NATIONAL BUREAU OF STANDARDS REPORT

5983

THE PROCESS OF IGNITION

by

A. F. Robertson

<NBp>

U. S. DEPARTMENT OF COMMERCE

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national bureau of standards report

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l002-ll-4d'73 July l8, 195^^

NBS REPORT

5983

THE PROCESS OF IGNITION

by

A. F. Robertson

^iATIONAL BUREAU ( Intended for use withi

to additional evaluation

listing of this Report) i the Office of the D!rec< however, by the Gover

to reproduce additions

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Ifically prepared If that agency wishes <NB^

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS
W 5 3 S ' w 3 -3 ; ? s f '? 3) « j. j ^ * y »5 j A 0 1 

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V. . 1 • 1
THE PROCESS OF IGNITION

ABSTRACT

A brief elementary description is presented of factors likely

to influence the process of ignition of combustible materials.

Nine mechanisms for promotion of ignition are mentioned and

described. A brief bibliography is included.

INTRODUCTION

The initiation of combustion or explosion of a fuel may be

described as the ignition process. Because of the large

variations in the combustible characteristics of fuels and

variations in ambient conditions, this ignition process may

assume a wide variety of forms. This paper has been written

with the objective of providing a brief review of some of the

conditions and manner in which ignition can take place.

MECHANISM OF IGNITION

The detailed chemical processes involved in an ignition

reaction are for most materials so complex that no detailed

explanation of the process can be given. There is, however,

rather general agreement that many such reactions are

of the chain type, reference 1 . The term chain is used to

describe the reaction because the process involves many different

steps for completion. Each of these steps may be dependent on

the prior completion of another stage of the reaction for

completion of the combustion process. The fact that many fuel

molecules are very complex, (cellulose molecules (C5H2q*^5^x 5 exist in multiples of 21 atom groups), further complicates the

process by requiring that a multiplicity of reactions take place

together.

In the case of low temperature heat generation, reactions

of the type associated with self ignition of materials, there

is evidence that the heating is largely a result of depolymerization or breaking up of the complex molecular structure of the

fuel and the resulting formation of new compounds. This process

of breaking large complex molecules down into smaller ones with

associated oxidation and other chemical reactions seems to be

taking place continually even at very low temperatures. The
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rate with which these reactions take place depends on the number

of molecules which are likely to have, or take on, energy

sufficient to cause a reaction to occur. The kinetic theory of

gases suggests that the probability that an individual molecule

will have energy in excess of an amount E, at temperature T, is

proportional to where k is the gas constant per molecule.

Although most combustion processes are exceedingly complex, it

is interesting to find that experimental data on slow combustion

reactions can often be correlated on the basis of this type of

relationship.

THERMAL CONSIDERATIONS

In order for an ignition reaction to be successful in

initiating a combustion or explosion reaction, it is necessary

that it cause the heated portion of the fuel to liberate heat

at a rate faster than thermal losses take place to the

surroundings. This may be illustrated by Figure 1 (see also

reference 2). Here heat loss and heat generation curves are

shown for a given fuel as a function of absolute temperature.

The heat generation curve is of an exponential form, the rate

of heating increasing as a power function of the temperature.

The heat loss is assumed to be Newtonian in character. This

implies that the rate of heat loss is a linear function of the

temperature difference between the heated surface and ambient.

This may be a considerable over-simplification but is satisfactory

for illustrative purposes. The heat loss curve has been indicated

for three different ambient temperatures Tj^, T3 , and Tc* For

ambient temperature Tj^ heat generation exceeds the loss rate

until Tj) is reached, at which time a stable surface temperature

rise (Td - T_^) is reached and no further heating occurs. For

ambient temperature T3 there is only one point of contact, that

of tangency, for the two curves. This results in an unstable

temperature rise (Tf - Tb) from which it is highly likely that

the fuel will continue to increase in temperature and be consumed.

For any ambient temperature higher than Tb there is no point of

contact of the two curves and the heating of fuel will continue

until it is consumed.

