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Smoldering Combustion

T. J. Ohiemiller

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

February 1986

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

“QC

100

.056

85-3294

1986

REFERENCE

PUBLICATIONS

NBSIR 85-3294

SMOLDERING COMBUSTION

T. J. Ohiemiller

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Fire Research

Gaithersburg, MD 20899

February 1986

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director

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Table of Contents

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List of F Igures v LlstofTables vl

1. INTRODUCTION 1

2. SELF-SUSTAINED SMOLDER PROPAGATION 3

2.1 One-Dimensional Smolder Spread 4

2.1.1 Reverse Propagation ... 4

2.1.2 Forward Propagation 9

2.2 Mu 1 1 i -Dimens i onal Smolder Sprea'^ 10

2.2.1 Horizontal Fuel Layer 10

2.2.2 Other Fuel Configurations 15

3. TRANSITION TO FLAMING 17

4. CONCLUDING REMARKS 20

5. REFERENCES 21

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List of Figures

Figure 1

Figure 2

Figure 3

Smoldering upward from bottom within thick

layers of mixed wood sawdust (from ref. 4);

initiating layer 2.5 cm deep, 30 cm square

box; 0>initiating layer 5.2 cm deep, 30 cm

square box; A initiating layer 5.2 cm deep,

60 cm square box; 0 initiating layer 5.2 cm

deep, 90 cm square box

Smolder velocity vs. air flow velocity into

reaction zone for nearly one-dimensional

reverse smolder. Symbol Pb is bulk density.

Material A - ref. 7; Material B - ref. 5;

Material C - ref. 8; Material D,E - ref. 9. . . Dependence of smolder propagation rate through

horizontal layers of beech sawdust on air

velocity over top of layer (from ref. 4).

0 120 ym mean particle size; A 190 ym mean

particle size; Q 480 ym mean particle size.

Page

26

. 27

. 28

V
List of Tables

Table 1

Page

Data on Mu 1 t

i -d i mens i ona 1 Smolder In

Various Fuels 25

vi
SMOLDERING COMBUSTION

T.J. Ohiemiller

Center for Fire Research

National Bureau of Standards

I) Introduction

Smoldering is a slow, low temperature, flameless form of combustion,

sustained by the heat evolved when oxygen directly attacks the surface of a

condensed phase fuel. It constitutes a serious fire hazard for two reasons.

First, it typically yields a substantially higher conversion of a fuel to

toxic compounds than does flaming (though more slowly}. Secondly, smoldering

provides a pathway to flaming that can be initiated by heat sources much too

weak to cause a flame directly.

A burning cigarette is a familiar example of this flameless mode of

combustion; it is also one of the most common initiators of smoldering in

other materials, especially upholstery and bedding [1]. A cigarette also has

several characteristics common to most materials which do smolder. The finely

divided fuel particles provide a large surface area per unit mass of fuel

which facilitates the surface attack by oxygen. The permeable nature of the

aggregate of fuel particles permits oxygen transport to the reaction site by

diffusion and convection. At the same time, such aggregates of particles

typically form fairly effective thermal insulators that help slow heat losses

and permit sustained combustion despite low rates of heat release.

These physical factors favoring smoldering must be complemented by

chemical factors as well. Like virtually all other cellulosic materials,

tobacco in a cigarette, when degraded thermally, forms a char. A char is not

- 1 -
a well-defined material but typically it is considerably richer in carbon

content than the original fuel and its surface area per unit mass is also

enhanced. This char has a rather high heat of oxidation and is susceptible to

rapid oxygen attack at moderate temperatures (> 400®C). The attack of oxygen

(to form mainly CO and CO

2

) is facilitated not only by the enhanced surface

area but also by alkali metal impurities (present in virtually all cellulosic

materials derived from plants) which catalyze the oxidation process [2]. Char

oxidation is the principal heat source in most self-sustained smolder propagation processes; the potential for smoldering combustion thus exists with any

material which forms a significant amount of char during thermal decomposition. (Char oxidation is not always the only heat source and it may not be

involved at all in some cases of smolder initiation, see below and reference

3.)

Various quantitative combinations of the above physical and chemical

factors can produce a material which will undergo sustained smoldering in some

conditions. The range of factors is enormous and results in materials which

will only smolder when formed into fuel aggregates many meters across, at one

extreme, to materials which smolder when formed into aggregates only a few

tens of microns across. Unfortunately, a body of theory which allows one to

calculate which materials and which conditions are conducive to smoldering has

been developed only for certain types of smolder initiation (see section on

Spontaneous Combustion). Conditions which are sufficient to yield smolder

initiation, especially near an external heat source, are not necessarily

sufficient to assure self-sustained smolder spread away from the initiation

region. The potential transition of the smolder process into flaming combustion is even less correlated with factors determining smolder initiation.

- 2 -
This section is restricted to consideration of post-initiation behavior

of smoldering. There are a few models of smolder propagation in the literature but none can yet be said to have any practical usefulness; the state of

modeling is reviewed elsewhere [4] . Lacking any definitive theoretical

description, we are largely restricted here to examining typical experimentally determined behavior. In this overview of smoldering, an attempt is made to

convey some of the qualitative interplay of processes that determine overall

behavior together with specific experimental results.

