Corrosion behavior of ferritic/martensitic steel P92 in supercritical water

The corrosion behavior of a ferritic/martensitic steel P92 exposed to supercritical water (SCW) at 500–600 °C and 25 MPa was investigated by means of gravimetry, scanning electron microscope/energy dispersive X-ray spectroscopy and X-ray diffraction. A dual-layered oxide scale, which was mainly composed of an outer magnetite layer and an inner magnetite/spinel-mixed layer, formed on P92. The initial oxide scale was rather porous, while the porosity decreased with an increase of exposure time. Oxidation rates at three different temperatures followed the parabolic law. The oxidation at 600 °C was so severe that cracks occurred along grain boundaries in the oxide scale. A probable corrosion mechanism for P92 exposed in SCW was proposed based on the above observations, focusing on oxide formation by oxygen absorption without any metallic dissolution.     

Corrosion behavior of ferritic stainless steel alloyed with different amounts of niobium in hydrochloric acid solution

The corrosion behavior of stainless steel alloys containing corrosion-resistant elements was investigated. Ferritic stainless steel (FSSs) electrodes were synthesized by applying a scan rate of 1 mV s⁻¹. Stainless steels were used unalloyed and alloyed with about 0.5, 1, and 3 wt% elemental Nb. The samples were obtained from casting and forging. The samples were classified into three groups. In the first group, samples were unhomogenized and remained in production condition. In the second and third groups, samples were exposed to homogenization at 1,100 °C for 30 min or 180 min, respectively, and then quenched. The corrosion performance of the FSSs was investigated in 0.3 M HCl acid solution using electrochemical impedance spectroscopy (EIS). Corrosion resistance was calculated using the Stearn–Geary equation. SEM investigations of samples immersed in 0.3 M HCl acid solution for 60 and 360 min were performed. SEM micrographs showed generalized pitting. Consequently, it was determined that niobium has a beneficial effect on the corrosion resistance of FSS since niobium reacts with carbon to form stable carbides.     

Corrosion control methods in supercritical water oxidation and gasification processes

Use of supercritical water (SCW) as a medium for oxidation reactions, conversion of organic materials to gaseous or liquid products, and for organic and inorganic synthesis processes, has been the subject of extensive research, development, and some commercial activity for over 25 years. A key aspect of the technology concerns the identification of materials, component designs, and operating techniques suitable for handling the moderately high temperatures and pressures and aggressive environments present in many SCW processes. Depending upon the particular application, or upon the particular location within a single process, the SCW process environment may be oxidizing, reducing, acidic, basic, nonionic, or highly ionic. Thus, it is difficult to find any one material or design that can withstand the effects of all feed types under all conditions. Nevertheless, several approaches have been developed to allow successful continuous processing with sufficient corrosion resistance for an acceptable period of time. The present paper reviews the experience to date for methods of corrosion control in the two most prevalent SCW processing applications: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).     

Synthesis of manganese oxide particles in supercritical water

The synthesis of manganese oxide and LiMn₂O₄ particles in supercritical water has been investigated with a residence time of 10–40 seconds. It was suggested that the reaction temperature for SCW process should be relatively higher than the critical temperature of water, to synthesize the particles of uniform size and shape. It was observed that the selective synthesis of LiMn₂O₄ was mainly dependent of the amount of OH^- ion in the reactants. We concluded that the size, shape and structure of particles were strongly influenced by a change in the reaction temperature, reactant composition and OH^- ion amount, and thus enabling to synthesize a specific metal oxide particles. The reaction mechanisms for manganese oxides and LiMn₂O₄ have been proposed with the oxidation, hydrolysis and dehydration steps.     

Conversion of sulfur-rich asphaltite in supercritical water and effect of metal additives

The conversions of sulfur-rich asphaltite (the gross-formula CH_1.23N_0.017S_0.037O_0.01) in supercritical water (SCW) flow at 400 °C, 30 MPa without and with addition of aluminum and zinc shavings to asphaltite have been studied. At SCW conversion of asphaltite without addition of metals the yields of volatile and liquid products were found to be equal to 10.3 and 46.0%, respectively. The amount of oil in the liquid product was by 1.6 times higher than that in raw asphaltite. Hydrogen evolution during the oxidation of 〈Al〉 and 〈Zn〉 by supercritical water provided for the hydrogenation of asphaltite in situ. When 〈Al〉 and 〈Zn〉 were added, the portion of the insoluble conversion residue decreased from 44.5 up to 11.3 and 26.3%, respectively. The degree and efficiency of asphaltite hydrogenation with addition of 〈Al〉 were higher than the ones with addition of 〈Zn〉. The amount of O-containing substances in the products and the conversion residue was found to have increased as compared with raw asphaltite. At conversion without addition of metals, the bulk of oxygen was mainly concentrated in the conversion residue, while with addition of 〈Al〉 and 〈Zn〉 it was detected in the composition of CO and CO₂. According to the GC–MS, IR and NMR ¹H spectroscopy data, addition of metals to asphaltite resulted in decrease in the content of sulfoxides and carbonyl-containing substances and in increase in the content of polyaromatic substances in the liquid products. When 〈Al〉 was added to asphaltite, more than 70% of sulfur passed into H₂S and when 〈Zn〉 was added, more than 60% of sulfur passed into ZnS.     

