Minimum Ignition Energy Values Research Articles

Improved Photo-Ignition of Carbon Nanotubes/Ferrocene Using a Lipophilic Porphyrin under White Power LED Irradiation.

The aim of this work is to investigate and characterize the photo-ignition process of dry multi-walled carbon nanotubes (MWCNTs) mixed with ferrocene (FeCp2) powder, using an LED (light-emitting diode) as the light source, a combination that has never been used, to the best of our knowledge. The ignition process was improved by adding a lipophilic porphyrin (H2Pp) in powder to the MWCNTs/FeCp2 mixtures—thus, a lower ignition threshold was obtained. The ignition tests were carried out by employing a continuous emission and a pulsed white LED in two test campaigns. In the first, two MWCNT typologies, high purity (HP) and industrial grade (IG), were used without porphyrin, obtaining, for both, similar ignition thresholds. Furthermore, comparing ignition thresholds obtained with the LED source with those previously obtained with a Xenon (Xe) lamp, a significant reduction was observed. In the second test campaign, ignition tests were carried out by means of a properly driven and controlled pulsed XHP70 LED source. The minimum ignition energy (MIE) of IG-MWCNTs/FeCp2 samples was determined by varying the duration of the light pulse. Experimental results show that ignition is obtained with a pulse duration of 110 ms and a MIE density of 266 mJ/cm2. The significant reduction of the MIE value (10–40%), observed when H2Pp in powder form was added to the MWCNTs/FeCp2 mixtures, was ascribed to the improved photoexcitation and charge transfer properties of the lipophilic porphyrin molecules.

Is turbulent facilitated ignition through differential diffusion independent of spark gap?

In 2014, Wu et al. discovered an unexpected result. Turbulence can facilitate ignition through differential diffusion when the effective Lewis number (Le) of mixtures is sufficiently larger than unity using small electrode gaps (dgap ≤ 0.8 mm) in near-isotropic turbulence generated by a fan-stirred burner. This suggested that the required minimum ignition energy (MIE) in intense turbulence can be smaller than that in quiescence (Wu et al. did not measure MIE). This work explores whether the aforesaid turbulent facilitated ignition (TFI) for Le > 1 is independent of dgap. We apply the same hydrogen mixtures at the equivalence ratio ϕ = 5.1 (Le ≈ 2.3) and ϕ = 0.18 (Le ≈ 0.3) as Wu et al. in our large fan-stirred cruciform bomb capable of generating near-isotropic turbulence to measure values of MIE as a function of dgap at both quiescence and intense turbulence (the rms turbulent fluctuating velocity u′ = 5.4 m/s) conditions. A drastic fall of values of laminar and turbulent MIE (MIEL and MIET) with increasing dgap is observed. TFI only occurs for Le > 1 (ϕ = 5.1) and it is restricted at smaller dgap = 0.58 mm, where MIEL = 61.5 mJ >> MIET = 26 mJ (0.25-mm tungsten electrodes) and MIEL = 255.5 mJ >> MIET = 36.8 mJ (2-mm tungsten electrodes) in support of Wu et al.’s finding. However, we discover that the MIEL and MIET curves versus dgap can cross each other at larger dgap, at which no TFI for Le > 1 at dgap = 2 mm where MIEL = 0.52 mJ << MIET = 17.3 mJ (2-mm tungsten electrodes). This interesting result depending on dgap should be disseminated in our combustion community for stimulating further research.

Photo-Induced Ignition of Different Gaseous Fuels Using Carbon Nanotubes Mixed with Metal Nanoparticles as Ignitor Agents

ABSTRACTThis article describes the photo-induced ignition process of multi-walled carbon nano-tubes (MWCNTs)/ferrocene mixtures by pulsed Xe lamps using programmable driving boards with adjustable parameters, such as variable flash rate and pulse’s energy/intensity. Varying the energy of incident light pulse, minimum ignition energy values were found as a function of mixture weight ratio, observing that a higher MWCNT amount with respect to metal nano-particles leads to lower ignition energy. The photo-induced ignition of CNTs mixed with nano-particles was then used in a properly realized experimental setup for triggering the combustion of CNT-enriched fuel mixtures. Different types of gaseous fuels mixed with air (CH4, liquid propane, and H2) were tested. The combustion process triggered by MWCNTs/ferrocene photo-ignition shows better performances, for all used gaseous fuels and for all tested air/fuel ratios, compared with those obtained by using a traditional spark plug. In particular, CNT-based photo-induced combustion evolves more rapidly with shorter ignition delays, higher peak pressure values, and a higher fuel burning rate as observed by reported experimental tests.

