Supercritical Water Research Articles

Degradation characteristics and reaction mechanism of supercritical water oxidation of oily wastewater

Degradation characteristics and reaction mechanism of supercritical water oxidation of oily wastewater

Supercritical water co-gasification of waste R134a and plastics to produce valuable chemicals and fuels: ReaxFF reactive molecular dynamic simulation

Supercritical water gasification of waste hydrofluorocarbon refrigerants was an effective treatment technology, but there is a problem of high yield of fluorine-containing products with complex types. Plastic has the potential to provide sufficient H atoms as hydrogen donor to facilitate defluorination of fluorine-containing products. In this study, ReaxFF reactive molecular dynamic simulation and density functional theory calculation were used to investigate the supercritical water co-gasification (SCWCG) mechanism of R134a and low-density polyethylene (LDPE). The results showed that the decomposition of LDPE was the initial reaction of SCWCG, the generated radicals promoted the dissociation of R134a and H2O. The dominant SCWCG products were valuable hydrocarbons, H2, CO and HF. The present of LDPE promoted the defluorination reaction of fluorine-containing products from the dissociation of R134a, reduced the yield of fluorine-containing products, and greatly promoted the yields of H2, HF, CO and hydrocarbon products. 99% of F atoms in R134a can be converted to HF, which indicated that plastics participating in supercritical water gasification of R134a has excellent defluorination effect. This study provided an efficient and potential route to conduct waste HFC refrigerants into valuable chemicals and fuels.

Study on microstructure and properties of Cu-SnO2 composites prepared by in-situ supercritical water reaction method

In this study, Cu-SnO2 composites are successfully prepared using supercritical water in-situ oxidation and powder metallurgy techniques. The results demonstrate that the Cu-4.5 wt.% SnO2 composite possessed excellent comprehensive performance. The tensile strength of the composite is 331 MPa, represents a 36.21 % increase compared to pure copper. The elongation is 43.21 %, and the hardness after treatment at 1000 °C is 87.26 HV, which is 85.76 % of the untreated hardness. These properties are attributed to the specific orientation relationship of SnO2 with the Cu matrix {[]SnO2//[]Cu, ()SnO2//()Cu}, with SnO2 particles homogeneously dispersed in the composite.

Techno-economic analysis of sewage sludge supercritical water gasification for hydrogen and electricity co-generation system

Techno-economic analysis of sewage sludge supercritical water gasification for hydrogen and electricity co-generation system

CO2 intensified supercritical water gasification of waste plastics

CO2 intensified supercritical water gasification of waste plastics

Technoeconomic analysis of supercritical water gasification of canola straw for hydrogen production

Production of hydrogen from renewable sources is gaining popularity to reduce our dependency on non-renewable fossil fuels to meet growing hydrogen demand. However, despite the great prospect of production of hydrogen from sustainable sources such as lignocellulosic biomass via supercritical water gasification (SCWG), it has not been commercialized at a large industrial scale. This is due to the lack of detailed economic analysis of SCWG of lignocellulosic biomass, owing to the complexity of the SCWG process and the heterogeneous nature of biomass. Therefore, to address this knowledge gap, in this study, a detailed technoeconomic analysis (TEA) of a conceptual SCWG pilot having the capacity to process 200 tons/day of canola straw for the production of green hydrogen was conducted. Mass and energy balance of conceptual pilot was performed using Aspen Plus ® simulation by utilizing experimental data and hydrogen yield of 41.62 mmol/g was obtained at optimized reaction conditions of 500 °C, 23 MPa, and 10 wt%. Economic analysis based on calculated mass and energy balance was performed using SuperPro software. Cash flow analysis for capital expenses (CAPEX) of 81 Million USD showed a high internal rate of return (IRR) of 38.9% and an undiscounted net present value (NPV) of 548 million USD. A minimum selling price (MSP) of 3.38 USD/kg H2 for produced hydrogen was estimated, which is lower than other renewable hydrogen production processes and comparable to non-renewable hydrogen production technologies. A high positive IRR and NPV, while a lower MSP showed that despite having a low technological readiness level (TRL) of 4, SCWG of lignocellulosic biomass is a technically feasible and economically viable process for the production of hydrogen. Furthermore, sensitivity analysis also revealed that capital expenses (CAPEX) and canola straw price had the highest influence on net present value (NPV) and MSP. However, overall NPV and MSP were highly stable to changes in parameters highlighting the robustness of the economic analysis.

