Spontaneous ignition: relation between ambient temperature and size of specimen

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Spontaneous ignition: relation between ambient temperature and size

of specimen

SPONTANEOUS IGNITION:

Rela t i o n Between Ambient Temperature and Size of Specimen

K. Sum$ and P. Tsuchiya

A combustible m a t e r i a l or a combination of reactive

materials may undergo s l o w chemical reaction w i t h t h e e v o l u t i o n

of heat. If t h e rate 05 heat _{generation is greater than the r a t e} of heat dissipation, t h e temperature of t h e specimen rises. The

rise in temperature accelerates t h e rate of reaction and causes fuxther temperature r i s e . T h i s self-heating process is one of

the primary causes of spontaneous ignition. Materials t h a t have

a tendency to heat spsntaneeusly include alfalfa meal, ftsh meal,

coal, charcoal and rags contaminated w i t h drying oils.

Some of the most important f a c t o r s t h a t c o n t r i b u t e to

self-heating reactions are: ambient temperature, size (or volume)

,

heat of combustion and insulatj-on p r o p e r t i e s of the material.

Finely d i v i d e d materials, e.g., sawdust, are more susceptible to

self-heating than solid materials. The reasons are two-fold: t h e

larger surface area of d i v i d e d materials results in greater rate of

heat generation and the lower, thermal conductivity (or greater

i n s u l a t i o n property) retards heat transfer and d i s s l l p a t i o n . The

self-heating tendency increases w i t h the amount of material because hear generation depends on the volume (which is p r o p o r t i o n a l to the

t h i r d power of the d i a m e t e r ) while heat d i s s 5 p a t i o n depends on the

surface area (which is proportional to the second power of the

diameter). For a g i v e n temperature, there is a m i n i m u m s i z e necessary

for spontaneous ignition. The tern " c r i t i c a l r a d i u s (or diameter) " is used to d e s c r i b e this minimum s i z e .

The e f f e c t of ambierit temperature is a l s o two-fold, A higher ambient temperature increases t h e rate of reaction and retards heat dissipation. For a given specimen size, there is a m i n i m u m

temperature or "critical. temperature'ht which spontaneous i g n i t i o n c o u l d occur. Knowledge of t h e r e l a t i o n between c r i t i c a l s i z e and

c r i t i c a l temperature of materials is important to the fire investigator.

Ir

experfments by varying ambient temperature and size of specfmen.

mere is, however, a theoretical relation between critical diameter (or radius) and critical temperature. This relation could

be calculated if the physical. p r o p e r t i e s of the material ( s p e c i f i c

kinetic constants o f the self-heating reaction are _{known, The l a t t e r}

may be determined by means o f an adiabatic furnace such as the one in t h e Fire Research Section of the National Research Council of

Canada.

h this paper the determination of k i n e t i c constants of a self-heating reaction is shown, and the r e s u l t s of calculations g i v i n g

t h e relatton between c r i t i c a l radios and surface temperature for four

d i f f e r e n t materials are presented.

A diagram of an adiabatic furnace is shorn in F i g u r e 1. The furnace d e s i g n is similar to that described by Gross and Robertson.*

A w i r e mesh holder containing about 100 cu cm of sample is positioned in the adiabatic furnace.

me

set temperature. The furnace is then switched to the adiabatic made

and t h e specimen

Is

the adiabatic h e a t i n g period, the temperature surrounding t h e specimen is maintained at the same temperature as t h e centre of the specimen. The

specimen temperature is recorded from t h e start of the a d i a b a t i c

heating period until i g n i t i o n occurs. If the temperature. increase . under the adiabatic mode is too slow or too fast the experiment is

r e p e a t e d after adjusting the p r e - s e t temperature.

The temperature-the curve f o r w h i t e p i n e sawdust obtafned from an adiabatic furnace experiment is shown len Figure 2. The kinetic constants are obtained by p l o t t i n g the n a t u r a l logarithm of t h e rate of heating as a function of the recfpcocal of absolute temperature. Activation energy, E, is calculated from the slope 05 the regression

l i n e and k n ( f ~ / c ) is o b t a h e d f r o m the intercept of the Y - a x i s , where f is t h e frequency factor, Q is the heat of reaction and c is the s p e c i f i c

heat of the sample. RESULTS

The r e l a t i o n bemeen critical radius and surface temperature

f o r rhe self-ignition aE a sphere of white pine sawdust arid three o t h e r

s a m p l e s has been calculated. These data are presented in Figure 3 . The sanple that showed t h e greatest tendency towards spontaneous S g n i t i o n was cellulose impregnated w i t h linseed o i l , f o l l o w e d by white p i n e

sawdust, r i g i d urethane foam, and cellulose.

*

61, 413 31958). -

CONCLUSIONS

The determination of k i n e t i c constants of a self-hearing reaction using an a d i a b a t i c furnace has been described. These data

combined w i t h t h e m o t e c m o n physical properties of the material

p e r m i t s t h e s o l u t i o n of general s e l f - h e a t h g equations including those

where size and temperature become c r i t i c a l and i g n i t i o n results. The relation between critical temperature and c r i t i c a l size o f a self-heating

material are thus obtained. Such information is extremely difficult

to o b t a i n d i r e c t l y from simple experiments -1.n which a m b i e n t temperature and sample size are _varied.

The Eire service in Canada is being made aware of the

c a p a b i l i t y available at t h e National Research Council o f Canada to

determine the relation between ambient temperature and size of specfmen f o r spontaneous i g n i t i o n to occur. When such information is needed

to establish t h e cause of a f i r e , the f i r e investigator is encouraged to

ask the cooperation of the EZze scientist. Exchange of information on self-hearing problems should be of mutual b e n e f i t to both.

The a u t h o r s are indebted to Dr. J.J. P a r k and D.R. Hoffman

f o r the design and construction of t h e c o n t r o l c i r c u i t s and to J.F.

Mathieu for t h e c m s t r u c t i o n of the furnace and assistance in c o n d u c t k g

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