Prediction of Minimum Ignition Energy from Molecular Structure Using Quantitative Structure–Property Relationship (QSPR) Models

Minimum ignition energy (MIE) is one of the most important and most widely used parameters when characterizing hazardous chemicals. However, it is extremely difficult to obtain experimental MIE data due to the high cost, time involved, and safety issues of experimental tests. In this work, two quantitative structure–property relationship (QSPR) models were built based on existing experimental data and molecular simulations through analysis of multiple linear regression (MLR) and support vector machine (SVM). Experimental MIE data of 61 chemicals were collected, and their molecular descriptors were derived solely from their molecular structures, which were optimized at B3LYP/6-31G(d) level using Gaussian 09. Both models were validated to have excellent performances in goodness-of-fit, internal robustness, and external predictive ability, and hence, they are qualified to predict MIE values for chemicals with no experimental MIE data available. These two validated models can also help gain a better understan...

Experimental determination of dust cloud combustion parameters of α-AlH3 powder in its charged and fully discharged states for H2 storage applications

This study discusses results of an experimental program for determination of dust cloud combustion parameters of charged and fully discharged states of metastable alane (aluminum hydride, α-AlH3 polymorph) powder in air. The measured characterization parameters include: maximum deflagration pressure rise (ΔPMAX), maximum rate of pressure rise (dP/dt)MAX, minimum ignition temperature (TC), minimum explosible concentration (MEC), and minimum ignition energy (MIE). These measured values are used for calculating the associated explosion severity (ES) index, and volume-normalized maximum rate of pressure rise (KSt). The experimental results show values of MEC and TC of fully discharged alane to be greater than those of the charged alane but measured MIE values are about the same. Moreover, the results show higher reactivity of fully discharged alane dust cloud in air compared to its charged state. For example, ES and KSt of discharged alane dust cloud in air are about 300% and 35% greater, respectively, than ES and KSt of charged alane dust. The higher air reactivity of fully-discharged (primarily Al powder) dust cloud compared to its charged state can be attributed to the higher surface energy (J/m2) of Al compared to that of α-AlH3. These experimental insights have safety implications in postulated risk scenarios involving light-duty vehicles powered by PEM fuel cells. The core insights and critical data provided by this contribution are useful for supporting development and promulgation of hydrogen safety standards and augmenting property databases of hydrogen storage materials.

Thermal analysis and dust explosion characteristics of spent coffee grounds and jatropha

The relationships of fatty oil content in spent coffee grounds (SCG) and jatropha kernels (JK) with minimum explosible concentration (MEC), minimum ignition energy (MIE), cohesion, dispersibility and thermal analysis were investigated. Both SCG and JK are carbon-rich [> 50% by weight (wt.%)] with higher total calorific values than cellulose. The MEC values for absolutely dry 75–105 μm particles of oil-retaining and oil-extracted SCG were 35 g m−3 (high explosion hazard) and 120 g m−3 (low explosion hazard), respectively. The low MEC value of oil-retaining SCG was despite a high cohesion result, suggesting that the oil content increased the likelihood of a dust explosion. Conversely, the MIE values for all SCG and JK were sufficiently high to consider them a low-level risk. Behavior analysis showed that decomposition temperature of SCG did not differ to that for its oil-extracted form. However, exothermic peak temperature of JK was 76 °C higher than for its oil-extracted form. The SCG (20 wt% oil) was little affected by oil content; however, jatropha (60 wt% oil) was greatly affected. Thermogravimetric results for all samples showed that the decomposition temperature increased with the heating rate, although the overall behavior of decomposition was not affected by heating rate.

