EPA-450/3-73-009
October 1973
DEVELOPMENT
OF EMISSION FACTORS
FOR ESTIMATING
ATMOSPHERIC EMISSIONS
FROM FOREST FIRES
5SSZ
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Water Programs
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
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EPA-450/3-73-009
DEVELOPMENT
OF EMISSION FACTORS
FOR ESTIMATING
ATMOSPHERIC EMISSIONS
FROM FOREST FIRES
by
George Yamate
IIT Research Institute
10 West 35th Street
Chicago, Illinois 60616
Contract Number 68-02-0641
EPA Project Officer: William M . Vatavuk
Prepared for
ENVIRONMENTAL PROTECTION AGENCY
Office of Air and Water Programs
Office of Air Quality Planning and Standards
Research Triangle Park, N. C. 27711
October 1973
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This report is issued by the Environmental Protection Agency to report
technical data of interest to a limited number of readers. Copies are available
free of charge to Federal employees, current contractors and grantees,
and nonprofit organizations - as supplies permit - from the Air Pollution
Technical Information Center, Environmental Protection Agency, Research
Triangle Park, North Carolina 27711, or from the National Technical Informa-
tion Service, 5285 Port Royal Road, Springfield, Virginia 22151.
This report was furnished to the Environmental Protection Agency by
IIT Research Institute, Chicago, Illinois in fulfillment of Contract No.
68-02-0641. The contents of this report are reproduced herein as received
from IIT Research Institute. The opinions, findings, and conclusions
expressed are those of the author and not necessarily those of the Environmen-
tal Protection Agency. Mention of company or product names is not to
be considered as an endorsement by the Environmental Protection Agency.
Publication No. EPA 450/3-73-009
11
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FOREWORD
This report has been prepared for the Environmental
Protection Agency, under Contract No. 68-02-0641, to describe
work performed on IITRI Project No. C6265, "Development of
Emission Factors for Estimating Atmospheric Emissions from
Forest Fires", during the period 29 June 1972 to 27 July 1973.
Significant amounts of pollutants, both natural and man-
made, are released into the atmosphere. In order to obtain
a detailed knowledge of all known sources, the Office of Air
Quality Planning and Standards, Monitoring and Data Analysis
Division, is responsible for developing and reporting emis-
sion factors that are used with production or consumption
data or other "activity level" indicators to make estimates
of the amounts of pollutants which are released to the atmos-
phere. The specific objective of this study is to develop
emission factors for estimating emissions from various types
of forest fires.
At the present time, much of the available information
concerns industrial (man-made) pollutants. However, in cer-
tain urban areas in near proximity to forest areas, an addi-
tive effect of increased concentration of pollutants or a
reactive effect due to the mixture of urban pollution with
combustion products of a forest fire may result. Examples
of this were seen in the Florida Everglades fires and the
tourist cities of the East Coast of Florida, or the brush
fires of southern California on the periphery of the Los
Angeles basin. Other less newsworthy occurrences are fre-
quent in residential towns located downwind of forest fires.
Many of the references on the total mass of emissions
from wildfires differ by several orders of magnitude, and
deficiencies can be found in their values. To correct these
deficiencies and to increase the overall air pollution
knowledge regarding the forest fire emissions, a compilation
m
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and documentation of the emission factors of various pollu-
tants as it relates to various types of forests undergoing
combustion is necessary.
No intentional attempt is made to justify, qualify, or
negate the use of "prescribed" forest fires by emphasis of
the emissions produced by forest fires. The attempt is made,
however, to develop the most realistic emission values based
on the present state-of-the-art. No bias "for-or-against"
prescribed burning is stated or implied.
The cooperation and help received from personnel in the
U.S. Forest Service, universities, research stations, and
other friends are gratefully acknowledged. A special thanks
goes to Dr. Robert W. Cooper and the staff of the Southern
Forest Fire Laboratory in Macon, Georgia; to Dr. Ellis F.
Darley and the staff of the Statewide Air Pollution Research
Center in Riverside, California; and to Dr. Edwin V. Komarek,
Sr., and the staff of Tall Timbers Research Station in
Tallahassee, Florida, for conducting demonstration burns,
emission testing of burning forest fuels, and informative
discussions on emissions from forest fires.
We gratefully acknowledge the guidance and many helpful
suggestions offered by William Vatavuk, Project Officer, and
James Southerland, Chief, Emissions Section, National Air
Data Branch, of the EPA.
Personnel working on this program were: Dave Becker and
Patricia Llewellen, Information Sciences; Arthur Takata and
Thomas Waterman, Fire Research; and John Stockham and George
Yamate, Fine Particles Research. Conceptual mathematical
models were initiated by D. Becker and A. Takata as a first
step in predicting output of emissions from fuel and fire
behavior models.
Respectfully submitted,
Approved by IIT RESEARCH INSTITUTE
'
.
D. Stockham George Yamate
Manager Associate Chemist
Fine Particles Research Fine Particles Research
IV
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DEVELOPMENT OF EMISSION FACTORS FOR ESTIMATING
ATMOSPHERIC EMISSIONS FROM FOREST FIRES
ABSTRACT
The objective of this project was to develop emission
factors (weight of pollutant per acre burned) for estimating
emissions from forest fires, especially wildfires. The pol-
lutants of interest were particulates (filterable and total),
hydrocarbons (reactive or unreactive), carbon monoxide, nit-
rogen oxides, and sulfur oxides. The effects of terrain,
'density of vegetation coverage, type of vegetation, wind
speed, and humidity were to—be expressed as adjustment fac-
tors to the average emission factors. Also, emission fac-
tors were to be developed for the five geographical forest
areas of the country. Information from the literature and
unpublished test work were to be xjsed to develop the emission
factors.
The information sought for each forest area were:
1. The number of acres burned by wildfires.
2. The quantity of fuel available, or preferably,
the fuel consumed by the average wildfire.
3. Measurements of emissions from burning forest
fuels.
4. The effect of fire behavior variables on emissions.
Acres consumed by wildfires for each geographical forest
area were obtained from a compilation of wildfire statistics
prepared by the Division of Cooperative Forest Fire Control
of the USDA Forest Service. No comparable set of statistics
was available for acreage burned by "managed" fires.
Fuel data were also obtained from Forest Service re-
ports and contacts with regional Forest Service laboratories
and personnel. The Forest Service periodically undertakes
a forest survey of commercial timber and studies forest fuels
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for fire prediction, prevention, and control. They now con-
sider fuel assessment to be of major importance. Past and
present fire reports describe fuels by forest tree type.
However, the National Fire Danger Rating System (NFDR) is
now operational and each region measures standing timber and
fuels for use in fire models. These models recognize the
importance of the size of forest fuels rather than type.
They emphasize the fine forest fuels with a tendency to burn,
rather than the amount potentially able to burn.
The emissions produced by the burning of forest fuels
were obtained through an intensive information search and
site visits to several fire laboratories. No emission data
were available from wildfires. Experimental measurements of
the emissions were tabulated for comparison and found to be
consistent, with the exception of the values for carbon
monoxide. Carbon monoxide is an anomaly, since burning be-
havior (temperature, duration, and oxygen supply) and the
point of sampling radically alter the results. During in-
formal discussions at the 13th Tall Timbers Fire Ecology
Conference, references were made to the close agreement among
emission measurements made at the Southern Forest Fire
Laboratory, the bushfire smoke studies by an Australian
study team, and the results reported by Dr. E. F. Darley at
the University of California, Riverside. Emissions per ton
of forest fuel appear to be independent of tree type and
other variables, from a practical viewpoint. Using our best
engineering judgement, the following emission values or
pollutant yields were selected as representative of average
forest fuels:
Pollutant Yield,
Pollutant Ibs/ton of fuel
Total Particulate 17
Carbon Monoxide 140
Total Hydrocarbon (as CH^) 24
Nitrogen Oxides (as NOX) 4
Sulfur Oxides (as S02) Neg.
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The emission factors and the total emissions produced
from a forest fire for each geographic region in the United
States were calculated from the formulas:
1. Emission Factor (Ib/acre) = Pollutant Yield
(Ibs/ton) x Fuel Consumed (tons/acre)
2. Total Emissions (tons) = Emission Factor (Ibs/acre) x
Number of Acres Burned (Acres) x Constant
(1 ton/2000 Ibs)
No data were available that distinguished between filter-
able and total particulates. Evidence, based on one sample
collected from burning wood chips and shavings, is that the
geometric mass mean radius of the particulates is about
0.035 M-m. Because of difficulties in interpreting analytical
and reporting procedures, an estimate of reactive and non-
reactive hydrocarbons was impossible.
Fire behavior variables have not been correlated with
emissions. The consumable fuel and moisture content appear
to have significant effects, while terrain and wind speed
have minimal effects. "Green" vegetation produces about
three times the quantities of particulates as "dead", dry
materials. However, the emission factors tabulated repre-
sent moisture levels common to fuels consumed by wildfires.
Our understanding of how fire behavior variables affect emis-
sions is insufficient to develop correction factors at this
time. Their use would only serve to complicate the emission
estimates without improving the reliability of the data.
The following table is a complete summary of the wild
fire emission data by geographical subdivision. The table
shows acres consumed by wildfires in 1971; wildfire fuel
consumption; emission factors for particulates, carbon mon-
oxide, hydrocarbons, and nitrogen oxides; and emissions for
1971.
VII
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COMPLETE SUMMARY OF EMISSIONS AND EMISSION FACTORS
Geoeraohic Area
1) Rocky Mountain
Group
Northern,
Region 1
Rocky Mountain,
Region 2
Southves tern,
Region 3
Intermountain,
Region 4
2) Pacific Group
California,
Region 5
Alaska,
Region 10
Pacific N.W.,
Region 6
3) Southern Group
Southern,
Region 8
(Group and
Region are the
same)
4) North Central
Group
Eastern,
Region 9
(Both Groups
are in
Region 9)
5) Eastern Group
Region 9)
6) Total United
States
.** Acreage
Consumed
by
Forest Vegetation of the U.S. Wildfire
(Appendix B) (acres)
Western Larch-Western White Pine; 774,405
Ponderosa Pine-Douglas Fir;
Lodgepole Pine; Pinyon-Juniper
Ponderosa Pine-Douglas Fir;
Pacific Douglas Fire; Redwood;
Ponderosa Pine-Sugar Pine;
Pinyon- Juniper-Chaparral
Oak-Hickory; Oak-Pine;
Longleaf-Loblolly-Slash Pine;
Cypres s-Tupe lo-Sweetgum;
Chestnut-Chestnut Oak-
Yellow Poplar; Mangrove
Spruce-Fir; Jack, Red, and
White Pine; Birch-Beech-Maple-
Hemlock; Oak-Hickory; Chestnut-
Chestnut Oak-Yellow Poplar
Chestnut-Chestnut Oak-Yellow
Poplar; Birch-Beech-Maple-
Hemlock; Spruce-Fir
351,563
162,795
206,983
Wildfire
Fuel |l
Consumption
(tons /acre)
37
60
30
10
53,064 8
1,161,1381 19
46,941
1,046,542
67,655
1,992,339
1,992,339
232,749
349,000
116,251
4,276,882
18
16
60
9
9
11
11
11
17
Pollutants
Emission Factors
Particulate
17 #/Ton*
(#/acre)
629
1,020
510
170
136
323
306
272
1,020
153
153
187
187
187
289
CO i
140 #/Ton*
(tf/acre)
5,180
H-C ***T NOX
24#/Ton*|4 #/Ton*
(#/acre) (#/acre)
888
8,400 , 1,440
4,200 720
1,400 i 240
1
1,120 1 192
2,660 456
2,520
2,240
432
384
8,400 I 1,440
1,260
1,260
1,540
1,540
1,540
2,380
216
216
264
264
264
408
148
240
120
40
32
76
72
64
240
36
36
44
44
44
68
** Emissions
Particulate
(tons)
243,550
179,297
41,513
17,593
3,608
187,524
7,182
142,330
34,504
152,414
152,414
21,762
32,632
10,870
618,009
CO
(tons)
2,005,709
1,476,560
341,870
144,887
29,716
1,544,314
59,144
1,172,127
284,147
1,255,179
1,255,179
179,217
268,730
89,513
5,089,490
H-C ***
(tons)
343,836
253,125
58,606
24,843
5,094
264,739
10,139
200,936
48,712
215,173
215,173
30,723
46,068
15,345
872,484
KOX
(tons)
57,306
42,187
9,768
4,140
849
44,123
1,690
33,489
8,118
35,862
35,862
5,120
7,678
2,558
145,414
* Pollutant Yield, Ib pollutant/ton fuel consumed
** Acreage Consumed by Wildfire and Emissions are for 1971.
*** Hydrocarbon as methane
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TABLE OF CONTENTS
Page No,
GLOSSARY xii
1. INTRODUCTION ..... . 1
1.1 Background 1
1.2 Overview Information .,.„...... 3
2. INFORMATION SEARCH 4
2.1 Literature Survey «,.„.' 4
2.2 Site Visits : 7
2.3 Letters and Telephone Calls of Inquiry . 8
2.4 Contributors ....... 9
3. ACREAGE BURNED 10
3.1 Wildfire ..... 10
3.2 Managed Fires 10
4. FUELS, FUEL TYPES, AND ESTIMATED QUANTITIES . 14
4.1 General Forest Fire Background 15
4.2 Overview of Fuels and Fuel Types .... 16
4.3 Fuel Models , . . « 17
4.4 Quantity of Fuel Consumed-Burned .... 20
4.5 Regional Values of Fuel Loadings .... 21
4.5.1 Region 1, Northern Region , .. . . 21
4.5.2 Region, Rocky Mountain Region . . 22
4.5.3 Region 3, Southwestern Region . 23
4,5.4 Region 4, Intermountain Region . . 26
4.5.5 Region 5, California Region ... 27
4.5.6 Region 6, Pacific Northwest Region 31
4.5.7 Region 8, Southern Region .... 33
4.5.8 Region 9, Eastern Region ..... 35
4.5.9 Region 10, Alaska Region , . . . . 37
4.5.10 Summary of Regional Fuel Loadings. 38
5. EMISSIONS AND EMISSION FACTORS 40
5.1 Wood Chemistry „ 41
5.101 Chemical Composition 41
5c2 Pyrolysis and Combustion . . 42
5.2.1 Pyrolysis , 43
5c2.2 Combustion . . . 46
5.3 Measured Emissions . 47
5.4 Summary of Emissions . 50
1x .
