Critical Oxygen Concentration Research Articles

Thermokinetic Characteristics of Coal Combustion under High Temperatures and Oxygen–Limited Atmospheres

ABSTRACT Coal fires are widespread in the coal–major producing countries, which causes serious environmental issues due to the high temperature and hazardous gases during coal combustion. Under oxygen–depleted atmospheres, the thermal behaviors and kinetic characteristics during coal combustion at high temperatures were paid more attention by the method of TG–DSC synchronous thermal analysis. The results indicate that the focused weightlessness and heat release processes became scattered under oxygen–depleted atmospheres, and the influence of oxygen concentration obviously increased when it was below 13 vol.%. The mass presented a linear correlation to the quantity of heat release, and the relationship between the mass and quantity of heat release showed stage characteristics with oxygen concentration. Moreover, the kinetic modes transformed from random nucleation and subsequent growth to contractive sphere, and the critical oxygen concentrations are consistent to these of thermal behaviors. These findings are significant for better understanding the characteristics of coal spontaneous combustion at high-temperature and under low-oxygen concentrations.

Flammability and critical parameters at low oxygen extinction limit of stagnation-point diffusion flames with varied stretch rates

Recently, to simulate microgravity flames in a terrestrial environment, low stretched stagnation-point diffusion flames have aroused wide interest, while the extinction behaviors of varied stretched flames at reduced oxygen concentration were scarcely investigated. This issue was accomplished by an investigation in this paper. Firstly, local and total extinction modes with ‘U’-shape boundaries were found to enlarge the unstable area in the flammability map. Furthermore, gas-phase radiation inversely proportional to stretch rate contributed to decreased critical flame temperature with dropped stretch rate. Two different power laws of critical flame standoff distance versus stretch rate existed at quenching and blowoff due to the coupling effects of boundary layer and critical oxygen concentration. Surface energy balance to evaluate the mechanisms of quenching and blowoff demonstrated that, both solid-phase conduction and surface radiation dominated at quenching, while they can be ignored at blowoff. Finally, critical burning rate was obtained from the surface energy balance and found to decrease with declined stretch rate. The similar changing tendencies of flammability and burning characteristics versus stretch rate in this work to the microgravity results implied a low-cost earth-based research method to simulate microgravity flames, and some new findings can provide instructions for fire protection in space.

Oxygen solutes induced anomalous hardening, toughening and embrittlement in body-centered cubic vanadium

Vanadium (V) is sensitive to minute quantity of oxygen interstitials, which induce pronounced hardening and embrittlement. Here, we utilize oxygen to synthesize V solid solutions in order to reveal the mechanism of oxygen solutes induced hardening. With increasing of oxygen solute concentrations, the fracture modes of V samples transform from dimple, to a mixture of dimple and cleavage, and to a fully transgranular cleavage. High density of dislocations and dislocation debris are produced in strained samples. The mobility of screw dislocations is reduced and the dislocation cross-slip events are promoted by oxygen solutes. In addition to oxygen solution hardening, the generation of high density of oxygen-vacancy complexes plays a dominant role in the strengthening. High quantity of loop-shaped dislocation debris are direct evidence for the formation of oxygen-vacancy complexes. Profuse oxygen-vacancy complexes trap dislocations, promote cross-slips and assist dislocation storage, thus give rise to a superior combination of strengthening, strain hardening, and ductility in V with 1.0 at% of oxygen. Once beyond a critical oxygen concentration (>1.6 at%), V shows catastrophic brittle failure due to the exceptional high density of oxygen-vacancy complexes. These findings provide insight to design high performance refractory metals utilizing oxygen solutes.