II) Self-Sustained Smolder Propagation

The smolder initiation process is dominated by the kinetics of the

oxidation of the solid; subsequent propagation of smolder is controlled,

however, to a large degree by the rate of oxygen transport to the reaction

zone. This comes about because the heat evolved during initiation raises the

local temperature, and thus the local reaction rate, until all of the neighboring oxygen is consumed. Subsequently, the reaction continues to consume

oxygen as fast as it reaches the reaction zone, yielding a very low oxygen

level locally which limits the rate.

The subsequent evolution of the smoldering zone away from the initiation

region is heavily influenced by oxygen supply conditions. If initiation

occurs deep within a layer of fine particles (e.g., sawdust, coal dust), for

example, it will slowly work its way to the nearest free surface at a rate

dictated by oxygen diffusion through the particle layer. (The more coarse and

loosely packed the particles, the greater the influence of buoyant flow

through the fuel leading to predominant upward spread.) When it reaches the

-3 -
free surface region it will spread more rapidly over this region in response

to local convective and diffusive oxygen supply conditions. As will be seen,

when smolder spread over the surface region of a fuel layer is forced by air

flow its response depends in addition on heat transfer considerations.

In examining self-sustained smolder propagation and its response to

oxygen supply conditions, dimensionality is important; it is necessary to

distinguish one-dimensional from multi-dimensional configurations. It is

further necessary to discern whether the smolder zone is spreading in the same

direction as the net movement of oxygen or in the opposite direction.

II-A) One-Dimensional Smolder Spread

This is an idealized situation that is sometimes approximated in real

fires; for example, the spread outward or upward from deep in a layer of fuel

particles, mentioned above, approaches this limit when oxygen diffusion

dominates convection and any curvature of the reaction front is small compared

to the reaction zone thickness. In practice this curvature requirement would

likely be met by spread away from ignition sources about 10-20 cm in extent.

Reverse Propagation . When oxygen diffuses to the reaction zone, in this

particular example, from the outer surface of the fuel layer, through the

unburned fuel, toward the reaction front, it is moving opposite to the

direction of smolder propagation; such a case of relative movement is termed

reverse smolder. It is one of two possibilities for movement of oxygen flow

relative to the smolder front; the second case is considered briefly later in

this discussion.

-4 -
Palmer [5] examined this diffusive reverse smolder case using layers of

wood sawdust of various depths; the configuration was only roughly onedimensional. Some of his results are shown in Fig. 1. Note that the time

scale is in hours; the time to smolder up through a layer 1 m deep is about

two weeks, a surprisingly long time. Palmer noted that in this configuration

the smoldering process gave little hint of its presence until it was close to

the surface of the fuel layer.

The slope in Fig. 1 indicates that the time for smolder to penetrate a

fuel layer in this mode is nearly proportional to the square of the layer

depth [5]. Palmer showed that one would expect exactly a second power dependence on layer depth if it is assumed that the smolder reaction zone propagation velocity is proportional to the one dimensional diffusion rate of oxygen

from the surroundings through the unburned fuel, to the reaction zone. From

this one gets

= AL^ (1)

where tj^ is the time for the smolder zone to penetrate the layer of thickness

L; A is a constant that can, at present, only be determined by experimental

measurement of at least one layer thickness. Note that this relation and

Figure 1 imply that a 10 m fuel layer depth, such as might be encountered in a

land fill or coal mine tailing pile, would require more than four years for

smolder penetration. In practice such a deep layer is unlikely to be uniform

and the smolder front movement would be dominated by buoyant convective flow

in regions of lesser flow resistance. However, this does illustrate how very

slow some smolder processes can be.

-5 -
A key factor in the existence of stable, self-sustaining smolder at such

extremely low rates is a well-insulated reaction zone. The heat loss rate

cannot exceed the heat generation rate. In this case, the same factor which

is slowing the oxygen supply rate and, therefore, the heat generation rate,

(i.e., the thick layer of wood particles over the reaction zone) is also

slowing the heat loss rate.

In the above example, the smolder propagation process is inherently

unsteady because of the time-dependent oxygen supply process. If oxygen is

instead continually supplied by a forced convective flow through the fuel

layer, nearly steady propagation occurs. Such a configuration is encountered

in some incinerators and coal burners but rarely in a smoldering fire. Its

main interest is that it has been examined experimentally [6] and modeled [7],

so it is a relatively well-understood smolder mode whose underlying mechanisms

are qualitatively similar to the transient case discussed above.

In this mode of reverse smolder propagation, oxygen surrounds the fuel

particles as they are heated by the advancing smolder reaction zone. Thermal

degradation of some fuels in the presence of oxygen is exothermic. This is

particularly true of cellulosic materials and this heat can be sufficient to

drive the smolder wave without any char oxidation [6]

.

In flexible polyurethane foams, the presence of oxygen during degradation plays another key

role. Without it many foams do not form any char [7] and here char oxidation

is a necessary source of heat. In any event, in the reverse smolder mode, the

net oxidation rate (and net heat release rate) is again directly proportional

to oxygen supply rate; the smolder zone spreads to adjacent material as fast

as this generated heat can be conducted to it. Increased oxygen supply rate

-6 -
causes a greater rate of heat release, increased peak temperature in the

reaction zone which, in turn, increases the heat conduction rate to adjacent

fuel, thus accelerating the smolder spread rate. This sequence implies that

the smolder reaction zone may well move through a layer of fuel without fully

consuming the solid at any point; this unconsumed material acts, in fact, like

an insulator for the reaction zone, increasing its stability, as indicated

previously.

