPO) and supercritical water oxidation (SCWO) technologies with increasing amounts of oxidants. Hydrogen-rich gases can be generated from MSS by SCWG or SCWPO technology using oxidants less than stoichiometric ratio while organic compounds can be completely degraded by SCWO technology with using an oxidant excess. For SCWG and SCWPO technologies, this paper reviews the influences of different process variables (MSS properties, moisture content, temperature, oxidant amount and catalysts) on the production of gases. For SCWO technology, this paper reviews research regarding the removal of organics with or without hydrothermal flames and the changes in heavy metal speciation and risk. Finally, typical systems for handling MSS are summarized and research needs and challenges are proposed.

A mixture of phenol, acetic acid, and naphthalene was partially oxidized in supercritical water in order to produce hydrogen. The effects of temperature, oxidation ratio (OR), reaction time, and reactant concentrations on gaseous distributions, gasification efficiencies, and reactants removal efficiencies were investigated. Furthermore, the effects of oxygen on the main intermediate products were analyzed, and possible degradation pathways were proposed. Results indicated that higher temperatures significantly promoted the H 2 yield. In addition, small amounts of oxygen (OR 2 and CH 4 . The maximum H 2 gasification efficiency (240.25%) and H 2 yield (70.16 mmol g −1 ) were obtained at 560 °C, 25 MPa, reaction time of 20 s and OR of 0.2. Longer reaction time enhanced the gasification efficiencies in 10 s, while the effect was little when reaction times longer than 10 s. The concentrations of the reactants did not significantly influence the gasification efficiencies.