Heat Transfer Limitations in Supercritical Water Gasification

Supercritical water gasification (SCWG) is a promising technology for the valorization of wet biomass with a high-water content, which has attracted increasing interest. Many experimental studies have been carried out using conventional heating equipment at lab scale, where researchers try to obtain insight into the process. However, heat transfer from the energy source to the fluid stream entering the reactor may be ineffective, so slow heating occurs that produces a series of undesirable reactions, especially char formation and tar formation. This paper reviews the limitations due to different factors affecting heat transfer, such as low Reynolds numbers or laminar flow regimes, unknown real fluid temperature as this is usually measured on the tubing surface, the strong change in physical properties of water from subcritical to supercritical that boosts a deterioration in heat transfer, and the insufficient mixing, among others. In addition, some troubleshooting and new perspectives in the design of efficient and effective devices are described and proposed to enhance heat transfer, which is an essential aspect in the experimental studies of SCWG to move it forward to a larger scale.

Study on the residual performance of RC beams exposed to processed temperatures and fire

PurposeThe Reinforced Concrete(RC) elements are known to perform well during exposure to elevated temperatures. Hence, RC elements are widely used to resist the extreme heat developing from accidental fires and other industrial processes. In both of the scenarios, the RC element is exposed to elevated temperatures. However, the primary differences between the fire and processed temperatures are the rate of temperature increase, mode of exposure and exposure durations. In order to determine the effect of two heating modalities, RC beams were exposed to processed temperatures with slow heating rates and fire with fast heating rates.Design/methodology/approachIn the present study, RC beam specimens were exposed to 200 °C, to 800 °C temperature at 200 °C intervals for 2 h' duration by adopting two heating modes; Fire and processed temperatures. An electrical furnace with low-temperature increment and a fire furnace with standard time-temperature increment is adapted to expose the RC elements to elevated temperatures.FindingsIt is observed from test results that, the reduction in load-carrying capacity, first crack load, and thermal crack widths of RC beams exposed to 200 °C, and 600 °C temperature at fire is significantly high from the RC beams exposed to the processed temperature having the same maximum temperature. As the exposure temperature increases to 800 °C, the performance of RC beams at all heating modes becomes approximately equal.Originality/valueIn this work, residual performance, and failure modes of RC beams exposed to elevated temperatures were achieved through two different heating modes are presented.

Features of the Thermolysis of Aromatic Compounds

The effect of structure of 25 aromatic compounds on the morphology of carbon formed from them during heating to a temperature of 970°C in an inert atmosphere is studied. The specific surface area is determined for a number of products via nitrogen adsorption (28–48 m2/g). Several aromatic compounds are shown to form carbon without melting stage. X-ray phase analysis nevertheless confirms the formation of just amorphous carbon in all cases, and a negligible amount of graphite in amorphous carbon in only two cases. The thermolysis of a number of compounds is studied via synchronous thermal analysis. It is shown that slow heating during thermolysis can reduce the temperatures of transformation by tens of degrees and even alter the nature of thermolysis.

Effects of Different Carbonization Conditions on the Color Change of Biochar

The aim of this research was to determine the effects of different carbonization temperature, gas flow rate and heating rates on biochar's color change. Three different carbonization temperatures (400°C, 500°C, and 600°C), two different gas flow rates (0.2 l min-1 and 0.5 l min-1) and two different heating temperature rates (30°C min-1 and 60°C min-1) were used in the experiments. The color changes of biochar were examined utilizing the international approved L*, a*, b* system. Atriplex nitens Schkuhr was used as a biomass source in the experiments. High carbonization temperature and high gas flow rate caused a decrease in the “L” value of biochar. It is an indication that the color is getting darker, when the L value approaches zero. In the study, only the effect of gas flow rate on the "a" value was found to be statistically significant (P≤0.05). The increase in gas flow rate caused the biochar to become darker by increasing the deep red tone. Heating rate and gas flow speed significantly influenced the "b" values of biochar. The slow heating rate and high gas flow rates made the biochar color darker. At end of the research, it can be said that the biochar produced at high carbonization temperature, low heating rate and high gas flow rates will have darker tones.

