Supercritical Water Environment Research Articles

Gasification of uric acid in supercritical water: Reaction characteristics, nitrogen transformation mechanisms, and kinetics analysis

Supercritical water gasification (SCWG) of biomass waste as a new clean waste conversion technology is increasingly valued worldwide attributable to the unique properties of supercritical water (water: T > 374 °C, P > 22.1 MPa). However, the fate of conventionally contaminated elements such as nitrogen in this process still warrants further research to control potential contamination. In this work, uric acid was used as the model compound of nitrogenous components in chicken manure to investigate the transformation mechanism of nitrogen during the SCWG process. The effects of different temperatures (550–650 °C) and residence times (2–30 min) on the gasification characteristic of uric acid and the behaviour of nitrogen were experimentally explored. The results indicated that the higher temperatures and longer residence time significantly facilitated the uric acid gasification and nitrogen transformation. The maximum nitrogen conversion efficiency was 84.83%, and the maximum yield of NH3, H2, and CO2 reached 20.20 mol/kg, 23.78 mol/kg, and 20.10 mol/kg at 650 °C and 30 min, respectively. Subsequently, the reaction pathways of nitrogen in uric acid were proposed in detail according to the product analysis. Finally, a kinetic model was developed to describe the SCWG process, especially the reaction kinetic of nitrogenous products. The results revealed that the hydrolysis reactions of uric acid led to a rapid increase in NH3 yield in the initial stage of the SCWG process. As the process continued, the increase in NH3 was mainly due to the deamidation reaction of small-molecule nitrogenous intermediates. Notably, NOx, NO2−, and NO3− were not detected in products, validating the inhibitory effects of the reducing environment of supercritical water on the generation of nitrogen oxides.

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.

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.

Fabrication and Supercritical Water Corrosion Resistance of TiN/TiNC/Al2O3 Multilayer Coating on Inconel 625 Alloy by CVD Method.

A TiN/TiNC/Al2O3 multilayer coating was deposited on an Inconel 625 alloy by the chemical vapor deposition method as a protective barrier to improve the corrosion resistance in supercritical water. The corrosion characteristics were evaluated in a reactor at 500 °C and 25 MPa for 72 h. The surface morphology of the coated samples was relatively dense with no obvious cracks or pores observed. The XRD analysis revealed that the coatings were composed of TiN, TiNC and α-Al2O3 phases. After exposure to supercritical water, the surface morphology of the coatings was still dense and kept integrity. The phase composition of the coatings was also not changed, with no obvious corrosion scales detected. This result demonstrates the effectiveness of TiN/TiNC/Al2O3 coatings as a protective coating under harsh supercritical water environments.

Evolution of oxide film of a new martensitic steel 9Cr3W3Co in supercritical water

The oxide evolution of 9Cr3W3Co martensitic steel in the supercritical water environment at 650 °C and 30 MPa was studied. The results indicated that a double-layer oxide film, composed of a columnar Fe3O4 outer layer and a fine grain FeCr2O4/Fe3O4 spinel mixed inner one, that thicknesses followed a sub-parabolic law, was formed. The FeCr2O4 first grew along various boundaries, then grew into lath along subgrain boundary, dislocation line or by bulk diffusion. After a continuous Cr-rich oxide layer was formed at the inner oxide layer/matrix interface, the inner layer was gradually completely oxidized, which significantly slowed down the oxide rate.

Co-gasification of waste lignin and plastics in supercritical liquids: Comparison of water and carbon dioxide

Waste lignin and plastic can be converted into useful syngas by co-gasified in supercritical liquids. In this work, lignin with β-O-4 linkages and two kinds of common plastics, polyethylene and polypropylene, were simulated in the process of co-gasification. The processes in supercritical water (SCW) or supercritical carbon dioxide (SCCO2) environment at different temperatures and ratios of reactants were studied comparatively using ReaxFF molecular dynamics simulations. The composition of products has been analyzed, the co-gasification synergistic effect has been explored, and the reaction mechanisms were provided and compared. The addition of plastic increased the production of H2 in both the SCW and SCCO2 systems The effect of adding plastics on the initial pyrolysis of lignin in SCW was greater than that in SCCO2, resulting in the production of more H2 and CH4 and less CO in the SCW system.