Experimental research on inerting characteristics of carbon dioxide used for fire extinguishment in a large sealed space

To accurately guide the quantitative injection of carbon dioxide for extinguishing the fire or diluting the accumulated toxic gases, 26 sensing devices used for detecting the multiple environmental parameters were firstly laid inside and outside of 210 m3 large sealed space, then each signal output from the sensing devices was transmitted successively to the A/D conversion module and the server with signal acquisition and analysis software, so the experimental research on inerting characteristics of carbon dioxide in different physical forms in this sealed space were carried out by using the mobile liquid storage and injection equipment, including fire source extinction, and accumulation and distribution of gas components, as well as cooling. Results show that: (1) For wood and gasoline, the critical oxygen concentration of combustion 16.98 % and the corresponding carbon dioxide concentration 28.6 % were determined comprehensively; (2) The concentration layering law in the stabilized gas was proved to be inconsistent with theoretical correspondence between specific gravity and height; (3) For direct liquid injection and gas perfusion at 2.2 MPa, the average temperature difference were 13.8 °C and 0.3 °C respectively, so the temperature drop characteristic of liquid injection was proved, and its optimal cooling distance from the nozzle is further determined from 5.6 m to 11.2 m.

Experimental study on the effect of acidity on coal spontaneous combustion at different oxygen concentrations

ABSTRACT In order to study the effect of acidity and different oxygen concentration on coal spontaneous combustion under carbon dioxide-oxygen conditions, Changcun coal was investigated by the Thermogravimetry analysis (TG). The combustion characteristics of neutral coal sample and hydrochloric acid coal sample were discussed under oxygen concentration of 20%, 15%, 11.25%, 7.5%, 3.75%, and 2.5%, respectively. The results showed that the TG-DSC curves moved to the high-temperature region, the characteristic temperature increased correspondingly, and the heat release rate became small with the decrease of oxygen concentration. As the temperature increased, the heat release increased rapidly and the difference of heat value between the different oxygen concentrations gradually became larger. At 800 K, the difference of heat release between the maximum and the minimum oxygen concentration was biggest, approximately 11701 J/g for neutral coal and 11510 J/g for acidic coal. At the same oxygen concentration, the heat release of the neutral coal was greater than that of the acidic coal, but it was not obvious when the oxygen concentration was lower than 7.5%. And according to the heat release, critical oxygen concentration of Changcun lean coal was 3.75%.

The Impact of the Strength of Roof Rocks on the Extent of the Zone with a High Risk of Spontaneous Coal Combustion for Fully Powered Longwalls Ventilated with the Y-Type System—A Case Study

During the ventilation of longwalls in hard coal mines, part of the air stream migrates to the goaves with caving. These goaves constitute a space (void) filled with rocks following coal extraction. In the case where these goaves contain coal susceptible to spontaneous combustion, the flow of such an air stream through the goaves may lead to the formation of favourable conditions for coal oxidation, self-heating and spontaneous combustion. Such an area is referred to as the zone with a particularly high risk of spontaneous coal combustion (endogenous fires). The location and extent of this zone depend on many factors, with one of the most important being the permeability of the goaves which determines the tensile strength of the roof rocks forming the caving. This strength determines the propensity of these rocks to transform into the state of caving and the degree of tightness of the cave-in rubble (treated as a porous medium). The purpose of the present paper is to determine how the tensile strength of roof rocks influences the extent of the zone with a particularly high risk of spontaneous coal combustion (endogenous fires) in caving goaves of the longwalls ventilated with the Y-type system. To achieve this goal, model-based tests were conducted for a region of the longwall mined with caving and ventilated with the Y-type system. Critical air speed and oxygen concentration values in the caving goaves of this longwall were determined for the actual conditions of exploitation. These parameters define the risk zone of spontaneous coal combustion. The tests also helped to determine the extent of this zone, depending on the strength of the rocks forming the caving. The results obtained unequivocally indicate that the type of rocks forming the caving affects its permeability and the extent of the risk zone for spontaneous coal combustion. At the same time, the distribution of this zone is substantially different than in the case of other ventilation systems. The results obtained are of real practical significance for preventive measures to reduce fire risks. The effectiveness of these measures significantly improves the safety of mining exploitation.

