Limiting Oxygen Concentration Research Articles

The cryo-EM structure of the bd oxidase from M. tuberculosis reveals a unique structural framework and enables rational drug design to combat TB

New drugs are urgently needed to combat the global TB epidemic. Targeting simultaneously multiple respiratory enzyme complexes of Mycobacterium tuberculosis is regarded as one of the most effective treatment options to shorten drug administration regimes, and reduce the opportunity for the emergence of drug resistance. During infection and proliferation, the cytochrome bd oxidase plays a crucial role for mycobacterial pathophysiology by maintaining aerobic respiration at limited oxygen concentrations. Here, we present the cryo-EM structure of the cytochrome bd oxidase from M. tuberculosis at 2.5 Å. In conjunction with atomistic molecular dynamics (MD) simulation studies we discovered a previously unknown MK-9-binding site, as well as a unique disulfide bond within the Q-loop domain that defines an inactive conformation of the canonical quinol oxidation site in Actinobacteria. Our detailed insights into the long-sought atomic framework of the cytochrome bd oxidase from M. tuberculosis will form the basis for the design of highly specific drugs to act on this enzyme.

Coming up for air

Coming up for air

Opposed flame spread over thick solid fuels under influence of sub-atmospheric pressure and low-velocity flow

The future generation of inhabited spacecraft will have a significantly different cabin environment from the present ones, characterized by low pressure and elevated oxygen concentration. This new atmosphere and the low-velocity gas flows in microgravity provide distinct conditions for the combustion of the solid materials used, and their influence on material flammability is of particular interest in the fire safety of spacecraft. Experiments have been conducted to investigate the effects of sub-atmospheric pressure and low flow velocity on the opposed flame spread and extinction behaviors over a thick PMMA. A flammability map was constructed that delineates the uniform regime, the flamelet regime, and extinction limits for thick PMMA under sub-atmospheric pressures. The limiting oxygen concentration increases with the reduced ambient pressure at a fixed opposed flow, while the flamelet regime becomes wider. Under low ambient pressure, the flame spread rate increases with the flow velocity, but the increasing rate slows down. At a constant oxygen concentration, the flame spread rate increases with the ambient pressure and gas-phase conduction dominates flame spread. At a constant oxygen partial pressure, the higher ignition temperature and less gas-phase conduction reduce the flame spread rate synchronously with the increased pressure.

A novel concept for grid Li-ion BESS safety: Integration of Vanadium-air flow battery technology in fire protection system

Li-ion battery is the most diffused technology among electrochemical energy storage systems. Installed capacity forecasts suggest a strong growth in the next years with renewable energy utilization to meet decarbonization purposes. Over 20 fire incidents in grid systems were reported during the last years in Korea and United States, with negative repercussions in market and safety. Thermal runaway results as the recurring high impact failure effect. A novel concept to prevent Li-ion battery fires in grid installations could be represented by the integration with Vanadium-air flow batteries (VAB), a hybrid energy storage system configuration capable of fire prevention through permanent oxygen reduction in the protected volume. This novel solution is presented and discussed with reference to the safety framework including standards, firefighting systems, batteries features, experimental findings and incidental experience. Protection strategy fundamentals, the limiting oxygen concentration to be achieved, system configuration and sizing are defined proving the feasibility of the application of VABs as oxygen reduction system. To guarantee safety conditions in commercial container modules for grid applications, the required VAB installed power is three orders of magnitude lower than the Li-ion battery, while VAB capacity is more than 30 times smaller. The occupied volume for the anolyte is two orders of magnitude smaller than the protected one.

