Comparison of the high-temperature corrosion of aluminium nitride,alumina, magnesia and zirconia ceramics by coal ashes

Higher-grade reactor lining materials are needed to withstand the higher working temperatures and gas pressures used in gasification reactors to improve their efficiency. Both conventional oxide materials and nonoxide materials such as SiC and AlN are suitable refractory materials for the reducing atmospheres prevailing in gasifiers. Interactions between the reactor lining and the slag constitute the main corrosion mechanism. In previous investigations on the high-temperature corrosion of aluminium nitride by coal ash [1] AlN materials were tested in basic and acidic coal ashes at temperatures of 900–1300 °C. In the present work the high-temperature corrosion of aluminium nitride by different coal ashes is compared with that of alumina, magnesia and zirconia.     

Raising the efficiency of heat-power units due to the use of modern refractory and heat-insulating materials

The timeliness and efficiency of the use of new refractory materials in modern heat-power units and the positive experience in the use of fiber refractory articles in heat-treatment furnaces and steam-generating units are considered. Novel design solutions for linings of heating furnaces operating at a temperature exceeding 1000°C are presented. __________ Translated from Novye Ogneupory, No. 12, pp. 6–9, December, 2007.     

Performance of a ZVI‐UASB reactor for azo dye wastewater treatment

BACKGROUND: Zero valent iron (ZVI) is expected to be helpful for creating an enhanced anaerobic environment that might improve the performance of the anaerobic process. Based on this idea, a ZVI packed upflow anaerobic sludge blanket reactor (ZVI‐UASB) was developed to enhance azo dye wastewater treatment. RESULTS: The ZVI‐UASB reactor was less influenced by a decrease in the operational temperature from 35 °C to 25 °C than a reference UASB reactor that did not contain ZVI. In addition, chemical oxygen demand (COD) and color removal efficiencies of the ZVI‐UASB reactor at an HRT of 12 h exceeded those of the reference reactor at an HRT of 24 h. The hydraulic circulation in the ZVI bed enhanced the function of ZVI so that it improved the COD and color removal efficiencies. Moreover, fluorescence in situ hybridization experiments revealed that the abundance of Archaea in the sludge of the ZVI bed was significantly higher than that at the reactor bottom, which made the reactor capable of greater COD removal under low temperature and short HRT conditions. CONCLUSION: This ZVI‐UASB reactor could adapt well to changes in the operational conditions during wastewater treatment. Copyright © 2010 Society of Chemical Industry     

Materials Challenges in Reverse‐Flow Pyrolysis Reactors for Petrochemical Applications

Currently, the pyrolysis of hydrocarbons for the production of light olefins is almost exclusively carried out in steam crackers operating around 900–1000°C. However, cracking hydrocarbons at much higher temperature results in high selectivity to acetylene, which can be converted into many petrochemical products including ethylene. The desired hydropyrolysis reaction from hydrocarbons to acetylene can be realized in a reverse‐flow reactor at very high temperatures (>1700°C) in a scalable manner. The reactor elements include ceramic components that are placed in the hottest regions of the reactor and must withstand a temperature that is in the range of 1500–2000°C. In addition, the temperature rises and falls with the reverse‐flow cycle; a fluctuation that could be as high as 100–500°C over a period of several seconds. Moreover, the materials in the hot zone are exposed alternately to a regeneration (heat addition) step that is mildly oxidizing, and a pyrolysis (cracking) step that is strongly reducing with a correspondingly high carbon activity. This article addresses the thermodynamic stability of selected ceramic materials based on alumina, zirconia, and yttria for such an application. Results from laboratory tests involving the exposure of these ceramic materials to simulated process conditions followed by their microstructural characterization are compared with expectations from thermodynamic predictions.     

