Supercritical Environment Research Articles

Analyzing single and multicomponent supercritical jets using volume-based and mass-based numerical approaches

The injections of cryogenic and non-cryogenic fluids in a supercritical environment, respectively, liquid N2 into gaseous N2 and n-dodecane into gaseous N2, are investigated. The two systems are analyzed under dynamic and thermal similarity (same reduced temperatures, reduced pressures, and Reynolds numbers) using the same simplified two-dimensional configuration for the totality of the simulations. This work contributes to provide insight into the interpretation of numerical studies on single- and multicomponent systems under supercritical conditions. A comprehensive comparison of the results obtained from two numerical approaches, based on the volume of fluid and on the homogeneous mixture assumption, making use of two distinct thermophysical and mixing rule frameworks, is presented. Results show very similar and consistent fluid mechanics and mass diffusion processes predicted by the two approaches, but different thermal behaviors for binary-species configurations. The two different mixing models are found to have the greatest impact on the temperature predictions. Also, isobaric–adiabatic mixing, which is obtained with the mass-based homogeneous approach, leads eventually to a larger extension of the predicted two-phase region. Such findings have large implications in energy systems operating at high pressure, where accurate local temperature predictions are crucial.

Corrosion Behavior of Alumina-Forming Austenitic Steel in Supercritical Carbon Dioxide Conditions: Effects of Nb Content and Temperature.

The corrosion behavior of alumina-forming austenitic (AFA) stainless steels with different Nb additions in a supercritical carbon dioxide environment at 500 °C, 600 °C, and 20 MPa was investigated. The steels with low Nb content were found to have a novel structure with a double oxide as an outer Cr2O3 oxide film and an inner Al2O3 oxide layer with discontinuous Fe-rich spinels on the outer surface and a transition layer consisting of Cr spinels and γ'-Ni3Al phases randomly distributed under the oxide layer. Oxidation resistance was improved by accelerating diffusion through refined grain boundaries after the addition of 0.6 wt.% Nb. However, the corrosion resistance decreased significantly at higher Nb content due to the formation of continuous thick outer Fe-rich nodules on the surface and an internal oxide zone, and Fe2(Mo, Nb) laves phases were also detected, which prevented the outward diffusion of Al ions and promoted the formation of cracks within the oxide layer, resulting in unfavorable effects on oxidation. After exposure at 500 °C, fewer spinels and thinner oxide scales were found. The specific mechanism was discussed.

The effects of sub/supercritical conditions on the spray interface characteristics of alkane fuel

ABSTRACT Alkane fuels are widely used in engines. The spray characteristics in supercritical environments are very different than in subcritical environments, and supercritical environments significantly impact the formation of the gas mixture in the engine. In this paper, the spray images of n-heptane and n-tetradecane in subcritical (500K-2MPa) and supercritical (700K-4MPa, 800K-6MPa) environments were obtained on the constant volume spray test platform. The effects of the supercritical environment and carbon chain length of alkane fuel on spray characteristic parameters were investigated. The results showed that the carbon chain length and ambient pressure are the main factors of penetration. The ambient pressure is the main factor of the spray cone angle and the spray non-liquid layer thickness. Compared with n-heptane, in the 500K-2MPa (subcritical) environment, the cone angle of n-tetradecane was reduced by 2°, the penetration was increased by 10 mm, and the spray non-liquid layer thickness was reduced by about 1 mm. In the 800K-6MPa (supercritical) environment, the cone angle of n-tetradecane was increased by 2°, the penetration was increased by 10 mm, and the spray non-liquid layer thickness was increased by about 1.2 mm. In addition, the calculation model of alkane fuel spray penetration in sub/supercritical environments was modified and achieved 95% prediction accuracy.

Testing Corrosion Resistance of Particulate Corundum, Titanium Nitride, and Eutectic Alloy TiB2/TiN in Supercritical Aqueous Fluid Environment

Testing Corrosion Resistance of Particulate Corundum, Titanium Nitride, and Eutectic Alloy TiB2/TiN in Supercritical Aqueous Fluid Environment

Investigation of Stress Corrosion Cracking Resistance of Irradiated 12Cr Ferritic-Martensitic Stainless Steel in Supercritical Water Environment

