Evaluation of lower flammability limits of fuel-air-diluent mixtures using calculated adiabatic flame temperatures

Estimation of the upper flammability limits for alkanes in air at increased pressures

A method is proposed to predict the upper flammability limits for alkanes in air at increased pressures. The upper flammability limits for methane, ethane, propane and n-butane/air mixtures at ambient temperature and initial pressure of 0.3 MPa–2.0 MPa are identified through the adiabatic flame temperature calculation model. The association of calculated adiabatic flame temperature with pressure is presented to determine the upper flammability limit. Research shows the good agreement between the forecast upper flammability limits with pressure dependence and the experimental upper flammability limit values. The average relative error of the estimated upper flammability limits for alkanes in air at high pressures reaches 2.52%.

Suppression of deflagration flame propagation of methane-air in tube by argon gas and explosion-eliminating chamber

To explore the inhibitory effect of argon gas and explosion-eliminating chamber on methane-air deflagration flame propagation in the tube, based on the Φ = 120 mm, L = 5.5 m stainless steel pipeline test system to measure methane-air deflagration flame structure, flame propagation speed, and deflagration pressure. The results show that: 10–30% argon is mixed into the methane-air premixed gas with different equivalent ratios. With the increase in the mixed argon content, the tensile distortion and instability of the flame front increase, and the average value of flame propagation speed decreases by 2.52–60.0%. The first and second deflagration pressure peaks are reduced by about 13.1–62% and 17.7–86.5% respectively. The average value of the methane-air deflagration flame propagation velocity was reduced by 5.7–37.0% with the explosion-eliminating chamber laid at the nozzle. The second and third deflagration pressure peaks are reduced by about 10–30% and 50–90% respectively. The inhibitory effect of argon on the propagation of methane-air flame is considered better than the laying of the explosion-eliminating chamber under the experimental conditions.

Recent Progress in Hydrogen Flammability Prediction for the Safe Energy Systems

Many countries consider hydrogen as a promising energy source to resolve the energy challenges over the global climate change. However, the potential of hydrogen explosions remains a technical issue to embrace hydrogen as an alternate solution since the Hindenburg disaster occurred in 1937. To ascertain safe hydrogen energy systems including production, storage, and transportation, securing the knowledge concerning hydrogen flammability is essential. In this paper, we addressed a comprehensive review of the studies related to predicting hydrogen flammability by dividing them into three types: experimental, numerical, and analytical. While the earlier experimental studies had focused only on measuring limit concentration, recent studies clarified the extinction mechanism of a hydrogen flame. In numerical studies, the continued advances in computer performance enabled even multi-dimensional stretched flame analysis following one-dimensional planar flame analysis. The different extinction mechanisms depending on the Lewis number of each fuel type could be observed by these advanced simulations. Finally, historical attempts to predict the limit concentration by analytical modeling of flammability characteristics were discussed. Developing an accurate model to predict the flammability limit of various hydrogen mixtures is our remaining issue.

Effect of gas disturbance on combustion characteristics of flammable refrigerants near LFLs

Refrigerants with low global warming potential (GWP), will play an important role in the research of alternative refrigerants and the development of new refrigerants, but most of them are flammable. And the actual environment of the refrigeration system varies with different factors, especially gas disturbance. In this paper, the combustion characteristics of four refrigerants, i.e., isobutane (R600a), ethylene (R1150), propane (R290), and 2,3,3,3-Tetrafluoroprop-1-ene (R1234yf) under the conventional static and the gas disturbance were studied. Firstly, the flame characteristics and lower flammability limits (LFLs) of the refrigerants were investigated. Then the special combustion phenomena were observed and analyzed in the presence of the gas disturbance, and an improved method for better defining the LFLs of the flammable gases under the disturbance was proposed. Finally, the experimental results show that the LFLs of the three natural refrigerants under specific low disturbance are lower than that in normal conditions, but except for R1234yf. It was found that the specific disturbance may enhance the combustion intensity by comparing the burning time of visible flame under the two conditions. The results have guiding significance for the safe application of flammable refrigerants.

