Supercritical Water Conditions Research Articles

Gasification characteristics of histidine and 4-methylimidazole under supercritical water conditions

In this study, gasification characteristics of histidine and 4-methylimidazole were investigated for the first time. A tubular flow reactor was employed, and experiments were conducted in the temperature range of 500–650 °C, at a fixed pressure of 25 MPa, with residence times of 86–119 s, and with 1.0 wt% aqueous solutions of histidine and 4-methylimidazole. The gaseous products were identified and quantified by gas chromatography (GC), and the aqueous phase was analyzed for total organic carbon (TOC). The gasification characteristics were compared with those of glycine and alanine, which represented the standard amino acid structure. The result showed that the carbon gasification efficiencies of both histidine and 4-methylimidazole increased with increasing reaction temperature. The gasification rate of 4-methylimidazole followed first-order kinetics and was explained well by the Arrhenius equation. The gasification rate for histidine could be predicted by the weighted summation of the adjusted gasification rates of glycine and 4-methylimidazole.

Gasification Characteristics of Aminobutyric Acid and Serine as Model Compounds of Proteins under Supercritical Water Conditions

The gasification characteristics of aminobutyric acid and serine were determined under supercritical water conditions using a tubular flow reactor. A 1.0 wt% aqueous solution of these two amino acids was gasified at temperatures ranging from 400 to 650 °C and a pressure of 25 MPa with residence time of 86-222 s. The products were identified and quantified by gas chromatography, and the total organic carbon in the aqueous phase was also determined. The gasification characteristics were compared with those of glycine and alanine. The carbon gasification efficiency increased with higher reaction temperature. The gasification rate followed first order kinetics and was explained well by the Arrhenius equation. The gasification rate of aminobutyric acid was similar to that of glycine and alanine but the gasification rate of serine is faster. The oxygen in the hydroxyl group of serine is highly electronegative, so serine is more reactive than glycine and alanine.

The effect of thermohydrolysis on biological properties of selected chemical compounds

One of the negative effects of the continuous development of civilization which is observed in recent decades is an increase in the number and variety of pollutants released into the environment. These substances have ecotoxic properties and pose a serious threat to humans and proper functioning of the ecosystem. Their presence in the environment forces the development of new pollution control techniques. One of the latest and very promising methods to destroy organic contaminants using the properties of water which exceeded the critical point is thermohydrolysis in supercritical water. The aim of this study was to investigate thermohydrolysis in supercritical water conditions and to check its suitability for removal of hardly biodegradable organic compounds present in wastewater. Tests for toxicity of these compounds to aquatic organisms such as sewage bacteria, crustaceans and guppy fish were also performed and the degree of their biological decomposition was determined. Biological studies were carried out on starting compounds and after the processes of thermohydrolysis. The research material included p-nitrotoluene sulfonic acid (PNTS), 4,4′-dinitrostilbene-2,2′-disulfonic acid (DNSDS) and 4,4′-diaminostilbene-2,2′-disulfonic acid (DASDS). The research leads to the conclusion that the reactions occurring during high-temperature thermohydrolysis of the three tested compounds are extremely complex. Results of biological studies indicate a significant decrease in toxicity and increase in the biodegradability of products obtained after the process of thermohydrolysis.

Removal of phenol from oil/gas produced water using supercritical water treatment with TiO2 supported MnO2 ctalyst

MnO2 catalyst supported on TiO2 was synthesized and evaluated for removal of phenol in supercritical water. Synthesized catalysts were characterized using temperature programmed reduction, pulse chemisorption, and X-ray powder diffraction. Catalyst activity for phenol conversion was evaluated in a continuous packed bed reactor at supercritical water conditions by analyzing gaseous and liquid products. Process variables evaluated include O2 stream concentration and Mn catalyst loading. Both variables had positive effects on phenol conversion. The process achieved complete conversion of phenol at oxygen ratio of 5 based on complete phenol combustion stoichiometry. Increasing catalyst Mn loading increased its active site concentration, enhancing the contribution of heterogeneous reaction kinetics for supercritical water oxidation (SCWO) of the phenol. A phenol conversion of 70% was reached at 12% (w/w) Mn in the catalyst with oxygen ratio of 1.