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TABLE OF CONTENTS (continued)
Page No,
6. ADJUSTMENT OR CORRECTION FACTORS 54
6.1 Fuel and Fuel Particle Characteristics . 55
7. RELATED STUDIES , 57
7.1 Proposed Design of An Incinerator or
Burn Chamber ....... 57
7.2 Remote Sensing 58
7.3 Parametric Study of the Combustion
Process ...... 59
8. RESULTS AND CONCLUSIONS 60
9. RECOMMENDATIONS FOR FUTURE WORK 62
9.1 Office of Air Quality Planning and
Standards R&D 62
9.1.1 Smaller Inventory Units 62
9.1.2 System of Map Overlays , 62
9.1.3 Inventory Needs and Fire Report
Forms . 63
9.1.4 Field Study . 63
9.2 General Recommendations 63
9.2,1 Wood Smoke Composition 63
9.2.2 Modeling Studies of Emissions from
Fires 64
REFERENCES . 65
Appendix A - LIST OF CONTRIBUTORS 74
Appendix B - MATHEMATICAL DESCRIPTION OF FOREST
FIRE EMISSIONS 78
Appendix C - CONCEPTUAL MODEL OF PYROLYSIS-COMBUSTION
OF FOREST FUELS 108
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LIST OF TABLES
Page No,
1. Summary of Wildfire Statistics for the United
States -- Calendar Year 1971 for Protected
Areas, by Groups of States 11
2. Compilation of Data Related to Size Classes
of Fire 12
3. Values for Input Parameters of 11 Preliminary
Fuel Models for the National Fire Danger Rating
System 19
4. Composition of Light Chaparral Fuel Type ... 29
5. Composition of Medium Chaparral Fuel Type . . 30
6. Composition of Heavy Chaparral Fuel 30
7. Fuel Models Used in Region 6 32
8. List of Fuel Types - Fuel Models 34
9. Fuel Models (NFDR) in the Eastern Region ... 36
10. Summary of Estimated Fuel Consumed by Forest
Fire 39
11. Published Emissions/Emission Factors (#,
#/Acre, #/Ton) 48
12. Complete Summary of Emissions and Emission
Factors 61
xi
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GLOSSARY
For a more extensive and complete terminology, the Society
of American Foresters, 1010 Sixteenth Street, N.W., Washington,
D.C. 20036, has a publication, "Terminology of Forest Science,
Technology, Practice and Products", that is used as a standard
for forestry concepts.
ambient
blow-up
branches
brush
burn
agricultural
burn
controlled burn
hazard
reduction burn
managed burn
orchard pruning
burn
the surrounding, enveloping conditions.
As they pertain to weather at the earth's
surface, the conditions measured in the
instrument shelter are considered to
be ambient.
the very rapid escalation of a surface
fire (ground fire) into a crown fire.
1" to 3" diameter material.
scrub vegetation and stands of tree
species that do not produce merchant-
able timber. (Not a synonym for slash.)
the application of fire to fuel.
a managed burn of residues derived from
an agricultural operation.
a directed and restrained application of
fire to fuel.
a managed burn of litter and underbrush
in order to prevent fuel buildup in the
forest and reduce the hazards of a
wildfire.
an overall designation for a man-adminis-
tered combustion (burning) operation for
some utilitarian purpose. Many times
the term is used synonymously with con-
trolled burning or regulated burning.
a specific type of agricultural burn as
applied to the burning of prunings and
other wastes derived from good orchard
practices.
xn
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prescribed burn
range
rehabi1itation
Burn
slash burn
burning index (BI)
chaparral
clearcutting
duff
emission factor
energy release
component (ERG)
the generally recognized managed burn
that is often called "prescription burning".
This is a low to medium intensity surface
fire initiated to carry out a variety of
sound forestry concepts and to simulate
a natural fire control method.
the managed burning of undesirable non-
timber trees and shrubs that reduce the
value of a range.
a managed burn of residue remaining after
a logging or thinning operation.
a number related to the amount of effort
needed to contain a fire of a particular
fuel type within a rating area. A
doubling of the BI indicates that twice
the effort will be needed to contain a
fire of that fuel type as was previously
required.
a loose term denoting a variety of brush
and shrub species; principally scrub
oak, manzanita, sumac, cliffrose, ceanothus,
and chamise.
a controversial type of forest management
practice in timber harvesting whereby an
entire area is cut completely.
the partially decomposed organic material
of the forest floor beneath the litter
of freshly fallen twigs, needles, and
leaves. (The F and H layers of the forest
floor.)
the pounds of a pollutant emitted to the
atmosphere from burning an acre of forest
land.
a number related to the rate of heat re-
lease (BTU per second) per unit area
(square foot) within the flaming front
at the head of a moving fire. The ex-
pression differs from that of intensity
(see intensity) but is indicative of how
"hot" a fire is burning.
xi ii
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equilibrium
moisture content
TEMC'
firebrand
crpwn fire
head fire
mass fire
surface fire
fire intensity
flaming front
forb
forest floor layer
the moisture content that a fuel particle
would attain if exposed for an infinite
period in an environment of specified
constant temperature and humidity. When
a fuel particle has reached its EMC,
there is no net exchange of moisture
between it and its environment.
the burning of a combustible body.
a managed burn utilized in the removal of
fuel concentrations, usually in forest
fire fighting in which the fire is set
to travel on the ground and against the
wind.
any source of heat, natural or man-made,
which is capable of igniting wildland
fue1s.
fire starting on the surface and burning
vertically into the crown layer (tree
tops) of the forest. A crown fire re-
quires the support of surface fires.
a managed burn set to travel on the
ground and with the wind.
large area actively burning at the same
time.
fire burning mostly in the duff or litter
of fallen leaves, needles, and twigs
and the undergrowth of the forest floor -•
a ground fire.
energy release per unit time per unit
length of fire front. (Includes the
depth of flame behind the fire front.)
that zone of a moving fire within which
the combustion is primarily flaming.
Behind the flaming front combustion is
primarily glowing.
a nongrasslike herbaceous plant.
accumulation of dead organic plant matter
above the mineral soil. The layer is
composed of parts called the L, F, and
H layers.
xiv
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L layer
F layer
H layer
fuel
fue1 assessment
available fuel
fuel class
dead fuel
fine fuel
fine fuel
moisture
herbaceous fuels
this is the litter layer and is composed
of unaltered organic plant matter --
top layer.
this is the fermentation layer and is
composed of partly decomposed plant
matter -- middle layer.
this is the humus layer and is composed
of well decomposed plant matter -- above
the mineral soil.
a body of combustible material.
an experienced evaluation (inventory) of
the type and quantity of combustible
material (fuel loading) present in a
forest area for' a "possible" fire
situation.
the portion of the total fuel that
actually burn?; fuel which will be con-
sumed under a given set of or prevailing
weather conditions.
.a group of fuels possessing common
characteristics. In the NFDR System,
dead fuels are grouped according to
their timelag (1-, 10-, and 100-Hr.)
and living fuels by whether they are
herbaceous or woody.
naturally occurring fuel in which the
moisture content is governed almost
entirely by atmospheric moisture (rela-
tive humidity and precipitation).
the complex of living and dead herbaceous
plants and dead woody plant materials
less than one-fourth inch in diameter.
the probably moisture content of fast-
drying fuels which have a timelag
constant of 1 hour or less; such as
grass, leaves, ferns, tree moss, draped
pine needles, and small twigs.,
undecomposed material, living or dead,
derived fr"om herbaceous plants.
xv'
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living fuel
fuel loading
fuel model
fuel moisture
content (also
fuel moisture)
one--hour time lag
fuel moisture
(1-hr. TL~FMT
one-hour timelag
fuels
one-hundred hour
timelag fuel
moisture
(100-hr, TL FM)
one-hundred hour
time lag fuels'
fuel particles
naturally occurring fuel in which the
moisture content is physiologically con-
trolled within the living plant. The
NFDR System considers only herbaceous
plants and woody plant material which is
small enough (leaves and needles, and
twigs less than one-fourth inch in
diameter) to be consumed in the flaming
front of an initiating fire.
amount of fuel (combustible matter) pre-
sent in a defined area --in the context
of this report, it is synonymous with
"available fuel" per acre in that it is
the specified estimated fuel tonnage
which will be consumed in a wildfire.
a simulated fuel complex for which all
the fuel descriptors required for the
solution of the mathematical fire
spread model have been specified.
the quantity of water in a fuel particle
expressed as a percent of the ovendry
weight of the fuel particle.
the moisture content of the 1-hour
timelag fuels.
fuels consisting of dead herbaceous
plants and roundwopd less than about
one-fourth inch in diameter. Also in-
cluded is the uppermost layer of needles
or leaves on the forest floor.
the moisture content of the 100-hour
timelag fuels.
dead fuels consisting of roundwood in
the size range of 1 to 3 inches in
diameter and very roughly the layer of
litter extending from approximately
three-fourths inch to 4 inches below
the surface.
the wide variety of living and dead
plant parts that exist in the forest.
xvi
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potential fuel
ten-hour timelag
fuel moisture
(10-hr. TL~FM)
ten-hour timelag
fuels
total fuel
herb
humidity
hydrocarbon,
reactive
hydrocarbon,
non-reactive
ignition component
mi
initiating fire
insolation
combustible material of large dimension
that is considered fuel but would not
burn completely even in an intense
wildfire,
the moisture content of the 10-hour
timelag roundwood fuels-
dead fuels consisting of roundwood in
the size range of one-fourth to 1 inch
and very roughly the layer of litter
extending from just below the surface
to approximately three-fourths inch
below the surface.
combustible material that wpuld burn
under the most severe weather and
burning conditions.
a plant which does not develop woody,
persistent tissue but is relatively
soft or succulent and sprouts from the
base or develops from seed (annuals)
each year. Included are grasses, forbs,
and ferns.
a measure of water-vapor content of
the air.
gas phase hydrocarbon (organic compound)
reactive in the photochemical reaction
system, for example, olefins.
gas phase hydrocarbon (organic compound)
inert in the photochemical reaction
system, for example, methane.
a number related to the probability that
a spreading fire will result if a fire-
brand encounters fine fuel.
a wildfire which exhibits reasonably
predictable Behavior (no crowning or
spotting).
solar radiation received at the earth's
surface.
xvn
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land classifications
commercial
forest land
noncommercial
forest land
litter
National Fire
Danger Rating
System (NFDR
System)
natural abatement
oven dry fuel
weight
oven drying
technique
forest land that is producing, or is
capable of producing, crops of industrial
wood and is not withdrawn from timber
use by statute or administrative regula-
tion. Includes areas suitable for man-
agement to grow crops of industrial wood
and generally capable of producing in
excess of 20 cubic feet per acre of an-
nual growth. Includes both accessible
and prospectively accessible areas and
both operable and prospectively operable
areas. (USFS Forest Survey.)
unproductive forest land incapable of
yielding crops of industrial wood because
of adverse site conditions, and produc-
tive forest land withdrawn from commer-
cial timber use through statute or ad-
ministrative regulation, (USFS Forest
Survey,)
the top layer of the forest floor, com-
posed of loose debris including dead
sticks, branches, twigs and recently
fallen leaves or needles; little altered
in structure by decomposition (The L
layer of the forest floor.)
the U.S Forest Service system using
fuel models to standardize fuel descrip-
tions from which to develop fire behavior
components and indices to aid in planning
supervisory fire control activities on a
fire protection unit.
treatment of residue by natural decay
and deterioration -- will require a long
period of fire protection so that it
will not be a large fuel source.
weight of fuel sample obtained by the
oven drying technique.
method to determine the moisture content
of fuel by oven-drying at 100-105°C
until constant weight is obtained.
xvm
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pollutant yield
protection unit
relative humidity
TRH!
residence time
shrub
silviculture
slash
spread component
IscT
sticks
the emissions associated with the burning
of the available forest fuel. The yield,
reported in Ibs/ton, appears to be inde-
pendent of tree type and other variables
associated with forest fires. Often
this relationship is termed the emission
factor. However, for this study, the
emission factors relate the pollutants
emitted per acre burned.
a geographical area which is administra-
tively defined and which is the smallest
area for which organized fire suppression
activities are formally planned.
the ratio of the actual amount of water
vapor in the air to the amount necessary
to saturate the air at that temperature
expressed as a percentage.
(1) the time required for the flaming
zone of a fire to pass a stationary
point, (2) the width of the flaming
zone divided by the rate of spread of
the fire.
a woody perennial plant differing from a
perennial herb by its persistent and
woody stem, and from a tree by its low
stature and habit of branching from the
base.
the cultivation of forest trees; forestry;
arboriculture.
branches, bark, tops, chunks, cull logs,
uprooted stumps, and broken or uprooted
trees left on the ground after logging;
also debris resulting from thinnings,
wind, or fire.
the variation of terrain from the hori-
zontal; the number of feet rise or fall
per 100 feet measured horizontally,
expressed as a percentage.
a number related to the forward rate of
spread of the head of a fire.
a piece of wood generally long and
slender.
xix
-------
timelag (TL) the time necessary for a fuel particle
to lose approximately 63 percent of the
difference between its initial moisture
content and its equilibrium moisture
content.
volatiles readily vaporized organic materials
which, when mixed with oxygen, are
easily ignited.
wildfire any fire that burns uncontrolled in
vegetative or associated flammable
material.
average wildfire wildfire that does not generate special
emergency measures by the fire control
officer.
xx
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DEVELOPMENT OF EMISSION FACTORS FOR ESTIMATING
ATMOSPHERIC EMISSIONS FROM FOREST FIRES
1. INTRODUCTION
The object of this project is to develop emission factors
for use in estimating atmospheric emissions from forest
fires, especially "wild fires". To develop reliable emission
factors, it is necessary to measure the emissions and to
determine the variables associated with "wild fires" and
how they affect the quantities of emissions released to the
atmosphere. Some of these variables are: fuel concentration,
arrangement, and type; moisture content of the fuel and en-
vironment; wind speed; and terrain.
1.1 Background
The Office of Air Quality Planning and Standards, Moni-
toring and Data Analysis Division of the Enviromental
Protection Agency is responsible for developing and reporting
emission factors. The Environmental Protection Agency (EPA)
defines the emission factor as, "a statistical average of a
quantitative estimate of the rate at which a pollutant is
released to the atmosphere as a result of an activity such
as combustion or industrial production, divided by the level
of that activity". The emission faptor thus relates the
quantity of pollutants emitted to some indicator such as
production capacity, quantity of fuel burned, or, in the case
of forest fires, to the number of acres burned. In general,
emission factors are not precise indicators of emissions
from a single source, but are more valid when applied to a
large number of sources and processes and can be useful in
conducting source inventories in pollution studies. A col-
lection of emission factors has been published by the EPA (1).
In order to define problem areas in air pollution, an
emission inventory is maintained to determine sources and
components of air pollution. The emission inventory is a
-------
detailed and descriptive information catalog regarding emis-
sion sources, pollutant types, location, quantities, fre-
quency, duration, and relative impact of these emissions to
air quality.
The emission inventory is an important planning tool
available to an air pollution control agency. It can be
used to design an air sampling network; to predict ambient
air quality; to design, evaluate, or modify a control pro-
gram; and provide information on major sources of pollution.
Forest fires are a major natural source of air pollu-
tants. A "wild fire", or uncontrolled fire, is a large scale
natural combustion process of various ages, sizes, and types
of botanical specimens growing outdoors in a defined geographi-
cal area. Thus, the burning characteristics (intensity,
spread, etc.) are closely interrelated to the fuel quality
and history, meteorological conditions and history, and the
physical locality in terms of climatology and physical
topography. Each factor is dependent on the others; a
change in one results in a change in others, with subsequent
reaction.
The fire behavior thus will be the sum total of the
various factors and will be regulated to the extent deter-
mined by the influence of the dominant factor. For example,
a high moisture level, or the lack of fuel, or a physical
obstacle may dominate and result in a fire being stopped.
In an extremely intense fire such as a "blow-up" fire, all
factors may be conducive to combustion and thus result in
extreme fire intensity and the consumption of numerous acres
of forest.
The atmospheric emissions from this complex combustion
process are thus difficult to measure and "control" as com-
pared to man-made mobile and stationary sources of combus-
tion (2,3,4,5,6,7,8).