MoSi2-based cylindrical susceptor for rapid high-temperature induction heating in air

Conductive susceptors are necessary for heating non-conductive materials in induction heating systems. Susceptor materials should have sufficient electrical conductivity and thermal/chemical stability under a range of environmental conditions. However, many susceptor materials oxidize in high-temperature environments, resulting in degradation and poor durability. Here, we used intermetallic MoSi₂ ceramics as a susceptor material and designed a cylindrical susceptor to apply to rapid high-temperature induction heating in an oxidizing environment. MoSi₂ was prepared by self-propagating high-temperature synthesis (SHS), and then used to fabricate cylindrical susceptors by slip casting. The optimal thickness of the susceptor was controlled by modelling. A MoSi₂-based cylindrical susceptor with SiO₂ protective layer showed higher heating rate (4.2–26.8 °C/s at 0.5–2.5 kW) than a commercial rod-type susceptor (7.7–13.6 °C/s at 1.0–2.5 kW) of the same material. In addition, our susceptor endured high temperatures below 1700 °C and severe thermal cycle (700–1600 °C, heating for 2min and cooling for 1min) during 36 cycles. In general, these results demonstrate that the MoSi₂-based susceptor can be applied to a rapid induction heating furnace that can be used in air at a high temperature of 1600 °C (equal to the available temperature of a commercial graphite susceptor in H₂).     

Steam‐air blown bubbling fluidized bed biomass gasification (BFBBG): Multi‐scale models and experimental validation

During fluidized bed biomass gasification, complex gas‐solid mixing patterns and numerous chemical and physical phenomena make identification of optimal operating conditions challenging. In this work, a parametric experimental campaign was carried out alongside the development of a coupled reactor network model which successfully integrates the individually validated sub‐models to predict steady‐state reactor performance metrics and outputs. The experiments utilized an integrated gasification system consisting of an externally‐heated, bench‐scale, 4‐in., 5 kWth, fluidized bed steam/air blown gasifier fed with woody biomass equipped with a molecular beam mass spectrometer to directly measure tar species. The operating temperature (750–850°C) and air/fuel equivalence ratio (ER = 0–0.157) were independently varied to isolate their effects. Elevating temperature is shown to improve the char gasification rate and reduce tar concentrations. Air strongly impacts the composition of tar, accelerating the conversion of lighter polycyclic‐aromatic hydrocarbons into soot precursors, while also improving the overall carbon conversion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 1543–1565, 2017     

Highly efficient two-stage ring-opening of epichlorohydrin with carboxylic acid in a microreaction system

Ring-opening of epoxides with carboxylic acids has been widely used to prepare many high value intermediates in the polymer and pharmaceutical industries. Most of conventional processes proceeded in batch stirred reactors. As such they always suffer from low productivity and selectivity. Here we developed an advanced technology to perform the ring-opening reaction of epichlorohydrin with neodecanoic acid (NDA) for continuous production of 3-chloro-2-hydroxypropyl neodecanoate in a more efficient and safer way. A microreaction system where a microreactor connected to a stirred reactor was established. When the conversion of NDA rapidly reaching around 90% in a microreactor at 110°C, the reaction solution was transferred to a stirred reactor at 90°C. This two-stage operating mode can reduce the reaction time and improve the selectivity through free switching of temperature in the consecutive two reactors, thus substantially reducing the consumption of energy and materials.     

Self‐catalyzed silicon‐containing phthalonitrile resins with low melting point,excellent solubility and thermal stability

A series of silicon‐containing self‐catalyzed phthalonitrile derivatives (SiPNs) have been successfully synthesized from reaction of 4‐(4‐aminophenoxy)phthalonitrile (APN) with corresponding chlorosilanes. The chemical structures of the SiPNs were confirmed by spectroscopic techniques. The introduction of silicon‐containing unit into the phthalonitrile structure has dramatically decreased the melting point from 143°C for APN to 40–60°C for the new SiPNs, which also exhibit improved solubility and are soluble in many common solvents. Differential scanning calorimetry analysis showed that they possess the self‐catalyzed behavior with the temperature of exothermic peak due to the self‐catalyzed reaction between 255 and 281°C. The cured SiPNs exhibit excellent thermal stability with glass transition temperature above 450°C, the temperature of 5% weight loss in range of 535–570°C under nitrogen, and 543–562°C under air. Their char yields at 1000°C are in the range of 80.2–82.6% in nitrogen, and 10.1–12.5% in air, respectively. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40919.     

