Supercritical Water Oxidation Reactor Research Articles

Experimental study on supercritical water oxidation of oily sludge with auxiliary fuels

Oily sludge, a natural by-product of the petroleum industry, is a hazardous waste that is challenging to dispose of. In this paper, degradation of pollutants and heavy metal migration in supercritical water oxidation (SCWO) were studied, illuminating the influence and behavior mechanisms of auxiliary fuels. First, it was determined how reaction parameters affected C and N degradation as well as products for oily sludge SCWO with methanol. With addition of 800 mmol/L methanol, the removal efficiencies of TOC and TN at 550 °C were 97.9% and 67.0%, respectively. Organic materials could not completely decompose due to long-chain alkanes. The evaluation of auxiliary fuel type and injection technique revealed excellent enhanced degradation effect of ethylene glycol and improvement result of two-stage injection. Finally, the distribution of heavy metals before and after SCWO reaction was investigated. Heavy metals were stabilized, and the aqueous product and solid residual both met heavy metal discharge criteria.

Destruction and defluorination of PFAS matrix in continuous-flow supercritical water oxidation reactor: Effect of operating temperature

Cleanup and disposal of stockpiles and waste streams containing per- and polyfluoroalkyl substances (PFAS) require effective end-of-life destruction/mineralization technologies. Two classes of PFAS, perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl sulfonic acids (PFSAs), are commonly found in legacy stockpiles, industrial waste streams, and as environmental pollutants. Continuous flow supercritical water oxidation (SCWO) reactors have been shown to destroy several PFAS and aqueous film-forming foams. However, a direct comparison of the SCWO efficacy for PFSAs and PFCAs has not been reported. We show the effectiveness of continuous flow SCWO treatment for a matrix of model PFCAs and PFSAs as a function of operating temperature. PFSAs appear to be significantly more recalcitrant than PFCAs in the SCWO environment. The SCWO treatment results in a destruction and removal efficiency of 99.999% at a T > 610 °C and at a residence time of ∼30 s. Fluoride recovery lags destruction PFAS at 510 °C and reaches >100% above 610 °C, confirming the formation of liquid and gaseous phase intermediate product during lower temperature oxidation. This paper establishes the threshold for destroying PFAS-containing liquids under SCWO conditions.

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.

Pfos Destruction in a Continuous Supercritical Water Oxidation Reactor

Pfos Destruction in a Continuous Supercritical Water Oxidation Reactor

Catalytic and non-catalytic degradation of acetaminophen in supercritical water

Pharmaceutical industrial wastewater is typical wastewater consisting of complex organic compounds with higher concentration, microbial toxicity, strenuous to deteriorate, and environmental threatening. The present work assesses the degradation of recalcitrant acetaminophen (ACM) by a green technology known as supercritical water oxidation (SCWO). Experiments were carried out in a continuous flow SCWO reactor by altering reaction conditions such as temperature 400–600 °C, oxidant coefficient (OC 0 to 3), and Fe(II) catalyst concentration (0.5 and 1 mg L−1) to study the technical feasibility of highly concentrated ACM. Liquid product analysis indicated the total organic carbon (TOC) removal efficiency could reach up to 99.5% without catalyst at 600 °C and 99.9% with Fe(II) at 500 °C. The addition of Fe not only suppressed the intermediate ring components but also promoted the formation of permanent gases via decarboxylation and reforming reactions. The reaction between Fe(II) and H2O2 in supercritical water is extremely fast, which has a direct impact on the system's operating conditions. The high activity exhibited by Fe(II) catalyst degraded the ACM completely at an operating condition of 500 °C. Maximum H2 fraction was attained without catalyst at 600 °C, OC 0.5, and with the catalyst at 500 °C, respectively, whereas, CO2 tends to rise significantly with both temperature and oxidant concentration. The catalytic process is efficient in comparison to the non-catalytic process. A possible reaction pathway was proposed based on the intermediates generated during the degradation.