A new model based on adiabatic flame temperature for evaluation of the upper flammable limit of alkane-air-CO2 mixtures

For security issue of alkane used in Organic Rankine Cycle, a new model to evaluate the upper flammability limits for mixtures of alkanes, carbon dioxide and air has been proposed in present study. The linear relationship was found at upper flammability limits between molar fraction of diluent in alkane-CO₂ mixture and calculated adiabatic flame temperature. The prediction ability of the variable calculated adiabatic flame temperature model that incorporated the linear relationship above is greatly better than the models that adopted the fixed calculated adiabatic flame temperature at upper flammability limit. The average relative differences between results predicted by the new model and observed values are less than 3.51% for upper flammability limit evaluation. In order to enhance persuasion of the new model, the observed values of n-butane-CO₂ and isopentane-CO₂ mixtures measured in this study were used to confirm the validity of the new model. The predicted results indicated that the new model possesses the capacity of practical application and can adequately provide safe non-flammable ranges for alkanes diluted with carbon dioxide.

The Evolution of Process Safety: Current Status and Future Direction

The advent of the industrial revolution in the nineteenth century increased the volume and variety of manufactured goods and enriched the quality of life for society as a whole. However, industrialization was also accompanied by new manufacturing and complex processes that brought about the use of hazardous chemicals and difficult-to-control operating conditions. Moreover, human-process-equipment interaction plus on-the-job learning resulted in further undesirable outcomes and associated consequences. These problems gave rise to many catastrophic process safety incidents that resulted in thousands of fatalities and injuries, losses of property, and environmental damages. These events led eventually to the necessity for a gradual development of a new multidisciplinary field, referred to as process safety. From its inception in the early 1970s to the current state of the art, process safety has come to represent a wide array of issues, including safety culture, process safety management systems, process safety engineering, loss prevention, risk assessment, risk management, and inherently safer technology. Governments and academic/research organizations have kept pace with regulatory programs and research initiatives, respectively. Understanding how major incidents impact regulations and contribute to industrial and academic technology development provides a firm foundation to address new challenges, and to continue applying science and engineering to develop and implement programs to keep hazardous materials within containment. Here the most significant incidents in terms of their impact on regulations and the overall development of the field of process safety are described.

Estimation of upper flammability limits of C-H compounds in air at standard atmospheric pressure and evaluation of temperature dependence

This study focuses on estimating the upper flammability limits of C-H compounds. A method was developed to determine the upper flammability limits in air at standard atmospheric pressure for the following cases: (a) estimation of the UFLs of pure C-H compounds at standard ambient temperature (25°C); (b) estimation of the UFLs of binary mixtures of C-H compounds at standard ambient temperature (25°C); (c) estimation of the UFLs of C-H compounds at different initial temperatures. The method was accurate in all cases. In case (a), for a total set of 115 compounds, the absolute average relative error was 7.27% and a squared correlation coefficient of 0.9248 was obtained. In case (b), the average absolute relative error was 5.55%; in case (c) it was 2.19%.

Flame temperature theory-based model for evaluation of the flammable zones of hydrocarbon-air-CO2 mixtures

Theoretical models to evaluate the flammable zones of mixtures made up of hydrocarbon, carbon dioxide and air have been proposed in present study. A three-step reaction hypothesis for hydrocarbon combustion was introduced for predicting the upper flammability limit. The method to predict the parameters at fuel inertization point was put forward as well. Validation of these models has been conducted on existing experimental data reported in the literature, including the cases of methane, propane, propylene and isobutane, and an acceptable precision has been achieved. The average relative differences between the estimated results and experimental ones, except for the results at fuel inertization point, are less than 8.8% and 3.3% for upper and lower flammability limit, respectively. This work also indicated that these models possess practical application capacity and can provide safe prediction limits for nonflammable ranges of hydrocarbon diluted with carbon dioxide.

Estimation of lower flammability limits of C-H compounds in air at atmospheric pressure, evaluation of temperature dependence and diluent effect

Estimation of the lower flammability limits of C-H compounds at 25 °C and 1 atm; at moderate temperatures and in presence of diluent was the objective of this study. A set of 120 C-H compounds was divided into a correlation set and a prediction set of 60 compounds each. The absolute average relative error for the total set was 7.89%; for the correlation set, it was 6.09%; and for the prediction set it was 9.68%. However, it was shown that by considering different sources of experimental data the values were reduced to 6.5% for the prediction set and to 6.29% for the total set. The method showed consistency with Le Chatelier's law for binary mixtures of C-H compounds. When tested for a temperature range from 5 °C to 100 °C, the absolute average relative errors were 2.41% for methane; 4.78% for propane; 0.29% for iso-butane and 3.86% for propylene. When nitrogen was added, the absolute average relative errors were 2.48% for methane; 5.13% for propane; 0.11% for iso-butane and 0.15% for propylene. When carbon dioxide was added, the absolute relative errors were 1.80% for methane; 5.38% for propane; 0.86% for iso-butane and 1.06% for propylene.

