Supercritical water gasification of glycerol: Intermediates and kinetics

In this paper, the liquid products from supercritical water gasification (SCWG) of glycerol were analyzed and some intermediates were identified. A simplified reaction pathway for gases production from SCWG of glycerol was proposed. The first quantitative kinetics model for describing the gaseous products (H₂, CO, CH₄ and CO₂) of SCWG of glycerol was developed. The model comprises seven reactions to describe the typical reactions in SCWG, and the reaction rate constant of each reaction was obtained by using the nonlinear least-square fitting method. The reaction rate analysis showed that the main sources of hydrogen yield were glycerol pyrolysis and steam reforming of intermediates, while the hydrogen yield from water–gas shift reaction (WGSR) was very small. The temperature estimated by the kinetics model for completely SCWG of glycerol solution was given. In addition, the sensitivity analysis of rate constant of WGSR was done based on the model.     

Reprint of: Application of subcritical water liquefaction as pretreatment for supercritical water gasification system in domestic waste water treatment plant

Applicability of pretreatment using subcritical water liquefaction (subCWL) for supercritical water gasification (SCWG) of sewage sludge was investigated. Effects of pretreatment conditions on liquefaction and viscosity were studied; the results revealed that treatment at 250 °C for 30 min was optimum in fluidizing the sewage sludge used herein. The pretreatment tended to enhance H₂ and CH₄ yields in the subsequent SCWG while suppressing CO₂ production. A continuous SCWG system assembled with a subCWL pretreatment process was designed. From energy balance and cost analysis on SCWG-installed municipal wastewater treatment plant (WWTP) with conventional activated sludge process, the system was capable of producing surplus energy by utilizing exhaust heat. Cost analyses indicated that the capital cost and treatment fees were cost competitive compared to conventional treatments.     

超临界水在降解废弃物及资源化中的应用

综述了超临界水的特性及其在降解废旧塑料、橡胶、纤维素等废弃物及资源化中的应用现状。与传统热分解方法相比,超临界水可实现对高分子废弃物的快速、有效分解,通过分解反应条件的控制,可以控制产物组成,是一种很有前途的废弃物资源化技术。     

Char and Coke Formation as Unwanted Side Reaction of the Hydrothermal Biomass Gasification

The hydrothermal biomass gasification is a promising technology to produce hydrogen and/or methane from wet biomass with a water content of ≥ 80 % (g/g). In the process, the coke formation usually is very low, but already low amounts may cause problems like, e.g., fouling in the heat exchanger. To learn more about the product formation, the results of the hydrothermal treatment (at 400, 500, 600 °C and 1 h) of different biomass feedstocks (artichoke stalk, pinecone, sawdust, and cellulose as model biomass) in a microreactor are compared. The gas composition and the total organic carbon content of the aqueous phase were determined after reaction. The gas formation rises with increasing temperature. The formation of carbon deposits and their characterization has been investigated by scanning electron microscopy (SEM). The variation of the solid morphology during the hydrothermal conversion is discussed based on chemical pathways occurring during hydrothermal biomass degradation.     

Hydrothermal gasification of olive mill wastewater as a biomass source in supercritical water

In this study, the hydrothermal gasification of biomass in supercritical water is investigated. The work is of peculiar value since a real biomass, olive mill wastewater (OMW), is used instead of model biomass compounds. OMW is a by-product obtained during olive oil production, which has a complex nature characterized by a high content of organic compounds and polyphenols. The high content of organics makes OMW a desirable biomass candidate as an energy source. The hydrothermal gasification experiments for OMW were conducted with five different reaction temperatures (400, 450, 500, 550 and 600 °C) and five different reaction times (30, 60, 90, 120 and 150 s), under a pressure of 25 MPa. The gaseous products are mainly composed of hydrogen, carbon dioxide, carbon monoxide and C₁-C₄ hydrocarbons, such as methane, ethane, propane and propylene. Maximum amount of the gas product obtained is 7.71 mL per mL OMW at a reaction temperature of 550 °C, with a reaction time of 30 s. The gas product composition is 9.23% for hydrogen, 34.84% for methane, 4.04% for ethane, 0.84% for propane, 0.83% for propylene, 49.34% for carbon dioxide, and 0.88% for minor components such as n-butane, i-butane, 1-butene, i-butene, t-2-butene, 1,3-butadiene and nitrogen at this reaction conditions.     

Hydrothermal synthesis of metal nanoparticles using glycerol as a reducing agent

Metal and metal oxide nanoparticles were synthesized using supercritical water (SCW) as a reaction medium and glycerol as a reducing agent at 400 °C and 300 bar. X-ray diffraction (XRD) patterns confirmed that silver, copper and nickel nitrates were reduced to zero-valent metal nanoparticles. On the other hand, cobalt, iron and manganese nitrates were partially reduced into low-valent metal oxides. Scanning electron microscopy (SEM) images showed that the reduced metals and metal oxides were smaller than the metal oxides formed without glycerol. The difference in reduction behavior of elements is explained using their reduction potentials. Glycerol proved to be an effective reducing agent for hydrothermal applications.     