Minimum ignition energy of hybrid mixtures of combustible dusts and gases

Mixtures of suspended combustible dust and flammable gas are usually encountered in various processes and systems where substances of different states of aggregate are handled. Knowing the lowest amount of energy needed to ignite such mixtures are critical to identify possibilities of accidental hazards in industry. Investigation of the minimum ignition energy (MIE) of a hybrid mixture of two flammable gases (methane and propane) and eight combustible dusts (wheat flour, starch, protein, polyethylene, peat, dextrin, wood coal and brown coal) were carried out in the modified Hartmann apparatus. The determination of the MIE of the dusts alone was in accordance with the European standard EN 50281. In the case of hybrid mixtures testing, this protocol had to be slightly modified, as hybrid mixtures are not included in the standard mentioned. The device used is limited to a lowest ignition energy of 4mJ. Thus, the MIE of pure gases could not be as tested directly, as their values are all below that energy. The MIE values as well as the lower explosible limits (LEL) for gases were taken from the literature. To determine the MIE of hybrid mixtures at different concentrations of gas below the respective LEL were added to the pressurized air that used to generate the dust cloud in the MIE apparatus. The experimental results demonstrated a significant decrease of the MIE of the dusts and an increase in the likelihood of explosion when a small amount of gas that was below its LEL was mixed with the dust. For example, the MIE of polypropylene was observed to decrease from 116 to 5mJ when only 1vol.% of propane (below its LEL) was added. Moreover, an empirical model to predict the MIE of hybrid mixtures was presented and further compared with the experimental results were done.

Minimum Ignition Energy Measurements for α-Pinene/Air Mixtures

The aim of this work is to determine the minimum ignition energy (MIE) of the α-pinene/air mixture using a laser-induced spark ignition. The α-pinene is a volatile organic compound (VOC) widely found in air freshener, home fragrance, and household products and also emitted by several vegetal species. However few studies deal with this material despite its flammability limits lower than 1% at various temperatures and its high laminar burning speeds. Moreover, some studies assume that the α-pinene/air mixture can play a role when an accelerating forest fire (AFF) occurs. This experimental work highlights the low values of MIE for seven equivalent ratios and also pinpoints the quick rise of ignition probability on a short range of energy absorbed. This study shows the potential threat of the α-pinene/air mixture.

Experimental analysis of laser-induced spark ignition of lean turbulent premixed flames

Spark ignition in lean and highly turbulent premixed flows is experimentally investigated by measuring the Minimum Ignition Energy (MIE), for laser-induced spark ignition in methane/air mixtures, as a function of the velocity fluctuation u′, the equivalence ratio and the focal length of the focusing lens. The burner provides a 2D stationary flow with a homogeneous and isotropic turbulence.A clear transition on the MIE values is observed, when u′ increases. Before the transition, the MIE increases gradually with u′, whereas across the transition (for u′∼1 ms−1), the MIE increases strongly. As the MIE is proportional to the flame thickness to the power of three, this slope breakdown could indicate the existence of two distinct modes of flame structure. From a certain threshold of turbulence intensity, the flame kernel no longer develops in a flamelet regime, rather in a regime where its structure is strongly modified by the turbulence: the flame front would undergo an intense mixing by all the eddies of the flow (Da<1) and a thickening by the small eddies (Ka∼10), leading to an intense stretch and possibly local extinctions. This ignition transition phenomenon in turbulent mixtures has already been reported by Shy et al. (2010) [1] and Huang et al. (2007) [2], but for different experimental conditions.The present study also reveals that there is a common criterion for the ignition transition phenomenon: whatever the focal length of the focusing lens and the equivalence ratio of the mixture, an ignition transition occurs when the turbulent flow reaches a Karlovitz number of the order of 10.