Determining the Spontaneous Combustion Period and Limit Parameters of Coal: A Large-Scale Furnace Experiment

ABSTRACT A large-scale experiment on coal spontaneous combustion (CSC) was conducted to investigate the experimental spontaneous combustion period (ESCP) and limit parameters. Coal samples were collected from the Wenjiapo coal mine in the Binchang mining area in Shaanxi Province, China. Specifically, the high-temperature point (HTP) migration of coal and the variations in the oxygen consumption rate, heat release intensity, lower limit oxygen concentration (LOC), maximum air leakage intensity (ALI), and minimum residual coal thickness (RCT) during CSC were investigated. The ESCP was discovered to be 35 days. During CSC, the HTP moved from the middle of the coal body to the bottom where air was supplied. Heat release intensity and oxygen consumption rate were directly proportional to coal temperature. Under the same conditions, minimum RCT increased with ALI, lower LOC decreased as RCT increased, and maximum ALI increased with RCT.

Effects of thermal radiation on near-limit flame spread in a low convective microgravity environment

A numerical study is conducted to investigate the effects of thermal radiation on flame spread phenomenon close to limiting oxygen conditions in a low convective spacecraft environment. To understand the role of radiative heat loss from the flame, a set of simulations is performed where gas radiation is completely suppressed. The results show that radiation exerts a dual influence on flame spread rate depending on oxygen concentration. If there is more radiative heat loss from the flame, the limiting oxygen concentration increases, and the size of the flamelet decreases. It was observed that in the absence of gas radiation, oscillating near-limit flamelets stabilise and merge into a single flame. Further, the effects of surface radiation were studied by varying emissivity of the solid fuel and it was noted that a more emissive fuel is more susceptible to the formation of flamelets.

Hazard evaluation, explosion risk, and thermal behaviour of magnesium- aluminium alloys during the polishing process by using a 20-L apparatus, MIEA, and TGA

Magnesium-aluminium alloy powders, although widely used in the polishing process on variable scales, also have a potential to be explosive. Having a thorough understanding of the thermal aspects of magnesium-aluminium alloy powder ensures process safety and is essential for developing an adequate emergency system. This study analysed the influence of the particle size, ranging between 105 and 420 μm on the thermal characteristics of magnesium-aluminium. The results revealed that at a dust concentration of 1750 g/m3, 105, 210, and 420 μm magnesium-aluminium alloy powders exhibited a maximum explosion pressure of 0.97, 0.86, and 0.78 MPa, respectively. The limiting oxygen concentration values of 105, 210, and 420 μm samples were 4, 13, and 15 vol.%, respectively. The apparent activation energies Ea of 105, 210, and 420 μm samples were 286.5–70.6, 165.2–39.9, and 224.8–168.8 kJ/mol, respectively. Furthermore, the flammability parameters exhibited greater reactivity for smaller particle sizes, i.e., the minimum ignition temperature sharply decreased from 700 to 550 °C. Thus, this study provides valuable insights into the combustion characteristics and the severity of the dust explosion hazards of magnesium-aluminium alloy powders in order to ensure better safety standards in the metal processing industry.

Effects of initial temperature and pressure on explosion characteristics of propane–diluent–air mixtures

The explosion characteristics of propane–diluent–air mixtures under various temperatures and pressures were investigated using a 20-L apparatus. The explosion limits of propane diluted with nitrogen or carbon dioxide were measured at high temperatures from 25 to 120 °C. The results showed that the upper explosion limit (UEL) increased, and the lower explosion limit (LEL) decreased with the rising temperature. The explosion limits of propane diluted with nitrogen or carbon dioxide were also measured at high pressures from 0.10 to 0.16 MPa. The results showed that the UEL increased, and the LEL almost remainedunchanged along with increased pressure. Under the same initial operating conditions, the concentration of nitrogen required to reach the minimum inerting concentration (MIC) point was higher than the concentration of carbon dioxide. Finally, the study investigated the limiting oxygen concentration (LOC) of propane under various initial temperatures, initial pressures, and inert gases. The LOC of propane decreased approximately linearly with increased temperature or pressure, and the LOC of propane dilution with carbon dioxide was greater than dilution with nitrogen from 25 to 120 °C or from 0.10 to 0.16 MPa, which indicated that the dilution effect of carbon dioxide was better than that of nitrogen.