The supercritical water-cooled reactors (SWCR) belong to Generation IV of reactors. These reactors have a number of advantages over currently operating WWERs and PWRs. These advantages include higher thermal efficiency, a more simplified unit design, and the possibility of incorporating it into a closed fuel cycle. It is therefore necessary to identify candidate materials for the SWCR and validate the safety and effectiveness of their use. 12Cr ferritic-martensitic (F/M) stainless steel is considered a candidate material for SWCR internals. Radiation embrittlement and corrosion cracking in the primary circuit coolant environment are the main mechanisms of F/M steels degradation during SWCR operation. Here, the stress corrosion cracking (SCC) in supercritical water at 390 and 550 °C of 12Cr F/M steel irradiated by neutrons to 12 dpa is investigated. Autoclave tests of specially designed disk specimens in supercritical water were performed. The tests were carried out under different constant load (CL), temperature 450 °C, and pressure in autoclave 25 MPa. The threshold stress, below which the SCC initiation of irradiated 12Cr F/M steel does not occur, was determined.

Carbonization of Class G well cement containing metakaolin under supercritical and saturated environments

A well-cement alteration in a CO2-rich environment can affect the well integrity, and, consequently, compromise the overall safety of an external ecosystem. The objective of this study was to evaluate the carbonization of Class G well cement and metakaolin modified Class G cement samples exposed to CO2 at 323 K and 15 MPa under supercritical state and in the saturated solution. Cement alternation was more rapid and extensive under a CO2 saturated solution than under supercritical CO2. Metakaolin amended Class G cement samples showed improved mechanical properties, while their carbonation resistance was impaired and reduced. X-ray Powder Diffraction confirmed that CaCO3 polymorphs were present in greater proportions in the metakaolin-modified samples and when exposed to a CO2-saturated salt solution. Moreover, samples containing metakaolin have a larger specific surface area, and, consequently, can absorb more CO2. The micrographs showed that the cement samples exposed to a CO2-rich environment have rough and porous surfaces containing CaCO3 polymorphic crystalline forms.

Microstructure and stability of SrCe0.9Yb0.1O3−α ceramics in H2/H2O/CO2 supercritical environment

Microstructure and stability of SrCe0.9Yb0.1O3−α ceramics in H2/H2O/CO2 supercritical environment

The mechanism of abnormal corrosive behavior of metallic materials in the pseudocritical region of supercritical water

To study the corrosive behavior of metal materials in the pseudocritical region of supercritical fluid and explain the abnormal mechanism in corrosive within the pseudocritical region, a corrosive experimental platform for supercritical fluids was built. Three different candidate materials for Supercritical Water Reactor (i.e. Q235 low carbon steel, 316 L austenite stainless steel and 3Cr13 martensitic steel) were selected and reacted for 200 h in the pseudocritical region (375.6 ℃, 22.5 MPa) and gas-like region (405.6 ℃, 22.5 MPa) respectively. The results show that 316 L exhibits abnormal corrosive exacerbation in the pseudocritical region with a corrosive rate 1.67 times that of the gas-like region. Considering the special variation of physical properties in pseudocritical region, the enhanced kinetic effect is further analyzed. It is found that the migration of corrosive media such as oxygen to the material matrix is accelerated, which aggravates the corrosive products shedding and freeing. The present study can provide a basis for improving the anti-corrosive performance of materials under supercritical fluid environments and providing a safe operation of the Supercritical Water Reactor.

Effect of Flow on the Corrosion Behavior of Pipeline Steel in Supercritical CO2 Environments with Impurities

Corrosion is a major concern in transmission pipelines that transport captured CO2. While dry CO2 is noncorrosive, significant corrosion has been reported in dense-phase CO2 with trace amounts of water and impurities such as O2, H2S, SOx, and NOx. The aim of this work is to improve our understanding of the physicochemical aspects of the corrosion of carbon steels in the high-pressure environments associated with CO2 transmission pipelines. The effect of flow on the corrosion of X65 carbon steel was investigated in a series of autoclave tests with different combinations of impurity concentrations in supercritical CO2 conditions (8 MPa and 35°C). The corrosion rate of specimens was determined by weight loss measurements. The surface morphology and composition of the corrosion product layers were characterized using surface analytical techniques (scanning electron microscopy, eneregy dispersive x-ray spectroscopy, and Raman microscopy). Localized corrosion was measured via surface profilometry after corrosion products were removed. Results showed that no corrosion was observed in the supercritical CO2 with 650 ppmv of water, 50 ppmv SO2, and 100 ppmv NO, but corrosion occurred when SO2 concentration was increased to 4,500 ppmv and 40,000 ppmv of O2 was added to the system. The presence of flow significantly accelerated the corrosion of carbon steel. Furthermore, localized corrosion was observed in the presence of both O2 and flow.

