Dust Explosions in the Process Industries Second Edition phần 5 pot

Generation of explosible dust clouds 255

Motion of Particles in a Turbulent, Particle-Laden Gas Flow. Fluid Mechanics - Soviet Research

Singer, J. M., Greninger, N. B., and Grumer, J. (1967) Some Aspects of the Aerodynamics of the

Formation of Float Coal Dust Clouds. 12th Znt. Conf. Mine Safety Res. Establ., Dortmund

Siwek, R. (1977) 20-I-Laborapparatur fiir die Bestimmung der Explosionskenngrossen brennbarer

Staube. Diploma Thesis (Sept), Technical University of Winterthur, Switzerland

Siwek, R. (1988) Zuverlassige Bestimmung explosionstechnischer Kenngrossen in der 20-Liter

Laborapparatur. VDI-Berichte 701 pp. 215-262

Smolyakov, A. V., and Tkachenko, V. M. (1983) The Measurement of Turbulent Fluctuations.

(English Translation) Springer-Verlag

Sokolovski, V. V. (1960) Statics of Soil Media, (Translated to English from Russian by D. H. Jones

and A. N. Schofield), Butterworths Scientific Publications, London

Tadmor, J., and Zur, I. (1981) Resuspension of Particles from a Horizontal Surface. Atmospheric

Environment, 15 pp. 141-149

Tomita, Y., Tashiro, H., Deguchi, K., et al. (1980) Sudden Expansion of Gas-Solid Two-Phase Flow

in a Pipe. Phys. Fluids, 23(4) pp. 663-666

Trostel, L. J., and Frevert, H. W. (1924) The Lower Limits of Concentration for Explosion of Dusts

in Air. Chem. Metall. Engng., 30 pp. 141-146

Ural, E. A. (1989) Dispersibility of Dusts Pertaining to their Explosion Hazard, Factory Mutual

Research Report J. I. OQ2E3.RK, (April), Norwood, Mass., USA

Ural, E. A. (1989a) Experimental Measurement of the Aerodynamic Entrainability of Dust

Deposits. 12th Int. Coll. Dyn. Expl. React. Syst. (July 24-28) Ann Arbor, Michigan, USA

Weber, R. (1878) Preisgekronte Abhandlung uber die Ursachen von Explosionen und Branden in

Muhlen, sowie uber die Sicherheitsmassregeln zur Verhiitung derselben. Verh. Ver. Gew. Fliess.,

Berl. pp. 83-103

Yamamoto, H., and Suganuma, A. (1984) Dispersion of Airborne Aggregated Dust by an Orifice.

International Chemical Engineering, 24 pp. 338-345

Yamamoto, H. (1990) Relationship between adhesive force of fine particles and their dispersibility

in gas. Proc. 2. World Congress in Particle Technology, Sept. 19-22, Kyoto, Japan, pp. 167-173

Zeleny, J., and McKeehan, L. W. (1910) Die Endgeschwindigkeit des Falles kleiner Kugeln in Luft.

Physik. Zeitschrifi XI pp. 78-93

17 pp. 27-34

Chapter 4

Propagation of flames in dust clouds

4.1

IGNITION AND COMBUSTION OF SINGLE PARTICLES

4.1.1

ALUMINIUM

Friedman and Macek (1962, 1963) studied the ignition and combustion of aluminium

particles in hot gases of varying oxygen content. They concluded that ignition occurred

only after melting of the oxide layer (melting point 2300 K) which coats the particle. The

process of ignition did not appear to be affected by the moisture content of the hot

ambient gas and was only slightly influenced by the oxygen content. At an oxygen content

of only 2-3 mole per cent, ignition occurred at 2300 K, whereas at 35 mole per cent

oxygen, it occurred at 2200 K. On the other hand, the concentrations of oxygen and water

vapour had significant influence on the combustion of the metal. Oxygen promoted

vigorous combustion, and, if its concentration was sufficiently high, there was fragmenta￾tion of particles. In the absence of moisture, diffusion and combustion took place freely in

the gas phase, whereas in the presence of moisture, the process was impeded and confined

to a small region, because the reactants had to diffuse through a condensed oxide layer on

the surface of the molten particle.