Figure 2 shows measured reverse smolder velocities for several types of

fuel as a function of air flow velocity through the fuel bed. The bulk

densities of the fuel bed are all low but typical for these types of

materials. Note that the air flow velocity range is also quite low but flows

much higher than this are sufficient to move the fuel particles in the bed

(i.e., an upward flow higher than about 1-2 cm/s would fluidize the fuel bed).

Despite the considerable variation in the chemical nature of these fuels,

_2

the smolder velocity is always of order 10 cm/s. For the same air supply

rate, the smolder velocities do not vary much more than a factor of two. This

is consistent with the idea that oxygen supply rate dominates the propagation

process, not reaction kinetics. The differences that do exist appear to

mainly reflect variations in available heat and effective thermal

conductivity.

Only limited information is available on toxic gas production from this

mode of smoldering. The molar percentage of carbon monoxide in the evolved

gases has been examined for two of the fuels in Fig. 2. For the flexible

polyurethane foam, the CO is 6-7% for an air velocity of 0.15 cm/s; the flow

- 7 -
rate dependency was not examined [8]. For the cellulosic insulation material

[9], the CO mole fraction varies from about 10% to 22% as one goes from the

lowest to the highest air velocity in Fig. 2. The mass flux of CO from such a

2 smoldering process (grams of CO/cm of smolder f ront/second) then is estimated

as follows:

CO

(m

AIR ( 2 )

or

Y f p V + ( I4*) V 1 CO ' AIR AIR ^ s s-* (3)

Here is the mass fraction of CO in the evolved product gases

(approximately equal to the mole fraction); is the mass flux of air

entering the smolder zone; m^_ is the mass flux of gaseous material evolved

from the solid fuel; is the density of the air at the point where its

velocity is measured; 4> is the initial void fraction of the fuel bed;

Ap^ is the change in density of the fuel bed (for reverse smolder, typically

65-95% of the original mass is gasified); v^ is the smolder front velocity.

Limited information is also available on the aerosol emitted by a reverse

smolder source [11]; this is pertinent to detection of a smoldering fire. The

source studied was essentially identical to that used to obtain the data for

curve B in Fig. 2; the fuel again was an unretarded cellulosic insulation.

The mass mean particle size of the aerosol was 2-3 pm; this is about 5 times

larger than cigarette smoke and 50-200 times larger than the sooty particulate

produced by flaming combustion. This large size explains the relatively poor

-8 -
sensitivity that ionization smoke detectors have for realistic smolder

sources. The residual solid left in the smolder wave and the original fuel

both were found to be effective filters for this aerosol; this helps explain

the observation by Palmer [5] that smoldering in a thick layer of fuel was not

detectable until it neared the surface exposed to the ambient atmosphere.

The rate of heat release for this mode of smolder can be estimated from

the total mass flux of products and their heat content (gas temperature

typically 400-600°C). The result is a few tenths of a watt per square centimeter of smolder front. This translates to a few tens of watts for a reverse

smolder source 10-15 cm in diameter. The strength of the heat source bears on

the behavior of the buoyant plume (see section on fire plumes). Sources as

weak as those considered here generate plumes that may not reach the ceiling

of a room.

Forward Propagation . The second limiting case of one-dimensional smolder

propagation is called forward smolder; in this case the oxygen flow is in the

same direction as the movement of the smolder front. The most familiar

example (though it is not one-dimensional) is a cigarette during a draw. This

case is encountered in some industrial combustion processes but is unlikely to

be found in its pure, one-dimensional form in a fire context; some elements of

this mode are encountered in realistic cases, however. Some of its characteristics are briefly mentioned here to convey a sense of the major effects that

reversing the oxygen flow direction can have on smolder propagation

characteristics.

-9 -
Forward and reverse smolder propagation were compared in references 6 and

9; the fuel was an unretarded cellulosic insulation. Forward smolder through

this same fuel at the same air supply rate is about ten times slower than

reverse smolder. The CO mole fraction is independent of air supply rate and

is about 9%. Forward smolder gives more complete combustion of the fuel.

These and other differences between the two smolder modes can be explained in

terras of the differing wave structures [6].

II-B) Multi-Dimensional Smolder Spread

Factors such as ignition source geometry, fuel geometry and the strong

influence of buoyant flow on oxygen supply usually interact to assure that a

smolder reaction zone has significant gradients of temperature and species in

two or three dimensions. The number of possible configurations is then

virtually limitless. The practical configurations that have been studied are

few and they are usually two dimensional; they do shed some light on most

cases likely to be of interest.

Horizontal Fuel Layer . The configuration that has been studied most

extensively is that of two-dimensional smolder propagation in a uniform horizontal layer of particles or fibers. Ohlemiller [12] examined the structure

of the smolder zone in a thick (18 cm) horizontal layer of cellulosic insulation in the absence of any forced air flow over the fuel layer. In these

conditions, the flow induced by the buoyant plume rising above the smolder

zone assures a constant supply of oxygen to the space above the layer; oxygen

penetration of the layer is largely by diffusion.

