Fast Heating Conditions Research Articles

DETERMINATION OF THE FEATURES OF THERMAL DECOMPOSITION OF SUNFLOWER HUSK IN A FLUIDIZED BED

Sunflower husk (SH) is a plant waste fuel. The carbon content in different samples of SH ranges from 40.5% to 54.5% in an operating state, with ash content ranging from 1.5% to 8.5%, moisture content ranging from 6.9% to 9.5%, chlorine content ranging from 0.05% to 0.3%, and lower heating value ranging from 14.5 MJ/kg to 20.5 MJ/kg. These characteristics make it a suitable substitute for fossil fuels in power boilers. Waste-to-energy (WTE) technologies are rapidly developing worldwide, offering the potential for generating renewable energy from waste, including agricultural and food industry waste such as SH. According to our estimates, Ukraine has an annual energy potential of approximately 3.4 million tons of SH or about two million tons of fuel equivalent. Approximately half of this volume is currently being burned in oil extraction plants' boilers; however, up to one million tons of SH end up in landfills annually, resulting in significant energy losses. To develop new and improve existing WTE technologies that utilize SH as a fuel source, it's essential to understand the thermal processing characteristics of SH under conditions similar to those found in different zones within real power boilers - specifically the heating of fuel particles at rates up to 500 °C/s over a temperature range of 500–1000 °C. In this study, we aimed to investigate the thermal processing characteristics by subjecting SH particles within a laboratory fluidized bed reactor to high-speed heating within the aforementioned temperature range. During rapid heating between 500–1000 °C temperatures range, SH particles undergo conversion into volatile compounds and solid carbon residue. Two distinct stages can be observed on the dynamic yield curves for volatiles. The release and burnout of volatiles occurs during the first stage while the second stage involves coke ash residue burnout. We obtained empirical temperature-dependents for total heat treatment time and carbon residue burnout time under fast heating conditions in the investigated temperature range. The stage of carbon residue combustion is the most enduring and determines the overall duration of thermal treatment. This stage determines the degree of fuel transformation, especially in cases where low-reactivity carbon residue enters the low-temperature combustion chamber area of the boiler. The obtained regularities have practical significance in designing combustion chambers for thermal processing of sunflower husk.

Numerical study on temperature response of microheater and evolution characteristics of superheated zone under fast transient heating

The nucleate boiling process that occurs on the surface of a microheater immersed in a liquid pool under fast transient heating conditions can be regarded as homogeneous or near-homogeneous nucleation, exhibiting a significant difference when compared to boiling at conventional time scales on large and even micro spatial scales. This paper numerically investigates the boiling process at both small spatial and temporal scale by using a validated model. Results indicate a significant difference between the average microheater temperature and the maximum temperature at the boiling surface, which can reach up to 80 °C within the scope of present study. Consequently, the nucleation temperature, typically determined by the average temperature of the microheater in most references, may not precisely represent the homogeneous (or nearly homogeneous) nucleation temperature. At the initiation of nucleation, the profiles at the boundary of superheated zone are approximately circular in the z-plane and arched in the x-plane, respectively. Meanwhile, the profile of the superheated zone boundary at the onset of nucleation under direct heating mode (DHM) appears approximately trapezoidal, exhibiting a significant difference from that observed under indirect heating mode (IHM). It is also noted that the local temperature distribution in the fluid region can be considerably affected by natural convection, potentially exerting a substantial influence on the bubble dynamic behaviors during the subsequent bubble growing phase. These findings may provide valuable references for future experimental study on the mechanisms involving in microscale boiling heat transfer process under rapid heating conditions.

Understanding mono- and bi-metallic Au and Ni nanoparticle responses to fast heating.

