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.     

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 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.     

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.     

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].     

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.     

Partial oxidative gasification of phenol for hydrogen in supercritical water

The paper reports partial oxidative gasification of phenol for hydrogen in supercritical water (SCW) at lower temperature (<753 K), at which cleavages of aromatic ring occur difficultly and tend to undesirable polymerization. The results showed that O₂ is effective to gasification of phenol in SCW. ∼76% of phenol was gasified and 2.7 mol/mol of hydrogen was produced within 180 s with Na₂CO₃ as catalyst at the selected process conditions, a molar ratio of oxygen-to-phenol, 7.5–1, 723 K, and 24 MPa. It was found that unstable opening-rings products oxalic and maleic acid and stable dimmerization compounds in liquid water were formed during partial oxidation process. The process also indicated phenol was rapidly converted, and some opening-rings products were slowly gasified, which also confirmed oxygen served as effective reactant for ring-opening. Based on the given reaction conditions, a treatment process using a real wastewater from coking industry was performed. The data showed that the present technology provides an effective way to gasification of phenol wastewater for high-value energy utilization.     

Application of the computer code TOUGH2 to the simulation of supercritical conditions in geothermal systems

At the high pressures and temperatures found in deep geothermal systems, supercritical conditions can occur. Current numerical geothermal simulators are either not capable of modelling these conditions, or can do so only at significantly reduced computation speed. This paper describes modifications to the TOUGH2 simulator to extend its applicability. It employs the updated IAPWS-97 thermodynamic formulation, and uses density and temperature as primary thermodynamic variables under supercritical conditions. Results from test problems are in agreement with results produced by other simulators, giving confidence that the simulator can be used for modelling deep geothermal reservoirs.     

Experimental study on the heat transfer characteristics during the pressure transients under supercritical pressures

Main objective of this study is to investigate an applicability of a steady-state heat transfer correlation to pressure transient sequences and an effect of the pressure transient rates on the overall heat transfer rates under the supercritical pressures. Heat transfer rates are brought in line for both the pressure increasing and the pressure decreasing transients. And effects of pressure transient rates on heat transfer rates are trivial. As for an applicability of steady-state heat transfer correlation to the pressure transient sequences, the heat transfer correlation always overestimates the Nusselt number measured in the pressure transient heat transfer experiments by average 30%.     

Modified potential theory for modeling supercritical gas adsorption

Theoretical modeling of adsorption plays a crucial role in providing better understanding of the adsorption phenomena, isotherms and isosteric heats. However, when modeling the adsorption of gas mixtures containing hydrogen, it is necessary to accommodate a wide temperature range because of hydrogen's low critical temperature. In this work, we extend the multicomponent potential theory of adsorption's (MPTA) capability of predicting adsorption isotherms to a wide temperature range by introducing a temperature dependent Dubinin potential parameter and use it to model adsorption isotherms of supercritical hydrogen, nitrogen and methane on various activated carbons. This extended MPTA can accurately predict the adsorption isotherms when used with NIST equation of state (EOS). The resulting isosteric heats of adsorption of hydrogen agree well with the experimental data for similar volume filling scenarios. Hydrogen's low temperature adsorbed-phase pressure inside the activated carbon's micropore volume reaches the melting pressure of solid hydrogen. This causes the transition of adsorbed hydrogen from supercritical gas to solid-like phase which is clearly observed in our model. Our study, thus, provides a better understanding of physisorption of hydrogen inside the micropores.     

Co-gasification of catechol and starch in supercritical water for hydrogen production

Hydrogen production from waste feedstocks using supercritical water gasification (SCWG) is a promising approach towards cleaner fuel production and a solution for hard to treat wastes. In this study, the catalytic co-gasification of starch and catechol as models of carbohydrates and phenol compounds was investigated in a batch reactor at 28 MPa, 400–500 °C, from 10 to 30 min. The effects of reaction conditions, and the addition of calcium oxide (CaO) as a carbon dioxide (CO₂) sorbent and TiO₂ as catalyst on the gas yields and product distribution were investigated. Employing TiO₂ as a catalyst alone had no significant effect on the H₂ yield but when combined with CaO increased the hydrogen yield by 35% and promoted higher total organic carbon (TOC) reduction efficiencies. The process liquid effluent was characterized using GC–MS, with the results showing that the major non-polar components were phenol, substituted phenols, and cresols. An overall reaction scheme is provided.     

Catalytic gasification of glycine and glycerol in supercritical water

Glycine and glycerol were used as the model compounds of protein and fattiness, respectively. A continuous tubular-flow reactor was used for the gasification experiments operated at 380–500 °C and 25 MPa with or without Na₂CO₃ catalyst. Compared with a negative effect on glycerol gasification, Na₂CO₃ could increase hydrogen yield and Chemical Oxygen Demand (COD) destruction efficiency, and the catalytic performance of 0.1 wt% Na₂CO₃ was better than that of 0.2 wt% for glycine gasification. When 1 wt% glycine solution with 0.1 wt% Na₂CO₃, or 1 wt% glycerol solution without Na₂CO₃ was gasified at 500 °C with the residence time of 0.98 min, their corresponding gasification efficiencies were up to 95.8% and 98%, and hydrogen yields could reach 4.14 and 5.08 mol/mol, respectively. Hydrogen molar fraction in gaseous product was about 60% and liquid effluents could be reutilized. Correspondingly, the ideal overall reaction equations for glycine and glycerol gasification were proposed.     

