Flammable Region Research Articles

Formaldehyde process intensification through gas heat capacity

The partial oxidation of methanol to formaldehyde is highly exothermic and reactors are operated with a hot spot in the range of 100 ∘ C above the desired operating temperature, which can affect selectivity considerably and can also reduce the catalyst life. Nitrogen-diluted air is a common feed gas but by replacing the nitrogen for a gas with a substantially higher heat capacity, the heat evolution may be more efficiently controlled. In this work, we have studied the potential of propane as a substitute for nitrogen. According to a simple reactor model using current commercial conditions as a “base” case, substituting propane for nitrogen could increase productivity by as much as 50%. Furthermore, propane would be safer to use because the gas mixture would be further away from the flammable region versus current operation.

Prediction of flammability speciation for the lower alkanes, carboxylic acids, and esters

AbstractA Gibbs energy minimization procedure is used to predict the flammability envelopes of alkanes, carboxylic acids, and acetates. In addition to providing the calculated adiabatic flame temperature (CAFT), the product profiles reveal regions of incomplete combustion products and the onset of methane formation above 0.0001 mole fraction. Temperatures at the predicted onset of methane production appear to be closely related to the temperature at the upper flammability limit (UFL). Although a fixed CAFT value could be related to the lower flammability limit (LFL), it was found that this was not the case with the UFL and only for acetic acid could a single CAFT value of 1200K be used to conservatively predict the flammable region. Rather, in general, a single CAFT value could not conservatively predict the upper flammable region. The predictions also reveal local maxima and minima in the concentrations of reaction products. These maps of incomplete combustion products for the flammability region predict that incomplete combustion products are produced at fuel/oxygen ratios significantly below the fuel/oxygen stoichiometric ratio. This region decreases as the limiting oxygen concentration is approached. © 2006 American Institute of Chemical Engineers Process Saf Prog, 2006

The effect of reformate gas enrichment on extinction limits and NO X formation in counterflow CH 4/air premixed flames

The reformate gas enriched counterflow lean premixed CH 4/air flames were studied by numerical simulation in this paper. The reformate gas was assumed to be the product of partial oxidation of methane by air, and it consists of H 2, CO and N 2. Detailed chemistry and complex thermal and transport properties were employed. The results indicate that the addition of the reformate gas enlarges the flammable region, and extends the lean flammability limit of counterflow CH 4/air premixed combustion. When the reformate gas is added, the formation of NO is reduced in a near-stoichiometric flame, and increased in an ultra-lean flame at a constant equivalence ratio. The more significant advantage of the reformate gas enriched lean premixed combustion is that it greatly reduces the formation of NO by allowing a combustor to operate at leaner condition without any effect on flammable range. Further, the addition of the reformate gas decreases the formation of NO 2 and N 2O at a constant equivalence ratio.

Measurements of ignition probability in turbulent non-premixed counterflow flames

Spark ignition of turbulent counterflow non-premixed flames has been examined in terms of the ignition probability P ign and evolving flame structure. It was found that in the case of successful ignition, the flame spread as an edge flame with a large scatter in its radial position between individual realizations at the same time from the spark. P ign decreased with bulk velocity, which suggests that the local strain rate can be detrimental to ignition so that, even with the strongest spark tested, ignition could not be achieved at a bulk velocity about 90% of the extinction velocity. P ign was greater than zero even in regions well into the fuel and air streams where the mixture fraction fluctuations were virtually zero, giving zero probability of finding flammable mixture at the spark location. This observation was attributed to non-local effects, for example the convection of the spark heat to the flammable region. The data can assist the validation of advanced combustion models.