Buoyant-flow downward flame spread over carbon fiber reinforced plastic in variable oxygen atmospheres

This study explored the downward flame spread over carbon fiber reinforced plastic (CFRP) sheets under buoyant flow in variable oxygen concentrations. Tested CFRP sheets were fabricated by laminating two unidirectional carbon fiber (CF) sheets impregnated with epoxy resins and curing them in a high-temperature furnace. The fabricated CFRP sheets were combusted in a glovebox which allowed oxygen concentration to vary. Epoxy resin sheets were also tested to investigate the effect of CFs. SEM images of burned and unburned CFRP sheets showed that the flame spread over the CFRP sheets was driven via the pyrolysis of the impregnated epoxy resins. The limiting oxygen concentration (LOC) of the CFRP sheets was 31%, while that of the epoxy resin sheets was 19%. The difference in flammability would be produced by the different thermal inertia. Contrary to the order of flammability, the flame spread rate of the CFRP sheets was higher than that of the epoxy resin sheets in more than 31% O2. In-plane temperature distributions visualized via an IR camera suggested that the CFs conductively transferred heat from flame forward, thereby accelerating the flame spread. In other words, the CFs acted as heat conductors, such as metal cores in electrical wire fires. To predict the flame spread behaviors of the CFRP sheets, this work developed a novel simplified flame spread model that involves solid-phase heat transfer. According to the model with a simulated surface temperature profile, the analytical solution of flame spread rate was derived by formulating energy balance in each zone. When the calculated flame spread rates were compared with the measured ones, a quantitative agreement was recognized. This model would be applicable to other thermally thin high-thermal-conductivity materials as well as the CFRP sheets and therefore contribute to the fire risk assessment of such materials.

Study on associated gas combustion characteristics and inert gas influence rules in air-foam flooding process

Air-foam flooding technology is a potential tertiary oil recovery technology. However, enhancing oil recovery through air injection still indicates risks of explosion. In this study, the upper and lower flammability limit (UFL and LFL) and limiting oxygen concentration (LOC) of the associated gas during air-foam flooding are investigated. The change rules of these combustion characteristics are tested under different initial pressures and temperatures using a high-pressure experimental device. The measurement results indicate that with the increase of initial temperature and pressure, the LFL and LOC decrease, whereas the UFL increases. The flammability limits of the associated gas measured at 15 MPa and 80 ℃ are ranging from 1.14% to 56.67%, far exceeding the calculated value under the room temperature and pressure. The empirical formula of the associated gas combustion performance is also fitted to immediately identify the risk levels of associated gases with similar components. The change in initial pressure and temperature only slightly affects the LOC when CO2 is the inert gas. By contrast, it greatly reduces the LOC when N2 is the inert gas. The maximum difference value of 2.55% between the LOC of the two inert gases is observed at 4 MPa and 80 ℃. Overall, the findings of this study can serve as test data for the combustion limits and limiting oxygen content of associated gases in the process of air-foam flooding.

Experimentation on minimum ignition temperature for boron blended fireworks flash powder dust cloud at normal and inert environmental conditions

SummaryIn fireworks industries, mixing and filling of chemicals are being done manually and hence, dust formation is inevitable. Due to inherent nature of ingredients like aluminium metal powder, which has more combustible and the possibilities of dust explosion are merely high. Nowadays, the use of boron as a replacement of aluminium in firecracker mixtures is being researched upon. Hence, this paper aims at measuring the minimum ignition temperature (MIT) of existing and boron blended firework composition by using Godbert Greenwald furnace. Nowadays, many firework industries are employing automation. In such places, by limiting the level of oxygen supply with Nitrogen gas, the accidents can be avoided. Hence, the limiting oxygen concentration (LOC) is also measured. It has been concluded that the probability of ignition is higher when the replacement level of aluminium with boron is less than 70% at a concentration of 1456 g/m3 with less than 75% inert gas level. The safe margin has been found which will be useful for risk assessment study.

