Mine fires caused by spontaneous coal combustion are major disasters in coal mines. The staged oxidation kinetic parameters of various coal samples at oxygen concentrations of 21 %, 15 %, 10 %, 5 %, ...and 3 % were analyzed using a programmed temperature testing system. Herein, the temperature increase rate of coal, the temperature difference between the furnace and coal, and the oxygen consumption characteristics were obtained. Based on the amount of CO produced and the temperature sensitivity coefficient, three characteristic temperatures and four stages of low-temperature oxidation (LTO) were identified. The results showed that at a critical temperature (TC), the amount of CO gas released from the coal samples increased with increasing oxygen concentration, and the difference in the oxygen consumption rate increased. After the limit temperature (Tu), the amount of CO gas increased steadily, and the increase in the oxygen consumption rate stagnated. CO production, the maximum heating rate, and the maximum heat release rate were positively correlated with the oxygen concentration. As the oxygen concentration increased, the activation energy during the oxygen absorption stage gradually decreased. The average reaction enthalpy (ΔH) of pre-oxidized water-immersed coal was 19.37 kJ/kg greater than that of raw coal. The equation for the conservation of energy of the coal oxidation warming process was normalized. The theoretical values of the awakening stage and the stable stage were τν and τν (1-B), respectively. When B was >1, pre-oxidized water-immersed coal at a low oxygen concentration was prone to crossover points during the oxygen absorption stage, which increased the risk of coal spontaneous combustion (CSC). The research results could provide a theoretical basis for the staged control of the spontaneous combustion of water-immersed coal in goaf areas.
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•Discovered the stage characteristics of oxidized water-soaked coal•The heat production characteristics of oxidized water-soaked coal were analyzed.•The kinetic law of coal sample stage combustion was obtained.•Discussed the dimensionless method of solving cross-point temperature
Isothermal oxidation experiments in the range of 450–525 °C were carried out to investigate the effect of different Er contents (0, 0.35, 0.96 and 2.27 wt.%) on the oxidation resistance of ...Mg-4Al-0.2Mn alloys. The addition of Er extended the incubation period of the alloys. The best oxidation resistance of the alloy was obtained with the Er content of 0.96 wt.%. This is due to (i) the formation of a stable oxide film containing Er2O3 on the surface of the alloy, which inhibits further oxidation of the substrate, and (ii) the improvement of the thermal stability of the substrate. The oxidation kinetics of these alloys at elevated temperatures follow parabolic, linear, and superlinear laws.
•The effect of Er addition on the high temperature oxidation resistance of Mg-4Al-0.2Mn alloy was investigated.•The addition of Er reduces the oxidation weight gain and oxidation rate of Mg-4Al-0.2Mn alloy.•The formation of Er2O3 contributes to the formation of a dense oxide film.
The isothermal steam oxidation experiments of Cr-coated Zr alloy cladding at 1350 ℃ were conducted, and the influence of Cr-Zr eutectic reaction on the oxidation behavior was investigated. The ...oxidation duration, ranging from 50 s to 3600 s, covers the cases from the initial formation of eutectic liquid to the complete oxidation of the cladding. The microstructures of the coating and the substrate were characterized. The eutectic-oxidation degradation mechanism of the coating is summarized. The migration of the eutectic liquid and the evolution of Cr-rich phases in the post-oxidation eutectic structure are studied and discussed, confirming the process of ZrCr2 reacting with α-Zr(O) to form Zr3Cr3O. The degraded Cr coating and the Cr retained in ZrO2 during the migration of the eutectic liquid can hardly affect the oxidation of cladding substrate. However, the Cr-rich phases, which eventually aggregated in the radial center of the cladding, can reduce the oxidation rate.
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•Systematic isothermal steam oxidation experiments of Cr-coated Zr alloy cladding at 1350 ℃ were conducted.•Microstructural evolution mechanism of Cr coating and Zr substrate induced by eutectic and oxidation reactions is summarized.•The formation of Zr3Cr3O during the oxidation of Cr-Zr eutectic liquid is confirmed.•The oxidation kinetics are summarized and the influence of Cr-rich phases is analyzed.