Energetic analysis of gasification of aqueous biomass in supercritical water

The aim of this work is to investigate the field of gasification of aqueous biomass in supercritical water. In order to achieve this analysis, a mathematical model (based on thermodynamical equilibrium assumption) is built. This model allows not only the computation of the solid, liquid and gas phases produced in a process composed of a gasification reactor and a separator unit, but also the computation of the energy requirements or yields in these specific units. The composition of these three phases is computed in terms of fractions of CH₄, H₂, H₂O, CO, CO₂, H₂S, NH₃, C₆H₅OH, CH₃COOH, CH₃CHO, C_(s) and minerals. The model also predicts the lower heating value of the gas leaving the process. The main problem that is encountered in the derivation of this mathematical model is the estimation of the activities of the chemical species present in the mixture as well as the enthalpy of this mixture. In this work, these evaluations are performed using Peng-Robinson equation of state. Results of the model are presented in the frame of gasification of vinasse, which is an aqueous residue from the alcohol production industry. It is shown that some specific conditions exist where it is not necessary to yield energy to the reactor to convert the incoming biomass into a gas for which the lower heating value can reach 5 MJ/kg with a gasification efficiency of 85%.     

Influence of oxidation coefficient on product properties in sewage sludge treatment by supercritical water

Oxidation coefficient plays a significant role in sewage sludge treatment in supercritical water, such as supercritical water oxidation (SCWO) and supercritical water partial oxidation (SWPO). In this work, the influences of oxidation coefficient (n) on sewage sludge treatment in supercritical water are investigated systematically and the corresponding reaction mechanisms are discussed objectively. Moreover, corrosion properties of stainless steel 316 considered as reactor material are also explored. The results show that H₂ yield first rises and then decreases with an increase in n. Its maximum value is approximately 190.4 ml/L when sewage sludge is disposed at 450 °C, 25 MPa, n = 0.6 and a residence time of 2.5 min. Under the reaction conditions of 540 °C, 25 MPa, n = 2.0 and 2.5 min, liquid product COD (Chemical Oxygen Demand) removal ratio, TOC (Total Organic Carbon) removal ratio and ammonia removal ratio of liquid product can reach up to 99.95%, 99.8% and 99.7%, respectively. However, if coupling SWPO and SCWO, we can obtain a certain amount of H₂ and CH₄, achieve the above removal ratios even at 450 °C and a lower total n (0.74), and meanwhile liquid products can meet corresponding discharge standards. However, special attention should be paid to reactor material corrosion in the above SWPO. We also confirm that stainless steel 316 undergoes more severe pitting corrosion in the absence of oxygen, compared with general corrosion in the excessive oxygen environment.     

Numerical simulation of supercritical carbon dioxide flows across critical point

Numerical study of supercritical carbon-dioxide flows across the critical point is presented. The present numerical method is based on the preconditioning method developed by Yamamoto and mathematical models of thermophysical properties for carbon dioxide programmed in the program package for thermophysical properties of fluids, developed by Kyushu University. First, the two-dimensional natural convection of carbon dioxide between two parallel plates is calculated while changing the bulk pressure. The calculated thermophysical properties of the carbon-dioxide flow under supercritical pressure are compared with those in a gas condition. Next, the natural convection of carbon dioxide in an O-shaped cyclic channel is calculated, and the effect of the density difference induced by the phase change to the flow is investigated. For application to high-speed flows, supercritical carbon dioxide flows through a nozzle with free-jet expansion (known as the process of rapid expansion of supercritical solutions) process are calculated. The calculated shock distance to the Mach disk generated in the free jet is compared with experiments and the density variations in the nozzle while changing the inlet temperature are numerically predicted.     

Thermal elongations in steam turbines with welded rotors made of advanced materials at supercritical steam parameters

This paper presents research results obtained for supercritical steam turbines. The analysis aims to develop data, knowledge bases and procedures to support the operational control of these turbines. The control involves thermal and strength states of the main components. The thermal states and axial elongation in turbine rotors and casings are modeled, and the results are analyzed. The components under investigation are made of more than one material.     

Activity of Ni/CeO2 catalyst for gasification of phenol in supercritical water

An effective Ni/CeO₂ catalyst prepared by the polyol reduction method for degrading phenol into CH₄, H₂ and CO₂ in supercritical water (SCW) was developed. About 80% carbon gasification efficiency can be achieved at 525 °C and 60 min with 5 wt% phenol, 0.098 kg/m³ water density and 0.5 g Ni/CeO₂/g phenol catalyst, forming CH₄ and H₂ as the main gaseous products. Comparison study indicated that the efficiency of present Ni/CeO₂ catalyst was about 20% higher than that of a commercial catalyst, i.e., Ni/SiO₂Al₂O₃ from Sigma-Aldrich with 65 wt%Ni, at a reaction conditions of 500 °C and 30 min. The characterization analyses of BET, TPR, XRD, XPS and TEM indicated that there was a NiCe alloy formed in Ni/CeO₂, which could be important to enhance the activities of the carbon gasification efficiencies and gas yields. A kinetic modelings were conducted and the results showed that the lnA and the activation energy (Ea) of gasification were 7.1 ± 0.5 and 58.1 ± 3.2 kJ/mol for the gaseous product, and were 2.6 ± 0.9 and Ea is 36.6 ± 5.6 kJ/mol for the char formation, respectively. The present Ni-based-metal Ni/CeO₂ catalyst is cheaper and has a potential application for the gasification to convert phenol into gases fuels in SCW process.