Supercritical water oxidation process under energetically self-sufficient operation

Catalytic supercritical water oxidation of o-chloroaniline over Ru/rGO: Reaction variables, conversion pathways and nitrogen distribution

To ascertain the reaction variables on o-chloroaniline (o-ClA) mineralization, total nitrogen (TN) removal rate, and N-species distribution, o-ClA was subjected to catalytic supercritical water oxidation (CSCWO) in a fused quartz tube reactor (FQTR). The findings demonstrated that when the temperature, reaction time, and excess oxidant were 400 °C, 90 min, and 150%, respectively, the mineralization rate of o-ClA could reach more than 95%. Moreover, potential degradation pathways of o-ClA in supercritical water oxidation (SCWO) was proposed according to the GC-MS results. TN removal rate is significantly impacted by Ru/rGO, despite the fact that its catalytic effect on the mineralization of o-ClA was not particularly noteworthy. Compared with no catalyst, the TN removal rate of o-ClA obviously increased from 44.1% to 90.3% at 400 °C, 10 wt% Ru loading, 90 min and 200% excess oxidant. In addition, N-species distribution in SCWO and CSCWO were also investigated. Results indicated that the Ru/rGO catalyst could accelerate the oxidation of ammonia-N and convert it to nitrate-N, promoting N₂ generation. Finally, the possible N transformation pathway in CSCWO of o-ClA was proposed. As a result, this work offers fundamental information about o-ClA catalytic oxidation removal in the SCWO process.

Removal of Azo Dyes from Wastewater through Heterogeneous Photocatalysis and Supercritical Water Oxidation

Azo dyes are synthetic organic dyes used in the textile, leather, and paper industries. They pose environmental problems due to their toxic and persistent nature. The toxicity is due to the presence of azo groups in the dye molecule that can break down into aromatic amines, which are highly toxic to aquatic organisms and humans. Various treatment methods have been developed to remove azo dyes from wastewater. Conventional wastewater treatments have some drawbacks, such as high operating costs, long processing times, generation of sludge, and the formation of toxic by-products. For these reasons, a valid alternative is constituted by advanced oxidation processes. Good results have been obtained using heterogeneous photocatalysis and supercritical water oxidation. In the former method, a photocatalyst is in contact with wastewater, a suitable light activates the catalyst, and generated reactive oxygen species that react with pollutants through oxidative reactions to their complete mineralization; the latter involves pressurizing and heating wastewater to supercritical conditions in a reactor vessel, adding an oxidizing agent to the supercritical water, and allowing the mixture to react. In this review paper, works in the literature that deal with processing wastewater containing azo dyes through photocatalysts immobilized on macroscopic supports (structured photocatalysts) and the supercritical water oxidation technique have been critically analyzed. In particular, advancement in the formulation of structured photocatalysts for the degradation of azo dyes has been shown, underlying different important features, such as the type of support for the photoactive phase, reactor configuration, and photocatalytic efficiency in terms of dye degradation and photocatalyst stability. In the case of supercritical water oxidation, the main results regarding COD and TOC removal from wastewater containing azo dyes have been reported, taking into account the reactor type, operating pressure, and temperature, as well as the reaction time.

Oily sludge treatment in subcritical and supercritical water: A review

Oily sludge, an inherent byproduct of the petroleum industry, presents dual characteristics of petroleum resources and hazardous waste. Owing to the unique physicochemical properties of sub-/supercritical water, hydrothermal technologies have been increasingly used for oily sludge treatment. This review is the first to focus on oily sludge treatment using sub-/supercritical water. Eight hydrothermal technologies used for different purposes are summarized herein: pressurized hot water extraction (PHWE) for hydrocarbon separation, thermal hydrolysis (TH) for dewaterability improvement, hydrothermal carbonization (HTC) for hydrochar production, wet air oxidation (WAO) for biodegradability improvement, hydrothermal liquefaction (HTL) for bio-oil production, supercritical water upgrading (SCWU) for light oil production, supercritical water oxidation (SCWO) for complete degradation, and supercritical water gasification (SCWG) for H₂-rich syngas production. Moreover, a general reaction pathway for sub-/supercritical water treatment of oily sludge is presented, with a particular focus on the chemical mechanism at temperatures above 350 °C. Lastly, two reaction maps are included to illustrate the reaction pathways of two groups of identifiable model compounds in oily sludge: aliphatic and aromatic hydrocarbons. This review provides detailed information that can promote a better understanding of various hydrothermal technologies, a guideline for selecting the suitable hydrothermal process for a particular oily sludge, and recommendations for further researches.

