Upper Flammability Limit Research Articles

Prediction of flammability limits of fuel-air and fuel-air-inert mixtures from explosivity parameters in closed vessels

Flammability limits of fuel-air and fuel-air-inert gaseous mixtures, especially at non-atmospheric conditions, are essential properties required for establishing safety operating conditions for handling and processing flammable gases. For pure fuels, an important data pool exists, formed by the flammability limits of fuel-air and fuel-air-inert gaseous mixtures at ambient initial conditions measured by standard methods. Such methods can be used for experimental determination of flammability limits for multi-fuels mixed with air, with or without additives, under non-atmospheric conditions. Their use is however a time- and material-consuming process; in addition, the flammability limits obtained by various standard methods may be scattered as a result of different choices in the operating parameters, for each standard method. It appears that a preliminary estimation of the flammability limits for fuel-air and fuel-air-inert gaseous mixtures can minimize the effort of measuring them in specific initial conditions.The present paper describes a new method for estimating the flammability range of fuel-oxidizer gaseous mixtures based on measurements of explosivity properties e.g. the peak explosion pressure and maximum rate of pressure rise recorded during closed vessel laminar explosions of fuel-oxidizer mixtures far from limits. Data obtained for several hydrocarbon-air gaseous mixtures with or without inert gas addition are used to examine the accuracy of estimated flammability limits (LFL – the lower and UFL – the upper flammability limit) as well as of the Limiting Oxygen Concentration (LOC) and the Minimum Inert Concentration (MIC). The predictive ability of the proposed method is examined against the predictive ability of other recently described methods.

Inert Effects on Flammability Limits and Flame Propagation of LPG by CO<sub>2</sub>

In this study, the inert effects of CO2 on the flammability limit and flame propagation of LPG has been investigated experimentally. The observation was done using cubic combustion bomb with the dimension of 500 mm x 200 mm x 10 mm. The results showed that the lower flammability limit (LFL) of LPG-Air mixtures is found to be 2.7% (by volume) and upper flammability limit (UFL) is 8.6% (by volume) with upward propagation of flame. The CO2 dilution effects on the flammability limits have been explored, the limits of flammability was narrowed by adding CO2 and propagation flame was reduced accordingly. The results indicated that to formulate an inflammable refrigerant mixture, using CO2, with substantial hydrocarbon content is not possible.

Flammability limits of hydrated and anhydrous ethanol at reduced pressures in aeronautical applications

There is interest in finding the flammability limits of ethanol at reduced pressures for the future use of this biofuel in aeronautical applications taking into account typical commercial aviation altitude (<40,000ft). The lower and upper flammability limits (LFL and UFL, respectively) for hydrated ethanol and anhydrous ethanol (92.6% and 99.5% p/p, respectively) were determined for a pressure of 101.3kPa at temperatures between 0 and 200°C. A heating chamber with a spherical 20-l vessel was used. First, LFL and the UFL were determined as functions of temperature and atmospheric pressure to compare results with data published in the scientific literature. Second, after checking the veracity of the data obtained for standard atmospheric pressure, the work proceeded with reduced pressures in the same temperature range. 295 experiments were carried out in total; the first 80 were to calibrate the heating chamber and compare the results with those given in the published scientific literature. 215 experiments were performed both at atmospheric and reduced pressures. The results had a correlation with the values obtained for the LFL, but values for the UFL had some differences. With respect to the water content in ethanol, it was shown that the water vapor contained in the fuel can act as an inert substance, narrowing flammability.

Ignition of filtration gas combustion waves by the flame of the filtered gas

Mathematical modeling of ignition of filtration gas combustion waves in a porous medium with external initiation of combustion by the filtered gas is performed. It is shown that the surface temperature of the porous medium at which the flame enters the latter is a function of system parameters. The existence of the lower and upper flammability limits in terms of the gas filtration rate is found. Dependences of the ignition time on parameters of the porous medium are obtained, and their interpretation is given.

