Minimum Ignition Temperature Research Articles

Ignition Studies on Aluminised Propellant.

An experimental investigation on the ignition of metallised propellants (APIHTPB/AI) has been carried out 10 determine the ignition delay, minimum ignition energy and corresponding heat flux,threshold heat flux for ignition and minimum ignition temperature, Ignition experiments were conductedusing a shock tube under convectiveheating conditions similar to those prevailingin a rocket motor. Heat flux at propellant location was measured by thin film heat flux gauge and also calculated from a ribbon thermocouple output under similar test conditions. The igntion delay was measured as the time lag between the arrival of hot gas at the propellant and the light emission due to actual ignition of the propellant. The experimental results indicate that the ignition delay characteristics are independent of pressure. The minimum energy for ignition obtained for the propellant is 1100J/m2 corresponding to the heat flux range of 80·120 WIcm2 for a gas velocity of 110 mls. The threshold heat flux required to ignite the propellant was 40 W/cm2 at a velocity of 110 mls. Heat flux corresponding to minimum ignition energy and the threshold heat flux increase with gas velocity. The threshold ignition temperature of the propellant was found to be 600 ± 20 K.

Ignition behavior of dusts: Meaning and interpretation

AbstractThe minimum ignition energy and minimum ignition temperature of dust‐air‐mixtures are important technical safety indices. They are used for the assessment of the efficacy of the ignition sources expected in dust‐air mixtures. First of all, the test apparatus and the determination procedures are introduced, including the significant parameters on the mentioned indices. Finally, the correlations are described between—the minimum ignition energy and the efficacy of electrostatic sparks, —the minimum ignition energy and the minimum ignition temperature and the mechanically generated sparks on the on hand, the limiting oxygen concentration and the limiting gap width of combustible dusts on the other hand, —the minimum ignition temperature and hot steel surfaces (mechanically generated hot surfaces) or glowing particle nests surfaces.

Auto-ignition of [formula omitted], [formula omitted], CH 4/C 3H 8/air and CH 4/CO 2/air using a 11 ignition bomb

The distinction between auto-ignition and hot-surface ignition of a given gas is emphasized. In ideal auto-ignition there is no diffusion of heat or matter. Published information on auto-ignition temperatures (AIT) of multi-component fuels in air is scarce. This also applies to North Sea natural gas, of which CH 4, higher alkanes and CO 2 are essential components. In the present experimental laboratory-scale study, AIT of four types of hydrocarbon mixtures ( CH 4 air , C 3H 8 air , CH 4/C 3H 8/air and CH 4/air/CO 2) have been measured using a 11 ignition bomb. The experimental method ensured that the gas mixtures studied were of known composition and homogeneous in concentration. The gas mixture was admitted to the pre-evacuated ignition bomb in the form of a turbulent jet when the bomb wall had reached the desired temperature. Ignition was recognized as a sudden pressure rise in the bomb a few seconds after the gas flow into the bomb had stopped. The minimum AITs for CH 4 air and C 3H 8 air were found to be 640 °C and 500 °C, respectively. The AIT of CH 4/C 3H 8/air decreased with increasing propane content and total fuel concentration. A fuel concentration region was discovered for which CH 4/C 3H 8/air and C 3H 8 air with the same ratio of propane to oxygen gave the same AIT. Reducing the oxygen content of a CH 4 air mixture by adding CO 2 gave, under the present experimental conditions, a systematic increase of AIT with increasing CO 2 content. The role of the CO 2 was probably essentially that of an inert diluent. It has been known for a long time that the ‘minimum hot-surface ignition temperature’ is not a constant for a given gas mixture, but highly dependent, by several hundred degrees centigrade, on the dynamic state of the gas, the geometry and material of the ignition surface, and the mode of heat supply to the surface. The direct application of AIT values to assess industrial hot-surface ignition risks may therefore be unduly conservative. Consequently there is a need for general mathematical models that can predict minimum ignition temperatures for various practical situations in industry. Such models will have to contain sub-models of ignition chemistry, fluid mechanics and heat and mass transfer.