Analysis of the Energy Flow in a Municipal Wastewater Treatment Plant Based on a Supercritical Water Oxidation Reactor Coupled to a Gas Turbine

Biological municipal wastewater treatments lead to high sludge generation and long retention times, and the possibilities for recovery of the energy content of the input waste stream are very limited due to the low operating temperature. As an alternative, we propose a sequence of exclusively physicochemical, non-biological stages that avoid sludge production, while producing high-grade energy outflows favoring recovery, all in shorter times. Ultrafiltration and evaporation units provide a front-end concentration block, while a supercritical water oxidation reactor serves as the main treatment unit. A new approach for energy recovery from the effluent of the reactor is proposed, based on its injection in a gas turbine, which presents advantages over simpler direct utilization methods from operational and efficiency points of view. A process layout and a numerical simulation to assess this proposal have been developed. Results show that the model process, characterized with proven operating parameters, found a range of feasible solutions to the treatment problem with similar energy costs, at a fast speed, without sludge production, while co-generating the municipality’s average electricity consumption.

Combined Gasification-Oxidation System for Waste Treatment with Supercritical Water: LCA and Performance Analysis

In this study the environmental performance of a first-of-its-kind integrated process based on supercritical water gasification and oxidation (SCW-GcO), was evaluated using life cycle assessment (LCA). The process was applied to the treatment of carbon black and used oil as model wastes. Mass and energy balances were performed using Aspen Plus, and the environmental assessment was carried out through SimaPro. A “from cradle to grave” approach was chosen for the analysis, considering impact categories such as climate change, ozone depletion, human toxicity, particulate matter, land use, resource depletion, and other relevant indicators. The environmental profile of the SCW-GcO process was compared to other technologies for the treatment of dangerous wastes, solvent mixtures, and exhaust mineral oils by using the Ecoinvent database. It is shown that SCW-GcO allows for reduced impacts in different categories and the obtention of a favorable positive life cycle energy balance, achieving good environmental performance.

Supercritical water oxidation and process enhancement of nitrogen-containing organics and ammonia

Supercritical water oxidation (SCWO), as a promising technology for treating organic wastewater and sludge, has attracted the attention of many scholars. Nitrogen-containing organics are refractory substances that widely exist in industrial waste, and their effective degradation is of great significance to the environment. In this paper, the treatment effects, reaction kinetics, and migration and transformation pathways of various nitrogen-containing organics (amino group, nitro group, mixed group, and nitrogen heteroatom) under SCWO conditions are summarized, and the influences of the reaction temperature, oxidant type and concentration, residence time, and initial concentration of organics on the degradation of organics are also discussed. NH₃-N is the primary intermediate product produced during the oxidation process of the amino group and nitrogen heteroatom organics, and the further degradation of NH₃-N is the limiting step for the whole reaction. This paper focuses on the relevant strengthening technologies used to enhance the degradation of NH₃-N, including heterogeneous catalytic oxidation with reactor wall or metal oxides; co-oxidation with auxiliary fuels such as methanol, ethanol, isopropanol, and glycol; strong oxidation with NO₃^- or NO₂^-; and segmented oxidation by multi-injection of oxidants or fuels. In addition, in order to achieve the complete removal of NH₃-N and COD synergistically under relatively mild SCWO conditions, avoid the formation of NO_x, NO₃^-, and NO₂^-, and convert organic nitrogen into environmentally friendly products such as N₂ and N₂O, further research requirements and challenges are introduced.

Modeling and simulation of hydrothermal oxidation of cutting oil in supercritical water

Supercritical water oxidation may be used as waste treatment technique that consists of oxidizing organic and inorganic matter using water at supercritical conditions. It was developed as an alternative technique in order to limit the risks of secondary pollution. The purpose of this study is the development of simulation tools in stationary state in supercritical water oxidation (SCWO) tubular reactor which has been numerically investigated basing on using a two-dimensional modeling approach applying the k-ε turbulence model, and on the international association for the properties of water and steam formulation (IAPWS-IF97). A Multiphysics simulation using Comsol Multiphysics 5.2 software for the supercritical water oxidation of “cutting oil Biocut 35” as a pollutant is reported. First, the heat transfer coefficient was estimated and the obtained temperature and species concentration profiles were compared to experimental data where a quite good agreement was obtained as confirmed by the acceptable coefficient of determination value. Finally, once the used model was validated, numerical simulations were performed to describe the behavior of the reactor investigating the effects of operating conditions such as temperature, pollutant feed concentration and reactor inlet flow rate, on dependent variables like outlet temperature, chemical oxygen demand removal (COD) and the highest temperature reached inside the reactor. This last parameter was considered in the study to take into account the operation safety. The significance of the studied factors and their interactions were quantified and analyzed by means of the full factorial design of experiment (DOE). The results showed that the effect of initial temperature was the most important for the three responses, followed by the feed concentration then to a lesser extent the flow rate. The effect of initial temperature was positive for outlet temperature, maximal temperature and cutting oil removal. The interaction temperature-feed concentration was the only significant one for the COD removal and the maximal temperature. The results defined an operation safety zone of the SCWO reactor based on the superposition of the three studied dependent variables and imposed constraints on the inside reactor temperature and the pollutant outlet concentration.