-------
From a naturalist's perspective, forest fires can be
considered natural phenomena. Dynamic and living, they re-
quire space, time, fuel (food), and the proper environment
to grow, sustain themselves, and finally die. The fuel
character (plant community) appears to be the least difficult
parameter for people to manipulate as an essential component
in the natural sequence of fire ignition, spread, and extin-
guishment. Nature itself spawns a forest fire, nurses it,
propagates it, and finally kills it.
Fuel, weather, and topographic factors are discussed
from a naturalistic viewpoint in several papers that treat
fire as a natural phenomenon (9,10,11,12).
1.2 Overview Information
There was a lack of published information on the quanti-
ties and types of pollutants generated from forest fires.
The information search revealed a lack of pertinent experi-
mental data on the emissions from wild fires. Data and
measurements reported in the literature have been obtained
on laboratory fires, burning towers, slash fires, and pre-
scribed burns. Other reported emission data have been
estimates derived from experience and knowledge of the com-
bustion of woody fuels and the estimated fuel tonnages in
the forest areas of the country.
The fundamental information needed for this project were;
a) acreage burned;
b) quantity of fuel (% consumed -- burned);
c) measured emissions per quantity of fuel; and
d) fire behavior factors affecting emissions.
-------
2. INFORMATION SEARCH
Prior to the initiation of the project, communication
links were established with sources that could provide infor-
mation pertinent to the program scope. A rapid literature
survey was made at the John Crerar Library, located on the
IIT Campus. Telephone calls were also made to colleagues
knowledgeable in the areas of forest fires, mass fires, and
atmospheric research to determine the sources and availa-
bility of relevant information.
Upon project initiation, an information retrieval
strategy was undertaken. Under an overall plan, the search
was divided into four interrelated categories to systematize
the intensive retrieval of related, useful information.
These categorie , //era. literature surveys, site visits,
letters and telephone calls of inquiry, and personal contacts.
2.1 Literature Survey
Although the program was to focus on "wild fires", data
on "prescribed burning", controlled outdoor fires, industrial
processing, and wood chemistry were also retrieved to broaden
the range of physical conditions represented and to describe
the processes at work. In addition, information relative to
forest fire behavior, forest fire meteorology, and forest
type vegetation in various geographic areas were sought.
The entire set of the Air Pollution Abstracts (and the
NAPCA Abstracts) were searched comprehensively. This search
yielded more than 100 relevant citations. Next, the Fire
Research Abstracts were searched, beginning with the 1961
issues and continuing through the present. This search
yielded about 70 additional citations. The abstracts of the
NASA Remote Sensing of Earth Resources Programs were searched
and yielded more than 20 additional citations. The Government
Research Abstracts yielded 8 citations.
-------
The Directory of Fire Research In the United States,
1967-1969, 5th Edition, was a valuable source of information
for subject matter, organizations, and personnel involved in
the various aspects of fire research (including forest fires).
Later, the earlier editions of the Directory as well as the
latest 6th edition (1969-71) were searched.
Environmental Science and Technology and the Air Pollution
Control Association journals were searched as well as the
preprints of papers presented at various American Chemical
Society -- Division of Water, Air, and Waste Chemistry
meetings.
The resources of The John Crerar Library, the IIT
Library, and the Chicago Public Libraries were also searched.
More than 300 forestry management and forestry research
volumes were identified, but less than 15 of these were
judged to be useful to the program.
The Chicago Office of the Government Printing Office
was visited. The current resources were examined and four
summary type publications were acquired. These identified
other personnel active in fields related to the project scope.
Abstracts, citations, journal articles, and microfiche
were obtained. These were screened and catalogued for use
in developing the emission factors.
Efforts were then concentrated on acquiring published
and unpublished material. These published materials were to
be found in symposias, proceedings, meetings, and conferences
relating to the subject area of interests. The unpublished
materials were those given at talks, data taken but not for-
mally published, data in the process of being published, or
data presently being taken.
The visit to the Forest Products Library and the lib-
raries of the University of Wisconsin Forestry and Agriculture
Departments in Madison, Wisconsin, was very productive. The
-------
Forest Products Library yielded about 10 new references^ in-
cluding pyrolysis data and an excellent report from the Rand
Corporation, entitled "The Simultaneous Flammability of
Wildland Fuels in the United States". This work contains
the construction of two "growth stages dependent" burning
indices, a calculation of the national distribution of wild-
land fuels, and an evaluation of them.
Over 433 primary abstract cards (references) were pre-
pared. These abstracts were descriptive of the best avail-
able literature pertinent to the program. Some of these
references were bibliographies and thus indicated a larger
number of source materials for information. Following an
ir.itial screening of the abstract cards and/or the documents
they represented, photocopies, microfiche, or original hard
copies were obtained of 154 of them. These materials were
secured from the John Crerar Library, IITRI resources, The
Forest Products Laboratory, The Government Printing Office,
The University of Illinois Library, and the EPA.
A special area of potential interest was the capability
of using remote sensing in forest fire emission studies.
Thus, references to organizations in the field of remote
sensing were obtained from work done on Contract NASW-2173,
Survey for Air Pollution Monitoring Instrumentation, IITRI
Project C6246,
the formal published literature, personnel known
to be active in the field were documented and categorized
into groups based on their degree of involvement in the study
of atmospheric emissions from forest fires. The author index
of the source documents and abstract cards that were prepared
were analyzed to uncover the organization and structure of
the research. A plot of the number of authors who had written
papers of interest, versus the number of papers selected for
each author, illustrated the diversity of the field as well
as the intensity of activity,, From this analysis and from
-------
consideration of the subject areas of the papers, a list of
primary contacts and secondary contacts were prepared. These
contacts were utilized in preparing site visits on the planned
survey trips.
The services of the Air Pollution Technical Information
Center at Research Triangle Park, North Carolina, was util-
ized in the early phase of the program to obtain pertinent
abstracts and later, near the end of the program, to obtain
material abstracted during the interim period. Information
was also requested from the Library of Congress, Science and
Technology Division, National Referral Center, Washington, B.C.
Trade industry groups were contacted for information and
leads to personnel working in the area of emissions from
forest fuels combustion.
2.2 Site Visits
From the pre-project information search, the locations
of personnel knowledgeable in the areas of forest fire be-
havior and emissions were determined. These people were
found to be either employed with schools of forestry or en-
vironmental science. Coincidentally, the analysis of author
and source documents reinforced the selection of people to
contact and visit. In addition, the close proximity of a
university tc a Forest Service facility was fortunate, in
that these site visits could be arranged with a minimum of
time delays.
The U.S. Forest Service maintains strategically located
offices, research units, regional forestry headquarters,
and three forest fire laboratories. These specialized fire
laboratories are: Northern Forest Fire Laboratory, Missoula,
Montana; Forest Fire Laboratory, Riverside, California; and
the Southern Forest Fire Laboratory, Macon, Georgia. Arrange-
ments were made to talk to staff members at each fire labora-
tory because each laboratory has a different fire-related
mission. Thus, three survey trips were taken to interview
-------
people in the Rocky Mountain area, Pacific area, and the
Southeastern area.
The site visits were very fruitful,. The discussions
with personnel active in the area of forest fires and air
quality resulted in information, literature, and suggestions
directly applicable to the objectives of the project. Demon-
stration burns, with concurrent emission testing, were also
witnessed at the Southern Forest Fire Laboratory and at the
University of California-Riverside.
2.3 Letters and Telephone Calls of Inquiry
As the information search progressed, telephone calls
were made for information or for clarification of information
The responses obtained were excellent. They primarily con-
sisted of suggestions for further research, information on
publications, and referrals to other persons knowledgeable
in the project area.
Various letters were written to obtain specific informa-
tion that would not have been available in the open, formal
publication services. This was especially important in ob-
taining information on fuel assessments from experienced
professionals.
Inquiries were made to firms with the capabilities of
remote sensing of pollutants. For example, Environmental
Measurements, Inc. (San Francisco, California) was very en-
thusiastic about the project and tried to get some data on
their Cospec II instrument. This firm is the U.S. licensee
of the Barringer Correlation Spectrometer that has been
utilized in the measurement of some of the airborne pollu-
tants (SOo and NC^) in the atmosphere. Unfortunately, the
availability of the instrument and the convenience of an
accessible forest fire could not be programmed for an experi-
mental test.
8
-------
2.4 Contributors
Personal contacts were very helpful in obtaining publi-
cations, references, and names of other people to contact.
An "inverted pyramidal effect" took place, in that all the
leads on emission studies from forest fuels eventually led to
personnel working in the laboratories in the Northwest,
California, and Southeast of the United States.
Since the U.S. Forest Service has recognized the assess-
ment of fuel in its new scheme of fire control, fuels
specialists were available in each geographic area of the
country. These sources were then contacted for data con-
tributions .
A list of the names of the people who generously contri-
buted their time and knowledge to this project is given in
Appendix A.
-------
3. ACREAGE BURNED
3.1 Wildfire
The wildfire acreage burned has been the most accurate
estimate of the information needed to obtain more reliable
pollutant emission values from forest fires. The Division
of Cooperative Forest Fire Control of the Forest Service-U.S.
Department of Agriculture has prepared a compilation of wild
fire statistics for the United States. This publication has
been utilized to obtain acreage consumed per geographic area.
No comparable set of statistics are available for managed burn
acreage, since reporting requirements vary among state
agencies and other federal agencies, such as the Bureau of
Land Management, National Park Service, Fish & Wildlife
Services, Bureau of Indian Affairs, and the Tennessee Valley
Authority. Also, a clarification in nomenclature must be
made between various types of managed burning such as con-
trolled burning, prescribed burning, slash burning, agricul-
tural burning, range rehabilitation, litter removal, etc.
Tables 1 and 2 together constitute a summary and com-
pilation of data on the number of wild fires and acres burned
to size classes of fire from the "1971 Wild Fire Statistics",
USDA Forest Service, 1972 (13).
3.2 Managed Fires
Managed fires are intentionally initiated for a variety
of purposes, objectives, and benefits. One form of managed
fires, prescribed burning, is based on the premise that fire
is natural and can be used as a valuable and essential man-
agement tool. Fire is used only when and where it is needed,
by controlling and managing it in such a way that it has bene-
ficial effects for the user. Prescribed fire is defined by
the Society of American Foresters as:
"The skillful application of fire to natural fuels under
conditions of weather, fuel moisture, soil moisture, etc-,
that will allow confinement of the fire to a predetermined
10
-------
Table 1
SUMMARY OF WILDFIRE STATISTICS FOR THE UNITED STATES — CALENDAR YEAR 1971
FOR PROTECTED AREAS, BY GROUPS OF STATES (13)
CROUPS
or
STATES
UNITED STATES
• ROCKY
MOUNTAIN
GROUP
PACIFIC
GROUP
NORTH
CENTRAL
CROUP
SOOTHERS
CROUP
EASTERN
GROUP
I/ Feder
OWNERSHIP
i/ i/
Federal
Stale & Private
TOTAL
Federal
State & Private
TOTAL
Federal
State & Private
TOTAL
i/
FOREST AREA
U
Heeding
M Acres
712,231
613,971
1,326,202
297,598
156,435
454,033
385,593
66,574
452,167
Federal 1 10,253
State & Private 79.820
TOTAL j 90,073
Federal ] 16,143
State & Private! 234,624
TOTAL 250,767
Federal
2,644
TOTAL 79,162
Pro-
H Acrea
646,694
574,443
1,221,137
297,598
133,340
430,938
320,056
63,522
383,578
10,253
76,126
86,379
16,143
224,937
241,080
2,644
76,518
79,162
M Acres
65,537
39.528
105,065
23,095
23,095
65,537
3.052
68,589
3,694
3,694
9,667
9,687
Percent
7.
90.80
93.56
92.08
100.00
85.24
94.91
83.00
100.00
84.83
100.00
95.37
95.90
100.00
95.67
96.14
100.00
100.00
100.00
TOTAL
Number
13,167
95,231
106,396
7,382
6,706
14,06"-
3,859
5,436
9.295
-'.30
11,382
11,632
1,413
58,744
60,157
1 63
12,963
13,026
al land areas shown as Needing Protection. Protected and Unprotected, art
Service; Bureau of Indian Affairs; and the Tennessee Vsll-y Aiitti
i' -->:•
:.. !!<»-'. ..;. f
rltv.
Protected Area '
.
Number j %
13,167 ! 100.00
91ffc73 I 96.26
:.04.0«W.! -' ?6.72
7,382 j 100.00
6,156 ' 91.80
!3,53fi 1 96.10
RES
AREA BURNED
TOTAL
Number " Acres
No DJHA N.D
3,558 I J.74
3,55« j 3.28
P t t d A Ic t t d A
i n
" ' !
Acres 7. i Acrea
1,719.315 1,719.315 IOC. 00 ! No Data
.',559,157 ; 1.626,580 71.37 : 732,577
4,278,472 j 3,545,895 82.88 73^,577
': — ., 539, i3b 539,538 100.00 1
550 | 8.20 | 234,8ft/ ] 224.94,2 95.77 9,925
550 3.90 . '74,405 , 764,480 98.72 9,925
II J 1
3,859 I 100.00 — {
5,436 : 100.00 No Data N.D.
9.295 IOC. 00 j
I
" I :
450 i 100.00 j
9.717 ! H5.37 \ 1,665 14.63
10.167 | P5.93 J 1,665 14.07
1,413 I 100.00
57,401 ' 97.71
5fi,fcl4 ! 97.77
63 | 100.00
12,963 ' 100.00
13,026 j 100.00
Laken from Official
rJi-ral-AgcneieB! V.
1,343 2.29
1.343 2.23
::: | :::
1.123.684
39,044
1,162.728
14.97"
217,770
232.749
40,419
1,951,920
1,992,339
695
115,556
116,251
1,123.684 ' 100.00 I
39,044 100.00 1 Mo Data
1,162, 728 100.00
14,979 100.00
115.270 ! 52.93 102,500
•_3 0.249 | 55.96 102,500
40,419 100.00
1,331,768 68.21 620.152
1.372,187 68.87 620.152
695 100.00 I
115,556 100.00
116,251 100.00
•;.
N.D.
28.63
17.12
4.23
1.28
S.D.
47.07
44.04
31.77
31.13
:::
•
BtfRNED FTOCTON
OF FOREST AREA
Pro-
tected
7
0.27
0.32
0.29
0.18
0.17
0.18
0.35
0.06
0.30
0.15
0.15
0.15
0.25
0.59
0.57
0.03
0.15
0. 15
Reports submitted by the report!",. Federal Agencies.
Unpro-
tected
t
N.D.