Optimal design and thermodynamic analysis on the hydrogen oxidation reactor in a combined hydrogen production and power generation system based on coal gasification in supercritical water

The novel power generation system based on coal gasification in supercritical water (SCW) holds great promise to meet the increasing energy demands in China, with less emissions in comparison to the traditional coal-fired power generation. The stable and complete oxidation of hydrogen occurring in the supercritical water oxidation (SCWO) reactor is paramount to the safety and overall energy efficiency of the system. In order to better understand and improve the performance of this reactor, optimal design and thermodynamic analysis were conducted using Aspen Plus. The outlet hydrogen concentration and the internal temperature of the reactor were taken as the two key indicators of the optimal design. The optimized results for the reactor were found to be the reactor diameter of 90 mm, axial length of 10.75 m, oxygen inlet number of 3 with the oxygen ratio of 11.0%/39.9%/49.1%, under the given inlet working fluid components and flow rates. The three-stage reactor design was confirmed to have good robustness when different types of coals are adopted for generating syngas. In addition, by analyzing the enthalpy and exergy flow distributions in the reactor, it was revealed that the total exergy destruction for the whole three-stage reactor is only about 6.29%, and that the exergy destruction is consistent with the oxidation process. These findings could be used as guidelines for the design and implementation of the SCWO reactor towards higher safety margin and energy efficiency for this new coal-boiled power generation technique.

Design of a Small-Scale Supercritical Water Oxidation Reactor. Part I: Experimental Characterization

Supercritical water oxidation (SCWO) has been previously studied in the context of the destruction of organic compounds; however, information regarding the practical design and the details about th...

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.

Intensification of supercritical water oxidation (ScWO) by ion exchange with zeolite for the reuse of landfill leachates

The disposal of solid residues in sanitary landfills results in the formation of a complex, variable, and recalcitrant wastewater, known as leachates. Supercritical water oxidation (ScWO) can be applied to treat leachates although most studies are based on removing the most relevant contaminants, such as organic matter and ammonia. Therefore, comprehensive analysis of this process is essential for large-scale applications. In this study, we investigated a system composed of ScWO and ion exchange using zeolite (ScWO/zeolite) for the reuse possibilities of treated leachates based on different regulations for municipal wastewater reuse. This system was applied to both raw leachate (RL) and leachate treated via conventional processes at the studied landfill (PL). The continuous ScWO reactor operated under a pressure of 23 MPa at 600 °C without the addition of oxidants. A commercial zeolite (clinoptilolite) in a fixed-bed glass column was used for ion exchange. The intensified system significantly improved the characteristics of RL by removing 89% of COD and 99% of NH3-N. Moreover, the contaminant concentrations of PL were within the limits for discharge and reuse, except arsenic and molybdenum contents. The unexpected high concentrations of arsenic in RL and PL necessitated the requirement of further investigation of the complex and toxic characteristics of leachates. Nevertheless, the intensified process was conducted without the addition of oxidants or auxiliary substances and resulted in a less expensive and more environmentally -friendly process that can be applied for the treatment of leachates with similar characteristics.

Destruction of perfluorooctanesulfonate (PFOS) in a batch supercritical water oxidation reactor

Effective technologies are needed for the destruction of per- and polyfluoroalkyl substances (PFAS). One promising technology is supercritical water oxidation (SCWO), which can be accommodated in batch or continuous reactors. Many PFAS-laden wastes consist primarily of solid particles, and batch SCWO processing may offer safe end-of-life PFAS destruction for these feedstocks. In this study, perfluorooctanesulfonate (PFOS) is reacted via supercritical water oxidation in a batch reactor at temperatures between 425 and 500 °C and residence times from 0 to 60 min, to determine the effect of both parameters on the extent of destruction and defluorination. Analysis of liquid products via targeted LC-QToF-MS does not indicate production of intermediate fluorocarbons. However, a low fluorine mass balance at temperatures of 425 and 450 °C may indicate the existence of fluorinated species in the gaseous and/or liquid product which are not detected by targeted analysis. Destruction and defluorination efficiencies are determined for each tested condition, with a maximum 70.0% PFOS destruction and 78.2% defluorination achieved after 60 min of reaction at 500 °C.

Supercritical water oxidation of glyphosate wastewater

In this paper, the treatment characteristics of glyphosate wastewater based on supercritical water oxidation (SCWO) technology is studied both in spiral coiled tube reactor and tank reactor, including the influences of operating parameters such as reaction temperature, reaction pressure, oxidation coefficient, residence time, and initial properties of wastewater such as organic concentration, pH value on the degradation of COD and NH3-N with H2O2 as oxidant, and the different reaction characteristics in different batch reactors are also analyzed in detail. Furthermore, the simplified SCWO reaction pathway of glyphosate wastewater is also proposed to understand the degradation process better. Finally, a comprehensive SCWO system for treating glyphosate wastewater efficiently, and transforming and utilizing organic nitrogen as well as organophosphorus directionally is proposed and designed, including SCWO unit, ammonia distillation unit, phosphate recovery unit, and biochemical treatment unit, which can enhance the treatment efficiency and economic benefits synergistically, and provide basic data for the industrialization of glyphosate wastewater degradation based on SCWO technology.