Effects of humidity on minimum ignition energy of gaseous epoxypropane/air mixtures

The minimum ignition energy (MIE) is an important property for designing safety standards and understanding the ignition process of combustible mixtures. The minimum ignition energy (MIE) of gaseous epoxypropane/air mixtures is measured using capacitive spark discharge. The effect of humidity on MIE is studied. It is shown that the MIE is not constant when the relative humidity increases from 40% to 88% at room temperature. The relative humidity has no significant influence on the MIE of gaseous epoxypropane/air mixtures at the lower volume fraction of gaseous epoxypropane in air. But, it has significant influence on that at the higher volume fraction. The MIEs of gaseous epoxypropane/air mixtures vary with the fraction of gaseous epoxypropane in air and the humidity. The lowest value of MIE (0.12 mJ) of gaseous epoxypropane/air mixtures is reached at around 10% in the examined ranges of the concentrations for the humidity 40%. The lowest values of MIE (0.1 mJ) of the mixtures are reached also at around 10% in the examined ranges of the concentrations for the humidity 66% and 88% respectively.

The effect of reduced oxygen levels on the electrostatic ignition sensitivity of dusts

When handling powders with low values of minimum ignition energy (MIE), it is often necessary to employ additional protective measures such as explosion venting, suppression, containment or inerting. Inerting generally involves reducing the oxygen concentration to around 5% v/v; however, it has been shown with gases that more modest reductions in the oxygen content can still have a significant effect on the MIE. Therefore, a test program was carried out to assess the impact of reduced oxygen levels on the MIE of a series of sensitive powders. In addition, this work was also used to investigate whether testing of such sensitive materials in the standard equipment but with reduced oxygen levels could enable the prediction of MIEs <1 mJ at standard atmospheric oxygen levels.

Statistical analysis of electrostatic spark ignition of lean H2/O2/Ar mixtures

The concept of minimum ignition energy (MIE) has traditionally formed the basis for studying ignition hazards of fuels. However, the viewpoint of ignition as a statistical phenomenon appears to be more consistent with the inherent variability in engineering test data. We have developed a very low-energy capacitive spark ignition system to produce short sparks with fixed lengths of 1–2 mm, and the ignition system is used to perform spark ignition tests using a range of spark energies in lean hydrogen–oxygen–argon test mixtures used in aviation safety testing. The test results are analyzed using statistical tools to obtain probability distributions for ignition versus spark energy. A second low-energy spark ignition system was also developed to generate longer sparks with varying lengths up to 10 mm. A second set of ignition tests is performed in one of the test mixtures using a range of spark energies and spark lengths. The results are analyzed to obtain a probability distribution for ignition versus the spark energy per unit spark length. Preliminary results show that a single threshold MIE value does not exist, but rather that ignition is statistical in nature and highly dependent on mixture composition and spark length.

A transition on minimum ignition energy for lean turbulent methane combustion in flamelet and distributed regimes

Minimum ignition energy (MIE) of lean methane–air mixtures is quantitatively measured using a high-power pulse generator which can vary ignition energies of a spark-electrode in the central position of a large fan-stirred cruciform burner. The burner equipped with a pair of counter-rotating fans and perforated plates can be used to generate isotropic turbulence having a very wide range of turbulent intensities ( u′) up to 8 m/s with negligible mean velocities. Observations of ignition, flame kernel development, and subsequent flame propagation in the central uniform region of the burner are recorded by a CMOS high-speed camera (5000 frames/s), showing distributed-like flames of very dispersive and fragmental structures with filiform edges for the first time. A complete MIE data set of lean methane–air mixtures at the equivalence ratio ϕ = 0.6 as a function of u′/ S L is obtained, where S L is the laminar burning velocity. It is found that there is a transition on values of MIE due to different modes of combustion. Before the transition, MIE only increases gradually with u′/ S L. Across the transition when u′/ S L > 24 corresponding to the commonly defined turbulent Karlovitz number Ka = ( u′/ S L) 2( Re T) −0.5 > 8, MIE increases abruptly, where Re T is the turbulent Reynolds number based on the integral length scale of turbulence. This transitional value of Ka is much greater than the Klimov–Williams criterion ( Ka = 1). Since values of MIE under different levels of turbulence should be relevant to the size of the reaction zone at least in the beginning of turbulent combustion, MIE ∼ δ 3 based on an order-of-magnitude criterion where δ is the reaction zone thickness. It is thus concluded that this new experimental finding proves the existence of both thin and broken reaction zones regimes proposed by Peters for a new regime diagram of premixed turbulent combustion.