Temperature Tb is for the conditions assumed in Figure 1, the lowest temperature from which self-ignition may be assumed

to occur. It should be noted, however, that this temperature

is not a^ unique property of the fuel studied but more so of the

combination of fuel and experimental conditions used for its

determination. Thus, if the fuel sample is changed in size o.r
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extent of thermal contact with conducting portions of the

apparatus the slope or form of the heat loss curve will be

changed. This will result in a new critical ambient temperature which will be higher if heat loss rate has been increased

or lower if losses are reduced. Thus a different self

ignition temperature results for the same fuel.

The preceding has assumed that all heating has been a

result of generation within or by the fuel. Many common

combustible materials are stable under ambient conditions

and the temperature rise (Tq - which they develop is

very small and seldom if ever noticed. These materials may

however ignite and burn if an additional heat input rate is

provided. Figure 2 illustrates such conditions. Here the

total heating rate is the sum of the generation rate and the

constant heat input rate which has been added. This results

in the total heating curve being greater than the heat loss

curve for all temperatures above Tq and ignition will have

been completed.

Successful ignition is thus seen to have required a total

heat gain rate greater than that of the losses to the surroundings.

IGNITION LAG

In the preceding discussion it was assumed that the time

available for the various stages of the ignition process was

not a determining factor. This assumption is often correct

for self-ignition .processes'^ butlin* the case of an externally

applied ignition source this situation may not exist. Figure

3 shows a diagram for an unsuccessful ignition attempt. Here

conditions were much the same as those shown in Figure 2.

However, in this case the duration of the heat pulse was too

short to prevent the heat generation curve from dropping below

the loss curve. A maximum temperature Tp would be reached.

When the heat input ceased, the loss rate would exceed the

generation rate and the temperature would return to Tq and

stabilize there.

This discussion of the ignition lag problem has been

greatly simplified and is largely illustrative. It has not

described one of the important causes of theL_delay which

involves the time required to form the necessary intermediate

chemicals required for completion of the chain combustion

reaction.
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SOURCES OF IGNITION

The variety in form and detail of successful ignition

reactions may be very great. It is thus almost impossible to

set forth all of them. Instead a few brief comments will be

made on the manner in which several types of ignition sources

may cause ignition. These include:

1. Bacteriological processes

2. Chemical heating

3. Frictional effects

4. High temperature exposure

5. Rapid compression

6. Shock wave

7. Open flame

d. Electric spark

9. Mechanical impact

Each of these processes will be discussed separately.

First, however, a brief discussion of the fuel types seems

desirable. The flammability or the ease and rapidity with

which ignition and combustion or explosion can take place, in

a given fuel, depends largely on the chemical properties of

the fuel and the shape and distribution or "geometry".

The chemical properties of interest here would include

chemical composition of the fuel and kinetic reaction data for

the combustion process involved. Many of the common fuels

require air as an oxidizer whereas most high explosives and

propellants are prepared as mixtures of fuel and oxidizer.

The geometry of the fuel is important for several reasons.

In the case of common fuels, particle size is very important

in determining the degree with which fuel and oxidizer are

mixed. An example of high flammability resulting from this

effect is evident in the occurrence of dust explosions. The

use of pulverized coal, and atomized liquid fuels for power and

heat production are evidences of the importance of proper fuel

and oxidizer mixtures for high heat release rates. Geometry

is important too in determining the heat transfer characteristics of the fuel. Thus the ease with which the flame produced

by the fuel feeds back heat to unignited portions is largely

influenced by its geometry or surface to volume ratio. This

is true even for explosives and propellents when burned. Although with these latter materials there is evidence of a flame

in close proximity with the fuel surface, it appears that the reaction rate is largely controlled by the rate of heat transfer to

the fuel and thus the rate with which a combustible vapor is

produced

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Most fuels will burn with flaming combustion only in the

gas phase. The process seems to be one of heat transfer from

the flame to the fuel which volatilizes and furnishes vapors

which when mixed with air or other oxidizers help sustain the

flaming. If any mechanism is devised to reduce or increase

the rate with which heat transfer takes place to the fuel,

the speed of the combustion process is either reduced or increased respectively.

Common liquid fuels evidence the same effects mentioned

previously. Those with low vapor pressure are more difficult

to ignite than highly volatile fuels. It is, however, the

vapors above the liquid surface which burn and a stable flame

can be produced above such a surface only after the liquid has

been heated sufficiently to provide a continuing supply of

vapor to the fire.