Experimental study of flammability limits of oxy-methane mixture and calculation based on thermal theory

The flammability limits of CH4/CO2/O2 were studied using theory calculations and experimental investigation. A calculation model was re-deduced based on thermal theory. In this model, the chemical effects of CO2 on flammability limits are concerned. Results show that the chemical effects of CO2 decrease the upper flammability limits (UFL) of mixtures, while have little effect on the lower flammability limits (LFL). Experimental measurements were performed using a cylinder reactor. The measurements of the LFL of CH4/O2/CO2 increase from 5% to 8.25% with the increase of CO2 concentration, and the UFL of mixture CH4/O2/CO2 decrease from 61% to 8.25% accordingly. The model predicted the LFL very well, but the calculated values are an average 10.7% higher than the measurements at the UFL points. The differences between the predictions and measurements are mainly due to the two assumptions of this model.

Modelling the effect of water fog on the upper flammability limit of hydrogen–oxygen–nitrogen mixtures

Nitrogen dilution and very fine water mist fogs have both been suggested as possible methods of controlling flammable hydrogen–air mixtures by modifying the flammability limits to prevent ignition. A simplified theoretical model, based upon the inhibition parameter concept, is developed allowing the effect of nitrogen dilution and very fine water fog upon the upper flammability limit (UFL) of hydrogen–oxygen–nitrogen mixtures to be predicted and compared with corresponding experimental test data. The model predicts similar general trends in behaviour with regard to the effect of water fog upon the UFL to those observed in the experimental tests. The results suggest that the combination of nitrogen dilution and high density water fog can produce a significant reduction in the range of the UFL and total extent of the flammable region.

Combustion parameters of gaseous epoxypropane/air in a confined vessel

We present an ignition energy measurement system, which comprises a 5L explosion vessel, a transient pressure measurement sub-system, and a transient temperature measurement sub-system. Through a series experiments carried out with this system, the influences of the concentration of gaseous epoxypropane on the combustion pressure and temperature and on the rate of combustion pressure rise and combustion temperature rise have been analyzed, and the results are discussed. The combustion pressure and temperature of gaseous epoxypropane/air mixtures reached their highest values at a concentration of 7% within the studied range. Variation of the ignition energy within the studied range was found to have little effect on the combustion pressure or temperature of the gaseous epoxypropane/air mixtures. However, it had a significant impact on the upper flammability limit. The variation trends in the combustion pressure and temperature and the rate of pressure rise of gaseous epoxypropane/air mixtures with volume fraction appear similar. When the volume fraction of gaseous epoxypropane lies in the range 3.5–7%, the combustion pressure and temperature and the rate of pressure rise of gaseous epoxypropane/air mixtures increase with the volume fraction, while in the range 7–30% these parameters decrease with the volume fraction. The rate of temperature rise of gaseous epoxypropane/air mixtures shows a slightly different trend. It reaches the highest value at a volume fraction of 10%. The combustion pressure and temperature of gaseous epoxypropane/air mixtures reach their highest values near the lower flammability limit, which is in marked contrast to previous results on gaseous nitromethane/air mixtures.

Upper Flammability Limits of Hydrogen and Light Hydrocarbons in Air at Subatmospheric Pressures

The upper flammability limits (UFL) of hydrogen–air, methane–air, ethane–air, n-butane–air, and ethylene–air were determined experimentally at room temperature (20 °C) and initial pressure of 1.0, 0.7, 0.5, 0.3, 0.1, and 0.05 atm. Experiments were conducted in a closed cylindrical stainless steel vessel (i.d. 10.22 cm, length 100 cm) with upward flame propagation. The UFL of hydrogen was observed to be inversely proportional to the initial pressure in the range from 1.0 to 0.3 atm and proportional to the initial pressure from 0.3 to 0.05 atm. In contrast, the UFLs of the lower alkanes and ethylene decreased with the initial pressure. The average flame propagation velocities at UFL concentrations of hydrogen, methane, ethane, n-butane, and ethylene in air at reduced pressures were also examined. It was found that the flame propagation velocity of hydrogen was larger than those of the hydrocarbons, increased when the initial pressure decreased from 1.0 to 0.3 atm, and then decreased with further decrease of...

