Sub- and supercritical liquefaction of rice straw in the presence of ethanol–water and 2-propanol–water mixture

The critical liquefaction of rice straw to bio-oil with sub- and supercritical mixtures (ethanol–water and 2-propanol–water mixture) was studied in a 1000 ml autoclave at 533–623 K, 6–18 MPa, respectively. The results showed that the maximum yield of bio-oil was 39.7% for the 2-propanol:water volume ratio of 5:5 at 573 K, while the higher heating value (HHV) of bio-oil increased with the reaction temperature and solvent volume ratio. The formation of low-boiling-point materials was reduced by a mixture. Using a mixture could inhibit the formation of residue and then promote the conversion of rice straw with the ratio of 1:9–5:5. The bio-oil was analyzed by GC/MS and Elemental Analyzer, while the morphological changes of residue were observed by a scanning electron microscope (SEM).     

Liquefaction of palm kernel shell to bio-oil using sub- and supercritical water: An overall kinetic study

Palm kernel shell was liquefied using sub- and supercritical water at 330–390 °C and 25 MPa for different reaction times. The overall kinetics of the liquefaction based on the conversion of biomass was analyzed using kinetic equation adopted from the literature, and the kinetic parameters were estimated from the evaluation of the kinetic equation. In this study, the rate constant (k) increased from 0.43 to 0.49 s^?1 with reaction temperature from 330 to 390 °C. The relationship of rate constant (k) and temperature agreed reasonably well with the Arrhenius equation. The activation energy (E) and pre-exponential factor (A) values for the liquefaction process were estimated to be 6.70 kJ/mol and 1.65 s^?1, respectively. In addition, the experimental bio-oil yields with respect to reaction time were well-fitted using the modified Reverchon-Sesti Osseo equation.     

Characterization of oxide scales formed on heating equipment in supercritical water gasification process for producing hydrogen

The external- and internal walls of stainless steel heating equipment installed in supercritical water gasification plants for converting fossil fuels or renewable biomass to produce hydrogen-rich gases, respectively, are exposed to high-temperature air and reducing subcritical aqueous systems, all confronting severe corrosion issues. In these two harsh environments, the oxidation characteristics of typical stainless steel 304 were investigated by series of analytical methods. The oxide scale formed on stainless steel 304 in air at 550–650 °C exhibits a three-layer structure: an outmost layer consisting of Fe-rich corundum-type oxides, an inner layer comprising Cr-rich oxides and unoxidized metals, and an intermediate layer. The reducing subcritical aqueous experiments indicate that the relatively severe corrosion occurs at 350 °C rather than at 250 °C and 425 °C, which can be attributed to the reduced H⁺ concentration at 425 °C comparing to that at 350 °C and the increased Arrhenius rate constant of corrosion reaction with further increasing temperature from 250 °C to 350 °C. A proper pH range (6.5–10.5) for improving the corrosion resistance of Fe/Ni-based alloys in reducing subcritical aqueous environments was obtained, of which the upper limit decreases with increasing oxidicability of solutions.     

Conversion of fern (Pteris vittata L.) biomass from a phytoremediation trial in sub- and supercritical water conditions

Uncontaminated and As-contaminated fronds of Pteris vittata L., an As-hyperaccumulator fern used to phytoextract As from contaminated soils and water, were converted by sub-critical water (300 °C, 25 Pa) and supercritical water (400 °C, 25 Pa) treatments. Frond biomass was reduced between 70 and 77%. Compared to sub-critical conditions, supercritical conditions decreased C and inorganic contents in both the solid and liquid phases for uncontaminated and contaminated fronds and promoted CH₄ formation. Higher As, Fe and Zn contents in contaminated fronds promoted decreasing C contents and the formations of cyclopentenones and benzenediols in the liquid phase. Al, Fe, P, Zn and Ca mainly remained in the solid phase whereas As and S were transferred to the liquid phase for both phytomasses. As the temperature increased from 300 °C to 400 °C, the concentrations of cyclopentenones and phenols in the liquid phase rose while those of guaiacols and other compounds decreased for both phytomasses. Arsenic in the liquid phase was removed by sorption on hydrous iron oxide.     