Stable aerobic and anaerobic coexistence in anoxic marine zones

Mechanistic description of the transition from aerobic to anaerobic metabolism is necessary for diagnostic and predictive modeling of fixed nitrogen loss in anoxic marine zones (AMZs). In a metabolic model where diverse oxygen- and nitrogen-cycling microbial metabolisms are described by underlying redox chemical reactions, we predict a transition from strictly aerobic to predominantly anaerobic regimes as the outcome of ecological interactions along an oxygen gradient, obviating the need for prescribed critical oxygen concentrations. Competing aerobic and anaerobic metabolisms can coexist in anoxic conditions whether these metabolisms represent obligate or facultative populations. In the coexistence regime, relative rates of aerobic and anaerobic activity are determined by the ratio of oxygen to electron donor supply. The model simulates key characteristics of AMZs, such as the accumulation of nitrite and the sustainability of anammox at higher oxygen concentrations than denitrification, and articulates how microbial biomass concentrations relate to associated water column transformation rates as a function of redox stoichiometry and energetics. Incorporating the metabolic model into an idealized two-dimensional ocean circulation results in a simulated AMZ, in which a secondary chlorophyll maximum emerges from oxygen-limited grazing, and where vertical mixing and dispersal in the oxycline also contribute to metabolic co-occurrence. The modeling approach is mechanistic yet computationally economical and suitable for global change applications.

Ignition resistance and mechanisms of TC11 titanium alloys with flame retardant coating

The effect of YSZ+NiCrAl-B.e composite coating on the ignition resistance of TC11 titanium alloy was studied by using the method of friction in oxygen-enriched atmosphere. Combining friction and wear analysis and unsteady heat conduction calculation, the fireproof mechanism of the composite coating was discussed. The results show that the YSZ+NiCrAl-B.e composite coating significantly improves the ignition resistance of the titanium alloy, and its critical ignition oxygen concentration is about 2.3 times than that of the bare titanium alloy. The friction property of the friction pair composed of the titanium alloy and the composite coating is higher than that of the friction pair composed of titanium and titanium alloy; And the effect of NiCrAl-B.e layer on the ignition resistance of the titanium alloy is not obvious. In the process of friction ignition, the YSZ layer can significantly reduce the temperature rise of the titanium alloy substrate though preventing the rapid heat transfer, delaying the ignition of the titanium alloy. In this sense, the YSZ layer is the flame retardant layer in the coating system, and heat barrier is the main fireproof mechanism.

Effects of Propane on the Flammability Limits and Chemical Kinetics of Methane–Air Explosions

ABSTRACT This work presents a comprehensive account of how adding C3H8 impacts the possibility of explosion for CH4–air mixtures. A series of experiments was carried out to determine the upper and lower flammability limits (UFL and LFL, respectively), and the explosive risk value for different mixtures of CH4 and C3H8. The critical oxygen concentration, the concentrations of CH4 and N2 at the point where the LFL and UFL merge, and the CH4 explosive triangle were obtained for nitrogen-diluted mixtures. In addition, we used a detailed mechanism (GRI-Mech3.0) in the CHEMKIN software package to acquire the impacts of adding C3H8 on the chemical kinetics of the explosion of CH4. The flammable limits of CH4 decrease linearly with the addition of C3H8 while the flammable range increases slightly. The explosive risk value for CH4 in air increases greatly upon adding C3H8 following a parabolic relationship. Nitrogen dilution noticeably decreases the UFL and slightly increases the LFL, ultimately resulting in these points merging. The concentration of CH4 and nitrogen at this merging point varies linearly when the C3H8 concentration is varied between 0% and 2.0%. The addition of C3H8 also expands and shifts leftwards and downwards CH4’s explosive triangle. Numerical analysis reveals that the maximum temperature and pressure during the explosion, and the formation rates of CO and NOx, noticeably increase after adding C3H8. Sensitivity coefficients of key dominant reactions show that overall more •H, •O, and •OH are consumed when C3H8 is added. Furthermore, the time between ignition and maximum pressure (the burning time) is evidently shortened by the addition of C3H8, which also shortens the explosion duration.

