Papers by Keyword: Supercritical Water

Abstract: Supercritical hydrothermal synthesis is a green synthesis method for metal and metal oxide ultra-fine particles. Ultra-fine copper particles are of great interests for the researchers because of the excellent performance in recent years. In this paper, supercritical hydrothermal synthesis of copper ultra-fine particles with three different precursors (CuSO₄, Cu(NO₃)₂, Cu(HCOO)₂) are reported. This thesis reports that different products are produced with different precursors. Also, three kinds of reaction mechanisms with different precursors in supercritical water were explained. The conversion of copper ions in the reaction of Cu(HCOO)₂ in supercritical water is the highest, the value reaches 100.0%. In the process of synthesizing ultra-fine copper particles, different additional HCOOH concentrations (0, 0.1 mol/L, 0.2 mol/L) and different reaction times (5 mins, 10 mins) were applied. Zero-valent ultra-fine copper particles without impurity were synthesized. The synthesized copper ultra-fine particles were cubic aggregations with micro-meter size

Abstract: 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.

Abstract: The energy analysis of hydrogen production from the methanol reforming and oxidation under atmospheric (ATM) pressure and supercritical water (SCW) conditions was performed. The equilibrium hydrogen was investigated by the minimization of the Gibbs free energy based on Peng-Robinson equation of state for high pressure and ideal gas equation for atmospheric pressure. An objective of this study was to obtain the optimum operating conditions to maximize the net hydrogen yield, defined as the hydrogen yield taking into account also the methanol consumed by combustion to generate heat. This was done by investigating the effect of operating parameters over the following ranges: temperatures between 773 and 1273 K, pressures between 0.1 and 25.0 MPa, water-to-methanol (H₂O:MeOH) ratios between 1 and 5, and oxygen-to-methanol (O₂:MeOH) ratios between 0 and 1.05. At ATM pressure, it was found that the equilibrium hydrogen yield increases with increasing H₂O:MeOH ratio but the peak of equilibrium H₂ yield is at 973 K for higher H₂O:MeOH ratio than 1:1. Additionally, the total heat load increases significantly as the reaction temperature and the water amount increase. Therefore, the optimum net H₂ yield is at the H₂O:MeOH ratio of 2:1 and the reaction temperature at 973 K. Under SCW conditions, an increase of temperature and water amount in the system constantly increases the equilibrium H₂ yield. It means that the high H₂O:MeOH ratio and temperature are required in SCW. The presence of oxygen in hydrogen production was investigated that an increase of O₂:MeOH ratio constantly decreases the H₂ yield and also the net H₂ yield for reaction at ATM pressure whereas under SCW conditions, the equilibrium H₂ yield and the net H₂ yield increase with increasing oxygen up to 0.42 and 0.84, respectively.

Abstract: This study investigated the application of supercritical water as green and environmentally friendly treatment medium for hydrolysis and decomposition lignin in order to obtain the relation the products of phenolic compounds and the lignin structural. The processes were studied using three different biomass feed-stocks: poplar alkali lignin (AL), corncob enzymatic/mild acidolysis lignin (EMAL), and cornstalk or benzene–ethanol-extracted lignin (BEL), and lignin were identified by elemental analysis, FTIR, ³¹P-NMR. Experiments were performed in a batch stainless steel reactor at supercritical water 375°C at residence time of 10min. Main phenolic compounds from decomposition of lignin were identified by external standard method. Results indicated that the functional groups of AL, EMAL and BEL had a marked influence on the produces. More phenol OH groups and more benzene and phenolic contents could be gained under supercritical water conditions. The total produces reached to 74mg/g of dry EMAL and which was a high quality raw material as the source of phenol and 4-ethylphenol. The AL compositions of guaiacol, 4-methylguaiacol, 3-methoxycatechol, 2,6-dimethoxyphenol, 1,2,3-trimethoxybenzene had more than EMAL and BEL.