A molecular dynamics simulation study on solubility behaviors of polycyclic aromatic hydrocarbons in supercritical water/hydrogen environment

Violates containing polycyclic aromatic hydrocarbons (PAHs) were precipitated in the process of fast pyrolysis and gasification of coal and organic substances. PAHs are one of bottlenecks of entire coal gasification for hydrogen production. In current work, the solubility of PAH oil droplets in supercritical water/hydrogen circumstances were investigated based on molecular dynamics simulation, which was beneficial for understanding the solubility behaviors of PAHs in supercritical water/hydrogen environment. The results showed that heavy PAHs were rather stable in the water phase. Supercritical water along with hydrogen promoted the miscibility of PAHs compared with that of pure supercritical water. Furthermore, high density and high temperature facilitated the rapid solvation of PAHs in supercritical water/hydrogen environment. This paper is expected to provide a theoretical support for the development of complete coal gasification technology for hydrogen production.     

Effects of temperature on corrosion performances of TiO2/SS316L in supercritical water for hydrogen production

Temperature is the most important factor for hydrogen generation during supercritical water gasification process. However, the increasing temperature could accelerate the corrosion of the reactor material, at the presence of oxygen, as less amount of oxygen can promote the hydrogen production. In this study, we prepared a 0.1 mm thick of TiO₂ coating on the surface of 316L stainless steel (SS316L) to enhance the corrosion resistance of SS316L during hydrogen production process in supercritical water. The influences of temperature (400–500 °C) on surface morphologies and corrosion depth and rate of TiO₂/SS316L were evaluated at 25 MPa with 1000 mg/L oxygen for 80h. Results showed that cracks and pores were present on the surface of TiO₂/SS316L after corroded in SCW for 80h. The crack width and corrosion rate was aggravated at higher temperature. The remained thickness of the coating at 400 °C, 450 °C, 500 °C were 0.08 mm, 0.05 mm and 0.03 mm, respectively. NiO and NiFe₂O₄ were generated around the crack on the surface of TiO₂/316L at 500 °C, the coating had a tendency to peel off the substrate.     

Gasification of landfill leachate in supercritical water: Effects on hydrogen yield and tar formation

Landfill leachate was gasified in supercritical water (SCW) in a batch reactor made of 316 SS. The effects of temperature, pressure, reaction time and oxidation coefficient (OC) on the pollutant removal efficiencies and gasification characteristics were investigated. To observe the formation of tar and char visually, a capillary quartz reactor was also used. Results indicated that CO₂, H₂ and CH₄ were the most abundant gaseous products. Temperature has an appreciable effect on the gasification process. Increasing temperature enhanced the H₂ yield (GY_H2) and TOC removal efficiency (TRE) significantly. Although the influence of reaction time on the fractions of gaseous products was negligible at time above 300 s, the yields of H₂, CH₄, and CO₂ increased with reaction time whereas the CO, C₂H₄ and C₂H₆ yields decreased. Tar and char formation was evident on the interior surface of capillary quartz reactor. Adding a little oxidant could increase H₂ and CH₄ yields and decrease tar and char formation. GY_H2 reached up to the maximum of 231.3 mmol L^?1 leachate at 500 °C, 25 MPa, 600 s and 0.2 OC, which was 2.4 times of that without oxidant.     

Effects of Lewis acid on catalyzing gasification of sewage sludge and model compounds in supercritical water

In this work, we investigated how different types and concentrations of Lewis acids effect gas yield and composition from supercritical water gasification (SCWG) of dewatered sewage sludge (DSS). Furthermore, the catalytic mechanism was investigated using AlCl₃ as a representative Lewis acid catalyst. Results showed that non-catalytic gasification of DSS produced a hydrogen yield of 0.13 mol/kg organic matter (OM) and total gas yield of 4.82 mol/kg OM; while the addition of 10 wt% Lewis acid resulted in a significantly improved H₂ and total gas yield of 0.77–7.76 mol/kg OM and 7.57–22.88 mol/kg OM, respectively. In addition, the Lewis acids tested herein depicted the following activity trend: AlCl₃＞FeCl₃＞NiCl₂＞ZnCl₂. The required temperature for generating hydrogen from SCWG of DSS was greatly reduced when AlCl₃ was present during the heating process. Furthermore, AlCl₃ significantly increased the hydrogen yield from SCWG of the model compounds. Finally, AlCl₃ catalyzed SCWG of guaiacol and glycerol showed the best hydrogen selectivity.     

