Investigation of thermochemical conversion of biomass in supercritical water using a batch reactor

This study focused on gasification of biomass and a biomass model compound. Data are presented that show the presence of supercritical water enhances gasification efficiency, as it participates as both a solvent and a reactant. It is established that biomass gasification efficiencies are in the same range for all types of biomass. The thermodynamic changes of state are functions of elemental composition, not biomass species. The oxidation state of carbon atom of biomass is a key variable in determining the changes in enthalpy during both conventional combustion and supercritical water gasification. The oxidation state of the feed (together with the reaction conditions that influence the degree to which water participates as a reactant) also determines the vapor product composition.Decomposition reactions to vapor products are rapid and complete at high temperature (?550 °C), catalytic mediation is not required. Temperature and residence time are important operating parameters for SCW gasification. Less important are the pressure of gasification (in the range of 40-67 MPa) and the presence of catalyst. The vapor yield, gas composition, the carbon and hydrogen balance of SCW gasification are functions of gasification temperature, residence time and biomass load (concentration).     

超临界水中葡萄糖气化制氢的热力学分析

引　言氢气是一种高效清洁的二次能源 ,具有无污染、可储存、可运输的特点 ,被公认为未来最有希望的能源载体 .在超临界水中进行生物质的气化制氢 ,气体产物中H2 的摩尔分数甚至可超过 5 0 % ,反应不生成焦油、木炭等副产品 ,不会造成二次污染 ,对于含水量高的湿生物质 ,不需要     

Lumped reaction kinetic models for pyrolysis of heavy oil in the presence of supercritical water

The reaction kinetics of the pyrolysis of heavy oil in the presence of supercritical water (SCW) and high pressure N₂ were measured. At any reaction temperature applied, the pyrolysis under SCW environments is faster than that under N₂ environments. Meanwhile, at lower temperatures the pyrolysis under both environments is accelerated by the introduction of coke into the feedstock. On the basis of a first‐order four‐lump reaction network consisting of the sequential condensation of maltenes and asphaltenes, the pyrolysis in whichever medium can be preferably described either by the lumped reaction kinetic model modified with autocatalysis and pseudoequilibrium or by the model modified solely with pseudoequilibrium. Benefited from the reduced limitation of diffusion to reaction kinetics, the pyrolysis in the SCW phase is more sensitive to the increase in reaction temperature than that in the oil phase, disengaging readily from the dependence on autocatalysis at a lower temperature. © 2015 American Institute of Chemical Engineers AIChE J, 62: 207–216, 2016     

玉米芯与羧甲基纤维素钠的超临界水气化制氢

采用Gibbs自由能最小原理,建立了生物质超临界水气化制氢的化学平衡模型。将该模型应用于玉米芯/羧甲基纤维素钠(CMC)的超临界水气化制氢,分析模拟和实验结果,得到反应温度对化学平衡产物的作用如下:在300—374℃的亚临界区,气体产物的摩尔分数排序为x(CO2)>x(CH4)>x(H2),在375—420℃的低温超临界区,气体产物排序为x(CO2)>x(H2)>x(CH4),在420℃以上的高温超临界区,H2摩尔分数跃居最高,可达65%以上。较高的反应温度有利于提高H2的摩尔分数和气化率,但降低了气体的高热值。获得玉米芯/CMC制氢的最佳温度范围为420—600℃。表明农业废弃物的超临界水气化制氢是一种极具发展前景的能源转化新技术。     

Coal gasification with water under supercritical conditions

The conversion of an array of coal particles in supercritical water (SCW) was studied in a semibatch reactor at a pressure of 30 MPa, 500–750°C, and a reaction time of 1–12 min. The bulk conversion, surface conversion, and random pore models were used to describe the conversion. The quantitative composition of reaction products was determined, and the dependence of the rate of reaction on the degree of coal conversion, reaction time, and reaction temperature was obtained on the assumption of a first-order reaction and the Arrhenius function (E = 103 kJ/mol; A ₀ = 7.7 × 10⁴ min^?1). It was found that the gasification of coal under SCW conditions without the addition of oxidizing agents is a weakly endothermic process. The addition of CO₂ to SCW decreased the rate of conversion and increased the yield of CO. It was found that, at a 90% conversion of the organic matter of coal (OMC) in a flow of SCW in a time of 2 min, the process power was 26 W/g per gram of OMC.     