Decomposition mechanism of furfural under supercritical water gasification conditions using ReaxFF simulations

The supercritical water gasification (SCWG) is a convenient and efficient method to utilize hydrogen stored in biomass. Furfural is an important platform during the conversion of biomass thus it is of great significance to understand the microscopic mechanism of the SCWG of furfural. In this work, the SCWG of furfural is investigated using reactive molecular dynamic (RMD) simulations under the temperature range from 1800 K to 2800 K. Based on the detailed reaction scheme of furfural pyrolysis and SCWG, it is found pyrolysis dominates the conversion of furfural under low temperature such as 2400 K and produces only a small amount of H2; while under higher temperature, H2O participates in SCWG as reactant and actively reacts with furfural and C3 fragments in forms of H and OH radicals, which promotes the production of H2. In addition, the combination of water and C3 fragments also effectively inhibits the coke formation. This work could provide a further insight for the conversion of biomass to hydrogen energy at the atomic level.

Hydrothermal Liquefaction of Post-Consumer Recycled Linear Low-Density Polyethylene with Different Molar Masses in Supercritical Water

Hydrothermal Liquefaction of Post-Consumer Recycled Linear Low-Density Polyethylene with Different Molar Masses in Supercritical Water

Surface facet engineering of ZnIn2S4 via supercritical hydrothermal synthesis for enhanced NO gas sensing performance

In this study, we investigate the novel application of ZnIn2S4 as an NO gas detection device by precisely modulating its surface facets through crystal growth control in a supercritical water environment. The supercritical hydrothermal synthesis successfully transforms ZnIn2S4 from a flower-like structure into a hexagonal plate morphology, driven by the preferential growth of the basal plane (003) surface facet. This morphological control, which is unattainable in a subcritical environment, is evidenced by a substantial increase in the (003)/(011) facet ratio from 0.52 to 1.98 with rising temperature. NO detection results indicate that this surface morphology modification significantly accelerates sensor response, attributed to enhanced interaction between the ZnIn2S4 surface and NO gas, as well as reduced diffusion limitations compared to the flower-like morphology. The hexagonal plates exhibit a remarkably fast response time of approximately 25 s, in contrast to 181 s for the flower-like counterpart. These findings underscore the crucial role of surface facet engineering in optimizing the gas-sensing properties of ZnIn2S4, highlighting its potential for advanced gas sensor applications.

Utilizing supercritical water gasification for hydrogen production combined with a waste heat recovery system for domestic households

Hydrogen from food waste can provide sustainable energy. Food waste can be gasified into clean, efficient hydrogen gas using cutting-edge techniques. This study is about the gasification of food waste under different settings to determine gas production and efficiency. The supercritical water gasification (SCWG) method was used in experiments at 400 °C, 450 °C, and 500 °C with reaction durations of 30, 60 and 90 min. The results showed peak values of CO2, CO, CH4, H2 at 90 min, equivalent to 6.2 % and 7.9 % hydrogen efficiency (HE) and cold gas efficiency (CGE). Peak CGE and HE were 6.7 % and 9.2 % at 500 °C, whereas peak CE and total gas output were 6.8 % and 8.6 %. To generate heat and power, a PEM cell was added. Gasification-generated hydrogen was warmed and fed in the PEM circuit at 0.7, 1 and 1.5 m/s. The peak velocity indicates a 23.1 % waste heat recovery rate and a higher hydrogen temperature difference of roughly 8.1 °C. At the temperature of peak gasification (500 °C), the values of CO2, CO, CH4, and H2 are approximately 3.6 mol/kg, 0.3 mol/kg, 2.6 mol/kg, and 4.7 mol/kg, respectively. At the peak gasification temperature (500 °C), As a result, the suggested system is ideal for combining hydrogen production to generate both power and heat energy for use in heating small homes or buildings (the proposed system can deliver that much energy or at least partially fulfill it).