Preparation of hierarchically porous PVP/ZIF-8 in supercritical CO2 by PVP-induced defect-formation method for high-efficiency gas adsorption

The fabrication of structural stable metal organic frameworks (MOFs) with large pore size by facile, rapid and environmentally friendly synthetic method remains a serious challenge. Herein, it is the first time to develop a novel synthetic strategy that rapidly enlarges the pore size of microporous ZIF-8 by employing polyvinyl pyrrolidone (PVP) as a modulator to induce defect formation in environmentally friendly supercritical CO2 (scCO2). Notably, the high solubility and low mass transfer resistance of reactants in scCO2 and low surface tension nature of scCO2 are the key factors for the rapid preparation of structural stable PVP/ZIF-8 with hierarchically pores. And the PVP modulator as a weak ligand not only facilitates pore size but also perturbs coordination to form coordination defects, exposing more coordination metal sites. These characteristics are beneficial to fast diffusion and adsorption of CO2 molecules. In addition, the influences of reaction conditions on the morphology, pore parameter and CO2 adsorption performance of PVP/ZIF-8 are investigated. The result indicates that the PVP/ZIF-8 prepared with 1:1 of Zn/PVP mass ratio at 90 °C and 30 MPa in supercritical environment have the highest CO2 adsorption equilibrium capacity (2.02 mmol CO2/g sorbent) and great cycle stability, which should be attributed to hierarchically porous structure with interconnected macropores and micropores.

Effect of temperature and medium environment on corrosion fatigue behavior of Inconel 625

AbstractInconel 625 is a nickel‐based alloy with exceptional creep and corrosion resistance properties. It has emerged as a candidate material for operational and structural use in critical and supercritical power generation plants. Supercritical power plants work at high temperatures, pressure and in an erosive environment that makes metallic materials more vulnerable to corrosion fatigue. This study aimed to evaluate the fatigue crack growth rate (FCGR) at elevated temperatures (550–650°C) in a steam and supercritical water environment. All the experiments were performed at a water oxygen level of 8000 ppb. A cyclic stress of 1 Hz frequency and sine waveform was applied for the test. A 0.6 stress ratio was applied with a maximum stress intensity factor of 32 MPa√m. A direct current potential drop method was applied to detect the crack growth rate (CGR). For the given parameters, fractographic analysis validated the role of corrosion‐fatigue interaction as the primary source of variation in the FCGR. Results revealed that the CGR was enhanced with a rise in temperature under all test environments. The influencing mechanism of temperature on CGR was also discussed.

Oil diffusion mathematical model in oily sludge particle under supercritical water environment.

Supercritical water (SCW, T>374.15°C, P>22.1MPa) treatment can achieve volume reduction, harmless disposal, and resource utilization of oily sludge. Herein, we investigated the oil removal efficiency (ORE) and oil diffusion characteristics in oily sludge particles under SCW environment. The experimental results showed that when the treatment duration was extended from 5min to 60min, the particle diameter decreased from 4mm to 2mm, and the ORE improved considerably; however, the treatment temperature (375°C ∼ 425°C) had little influence. Based on these findings, an oil diffusion mechanism in oily sludge particles under SCW environment was proposed. Subsequently, a reasonable mathematical model of diffusion was developed to represent the heat and mass transfer in oily sludge particles characterized by porous, high moisture, and oil content. Finally, by analyzing the oil diffusion process in sludge particles within this model, it was found that the oil concentration in SCW and particle diameter had a considerable influence on ORE, while the effect can be ignored when the diameter <0.2mm. This research serves as a guide for effectively using SCW to remove oil from oily sludge.

Polycyclic aromatic hydrocarbons production from the supercritical pyrolysis of n-decane, ethylcyclohexane, and n-decane/ethylcyclohexane blends