Corrosion resistance of stainless steels in chloride containing supercritical water oxidation system

As the science and process applications of supercritical water (SCW) and supercritical water oxidation (SCWO) become more thoroughly understood, it is logical to envision the use of the SCWO process by diverse industries and public wastewater and sludge generators. This technology can be adapted to accomplish either pre or end-of-pipe wastewater treatment. There is a need to destroy both military and civilian hazardous waste, and urgency, mandated by public concern over traditional waste handling methodologies, to identify safe and efficient alternative technologies. By capitalizing on the properties of water above its critical point, 374 °C and 22.4 MPa for pure water, this technology provides rapid and complete oxidation with high destruction efficiencies at typical operating temperatures. Nevertheless, corrosion of the materials of fabrication is a serious concern. While iron-based alloys and nickel-based alloys are generally considered important for service applications, results from laboratory and pilot-scale SCWO systems presently in operation indicate that they will not withstand some aggressive feeds. Significant weight loss and localized effects, including stress corrosion cracking (SCC) and dealloying, are seen in chlorinated environments. This work assesses the corrosion characteristics of iron-based stainless steels exposed to high supercritical temperatures in a chlorinated military waste containing salts.     

Fast catalytic oxidation of phenol in supercritical water

The catalytic oxidation of phenol in water over a commercial oxidation catalyst, CARULITE 150, was investigated in a fixed bed flow reactor at 250 atm and temperatures from 380°C to 430°C. The phenol and oxygen concentrations at the reactor entrance varied between 0.070 and 1.24 mmol/l, and 9.60 and 39.6 mmol/l, respectively. The reaction conditions produced phenol conversions and selectivities to CO₂ much higher than those produced by non-catalytic oxidation. The kinetics of phenol disappearance and of CO₂ formation were both roughly first-order, and the activation energies were 31 and 47 kcal/mol, respectively. The catalyst did not undergo continuous deactivation during the catalytic oxidation, but rather maintained a high activity even after several days of continuous operation.     

Electrochemical corrosion behavior of hot-rolled steel under oxide scale in chloride solution

In this work, electrochemical corrosion behavior of a hot-rolled steel with oxide scale was investigated in chloride solution through electrochemical measurements and surface characterization. Results demonstrated that the presence of a layer of compact oxide scale would protect the underlying steel from corrosion attack. Upon immersion in chloride solution, oxide scale on the steel surface is reduced, resulting in increase of electrode activity and corrosion rate of the steel. With the further immersion, the insolvable corrosion product and/or new oxide form to accumulate on the electrode surface, enhancing the electrode resistance and decreasing the corrosion of the steel. The amount of dissolved oxygen affects the corrosion of steel. With the increasing concentration of oxygen, there is more compact corrosion product and/or new oxide generating on electrode surface.     

Sulfur transformations during supercritical water oxidation of a Chinese coal

The oxidation of coal in supercritical water was explored by using H₂O₂ as the oxidant. The sulfur-containing components in the effluents were identified. The experiments, which were conducted in a bench scale semi-continuous Supercritical Water Oxidation (SCWO) installation, indicated that the sulfur contained in coal could be gradually oxidized to sulfate in supercritical water medium. The main species containing sulfur in the effluents of coal SCWO were determined as sulfide, thiosulfate, sulfite and sulfate, in which thiosulfate and sulfate were predominant. The effects of the reaction temperature and time on the sulfur transformations during SCWO of coal were also investigated.     

Transformation of petroleum asphaltenes in supercritical water

The transformation of petroleum asphaltenes in supercritical water was studied. The experiments were performed in autoclave at temperature 380 °C and pressure 226 atm with stirring for 3 h, medium density was about 0.33 g/cm³. The reaction resulted in the formation of gas products, about 4.3%, and an insoluble residue (coke) with about 48.6% yield. The remaining products were separated into fractions by consecutive dissolution in hexane (30.0%), benzene (10.6%), and chloroform (5.7%). The properties of the obtained products were studied with FT-IR spectrometry and ¹H NMR spectroscopy. The method of simulated distillation was used to demonstrate that the fractional composition of the hexane-soluble part of the products is close to the fractional composition of a mixture of the diesel fraction and vacuum gas oil of the corresponding oil in 1:1 ratio. The obtained data support the conclusion that asphaltene cracking proceeds in SCW, with most probable main processes being dealkylation of substituents in the aromatic fragments of molecules and aromatization. This leads to formation of gaseous products and hexane-soluble fraction consisting of lighter aliphatic and aromatic compounds, as well as carbonized solid residue.     