Fundamental investigation of stress corrosion cracking of E690 steel in simulated marine thin electrolyte layer

The mechanism of stress corrosion cracking (SCC) of E690 high-strength steel in a marine thin electrolyte layer (TEL) was investigated by performing in-situ mechanical–electrochemical tests, slow strain rate tensile (SSRT) tests, and characterization of corrosion morphology. It was concluded that E690 steel was highly sensitive to SCC, which was jointly determined by local anodic dissolution (AD) and hydrogen embrittlement (HE) both caused by dissolved O2. In addition to these functions, hydrogen oxidation catalyzed by ferric ion was found. There was a critical oxygen concentration, approximately 21% by volume, between these two different roles. Below this value, the increase in the oxygen concentration promoted the synergistic effect of AD and HE, resulting in the increase in SCC susceptibility. However, above this value, worse general corrosion offset crack initiation as well as the oxidation of hydrogen catalyzed by ferric ions reduced the SCC susceptibility.

Degradation of ozonized tire rubber by aniline - Degrading Candida methanosorbosa BP6 strain.

Aniline-degrading yeast strain - Candida methanosorbosa BP-6 was tested for its ability to degrade ground tire rubber, treated and non-treated with ozone. The protein content, respiratory activity, critical oxygen concentration (COC) and emulsifying activity of the yeast strain were monitored during 21 day degradation process. The effects of biodegradation were evaluated using aldehyde detection, Scanning Electrone Microscope (SEM) and Fourier-transform infrared spectroscopy (FTIR) analysis. Pre-treatment of ground tire rubber with ozone resulted in lower microbial growth. However, metabolic condition of the C. methanosorbosa BP-6 yeast strain was higher in sample with ozonized tire rubber. Furthermore, the COC values in the last days of the process were about 30% lower regarding non-ozonized polymer. Also, the ozonization of tire rubber resulted in higher biosurfactant production of the yeast strain. The roughness and visible gaps in rubber matrix (SEM analysis) confirmed the ability of Candida methanosorbosa BP-6 yeast strain for tire rubber biodegradation.

Ignition and combustion behaviors of single coal slime particles in CO2/O2 atmosphere

Coal slime, a low-calorific-value fuel, is a by-product during coal washing, which can be disposed in quantity and fully utilized in circulating fluidized bed (CFB) boiler. This research paid attention to the ignition and combustion behaviors of single coal slime particles, which affect the normal combustion, operation stability and burning efficiency in circulating fluidized bed boiler. The ignition and combustion behaviors including ignition mechanism, ignition delay, ignition temperature and combustion process of single coal slime particles in a vertical heating tube furnace in CO2/O2 atmosphere were researched under different operation condition parameters, for different gas temperatures (Tg = 923, 1073, and 1173 K), gas flow rates (V = 0–20 L/min), and oxygen mole concentrations (O2% = 5–80%). Coal slime particle had three ignition mechanisms in CO2/O2 atmosphere, namely, homogeneous ignition of volatiles at the windward and leeside of particle, heterogeneous ignition of char and heterogeneous ignition of coal. Only heterogeneous ignition of char and homogeneous ignition of volatiles at the windward of particle occurred in quiescent atmosphere. However, homogeneous ignition region decreased while heterogeneous ignition region increased gradually with the increasing flow rates in the oxygen concentration-gas temperature plane. Different ignition mechanisms were accompanied with various combustion processes. The combustion processes corresponding to heterogeneous ignition of char changed from flameless combustion to flaming combustion as oxygen concentration increased. Moreover, the critical oxygen concentration elevated from 30% to 50% with the increasing flow rate. The ignition temperatures and ignition delays decreased with the increasing gas temperature and oxygen concentration. As the flow rate increased, the trends became more obvious in medium-to-low oxygen concentrations. Compared with the ignition characteristics and combustion processes of coal slime particles in N2/O2 atmosphere, those in CO2/O2 atmosphere were suppressed due to the higher mole heat capacity and lower diffusion rate of oxygen molecule in CO2.

Soil Evolution par for the [golf] Course

Soil Evolution par for the [golf] Course

The secondary explosion phenomenon of gasoline-air mixture in a confined tunnel

In this article, experimental evidences of the special phenomenon of the secondary explosion of gasoline-air mixture were presented in the first place, and then, the causes for the secondary explosion were discussed. At last, effects of concentration on the secondary explosion were studied in detail through multiple experiments carried out in a cylinder tunnel with a solid heating device. Based on the experimental results analysis, the critical concentration for the secondary explosion of the gasoline-air mixture in the confined tunnel were discussed. Results indicated that main causes for the secondary explosion could be summarized as 4 points. Whether the secondary explosion occurred or not was determined by critical gasoline vapour and oxygen concentration even if the temperature was maintained in a reasonable scope. When the concentration of the gasoline vapour was as low as 0.45% and the oxygen as low as 10.4%, the secondary explosion still could be triggered.

