Prog.EneroyCombust.Sci. 1982, Vol. 8, pp. 317-354.                                                  0360-1285/82/040317 38...
F. A. WILLIAMS
318

                                                TAnLE 1. Annual fire losses

                         ...
Urban and wildland fire phenomenology                                           319

                                     ...
320                                                F.A. WILLIAMS

                                                        ...
Urban and wildland fire phenomenology                                  321

                            3.2. Polymers     ...
322                                                  F.A. WILLIAMS

                                                      ...
Urban and wildland fire phenomenology                                                      323

table of adiabatic flame t...
F. A. WILLIAMS
324

                     TABLE6. Yield of monomer in the pyrolysis of some organic polymers in a
         ...
Urban and wildland fire phenomenology                                             325

                                   ...
326                                               F.A. WILLIAMS

first by the oxygen radical and next by the hydroxyl.
The...
Urban and wildland fire phenomenology                                       327

                                         ...
328                                                    F.A. WILLIAMS

                                                    ...
Urban and wildland fire phenomenology                                         329

                                       ...
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Sdarticle
Upcoming SlideShare
Loading in …5
×

Sdarticle

2,024 views

Published on

Published in: Technology, Education
0 Comments
0 Likes
Statistics
Notes
  • Be the first to comment

  • Be the first to like this

No Downloads
Views
Total views
2,024
On SlideShare
0
From Embeds
0
Number of Embeds
1
Actions
Shares
0
Downloads
23
Comments
0
Likes
0
Embeds 0
No embeds