Novel designs for the reliability and safety of supercritical water oxidation process for sludge treatment

For strengthening the safety and reliability for supercritical water oxidation (SCWO) process, the essential causes and solutions of key challenges such as the high corrosion risk of the heat-exchanger and the entrance part of SCWO reactor, the blockage problems resulted from the solid deposition, and the safety and environmental issues caused by the emergency discharge of supercritical aqueous fluids, were investigated theoretically and experimentally. The thermodynamic state division between chemical- and electrochemical corrosion and the corrosion sensitive temperature range of 280−450 °C featuring the predominant electrochemical corrosion were proposed. A set of optimized parameters for the petrochemical sludge treatment such as a temperature of 520–580 ℃, a pressure of 23−25 MPa, and an oxidation coefficient of 1.1–1.3, which not only can ensure the high enough removal rate of organic pollutants but also prevent the corrosion of construction materials as much as possible; the critical velocity of conveying fluid for avoiding the deposition of solid particles were also obtained. On the bases of innovatively developing the “indirect heat transfer” unit, the mixing-preoxidation equipment, the oxygen removal process for SCWO effluents, and the emergency discharge protection device, a novel SCWO process and its corresponding control scheme were proposed and analyzed.

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

AbstractSupercritical 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.

Study on the uniformity of water film in a transpiring wall reactor for supercritical water oxidation

AbstractA three‐dimensional computational fluid dynamic model of a transpiring wall reactor for supercritical water oxidation has been built to optimize the uniformity of water film. Results show that the temperature and species distributions at the nozzle outlet deviate from the reactor centre. The inner wall of the porous tube near the transpiring water injection tube displays low temperatures, while high temperatures are recorded far from the injection tube. The circumferential temperature distribution on the inner wall of the porous tube is uneven. This phenomenon is due to the uneven injection of the transpiring water, leading to the uneven protection of the water film and local overheating of the porous wall. The injection velocity of the transpiring water significantly decreases when the number of injection tubes is increased, and the circumferential velocity and temperature distributions on the porous wall gradually become even. Moreover, high pressure drops across the porous wall at low porosities are useful for the uniform injection of the transpiring water. This characteristic is also conducive to obtaining a more uniform water film protection.

Assessment of carbon recovery from solid organic wastes by supercritical water oxidation for a regenerative life support system

The carbon recovery from organic space waste by supercritical water oxidation (SCWO) was studied to support resource recovery in a regenerative life support system. Resource recovery is of utmost importance in such systems which only have a limited total amount of mass. However, the practical waste treatment strategies for solid space wastes employed today are only storing and disposal without further recovery. This work assesses the performance of SCWO at recovering organic wastes as CO2 and water, to discuss the superiority of SCWO over most present strategies, and to evaluate the different SCWO reactor systems for space application. Experiments were carried out with a batch and a continuous reactor at different reaction conditions. The liquid and gas products distribution were analyzed to understand the conversion of organics in SCWO. Up to 97% and 93% of the feed carbon were recovered as CO2 in the continuous and the batch reactor, respectively. Residual carbon was mostly found as soluble organics in the effluent. Compared with the batch reactor, the continuous reactor system demonstrated a ten times higher capacity within the same reactor volume, while the batch reactor system was capable of handling feeds that contained particulate matter though suffering from poor heat integration (hence low-energy efficiency) and inter-batch variability. It was concluded that SCWO could be a promising technology to treat solid wastes for space applications. A continuous reactor would be more suitable for a regenerative life support system.

Experimental study on the mixing characteristics inside an inner preheating transpiring-wall reactor for supercritical water oxidation

The mixing characteristics between bulk flows and transpiring water are critical to feed degradation and water film formation inside a transpiring-wall reactor (TWR). In this study, a supercritical water oxidation system with an inner preheating TWR is established, and special experiments are designed by adding organic matter (glycerol) to the transpiring water to evaluate the mixing degree between the bulk flows and transpiring water in the hydrothermal flame zone. Although the chemical oxygen demand concentration of the reactor effluent increases significantly due to the addition of glycerol to the transpiring water, a certain amount of glycerol is entrapped into the flame zone and degraded. The increase in jet velocity with increased heat source flow or temperature is favorable for mixing and enhances jet entrainment. Moreover, increasing the transpiring coefficient and transpiring water temperature can increase the radial velocity of transpiring water, which also benefits mixing. Possible applications utilizing the mixing characteristics are further proposed.