Comparison of biodiesel production from crude Jatropha oil and Krating oil by supercritical methanol transesterification

This work compared the production of biodiesel from two different non-edible oils with relatively high acid values (Jatropha oil and Krating oil). Using non-catalytic supercritical methanol transesterification, high methyl ester yield (85–90%) can be obtained in a very short time (5–10 min). However, the dependence of fatty acid methyl ester yield on reaction conditions (i.e., temperature and pressure) and the optimum conditions were different by the source of oils and were correlated to the amount of free fatty acids (FFAs) and unsaturated fatty acid content in oils. Krating oil, which has higher FFAs and unsaturated fatty acid content, gave higher fatty acid methyl ester yield of 90.4% at 260 °C, 16 MPa, and 10 min whereas biodiesel from Jatropha oil gave fatty acid methyl ester yield of 84.6% at 320 °C, 15 MPa and 5 min using the same molar ratio of methanol to oil 40:1. The product quality from crude Krating oil met the biodiesel standard. Pre-processing steps such as degumming or oil purification are not necessary.     

Thermal analysis of cooled supercritical steam turbine components

This paper describes thermal analysis methodology results for the supercritical steam turbines. The analysis presented here concerns the external cooling of the turbine components. Due to the supercritical parameters of the live steam, the inlet areas of the high and intermediate pressure parts are exposed to the steam at high temperature level. The design solutions applied to the turbines so far aim to protect the inlet areas. Basic solution incorporate protective screens, which are made of a material more resistant to the high temperature than the rest of the components. Additional protection is provided by the external cooling. The applications of both methods described above allow to increase the live steam temperature. The conducted analysis determined the temperature field in the steam, which cools the rotor, as well as the distribution of the temperature and stresses in the rotor. The obtained results were then applied to the investigations concerning the durability of the rotor.     

Thermodynamic study on the integrated supercritical water gasification with reforming process for hydrogen production: Effects of operating parameters

In this paper, a conceptual process design of the integrated supercritical water gasification (SCWG) and reforming process for enhancing H₂ production has been developed. The influence of several operating parameters including SCWG temperature, SCWG pressure, reforming temperature, reforming pressure and feed concentration on the syngas composition and process efficiency was investigated. In addition, the thermodynamic equilibrium calculations have been carried out based on Gibbs free energy minimization by using Aspen Plus. The results showed that the higher H₂ production could be obtained at higher SCWG temperature, the H₂ concentration increased from 5.40% at 400 °C to 38.95% at 600 °C. The lower feed concentration was found to be favorable for achieving hydrogen-rich gas. However, pressure of SCWG had insignificant effect on the syngas composition. The addition of reformer to the SCWG system enhanced H₂ yield by converting high methane content in the syngas into H₂. The modified SCWG enhanced the productivity of syngas to 151.12 kg/100kg_feed compared to 120.61 kg/100kg_feed of the conventional SCWG system. Furthermore, H₂ yield and system efficiency increased significantly from 1.81 kg/100kg_feed and 9.18% to 8.91 kg/100kg_feed, and 45.09%, respectively, after the modification.     

Molecular dynamics investigation on the gasification of a coal particle in supercritical water

Coal gasification technology in supercritical water provides a clean and efficient way to convert coal to H₂. In the present paper, the whole supercritical water（SWC）gasification process of a coal particle is studied with the reactive force field (ReaxFF) molecular dynamics (MD) method for the first time. First, the detailed reaction mechanism which can't be clearly illustrated in experiments, such as the evolution of the carbon structure during the gasification process and the detailed reaction mechanism of the main products, is obtained. According to the generation mechanism of H₂, it is found that the supercritical water gasification process of a coal particle can be divided into two stages with different reaction mechanisms, namely the rapid reaction stage and the stable reaction stage. Then, the effects of temperature and coal concentration in the reaction system on the yield of H₂ are studied. Finally, the transition of N in the coal particle is revealed, in which the precursors of NH₃ such as CN, CHN, and CHON are the basic molecular structures for nitrogen atoms during the gasification process at high temperature.     