Experimental study of the spill and vaporization of a volatile liquid

Pool and vapor cloud characteristics of an acetone spill issuing from the downstream wall of a flow obstruction oriented perpendicular to a uniform flow were investigated experimentally. Data indicate that the spill event was largely governed by the temperature of the surface in relation to the boiling point of the spilled liquid. The free stream velocity (ranging from 0.75 to 3.0 m/s) also impacted the spreading of the spill. Planar laser-induced fluorescence (PLIF) was used to measure acetone vapor concentrations during the transient pool spreading and vaporization in a window 60 cm long by 50 cm high and located downstream of the 16 cm high obstruction. The recirculation region induced by the flow obstruction caused upstream transport of the acetone vapor along the spill surface, after which it was convected vertically along the obstruction wall before being entrained into the flow and convected downstream. The recirculating flow caused regions of vapor within the flammability limits to be localized near the flow obstruction. These regions moved into and out of the measurement plane by large three-dimensional flow structures. The flammable region of the evolved vapor cloud was observed to grow well past the downstream edge of the measurement domain. With decreasing wind speeds, both the mass of acetone vapor within the flammability limits and the total spill event time increased significantly. The data presented herein provides a basis for validating future spill models of hazardous chemical releases, where complex turbulent flow modeling must be coupled with spill spreading and vaporization dynamics.

Approximation of flammability region for natural gas–air–diluent mixture

The growing implementation of exhaust gas recirculation (EGR) in reducing NO x emissions of engine is of paramount motivation to perform a fundamental research on the flammability characteristics of fuel–air–diluent mixtures. In this work, the influences of EGR on the flammability region of natural gas–air–diluent flames were experimentally studied in a constant volume bomb. An assumption of critical burning velocity at flammability limit is proposed to approximately determine the flammability region of these mixtures. Based on this assumption, an estimation of the flammability map for natural gas–air–diluent mixtures was obtained by using the empirical formula of burning velocity data. The flammability regions of natural gas–air mixtures with EGR are plotted versus the EGR rate. From the comparison of estimated results and experimental measurements, it is suggested that the accuracy of prediction is largely dependent upon the formula of burning velocity used. Meanwhile, the influence of pressure on the critical burning velocity at flammability limit is also investigated. On the basis of the pressure dependence criterion, the estimation was performed for the circumstance of high temperature and pressure, and the prediction results still agree well with those of experiments.

Pulsating Mode of Flame Propagation in Two-Dimensional Channels

Flame propagation in channels and cracks is a problem of considerable interest with applications in many practical combustion devices, in fire hazard scenarios, and in the emerging micropropulsion technologies. Understanding the dynamics and stability characteristics of flame propagation in channels is, therefore, important both for fundamental research as well as for practical applications. In this work, we examine the propagation of a premixed flame front in a two-dimensional channel in the presence of a Poiseuille flow. Our primary objective is to determine within the flammable regions the structure of the flame front and the conditions that result in steady propagation and those leading to a pulsating mode of propagation. Special attention is given to the difference between propagation in narrow and wide channels, heat losses to the channel’s walls, and an imposed flow that either supports or opposes the propagation. In general, flame oscillations are found to occur in mixtures for which the effective Lewis number is sufficiently large. They are more likely to occur in narrow or wide channels and particularly at near-extinction conditions where the critical Lewis number is reduced to physically accessible values.

The effect of hydrogen addition on flammability limit and NO x emission in ultra-lean counterflow CH 4/air premixed flames

The effect of hydrogen addition to ultra lean counterflow CH 4/air premixed flames on the extinction limits and the characteristics of NO x emission was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. The results show that the addition of hydrogen can significantly enlarge the flammable region and extend the flammability limit to lower equivalence ratios. If the equivalence ratio is kept constant, the addition of hydrogen increases the emission of NO in a flame due to the enhancement in the rate of the NNH or N 2O intermediate NO formation routes. The addition of hydrogen causes a monotonic decrease in the formation of NO 2 and N 2O, except flames near the extinction limits, where the emission of NO 2 and N 2O first increases, and then decreases with the increase in the fraction of hydrogen. Overall, hydrogen enrichment technology allows stable combustion under ultra lean conditions, resulting in significant CO 2 and NO emission reduction.