Gasification studies of onakawana lignite

Onakawana lignite was gasified in air, steam and an air + steam mixture in a fixed bed reactor. The extent of devolatilization was determined by pyrolysis in nitrogen. The composition of products, expressed in terms of H₂/CO ratio, was temperature dependent. The ratio decreased with increasing temperature. During steam gasification the ratio decreased from 4.6 to 2.6 when temperature increased from 700° to 990°C. The addition of air to steam resulted in a marked decrease of this ratio. Steam gasification reactivity of chars prepared from Onakawana lignite at 500°C and 800°C were studied in the temperature range of 650°C to 1000°C. The carbon conversion results were fitted into equations describing the continuous and shrinking core models. The char prepared at 500°C was much more reactive than the one prepared at 800°C. Product distribution expressed as the H₂/CO ratio, was favourable in the temperature range. For comparison, the Kentucky #9 coal and chars derived from this coal were used as referee materials. The reactivity of these chars was markedly lower than that of chars derived from Onakawana lignite.     

Manufacture and thermomechanical characterization of wet filament wound C/C-SiC composites

The paper presents manufacture of C/C-SiC composite materials by wet filament winding of C fibers with a water-based phenolic resin with subsequent curing via autoclave as well as pyrolysis and liquid silicon infiltration (LSI). Almost dense C/C-SiC composite materials with different winding angles ranging from ±15° to ±75° could be obtained with porosities lower than 3% and densities in the range of 2 g/cm³. Thermomechanical characterization via tensile testing at room temperature and at 1300°C revealed higher tensile strength at elevated temperature than at room temperature. Thus, C/C-SiC material obtained by wet filament winding and LSI-processing has excellent high-temperature strength for high-temperature applications. Crack patterns during pyrolysis, microstructure after siliconization, and tensile strength strongly depend on the fiber/matrix interface strength and winding angle. Moreover, calculation tools for composites, such as classical laminate and inverse laminate theory, can be applied for structural evaluation and prediction of mechanical performance of C/C-SiC structures.     

High temperature polyimides,chemistry and properties

Selected polyimide resins are capable of long term use at 316°C for over thousands of hours in an inert atmosphere, and approximately 2000 hours in air. This attractive feature of polyimides has provided an avenue for applications at elevated temperature (316°C) as adhesives, coatings, and secondary structures in the form of fiber reinforced polyimide composites. The chemistry and properties of thermoplastic and crosslinked polyimides, including addition-type polyimides, will be discussed. The chemical considerations of both commercial and experimental polymer materials, Avimid N, PMR-15, LARC TPI, IP-600, L-20, and L-30 as they affect processing are reviewed. General physical, thermal, mechanical, and thermo-oxidative stability properties of polyimides are presented Room temperature and 316°C mechanical properties of unaged and 1000 hr 316°C aged high temperature polymer composites will also be discussed.     

Experimental study of the ignition behaviour of an adiabatic SRC-II coal liquefaction reactor

Experimental data are presented which describe the thermal behaviour of a bench-scale adiabatic coal liquefaction reactor operating in an open loop. The reactor employed external feedback control for maintaining adiabaticity. Conditions for both ignition and quench have been found for coal liquefaction at SRC-II operating conditions where repeated ignition/quench behaviour was demonstrated. No stable steady states were found between 450 °C and 475 °C for SRC-II operation. Ignition occurred at a feed temperature of ≈415 °C. The low steady states occurred at conditions of essentially no heat generation; at a very low extent of reaction. Some unexpected evidence of a preheater effect on reactor ignition was observed; however, the effect of preheater temperature profiles on reactor performance was not systematically studied.     

Oxidation behavior and protection of carbon/carbon composites

The oxidation behavior of C/C composite sheet materials in air has been studied over a wide range of temperature. Gasification was detectable at around 500°C and above about 900°C, under the flow conditions used in the experiments, the overall rates of gasification were controlled by gas phase diffusion. The presence of catalysts reduced the temperature for the onset of gasification but had no effect on the kinetics in the diffusion-controlled region. Borate-based coatings containing refractory particulates and silicon carbide coatings sealed with borates have been found capable of protecting C/C composites against air oxidation for extended periods to temperatures of at least 1200°C.     