No notes for slide

Sdarticle

  1. 1. Prog.EneroyCombust.Sci. 1982, Vol. 8, pp. 317-354. 0360-1285/82/040317 38519.00/0 Printed in Great Britain. All rights reserved. Copyright © Pergamon Press Ltd. URBAN A N D WILDLAND FIRE P H E N O M E N O L O G Y F. A. WILLIAMS Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, U.S.A. to fire science. References 1 through 5 are examples of 1. I N T R O D U C T I O N the type of material that is available. These references Mankind's concern with unwanted fires likely pre- are directed toward urban fires; fewer books are avail- dates the first practical use of combustion in unre- able concerning wildland fires. Reference 6 is a collec- corded history. Yet the science of fire protection has tion of articles concerning both urban and wildland progressed more slowly than other aspects of com- fires. Reference 7 often is cited as a forest-fire text. bustion science. This state of affairs is due partially to Reference 8 is a detailed documentation of many the complexity of the problem and partially to the fact forest fires that have occurred in North America. that relatively large technological payoffs generally Reference 9 is a fictional novel which nevertheless is are not anticipated to be obtained from scientific rather accurate technically, concerning a forest fire. investigations of fires. Numerous promises and prob- With the exception of the material in Ref. 6, the lems in the use of combustion in heat and power pro- level of scientific description in the books cited is not duction, in locomotion and in industrial endeavors very advanced. Typical undergraduates in engineering have generated intensive scientific efforts. By way of have backgrounds in physics, chemistry, thermo- contrast, the ever-present fire problems have attracted dynamics, fluid mechanics and heat and mass transfer fluctuating interest with a relatively low average level that would enable them to appreciate a more sophisti- of concern. cated treatment. No book exists giving a unified Although periodic disasters engender beliefs that exposition of fire science at a more advanced level. more should be done in fire science, these beliefs often The variety of authors of Ref. 6 provide material at an are short-lived and are replaced by more immediate advanced level with uneven coverage. concerns. Rarely has a disaster or a series of disasters A source of references on continuing research in fire provoked a program of scientific study of fire phenom- science is provided by Ref. 10. This periodic publi- ena. The long-term efforts that have been mounted in cation contains numerous good review articles on the field instead have been motivated mainly by various aspects of fire problems. Much of the informa- detailed comparative evaluations of the magnitude of tion underlying the present article has been obtained the fire problem. from Ref. 10. The biennial combustion symposia, There is reason to believe that today's rapid techno- starting approximately with the tenth,11 contain many logical advances intensify problems of unwanted fires. original research papers in the field. Hosts of new processes and new combustible materials are emerging and finding their way into widespread 1.2. Magnitudes of Fire Problems use. Too often these innovations become common- place before their fire hazards are properly under- There have been many compilations of losses at- stood. Therefore there seems to be justification for tributable to unwanted fires. The accuracy of such information always is open to question because of more extensive development and dissemination of uncertainties in reporting and possible errors in collec- knowledge in fire science. tion and tabulation of data. Nevertheless, published The present article has been prepared with the numbers are at least roughly indicative of reality. objective of bringing together many aspects of fire science for a nonspecialized audience. It is based on an Table 1 lists some loss information obtained in the undergraduate course by the same name, given only middle of the decade 1970. Some of the numbers given once, at the University of California, San Diego. It have been rounded here in an effort to reflect uncer- focuses on basic aspects of fire as a phenomenon, tainties. Annual fire deaths per capita depend on many presented in an elementary but unified manner. It is factors, such as living conditions and degree of indus- more restricted than other treatments of the subject in trialization; they may vary over an order of magni- that physiological, social, economic, organizational tude. The relative significance of deaths and of prop- and operational aspects are not covered. However, it erty loss is difficult to assess rationally. In the United is broader in that both urban and wildland fires are States, most of the fires are residential fires, and most considered equally; most presentations are slanted of the fire deaths occur in residential fires, but most of toward one or the other of these classes of fire the property loss occurs in industrial and commercial problems. I hope that this article will help to introduce fires. In the figures for 1972, obtained by the Presi- nonspecialists to the scientific phenomenology of fires. dent's Commission on Fire Protection and Control, 1.1. Books on Fire Science the total cost is calculated as roughly equal contribu- tions from property loss, fire-related expenses in build- A number of books have been devoted specifically 317
  2. 2. F. A. WILLIAMS 318 TAnLE 1. Annual fire losses US UK USSR Japan Deaths 10,000 1,000 Deaths per million 40 12.6 1.2 Serious injuries 300,000 1972 property loss $3 X 10 9 Property loss capita $20 $2.5 $0.2 $2.6 1972 total cost $10 x 109 Number of fires 5 x 106 Fires fought by fire service per 1000 population 4.5 3.1 0.16 Forest acres burned (1973-78 average) 3.8 x 106 was record-breaking, but nevertheless about 80~o of ing construction, costs of fire departments and fire the population in the fire area survived in Hamburg. insurance costs. The forest acres burned are approxi- Large forest fires continue to occur when atmospheric mately the size of the land area of the state of New conditions favor them; the last entry in Table 2 repre- Jersey. It appears from these figures that fire problems sents 1260 separate fires in a two-month period, with are significant, particularly in the United States. total losses placed in excess of $2 x 1 0 6. 