Continuous Supercritical Water Research Articles

The high-concentration and pumpable pig manure slurry: Preparation, optimization, and evaluation for continuous supercritical water gasification

The preparation of pumpable slurry with high solid content has emerged as one of the obstacles in the continuous supercritical water gasification of pig manure. This study aims to address this challenge by employing K2CO3 and thermal treatments. Initially, evaluating the effect of parameters on the pumpability, stability, and viscosity of the slurry and optimizing the pulping conditions were carried out. The findings indicate that a 40 wt% slurry can be obtained at a K2CO3 ratio of 0.10, a pulping temperature of 160 °C, a reaction time of 30 min, and a premixing temperature of 80 °C. For a 50 wt% slurry, the corresponding conditions increased to 0.20, 180 °C, and 90 min, separately. Then, an insight into pulping mechanism reveals that the viscosity and stability of slurry are determined by the particle size and surface functional groups. The functional groups such as -C=O and -OH on the particle surface are matched with the organic components in the slurry, which is beneficial for reducing the viscosity of the slurry and improving its stability. Finally, a 40 wt% concentration slurry is successfully pumped. At a temperature of 620 °C and a preheating water ratio of 4, gasification efficiency reaches 74.43 %.

Macro-characteristics and micro-mechanisms of Na2SO4 precipitation dissolved by KNO3 molten salt in a continuous supercritical water system

This work aims to explore the ability of molten salt to solve salt deposition in supercritical water (SCW) related technologies including supercritical water oxidation and supercritical water gasification, with KNO3 and Na2SO4 as examples. In the pure KNO3 solution, the two-phase layering of high-density KNO3 molten salt (settling at the reactor bottom) and low-density saturated KNO3-SCW salt solution (flowing out at the top outlet of the reactor) was formed in a kettle-reactor with about 6.5 ratio of depth to inner diameter, thereby improving the accuracy of measured solubilities. The precipitation macro-characteristics of mixed KNO3 and Na2SO4 in SCW were investigated under different feed concentration conditions. The results showed that Na2SO4 deposition on the reactor sidewall could be reduced by more than 90 % when the mass ratio of KNO3 to Na2SO4 in the feed was only 0.167. No visible salt deposition was observed on the sidewall when the ratio was 0.374. All solid deposited salts were converted into the liquid molten salt as the ratio reached 3.341, and thus could easily flow out of the reactor, without plugging. ‘Molten salt dissolution’ mechanism may provide a more plausible explanation for mixed KNO3 and Na2SO4 in SCW. In addition, the precipitation micro-mechanisms of mixed KNO3 and Na2SO4, and the critical conditions of avoiding sidewall deposition and reactor plugging were proposed. This work is valuable for overcoming the salt deposition problem in SCW-related technologies.

Investigating Salt Precipitation in Continuous Supercritical Water Gasification of Biomass

The formation of solid deposits in the process of supercritical water gasification (SCWG) is one of the main problems hindering the commercial application of the process. Seven experiments were conducted with the grass Reed Canary Grass with different preheating temperatures, but all ended early due to the formation of solid deposits (maximum operation of 3.8 h). The position of solid deposits in the lab plant changed with the variation in the temperature profile. Since the formation of solid deposits consisting of salts, coke, and corrosion products is a severe issue that needs to be resolved in order to enable long-time operation, inner temperature measurements were conducted to determine the temperature range that corresponds with the zone of solid formation. The temperature range was found to be 400 to 440 °C. Wherever this temperature was first reached solid deposits occurred in the system that led to blockage of the flow. Additional to the influence of the temperature, the influence of the flow direction (up-flow or down-flow) on the operation of the continuous SCWG plant was examined. If salts are not separated from the system sufficiently, up-flow reactors should be avoided because they amplify the accumulation of solid deposits leading to a shortened operation time. The heating concept coupled with the salt separation needs to be redesigned in order to separate the salts before entering the gasification reactors. Outside of the determined temperature zone no deposition was visible. Thus, even though the gasification efficiency was low it could be shown that the operation was limited to the deposits forming in the heating section and not by incomplete gasification in the reactor where T > 600 °C.

