Supercritical Water Oxidation Research Articles

The Effect of Molten Phosphate on Corrosion of 316 Stainless Steel, Alloy 625, and Titanium TA8 in Supercritical Water Oxidation Conditions

The early formation of phosphate oxide formed on 316 stainless steel (316 SS), nickel-based Alloy 625, and titanium alloy TA8 exposed in supercritical water (400 °C, 25 MPa) containing phosphate, chloride, and oxygen was investigated. Phosphate corrosion products of austenitic stainless steel displayed the severest spallation. Stable phosphates oxide films were inclined to form on Alloy 625. TiO2 and Ti2O3 are the two main components of oxide films on TA8. There is a strong synergistic effect between phosphates, oxygen, and supercritical water, leading to severe corrosion. The corrosion behavior of the three alloys at the top and bottom of the reaction tube was compared. Both at the top of the reaction tube and at the bottom of the reaction tube, TA8 showed an increase in mass. 316 SS and alloy 625 showed mass gain at the top and mass loss at the bottom. The alloys' detailed molten corrosion mechanism after exposure to supercritical water is discussed.

Proposal and thermo-economic analysis of a novel multi-generation system producing heating, power and clean water under energetically self-sufficient operation

ABSTRACT In this work, by using the energy released during the supercritical water oxidation process (SCWO) as the heat source, a cogeneration system producing heating, power and clean water is proposed. The system is modeled, and the performances under energetically self-sufficient operation are simulated. For the proposed system, parametric sensitivity analysis is first performed. Then the system is evaluated from exergy and economic aspects. The results show that under the given working condition, the proposed system can produce 1832.2 kW of heat and 245.2 kW of power simultaneously. The proposed system is a “heat dominant” system, for a larger part of the energy is utilized as heat, with the thermal efficiency reaching 85.9%. The exergy efficiency of the system reaches 40.1%. The total investment cost is 9053.0 k$. The equipment cost of turbines is 5703.5 k$, which accounts for 60% of the total investment cost. The electricity revenue of the system is 252.21 k$. The heat revenue is 867.94 k$. The clean water revenue is 31.73 k$. The obtained net annual income is 1124.78 k$. The dynamic payback period (DPP) is 12.31 years. While the simple payback period (SPP) is 9.03 years. This study may contribute to provide data support and theoretical basis for using the energy contained in organic wastewater.

Effect of hydrothermal flame generation methods on energy consumption and economic performance of supercritical water oxidation systems

Hydrothermal flames are critical to solving preheating problems and enhancing feed degradation in supercritical water oxidation (SCWO) systems. In this work, different hydrothermal flame generation methods including hot water injection, evaporation concentration, and addition of auxiliary fuel are proposed for SCWO of cutting fluids. The proposed systems are simulated using Aspen Plus 8.2, and the simulation model is validated by comparisons with experimental temperature profiles and product properties. A low ratio between the heat source flow and the feed flow (FR) requires a high heat source temperature to keep the hydrothermal flame stable and results in a high electricity consumption, and FR = 2 is chosen as the optimum condition for the SCWO system with hot water injection (SCWOH). The increase in concentration ratio and fuel concentration can effectively reduce the energy consumption for the SCWO systems with evaporation concentration (SCWOE) and addition of auxiliary fuel (SCWOF), respectively. The exergy analysis results indicate that most of exergy inputs are destructed inside the system, and the exergy distribution coefficients of the reactor exceed 80% in the SCWOE and SCWOF. Economic analysis results show the SCWOE has the lowest treatment cost of 95.9 USD/t.

Performance comparison of black liquor gasification and oxidation in supercritical water from thermodynamic, environmental, and techno-economic perspectives

The black liquor poses a great threat to the environment and it is also a kind of available biomass resources. By the supercritical water gasification (SCWG) and supercritical water oxidation (SCWO) technology, the black liquid with high water content can be converted into hydrogen-rich gas or heat energy. In this study, a pulp mill integrated with black liquor SCWG and SCWO subsystems, which have a processing capacity of 11.2 t/h of black liquor with a concentration of 15.2 wt%, is designed. The thermodynamic analysis, techno-economic analysis (TEA), and life cycle assessment on the system are conducted. The gasification characteristics and thermodynamic performance of the SCWG and SCWO systems under different working conditions are studied. The thermodynamic analysis shows that the highest energy efficiencies of SCWG and SCWO systems are 41.53 % and 19.89 %, respectively. The lowest processing cost of black liquor is 398.68 RMB/t, which is obtained in the SCWO system at 600 °C. At 800 °C, the minimum hydrogen production cost of SCWG system is 23.28 RMB/kg H2. The LCA results show that the environmental benefits of SCW system at high temperatures are significantly better than those of integrated black liquor combustion systems. Finally, a multi-attribute decision making results of the SCWG and SCWO systems show that the optimal reaction temperature is 800 °C.

