Supercritical Water Environment Research Articles

Heterogeneous Yielding Mechanisms of Body Center Cubic Iron for High Resistance to Chemical Reaction-Induced Deterioration in Supercritical Water Environments: A Reactive Molecular Dynamics Study

The atomic-scale yielding mechanisms of body center cubic (BCC) iron in supercritical water have been an elusive problem that requires understanding the roles of chemical reactions with supercritical water in the mechanical behavior of iron. This work shows the combined effect of the supercritical water and tensile direction on the yielding mechanism of BCC iron using reactive molecular dynamics simulations. Our simulation results show that tensile strain along the [110] direction of BCC iron may potentially exhibit much higher tensile strength and lower sensitivity to the environment compared with other directions. This is because yielding of iron along the [110] direction originates from the homogenous generation of HCP precursors inside the iron bulk rather than at the surface, which limits the effects of surficial chemical reactions with supercritical water on the yielding behavior. This work is expected to contribute to the theoretical design of high-strength alloys in supercritical water.

Deposition of iron oxides in supercritical water reactor: A review

Low-temperature corrosion of the system and external input make it difficult to avoid the entering Fe ions in the supercritical reactor. Fe ions crystallize into particles in supercritical water and deposit on the surface of the reactor, causing safety threats and economic losses. In this paper, research on the Fe ion deposition process, from crystallization, to oxide particle formulation and to completely adhering to the wall surface, is reviewed. The literature mainly includes theoretical and experimental research on the nucleation and growth process of Fe oxide particles in a supercritical water (SCW) environment, particle movement and experimental observation methods in trans-critical turbulence, and the effects of Derjaguin-Landau-Verwey-Overbeek (DLVO) and collisions considered in the process of particle attachment. The particle deposition process involves particle formation, particle movement and surface adhesion. These three dynamic processes have cross-influencing factors that include geometric factors (the size of the particles themselves, the roughness of the wall), hydrodynamic factors (turbulence and transcritical conditions), and environmental parameters (temperature, pH). Independent research on dynamic processes is no longer suitable. The review aims to clarify the mechanism of three dynamic processes and their roles in the particle deposition process so that they can be comprehensively considered in the study of particle deposition in actual operating systems in the future.

An overview of hydrogen-rich gas production from biomass by using thermal technologies

Studies on the production of hydrogen energy are at the forefront of reducing the dependence on fossil fuel energy. This work is a brief summary of hydrogen production systems especially in a supercritical water environment. Hydrogen production by the gasification of organic wastes in a supercritical water environment has been preferred more than other methods in recent years. In this study, the common biomass resources used for the hydrogen production have been explored, evaluating their properties. Moreover, catalysts which are used for the gasification of biomass have been presented in-details. Therefore, a review of hydrogen production system by supercritical water gasification is summarized with the remarkable features.

A molecular simulation study on solvation free energy and structural properties of polycyclic aromatic hydrocarbons in supercritical water environment

Molecular simulations have been successfully applied for studying the kinetic properties of supercritical fluids, extremely in exploring the microscopic mechanisms of high-pressure and high-temperature fluids. Molecular simulation is an efficient and convenient way to investigate complex molecular systems with supercritical water. Polycyclic aromatic hydrocarbons (PAHs) are typical coal chemical pollutants. Molecular simulations using PAH model compounds were carried out to explore the effects of supercritical water on solvation free energies and structural properties. The water density was ranging from 0.1 g/cm3 to 0.3 g/cm3 and temperature was increased from 380 °C to 440 °C. In terms of PAHs in this study, the simulations and calculations revealed that the solvation free energy remained negative and simultaneously varied with water density and temperature. This work is expected to provide a theoretical support for understanding the dynamic, structural properties and the solvation properties of different aromatic hydrocarbons in supercritical water environment.

