J. Loss Prev. Process lad. Vol. 10. No. 4. pp. 231-242, 1997 0 1997 Elsevier Science Ltd

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Initiation of smouldering fires in combustible bulk materials by glowing nests and embedded hot bodies

Ulrich Krause*? and Martin Schmidt?+ tFederal Institute for Materials Research and Testing (BAM), D-12200 Berlin, Germany SOtto-von-Guericke University, D-39016 Magdeburg, Germany

Smouldering fires in large storage facilities may be caused by glowing nests or embedded hot bodies and remain undiscovered for very long times. Due to large temperature gradients near the glowing nest in a deposit of bulk material the detection of a smouldering fire is difficult. This paper reports experimental investigations on thermal conditions, which may cause or pro- mote an ongoing smouldering process, e.g. critical initial temperatures of embedded hot bodies or critical initial sizes of glowing nests. Propagation velocities of smouldering fires were found to be independent of calorific properties of three combustible dusts. 0 1997 Elsevier Science Ltd. All rights reserved

Keywords: Smouldering fires; glowing nests; smouldering temperatures; fire propagation velo- city

1. Introduction Recent fire accidents in industrial sites in Germany- each causing losses of millions of DM, several injuries and in one case even death of fire fighters-show a con- siderable hazard due to nests of glowing material or glowing parts from machinery, which may uninten- tionally enter into deposits of combustible solid material and initiate catastrophic fires, see ref. [l].

Moreover a statistical review of about 600 dust explosion events in Germany since 1970 indicates that for about 13% of them glowing nests were assumed to be the ignition source, see Beck [2].

An especially difficult problem in preventing fires and explosions is, that glowing nests are hard to detect in large storage facilities and may sustain smouldering processes over periods of days or even weeks.

The characteristic parameters to evaluate the risk attributable to the existence of heaps or layers of com-

*To whom correspondence should be addressed. Phone + 49 30 8104 3412; Fax + 49 30 8104 1217; E-mail: 4120krause@- axpl.rz.bam-berlin.de

bustible bulk material are the self-ignition temperature and the glow temperature, see VDI guideline 2263 [3].

For glowing nests or hot bodies appearing in a deposit of bulk material the initial or boundary con- ditions for the heat transfer through the deposit deviate from those for self-ignition or glowing. For glowing nests in a powder deposit time-dependent boundary con- ditions are valid, while for embedded hot bodies non- uniform initial conditions exist. Therefore experimental results from the usual hot storage tests (self-ignition) or hot plate tests (glowing) cannot give sufficient infor- m&on about the thermal transport behaviourin the case of glowing nests or embedded hot bodies.

The present work was intended to investigate the ‘critical’ conditions for bulk materials, which may result in a smouldering fire initiated by glowing nests or hot bodies. Special items under consideration were:

critical size of the glowing nest or the hot body, ‘lifetime’ of glowing nests, influence of the size of the deposit, initial temperature of the hot bodies entering the deposit,

237

238 Initiation of smouldering fires: U. Krause and M. Schmidt

properties of the bulk material (calorific value, ther- Figure 1 shows a schematic view of the experi- mal conductivity, apparent activation energy, etc.), mental set-up also containing the positions of thermo- transition to open fire. couples inside the cylinder.

3. Experimental results 3.1. Cork dust

The results obtained promise to characterise the process of smouldering in terms of smouldering propa- gation velocities depending on the above parameters. The present paper reports on experiments with three dif- ferent powder materials being stored in cylindrical vol- umes of different sizes. Ignition was caused by introduc- ing glowing nests or heated spherical bodies into the powder deposit at a fixed moment.

2. Experimental set-up

The experimental arrangement consists of cylindrical mesh wire baskets with volumes of 6, 12 and 50 1 (height to diameter ratio equal to 1). The smallest volume of 6 1 was chosen considering that it is still two orders of magnitude larger than the largest glowing nest used. On the other hand the volumes of the cylinders had to be small enough to carry out experiments within a reason- able span of time.