Cassel (1964) injected single 60 pm diameter aluminium particles into the centre of a

laminar aluminium dust flame of known spatial temperature distribution. Ignition of the

particles occurred at 2570 K, but this was probably higher than the minimum temperature

required for ignition, because the residence time of the particle in the hot environment

was not more than 2 ms. This is shorter than the induction period required for self-heating

of the particle from its minimum ignition temperature to the minimum temperature for

self-sustained oxidation.

Cassel further observed that within 2 ms after ignition a concentric burning zone, of

diameter about nine times the original particle diameter, developed around the particle.

After 3 ms, a detached envelope appeared, which at first surrounded the particle

concentrically, but then became elongated and gradually developed into a cylinder of

length more than 10 times its diameter. This expanding oxide envelope, being in the liquid

state, followed the relative motion of the ambient atmosphere.

Burning times of 60 pm aluminium particles located between the lobes of the

aluminium-dust flame were found to be of the order of 10.5 ms (about 4.5 times longer

than for magnesium particles burning under the same conditions). Cassel attributed this to

the greater oxygen requirement for the oxidation of aluminium.

Prentice (1970) studied the ignition and combustion of single 300-500 pm aluminium

particles in dry air, following initial heating and melting by a light flash from either a

pulsed Nd-glass laser or a xenon-flash discharge lamp. In air (as opposed to in Ado2)

Propagation of flames in dust clouds 257

oxide accumulated on the burning aluminium droplet. Because of this, the combustion

process was terminated by fragmentation of the droplet (as shown by Nelson, 1965 for

zirconium). The very fast flash-heating method generated fully developed metal droplets

with practically no oxide on the surface. This presented initial conditions for studying the

subsequent ignition and combustion processes, when the virgin droplets interacted with

the surrounding air. Detailed SEM studies of the oxide layer build-up revealed a porous

structure with a great number of fumaroles. Over the experimental range, the burning

time to fragmentation increased linearly with the particle diameter from about 200 ms at

300 pm to 600 ms at 500 pm. Prentice studied the combustion of aluminium droplets in

dry air over a range of pressures up to 4.5 bar (abs.). The particles were found to fragment

in dry air at pressures up to about 2.4 bar (abs.). Fragmentation became quite weak and

sporadic at this pressure and finally ceased as the pressure was raised to approximately 4.0

bar (abs.). The time to fragmentation was found to be inversely proportional to the air

pressure, i.e. to the oxygen concentration.

Prentice also found that the nitrogen in the air played an active role in the combustion

process, causing the oxide generated to adhere to the droplet surface and form an

asymmetrical, spin-generating oxide layer that appeared to be a pre-condition for

fragmentation. The driving gas causing particle fragmentation is in part aluminium

vapour, but for combustion in air the major constituent is nitrogen from nitride.

Frolov et af. (1972) studied ignition and combustion of single aluminium particles in

high-temperature oxidizing gases, as a function of particle size and state of the gas.

Various theories were reviewed.

Grigorev and Grigoreva (1974) modified the theory of aluminium particle ignition by

Khaikin et al. (1970), by including a fractional oxidation law accounting for possible

changes of the structure of the oxide film during the pre-flame heating period. Exper￾iments had revealed that the minimum ignition temperature of aluminium particles was

independent of particle size, and Grigorev and Grigoreva attributed this to the oxidation

rate depending very little on the thickness of the oxide layer.

Razdobreev et af. (1976) studied the ignition and combustion of individual 230-680 pm

diameter aluminium particles in air, following exposure to stationary laser light fluxes. At

incident fluxes approaching 150 W/cm2 melting of the particle took place, but ignition

occurred only at fluxes higher than 250 W/cm2. Coefficients of reflection were not

measured, but were assumed to be in the range 96 to 50%, which means that less than half

of the incident light flux was absorbed by the particle. The time from onset of radiant

heating to ignition increased with particle diameter from 100 ms for 230 pm, via 270 ms

for 400 pm, to 330 ms for 680 pm.