Supercritical water oxidation of semi-coke wastewater: Effects of operating parameters, reaction mechanism and process enhancement

Semi-coke wastewater is a kind of industrial wastewater with complex composition, high concentration of organic pollutants and high chroma, seriously threatening the ecological environment and requiring to be effectively degraded. Supercritical water oxidation (SCWO), as for a promising environmental technology, was applied to treat semi-coke wastewater in this work. The influences of key operating parameters such as reaction temperature (400-600 °C), oxidation coefficient (1.0-4.0) and residence time (0.5-10 min), the reaction mechanism for organics in semi-coke wastewater and the process enhancement methods like catalytic oxidation and segmented oxidation were systematically investigated. Experimental results showed that the removal efficiency of COD and NH₃-N both significantly increased with the increasing of temperature, oxidation coefficient and residence time, the COD removal efficiency and NH₃-N removal efficiency could be 99.02% and 63.94% obtained under the condition of 600 °C, 25 MPa, 1.3 times oxidation coefficient and 10 min. The residual organics in liquid products were mainly phenols, ketones, imidazoles, esters and pyridines, which produced from the cyclization and esterification reaction between intermediate products such as alcohols, aldehydes, acids and NH₃-N, etc. What's more, NH₃-N was proved to have inhibitory effect on the degradation of phenol by generating more stubborn nitrogen-containing compounds with that. Besides, compared with single catalyst, the composite catalyst of MnO₂/CeO₂ exhibited the highest catalytic activity, which could synergistically degrade 98.52% COD and 67.18% NH₃-N under a relatively mild reaction condition (550 °C, 25 MPa, 1.3 times oxidation coefficient, 2 min). Moreover, the segmented oxidation, combining the pre-oxidation in preheater and oxidation in reactor, was firstly observed and analyzed here, could achieve a higher COD removal efficiency with a shorter length of the reactor. The results obtained in this paper proved the technical feasibility and could provide basic data support for the industrialization of semi-coke wastewater treatment by SCWO.

Review on an Advanced Combustion Technology: Supercritical Hydrothermal Combustion

Supercritical hydrothermal combustion, a new and promising homogeneous combustion technology with a wide range of application scenarios and broad development prospects, provides creative ideas and means for the enhanced degradation of organic wastes, hydrothermal spallation drilling, thermal recovery of heavy oil, etc. This technology is elaborated upon in five parts: (1) introducing the main devices including semi-batch reactor and continuous reactor to study the hydrothermal flame in accordance with research institutions, (2) presenting the research status of related numerical simulation from the angles of reaction kinetics and flow-reaction, (3) summarizing the characteristics of hydrothermal flame and combustion by five key parameters, (4) dividing up ignition process and explaining ignition mechanism from the perspectives of critical physical properties of water and heat transfer and mixing conditions, (5) discussing and forecasting its industrial applications including hydrothermal spallation drilling, the thermal recovery of heavy oil, the clean conversion and utilization of coal-based fuel, and the harmless treatment of pollutants. By and large, this paper analyzed in detail everything from experimental equipment to industrial applications, from combustion characteristics to ignition mechanisms, and from summary conclusions to prospect prediction. In the end, herein is summarized a couple of existing paramount scientific and technical obstacles in hydrothermal combustion. Further significant studies in the future should include excellent reactors, advanced monitoring techniques, and powerful computational fluid dynamics.

Novel design concept for a commercial-scale plant for supercritical water oxidation of industrial and sewage sludge

Supercritical water oxidation (SCWO) is a promising chemical technology for organic waste water and sludge treatment. Our team has successfully constructed the first pilot-scale SCWO plant in China, and the design concept for our first commercial-scale plant is reported in this paper. The challenges that hinder the commercial development of SCWO are introduced, including corrosion, plugging, high investment and operating costs. Some important lab-scale and pilot-scale experimental results are shown, and some key design parameters for the commercial plant are proposed. The technological process, specialized equipment design and new system flowsheet are described objectively. Moreover, an estimate of the equipment investment and operating costs of this commercial plant is carried out, and a comparison is made with other commercial sludge SCWO plants. This information is valuable for guiding how to best design commercial SCWO plants for the treatment of sludge and other feedstocks including solid particles.