04/03052 Control of anthropogenic CH 4 and N 2O emissionsfrom several industrial sources and from daily human life : Inamori, Y. et al. Journal of Chemical Engineering of Japan, 2003, 36, (4), 449–457

The extinction, and flame bifurcations of radiative CH4/air and C3H5/air free propagating flames and counterflow flames are investigated numerically with detailed chemistry as well as the optically thin and spectral emission-absorption statistical narrow band models. For counterflow CH4/air flame, a G-shaped extinction curve is obtained. It is shown that flame bifurcations and extension of the flammability limit due to the interaction of radiation and stretch at subunity Lewis number exist regardless the radiation model used. Furthermore, it is shown that even for small-scale flame, radiation reabsorption greatly extends the flammable regions of the weak flame (WF) and the near-stagnation flame (NSF). For counterflow C3H8/air flame, a K-shaped extinction curve is obtained. It is concluded that multiple flame bifurcations are the physically intrinsic phenomena of the pratical stretched flames. It is shown that the extrapolation of the stretched extinction limit to zero stretch rate is possible only when the extinction curve is K-shaped. Transition from the G-shaped extinction curve to the K-shaped one, as a result of the competition between radiation heat loss and the Lewis number effect, is examined by replacing the nitrogen gas in CH4/air mixture with helium. The critical Lewis number for the transition of the extinction curve is obtained. Results of the present study show good agreement with theory and experiment.

Characteristics of Microscale Combustion in a Narrow Heated Channel

The characteristics of microscale combustion were investigated by using a microchannel heated by an external source. The inner diameter of the channel was 2 mm, which was slightly smaller than the quenching distance of the stoichiometric methane–air mixture under normal conditions. The effects of the equivalence ratio and the averaged flow velocity on the characteristics of combustion in the microchannel were examined. At a channel‐wall temperature of ≈ 1000°C, flames could be stabilized at equivalence ratios of 0.05–1.9 and mixture velocities up to 150 cm/sec in a U‐shaped quartz‐glass channel. At moderate equivalence ratios and lower velocity conditions within the flammability region, oscillatory combustion was observed. A simple analytical model predicting flame oscillations on the basis of the linear analysis of steady solutions is proposed.

Interactions for flames in a coaxial flow with a stagnation point

The possible burning structures existing in two co-flowing combustible mixtures with different compositions, and their implications to the field of turbulent combustion are examined in this study. A coaxial burner with a quartz plate was used to experimentally investigate the flames of methane/air and propane/air mixtures propagating in a coaxial flow impinging onto a stagnation surface. The possible burning structures were observed to be: (1) a single-flame (a lean or rich premixed flame); (2) a double-flame (two lean or rich premixed flames, or a rich premixed flame and a diffusion flame); and (3) a triple-flame (a rich premixed flame, a diffusion flame and a lean premixed flame). An inner (or outer) mixture, far beyond the flammability limit, can still burn if a stronger outer (or inner) flame supports it. The extinction limit of the top part of the inner hat-shaped premixed flame is nearly independent of the burning intensity of the outer flame. It was found that the inner flame has a wider flammable region than the outer flame, and that the latter has a narrower flashback region than the former. Both propane and methane flames may exhibit flame-front instability, although the former displays much more clearly than the latter. Cellular and polyhedral instabilities can exist individually or appear simultaneously in the inner flame. However, only polyhedral (stripped-pattern) instability was observed in the outer flame. Finally, the experiments were analyzed theoretically using a simple geometrical model incorporated with the numerical simulations. The predicted shapes and locations of the flames are in good agreement with the experimental observations qualitatively.