Sol-Gel-Derived Ni3Al Coating on Nickel Alloy for Oxidation Resistance in Supercritical Water Environments.

Although nickel-based alloys are widely used in industries due to their oxidation and corrosion resistance, the pursuit of better performance in harsh environments is still a great challenge. In this work, we developed a sol–gel method to synthesize Ni3Al coating on a nickel alloy, assisted by a post-annealing process, and investigated the oxidation-resistant performance. The coating thickness can be controlled by designing the deposition times, which keep the pure Ni3Al phase stable. In addition, the surface morphologies indicate that the coating is compact without obvious voids or cracks. Furthermore, the oxidation-resistant property of the coating was investigated by carrying out a supercritical water oxidation experiment. The crystalline structure and surface morphology of the samples before and after 72-h oxidation demonstrated the superior oxidation resistance of the coating. This work provides a convenient method to fabricate an oxidation-resistant coating on a nickel-based alloy, which would be significant for prolonging the service life of vessels under oxidation conditions, especially for supercritical water reactions.

Erosion-corrosion behavior of Nickel based alloys in supercritical water oxidation environment

In this study, the erosion-corrosion experiment of Ni based alloys in high-temperature and high-pressure supercritical water environment was performed. SiO2 and Al2O3 particles with the average diameter of 30 µm and 50 µm were used as erodent. The special designed specimens were fixed on a blade impeller, and the erosion tests were performed in a high pressure autoclave with a magnetic stirrer system. Results show that the basic corrosion mechanism is not changed by the erosion process. A Cr-rich inner oxide layer and a Fe/Ni rich outer oxide layer were formed on the alloy surface. However, the oxidation rates of the alloys were affected by the erosion process. Parts of the outer Fe and Ni oxides were removed by the erodent particles during the tests. The inner Cr oxide layer still exists owing to the self-healing ability of the Cr2O3 layer, acting as a barrier to resist further corrosion of the alloys. The factors that affect the degree of erosion and the erosion-corrosion mechanism in SCW environment is analyzed.

Corrosion characteristics and mechanisms of typical iron/nickel-based alloys in reductive supercritical water environments containing sulfides

The corrosion characteristics of stainless steel 316 and typical nickel-based alloys in supercritical water containing sulfides were investigated by a series of detection methods. Breakaway corrosion occurred on Incoloy 825 and Inconel 600 due to sulfide-formed tunnels or enriched microlayers in the inner layer of corrosion scales. In supercritical water containing sulfides, we find that the alloys with higher chromium and iron contents exhibited superior corrosion resistance than the nickel-rich alloys, which is further confirmed by an analysis of the thermodynamics and the effects of temperature and alloying elements. The corrosion mechanism of iron/nickel-based alloys in reductive supercritical water systems containing sulfides was proposed, especially the related breakaway corrosion mechanism and the effects of temperature and alloying elements. Additionally, prediction models of corrosion products are established to guide material selection.

Superior protective performance of Ni Cr coating on stainless steel 316L in pure, oxygenated and chlorinated supercritical water

The protective effect of Ni Cr coating in supercritical water (SCW) was studied in this work. Ni Cr coated 316 L stainless steel showed an excellent microstructure and performance stability in SCW. The corrosion weight gains of as-received 316L were 2–3.6 times of that of Ni Cr coated sample in SCW with 5000 ppm oxygen content. The coating greatly reduced the corrosion weight gain of 316L in a SCW environment after 144 h exposure. The surface of Ni Cr coating was composed of the outermost hydration layer (Ni(OH) 2 ), oxide layer (NiO, NiCr 2 O 4 and Cr 2 O 3 ) and metallic layer (Ni 0 ) mixed in oxides. NiO on the surface of the Ni Cr coating decomposed with the addition of oxygen or chloride ions in SCW. • The corrosion weight gains of as-received 316L were 2–3.6 times of that of the two coated samples. • The coating greatly reduced the corrosion weight gain of 316L in SCW environment after 144 h exposure. • Small amount of Fe diffused to the coating surface along surface defects on the Ni-Cr coating. • Ni-Cr coating contains the outermost hydration layer, oxide layer and metal elementary layer.

