Supercritical Water Conditions Research Articles

Conversion of Residual Palm Oil into Green Diesel and Biokerosene Fuels under Sub- and Supercritical Conditions Employing Raney Nickel as Catalyst

A comprehensive study of the thermal deoxygenation of palm residue under sub- and supercritical water conditions using Raney nickel as a heterogeneous catalyst is presented in this paper. Hydrothermal technology was chosen to replace the need for hydrogen as a reactant, as happens, for example, in catalytic hydrotreatment. Several experiments were carried out at different reaction temperatures (350, 370, and 390 °C) and were analyzed with different times of reaction (1, 3.5, and 6 h) and catalyst loads (5, 7.5, 10 wt.%). No hydrogen was introduced in the reactions, but it was produced in situ. The results showed the selectivity of biokerosene ranged from 2% to 67%, and the selectivity of diesel ranged from 5% to 98%. The best result was achieved for 390 °C, 10 wt.% catalyst load, and 3.5 h of reaction, when the selectivities equal to 67% for biokerosene and 98% for diesel were obtained. The Raney nickel catalyst demonstrated a tendency to promote the decarboxylation reaction and/or decarbonylation reaction over the hydrodeoxygenation reaction. Moreover, the fatty acid and glycerol reforming reaction and the water−gas shift reaction were the main reactions for the in situ H2 generation. This study demonstrated that a hydrothermal catalytic process is a promising approach for producing liquid paraffin (C11−C17) from palm residue under the conditions of no H2 supply.

Sub- and Supercritical Water Gasification of Rice Husk: Parametric Optimization Using the I-Optimality Criterion.

In this study, rice husk biomass was gasified under sub- and supercritical water conditions in an autoclave reactor. The effect of temperature (350–500 °C), residence time (30–120 min), and feed concentration (3–10 wt %) was experimentally studied using the response surface methodology in relation to the yield of gasification products. The quadratic models have been suggested for both responses. Based on the models, the quantitative relationship between various operational conditions and the responses will reliably forecast the experimental outcomes. The findings revealed that higher temperatures, longer residence times, and lower feed concentrations favored high gas yields. The lowest tar yield obtained was 2.98 wt %, while the highest gasification efficiency and gas volume attained were 64.27% and 423 mL/g, respectively. The ANOVA test showed that the order of the effects of the factors on all responses except gravimetric tar yield follows temperature > feed concentration > residence time. The gravimetric tar yield followed a different trend: temperature > residence time > feed concentration. The results revealed that SCW gasification could provide an effective mechanism for transforming the energy content of RH into a substantial fuel product.

Gasification of Psidium guajava L. Waste Using Supercritical Water: Evaluation of Feed Ratio and Moderate Temperatures

Biomass waste, as raw material for renewable energy, is an attractive alternative since it does not compete with human food supply. An emerging alternative for its treatment is supercritical water gasification (SCWG), due to the high moisture content of some types of biomass. On this regards, guava fruit (Psidium guajava L.) is one of the most wasted agro-food products in Mexico. This motivated us to evaluate gasification of guava waste on dry biomass base under supercritical water conditions for the first time, with the aim of analyzing the impact of moderate temperatures and feed ratios as reaction parameters on gas products. Temperature was varied in the range of 673.15–773.15 K and using a batch reactor loaded with biomass:water (B:W) mass ratios of 1:1, 1:4, and 1:6. Furthermore, the obtained solid, liquid, and gas phase products were characterized. Hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO), methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10) were identified in gas phase and quantified by means of a gas chromatograph equipped with a TCD detector. Liquid and solid phase products were subjected to Fourier Transform Infrared spectroscopy analyses. This preliminary research indicated that high temperature operation and high biomass:water mass ratio enhanced gas yields (mol/kg) of about 4.137 for CH4, 6.705 for CO2, and 7.743 for H2; whereas the selectivity and gas efficiency for hydrogen was 65.26% and 58.94%, respectively.

Efficiency and energetic analysis of the production of gaseous green fuels from the compressed steam and supercritical water gasification of waste lube oils

The gasification of a waste lube oil (WLO), using water under vapor and supercritical states leads to its valorization into green fuels with high energetic power like H2 and CH4. This work investigates how the most important variables of a flow continuous gasification process, temperature, pressure and reaction time, influence the efficiency of gasification and the amounts and characteristics of the gases produced. The study of the influence of these variables on the energetic efficiency of the process was also completed. The highest production yields, 2.4 10−2 molH2goil−1 and 3.0 10−2 molCH4goil−1, were registered at supercritical water conditions (750°C, 250 bar and 1.87 min). The energetic study revealed that the energetic efficiency increased as the reaction time was lengthened. Gasification kinetics were slower as pressure increased but the reaction times inside the flow reactor were longer. A pressure range from 150 bar, steam region, to 250 bar, supercritical region, was identified as optimal since allowed the most suitable balance between kinetics and the reaction times achieved.