1.3. Historic Fires 1.4. Definitions of Fires There are many well-known fires in recorded his- Certain terms peculiar to fire studies deserve defini- tory. A few of these are listed in Table 2, along with tion at the outset. As a general definition, a fire may some other fires that are not so well known. It is seen be taken to be a chemical reaction of fuel with oxygen that the famous fire of London burned over a relatively to produce heat, thereby involving heat transfer and small area. Fires often are associated with military fluid flow. This definition is intended to exclude very events; the Moscow fire coincided with Napoleon's slow oxidations, such as rusting, but to allow for occupation of the city. It is not generally known that gaseous, liquid or solid fuels, polymers or metals, in October, 1871, on the same day as the famous burning under controlled or uncontrolled conditions. Chicago fire, fires began relatively nearby, in the A mass fire may be defined as any large fire involv- Peshtigo area of Wisconsin and in central Michigan, ing more than one sizeable structure and taxing that burned through 17 towns and 5000 square miles, resources of fire-fighting agencies. Mass fires may be killing nearly four times the number of people who divided into subcategories, depending on their charac- perished in the Chicago fire; the coincidence likely teristics. For example, a conflagration is a large propa- reflects the occurrence of optimal weather conditions gating fire; spread of the fire is a key aspect of this for burning in the area. definition. Large forest fires are typical examples of Although the San Francisco fire, associated with an conflagrations, but conflagrations also may occur in earthquake, is well-known near the turn of the century, cities. A fire storm may be defined as a large, intense, the Baltimore fire was considerably more instructive in revealing fallacious fire-fighting practice, x2 The localized fire, usually with a single convection column above it, nonspreading and having high-velocity fire- fires in Hamburg, Tokyo and Dresden, during World War II, were caused intentionally by incendiary bomb- induced winds. Specific definitions of fire storms often require the velocity somewhere to exceed a specified ing; fire storms were established, and the loss of life TABLE2. Some historic fires Location Date Acres burned Homes lost Deaths London 1666 336 13,200 Moscow 1812 Chicago 10/8/1871 2,124 300 Peshtigo 10/8/1871 3,600,000 1,000 Baltimore 1904 San Francisco 1906 Idaho and Montana 8/1910 3,000,000 Tokyo 1923 1,200 Hamburg 1943 2,500 40,000 Tokyo 1945 9,600 85,000 Dresden 1945 150,000 Hiroshima 1945 3,000 Ft. Yukon, Alaska 1950 2,000,000 Laguna, California 10/1970 175,000 382 3 California 9/15 11/15/70 600,000 885 14
  3. 3. Urban and wildland fire phenomenology 319 Table 3. A few combustible materials Heat of combustion Material Formula Flame color kcal/mole of fuel cal/g of fuel Gases Hydrogen H2 invisible 68.3 34,150 Carbon monoxide CO blue 67.6 2,410 Natural gas (methane) CH 4 blue 210.8 13,180 Propane C3 H8 blue yellow 526.3 11,960 Ethylene C2 H4 blue-yellow 337.3 12,050 liquids blue-yellow 1149.9 11,500 Heptane C7H16 blue yellow 1302.7 11,430 Octane Cs H t s Benzene yellow green 782.3 10,030 C6H 6 Gasoline HC 11,530 Kerosene HC 11,000 Methyl alcohol CH3 OH blue 170.9 5,340 yellow 1047.1 10,070 Styrene Cs Hs Solids Carbon (graphite) C yellow 93.9 7,830 C12H22011 1349.6 4,000 Sugar (sucrose) 397.2 4,460 Urethane C3H7NO 2 4,200 Cellulose (~ glucosan) C 6H 10O 5 Wood (birch, oak, etc. under average conditions in nature) 4,000 Charcoal CH~(c~ < 1) 7,260 Steel (iron) Fe 1,580 Magnesium Mg 6,080 2.1. Hazard Aspects value, e.g. 75 mph. Fire storms with well-developed convection columns may generate clouds of water Many different properties of fuels have bearing on droplets from condensation of cooled reaction pro- their fire hazards. One is their ease of ignition; even if ducts; in extreme cases rain may fall from the clouds. its heat release is low, a material that can be ignited A fire whirl or a fire vortex may be defined as a easily may pose a severe fire hazard. Another relevant fluid-mechanical vortex with fire in it, generated or property is the heat of combustion, listed in Table 3; intensified at least partially as a consequence of the materials with high heats of combustion can be rela- fire. Fire whirls typically may be elements of fire tively more effective in sustaining fires. Flame spread storms or of mass fires in general; they have been sug- is a third aspect of fire hazards; materials that are gested as small-scale models for some types of fire difficult to ignite and that have low heats of combus- storms. Intense whirls, sometimes called fire tornadoes, tion may nevertheless spread flames relatively rapidly can be destructive. and thereby be dangerous. Subsidiary aspects of behaviors of materials in fires 1.5. The Fire Triangle also influence their fire hazards. Smoke can cause Books on fire science often employ a triangle to damage and also can interfere with escape from fires represent the key elements of a fire. The triangle has and with fire fighting; propensity of a material for three legs, representing heat, air and fuel. Strategies smoke production therefore is relevant in assessing its for fire suppression through flame extinguishment fire hazard. Materials capable of generating toxic pro- often are viewed as attempts to remove one of these ducts in fires are of particular concern. three elements. The fire triangle is intended to provide Finally, ease of extinction is a significant aspect an intuitive feeling for essentials of fire at an elemen- of a material's fire hazard. An otherwise dangerous tary level. material may be acceptable if its flames can be extin- guished readily. There are many tricky aspects to the 2. COMBUSTIBLE MATERIALS evaluation of fire hazards. Some will be considered Of basic concern in fire problems is the identifica- later in connection with estimates of flammability. tion of materials that can serve as fuel. Most things, 2.2. Fire Categories even steel, will burn under suitable conditions; carbon There is a partial correspondence of the states listed dioxide, water and sand are examples of materials that in Table 3 with the categories of fires employed in fire cannot burn. Table 3 lists some c o m m o n combustible protection. Fire classes are: Class A, Solid; Class B, materials and gives some of their combustion proper- Liquid; Class C, Electrical. These classes are defined ties, notably the energy released when they burn.
  4. 4. 320 F.A. WILLIAMS Pyrolysis means a chemical transformation pro- roughly in increasing order of the danger associated duced by application of heat. In finite-rate pyrolysis, with the fire and call for different techniques of fire molecules from the gas that strike the surface of the fighting. There is no class corresponding to gaseous fuel enter the condensed phase to a negligible extent, fuels because fires with such fuels are encountered and approximate formulas may be written directly for relatively infrequently and because when they do occur the duration of the fire usually is too short for the rate of gasification as a function of T. 15 A useful counter-measures to be taken. This does not mean approximate formula for the gasification rate m, the that gaseous fuels are less dangerous; on the contrary, mass per unit area per second of vapors leaving the rapid flame spread through gases often generates surface of the fuel, is pressure increases characteristic of explosions or buoy- m -- msex p [ - E s / ( R T ) ] , (2) antly rising clouds of burning gases with damaging levels of radiant energy transfer. where ms and Es are constants, the latter being an In all three fire classes usually gases actually burn. effective overall energy of activation for surface pyro- These gases are secondary, not primary fuels and are lysis. Values of ms and E~ are difficult to find in the liberated from the liquid or solid fuels in the fire literature, although some information is available.6 environment. There are a few exceptional cases in All of the gasification parameters that have been which the liquid or solid fuels burn directly without introduced here are properties of fuels. Understanding previous liberation of combustible gases. Carbon, of these properties and of pyrolysis processes requires some explosives and solid propellants, and certain knowledge of chemical bonding, chemical conversion metals are examples of fuels that burn directly, and and heat liberation. glowing combustion of wood or tobacco is a burning