1.85
0.70
0.04
0.04
-~
2.77
2.77
6.40
6.40
-------
Table 2
COMPILATION OF DATA RELATED TO SIZE CLASSES OF FIRE - 1971 (13)
1) Geographic Groups of States
2) Listing of Regions within Group
3) Listing of States within (1) and (2)
4) Number of Fires (CY-1971)
5) Area Burned (acres) (CY-1971)
6) Number of Wildfires by Size Classes
(Protected Area Only) (CY-1971)
Size Classes
A - 0.25 acres or less
B - 0.26 to 9 acres
C - 10 to 99 acres
D - 100 to 299 acres
D - 300 to 999 acres
F - 1,000 to 4,999 acres
G - 5,000 acres or more
7) Acres Burned by Wildfires by Size Classes
(Protected Area Only) (CY-1971)
Size Classes
A - 0.25 acres or less
B - 0.26 to 9 acres
C - 10 to 99 acres
D - 100 to 299 acres
E - 300 to 999 acres
F - 1,000 to 4,999 acres
G - 5,000 acres or more
Rocky Mountain
a) Northern,
b) Rocky Mountain,
c) Southwestern
d) Intennountain
a) Idaho, Montana,
N. Dakota
b) Colorado, Kansas,
Nebraska,
South Dekota,
Wyoming
c) Arizona,
New Mexico
d) Nevada, Utah
14,088
774,405
7,647
4,191
1,237
215
147
74
27
124
12,414
39,736
37,455
91,741
183,565
399,445
Pacific
a) California
b) Alaska
c) Pacific Northwest
a) California,
Hawaii
b) Alaska
c) Oregon, Washington
9,295
1,162,728
7,168
1,556
361
84
52
41
33
56
3,527
13,572
14,266
29,014
97,733
1,004,560
North
Central
North
Central
Illinois,
Indiana,
Iowa,
Michigan,
Minnesota,
Missouri,
Ohio,
Wisconsin
11,832
232,749
2,669
5,572
1,698
185
35
7
1
55
15,006
49,933
28,904
18,793
8,238
9,320
Southern
Southern
Alabama,
Arkansas,
Florida,
Georgia,
Kentucky,
Lou i s i ana ,
Mississippi,
North Carolina,
Oklahoma,
South Carolina,
Tennessee,
Texas, Virginia
60,157
1,992,339
7,729
37,886
11,682
1,071
318
100
28
8
188,897
342,986
170,123
163,273
194,522
312,278
Eastern
Eastern
Connecticut,
Delaware,
Main, Maryland,
Massachusetts,
New Hampshire,
New Jersey,
New York,
Pennsylvania,
Rhode Island,
Vermont,
West Virginia
13,026
116,251
5,277
6,626
971
106
35
10
1
155
13,400
25,817
18,124
16,607
21,148
21,000
United States
108,398
4,278,472
30,490
55,831
15,949
1,661
587
232
90
398
233,244
472,044
268,872
319,428
505,206
1,746,703
-------
area, and at the same time will produce the intensity
of heat and rate of spread required to accomplish cer-
tain planned benefits to one or more objectives of sil-
viculture, wildlife management, grazing, hazard reduc-
tion, etc. Its objective is to employ a fire scienti-
fically to realize maximum net benefits at minimum
damage and acceptable cost."
Managed burning is used as a simulated "ecological" or
"natural" fire control method. The periodic low intensity
surface fires kept the forest free of debris, unwanted
understory trees, and minimized high intensity crown fires
by removal of the ground litter (fuel accumulation). The
use of managed fires has been expanded and applied to natural
fire risk areas, other fuel types, and management practices.
From the point of view of the purist, debris burning,
such as slash burning after clear-cutting, agricultural
burning, railroad and road right-of-way burning, orchard
prunings, etc., have not been considered prescribed burnings.
In the South, managed burning is widely used for
hazard reduction, insect and disease control, site prepara-
tion, slash disposal, wildlife habitat management, and to
improve recreational value. In the West and Southwest brush
control for range rehabilitation and management, hazard
reduction, and watershed improvement are some of the pur-
poses of managed fires (14,15,16).
13
-------
4. FUELS. FUEL TYPES. AND ESTIMATED QUANTITIES
The combustion of wood is a continuing study on a theo~
retical, as well as an applied basis. These studies have
been based on beneficial or deleterious effects, depending
on the outlook of the viewer At present, the mechanism and
chemistry of wood combustion, especially of forest fuels, is
still considered a complex phenomenon.
The U.S. Forest Service considers the assessment of fuel
to be of major importance and each district measures standing
timber and fuels. The present mission of the Rocky Mountain
Forest & Range Experiment Station is the development of the
National Fire Danger Rating System (NFDR) (17) . The NFDR
system uses fuel models to develop fire behavior components
to aid in planning fire control activities Fuel models were
devised (18) to organize fuels information as inputs for
solution of a fire spread model. The fuel models in the
NFDR system are concerned with ignition, and thus only those
fuels involved in combustion within the immediate flame front
are considered (19). The length of the flames at the head
of the fire was assumed to be directly related to the contri-
bution that fire behavior makes to the job of containment.
The fuel loading as it relates to the various fuel
models will be correlated to regional forest cover types^
timber types, or fuel types.
However, for air quality assessment, fuel loadings will
require adjustments to obtain total material consumed by a
wildfire (70 burned) . As a first approximation, the fuel
loadings designated in the NFDR fuel model will be totally
consumed in a wildfire. Some foresters feel that fuel models
and emissions can be related. Thus, a continuing study of
this relationship may lead to better fire management and
emission output.
14
-------
4.1 General Forest Fire Background
In the United States, the forested area comprises ap-
proximately 786 million acres or 34.4% of the land area.
Forest fires are quite prevalent and usually seasonal for
certain geographical areas. The U.S. Forest Service has con-
ducted research for many years on free-burning forest fires
under real as well as simulated natural conditions. Fires
that sweep through the forest usually do not destroy all the
trees. Many variables are inherent and characteristic of a
forest fire. The topography or terrain of the forest, the
vegetation composing the forest, and the atmosphere above
the forest are well interrelated to the burn rate and spread,
intensity of burn, and the size of the burn. These variables
may be altered by fire suppression and control technology,
with the possibility of increased emission of combustion
by-products.
Large forest fires produce great volumes of smoke which
vary in character and color in accordance with variation in
fuels (vegetation) and rates of combustion. The main smoke
column, if present, exhibits an almost continuous corkscrew
motion, but at the base, variation in smoke color and density
occurs due to the variation in intensity and type of com-
bustion of the fuel supply.
Of the major North American forest fires, the worst was
the Peshtigo Fires in Wisconsin and Michigan in 1871 when
3,780,000 acres of timber were burned. The Dudley Lake Fire,
Arizona, in a 48-hour period consumed 20,000 acres of mature
timber and slash which approximated 300,000 tons by weight.
At the peak period, about 22,500 tons of dry weight were cor\r
sumed1 per hour. The Sundance Fire (20) in northern Idaho in
the summer of 1967 was pne of the better documented forest
fires. The convection column reached an altitude of 31,000
ft, traveled 16 miles in 9 hours and consumed over 50,000
acres of mature trees, ground litter, brush, and crown material,
15
-------
Meteorological, topographical and fuel determinations were
carefully analyzed to study the fire behavior characteristics
Historically, men and materials have been utilized for
prevention, suppression, and control of forest fires rather
than the measurement of pollutant parameters. We hope that
time will modify these efforts enough to obtain a more com-
plete knowledge of the total picture of a forest fire.
4.2 Overview of Fuels and Fuel Types
The importance of fuel type on the fire process and the
emissions therefrom cannot be overestimated. Therefore,
categorizing forest regions by fuel types was an essential
task of the program. Historically, the geographic distribu-
tion of forest trees has been made by botanists, and thus
the forested areas were grouped in vegetation zones. The
Society of American Foresters' booklet (21) names forest
types from the predominant stand of trees. They recognize
106 forest type groups in the eastern part of North America
and 50 type groups in the western part. The U.S. Forest
Service classifies forest types into several major type
groups; 10 Western and 10 Eastern. The classification
stresses the commercial value of forest cover types. The
American Forest Institute, a trade group of forest products
industries, have produced a full color educational map of
the United States outlining geographic areas and their re-
lated three coverage. This map is appendixed to this report,
Appendix D.
Shroeder (22) divided the country into homogeneous areas
based on the combination of fire-climate and fuels. This was
the first instance of a distribution pattern which was based
on natural forces rather than one of taxonomic, topographical
or political demarcation of an area. Komarek (9) has also
proposed seven lightning, bio-climatic regions for North
America. Recently, Fahnestock (23) characterized forest
fuels using two keys: the fire spread potential and the
16
-------
crowning potential. The keys are easily recognized and
evaluated in the field and can be used by persons .without
technical knowledge of vegetation or experience with fire.
Color photographs are used to illustrate descriptions used
in evaluating the vegetation in terms of using the keys«
During a study of the biological and environmental con-
sequences of nuclear war, The Rand Corporation (24) made a
study of the possible extent of wildland fires that might re-
sult from a large scale nuclear attack^ A simultaneous
natural fuel condition for the entire nation was considered
and for this purpose an assessment of the national wildland
flammability conditions were needed. Utilizing Kxichler's map
and description, "Potential Natural Vegetation of the
Coterminous United States", 1964, a national wildland fuel
distribution data summary was prepared by reducing all wild-
land vegetation to 15 fuel types. This study is potentially
of great value but at the moment not related and pertinent
to the project.
A few regions in the U.S. Forest Service have a fuel
classification system. An example is the Region 6 Guide for
Fuel Type Identification, 1968 (25). Each fuel-type is der
fined by one of four rate-of-spread and one of four resistance
to control classes, or a possible 16 combinations. Black and
white photographs of examples of typical fuel types in the
region are included for reference. However, no estimates of
fuel loadings by size class in tons per acre are included.
4.3 Fuel Models
A new, country-wide fuel identification system is pre-
sently being developed£ This' system will attempt to include
estimates of fuel loading by size class, in tons per acre.
The Director of the Division of Forest Fire and Atmospheric
Science Research of the U.S. Forest Service states (26) that
research in fuel science has recently developed descriptive
fuel models for use in all forest regions.
17
-------
Rothermel introduced the concept of fuel models tailored
to the vegetation patterns in the field (18). The fuel model
is a simulated fuel complex for which all the required fuel
descriptors have been determined and describes the vegetation
pattern on the basis that fuels have inherently similar
characteristics. The fuel model would then represent typical
i
field situations. A knowledge of the characteristics of the
fuels in the field appears basic to a study of fire behavior.
Eleven fuel models have been assembled to represent forests,
brush, and grasslands found in the temperate climate of North
America. Refinements of the models could be made for more
specific fuel situations. The input parameters for the fuel
mo.dels were grouped on the basis of current, knowledge of
mechanisms of fire spread and the propagation of a fire.
Table 3 is Rothermel1s (18) values for the National Fire
Danger Rating System.
The environmental parameters of wind, slope, and ex-
pected moisture changes were superimposed on the fuel model
and incorporated into a National Fire Danger Rating System
(NFDR) (17,19). The NFDR System considers five classes of
fuels, three dead and two living. These fuels were selected
on the basis of combustibility; response to moisture for the
dead fuels (1-hr, 10-hr, and 100-hr timelag class) and whether
the living fuels were herbaceous or woody. For this project,
the goal and purpose of the NFDR which is the rating of fire
danger (prediction of the behavior of a potential fire) is
of value in its relation to the development of a uniform
national estimate of fuel models and loadings in tons per
acre.
The previous sections give various methods or systems of
identifying the fuel types and also fuel loadings for various
fuel models. These have been included to indicate the develop-
ment of the importance of understanding the role of fuels in
the total forest fire behavior from point of ignition, spread
and extinction, and the overall production of emissions in
the process.
18
-------
VALUES FOR INPUT PARAMETERS OF 11 PRELIMINARY FUEL MODELS FOR THE
NATIONAL FIRE DANGER RATING SYSTEM1 (18)
Fuel
types
Total
loading
Tons /acre
Grass (short)
Grass (tall)
Brush (not chaparral)
Chaparral
Timber (grass and
understory)
Timber (Utter)
Timber (litter and
understory)
Hardwood (litter)
Logging slash (light)
Logging slash (medium)
Logging slash (heavy.!
. 0
3
6
23
4
15
30
15
40
120
200
.75
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
Dead fuel
Kirie Medium Large
F
3
1
2
2
3
2
2
2
I
I
1
a
t.-1
,500
,500
,000
,000
,000
,000
,000
,500
,500
,500
,500
K0 C Wp 0 KO
Lb./ Ft.-1 Lb./ Ft.-1 Lb./
> _ 2 -•- _ 2 ,-v _ 2
0.034
.138
.046 109 0.023
.230 109 .184 30 .092
.092 109 .04(3 30 .023
.069 109 .046 30 .115
.138 109 .092 30 .230
.134 109 .019 30 .007
060 10° '''f "o rss
.184 jp'i .o44 50 .759
.5:: I'U: 1.05S 30 1.2SS
Living fuel Fi
dei
0 W0
Ft.-1 Lb./
ft*
1
2
1,500 0.092 2
1,500 .230 6
1,500 .023 1
0
1,500 .092 1
0
3
icl
pth
Ft.
.0
.5
.0
.0
.5
O
.0
.2
0
.0
1For all models, h = 8,000 B.t.u./lb., p = 32.0 lb/ft , ~, fuel particle surface area-
to-volume ratio, I/ft. "
2
W , ovendry fuel loading, lb/ft
h, fuel particle low heat content, B.t.u./lb.
o
p , ovendry particle density, lb/ft .
-------
4.4 Quantity of Fuel Consumed-Burned
Many descriptors have been used to describe forest fuels.
The terminology and concept of fuel loading (quantity of fuel
in a defined area) in relation to fuel consumption by fire is
at present not firmly and clearly defined. This difficulty
has tended to cause confusion even among forestry personnel.
In this report, fuel loadings have been reported as estimates
of the tonnages of fuel that will be consumed in an average
wildfire.
The intensity of the wildfire can be approximately
scaled by experienced observers; for example, light surface
fire consuming only the top litter layer; brush fire con-
suming most of the litter and brush; to very high tensity
fires and crown fires burning off all litter, fire fuel
particles, and the entire organic soil mantle. However, even
in the most intense fire, the greater proportion of the lar-
ger woody material will not be consumed. The larger fuels
do not contribute to fire intensity or to the rate of spread
since these burn after the main fire front has passed and
are difficult to extinguish.
In this report, the stated estimated fuel loadings will
be synonymous to the "available fuel" (fuel which will be
consumed in a wildfire under a given set of or prevailing
weather conditions). "Total fuel" has been defined in dif-
ferent ways by forestry personnel to indicate the complete
consumption of the fuel. "Total fuel" will be defined in
this report as all the combustible material that would burn
under the most severe weather and burning conditions.
"Potential fuel" would then be all the larger woody material
that is referred to as fuel, but remains even after an ex-
tremely high-intensity wildfire.
The U.S. Forest Service is the single organization
administering the largest forest acreage, over 187 million
acres in 41 states and Puerto Rico. Since they are responsible
20
-------
for the reporting, compiling,, and publication of forest fire
statistics, their system of fuel identification and assess-
ments will be utilized in this report.
The various methods of fuel identification are of value
only when they are related to the existing fuel quantity, the
quantity consumed by the fire, and finally the geographic
area and conditions in which the fire occurred. These re-
lationships are discussed in the following sections.
4.5 Regional Values of Fuel Loadings
Since the U.S. Forest Service Regions are in the process
of assessing fuel, a more reliable fuel inventory should be
forthcoming. Eventually, the fuel loadings should reflect
the breakdown given for acreage burned by states as in the
compilation on "Wildfire Statistics" (13).
The U.S. Forest Service has nine National Forest Regions.
Fuel loadings for these regions as well as fuel loadings for
smaller geographic areas were obtained where these values
were reported in the literature.
