Upper Explosion Limit Research Articles

A new approach to utilize Hydrogen as a safe fuel

Fundamental to the creation of a hydrogen economy is a viable, safe and affordable hydrogen-energy-system. Examining carefully some of the key properties of hydrogen that are related to fire and explosion, it is found that hydrogen is combustible over a wide range of concentrations. At atmospheric pressure, it is combustible at concentrations from 4% to 74.2% by volume. It has the highest flame velocity of any gas and its ignition energy is very low, which is 32% less than methane gas. In this paper, the problem of “safe hydrogen” is tackled using a new theoretical approach. Hydrogen is mixed with predetermined amounts of methane gas and to be sold as “Hydrothane”. The properties of this mixture—most important are the flame speed, lower explosion limit (LEL) and upper explosion limit (UEL) are to be developed as a function of the ratio of the hydrogen–methane. The maximum flame speed, cm/s, for a selected number of hydrocarbons along with the corresponding volume percentage of combustible mixture (fuel in air) are used in the proposed analysis. In addition, Le Chatelier's law is used to predict limits of flammability of the Hydrothane.

Explosions Limits of H2S/CO2>/Air and H2S/N2/Air

In chemical processes H2S in higher concentrations is used or arises during the process. The detailed knowledge of the lower respectively the upper explosion limit (LEL and UEL) is therefore necessary for a safety evaluation. The bomb method of the new European Norm EN 1839 represents the state of the art and serves for a revision of data from elder literature. The determined explosion limits of the two ternary gas systems H2S/CO2/air and H2S/N2/air show, for example, that for the UEL higher amounts of H2S were found than the reported ones. All new data will be recorded in the database “CHEMSAFE”.

Application of a catalytic combustion sensor (Pellistor) for the monitoring of the explosiveness of a hydrogen-air mixture in the upper explosive limit range

Application of a catalytic combustion sensor (Pellistor) for the monitoring of the explosiveness of a hydrogen-air mixture in the upper explosive limit range

Investigations on flammability models and zones for o-xylene under various initial pressures, temperatures and oxygen concentrations

The main purpose of this study is to investigate various initial temperatures (100–230°C) and pressures (760–2,280mmHg) for fire and explosion characteristics of o-xylene (OX)/air mixtures, which are commercially used in the production of phthalic anhydride (PA), so that empirical flammability models can be established and used in calculations for flammability zones.According to the results of the experiment, while the experimental condition of OX is controlled by the same oxygen concentration, if the initial temperature is increased, the upper explosion limit (UEL), flammability zones of OX are correspondingly increased, but the maximum explosion pressure (Pmax) gas or vapour explosion constant (Kg) of OX is decreased. In addition, if the initial pressure is increased, the explosion characteristic parameters are all increased.However, while OX is controlled by the same oxygen concentration, if the initial temperature is increased, the lower explosion limit (LEL) of OX is decreased. Nevertheless, if the initial pressure is increased, the LEL has various changes under various temperatures. In general, the LEL of OX has little change at higher temperature.From the experimental results, the minimum oxygen concentration (MOC) does not significantly vary while the initial temperature is increased, whereas it will be decreased by enhancing initial pressure. Therefore, the MOC of OX is determined by initial pressure instead of initial temperature.It is crucially important that if the initial temperature is lower than the normal boiling point of OX (144°C), then the UEL, Pmax and Kg of OX are higher under 100°C than 150°C and even higher temperatures. Consequently, the liquid–vapour co-existing phase will demonstrate higher degree of hazard. Therefore, it is important that the loading of the volume concentration of OX should not fall into the flammability zone.

Pressure dependence of the auto-ignition temperature of methane/air mixtures

The results of experiments conducted to determine the auto-ignition temperature (AIT) and the cool flame temperature (CFT) of methane/air mixtures at elevated pressures (200–4700 kPa) and for concentrations from 30 up to 83 vol.% are reported. The experiments were performed in a closed spherical vessel with a volume of 8 dm 3. It is shown that methane/air mixtures can react spontaneously for methane concentrations far outside the flammability limits. Cool flames are observed for methane concentrations higher than 40 vol.%. The AIT and CFT are strongly pressure-dependent and decrease with increasing pressures. A Semenov correlation is developed for the CFT as a function of the pressure. The upper explosion limit (UEL) of methane/air mixtures at elevated pressure and temperature is linked with the CFT. It is shown that for certain conditions of pressure and temperature, the flammability range increases considerably even for a small temperature rise.

Numerical study of low- and high-temperature silane combustion

Self-ignition and flame propagation properties of silane combustion systems have been studied through computer simulations using a database of kinetic and thermodynamic information that is consistent with current understanding of the elementary processes. These new inputs include the mechanism for chain branching through the SiH3 radical, rate constants for the reactions of HO2 with silane and its breakdown products, and the reaction of SiO with oxygen. Over the entire temperature range, the simulations show two distinct mechanisms. At low temperatures, the kinetics of SiH3 is controlling, whereas at high temperatures, SiH2 chemistry is of key importance. The results demonstrate that the upper explosion limit and ignition at room temperature and 1 bar can be described by the same set of reactions. With the new database, many of the experimental observations can be reproduced, and predictions are made regarding dependencies on process parameters. These include the critical conditions for chain ignition, the dependence of the critical pressure on the ratio of silane and oxygen concentration, and the temperature dependence of the critical ratio of silane to oxygen concentration. A scenario for low-temperature ignition is presented. At high temperatures, the importance of condensation processes for accurate prediction of flame velocities is clear. For very lean flames, the maximum reaction rate occurs at the lower temperature region of the flame zone.