Surface wettability of lignin materials from supercritical water hydrolysis of wood

To meet the demands of the evolving circular economy, there is a growing need for renewable resources as base materials for innovative, easily recyclable products. Lignin, the second most abundant biopolymer, has emerged as a promising source of aromatics and reinforcing agent in polymer composites. For the successful manufacturing of homogeneous composite materials, good bonding between the coexisting phases is essential to prevent the formation of voids and agglomerates. Therefore, understanding the surface properties of these materials is crucial for designing optimal composite compounds. In this study, the wettability of lignin-cellulose composites and lignin samples obtained through supercritical water hydrolysis (SCWH) of birch wood is investigated. The contact angle (CA) technique, specifically the sessile drop method, was employed to assess and compare the wettability of SCWH lignin with commercially available lignin and raw birch wood. The results provide insights into their surface energy, adhesion, and hydrophilic or hydrophobic characteristics under processing conditions. All samples exhibited hydrophilicity, with an initial CA approximately 40 °, except for raw birch wood, which had a higher initial CA of ∼ 64°. Notably, when lignin is accompanied by significant amounts of cellulose, different trends in CA changes over time were observed. The influence of pressure on the CA between water and these polymers was also analyzed, but no significant impact was detected. This research advances the development of lignin-based materials with tailored surface properties for various industrial applications.

Evaluating sustainable energy pathways: Economic perspective on advanced hydrogen production

This investigation seeks to evaluate hydrogen generation technologies in China with a focus on two commercial processes, namely hydrothermal oxidation and supercritical water gasification of organic waste from 2011 to 2021. This period witnessing major development, and highlight the key determinants of hydrogen production. The present study was mainly concerned with the enhancement of SCWG processes for hydrogen generation from microalgae and other potential feedstock. Some of the significant observations made in this period are the critical role of biomass composition, choice of catalyst, and operating conditions in increasing H-producing capabilities of reactor systems. The authors of research affirm that the feedstock with a high carbohydrate content and low levels of protein are the most suitable for hydrogen generation. Thus, expanding research on Supercritical Water Gasification, there is the development of HTO methods to consider various pathways of hydrogen production and their connection with existing energy networks. It also studied the effects that several types of catalysts embracing the heterogeneous transition metals and the homogeneous alkali metal compounds have on the efficiency of hydrogen production. These changes have rendered the hydrogen generation process to make more precise predictions and/or course corrections on the process. The studies that have been done between 2011 and 2021 show significant improvements in the ways that sustainable and efficient hydrogen generation technology can be created in China. The results provide a solid platform for future improvements and stress positive impacts that can be provided by these technologies for China's sustainability and energy transition progress.

Inhibition of Polycyclic Aromatic Hydrocarbons Formation During Supercritical Water Gasification of Sewage Sludge by H2O2 Combined with Catalyst

This work evaluated the alterations in the levels and types of polycyclic aromatic hydrocarbons (PAHs) within both liquid and solid products throughout the process of the catalytic supercritical water gasification of dewatered sewage sludge to examine the catalytic effect of various catalysts and the inhibit reaction pathways. The addition of Ni, NaOH, Na2CO3, H2O2, and KMnO4 reduced the concentrations of PAHs, with Ni and H2O2 showing the best performance. The concentrations of PAHs, especially higher-molecular-weight compounds in the residues, decreased sharply as the H2O2 amount increased. At a 10 wt% H2O2 addition, the levels of PAHs in the liquid and solid products were reduced by 91% and 88%, respectively. High-ring PAHs were not detected in the residues as the H2O2 amount increased to an 8 wt%. H2O2 addition evidently inhibits PAH formation by promoting the ring-opening reactions of initial aromatic compounds in raw sludge and inhibiting the polymerization of open-chain intermediate products. The addition of NaOH + H2O2 or Ni + H2O2 as combined catalysts significantly lowered PAH concentrations while increasing the H2 yield. The addition of 5 wt% Ni + H2O2 reduced PAH concentrations in the liquid and solid residues by 70% and 44%, respectively, while the H2 yield escalated from 0.13 mol/kg OM to 3.88 mol/kg OM. Possible mechanisms associated with the reaction pathways of these combined catalysts are proposed.