Nanoparticle assembly, alloying and fragmentation are fundamental processes with significant implications in various fields such as catalysis, materials science, and nanotechnology. Understanding these processes under fast heating conditions is crucial for tailoring nanoparticle properties and optimizing their applications. For this, we employ molecular dynamics simulations to obtain atomic-level insights into nanoparticle behavior. The performed simulations reveal intricate details of sintering, alloying and fragmentation mechanisms shedding light on the underlying physical phenomena governing these processes. The calculation results help to visualize nanoparticle evolution upon undercritical and supercritical heating elucidating not only the role of temperature, but also of nanoparticle sizes and composition. In particular, it is shown that surface tension and surface energy play important roles not only in nanoparticle melting but also in its fragmentation. When the added energy exceeds a critical threshold, the nanoparticle begins to experience alternating compression and expansion. If the tensile stress surpasses the material's strength limit, fragmentation becomes prominent. For very small particles (with radius smaller than ∼10 nm), this occurs more rapidly, whereas sub-nano-cavitation precedes the final fragmentation in larger particles, which behave more like droplets. Interestingly, this effect depends on composition in the case of AuNi alloy nanoparticles, as expected from the phase diagrams and excess energy. The heating level required to overcome the mixing barrier is also determined and is shown to play an important role in the evolution of AuNi nanoparticles, in addition to their size. Furthermore, our findings provide insights into controlling nanoparticle synthesis for various applications in numerous nanotechnological domains, such as catalysis, sensors, material analysis, as well as deseas diagnostics and treatment. This study bridges the gap between experimental observations and theoretical predictions paving the way for designing advanced nanomaterials with enhanced functionalities.

Exfoliated Magnesium Diboride (MgB2) Nanosheets as Solid Fuels.

Magnesium diboride (MgB2) has been explored as an alternative fuel to boron (B) due to its high energy density and the additive effect of magnesium (Mg) to promote B combustion. However, the primary oxidation of MgB2 does not occur unless it decomposes at a high temperature (830 °C), which makes ignition difficult and the reaction slow. Recently, two-dimensional (2D) exfoliated MgB2 nanosheets have attracted increasing attention due to their unique properties and potential applications in various fields. In this study, we investigate the potential of 2D exfoliated MgB2 nanosheets as solid fuels for overcoming the challenges of MgB2 combustion. We analyzed their oxidation behavior and energetic performance through material characterization and combustion tests under slow- and fast-heating conditions and compared their performance with those of bulk MgB2, B nanoparticles, and a B/Mg nanoparticle mixture. This study highlights the potential of MgB2 nanosheets as promising solid fuels with superior energetic properties.

Green coal substitutes for boilers through hydrothermal carbonization of biomass: pyrolysis and combustion behavior

Hydrothermal carbonization (HTC) is a thermochemical conversion process in aqueous phase that is well suited for the valorization of waste biomass. The carbonaceous solid phase produced by the HTC process (the so-called hydrochar) shows interesting properties that make it similar to conventional solid fuels. Thus, it may be used in existing coal-handling infrastructures, rendering it particularly attractive for co-firing. Two different biomasses are investigated: walnut shells as biomass richer in lignin, and miscanthus as biomass richer in hemicellulose/cellulose. The properties of the resulting hydrochars are analyzed and compared with those of untreated biomass and of coals typically used in industrial boilers. The HTC treatment results in the reduction of the cellulose/hemicellulose contribution and O/C ratio, and in an increase of aromatic structures, heating value and grindability. The behavior of the hydrochars upon pyrolysis under fast heating conditions typical of pulverized fuel boilers is investigated using a heated strip reactor (HSR). The combustion reactivity tests in a thermogravimetric apparatus show that the HTC treatment levels off the differences between various biomasses, while the HSR treatment generates chars with the same combustion reactivity as char obtained from coal erasing the differences between HTC-derived biomass and fossil fuels. As for char reactivity, the HTC process reduces the impact of the different biomass compositions on tar formation, but few differences are still observed regarding the volatile phase between biomass hydrochar and coal samples. Results are analyzed and discussed with the goal of evaluating the potential of HTC of lignocellulosic biomass to generate green fuels suitable for industrial boilers.

Ignition and combustion of Perfluoroalkyl-functionalized aluminum nanoparticles and nanothermite