A Study of Thermal Stability of Hydroxyapatite

High-temperature powder sintering is an integral part of the dense ceramic manufacturing process. In order to find the optimal conditions for producing a ceramic product, the information about its behavior at high temperatures is required. However, the data available in the literature are very contradictory. In this work, the thermal stability of hydroxyapatite prepared by a solid-state mechanochemical method and structural changes occurring during sintering were studied. Stoichiometric hydroxyapatite was found to remain as a single-phase apatite structure with the space group P63/m up to 1300 °C inclusively. A further increase in the sintering temperature leads to its partial decomposition, a decrease in the crystallite size of the apatite phase, and the appearance of significant structural strains. It was shown that small deviations from stoichiometry in the Ca/P ratio upward or downward during the hydroxyapatite synthesis lead to a significant decrease in the thermal stability of hydroxyapatite. An apatite containing almost no hydroxyl groups, which is close to the composition of oxyapatite, was prepared. It was shown that the congruent melting of stoichiometric hydroxyapatite upon slow heating in a high-temperature furnace does not occur. At the same time, the fast heating of hydroxyapatite by laser radiation allows, under certain conditions, its congruent melting with the formation of a recrystallized monolayer of oxyhydroxyapatite. The data obtained in this study can be used when choosing sintering conditions to produce hydroxyapatite-based ceramics.

Explosive Spalling Behavior of Single-Sided Heated Concrete According to Compressive Strength and Heating Rate

In this study, the effects of heating rate and compressive strength on the spalling behavior of single-sided heated ring-restrained concrete with compressive strengths of 60 and 100 MPa were investigated. The vapor pressure and restrained stress inside the concrete were evaluated under fast- and slow-heating conditions. Regardless of the heating rate, the concrete vapor pressure and restrained stress increased as the temperature increased, and it was confirmed that spalling occurred in the 100-MPa concrete. Specifically, it was found that moisture migration and restrained stress inside the concrete varied depending on the heating rate. Under fast heating, moisture clogging and restrained stress occurred across the concrete surface, causing continuous surface spalling for the 100-MPa concrete. Under slow heating, moisture clogging occurred, and restrained stress continuously increased in the deep area of the concrete cross-section owing to the small internal temperature difference, resulting in explosive spalling for the 100-MPa concrete with a dense internal structure. Additionally, while the tensile strength of concrete is reduced by heating, stress in the heated surface direction is generated by restrained stress. The combination of stress in the heated concrete surface and the internal vapor pressure generates spalling. The experimental results confirm that heating rate has a significant influence on moisture migration and restrained stress occurrence inside concrete, which are important factors that determine the type of spalling.

Complete Goss Secondary Recrystallization by Control of the Grain Size and Texture of Primary Recrystallization in Grain-Oriented Silicon Steel

Matrix microstructure and texture controlling is an important way to optimize Goss ({110}<001>) abnormal grain growth (AGG) in high magnetic induction grain-oriented silicon (Hi-B) steel during primary recrystallization. In the present work, a matrix with homogeneous grain size and favorable texture components was obtained through two-stage normalized annealing followed by primary recrystallization. Furthermore, secondary recrystallization was performed for sharp Goss orientation by slow heating and purified annealing. It was found that plenty of island grains, which occurred and disappeared gradually, accompanied the process of AGG. Through analyzing the evolution of microstructure and texture, we realized that the formation of island grains was related to the large-size grains in matrix, and the elimination of that was attributed to the special grain boundaries which satisfied both coincident site lattice (CSL) and high-energy (HE) models. It was essential to control grain size and favorable orientations in matrix comprehensively for the high-efficient abnormal growing of sharp Goss orientation, through which excellent magnetic properties could be obtained simultaneously.