Domain knowledge-guided interpretive machine learning: formula discovery for the oxidation behavior of ferritic-martensitic steels in supercritical water

A general formula with high generalization and accurate prediction power is highly desirable for science, technology and engineering. In addition to human beings, artificial intelligence algorithms show great promise for the discovery of formulas. In this study, we propose a domain knowledge-guided interpretive machine learning strategy and demonstrate it by studying the oxidation behavior of ferritic-martensitic steels in supercritical water. The oxidation Cr equivalent is, for the first time, proposed in the present work to represent all contributions of alloying elements to oxidation, derived by our domain knowledge and interpretive machine learning algorithms. An open-source tree classifier for linear regression algorithm is also, for the first time, developed to materialize the formula with collected data. This algorithm effectively captures the linear correlation between compositions, testing environments and oxidation behaviors from the data. The sure independence screening and sparsifying operator algorithm finally assembles the information derived from the tree classifier for linear regression algorithm, resulting in a general formula. The general formula with the determined parameters has the power to predict, quantitatively and accurately, the oxidation behavior of ferritic-martensitic steels with multiple alloying elements exposed to various supercritical water environments, thereby providing guidance for the design of anti-oxidation steels and hence promoting the development of power plants with improved safety. The present work demonstrates the power of domain knowledge-guided interpretive machine learning with respect to the data-driven discovery of physics-informed formulas and the acceleration of materials informatics development.

Atomic insight into iron corrosion exposed to supercritical water environment with an improved Fe-H2O reactive force field

Waterside anti-corrosion of Fe-based alloys is one of the great challenges for supercritical water reactors. Herein, an improved Fe-H2O reactive force field (ReaxFF) is constructed to understanding the Fe corrosion mechanism at the atomic scale. Using molecular dynamics simulations, the corrosion of Fe (100) and Fe (110) in supercritical water was investigated. It was found that oxidation and hydrogen evolution reactions occurred along with Fe dissolution; the corrosion rate on Fe (100) was faster than that on Fe (110) in the early stage, but the reverse occurred on a longer time scale; and the precipitation of ferrous hydroxide at the initial stage led to the formation of an oxidation layer, where FeO and Fe3O4 were the dominant products. The present ReaxFF can become a crucial tool for corrosion research on Fe-based alloys.

Precipitation of sodium sulfate and sodium carbonate during supercritical water oxidation/gasification of ethanol

Reactor plugging induced by salt precipitation in supercritical water (SCW) is a major problem during the hydrothermal treatment of hypersaline waste. Considering that current investigations have focused strictly on pure water systems, a continuous tube reactor was designed herein to study the precipitation of Na2SO4 and Na2CO3 in a complex SCW environment, which was generated using C2H5OH and H2O2 in order to reproduce real SCW conditions. Results indicate that salt precipitation was accelerated in the presence of C2H5OH and H2O2. The passage rate was only 87% for Na2SO4 and 43% for Na2CO3 in an aqueous solution of 0.1 wt% C2H5OH and 0.5 times of H2O2, compared to the solubility of these salts in pure water at 385 °C and 25 MPa. Organics and gases reduce the fluid density and further aggravate salt precipitation. Salt precipitation occurred mainly in the area around gas inlet and mixer in a real SCW process.

Oxidation behavior of Al0.2CoCrFeNi high-entropy alloy film in supercritical water environment

Oxidation behavior of Al0.2CoCrFeNi high-entropy alloy film in supercritical water environment

Oxidative coupling of methane over MnO2/ZrO2 catalyst in supercritical water: A parametric study

Hydrocarbons production with carbon numbers above C2 through methane coupling has been a target of many researches in the past decades. Recently we discovered that the reaction also occurs under supercritical water conditions. In this work, effects of reaction parameters such as catalyst Mn loading, charged catalyst amount, reaction time as well as reaction pressures and temperatures on the catalytic methane coupling under sub- and supercritical water environment were explored over MnO2/ZrO2 catalyst using a batch reactor. It was found within the investigated parameters that methane coupling activity was maximum at 12–35 MPa, 365–400 °C, 10 wt% Mn loading on ZrO2, reaction time 5 min and 1.0 g catalyst charging. It was conjectured from the temperature–pressure mapping of products that the reaction pathway of hydrocarbon formation over MnO2/ZrO2 is not the same with methanol formation pathway.