Constructing the Pourbaix diagram of Fe-Cl−-H2O ternary system under supercritical water conditions

Supercritical water oxidation (SCWO) technology, which has outstanding performance, is impeded commercial development due to serious corrosion problems. In this work, the first Pourbaix diagrams of the Fe-Cl−-H2O ternary system under supercritical water conditions was established to predict the evolution process of metals in thermodynamics equilibrium. The fifteen species were rationally screened by the ion distribution coefficient, and thermodynamics parameters under supercritical conditions were calculated by the extrapolation method of the revised Helgeson-Kirkham Flowers (R-HKF) model. The results show that hematite and magnetite passivation film and ferrous iron are continuously eroded by FeCl3 (aq) corrosion zone with the increase of chloride concentration, which greatly expands the area. Interestingly, the range of the immunization zone is almost unchanged. On the other hand, most of the Fe(OH)- 3 corrosion zone, passivation zone, and immunization zone to be converted to ferrous iron caused by temperature rise when FeCl3 (aq) reaches a certain concentration. Metals often form a hematite and magnetite passivation film to reduce the dissolution rate of the substrate under alkaline conditions with relatively high potentials, so the pre-neutralization method can be used as an anti-corrosion measure.

Gasification of Biomass in Supercritical Water, Challenges for the Process Design—Lessons Learned from the Operation Experience of the First Dedicated Pilot Plant

Gasification of organic matter under the conditions of supercritical water (T > 374 °C, p > 221 bar) is an allothermal, continuous flow process suitable to convert materials with high moisture content (<20 wt.% dry matter) into a combustible gas. The gasification of organic matter with water as a solvent offers several benefits, particularly the omission of an energy-intensive drying process. The reactions are fast, and mean residence times inside the reactor are consequently low (less than 5 min). However, there are still various challenges to be met. The combination of high temperature and pressure and the low concentration of organic matter require a robust process design. Additionally, the low value of the feed and the product predestinate the process for decentralized applications, which is a challenge for the economics of an application. The present contribution summarizes the experience gained during more than 10 years of operation of the first dedicated pilot plant for supercritical water gasification of biomass. The emphasis lies on highlighting the challenges in process design. In addition to some fundamental results gained from comparable laboratory plants, selected experimental results of the pilot plant “VERENA” (acronym for the German expression “experimental facility for the energetic exploitation of agricultural matter”) are presented.

Syngas Production via Supercritical Gasification of Leachate: A Review

Leachate is characterized as heterogeneous, recalcitrant wastewater, and potentially harmful pollutant to the environment and human health. It is encountered in landfills municipal disposal, which contains high concentrations of many organic and inorganic compounds such as Chemical Oxygen Demand (COD), Biochemical Oxygen Demand (BOD), Total Organic Carbon (TOC), and Total Nitrogen (TN). This report provides a comprehensive research review of syngas production characteristics via supercritical gasification (SCG) technology as a treatment before leachate is released to final waste disposal. This technology is worked by gasifying the leachate under supercritical water condition (500°C, 23 MPa) followed by syngas production such as H2, CO, CO2, CH4. First, we compare the essential characteristic of leachate by addressing the value of COD, TOC, pH, TN to understand effect contaminants that will be harmful. Second, the effect of gasification techniques are evaluated to syngas production and carbon gasification ratio. Finally, the landfill leachate treatment using supercritical gasification indicates that this method plays a vital role in treating leachate with syngas production that hydrogen is produced as abundant gas compares to the other. The result also summarizes future development that supercritical gasification technology is feasible to reduce leachate pollutants; therefore, it has the possibility to release into surface water.