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).     

矿物燃料在超临界水中的转化和改质

综述了近年来矿物燃料和烃的模型化合物在超临界水（ＳＣＷ）作用下的转化和改质。用超临界水对诸如煤、重质油及油页岩等矿物燃料的转化和改质,可生产出轻质清洁的液体燃料,该法是轻质化矿物燃料非常有前途的方法。尤其应该引起注意的是,通过ＣＯ／Ｈ２Ｏ的水气转移反应可产生活性氢,用产生的氢对矿物燃料进行转化和改南,ＣＯ可由矿物燃料的部分氧化直接获得。有关超临界水中进行的水解反应、水气转移反应及部分氧化反应需进一     

Chemicals from heavy oils by ZSM-5 catalysis in supercritical water: Model compound and reaction engineering

Dodecane cracking and aromatization over ZSM-5 was studied in the presence and absence of supercritical water (SCW). A group-type model was used to determine five best-fit rate constants to describe yields to aliphatics, aromatics, coke, and gases. SCW accelerated gas formation while suppressing coke formation. CO and CO₂ were formed in the presence of SCW, but not in its absence; a new, low-temperature coke gasification pathway was suggested to account for this observation. Similarly, a low-temperature alkane reforming pathway was hypothesized to explain the increased relative rate constant for production of gases in the presence of SCW compared with its absence. Additional tests and analysis indicated that these effects could not be ascribed solely to zeolite degradation in the presence of SCW, implying that water directly influences the reaction mechanism. These results provide new insights into the role(s) of water during oil cracking under supercritical conditions.     

Pyrolysis of heavy oil in the presence of supercritical water: The reaction kinetics in different phases

In the presence of supercritical water (SCW) and N₂, the pyrolysis of heavy oil was investigated to distinguish the difference in the reaction kinetics between the upgrading in the SCW and oil phases. The pyrolysis in the SCW phase is faster than that in the oil phase, but the reaction in whichever phase is retarded by vigorous stirring. The pyrolysis can be preferably described by a four‐lump kinetic model consisting of the condensation of maltenes and asphaltenes in series. In the SCW phase, highly dispersed asphaltenes are isolated by water clusters from maltenes dissolved in SCW surroundings, by which the condensation of asphaltenes is drastically accelerated. Benefited from excellent mass transfer environments in SCW, the condensation of maltenes is promoted simultaneously. The introduction of SCW into the pyrolysis of heavy oil results in an effectively increased upgrading efficiency, but its influence on the properties of equilibrium liquid products is minor. © 2014 American Institute of Chemical Engineers AIChE J, 61: 857–866, 2015     

Effect of supercritical water on upgrading reaction of oil sand bitumen

The advantages of supercritical water (SCW) as a reaction medium for upgrading oil sand bitumen were investigated through a comprehensive analysis of the output product, which includes gaseous products, middle distillate, distillation residue, and coke. Canadian oil sand bitumen mined by the steam assisted gravity drainage method was treated in an autoclave at 420-450 °C and 20-30 MPa for up to 120 min with three kinds of reaction media: SCW, high-pressure nitrogen, and supercritical toluene. The yields of gaseous products indicated that a very small amount of water was involved in the upgrading reaction. The analytical results of the middle distillate fractions were almost the same using water and nitrogen at 450 °C. The distillation residues produced in SCW had lower molecular weight distributions, lower H/C atomic ratios, higher aromaticities, and consequently more condensed structures compared to those produced in nitrogen. The coke produced using SCW also had lower H/C values and higher aromaticities. Judging from all the analytical results, the upgrading of bitumen by SCW reaction was primarily considered to be physical in nature. As a result, it is possible to highly disperse the heavy fractions by SCW. This dispersion effect of SCW led to intramolecular dehydrogenation of the heavier component and prevention of recombination reactions, and consequently gave the highest conversion.     