Aluminum (Al) is an excellent metal fuel for combustion applications because of its high energy density and low cost. Nano-sized Al (nAl) particles have relatively low ignition temperature and high reactivity, but their performance is limited by the presence of the native oxide (Al2O3) layer surrounding the active Al core. To overcome this problem, nAl coated or mixed with fluorocarbon species have been intensively studied recently. In this work, we demonstrate a facile method to functionalize the surface of nAl particles with perfluoroalkylsilane and investigate the thermochemical and combustion behaviors of perfluoroalkyl-functionalized nAl particles at both slow and fast heating rates. At slow heating rates, the covalently-bonded perfluoroalkylsilane molecules partially undergo debonding at low temperatures before reacting with Al2O3. The remaining perfluoroalkyl molecules slowly react with surface oxide to thicken the surface shell, which improves the thermal stability of nAl particles. Nevertheless, under fast heating conditions, the dissociated fluorine species can readily react with the oxide on the nAl surface, enhancing the ignition and combustion performance of both nAl particles in the air and Al/copper (II) oxide (CuO) nanothermite. These results advance the understanding of the thermochemical behaviors of the fluorocarbon species and their effects on the ignition and combustion of nAl particles and Al-based energetic materials.

Thermoplastic embossing device to probe rheological changes of supercooled metallic liquids during rapid heating.

The setup and working principle of a device designed for thermoplastic embossing and investigating rheological changes during fast-resistive heating of bulk metallic glasses (BMGs) is presented. The device was developed and custom-built at the Leibniz Institute for Solid State and Materials Research Dresden (Leibniz IFW Dresden) and is integrated into a universal testing machine. By varying the electrical-current pulse signal, the rate at which BMGs are resistively heated is controlled. Next to temperature and electrical resistance, the displacement of the punch, which penetrates the sample during rapid heating, is monitored. Additional pre-heating controlled by thermocouples allows for stable heating conditions to minimize heat extraction from the resistively heated specimen, which could eventually lead to damage of the device. The main focus of this device is to study fundamental phenomena under kinetic constraints evoked by fast heating conditions and to structure the surface of BMGs by thermoplastic embossing. A case study, which is carried out with Zr52.5Cu17.9Ni14.6Al10Ti5 BMG, demonstrates the device's performance covering heating rate ranges of three magnitudes and the access to rheological changes during the devitrification and subsequent crystallization of the BMG during rapid heating. Furthermore, the present device was successfully used to nanostructure the surface of the BMG at elevated heating rates.

C1∼C2 hydrocarbons generation and mutual conversion behavior in coal pyrolysis process

Different hydrocarbon species in coal pyrolysis gases have the NOx reduction potential. To investigate hydrocarbons CxHy mutual conversion in the coal pyrolysis gases, four typical coals were pyrolyzed under slow and fast heating rate conditions below 1200 °C. Results show that the yield of the hydrocarbon species has positive relation to coal volatiles. In fast heating condition CH4 is the major pyrolysis products. In addition, the alkanes generation behavior is quite different from that of olefins and alkynes. In the range of 600–1200 °C, as the pyrolysis temperature increases, the yield of C2H4 and C2H2 increases while CH4 and C2H6 decreases. The yield of C2H2 is significant at the temperature above than 1000 °C. Fourier transform infrared (FTIR) analysis show a close relationship between the coal functional group from 2800 to 3000 cm−1 and the alkanes yield. In the slow heating condition, five CH4 and two C2H6 evolution paths are proposed by the mathematic fitting method. The ethylene in the high temperature produced from the decomposition of C2H6. When the heating rate is low, no C2H2 is detected which illustrates that the formation of acetylene is closely related with the heating rate.

Enhanced formability and forming efficiency for two-phase titanium alloys by Fast light Alloys Stamping Technology (FAST)

During hot stamping of titanium alloys, insufficient forming temperatures result in limited material formability, whereas temperatures approaching the β phase transus also result in reduced formability due to phase transformation, grain coarsening and oxidation during the long-time heating. To solve this problem, Fast light Alloys Stamping Technology (FAST) is proposed in this paper, where fast heating is employed. Effects of heating parameters on the formability and post-form strength were studied by tensile tests. Forming of a wing stiffener was performed to validate this new process. Results show that microstructure of TC4 alloy after fast heating was in nonequilibrium state, which could enhance ductility significantly compared with the equilibrium state. When TC4 alloy was first heated to 950 °C with heating rate of 100 °C/s and then cooled to 700 °C, the elongation at 700 °C was more than 3 times that of a slow heating rate with soaking. Nano-scaled martensite with high dislocation density transformed from β phase was observed under fast heating condition. A complex shaped wing stiffener was successfully formed from TC4 titanium alloy in less than 70 s including heating, transfer and forming, and the post-form strength was almost the same with the initial blank.