Effects of key parameters on water film properties and temperature field distribution inside transpiring wall reactor

Transpiring wall reactor (TWR) can efficiently weaken corrosion and salt deposition problems during supercritical water oxidation. Its corrosion and salt deposition resistance are mainly affected by water film properties and temperature field distribution. Therefore, the effects of crucial parameters on water film properties and temperature field distribution are investigated via numerical simulation in this work. The results indicate that the upper supercritical oxidation zone is mainly affected by the second layer of transpiration water instead of the third layer. The second, third layer optimal injection locations are at 0.036 m, 0.060 m in this study, respectively. Excessively thick transpiring wall decreases water film coverage rate thereby damaging TWR, conversely makes supercritical zone shrink and thus weakening degradation ability. Annular gap primarily affects the temperature distribution near the transpiring wall. Under the conditions of the work, transpiring wall thickness of 0.002 m and annular gap of 0.004 m are recommended.

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.

Fate of phthalate esters in landfill leachate under subcritical and supercritical conditions and determination of transformation products

The hypothesis of this study is that the complex organic load of landfill leachate could be reduced by supercritical water oxidation (SCWO) in a single stage, but this operation could lead to the formation of some undesired by-products of phthalate esters (PAEs). In this context, the fate of selected PAEs, butyl benzyl phthalate (BBP), di-2-ethylhexyl phthalate (DEHP) and di-n-octyl phthalate (DNOP), was investigated during the oxidation of leachate under subcritical and supercritical conditions. Experiments were conducted at various temperatures (250–500 °C), pressures (10–35 MPa), residence times (2–18 min) and dimensionless oxidant doses (DOD: 0.2–2.3). The SCWO process decreased the leachate’s chemical oxygen demand (COD) from 34,400 mg/L to 1,120 mg/L (97%). Removal efficiencies of DEHP and DNOP with longer chains were higher than BBP. The DEHP, DNOP and BBP compounds were removed in the range of −35 to 100%, −18 to 92%, and 28 to 36%, respectively, by the SCWO process. Many non-target PAEs were qualitatively detected in the raw leachate apart from the selected PAEs. Besides, 97% of total PAEs including both target and non-target PAEs was mineralized at 15 MPa, 300 °C and 5 min. Although PAEs were highly mineralized during SCWO of the leachate, aldehyde, ester, amide and amine-based phthalic substances were frequently detected as by-products. These by-products have transformed into higher molecular weight by-products with binding reactions as a result of complex SCWO process chemistry. It has also been determined that some non-target PAEs such as 1,2-benzenedicarboxylic acid bis(2-methylpropyl)ester and bis(2-ethylhexyl) isophthalate can transform to the DEHP. Therefore, the suggested pathway in this study for PAEs degradation during the SCWO of the leachate includes substitution and binding reactions as well as an oxidation reaction.

Effect of oxygen concentration on the corrosion behavior of Cr-coated Ni alloy in supercritical water oxidation environment

To improve the corrosion resistance of component materials in supercritical water oxidation (SCWO) environment, a 15 µm Cr coating was prepared on Alloy 625 by magnetron sputtering, and the effect of oxygen concentration (0 M, 0.16 M, and 0.94 M) on the stability of the Cr coating in an SCW environment was investigated. Results show that the stability of the Cr2O3 layer strongly depends on the oxygen concentration in the exposure environment. The protective Cr2O3 layer was transformed to soluble Cr(VI) species and dissolved in SCW containing 0.94 M oxygen. Except for Cr2O3, some Fe/Ni containing oxides were also detected. The stability of the Cr coating in SCW with different concentrations of oxygen is discussed.

