Combustion Science and Technology

This article was downloaded by:[China Science & Technology University] On: 26 October 2007 Access Details: [subscription number 780787262] Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK

Combustion Science and Technology Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t713456315

Combustion Behavior of Iron Particles Suspended in Air Jin-Hua Sun a; Ritsu Dobashi a; Toshisuke Hirano a a Department of Chemical System Engineering, School of Engineering, The University of Tokyo,

Online Publication Date: 01 March 1990 To cite this Article: Sun, Jin-Hua, Dobashi, Ritsu and Hirano, Toshisuke (1990) 'Combustion Behavior of Iron Particles Suspended in Air', Combustion Science and Technology, 150:1, 99 - 114 To link to this article: DOI: 10.1080/00102200008952119 URL: http://dx.doi.org/10.1080/00102200008952119

PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article maybe used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

Downloaded By: [China Science & Technology University] At: 09:43 26 October 2007

e 20CK1 OPA (Overseas Publishers Association) N.V.

Combust. Sci. and Tech., 20CK1, Vol. 150. pp. 99·114 Reprints available directly from the publisher Photocopying permitted by license only

Published by license under the Gordon and Breach Science Publishers imprim. Printed in Malaysia

Combustion Behavior of Iron Particles Suspended in Air JIN-HUA SUN, RITSU DOBASH( and TOSHISUKE HIRANO Department of Chemical System Engineering, School of Engineering, The University of Tokyo (Received November 02, 1998; Revised March 02, 1999) The combustion zone propagating through an iron particle cloud and the combustion behavior of individual iron particles have been examined by using high-speed photomicrographs. Propagation of the combustion zone of 4-5 mm in width was observed as the movement of a luminous zone which consists of burning iron particles. In the region just behind the leading edge, burning particles of various diameters are examined. As the distance from the leading edge becomes larger, smaller particles are fading away, and then only large particles are observed to remain luminous in the region where the distance is larger than 2 mm. Each iron particle bums at the combustion zone without gas phase flame. The bum-out time (the duration of light emission) is proportional to the diameter of iron particle when the particle diameter is not so large. It agrees well with the result of a simple analysis. As the particle diameter becomes larger, the burn-out time becomes much larger than that predicted by the simple analysis.

Keywords: Iron particle cloud; Structure of combustion zone; Propagation of combustion zone; High-speed photomicrograph

INTRODUCTION Accidental dust explosions have happened in the mining of coal and other industries. The hazards 'of combustible dust clouds have been well recognized, and many practical studies for prevention of the losses by such explosions have been performed for past few decades (Hertzberg et al., 1982, 1986, 1992; Cashdollar et aI., 1988, Cashdollar, 1996; Dahoe et aI., 1996; Tamanini and Valiulis, 1996; Siwek, 1996). In these studies, the explosion pressure, rate of pressure rise, explosion temperature, minimum ignition energy, and explosion limits are measured by using closed experimental chambers. Through those studies, various practical data of dust explosions became available. Indeed, these data are useful

* E-mail: [email protected] 99


100

JIN-HUA SUN et al.

for evaluating the hazards involved in the manufacture, transport, storage and use of combustible partices. However, in order to appropriately take measure against dust explosion, only applied studies of dust explosion are not enough. Basic knowledge of dust explosion phenomena, such as structure and propagation mechanisms of flames (Seshadri et aI., 1992; Chen et aI., 1996; Ju et aI., 1997) or combustion zones and behavior of particles during the flames or combustion zones propagating through metal dust clouds would. be also important. Especially, the burning behavior of metal particles without flam~ in ihe gas phase is not clear, despite of the importance to mitigate losses cause.d, by explosion of a cloud composed of such metal particles. Only a few studies have been conducted to elucidate the burning behavior of metal particles during explosion (Sun et aI., 1998a, 1998b; Rozenband and Vaganova, 1992). In our previous studies, the structure of combustion zones propagating through iron particle clouds and behavior of the particles near the combustion zones have been examined experimentally (Sun et aI., 1988a, 1988b). However, the results obtained in those studies are not sufficient to reasonably explain the mechanisms of the combustion zone propagation. Thus, in the present study, the burning behavior of individual iron particles in the combustion zone is examined in detail.