The question of explosive limits is one of importance in

connection with many fuels. Thus in a poorly ventilated compartment the open storage of gasoline while not advisable may

not present a serious explosion hazard while similar storage

of' kerosene would. The apparent anomaly here results from

the difference in vapor pressures of the two fuels. The vapor

pressure of gasoline is so high that the fuel vapor concentration at normal temperatures may be above the explosive limit.

The vapors from kerosene on the other hand develop only a low

partial pressure and thus permit maintenance of an explosive

mixture with attendant hazard.

A discussion of the various ignition processes may now be

presented

.

Bacteriological processes ; The production of microorganisms under the rather ideal conditions provided by stacks

of agricultural products which have not been properly dried is

considered as a likely source of initial heating to about 75°C,

references 3 and h. Such bacteriological activity is reported

as stopping at temperatures much above this and it seems likely

that further heating, when it takes place, must result from the

previously _ mentioned depolymerization and oxidation reactions.

Such reactions are only likely to occur when stacks of sufficient size are exposed in a position to reduce heat losses from

the remote central regions. Bacteriological heating is reported

to require both moisture and oxygen and apparently does not take

place in their absence, reference V.
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Chemical Heating ; The production of heat as the result of

a chemical reaction is a common occurrence. The rate of release

of heat will depend on the process involved and will thus determine the possible form the ignition process will assume. Thus

absorption of water from the air may take place with liberation

of heat but for most materials this is, under normal conditions,

such a gradual process that it seems likely to present a serious

ignition hazard only when large bulks of combustible materials

are involved and other self heating phenomena can take over after

this process has neared completion.

Other chemical reactions involving the production of new

compounds can often result in very high heat release rates.

References 5 and 6 present useful information on this type

of ignition hazard.

Frictional Heating : The energy expended in rubbing two

materials together is a well known source of fire. It has

long been used by man as a primary source of ignition. The

fire drill used by the American Indians is but one of a wide

variety of friction ignition systems which have been used,

see reference 7*

It seems likely that all frictional ignition phenomena

will involve relative motion between solids. As in the case

of the fire drill, although the rubbing members may ignite,

some tinder is often involved which provides a more flammable

mixture of fuel and air. Thus fires in cotton bales have

been attributed to frictional heating between adjacent metal

ties.

High Temperature Exposure : Many materials will ignite

spontaneously after prolonged exposure to elevated temperatures,

references 8 and 9* Most cases of such ignition actually

appear to take place in the gaseous or vapor phase. Thus the

distillation products resulting from heating wood provide

vapors which are subject to self ignition, producing a flame

which may become stabilized on the fuel itself. The ignition

of fuels by radiant energy sources should be considered as

a special case of ignition by high temperatures.

Rapid Compression: The Diesel engine presents one of the

common examples of the use of rapidly compressed air for ignition of fuel. Although a dispersed oil is usually used,

engines have been operated on pulverized solids such as coal.

Earlier use of compression of air as an ignition source is
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exemplified by the fire syringe or fire piston as used by

natives of Burma and Siam, reference 7* Here the rapid compression of air was used for ignition of tinder. The compression of air within which a suitably prepared fuel charge

is placed seems to present a reliable and often useful source

of ignition.

Shock Wave : Under certain conditions shock waves can be

developed in gases. Such waves result from a portion of the

gas achieving a high velocity of such a character that the

front of the wave assumes a discontinous aspect:. Such waves

may be formed when a compressed gas is suddenly allowed to

expand into a region of gas at a lower pressure. When this

happens, a shock is formed which travels through the low

pressure gas and raises the pressure and temperature in the

region through which it has traveled. When such a shock is

reflected from a material boundry the pressure and temperature

are again raised and material behind the shock still further

heated. Combustible materials exposed to these conditions

may thus become ignited; however, because of the transient

nature of this process, ignition delays must be very short.

This is however often possible because of the very high temperatures which may be experienced behind such shocks. Under

certain conditions an oscillating gas discharge can be established in such a manner that high temperatures resulting from

the shock can be maintained for an extended period. In this

case the short ignition delay period is not required and ignition of massive combustibles can be directly achieved. An

example of ^this is the wooden block and compressed air jet

experiment in which a hole drilled in the block is used with

the air jet to form a Hartman generator. During operation of

this device a cavity is burned within the block and a fire may

be started.