4.5.1 Region 1. Northern Region
The Northern Region covers Montana, northeastern
Washington, northern Idaho, North Dakota, and northwestern
South Dakota. This region has 16.8 million acres of commer-
cial forest land of which 7 million acres have a cover type
dominated by pine (western white pine, ponderosa pine, and
lodgepole pine). Other general tree types in this northern
Rocky Mountain forest are western larch and Engelman spruce.
Beaufait (27) refers to a study by Bray and Gorham (1964,
p 106) that the organic mantle is enriched at the rate of
3 to 4 tons per acre per year in the North Temperate Zpne,
The decomposition rate of organic matter is much less than
the production rate as compared with the Tropical Zone, Thus,
fuel accumulates in this region to a hazardous level unless
fire removes it.
21
-------
Research studies conducted on logged areas in western
Montana (western larch-interior Douglas-fir type) indicated
that the weight of logging debris and residual debris (dead
and down material, including duff on the site before logging)
ranged from 50 to 150 tons per acre, averaging just over
100 tons of organic matter per acre (28). The atmospheric
emissions from burning logging and residual debris depends
on how it is burned. Slash pile burning results in rela-
tively complete removal of the debris. Broadcast burning,
i.e., burning the slash where it fell rather than gathering it
in piles, reduces the fire hazard by removal of the "fine
fuels" but leave considerable material that makes reforesta-
tion difficult (11). In another study (29) at the University
of Montana, Missoula County, Montana, the total organic mat-
ter of Douglas fir slash, non-commercial residue, and duff
(after logging) averaged 64 tons per acre.
Region 1 does not have a guide for fuel type identifica-
tion but does have estimates of available organic residues.
The region does have acreage and biomass information avail-
able in their files which could be assembled for use at a
later date. Unpublished data at the nearby Northern Forest
Fire Laboratory would also be accessible (30).
An average fuel loading of 60 tons per acre can be esti-
mated for this region.
4.5.2 Region 2, Rocky Mountain Region
The Rocky Mountain Region covers Colorado, Kansas,
Nebraska, South Dakota, and Wyoming and encompasses approxi-
mately 22 million acres of forest land. The following is a
broad, subjective estimate of fuel loadings by type, from
readily available information (31).
Region 2 has not prepared a "Guide for Fuel Type
Identification". An average fuel loading of 30 tons per acre
can be estimated for this region.
22
-------
Fuel Types Rased on
the National Fire
System
Unburnable
Fuel Model A
Fuel Model C
Fuel Model G
Fuel Model H
Others
Estimated Acres
in
1,000,000
2,500,000
7,800,000
4,800,000
5,000,000
900,000
Tons/Acre of
Vege tat ive_Mat ter
1-3 tons/acre
20-40 tons/acre
50-100 tons/acre
100-150 tons/acre
5-20 tons/acre
4.5.3 Region 3. Southwestern Region
The Southwestern Region covers Arizona and New Mexico.
The following are approximate tonnages per acre in Region 3
(32) based on the National Fire Danger Rating System fuel
models.
FueI Model
A
A
I1,
B
C
C
C
G
II
Crass and herbaceous plants
Crasr;, herbaceous plants and
Jess than 'J./'.< piiiyon pi uo,
juniper or ponderosa piuc>
Chaparral
Coniferous slash with needles
attached
Open ponderosa pine
Ponderosa pitio
Piriyon pine or juniper
Spruce
Mixed conifer
Approximate
Tonnage of l'"uej /Acre *
0.5
L.O
20.0
100.0
8.0
20.0
5.0
20.0
50.0
A system to inventory fuels is being developed for the region.
* We interpret these values to be available fuel.
23
-------
In Arizona, the forest area covers 20.6 million acres,
of which 4 million acres consists of commercial forest.
Ponderosa pine is the dominant timber type covering 927o of
the commercial forest area. The pinyon-juniper and chaparral
types cover 12.2 million and approximately 4.4 million acres,
respectively. Of the commercial forest, ponderosa pine pre-
dominates on 3.66 million acres, Douglas-fir on 129,900
acres (370), aspen on 79,000 acres (2%), and the rest con-
sists of fir-spruce at the higher elevations (33).
In New Mexico, the forest area covers 18.2 million acres
of which 6.3 million acres are commercial forest. Eighty-
nine percent of the 11.9 million acres of non-commercial
forest is classified as pinyon-juniper type and the rest is
chaparral (oak brush and woodland). Ponderosa pine type
occupies 4.3 million acres (697,) of the commercial forest
area, Douglas-fir type on 1 million acres (1670), fir-spruce
type on 525,000 acres (87o), and aspen type on 367,000 acres
(67=) (34).
The forest floor is an important fuel component. Ffolliott,
et al. (35), obtained depth, weight, and density of the for-
est floor under Ponderosa pine in Arizona. They obtained a
mean depth of 1.3 inches, mean weight of 9.3 tons per acre,
and a mean density of 6.0 tons per acre-inch. The bulk of
the weight was in the H layer (7.3 tons per acre).
Litter production under chaparral in central Arizona
was measured by Pase (36). The chaparral community produced
an annual litter mass of 215.3 gm/m in northerly slopes and
o
193.2 gm/m in southerly slopes. Leaves comprised the great-
est percent of annual litter shed; for example, 1007o of the
leaves on healthy shrub live oaks were replaced each year.
The forest floor varied from 4.1 to 12.1 tons per acre.
Besides knowing the average amount of fuel on the forest
floor, a measure of the amount of fuel consumed in a moderately
high intensity surface fire would indicate the approximate
24
-------
amount of consumable material. Davis, et al. (37), described
a prescribed burn that took place in the Coconino National
Forest in northern Arizona. Duff weights were an average of
10.2 tons per acre in a flat area and 17.6 tons per acre in
a sloped area. Burning resulted in 3.4 tons per acre con-
sumed in the flat area and 6.3 tons per acre in the sloped
area. Besides the 30% slope, the area with 17.6 tons/acre
had a 5% higher air temperature and 2 miles per hour higher
wind.
Since chaparral, or evergreen brush, covers about 8% of
the total area of Arizona, wildfire in such an area can be
potentially very extensive. Fosberg and Davis (38), and
Lindenmuth and Davis (39), summarized and interpreted the
first research study of fire behavior in Arizona oak chapar-
ral, in the vicinity of Prescott, Arizona, in the Prescott
National Forest.
CHAPARRAL FUEL DATA (38)
Loading,
Surface
in
tons /acre
area /vol. ,
Leaves
Total
Leaves
0-1/8"
1/8-1/4"
Litter bed depth,
inches
Low
0.3
5.0
1918
2.42
Mean
2.1
13.9
2519
5102
1915
2.81
High
6.2
34.8
3314
3.58
SHRUB LIVE OAK CHAPARRAL ,(38)
Total loading before burn 0.633 Ib/ft2
Total consumed by fire 0.407 Ib/ft2,
Flaming combustion 0.287 Ib/ft2,
Glowing combustion 0.120 lb/ft2
25
-------
7a FUEL LOADING BY CLASS (38)
Total 0.637 lb/ft2 10070
less than 1/4" 0.325 lb/ft2 51
l/4"-l" 0.115 lb/ft2 18
over 1" 0.102 lb/ft2 16
leaves 0.095 lb/ft2 15
The fire season in the southwest is May and June and
sometimes August to September. The problem areas are chapar-
ral, ponderosa pine, and lodgepole pine. An average fuel
loading of 10 tons per acre can be estimated for this region.
4.5.4 Region 4. Intermountain Region
The Intermountain Region covers Utah, southern Idaho,
western Wyoming, and Nevada. Qualitative data is not readily
available (40). The Northern Forest Fire Laboratory in
Missoula, Montana, was referred to as a location for informa-
tion and daca relative to forest fuels in the Intermountain
area.
Utah's forested area is 14.9 million acres, of which 4
million acres are commercial forest. Of the non-commercial
forest, 9.3 million acres or 86% is classed as pinyon-juniper
type, and 1 million acres as mountain brush (Gambel oak).
Aspen (33%) and fir-spruce (25%) are the largest timber types
in the commercial forest. Others are Douglas-fir, lodgepole
pine, and ponderosa pine (41).
The nearest fuel loading estimates that can be utilized
is based on Region 3's data.
Pinyon-Juniper 9.3 million acres 5 tons/acre
Mountain Brush 1.0 million acres 8.8 tons/acre
Other Non-Commercial 0.6 million acres 5 tons/acre
Aspen 1.3 million acres 12-20 tons/acre
Fir-Spruce 1.0 million acres 20 tons/acre
Other Conifers 1.7 million acres 8-20 tons/acre
An average fuel loading of 8 tons per acre can be
estimated for this region.
26
-------
4.5.5 Region 5. California Region
This region covers the states of California and Hawaii.
The fuel models developed for the National Fire Danger Rating
System are used to describe the forest fuels. The tonnage
figures furnished by the NFDR project are used for most pur-
poses. Therefore, adjustments must be made to the NFDR
tonnage figures to obtain a better estimate of the total
material available and the total consumed. In Region 5, the
available fuel models do not describe the fuel types ac-
curately, especially in uneven aged timber stands with signi-
ficant amounts of logging debris which have not been dis-
posed of or treated (42).
Funl Modnl & Name Tonnage Per Acre *
A Grass 3
B Brush .19
C Open Pine Timber
West side of .Sierra
Nevada Mountains 20 + (20) = 40
East side of Sierra
Nevada Mountains 11 + (9) =20
D Young Brush 12
G Dense Conifer 30 + (50) = 80
I Logging Slash **
Light 30
Medium 35-70
Heavy 80 - 150
* We interpret these values to be available fuel.
** California Region Estimate.
27
-------
The figures within the parenthesis in the above tabulation
were Region 5's estimates of the additional tonnage consumed
in a wildfire of the fuel types. Fuel Model B was not used
for logging slash (NFDR System). Fuel Model C or D was used
so that the significant differences between the fire behavior
in chaparral (Model B) and conifer stands could be exhibited.
Little or no use was made of Fuel Models E, F, or H in
Region 5 (42).
One-half of California is covered by vegetation; coni-
ferous forest, woodland savanna, chaparral, and grassland.
It is estimated that 15-24 million acres of brush (of which
8 million acres are chaparral) exist in California. The
fire potential is tremendous, in that living fuel in drought
conditions can act like dead fuels in combustibility.
Countryman (43) refers to studies conducted on certain
chaparral fuels in southern California. In that study, plots
of chaparral (fuel) were dissected to obtain amounts and
distribution of fuel particles within the fuel bed. Results
from typical plots in three different chaparral weights were
given and reproduced in Tables 4, 5, and 6.
Kilgore (44) made a study of managed burning on a
Sequoia-mixed conifer forest in Kings Canyon National Park,
in which the white fir is the dominant tree in density.
Before burn, there were 50 tons/acre of total litter and
duff fuel. After burn, there was a 75% reduction in litter
fuels and 857o reduction in duff fuels. Remaining after the
burn was 7.7 tons/acre. The estimated log fuel weights
(understory trees) decreased from 12.8 to 2.8 tons per acre.
In the California region, the potential hazards from
wildfire due to fuel accumulation have appeared in the news-
papers and national weeklies. The hazard developed as a
direct result of the extremely cold winter (1972-73) in
the California Bay Area (Berkeley) which killed and injured
2-3 million Eucalyptus trees (approximately 3,000 acres)
28
-------
Table 4
COMPOSITION OF LIGHT CHAPARRAL FUEL TYPE (43)
Item
Dry weight
(tons per acre)
Percent of
class weight
Total fuel:
Living
Dead
Duff and litter
Total
Predominant species:
California sage
(Artemisia californica)
White sage
(Salvia apiana)
Deerweed
(Lotus scoparius)
Height(feetj:
Over 6
4 - 6
2 - 4
0 - 2
Size class:
Flowers
Leaves
Twigs to 1/4 in.
Stems I/if - 1/2 in.
Stems 1/2 - 1 in.
Stems 1 - 2 in.
Stems 2 in. or over
2.19
4.10
6.13
17.6
33-0
49.4
12. k2
0.59
4.1*5
1.25
9A
70.7
19-9
.00
.02
5.80
.00
• 76
2.43
1.45
• 98
• 76
.00,
.o
• 3
7.5
92.2
.0
12.1
38.5
23.1
15.6
10.7
.0
29
-------
Table 5
COMPOSITION OF MEDIUM CHAPARRAL FUEL TYPE (43)
Item
Total fuel:
Living
Dead
Duff
Total
Predominant species : ~
Chamise (Adenostoma
fasciculatum)
Dry weight
(tons per acre)
9.76
5-32
6.06
21. 1U
11.02
Buckbrush (Ceanothus cuneatus) 3-02
Sumac (Khus laurina)
Height (feet):
Over 6
1+ - 6
2 - 1+
0-2
Size class:
Leaves
Twigs to 1/4 in.
Steir.s 1/1+ - 1/2 in.
Stems 1/2 - 1 in.
Stems 1-2 in.
Sterna over 2 in.
1.05
.92
2.90
5-09
6.19
1.20
3.80
2.78
5-51
1.81
.00
Percent of
class weight
1+6.1
25-2
28.7
—
73-0
20.0
7.0
6.1
19-2
33-7
1+1.0
8.0
25.2
' ISA
36.1+
12. 0
.0
Table 6
COMPOSITION OF HEAVY CHAPARRAL FUEL (43)
Itf:m
Tot:il fuel:
Living
Dead
Duff
Total
Predominant species:
Dry weight
(tons per aci-e)
28.62
2.56
8.25
39- '*3
Scrub oak (Quevcus donoua) 22.80
Buckbrush (Ceanothus o>.
Height (feet):
Over 6
1+ - 6
2 - 1+
0-2
Size class:
Leaves
Twigs to !/!+ in.
Stems 1/h - 1/2 in.
Stems 1/2 - 1 in.
Ste-ma 1 - 2 in.
Stems over 2 in.
ncatus) 8.38
7-96
8.1*6
11.1+2
3-35
2.61+
1+.01+
1+.08
1+.1+5
11.36
1+.61
Percent of
class wejght
72.6
6.5
20.9
—
73-1
26.9
25-5
27.1
36.7
10.7
8.5
13-0
13.1
H+.3
36.3
lit. 8
30
-------
creating litter problems (as much as 50 tons of debris per
acre) and potential wildfire problems with or without a hot,
dry summer. A large part of the fuel produced by eucalypts
is leaf litter which, in most forests, accumulates to about
1/2 £o 1 ton per acre per year for at least 25 years (45).
Additional fuel are the dead twigs, branches, and a unique
fuel which is the dead bark that falls off in sheets and rib-
bons. Mount (46), as an Australian visiting silviculturist
in 1969, viewed fires in the Berkeley area as a far greater
threat than the San Andreas Fault. His observation of an
id?al fire situation: a combination of topography, climate,
venatation, and houses buried in the vegetation (fuel accumu-
lation under i:ai auealypts up to their stems, and a "perfect"
intermixing of the fuels -- pine and eucalypts), leads him
to foresee e •;.:. :•*strophe far worse than the Hobart fires of
1967 in Tasmania.
The annual accumulation of litter is a growing problem.