To investigate the effects of fuel composition on the production and growth of polycyclic aromatic hydrocarbons (PAH) at conditions relevant to the pre-combustion environment of fuels in future high-speed aircraft, we have conducted supercritical pyrolysis experiments in an isothermal, silica-lined stainless-steel flow reactor at 568 °C, 94.6 atm, and 133 s, with the model fuels n-decane and ethylcyclohexane—as well as n-decane/ethylcyclohexane blends in which the fraction of fuel carbon coming from ethylcyclohexane is 0.25, 0.50, and 0.75. High-pressure liquid-chromatographic analyses of the reaction products have led to the isomer-specific identification and quantification of 169 three- to nine-ring PAH—159 of which have never before been reported as products of a cyclic-alkane fuel. Quantification of the aliphatic products by gas chromatographic techniques reveals that n-decane produces mostly 1-alkenes and n-alkanes, whose radicals generate more radicals; whereas ethylcyclohexane produces mostly C5-ring and C6-ring alkanes and alkenes, whose radicals show a propensity for ring dehydrogenation. These contrasts in the aliphatic-product distributions have major impacts on PAH growth, which, in this reaction environment, occurs chiefly through the reactions of resonance-stabilized arylmethyl radicals with the C2-C4 1-alkenes. For fuel compositions rich in n-decane, all the ingredients necessary for these PAH-growth reactions to thrive are in place: methyl-substituted PAH, as sources of arylmethyl radicals; a radical-rich environment that fosters H abstraction and arylmethyl-radical formation; and C2-C4 1-alkenes, as growth species. However, an increase in the level of ethylcyclohexane in the fuel brings about substantial reductions in the levels of H-abstracting radicals and C2-C4 1-alkenes as well as an increase in the supply of “donated” hydrogen for stabilizing radicals. These factors combine to bring about marked reductions in PAH growth at fuel compositions rich in ethylcyclohexane, resulting in much lower production of the high-ring-number PAH that are the precursors to solids in the supercritical fuel-pyrolysis environment.

Supercritical CO2‐assisted Ru‐Pd/USY synchronised isomerisation to produce rice bran oil rich in conjugated linoleic acid

SummaryIn this work, the catalytic efficiency and influencing factors of supported bimetallic catalysts for the synchronised isomerisation of rice bran oil (RBO) to produce conjugated linoleic acid (CLA)‐rich RBO under supercritical conditions were investigated. The morphology and internal composition of the metal catalysts were analysed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), which revealed that Ru and Pd were successfully supported by ultra‐stable Y(USY). The catalyst and oil were synchronously isomerised in the supercritical CO2 environment, and the change in the fatty acid composition of the oil was measured by gas chromatography. The results showed that when the CO2 pressure was 8.0 MPa, the catalyst addition was 5% and the reaction temperature was 160 °C, the conversion of linoleic acid in the RBO catalysed by Ru‐Pd/USY was 68.25 ± 0.08%, and the produced oil met the edible standard.

The mechanism study of laser peeling of ultra-thin polyimide film from the transparent substrate

Laser peeling of ultra-thin polyimide (PI) film from transparent substrate is a critical technology in the flexible electronics industry. This paper studies the laser ablation and interface separation of the PI film to reveal the mechanism of laser peeling. In the laser ablation process, the supercritical environment is formed, and gas bubbles appear. Then the gas bubbles fuse together and become cavitation holes. These cavitation holes fuse together and produce sparse nanopillar structures that can be seen as a blister. In the interface separation process, the blister absorbs thermal energy and expands under laser irradiation. This paper proposes a discrete quasi-static method to analyze the separation of the PI film from the transparent substrate during the blister expansion. The cohesive zone model is also introduced to predict the peeling behaviors of the PI film. The results show that the blister profile is related to the laser influence, and the blister profile obtained by the theoretical model can fit well with the experiment. When the laser fluence is larger, the blister profile is steeper, and the pressure inside the blister is larger. This paper provides a new perspective for understanding the laser peeling mechanism of ultra-thin PI film from a transparent substrate.

An Experimental Investigation of Supercritical Methane Injection Characteristics in a CO2 Environment

Abstract Clean energy generation is gaining significant attention from industries, academia, and governments across the globe. The Allam cycle is one such technology that has been under focus due to its efficiency, environmental friendliness, and economics. This is a direct-fired cycle operating at supercritical conditions using carbon dioxide as a working fluid. Fuel or oxidizer jet mixing with CO2 is a vital phenomenon that governs combustion efficiency, and it is not well understood for the Allam cycle conditions. This paper experimentally investigated the jet characteristics of a methane jet injected into a subcritical to supercritical carbon dioxide environment. A wide range of injection pressures and temperatures were targeted between subcritical to supercritical conditions. Unlike previous studies, the current work focused on injecting lower-density (methane) jets into higher-density (carbon dioxide) environments. Schlieren imaging and methane absorption measurements were simultaneously performed with a CMOS high-speed camera and a 3.39 μm infrared laser. Specifically, we looked at the classical injection parameter of jet spreading angle, which was classically established to be mainly a density ratio function. Here, the jet cone angle was obtained from the postprocessed schlieren imaging. The jet cone angle is a critical characteristic parameter that describes the entrainment rate in a jet; thus, it is a crucial parameter in understanding the nature of the jet. The laser measurements were only used as an additional check to confirm the entry time of methane into the chamber filled with carbon dioxide. Notably, this paper makes a detailed comparison between the jet cone angles of jets with a density ratio. The result showed that the classical correlations, such as Abramovich's theory applied to submerged turbulent gas jets developed for low-density ratio jets, were unsuitable for higher-density ratio jets. It was also observed that the divergence angles were dependent not only on density ratio but also on other parameters such as pressure ratios and reduced pressures.