In the first series of experiments a porcelain sphere of 40 mm in diameter was introduced into a deposit of cork dust contained in the 12 1 cylinder. The initial tempera- ture of the sphere was varied while all other parameters remained constant. As mentioned above the powder sample was initially at room temperature. It was observed that ignition and self-sustaining combustion of the sample occurred with an initial temperature of the sphere of 435T, whilst with 420°C this was not the case. In the 6 1 cylinder ignition was observed for a tem- perature of the sphere of 437°C while at 419°C ignition of the powder sample did not occur. Hence, it seems that for this size of a hot body the ‘critical’ initial tempera- ture for the onset of a smouldering combustion process lies between 420 and 435°C and is largely independent from the volume of the sample, i.e. its thermal resistance.

The preparation of the glowing nests or the hot bod- ies took place in an oven, which could be heated up to 1200°C. In order to minimize the heat loss of the ignition source the following procedure was chosen.

A removable tube was introduced from the side wall into the cylinder in a way that the glowing nest or the hot body could be placed exactly in the centre of the cylinder. Then the cylinder was completely filled with the powder material which was initially at room tem- perature.

Further experiments were undertaken to investigate the temperature evolution for initial temperatures below the ‘critical’ temperature level. Figure 2 shows the tem- perature dependence on time for the 40 mm porcelain sphere in the 12 1 cylinder. The thermocouple was placed in the center of the sphere. It is interesting to note that for lower initial temperatures (from 363°C to 388°C) the sphere temperature decays exponentially as it is expected for an ordinary cooling process. In these cases heat trans- fer from the powder deposit into the sphere caused by an exothermic reaction did not occur.

At the start of an experiment the ignition source preheated in the oven was quickly introduced through the removable tube into the centre of the cylinder. Then the tube was pulled out of the powder sample. In this way a quick placement (within less than 5 s) of the ignition source could be achieved and the heat loss could be kept low.

The time-dependent temperature evolution within the cylinder was observed by up to 15 thermocouples at different radii and heights in the cylinder. In addition, one thermocouple was placed in the centre of the ignition source.

For initial temperatures from 400°C to 420°C the curves show a weaker descent which means that the coo- ling is delayed considerably. In a certain part of the pow- der deposit adjacent to the hot sphere a partial combus- tion process connected with a small production of heat had already started. However, the total amount of heat release was not sufficient to cause a self-sustaining com- bustion throughout the entire sample.

As ‘hot bodies’ porcelain spheres with diameters of 25,30,35 and 40 mm were used for one type of experi- ment.

Figure 3a and b show the temperature evolution with time at different locations in the powder sample for initial temperatures of the sphere of 363°C and 420°C in the 12 1 cylinder. As visible from Figure 3a, the half- value decay time of the sphere temperature was in a range of 45 min. After 2 h the sphere temperature was still at a level of 1OOT.

For the second type of experiment small cylindrical wire mesh baskets of 11, 15, 25, 34, 40 and 45 mm in diameter were applied (height to diameter ratio equal to one). Dust of the same material, just as in the large cylin- der, was filled into the small baskets by this way forming a realistic glowing nest.

The temperature of the thermocouple nearest to the sphere (r = 25 mm) reached a maximum value of 150°C after 17 min, while after 2 h 75°C were still observed. At larger distances from the hot sphere the temperature of 50°C was not exceeded.

The materials under consideration were cork dust, beech wood dust and cocoa powder. Table 1 contains selected material properties essential for the present investigations.

For a starting temperature of the hot sphere of 42OT, see Figure 3 b, the half-life decay time was 82 min. The curve shows two points of inflexion which indicate the onset and the extinction of a limited smould- ering process.