Development of Safety Analysis Code TACOS and Application to Fuel Qualification Test Loop

In this study, transient analysis code of SCWRs (TACOS), with the ability of simulating transients or accidents under both supercritical water (SCW) conditions and subcritical water conditions, has been developed with fortran 90 language, and simulation has been performed to the European SCWR fuel qualification test (FQT) system. The semi-implicit finite difference technique was adopted for the solution of coolant dynamic behavior in the loop. Furthermore, an illustration of numerical solution for the heat structure model and other models was presented. The code TACOS is then applied to simulate the Edward-O’Brian blow-down experiment to evaluate its capacity in simulating the fast blow-down progress. Therefore, the design basis accidents (DBAs) with the trans-critical transient were investigated for the SCWR-FQT system. The results by TACOS indicate that the SCWR-FQT with the existing safety system can be cooled effectively.

Biofuels and Bio-based Production via Supercritical Water Gasification of Peach Scraps

The aim of the paper is to evaluate the bioenergy and bio-based material recovery from fruit scraps through an innovative process, e.g., supercritical water gasification, which presents several advantages in comparison to the traditional processes for energy and matter recovery. In particular, experimental tests of peach scraps were carried out using bench-scale plant plug flow reactor type, in which the selected liquid matter can be pumped until 300 bar and heated until 600 °C to achieve the supercritical water condition of the water. The main results showed that, in the range of the feed flow rate of 5–30 mL/min and at fixed operative conditions of T and P (T = 550 °C, and P = 250 bar), it is possible to obtain tar-free syngas with a higher heating value of 14–16 MJ/kgdry basis. The influence of the residence time was studied for the gaseous and liquid phases. For each one of them, it was possible to highlight a specific trend. In the liquid phase, the main components were acetic acid, hydroquinone, and syringaldehyde that are intermediates for the chemical synthesis of interesting bio-based chemicals and biochemicals.

Effect of Pressures on the Corrosion Behaviours of Materials at 625°C

The corrosion behaviors of austenitic stainless steels (SS) 310, 304 and Ni- and Fe-based A-286 exposed to 0.1 MPa, 8 MPa and 29 MPa at 625°C for 1000 h were investigated. These represent exposure to superheated steam, subcritical and supercritical water (SCW) at 625°C, respectively. As SS 310 showed the smallest weight change, the oxide cross-sections made from 310 samples were examined by transmission electron microscopy. The results revealed a single-layer oxide at 0.1 MPa and dual-layer oxides at 8 MPa and 29 MPa, followed by a Cr-depleted region into the austenite substrate. The compositions of the inner oxides at 8 MPa and 29 MPa are Cr-rich and largely similar to those of the single-layer oxides at 0.1 MPa exposure. These results suggest that corrosion testing in superheated steam may be a suitable surrogate for scoping tests of materials under SCW conditions at >650°C.

Catalytic supercritical water gasification: Interaction of sulfur with ZnO and the ruthenium catalyst

Continuous catalytic supercritical water gasification (CSCWG; 400°C, 28MPa) of microalgal biomass (Chlorella vulgaris) was carried out at the microalgae production site of ZHAW in Wädenswil (Switzerland) non-stop over a period of 100h. Characterization of the spent catalyst showed that mainly sulfur poisoning, and to a lesser extent coking, salt deposits, and some sintering of the Ru nanoparticles were responsible for the deactivation of the catalyst after 55h of time on-stream. The commercial zinc oxide adsorbent exhibited a high mechanical stability and good sulfur adsorption performance under supercritical water conditions although its specific surface area collapsed. In summary, the use of a zinc oxide adsorbent upstream of the catalyst bed, together with a higher ruthenium loading of the catalyst, improved the long-term performance of the CSCWG process significantly.

A new flexible titanium foil cell for hydrothermal experiments and fluid sampling.

This paper describes the design of a flexible titanium foil cell, as well as its applications in hydrothermal experiments and in non-contaminating storage of seafloor hydrothermal fluids. A flexible cell constructed totally from pure titanium (Grade 1) can be used in corrosive environment because of the excellent chemical stability and temperature tolerance of the material. Theoretical calculation and finite element analysis of the titanium foil cell have been conducted to identify its flexibility and deformation mode. Two applications, i.e., hydrothermal reaction and non-contaminating fluid sampling, were introduced subsequently. The flexible titanium foil cell was successfully tested at elevated temperature and pressure of up to 400 °C and 40 MPa, respectively, demonstrating that it could be widely used under supercritical water conditions.