Comparative study of the inherent combustion reactivity of sawdust chars produced by TGA and in the drop tube furnace

Thermogravimetric analysis (TGA) has been widely used to obtain inherent combustion reactivity data for biomass fuels. However, fundamental information relating such data to systems that operate under conditions closer to those encountered in pulverized fuel (PF) combustion is relatively scarce. This study therefore, compares the combustion reactivity of sawdust devolatilized under slow heating conditions by TGA and fast heating conditions in a drop tube furnace (DTF) representative of PF combustion. The implications of devolatilization temperature, heating rate and residence time were considered. It was established that the char burnout profiles of the DTF char prepared at 1300 °C and 600 ms was similar to the 700 °C TGA char although, the DTF char showed a lower maximum rate. Scanning electron microscope (SEM) images revealed that the TGA chars retained a cell matrix structure with visible macropore network. In contrast, the DTF chars displayed swollen surfaces with distributed pore networks, resulting in higher BET surface areas. The activation energy of the DTF chars was smaller than those of TGA char, demonstrating the presence of greater internal diffusion control. Finally, it is demonstrated for the first time that the compensation effect for biomass char is considerably less than that reported previously for coal chars.

Generalized thermoelastic diffusion problems with fractional order strain

The studies of the coupling of thermodiffusion and mechanical deformation in solids have aroused great interests with miniaturization of the micro/nano-electromechanical devices and wide application of ultrafast heating technologies. In such situation, it is commonly accompanied with high heat/diffusion flux and thermal/diffusion induced stress accordingly. So far, although there have been many developments and extensions of research and theorizing within the generalized thermoelastic diffusion problems, these theoretical formulations are only applicable when the strain rate is assumed to be relatively small, but this is questionable under ultrafast heating condition. The present work aims to investigate generalized thermoelastic diffusion problems with fractional order strain within the extended thermodynamic framework. For numerical evaluation, the analytical model of a sandwich structure with ideal interfacial conditions is investigated whilst its bounding surfaces are subjected to symmetrical transient heat and chemical potential shock loads. From numerical results, the effects of the fractional order parameters and material characteristic parameters of each layer on structural transient thermo-elasto-diffusive responses are also analyzed and discussed, which is expected to provide a comprehensive understanding on thermoelastic diffusive coupling and optimal design of sandwich structures under extreme fast heating condition.

Effect of moisture migration and water vapor pressure build-up with the heating rate on concrete spalling type

In this study, the effect of moisture migration and water vapor pressure build-up within high-strength concrete (HSC) on its spalling property is empirically investigated at various heating rates and compressive strengths with different water-binder ratios (W/B). An experiment is conducted on concrete specimens with various W/Bs by applying two different heating rates: fast and slow heating. It is confirmed that the moisture migration and water vapor pressure within the concrete differ, depending upon the W/B and heating rate. For HSC with W/B less than 0.33, surface spalling occurs due to the formation of moisture clogs on the surfaces of the concrete specimens, under the fast heating condition. For HSC with dense microstructures, although moisture clogs are not formed at specific positions within the concrete specimens, explosive spalling occurs due to boiling liquid expanding vapor explosion (BLEVE) in the concrete pores, under the slow heating condition.

Study on the physical properties of indium tin oxide thin films deposited by microwave-assisted spray pyrolysis

Transparent and conductive indium tin oxide (ITO) thin films were successfully deposited on quartz substrates by the microwave assisted spray pyrolysis method. Microwave heating favors the pyrolysis process to prepare high quality films due to the novel conditions of internal, fast and uniform heating. The effects of microwave pyrolysis temperatures (300–700 °C) on crystal structure, morphology, optical and electrical properties were investigated. X-ray diffraction results confirmed the formation of pure and tetragonal poly-crystalline phase for all the deposited films whose predominant orientation of growth along (222) or (400) direction at different temperatures. By optimizing microwave pyrolysis temperature at 500 °C, scanning electron microscopy and atomic force microscopy results displayed the formation of 318 nm thick film composed of nanograins with narrow sizes distribution and low roughness. The as-deposited ITO thin films showed higher optical and electrical properties, which the average optical transmittance was 86.4% (including 90.1% of the bare substrate) in the visible region, and the sheet resistance and resistivity were 11.11 Ω/□ and 3.54 × 10−4 Ω cm, respectively. The band gaps were determined in the range of 3.78–3.91 eV. Moreover, Haacke's figure of merit reached a maximum value of 33.73 × 10−3 Ω−1 for the ITO film deposited at 500 °C.