The isothermal steam oxidation behavior of Cr-coated Zr-1Nb alloy at 1350 ℃ and 1400 ℃ was investigated. The temperatures are above the Cr-Zr eutectic temperature. The reliability of the experimental ...setup and methodology was confirmed by conducting isothermal steam oxidation experiments on bare Zr-1Nb specimens at 1100∼1400 ℃, and the corresponding oxidation kinetics correlation was obtained. After the eutectic-oxidation reaction, the surface of the coated specimens exhibited a “crocodile skin” morphology composed of “ridges” and “hollows”, which was attributed to the non-uniform eutectic reaction. Due to the much higher O affinity of Zr compared to Cr, and the higher solubility of Cr in β-Zr compared to α-Zr(O) and ZrO2, the ZrO2 and α-Zr(O) layers initially grew on the Cr-Zr eutectic during oxidation. Cr continuously diffused inward and eventually precipitated as Cr-rich phases within the metallic matrix. Three types of Cr-rich phases were observed, with varying oxygen content and reflecting the oxidation process of the Cr-rich phases. The Cr-rich phases were ultimately oxidized into a mixture of Cr2O3 and ZrO2. The parabolic weight gain rates for the oxidation of Cr-Zr eutectic are compared with the oxidation kinetics correlation derived from the bare Zr-1Nb specimens, and the model applicability of the oxidation kinetics correlations of Zr alloys in the simulations of Cr-Zr eutectic-oxidation reaction is discussed.
This study aimed to analyze how the addition of titanium (Ti) to TiAlTa alloys affects their mechanical properties (Young’s modulus and hardness) and oxidation behavior. Alloys with three different ...Ti additions and equi-atomic Al and Ta were fabricated with nominal compositions of 33Ti-33Al-33Ta at%, 50Ti-25Al-25Ta at%, and 70Ti-15Al-15Ta at%. Phase identification of as-processed and compositionally homogenized alloys was conducted by coupling various electron microscopy, diffraction, and chemical analysis techniques. Composition and structures were modeled using CALPHAD-based phase predictions and compared with experimental results. Each of the alloys exhibited a unique set of microstructures with distinctly different ordered precipitates, dependent on cooling rate and chemical composition. Subsequently, nanoindentation tests were performed at temperatures of 25°C, 250°C, 500°C, and 750°C. Oxidation studies were conducted under static air at 750°C for up to 200 h. 33Ti-33Al-33Ta at% and 70Ti-15Al-15Ta at% alloys showed higher hardness and modulus than 50Ti-25Al-25Ta at% at room temperature. The 50Ti-25Al-25Ta at% alloy exhibited higher hardness and modulus values but lower oxidation resistance at 750°C when compared with the other specimens. The X-ray photoelectron spectroscopy experiments were conducted to further analyze the oxides present in the samples. Despite each sample displaying combinations of mixed oxide layers, TiO2 was the preferred oxide forming in the 50Ti-25Al-25Ta composition, while the other samples preferred to form Al2O3. In conclusion, the findings from this systematic exploration of phase evolution in the TiAlTa alloy space provide insights for optimized material chemistries, processing, and properties.
The isothermal oxidation behavior of magnetron-sputtered Cr-coated Zr-1Nb alloy cladding in 1000–1300 ℃ steam environment is studied. The phenomena at each temperature are summarized and the ...oxidation kinetics are discussed. Cr coating can maintain the oxidation resistance for more than 6 h at 1000 ℃ and 1100 ℃ under double-sided oxidation condition, while suffered degradation during long-duration oxidation at 1200 ℃ and 1300 ℃. After coating degradation, Zr substrate was oxidized prior to residual metallic Cr and the metallic Cr was not completely oxidized until the ZrO2 layer beneath the coating had grown to a certain thickness. Mainly due to the redox reaction between Zr and Cr2O3 and the preferential oxidation of Zr substrate, the thickness of Cr2O3 film decreased after a certain oxidation duration. The Cr coating after degradation at 1200 ℃ can still hinder the oxidation of Zr substrate to a certain extent, while that at 1300 ℃ almost lost the protective effect.
•Isothermal steam oxidation of Cr-coated Zr alloy was performed in temperature range of 1000–1300 ℃.•Oxidation kinetics of Cr-coated Zr alloy cladding was summarized.•Oxidation behavior of Zr substrate after coating degradation were discussed.