process that does not involve a gaseous combustible 3. CHEMICAL CONVERSION A N D HEAT LIBERATION intermediary. Chemical conversion is a process whereby a chemi- cal in one form is transformed into chemicals in other 2.3. Burning Mechanisms of Solid and Liquid Fuels forms. The many different types of chemical con- It is important to understand the usual mechanism, version that are possible are dictated by molecular alluded to above, by which condensed-phase fuels structure, which is determined by chemical bonding. (solids and liquids) burn. The heat is released in the 3.1. ChemicaI Bonding gas phase by the exothermic combustion of the secon- dary gaseous fuels. Some of this heat is transferred Molecules are formed by establishment of chemical back to the condensed phase to cause gasification of bonds among atoms. These bonds may be ionic (i.e. the primary fuel. This gasification usually is an endo- involve exchange of electrons) or covalent (i.e. involve thermic process (requiring heat) which releases the sharing of electrons). For combustible materials co- gaseous combustibles to burn. Thus, feedback of heat valent bonds are by far of greatest importance. Ar- from the gas-phase flames to the condensed-phase rangements of bonds formed in stable molecules de- fuels usually is an essential aspect in maintaining a pend on the valences of the atoms involved. A covalent fire. bond, two shared electrons, conventionally is indi- cated by a bar; for example, H 2 is H--H. Some of the 2.4. Gasification Processes fuels in Table 3 are Two fundamentally different types of gasification H H H processes occur in fire. One, encountered most often I I I for liquid fuels, is equilibrium evaporation, and the methane, H--C--H ethylene, H--C=C--H other, encountered most often for solid fuels, is finite- I I [ rate pyrolysis. H H H In equilibrium evaporation, interphase equilibrium is maintained at the surface of the fuel. This equi- H I librium may be described by a useful approximate methanol, H--C--O--H formula for the mole fraction X~ of fuel vapor in the I gas at the surface of the condensed fuel. 13 If T(K) is H the surface temperature, Tb(K) the normal boiling temperature, R ~ 2 cal/mol K the universal gas con- H H I I stant and L the latent heat of vaporization, then CmC // % L 1 1 and benzene, H - - C C--H, / C=C Tables of L and Tb are available; 13'14 for example I L ~ 10kcal/mole and Tb = 373K for water. Equa- H H tion (1) thus provides a relationship between Xe and T. This relationship must be used in formulas for which will be represented as H - - © for brevity. Note transfer rates to obtain gasification rates under con- that ethylene, for example, has a C = C double bond, ditions of equilibrium evaporation. representing four shared electrons.
  5. 5. Urban and wildland fire phenomenology 321 3.2. Polymers H I Many of the solid fuels of concern in fires are H--C--O--H i polymers. Polymers are large molecules, formed con- C O ceptually (in the simplest cases) by breaking a double bond in identical molecules and interconnecting them. 16 Thus, polyethylene is C C -o / ?-H H H H HIH H C C J lli i I I .... C m C - ~-C--C ,--~-C--C • •., H O--H i II I:1 I H H',H H H H Qualitative feelings for burning behaviors of cellulosic materials are common. There is less in- where the broken vertical lines separate the ethylene tuition concerning burning behaviors of synthetic quot;monomersquot;. The degree of polymerization is the polymers. Many, such as polyethylene, polystyrene number of monomers in the polymer chain; the chains and poly(methyl methacrylate) soften and form a may be terminated in various ways, e.g. by placing an liquid-like quot;meltquot; when they burn. Thus, it becomes H at the end. unclear as to whether they should fall in fire class Styrene is A or B. Polyvinylchloride may form corrosive HCI during combustion, while acrylonitrile may produce H H measureable amounts of highly toxic HCN upon pyrolysis. Thus, the advent of synthetic polymers C=C , f J raises new fire problems. © H 3.3. Bond Energies and therefore polystyrene is Energetic aspects are of importance for chemical conversions that occur in fires. Energies liberated in H H H H chemical processes, such as heats of combustion, need I I I I to be known. Energies absorbed, such as heats of .... C--C--C--C .... I I i i pyrolysis of polymers, energies required to convert 0 H 0 H specified polymers to gases at a given temperature, also must be known. There are many tables 14 of these In addition to polyethylene and polystyrene, many heats of reaction. However, often it is of interest other synthetic polymers are experiencing increasingly to calculate energy changes for processes that are widespread use. These include polyvinylchloride difficult to find in tables. Bond-energy methods enable such calculations to be performed, with accuracies that although typically are not high nevertheless are sufficient for many purposes. H H The bond-energy approach rests on the idea that a I I (monomer unit - - c - - c - - ), definite amount of energy liberation is associated with I I the formation of a given chemical bond. The idea is H C1 not precisely correct in that energy liberated may depend also on the molecule in which the bond occurs acrylonitrile and on the location of the bond within the molecule. In fact there are correction procedures to account for H H these effects, which may be quite substantial. In a I I rough first approximation the corrections may be --c--c-- ) (monomer unit neglected, and the energy liberated in forming a I I H C~N gaseous molecule from its constituent atoms may be calculated simply by adding the energies associated and poly(methyl methacrylate), quot;plexiglasquot; or quot;lucitequot;, with each bond formed. A list of the bond energies with monomer unit needed for this calculation is given in Table 4, which has been taken from information in Ref. 17 and does I not necessarily represent the most up-to-date infor- H H--C--H H mation, although it is useful for illustrative examples. I I I Accuracies in energy calculations better than 5 0 ~ H--C C C--O--C--H. may be anticipated when using Table 4. i f fi i H O H 3.4. Combustion Reactions The combustion reaction which occurs in the flames Cellulose, the principal polymeric constituent of of fires is a chemical combination of fuel with air natural wood, is built from a glucosan monomer,
  6. 6. 322 F.A. WILLIAMS 3.6. Flame Temperature TABLE4. Mean bond energies (kcal/mole) Temperatures of flames exceed ambient tempera- Bond Energy Bond Energy ture because the heat released in combustion goes into C--C 85 N=-N 225 raising the temperature of the combustion products. C~---C 143 H--H 103 The extent to which the temperature is raised depends C~C 198 O--H 109 on the heat capacity Cp of the products. Tables of cp C--H 98 O--N 150 are available.'4 In fact cp varies with temperature, but C--O 86 N--H 88 as a first approximation it may be taken as constant. C=O 173 S--S 50 C--N 81 C1--C1 57 For gases cp generally lies between 0.2 and 0.5 cal/gK; C~N 210 Br--Br 46 in a very rough approximation it may be taken as C--CI 78 I--I 36 0.3 cal/gK for all gases. For liquid water % - 1 cal/gK; C--Br 67 F--F 36 for most other liquids and for solid combustibles it C--I 64 H - - C1 103 C--F 102 H--Br 88 typically lies between 0.3 and 0,7 cal/gK. C--S 64 H--I 72 F r o m the molar heat of combustion Q, the heat O--O 33 H--F 135 release per unit mass of products may be calculated as O=O 117 H--P 76 Q/W, where W is the sum of the molecular