A Numerical Study on the Effect of the Equivalence Ratio of Hydrogen/Air or Methane/Air Mixtures on Minimum Ignition Energy in Spark Ignition Process

Two dimensional calculations of the spark ignition process for hydrogen/air or methane/air combustible mixtures were conducted. Two dimensional equations with chemical reactions were solved by fourth-order Runge-Kutta method in Cartesian coordinate system. Harten Yee's upwind TVD scheme was used to capture shock waves produced by a spark discharge. The spark energy was applied into the electrode gap for 1.0 micro second, which is equivalent to capacity spark discharge. Detailed reaction mechanisms for hydrogen and methane combustion chemistries were employed. Equivalence ratio of combustible mixtures were varied, and minimum ignition energy for hydrogen/air or methane/air combustible mixtures was calculated. Early stages of flame development were investigated. Success or failure of the ignition was measured by increase rate in the maxmum temperature after the end of spark discharge. The maximum temperature in the flame kernel decreases after the end of spark discharge and then reaches the minimum value. In the case of ignition, the maximum temperature increases after a certain period of time. In the success of ignitions in hydrogen/air and methane/air mixtures, the maximum temperature in methane/air mixtures is greater that in hydrogen/air ones. Profiles of minimum ignition energy as a function of the equivalence ratio of hydrogen/air and methane/air mixture gases were obtained. Peak values of minimum ignition energy for both of hydrogen/air and methane/air mixtures are observed under lean mixture conditions.

04/01479 Prediction of the burnout performance of some South American coals using a drop-tube furnace: Barranco, R. et al. Fuel, 2003, 82, (15–17), 1893–1899

Reducing engine pollutant emissions and fuel consumption is an important challenge. Lean-burning engines are a promising development; however, such engines require high-energy ignition systems for typical working conditions (equivalence ratio, Φ < 0.7). Laser-induced ignition is envisaged as a way to obtain high-energy ignition as a result of progress that has been made in laser beam technology in terms of stability, size, and energy. This study investigated the minimum energy necessary to ignite a laminar premixed methane air mixture experimentally. A parametrical study was performed to characterize the effects of the flow velocity, equivalence ratio, and lens focal length on the minimum energy required for ignition. Experiments were conducted using a premixed laminar CH4/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser with an anti-reflection-coated lens. Mixture ignition and the early stages of flame propagation were studied using a high speed Schlieren technique. Despite the stochastic characteristic of the laser breakdown phenomena, good reproducibility in the minimum energy required for the ignition measurements was observed. The cases in which the CH4/Air mixture flow ignites are defined as those with a laminar flame front propagation visible in the Schlieren images 10 ms after the energy deposition. The same minimum ignition energy (MIE) versus equivalence ratio (Φ) type of curves were obtained with a laser-induced spark and with a spark plug. Due to the threshold of energy required to obtain breakdown and the stochastic character of the energy absorption by the spark, a constant value was obtained (corresponding to the breakdown threshold) when the minimum ignition energy was lower than the breakdown threshold. As already noticed by several authors, MIE values higher than those observed using spark plugs were obtained. However, these differences tended to disappear at the lean and rich fuel limits.

Measurements of minimum ignition energy in premixed laminar methane/air flow by using laser induced spark