Sulfur transformation in coal during supercritical water gasification

The efforts of supercritical water (SCW) on sulfur transformation during coal gasification process were studied in a batch autoclave. Sulfur morphological distribution of two different rank coals on solid, liquid and gas phase at different temperatures were thoroughly investigated during coal SCW gasification compared with coal pyrolysis. Compared with coal pyrolysis, the removal of sulfate-sulfur and pyritic-sulfur in solid state were enhanced during coal SCW gasification, especially with temperature rising. Both in coal pyrolysis and coal SCW gasification, the XRD analysis showed that the pyrite which blended with coal transformed into pyrrhotite, whereas pyrrhotite further converted into magnetite during SCW gasification. Supercritical water had the ability not only restrict thiophene forming, but also oxidize organic-sulfur to produce sulfone, causing that the forming of organic-sulfur was restrained. The sulfate and sulfite were the main liquid-sulfur composition after coal pyrolysis, whereas sulfide also existed in spite of sulfate and sulfite after coal SCW gasification. The sulfide may be produced via H2S absorbing in water. Because the sulfide can be oxidized into sulfate and sulfite by OH radicals in SCW condition, the amount of sulfate and sulfite was much larger than that after coal pyrolysis. The gas sulfur after coal pyrolysis included H2S and SO2, but the abundant H radicals provided by SCW system caused much more H2S produced and inhibited the formation of SO2. The sulfur transformation could be explained by free radical mechanism. Those transformation properties were important for further environmental evaluation and the better using of coal SCW gasification technology with low sulfur pollution.

Diffusion and hydration of oxygen confined in Fe(OH)2 nano-cracks at high temperatures

Molecular dynamics simulations were conducted of an oxygen molecule in high temperature water confined between parallel iron hydroxide surfaces spaced at 10, 20 and 80Å apart. Radial distribution functions, coordination numbers, diffusion coefficients, and density profiles across the gap are provided. At supercritical conditions, the oxygen was found to be most likely situated in the centre region of the gap. Hydration numbers increased as the spacing of the surfaces increased and the mean diffusion coefficients for both oxygen and water were seen to be lower than their values in the bulk systems. However, the lateral diffusion coefficients of oxygen through the nano-cracks at supercritical water conditions were greater than the mean diffusion coefficients in the bulk. The effect of confinement on the water structure was seen to diminish with 20Å of surface spacing and was absent with 80Å of separation.

Primary reactions of lignite-water slurry gasification under the supercritical conditions

The experimental research of lignite-water suspension gasification under the supercritical conditions of water was carried out. The conversion of lignite was studied under the pressure 24MPa in two temperature range: 290–360 and 390–500°C without and with additives. Calculation of the activation energies of the reactions of gases formations had shown that NiO-MoO3-Al2O3 had acted as a catalyst, sodium and calcium hydroxides had behaved as reactants. The general scheme of the mechanism on formation of the gas products containing H2, CH4 and CO2 was examined. It was shown that at the subcritical temperature the gas products were formed mainly from hydrogen of coal and oxygen of mineralized water. Carbon of lignite behaves like an acid, and water is as a base at the subcritical temperature; they switch their roles at the supercritical temperature. 80% of hydrogen and oxygen of gas products were of water origin at the supercritical temperature. C, O and H conversions at initial rates of reactions with gases distribution have been synchronously analyzed for primary reactions clearing. It had found that the gasification of lignite-water suspension under the supercritical pressure had at least four types of interactions: thermal decomposition of coal, carbon oxidation by oxygen of water as well as hydrolysis and hydrocracking of coal. Oxidation of carbon was dominant for the process at the subcritical temperature on water. Hydrolysis of CC bonds of coal was the main at the supercritical parameters of water.

European Project “Supercritical Water Reactor-Fuel Qualification Test”: Summary of General Corrosion Tests