- 10 -
If such a layer is ignited uniformly on one end, the smolder reaction

zone soon evolves into a new shape dictated by oxygen supply rates [12]. The

uppermost elements of the reaction zone, being closest to the free surface

and, hence, ambient air, spread away from the ignition source the fastest;

successively deeper elements spread in the same direction more slowly. The

result is a smolder reaction zone that, viewed in vertical cross-section,

slopes upward, from the bottom of the layer to the top, in the direction of

movement. The steady-state length of this inclined smolder front is roughly

twice the depth of the original fuel layer. This inclined reaction zone is

several centimeters thick and across this thickness there is a smooth transition from unburned fuel to ash. On the ash side (the free surface adjacent to

air) oxygen diffuses down and inward in the same direction as the smolder

front is moving and attacks the charred fuel; this aspect is analogous to

forward smolder discussed above. On the unburned fuel side of the inclined

smolder front, oxygen diffuses in from the region ahead of the front to react

with the fuel as it is thermally degraded by heat conducted from the char

oxidation region. Oxygen here is moving opposite to the direction of smolder

propagation so this aspect of the overall reaction zone is analogous to

reverse smolder. It was pointed out previously that in cellulosic materials,

this oxidative/thermal degradation is exothermic. Thus the two dimensional

horizontal smolder zone incorporates features of both forward and reverse

smolder and is driven forward by the combined heat release from char oxidation

and oxidative/thermal degradation.

The participation of oxidative/thermal degradation in driving the smolder

process requires that oxygen have free access to the thermal degradation

region. For a low permeability fuel such as solid wood this is not the case.

- 11 -
Even though solid wood has basically the same reaction chemistry as cellulosic

insulation (which consists mostly of wood fibers) and smolders with a qualitatively similar inclined reaction zone, it must be driven solely by char

oxidation.

The low permeability and corresponding high density of solid wood has

another consequence with regard to smolder. The self-insulating quality of

the reaction zone is much less than it is with a low density layer of fuel

particles or fibers. A single layer of wood will not sustain smolder unless

it is subjected to an additional heat input of about 1 W/cm [13]; this could

come from some external radiant source or from another piece of smoldering

wood which has an adequate radiative view factor with respect to the first.

In view of the previously noted, strong role of oxygen supply rate in

shaping the smolder process in a horizontal fuel layer, it is not surprising

that it also accelerates in response to an increased oxygen supply rate

produced by an air flow over the top of the smoldering layer. As with the

one-dimensional propagation situation, one again has two possibilities: the

air flow can go in the same direction as the smolder front (again called

forward smolder) or the opposite direction (reverse smolder). Note, however,

that now the actual fluxes of oxygen within the smoldering fuel bed may go in

various directions; they are no longer constrained to be parallel to the

smolder wave movement, as in the one-dimensional cases.

Palmer [5] examined both of the flow direction possibilities for.

relatively thin horizontal layers (0.3 to 5.7 cm) of various cellulosic

particles (cork, deal, beech, grass); Fig. 3 shows some typical results. Note

- 12 -
that the smolder velocities are generally much slower than in Fig. 2 despite

the much higher air velocities. Much of the difference is probably due to

differing rates of actual oxygen delivery to the reaction zone and to the fact

that the near-surface region which gets the best oxygen supply is also

subjected to the highest heat losses.

The influence of two factors is shown in Fig. 3, fuel particle size and

relative direction of air flow and smolder propagation. Particle size has a

relatively weak effect on smolder velocity but its effect depends on whether

the smolder configuration is forward or reverse. The configuration itself has

a much greater effect.

Ohlemiller [14] obtained comparable smolder velocities and dependence on

configuration for 10-11 cm thick layers of cellulosic insulation. It was

found that the configuration dependence cannot be explained solely on the

basis of oxygen supply rates. The mass transfer rate to the surface of the

fuel bed was measured for forward and reverse configurations; it differs by

only 20-30% (these differences are caused by changes in the bed shape due to

shrinkage during smolder). It was pointed out that the observed dependence on

relative direction of the air flow is consistent with there being a prominent

role for convective heat transfer along the top surface of the fuel layer.

This can only be so if part of the smolder wave, i.e., the region near the

leading edge, is kinetically-limited (and therefore highly temperature

sensitive) rather than oxygen supply rate-limited. This idea explains the

qualitative impact of both relative air flow direction and combustion

retardants on smolder velocity; it also explains why forward smolder is faster

than reverse smolder in the horizontal layer configuration whereas the

- 13-
opposite was seen to be true for one-dimensional propagation. The role played

by fuel particle size may be implicit in this view but a quantitative model is

not yet available.