Investigations into the distribution of hydrocarbon concentrations in underground tanks of petrol stations

Underground tanks in petrol stations for the storage of petrol need not be equipped with flame-arresting devices provided some conditions stipulated in the relevant technical rules are met. These conditions are to ensure that the gas atmosphere in storage tanks is always overrich. Investigations carried out in an arrangement of tanks corresponding to the practice showed that the air entering the tank during the delivery of petrol leads to a dilution of the gas atmosphere in the tank and that the upper explosion limit is not reached in partial volumes. These partial volumes are thus explosive. The extent of the explosive partial volumes essentially depends on the impulse associated with the inflowing air. The greater this impulse, the stronger the evaporation of petrol and the smaller the explosive partial volume to be expected. The limitation of the delivery rate to 2001/min as required in the Technische Regeln für brennbare Flüssigkeiten (TRbF, Technical rules for flammable liquids) [4] is therefore an unsuitable criterion to ensure an overrich tank atmosphere.

Effect of inhibitors on flammability range of flames produced from LPG/air mixtures

Burning velocities of Liquefied Petroleum Gas/air flames of different fuel/air compositions have been measured by the flat-flame method originally developed by Egerton and Powling, and the effects of inhibitors such as chlorinated hydrocarbons (chloroform, methylene dichloride, carbon tetrachloride) on the burning velocities have been investigated. The flammability limits at different fuel/air compositions have also been measured. The difference between upper and lower limits of flammability was narrowed by the addition of inhibitors in the order carbon tetrachloride chloroform methylene dichloride, i.e. the inhibiting effect increased with increase in the number of chlorine atoms. The relative effectiveness of different inhibitors has been expressed as an inhibition efficiency, and it has been found that an interesting correlation exists between the inhibition efficiency (also the narrowing of limits, and the maximum burning velocity) and the number of (dissociable) chlorine atoms present in the molecule of inhibiting compound. Other factors were the degree of dissociation of the inhibiting molecule in the flame, and the concentration of the inhibitor in the fuel/air mixture; the effect of small concentrations was that of specific chemical inhibition whereas at higher concentrations dilution effects were dominant. At the upper limit the flame has maximum tendency to flash back in an ordinary burner and the combustion wave may develop into a detonation wave inducing an explosion hazard. Hence upper explosion limit may be considered an index of explosion risk. The inhibitors lower this largely by reducing the concentration of free radicals in the flame boundary, increasing the difficulty of ignition, and thus increasing safety. A large difference has been found between the experimental and calculated value of the upper limit for the pure gas, probably owing to the chemical effects of ethylene via production of acetaldehyde which catalyses the combustion.

The oxidation of phosphine studied by flash photolysis and kinetic spectroscopy

Mixtures of phosphine and oxygen at pressures both above the upper explosion limit and below the lower limit have been flashed, and the variation with time of the spectra of PH 2 , PH, OH and PO ( β and γ systems) studied. The absorption spectrum of the β bands of PO indicates that only one transition ( 2 Π — 2 Π ) is involved, as proposed by Dressier (1955). A revision of the mechanism for the oxidation of phosphine is proposed, involving OH and PH 2 radicals as carriers of the straight chain and O atoms only in branching and termination steps. The mechanism is Compared with the experimental results of Melville & Roxburgh (1934).

The Reaction Between Hydrogen and Oxygen: The Upper Explosion Limit and the Reaction in Its Vicinity

The Reaction Between Hydrogen and Oxygen: The Upper Explosion Limit and the Reaction in Its Vicinity

Kinetics of the Dry and Water-Catalyzed Reaction between Carbon Monoxide and Oxygen at and above the Upper Explosion Limit1

Kinetics of the Dry and Water-Catalyzed Reaction between Carbon Monoxide and Oxygen at and above the Upper Explosion Limit¹

The Reaction between Hydrogen and Oxygen above the Upper Explosion Limit1

The Reaction between Hydrogen and Oxygen above the Upper Explosion Limit¹

The Photochemical Oxidation of Phosphine above the Upper Explosion Limit

An investigation has been made of the kinetics of the photochemical oxidation of phosphine above the upper explosion limit. The results are summarized by the equations Mercury sensitized reaction — d[PH3]/dt = const. [PH3]/[O2]2 intensity. Direct photo-reaction — d[PH3]/dt = const. [PH3]2/[O2]2. The reactions are shown to be of the chain type and inhibited by oxygen. On the basis of previous knowledge of the propagation of the chains, the question of termination was examined and it is shown that out of six possible cases, only one is in agreement with the experimental facts. This is that the chain carrier, which normally reacts with phosphine, is destroyed in a ternary collision with two oxygen molecules or with one oxygen and one nitrogen molecule (or an argon atom). The efficiency of the collision is about 0.5. In view of these results, it is highly probable that this carrier is an oxygen atom. In the photosensitized reaction one-fifth of the collisions between oxygen molecules and excited mercury atoms results in the initiation of a chain. These results are shown to be quantitatively consistent with the shape and also the position of the upper limiting explosion curve, except that the limiting pressure is a little higher than that calculated by a strict application of the theory of branched chains. The upper limit is displaced to higher pressures by illumination of the mixture, a phenomenon not predicted by theory. The effect is of short duration, <2 sec. and it is not exhibited at the lower limit. The significance of this result is discussed in relation to the theory of the reaction.