Estimating the flammability of vapours above refinery wastewater laden with hydrocarbon mixtures

In this study, the likelihood of fire hazards attributed to the vapourisation of hydrocarbon components derived from refinery wastewater drainage systems was assessed. Liquid samples containing mixtures of hydrocarbon products and water were collected from a refinery drainage system and subjected to a distillation process to separate oil and water. The oil–liquid phase was analysed using gas chromatography–mass spectrometry (GC–MS) to examine the composition of the sample. Hydrocarbon compounds ranging from C9 to C16 were detected. Mole fractions of 28 selected components in the liquid phase were obtained from the GC–FID data and used to calculate mole fractions of components in the gas phase via modified Raoult’s law. Lower Flammability Limits (LFLs) and Upper Flammability Limits (UFLs) for individual components were calculated using a stoichiometric concentration method, while the LFL and UFL values for the mixture (LFL mix and UFLmix) were calculated using the Le Chatelier equation. The LFL mix and UFLmix values were used to construct a flammability diagram and subsequently used to determine the flammability of the mixture. The findings of this study may assist in minimising fire hazards associated with the presence of hydrocarbon vapours derived from refinery wastewater streams.

An Experimental Investigation of Flammability Limits and Autoignition Temperatures of Petrofuels and Biofuels in a Tubular Burner

A set of experiments to determine the flammability limits of pure and binary blends of fuels was carried out in a combustion tube operated at atmospheric pressure. The lowest and highest volumetric fraction of vapor fuels that allowed flame propagation when the air/fuel mixture was submitted to a high voltage spark with energy higher than the minimum ignition energy defined the lower and upper flammability limit, respectively. The obtained experimental results for liquefied petroleum gas (LPG), acetone, ethylic alcohol, gasoline, diesel, and bioethanol are in close agreement with results reported in the literature. Diagrams of flammability limits of mixtures of gasoline/bioethanol and diesel/biodiesel were found within the range of composition of standard commercial blends of these fuels available for market in Brazil. The autoignition temperature and the influence of temperature on flammability limits of pure ethanol and a commercial mixture with 4% of biodiesel and 96% of diesel were experimentally determined.

Effect of low temperature on the flammability limits of methane/nitrogen mixtures

An experimental setup was established to measure the flammability limits of flammable gases at low temperatures. The flammability limits of methane/nitrogen mixtures in air were measured at atmospheric pressure and wide temperature range from 150 to 300 K. The estimated uncertainty of the experimental values was 0.2 vol%. The results reveal that as initial temperature decreases, the upper flammability limits (UFL) decrease while the lower flammability limits (LFL) increase. Therefore, the critical flammability ratio (CFR) decreases as the initial temperature decreases. In addition, the variation in the UFL is more sensitive than that in the LFL as the molar ratio of nitrogen to methane increases. In addition, an extended Le Chatelier’s formula with higher-order and temperature terms was proposed to correlate and predict the flammability limits at each temperature. The calculated values conformed to the experimental results for both the LFL and the UFL.

Analysis of flammability limits for the liquefaction process of oxygen-bearing coal-bed methane

A novel liquefaction and distillation process has been proposed and designed for the typical oxygen-bearing coal-bed methane (CBM), in which the impurities of the oxygen and nitrogen components are removed in the distillation column. The flammability limit theory combining with HYSYS simulation results are employed to analyze and calculate the flammability limits and the results indicate that no flammability hazard exists in the stages of compression, liquefaction and throttling. However, flammability hazard exists at the top the distillation column because the methane mole fraction decreases to the value below the upper flammability limit (UFL). The safety measures of initially removing oxygen content from the feed gas combining with the control of the bottom flowrate (flowrate of the liquid product at column bottom) are proposed to ensure the operation safety of the liquefaction process. The results reveal that the operation safety of the whole process can be guaranteed, together with high methane recovery rate and high purity of the liquid product. The applicability of the liquefaction process has also been analyzed in this paper. The simulation results can offer references for the separation of oxygen from CBM, the analysis of flammability limits and the safety measures for the whole process.