There is a minimum thickness below which a horizontal fuel layer will not

undergo self-sustained smolder propagation. As the thickness of a fuel layer

decreases, its surface to volume ratio increases (inversely with thickness to

the first power). The ratio of the rate of heat loss to the rate of heat

generation varies in this manner also so that ultimately the losses are overwhelming and extinction occurs. The exact thickness will depend on factors

influencing the heat generation per unit volume at a given thickness such as

bulk density, fuel type and particle size, rate of oxygen supply, etc. The

same considerations apply to other thin layers of fuel such as fabrics on

upholstery and sheets of paper, wood or particle board. Palmer [5] found that

the minimum depth for sustained smolder in still air increased linearly with

particle size for beech, deal and cork; for cork this dependence ceased above

0.2 cm, apparently because more complete oxidation of the char stabilized the

process in the layers of larger particles. For very small particles,

(< 100 Mm) the minimum depth dropped as low as 1 mm for cork dust; 1 cm was

typical of small particles of beech or deal sawdust. Ohlemiller and Rogers

[15] found the minimum depth in still air for an unretarded cellulosic insulation to be 3-1/2 cm; a heavy loading of the smolder retardant boric acid

roughly doubled this value. Since the insulation has a very small effective

particle size and essentially the same chemistry as Palmer's sawdusts, most of

the difference in minimum depth (for the unretarded material) probably lies in

the bulk density which is about four to five times less for the insulation

compared to the sawdusts (O.OA g/cc vs. 0.18 g/cc). Palmer found that the

- 14 -
minimum depth dropped rapidly with increased air flow over the sawdust layers,

in keeping with the idea that a greater rate of heat release per unit volume

stabilizes the smolder process.

Smolder propagation data on a few other fuels (including some that are

inorganic) in horizontal layers can be found in reference 15. Unfortunately

neither here nor in the previous references on horizontal layer smolder is

there any data on the evolved products. For crude estimates on cellulosic

materials the previous results for reverse smolder are adequate but they

should be applied here with caution.

Other Fuel Configurations . Data on a few other multi-dimensional smolder

configurations are summarized in Table 1. Again there is little more available than the rate of smolder propagation. All the materials in Table I are

fairly porous. As noted previously, solid wood also smolders given a configuration that limits heat losses; available data on rates are extremely limited,

however. Reference 32 reports rough measurements for smolder spread on 3.8 cm

thick slabs of white pine. Two vertical slabs formed a gap up which air

_2 flowed at 10 cm/s. Upward (forward) spread was at about 10 cm/s; downward

(reverse) spread was at 2.10 cm/s.

The last type of smolder configuration mentioned in Table 1 is quite

pertinent to the scenario which makes smoldering a major contributor to

residential fire deaths, i.e. , upholstery and bedding fires initiated by

cigarettes. This is frequently a composite problem with the smoldering

tendency of both the fabric and the substrate (polyurethane foam, cotton

batting) pertinent to the overall smolder behavior of the combined assembly

- 15 -
[23]. Ortiz-Molina, et al. have shown that the combination of a cellulosic

fabric plus a polyurethane foam can smolder over a substantially wider range

of conditions than can the foam alone [24]. The fabric smolder process

supplies added heat to the foam smolder zone while competing for oxygen at the

same time. The full complexity of this interaction is yet to be explored. A

considerable amount of empirical data on the tendency of cigarettes to

initiate this type of smolder is available [25-28,40].

The life hazard posed by smoldering bedding or upholstery within a closed

room has been studied to some extent [29-31]. In ref. 30 data are presented

on the build-up of CO (near the ceiling) in a room 2.4m on a side due to

cigarette-initiated smolder in a cotton mattress. The smolder front was

_3 reported to spread radially at a rate of 6.3 10 cm/s independently of the

size of the smoldering area. In two out of five tests the smolder process

underwent a transition to flaming combustion after 65-80 minutes. This is

close to the time at which the authors estimated that the total carbon

monoxide exposure was lethal. In ref. 30 similar data are reported for a

greater variety of bedding and upholstery materials; these were ignited by

cigarettes (and by flaming sources) in a room 4.3 x 3.6 x 2.4m. Carbon

monoxide and several other gases were sampled at three locations. Flaming

developed from smoldering in several of the tests usually requiring 2-3 hours

of smoldering first; again the total exposure to carbon monoxide from the

smolder smoke was near or exceeded lethality. Lethal conditions due to carbon

monoxide were reached in much shorter times in some cases.

In ref. 31, all available data on the hazards of smoldering in a closed

room were evaluated; it was concluded that the probability of a lethal CO dose

- 16 -
and ot transition to flaming are comparable for a period from 1 to 2 1/2 hours

after cigarette initiation of smoldering. A model is presented for build-up

of CO due to a smoldering fire; the results generally show reasonable agreement with experiment though some of the input parameters must be forced a bit.

Ill) Transition to Flaming

The transition process from smolder to flaming in the above bedding and

upholstery fires is essentially spontaneous. At room conditions both

smoldering and flaming are possible in many such systems; Sato and Sega [36]

explored the domain of overlapping smolder and flaming potential for

cellulosic materials and noted a hysteresis in the spontaneous transition

between these two combustion modes. The mechanism of such a spontaneous

transition has not been investigated in detail. It has been suggested, on the

basis of small mock-up studies, that a chimney-like effect develops in the

crevice between the horizontal and vertical cushions of a smoldering chair

[33]; the enhanced air supply presumably accelerates local char oxidation,

heating the char to the point where it can ignite pyrolysis gases. Such a

mechanism is plausible but it has not been demonstrated to be operable in real

upholstery or bedding where the chimney effect may not develop so readily.