Interactive processes in gasification and combustion—Part III: Coal/char particle arrays, streams and clouds

A comprehensive review is presented on the interactive transport processes in the gasification and combustion of a cloud of drops and solid particles. The review is divided into three parts. Part I is concerned with the interactive processes for arrays, streams and clouds of drops, Part II presents a review of isolated coal, carbon and porous char particles pertaining to interactive processes, and finally Part III deals with the interactive processes for solid particle arrays, streams and clouds. Isolated particle gasification (pyrolysis and heterogeneous) and combustion were briefly reviewed in Part II. Because of strong analogy of the group ignition and combustion, to porous char ignition and combustion, the literature on porous char combustion was also included in Part II and new results were presented on the internal ignition of porous char particle using Frank-Kamanetsky type of analysis. Part III presents an integrated approach starting from arrays to clouds and gasification to combustion. The interactions occur through processes ranging from pure diffusive to convective transport processes. Approximate criteria for interactive processes are given. As opposed to liquid drop arrays and clouds, there is no systematic study for arrays of char or coal particles. Due to the similarity between droplet evaporation and char combustion, new results are presented for the combustion of char arrays in quiescent atmosphere. Convective effects are also briefly discussed. Expanding the Frank-Kamenetsky analysis to ignition of clouds, simple solutions are presented for cloud ignition temperatures. A comparison of results between different techniques and between theory and experiment is given. Interesting results for the ignition characteristics of coal dispersions are obtained in that the particles with relatively small or low volatile matter which ignite heterogeneously when isolated are found to ignite homogeneously under cloud conditions. The minimum ignition temperature is found to increase with decrease in size under isolated mode while the opposite is true under interactions. The problems of the gasification, ignition, and combustion of clouds in confined and unconfined volumes are reviewed. Experiments conducted with streams (laminar flow reactors, LFR) and clouds (TGA, heated grids, shock tubes, batch of particles in LFR, Hertzberg Ignition apparatus) are reviewed. Following the drop literature, the relation between array and group combustion is presented. Finally, the relevance of the reviewed literature to pollutants' formation and destruction and spray combustion modeling is briefly discussed.

Ignition characteristics of spray-dried milk product powders in oven tests

A range of seven spray-dried milk product powders, commercially manufactured in Ireland, is tested for minimum ignition temperature (MIT) in an oven (all-round heating). Samples are tested as cubes with side dimensions of 25, 38, 51, 76 and 102 mm. The relationship obtained between MIT (T) and r, which is half the characteristic dimension of the sample (cube side), is given, with the coefficient of correlation obtained. In all cases, MIT decreases with increasing sample size and the equation In (δ cT 2/r 2) = M - N/T is found to describe the relationship accurately when using modified values of δ c for each cube size. The value of the constant, N, in each case and its relationships with the percentage fat content, total unsaturation of the powders and percentage protein content are examined. Excellent correlation is found for N as a joint function of percentage of fat and percentage of protein.

Extinction of methaneoxygen mixtures by nitrogen dilution

This study refers to the minimum ignition energy (MIE) of dusts as determined by means of the Hartmann tube. To do so, six different dust with different particle size and properties are employed. A theoretical model is developed to predict the MIE based upon the physical and chemical properties of the dust. The model clearly shows the relationship between the minimum ignition energy and the minimum ignition temperature of a dust cloud. The model results in a method to calculate the MIE of dusts. Also, the time necessary for ignition can be calculated with the model. The results show that despite some limitations, the model predicts the MIE for dusts rather well. It is proposed that 40–60% energy loss would be considered when applying the model.

Ignition temperatures of dust layers: Flaming and non‐flaming

AbstractCombustible dusts, based on tests conducted in the Setchkin Furnace on five representative dusts, are shown to have two distinct layer minimum ignition temperatures, designated MITnon‐flaming and MITflaming. The MITnon‐flaming is the one usually reported in the literature; the MITflaming is determined by starting at a much higher temperature and working downwards. MITflaming can yield a different ranking of dust flammability than MITnon‐flaming. The MITflaming appears to be more closely correlated to the MIT of dust clouds and to the test prescribed for flammable solids in the US Code of Federal Regulations than the MITnon‐flaming.

DETERMINATION OF GRAIN DUST EXPLOSIBILITY PARAMETERS

ABSTRACT Three fractions of wheat and grain sorghum dust were tested at five concentrations for explosibility. Explosibility of one lignite dust fraction was determined as a reference. Parameters determined were minimum explosive concentration, minimum ignition temperature, maximum pressure, and maximum rate of pressure rise. Minimum explosive concentration tests demonstrated that currently accepted minimum explosive concentrations are lower than actual concentrations required for propagation of an explosion. Differences between maximum pressure and maximum rate of pressure rise for primary and secondary chambers demonstrated that a secondary explosion occurred in the secondary chamber. Since residual dust was not initially present in the secondary chamber, fuel for the secondary explosion had to have been transported by the pressure front from the primary chamber. This event demonstrated that residual dust is not essential for occurrence of secondary explosions. KEYWORDS. Grain dust, Explosibility.