Mechanism on heavy metals vaporization from municipal solid waste fly ash by MgCl2⋅6H2O

This work aims to study the mechanism of heavy metals vaporization by MgCl2⋅6H2O. Firstly, the decomposition mechanism of MgCl2⋅6H2O was investigated by thermodynamic equilibrium calculations, XRD and TG. Upon heating, MgCl2⋅6H2O went through the processes of dehydration and hydrolysis simultaneously accompanied by the release of HCl between 150 and 500°C. At temperature higher than 500°C, Mg(OH)Cl gradually release part of HCl. MgCl2⋅6H2O followed the similar processes of decomposition at both oxidative and reductive atmospheres. In oxidative atmosphere, vaporization of Zn and Cu was significantly accelerated by MgCl2⋅6H2O. However, in inert atmosphere, vaporization of Cu was not promoted since copper chloride was only stable in oxidative atmosphere. Under slow heating condition, vaporization of heavy metals were close to that under fast heating condition. This may be partially attributed to that most heavy metals already reacted with HCl forming metal chlorides below 500°C, which can be vaporized at higher temperature. Moreover, the Mg(OH)Cl contributed to release HCl up to 800°C. At such high temperature, the metal chlorides continue to be formed and then vaporized. After treatment, the leaching concentration of heavy metals from treated fly ashes were much lower than that from raw fly ash and met the regulatory limit of leachate. Since a large amount of MgSiO3 were formed during thermal treatment, the fly ash treated with MgCl2⋅6H2O can be used as raw materials for glass–ceramics production.

Desensitizing ignition of energetic materials when exposed to accidental fire

Composite energetic materials combine fuel and oxidizers for high energy density exothermic reactions and are used for ordnance, industrial and localized power generation applications. This study focuses on examining an additive to a mixture of aluminum (Al) and copper oxide (CuO) to decrease ignition sensitivity under accidental fire exposure conditions. Ammonium nitrate (AN) was incorporated into Al+CuO, as a 1:1 replacement for CuO, for varied equivalence ratios and examined for ignition and combustion when exposed to slow and fast heating rate ignition conditions. The goal was to develop an Al+CuO+AN formulation that would perform comparable to Al+CuO when intentionally ignited, but would not ignite in an accidental fire. Experimental results show that Al+CuO+AN with an equivalence ratio (ER) ranging from 4.0–5.5 inerts the reactants when exposed to slow heating conditions, yet ignites with comparative combustion performance to the baseline Al+CuO mixture when exposed to fast heating conditions. These results are consistent with thermochemical simulations of the heat of combustion and adiabatic flame temperature for the respective reactions. This study presents a new approach for tailoring composite energetic materials toward accidental fire safety by exploiting the early stage decomposition kinetics of AN, which are activated only by slow heating ignition conditions.

Microstructural Characterization and Hardening Mechanism of Steel for Large Size Bearing Ring Under Fast Heating and Short Soaking Time Condition

AbstractThe microstructure changes of steel for large size bearing ring with different Ni contents at different cooling rates under fast heating and short soaking time condition are analyzed by using thermal simulation test, scanning electron microscope (SEM), field emission scanning electron microscope (FESEM), electron‐backscattering diffraction (EBSD), and transmission electron microscope (TEM). The hardness is evaluated by Rockwell hardness test and the hardening mechanism is discussed. Full martensite cooling rate can be lowered from 10 to 3 °C s−1 by the addition of 0.59% Ni. At full martensite cooling rate range, the martensitic lath structures are refined by adding 0.59% Ni or increasing the cooling rate. The hardness increases progressively with the cooling rate due to the refinement of martensitic lath structures and the precipitation strengthening of M23C6‐type carbides. But the grain boundary strengthening has predominance. The highest hardness values of steel No. 2 with higher Ni is 60 and 2.5 HRC higher than that of steel No. 1 at the cooling rate of 100 °C s−1. This increase of hardness in steel No. 2 can be attributed to the finer martensitic lath structures and the solid solution strengthening. The grain boundary strengthening is the main influencing factor.