Abstract: The hydrogen production via methanol reforming in supercritical water (SCW) condition has been thermodynamically investigated by a Gibbs free energy minimization method to determine equilibrium yields and energy requirements in fuel processor over the temperature range between 673 and 1273 K and water-to-methanol (H₂O:MeOH) ratio of 1 – 5 corresponding to the methanol concentration from 64 to 26 wt%, respectively. In this research, the main objective is to compare the influence of operating pressure from atmospheric (ATM) pressure to SCW condition on the product yields. At ATM pressure, a hydrogen production increases with increasing reaction temperature up to 973 K while under SCW condition, an increase of temperature constantly increases the formation of hydrogen. As the reaction temperature increases, the hydrogen production and the total heat load increase but the hydrogen to carbon monoxide (H₂/CO) ratio decreases. Although the hydrogen yield is constantly higher at ATM pressure than under SCW condition, the H₂/CO ratios are always greater in SCW at temperatures above approximately 700 K. The amount of hydrogen and the heat load of both the preheater and the reactor also increase significantly as the H₂O:MeOH ratio increases.

Abstract: Co-pyrolysis of polyethylene plastic and cellulose as models for medical waste had been studied on a supercritical water batch reactor. The results show that temperature, reaction time, pressure and the mass ratio of water to organic matter have some degree impact on the conversion rate, oil yield and gasification efficiency. Conversion and gasification efficiency reached the maximum values at 440 °C. The content of H₂ in the gaseous products rose significantly between 25 MPa~27 MPa. As reaction time increased, conversion and gasification efficiency increased, but oil yield decreased. The composition of gaseous products was affected greatly by the mass ratio of water to organic matter. Adding K₂CO₃ and Ca (OH)₂ as catalyst, the reaction was promoted obviously.

Abstract: In the present work, the heat transfer characteristics of supercritical pressure water are numerically investigated in an upward flow vertical smooth tube. The numerical simulations are carried out by using Ansys-Fluent solver. The objective of the present work is to investigate the effect of heat flux and mass flux on heat transfer characteristics in supercritical water. In order to perform numerical simulation, experimental data of Mokry et al. [2] is considered. Various simulations were carried out for the inlet parameters of temperature 350°C, pressure 240bar; heat flux values ranging from 190 to 884kW/m² and mass flux values ranging from 498 to 1499kg/m²s. Based on the available parameters of heat flux and mass flux, they are segregated as groups with heat flux to mass flux ratios of 0.39 and 0.67. According to computational data, the heat transfer enhancement and heat transfer deterioration phenomenon of supercritical water were analyzed and based on the comparison with experimental data; their occurrence and mechanism were addressed.

Abstract: Lithium iron phosphate particles were synthesized with supercritical water as a reaction medium, and the effects of temperature and pressure on particle purity and size were examined. A flow-through reactor was designed and set up with a SUS316L stainless steel material, and a large yield could be obtained with no preheated water involved. The products were recovered and characterized by X-Ray Diffration (XRD), Laser Particle Analyzer (LPA), and Scanning Electron Microscopy (SEM). Particles with an average size of 500nm could be obtained continuously.

Abstract: Technical and economic conditions of hydrogen production by biomass gasification in supercritical water and in air-steam media have been evaluated. Reaction mechanism, technological process, product gas composition and operation costs of two processes were compared. Results indicate that biomass gasification in supercritical water has much more advantages than in air-steam medium, such as easier operation process, smaller machine area, high gasification efficiency, and less pollution, etc. There are no needs of biomass pretreatment and post-processing for product gas in supercritical water. Moreover, the proportions of hydrogen, carbon dioxide, methane are high, so these kinds of product gas all can be utilized. However, the cost of producing 1Nm3hydrogen in supercritical water is $0.6537, which is a little higher than $0.4228 in air-steam media. With the construction of more supercritical water unit and accumulation of more experience, hydrogen production by biomass gasification in supercritical water will have a more bright future.

Abstract: Supercritical water oxidation (SCWO) is an alternative to effectively dispose many varieties of organic wastewaters. In this article, a high concentration pesticide wastewater with very complicated components was handled by SCWO in a batch experimental plant at 25 MPa, 410–580 °C within the oxidant coefficient of 1.1–4.0 and the residence time of 1.0–10.0 min. The results show that reaction temperature, oxidant coefficient, residence time can improve X_COD (removal efficiency of chemical oxygen demand) of reactor effluent. X_COD reaches up to 99.89% at 550 °C, 25 MPa with the oxidant coefficient of 3.0 and the residence time of 5.0 min, and the corresponding COD concentration is 73 mg/L. Residence time indicates a relatively more important influence on COD₁ at higher reaction temperatures and OCs. Furthermore, possible reaction pathways for SCWO of organic matters in the pesticide wastewater were also proposed primarily.