Supercritical Water Oxidation Research Articles

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.

Investigation of hydrogen oxidation in supercritical H2O/CO2 mixtures using ReaxFF molecular dynamics simulation

Hydrogen oxidation kinetics in supercritical H2O/CO2 mixtures is a fundamental topic in supercritical water oxidation (SCWO) technology. A series of reactive molecular dynamics (ReaxFF-MD) simulations were performed to investigate hydrogen oxidation process in supercritical mixtures. The results showed that HO2 and H2O2 radicals played key roles in the reaction kinetics. High concentration H2O suppressed the production of OH radical and increased steric hindrance for effective collisions, exerting negative influence on hydrogen oxidation. The presence of CO2 advanced the oxidation rate of hydrogen mainly through the elementary reaction CO2 + H → CO + OH. It was found that high O2 concentration promoted the oxidation of hydrogen and also affected the reaction induction time. The hydrogen reaction mechanisms under supercritical conditions were illustrated, showing different characteristics from those at atmospheric condition. The results will facilitate the development of continuum-scale kinetic models for accurate prediction of hydrogen oxidation behavior in supercritical systems.

A novel synergistic denitrification mechanism during supercritical water oxidation

Nitrogen transformation processes have an important impact on human life. In this work, the synergistic denitrification mechanism in the process of supercritical water oxidation (SCWO) was studied. N-containing organic compounds had better denitrification effects than the inorganics because they could produce more active radicals. Nitroaniline had higher total nitrogen (TN) removal rates than those of compounds containing only amino or nitro group due to the intramolecular synergistic denitrification. Meanwhile, intermolecular synergistic denitrification was found between nitro and amino/N-heterocyclic compounds, but its denitrification efficiency was not as good as intramolecular synergistic denitrification. The reaction mechanism of synergistic denitrification was proposed, which was similar to anaerobic ammonia oxidation. In addition, the overall equilibrium process of intermolecular synergistic denitrification was established. It included the rapid reduction of org-N at the early stage of SCWO process, as well as the limiting step of slow denitrification rate of NH4+ and NO3− in the late stage. The relationship between TN removal rate of mixture (kMIX-TN) and monomer compound (kNO2-TN or kNH2-TN) could be expressed by an equation, that was kMIX-TN = 3.3698 kNO2-TN·kNH2-TN + 0.0901, with R2 of 0.7788, which demonstrated that kMIX-TN was linearly related to kNO2-TN·kNH2-TN. All these works provided theoretical basis for nitrogen removal in water by SCWO technology.

Combined effects of protective film and oxidant on the performance of the supercritical water oxidation system with a film protective reactor: A simulation study

Supercritical water oxidation (SCWO) is a promising technology for treating organic waste and recovering energy. Water film protective reactor (WFPR), which is also referred to as transpiring wall reactor, provides a solution to corrosion and salt plugging issues. Three novel film protective reactors (FPRs) using air, nitrogen, and hydrogen peroxide as protective fluids were proposed to improve the corrosion and cost issues in WFPRs. Detailed optimized process parameters and flow field characteristics within the FPRs were obtained using Aspen Plus and computational fluid dynamics, respectively. The anti-corrosion performance and prevention of salt plugging in the FPRs were analyzed, and economic analyses for SCWO systems with different FPRs were conducted. Results indicate that the WFPR and hydrogen peroxide FPR present good solubility for inorganic salts but a high probability of corrosion. An SCWO system with the WFPR or nitrogen FPR is more competitive for industrial plants compared with other SCWO systems.

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.

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.

Effect of Salt Deposits on Corrosion Behavior of Ni-Based Alloys in Supercritical Water Oxidation of High-Salinity Organic Wastewater

AbstractIn this study, the corrosion behavior of nickel-based alloys 625, 825, and 800 with a salt deposit layer was investigated in supercritical water containing a dissolved oxygen of 5,000 ppm a...

The treatment of detergent industry wastewaters by supercritical water oxidation

The treatment of detergent industry wastewaters by supercritical water oxidation

Experimental investigation and simulation optimization of a pilot-scale supercritical water oxidation system

Supercritical water oxidation (SCWO) technology has an excellent performance in treating high concentration and refractory organic wastes. While the high treatment cost is one of the key drawbacks currently restricting its large-scale application. In this study, a SCWO pilot system with a novel jet reactor (treatment capacity of 200 kg/h) was experimentally investigated, and the process model was developed to optimize the system by Aspen Plus. The energy and exergy efficiency of the system was evaluated, and the wastewater treatment cost in different operating conditions was estimated. The results show that chemical oxygen demand (COD) removal efficiency could be increased with the increase of the temperature, pressure, and the excess oxygen coefficient. By locating the electric heater 1 (H1) before the heat exchanger 2 (H2), and improving the outlet temperature of the oxygen compressor (MP1), the wastewater treatment cost could be reduced to 53.5 $/t, and the energy and exergy efficiency of the SCWO system could be improved to 81.61% and 8.91%, which is 26.28% and 6.23% higher than that of the current system, respectively. Both the reaction temperature and COD concentration of the wastewater have positive effects on the wastewater treatment cost, e.g. the wastewater treatment cost could decrease from 80.88 $/t to 59.27 $/t when the reaction temperature increases from 380 °C to 420 °C, and the SCWO system could achieve the energy self-sufficient operation when the COD concentration is over 60,000 mg/L.