Initiation of smouldering fires: U. Krause and M. Schmidt 239

Table 1 Thermal and chemical properties of the materials under investigation

Substance Grain size Bulk density Calorific value App. activation energy

Heat conductivity

Cork dust Beech wood Cocoa

m kg/m3 < 500 200 < 150 382

< 32 337

MJlkg J/mol W/(m.K) 27.1 99 362 0.105 17.8 97 501 0.225 20.3 110905 0.1

k material sample

Figure 1 Experimental setup for the investigation of smoulder- ing combustion caused by glowing nests or embedded hot bod- ies

450

400

0 350

E f 3W

1 2 250

5 200

loo 1 I 0 20 40 60 80 100

lime in min

Figure2 Temperature decay of a hot sphere (40 mm in diameter) at different initial temperatures in a 12 liter cylindrical deposit of cork dust

In fact after the end of the experiment a layer of char with a thickness of about 4 mm was found around the porcelain sphere. This had not been the case for a starting temperature of 363°C.

Figure 3c shows the temperature evolution with time at different locations in the powder deposit for an initial temperature of the hot sphere of 435°C. In this case the temperature in the centre of the sphere decayed to a value of 335°C after 33 min and remained nearly constant over a period of another 220 min. After this the sphere was heated up to more than 500°C by the heat of reaction released from the powder sample.

At the location r = 25 mm from the hot sphere the temperature rose to a value of 150% after about 10 min

I 0 20 40 60 80 100 120

400

0

7w

600

loo

0

1 0 50 loo 150 2m 250 300

1

1 -I

0 loo 2w_hmh3W 400 SW

Figure 3 a. Temperature development in a 12 I deposit of cork dust at different radial positions-initial temperature of the embedded hot sphere 363°C. b. Initial temperature of the embedded hot sphere 420°C. c. Initial temperature of the embedded hot sphere 435°C

and to 330°C after 100 min. After this it remained con- stant at this level for another 150 min. The temperatures at growing distances,from the hot sphere followed con- secutively. It seems that the temperature of 330°C is a characteristic value for the propagation of the smoulder- ing process, which is presumably influenced by the avail- ability of oxygen in the powder sample. This assumption

240 Initiation of smouldering fires: U. Krause and M. Schmidt

is supported by the fact that a temperature rise above this level of 330°C appears after about 275 min first at the thermocouple located at the surface of the powder deposit (r = 12.5 cm).

It was observed in the experiments that during the propagation of the smouldering process inside the pow- der sample the sample volume remained constant. No observations of an ongoing combustion process in the sample could be made from the outside until the com- bustion front had reached the surface. After this flue gases were released and glowing occurred. The combus- tion now propagated reversely from the surface back into the sample. Due to a sufficient supply of oxygen the combustion was at this stage presumed to be complete and only a very small ash layer remained as a solid resi- due.

The maximum temperature observed in the experi- ments where only glowing occurred was 900°C. In a number of experiments transition to open flames took place after the combustion front had reached the deposit surface. For these cases temperatures up to 1200°C could be measured.

After about 7 h the combustion process was com- pleted and ambient temperatures were detected by the thermocouples, which were now hanging freely above the ash layer.

The described experiments were repeated for the 6 1 cylinder. The results obtained are analogous to those for the 12 1 sample, however, the time scales for cooling and, respectively, combustion are somewhat smaller. The whole combustion process in the 6 1 cylinder was completed after about 6 h.

Further experiments were carried out to ignite cork dust in the 6 1 and the 12 1 cylinder by glowing nests produced in the way described in Section 2. Ignition could be observed for a glowing nest of 40 mm in diam- eter, while for smaller nests extinction occurred.

Figure 4 shows a typical evolution of the tempera- ture field with time along the radius of the cylinder in the centre cross section. The volume of the cork dust sample in question was 12 1. The temperature in the centre of the glowing nest first rose for about 1 h on forming char to a level between 300 and 400°C. How-

700 , I

ml

VW . .s 400 i

g 300

E 200

0 100 200 300 4OLl time in min

Figure4 Temperature development in a 12 I deposit of cork dust at different radial positions initiated by a preheated cylin- drical glowing nest of 40 mm in length and diameter

ever, presumably due to a lack of oxygen in the deposit no runaway reaction occurred and the nest temperature decayed to the level of about 330°C which was found to be characteristic for the propagation of the smoulder- ing process.