Dodge (47) refers to a few studies in California. Accumula-
tion rates of 0.45 to 1.3 tons per acre were found in the
chaparral of southern California; 0.89 to 2.8 tons of litter
per acre were contributed each year to the forest litter by
several species in the central Sierra Nevada; and in studies
of the giant sequoias (Sequoiadendron giganteum) groves, as
much as 23 to 38 tons of dead fuel per acre were found. An
average fuel loading of 18 tons per acre can be estimated
for this region.
4.5.6 Region 6, Pacific Northwest Region
The Pacific Northwest Region covers the states of
Washington and Oregon. This region has just completed a
summary of the total acres for each fuel model (described
in the National Fire Danger Rating System) found in this
region. There is only a very small percent of National
Forest land containing Fuel Models E, F, and H. Table 7
lists forest vegetation in the region and the acreage for
31
-------
Table 7 - FUEL MODELS USED IN REGION 6 (48)
(Showing Acres in Each Model)
u»
to
R6-6200-3 (9/67)
UNIT
01 PCSCHOT'S
J
02 rrcMCM-r . 1337,634
C^ G- PlMCHOT JJ
*
O'i VlLV'SU i
• •(
.Co »r H.MS
(232,448
j"
r'"~' '
C7 r«,c-.o jj 199, 731 '
ra 3 •
^ CKANIX.A'! £
C9 OLYMPIC '(<
10 2. PIVES 0/C li
11 SISK;VO-J C/C '!
12 SIOSUAV
i
13 £'ICK-.'/'.M!C 'I 3,870
"«'•• SJMATI1.1.A 0/V
15 l!M»CJA
156,733
-.<5 V-W^TMAN " 1 786, 890
17 WCNATCHCE 8202,907
'? WlUtAMCTTE ?,
?0 WlSCMA
TOTALS
5,200
B
•208,215
_44,320
108,288*1
HZ1
40,000
•- -• "
195,000
393,024
14,570
181,383
241 ,000
,925,413 1,425,80
c
276^400 i
•-*-•• **&-**• r*-«"^
735^541 !
— j — __
725,873
*~"
704,^61
150",455
136,515
D
r
l,155,70ff
' !
57^4401
i
i
- _ _ „
•i
122,120
p .
35^320
L-^. -^-
. i
— i
^_.™^.._.
145,897
[ ' -1
*
403,200
1
,286,48C|
f87*,*394
JB47.00CJ
125, 15^
1,082,883
262,542
H
i
i
• !
' i
I
• • i.
•2,200
139,000
763,113
\
... £--—
78,824 I 48,712
1
1 , ' ,' •
_:
;
- 489, 498J' •
i 1 1 727.746J
1,096,20*6
3,800
1,108,30
531,200
"^
28,400
' 5,497,5
l I
)
801 ,464]
993,536
309,228
701,280
996,000
i
j i
51 2, 585.) 77 35.32J) 145,897
. ,_ .. _. i _.
— i
98,688
-
i
j
~: "" r
157,250| _ J j _ ,
,211,098)
,485,000!
226,520
11,999,8
•
51,275
215,000
1
i
i
JS 78,824 1,317,98^
i i
'
-------
each fuel model (48). This region does have a "Guide for
Fuel Type Identification", 1968 (25), with black and white
photographs of typical fuel situations.
An average fuel loading of 60 tons per acre can be
estimated for this region.
Slash was broadcast burned in the Cascade Mountains of
western Oregon and in the southern districts of Gifford
Pinchot National Forest, Washington. The fuel data (49)
was reported as follows:
Date Acres Burned Fuel Consumed. Tons
October 21, 1969 819 54,850
October 21, 1969 2,936 192,200
October 22, 1969 2,225 139,250
Total 5,980 386,300
The fuel burned averaged about 65 tons per acre.
4.5.7 Region 8. Southern Region
The Southern Region is composed of the following states:
Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana,
Mississippi, North Carolina, Oklahoma, South Carolina,
Tennessee, Texas, and Virginia. A regional guide for fuel
type identification has not been developed. Table 8, a
list of fuel types as it relates to the fuel models in the
National Fire Danger Rating System, was available (50). The
region was divided into 3 areas: Plains Area, Mountain
Area, and Intermediate Area.
An average fuel loading of 9 tons per acre can be
estimated for this region.
This region has developed the expertise on the use of
fire to a highly sophisticated degree (51,52,53). Prescribed
burnings as a pine management tool have resulted in numerous
benefits, one of which has been the reduction of wildfires.
In a 4-year study period, the number, size, and intensity of
33
-------
A11 Plains Area Units (Fxcept Florida
Tjmber Type
Pine (except Sand Pine)
Bottomland Hardwood
Pine & Hardwood
Scrub Oak
Orqani c Soi 1
Grass
Pocosi n - Hi gh
Pocosin - Low-
Florida
Pine-LL-S 1 ash Pine-
Palmetto Flatwoods
Pi ne-Lnnq I eaf-Scruo Oak
Pond Pinc'-TLti.
Sand tjinc
Sand Pine Double Burn
Pine-Hnrdwood
Grass
Sawgrass (Organic Soil)
Bottomland Hardwoods
Cypress-Tupelo
Mo u n t n i n Area. All of * h e C h a 1 1 a
Njntah:;ld Units of 'ic'rt; uarclina
Tallddega Districts o T ;- 1 ;oan..i : I
Timber Type
Upland Hardwood
Conifers & Hardwoods
Pine (all, except White Pine)
Pine H.-Jw. (except White Pine)
Brush (Rhod., Laurel, etc.)
Scrub Hardwood
White Pine
Intermediate Area. All areas not
fall in the Intermediate Area:
Timber Type
Grass
Scrub Hardwoods
Pine (all)
Pine-Hardwood
Pine V a u p c n
A | I Areas
Symbo 1 s
P
BH
PH
ScO
OS
GR
Poc
Poc
PF
PLS
PT
SP
SPD
PH
GR
GRO
BH
CT
hoochec, Cherokee
: B 1 jck v;arri t.r ,
he Andrew PICK 0:15
Sjjritio l.s
UpH
Cll
P
PH
BR
ScH
inc 1 uded in the
Symbol s
GR
ScH
P
PH
PYO
? v >' r o l b
Fuel Models
C,E,l)
H
E,H
0
Choose on the basis of the
surface fuels in which the
fire wi 1 ! spread
A
B
D
D
C
0
r.
D
C,H
rt
C
H
H
, li/ark. Ouacnita: Pi;r]2n a-"
B a r N n e j 1 , o h o a i C r e c K -.2 n ••
District of South Cjroiini).
Fuel Models
F_,H
E,H
C,D,E
E-.H
F
E,H
H
Plains or Mountain Areas will
Fuel Mode 1 s
A
E,H
C,D,E
E.H
H
Fuel Models
Slash (C I ear cut ) Pi ne
Slash (1h L n n i n q ) r i r p
Table 8. LIST OF FUEL TYPES - FUEL MODELS (50)
34
-------
wildfires that occurred generally increased with increasing
age of the fire protected forest. In the pine flatwoods of
Georgia and Florida, wildfire burn percentage amounted to less
than 0.1% with sound prescribed burning programs. With no pre-
scribed burning, 7% of the forest acreage was burned during
the same time period. In the South, fuel accumulation become
hazardous when it exceeds about 3 tons per acre (54,55,56).
From the extensive studies and practice of prescribed
burning in this region, the fuel loadings have been measured
and reported in the literature (57,58,59,60). Managed fires
(prescribed fires in the South) consume approximately 2 to
2.5 million acres with a fuel loading of 2 to 4 tons per acre
(average of 3 tons per acre). In prescribed burning, the
brushy fuel and the upper litter layers are the intended fire
removal goal, and this is the portion of the fuel that is
consumed.
In the central hardwoods area (Tennessee) the forest
floor litter averages 4 tons per acre or about 1/5 the amount
in a pine forest (61). The approximate composition of the
litter is: hardwood leaves (64-74%), twigs (15%), grass (8%),
and miscellaneous parts (3%).
4.5.8 Region 9. Eastern Region
The Eastern Region is composed of the following states:
Connecticut, Delaware, Illinois, Iowa, Indiana, Maine,
Maryland, Massachusetts, Michigan, Minnesota, Missouri,
New Hampshire, New Jersey, New York, Ohio, Pennsylvania,
Rhode Island, Vermont, West Virginia, and Wisconsin. This
region is unique in that it covers two of the geographic
groups of states in the United States (13), the Eastern and
the North Central.
The information in Table 9 (62) is a broad classification
of fuel models used in the Eastern Region National Forests.
These fuel loadings, as defined in the NFDR System, are the
35
-------
Table 9
FUEL MODELS (NFDR) IN THE EASTERN REGION (62)
Units
Allegheny
Chequamegon
Chippewa
Clark
Green Mountain
Hiawatha
iHuron-Manis tee
Nark Twain
Monongahela
TCicolet
Ottawa
National Fire Danger Rating Approx. Avail. Fuel
System Fuel Models Loading, Total Tons,
Primary \ Secondary . Tertiary Primary
H
H
C
E
H
C
C
E
C
A
H
H E
H
3.0
3.0
2.5
2.5
| 3.0
2.5
2.5
: 2.5
3.0
C 3.0
1 '
H
Shawnee E
Superior
Wayne-Hoosier
White Mountain
c
H
E i C
H
4J
C
CO
C
•H
E
O
'U
QJ
V-4 *
PL, ,— |
OJ
*»*O
S»i O
H ^
CO
E r-l
U 3
o
C
•r-\
)>i •
V>
C cO
•r4 QJ
J_(
4J -^1
3
QJ
„ }_|
>-l 01
^3 -M
0 4J
3.0
i 2'5
I 2.5
, — i
i-H
cO
e •
en en
QJ
•» CX
*X3 t>^
QJ H
^i
2.5
3.0
QJ C
J-t 0
4_) .p^
cO -P
U -H
cn en
C
O CO >-, cfl
O O iH }-l
cy^
QJE-i
Cfl J4 -H 1^
P-I ii ; JS b !3 b
Secondary
2.5
1.25
t
3.0
•
2.5
2.5
3.0 i
2.5
36
-------
timated typical figures of the fuel that actually burns.
For this region, a very crude estimate of the fuel
1 ading of a NE mixed conifers is 7 to 12 tons gross per acre
£ available fuel (the fuels consumed in a forest fire, gen-
rally from needle size up to 4" diameter stem), and for NE
mixed hardwoods about 12 tons gross per acre of available
fuel (62). An average fuel loading of 11 tons per acre can
be estimated for this region.
The region is at present working on a project correlat-
ing the Eastern Region timber types with the NFDR fuel models,
along with quantified fuel loading by type and other factors.
Hopefully, their project and "Guide for Fuel Type Identifi-
cation" should be completed by the end of 1973.
4.5.9 Region 10. Alaska Region - (63)
Alaska, the largest state in the United States, has a
total land area of 365.5 million acres (16% of the total
are in the United States) and extensive forest areas (16% of
the forest land in the United States or 119 million acres).
For forest inventory purpose, the state is divided into two
regions, Coastal and Interior.
The Coastal forest comprise 13,247,000 acres and is an
extension of the rain belt forests of Oregon, Washington,
and California. An important difference is that the Douglas-
fir is not found in Alaska. Western hemlock and Sitka
spruce forest types together account for 967» of the Coastal
forest area. Western hemlock is dominant on about 2.7 mil-
lion acres and Sitka spruce on a million acres. On another
1.5 million acres, hemlock and spruce are mixed, with spruce
making up 30 to 49% of the stand. Some cedar, western red-
cedar and Alaska-cedar, and hardwoods such as black cotton-
wood and birch are also found. In the transition area from
coastal to interior forest types, the western hemlock-Sitka
spruce type is displaced by white spruce and mountain hemlock
in mixture with aspen and paper birch. The average volume
37
-------
per acre of saw timber stands in Coastal Alaska compares
favorably with similar stands in Oregon and Washington. The
Coastal forests are well protected from fire by the heavy
rainfall which is well distributed throughout the seasons.
Alaska's Interior forested area is about 106 million
acres. The interior forest types are mixtures of four major
commercial species: white spruce, paper birch, aspen, and
balsam poplar. The Interior stands are similar to some in
the Lake States (Minnesota, Michigan, and Wisconsin), and
compare favorably with such stands in volume and quality.
Fire consumes an average of more than a million acres each
year of the Interior forests. Accurate information concerning
the total area burned, the types of vegetation destroyed,
the amount of timber killed, and other damage appraisal in-
formation is difficult to obtain because of the remoteness
of the region and the difficulty required to fight the fire.
The weather changes usually extinguish these remote, inac-
cessible fires.
For fuel loadings, the estimates utilized for the
Pacific Northwest Region can be projected for the Coastal
area (60 tons/acre), and the estimates for the Lake States
projected for the Interior forest area (11 tons/acre). Tak-
ing into account the acreage of each forest area, the aver-
age fuel loading for Alaska is 16 tons/acre.
4.5.10 Summary of Regional Fuel Loadings
It is very difficult to assess a so-called average fuel
loading or average fuel consumption by wildfire for a large
geographic area. This is especially true when there is a
diverse and heterogeneous vegetation cover. Conversely, if
a relatively homogeneous fuel cover type exists, a fairly
accurate fuel assessment can be projected„
A subdivision of geographic areas into regional units
reduces the bias of the extremes of the averge loadings to
38
-------
a lower order of magnitude. Eventually, as the geographic
units or blocks become smaller, the reliability or accuracy
of the estimated fuel loadings and/or fuel consumption would
be more closely related to the actual field condition and
may even reflect seasonal variations.
In this report, estimates were made for the U.S. Forest
Service's geographic areas and regional units. The use of
an estimated fuel loading is recognized as a first step in
obtaining values relating actual fuel load conditions to a
geographic unit.
Table 10 is a summary of the estimated fuel consumed by
wildfire for the geographic areas of states and the regions
within the area. The estimate of the quantity of fuel con-
sumed by a fire (or estimate of fuel remaining) varies with
the intensity (flaming combustion) and duration (glowing
combustion) of the wildfire.
Table 10
SUMMARY OF ESTIMATED FUEL CONSUMED BY FOREST FIRES
Average Fuel Loading
Area and Region (estimated) tons/acre^
A. Rocky Mountain Area 37
Northern, Region 1 60
Rocky Mountain, Region 2 30
Southwestern, Region 3 10
Intermountain, Region 4 8
B. Pacific Area 19
California, Region 5 18
Pacific Northwest, Region 6 60
Alaska, Region 10 16
coastal 60
interior 11
C. Southern Area 9
Southern, Region 8
wildfire 9
prescribed fire 3
D. Eastern Area 11
E. North Central Area 11
Eastern, Region 9
conifers 10
hardwoods 12
39
-------
5. EMISSIONS AND EMISSION FACTORS
The information search has revealed a lack of pertinent
experimental data on the emissions from wildfires. Data and
measurements reported in the literature have been obtained
on laboratory fires, burning towers and from prescribed
burns. Although scaling of such data to wildfires is frowned
on by experienced forest fire personnel, all collected data
and measurements were assessed and categorized into original
and quoted data.