Open Flame : The open flame provides another means for

application of intense local heating to a combustible and

serves as an ignition source for distillation products. The

question of whether or not a liquid or solid fuel will sustain

combustion on removal of the flame depends largely on the composition of the fuel and the manner in which it is dispersed

(geometry). The term flammability has frequently been used as

indicative of the ease which which a material may become inflamed. This property is usually largely dependent on both

the chemical nature of the reacting materials and the rate with

which the materials become heated by the flames which are

developed. The rate with which this feedback of energy in the
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form of heat takes place will depend not only on the reaction

under way but also on ambient conditions such as temperature

and pressure. Thus fires involving materials such as propellants

can often be extinguished by simply lowering the atmospheric

pressure.

Electric Sparks : Sparks are frequently used as ignition

sources for gaseous or liquid fuels, reference 2. The automobile serves as an excellent indication of the reliability

of such ignition sources. Dispersed solids can also be ignited

as evidenced by ignition of forest fuels by lightning. Static

electricity can serve as a source of such sparks. The manner

in which such electrostatic charges can be collected in_a form

suitable for production of a spark is almost infinitely varied

and has been the subject of extensive study; references 10 and

11 present good introductions to some of the problems involved.

Mechanical Impact : Mechanical impact may provide a source

of ignition for many types of fuels. It appears however that

in most instances such impact acts through other previously

described ignition sources as intermediate mechanisms. Thus

impact initiation of detonation in liquid explosives is currently

considered to result from rapid compression of small gas inclusions in the explosive, reference 12. Similarly the percussion

ignition of tinder and gases is usually found to result from

the formation of small highly heated particles which when introduced into the fuel serve as ignition sources, reference 13.

Impact can also cause frictional heating by rubbing grains of

an explosive together providing another source of Ignition.

The fac^ that direct impact does not serve as a common ignition

source for other than explosive materials probably results from

the low compressibility of most combustible solids and the

resulting small amount of compression energy that can appear

in the form of heat. This together with the fact that direct

mechanical impact so often causes dispersion of the material

struck and thus rapid dissipation of the heated spots causes

this source of ignition to be rather ineffective for most common

materials. However, for sensitive materials such as explosives,

propellents, etc., this source can oft-en provide a good mechanism

for the initiation of an ignition reaction.

SUMMARY

The process of ignition of combustible material may take

place in an almost endless variety of ways. To be successful

an ignition process must result in the establishment of a
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heat generation rate greater than the loss rate to the

surroundings. Once this has been achieved the process, may

become self-sustaining until either the readily combustible

portions of the fuel or, the oxidizing agent has been consumed.

Because of the dependence of the ignition reaction on the

rate of heat loss to the surroundings it is difficult if not

impossible to define an ignition temperature which may be

considered a unique property of the material being studied.
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10

REFERENCES

1. B. Lewis and G. von Elbe, "Combustion, Flames and

Explosions of Gases", Academic Press, Inc., 1951*

2. W. lost, "Explosion and Combustion Processes in Gases",

translated by H. 0. Croft, McGraw-Hill Book Co. , 19^6.

3. P. C. Bowes, J. H. Burgoyne, T. P. Hilditch, and A. Thomas,

"Spontaneous Heating and Ignition in Stored Palm Kernels",

J. Sci. Food and Agric. , N12, pp. 360-3o6, 1950.

h. C. A. Browne, "The Spontaneous Combustion of Hay", U. S.

Department of Agriculture, Tech. Bull. 1^1, pp. 1-395 1929.

5. C. W. Bohme, "Fire Protection for Chemicals", National Fire

Protection Association International, 1956.

6. Anon, "Fire and ^Explosion Hazards of Organic Peroxides"-,

NFBU Research Report No. 11, 1956.

7. W. Hough, "Fire As An Agent in Human.'Xulture " , Bulletin

1395 Smithsonian Institution, U. S. Govt. Printing Office,

1926.

8. C. R. Brown, "The Determination of the Ignition Temperatures of Solid Materials", PhD Thesis 193^, Catholic

University of America.

9 . N. P. Setchkin, "Self Ignition Temperatures of Combustible

Liquids", J. Res. NBS, V53, N1 , July 195^.