Boiling point of levoglucosan and devolatilization temperatures in cellulose pyrolysis measured at different heating area temperatures

Fast heating conditions are known to improve the yields of volatile products in cellulose pyrolysis. The mechanism, however, is not yet well understood. For better understanding of the cellulose fast pyrolysis, the devolatilization temperature was directly measured during the pyrolysis of cellulose. An in situ measurement method employing a very fine thermocouple was first developed. A sample in a ceramic boat was pushed into the heating area, preheated at 430–700°C under the flow of N2 (150mL/min). Employing this method, the boiling point (385°C) of levoglucosan, the major volatile product derived from cellulose fast pyrolysis, was measured directly for the first time, and was found to reduce to 345 and 292°C under reduced pressures of 0.5 and 0.1atm, respectively. High heating area temperatures increased the selectivity of levoglucosan evaporation by shortening the period required for the completion of evaporation. With these results in mind, pyrolysis of cellulose powder (Ceolus FD-F20) was studied using a similar procedure. The results obtained under the reduced pressures suggested that the depolymerization of cellulose, not evaporation of the volatiles products, is the rate-determining step for the pyrolytic devolatilization of cellulose. Interestingly, the depolymerization temperature varied depending on the heating area temperature as 360–385°C (for 430–500°C) and 400–450°C (for 500–700°C) (the values in the parentheses: heating area temperatures). The char morphology also changed; the pyrolysis conditions provided powder-like and film-like char materials, respectively. According to these results, the effects of heating area temperature on the cellulose pyrolysis mechanism are discussed, focusing on the behavior of cellulose crystallites.

Thermal Decomposition and Ignition of PBXN-110 Plastic-Bonded Explosive

Due to safety requirements, insensitive behavior in slow or fast thermal heating (cook-off) conditions is a desired behavior for today’s munitions. The ignition time of munitions under slow or fast cook-off conditions is an important parameter in the design of insensitive munitions. The critical temperature, which mainly depends on the chemical, physical, and the geometrical properties of the energetic material, is the determining factor whether the material will end up with thermal initiation or not, when it is exposed to an external heat source. In this study a slow cook-off test setup is designed and developed and the tests for a generic munition containing PBXN-110 plastic-bonded explosive are performed in order to obtain temperature distribution in the test item, ignition time, ignition temperature, and ignition location. In this paper the development procedure and the experimental results of the slow cook-off tests are explained. Moreover, the kinetic parameters such as activation energy and pre-exponential factor for the plastic-bonded explosive obtained from the TGA tests are presented.

Enhanced thermomechanical properties of a nematic liquid crystal elastomer doped with gold nanoparticles

Here we report the development of a thermomechanical nematic liquid crystal elastomer (LCE) actuator containing gold nanoparticles (AuNPs). Embedding a low concentration of AuNPs enhances the thermal conductivity of the actuator with minimal effect on the elasticity of the cross-linked polymer. Doping LCEs with AuNPs provides a means to improve the material response time to external stimuli. Under fast heating conditions, the AuNP-doped LCE actuators exhibited more than a 100% increase in the rate of change of strain with respect to time.

Fracture of metals samples under conditions of fast heating by intensive X-ray radiation

Results on studying the fracture of metals samples in the form of thin disks under fast heating by the X-ray pulse with the complete spectrum are presented in the paper. The samples of such metals as iron, copper, AMg6 aluminum, VT14 titanium, molybdenum, tungsten, cadmium, lead and zinc were tested. The samples were fixed in the special cartridges equipped with the gauges of a mechanical recoil momentum. The cartridges with samples were placed at such distances from the X-ray irradiator where the energy fluxes were 1.38, 0.90 and 0.29kJ/cm2. The irradiating X-ray pulse was about 2 ns in duration. After testing, the depth of material ablation from a sample front surface and the degree and character of its spall damage were determined. The method of metallographic analysis was used for these purposes. Numerical calculations of loading conditions were made with the use of an equation of state taking into account the process of evaporation. The calculated value of maximum negative pressure in the sample at the coordinate corresponding to the formation of spallation zones or spall cracks was conventionally accepted as the material resistance to spall fracture. The comparison of obtained results with the data on the fracture of examined materials in the conditions of fast heating by the X-ray pulse with the hard spectrum and a high-current electron beam was conducted.