Treatment of Wastewater Containing Medicinal Compounds, Sorption-Oxidative Method

Presents the result of testing a hybrid wastewater treatment method with contaminants of organic nature, in particular, effluents of pharmaceutical enterprises. The method combines reagent-membrane concentration of aqueous solutions and the subsequent oxidation of organic components at supercritical water parameters. Primary concentration is based on the introduction into solution of sorption materials of organic nature and microfiltration of the resulting suspension using ceramic membranes. It is shown that supercritical water oxidation of concentrates reduces their COD to the level acceptable for discharge.

Degradation of Ethylene Diaminetetra Acetic Acid With Ferrous Ions Using Fenton’s Reagent in Absence of Light

Accumulation of deposit corrosion products inside secondary side of steam generator can lead to serious issue for pressurized water reactor in operation.One way to avoid accumulation of these deposits is to perform chemical cleaning which removes certain amount of deposits but this kind of operation generates high amount of liquid waste which contains process chemicals and metal ions in complex state. Oxidation of complexes will render radioactive ions in free state enabling us for further treatment to innocuous level and condition. In waste treatment there are various treatment technologies (advance oxidation process, ozone, hydrogen peroxide, electrochemical oxidation, supercritical water oxidation, electro flocculation etc to remove the accumulated deposit.).In decontamination process removal of the deposits was carried out using EDTA. which complexes the metal ions but it generates secondary waste with EDTA. We have made an attempt to degrade ethylene diaminetetra acetic acid (with ferrousions) using Fenton’s reagent in absence of light. This will free the radioactive ion for further treatment. We have studied different volume of hydrogen peroxide (H2O2) and different concentration of ethylene diamine tetra acetic acid (EDTA). Study was also done to find the effect of pH and temp on the degradation of EDTA. In the range of concentration of ethylene diamine tetra acetic acid 400ppm to 22500 ppm % degradation was found to change from 57 to 98% in 180 min. The pH was found to change from 4.0 to 8.2. The temperature was found to change from 298K to 328K. The rate constant of the degradation reaction was found to be in the range 10-4 to 10-3 in temperature range 298K to 328K. Activation energy for degradation reaction was found to be 33-88 KJ/mol.

Influence of multiphasic systems on salt(s) solubility in supercritical water: the case of NaCl and NaCl-Na2SO4

Supercritical water oxidation processes (SCWO) have been developed as an alternative technology to treat toxic and/or complex chemical wastes with a very good efficiency. However, one main limitation of the SCWO process comes from the precipitation of inorganic compounds. When dealing with supercritical water conditions (T ≥ 374 °C, p ≥ 22.1 MPa), the polarity of water drops and inorganics, such as salts, are no longer soluble. This precipitation phenomenon results in a solid salt deposition in the reactor, which can lead to clogging and interruption of the continuous process. Considering a specific salt, beside its intrinsic solubility in supercritical water, the presence of other salts or other compounds in the media can modify its precipitation behaviour. With the use of a specific experimental set-up, we propose in the present work to study the solubility of NaCl (type I salt) in supercritical water, and the influence of its phase diagram on its precipitation behaviour as well as on the solubility of a second salt, Na2SO4 (type II salt). The complex and dynamic aspect of NaCl precipitation has been studied through the use of a continuous solubility measurement set-up, revealing unexpected results.

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.

Role of Dissolved Oxygen in Iron Oxidation in Supercritical Water: Insights from Reactive Dynamics Simulations

Oxygenated treatment is considered to be an effective chemical water treatment method and is widely used in supercritical power plants. Previous isotope tracer experiments reported that dissolved o...

Sub and supercritical water oxidation of pharmaceutical wastewater

Sub and supercritical water oxidation (Sub- CWO & SCWO) technology has gained attention over the past few years due to near complete conversions at shorter residences times. A pharmaceutical sample containing variety of analgesics, antibiotics, antipyretics, antifungal, β-blockers and drug intermediates contributing to overall total organic carbon (TOC) of 2017 ± 49 mg/L was collected from a nearby pharmaceutical industry for this research. The sample was subjected to sub and supercritical water oxidation both in batch and continuous processes. A maximum removal efficiency of 80.1% was achieved with excess oxidant (oxidation coefficient (OC 3)) at 400 °C for 60 min residence time in batch reactor. The pharmaceutical sample was also processed in continuous supercritical water reactor by varying both temperature (400–550 °C) and oxidant coefficient (0–2) at 23 MPa for 60 s residence time. With the operating temperature of 550 °C and OC 2 resulted in maximum removal of the pharmaceutics with efficiency of 97.8%.