Corrosion characteristic comparisons of ZrO2-, TiO2-coated and uncoated 316 stainless steel samples in supercritical water oxidation of municipal sludge

Supercritical water oxidation (SCWO) can efficiently and harmlessly dispose municipal sludge (MS), but typically faces severe reactor corrosion problems. In this work, 316 stainless steel (316SS) sample surface was processed by atmospheric plasma spraying to form a protective ZrO2 or TiO2 coating. The 316SS, ZrO2/316SS and TiO2/316SS samples were exposed to a typical MS SCWO environment (at 450 °C, 24 MPa and 1.2 of oxidation coefficient) for 20 h–150 h. The results show that prolonging exposure time increased corrosion and surface coating could inhibit 316SS corrosion. ZrO2/316SS showed the best corrosion resistance among 316SS, ZrO2/316SS and TiO2/316SS, and its maximum corrosion rate was 0.0097 mg·cm−2·h-1. The oxide film of ZrO2/316SS after 150 h of corrosion exposure consisted of ZrO2, FeO, Fe2O3, NiO and Cr2O3, and its inner layer was rich in Cr2O3. The ZrO2 coating was much more stable than the TiO2 coating, and can be considered as the coating of the 316SS structure material for a MS SCWO reactor.

Continuous supercritical water oxidation treatment of oil-based drill cuttings using municipal sewage sludge as diluent

Oil-based drill cuttings (OBDC) is a characteristic hazardous waste that is generated in oil and gas exploration. In this study, two typical OBDCs from shale gas fields were treated in a continuous supercritical water oxidation (SCWO) for the first time. Because both heat value and ash content (AC) in the OBDCs were well beyond the capacity of continuous operation, municipal sewage sludge (MSS) was innovatively adapted as the diluent. A mixed sludge with OBDC addition levels of 10%, 20%, and 30% was tested using a novel SCWO reactor. Mean residence times of reactants in different reaction zones were specifically calculated. Results indicated the organic carbon removal efficiency could reach up to 98.44%. Eight detected heavy metals were found to be almost completely removed into solid products, and the concentrations in liquid products were all below the discharge limits. It was also found that the SCWO reactor exhibited good anti-plugging and anti-corrosion performance. The AC in the feedstock was up to 28.58%. To the best of our knowledge, this has, hitherto, not been achieved in a continuous SCWO operation. This study provides a new approach for harmlessly and completely degrading OBDC, and is also helpful for the industrialization of SCWO technology.

Global direct methanol fuel cells market will reach $367 M by 2025: Zion Market Research

Systematical corrosion tests of austenitic stainless steel 316L exposed to sewage sludge SCWO (supercritical water oxidation) were conducted in a batch stirred reactor with hydrogen peroxide as oxidant. Experiment conditions such as temperature, oxidation coefficient, pH value, corrosion medium, were chosen mainly keeping in mind the place and environment of reactions (i.e. surrounding transpiring wall). The exposed samples were ultimately analyzed by weight measurement, scanning electron microscopy in conjunction with energy dispersive spectroscopy, and X-ray diffraction analysis. The results show that severe pitting corrosion occurred as the sample was exposed to complicated environments, and different oxides including Fe3O4, FeCr2O4 and MoO3 were found on the sample surface. The corrosion rate at all test conditions (360–450 °C pH = 5.2–10.05, oxidation coefficient of 0–2.0, sewage sludge or its SCWO reactor effluent) was in the range of 0.12–0.66 mm/y, and it increased as temperature and OC increased at supercritical conditions. Moreover, potential corrosion mechanism of 316L in sewage sludge SCWO is proposed, and influences of operating parameters on 316L corrosion properties are summarized. 316L and reactor effluent could be considered as transpiring wall material and transpiring water in sewage sludge SCWO with transpiring wall reactor, respectively.

Hydrothermal ethanol flames in Co-flow jets

Results on the autoignition and stabilization of ethanol hydrothermal flames in a Supercritical Water Oxidation (SCWO) reactor operating at constant pressure are reported. The flames are observed as luminous reaction zones occurring in supercritical water; i.e., water at conditions above its critical point (approximately 22 MPa and 374 °C). A co-flow injector is used to inject fuel (inner flow), comprising an aqueous solution ranging from 20%-v to 50%-v ethanol, and air (annular flow) into a reactor filled with supercritical water at approximately 24.3 MPa and 425 °C. Results show hydrothermal flames are autoignited and form diffusion flames which exhibit laminar and/or turbulent features depending upon flow conditions. Two orthogonal camera views are used; one providing a backlit shadowgraphic image of the co-flow jet and the other providing color images of the flame. In addition, spectroscopic measurements of flame emissions in the UV and visible spectrum are discussed.