Transition to flaming (fast exothermic gas phase reactions) requires both

a mixture of gases and air that are within their flammability limits and a

sufficient heat source to ignite this mixture. Furthermore these two requirements must be realized at the same locus in space and at the same time.

Anything which either enhances the net rate of heat generation or decreases

the net rate of heat loss will move the smoldering material toward flaming

- 17 -
ignition both by increasing local temperature and rate of pyrolysis gas generation. Such factors include an enhanced oxygen supply, an increase in scale

(which usually implies lesser surface heat losses per unit volume of

smoldering material) or an increasingly "concave" smolder front geometry,

which reduces radiative losses to the surroundings and enhances gaseous fuel

concentration build-up. All of these factors may be operating simultaneously

in the case of upholstery and bedding smolder; sequential photos in ref. 33 of

smolder initiation, growth and transition to flaming in an upholstered chair

appear consistent with this idea.

A further factor, in the above case and in other systems involving

cellulosic materials is secondary char oxidation. This process is quite

similar to after-glow seen in cellulosic chars left by flaming combustion.

Intense, high temperature (probably > 800°C) reaction fronts propagate intermittently in seemingly random directions through the fibrous low density char

left by the main lower temperature smolder front. In charred fabrics these

glowing fronts can sometimes progress stably along the charred residue of a

single fiber, in spite of very high heat losses per unit volume of fuel. Such

a process requires the catalytic action of alkali metals frequently found

naturally in cellulosics or left there during manufacture [35]. While a very

hot smolder front the size of a single fiber is unlikely to be sufficiently

energetic to ignite flammable gases, the larger fronts (few mm. to cm. in

scale) may well be. A process analogous to this has been found to cause

occasional flaming ignition of smoldering, unretarded cellulosic insulation

[ 12 ] .

- 18 -
The transition from smolder to flaming can also be induced by, for

example, a forced increase in oxygen supply rate to the smolder reaction zone

[5,8,36,37,38]. This was first studied quantitatively by Palmer [5] for air

flow over horizontal layers of wood sawdust; it is, of course, a process that

is familiar to anyone who has started a camp fire from tinder and sparks.

Transition to flaming was noted by Palmer only for air flow in the same direction as smolder propagation (forward smolder); depending on the material, the

transition occurred at air flow velocities from about 90 to 170 cm/s. For

these materials flaming did not develop when the mean particle size was less

than 1 mm. Ohlemiller [37] did obtain transition to flaming in fibrous

insulation material of very small diameter (~ 25 pm) but again only with

forward smolder; this occurred at air velocities of about 200 cm/s for

unretarded insulation. Leisch [38] utilized ignition sources placed midway

along the length of his grain and wood particle fuel layers so that he simultaneously obtained forward and reverse smolder zones; flaming was noted at 4

m/s air velocity only after the smoldering process produced a substantial

depression or cavity in the surface of the fuel layer.

Ohlemiller [37] explained the weak response and lack of flaming

transition in reverse smolder on the basis of heat transfer effects

influencing the leading edge of the smolder reaction zone. These heat

transfer effects intensify the smolder in the leading edge region for forward

smolder. In the case of cellulosic insulation the intensification leads to

random development of small (few cm) cavities near the leading edge which act

as flame initiation regions and flame holders.

- 19 -
Ohlemiller also found that both boric acid (a smolder retardant) and

borax (a flame retardant) could each eliminate the transition to flaming when

the retarded cellulosic insulation was the only fuel. However, their

effectiveness was substantially reduced if the smoldering fuel abutted

unretarded wood; heat transferred from the smolder zone readily ignited the

wood. Palmer [39] noted similarly that layers of fine dust that would not

themselves undergo transition to flaming readily ignited adjacent flammable

materials.

IV ) Concluding Remarks

Smoldering is a branch of solid fuel combustion quite distinct from

flaming in many aspects but equally diverse and complex. Unfortunately it has

not been studied nearly to the same extent. This is quite apparent in the

lack of quantitative guidelines provided here for estimating the behavior of

realistic smolder propagation processes, smolder detection, toxic gas production and the transition to flaming. The experimental data provided can be

readily used for closely analogous situations; they must be used cautiously

for dissimilar conditions. The reader should always bear in mind the strong

role that oxygen supply rate plays with smolder processes. The other most

important factor is the relative direction of movement of oxygen supply and

smolder propagation; this can be somewhat obscure in many realistic configurations. The actual chemical nature of the fuel is relatively secondary at

least with regard to smolder rate; it may be important for toxic gas

production rates, but the data here are quite limited.

-20-
References

1. Clarke, F. and Ottoson, J., Fire J. , May 1976, p. 20.

2. McCarter, R., "Smoldering Combustion of Cotton and Rayon", J. Cons. Prod.

Flamm . ^ (1977) 346.

3. bowes, P., Self Heating: Evaluating and Controlling the Hazards , Elsevier, New York, 1984, chap. 7.

4. Ohlemiller, T. , Modeling of Smoldering Combustion Propagation, to appear

in Progress in Energy and Combustion Science .

5. Palmer, K., Smoldering Combustion in Dusts and Fibrous Materials,

Combustion and Flame 1, (1957) 129-154.

6. Ohlemiller, T. and Lucca, D., An Experimental Comparison of Forward and

Reverse Smolder Propagation in Permeable Fuel beos. Combustion and Flame

(1983) 131-147.