Experimental study on the catalytic supercritical water oxidation of oilfield sludge

Oilfield sludge is a kind of hazardous waste. In this study, supercritical water oxidation (SCWO) was used to treat oilfield sludge. The effect of operating parameters was investigated, including temperature (390–450 °C), reaction time (5–30 min), oxidation coefficients (OC, 1.0–5.0) and Ni/Al2O3 catalyst. The experimental results showed that higher temperature and higher oxidation coefficient favored the degradation of oilfield sludge. Carbon in oilfield sludge was mainly converted to gas phase, for instance, 63.6% of carbon was converted to gas at 5.0 OC, 20 min and 450 °C without catalyst. The removal efficiency of total organic carbon (TRE) could be up to 96.0% at 450 °C, 20 min, and 4.0 OC without catalyst. Furthermore, the addition of Ni-based catalysts could also improve TRE. TRE and carbon conversion efficiency (CE) were 95.2% and 68.2% at 1.5 OC, 20 min and 450 °C with 20Ni/Al catalyst addition, respectively, which were higher than that of without catalyst (89.5% and 33.8%). The characterization and regeneration of catalyst were also carried out. Catalyst Ni/Al2O3 had good reliability, the TRE of 92.5% and the catalytic recovery efficiency of 52.6% were observed after three cycles.

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.

Purification of synthetic diamond and graphite mixtures in catalytic supercritical water oxidation processes

Purifying diamonds after high-pressure and high-temperature synthesis is essential for their industrial applications, but classical purification methods usually generate a massive amount of toxic liquid wastes or cannot achieve good separation efficiency in a short time. This work proposes to utilize a catalytic supercritical water oxidation process as an environmentally friendly, fast, and highly selective alternative. To test the technical feasibility, we first examined the oxidation performance of non-catalytic and catalytic processes and discovered that potassium hydroxide and potassium carbonate accelerate oxidation kinetics. We also examine the influence of the oxidation process on the structural and surface characteristics of the synthesized diamonds. Potassium carbonate was chosen as the best homogeneous catalyst that does not deteriorate the synthesized diamond but effectively removes graphite residues. These results demonstrate that catalytic supercritical water oxidation is suitable for purifying synthesized diamonds in industrial mixtures.

PFOS destruction in a continuous supercritical water oxidation reactor

Effective end-of-life technologies are needed for remediation of PFAS-containing byproducts and wastewater effluent streams. We describe a continuous flow supercritical water oxidation (SCWO) reactor operated in an autogenic regime achieved by introducing pilot fuel and compressed air; this system can achieve sufficient temperatures to destroy-one of the most recalcitrant PFAS -- perfluorooctane sulfonic acid (PFOS). We parametrically investigate PFOS and perfluorooctanoic acid (PFOA) destruction at temperatures between 410 °C and 650 °C. Targeted LC-MS/MS analysis suggests that oxidative CS bond cleavage initiates PFOS degradation, followed by decarboxylation to facilitate sequential -CF2 elimination and chain shortening. Trifluoroacetic acid (TFA) was found to be the most recalcitrant intermediate product in the effluent, persisting at T = 600 °C and residence time of ∼ 32 s, emphasizing the need for TFA monitoring in SCWO treatment of PFAS-impacted matrices. Analysis of gaseous products shows the presence of the volatile organofluorine (VOF) such as 1H-perfluoroalkane at T = 420–530 °C; the GS/MS spectra of gaseous reaction products suggest that other VOFs may be present. Free fluoride measurement by ion chromatography shows complete PFOS defluorination at T = 650 °C. Overall, this work establishes the operational envelope for SCWO treatment of PFAS-impacted liquids and suggests the chemical routes for PFAS destruction. Additional characterization of intermediate species in gas and liquid effluent is required to fully elucidate a chemical reaction mechanism for PFAS destruction in SCWO environments.

Catalytic supercritical water oxidation of tri-n-butyl phosphate: Process optimization by response surface methodology and cytotoxicity assessment

Catalytic supercritical water oxidation (SCWO) of an organophosphate flame retardant, namely tri-n-butyl phosphate (TNBP) was studied. Firstly, copper oxide nanoparticles (NPs) were synthesized in SCW and their properties were characterized by various analyses. Afterwards, their catalytic performance was investigated under different conditions including reaction temperature (400–500 °C), TNBP volume percentage in the feed (1–4%), oxidant ratio (0–2) and reaction time (50–150 min) based on response surface methodology (RSM). The synthesized CuO NPs had an average particle size of 30 nm with a narrow distribution. According to RSM analysis, the reaction temperature and time are the most significant factors; whereas, the impact of the other factors, especially TNBP volume percentage in the feed, was found to be negligible. Overall, excellent performance was achieved under optimal conditions found by the RSM, which was reaction temperature of 500 °C, TNBP volume percentage of 4%, oxidant ratio of 1.5, and reaction time of 90 min. The TOC removal efficiency as an indicator of TNBP degradation was about 99%. Finally, in vitro cell viability assays for the cytotoxicity evaluation of fresh and SCW-treated solution were applied. The results of MTT showed that SCWO converts TNBP into by-product that did not induce any cytotoxicity.