Changes in Thermal Kinetics Characteristics during Low-Temperature Oxidation of Low-Rank Coals under Lean-Oxygen Conditions

In the present work, we derive the heat-flow and mass-loss curves for the oxidation of three low-rank coals under Oxygen-Lean conditions using thermogravimetry and differential scanning calorimetry. Relationships between the exothermic character of the low-temperature oxidation (the stage before the volatile combustion of coal) and the oxygen concentration were analyzed. Kinetic parameters for low-temperature coal oxidation under oxygen-lean conditions were obtained, based on a derived Arrhenius equation. The results demonstrate that there is a critical oxygen concentration above which the full oxidative combustion reaction of low-rank coal will be reached under oxygen-lean conditions of continuous ventilation; if the oxygen concentration exceeds the critical value, the temperature at which coal oxidative combustion reaction is completed is pushed back as the oxygen concentration decreases. The amount of heat released during low-temperature oxidation decreases with decreasing oxygen concentration, but the...

Application of oxygen-sensing technology to measure seed quality of Ginkgo biloba

Germination tests are currently the most widely used method to evaluate seed quality of Ginkgo biloba L., but they are time-consuming and labor intensive. Oxygen-sensing technology, based on the principle of fluorescence quenching to detect oxygen and assess seed quality was used to rapidly evaluate seed quality of two varieties (Shandong Tancheng 202 and Zhejiang Dafoshou) of G. biloba from five mother plants. Fifteen samples of three vigor levels were produced by accelerated aging treatments. This process was applied in duplicate. A portable oxygen-sensing detector was employed to measure oxygen content during seed germination in a closed system at 25 °C each day until day 14. Four oxygen metabolism indices were calculated: oxygen consumption index, oxygen consumption rate, critical oxygen concentration, and theoretical germination time (TGT). Additionally, we tested laboratory germination and field emergence. The results of a one-way analysis of variance and correlation analysis showed that TGT was the candidate index to evaluate seed quality of G. biloba. Therefore, the TGT value was used to validate the reliability of oxygen-sensing evaluation for Zhejiang Dafoshou seeds kept under four storage conditions. The trend in the change in oxygen metabolism agreed completely agreement with that of seed germination under all storage conditions. The oxygen-sensing test reliably and rapidly assessed seed quality of G. biloba. The germination rate of Zhejiang Dafoshou was accurately predicted by TGT.

A theoretical basis for a nanomolar critical oxygen concentration

AbstractWhen aerobic microbes deplete oxygen sufficiently, anaerobic metabolisms activate, driving losses of fixed nitrogen from marine oxygen minimum zones. Biogeochemical models commonly prescribe a 1–10μM critical oxygen concentration for this transition, a range consistent with previous empirical and recent theoretical work. However, the recently developed STOX sensor has revealed large regions with much lower oxygen concentrations, at or below its 1–10 nM detection limit. Here, we develop a simplified metabolic model of an aerobic microbe to provide a theoretical interpretation of this observed depletion. We frame the threshold as, the subsistence oxygen concentration of an aerobic microbial metabolism, at which anaerobic metabolisms can coexist with or outcompete aerobic growth. The framework predicts that this minimum oxygen concentration varies with environmental and physiological factors and is in the nanomolar range for most marine environments, consistent with observed concentrations. Using observed grazing rates to calibrate the model, we predict a minimum oxygen concentration of order 0.1–10 nM in the core of a coastal anoxic zone. We also present an argument for why anammox may be energetically favorable at a higher oxygen concentration than denitrification, as some observations suggest. The model generates hypotheses that could be tested in the field and provides a simple, mechanistic, and dynamic parameterization of oxygen depletion for biogeochemical models, without prescription of a fixed critical oxygen concentration.