7. Ohlemiller, T. , Bellan, J., and Roger, F., A Model of Smoldering

Combustion Applied to Flexible Polyurethanes, Combustion and Flame 36 , (1979) 197-215.

8. Rogers, F., and Ohlemiller, T. , Smolder Characteristics of Polyurethane

Foams, J. Fire Flamm . 1

, (1980) 32-44.

9. Lucca, D. , An Investigation of Co-Current and Counter-Current Smoldering

Combustion in Particulated Fuel Beds, MSE Thesis, Dept, of Mech. and

Aero. Eng., Princeton Univ., Oct. 1979.

10. Davidson, B., Keitelman, E., O’Such, W., Smoldering Aspects of Rigid

Polymeric Insulation Foams, Rutgers Univ., Dept, of Chem. and Biochem.

Eng., Final Report to the Products Research Committee, June 1979.

11. Mulholland, G., and Ohlemiller, T., Aerosol Characterization of a Smoldering Source, Aerosol Sci. and Tech., 1, (1982) 59-71 .

12. Ohlemiller, T., Smoldering Combustion Hazards of Thermal Insulation

Materials, National Bureau of Standards, NBSIR 81-2350 (1981).

13. Ohlemiller, T., Unpublished results on yellow pine.

14. Ohlemiller, T., Forced Smolder Propagation and the Transition to Flaming

in Cellulosic Insulation, National Bureau of Standards, NBSIR 85-3212,

August 1985.

15. Ohlemiller, T. and Rogers, F., Cellulosic Insulation Material II. Effect

of Additives on Some Smolder Characteristics, Combust. Sci. Technol. 24

(1980) 139-152.

-21 -
16. Cohen, L. and Luft, N. , Combustion of Dust Layers in Still Air, Fuel 34

(1955) 154-163.

17. Brenden, J. and Schaffer, E., Smoldering Wave-Front Velocity in

Fiberboard, USDA Forest Products Laboratory Research Paper, FPL-367,

Sept. 1980.

18. Kinbara, T., Endo, H. , and Sega, S., Downward Propagation of Smoldering

Combustion through Solid Materials, Proceedings of the Eleventh Symposium

(International) on Combustion, The Combustion Institute, Pittsburgh,

(1967) 525-531.

19. Egerton, A., Gugan, K. , and Weinberg, F., The Mechanism of Smoldering in

Cigarettes, Combust. Flame 7 (1963) 63-78.

20. Gann, R. , Earl, W., Manka, M. , and Miles L., Mechanism of Cellulosic

Smoldering Retardance by Sulfur, Proceedings of the Eighteenth Symposium

(International) on Combustion , The Combustion Institute, Pittsburgh

(1980) 571-577.

21. Donaldson, D., Yeadon, D. , and Harper, R. , Smoldering Characteristics of

Cotton Upholstery Fabrics, Text. Res. J. , March (1983) 160-164.

22. Donaldson, D. , Yeadon, D. , and Harper, R. , Smoldering Phenomenon

Associated with Cotton, Text. Res. J. , March (1983) 160-164.

23. Tesoro, G. and Toong, T.-Y., Smoldering in Cotton Upholstery Fabrics and

Fabric/Cushioning Assemblies, Final Report to Cotton Inc. from MIT Dept,

of Mechanical Engineering, Cambridge, Mass. (Feb. 1981).

24. Ortiz-Molina, M., Toong, T-Y., Moussa, N. , and Tesoro, G., Smoldering

Combustion of Flexible Polyurethane Foams and Its Transition to Flaming

or Extinguishment, Proceedings of the Seventeenth Symposium

(International) on Combustion , The Combustion Institute, Pittsburgh

(1979) 1191-1200.

25. Damant, G. , A Survey of Upholstery Fabrics and Their Flammability

Characteristics, J. F. F. Consumer Prod. Flamm. 2, (March 1975).

26. Damant, G., Cigarette Induced Smoldering of Uncovered Flexible

Polyurethane Foams, J. F. F. Consumer Prod. Flamm. 2 (June 1975) 140.

27. Damant, G., Home Furnishings Fire Retardant Requirements: The California

Experience, J. Consumer Prod. Flamm. 6, (June 1979) 95.

28. Palmer, K. and Taylor, W., Fire Hazards of Plastics in Furniture and

Furnishings: Ignition Studies, J. F. F. Consumer Prod. Flamm. 1 (June

1974) 186.

29. Sumi , K. and Williams-Leir , G., Lethal Effects of Mattress Fires,

National Research Council of Canada, Division of Building Research,

Research Paper No. 402, Ottawa, Canada (May 1969).

-22 -
3U. Wafer, C. and Yuill, C. , Characterization of Bedding and Upholstery

Fires, Southwest Research Institute Final Report on National Bureau of

Standards Contract No. CST-7y2-5-69 , San Antonio, Texas (March 1970).

31. Quintiere, J., Birky, M., McDonald, F., and Smith, G., An Analysis of

Smoldering Fires in Closed Compartments and Their Hazard Due to Carbon

Monoxide, National Bureau of Standards, NBSIR 82-2556 (July 1982).