Oxidation Behavior of Nickel Based Alloys in Supercritical Water Environment

Corrosion resistance is a key performance factor for engineering materials used in the supercritical water-cooled reactors (SCWR). Choosing the materials with the best characteristics in the corrosive environment for SCWR, at high temperatures and pressures, is a real challenge for many researchers. Therefore, this paper aims to test two nickel-based alloys (Incoloy 800HT and Inconel 718) and to analyze the microstructures of the samples following exposure in a supercritical environment. Oxidation tests were performed at high temperature and pressure (550oC, 25MPa) for up to 70 days. The samples were investigated using gravimetric corrosion test and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX). The results obtained can be used in future research of the test protocol to identify alloys that could be used for SCWR components.

Heavy Oil Hydrocarbons and Kerogen Destruction of Carbonate–Siliceous Domanic Shale Rock in Sub- and Supercritical Water

This paper discusses the results of the influences of subcritical (T = 320 °C; P = 17 MPa) and supercritical water (T = 374 °C; P = 24.6 MPa) on the yield and composition of oil hydrocarbons generated from carbonaceous–siliceous Domanic shale rocks with total organic content (Corg) of 7.07%. It was revealed that the treatment of the given shale rock in sub- and supercritical water environments resulted in the decrease of oil content due to the intensive gas formation. The content of light hydrocarbon fractions (saturated and aromatic hydrocarbons) increased at 320 °C from 33.98 to 39.63%, while at 374 °C to 48.24%. Moreover, the content of resins decreased by almost twice. Insoluble coke-like compounds such as carbene–carboids were formed due to decomposition of kerogen after supercritical water treatment. Analysis of oil hydrocarbons with FTIR method revealed a significant number of oxygen-containing compounds, which are the hydrogenolysis products of structural fragments formed after destruction of kerogen and high-molecular components of oil. The gas chromatography–mass spectroscopy (GC–MS) method was applied to present the changes in the composition of mono- and dibenzothiophenes, which indicate conversion of heavy components into lighter aromatic hydrocarbons. The specific features of transforming trace elements in rock samples, asphaltenes, and carbene–carboids were observed by using the isotopic mass-spectrometry method.

Demetallization of Heavy Oils of High Metal Content through Pyrolysis under Supercritical Water Environment

Through pyrolysis under a supercritical water (SCW) environment, the demetallization of Tsingtao vacuum residuum (VR) and Tahe atmospheric residuum (AR) with a total vanadium and nickel content above 300 ppm was studied. By introducing SCW, pyrolysis of heavy oil can be carried out in an emulsion structure in which heavy fractions rich in metals are concentrated in oil droplets. A metal removal rate higher than 90 % could be obtained in 2 h, which is faster than the pyrolysis under the nitrogen environment. Although two heavy oils have similar apparent properties, Tsingtao VR is characterized by the metal enrichment in the heaviest asphaltenes and the higher condensation activity of asphaltenes. Because of the preferential condensation of heavy fractions rich in metals, the liquid product yield of Tsingtao VR could be 15 wt% higher than that of Tahe AR at the metal removal rate of 70 to 90%.

Effects of Si, Mn on the corrosion behavior of ferritic–martensitic steels in supercritical water (SCW) environments

Ferritic–martensitic (F/M) steels with various amounts of alloying elements (Cr: 12 wt. %, Si: 0.6–2.2 wt. %, Mn: 0.6–1.8 wt. %) were exposed to supercritical water (SCW) environments at 500 °C and 25 MPa for up to 1000 h. Gravimetric, Scanning Electron Microscope, Transmission Electron Microscopy (TEM), Energy-dispersive X-ray Spectroscopy (EDX) and X-ray Diffraction (XRD) analyses were conducted to characterize the corrosion behavior of F/M steels in supercritical water. The results showed that the contents of the alloying elements (Si, Mn) affected both the oxidation kinetics and the oxidation scales formed on F/M steels. The F/M steel (Si: 0.6 wt. %, Mn: 1.8 wt.%) showed the highest corrosion rate in SCW with an intermittent oxide scale formed on the surface. Increasing the Si content in F/M steel improved the oxidation resistance and promoted the formation of a uniform oxide scale. The mechanism associated with the corrosion of these F/M steels under SCW conditions is discussed.