Ermakov et af. (1982) measured the surface temperature of 400-1200 pm diameter

aluminium particles at the moment of ignition. The heating was performed by a

continuous laser of wavelength 10.6 pm at a constant flux incident on the particle in the

range 1500-4500 W/cm2, i.e. much higher than the experimental range of Razdobreev et

al. (1976). The particle temperature was measured by a tungsten-rhenium thermocouple,

whose junction of thickness 18-20 pm was located at the centre of the particle.

Microscopic high-speed film records were made synchronously with the recording of the

particle temperature at a rate up to 4500 frameds. The simultaneous recording permitted

detailed simultaneous comparison of the temperature of the particle with physical

phenomena observed on the particle surface. The appearance of a flame in the form of a

tongue on a limited section of the surface was noted at a particle temperature of

258 Dust Explosions in the Process Industries

2070 k 50 K. With further heating to 2170 K, the flame tongue propagated to the entire

particle surface, and the particle temperature remained constant at 2170 K during the

subsequent burning. This temperature is slightly lower than the melting point of the oxide,

and Ermakov et al. challenged the oxide melting point hypothesis. They concluded that

the ignition temperature obtained in their experiments showed that ignition is not caused

by melting of the oxide film, but is a result of the destruction of the integrity of the film

due to thermomechanical stresses arising during the heating process. This was indicated by

photographs of the particle surface at the time that the flame tongue appeared. No

influence of the incident heating flux density on the stationary combustion temperature of

the particle was detected.

4.1.2

MAG N ESI UM

Cassel and Liebman (1959) found that ignition temperatures of magnesium particles in air

did not differ from those in pure oxygen. Therefore they excluded oxygen diffusion as the

reaction rate controlling mechanism in the ignition process, and proposed a theory based

on a simple chemical control Arrhenius term for describing the rate of heat generation per

unit of particle surface area. An average value of the activation energy of 160 f 13 J/mole

was derived from the available experimental data.

Cassel and Liebman (1963) measured the ignition temperatures of single magnesium

particles of 20 to 120 pm diameter by dropping the particles into a furnace containing hot

air of known temperature. They found that the minimum air temperature for ignition

decreased systematically with increasing particle size, being 1015 K for a 20 pm diameter

particle, 950 K for 50 pm, and 910 K for 120 pm.

Cassel (1964) proposed a physical model for the combustion of individual magnesium

particles, as illustrated in Figure 4.1. After ignition, the oxide layer that coats the particle

prior to ignition, is preserved, only growing slightly in thickness. During combustion, the

oxide shell encloses the evaporating metal drop, while superheated metal vapour diffuses

through the semi-permeable shell to the outside and reacts with oxygen that diffuses

toward the particle from the ambient atmosphere. The rate of burning of the particle is

therefore governed by the rate of oxygen diffusion towards the reaction zone. In the initial

stage of combustion the site of reaction is close to the outer surface of the oxide layer.

However, owing to depletion of oxygen, this zone is detached from the oxide surface and

shifted to a distance, L, from the particle shell. The rate of oxygen diffusion and the rate

of combustion are determined by the gradient of oxygen partial pressure at ro + L. This

gradient remains approximately constant over the lifetime of the burning particle, except

for the final stage, when the reaction zone withdraws to the oxide shell.

Cassel (1964) also suggested a theoretical model for the combustion of a magnesium

particle. On the assumption that the location of the liquid drop inside the oxide shell is

unimportant, and that the rate of oxygen diffusion is always slower than the rate of the

chemical reaction, the burning rate of a magnesium particle is given by the quasi￾stationary balance of the oxygen diffusion rate:

- DP P -PL w,, = 4.rr(ro + L) - In -,

RT p-pp

Propagation of flames in dust clouds 259

and the rate of metal vaporization:

- 4np? dr

ME dt

w,, (44 = - --

Here D is the average oxygen diffusion coefficient at average temperature T, M is mole

weight of magnesium, p is density of magnesium, E is oxygen equivalent (=2 for oxidation

of magnesium), p is absolute total pressure at distance ro (just outside of the oxide shell),

and pL and px are the partial pressures of oxygen at distances L and infinity.