weights of N--N 60 H--S 81 the species on the right-hand side of the equation for the chemical conversion of one mole of fuel, i.e. the stoichiometric mass of all products per mole of fuel consumed. The flame temperature TI is then found to produce CO2, H 2 0 , N2 and heat. Air, in a from the adiabatic energy balance Q/W = cp(Ty- Ti), first approximation, is 0 2 + 4N 2. Thus, for example, where T~ is the initial temperature, typically r o o m the combustion of hydrogen in air is represented temperature, about 300K. Thus as H z + ½ O z + 2 N 2 ~ H z O + 2 N a + Q H 2, where Qn~ is the heat of combustion per mole for hydrogen. (3) Ts = T , + Q / t W c p ) . Similarly, for carbon monoxide, C O + ½02 + 2N2 Corrections to this for phase changes may be included CO2 + 2 N z + Q c o . These equations are balanced by suitably revising Q. chemically in that there is no fuel or oxygen left over; As an example, consider the combustion of propane such chemical conversions are termed stoichiometric. in air, C3H s + x(O 2 + 4N 2) --~3CO 2 + 4 H 2 0 + 4xN 2 + Balancing a chemical reaction to achieve stoi- QC3H~ with x = 5 from the chemical balance. F r o m chiometry may be illustrated by considering the Table 3, since the molecular weight of propane is combustion of heptane. Write the reaction as C7 H ~6 + 44 g/mole, QC3H, = 11,960 × 44 = 526,000 cal/mole, x(O2 + 4N2) ~ 7CO2 + 8 H 2 0 + 4xN2 + QCTH~6, and W = 3 × 4 4 + 4 × 1 8 + 2 0 × 28 = 764 g/mole. Hence, where x is unknown. The coefficients of C O 2 and of with ee = 0.3g/mole K, eq. (3) gives Ts = 3 0 0 + 6 9 0 / H 2 0 have been determined from the chemical formula 0.3 = 2600K, which is about 300K too large. This pro- of the fuel. An oxygen balance then is used to find that cedure usually overestimates TI because it neglects x = 11, thereby completing the stoichiometry. effects of dissociation of reaction products, which occurs above about 2000K; dissociation involves, for ex- ample, C O 2 ~ - C O + ½0 2. There are iterative methods 3.5. Calculation of Heat of Combustion and computer programs for calculating Ty with dis- The energies Q in the preceding equations are sociation included (see, for example, Ref. 18). A short best calculated from tables of standard heats of for- mation, the energies liberated when molecules are formed from their constituent elements in their stan- TABLE5. Approximate flame temperatures of various dard states. A somewhat less involved approach is stoichiometric mixtures having initial temperature 298K to use the bond energies listed in Table 4. As a simple example consider the combustion of hydro- Pressure Fuel Oxidizer (atm) Tf(K) gen. Write the equation for chemical conversion as H--H+½0 = O~H--O--H+Qrc F r o m Table 4, Acetylene Air 1 2600* this implies 103+½× 117 = 2× 109-QH2, where Acetylene Oxygen 1 3410quot; additivity of energies in reactions has been employed. Carbon monoxide Air 1 2400 The negative sign occurs because the heat of combus- Carbon monoxide Oxygen 1 3220 Heptane Air 1 2290 tion is positive if the total bond energies of the Heptane Oxygen 1 3100 products exceed those of the reactants. The result that Hydrogen Air 1 2400 QH~ = 56.5kcal/mole for combustion of gaseous H 2 Hydrogen Oxygen 1 3080 with gaseous 0 2 to form gaseous H 2 0 is within 5 ~o of Methane Air 1 2210 Methane Air 20 2270 the correct value. To find QH2 for combustion to Methane Oxygen 1 3030 liquid H 2 0 , the latent heat of vaporization L must be Methane Oxygen 20 3460 added to this result. The accuracy obtained here is better than average; it is preferable to use tables for Q *A maximum temperature that occurs under fuel-rich if they are available. rather than stoichiometric conditions.
  7. 7. Urban and wildland fire phenomenology 323 table of adiabatic flame temperatures, taken from may break another, stable polymer somewhere in the Ref. 18, is shown in Table 5. It is seen that tempera- middle, forming a new stable polymer with half of the tures in the range of 2300K are typical for burning in attacked chain and leaving the other half active. air. These temperatures are of importance in calculat- Among the possible termination steps is direct ing heat transfer in fires. The method for calculating combination of the radicals at the ends of two active Ts that has been outlined here is useful for obtaining chains to form a single stable polymer. Another type quick rough estimates. of termination step is disproportionation, in which two active radicals deactivate each other by an ex- change at the end of the chain. For polystyrene, an 4. CHEMICAL KINETICS OF PYROLYSIS example of disproportionation may be Other chemical conversions, in addition to combus- H H H H H H H H tion reactions, can be of significance in fires. These I I [ I I I J I include the formation of smoke and of toxic products ...--C--C--C--C-- +--C--C--C--C--... , I I I I I I I I in flames as well as conversions of solid fuels to OH OH 0 H 0 H gaseous combustibles. Many of these reactions are pyrolysis processes. For example, smoke may be pro- H H H H H H H I I I I I I I duced through a sequence of pyrolysis reactions of ...--C--C--C=C + H--C--C--C--C--.... gas-phase fuels in fuel-rich regions, and carbonaceous I I I I I I [ I residues may arise from liquid-phase pyrolysis of © H © H © H © H heavier liquid fuels. Attention here is focused on pyro- Clews concerning pyrolysis mechanisms for specific lysis of solid fuels to produce gaseous reactants. polymers are obtained from many different experi- 4.1. Chain Reactions in Polymer Pyrolysis mental observations. 15 One such measurement is the percentage of product volatiles composed of monomer, The chemistry of polymer pyrolysis is complex and found when the polymer is heated in a vacuum. Some differs for different polymers. Simplified descriptions data of this type are given in Table 6, taken from are needed to achieve understanding. A useful simplifi- Ref. 15. If the monomer yield is low then unzipping cation for many processes of polymer pyrolysis (as is unlikely, while high monomer yields are consistent well as for the kinetics of combustion reactions them- with unzipping. selves) is the idea of a chain reaction. Chain reactions have active intermediate species, chain carriers, whose 4.2. Simplified Kinetic Expressions presence cause the reaction to proceed more rapidly than it otherwise would. The chain carriers are formed Rates of polymer pyrolysis may be described by in initiation steps, cause the reaction to proceed in expressions for dM/dt, the time rate of change of the chain-carrying or propagation steps, and are con- mass M of the condensed phase in a homogeneous sumed in termination steps. system. Such expressions may be complicated for For polymer pyrolysis, there are many types of ini- chain reactions. There are conditions under which tiation steps. In end initiation, the monomer at the useful simplified approximations may be obtained. end of the polymer chain splits off, leaving a radical (a For example, for an unzipping process with a kinetic species with an unsatisfied chemical bond) at the chain length (or zip length, i.e. the number of propa- chain end. In random-scission initiation, thermal fluc- gation steps that occur prior to termination) com- tuations break the polymer at random points along its parable with the degree of polymerization, each initia- chain, producing radicals on each side of the scission. tion effectively results in unzipping of an entire chain. In weak-links initiation, the polymer is broken in- The rate of mass loss then is controlled by the rate of ternally at preferred high-strain spots, again leaving initiation, and radical-ended chains. dM/dt = - k M , (4) There are also many types of propagation steps. A relatively easy type to understand is unzipping, in where k is a specific reaction-rate constant for initia- tion. Often this first-order reaction-rate expression which a single monomer unit is formed and detached provides a reasonable approximation under more at the radical end of the chain. An illustration of un- zipping for polystyrene is complex circumstances, in which k becomes an effec- tive rate constant that includes influences of many H H H H H H H H different steps. I [ I I I I [ I The rate constant k depends on temperature T. , ...