Effect of intergranular carbides on the cracking behavior of cold worked alloy 690 in subcritical and supercritical water

The effect of intergranular (IG) carbides on the cracking behavior of cold worked Alloy 690 was studied in both subcritical and supercritical water (SCW) environments. The crack growth rate (CGR) was lower when IG carbides were removed by prior solution annealed (SA) treatment, indicating a detrimental effect of IG carbides. The presence of IG carbides enhances the local strain accumulation at the grain boundary due to the lattice mismatch, thus promotes the crack tip strain rate and increases the CGR. The detrimental effect of IG carbides is more significant in subcritical water than in SCW, which can be attributed to the different controlling parameters of cracking between these two environments. The widely recognized beneficial effect of IG carbides on stress corrosion cracking in Alloy 600 is detrimental in cold worked Alloy 690, which might be related to the different degrees of grain boundary oxidation that contributed to IG cracking.

Effect of salt deposit on corrosion behavior of Ni-based alloys and stainless steels in supercritical water

The under-deposit corrosion behavior of Ni-based alloys 625, 825 and Fe-based alloy 800 in supercritical water (SCW) at 400 °C and 500 °C was investigated. Newly designed experimental apparatus was used to ensure that the surface of alloy specimens was covered by NaCl or KCl deposit layers during the corrosion test in SCW environment. Based on the cross-sectional observation and surface chemical analyses, the oxide film on the samples after exposure contains an outer layer containing Fe-Ni-Cr spinel and Fe3O4, and an inner layer of Cr2O3. NiCl2 was also found on the alloy surface. At 400 °C, hydrolysis of the metallic ions occurred, forming HCl with chloride ions. At 500 °C, the oxides react with the solid-phase salt deposit via the formation and transformation of the HCl vapor. The salt deposit layer acted as a barrier for the diffusion of SCW and HCl vapor at 500 °C.

Aquathermolysis of heavy oil in the presence of supercritical water

The purpose of the work is to identify patterns of transformation of high-molecular components of super-heavy oil by hydrothermal-catalytic processes, namely in the presence of a supercritical state of water. The regularities of changes of the component, structural-group, fractional and elemental compositions of heavy oil during the conversion under the above conditions were established, rheological characteristics of the initial crude oil and converted oil were studied as well. As a result of carrying out aquathermolysis in the supercritical water environment and in the presence of carbonaceous substances, the high-molecular weight components of the initial crude oil were degraded with the formation of light distillate fractions, which were scarcely present in the initial crude oil.

Characterizing the effects of in-situ sensitization on stress corrosion cracking of austenitic steels in supercritical water

This work compared the stress corrosion cracking (SCC) susceptibility of two austenitic alloys in supercritical water (SCW), and revealed the quantitative difference of SCC growth rates through scanning transmission electron microscopy (STEM). Results showed that in-situ sensitization and creep occurred on the specimens during the SCC tests in SCW environment. Higher degree of Cr-depletion at the grain boundary ahead of the SCC crack tip was identified in the Fe-25Cr steel specimen, which has suffered severer in-situ sensitization because of the lower Cr content than the Ni-29.3Cr alloy specimen, and this in turn increased the creep susceptibility.

Effect of maximum stress intensity factor, loading mode, and temperature on corrosion fatigue cracking behavior of Inconel 617 in supercritical water

Inconel 617 is a candidate material to be used as a structural material in ultra-supercritical power plants. In such an extreme corrosive medium environment, metallic materials are susceptible to corrosion fatigue. In the present study, the aim is to evaluate the effect of maximum stress intensity factor, loading mode and temperature on the corrosion fatigue cracking behavior of Inconel 617 in a deaerated (<10 ppb dissolved oxygen) supercritical water environment. Water pressure was kept constant at 25 MPa while its temperature was maintained at 600, 620 and 650 °C in the applied test conditions. For the selected parameters, microscopic investigations and detailed analysis affirmed corrosion-fatigue interaction to be leading factor for change in fatigue crack growth rate. The crack propagation mechanism for the selected parameters in the applied test conditions is further discussed.