Deposition characteristics of diesel and bio-diesel fuels

The aim of this study is to investigate the deposition characteristics of different types of fuels by using the hot surface deposition test (HSDT) as a substitute procedure for real engine deposit tests. Deposit development, deposit compositions and deposit surface temperature fluctuation for diesel fuels and bio-diesel fuels (palm oil based and coconut oil based) are discussed. Deposit development depended on hot surface temperature, overlapping conditions, fuels, deposit properties, initial stage of deposition and competition phenomena during deposit formation. Results show DFP having 1% B100C in composition, showed a greater deposit development rate compared to DF, which resulted in a relatively large amount of deposits for DFP. However, for bio-diesel fuels, B100C obtained a slower deposit development rate compared to B100 although the test conditions were changed. Due to the lower value of MEP and shorter droplet lifetime before MEP, utilization of B100C had a greater potential in reducing deposit formation compared to B100.     

High energy syngas production by waste tyres steam gasification in a rotary kiln pilot plant. Experimental and numerical investigations

This paper presents experimental and numerical results on steam gasification of waste tyres in a rotary kiln pilot plant. Both the process performance and the gas features have been evaluated varying the feeding ratio (FR), defined as the steam/tyres mass ratio. First, several experimental tests have been performed. Then, the obtained experimental results have been used to verify the consistency of a numerical model developed with the aid of the commercial code ChemCAD®. Once done, the effect of increasing the FR on the gas energy content has been evaluated.Numerical results showed that the gas energy content increases as the FR increases as well, achieving a maximum value for FR = 0.33 that produced a gas which composition N₂ free is (H₂ = 52.7%_vol, CO = 18.1%_vol, CO₂ = 7.0%_vol, CH₄ = 22.2%_vol) in correspondence of which the lower heating value (LHV) is equal to 29.5 MJ kg_gas⁻¹. Higher FR values do not produce a further increase of the gas energy content, rather require a greater amount of input energy for heating the steam from the atmospheric to the process temperature.     

Production and fuel properties of fast pyrolysis oil/bio-diesel blends

This paper describes the production and fuel properties of fast pyrolysis oil/bio-diesel blends. The bio-oils used in this study were produced from the fast pyrolysis of woody biomasses, oil mallee and pine. The bio-diesel employed was derived from canola vegetable oil. The conditions used to prepare the bio-oil/bio-diesel blends, as well as some of the fuel properties of the resulting bio-diesel rich phase, are reported. The experimental results show that the solubility of fast pyrolysis oils in bio-diesel is not as high as was previously reported for decanted oils obtained by Auger pyrolysis. The carboxylic acids, mono-phenols, furans and lignin derived oligomers were the compounds most soluble in bio-diesel, while the sugars, on the other hand, showed poor solubility. Although the presence of phenols enhances the oxidation stability of the bio-diesel rich phases, other fuel properties deteriorate. For example, the content of solid residues increased primarily because of the solubilisation of lignin derived oligomers, which were quantified by UV-fluorescence. Concentrations as high as 3.5 mass % of these compounds were observed in the bio-diesel rich phase. The solubility of bio-oil in bio-diesel was enhanced by using ethyl acetate/bio-diesel blends. Some fuel properties of the bio-diesel rich phase, after the removal of ethyl acetate, are reported.     

Biomass gasification in supercritical water: Part 1. Effect of the nature of biomass

In this study, biomass feedstocks, including lignocellulosic materials and the tannery wastes, were gasified in supercritical water. Gasification experiments were performed in a batch autoclave at 500 °C. The amount of gases, the gas compositions and the amount of water soluble compounds from gasification were determined. The hydrogen yields ranging between 4.05 and 4.65 mol H₂/kg biomass have been obtained. The results showed that the yields and composition of gases depend also on the organic materials other than cellulose and lignin in lignocellulosic material. In addition to this, it was concluded that the kind of lignin may also have an effect on gasification products. In the case of tannery wastes, the type of tannen agent used in leather production considerably effected the gasification results.     

Corrosion control methods in supercritical water oxidation and gasification processes

Use of supercritical water (SCW) as a medium for oxidation reactions, conversion of organic materials to gaseous or liquid products, and for organic and inorganic synthesis processes, has been the subject of extensive research, development, and some commercial activity for over 25 years. A key aspect of the technology concerns the identification of materials, component designs, and operating techniques suitable for handling the moderately high temperatures and pressures and aggressive environments present in many SCW processes. Depending upon the particular application, or upon the particular location within a single process, the SCW process environment may be oxidizing, reducing, acidic, basic, nonionic, or highly ionic. Thus, it is difficult to find any one material or design that can withstand the effects of all feed types under all conditions. Nevertheless, several approaches have been developed to allow successful continuous processing with sufficient corrosion resistance for an acceptable period of time. The present paper reviews the experience to date for methods of corrosion control in the two most prevalent SCW processing applications: supercritical water oxidation (SCWO) and supercritical water gasification (SCWG).     