EXPERIMENTAL APPARATUS AND PARAMETERS Most of explosibility data of dusts were measured by using small-scale and intermediate-scale test chambers, such as the ISO dust explosion chamber, Hartmann pressure bomb, U. S. Burean of Mines 20 liter laboratory explosibility test chamber, which are all closed chambers (Baker and Tang, 1991). They are suitable for measurements of the explosion pressure, maximum rate of pressure rise, explosion temperature and lean limit concentration. However, it is difficult to examine the flame structure and flame propagation mechanisms by using such apparatus. In this study, an experimental system was designed so as to make the structure and the flame propagation mechanisms. of metal particles in a flame of their cloud clear. Figure I shows the experimental system composed of an air supplying part, a controller part, a combustion chamber of Lliter in size, an ignition part and a recorder part. The combustion chamber shown in Fig. I is provided with an air nozzle, a sample dish, a pair of ignition electrodes, a movable tube, and a vent with mesh. Before the movable tube starts to move down, the. iron dust is dispersed by air into the combustion chamber, Just after the movable tube moves down to its bottom position, the suspended iron dust is ignited by an electric spark. A flame starts to propagate through the metal particle cloud. Using this chamber, the flame can propagate in an open field without any influence of the


101

COMBUSTION OF AIR

chamber wall, and observation can be achieved directly. The ignition pan consists of a pair of the electrodes and a neon transformer with the output voltage of 15,000 V. Mesh Monitor

Ignition electrode

High speed video camera

)

Controller

Air tank

\

i

__

---....._._.. ..---.J

- - _...

I

\~._._..._ /

FIGURE I Experimental setup

In order to explore the detailed structure of the combustion zone propagating through an iron particle cloud and combustion process of iron particles, the pure iron dust was used, because it is suitable for fundamental studies of combustion and explosion to use pure uniform substances with well-characterized phase transitions. The iron particles were dispersed by air and ignited by an electric spark. The combustion zone propagation process and the behavior of burning iron particles were recorded on photomicrographs by using a high-speed video camera (recording speed is 10D-2000 frames/s) with a microscopic optical system, and the combustion product of iron particles were observed by using a scanning electron microscope. In this study, two kinds of iron particles of different diameter distributions (iron particles(l) and iron paniclest ll) were used (purity: 99.5%). The diameter of particles and its distribution were measured by using the scanning electron microscope photographs. It was observed that the iron particles were almost spherical. The measured diameter distribution is shown in Fig. 2. The diameters


102

JlN-HUA SUN el al.

of the particles are distributed mostly from 111m to 311m for iron particles(l), from 211m to 4.Sl1m for iron particles(I1). The particle concentration was measured by a simple sampling apparatus. The particles were sampled in a confined apace using shutter at the ignition moment, and their mass was measured by a balance.

25-r-----

_ Eill

Iron particle(l)

III

Iron particle(ll)

Particle diameter, I'm

8

FIGURE 2 Iron particle diameter distribution

The experimental procedure was as follows: At first, the iron particles to be tested were placed on the sample dish and were dispersed by an air blow supplied from the air tank of 0.10-0.11 MPa in gauge pressure. The duration of air blowing time was 0.1 s. The iron particle cloud was ignited by an electric spark just after the movable tube moved down(the delay time from the end of iron particle injection to ignition was 0.4 s). The discharge period of the spark was 0.03 s, and the ignition energy was about 4.8 J.

RESULTS AND DISCUSSION Detailed structure of combustion zone propagation through an iron particle cloud Figure 3 shows the direct photographs of the flame propagating through iron paruclerl) cloud of 0.79kg/m 3 in concentration recorded by a normal speed (30fps)


COMBUSTION OF AIR

103

CCO video camera. It is seen that at the ignition stage the flame shape is irregular, when the spark may influence the flame propagation. At about 40ms after ignition (i.e., about 15 mm from the ignition point) the flame becomes to propagate spherically and it's propagating velocity is almost constant with the time during the measurement (1=70 ms-90 ms) (Sun et al., 1999).