--C--C--+ C = C . ...--C--C--C--C-- I 1 I I 1 1 J I Often an Arrhenius expression for this dependence 0 H 0 H 0 H 0 H provides a good approximation. Thus, Propagation steps other than unzipping could be k = Bexp [-Eb/(RT)], (5) intramolecular transfer steps, namely detachment of where B and E b are constants, the latter being the higher units of the monomer, e.g. dimers or trimers, overall activation energy for bulk degradation. A from the radical end. Intermolecular transfers, inter- table of some measured values of Eb is shown as chain propagation processes, also are possible. For Table 7, again taken from Ref. 15. There have been a example, the radical at the end of an active chain
  8. 8. F. A. WILLIAMS 324 TABLE6. Yield of monomer in the pyrolysis of some organic polymers in a vacuum Temperature Yield of range monomer, ~o °C of volatiles Polymer Polymethylene 335-450 0.03 Polyethylene 393-444 0.03 Polypropylene 328-410 0.17 Polymethylacrylate 292 399 0.7 Hydrogenated polystyrene 335 390 1 Poly(propylene oxide), atactic 270-550 2.8 Poly(propylene oxide), isotactic 295-355 3.6 Poly(ethylene oxide) 324-363 3.9 Polyisobutylene 288-425 18.1 Polychlorotrifluoroethylene 347-415 25.8 Poly-fl-deuterostyrene 345 384 39.7 Polystyrene 366-375 40.6 Poly-m-methylstyrene 309 399 44.4 Poly-~-deuterostyrene 334-387 68.4 Poly-~,fl, fl-trifluorostyrene 333-382 72.0 Poly(methyl methacrylate) 246 354 91.4 Polytetrafluoroethylene 504-517 96.6 Poly-ct-methylstyrene 259 349 100 Polyoxymethylene Below 200 100 number of studies in which expressions for k have Two conceivable paths are been derived for more complex mechanisms. ~9 It can k~ nCO+nH 2 +nO z often be shown for steady-state pyrolysis that the E~ in eq. (2)of Section 2.4 is Eb/2. ....-/'/' ~ nCO2 + n H 2 0 (CH20)n k 2 quot; ~ n C+nH20 +.~nO2 4.3. Competition in Pyrolysis Certain materials such as wood and paper exhibit The rate constant for the initial step is kl in the upper two types of combustion, flaming and glowing. The path and k 2 in the lower. The final two arrows occurrence of these two types may be traceable to the represent oxidation, involving combination with 0 2 existence of two competing pyrolysis mechanisms for to produce combustion products. Although the final products of combustion are the the fuel. Such competition may be illustrated most simply by considering pyrolysis of a carbohydrate, the same, the different intermediaries can cause the burn- formula for which is (CH20)n, with n = 6 for glucose. ing mechanisms to differ. The species C O and H 2 are TABLE7. Activation energies of thermal degradation of some organic polymers in a vacuum Temperature Activation Molecular range, energy Polymer weight °C kcal/mole Phenolic resin -- 332-355 18 Atactic poly (propylene oxide) 16,000 265-285 20 Poly(methyl methacrylate) 150,000 226-256 30 Polymethylacrylate -- 271-286 34 Isotactic poly(propylene oxide) 215,000 285-300 45 Cellulose triacetate -- 283-306 45 Poly(ethylene oxide) 10,000 320 335 46 Polyisobutylene 1,500,000 306 326 49 Hydrogenated polystyrene 82,000 321-336 49 Cellulose -- 261 291 50 Polybenzyl 4,300 386-416 50 Polystyrene 230,000 318 348 55 Poly-~-methylstyrene 350,000 229 275 55 Poly-m-methylstyrene 450,000 319-338 56 Polyisoprene -- 291 306 57 Polychlorotrifluoroethylene 100,000 332 371 57 Polypropylene -- 336-366 58 Polyethylene 20,000 360-392 63 Poly-e-fl-fl-trifluorostyrene 300,000 333-382 64 Polymethylene High 345-396 72 Poly-p-xylyene -- 401-411 73
  9. 9. Urban and wildland fire phenomenology 325 bustible, and the char that remains can support only a surface oxidation, glowing combustion. Estimates of rate constants, according to eq. (5), are B2 = 10 ~2 s- and Eb2 = 40 kcal/mole for k2, the char process, and B 1 = 1 0 1 7 s - 1 and Ebl = 53kcal/mole for kl, the tar process.* A reasonable mechanism has been suggested for the tar-production path. 2° The yield of levoglucosan is so high that probably some sort of an unzipping process is indicated. It has been proposed that the chain may rc quot;r be initiated either by random scission or by end- initiation, through attacks by a hydroxyl group, OH, FIG. 1. Illustration of competing rates of pyrolysis. one of which is attached to the C atom at the end of each chain. After the monomer breaks off, propagation gaseous fuels and therefore may escape from the solid could be sustained by the free oxygen bond. It is the and support flaming combustion. By contrast, in the reason for the monomer appearing as levoglucosan lower (dehydration) path H 2 0 is noncombustible which requires explanation. A proposed model for while C is a solid. The lower path therefore does not this process is a two-step attack, 2° viz., liberate gaseous combustibles but instead forms C which experiences surface burning, a type of glowing combustion process of the solid fuel. While tobacco H2COH burns by a process analogous to the lower path, [ 0 matches burn by processes corresponding to both /t C paths, the flaming resulting from a process like the /H /~Cellulose upper path. H C C H/I H With the two competing processes illustrated, the _O~/ O1-1 rate of conversion of the fuel is C (6) C dM/dt = - (k 1 + k2)M, J I in which kl and k 2 are given by separate expression of H OH the type shown in eq. (5). It may be seen that if the activation energies differ, Eb~ ~ Eb2, then different H2COH reactions may predominate at different temperatures. f This is illustrated in Fig. 1. At sufficiently low T, both rates are negligibly small. Typically k doubles when T increases by an amount on the order of only 10°C. At slightly elevated temperatures, k 2 may be appreciable while kl is negligible. Above T~, k 2 soon becomes H C H~-'~ 0 ~ C d Cellulose yf negligibly small compared with k 1. For cellulosics, k2 corresponds to dehydration and k I to production of secondary fuels capable of burning in the gas phase. C C I I 4.4. Pyrolysis of Cellulosics H OH Pyrolysis mechanisms of cellulose have been sub- jected to detailed investigation. Numerous techniques have been employed, and a multitude of facts have H2 C -O o been established. Although the current situation is I complex, a few unifying principles have been de- C vel°ped'2°'21 In particular' there appear t° be tw° /H I principal competing paths, which may be represented H ,/ as , C C ~ + Cellulose H H quot;dehydro- HO / ~OH t200- L-,~ll, lnse '' +HzO----~har + H 2 + C O 2 +... (exothermic) C 54~¢~oc'~t. / ~'~i~,'~i, C . . . . , ~2/ en~to.i~ermic) I I cellulose- OH (280- k ~ (endothermic) H 340°C) quot; t a r ' (primarily --levoglucosan) * These values are approximations to those of A. Broido, reported in quot;Kinetics of Solid-Phase Cellulose Pyrolysisquot;, The quot;'tarquot; is volatile and vaporizes to form a major (see Thermal Uses and Properties of Carbohydrates and Lignins gaseous fuel to support a gas-phase flame. The gases (F. Shafizadeh, K. V. Sarkanen and D. A. Tillman, eds.), Academic Press, New York, 1976). evolved in the dehydration path are mainly noncom-