Catalytic oxidative coupling of methane in supercritical water: Investigations on a catalytically active species

Catalyst survey for oxidative coupling of methane (OCM) under supercritical water (SCW) environment at 658 K and 26 MPa using an 8.8-mL batch reactor and H2O2 as a molecular oxygen source was performed at a feed concentration molar ratios of 0.10 CH4/H2O and 0.15 O2/CH4. First and second transition metal oxides (V, Cr, Co, Cu, Zr and Mo) and some lanthanide oxides (La and Ce) showed relatively high OCM activities of ca. 0.5–1 % C yield at a very low catalyst loading of 0.05 g. Exploration of catalyst formulations found that oxides of transition metals combined with ZrO2 were effective, with MnO2–ZrO2 showing a high synergy effect giving almost twice as much methane coupling yield. The OCM activity was correlated with the surface amount of a low 1s binding energy (O1s, 526–527 eV) oxygen species on the catalyst, as well as its energy separation from the lattice oxygen. In terms of this oxygen species, SCW-OCM activity of Mn/Na2WO4/SiO2, known as a good gas phase OCM catalyst and also showed a high turnover efficiency for SCW-OCM was further improved by supporting Mn/Na2WO4 on ZrO2 instead of SiO2.

Evidence of heterogeneous catalytic activity of ZSM-5 in supercritical water for dodecane cracking

ZSM-5, HY, and β zeolites were tested for cracking supercritical dodecane (SCD) in the presence and absence of supercritical water (SCW). A key experimental advantage of operating at supercritical conditions is that SCW and SCD are completely miscible, eliminating uncertainty introduced by liquid-liquid mass transport. As expected, all three zeolites were active in the absence of SCW, resulting in 70–80% dodecane conversion after 2 h at 400 °C. In the presence of SCW, only ZSM-5 retained activity, with a measured dodecane conversion of approximately 35%. Post-reaction characterization of the zeolites indicated that only ZSM-5 retained crystallinity, suggesting that access to acid sites decreases when the Y and β zeolites decrystallize. Although ZSM-5 retained crystallinity after exposure to the SCD/SCW reaction mixture, its BAS density decreased by >95% and its micropore area decreased by 80%. Interestingly, while the SCW environment decreases ZSM-5 activity, the reduction is not consistent with the magnitude of the reduction in BAS or micropore area. The apparent contradiction suggested that some material other than ZSM-5 may play the role of catalyst and extensive tests were performed to verify that acid sites on ZSM-5 remained catalytically active in the presence of SCW. Catalyst reuse experiments indicated that ZSM-5 can be reused in SCW. Additional tests with Na-ZSM-5, silicalite, alumina, coke, silicic acid, Al(NO3)3, HNO3, and the supernatant liquid recovered after reaction failed to identify any material other than ZSM-5 that could act as a catalyst. Therefore, ZSM-5 must act as an acid catalyst, even in the presence of SCW and despite loss of many of its acid sites. In contrast, HY and β do not. This work helps guide future work on the use of zeolites in the presence of liquid water and dense supercritical water phases.

Corrosion behavior of pre-oxidized HR-224 superalloy in supercritical water environment at 700 °C

Corrosion of cladding materials in supercritical water (SCW) environment is a key reliability issue for the safety of nuclear power plant. In this study, we propose to use the pre-oxidized treatment to provide better corrosion resistance of cladding materials in the SCW environment. The nickel-based superalloy HR-224 was first pre-oxidized in flowing air at 982 °C for 100 h to establish a continuous and dense scales and then exposed in the SCW environment with high oxygen content (8.3 ppm) at 700 °C and 24.8 MPa for cyclic oxidation testing up to 1300 h. The pre-oxidized samples exhibit better corrosion resistance than as-received samples in the SCW environment. After pre-oxidizing process, triple scales, Ni(Cr, Fe)2O4 spinel/Cr2O3/α-Al2O3 were observed, and the growth of inner α-Al2O3 layer is predominant in the SCW environment. The α-Al2O3 layer in pre-oxidized samples was found to effectively decrease outward migration of metal ions in Alloy HR-224 and thus provides better corrosion resistance than as-received samples in SCW environment.