The mechanism of methyl hydrazine degradation by supercritical water oxidation using ReaxFF simulation

Methyl hydrazine (MMH) is a key ingredient in rocket fuel as well as an important intermediate and solvent in organic synthesis. However, any remaining MMH in environment could be harmful to both people and the environment. In this work, we proposed a method for treating MMH pollution by supercritical water oxidation (SCWO) technology. Reactive molecular dynamics were used to reveal the microscopic mechanism of the oxidation decomposition of MMH. The results showed that the principle of SCWO is that the OH radicals are formed by the reaction between H2O and O2. It can effectively capture the H both in MMH and its decomposition fragments, thus speeding up the decomposition. At low temperature and low oxygen content, the initial decomposition of MMH has two main pathways: (1) CH3NHNH2 → CH3NH + NH2 (R1), and (2) CH3NHNH2 + OH → CH3NNH2 + H2O (R7). N2, CO2, and H2O are the final products. At high temperatures, the intermediate NH3 can be converted to N2. The activation energy of MMH decomposition with excess oxygen was also obtained by the Arrhenius equation as 29.3 kcal/mol.

Insights into carbon monoxide oxidation in supercritical H2O/CO2 mixtures using reactive molecular dynamics simulations

Carbon monoxide is an important intermediate in supercritical water oxidation processes. Carbon monoxide oxidation in supercritical H2O/CO2 mixtures was investigated using reactive molecular dynamics simulations. The results show that CO mainly converts into CO2 through the reaction CO + OH → HOCO → CO2 + H. The intermediate HOCO can also react with OH or O2 to form CO2. The overall activation energies of CO oxidation in the supercritical medium range from 125.3 ± 4.0–159.4 ± 3.6 kJ/mol, which are roughly consistent with the experimental results. Supercritical water not only provides reactive free radicals for CO conversion but also acts as a third body to induce some chemical events. The high concentration of CO2 inhibits the production of OH but highlights the oxidizing action of O and HO2. Oxygen increases the amounts of OH and HO2 through the reaction H2O + O2, thereby promoting the conversion of CO.

Corrosion behaviour characterisation of 316L stainless steel and Inconel 625 in supercritical water containing hydrochloric acid and high oxygen

ABSTRACT Corrosion was currently the most crucial factor hindering the commercial development of supercritical water oxidation (SCWO) technology. Stainless steel represented by 316L and Ni-based alloy represented by Inconel 625 were performed and compared with corrosion resistance in supercritical environment with high concentrations of oxygen and chlorides. The results shown that the duplex layer was formed on two samples. The surface of 316L SS was mainly composed of loose magnetite and haematite, resulted in pitting corrosion, microcracks, and the exfoliation of oxide. The inner layer were separately a small amount of Cr2O3 oxide and FeCr2O4 spinel, which were caused by the lower content of chromium. Compared with 316L SS, the oxide film structure of Inconel 625 was composed of the outer layer of spinel and the inner layer of Cr-rich oxide. Some nodular oxides were formed, and it can be explained by the volatilisation process of chromium.

Concept of PPI energy technology complex upgrading

THE PURPOSE. Development and implementation of a set of theoretical and practical recommendations for creating scientifically founded heat flow diagrams for pulp and paper processes, intensifying heat exchanging processes under the economical use of energy resources. METHODS. The method of thermodynamic analysis on the basis of exergy increments in the process of interaction of heat flows is proposed, it allows evaluating the energy efficiency of both individual PPI units and the entire process flow; mathematical relationship between exergy efficiency of process elements and that of a complex heat flow diagram; schematic diagram of a heat recuperator of PBM drying section with a closed loop for the air delivered to drying. RESULTS. Thermodynamic analysis was made to evaluate energy efficiency of PPI equipment; the relationship between conversion factor of thermal pumping plant and thermodynamic properties of actuating media was obtained. Sulfate pulp production was considered in detail. The low energy efficiency of the existing process flow diagram is shown, with its exergy efficiency being equal to 48%. A main share of the consumed exergy (71% of the incoming one) relates to the recovery boiler processes. The process flow diagram consisting of hydrochemical pulpchips processing is suggested. The recovery boiler and rotary-type lime kiln are removed from that flow diagram. Risks of air and water basins pollution fall. The main ways for increasing energy efficiency of lignosulfonates production at sulfite pulp plants are mentioned. DISCUSSION. Further development of energy efficient pulp technologies can be forecasted on the basis of application of fluid technologies such as supercritical water oxidation (SCWO). The positive results of research in water chemistry under supercritical parameters give promise to predict success in developing the energy efficient pulp technology by exposing pulpchips to water at the subcritical and supercritical parameters. Paper and board production relates to the complex physical–chemical and heat-wet processes. The thermodynamic studies allowed determining low exergy efficiency of a drying section of paper and board machine that is because of inefficient operation of the heat recuperation unit. This circumstance is responsible for the bulk supply of low pressure steam from a heat and power plant. The main ways for improving the heat flow diagram of PBM drying section are shown. Cogeneration issues in the pulp and paper industry are considered. The essential attention is given to environmental safety of energy efficient technologies. CONCLUSION. The paper presents main ways for increasing energy efficiency of basic production processes at pulp and paper enterprises. Based on the analysis of exergy efficiency of individual equipment elements and the entire process flow diagrams bottlenecks were revealed and measures for their removal were suggested.