Biodegradation of diesel oil hydrocarbons enhanced with Mucor circinelloides enzyme preparation

The aim of the study was to use immobilized in situ Mucor circinelloides (MC) enzymes preparation as an agent for enhanced microbial hydrocarbons biodegradation. It is a new element of complex biological degradation of diesel oil hydrocarbons (8% v/v) performed by bacterial consortia (Gordonia alkanivorans S7 (S7), Achromobacter xylosoxidans G21 (G21), Pseudomonas sp. A34 (A34)). After 14 days of biodegradation experiments, addition of M. circinelloides enzyme preparation not only increased the efficiency of hydrocarbons degradation in about 20 %–30 % compared to samples without the preparation, but also resulted in higher metabolic activity of microorganisms (higher respiratory activity, 30% decrease in critical oxygen concentration). The use of M. circinelloides enzyme preparation has significant influence on the emulsifying activity of the strains, which is very important in case of utilization of hydrophobic substances, such as hydrocarbons. Values of the parameter were relatively higher in the presence of the preparation (trail with A. xylosoxidans G21 and MC OD500 = 1.3 ± 20.8 × 10−3 at the same time trail with A. xylosoxidans G21 OD500 = 0.925 ± 0.5 × 10−3). Data obtained with GC chromatographic analysis confirmed the positive impact of fungal enzyme preparation on the process of diesel oil hydrocarbons degradation.

The combined effect of hypoxia and nutritional status on metabolic and ionoregulatory responses of common carp (Cyprinus carpio)

In the present study, the combined effects of hypoxia and nutritional status were examined in common carp (Cyprinus carpio), a relatively hypoxia tolerant cyprinid. Fish were either fed or fasted and were exposed to hypoxia (1.5–1.8mg O2L−1) at or slightly above their critical oxygen concentration during 1, 3 or 7days followed by a 7day recovery period. Ventilation initially increased during hypoxia, but fasted fish had lower ventilation frequencies than fed fish. In fed fish, ventilation returned to control levels during hypoxia, while in fasted fish recovery only occurred after reoxygenation. Due to this, C. carpio managed, at least in part, to maintain aerobic metabolism during hypoxia: muscle and plasma lactate levels remained relatively stable although they tended to be higher in fed fish (despite higher ventilation rates). However, during recovery, compensatory responses differed greatly between both feeding regimes: plasma lactate in fed fish increased with a simultaneous breakdown of liver glycogen indicating increased energy use, while fasted fish seemed to economize energy and recycle decreasing plasma lactate levels into increasing liver glycogen levels. Protein was used under both feeding regimes during hypoxia and subsequent recovery: protein levels reduced mainly in liver for fed fish and in muscle for fasted fish. Overall, nutritional status had a greater impact on energy reserves than the lack of oxygen with a lower hepatosomatic index and lower glycogen stores in fasted fish. Fasted fish transiently increased Na+/K+-ATPase activity under hypoxia, but in general ionoregulatory balance proved to be only slightly disturbed, showing that sufficient energy was left for ion regulation.

Influence of substance πQ1983 on evoked potentials of somatosensory cortex during acute hypoxia development

The influence of selenium-containing metal-complex substance πQ1983 on somatosensory cortex bioelectrical activity has been studied on cats (n = 22) during development of acute hypoxia. The condition of acute hypoxia was performed by making of loop including an animal, an apparatus for artificial lung ventilation, and a reservoir for respiration with 5 liter volume. Evoked activities of neurons were provided by direct electrical current irritations applied to the radial nerve of right superior extremity. Couple needle-shaped electrodes were used for mean evoked potentials registration. 100 mg/kg of the substance πQ1983 was introduced in the stomach 180 min before experiment. Also on any steps of the experiments ECG was registered. Containing of oxygen and carbon dioxide were determined by method of gas analysis in air enabled for respiration. It have been established that the substance πQ1983 taking internally increases cortex neurons functional activity period more than triple during acute hypoxia, stabilizes myocardial electrical activity, and helps animals to sustain the action of critical oxygen and carbon dioxide concentrations. Finally, the substance πQ1983 may be related to the high effectively antihypoxants because it significantly rises the resistance of animals with high CNS organization level (cats) to the acute hypoxia even after introduction per os.