/=33 ms

1=17 ms

1=50 ms

scale

o

1=67 ms

20mm

FlGURE 3 Images of an iron particle cloud flame recorded using a normal speed CeD video camera. Sample: iron particletl); cloud concentration: O.79kg/m 3; t: time from ignition (See Color Plate I at

the back of this issue)

Figure 4 shows a typical series of photomicrographs of combustion zone propagating through a cloud of iron particles(lI) of 1.05kg/m 3 in concentration. In Fig.4(a), many luminous points can be observed on each high-speed photomicrograph. The luminous points represent the direct emission images of burning iron particles. Some of them are clear and others are fuzzy. The clear points must be those near the focal point of the optical system, and the fuzzy ones must be out of the focal point. The behavior of combustion zone through an iron particle cloud can be identified as the movement of a luminous zone which consists of


104

JlN-HUA SUN et al.

luminous particles (burning iron particles). The width of the combustion zone can be easily determined to be 4-5 mm by measuring the distance from the leading edge of combustion zone to the end line of clear luminous points on the high-speed photomicrographs shown in Fig. 4. The leading edge of the combustion zone is clear and smooth. It is seen that just after the unburned iron particles enter into the combustion zone, they immediately start to bum and become luminous as recorded on the photomicrographs. When they bum out, their temperature decreases due to heat loss and they fade out. In the region of 1-2 mm from the leading edge, burning particles of various diameters are observed (Fig. 4 (b)). As the distance from the leading edge increases, the number of luminous particles decreases. In the region at the distance larger than 2 mm from the leading edge, only large particles remain observable: The burn-out time of each iron particle increases with its diameter.' According to a series of high-speed video images, the. burning iron particles move in the same direction with the combustion zone. The concept of combustion zone propagation and particle motion in this case is shown in Fig. 5. The combustion zone propagates upward at an almost constant velocity. However, the velocity of each burning iron particle is not constant. The movement of burning iron particles have been examined in detail. Figure 6 shows variation of the velocity of burning iron particles with the distance x to the leading edge of the combustion zone. It is seen that the burning iron particle velocity decreases with the increase of x. The burning iron particles change their direction of movement from upward to downward at x about 3.5 mm. In this case, the combustion zone velocity is about 75cmls for iron particles (Iljof 1.05 kg/rrr' in concentration.

Iron particle combustion No flame is seen in the gas phase. This would be attributable to the high boiling point of iron (3023K) (Weast, 1979), which is much higher than the adiabatic temperature of iron particle cloud combustion (2285K). This implies that iron vapor sufficient for sustaining a flame in the gas phase is not ejected from the particle surface even during its combustion. Figure 7 shows the scanning electron microscopic photographs of unburned and burned iron particles. It is seen that both the unburned and burned iron particles are spherical. However-the surfaces of burned iron particles are less smooth than those of unburned ones. The average diameter of burned iron particles is larger than that of unburned ones. This increase of the diameter must result from combustion reaction. Figure 8(a) shows the distribution of burned and unburned iron particle diameters. The average diameter of burned iron particles are 1.2 times larger than that of unburned iron particles. The density of iron oxide is


COMBUSTION OF AIR

1=74ms

t=78ms

1=86ms

1=90ms

105

te82ms

1'="94ms

Scale '-------' 2mm

o

(a) Combustion zone propagation process

Luminous zone 4-5mm

Small and large iron particles burning region 1-2mm Only large iron particles burning region

Clear luminous points

Fuzzy luminous points

Scale

o

2mm

(b) Structure of iron particle cloud combustion zone FIGURE 4 High-speed photomicrographs of combustion zone propagating through an iron particle cloud. Sample: iron particle(II); cloud concentration: 1.05kglm 3; framing rate: 1,000 frames/s; I: rime from ignition (See Color Plate II at the back of this issue)


106

JIN-HUA SUN et al.

t:!

/

Particle B Leading edge of combustion zone FIGURE 5 Conceptof combustion zone propagation andpanicle motion

smaller than that of iron, and the mass of each particle increases due to oxidation by combustion. Diameters of a few burned iron particles are found to be larger than 10 J.1m. Despite of inclusion of such larger particles, the measured diameter distribution of burned iron particles agree fairly well with the diameter distribution calculated on the bases of the measured unburned particle diameter distribution assuming the iron oxide as FeO (Fig. 8 (b)). A few exceptions found in the range of larger particles are inferred to be caused by agglomeration. The combustion mechanism of solid iron burning in pure oxygen have been studied (Hirano et al., 1984, 1993; Sato et al., 1983). They proposed a model of the iron oxidation process at combustion. The oxidation process consists of the following three steps.