Dynamics and flammability limit of stretched premixed flames stabilized by a hot wall

Dynamics and flammability limit of stretched, ultralean methane/air premixed flames stabilized by a hot wall were investigated theoretically and numerically using the large activation energy asymptotic approach and the detailed chemistry approach, respectively. An analytical expression describing the flame temperature and location was obtained. Several new findings on the flame regimes, bifurcation, and flammability limit are demonstrated. The analytical results showed that the appearance of hot wall results in new flame bifurcations. Except for the high-temperature flame, there exists a hot-wall-stabilized low-temperature flame which dramatically extends the flammable region of stretched flame to low stretch. Sublimit flame can be stabilized by a hot wall and has a non-zero flame speed even with a wall temperature less than its adiabatic flame temperature. Furthermore, the results also showed that the effects of radiation heat loss and the Lewis number effect are suppressed by the hot wall, and that stretched flames at a Lewis number below unity can extinguish at a finite distance from the wall. Numerical calculations with detailed chemistry well reproduced the predictions of the analytical results. In addition, numerical results showed that except for the low-temperature flame on the low-stretch-rate side, there exists another low-temperature flame regime on the high-stretch-rate side, which dramatically extends the flammable region to the high stretch rate and yields multiple flame extinctions. Theoretical analysis qualitatively agreed well with the numerical calculations, but failed in the prediction of the low-temperature flame at a high stretch rate. The present results provided useful guidance for the application of ultralean combustion in new combustion technology.

Effects of radiative heat loss on the extinction of counterflow premixed H2–air flames

Radiation heat loss has an important impact on near-limit flames. It has been shown that radiation heat loss can make stretched CH4–air flames extinguish at a lower stretch rate. Numerical calculations of counterflow premixed H2–air flames were conducted using an accurate description of the chemical kinetics and transport properties. Radiation heat loss was considered. The results show that in addition to the stretch extinction limit, radiation heat loss also allows the lower and middle equivalence ratio counterflow premixed H2–air flames to extinguish at a lower stretch rate. For middle equivalence ratio counterflow H2–air flames, the closed temperature profiles become distorted O–shaped curves due to the lower Lewis number, being different from those of CH4–air flames. For higher equivalence ratio counterflow premixed H2–air flames, there are two stable flame branches—a normal flame branch and a weak flame branch. When the equivalence ratio is greater than a critical value, the closed temperature profile curve of every equivalence ratio flame opens and the normal flame curve can be extended to zero stretch rate. The calculation of the concentration limit of one-dimensional planar premixed H2–air flames was also conducted. The results show that the critical equivalence ratio corresponds to the concentration limit of the one-dimensional planar premixed H2–air flame. The extension of the flammable region due to the stretch is amplified for the counterflow H2–air flame because of its much lower Lewis number than that of the counterflow CH4–air flame.

Reaction zone structure in flameless combustion

We present a study of the combustion limit and reaction zone structure of non-premixed counterflow flames of highly preheated air and methane diluted with nitrogen. First, we measured the flammable region experimentally in the range of air temperatures up to 800 K. In the case of flame stretch rates smaller than 20 s −1 , experiments were conducted under microgravity. All the microgravity tests were conducted at the JAMIC drop tower facility in Hokkaido, Japan. The results show that the extinction limits become broader with increasing air temperature. All the extinction curves are C-shaped and exhibit the radiation extinction branch. That is, the configurations of the extinction curves are the same as those of conventional combustion at air temperatures from 300 K to 800 K. Second, we computed the flammable region and reaction zone structure of these flames by using the conventional one dimensional flame code with detailed chemistry. The flammable regions obtained by experiment agree with the computated ones. This shows that the present computation could also represent the phenomena well for high-temperature air combustion. Based on this, computation was extended to higher air temperatures (up to 1900 K) at which microgravity experimentation is not possible. When the air temperature was higher than 1300 K, extinction limits disappeared. In this temperature range, combustion continues even under extremely fuel-lean conditions such as 1% methane in nitrogen. Reaction zones without any temperature peaks were observed. Methane and oxygen leakage through the flame occurred, and they coexisted there. This is like the Linan's premixed flame regime, in other words, a reaction-time-dominated reaction zone structure. In these regions, NO x emission is very low, and this may lead to the low levels of NO x emission of high-temperature air combustion.