Non-catalytic Oppenauer oxidation of alcohols in supercritical water

Non-catalytic Oppenauer oxidation was applied for alcohols, such as benzyl alcohol (4) and benzhydrol (1), in the presence of an excess amount of carbonyl compound, formaldehyde (5a), as an oxidant with and without water. Oppenauer oxidation took place in both reactions of 4 and 1 to afford the oxidation products, benzaldehyde (6) (95%) and benzophenone (2) (64%), concomitant with relatively small amounts of reduction products, toluene (7) (1%) and diphenylmethane (3) (13%), respectively, at 400 °C for 10 min without water in an SUS 316 batch-type tubular reactor. Lower yields of oxidation products 6 (68%) and 2 (30%) were obtained in supercritical water under the conditions of 400 °C, 10 min, and 0.35 g/mL water density, while the formation of the reduction products 7 and 3 was completely suppressed. Thus, water was indispensable for the clean and highly selective Oppenauer oxidation of 4 and 1 to yield 6 and 2.     

烃类在超临界水中的化学转化

超临界水是一种新型反应介质,烃类在超临界水中化学转化效率高。对烃类在超临界水反应制氢气、重油改质和合成含氧化合物方面的研究进展进行了综述,同时简要介绍了各种技术产生的背景,对研究重点进行了必要评述,展望了该领域的发展前景。     

Decomposition of 2-chlorophenol by supercritical water oxidation with zirconium corrosion

The 2-chlorophenol (2-CP) was oxidized in a continuous anti-corrosive supercritical water system. The variation of decomposition efficiency by the corrosion of zirconium 702 was also studied at the variation of feed concentration and reaction time. According to AES depth profile, the oxygen penetration depth to zirconium was not proportional to the exposure time. It might stem from the formation of zirconium oxide layer on the surface delaying the corrosion. However, the increase in feed concentration accelerated the corrosion of zirconium. The corrosion of zirconium at low feed concentration led to the improvement of decomposition efficiency due to the catalytic effect of formed zirconium oxides, while that at high feed concentration deteriorated the decomposition efficiency owing to large consumption of oxidant in corrosion.     

超临界水降解聚丙烯的工艺研究

采用间歇式管式反应器进行了超临界水降解聚丙烯实验,研究了影响聚丙烯降解的因素。实验结果表明,在温度400－450℃、压力23－35MPa及反应时间60－120min的条件下,超临界水能有效地降解聚丙烯。反应温度和反应时间是影响聚丙烯降解的主要因素,温度越高、时间越长,聚丙烯降解越彻底;聚丙烯颗粒度越小降解速率越快,粉末原料在温度400℃、反应时间60min时,以油相产物为主;在温度450℃、反应时间120min时,有利于得到气相产物。     

High Cr ODS steels performance under supercritical water environment

This paper summarizes the results of supercritical water corrosion studies of two ferritic oxide dispersion strengthen (ODS) steels MA956 and PM2000 at the temperature of the upper limit of potential peak cladding temperature under normal operation, according to the conceptual design being developed in the EU. As the high temperature and pressure above the thermodynamic critical point of water result in higher oxidation rate for conventional austenitic alloys than observed in sub-critical light water reactor (LWR) conditions, ensuring adequate corrosion resistance is critical for thin-wall components like fuel cladding. This study concentrated on the investigation of two effects, surface finish and orientation of the cuts. Two different surface treated coupons were prepared in order to study the effect of cold work in sample surface on corrosion resistance. Samples were exposed in supercritical water at 650 °C and 25 MPa, for up to 1800 h. The corrosion rate was evaluated by measuring the weight change of the samples and by cross-section examinations. The microstructure of the oxide layers was analyzed using a scanning electron microscope (SEM) in conjunction with energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). Weight gain results of both ODS steels proved a good resistance to general corrosion. Nevertheless the cross-sectional SEM study showed signs of nodular corrosion, observed mostly on the ground specimens after long exposure times.     