  10. 10. 326 F.A. WILLIAMS first by the oxygen radical and next by the hydroxyl. The final molecule shown is levoglucosan (fl-glucosan or 1,6 anhydroglucose). The first step is endothermic and the second exothermic, releasing less heat than is required for the first step. For the dehydration process, it has been reasoned 2° that an out-of-plane, interrnolecular interaction must be the cause. The hydroxyl in an H2COH group of one chain can attack the carbon-oxygen linkage of an adjacent chain, breaking that chain in such a way that half of it is linked to the attacking chain while the other half gives up H 2 0 in forming a stable end- group. Hypotheses for the mechanism of the further decomposition toward char through production of HzO and CO have also been developed. 2° Thus, the dominant features of the pyrolysis of pure cellu- lose can be understood self-consistently. --- WICK Although cellulose is the major constituent of cellu- losics such as natural woods, there are other impor- WAX tant constituents, notably hemicellulose and, typically in somewhat lower concentration, lignin. 22 These materials have less regular structures than cellulose and show more complex behavior upon pyrolysis. Even cellulose has a macrostructure, exhibiting amor- phous regions and more regular crystalline segments. This macrostructure may affect pyrolysis behavior. Small amounts of inorganic constituents also have FIG. 2. Schematic illustration of burning candle. measurable influences on pyrolysis. Therefore the overall kinetics of thermal degradation of natural reactant molecules. For example, for A + B ~ p r o - cellulosics vary. Nevertheless, the pyrolysis properties ducts, the rate co (moles of A consumed/vol, s) is co = of cellulose always exert an influence on the rates of kCACB, where the rate constant k may be given by an breakdown of cellulosics subjected to heat, and cellu- expression like eq. (5). Table 8, taken largely from Ref. lose provides the best model currently available for 18, lists approximate rate constants for a few elemen- these natural substances with respect to their pyrolysis tary steps. kinetics. The species CH 3 and H are radicals that serve as chain carriers. The first two reactions in Table 8 are representative initiation steps, with M denoting any 5. CHEMICALKINETICSOF COMBUSTION stable molecule. In established flames these steps may The mechanisms of gas-phase reactions occurring be relatively unimportant since radicals H, O and OH in fires may be discussed by reference to the burning may reach the fuel molecules by diffusion and consume of a candle, illustrated in Fig. 2. The hydrocarbon fuel them more rapidly by propagation steps such as 3, 4 (wax) vaporizes from the wick under the influence of and 5. It is known that formaldehyde, H2CO , plays a the heat from the flame. The dark region is fuel rich role in hydrocarbon oxidation, and step 6 is a potential with insufficient oxygen for appreciable oxidation. means for producing it. Steps 7 and 8 describe a path The blue is characteristic of the burning zone where for production of CO through the formyl radical gaseous fuel meets oxygen; the blue colour is chemi- (HCO). Oxidation of CO to CO 2 occurs by step 9, luminescent, not thermal or equilibrium radiation but which may proceed more slowly than other steps, rather nonequilibrium radiation from species that leaving unburnt CO if reactions are quenched by have achieved excited states through the chemical rapid cooling. Steps 10 through 13 are part of the reactions of combustion. The yellow is mostly equi- chain mechanism for hydrogen oxidation and are librium radiation from fine, hot soot particles that quite relevant to hydrocarbon oxidation. The last may be burning with oxygen; the soot has been reaction listed is a representative termination step, formed by pyrolysis of fuel gases. Chemical processes involving three-body collisions and having a rate pro- that occur in the blue flame have been subjected to portional to the product of the concentrations of the detailed investigation. three reactants. 5.1. Mechanisms and Rates in Methane Flames 5.2. Simplified Rate Expressions Combustion reactions fundamentally are chain re- Many steps not shown in Table 8 are known to actions involving many elementary steps. Each step occur in methane oxidation. Gas-phase oxidations of proceeds at a rate proportional to the product of other fuels involve many additional steps as well. the concentrations c (moles/vol.) of the colliding Knowledge of rates of elementary steps and computer
  11. 11. Urban and wildland fire phenomenology 327 TABLE 8. A few rate constants for reaction steps Reaction k-Rate constant* 1.5 x 1019exp ( - 100,600/RT) 1. CH4+M~CHa+H+M 1.0 x 1014exp ( - 45,400/RT) 2. CH4+O2--*CH3 + HO2 3. CH4+O~CH3 +OH 1.7 × l0 la exp ( - 8,760/RT) 6.3 x 10la exp ( - 12,700/RT) 4. CH4+H~CH3+H 2 2.8 x 1013exp ( - 5,000/RT) 5. CH4+OH~CHa +H20 1.3 x 1014exp ( - 2,000/RT) 6. CH3 + O ~ H 2 C O + H 2.3 x 1013exp(- 1,570/RT) 7. H2CO + O H ~ H C O + H 2 0 1.0 x 1014 8. HCO+OH~CO +H20 3.1 x 1011exp ( - 600/RT) 9. CO+OH~CO2+H 2.2 × 1014exp ( - 16,600/RT) 10. H+O2-,OH+O 4.0 × 1014exp ( - 9,460/RT) It. O+H2~OH+H 8.4 × 10X'~exp( - 18,240/RT) 12. O+H20~2OH 1.0 x 10X4exp( - 20,400/RT) 13. H + H 2 0 ~ H 2 +OH 14. H+OH+M~H20+M 2.0x 10~ T -l** * Units are cm3/mole s. ** Units are cm6/mole2s for k and K for T. Complete chemical equilibrium would involve equi- capacities are becoming sufficient to enable compu- librium for every step, a condition seldom achieved. tations of histories of chemical conversions to be However, equilibrium often is a good approximation made with full chemistry for most fuels. However, for for certain steps involving major species such as H 2 0 , many purposes it is helpful to have simplified expres- sions for overall rates of heat release involving a small CO2 and CO. Equating forward and backward rates n u m b e r of lumped steps that are not elementary, e.g. results in a relationship between concentrations and expressions corresponding to two overall steps, first temperature for equilibrium (see Ref. 18, for example) that involves an equilibrium constant, K c = kl/kb, combustion of fuel to CO and H 2 0 then oxidation of CO to CO,. Overall rate parameters for such simpli- where k s and k b are the previously defined rate constants for the forward and backward elementary fied descriptions are becoming available (e.g. Ref. 23). steps. Combining such equilibrium equations with For many purposes, a one-step approximation to equations for element conservation (stating that the complex chemistry is sufficient. The molar rate of chemical elements are neither created nor destroyed consumption of fuel F by oxidizer O is represented, for in chemical reactions) and for energy conservation example, as results in expressions for temperature and for concen- dcF/dt = - w = - c F c o B e x p [ - E / ( R T ) ] , (7) trations of major species as functions of a local mixture ratio (total local concentration of an element in which the overall activation energy E and the contained in the fuel, divided by total local concen- overall prefactor B are constants. Over a sufficiently tration of the element oxygen) in diffusion flames. limited range of conditions, a representation of the These expressions often are obeyed, in a rough ap- type shown in eq. (7) often is acceptable. proximation, in fires. 