Isolation of β-O-4-Rich Lignin From Birch in High Yields Enabled by Continuous-Flow Supercritical Water Treatment.

The continuous flow supercritical water (scH2O) treatment of Birch wood (T=372-382 °C; t=0.3-0.7 s; p=260 bar) followed by alkali extraction of lignin allowed for the isolation of lignin and lignin carbohydrate complexes (LCCs) with a high number of β-O-4 moieties in the range 29-57/100 Ar (evaluated by quantitative 13C NMR analysis) in yields ranging between 13-19 wt % with respect to the initial wood. A "lightning rod effect" of carbohydrates has been claimed to explain the low degradation of β-O-4 bonds during the process. The structure of the isolated lignin was thoroughly elucidated via comprehensive NMR studies (HSQC, 13C and 31P). A low degree of condensation (DC)<5 % was found for all the lignin samples, which was only slightly dependent on the reaction severity. The number of aliphatic -OH, phenolic -OH, and -COOH groups was in the range 3.37-5.25, 1.41-2.31 and 0.39-0.73 mmol/g, respectively. The number of -COOH groups increased with increased severity, suggesting that oxidation can occur during the scH2O treatment. Furthermore, by simply varying the reaction severity, it was possible to tune important lignin properties, like the molar mass and the glass transition temperature (Tg).

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 %.

Hydrogen production and elemental migration during supercritical water gasification of food waste digestate

The rapid growth of food waste (FW) is a huge challenge on a global scale, and anaerobic digestion is one of the most commonly used methods to deal with food waste, and the increasing amount of food waste digestate (FWD) produced by anaerobic digestion also poses a huge challenge to waste management. This paper explores supercritical water gasification (SCWG) as a valuable and innovative strategy for the conversion of FWD into H2-rich syngas. The research focuses on analyzing the effects of reaction temperature and residence time on syngas production, gas composition, element migration (C, N and P) during the SCWG process. Experimental results show that as the reaction temperature increases from 400 °C to 500 °C, the total syngas yield increases significantly, from 2.4 mol/kg to 9.7 mol/kg, especially the yields of H2, CO2, and CH4. As the reaction temperature increases and the residence time increases, the migration of carbon from the solid and liquid phases to the gas phase accelerates with increasing temperature and residence time, resulting in a higher proportion of carbon in the gas phase. In terms of liquid phase composition, nitrogenous compounds are significantly converted into ammonium (NH4+-N) at higher temperatures. In addition, the organic phosphorus is observed transferring into inorganic phosphorus, which are mainly apatite. This study explores the scalability of SCWG and its potential for the production of clean fuels, thereby contributing to the sustainable management of FWD.

Hydroxide-ion induced complete mineralization of poly(tetrafluoroethylene-co-hexafluoropropylene) copolymer (FEP) in subcritical water

Decomposition of poly(tetrafluoroethylene-co-hexafluoropropylene) copolymer (FEP) in supercritical and subcritical water was investigated with the aim of waste treatment. The efficiency of such decomposition was examined by using either oxidizing agents (O2, H2O2) or an alkaline reagent (KOH). When O2 was chosen, the highest F– and CO2 yields, 75 % and 64 %, respectively, were obtained from a reaction of FEP in supercritical water at 384 °C for 24 h. Under these conditions, traces of CHF3 were detected in the gas phase, the amount of which decreased with increasing the reaction time·H2O2 gave slightly higher reactivity than O2. In contrast, reactions with KOH induced efficient fluorine mineralization. When FEP reacted in 3.0 M KOH solution at 360 °C for 18 h, the F– yield reached 98 %. Hence, complete fluorine mineralization was achieved, where very little CO2 (∼0% yield) and no CHF3 were generated in the gas phase. Furthermore,19F NMR spectroscopy and combustion-ion chromatography revealed that the reaction solutions did not contain any organofluorine compounds during the reactions. FEP decomposed, releasing F– into the reaction solution, resulting in the formation of amorphous carbon in the residue.