Flammability limits: a review with emphasis on ethanol for aeronautical applications and description of the experimental procedure

The lower and upper flammability limits of a fuel are key tools for predicting fire, assessing the possibility of explosion, and designing protection systems. Knowledge about the risks involved with the explosion of both gaseous and vaporized liquid fuel mixtures with air is very important to guarantee safety in industrial, domestic, and aeronautical applications. Currently, most countries use various standard experimental tests, which lead to different experimental values for these limits. A comprehensive literature review of the flammability limits of combustible mixtures is developed here in order to organize the theoretical and practical knowledge of the subject. The main focus of this paper is the review of the flammability data of ethanol-air mixtures available in the literature. In addition, the description of methodology for experiments to find the upper and lower limits of flammability of ethanol for aeronautical applications is discussed. A heated spherical 20L vessel was used. The mixtures were ignited with electrode rods placed in the center of the vessel, and the spark gap was 6.4mm. LFL and the UFL were determined for ethanol (hydrated ethanol 96% °INPM) as functions of temperature for atmospheric pressure to compare results with data published in the scientific literature.

Estimation of the lower flammability limit of organic compounds as a function of temperature

A new method of estimating the lower flammability limit (LFL) of general organic compounds is presented. The LFL is predicted at 298 K for gases and the lower temperature limit for solids and liquids from structural contributions and the ideal gas heat of formation of the fuel. The average absolute deviation from more than 500 experimental data points is 10.7%. In a previous study, the widely used modified Burgess-Wheeler law was shown to underestimate the effect of temperature on the lower flammability limit when determined in a large-diameter vessel. An improved version of the modified Burgess-Wheeler law is presented that represents the temperature dependence of LFL data determined in large-diameter vessels more accurately. When the LFL is estimated at increased temperatures using a combination of this model and the proposed structural-contribution method, an average absolute deviation of 3.3% is returned when compared with 65 data points for 17 organic compounds determined in an ASHRAE-style apparatus.

Calculated flame temperature (CFT) modeling of fuel mixture lower flammability limits

Heat loss can affect experimental flammability limits, and it becomes indispensable to quantify flammability limits when apparatus quenching effect becomes significant. In this research, the lower flammability limits of binary hydrocarbon mixtures are predicted using calculated flame temperature (CFT) modeling, which is based on the principle of energy conservation. Specifically, the hydrocarbon mixture lower flammability limit is quantitatively correlated to its final flame temperature at non-adiabatic conditions. The modeling predictions are compared with experimental observations to verify the validity of CFT modeling, and the minor deviations between them indicated that CFT modeling can represent experimental measurements very well. Moreover, the CFT modeling results and Le Chatelier's Law predictions are also compared, and the agreement between them indicates that CFT modeling provides a theoretical justification for the Le Chatelier's Law.

Carbon dioxide dilution effect on flammability limits for hydrocarbons

Theoretical models to predict the upper/lower flammability limits of a mixture composed of hydrocarbon and inert carbon dioxide are proposed in this study. It is found theoretically 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 gas mixture. These theoretical linear relations are examined by existing experimental results reported in the literature, which include the cases of methane, propane, ethylene, and propylene. The coefficients of determination (R(2)) of the regression lines are found to be larger than 0.959 for all aforementioned cases. Thus, the proposed models are highly supported by existing experimental results. A preliminary study also shows the conclusions in present work have the possibility to extend to non-hydrocarbon flammable materials or to inert gas other than carbon dioxide. It is coincident that the theoretical model for the lower flammability limit (LFL) in present work is the same as the empirical model conjectured by Kondo et al.

Flammability limits of isobutane and its mixtures with various gases

Flammability limits of isobutane and five kinds of binary mixtures of isobutane were measured by the ASHRAE method. Propane, nitrogen, carbon dioxide, chloroform, and HFC-125 (1,1,1,2,2-pentafluoroethane) were used as the counter part gases in the mixtures. The observed data were analyzed using the equations based on Le Chatelier's formula. The flammability limits of mixtures with propane were well explained by the original Le Chatelier's formula. The flammability limits of mixtures with nitrogen and the ones with carbon dioxide were adequately analyzed by the extended Le Chatelier's formula. It was found that the extended Le Chatelier's formula is also applicable to the flammability limits of mixtures with chloroform and HFC-125.