Criteria for spontaneous ignition in exothermic, autocatalytic reactions: Chain branching and self-heating in the oxidation of cyclohexane in closed vessels

The combustion of fuel-rich hydrocarbon + air mixtures constitutes an autoignition hazard that is of concern in industrial processed. In the present article we report studies of the combustion of cyclohexane + air, in the molar proportions cC 6H 12:air = 1:2, using five closed vessels covering a very extensive range of volume (0.2–20 dm 3). We use the minimum ignition temperature measured in each vessel to test scaling rules for the prediction of conditions for criticality. Measurements were also made of the yields of some products, the extent of reaction (and from it the rate), and the accompanying temperature change under both subcritical and supercritical conditions. Characteristic cooling times from hot gases in each vessel were also assessed experimentally. We show that fuel-rich compositions undergo isothermal oxidation that follows a quadratic, autocatalytic rate law. Departures from this due to self-heating of the reactants are also reported Spontaneous ignition occurs as a result of chemical autocatalysis augmented by self-heating. On the basis of the Semenov parameter for criticality in a corrected form of the condition for thermal ignition, as derived by Melentiev and Todes, the autoignition temperatures in vessels of different size are found to match a linear scaling relationship. This would be expected in systems in which spatially uniform temperatures prevail. Although there is no forced gas motion in the present experiments, the Rayleigh numbers associated with the reactants on the verge of ignition confirm that natural convection may be playing a significant part in smoothing out temperature distributions and aiding heat loss, especially from the largest vessels. Ab initio calculations of a criterion for criticality reveal the need for measurements of activation energy that are more precise than the limits of accuracy normally encountered in global kinetic studies.

Hot‐surface ignition temperatures of dust layers

AbstractMinimum dust layer ignition temperatures on a hot surface were determined for several dusts, using a test procedure recommended by the National Academy of Sciences. The dusts included coal, three oil shales, lycopodium spores, corn starch, grain and brass powder. For a few of the dusts the effects of particle size and layer thickness on the minimum ignition temperatures were examined. Test results were repeatable and reliable for the fuels, the lycopodium and the brass powder. The minimum hot‐surface ignition temperatures of 12.7‐mm thick layers of these dusts ranged from 160°C for brass to 290°C for 20‐gal ton−1 oil shale. Flaming combustion was observed only with the brass powder. The minimum ignition temperatures decreased with thicker layers and with smaller particle sizes. Some difficulties were encountered with the corn starch and grain dusts. During heating, the starch charred and expanded; the grain dust swelled and distorted. The test was found acceptable for the purpose of determining the minimum layer ignition temperature of a variety of dusts. To prevent fire hazards due to smoldering or flaming dust layers the temperatures of surfaces on which combustible dusts accumulate should be lower than the minimum hot‐surface ignition temperatures of the dusts.

Influence of sample shape and size on self-ignition of a fat-filled milk powder

A lecithinated, 30% tallow fat-filled, milk-product powder, commercially-manufactured in Ireland, was used as the test material to assess the effect of sample shape and size on self-ignition in a thermostatically-controlled oven. Cubes of edge 25, 38 and 51 mm, spheres of diameter 26, 50, 77 and 102 mm, cylinders with diameter equal to length of 51, 76 and 100 mm and layers of 100 mm diameter and thickness of 10, 15 and 20 mm were tested. The minimum ignition temperature (MIT) varied from 206°C for the thinnest layer to 131°C for the largest sphere. For each shape, the results fitted the linear relationship ln( δ c T 2 r 2 ) = M− (N/T) , with close correlation. However, contrary to theory, the four straight lines did not coincide. Those for the spheres and layers were very close while those for cubes and cylinders had approximately 25% greater values for their slope (N). These results are shown to introduce significant differences when the values of M and N for the two most divergent lines within the test range are used to extrapolate estimates of critical layer thickness for the lower operating temperatures in parts of a milk spray dryer. M and N obtained from spheres gave the lowest estimates while those for cubes gave values up to 2·32 times, and those for cylinders, 2·85 times greater at 50°C. These results could also affect the safe design of silos for the storage of powder in the 40–50°C range to avoid self-ignition and allow the contents to cool to ambient temperature.