Flammability Limits of Flat Materials with Moderate Thickness in Microgravity

The flammability limits of flat materials with different moderate thickness in opposed flow were investigated to clarify the effect of sample thickness on flammability limit. Thin poly-methyl methacrylate (PMMA) sheets and polycarbonate (PC) sheets were used as samples. The thickness was varied as 0.2 mm, 0.5 mm and 1.0 mm for PMMA, and 0.1 mm, 0.2 mm, 0.5 mm and 1.0 mm for PC. The limiting oxygen concentration (LOC) of each sample was obtained by 20 s parabolic flight experiments in mild and low flow velocity condition, and by blow-off tests on ground in high flow velocity condition. The flight experiments showed that there existed significant difference in LOC according to the sample thickness especially in mild flow condition; the thicker the sample was, the larger the LOC became. We applied a hypothesis of heat penetration zone shape into the heat balance equation around the preheat zone to account the effect of sample thickness on quenching phenomena. Although the hypothesis was not confirmed by experimentally, the obtained empirical model based on the hypothesis well represented the trend of the change of LOC when the sample thickness varied not only for the PMMA and PC samples, but also for low-density polyethylene (LDPE) and high-density polyethylene (HDPE) samples with relatively thick thickness. The modified model states that increase in the sample thickness has equivalent effect to increase in the radiative heat loss and such thickness effect is amplified when the pyrolysis temperature of the material is high.

Machine learning applied for metabolic flux-based control of micro-aerated fermentations in bioreactors.

Various bio-based processes depend on controlled micro-aerobic conditions to achieve a satisfactory product yield. However, the limiting oxygen concentration varies according to the micro-organism employed, while for industrial applications, there is no cost-effective way of measuring it at low levels. This study proposes a machine learning procedure within a metabolic flux-based control strategy (SUPERSYS_MCU) to address this issue. The control strategy used simulations of a genome-scale metabolic model to generate a surrogate model in the form of an artificial neural network, to be used in a micro-aerobic fermentation strategy (MF-ANN). The meta-model provided setpoints to the controller, allowing adjustment of the inlet air flow to control the oxygen uptake rate. The strategy was evaluated in micro-aerobic batch cultures employing industrial Saccharomyces cerevisiae yeast, with defined medium and glucose as the carbon source, as a case study. The performance of the proposed control scheme was compared with a conventional fermentation and with three previously reported micro-aeration strategies, including respiratory quotient-based control and constant air flow rate. Due to maintenance of the oxidative balance at the anaerobiosis threshold, the MF-ANN provided volumetric ethanol productivity of 4.16 g·L-1 ·h-1 and a yield of 0.48 gethanol .gsubstrate-1 , which were higher than the values achieved for the other conditions studied (maximum of 3.4 g·L-1 ·h-1 and 0.35-0.40 gethanol ·gsubstrate-1 , respectively). Due to its modular character, the MF-ANN strategy could be adapted to other micro-aerated bioprocesses.

Inerting effect of N2 on explosion of LDPE dust/ethylene hybrid mixtures

Explosion flame propagation characteristics and overpressure distribution of low density polyethylene (LDPE) dust and ethylene hybrid mixture were investigated under N2 inerting conditions using a custom-designed 12 L cylindrical explosion tank. The results showed that a small amount of ethylene could promote the explosion characteristics of LDPE dust. N2 inerting had different inhibitory effects on the explosion flame of LDPE dust and its mixture with ethylene. The explosion overpressure strength of the LDPE dust/ethylene hybrid mixture decreased with increasing N2 concentration. The overall suppression effect of N2 on the explosion overpressure of the LDPE dust was better than that of the LDPE dust/ethylene hybrid mixture explosion. As the ethylene concentration increased from 0% to 2.5%, the limiting oxygen concentration decreased by 13% oxygen. This small amount of ethylene restricted the traditional inerting process. The study conclusions can provide further scientific basis for the inerting and explosion proofing design of production process equipment involving LDPE dust.