On the temperature dependence of flammability limits of gases

Flammability limits of several combustible gases were measured at temperatures from 5 to 100°C in a 12-l spherical flask basically following ASHRAE method. The measurements were done for methane, propane, isobutane, ethylene, propylene, dimethyl ether, methyl formate, 1,1-difluoroethane, ammonia, and carbon monoxide. As the temperature rises, the lower flammability limits are gradually shifted down and the upper limits are shifted up. Both the limits shift almost linearly to temperature within the range examined. The linear temperature dependence of the lower flammability limits is explained well using a limiting flame temperature concept at the lower concentration limit (LFL) – ‘White's rule’. The geometric mean of the flammability limits has been found to be relatively constant for many compounds over the temperature range studied (5–100°C). Based on this fact, the temperature dependence of the upper flammability limit (UFL) can be predicted reasonably using the temperature coefficient calculated for the LFL. However, some compounds such as ethylene and dimethyl ether, in particular, have a more complex temperature dependence.

Chemical Structure-Based Model for Estimation of the Upper Flammability Limit of Pure Compounds

In the present work, a new molecular-based model is presented for estimation of the upper flammability limit (UFL) of pure compounds. The parameters of the model are the number of occurrences of a new collection of 113 functional groups. On the basis of these 113 functional groups, a feed-forward neural network is presented to estimate the UFL of pure compounds. The squared correlation coefficient, absolute percent error, standard deviation, and root-mean-square error of the model over the 867 pure compounds used for the development of the model are 0.9469, 7.07%, 0.883, 0.882, respectively. Therefore, the model is accurate and can be used to predict the UFL for a wide range of pure compounds.

Nitrogen and carbon dioxide dilution effect on upper flammability limits for organic compound containing carbon, hydrogen and oxygen atoms

In this work semi-empirical models for predicating upper flammability limit (UFL) for flammable gases diluted with inert nitrogen and with carbon dioxide are developed, respectively. Experimental data for 20 flammable gases are collected and used to determine the unspecified parameter in our earlier theoretical model. It is found that if we divide these 20 organic compounds into three groups—C n H m , C n H m O, and C n H m O 2, the coefficients of determination ( R 2) for these three categories for the case of inert nitrogen will be 0.9969, 0.9940 and 0.9971, respectively; and for the case of inert dioxide will be 0.9922, 0.9811 and 0.9877, respectively. If all the compounds are unified in a same model, it is found the R 2 values are 0.9954 and 0.9897 for inert nitrogen and inert carbon dioxide, respectively. Experimental data of cyclopropane, ammonia, benzene, acetylene, carbon disulfide, carbon monoxide, diethyl ether, and hydrogen, which are not included in the original semi-empirical model, is used to validate the predictive ability for the proposed unified model, and it is found that the average predictive errors for each component are lower than 10% for inert gases of both nitrogen and carbon dioxide.

Prediction of the Upper Flammability Limits of Organic Compounds from Molecular Structures

A quantitative structure−property relationship (QSPR) study is performed to develop mathematical models for prediction of the upper flammability limits (UFL) of organic compounds from their molecular structures. The structural features of the compounds are numerically represented by various kinds of calculated molecular descriptors such as topological, charge, and geometric descriptors. The genetic algorithm combined with multiple linear regression (GA-MLR) is used to select an optimal subset of descriptors that have significant contribution to the overall UFL property from the large pool of calculated descriptors. The best resulted model is a four-variable multilinear model with a defined applicability range. The average absolute error and root-mean-square error obtained for the external test set are 1.75 vol % and 2.77, respectively. The proposed model can be used to predict the UFL of organic compounds with only four preselected theoretical descriptors which can be directly calculated from molecular structure alone.