High concentrations of CO and CO2 inhibit lignite gasification in supercritical water

Supercritical water gasification (SCWG) technology enables the efficient utilisation of lignite to produce H2, CO, and CO2. Coal gasification differs under different atmospheres and H2 promotes the pyrolysis of carbon structures. While simultaneous increases in H2, CO, and CO2 are observed, their combined effect on the SCWG of coal remains unknown. Formic acid (FA) was used to produce H2, CO, and CO2 in the supercritical water, and the impact of the gaseous product generated from high-concentration (>30wt%) FA decomposition on the SCWG of lignite was experimentally investigated. In comparison to lignite gasification in pure supercritical water, the carbon efficiency of lignite gasification in 30wt% FA solution decreased by 7.66%, and the mass conversion of lignite decreased by 4.0%. Higher concentrations of CO and CO2 yielded stronger inhibition of lignite SCWG.

High-efficiency co-liquefaction of brominated epoxy resin and low rank coal in supercritical water: Waste recycling and products upgrading

With the acceleration of upgrading electronic information products, a large amount of electronic waste was generated. Brominated epoxy resins (BERs) are the main nonmetallic components of e-waste. The recycling of BERs has attracted people's attention due to their potential environmental risks. Low rank coal is difficult to efficiently utilize due to its high moisture content, oxygen content and low calorific value. This study developed an efficient co-liquefaction strategy for BERs and low rank coal by supercritical water (SCW) process. Under optimized conditions (temperature of 425 °C, reaction time of 60 min, solid-to-liquid ratio of 1:10 g/mL, and BERs-coal mass ratio of 1:4 g/g), the total conversion efficiency of co-liquefaction reached 56.76 %. When the temperature was 325–425 °C, there was a positive synergistic effect between BERs and low rank coal co-liquefaction, and the highest synergy efficiency of 11.13 % was reached at 400 °C. The introduction of BERs could significantly reduce the oxygenated compounds and the heteroatomic compounds in the liquefied oil products, improving the quality of oil products. The co-liquefaction promoted the production of phenolic chemicals and inhibited the cross-linking reaction of the decomposition products from low rank coal, and realized the synchronous conversion of BERs and low rank coal. The C-Br bond in the residue was significantly weakened after co-liquefaction. The oil products did not contain brominated organic compounds, also demonstrating the potential of the co-liquefaction process for debromination of BERs.

Effects of supercritical CO2, water, and supercritical CO2-based water on the mechanical properties and fracturability of shale

The study of the effects of supercritical CO2 (ScCO2) under high temperature and high pressure on the mechanical properties and fracturing potential of shale holds significant implications for advancing our understanding of enhanced shale gas extraction and reservoir exploration and development. This study examines the influence of three fluids, i.e. ScCO2, deionized water (DW), and ScCO2+DW, on the mechanical properties and fracturability of shale at immersion pressures of 15 MPa and 45 MPa, with a constant temperature of 100 °C. The key findings are as follows: (1) Uniaxial compressive strength (UCS) of shale decreased by 10.72%, 11.95%, and 23.67% at 15 MPa, and by 42.40%, 46.84%, and 51.65% at 45 MPa after immersion in ScCO2, DW, and ScCO2+DW, respectively, with the most pronounced effect observed in ScCO2+DW; (2) Microstructural analysis revealed that while ScCO2 and DW do not significantly alter the microstructure, immersion in ScCO2+DW results in a more complex surface morphology; (3) Acoustic emission (AE) analysis indicates a reduction in stress for crack damage, with a decreased fractal dimension of AE signals in different fluids. AE energy is primarily generated during the unstable crack propagation stage; (4) A quantitative method employing a multi-factor approach combined with the brittleness index (BI) effectively characterizes shale fracturability. Evaluation results show that ScCO2+DW has a more significant effect on shale fracturability, with fracturability indices of 0.833% and 1.180% following soaking at 15 MPa and 45 MPa, respectively. Higher immersion pressure correlates positively with increased shale fracturability.