Study on the Characteristics of a TBC System Containing a PVD-Al Interlayer under Isothermal Loading

In this work, the oxidation behavior of an atmospheric plasma-sprayed thermal barrier coating (TBC) system with a thin Al physical vapor deposition (PVD) film deposited over the bond coat is discussed. The TBC consisted of: (i) CoNiCrAlY bond coat sprayed on the Inconel 600 substrate; (ii) a thin Al interlayer deposited by direct current DC magnetron sputtering; and (iii) yttria-stabilized zirconia (YSZ) sprayed as the top coat. Such thermal barrier coatings (Al-TBC) were isothermally oxidized at 1150 °C with different holding times, and then they were compared with the reference TBC (R-TBC) systems without an Al interlayer (R-TBC). Scanning electron microscopy with energy-dispersive X-ray analysis was used to study the oxide formation along the bond coat (BC) and top coat (TC) interface, as well as crack formation in the yttria-stabilized zirconia top coat. Then, using Image Analysis, the oxide formation and crack formation were characterized in all specimens after a slow heating and cooling cycle, and after 100, 300, and 600 h of isothermal exposure. The results showed that the Al-TBC system proposed here exhibits higher oxidation resistance at the bond coat and top coat interface, less crack formation in the YSZ top coat, and enhanced mechanical stability compared to the conventional TBCs. It was found that enrichment of the bond coat and top coat interface with Al limited the formation of detrimental transition metal oxides during isothermal loading. Finally, the corresponding failure caused by thermally grown oxide (TGO) phenomena is “mixed failure mode” for both studied TBCs.

Hollow Graphitic Carbon Nanospheres Synthesized by Rapid Pyrolytic Carbonization

Hollow graphitic porous carbon nanosphere (CNS) materials are synthesized from polymerization of resorcinol (R) and formaldehyde (F) in the presence of templating iron polymeric complex (IPC), followed by carbonization treatment. The effect of rapid heating in the carbonization process is investigated for the formation of hollow graphitic carbon nanospheres. The resulting CNS from rapid heating was characterized for its structure and properties by transmission electron microscope (TEM), x-ray diffraction (XRD), Raman spectroscopy, bulk conductivity measurement and Brunauer-Emmett-Teller (BET) surface area. Hollow graphitic CNS with reduced degree of agglomeration is observed under rapid heating during the carbonization process when compared to the CNS synthesized using the standard slow heating approach. Key words: carbon nanosphere (CNS), rapid pyrolytic carbonization, agglomeration

Facile Synthesis of Sponge-Like Porous Nano Carbon-Coated Silicon Anode with Tunable Pore Structure for High-Stability Lithium-Ion Batteries

To address the challenge of the huge volume expansion of silicon anode, carbon-coated silicon has been developed as an effective design strategy due to the improved conductivity and stable electrochemical interface. However, although carbon-coated silicon anodes exhibit improved cycling stability, the complex synthesis methods and uncontrollable structure adjustment still make the carbon-coated silicon anodes hard to popularize in practical application. Herein, we propose a facile method to fabricate sponge-like porous nano carbon-coated silicon (sCCSi) with a tunable pore structure. Through the strategy of adding water into precursor solution combined with a slow heating rate of pre-oxidation, a sponge-like porous structure can be formed. Furthermore, the porous structure can be controlled through stirring temperature and oscillation methods. Owing to the inherent material properties and the sponge-like porous structure, sCCSi shows high conductivity, high specific surface area, and stable chemical bonding. As a result, the sCCSi with normal and excessive silicon-to-carbon ratios all exhibit excellent cycling stability, with 70.6% and 70.2% capacity retentions after 300 cycles at 500 mA g−1, respectively. Furthermore, the enhanced buffering effect on pressure between silicon nanoparticles and carbon material due to the sponge-like porous structure in sCCSi is further revealed through mechanical simulation. Considering the facile synthesis method, flexible regulation of porous structure, and high cycling stability, the design of the sCCSi paves a way for the synthesis of high-stability carbon-coated silicon anodes.