EFFECT of surface treatment on corrosion control of P91 hydrothermal aqueous solutions

ABSTRACT The proven efficacy of surface treatment on ferritic-martensitic (F/M) steels in subcritical and supercritical water environments has spawned interest in its application for the construction of biomass hydrothermal liquefaction (HTL) conversion reactors. The objective of this paper is to determine the relative extent to which well-established surface treatments, including mechanical abrasion (baseline), sandblasting, shot peening, and pickling, affect corrosion of the steels in a simulated HTL biomass conversion environment. These treatments were administered to P91 steel for the purpose of improving corrosion resistance towards that exhibited by Type 304 austenitic stainless steel. Samples were immersed in a simulated HTL conversion environment at 310 °C and 10 MPa for 10 days using a static autoclave testing system. None of the surface treatments administered improved corrosion control of P91. The structure and chemistry of the oxide scale formed during immersion were not altered by the surface treatments administered, i/e., none of which promoted of the formation of a more protective Cr-enriched scale. Shot peening was particularly problematic as it promoted the formation of Cr-rich carbides in the near-surface region, which, when incorporated into the oxide scale that formed during high temperature immersion, served to reduce the amount Cr available for oxidization.

ИССЛЕДОВАНИЕ МЕТОДОВ ПОДГОТОВКИ ОРГАНОСОДЕРЖАЩИХ СЕЛЬСКОХОЗЯЙСТВЕННЫХ ОТХОДОВ ДЛЯ ИХ ДАЛЬНЕЙШЕЙ ПЕРЕРАБОТКИ В СВЕРХКРИТИЧЕСКОЙ ВОДНОЙ СРЕДЕ

The problem of waste disposal is always acute in all areas of production activity, especially for the agro-industrial complex. Supercritical water oxidation is the most environmentally friendly method for processing organic waste and persistent organic pollutants. In the process of recycling and disposal, along with the harmfulness, one of the main indicators is the initial state of the waste and its suitability for different disposal technologies. (Research purpose) The research purpose is in choosing and justifying the most energy-efficient method of grinding, taking into account the fact that the organ-containing waste under study (buckwheat husk, sunflower, rice) is soft, elastic and fibrous materials. (Materials and methods) The crushing of organic waste to prepare it for further processing was carried out by different methods in roller, ball, knife and colloid mills. A set of sieves with square cells according to GOST 2715-75, a drying cabinet SHS-80-01 SPU, analytical scales Acculab ALC-210d4 (weighing error of no more than 0.5 percent of the sample weight) were used to evaluate the fractional composition of the obtained material. (Results and discussion) At a concentration of 100 grams of husk per 1 liter of water after 30 minutes of wet grinding, the passage of particles through the sieve is 87.1 percent. The article presents the advantage of wet grinding in a knife mill, reveals mathematical dependencies that allow predicting the grinding process to obtain the product of the required dispersion. (Conclusions) The grinding of organ-containing waste in a knife mill by wet grinding is the most optimal technology for preparing this waste for further processing in a supercritical water environment. The obtained mathematical dependencies allow us to predict the grinding process.

Heterogeneous yielding mechanisms of body centered 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 centered 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 [11¯0] 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 [11¯0] 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.

Supercritical water partial oxidation mechanism of ethanol

In recent years, supercritical water as a green reaction medium to convert organic matter to produce hydrogen has attracted significant attention. At present, the mechanism of the supercritical water partial oxidation (SCWPO) process is still unknown when complete gasification is achieved. In this paper, a detailed mechanism for SCWPO of ethanol was proposed by establishing a novel kinetic model with a wide reaction temperature range. This model described the formation and consumption of gas products (H2, CO, CH4, and CO2) from ethanol by partial oxidation in the supercritical water environment through nine reactions. The results showed that the dehydrogenation and pyrolysis of ethanol were the main ways to generate H2 in the early stage, and the water-gas shift reaction had the most significant impact on hydrogen production in the later stage. The hydrogenation reaction of the intermediate product ethane was a key step for complete conversion into combustible gas products. The steam reforming reaction of a large amount of CH4 produced by ethanol and intermediate product will become the rate-determining step for hydrogen production.