Population Balance and CFD Simulation of Particle Aggregation and Growth in a Continuous Confined Jet Mixer for Hydrothermal Synthesis of Nanocrystals

Population balance and computational fluid dynamics models are built and integrated to carry out a simulation study of the reactive crystallisation process in a confined jet mixer (CJM) for the continuous flow synthesis of TiO2 nanoparticles at a supercritical water condition. In the population balance model, the crystal growth in size is modelled as being due to combined nanocrystal aggregation as well as surface growth. A free molecular model is used to predict the particle aggregation. The performance of the combined aggregation and surface growth models is compared with models that only consider surface growth as the only mechanism for particle size enlargement. It was found that the combined model gives a more accurate prediction of particle size distribution.

Kinetic modeling of carbon monoxide oxidation and water gas shift reaction in supercritical water

Carbon monoxide is a primary intermediate in supercritical water oxidation (SCWO) processes. A validated chemical kinetic model for hydrogen oxidation in supercritical H2O/CO2 mixtures was expanded to describe carbon monoxide oxidation and water gas shift reaction in supercritical water (SCW). The model performance was evaluated against experimental data in literature. Data for carbon monoxide oxidation were well reproduced by the model. Underprediction of hydrogen formation was attributed mostly to hydrolysis reactions occurring on the reactor surface. The rate of water gas shift reaction observed experimentally under SCW conditions strongly exceeds that predicted by the model. Analysis indicates that the observed WGS reaction is largely due to catalytic effects on the reactor surface, even though trace impurities of oxygen present in the experiments may also enhance reaction. Pressure corrections on thermodynamic properties and inclusion of HOCHO subset were found to have negligible influence on modeling results. CO+OH=CO2+H, HO2+H2O=OH+H2O2 and OH+HO2=O2+H2O were identified as the most important elementary reactions by sensitivity analysis.

Supercritical hydrothermal synthesis of UO2+x: stoichiometry, crystal shape and size, and homogeneity observed using 23Na-NMR spectroscopy of (U, Na)O2+x

The hydrothermal synthesis of pure uranium dioxide under supercritical water (SCW) conditions was investigated using a starting material composed of a uranyl(vi) nitrate solution at 450 °C.

Activity and stability of TiO2 samples with different phase compositions in the decomposition of formaldehyde in SCW

TiO2 samples with different crystal sizes and compositions were synthesized using a sol-gel method at different calcination temperatures (350–900 °C). The activity and stability of TiO2 samples were determined by the gasification of formaldehyde in supercritical water (SCW) and by treatment in SCW. Increasing calcination temperature and SCW gasification (SCWG)/SCW treatment decreased the surface area of anatase TiO2 samples due to growing crystallite size via agglomeration and sintering. Among anatase TiO2 samples, the TiO2 calcinated at 450 °C was found as the most suitable material under SCW conditions. However, the surface area of rutile TiO2 slightly increased from 17.2 m2 g−1 to 19.8 m2 g−1 with the weakly crumbling of particles during SCWG. The highest hydrogen formation (63%) from formaldehyde in the SCW was obtained in the presence of anatase TiO2 calcined at 350 °C and rutile TiO2 calcined at 800 °C. CO2 formation in the presence of anatase TiO2 is higher than rutile TiO2 because of the presence of active lattice oxygen species (O−, O2−) based on O2-TPD.

Application of supercritical water conditions to improve the flowback of fracturing fluid in ultra-low-permeability sandstone formations

Hydraulic fracturing is an important approach for stimulating the production of ultra-low-permeability oil reservoirs; however, the low flowback efficiency of the fracturing fluid (FF) may cause contamination of the groundwater and reduce oil recovery. This study experimentally investigates the possibility of improving the flowback efficiency of water-based FF and oil production through the achievement of supercritical water (SCW) conditions after the injection of FF in ultra-low-permeability oil reservoirs. Possible mechanisms are explored through measurement of the capillary pressure, visbreaking of oil, pore-fracture distribution, and mineral components. The results show that the FF flowback efficiency and oil recovery after the SCW condition treatment are an average of 10.2% and 19% higher than those after regular hydraulic fracturing (RHF), respectively. The lower capillary pressure after the SCW condition treatment indicates a lower flow resistance. Furthermore, the increase in the proportion of large pores ranges from 6.6% to 9.0% for the fracturing process after the SCW condition treatment, whereas the increase in large pores after RHF ranges from 2.8% to 4.5%. The increases in the porosity of the core samples after the SCW condition treatment are 0.87%, 1.08%, and 1.43%, which are markedly higher than those after RHF (0.33%, 0.24%, and 0.26%, respectively). The percentage of kaolinite in the rock samples increases, whereas that of interstratified illite–montmorillonite decreases after the SCW condition treatment, thus increasing the stability of the minerals in water. The reduced dynamic viscosity and lighter components of the recovered oil from the SCW condition treatment reveal the occurrence of visbreaking of the crude oil.