Continuous Supercritical Water Impregnation Method for the Preparation of Metal Oxide on Activated Carbon Composite Materials

Metal oxide (MexOy) nanomaterials are used as catalysts and/or sorbents in processes taking place in supercritical water (scH2O), which is the “green” solvent needed to obtain energy-relevant products. Their properties are significantly influenced by the synthesis method used to prepare active MexOy. In addition, the use of supported MexOy nanoparticles is more practical and cost-effective in terms of their performance maintenance. Within this context, the present study reports on the preparation of carbon-supported ZnO and CuO composites using an innovative scH2O impregnation method. Metal oxides were impregnated on a carbon (C) support using a continuous-flow tubular reactor. The results show that impregnation in scH2O is a promising approach for the preparation of ZnO/C and CuO/C composite materials. This one-step synthesis method, in a continuous flow, uses neither a seed layer nor a mineralizer, and it needs substantially lower preparation times than conventional impregnation methods.

Tube explosion during continuous supercritical water oxidation of sludge: An incident investigation

Supercritical water oxidation (SCWO) is well known as a promising end-of-pipe technology for treatment of hazardous wastes. Safety is undoubtedly the first concern as the SCWO should be operated under a harsh chemical and physical environment, including high temperature, high pressure, and high corrosive medium and plugging risk. This paper is the first report about a tube explosion incident that encountered during continuous SCWO treatment of industrial sludge. 3D imaging, Scanning Electron Microscope - Energy Dispersive Spectroscopy (SEM-EDS) and 3D finite element analysis are used to analyze the incident tube comprehensively. The results indicate that the incident tube was working at a hash corrosive environment of subcritical water and chloride, which leads to all of general, pitting and intergranular corrosions. Of which, the pitting is the most severe one that finally results to perforation. Then, leakage took place. Soon afterwards, the hard inorganic particle from sludge plugged the leaking pore for a second, but immediately split it as a big hole due to the local high pressure around the leaking pore. Finally, explosion occurred. Three lessons are drawn for the future safe design and operation of SCWO, i.e., selecting suitable pressure bearing material, checking regularly, and executing safety regulations.

Comparison study of supercritical water gasification for hydrogen production on a continuous flow versus a batch reactor

This study compares batch and continuous supercritical water gasification (SCWG) processes for green hydrogen production from biomass. It offers insights for optimizing processes, enhancing yields, quality, and energy efficiency, assessing scale-up feasibility, and supporting techno-economic analyses. Glucose, glycerol, and black liquor were SCWG-treated at 500 °C with K2CO3 catalyst in a self-built continuous-flow reactor (150 g/h) and a batch reactor (75 mL). Comparisons primarily focused on gas product yields. Batch reactors outperformed continuous-flow reactors in hydrogen (glucose: 1.53 to 0.9 mmol/g, glycerol: 7.22 to 1.14 mmol/g, black liquor: 2.88 to 1.74 mmol/g) and total gas yields due to differences in reaction time and heating behavior. Temperature effects on continuous operation (450–600 °C) were studied, with glycerol showing the highest hydrogen yield increase (from 1.21 to 4.30 mmol/g). The study discusses the applicability of both reactors for biomass SCWG processes and their implications for sustainable green hydrogen production from renewable feedstocks.

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.

Particle size effects in Ru/CNF catalysts during supercritical water gasification of glycerol

Ru/CNF catalysts of different Ru nanoparticle (NP) sizes (0.9–2.7 nm) were assessed for their performance in continuous supercritical water gasification (SCWG) of glycerol. A structure sensitivity of Ru was demonstrated, with high initial turnover frequencies (TOF) for Ru NPs smaller than 1.2 nm. Deactivation happens essentially through: 1) coking, which occurs more readily at low Ru surface densities and is limited to nanometric layers, and 2) sintering, which led to final NP sizes ranging from 2 to 3 nm independent of the catalyst loading (1–30 wt% Ru). A correlation between TOF and the density of surface Ru atoms was found, with an optimal surface density (0.4–0.7 atomRu,sfc nm−2) resulting in high initial and high steady-state TOFs, allowing longer lifetimes through the delay of coke formation. CNF as a support enables high metal loadings with optimal performance, which could help in decreasing the volume of high-pressure vessels and hence the cost of SCWG plants, making this technology even more attractive.

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.