Hydrogen production by biomass gasification in supercritical water: A systematic experimental and analytical study

Hydrogen production from biomass gasification in supercritical water is a new technology, which was developed in last two decades. Biomass energy of low quality can be converted to hydrogen energy of high quality by supercritical water gasification. Particularly, supercritical water gasification is an elegant way of wet biomass utilization. Up to now, many important progresses have been made in supercritical water gasification technology by the studies of researchers around the world. Since 1997, supercritical water gasification, which include reaction system, rule of biomass gasification and theory, have been studied in State Key Laboratory of Multiphase Flow in Power Engineering of Xi’an Jiaotong University. In this paper, we summarize the results from systematic experimental and analytical study on biomass gasification in supercritical water in our laboratory. Also, the development status and future prospect on supercritical water gasification is evaluated.     

焦化废水处理工艺升级改造的实践

对邯钢焦化厂的废水处理工艺进行技术升级改造,改造主要针对预处理系统和生化系统,将A—A／O法工艺改造成A／O法工艺后,废水处理达到了国家一级排放标准。同时对改造前后工艺的优缺点及生产过程中的影响因素进行了分析。     

超临界方法在塑料分解回收中的应用研究

综述了超临界水在废旧塑料如聚对苯二甲酸乙二醇酯,聚碳酸酯,聚乙烯等回收利用中的研究进展。对比于传统的废旧塑料处理方法,利用超临界水所具有的特殊的化学,物理性能对上述废旧塑料的处理,同时可达到效率,原材料回收比例高且后处理工序简便,无公害等多优点,为废旧塑料的回收利用开辟了新的途径。     

High-yield hydrogen production by supercritical water gasification of various feedstocks: Alcohols,glucose, glycerol and long-chain alkanes

Continuous supercritical water gasification (SCWG) of various feedstocks of C1–C16 was conducted to produce hydrogen-rich gas. These feedstocks represent model compounds of biomass such as methanol/ethanol (alcohol-type), glucose and glycerol (byproducts of biodiesel synthesis), and model compounds of petroleum fuels such as iso-octane/n-octane (gasoline), n-decane/n-dodecane (jet fuels) and n-hexadecane (diesel). Almost complete gasification of all the feedstocks was achieved at 25 MPa, 740 °C and 10 wt% with low total organic carbon values of their liquid effluents. The hydrogen gas yields of each feedstock were very similar to the theoretical equilibrium yields estimated by Gibbs free energy minimization. SCWG at different gasification temperatures (650 and 740 °C) and concentrations (10 and 20 wt%) revealed that methanol and ethanol (alcohols), the simple oxygenated hydrocarbons, were easier to be gasified, producing negligible amounts of liquid products, when compared with long-chain hydrocarbons (iso-octane and n-decane) under the identical conditions. When the feedstock concentration was increased from 10 to 20 wt%, the equilibrium hydrogen ratio from iso-octane gasification decreased from 1.02 to 0.79 while that of n-decane increased from 1.12 to 1.50, implying that a branched hydrocarbon may be more resistant to gasification in supercritical water.     

Reforming of methanol and glycerol in supercritical water

Reforming of pure glycerol, crude glycerin, and methanol (pure and in the presence of Na₂CO₃) in supercritical water was investigated. Continuous experiments were carried out at temperatures between 450 and 650 °C, residence times between 6 and 173 s, and feed concentrations of 3-20 wt%. For methanol the gas products are mainly H₂, CO₂, and CO. The carbon-to-gas efficiency and the observed activation energy for pure methanol are higher than for methanol with Na₂CO₃. This can be explained by assuming different decomposition mechanisms for pure methanol and methanol with Na₂CO₃. For glycerol, H₂, CO, CO₂, CH₄, and higher hydrocarbons are produced. The carbon-to-gas efficiencies of crude glycerin and pure glycerol are comparable. Overall, 2 of the 3 carbon atoms present in glycerol end up in carbon oxides, while 1 carbon atom becomes C_xH_y. The overall mechanism of glycerol decomposition involves the dehydration of 1 mole of H₂O/mole glycerol. For both, methanol and glycerol at carbon-to-gas efficiencies below 70%, the gas yields (mole/mole feed) and carbon-to-gas efficiency correlate well.     