Corrosion degradation of mullite subject to carbon monoxide atmosphere at 1000oC–1600°C

Thus far, studies on the damage to refractory materials under carbon monoxide atmospheres have mostly concentrated on the effects of carbon deposition, and the testing temperature was always set at approximately 500°C to promote the deposition of carbon. However, this testing temperature is far below the operating temperature of most refractories. In this study, mullite, a widely used high-temperature structural material, was subjected to a carbon monoxide atmosphere at 1000°C-1600°C to investigate its phase and microstructural evolutions. Changes to the grain boundaries were initially observed in mullite specimens treated at 1000°C and 1200°C. After treatment at 1400°C, the specimen surface comprised α-Al₂O₃, a glass phase, and a small amount of mullite. However, treatment at 1600°C resulted in only α-Al₂O₃ and a small amount of glass phase on the surface. Additionally, pores and voids were found in the glass phase on the surface and in the bulk of the specimens treated at 1400°C and 1600°C. This study demonstrated the stability of pure mullite in a carbon monoxide atmosphere and revealed that impurities accelerating generation of the liquid phase in Al₂O₃–SiO₂ system significantly affect the stability of mullite in a carbon monoxide atmosphere.     

Stability of structural materials based on ZrO2

The thermal and mechanical stability of some high-strength ceramic materials from partially stabilized ZrO₂ manufactured from various domestic and imported powders, including coprecipitated, sol-gel, and hydrothermal ones, with the use of CIP and sintering is considered. The thermal stability is tested under conditions close to the operating ones, i.e., under long-duration holds at 1000 and 1550°C and in water quenching. The mechanical stability is determined in impact-erosion wear and under combined loads of high pressure and multiple indentations by solid particles. It is shown that all the materials undergo degradation of various degrees but those most durable under normal conditions (hydrothermal and sol-gel materials, ceramics manufactured from imported press powders) are least stable. They have widely fluctuating properties under cyclic high-temperature loads, endure 900-1400°C, and withstand a pressure of at most 1.0-2.0 GPa in an abrasive, just like standard corundum ceramics; however, they are characterized by maximum wear resistance. At the same time, an original material from commercial coprecipitated PSZ powder has quite different features; its thermal stability allows it to withstand repeated quenchings from 1550°C in water, and the mechanical strength can attain 2.6-2.8 GPa, exceeding the strength of quenched tool steels in similar situations. Due to its refractoriness (2700°C) and chemical stability this material is the most versatile in operating under extreme conditions.     

A replacement of traditional insulation refractory brick by a waste-derived lightweight refractory castable

Lightweight insulation refractories are essential for high-temperature performance to reduce energy consumption. This study investigates a new insulation material, that is, solid waste rice husk ash (RHA) derived lightweight refractory castable, replacing traditional insulation refractory brick. The RHA is generated after the burning of rice husk as biomass fuel. The RHA is used as an aggregate and alkali-extracted silica sol from RHA as a binder to fabricate the insulation castable. The nanosilica containing (~30 wt%) sol is employed to synthesize the refractory castable by varying the sol amount (2.5-12.5 wt% silica from sol). The castable specimens are cast by a vibro-caster and fired at 900-1200°C in a muffle furnace. The physic-mechanical and thermal conductivity (κ) of the castable is investigated. At 1100°C with 10 wt% dry sol retaining sample shows an excellent apparent porosity (~65%), low bulk density (~ 0.8 g/cm³), and κ (0.136 W/m k) with sustainable compressive strength (6 MPa). The acquired results are a good match with the literature (other wastes-derived insulation materials) and industrial (silica insulation brick) obtained data. These promising outcomes may inspire the refractory industries for using RHA as an aggregate and RHA extracted sol as a binder for making insulation castable.     

The kinetics modeling and reactor simulation of propylene chlorination reaction process

The kinetics experiments of fast reaction process of propylene chlorination at low temperature (30–90°C) and high temperature (420–480°C) are respectively conducted, and the corresponding reaction mechanisms and kinetics models are proposed. The radical mechanism at high temperature and the molecular mechanism at low temperature are found to be most likely with the experimental results. Specifically, the kinetics model, firstly considering the reversible reaction step of forming C₃H₆Cl · and direct hydrogen abstraction of forming C₃H₅ · , shows better agreement with the experimental data. Furthermore, the critical reaction temperature T_critical is firstly proposed to determine the dominant reaction mechanism in different conditions, and correspondingly the combination method of the high-temperature and low-temperature kinetics models has been adopted for tubular reactor simulation, which can reasonably reflect the influence of wide variation range of temperature in the reactor and guide the industrial reactor design in the further work.     