Phenol oxidation over CuO/Al2O3 in supercritical water

Phenol was oxidized in supercritical water at 380–450°C and 219–300 atm, using CuO/Al₂O₃ as a catalyst in a packed-bed flow reactor. The CuO catalyst has the desired effects of accelerating the phenol disappearance and CO₂ formation rates relative to non-catalytic supercritical water oxidation (SCWO). It also simultaneously reduced the yield of undesired phenol dimers at a given phenol conversion. The rates of phenol disappearance and CO₂ formation are sensitive to the phenol and O₂ concentrations, but insensitive to the water density. A dual-site Langmuir–Hinshelwood–Hougen–Watson rate law used previously for catalytic SCWO of phenol over other transition metal oxides and the Mars–van Krevelen rate law can correlate the catalytic kinetics for phenol disappearance over CuO. The supported CuO catalyst exhibited a higher activity, on a mass of catalyst basis, for phenol disappearance and CO₂ formation than did bulk MnO₂ or bulk TiO₂. The CuO catalyst had the lowest activity, however, when expressed on the basis of fresh catalyst surface area. The CuO catalyst exhibited some initial deactivation, but otherwise maintained its activity throughout 100 h of continuous use. Both Cu and Al were detected in the reactor effluent, however, which indicates the dissolution or erosion of the catalyst at reaction conditions.     

Catalytic denitrogenation of hydrocarbons through partial oxidation in supercritical water

In this work, the denitrogenation of hydrocarbons under supercritical water oxidation environment was investigated in a rotated bomb reactor at 623-723 K and 25-35 MPa over sulfided NiMo catalyst. Quinoline was used as a model nitrogen-containing compound. A high reduction of total nitrogen up to about 85% was obtained. The denitrogenation pathway is composed of two consecutive steps: in situ H₂ generation and the hydrogenation of quinoline. The hydrogenation mechanism of quinoline varies with reaction temperature because of the participation of supercritical water in HDN step. The strong adsorption of quinoline and its hydrogenation intermediates on catalyst surface has an adverse influence on total nitrogen reduction rate.     

Reforming of methanol and glycerol in supercritical water

Reforming of pure glycerol, crude glycerin, and methanol (pure and in the presence of Na₂CO₃) in supercritical water was investigated. Continuous experiments were carried out at temperatures between 450 and 650 °C, residence times between 6 and 173 s, and feed concentrations of 3-20 wt%. For methanol the gas products are mainly H₂, CO₂, and CO. The carbon-to-gas efficiency and the observed activation energy for pure methanol are higher than for methanol with Na₂CO₃. This can be explained by assuming different decomposition mechanisms for pure methanol and methanol with Na₂CO₃. For glycerol, H₂, CO, CO₂, CH₄, and higher hydrocarbons are produced. The carbon-to-gas efficiencies of crude glycerin and pure glycerol are comparable. Overall, 2 of the 3 carbon atoms present in glycerol end up in carbon oxides, while 1 carbon atom becomes C_xH_y. The overall mechanism of glycerol decomposition involves the dehydration of 1 mole of H₂O/mole glycerol. For both, methanol and glycerol at carbon-to-gas efficiencies below 70%, the gas yields (mole/mole feed) and carbon-to-gas efficiency correlate well.     

The solubilities of phosphate and sulfate salts in supercritical water

Inorganic compounds are regularly present in aqueous streams. To understand their influence and behavior on these streams at supercritical conditions, little to no property data is available, which can be used as starting point for further research or application design. Since inorganic compounds tend to precipitate at these conditions, scaling, blocking and erosion can occur as a consequence. Furthermore, a separation of (precious) compounds from the bulk stream due to the precipitation is possible. Here, phosphate compounds are regarded as interesting for further investigation since resources are assumed to be depleted in future. As phosphate is present in many waste streams, these could be used as sources for recoverable phosphate. Resulting from these facts and options, a proper understanding and knowledge of these systems is important for later industrial applications. Therefore, the authors have investigated the behavior of salts (e.g. NaCl, NaNO₃ and MgCl₂) in supercritical water in previous works.To extend this knowledge, the solubilities of the sulfate salts MgSO₄ and CaSO₄ in a range of 18.8-23.2 MPa and 655-675 K as well as of the phosphate salts Na₂HPO₄, NaH₂PO₄ and CaHPO₄ in a range of 20.5-24.2 MPa and 665-690 K were investigated in this work with a continuous flow method in continuation of former work of the authors. The solubilities were compared with existing data available from open literature. A quantitative correlation on base of a phase equilibrium between the present phases was used to describe the behavior and to compare it with previous results. For the investigated calcium salts, CaSO₄ and CaHPO₄, it was found that a significant solubility decrease already happens at subcritical conditions resulting in precipitation in unwanted locations. For the remaining compounds, a parallel hydrolysis reaction was found as could be seen from a change in pH in the effluent stream.