Flame structure of supercritical ethanol/water combustion in a co-flow air stream characterized by Raman chemical analysis

The thermochemical properties of the ethanol hydrothermal flame (30%-v ethanol-water solution), established in air co-flow under supercritical water conditions, are measured by a quantitative Raman diagnostic technique. The unique spectroscopic features of the chemical compounds are revealed to aid understanding of the fuel decomposition and oxidation process in supercritical phase. A point-wise fiber-optic probe is designed to resolve the needle-like thin flame and identify the spatial profiles – across the flame thickness and over the height of the visible flame – of the combustion species at different fuel/air ratios and airflow rates. An attempt is made to analyze the steep thermal gradients in the primary reaction zone near the fuel nozzle by estimating thermodynamic temperatures from the measured fluid density. The test cell was held at conditions above the critical point of water, at a nominal pressure of 25 MPa and a nominal temperature of 723 K (450 °C). The flames were over ventilated with an excess air:fuel ratio between two and five times stoichiometric. Depending upon the air:fuel ratio non-sooting blue flames or yellow sooting flames are observed. The results of the Raman diagnostic provide temperature and species profiles that will prove useful for validating numerical models using an idealized laboratory hydrothermal flame. The Raman diagnostic results are augmented with imaging from two orthogonal camera views: a backlit shadow-graphic image of the co-flow jet and a color image of the flame.

Sub- and Supercritical Water Liquefaction of Kraft Lignin and Black Liquor Derived Lignin

To mitigate global warming, humankind has been forced to develop new efficient energy solutions based on renewable energy sources. Hydrothermal liquefaction (HTL) is a promising technology that can efficiently produce bio-oil from several biomass sources. The HTL process uses sub- or supercritical water for producing bio-oil, water-soluble organics, gaseous products and char. Black liquor mainly contains cooking chemicals (mainly alkali salts) lignin and the hemicellulose parts of the wood chips used for cellulose digestion. This review explores the effects of different process parameters, solvents and catalysts for the HTL of black liquor or black liquor-derived lignin. Using short residence times under near- or supercritical water conditions may improve both the quality and the quantity of the bio-oil yield. The quality and yield of bio-oil can be further improved by using solvents (e.g., phenol) and catalysts (e.g., alkali salts, zirconia). However, the solubility of alkali salts present in black liquor can lead to clogging problem in the HTL reactor and process tubes when approaching supercritical water conditions.

Kinetics of extra-heavy oil upgrading in supercritical water with and without zinc nitrate using the phase separation kinetic model

Kinetic study of upgrading extra-heavy oil in supercritical water conditions with and without a zinc nitrate precursor was investigated. Reactions were carried out at 653 K, 673 K, 693 K, and 713 K in a batch reactor. Kinetic parameters including reaction rate constants were calculated based on a phase separation kinetic (PSK) model. Compared to pyrolysis, the rate of maltene decomposition and solubility limits were significantly low. Addition of the zinc nitrate precursor changed the amount of volatile from reacted maltene and the amount of coke from excess asphaltene cores. Coke induction period where coke formation is suppressed was obtained. Maltene selectivity was calculated based on the amount of maltene and volatile and the highest maltene selectivity value was 6.96 at 693 K, without the zinc nitrate precursor.

Evaluation of Friction-Factor Correlations at Supercritical Water Conditions in Support of the Canadian SCWR

Abstract Canada is participating in the Generation IV International Forum with the main focus on the pressure-tube-type supercritical water-cooled reactor (SCWR) concept. The Canadian SCWR concept is a heavy-water moderated and light-water-cooled reactor. The R&D framework for the development of the Canadian SCWR fuel-assembly concept includes experiments and analyses, including subchannel code development and applications. This paper focuses on the modeling of the hydraulic resistance under supercritical conditions, with or without the wire-wrap spacers. More specifically, it presents an assessment of three friction factor correlations developed for supercritical conditions. A literature survey of wire-wrap hydraulic models is presented. The assessment of the supercritical friction factor correlations and wire-wrap hydraulic models is carried out using three pressure drop experimental datasets, using the subchannel code ASSERT-PV V3.2m2.

Formation of Local, Transient “Acid Spikes” in the Fast Neutron Radiolysis of Supercritical Water at 400 °C: A Potential Source of Corrosion in Supercritical Water-Cooled Reactors?