Explosion regions and limiting oxygen concentrations of methyl propionate, methyl acetate, dimethyl carbonate with air and inert gas mixtures

New data about explosion regions with special focus on limiting oxygen concentrations for methyl propionate, methyl acetate, dimethyl carbonate with air in the presence of nitrogen, helium and carbon dioxide were determined at ambient initial pressure and 423 K. The measurements were executed according to EN 1839 method T. The changes of the explosion regions with temperature and type of inert gas were also modeled mathematically using an extended calculated adiabatic flame temperature profile (CAFTP) method. The shift of the explosion region boundaries with temperature when switching from nitrogen to carbon dioxide were reproduced well. For a switch to helium a good agreement could be reached only if the very high thermal conductivity of helium had been considered properly by using the Lewis number. This requires the knowledge of the respective Lennard-Jones parameters. The LOC of helium-containing mixtures can, however, be calculated with acceptable accuracy even if the Lennard-Jones parameters of the flammable substance are not known exactly by using reasonable estimations.

Investigation on the limiting oxygen concentration of combustible gas at high pressures and temperatures during oil recovery process

High pressure air injection is an important method for oil recovery. In this process, due to exist of combustible gas and oxygen, the wellbore explosion accident may happen. To solve the problem, the optimal method is to control the oxygen concentration in the wellbore. Thus, it needs to investigate the limiting oxygen concentration (LOC) of combustible gas at high pressures and temperatures. In this study, experiments were carried out to investigate the LOC of combustible gas at pressures from 1 MPa to 15 MPa and temperatures from 40 °C to 120 °C. The influence of pressure and temperature on LOC was analyzed. The results showed that the LOC of combustible gas declines a lot as the initial pressure and temperature increases. At 15 MPa and 120 °C, the LOC drops up to 8.21%. It was also found that the LOC is exponentially and linearly dependent on initial pressure and temperature, respectively. Besides, the empirical formulas were proposed to calculate the LOC at different conditions. Based on the present results, the standard for controlling the oxygen concentration in the wellbore was confirmed. As a result, the allowable oxygen concentration in the wellbore at different conditions can be determined. Therefore, the results of this study can help to avoid the wellbore explosion and ensure safety during air injection process.

Applications of dust explosion hazard and disaster prevention technology

Powdered materials are widely used in industrial processes, chemical processing, and nanoscience. Because most flammable powders and chemicals are not pure substances, their flammability and self-heating characteristics cannot be accurately identified using safety data sheets. Therefore, site staff can easily underestimate the risks they pose. Flammable dust accidents are frequent and force industrial process managers to pay attention to the characteristics of flammable powders and create inherently safer designs.This study verified that although the flammable powders used by petrochemical plants have been tested, some powders have different minimum ignition energies (MIEs) before and after drying, whereas some of the powders are released of flammable gases. These hazard characteristics are usually neglected, leading to the neglect of preventive parameters for fires and explosions, such as dust particle size specified by NFPA-654, MIE, the minimum ignition temperature of the dust cloud, the minimum ignition temperature of the dust layer, and limiting oxygen concentration. Unless these parameters are fully integrated into process hazard analysis and process safety management, the risks cannot be fully identified, and the reliability of process hazard analysis cannot be improved to facilitate the development of appropriate countermeasures. Preventing the underestimation of process risk severity due to the fire and explosion parameters of unknown flammable dusts and overestimation of existing safety measures is crucial for effective accident prevention.

Opposed-flow flame spread over carbon fiber reinforced plastic under variable flow velocity and oxygen concentration: The effect of in-plane thermal isotropy and anisotropy