Densification behavior and properties of alumina–chrome ceramics: Effect of TiO2

Highly dense alumina–chrome bodies with low porosity are usually used as corrosion and thermal resistant refractories. Alumina–chrome refractory with molar ratio 1:1 was developed using chemical grade hydrated alumina and chromium (III) oxide by conventional sintering route. Batch materials were attrition milled, isostatically pressed and sintered in the temperature range from 1000 °C to 1700 °C with 2 h soaking at peak temperature. Phase development of the sintered materials with temperature was studied by X-ray diffraction. Sintering temperature, sintering condition and addition of sintering aid (TiO₂) have immense effect on the densification of the alumina–chrome refractory. Highly dense alumina–chrome refractory with almost nil apparent porosity was developed at 1500 °C in reducing atmosphere. Flexural strength of the sintered materials at room temperature and at 1200 °C was also measured. 1 wt% TiO₂ gives the optimum result with respect to densification and flexural strength.     

Effect of sonication assisted by titanium dioxide and ferrous ions on polyaromatic hydrocarbons (PAHs) and toxicity removals from a petrochemical industry wastewater in Turkey

BACKGROUND: In Izmir (Turkey) polyaromatic hydracarbon (PAH) removal efficiencies are low in petrochemical industry aerobic biological wastewater treatment plants because bacteria are not able to overcome the inhibition of these toxic and refractory organics. In order to increase PAHs removal, sonication process was chosen among other advanced treatment processes include sonication processes. The effects of ambient conditions, increasing sonication time, sonication temperature, TiO₂ and Fe⁺² concentrations on sonication at a petrochemical industry wastewater treatment plant in Izmir (Turkey) was investigated in a 650 W sonicator, at a frequency of 35 kHz and a 500 mL glass reactor. RESULTS: Increasing the temperature improved PAH removal after 150 min sonication at 30 °C and 60 °C. The maximum total PAH removal efficiencies were the same in a reactor containing 20 mg L^?1 TiO₂ and in a TiO₂‐free reactor at 30 °C and 60 °C after 150 min sonication. Maximum 91% and 97% total PAH removals were obtained in a control reactor and a reactor containing 20 mg L^?1 Fe⁺² at 30 °C and 60 °C, respectively, after 150 min sonication. The PAH concentration was toxic to Daphnia magna, so that the EC₅₀ value decreased significantly from 342.56 ng mL^?1 to EC₅₀ = 9.88 ng mL^?1 and to EC₅₀ = 3.35 ng mL^?1, at the lowest TiO₂ (0.1 mg L^?1) and Fe⁺² (2 mg L^?1) concentrations, respectively, after 150 min sonication at 30 °C. CONCLUSION: PAHs and the acute toxicity in a petrochemical industry wastewater were removed efficiently through sonication. Copyright © 2010 Society of Chemical Industry     

High temperature gaseous corrosion resistance of MgO-containing refractories – A comparative study

In the work the results of investigations into corrosive resistance to the attack of sulphur oxides of six commercially available basic refractories containing magnesium oxide have been presented. The research was conducted for materials applied in glass furnace regenerators: three types of magnesia-zirconia-forsterite products, one magnesia-forsterite and one magnesia product as well as a magnesia-chromite product to be applied in copper convertors. Investigations were carried out in a semi-flow chemical reactor. Investigations into corrosive resistance were conducted at 600 °C, 800 °C and 1000 °C. The real and equilibrium composition of gaseous phase in the reactor versus temperature was determined. It was assumed that the relative increment of the tested products' mass after a certain time of their staying in the reactor would be and indicator of corrosive resistance. Based on investigations into the phase composition of the products of reaction, it was found that at 600 °C and 800 °C the product's corrosion resulted from the formation of MgSO₄, as the main product of reaction, whereas at 1000 °C – it was the formation of CaMg₂(SO₄)₃. In products with an addition of ZrO₂ destabilisation of the regular structure of c-ZrO₂ was observed as well as its change into the monoclinic variety - m-ZrO₂. The examined refractory materials were ordered according to their corrosive resistance to sulphur oxides at particular temperatures applied in the test. The conducted investigations qualitatively describe the behaviour of refractories exposed to the corrosive attack of sulphur oxides and can be useful in the selection of refractory materials for particular industrial applications.