Supercritical water oxidation of ammonia with methanol as the auxiliary fuel: Comparing with isopropanol

The co-oxidation behavior of methanol on ammonia in supercritical water was investigated in a continuous tubular reactor under a range of methanol concentration (10–800 mmol/L), ammonia concentration (50–400 mmol/L), oxidation coefficient (1.05–1.7) and preheating temperature of 400 and 415 °C. Calculation based on a mechanism-based model for supercritical water oxidation of methanol/ammonia showed that the synergy between the kinetics action to supply radicals and the thermal action to elevate the reaction temperature of methanol would improve the efficient decomposition of ammonia at relatively moderate reaction conditions. Specifically, near complete conversion of ammonia (97.5%) can be achieved at [CH3OH]0 = 800 mmol/L, preheating temperature of 415 °C and oxidation coefficient of 1.5. The influence of reaction conditions on the co-oxidation behavior and the formation of nitrate were examined experimentally. Methanol and isopropanol served as the auxiliary fuel for ammonia were compared by the positive effect and exothermic performance, which reached that methanol was superior to isopropanol.

Catalytic oxidation of various aromatic compounds in supercritical water: Experimental and DFT study

Abstract Catalytic supercritical water oxidation (CSCWO) has sprung up as a promising technique to eliminate refractory aromatic compounds, which are difficult to be removed by conventional wastewater treatment processes. Decompositions and degradation mechanisms of 18 aromatic compounds, including benzene, phenol, bisphenol A (BPA) etc., were investigated using CSCWO in this study. It was found that TOC removal efficiencies of the aromatic compounds were significantly enhanced, when CSCWO temperatures raised from 325 to 525 °C and reaction time prolonged from 0.5 to 6 min, with the addition of Cu(NO3)2. The correlationships among TOC degradation rates (kTOC), temperature at TOC removal rate of 90% (T90) and 19 molecular descriptors were studied. The results indicated that BOn and q(H)x could greatly impact the degradation behaviors. Fukui indices based density functional theory (DFT) method was applied to reveal the conceivable degradation pathway of BPA during CSCWO process.

Hydrothermal treatment of pharmaceutical wastewater

ABSTRACTThe effective disposal of industrial pharmaceutical waste waters with the desired quality has become a challenge. Supercritical water oxidation which involves water beyond critical point has been preferred due to its near complete conversions. This research work made an attempt to degrade the organic matter into fuel gases by gasifying the pharmaceutical waste water stream over sub- to supercritical temperature range (200–400°C). A maximum of 126.5 mmol/L of H2 gas yield was obtained at 400°C for 45 min. To achieve complete conversion, supercritical experiments have been performed in the presence of H2O2. The experiments resulted near complete conversion of 99.5% for an oxidation coefficient of 4 at 400°C in 5 min. The experimental results conveyed that useful products (H2, CO and CH4) can be attained with gasification in the absence of oxidant, while complete conversion can be achieved in the presence of oxidant in supercritical water.

Applications of Advanced Oxidation Technologies for Sludge Treatment

Treatment of sludge is an important issue in water treatment. At present, the traditional sludge treatment process still faces problems such as high sludge production and low solid content, and it is impossible to achieve environmental and economic benefits. Advanced oxidation technology can not only reduce sludge dry base production, but also improve sludge dewatering performance, which has attracted much attention in the field of sludge treatment. On the basis of a brief description of Fenton reagent oxidation, ozone oxidation, wet air oxidation and supercritical water oxidation mechanism, the effects of several advanced oxidation techniques on sludge dry yield and dewatering performance are discussed, and various oxidation methods are introduced. Finally, the advantages and disadvantages of various advanced oxidation technologies are discussed, and the main research directions of advanced sludge oxidation are pointed out.

Oxidation of high iron content electroplating sludge in supercritical water: stabilization of zinc and chromium.

The stabilization of heavy metals (zinc and chromium) and the degradation of organic pollutants during supercritical water (SCW) and supercritical water oxidation (SCWO) treatment of electroplating sludge (EPS) with a high iron content were studied. Experiments were performed in a batch reactor at temperatures in the range from 623.15 to 823.15K with an oxygen coefficient (OE) from 0 to 2.0, a reaction time of 7min and pressure of 25MPa to examine the effect of the operation conditions. Chemical oxygen demand (COD) and total organic carbon (TOC) in raw sludge and liquid products under different reaction conditions were detected. The results indicated that more organic pollutant degradation occurred under supercritical conditions than in subcritical water. Additionally, as the temperature and amount of oxidant increased, the organic pollutant removal rate increased. In addition, the Zn and Cr removal efficiency from sludge was more than 98% under all conditions. Temperatures under 773.15K had a positive effect, whereas the oxygen ratio was more significant than the other factors above 773.15K. Furthermore, leaching toxicity tests of the heavy metals in solid products were conducted based on the toxicity characteristic leaching procedure (TCLP). All heavy metals showed greatly reduced leaching toxicity due to their stabilization. The Zn in the EPS is more easily converted into a solid product after SCWO treatment; however, Cr is more difficult to leach from the solid product. Oxides of iron, zinc, and chromium were detected by X-ray diffraction and an electron probe microanalyzer, and the yield of the oxides increased with increasing temperature and oxidant amount. Using the obtained data and analysis results, the effect of Fe on the stabilization of Zn and Cr was studied.