Process route index (PRI) to assess level of explosiveness for inherent safety quantification

The fundamental concept of quantification of inherent safety level is based on the ranking of chemical process routes. Three pioneering inherent safety indices (i.e. PIIS, ISI and i-Safe) are based on this concept and have treated the chemicals in the process system as individual components and not as mixture. These indices lack of ability to reflect the contribution of individual components in the mixture which may affect the quantification of the inherent safety level for process route selection. A new index known as process route index (PRI) is proposed for inherent safety quantification to address the highlighted issues. Level of explosiveness is used for quantification of the inherent safety level for process route selection to illustrate the importance of the individual contribution of the components in the mixture. PRI is developed based on fundamental process parameters that influence the explosion of chemical processes. One of the important criteria for the quantification of the level of explosiveness is to determine the combustibility of the chemicals based on the difference between lower flammability limit (LFL) and upper flammability limit (UFL). The current available indices do not consider the influence of process temperature and pressure on the UFL and LFL. The PRI is developed with the function of temperature and pressure for the quantification of the explosiveness level. The PRI is benchmarked against the published results of the other indices using HYSYS simulation case studies to produce methyl methacrylate acid (MMA) by various process routes. Benchmarking results conclude that PRI is in close agreement with other inherent safety indices and also able to make differentiation between inherent safety level of process routes which were previously indistinguishable.

Nitrogen dilution effect on the flammability limits for hydrocarbons

Theoretical models to predict the upper/lower flammability limits of hydrocarbons diluted with inert nitrogen gas are proposed in this study. It is found that there are linear relations between the reciprocal of the upper/lower flammability limits and the reciprocal of the molar fraction of hydrocarbon in the hydrocarbon/inert nitrogen mixture. Such linearity is examined by experimental data reported in the literature, which include the cases of methane, propane, ethylene and propylene. The R-squared values ( R 2) of the regression lines of the cases explored are all greater than 0.989 for upper flammability limit (UFL). The theoretical slope of the predictive line for lower flammability limit (LFL) is found to be very close to zero for all explored cases; and this result successfully explains the experimental fact that adding inert nitrogen to a flammable material has very limited effect on LFL. Because limit oxygen concentration (LOC) could be taken as the intersectional point of the UFL curve and LFL curve, a LOC-based method is proposed to predict the slope of the UFL curve when experimental data of UFL are not available. This LOC-based method predicts the UFL with average error ranging from 2.17% to 5.84% and maximum error ranging from 8.58% to 12.18% for the cases explored. The predictive models for inert gas of nitrogen are also extended to the case of inert gas other than nitrogen. Through the extended models, it was found that the inert ability of an inert gas depends on its mean molar heat capacity at the adiabatic flame temperature. Theoretical calculation shows that the inert abilities of carbon dioxide, steam, nitrogen and helium are in the following order: carbon dioxide > steam > nitrogen > helium; and this sequence conforms to the existing experimental data reported in the literature.

Explosion Safety Aspects of Shock Wave-Induced Condensation in Fuel-Rich Gaseous Mixtures

The possibility of fuel vapor condensation in shock waves in fuel-rich cyclohexane-oxygen gaseous mixtures is shown. The influence of fuel vapor condensation on shock waves parameters and explosion behavior of corresponding mixtures is analyzed. It is shown, that condensation process led to the increase of pressure and temperature and can essentially change the chemical composition of the gas. For example, the molar fraction of the oxidizer can increase in a few times. This possibility should be taken into account in modeling of shock waves propagation and detonation wave initiation in corresponding gaseous mixtures. For example, the mixtures, which have an initial gas composition above the upper flammability limit, can, nevertheless, explode. It can happen, if the chemical composition of the gas starts to be inside the flammability range owing to fuel vapor condensation. The rate of the condensation process is estimated. According to estimations, this process can be fast enough to significantly change the chemical composition of the gas and shift it into the flammable range during the compression phase of blast waves, generated by explosions of typical fuel-vapor clouds.

Calculation of the upper flammability limit of methane/hydrogen/air mixtures at elevated pressures and temperatures

The results of three different numerical methods to calculate flammability limits—namely (1) the calculation of planar flames with the inclusion of a (radiation) heat loss term in the energy conservation equation, and the application of (2) a limiting burning velocity and of (3) a limiting flame temperature—are compared with experimental data on the upper flammability limit (UFL) of methane/hydrogen/air mixtures with hydrogen fuel molar fractions of 20% and 40%, at initial pressures up to 10bar and initial temperatures up to 200°C. The application of a limiting burning velocity is found to predict the pressure dependence of the UFL well, while the application of a limiting flame temperature generally is found to slightly underestimate the temperature dependence of the UFL.