Energy Consumption and Economic Analyses of a Supercritical Water Oxidation System with Oxygen Recovery

Oxygen consumption is one of the factors that contributes to the high treatment cost of a supercritical water oxidation (SCWO) system. In this work, we proposed an oxygen recovery (OR) process for an SCWO system based on the solubility difference between oxygen and CO2 in high-pressure water. A two-stage gas–liquid separation process was established using Aspen Plus software to obtain the optimized separation parameters. Accordingly, energy consumption and economic analyses were conducted for the SCWO process with and without OR. Electricity, depreciation, and oxygen costs contribute to the major cost of the SCWO system without OR, accounting for 46.18, 30.24, and 18.01 $·t−1, respectively. When OR was introduced, the total treatment cost decreased from 56.80 $·t−1 to 46.17 $·t−1, with a reduction of 18.82%. Operating cost can be significantly reduced at higher values of the stoichiometric oxygen excess for the SCWO system with OR. Moreover, the treatment cost for the SCWO system with OR decreases with increasing feed concentration for more reaction heat and oxygen recovery.

Thermolysis of scrap tire and rubber in sub/super-critical water

The rapid growth of waste tires has become a serious environmental issue. Energy and material recovery is regarded as a promising use for waste tires. Thermolysis of scrap tire (ST), natural rubber (NR), and styrene-butadiene rubber (SBR) was carried out in subcritical and supercritical water using a temperature-pressure independent adjustable batch tubular reactor. As a result, oil yields increased as temperature and pressure increased, and they reached maximum values as the state of water was near the critical point. However, further increases in water temperature and pressure reduced the oil yields. The maximum oil yield of 21.21% was obtained at 420 °C and 18 MPa with a reaction time of 40 min. The relative molecular weights of the chemicals in the oil products were in the range of 70-140 g/mole. The oil produced from ST, NR, and SBR contained similar chemical compounds, but the oil yield of SR was between those of NR and SBR. The oil yield from thermolysis of subcritical or supercritical water should be further improved. The main gaseous products, including CH₄, C₂H₂, C₂H₄, C₂H₆, and C₃H₈, increased with reaction time, temperature, and pressure, whereas the solid residues, including carbon black and impurities, decreased. These results provide useful information to develop a sub/super-critical water thermolysis process for energy and material regeneration from waste tires.

Application of red mud as both neutralizer and catalyst in supercritical water oxidation (SCWO) disposal of sewage sludge

Red mud was used in the supercritical water oxidation (SCWO) disposal of sewage sludge, not only as a neutralizer for acidic substances produced in situ, but also as a catalyst for decomposition of pollutants.

Current and foreseeable applications of supercritical water for energy and the environment

It is crucial to develop economical and energy-efficient processes for the sustainable transformation of biomass into fuels and chemicals. In this context, supercritical water biomass valorization (SCBV) processes are an alternative way to produce biogas, biofuels, and valuable chemicals. Supercritical water technology has seen much progress over the last fifteen years and an industrial application has merged: the supercritical water oxidation of wastes. The evolution from lab-scale to pilot-scale facilities has provided data on reaction mechanisms, kinetics, modeling, and reactor technology as well as an important know-how, which can now be exploited to use the reactivity in supercritical water to transform biomass into gases (CO, H(2), CO(2), CH(4), and N(2)) or into liquids (liquid fuel and valuable chemicals) with the supercritical water biomass gasification and liquefaction processes, respectively. This Review highlights the potential of SCBV processes to transform biomass into gas and liquid energy sources and highlights the developments that are still necessary to push this technology onto the market.

Hydrothermal oxidation: application to the treatment of different cutting fluid wastes

The aim of this work is to present the application of the hydrothermal oxidation in supercritical conditions, also named supercritical water oxidation (SCWO) to the treatment of two commercial cutting fluids: Biocut and Servol. Experiments were carried out in a continuous flow system at a constant pressure of 25 MPa, using pure oxygen as oxidant in excess, and different temperatures ranging from 673 to 773 K. Both semi-synthetic cutting fluids are a mixture of several compounds so the efficiency of the oxidation process was followed in terms of the reduction in chemical oxygen demand (COD) and total organic carbon (TOC). A comparison of the results obtained in the study showed that it is possible to apply successfully SCWO for both cutting fluids, obtaining more than 95% for both COD and TOC removal at 773 K. However, the results also show that different residence times are needed to obtain the same percentage of COD or TOC removal depending on the cutting fluid treated, being in all cases Servol easier to oxidize than Biocut. A kinetic model to predict COD and TOC conversion has been proposed for both cutting fluids. A two-parameter mathematical model involving two steps (a fast reaction followed by a slow reaction) was used to describe the Biocut SCWO kinetics and to calculate the kinetic constants.