Initiation of low-temperature filtration combustion waves for porous systems

In this work a numerical modeling method was used to study different variations of realizing this method: local ignition in the central part of a porous specimen of "infinite" (from the point of view of its excluding the effect of boundary conditions) length; initiation of a counter FC wave by local warm-up of the rear (in relation to the direction of gas filtration) part of the specimen; formation of an associated inverse FC wave with warm-up of its frontal part. The aim of this study is to clarify the features of FC wave dynamics, the effect on it of temperature @p and width of the firing zone 6warm-up) Lp; to establish the relationship between the minimum (critical) ignition temperature @i and Lp with different regime parameters and characteristics of the physical system; to determine the ignition time Ti and the amount of "flashback" of the oxidant (instantaneous ~L/~0 and integral ~(~L/~0)e~ during firing, o

Low temperature ignition of acetaldehydeoxygen mixtures initiated by organic peroxides adsorbed on the surface of a reaction vessel

The phenomenon of low temperature (up to room temperature) ignition of acetaldehydeoxygen mixtures, initiated by the products of heterogeneous radical decomposition of peroxide compounds, adsorbed at low temperature on a surface of the reaction vessel, has been discovered and studied. It has been shown that the minimum ignition temperature strongly depends on the character of the reaction vessel surface, as well as the nature and the concentration of the peroxide (peracetic acid, t-butyl hydroperoxide, and di-t-butyl peroxide). It has been discovered, that a blue-colored flame propagates along the cold reaction vessel as a result of the adsorption of the peroxy compounds on the wall and the subsequent rapid heating. The speed of the propagation of the flame depends on the peroxy compound adsorbed.

The effect of mixtures of particle sizes on the minimum ignition temperature of a dust cloud

The ignition temperature in the air atmosphere at the separating line between an explosion and no explosion in a dust cloud, T i b, has been investigated for mixtures of fine and coarse dusts of the same material. The minimum ignition temperature, T i m, which is the lowest temperature at which an explosion is obtained has also been determined. Measurements were made in a Godbert-Greenwald Furnace apparatus. The results obtained indicated that the ignition temperature is dependent on the particle size, and an admixture of fine dust of 30% to coarse dust is sufficient to reduce the T i m values significantly.

Investigations into the characteristics of irradiated zircaloy cladding material in connection with reactor operation and spent fuel reprocessing

To monitor and improve the performance of nuclear fuel assemblies, the uptake of gases (oxygen, nitrogen and hydrogen) is determined in zircaloy cladding materials. Gas chromatography and/or mass spectrometry combined with hot vacuum fusion were applied. Deviations from the initial concentrations (ca. 5 μg g −1 H 2, 30 μg g −1 N 2 and 1200 μg g −1 O 2) in the “as fabricated” condition, are important in estimating cladding corrosion. The material characteristics of zircaloy are altered substantially by the neutron irradiation and the chemical environment in the reactor coolant, thus the irradiated material must also be studied. For reprocessing safety consideration, the ignition and explosion parameters of unirradiated and irradiated zircaloy dusts were examined. Standard methods, tailored to hot-cell operation, were used to evaluate the minimum ignition temperature of a dust layer on a heated surface at constant temperature, the ignition temperature of a dust cloud, the auto-ignition temperature of a cylindrical dust formation as a function of sample volume, and the explosion pressure and pressure rise in a 20-l spherical chamber. Samples of fines (<100 μm diameter) were characterized by measuring their density and particle-size distribution, and by scanning electron microscopy. For samples of irradiated zircaloy, the ignition temperatures were lower and the explosion pressures and pressure rises higher than for unirradiated zircaloy. These findings can be explained by the different particle-size distribution of irradiated material samples. The increased brittleness of the irradiated material produces more small particles (<20 μm) which favour ignition and explosion.

Minimum ignition temperature of a self-raising flour on a standard hot-plate

A hot-plate to the draft specification of the International Electrotechnical Commission (Committee 31H) was constructed and located in a chamber with an air extractor and a transparent door. A commercial self-raising flour was tested to assess the relationship between minimum ignition temperature (MIT) and layer depth (5–40 mm in steps of 5 mm). MIT varied from 311°C for 5 mm to 265°C for 20 mm and to 238°C for 40 mm layers. The results fitted the relationship ln ( δ cT 2 r 2 )= M − ( N T ) , with a correlation coefficient of 0·994 for 4 points above the recommended minimum ratio of sample diameter-to-depth of 5:1 and 0·995 for 8 points above the ratio of 2·5:1. The latter relationship had a slightly flatter slope than the former. The variation in results from the two lines was examined. It is recommended for self-raising flour that more reliable results are obtained from 15 mm layer depth than 5 mm because of the tendency of the latter layers to buckle and crack under test. Relationships between hot plate temperature and (a) time taken for layer to reach plate temperature, (b) maximum temperature generated in sample and (c) percent residue after each test, were obtained for the 15 mm layer at plate temperatures up to 410°C.