In the case shown in Figure 4 the self-heating of the glowing nest was sufficient to initiate a self-sustaining smouldering process in the adjacent region of the nest. At a radial distance of 25 mm from the centre of the dust sample a temperature of 160°C was measured after one hour. After about 2 h a temperature of 3OOT was exceeded at the same location, while after about 3 h the combustion front had propagated to the location of ther- mocouple 1, 50 mm apart from the centre.

After about 230 min the combustion front had reached the surface of the sample cylinder (r = 12.5 cm), where now a comparatively steep temperature rise from 50°C to about 580°C within 30 min was observed.

The maximum temperature level of 730°C in this experiment (not depicted in Figure 4) indicated that a glowing fire without transition to flame took place. The combustion of the solid material was completed after 6 h, which was exactly the same time as found for the case of ignition by the hot sphere.

However, it has to be mentioned that the process departed considerably from the one-dimensional case, which had previously been assumed. The combustion front tended to move quicker in the upward direction than to the bottom. In addition during the first experi- ments it seemed that the smouldering process was influ- enced by the existence of the removable tube (chimney effect) and by the ventilation system of the room where the experiments were carried out. Therefore efforts were undertaken to mitigate disturbances and a better approach to one-dimensionality was achieved in sub- sequent experiments.

Beech wood dust For beech wood saw dust ignition experiments were car- ried out in the 12 1 cylinder using a hot sphere with 30 mm in diameter as an ignition source. The dust sam- ple could be ignited by a sphere with an initial tempera- ture of 550°C. In a sequence of experiments with initial temperatures from 295°C to 490°C a self-sustaining smouldering fire could not be detected. However, for an initial temperature of 490°C a partial combustion took place forming a char layer around the hot sphere.

The maximum temperature detected was 850°C which means that an open flame may have occurred, being obviously extinguished while the combustion propagated from the surface back into the cylinder. Maximum temperatures were measured last at the bot- tom of the cylinder.

Again, the combustion front preferably propagated upwards. The combustion process was completed after about 15 h and hence it was considerably longer than found for cork dust. However, a direct comparison is difficult because the hot sphere, which initiated the

Initiation of smouldering fires: U. Krause and M. Schmidt 241

smouldering fire, was smaller in the case of the beech wood dust.

Further experiments were conducted by introducing glowing nests into a cylindrical heap of 12 1. Ignition could be achieved by glowing nests with 34 mm and 40 mm in diameter, while a 24 mm glowing nest did not lead to an ignition. The behaviour of the temperature was similar to that of cork dust. After a self-heating pro- cess had started in the nest a temperature of 340°C in the centre of the nest remained constant for about 300 min. During this time the parts of the dust sample adjacent to the glowing nest showed a temperature increase up to the characteristic ‘smouldering temperature’.

Cocoa powder For cocoa powder only a few experiments were carried out. Glowing nests with diameters 11, 15, 24 and 40 mm were introduceded into a powder sample of 6 1. It was found that all the glowing nests except that of 11 mm were able to initiatiate a combustion process within the cocoa powder. A typical temperature evolution with time is given for the 40 mm glowing nest in Figure 5. Here also a nearly constant temperature of the smouldering process in the glowing nest of 340°C was obtained for a period of upto 6 h.

For all the experiments with cocoa powder which ended in a self-sustaining combustion transition to flame was observed. Therefore the temperatures measured were higher than for the other substances and reached values from 900°C up to 1100°C.

For the 40 mm glowing nest the combustion process ended after about 600 min. While for the 24 mm glowing nest a remarkable difference in the time scale did not occur, for the 15 mm glowing nest the process was delayed for approximately 1 h.

Discussion It was found that deposits of solid powder materials can be ignited by glowing nests or embedded hot bodies, if these satisfy certain critical conditions. For glowing nests a critical size, represented by the radius seems to exist. For cork dust ignition of the sample occured for a glowing nest of 40 mm in diameter. Beech wood dust

0 100 200 300 400 SW 6CUl 700 time in min

Figure 5 Temperature development in a 6 I deposit of cocoa powder at different radial positions initiated by a preheated cyl- indrical glowing nest of 40 mm in length and diameter

could be ignited with a nest of 34 mm, while for cocoa powder a nest of only 15 mm in diameter was sufficient to cause an ignition.