The main target of the EUROATOM FP7 project “Fuel Qualification Test for SCWR” is to make significant progress toward the design, analysis, and licensing of a fuel assembly cooled with supercritical water in a research reactor. The program of dedicated Work Package (WP4)-Prequalification was focused on evaluation of general corrosion resistance of three preselected austenitic stainless steels, 08Cr18Ni10Ti, AISI 347H, and AISI 316L, which should be prequalified for application as a cladding material for fuel qualification tests in supercritical water. Therefore, the experiments in support of WP4 concentrated on 2000-hr corrosion exposures in 25-MPa supercritical water (SCW) at two different temperatures 550°C and 500°C dosed with both 150 and 2000 ppb of dissolved oxygen content. Moreover, the water chemistry effect was investigated by conducting tests in 550°C SCW with 1.5 ppm of dissolved hydrogen content. At first, corrosion coupons were exposed for 600, 1400, and 2000 hrs in Joint Research Centre-Institute for Energy and Transport (JRC-IET), VTT Technical Research Centre of Finland Ltd. (VTT), and Shanghai Jiao Tong University (SJTU) autoclaves connected to the recirculation loop, allowing continual water chemistry control during the test. The following examination of exposed specimens consisted of weight-change calculations and detailed macro- and microscopic investigation of oxide layers using scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX). With respect to general corrosion results, all tested steels showed sufficient corrosion resistance in SCW conditions taking into account the conditions foreseen for future fuel qualification test in the research reactor in CVR Rez. When the results of weight-change calculations were compared for all three materials, it was found that the corrosion resistance increased in the following order: 316L<347H<08Cr18Ni10Ti. Results obtained in hydrogen water chemistry (HWC) did not indicate any significant beneficial effect compared to tests in SCW with 150 or 2000 ppb dissolved oxygen content. Additional tests were dedicated to investigation of the surface-finish effect. In these exposures, polished, sand-blasted, and plane-milled surface-finish techniques were investigated. The beneficial effect of surface cold work in particular of sand-blasting was clearly demonstrated.

Anthracene aquacracking using NiMo/SiO2 catalysts in supercritical water conditions

A series of effective NiMo/SiO2 catalysts for heavy oil upgrading in supercritical water have been developed. Experimental results with anthracene as model compound resembling structures present in heavy oils showed that the catalytic activity as well as the liquid and gas product distributions are governed by catalyst composition. In particular by adjusting the Ni/Mo ratio different physicochemical properties (crystalline phase composition, particle size and catalysts reducibility) are obtained, which have influence on catalytic behavior. A variety of liquid products together with a valuable gas (rich in H2) are produced in this process, which takes place with remarkably low coke deposition on the catalysts. Overall, the results derived from this work confirm the viability of upgrading polyaromatic structures in supercritical water using Ni-Mo catalysts and provides an insight on the main parameters to control in catalyst design.

Hydrothermal Gasification Performance of Iranian Rice Straw in Supercritical Water Media for Hydrogen-Rich Gas Production

As a clean and green alternative fuel to replace fossil fuels, hydrogen could be an ideal fuel for the future. Supercritical water gasification of lignocellulosic agricultural residues results in H2 production with zero CO2 emission, which makes this technique an attractive technology for hydrogen generation from biomass. Structural analyses were performed to determine the lignin, cellulose, and hemicellulose contents in feedstock. The effects of different process variables (temperature, reaction time, and feed concentration) on supercritical water gasification of Iranian rice straw (IRS) were evaluated. IRS, which has a high content of cellulose and hemicellulose, has significant potential for gaseous product generation under the supercritical water condition. The maximum H2 production of 5.56 mmol/gr of biomass was achieved at 440 °C (temperature), 20 min (reaction time), and 2 wt. % (feed concentration).

Methanol formation from the treatment of glycerol in supercritical water and with ethylsulfide

Methanol is found to be a major product when glycerol decomposes under supercritical water conditions. Glycerol is a significant byproduct of biodiesel waste streams, the value of which is decreasing as biodiesel output increases. The transformation of glycerol into methanol, which is a feedstock for biodiesel production, would potentially reduce the costs and climate impact of biodiesel production. The highest yield of methanol was found at 450°C, 300bar after 30min residence time. Addition of ethylsulfide increases the rate but does not improve the methanol yield. At longer residence times, methanol and acetaldehyde were consumed and acetic acid was produced. The generation of significant quantities of methanol (non-catalytically) from glycerol was not previously observed, and thus no mechanism for methanol production had been proposed in the published literature. A detailed kinetic model for the glycerol plus supercritical water system was constructed using the Reaction Mechanism Generator (RMG) software package. All the parameter values in this large model were predicted a priori based on quantum chemistry calculations; none were adjusted to try to match our data, which serves as a test set for the model predictions. The a priori kinetic model predicts yields of acrolein, acetic acid and acetaldehyde similar to those measured by previous researchers. However, the model underpredicted the yield of methanol measured in our experiments. We speculate that the generation of higher concentrations of methanol in our experiments than in prior work might be due to catalytic reactions on the reactor walls, since our reactor has different metallurgy than those used in prior studies.