5.3. Chemical Equilibrium 5.4. An Example of Diffusion-Flame Structure There are situations in fires under which chemical These ideas of chemical equilibrium help to ex- rates for combustion need not be considered at all plain some major observed characteristics of diffusion because, in a first approximation, chemical equi- flames. The shape of the blue flame in Fig. 2 causes it librium is attained locally at each point in the gas. to be difficult to probe. Measurements are easier to These situations may occur only in nonpremixed perform in flat diffusion flames, which may be estab- systems (systems in which the fuel and air are not lished with the apparatus illustrated in Fig. 3. 24 A mixed prior to burning), often termed diffusion flames liquid fuel is contained in a pool (shaded), and an since burning then involves diffusion of fuel and oxidizing gas stream is directed downward onto the oxidizer toward each other. They cannot occur every- surface of the liquid. When the fuel is ignited, con- where in premixed systems (systems in which fuel and ditions can be adjusted so that a flat flame remains oxidizer are mixed at a molecular level) because the stationary a few millimeters above the surface of the equilibrium state involves negligible concentrations of fuel, as illustrated. Quantities vary only in the vertical either fuel or oxidizer. The system illustrated in Fig. 2 direction, and the flame structure may be studied by is nonpremixed and therefore subject to approxima- thermocouples and by gas sampling. The liquid fuel tion by chemical equilibrium; in fact, most fires involve may be replaced by a gaseous fuel jet or by a solid fuel. diffusion flames. Representative results for the flame structure in At chemical equilibrium for a reaction step, the such an apparatus are shown in Fig. 4, for the solid forward rate equals the rate of the backward reaction fuel poly(methyl methacrylate). The gas stream had (defined by reversing the arrow, e.g. in Table 8).
  12. 12. 328 F.A. WILLIAMS AND BAFFLE l AIRDUCT I I S N D~x~ I~l A W VL E HE,G.T rm SCREENS CONTROL 1 72~S--S------~- ............a FUEL O-RINGSEAL ~ ~ j 'r'~ ~/jOVERFLOWDUCT ~ f~21/[..SJ~ WATER SPRAY~ quot;i quot;/////quot;// SUCTION SUCT'OI~I EXHAUST l~J ~____~7~, ~,~L,I EXHAUST RIG CONTROL~j~ !!~ I1~~ FuWALTERAI~ ' / / ' / N, N ~ F L TE I W A LINN R WATEROUT i . ~ POOLDEPTH CONTROL lOmm SCALE , , FIG. 3. Schematicdiagram of diffusion-flameapparatus. gently diffuses toward the fuel surface from the oxid- values of the exit velocity U and of the ratio of oxygen izing stream. mass to total mass in the oxygen-nitrogen stream, This behavior of the main constituents is roughly Yo2, listed in the figure. There is a two-phase, gas- consistent with the ideas of chemical equilibrium. The liquid layer on the order of I mm thick at the surface mixture ratio, measured on the basis of the ratio of of the polymer under these burning conditions; the location of the outer edge of this layer is indicated in carbon to oxygen or of hydrogen to oxygen, decreases as the distance from the polymer surface increases. If the figure, as is the location of the center of the equilibrium calculations are made of temperatures luminous blue zone, whose thickness is less than 1 mm. and of concentrations of 02, N2, C O 2 and H20 at The monomer, methyl methacrylate, has the chemi- each point on the basis of the local mixture ratio, then at least qualitative agreement with measurements is cal formula CsH802 and is the major species liberated obtained. There are quantitative discrepancies; for in polymer pyrolysis (see Table 6). It is seen from Fig. example the flame temperature is nearly 500K below 4 that this is the major fuel present at the outer edge of the dispersed layer. This material diffuses into the the theoretical flame temperature. The magnitudes of these discrepancies are indicative of the extent to blue zone from below, while oxygen diffuses into the blue zone from above. The heat release is greatest in which departures from equilibrium occur. the center of the blue zone, where these two species As an extreme idealization, it may be considered meet, as may be seen by the occurrence of the peak in that there is essentially no 02 on the fuel side of a the temperature profile at the center of this zone. The sheet of negligible thickness located at zero, the center concentrations of the major products CO 2 and H20 of the blue zone, and that there are essentially no fuel species (CsHsO2, CO, HE, etc.) present on the oxygen also peak near the center of the blue zone, and these diffuse away on each side of this zone. Nitrogen, side of this sheet. This quot;flame-sheetquot; approximation is which does not participate in the reaction, exhibits no useful conceptually as well as for approximate burning- distinctive behavior at the blue zone but instead rate calculations, even though the information in Fig.
  13. 13. Urban and wildland fire phenomenology 329 POLY ( METHYL METHACRYLATE) N 2 in 0 2 Yo2 =0.178 U = O.315m/s 300 9O 18 9 I,-.- Z w ~ 500 8O 8 16 I--- a. Z hi ,n,quot; ._1 o W 70 o_ ~00 14 7 ~ W quot;rquot; J O m re N T 60 z 6 ~ ~.00 12 I-- & z Z w I ,,y (.) rr ,i w w O_ 0_ 50 )00 IO w t.d J I ._1 O 0 ~E N T O re 8 1- 4 ~ ;00 d O T -rquot; re U O 3 r, O0 6 ~ -r t~ U T 2 a O0 4 u T U O0 d T quot;r I 0 I 2 DISTANCE FROM LUMINOUS FLAME ZONE (mm) FIG. 4. Representative concentration and temperature profiles in a diffusion flame. The pyrolysis of gaseous fuel proceeds in the dark 4 shows clearly that it is not very accurate in detail. fuel-rich zone between the fuel surface and the blue The flame-sheet approximation is a limiting form of zone. Occurrence of the gaseous fuel species observed, the equilibrium approximation. rather than other fuel species, can be understood The many species shown in lesser concentrations in from concepts of kinetic mechanisms of pyrolysis of Fig. 4, primarily on the fuel side, are not at all con- C 5 H 8 0 2 .24 It is seen that many of the fuel species sistent with chemical equilibrium. In addition to the produced in dark-zone pyrolysis have higher ratios of product CO of partial oxidation, these species include carbon to hydrogen than the parent fuel. the gaseous fuels hydrogen, methane, ethane, propane, ethylene (C2H4) , acetylene (C2H2) , propylene (C3H6) , allene (CH 2 = C = CH2), propyne (CH3C = CH) and 5.5. Kinetics of Gaseous Fuel Pyrolysis formaldehyde (HCHO). These latter species must be Numerous chemical reactions occur in the dark produced by finite-rate chemical processes. They are pyrolysis zone containing gaseous fuel. These reactions in no way representative of the species expected from are complex and differ for different fuels; they are not combustion kinetics, such as those discussed in Sec- understood thoroughly. 25 If allowed to proceed for a tion 5.1quot;, and they extend well beyond the blue sufficient length of time, they result in production of reaction zone. Instead, they are formed by pyrolysis of soot. In the experiment of Fig. 4 there is insufficient the secondary (gaseous) fuel C 5 H802. residence time in the fuel-rich zone for this to occur. However, in Fig. 2 there is sufficient time, and the soot becomes visible as the yellow zone of the flame. The * More sophisticated experimental techniques are needed soot also burns and finally is consumed completely at to measure most of the nonequilibrium species of the com- the upper boundary of the yellow region. bustion kinetics.

×