Reducing engine pollutant emissions and fuel consumption is an important challenge. Lean-burning engines are a promising development; however, such engines require high-energy ignition systems for typical working conditions (equivalence ratio, Φ < 0.7). Laser-induced ignition is envisaged as a way to obtain high-energy ignition as a result of progress that has been made in laser beam technology in terms of stability, size, and energy. This study investigated the minimum energy necessary to ignite a laminar premixed methane air mixture experimentally. A parametrical study was performed to characterize the effects of the flow velocity, equivalence ratio, and lens focal length on the minimum energy required for ignition. Experiments were conducted using a premixed laminar CH 4/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser with an anti-reflection-coated lens. Mixture ignition and the early stages of flame propagation were studied using a high speed Schlieren technique. Despite the stochastic characteristic of the laser breakdown phenomena, good reproducibility in the minimum energy required for the ignition measurements was observed. The cases in which the CH 4/Air mixture flow ignites are defined as those with a laminar flame front propagation visible in the Schlieren images 10 ms after the energy deposition. The same minimum ignition energy (MIE) versus equivalence ratio (Φ) type of curves were obtained with a laser-induced spark and with a spark plug. Due to the threshold of energy required to obtain breakdown and the stochastic character of the energy absorption by the spark, a constant value was obtained (corresponding to the breakdown threshold) when the minimum ignition energy was lower than the breakdown threshold. As already noticed by several authors, MIE values higher than those observed using spark plugs were obtained. However, these differences tended to disappear at the lean and rich fuel limits.

Dependence of Minimum Ignition Energy on Ignition Parameters

Abstract Using two mathematical models (one allowing for pressure variations, and the other assuming the pressure to be uniform), the minimum ignition energies of a stoichiometric methane-air mixture are calculated. The effect of the initial pressure wave generated by the spark on minimum ignition energy is studied by comparing the results of the two models. The pressure wave is found to affect the values of minimum ignition energy for ignition times shorter than 2 (is. The dependence of minimum ignition energy on the kernel radius and ignition time are also investigated. The results show that as the kernel radius is decreased to small values, the minimum ignition energy reaches a constant value. The ignition times that give an optimum value of minimum ignition energy are in the range of 3μs to 100μs. Our results show that minimum ignition energy is strongly dependent on the ignition parameters, and a minimum value of 0.005 mJ is found for an ignition kernel radius of 0.0635 mm and an ignition time of 27....

Effect of gravity on laminar premixed gas combustion II: Ignition and extinction phenomena

Minimum ignition energies and flame radii as a function of time were measured for near-limit, limit, and sublimit fuel-lean methaneair mixtures burning at one-g and zero-g. Minimum ignition energy values were the same at one-g and zero-g except for mixtures very near the zero-g flammability limit and leaner, where the zero-g values were much higher than the one-g values. For sublimit mixtures at zero-g a previously unreported mode of unstable flame propagation was observed; this mode was characterized by a flame radius increasing in proportion to the square root of the time lapse from ignition, an energy release often orders of magnitude greater than the spark energy input, and sudden extinction. This mode of flame propagation was observed at all gas pressures tested but was more pronounced at higher pressures. All zero-g propagation was spherically symmetric except for a few unusual flame extinguishments at high pressures. The principal conclusions are that flame extinguishment at zero-g is caused by a flame-front instability and that gravitational forces have a stabilizing effect on upward flame propagation. The cause of the instability could not be determined; further experiments which might aid in determining the cause are suggested.

Ignition phenomena in hydrogen-air mixtures

This paper presents an experimental investigation into the effect of spark gap and spark duration on the minimum ignition energy (MIE) of partially dissociated NH3 in air at 295 K and 1 atm. The measurements were carried out using a Hartmann bomb apparatus with the degree of NH3 dissociation varying from 0 to 10%, corresponding to 0 to 7.5%v/v H2 in the fuel mixture with a fixed H2/N2 ratio of 3, and equivalence ratio varying from 0.7 to 1.2, assuming complete oxidation. Axisymmetric two-dimensional CFD modelling was performed using Ansys Fluent incorporating the Okafor mechanism to predict MIE. At a given equivalence ratio, both measured and predicted MIE values decreased significantly with increasing the degree of NH3 dissociation, due to increased presence of H2 in the mixture. There were optimum spark gap and spark duration for a given degree of NH3 dissociation and equivalence ratio at which the MIE was the lowest. The increased presence of H2 due to rising degree of NH3 dissociation enhances the mixture's oxidation reactivity and combustion intensity, which in turn increases the flame kernel temperature and size, the rate of production of key radicals (OH, NH2, H, and O) and rate of heat generation, leading to a more self-sustaining flame and significantly reduced MIE.