32. Ohlemiller, T. and Rogers, F., Smoldering Combustion Studies of Rigid

Cellular Plastics, Princeton University, Dept, of Mechanical and

Aerospace Engineering Report No. 1432 (May 1979).

33. Salig, R., Smoldering Behavior of Upholstered Polyurethane Cushioning and

Its Relevance to Home Furnishings Fires, Masters Thesis, Massachusetts

Institute of Technology, Dept, of Mechanical Eng. (1981).

34. Palmer, K. and Taylor, W., Fire Hazards of Plastics in Furniture and

Furnishings; Ignition Studies, Great Britain Building Research

Establishment Current Paper CP 18/74, (Feb. 1974'),

35. McCarter, R. , Smoldering Combustion of Cotton and Rayon, J. Consumer

Prod. Flamm. 4 (1977) 346-358.

36. Sato, K. and Sega, S., The Mode of Burning Zone Spread Along Cylindrical

Cellulosic Material, J. Fire Sci. 3, (Jan/Feb. 1985) 26-34.

37. Ohlemiller, T., Forced Smolder Propagation and the Transition to Flaming

in Cellulosic Insulation, National Bureau of Standards, NBSIR 85-3212

(August 1985).

38. Leisch, S., Smoldering Combustion in Horizontal Dust Layers, Ph.D.

Thesis, University of Michigan, Dept, of Aerospace Engineering (1983).

39. Palmer, K. , Dust Explosions and Fires, Chapman and Hall, London (1973),

287.

40. Loftus, J., "Back-up Report for the Proposed Standard for the

Flammability (Cigarette Ignition Resistance) of Upholstered Furniture",

PFF-6-76, National Bureau of Standards, NBSIR 78-1438, June 1978.

-23 -
Data

on

Multi-dimensional

Smolder

in

Various

Fuels

c

0)

oo

>* £

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73 Lt

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0) V) 73 O

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24

20,21

Cellulosic

fabric

Various

weight

fabrics

natural

convection,

=

3.0-75

•

10"^

cm/s

Reported

values

Smolder

fastest

on

on

substrates

horizontal

on

fiberglass,

diffusion

dependent

on

subsuspiciously

inert

fiberglass

PU

foam,

cotton

batting

strate

and

fabric

low

substrate
TIME

FOR

SMOLDERING

TO

PENETRATE

LAYER,

hours

Figure 1. Smoldering upward from bottom within thick layers of mixed wood

sawdust (from ref. 4); initiating layer 2.5 cm deep, 30 cm square box; initiating layer 5.2 cm deep, 30 cm square box;

A initiating layer 5.2 cm deep, 60 cm square box; 0 initiating

layer 5.2 cm deep, 90 cm square box.

25
o

00o

CDa Oa o

CNJa OO

a

O

B o S <1) iH CO -u

>-l V-l ‘X

<U (U 00

T3 T3 O O M-l e e 0) CO CO

CM

0)3M

•H

26

AIR

FLOW

VELOCITY

THROUGH

FUEL

LAYER

(cm/s)
e-oi- X s/uio ‘3iva ONiuaano^^s

j-i (U> c

O to

(1) e

4-1 •H E U 3. 0

iH O

01 OO > O' M

•H

CO •-

CJ C N

O -H

cn

4-1 CO 0) 3 .H

T3 U

13 "H

CO 4-1 to (-1 CO x; a

ua • U-l

)-4 (U

OJ w

T3 E O O

a >-< CO U-i

U-l 0) O U N

tu •H

0) >. cn

CJ CO C r—

<v

0) •3 U-t u

C o •r^ (U U

D. D. u

(J O cr Q 4-1 a.

0)u

3

oc

27

AIR

VELOCITY

OVER

FUEL

LAYER,

cm/s
NBS-114A (REV. 2 -80 ) U.S. DEPT. OF COMM. 1. BIBLIOGRAPHIC DATA

SHEET (See instructions) I

PUBLICATION OR

REPORT NO.

2. Performing Organ. Report NoJ 3. NBSIR 85-3294

. Publication Date February 1986

4. TITLE AND SUBTITLE

Smoldering Combustion

5. AUTHOR(S)

T. J. Ohlemiller

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. national bureau of standards

DEPARTMENT OF COMMERCE 8 . Type of Report & Period Covered

WASHINGTON, D.C. 20234

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10.

SUPPLEMENTARY NOTES

Q21 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200~word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here)

Smoldering combustion is a common fire safety hazard that contributes

substantially to the fire losses in the United States; fire protection

gngineers thus have a need to be familiar with its characteristics. The

post-initiation behavior of smoldering (self-sustaining propagation and

transition to flaming) is described here. The most studied cases of propagation are one-dimensional; the underlying mechanisms are described

qualitatively and spread rates for several materials are presented along

with limited information on toxic gas evolution. Similar, though more

limited, information is presented for more realistic multi-dimensional

smolder spread, particularly in horizontal fuel layers. Mechanisms for

the transition from smolder to flaming are briefly described; limited data

on requisite air flows to force the transition are given.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

cellulosic materials; flaming combustion; smoldering combustion; propagation;

toxic hazards.

13. AVAILABILITY 14. NO. OF

PRINTED PAGES

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I [ For Official Distribution. Do Not Release to NTIS

r~] Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402.

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