Boundary Conditions for Grain and Soybean Dust Explosion

Grain and soybean dust explosion was investigated in order to determine the conditions that can cause an explosion to take place in a closed space such as in side a grain elevator, conveyor, miller and silo. The main parameters, which were investigated, were minimum ignition temperature, minimum explosible dust concentration, explosion pressure, dust particle size and moisture content. In all the materials and at all moisture contents, a concentration of about 1,000 g/m3 was observed to give the maximum explosion pressure and maximum rate of pressure rise. The delayed time at this concentration was minimum. At any given dust particle size the ignition temperature was highest in rice husks followed by wheat, corn, milled rice and soybean in ascending order. However, in terms of maximum pressure and maximum rate of pressure rise, the order was from wheat, rice husks, soybean, corn to milled rice in descending order. It was observed that dust particle size played a major role on the explosibility of any dust material. Moisture content affected the ignition temperature and the maximum rate of pressure rise, but did not affect the maximum explosion pressure and delayed time of explosion.

CO2-laser-induced deflagration of fuel/oxygen mixtures

Weakly focused pulsed-CO2-laser radiation has been examined as an ignition source for low-pressure (85–100 Torr), near-stoichiometric fuel/oxygen mixtures containing the following fuels: ethylene, methyl fluoride, methanol, ethanol, dimethyl ether, p-dioxane, n-propyl nitrate, and iso-propyl nitrate. The data analysis includes characterization of the spatially dependent ir absorption in the test cell, calculation of appropriate medium temperatures from the absorbed laser energy, and correlation of observed ignition delay times tig with the calculated temperatures. Effects of hydrodynamic motion on the pressure, density, and temperature profiles of the irradiated samples were modeled using a computer code for two-dimensional wave propagation. Code predictions are in accordance with experimental pressure-time histories obtained using a piezoelectric transducer. Minimum ignition temperatures ranged from 590 K for the iso-propyl nitrate/O2 system to 1645 K for CH3F/O2. The observed functional relationship between tig and temperature T was generally of the form ln tig =A/T+B, where A and B are constants. This relationship follows the form predicted by thermal and degenerate chain ignition theories, viz., ln tig =Eact/ RT+constant, where Eact corresponds to an overall activation energy. Using this relation, derived activation energies are 86, 57, 42, 47, ∼40, 13, and 12 kcal/mole for the CH3F/O2, CH3OCH3/O2, CH3OH/O2, C2H5OH/O2, C4H8O2/O2, n-propyl nitrate/O2, and iso-propyl nitrate/O2 mixtures, respectively. These results are in reasonable agreement with available data from shock-tube and hot air stream injection techniques. In contrast, an anomalous, threshold-like effect was observed for laser ignition of C2H4/O2. The applicability of ignition schemes of this type to time-resolved kinetic spectroscopic studies is briefly discussed.

A calorimetric study of self‐heating in coffee and chicory

SummaryThe technique of heat flow calorimetry was used to determine the specific heat of coffee and chicory products and to study their thermal behaviour above 20°C. Intensive exothermic reactions were particularly evident when measurements were made with sealed cells. The data obtained help in the understanding of how exothermic reactions (and self‐heating) occurring in such foodstuffs can bring them above their minimum ignition temperature.

An experiment on the ignition of hydrogen injected into a high temperature oxidizer

Experiments were performed to investigate the combustion process which occurs when hydrogen is injected into a high temperature oxidizer. Hydrogen was injected through a circular nozzle into an oxygen-argon mixture in a temperature range 950–1500 K produced behind a reflected shock wave. The effects of oxidizer temperature, oxygen concentration and size of nozzle diameter on the ignition delay were studied. Also, the flow field was observed with a conventional schlieren system and a high speed camera. Consequently it has been found that the ignition delay is influenced mainly by the oxidizer temperature, that the minimum ignition temperature is around 980 K irrespective of the oxygen concentration and the size of the nozzle diameter, that the ignition takes place at a position near the nozzle exit and that the resulting flame propagates toward the tip of the jet. This is followed by a discussion of the ignition process and the applicability of hydrogen to diesel engines based on the present experiment results.