超临界压力下航空煤油在竖直管内的传热研究

对竖直上升管内超临界压力下航空煤油的传热特性进行了实验研究。分析了不同质量流量、热流密度、压力和进口温度对超临界压力下航空煤油传热特性的影响。实验结果表明,提高质量流量或进口温度均使煤油传热效果变好。而热流密度对流体传热的影响主要在于改变了流体和壁面温度,热流密度越大,传热系数越高。压力对煤油传热影响不大,一般情况下,提高压力会恶化传热。超临界状态下,煤油物性变化很大,因此对煤油的传输和热力学性质的准确计算是研究超临界压力下传热现象的关键。利用拓展的对比态法来计算煤油的密度和传输特性,如黏度、热导率等。给出了煤油在超临界压力下的传热关联式,其计算值和实验值吻合良好。     

Study on gasification mechanism of biomass waste in supercritical water based on product distribution

Supercritical water gasification technology is widely applied to convert organic waste into valuable substances as a clean and efficient method. Biomass gasification in SCW is a complex process and complicated chemical reactions like decomposition and poly-condensation take place, thus, reaction mechanism of real biomass needs to be further investigated. In this paper, experimental study on cornstalk gasification in SCW was conducted at the temperature of 500–800 °C, reaction time of 1–15min and feedstock concentration of 1–9%. The effects of various operating parameters on evolution of gas, liquid and solid products were conducted. It was discovered that pore structure and carbon microspheres appeared successively on the surface of solid residue. Mechanism study showed that the biomass was first depolymerized into monomer and its derivatives, then cracked and poly-condensed into a nuclear to generate carbon microspheres as its concentration reached the critical concentration. As the reaction proceeds, reduction reaction, coke combustion and secondary reaction occurred, thus carbon microspheres decreased. The results indicated that higher reaction temperature, longer reaction time and lower feed concentration were conducive to improving reaction performance of biomass. Finally, it was discovered that carbon gasification efficiency reached 99% at the temperature of 700 °C, reaction time of 15 min and biomass concentration of 3%.     

Direct gasification of dewatered sewage sludge in supercritical water. Part 1: Effects of alkali salts

In the present study, the catalytic effects of alkali salts [NaOH, KOH, K₂CO₃, Na₂CO₃ and Ca(OH)₂] on the direct gasification of dewatered sludge in supercritical water were investigated by using a high-pressure autoclave at a constant temperature of 450 °C and a residence time of 30 min. The hydrogen yield increased in the presence of the alkali salts, except for Ca(OH)₂. Specifically, the hydrogen yield increased from 0.68 to 3.45 mol/(kg OM) as the K₂CO₃ concentration increased from 0 to 8 wt%. Although Ca(OH)₂ did not significantly impact the catalytic effect on the hydrogen yield, it did impact the CO₂ yield. Generally, the addition of alkali salts did not affect the organic matter or total phenol concentrations in the liquid residue. Moreover, char formation was considerably suppressed by the alkaline catalyzed hydrolysis of the dewatered sludge [except in the case of Ca(OH)₂].     

Experimental study on solar thermal conversion based on supercritical natural convection

In this paper, experimental investigation into the basic characteristics of solar thermal conversion using supercritical CO₂ natural convection are presented. Natural circulation of supercritical fluids can be easily induced and even a small change in temperature can result in large change in density close to the critical point. The supercritical experimental system carefully designed and operated in this study. It is found that an obvious and continuous long-time drop of solar radiation would not affect the CO₂ flow rate, temperature and pressure very much, if the solar radiation is in a relatively high-value level. This continuous drop can induce obvious drops in the CO₂ flow rate, temperature and pressure only when the solar radiation is in a low-value level. Furthermore, it is observed that a long-time drop and low-value in the solar radiation may make the flow rate temporarily become zero, which should be paid more attention in future system design and operation. The collecting efficiency increases with the comprehensive coefficient and this pattern is contrary to that of water based system. In addition, it is found that there exist optimal flow rate and CO₂ charge amount for system overall performance. This kind of solar thermal conversion has a higher collecting efficiency in spring and winter than summer and autumn; a better performance in cold and low-radiation region than hot and high-radiation region.     