This work explores opposed-flow flame spread over carbon fiber reinforced plastics (CFRPs) with an emphasis on the in-plane thermal isotropy and anisotropy depending on carbon fiber (CF) orientation. CFRP sheets with different CF orientations were fabricated by laminating unidirectional prepregs in different directions and combusted via a chimney where flow velocity and oxygen concentration were variable. The limiting oxygen concentration (LOC) under buoyant flow significantly increased with CF orientation angle. At more than 45°, a flame did not spread at all but was extinguished immediately after ignition even at 60% O2. The flammability of CFRPs with CF orientation angles of 0°, 15°, and 30° was further investigated with varying opposed-flow velocities and oxygen concentrations. The LOC of CFRP was higher than that of polymethyl methacrylate (PMMA) over all tested opposed-flow velocities regardless of CF orientation. The response of LOC to opposed-flow velocity varied with CF orientation angle. At 0°, the LOC was not overly sensitive to opposed-flow velocity but almost constant at 31% O2. Meanwhile, the LOCs at 15° and 30° linearly decreased with increased opposed-flow velocity. Flame spread for CFRP was much faster than that for PMMA because of the high thermal conductivity of carbon fibers. The flame spread rate of CFRPs was affected by CF orientation and that at 0° was highest among the three tested CFRP sheets. IR images used to visualize the in-plane temperature distribution during flame spread showed that the preheat region was significantly varied according to CF orientation. Varying CF orientation, therefore, causes the in-plane thermal anisotropy, resulting in the differences in the LOC and flame spread rate. The findings from this work help us evaluate the fire risk and hazard of CFRP and serve to discuss flame spread behaviors of other anisotropic materials.

Evaluating the effect of multiple flammable gases on the flammability limit of CH4: Experimental study and theoretical calculation

As the main component of natural gas and an important chemical raw material, methane easily forms a mixture of flammable gases during production and processing. Therefore, understanding the flammability limit and limiting oxygen concentration of methane is very important in predicting the possibility of fire and explosion in industrial production and designing the corresponding monitoring parameters. To solve this problem, the effects of the addition of a C2H6/C2H4/CO/H2 mixture on the flammability limit of methane and the limiting oxygen concentration under N2 dilution were systematically studied. In addition, the flammability limit was evaluated using the limiting burning velocity theory and detailed reaction kinetic model, and a sensitivity analysis was conducted to study the chemical kinetics near the limit. The experimental results show that when the volume fraction of mixed gas gradually increases, the upper and lower flammability limit of methane both decrease, and the decrease in the lower flammability limit is larger than that of the upper flammability limit. Under the condition of nitrogen dilution, the limiting oxygen concentration gradually decreases with an increase in mixed gas concentration. The explosion triangle expands and moves to the lower-left corner, the flammability limit range increases, and a greater danger of explosion exists. The calculated results based on the limit laminar burning velocity are in good agreement with the experimental data. The sensitivity analysis shows that elementary reactions involving the active free radicals of OH, H and O have higher sensitivity. With the addition of mixed gas, the chain-branching reaction R31 is promoted, the chain termination reactions R12 and R41 are inhibited, the competition between the chain-branching reaction and chain termination reaction changes, and the flammability limit range of methane expands.

Near-limit oscillatory behaviors on wick flames of dimethyl carbonate with trimethyl phosphate additions

The near-limit oscillatory behaviors on wick flames of dimethyl carbonate (DMC) with trimethyl phosphate (TMP) additions have been investigated experimentally. The experiments were conducted under a wick burner in conjunction with the limiting oxygen concentration test (wick-LOC), and the fuels were selected as typical examples of electrolyte solvents and organophosphorus compounds (OPCs) for lithium-ion batteries. The near-limit oscillating flames on the wick configuration were observed into two main characters: side-to-wake or side oscillation in for the side-stabilized flame (full flame) and wake oscillation for the wake-stabilized flame (wake flame). By tracing the flame base movement using a 240-fps camera, stable limit cycle oscillations were found in full flame cases, while the wake flame cannot sustain the oscillation for long and finally leads to global extinction. With the addition of TMP in DMC, the transition from the side-to-wake oscillation to the side oscillation was found in the unstable full flames with a linear increase in frequency and a significant drop in amplitude, while the wake oscillating flames performed increased trends for both. To clarify the dominant mechanism of TMP-added flame oscillations, laminar burning velocities of DMC+TMP mixtures were calculated at the oxygen level of each near-limit oscillating flame. The weakened flame speed with TMP addition revealed the inadequacy of buoyancy-driven mechanisms for the OPC added wick-flame. Then the wick surface temperature was measured adopting a special thermocouple arrangement to validate the thermal-diffusive promotion by TMP addition. Results showed that the heat feedback from TMP-added flames compensated for the low reactivity and provided a faster oscillation near the extinction limit.