Experimental investigation of high-temperature coal tar upgrading in supercritical water

The upgrading of high-temperature coal tar in supercritical water (SCW) was investigated using an autoclave reactor. The effects of temperature (673–753 K), pressure (24–38 MPa) and residence time (1–80 min) on product distribution were studied. The yields and characteristics of products in SCW were compared with those obtained in N₂. In order to study the reaction mechanism of coal tar upgrading, the experiments of model compounds were carried out in SCW at 673 K and 34 MPa for 20 min. The results indicate that the asphaltene conversion and the maltene yield are significantly higher in SCW. The H/C atom ratios of oil obtained in SCW are higher than those obtained in N₂. More valuable aromatic compounds such as naphthalene, fluorene and anthracene are obtained in SCW. The results of model compounds suggest that water in SCW could participate in coal tar upgrading and promote the formation of light products.     

Partial oxidation of n-hexadecane and polyethylene in supercritical water

In this study, we show the results of partial oxidation experiments of n-hexadecane (n-C₁₆) and polyethylene (PE) in supercritical water (SCW). The experiments were carried out at 673 or 693 K of reaction temperature and 5 or 30 min of reaction time using a 6 cm³ of a batch type reactor. Water density ranged from 0.1 to 0.52 g/cm³ (water pressure: 20–40 MPa). The loaded amount of oxygen was set to 0.3 of the ratio of oxygen atom to carbon atom. Some experiments were made using CO instead of oxygen for the partial oxidation of n-C₁₆ and PE to explore the effect of water gas shift reaction. In the results of partial oxidation of n-C₁₆, the yield of CO and some compounds containing oxygen atoms, such as aldehydes and ketones increased with increasing water density. Moreover, 1-alkene/ n-alkane ratio in the products decreased with increasing water density. The 1-alkene/n-alkane ratio was lower than that of pyrolysis in SCW. Also for the case of PE experiments, in dense SCW (0.42 g/cm³), the 1-alkene/n-alkane ratio in partial oxidation was lower than that in SCW pyrolysis. In the case of CO experiments for n-C₁₆ and PE, 1-alkene/n-alkane ratio was a little lower than that of pyrolysis in SCW. These results show that the yield of n-alkane, which is a hydrogenated compound, was higher through water gas shift reaction in SCW and also through partial oxidation in SCW. Therefore, these results suggest the possibility of hydrogenation of hydrocarbon through partial oxidation followed by the water gas shift reaction.     

SEPARATION OF IONIC SPECIES UNDER SUPERCRITICAL WATER CONDITIONS

ABSTRACT

The unique characteristics of supercritical water (SCW) offer potentially attractive processing options that can be explored for reaction and separation purposes. While supercritical water oxidation (SCWO) can achieve high organic conversion efficiencies, low and relative solubilities of inorganic species in SCW may be further utilized for in situ separation of potential by-products from the SCWO process effluent.

This paper describes a novel method for separating ionic species under SCW conditions. The concept is based on relative solubilities of different ionic species in SCW. Laboratory-scale demonstration tests were conducted with a Nylon monomer manufacturing process wastewater containing sodium hydroxide, sodium borate, carboxylic acids, and water. The process achieved (1) effective destruction (>99%) of organic components in the wastewater; (2) selective precipitation of sodium (>99.5%) as carbonates produced from oxidation of the organic components; and (3) efficient recovery of boron (>90%) as boric acid in the reactor effluent. The sodium removal efficiency is governed by the solubilities of sodium carbonates in SCW and, therefore, can be directly improved by increasing process temperature. As a result of the temperature increase, both organic destruction and boron recovery efficiencies may be enhanced.

This method of selective separation of ionic species in SCW has potential for a wide range of processing applications.     

Kinetics analysis of phenol and benzene decomposition in supercritical water

Decomposition of phenol and benzene was studied in supercritical water (SCW) at 370–450 °C and 25 MPa over very short residence times (0.5–100 s). The study of simple model compounds such as phenol and benzene is an essential preliminary step to elucidate the primary mechanism of char and gas formation from lignin compounds. A quantitative detailed chemical kinetics model for the primary pathways of phenol and benzene decomposition in SCW was determined using the reaction pathways for its decomposition under supercritical conditions. The activation energy of benzene decomposition (91.16 kJ mol⁻¹) in SCW is much higher than that of phenol (54.17 kJ mol⁻¹) under similar experimental conditions. This emphasized the importance of the substituent group (hydroxyl group) in the benzene ring to enhance its decomposition rate. In addition, the reaction rate parameters, which are deduced for the overall reaction network of its decomposition under similar conditions, show good agreement with each another. Hence, the reaction rates of these reaction pathways are successfully described in this study.     