10. F. L. Silsbee, "Static Electricity", U. S. Department of

Commerce, NBS Circular C438, U. S. Govt. Printing Office,

19^2.

11. P. Guest, "Static Electricity in Nature and Industry",

U. S. Department of Interior, Bureau of Mines Bulletin

No. 368 , U. S. Govt. Printing Office 19^.

12. F. P. Bowden and A. D. Yoffe, "Initiation and Growth of

Explosion in Liquids and Solids", Cambridge University

Press, 1952 .

13* Titman, "A Review of Experiments on Ignition of Inflammable

Gases by Frictional Sparking", Trans Inst. Mining, V115,

PP- 535, April 1956.

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U. S. DEPARTMENT OF COMMERCE

Sinclair Weeks, Secretary

NATIONAL BUREAU OF STANDARDS

A. V. Astin, Director THE NATIONAL BUREAU OF STANDARDS

The scope of activities of the National Bureau of Standards at its headquarters in Washington,

D. C., and its major laboratories in Boulder, Colo., is suggested in the following listing of the

divisions and sections engaged in technical work. In general, each section carries out specialized

research, development, and engineering in the field indicated by its title. A brief description of

the activities, and of the resultant publications, appears on the inside front cover. WASHINGTON, D. C.

Electricity and Electronics. Resistance and Reactance. Electron Devices. Electrical Instru- ments. Magnetic Measurements. Dielectrics. Engineering Electronics. Electronic Instrumentation. Electrochemistry.

Optics and Metrology. Photometry and Colorimetry. Optical Instruments. Photographic

Technology. Length. Engineering Metrology.

Heat. Temperature Physics. Thermodynamics. Cryogenic Physics. Rheology. Engine

Fuels. Free Radicals Research.

Atomic and Radiation Physics. Spectroscopy. Radiometry. Mass Spectrometry. Solid State Physics. Electron Physics. Atomic Physics. Neutron Physics. Nuclear Physics. Radioactivity. X-rays. Betatron. Nucleonic Instrumentation. Radiological Equipment.

Chemistry. Organic Coatings. Surface Chemistry. Organic Chemistry. Analytical Chemistry. Inorganic Chemistry. Electrodeposition. Molecular Structure and Properties of Gases. Physical Chemistry. Thermochemistry. Spectrochemistry. Pure Substances.

Mechanics. Sound. Mechanical Instruments. Fluid Mechanics. Engineering Mechanics.

Mass and Scale. Capacity, Density, and Fluid Meters. Combustion Controls.

Organic and Fibrous Materials. Rubber. Textiles. Paper. Leather. Testing and Specifications. Polymer Structure. Plastics. Dental Research.

Metallurgy. Thermal Metallurgy. Chemical Metallurgy. Mechanical Metallurgy. Corrosion.

Metal Physics.

Mineral Products. Engineering Ceramics. Glass. Refractories. Enameled Metals. Concreting Materials. Constitution and Microstructure.

Building Technology. Structural Engineering. Fire Protection. Air Conditioning, Heating,

and Refrigeration. Floor, Roof, and Wall Coverings. Codes and Safety Standards. Heat

Transfer.

Applied Mathematics. Numerical Analysis. Computation. Statistical Engineering,

matical Physics.

Data Processing Systems. SEAC Engineering Group. Components and Techniques

Circuitry. Digital Systems. Analog Systems. Application Engineering.

• Office of Basie Instrumentation. • Office of Weights and Measures. BOULDER, COLORADO

Cryogenic Engineering. Cryogenic Equipment. Cryogenic Processes. Properties of Materials.

Gas Liquefaction.

Radio Propagation Physics. Upper Atmosphere Research. Ionospheric Research. Regular

Propagation Services. Sun-Earth Relationships. VHF Research.

Radio Propagation Engineering. Data Reduction Instrumentation. Modulation Systems.

Navigation Systems. Radio Noise. Tropospheric Measurements. Tropospheric Analysis.

Radio Systems Application Engineering. Radio Meteorology.

Radio Standards. High Frequency Electrical Standards. Radio Broadcast Service. High

Frequency Impedance Standards. Calibration Center. Microwave Physics. Microwave Circuit

Standards.

. Mathe-

. Digital
NBS