Karst environments are an important but often overlooked global sink of atmospheric CH4. Little is known about methanotrophic bacterial communities in karst soils; consequently, the effects of land ...use change (LUC) and soil management practices on them and how they function as part of the soil CH4 sink are unclear. In this study, we compared net soil CH4 fluxes, kinetic parameters (Km and Vmax) of CH4 oxidation, and the pmoA gene of CH4 oxidizers at five test soil plots in karst areas that have different soil types and land use types. The maximum atmospheric CH4 uptake rate (−5.62 ± 3.14 kg ha-1 y-1) occurred in reverting scrublands just 10 years after they had been utilized for crop cultivation; followed by natural forest (−4.30 ± 4.02 kg ha-1 y-1) in limestone soils. The cultivated soils in the cropland emitted CH4 into the atmosphere. The difference in net CH4 fluxes (−8.21 kg ha–1 y–1) between soils in cropland and soils in reverting scrubland from cropland was 4–8 times higher than the global average values, suggesting that reforestation on limestone soils in karst cropland areas has much greater potential to enhance soil CH4 oxidation than in non-karst areas. Soil moisture is the most likely environmental factor to trigger the enhancement effect of reforestation. The remarkable corresponding relationships between Km, net soil CH4 fluxes, and methanotrophic compositions in these plots suggest that bacteria play an essential role in regulating the effects of LUC on soil CH4 oxidation rates. It is type Ⅰ methanotroph USCγ and JR3 (USCγ sensu lato), not type II methanotrophs USCα as in previous studies, that dominate in enhancing soil oxidation of atmospheric CH4 since reforestation on limestone soils. The specific unstable habitats in karst settings favor USCγ who, as r-strategists, can change rapidly to adapt to new conditions formed from LUC and then grow presumably at an exponential rate. This coupling of environmental and microbial factors can well explain the great and fast response of CH4 oxidation in limestone soils. Our study provides new insights that reforestation of cropland in karst areas may be a noticeably more efficient way to potentially mitigate atmospheric CH4 build-up and thus should be strongly encouraged.
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The oxidation behavior of a Mo51Co34B15 metallic glass was investigated at 873–973 K in dry air. The oxidation kinetics generally followed a parabolic-rate law, which indicated the rate-controlling ...step in the oxidation process is diffusion, and the parabolic-rate constants increased with increasing temperature, showing the oxidation rate is closely related to temperature. The thinner oxide layer was mainly composed of CoMoO4, MoO3, CoO, B2O3, and MoB, while the CoMoO4 and B2O3 oxides resulted in a lower oxidation rate, and the presence of MoB also suggested that small amounts of crystallization were taking place beneath the inner layer.
•The oxidation behavior of refractory MoCoB metallic glass is studied initially.•This system shows a superior oxidation resistance at higher temperatures.•Spinel-structured oxide provides a beneficial effect on reduced oxidation rates.
The oxidation process of H2/O2 mixtures has received a lot of attention since it represents the first step for the development of detailed kinetic schemes for complex fuels within the idea of their ...construction in a hierarchical structure. Although much effort has been dedicated to the definition of reliable and robust kinetic mechanisms for the H2–O2 sub-system, still recently updated kinetic mechanisms for the H2–O2 system have been proposed by several authors. While under traditional conditions, available kinetic schemes seem to perform properly, when moving towards the simulation of new combustion modes (i.e. MILD, Oxyfuel, LTC engine conditions) their performance is still under discussion. In particular, the presence of large amount of non-inert diluent species as H2O and CO2 with high collisional efficiencies poses several problems, such as uncertainties on third body collisional efficiencies, the chemical kinetic description of reaction pressure dependence and “mixing rules”.
In this work, new experimental tests have been run for a lean H2–O2 mixture in several bath gases (mainly N2, CO2 and H2O) in a Jet Stirred Flow Reactor. Dynamic behaviors have been experimentally detected, with temperature oscillations within the control volume for N2 diluted mixtures. CO2 and H2O act as inhibitors of such phenomenologies.
The performance of updated kinetic schemes in different environments is satisfactory when the diluent is N2, while some relevant discrepancies occur for mixtures diluted in other bath gases.
Analyses of Rates of Reactions (RR) have allowed to identify the kinetic mechanism responsible for the onset and termination of instabilities. CO2 and H2O may suppress instabilities because of the enhanced role of termolecular reactions in virtue of their high third body collisional efficiencies.
•New tests were run for H2–O2/N2–CO2 and H2–O2/N2–H2O mixtures in a JSFR.•Dynamic behaviors with temperature oscillations were recognized.•H2O and CO2 decrease the system reactivity and suppress instabilities.•H2O and CO2 promote termolecular reactions thus suppress instabilities.•A critical analysis on H2O/CO2 as bath gas in chemical kinetics is provided.