Supercritical water gasification of RDF and its components over RuO2/γ-Al2O3 catalyst: New insights into RuO2 catalytic reaction mechanisms

Five samples including a composite refuse derived fuel (RDF) and four combustible components of municipal solid wastes (MSW) have been reacted under supercritical water conditions in a batch reactor. The reactions have been carried out at 450°C for 60min reaction time, with or without 20wt% RuO2/gamma-alumina catalyst. The reactivities of the samples depended on their compositions; with the plastic-rich samples, RDF and mixed waste plastics (MWP), giving similar product yields and compositions, while the biogenic samples including mixed waste wood (MWW) and textile waste (TXT) also gave similar reaction products. The use of the heterogeneous ruthenium-based catalyst gave carbon gasification efficiencies (CGE) of up to 99wt%, which was up by at least 83% compared to the non-catalytic tests. In the presence of RuO2 catalyst, methane, hydrogen and carbon dioxide became the dominant gas products for all five samples. The higher heating values (HHV) of the gas products increased at least two-fold in the presence of the catalyst compared to non-catalytic tests. Results show that the ruthenium-based catalyst was active in feedstock steam reforming, methanation and possible direct hydrogenolysis of C–C bonds. This work provides new insights into the catalytic mechanisms of RuO2 during SCWG of carbonaceous materials, along with the possibility of producing high yields of methane from MSW fractions.

Hydrothermal gasification of biomass model compounds (cellulose and lignin alkali) and model mixtures

Biomass model compounds (cellulose and lignin alkali) and their mixtures were gasified at sub- and super-critical water conditions in the absence and presence of alkali catalyst (K2CO3). Hydrothermal gasification was performed in batch reactor at temperature range of 300–600°C and pressure range of 90–410bar with a reaction time of 1h. Product yields of gaseous, aqueous and residue were investigated for model compounds (cellulose/lignin alkali) and their mixtures with weight ratios of 80/20, 60/40, 40/60 and 20/80 (wt./wt.). These mixtures were arranged to define the interaction between degradation products of model compounds of lignocellulosic biomass. In addition, gaseous and aqueous product compositions were identified to highlight prevailing reaction pathways during decomposition of model compounds. The highest hydrogen and methane yields were reached at 600°C in the presence of K2CO3.

Continuous Flow Supercritical Water Synthesis and Temperature-Dependent Defect Structure Analysis of YAG and YbAG Nanoparticles

Yttrium and ytterbium aluminum garnet (Y3Al5O12 and Yb3Al5O12) nanoparticles have been synthesized under sub- and supercritical water conditions in a continuous flow reactor. The particle size has been investigated by scanning and transmission electron microscopy. The heat-induced structural changes of the garnet structure have been investigated by multitemperature powder X-ray diffraction at a synchrotron source (100–1000 K). In combination, Rietveld refinement of these multitemperature data and thermal analysis indicate a proposed diffusion mechanism for the aluminum and yttrium/ytterbium atoms in the garnet structure, which leads to fewer defects at higher temperature. Hence, hydrothermally synthesized nanoparticles give novel knowledge about the disordered internal structure important for their use in optical applications.

Solid Oxide Fuel Cell Integrated with Supercritical Water Fueled by Methanol

A solid oxide fuel cell (SOFC) is known as an interesting energy conversion device because of its fuel flexibility and high efficiency. The hydrogen-rich stream is used as fuel carrier converting to generate electrical energy. A non-stoichiometric thermodynamic model based on minimum free energy was performed to predict the amount of hydrogen production via the methanol reforming under supercritical water (SCW) condition. The effects of SCW reaction temperature and water-to-methanol molar ratio on the SOFC power generation integrated with SCW reforming from methanol were investigated. The hydrogen yield, the required heat duty for a feed preheater and a SCW reactor and the SOFC power generation increase with increasing the SCW reaction temperature and the amount of water fed in SCW reactor. Under operating parameters of SCW reformer based on 1 mole/sec of methanol fed at the high temperature of 1273 K and water-to-methanol molar ratio of 5, the SOFC electrical power of 246 kW was produced with the maximum fuel utilization of 0.7.