Exploring the influence of flue gas impurities on the electrochemical corrosion mechanism of X80 steel in a supercritical CO2-saturated aqueous environment

The influence of flue gas (100 ppmv of O2, NO2 or SO2 in CO2 steam) on the corrosion mechanism of X80 steel in a supercritical CO2-saturated aqueous (water-rich phase) environment at 8.0 MPa and 35 ℃ was explored via electrochemical methods combined with off-line analyses. While O2 mainly promoted the anodic reaction process, NO2 and SO2 primarily enhanced the cathodic reaction process. Compared with SO2, NO2 and O2 dramatically reduced the protection offered by X80 steel’s corrosion product layer in the water-rich phase. In addition, the study also found that O2 significantly promoted the localized corrosion of X80 steel in the water-rich phase.

Corrosion behavior of T91 tubing in high temperature supercritical carbon dioxide environment

The effect of stress on the corrosion behavior and mechanical properties of T91 tube exposed to high-temperature supercritical carbon dioxide (S-CO2) was investigated. Both yield strength, ultimate tensile strength, and elongation of T91 tube was decreased after being exposed to S-CO2. Besides, a large number of carbide precipitates were observed in the Fe-Cr-rich corrosion layer of T91 tube, which was attributed to higher ion diffusion and carburizing rate under the action of stress. As a result, the corrosion rate of T91 tube was significantly faster than that of T91 coupon.

Thermal stress stability of hydrocarbon fuels under supercritical environments

This study investigates the thermal stressing characteristics of hydrocarbon fuels to develop thermally stable fuels for supersonic vehicle applications. The thermal stability characteristics of two multicomponent hydrocarbon fuels, namely F-1 and F-2 fuels, under subcritical and supercritical conditions, are examined for a wide range of temperatures (30–500 °C) and pressures (15–50 bar) using a semi-batch reactor. Various analysis techniques, such as UV-Vis spectrophotometry, ASTM D86 distillation, gas chromatography, scanning electron microscopy (SEM), etc., are used to characterize the feed and product properties. The study showed that the fuels are reasonably stable up to around 400 °C and above 40 bar pressure. The stability of the fuels is relatively less under subcritical pressure at elevated temperatures. The amount of gum formation increased with the increase of both temperature and pressure parameters. Further, the SEM analysis showed oval-shaped and ribbon-like structures in the solid deposits. The present investigation may be useful in developing an appropriate fuel for regenerative cooling in supersonic applications.

Preparation of a thick NiAl/Al2O3 coating by embedding method and its corrosion resistance under supercritical water environment

One of the critical issues in the application of supercritical water oxidation technology is to improve the corrosion resistance of reactor materials. Use of Al2O3 coating is one of the most promising methods to address this issue. In this study, thick NiAl/Al2O3 coatings on Inconel 625 substrates were prepared by a consecutive pack embedding and in-situ thermal oxidation process. The effect of aluminizing and oxidation temperature on phase structure and coating thickness is studied. Results show the diffusion of Al from the exterior to the interior of the alloy matrix to form intermetallic compounds between Al and metal elements in the matrix (Ni, Cr, Mo, etc.). Moreover, the coating thickness can reach above 300 μm at the aluminizing temperature of 950 °C. Increasing the aluminizing temperature above 950 °C will not increase the coating thickness further. After high temperature oxidation subsequently, only phases of NiAl and Al2O3 were detected. The formation of Al2O3 layer can be ascribed to the surface oxidization of Al. And the NiAl between the alloy substrate and Al2O3 coating provides an interfacial layer that can alleviate the crack or exfoliation of ceramic coating due to the mismatching of thermal expand coefficient. The thick NiAl/Al2O3 coatings prepared by aluminizing 950 °C and oxidizing at 1100 °C exhibit satisfied corrosion resistance after supercritical water test. This work would provide a significant method to develop advanced ceramics coating for the corrosion resistance of alloys.