Removal of contaminants of emerging concern from secondary-effluent reverse osmosis retentates by continuous supercritical water oxidation- parametric study and conceptual design

The continuous removal of TOC and the degradation efficiency of carbamazepine and 17β-estradiol were investigated using actual secondary municipal-effluent RO-retentate solutions. A specific set of operating parameters were applied within the supercritical water oxidizing conditions: temperature range 420–480 °C, 25.1 MPa, hydraulic retention time (HRT) of 1–2 min, excess oxidant molar-ratio of 3–10 and presence of a homogenous catalyst (IPA) at 50–100 mg/L. > 99% organic carbon mineralization, along with complete degradation of model pollutants, was observed at 450 °C/1 min/OC= 5–10 and 100 mgIPA/L. The outlet estrone concentration, 1.03 ± 1.14 ng/L, representing estrogenic pollutants, dropped to the "no effect" range. A model for a SCWO plant treating secondary-municipal-effluent-RO-retentate for a city of 100,000 capita-equivalent was developed, based on a shell & tube SCWO flow reactor, showing > 75% energy-efficiency. The model yielded that for the extreme case of a zero caloric-value feed-solution, the total OPEX and CAPEX would be < $6.0 ± 2.5 per m3 of secondary effluents, i.e., two orders of magnitude lower than the reported environmental shadow-price associated with CECs (contaminants of emerging concern). Further work is required on the continuous and efficient separation of the salt-matrix, which can lead to higher overall heat transfer coefficients and enable further reduction in capital costs.

Pfos Destruction in a Continuous Supercritical Water Oxidation Reactor

Pfos Destruction in a Continuous Supercritical Water Oxidation Reactor

Determining the kinetic constants leading to mineralization of dilute carbamazepine and estradiol-containing solutions under continuous supercritical water oxidation conditions

Continuous subcritical and supercritical water oxidation experiments were conducted on dilute carbamazepine- and estradiol-containing synthetic solutions used to simulate the removal of model emerging pollutants from secondary municipal effluents. The operating conditions comprised 340–500 °C, retention time of 24–453 s and a stoichiometric oxidant ratio (O.C.) between 4 and 64. The transformation of the various species was determined at the outlet and by modeling a segmented non-isothermal reaction system. Four empirical power law kinetic models were established to represent both the pollutants' degradation and TOC removal efficiencies, using nonlinear multiple regression coupled with bootstrapping and K-fold cross-validation. The mineralization and degradation models for both pollutants yielded a R2 of 67–80.5% vs. the experimental results. Discussion on the various model assumptions revealed that attributing full model deviations to the constant oxygen concentration or to the laminar reactor flow, yielded a deviation of 6% and 15% in the removal efficiencies, respectively. However, the expected deviation of the models was lower than 0.32% at conditions leading to (almost) full mineralization (45–60 s, 480–500 °C and O.C.s of 5–10). The methodologies developed in the study are useful for interpreting future results obtained from SCWO of actual secondary effluent solutions.

Investigating active phase loss from supported ruthenium catalysts during supercritical water gasification.

Active phase loss mechanisms from Ru/AC catalysts were studied in continuous supercritical water gasification (SCWG) for the first time by analysing the Ru content in process water with low limit-of-detection time-resolved ICP-MS. Ru loss was investigated alongside the activity of commercial and in-house Ru-based catalysts, showing very low Ru loss rates compared to Ru/metal-oxides (0.2–1.2 vs. 10–24 μg gRu−1 h−1, respectively). Furthermore, AC-supported Ru catalysts showed superior long-term SCWG activity to their oxide-based analogues. The impact on Ru loss of several parameters relevant for catalytic SCWG (temperature, feed concentration or feed rate) was also studied and was shown to have no effect on the Ru concentration in the process water, as it systematically stabilised to 0.01–0.2 μgRu L−1 for Ru/AC. Looking into the type of Ru loss in steady-state operation, time-resolved ICP-MS confirmed a high probability of finding Ru in the ionic form, suggesting that leaching is the main steady-state Ru loss mechanism. In non-steady-state operation, abrupt changes in the pressure and flow rate induced important Ru losses, which were assigned to catalyst fragments. This is directly linked to irreversible mechanical damage to the catalyst. Taking the different observations into consideration, the following Ru loss mechanisms are suggested: 1) constant Ru dissolution (leaching) until solubility equilibrium is reached; 2) minor nanoparticle uncoupling from the support (both at steady state); 3) support disintegration leading to the loss of larger amounts of Ru in the form of catalyst fragments (abrupt feed rate or pressure variations). The very low Ru concentrations detected in process water at steady state (0.01–0.2 μgRu L−1) are close to the thermodynamic equilibrium and indicated that leaching did not contribute to Ru/AC deactivation in SCWG.

Partial Oxidation of Ethanol in Supercritical Water

Ethanol is partially oxidized in a continuous supercritical water reactor at temperatures from 500 to 530 °C, constant pressure of 25 MPa, initial ethanol concentration of 5 wt %, residence times o...