Inference of reaction kinetics for supercritical water heavy oil upgrading with a two-phase stirred reactor model

We present the development and application of a two-phase stirred reactor model for heavy oil upgrading in the presence of supercritical water (SCW), with coupled phase-specific thermolysis reaction kinetics and multicomponent hydrocarbon–water phase equilibrium. We demonstrate the inference of oil and water phase kinetics parameters for a compact lumped reaction kinetics scheme through the application of this model to two different sets of batch reactor experiments reported in the literature. We infer that, although SCW can suppress the formation of newer polynuclear aromatics (PNA) from distillate range species, it is broadly ineffective in deterring the combination of pre-existing PNA fragments in the oil feed. Quantification of the conversion to distillate liquids before the onset of coke formation helps arrive at a clear conclusion on whether the use of SCW in the batch reactor leads to better product outcomes for different oil feeds and operating conditions.     

氨在超临界水中氧化的平衡热力学研究

城市污泥中含有大量的蛋白质等含氮有机物,而氨是含氮有机物超临界水氧化的中间产物。文章使用热力学方法研究了氨在超临界水中氧化构成的平衡体系。采用Peng-Rob inson状态方程和最小自由能法计算了反应体系中各组分的平衡组成。结果表明,当反应体系达到平衡时,氨在超临界水中可以被完全氧化;其中大部分的氨转化成了氮气,只产生少量的NO和NO2,没有N2O的生成。提高温度和过氧量可以增加NOx的平衡产量,但NOx的选择性最高不超过2%。比较热力学平衡计算结果和前人实验结果发现,N2O只是氨在超临界水中氧化的中间产物。氨在超临界水中的降解效果主要受动力学因素影响。     

超临界水中萘酚氧化分解的研究

建立了超临界水中有机物连续氧化分解的实验装置。初步探索了超临界水中β－萘酚氧化分解的影响因素及主要残留物。     

超临界水的特性及应用

介绍了水在高温高压下的热力学性质、氢键、离子积、扩散系数和粘度等在超临界区域的特殊性，以及超临界水溶液在介电常数、偏摩尔体积、溶解性和极性等方面的特殊性质，并阐述了超临界水在化学反应和废物处理中的特殊应用。     

Heterogeneous Catalysis in Supercritical Media: 2. Near‐Critical and Supercritical Water

It was demonstrated in part I, also published in this journal, that supercritical carbon dioxide may be advantageous for the use as a reaction medium in heterogeneously catalyzed reactions. Due to its miscibility with hydrogen, hydrogenations of increased selectivity can be carried out or carbonization of catalysts can be reduced by the dissolution and removal of coke precursors. The drawback of supercritical H₂O (T_crit = 374 °C) as compared to supercritical CO₂ (T_crit = 31 °C) is its relatively high critical point. At these temperatures, organic substances decompose within minutes. Consequently, many reactions are carried out below this critical point, in the so‐called “near‐critical water” or “hot compressed water.” Reaction conditions in near‐critical and supercritical water are also referred to as “hydrothermal.” In spite of the relatively high pressures and temperatures required and the associated high experimental expenditure, many types of reactions have already been studied, as the strong dependence of physical and chemical properties on pressure and temperature makes near‐critical and supercritical water a unique and extraordinary solvent.     

The Partial Oxidation of Isobutene in Sub‐ and Supercritical Water

The task of this work was to investigate which kind of reaction pathway takes place under supercritical water conditions in the oxidation of isobutene with molecular oxygen. The reaction was examined at temperatures of 350–20 °C, pressures of 25–35 MPa, uncatalyzed and under platinum‐catalyses. Isobutyraldehyde was the main product. Tert‐butanol was used as a source for isobutene, which dehydrates to isobutene quantitatively under the conditions used.