The experimental data obtained from non-wildfires have
limitations of not being realistic. In a non-wildfire, the
fuel history, fuel, fuel arrangement, and environment is a
simulated, controlled area of combustion. The forest is ar-
tificial although the emissions that are generated are real
and tested with standardized and recognized sampling methods
for the fuel under study. These small scale studies were an
extension of past experiments on the pyrolysis or combustion
of wood and could be utilized to study the emissions of the
various components that make up the forest fuel. The emis-
sions from the burning of selected components of forest
fuels under varying simulated field conditions would provide
an insight to the total forest fire picture. Such emission
measurements, although of an arbitrary source, have an
essential experimental value.
On the legal side, control of a wildfire situation is
under the jurisdiction of governmental agencies with a com-
mitment to suppress and extinguish a wildfire. The inter-
action of an adequate field study of forest fire emissions
with present day available technology would probably inter-
fere with the operation of fire control personnel.
However, in the future, the emissions from a real wild-
fire would require testing in order to verify the laboratory
studies. This will be a case of a real fuel situation with
an arbitrary testing method. The fuel consumed and quantity
40
-------
of emissions produced under the prevailing measured environ-
mental and meteorological conditions would need to be deter-
mined. The sampling of the emissions would be difficult to
do in a real forest fire situation without the assistance of
Forest Service personnel.
5.1 Wood Chemistry
Although the burning of wood is a complex process, the
development of emission factors requires a more general out-
look of the process as a first approximation. A detailed
analysis of the various reaction mechanisms, burning stages,
or the composition of the wood, bark, and leaves that go
into the total combustion process could not be found in the
information search.
Wood as a forest fuel is in various stages of development
from living, growing vegetation; dormant, live vegetation;
dead and dry vegetation; and finally the natural decomposing
vegetative matter. Thus, combustion of forest fuel is com-
plex or simple depending on the outlook and degree of quan-
tification that is sought. An analogous description would
be the perspective to an amoeba in biology. From one view-
point, it is a simple single-celled organism; but from an-
other viewpoint, it is a complex animal in that all body
functions involve a single cell with no obvious differentiation.
5.1.1 Chemical Composition
The following is a general description of the chemistry
of wood. A description in much greater detail may be ob-
tained from references and textbooks (64,65).
Wood is a complex material of chemically different com-
ponents called cellulose, hemicellulose, lignin and a group
called extractables (oils, pigments, minerals, and other
organic substances). The cellulose, hemicellulose, and
lignin constitute up to 90-95% of the weight of oven-dry
wood. The woods of various species consist of the same main
41
-------
components but in different proportions; the largest differ-
ence exists between the two main groups, conifers and decidu-
ous trees. Conifers have a higher proportion of lignin
(28-34%) while deciduous trees average less (18-27%). The
extractables or the extraneous components (5-10%, are the
constituents that vary greatly between species, between
trees in the same species, and even within parts in the same
tree. The organic fraction of the extractables consist of
many classes of compounds; aliphatic and aromatic hydrocar-
bons, alcohols, aldehydes, gums, sugars, etc.
Although the number of organic compounds is large, 90-95%
of the dry weight of wood is composed of the three components
with cellulose predominating, and thus there is a chemical
limitation in its usage as a fuel that would minimize the
kinds and quantities of emissions during the pyrolytic decom-
position and combustion of wood. The bulk of the emissions
generated will be from the three components.
The moisture content of green wood (freshly cut) varies
from 30 to 60 percent. Air drying for approximately one
year reduces the moisture content to 18-257,. Wet bark may
contain 80%, or more moisture while the air dried bark approx-
imates 5-107,. Thus, in burning green twigs and branches,
approximately 50 to 607o of the moisture is evaporated and
released into the atmosphere. The dry litter containing
5 to 207o moisture requires less of a heat flux to attain the
temperature where exothermic reaction takes place.
5.2 Pyrolysis and Combustion
In a forest fire, the dry combustible material is con-
sumed first, and if the energy release is large and of long
enough duration, drying of green, live material takes place
with subsequent burning of this material as well as the
larger, dry material. Under proper conditions, this process
may develop into a chain reaction with a resultant forest
fire, With optimum environmental and fuel conditions, a
42
-------
conflagration may result.
The pyrolysis and combustion of wood have been exten-
sively studied for many years. Both are complex reactions
and are continuously studied by fuel chemists. However, for
this program, only a general description of the process of
pyrolysis-combustion of the forest fuels will be used to pro-
vide background to the development of emission factors.
In pyrolysis, heating is done in the absence of oxygen
with resultant charcoal, organic tars, and gaseous emission
products. Variations exist in the pyrolysis products of
different t :•>.£.' and tree components but the greatest varia-
tion exists between hardwoods and conifers.
In combustion, heating is done in the presence of oxygen
and with complete combustion, the hot volatile combustible
gases are in contact with sufficient oxygen producing carbon
dioxide, water vapor, and inorganic ash.
There are several properties of wood that relate to
combustion and its attendant emissions. Dry wood is very
hygroscopic and the amount of moisture adsorbed depends
mainly on the relative humidity and temperature. The excep-
tions are species having high extractives content, such as
cedars and redwoods. In greenwood, the cell walls are satur-
ated with moisture, while the cell cavities may be incom-
pletely or completely full of water. The moisture in the
cell walls are called "bound" water, while moisture in the
cell cavities is called "free" water. "Free" water removal
has little or no effect on many of the properties of the wood,
while the removal of "bound" water affects its properties.
5=2.1 Pyrolysis
Decomposition of wood by heating in the absence of air
is an ancient process. The decomposition and the amount of
resultant products depend mainly on the heating temperature,
duration of heating, the surrounding medium, and the wood
43
-------
species. When wood is exposed to a heat flux, the wood
heats up by conduction and when the surface layer becomes
hot enough, water vapor starts to evolve. As heating pro-
gresses, the surface layer starts to char and other gaseous
volatiles evolve. On large pieces of wood, as the interior
becomes hotter, the pyrolysis effects go deeper, but the
outward flow of volatiles convect heat back to the surface.
Thus, on very large pieces of wood, charring effects are
observed on the surface, which acts as a thermal insulating
barrier.
The hemicelluloses decompose first, then the cellulose,
and then the lignin. The extractables evolve on the basis
of their volatility and reactivity at the higher temperature.
The time-temperature relationship of heating and their pro-
ducts has been extensively studied. The course of pyrolysis
can be presented on the basis of zones of heating in rela-
tion to the temperature applied. The initial heating re-
sults in an endothermic reaction where the gaseous products
are largely non-combustible. Further heating results in an
exothermic reaction (about 280-300°C) with the liberation of
large amounts of carbon dioxide, carbon monoxide, and a
liquid distillate containing acetic acid an4 its homologs,
methanol, and light tars. Further high temperature heating
results in the production of hydrogen and heavy tars.
The pyrolysis of wood yields carbon (charcoal), an
aqueous distillate, an oil distillate, and gases (carbon
dioxide, carbon monoxide, various hydrocarbons, and hydro-
gen) . The products of pyrolysis vary considerably and de-
pend on the tree, the conditions of the reaction, the final
temperature, and the duration of heating. The extensive
laboratory studies (64,65) in the pyrolysis of wood have
been extended to include mathematical models of wood pyrolysis
(66), and a critique of the present state of knowledge (67).
44
-------
Broido and Martin (68), in their study on the addition
of inorganic solids to enhance the tendency of cellulosic
solids to glow and thus minimize the tendency of the flaming
reaction to occur, also analyzed the volatile pyrolysis pro-
ducts of treated and untreated alpha-cellulose samples by
means of gas chromatography and mass spectrometry at two dif-
ferent irradiance exposures. Volatiles measured were carbon
dioxide^ carbon monoxide, hydrogen, methane, ethylene, and
ethane. The alpha-cellulose samples treated with potassium
bicarbonate (flame retardant) produced more of the gases
than the untreated samples.
7h-2 Nortr»fc..n Forest Fire Laboratory has a broad study
in the relative importance of the chemical constituents of
wildland fuels. The wildland fuels differing in mineral com-
positions showed quite dissimilar emission properties (69).
The silica fraction was found to be unimportant when relating
ash content to pyrolysis and ignition of two highly combus-
tible grasses (70).
In the disposal of logging slash by burning, selection
of optimal burning methods must be considered as it relates
to cost, fire control, and air pollution potential. A lab-
oratory simulation of wood pyrolysis under field burning con-
ditions was made in which the thermal environment could be
controlled and the outputs such as specimen weight loss and
particulate production rate could be measured. Thus, various
chemical treatments of fuel with chemicals, such as retardants,
could be studied as possible reductants to air pollution.
On a limited basis, as yet no evidence has been found that
flame retardant treatment under otherwise controlled con-
ditions result in the reduction of air pollutants (71).
On the contrary, a study of two flame retardants on
particulate and residue production (72) indicated that am-
monium sulfate (AS) had little effect on particulate produc-
tion but diammonium phosphate (DAP) produces substantial
45
-------
increases in particulates on burning. The range of particu-
lates produced per unit weight consumed was 8 to 94 Ibs/ton
for DAP, 2.8 to 7.2 Ibs/ton for AS, and about 5.5 Ibs/ton
for the controls where no retardant was used. The fuel was
a mixture of Douglas-fir lumber and ponderosa pine sticks
at an equivalent loading of 227 tons/acre (slash pile).
5.2.2 Combustion
Combustion is the thermal degradation of the material in
air. The volatile vapors escape from the surface and mix
with oxygen yielding a flame if conditions are right. A low
energy fire (270°C or less) undergoes intermolecular dehy-
dration resulting in a phenomenon called glowing combustion
whereby char and water vapor is produced. In the presence
of oxygen, the char sustains glowing combustion with the
final products of carbon dioxide and more water vapor. A
high energy fire (340°C or higher) undergoes depolymeriza-
tion resulting in the phenomenon called flaming combustion
whereby carbon monoxide and hydrogen is produced. In the
presence of oxygen and an ignition source, a highly exother-
mic gas-phase reaction takes place with extensive flaming.
In an intense turbulent wildfire, pockets of combustible
gases develop which can flame violently at heights of several
magnitudes above the combustion zone.
The complete combustion of a forest fuel will require
a heat flux (temperature gradient), adequate oxygen content
(air supply), and a long enough duration of time. The dis-
tribution of forest fuels vertically and laterally, meteo-
rological conditions, and topographic features interact to
modify and change the burning behavior, and thus the fire
will attain different degrees of combustion over the period
of the lifetime of the wildfire.
Pyrolysis and combustion go hand in hand, and the
burning conditions will dictate the proportion of the types
of emissions released into the atmosphere. Glowing combustion,
46
-------
backfires, and intense, dry fires have a tendency to produce
carbon dioxide and water vapor with less particulates, car-
bon monoxide, and hydrocarbons.
The U.S. Forest Service has a publication describing
forest fuels, prescribed fire and air quality. The informa-
tion on the physical and chemical properties of smoke re-
sulting from combustion of forest fuels, primarily wood, is
presented to indicate the complex nature of the emissions
that can be produced (73).
5.3 Measured Emissions
The in"trration search revealed that research in the
emissions produced from forest fuels developed within the
past 15 years. The early investigations covered the area of
burning in an incinerator. As concern for air quality de-
veloped, studies were directed on open field burning of
grasses and stubble, agricultural burning of orchard trim-
mings and landscape materials and the burning of forest fuels.
General articles reviewing the quantities of the emis-
sions produced by forest fuels were available in references
to a single pollutant or to the recognized pollutants as
defined in the Clean Air Act of 1970. These emission values
were summarized in the EPA publications (1,74) and were
first approximations, rough estimates, or small-scale lab-
oratory studies. Efforts to locate real wildfire data were
not successful.
The following data in Table 11 is a compilation of the
information gathered on emissions that were published on
forest fuels or related fuels. All data were referenced on
the basis of actual experimental data, except for the EPA
nationwide values which are composite values of the best
available data and information at that time.
Review articles as well as approximations of emissions
produced by forest fuels were available. Some of the more
47
-------
Table 11
PUBLISHED FJCSSIONS /EMISSION FACTORS (#, #/ACRE. #/TON)
o.
1
2
3
4
5
6
7
8
9
1
1
Geographic
Area
Nationwide
Nationwide
California
California
California
Oregon
Washington
North
Carolina
South
Carolina
Montana
Fuel Type - Quantity
Uncontrolled Fires
4.57 million acres
(145.6 million tons)
ontrolled Fires
3.52 million acres
(76.4 million tons)
Open Burning
-As Agric. Field
-As Landscape
-As Wood
.F. Bay Area
>ay Area
Fruit Primings, 111 MC
Fruit Primings, 351 MC
Native Brush, 51 MC
Native Brush. 131 MC
Fir Chips, 51 MC
Redwood
San Joaquin Valley
Native Brush, Dry
Native Brush, Dry&Green
Native Brush, Green
Lgricultural and Forest
Fuels, Dry
Green Fuels & Wet Dead
Fuels
Grass
Woody Materials
Williamette Valley
Lab-Straw & Stubble
Residue, Grass
Field Studies
Slash-Broadcast Fire
Ground-Stage I
Ground-Stage II
Ground-Stage III
Aircraft-Stage II
Lab-Hemlock
Lab-Douglas Fir
Lab-Red Cedar
Landscape Refuse
(lawn clippings,
leaves, and tree
branches)
Loblolly Pine Utter,
Green Needles
Coastal Plain
Forest Floor
Douglas-Fir Lumber
Ponderosa Pine Sticks
"Average" Forest Fuel
(Litter, Under story,
Crown)
Emission of Interest
Particu ates
itterable
Total
6.7 million
tons /year
17 #/ton
17 */ton
17 #/ton
24 #/ton
16 */toti
11-17 #/ton
10-17#/ton
Ave.
15.6 */ton
Ave.
15.55 tftan
1,150 ng/Bt
1 510 )ig/m-
950 ng/m3
960 ug/mj
2.0 gm/kg
2.3 gm/kg
2.0 gm/kg
17 #/ton
5.5 #/ton
17 */ton
Reactive
30 #/ton
As Unsat.)
2.1 */ton
6.8 */ton
3.0 #/ton
2.4 */toti
0.5 #/ton
0.5 */ton
(As Unsat.)
1.9 #/ton
8.7 */ton
14.2 #/ton
4. "48 #/ton
(Total Olefins)
vdrocarbons
Non-Reactive
166 */ton
(As Sat.)
0.5 #/ton
1.3 */ton
0.5 #/ton
0.6 t/ton
0.1 If/ton
0.1 #/ton
(As Sat.)
0.5 */ton
1.1 #/ton
2.1 */ton
1.74 #/ton
(as Sat.)
30 #/ton (as
methane)
Total
2.2 million
tons/yr
:0 #/ton
20 #/ton
4 */ton
4.2 t/ton as C
9.7 #/ton a C
4.7 t/ton & C
-.4 t/ton a C
2.8 t/ton a C
2.2 */ton a C
4.7 t/ton as C
15.2 #/ton as C
27.4 #/ton as C
4-18 t/ton Ave.
14 */ton
Increases
(as C)
4-19 #/£on Ave.
12.3 #/ton
Ave. 10. 55 */ton
6 PPM
2 PPM
2 PPM
1.2 gm/kg
1.6 gm/kg
2.2 gm/kg
(as C)
55 */ton
L8 */ton
CO
7.2 million
tons /yr
100 #/ton
60 */ton
50 #/ton
600 #/ton
46 if/ton
66 t/ton
65 #/ton
55 #/ton
35 #/ton
70 #/ton
70 #/ton
81 #/ton
134 #/ton
40-140 #/ton
Ave. 92 #/ton
Increases
56-147 #/ton
Ave.