Combined effects of radiation, flame curvature, and stretch on the extinction and bifurcations of cylindrical CH 4/air premixed flame

The combined effects of flame radiation, stretch, and curvature on the extinction and flame bifurcations of the cylindrical premixed CH 4/air flames are numerically investigated with a detailed chemistry. The interaction between radiation heat loss and flame curvature is emphasized. The results show that a mixture below the standard limit can burn in the cylindrical flame configuration by imposing a moderate stretch rate. This flame is quenched either by radiation heat loss at low stretch rate or by incomplete combustion with an excessive stretch. As the fuel concentration increases, it is found that two kinds of flames, normal flame and weak flame, can exist at the same boundary conditions. A G-shaped extinction curve showing the flammable regions of these two flame regimes is obtained. The relation between the flammability limit of the cylindrical flame and the standard limit is discussed. Furthermore, comparisons between the cylindrical flame and the counterflow flame are made. The results show that the interaction of flame curvature and radiation heat loss greatly affects the flame strength and extinction. It is shown that flame curvature extends the radiation extinction limit and accelerates the opening up of the sublimit flame branch. Comparisons between the predicted results with the experimental data show good agreements both in the extinction limit and in the extinction flame diameter.

Application of the flammability diagram for evaluation of fire and explosion hazards of flammable vapors

AbstractThe safest method to prevent fires and explosions of flammable mixtures in the first place. This method requires detailed knowledge of the flammability region as a function of the fuel, oxygen, and nitrogen concentrations. A triangular flammability diagram is the most useful tool to display the flammability region, and to determine if a flammable mixture is present during plant operations.This paper describes how to draw and use a flammability diagram. A procedure to estimate the flammability region using the available and sometimes limited data is discussed. The paper also shows how to use the flammability diagram with plant operations involving inerting and purging, and from bringing vessels into and out of service. A compilation of flammability diagrams for 30 materials, based on previously published data is provided.An automated apparatus for acquiring data for a flammability diagram is described. The apparatus consists of a 20‐L sphere with an automated gas mixing system, a fuse‐wire ignition system, and a high speed pressure measurement and data acquisition system. Data derived from the apparatus includes flammability limits, maximum pressure during combustion, and the maximum pressure rate. The effect of fuse‐wire ignitor dynamics on the results is studied. A flammability diagram for methane drawn from data obtained from the apparatus, is presented.

Flame Bifurcations and Flammable Regions of Radiative Counterflow Premixed Flames with General Lewis Numbers

Flame bifurcations, extinction, and flammable regions of radiative counterflow premixed flames and the planar propagating flames are investigated numerically with general Lewis numbers. New insights and phenomena are presented by examining the interaction of radiation heat loss, Lewis number effect, and stretch. At a low Lewis number, the results show that for equivalence ratio lower than the standard flammability limit, radiation extinction limit and stretch extinction limit exist, respectively, at low and high stretch rates. For a higher equivalence ratio, this flame isola opens up and yields two stable flame branches, a normal flame branch, and a weak flame branch. A G-shaped curve showing the limits and flammable regions of these two flame regimes is obtained. The results show that the inferior limit of counterflow flame is lower than the standard limit. At unity Lewis number, the results show that, for equivalence ratio lower than the standard limit, there is also a flame isola with two distinct extinction limits at low and high stretch rates. For equivalence ratio larger than the standard limit, a new flame branch with flame standing far from the stagnation plane emerges on the low stretch rate side of the flame isola. This flame is related to the standard limit with the decrease of fuel concentration and merges with the flame isola when the fuel concentration is larger than a critical value. The extinction curve is also a G-shaped curve and the inferior limit is lower than the standard limit. At Lewis number larger than a critical value, for equivalence ratio near the standard limit, there is only the weakly stretched flame branch with a single stretch extinction limit. With an increase of fuel concentration, a weak flame isola emerges. As the fuel concentration further increases, this weak flame isola merges with the weakly stretched flame branch yielding three kinds of flame regimes. The resulting extinction curve is a K-shaped curve and the inferior limit is equal to the standard limit. The present results show that flame bifurcations and the existence of weak flames are physically intrinsic phenomena of radiative counterflow premixed flames. It is found that the normal flame and the weak flame can jump to each other at their limits. The obtained G and K-shaped curves show a clear relation between the counterflow flame and the planar propagating flame. The occurrence of G–K transition with the increase of Lewis number also gives a good explanation to the physics of the experimental results.