Supercritical hydrothermal combustion of nitrogen-containing compounds in a tubular reactor

Supercritical hydrothermal combustion is a promising approach for disposal of nitrogen-containing pollutants. The hydrothermal combustion characteristics of two typical nitrogen-containing compounds, ammonia and quinoline, were systematically explored in a continuous supercritical water tubular reactor. The ignition and combustion of two auxiliary fuels of methanol and ethanol were compared first. The combustion reaction for ammonia and quinoline with the presence of methanol was investigated, respectively. The result indicated that ethanol could ignite more easily than methanol, while the combustion rate of methanol was faster than that of ethanol. Ammonia/methanol was capable of ignition in supercritical water, depending on the initial fuel concentration, preheating temperature, and ammonia ratio. And ammonia could promote the overall combustion process at fuel COD concentration of 73.0 g/L, preheating temperature of 420 ℃ and ammonia ratio of 0.05–0.19. Moreover, this experiment also verified that 2.74 wt% quinoline could undergo ignition in dynamic flow of the supercritical water reactor, and the oxidation reaction mechanism of quinoline was further analyzed.

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.

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%.

Stability and activity maintenance of sol-gel Ni-MxOy (M=Ti, Zr, Ta) catalysts during continuous gasification of glycerol in supercritical water

The long-term stability of activities and physicochemical properties for nickel nanoparticles supported on TiO2, ZrO2 and Ta2O5 prepared by sol-gel methods, was evaluated for continuous supercritical water gasification (SCWG) of 3 wt.% glycerol at 425 °C and 25.2 MPa. The Ni/TiO2 catalyst bed gradually deactivated while the Ni/ZrO2 and Ni/Ta2O5 catalyst beds showed persistent and high carbon gasification efficiency for 80 h operation. Characterization results verified that property stabilization of the sol-gel Ni/ZrO2 and Ni/Ta2O5 catalysts occurred in the long-term SCWG flow, indicating a “catalyst in-situ activation” effect that enhanced the activity and coking resistance of the as-prepared sol-gel catalysts for continuous SCWG. Results also indicate that sintering of Ni with the supports to form new phases and metal leaching of the catalyst materials in the SCW flow may be key deactivation factors of the catalyst beds.

Effect of ammonia on gasification performances of phenol in supercritical water

The solutions of phenol and the mixture of phenol and ammonia were gasified in a continuous supercritical water reactor to evaluate the influences of ammonia on the hydrogen production and the degradation pathways of phenol. The effects of temperature (560–640°C), reaction time (2–20s) and concentration of ammonia (500–2000mg/L) on gaseous distributions, gasification efficiencies and reactants removal efficiencies were investigated. In addition, the effects of ammonia on the kinds of intermediate products were analyzed. Results showed that the increasing temperature greatly promoted the hydrogen production and degradation of phenol. Longer reaction time gave a positive effect on gasification efficiencies in 10s, while the effect was little when the reaction time longer than 10s. Ammonia prevented the production of hydrogen and the degradation of phenol, which was mainly due to the generation of more stable nitrogen polycyclic compounds. The possible degradation pathways for the mixture of phenol and ammonia were proposed.

An inclined plug-flow reactor design for supercritical water oxidation

A novel reactor, combining the advantages of the MODAR reactor, horizontal stirred reactor, dynamic gas seal wall reactor, and cool wall reactor, was designed and manufactured to overcome the problems encountered in continuous supercritical water oxidation of real waste, especially in semi-solid state. It is a Y-shaped reactor consisting of an inclined reaction section on the left and a vertical separation section on the right. Nine experiments were carried out to test the performance of the reactor using municipal sewage sludge (MSS), with 12% dry solid (DS) content, as a representative sample of real waste. The results suggest that MSS, without preheating, can be safely and efficiently treated by adding 12.5% isopropyl alcohol (IPA) and pure supercritical water, the volume of which is at least 2.5 times that of the MSS. The organic carbon removal efficiency is up to 99.94%. Inorganic solid particles from the MSS are continually pushed forward in the inclined reaction section, and are completely separated and stored at the bottom of the vertical separation section. Clean effluents, comprising water and gases as the main products, are released from the top outlet. The flow regime in the inclined reaction section can be considered to be plug flow. Counter-current flow was not observed during the tests. The excellent in situ solid separation efficiency suggests that this reactor has potential for application in various hydrothermal processes that involve high content of inorganic solid particles.