Continuous hydrothermal gasification of glycerol mixtures: Effect of glycerol and its degradation products on the continuous salt separation and the enhancing effect of K3PO4 on the glycerol degradation

At the Paul Scherrer Institut (PSI) a continuous process for the catalytic hydrothermal gasification of wet biomass to synthetic natural gas (SNG) has been developed. The catalytic reactor is operated at temperatures of 400–450 °C and pressures of 25–30 MPa. Salts contained in the biomass and released during the liquefaction step are continuously withdrawn in the supercritical salt separation step and recovered as a concentrated brine upstream of the catalytic reactor. The recovered salts may be recycled as valuable nutrients or fertilizers after a certain work-up.Salt management was identified as critical issue in many different hydrothermal processes such as supercritical water oxidation (SCWO) and in catalyzed or non-catalyzed gasification technologies in near- and supercritical water. In this article we focus on the influence of organics, in this case glycerol and its hydrothermal degradation products, on the continuous salt separation performance. In the presence of organics higher temperatures are needed in the salt separator for an efficient salt separation and recovery due to a higher overall fluid density in the presence of glycerol compared to the density of pure water at the same conditions. Increasing temperatures in the salt separator lead to an increased degradation and, in particular, gasification of the glycerol. The salt studied, i.e. K₃PO₄, catalyzed the gasification of the glycerol to CO, H₂, CO₂, and CH₄ as well as the water gas shift reaction. Due to the increased glycerol gasification at 460 °C in the salt separator, the fluid mixture density was lowered to similar values of pure water under the same conditions. Hence, at the fluid temperature of 460 °C in the salt separator the same salt separation performance was observed for water–K₃PO₄ and for an aqueous mixture of 20 wt.% glycerol with K₃PO₄.     

Conversion of waste cooking oil to biodiesel using ferric sulfate and supercritical methanol processes

In this comparative study, conversion of waste cooking oil to methyl esters was carried out using the ferric sulfate and the supercritical methanol processes. A two-step transesterification process was used to remove the high free fatty acid contents in the waste cooking oil (WCO). This process resulted in a feedstock to biodiesel conversion yield of about 85-96% using a ferric sulfate catalyst. In the supercritical methanol transesterification method, the yield of biodiesel was about 50-65% in only 15 min of reaction time. The test results revealed that supercritical process method is probably a promising alternative method to the traditional two-step transesterification process using a ferric sulfate catalyst for waste cooking oil conversion. The important variables affecting the methyl ester yield during the transesterification reaction are the molar ratio of alcohol to oil, the catalyst amount and the reaction temperature. The analysis of oil properties, fuel properties and process parameter optimization for the waste cooking oil conversion are also presented.     

Alkali-activated waste ceramics: Importance of precursor particle size distribution

The waste ceramics belongs to wide range of aluminosilicate materials which can be alkaline-activated to geopolymer cement – possible “green” alternative to conventional Portland cement. The studied ceramic material is generated during the size adjustment of ceramic building blocks by means of grinding. It means that most of the material is very fine, but it contains also some larger shards. This ceramic powder was used as geopolymer precursor “as received” and after removal of particles retained on 1, 0.5 and 0.125 mm sieves. These four types of precursor were activated by sodium silicate (SiO₂/Na₂O = 1) solution. The prepared mortars were tested for strength, basic physical properties, transport parameters and characterized by help of XRD and thermal analysis. It was found that the best mechanical performance provided the precursor after removal of particles retained on 0.5 mm sieve thanks to the highest geopolymerization rate. The presence of coarser particles in precursor gave rise to porosity, what consequently influenced transport parameter of geopolymers towards the lower thermal conductivity and faster moisture transport.     

Catalytic hydrogen production from 2-propanol in supercritical water: Comparison of some metal catalysts

The gasification of organics in supercritical water is a promising method for the direct production of hydrogen at high pressures, and in order to improve the hydrogen yield or selectivity, activities of various catalysts are evaluated. In this study, hydrogen production from 2-propanol over Ni/Al₂O₃ and Fe–Cr catalysts was investigated in supercritical water. The experiments were carried out in the temperature range of 400–600 °C and in the reaction time range of 10–30 s, under a pressure of 25 MPa. The hydrogen yields and selectivities of Ni/Al₂O₃ and Fe–Cr used in this study, and those of Pt/Al₂O₃ and Ru/Al₂O₃ used in our previous work were compared. The hydrogen contents of the gaseous products obtained by using Ni/Al₂O₃ and Fe–Cr were measured as 62 mol% and 70 mol%, respectively, at low temperatures and reaction times. However, the hydrogen yields remained in low levels when compared with that of Pt/Al₂O₃ used in previous study. Pt/Al₂O₃ was established to be the most effective and selective catalyst for hydrogen production. During the catalytic gasification of a 0.5 M solution of 2-propanol, hydrogen content up to 96 mol% and hydrogen yield of 1.05 mol/mol 2-propanol were obtained.