I. The reaction at the oxygen-oxide boundary, which consists of the following elementary steps: l-a Oxygen molecules are physically adsorbed on the oxide surface. l-b Physically adsorbed oxygen molecules dissociate and the oxygen atoms are chemically adsorbed on particular sites on the oxide surface. I-c The chemically adsorbed oxygen atoms are incorporated in the oxide.


COMBUSTION OF AIR

o

1

2

107

3

4

Distance from the leading edge of the combustion zone, x, mm FIGURE 6 Change of burning iron particle velocity with x

II. The diffusion of oxygen ions through the oxide layer. Ill. The reaction at the metal-oxide boundary. Among above three steps, step (I) is much slower than the other steps. In their model, an oxygen consumption rate ,ito is used to express the oxidation reaction rate of iron[20].

,i,· o = CP~'Tmexp(- ::T)'

(1)

where C, n, and m are constants. T is the temperature, Po is the pressure of oxygen.

R is the universal gas constant, E is the activation energy of step (I), and ,il.o is mass consumption rate of oxygen for per unit area at molten oxide surface. According to their model and experimental results of this study, the combustion model of an individual iron panicle can be illustrated as Fig. 9. In the case of a spherical single iron panicle, the equation representing the mass balance at the panicle surface is given by,

(2)


lOR

JIN-HUA SUN et al.

Unburned iron particles

Burned iron particles FIGURE 7 Scanning electron microscopic photographs


109

COMBUSTiON OF AIR

Gl1 Unburned •

Burned

4

8

10

Diameter, urn

25

* rii

..9l o

1m

(b)

20

'.;:l

....

12

Calculated Measured

ell

.....e,0 I::

0 '.;:l

'-' ell

J::

.... Q)

.D.

e;:l z

6

8

Diameter, 11 m

10

12

FIGURE 8 Diameter distribution of bumed and unburned iron particles, (a): Bumed and unbumed iron particle(ll) diameter distribution; (b): Measured and calculated diameter distribution of bumed iron particle(ll)

where s is the surface area of the molten oxide, t is the bum-out time, b is the mass required for oxidation reaction of iron per unit mass, Ro and p are the iron particle initial radius and iron density, respectively.


110

JlN-HUA SUN et al.

Molten oxide surface

Diffusion of oxygen

Reaction region FIGURE 9 Model of single iron particle combustion

If ,i,."s can be approximated to be constant and equal to ,;,,,,,s,, (,i,."" is the initial oxygen mass consumption rate, So is the initial surface area of the iron particle). Equation (2) can be written as, (3)

Equation (3) means that the bum-out time of an individual iron particle is almost proportional to its initial radius. Thus, the larger iron particles have longer bum-out times as observed in the experiments. The burn-out times for iron particles of various diameters were measured by analyzing the images on high-speed photomicrographs. The bum-out time of a specified iron particle was determined by the duration of its light emission, and the iron particle diameter was determined on the basis of that of the luminous point. The average diameter of luminous points just behind the leading edge of combustion zone is a few times larger than that of unburned iron particle meas-


III

COMBUSTION OF AIR

ured using a scanning electron microscope. This discrepancy might be caused by the aberration of the optical system. The images of luminous points should be expanded by the aberration effect. Since the diameter of the iron particle image is expected to be proportional to that of the object according to the geometric optics theory (Jenkins and Harvey, 1976), a non-dimensional diameter d/d m (d is measured diameter and dm is the measured minimum diameter) was introduced for discussion. Figure 10 shows the relationship between the bum-out time of iron particle and non-dimensional diameter. The bum-out time of iron particle is seen proportional to the non-dimensional diameter when the iron particle diameter is not so large. This result agrees well with that of our simple analysis. However, as the particle diameter becomes larger, the bum-out time becomes much larger than that predicted by the analysis. This phenomenon is inferred to be caused by the heat loss reduction per mass as the particle diameter becomes larger. Also the reduction of oxygen partial pressure around larger particles must be another reason for this phenomenon. The reduction of the oxygen consumption rate ,ito should be caused by the decrease of oxygen partial press are (Hirano et aI., 1984), so that the bum-out time of a larger iron particle would be longer than that predicted by the present simple analysis.

30, Experiment ~

Analysis ............•...•......

CF.

E c

....

20 l-

E

:l