Catalyst Oxidation and Dissolution in Supercritical Water

We use thermodynamic models to predict catalyst oxidation and dissolution in supercritical water (SCW) and use experiments to assess the viability of the models for practical SCW reaction systems and provide relative rates for these mechanisms. We examined the oxidation and dissolution of noble and transition metals, metal oxide catalyst supports, and transition metal carbides and nitrides under SCW conditions. The materials were tested in batch reactors at 400 °C for 60 min, and the SCW density was varied from 0 to 0.5 g/mL to observe the influence of the solvent properties on stability. Oxidation and dissolution were determined by comparing the initial catalyst composition and structure with those of the catalysts recovered from the reactors after exposure to the SCW environment. The gas-phase recovered from the reactors was analyzed for H2 produced from oxidation. The aqueous phase was analyzed for metals from dissolution. The ΔGrxn for oxidation and the solubility of the catalysts in SCW at the experi...

Visbreaking of Heavy Oil under Supercritical Water Environment

The visbreaking of heavy oil under high-pressure N2 or supercritical water (SCW) environment was experimentally investigated. Despite the difference in the reaction media, the visbreaking follows the same mechanism, that is, dealkylation and condensation of aromatics. The presence of SCW makes it possible that the visbreaking of heavy oil is transferred to the SCW phase with superior diffusivity by which the visbreaking tends to be controlled by intrinsic reaction kinetics rather than by diffusion. Accordingly, dealkylation occurring in the SCW phase, which is vital to the viscosity reduction of heavy oil, responds sensitively to the increase in reaction temperature. Being the secondary reaction of dealkylation at moderate temperatures, condensation is effectively suppressed with reduced reaction time required for dealkylation. By the introduction of SCW and the adoption of an appropriate reaction temperature, the visbreaking efficiency could be drastically improved together with guaranteed stability of v...

Miniature Autoclave and Double Bellows Loading Device for Material Testing in Future Reactor Concept Conditions—Case Supercritical Water

The presented work consists of a test setup study of a new pneumatic material testing device based on double bellows (DBs) loading device and with miniature autoclaves enabling applications at temperature and pressure up to 650 °C and 35 MPa, respectively. It has been demonstrated that it is technically feasible to carry out well defined and controlled material testing in the supercritical water (SCW) environment using this testing system. By using this type of system, it makes possible to investigate the intrinsic role of the applied stress on the deformation behavior of material in light water reactor (LWR) conditions and also in other harsh environments like SCW conditions. In addition, the compactness and versatility of the setup makes this system particularly attractive for deployment in a hot-cell for testing of irradiated materials.

Corrosion behavior of ceramic-coated ZIRLO™ exposed to supercritical water

The corrosion behavior of ceramic coated ZIRLO™ tubing was evaluated in a supercritical water (SCW) environment to determine its behavior in high temperature water. Two coating architectures were analyzed; a 4 bi-layer TiAlN/TiN coating with Ti bond coat, and a TiN monolithic coating with Ti bond layer on ZIRLO™ tubes using cathodic arc physical vapor deposition (CA-PVD) technique. Femtosecond laser ablation was used to introduce reproducible defects in some of the coated tubes. On exposure to deaerated supercritical water at 542 °C for 48 h, coated tubes exhibited significantly higher weight gain compared to uncoated ZIRLO™. Examination revealed formation of a uniform ZrO2 layer beneath the coating of a thickness similar to that on the uncoated tube inner surface. The defects generated during the coating process acted as preferential paths for diffusion of oxygen resulting in the oxidation of substrate ZIRLO™. However, there was no delamination of the coating. There were insignificant differences in the oxidation weight gain between laser ablated and non-ablated tubes and the laser induced defects did not spread beyond their original size.