Supercritical water synthesized Ni/ZrO2 catalyst for hydrogen production from supercritical water gasification of glycerol

Catalysts are crucial to promote the technical feasibility of supercritical water gasification (SCWG) for H₂ production from wet biomass, yet catalysts prepared by conventional methods normally encounter sintering problems in supercritical water. Herein, a series of ZrO₂-supported Ni catalysts were tried to be prepared by supercritical water synthesis (SCWS) and evaluated for SCWG in terms of activity and property stability. The SCWS was conducted at 500 °C and 23 MPa using metal nitrates as starting materials. Effect of precursor concentration on property and catalytic performance of the SCWS-prepared catalysts for SCWG of 20 wt% glycerol were systematically studied. XRD, SEM-EDS, TEM and TGA were applied for catalyst characterization. Results verified the successful obtaining of Ni/ZrO₂ nanocatalysts with Ni crystals of 30–70 nm and ZrO₂ crystals of ~11 nm by the SCWS process, which were found to be active on the WGSR for SCWG to increase the H₂ yield as high as 155%. Importantly, the SCWS-prepared Ni/ZrO₂ catalysts exhibited excellent property stability and anti-coking ability for SCWG of glycerol.     

Catalytic supercritical water gasification mechanism of coal

Catalytic supercritical water gasification (SCWG) for H₂ production is a hopeful way of coal conversion to replace the traditional coal utilization mode. At present, the detailed catalytic mechanism in the process remains unknown. Herein, a comprehensive catalytic SCWG mechanism of coal is proposed by establishing a novel catalytic kinetic model. It shows that catalysts (K₂CO₃) break up the coal matrix by a cyclic redox reaction to produce plenty of mesopores, accelerating steam reforming of fixed carbon and coal pyrolysis. Water-gas shift reaction is facilitated by K₂CO₃ via formation of formate, which then promotes steam reforming of CH₄ at high temperature (≥700 °C) due to the decreasing CO. The proposed mechanism provides important insights in catalytic SCWG process of coal.     

Fluid-to-fluid modeling of supercritical water loops for stability analysis

The use of supercritical water as coolant/moderator may induce oscillations in the supercritical light water reactor similar to the density wave oscillations observed in boiling water reactors (BWRs). In order to experimentally investigate the stability of supercritical reactors, a fluid-to-fluid downscaled facility is proposed. It is found that with an appropriate mixture of refrigerants R-125 and R-32, the dimensionless enthalpy and density of the supercritical water can be accurately matched for all relevant operational conditions of the reactor. Moreover, the inertia distribution, the friction factor distribution and the heat transfer mechanism are taken into account in the modeling. As a result of the proposed downscaling, the operational pressure, temperature and power are considerably smaller than those of a water-based system, which in turn helps reducing the construction and operational costs of a test facility. Finally, it is found that the often used modeling fluid supercritical CO₂ cannot accurately represent supercritical water at reactor conditions.     

Hydrogen generation by gasification of phenol and alcohols in supercritical water

The conversion of phenol, cyclohexanol (a hydrogenated analog of phenol for comparison with phenol), and ethanol into gas products in supercritical water (SCW) was studied with the goal to compare the reactivity of their aqueous solutions with the structural features obtained by the method of classical molecular dynamics. Transformation of phenol and alcohols occurs in different ways. In the case of alcohols, the conversion of 75–100% is achieved at 600 °C with noticeable gasification. At the same time, the conversion of phenol is only 47% and no gas products are formed at all. The complete conversion of phenol is achieved at a temperature of 750 °C, while the degree of gasification does not exceed 30%. It is shown that an increase in the phenol gasification degree is possible by pre-catalytic hydrogenation of phenol into cyclohexanol.