Supercritical water co-oxidation behavior in the different monohydric alcohol-ammonia reaction environment

The degradation of ammonia is a key rate-limiting step during the supercritical water oxidation of nitrogen-containing organics. This paper studied the co-oxidation behavior between different ammonia-alcohol environments, including the influence of reaction parameters and the co-oxidation mechanism. The results showed that increasing temperature, oxidation coefficient, residence time, and alcohol concentration significantly promoted the degradation of NH3–N and TOC, while rising the ammonia concentration enhanced the NH3–N destruction but inhibited the TOC degradation. Alcohols were oxidized first in the co-oxidation system to produce more OH* and HO2* radicals. Ethanol generated the highest concentration of HO2* in the shortest time, leading to more significant ammonia removal than isopropanol and methanol; however, the produced intermediate products like aldehydes and ketones reacted with residual ammonia to generate a small amount of organics at lower temperatures, inhibiting the degradation of alcohols slightly, and combined catalyst or nitrate in the batch reactor or used continuous supercritical water oxidation or supercritical hydrothermal combustion system without controlling the exotherm of fuels could improve this.

Management of waste printed circuit boards via supercritical water technology

In this study, the application of the supercritical water technology on the management of waste printed circuit boards (PCBs) obtained from small information technology (IT) and communication equipment was conducted. Initially, an extensive recycling-oriented characterization of waste printed circuit boards was conducted through a combination of several physicochemical analyzes such as MP-AES, XRF, TG/DTA, CHNS elemental analysis, SEM and XRD. Afterwards, at supercritical conditions of water (Tc > 374.29 °C and Pc > 22.089 MPa), the optimal conditions for the degradation of the organic polymers present in the waste PCBs were defined by a response surface methodology. A complete organic degradation rate (ODR) was achieved at 600 °C, reaction time of 60 min, waste PCBs load of 15 g and flow-rate of 5 mL min−1. After the removal of the organic polymers, the metals were liberated, and the metal recovery efficiency (MRE) reached values higher than 90% at all evaluated conditions. Then, initial insights on the subsequent treatment of the liquid decomposition by-products generated during the supercritical water (ScW) processing of waste PCBs were provided. A total organic carbon reduction of 99.88% was achieved via the ScW oxidation process using the same experimental apparatus. The treated solution was successfully re-used in the ScW processing of waste PCBs instead of clean water. Moreover, hydrogen, methane, CO2, and CO were identified as the major gaseous products associated to the supercritical water treatment of waste PCBs. Finally, a novel strategy to enhance the production of combustible gases, through the addition of ethanol and glycerol, and increase the economic feasibility of the supercritical water processing of waste PCBs was proposed.

Formation and evolution characteristics of hydrothermal flames inside a transpiring wall reactor: A transient numerical investigation

Transpiring-wall reactors with hydrothermal flames are effective in preventing corrosion and salt deposition in supercritical water oxidation systems. To uncover the not-well-understood evolution characteristics of the hydrothermal flame surrounded by a transpiring water film, a 2-D unsteady simulation is firstly conducted in this study. The transient results show that the variations of flow fields and flame shapes are attributed to the competition between inertial forces and buoyant forces. And the quasi-steady results present that high feed concentration may damage transpiring wall and delay the evolution of hydrothermal flames. In addition, a low feed temperature or a big flow area of jet can lead to open hydrothermal flames, which adversely affect the reactor. Furthermore, Froude Number, Fr, shows effectiveness in predicting quasi-steady flame shapes. For large-Fr conditions, the inertial force outweighs the buoyant force and hence closed flames are developed; while for low-Fr cases, open flames are developed.