From first qualitative experiments it seems that the critical size of a glowing nest depends on the grain size and the grain size distribution of the powder; however, this has not yet been investigated systematically.

On the other hand, a comparison of propagation velocities of the smouldering process shows the follow- ing results:

For cork dust a propagation velocity of 0.5 + 0.05 mm/mm was measured independently of the kind (glowing nest or hot body) and the size of the ignition source. Only for a diameter of the hot sphere of 30 mm the smouldering propagation velocity reached a value of 0.39 mm/mm.

For beechwood dust a smouldering propagation velocity of 0.35 f 0.03 mm/mm was detected, while for the cocoa powder 0.21 & 0.03 mm/min occurred. Con- sidering Table I it is not possible from the data available to explain the smouldering propagation velocity by one of the material properties only.

However, the following relations are remarkable: Cocoa powder possessing the highest apparent acti-

vation energy showed the lowest value of the smoulder- ing propagation velocity. The apparent activation ener- gies of the two other substances are approximately the same. On the other hand, cork dust having the highest calorific value reached the highest propagation velocity, while a ranking of the propagation velocities of beech- wood dust and cocoa powder according to their calorific values is not possible.

However, the influence of other properties, e.g. bulk density, grain size, thermal conductivity and specific heat capacity has to be a subject of subsequent investi- gations. Especially the amount of smouldering gases developed during the process and diluting the oxygen content in the intragranular volume should be taken into account.

For the purposes of risk assessment under practical conditions the following conclusions are drawn:

1. The onset of a smouldering process may be caused by glowing nests or embedded hot bodies with initial temperatures above 300°C.

2. The propagation velocity of the smouldering combus- tion in a pile of solid bulk material seems to be inde- pendent from the energy of the ignition source. Once

Table 2 Computed initial temperatures of hot bodies initiating smouldering

Diameter of the hot sphere

mm 25 30 40

Cork dust Beechwood dust

“C “C 326 315 318 312 304 305

242 Initiation of smouldering fires: U. Krause and M. Schmidt

3.

4.

5.

initiated, smouldering processes with a velocity, which may be attributed to the properties of the material and the availability of oxygen. While according to item (2) the propagation velocity is independent from the ignition source the initiation of the smouldering reaction depends on the size and the initial temperature of the ignition source (see Table 2). For embedded hot bodies the following is valid: the larger the size of the hot body the smaller the initial temperature for ignition may be. However, it is still questionable, if an ignition can be achieved at all for initial temperatures of the hot body below 300°C. The onset of the combustion in the bulk material depends also on the temperature distribution in the hot body, i.e. on the total amount of heat energy transported into the powder sample per unit of time. However, this effect has to be investigated further in detail. The glowing nests, once introduced into the powder sample, sustain a smouldering process at a nearly constant temperature of 330°C to 350°C over periods

of up to ten hours in the laboratory-scale bulk vol- umes. It is obvious from the experiments that the life- time of glowing nests is longer the lower the grain size of the powder is.

Glowing nests as a consequence of thermal loads of a storage tank or formed around embedded hot bodies are likely to have lifetimes of days or even weeks. How- ever, since the smouldering combustion usually propa- gates through the material stored and therefore the vol- ume of a glowing nest changes with time it seems more appropriate to define a propagation velocity instead of a ‘lifetime’. Future investigations will therefore be dedi- cated to a more systematic analysis of the influences of material properties, the oxygen content and the propa- gation velocity of the smouldering combustion process.

References [l] Schubert, J. and Bijhme, H., Feuewehr-Z&rung, 1991, 6, 288-

293. [2] Beck, H. and Jeske, A., VDI-Report 1272. Diisseldorf, 1996, pp.

365-388. [3] VDI guideline 2263, Dust Fires-Dust Explosions. Beuth,

Berlin, 1992.