Figure 4.1 Model of burning magnesium particle (From Cassel, 19641

The time T required for complete combustion of a particle is obtained by combining

equations (4.1) and (4.2) and integrating from the initial drop radius ro to zero. The

resulting equation is:

(4.3)

7=- PRT 4 Iln (P-PL)

MEDP 3(ro + L) P -P=

Equation (4.3) was used to derive values of (DIT) from observed T values. It should be

noted that p, pm, and D refer to different temperatures, namely the boiling point of the

metal, the ambient gas temperature, and the temperature in the diffusion zone near the

reaction front, T. The estimates of D assuming molecular diffusion, gave an unrealistically

high T value of 4860 K for a magnesium particle burning in air. Cassel suggested therefore

that the combustion of magnesium particles is governed predominantly by diffusion of

atomic oxygen. He also suggested that the same must be true in any dust flame burning at

3000°K or more.

Liebman et al. (1972) studied experimentally the ignition of individual 28-120 pm

diameter magnesium particles suspended in cold air, by an approximately square laser

light pulse of 1.06 or 0.69 pm wavelength and 0.9 ms duration. The results suggest that

during heating of a magnesium particle by a short flash of thermal radiation, the particle

temperature first rises rapidly to the boiling point. Vaporized metal then expands rapidly

from the particle surface, and vapour-phase ignition may occur near the end of the radiant

260 Dust Explosions in the Process Industries

pulse. In accordance with the model proposed by Cassel (Figure 4.1), ignition is assumed

to occur at some distance from the particle surface where conditions (magnesium and

oxygen concentrations, and temperature) are optimal. The onset of ignition was character￾ized by the rapid appearance of a large luminous zone. Radiant intensities required to

ignite the particles were found to increase with particle size and the thermal conductivity

of the ambient gas environment. In accordance with the results from hot gas ignition,

there was little change in the radiant intensities required for ignition when replacing air by

pure oxygen.

Florko et al. (1982) investigated the structure of the combustion zone of individual

magnesium particles using various techniques of spectral analysis. They claimed that their

results confirm the assumption that the oxide, after having been generated in the gas phase

in the reaction zone, condenses between this zone and the surface of the burning particle.

This observation is an interesting supplement to the observation made and the physical

model proposed by Cassel (1964).

Florko et af. (1986) estimated the temperature in the reaction zone of burning

magnesium particles as a function of the pressure of the ambient gas, by analysing the

spectrum of the unresolved electron-vibration bands of the MgO molecules in the reaction

zone. For large particles of 1.5-3 mm diameter, the reaction zone temperature was

practically independent of the gas pressure and equal to 2700-2800 K in the range 0.3 to 1

bar (abs.). When the pressure was reduced to 0.05 bar (abs.) the reaction zone

temperature dropped only slightly, to about 2600 K. The burning time of 1.5-3 mm

diameter particles was proportional to the square of the particle diameter. For a 2 mm

diameter particle at atmosphere pressure, the burning time was about 6 s. Extrapolation

to 60 pm particle diameter gives a burning time of 5.4 ms, which is quite close to the times

of a few ms found by Cassel (1964) for Mg particles of this size. When the pressure was

reduced to 0.2 bar (abs.), Florko et al. found a slight reduction, by about lo%, of the

burning time.

4.1.3

ZIRCON I UM

Nelson and Richardson (1964) and Nelson (1965) introduced the flash light heating

technique for melting small square pieces of freely falling metal flakes to spherical

droplets. They applied this method for generating droplets of zirconium, which were

subsequently studied during free fall in mixtures of oxygen and nitrogen, and oxygen and

argon. The duration of the light flash was only of the order of a few ms. A characteristic

feature was the sparking or explosive fragmentation of the drop after some time of free

fall. This was supposed to be due to the forcing out of solution of nitrogen, hydrogen, and

carbon monoxide that had been chemically combined with the metal earlier in the

combustion process. The experimental results for air at atmospheric pressure showed, as a

first order approximation, that the time from droplet formation to explosive fragmen￾tation was proportional to the initial particle diameter. The relative humidity of the air had

only marginal influence on this time. The heat initially received by a given particle by

the flash was not specified.