Abstract The use of supercritical water (SCW) in GEN IV reactors is a logical approach to the ongoing development of nuclear energy. A proper understanding of the radiation chemistry and reactivities of transients in a reactor core under SCW conditions is required to achieve optimal water chemistry control and safety. A Monte Carlo simulation study of the radiolysis of SCW at 400 °C by incident 2 MeV monoenergetic neutrons (taken as representative of a fast neutron flux in a reactor) was carried out as a function of water density between ∼150 and 600 kg/m3. The in situ formation of H3O+ by the generated recoil protons was shown to render the “native” track regions temporarily very acidic (pH ∼ 1). This acidity, though local and transitory (“acid spikes”), raises the question whether it may promote a corrosive environment under proposed SCW-cooled reactor operating conditions that would lead to progressive degradation of reactor components.

Electroluminescence in perovskite oxide nanocrystals

Chemically stable and easily printable inorganic surface-emitting devices are essential for portable, disposable, outdoor-safe, and wearable information display devices. However, inorganic electroluminescent devices with both chemical stability and printability have never been realized. We present the first report of an oxide-nanocrystalline powder-type self-emitting device without both capping and dispersion treatments. The Ca0.6Sr0.4TiO3:Pr nanocrystal was prepared by a hydrothermal synthesis method under supercritical water conditions. The red light-emitting electroluminescence intensity increased with an increase in the driving voltage. The starting electric field for electroluminescence generation is sufficiently low (6.9 × 104 V cm−1). The proposed electroluminescent device is achieved by minimizing the voids into the inside of the light-emitting layer and the interface between the light-emitting and insulating layers by using nanometer-sized oxide-phosphor-crystals. The oxide-nanocrystal electroluminescent device will be developed into future powder-type surface-emitting devices that may contribute in inventing numerous applications, such as outdoor information displays and wearable information terminals.

Two-step sub/supercritical water and ethanol processes for non- catalytic biodiesel production

The catalyst-free two-step process has been developed for biodiesel production using low-grade feedstocks. The first step consists of triglycerides hydrolysis under subcritical water conditions to generate and increase free fatty acid (FFA) content for ethyl ester production. In its subcritical state, water can be used as both a solvent and a reactant for the hydrolysis of triglycerides. The hydrolyzed product mixture is separated by decantation into the oil phase of FFA (upper layer) and a water phase that contains glycerol (lower layer). In the second step, the hydrolyzed products of free fatty acids were successfully esterified to their ethyl ester in supercritical ethanol conditions without any catalyst. Under the sub- and supercritical conditions of water and ethanol, the hydrolysis and the esterification reactions proceed quickly, with a conversion of greater than 98 % after 10−20 min. This two-step process for biodiesel production offers several advantages, such as milder reaction conditions and pollution reduction due to the use of water instead of organic solvents. Also, the glycerol is removed after the hydrolysis reaction so that the backward reaction between the glycerol and the ethyl ester disappears, and lead to the biodiesel yield and quality improvement. The aim of this study is making a comparison between our previous one-step process and the two-step reaction process to find the best pathway for designing and building an integrated reactor. Indeed, the two-step process is more applicable for low-grade feedstocks with a high amount of FFA and water.

Effects of Si, Mn on the corrosion behavior of ferritic–martensitic steels in supercritical water (SCW) environments

Ferritic–martensitic (F/M) steels with various amounts of alloying elements (Cr: 12 wt. %, Si: 0.6–2.2 wt. %, Mn: 0.6–1.8 wt. %) were exposed to supercritical water (SCW) environments at 500 °C and 25 MPa for up to 1000 h. Gravimetric, Scanning Electron Microscope, Transmission Electron Microscopy (TEM), Energy-dispersive X-ray Spectroscopy (EDX) and X-ray Diffraction (XRD) analyses were conducted to characterize the corrosion behavior of F/M steels in supercritical water. The results showed that the contents of the alloying elements (Si, Mn) affected both the oxidation kinetics and the oxidation scales formed on F/M steels. The F/M steel (Si: 0.6 wt. %, Mn: 1.8 wt.%) showed the highest corrosion rate in SCW with an intermittent oxide scale formed on the surface. Increasing the Si content in F/M steel improved the oxidation resistance and promoted the formation of a uniform oxide scale. The mechanism associated with the corrosion of these F/M steels under SCW conditions is discussed.