Mechanism of flame stabilization in turbulent, lifted-jet flames

Particle image velocimetry was used to study the velocity field in the stabilization region of lifted, turbulent CH4-jet flames over a range of Reynolds numbers from 7000 to 19,500. Measured velocities at the flame base are considerably below the turbulent flame speeds derived from previous studies and show a dependence on the Reynolds number. The average velocity at the stabilization point is nearly a factor of five below the premixed laminar burning velocity at the lowest Reynolds number and asymptotes to a value about 20% higher as the Reynolds number is increased. Planar images of OH show that the flame zone structure near the stabilization point is also highly dependent on the Reynolds number. Comparison of the present OH images with previous CH4 Raman imaging results shows that the flame thickness is determined by the width of the flammable region. At a low Reynolds number, the flame is stabilized near the jet exit where the flammable layer is thin, resulting in a thin flame zone. At an increased Reynolds number, the stabilization point is located farther downstream where the flammable region is wider, resulting in a correspondingly wider flame zone. It is proposed that the lower velocities observed at the flame base are related to thinning of the flame zone at low Reynolds, which results in greater curvature of the flame base. The increased flame curvature effectively defocuses the transport of heat and flame radicals to reactants upstream of the propagating flame front, resulting in reduced burning velocities. The implications of these results for mechanisms controlling turbulent flame stabilization, with an emphasis on the applicability of triple flame concepts to turbulent flows, are discussed.

Further examinations on extinction and bifurcations of radiative CH 4/air and C 3H 8/air premixed flames

The extinction, and flame bifurcations of radiative CH4/air and C3H5/air free propagating flames and counterflow flames are investigated numerically with detailed chemistry as well as the optically thin and spectral emission-absorption statistical narrow band models. For counterflow CH4/air flame, a G-shaped extinction curve is obtained. It is shown that flame bifurcations and extension of the flammability limit due to the interaction of radiation and stretch at subunity Lewis number exist regardless the radiation model used. Furthermore, it is shown that even for small-scale flame, radiation reabsorption greatly extends the flammable regions of the weak flame (WF) and the near-stagnation flame (NSF). For counterflow C3H8/air flame, a K-shaped extinction curve is obtained. It is concluded that multiple flame bifurcations are the physically intrinsic phenomena of the pratical stretched flames. It is shown that the extrapolation of the stretched extinction limit to zero stretch rate is possible only when the extinction curve is K-shaped. Transition from the G-shaped extinction curve to the K-shaped one, as a result of the competition between radiation heat loss and the Lewis number effect, is examined by replacing the nitrogen gas in CH4/air mixture with helium. The critical Lewis number for the transition of the extinction curve is obtained. Results of the present study show good agreement with theory and experiment.

Burner diameter and flammability limit measured by tubular flame burner

Flammability limits of combustible mixtures are important for preventing explosion and fire hazards. The authors showed in previous work that a tubular flame burner gives the equivalent flammability limits obtained by the explosion vessel method recently and concluded that the tubular flame burner method is superior to the explosion vessel method in measuring the flammability limits, because it is more reproducible, gives explicit results, and is easier to operate. In this work, to investigate whether the data obtained by the tubular flame burner method are general and comparable with one another, the flammability limits and flame temperatures at extinction points were measured by using three tubular flame burners with different diameters, and the effect of burner diameter on the limit was studied. As a result, if a nominal stretch rate is kept constant, the flammability limits and the flame temperatures were not affected by burner diameter except in the very low stretch rate region where the heat loss from the flame to the burner wall is important. Then, it was concluded that the tubular flame burners with different dimensions give not only the same limits of flammability but also the same flammable regions of ternary mixtures under the same stretch rate except in the very low stretch rate region.