101 #/ton
Ave.
132.2 t/ton
6.5 PPM
13.8 PPM
6.6 PPM
0
38 gm/kg
32 gm/kg
57 gm/kg
140 */ton
Ox
eg
eg.
eg.
eg.
.
NOX or N,
1.2 million
tons/yr(NOx)
2 */ton(NO-)
2 #/ton(NCQ
2 #/ton(NOJ)
.
50 PPM(NO )
at Temp. Peak
29.3 PPM
as(NOx)
4 #/ton(NOx)
10 */ton(N.)
24 */ten(N|)
20 #/toti(N2)
i #/ton(NOx)
C02
2258 #/ton
1995 #/ton
2620 */ton
2394 #/ton
1526 */ton
3742 #/ton
2733 #/ton
1990 #/ton
1528 if/ton
1600 to
2500 t/ton
Decreases
555 PPM
625 PPM
543 PPM
330 PPM
1121 gm/kg
1143 gm/kg
1474 gm/kg
700 #/ton
Ref.
AP-73,
1971 (74)
AP-42
(revised)
1972 (1)
Feldsteli
et al,
1963 (75)
Darley,
et al,
1966(76)
Darley.
et al,'
1972 (77)
Boubel ,
etal,
1969 (78)
Fritscheti
et al,
1970 Cl9)
Gerstle,
et al,
1967 (80)
Detail,
et al,
1970 (81)
Wells,
1971 (82)
Philpots,
1972 (n)
-------
complete reviews with discussion and bibliography were very
helpful in the information search (16,73,83,84,85,86,87).
An overall discussion and review on the emissions, con-
centrations, and fate of gaseous atmospheric pollutants from
sources including forest fires were prepared by Robinson and
Robbins (88).
The particulates produced by forest fires range in size
from the invisible (less than 0.1 M,IH) to large fire brands.
Below 0.1 |im in diameter, the particles are incapable of ef-
fectively scattering light, but may be potentially more un-
desirable d
-------
aircraft was used to collect smoke samples, which were
analyzed for particulate matter and gaseous products. The
results suggest the bushfire smoke is not involved to any
serious extent in the production of photochemical smog over
the Australian continent, but that the yield of particulate
matter from a fire is large, and the build-up of smoke re-
duces vision through the atmosphere (93).
For the fuels of Western Australia, the smoke composition
averaged: tar 5570, carbonaceous residue 257<>, and ash 207<>.
The total weight of solid particulate matter arising from a
typical prescribed fire was 1.5% of the litter quantities on
the ground. Most of the particles were less than 1 iim in
diameter, with the majority of these approximately 0.1 y,m in
size. The larger particles (<. 50 |im) seemed to be agglomer-
ates of a tarry nature. Single tar particles were smooth
and spherical and were usually slightly more than 0.1 |j.m in
diameter, but the other "crystalline" particles were smaller
than 0,1 M-m. The concentration of particles in the thicker
smokes
(93).
r f O
smokes appeared to be between 10 and 10 particles per cm
The natural organic emissions from forest trees have
been reported in the literature. These and the other com-
ponents of the leaves and needles are the most vulnerable
in a forest fire. At the present time, the Southern Forest
Fire Laboratory in Macon, Georgia, is conducting emission
tests on various conifer needles and hardwood leaves. Pre-
liminary results on these fuels, wind tests, and slope
tests have been obtained. Further tests are anticipated
before the results will be available for use.
5.4 Summary of Emissions
From the information search, the available data on
emissions from forest fires were compiled and tabulated.
Analysis of the tabulation indicated that the measured
emissions did not vary greatly in magnitude except for the
50
-------
values obtained for carbon monoxide. Variability existed in
the amount of measured carbon monoxide, since the degree of
the pyrolysis-combustion stage, temperature level, residence
time, and point of sampling all have a direct relation to
the measured value.
Nitrogen oxide production is related to the temperature
of the fire and residence time and is not frequently measured
in a laboratory fire. A general rule of thumb applicable to
particulates from a forest fire would be: the total weight
of solid particulate matter emitted to the atmosphere is
1 to 2% of the original fuel quantities found on the ground.
No investigator has separated the particulate catch from
forest fires into "filterable" and "total" fractions. Thus,
no estimate of these fractions can be reported.
The hydrocarbons were not adequately categorized in the
literature by the researchers on forest fuels. The data
available were for brush and grass residue, and the esti-
mated emissions were 9 Ibs per ton of reactive hydrocarbons
and 2 Ibs per ton of non-reactive hydrocarbons. Until more
data are reported in the literature, only total hydrocarbons
will be reported here.
Discussion with the researchers indicated that the pre-
liminary experimental results of the Southern Forest Fire
Laboratory and the bush fire smoke results obtained in
Australia compare quite favorably with Dr. Darley's numerous
reported results. The emission values taken from Table 11
for use in determining the emission factors were measured and
reported by Dr. Darley and co-workers. Darley's results are
from small samples that were test burned at the University
of California at Riverside. These burning tower studies were
either reported by his group or referenced by others in pre-
paring estimates of forest fire emissions into the atmos-
phere. Darley's data indicate that the burning of forest
fuel results in the following estimated amounts of solid
51
-------
and gaseous emissions: for each ton of forest fuel; one ton
of carbon dioxide, 140 Ibs of carbon monoxide, 24 Ibs of
hydrocarbons as methane, 17 Ibs of particulates, 4 Ibs of
nitrogen oxides, and essentially no sulfur oxides are produced.
Knowing the yield of pollutants emitted from the burning
of a. ton of forest fuels and the amount of fuel consumed by
wildfire per acre for the defined geographic area, the emis-
sion factors can be calculated. The amount of pollutants
emitted by burning forest fuels is relatively constant but
the fuel amount can vary considerably with the geographical
area. For this report, the fuel quantity selected in deter-
mining the emission factor is the amount of fuel consumed
under a set of average fire conditions for that area. This
is usually called the "available fuel".
The emission factors and the total emissions produced
from a forest fire for each geographic region is calculated
from the formulas:
1. Emission Factor (Ib/acre) « Emission (Yield of
Pollutant) (lb/ton) x Fuel Consumed (tons/acre)
2. Total Emissions (tons) = Emission Factor (Ib/acre) x
Number of Acres Burned (acres) x a Constant
(1 ton/2000 Ib)
The emission values presented in Table 11 are the
highest in the range of values reported by Darley, et al.,
for each pollutant. The reason for this was that the most
test fuels and burning conditions were not typical of field
conditions. The field and laboratory studies on grass and
stubble fires correlated well because burning conditions
were closely simulated. Experienced forest fire personnel
indicate that field fires appear "dirtier" than laboratory
fires.
The forest fuel situation, especially for a fire over
five acres in size, would also be radically different from
the laboratory fire/fuel problem since laboratory burning
52
-------
illustrates the diffusion burning phenomena. The fire be-
havior of a large wildfire is difficult to simulate and is
unique and different from a diffuse type of laboratory
fire (94). The heat flux, flaming front, convection forces,
and the greater mass of green vegetation associated with a
wildfire would result in a much higher quantity of pyrolysis
products being generated.
Until measurements are made on a wildfire, the present
reported data are the best available for the development of
emission factors for estimating atmospheric emissions from
forest fires.
53
-------
6. ADJUSTMENT OR CORRECTION FACTORS
After an extensive search of the literature, no reliable
analytical data were obtained that could relate emissions to
fire behavior parameters. Terrain, density of vegetation
coverage, type of vegetation, wind speed, and humidity were
related to fire spread, risk, and danger ratings, etc., but
not to emission values.
Considerable literature on the parameters that affect
the spread of forest fires have been published (95) . Three
criteria were used to measure rate of spread in forest fires:
(a) rate of area growth (acres per hour); (b) rate of peri-
meter increase (feet per hour); and (c) forward rate of spread
of the head or fastest moving portion of the fire (feet per
hour). Comprehensive studies have been made on the rate of
spread of fires in the field and in the laboratory (18,96,97).
The factors that affect the rate of spread are: (a)
weather (wind velocity, temperature, relative humidity);
(b) fuels (fuel type, fuel bed array, moisture content, fuel
particle size); and (c) topography (solar radiation, slope,
profile).
Preliminary studies indicate that fire intensity and
direction of fire relative to wind direction does have an
influence on emissions, whereas fire spread or rate of spread
have an indirect effect on the output of emissions. Fire
intensity and rate of spread are not directly related. The
U.S. Forest Service studies have been guided and directed
for the purposes of learning more about forest fire occur-
rence, prevention, spread, and control. Recently, studies
have been generated on emissions from burning forest fuels
as they are related to air quality. The fire behavior and
spread studies have indicated the interrelationships among
fuel, fuel properties, and environmental conditions in the
laboratory and in the field.
54
-------
Some of these observations are:
1. Rate of spread, flame heights, and fire intensity
increase as the amount of fuel increases.
2, Relatively little smoke is produced in burning dry
fuel in comparison to fires burning in green
vegetation.
3. Changes in wind direction are of major importance
in fire behavior,
4o Dryness of the atmosphere has significant effect
on combustion rates.
5, Steepness of slope, altitude, aspect, position of
fire on slopes and the shapes of mountains and
canyons are all factors of topography which have
an influence on fire behavior.
6. Large forest fires produce great volumes of smoke
which vary in character and color in accordance
with variations in fuels and rates of combustion.
7. Rate of spread of wildfires has been estimated to
approximately double for each 15° increase in
slope, if there is no local downslope wind.
8. Head fires are dirtier than back fires (more
flaming combustion).
9. Back fires are copier and cleaner (more glowing
combustion).
The recognition of the contribution of forest fuel com-
bustion products to air quality has thus initiated studies
to relate the various fire behavior parameters to emissions
output. Our extensive reading of the literature indicates
that the fuel and fuel particle characteristics are the main
factors affecting the quantities and types of emission pro-
ducts produced by wildfires. Topography and wind are in-
directly related to the generation of emission products.
6.1 Fuel and Fuel Particle Characteristics
The physical characteristics of fuels and fuel beds
influencing fire behavior have been studied intensively
(18,98). Besides the fuel type, fuel characteristics of
55
-------
importance are:
1. Distribution of fuel in the fuel bed, by size and
condition (live or dead)„
2. Fuel loading: fuel weight per unit of fuel bed
area.
3. Fuel density: fuel weight per unit of fuel volume.
4. Fuel surface-to-volume ratio.
5. Fuel bed porosity: ratio of fuel bed volume to
fuel volume.
6. Moisture content of fuel.
Many of these characteristics will require further
field measurements to specify the direct contribution of
each factor to the overall average emission value assigned
to the geographic area.
At the present time, only a few preliminary data were
obtained on emissions as they relate to green vegetation,
slope, and wind speed. These preliminary data were obtained
in defining laboratory experimental conditions and as such
were not released for publication. However, study of the
data and related literature indicated that a few conclusions
could be inferred for preliminary use. The study shows that
the burning of fine green fuels produces approximately three
times the amounts of particulates, carbon monoxide, and
hydrocarbons than the combustion of a fine dead dry fuel.
56
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7. RELATED STUDIES
During the course of the program, several related areas
of study were investigated. These studies were used as a
means of obtaining data or a better understanding of the con-
tribution of forest fire emissions to air quality. These
study areas were the design of an incinerator, remote sensing,
and parametric studies.
7.1 Proposed Design of An Incinerator or Burn Chamber
Data will be needed to correlate fire behavior parameters
to actual emissions. The various site visits and literature
search shoved that the burning towers and the laboratory fire
set-ups will require modification for use in determining
emissions from various forest fuels and fuel types under
differing environmental conditions to reflect an open burning
situation. The design must be able to accommodate various
fuel and fuel arrangements, as they relate to slope and wind
speed. At the present time, it is believed that fuel and
fuel properties have a greater influence on the amounts of
emissions produced than either wind or terrain,
The preliminary design will incorporate the wind tunnel,
the environmental conditioning system, moving grates and a
movable flue exhaust hood of the Northern Forest Fire
Laboratory, Missoula, Montana, as described in Rothermel
and Anderson's paper (96); with the design and usage of the
instrumentation at the Statewide Air Pollution Research
Center in Riverside,, California (76). Modification will be
required in all parts of the combined design in relation to
dimensions, laminar flow control3 and the exhaust portion
to obtain open burning conditions in the burn chamber, The
structure should be large enough to incorporate the burning
of complete shrubs, including chaparral.
One of the many problems to be resolved will be the
selection of the proper point or area of sampling^ to assure
57
-------
that interreaction in the convection column (chemical reaction
and physical agglomeration) is balanced by diffusion and
dilution effects. References were made during discussions
with forestry personnel to the unstable state of the emission
products generated during combustion. Small scale combustion
studies show that wood smoke particles vary in kind, number,
and size of agglomerates depending on the sampling point.
Another area of concern is that the gaseous emission products
undergo chemical reaction within the high temperature zone
prior to reaching ambient conditions.
The sampling locations must be selected to obtain a
representative sample of the emission products entering the
atmosphere and still be a physically realistic sample that
can be collected or analyzed by present instrumental capa-
bilities. Suggestions have been made to sample at the top
of the plume, the top of the convection area, and at the top
of the diffuse layer over the fire. Present research studies
may clarify or indicate the optimum sampling location in an
open combustion process.
7.2 Remote Sensing
Because of the difficulty in obtaining access to a
forest fire area, remote sensing of emissions has been the
subject of numerous studies and discussions. Ground-based
remote sensing, airborne sensing, and even satellite sensing
have been referenced in the literature. At the present
time, SOo and NC^ surveys have been accomplished by mobile
crews on land and in small aircraft by use of sensing de-
vices such as the Barringer Correlation Spectrometer and
the newer Cospec II, the Dual-Gas Remote Sensing Correlation
Spectrometer, of the Environmental Measurements, Inc. (99).
Airborne sensing has been utilized with instrumented
light aircraft -- single engine (100) and twin engine (101) --
to study plumes from slash fires and prescribed burning in
this country. Thermal infrared imagery has been utilized by
58
-------
the U.S. Forest Service and others for fire detection, fire
location, rate of spread,and direction of spread (102,103).
For high altitude atmospheric and meteorological studies,
high flying aircraft such as the U-2 have been utilized to
obtain samples. At still higher altitudes, satellite pic-
tures from the Gemini-VII have been taken of forest fire
plumes (104).
These remote sensing methods are potentially useful to
relate forest fire emissions to the fuel source and its
characteristics. The use of remote-controlled drones, heli-
copters, and other ground-related or controlled sampling and
detecting devices will require extensive research and develop-
ment.
7.3 Parametric Study of the Combustion Process
Two conceptual mathematical studies of the parameters
that relate to the wildfire combustion process were made.
These have been included in this report as Appendices B and
C under the names of the respective authors since their
efforts were not completely funded by this project=
The study effort was made to illustrate the complex
nature of the fuel to fire behavior and, possibly, to the
emission output of a forest fire. One approach was based
on data to be obtained while the fire was in progress (by
the fire control officer) and the other uses data to be ob-
tained after a fire (by the fire survey team). However, due
to the shortage of time, the development of both models has
been limited to the investigatory and preliminary stages.
The results were very preliminary, since no concurrent ex-
perimental data were employed. A definite need exists for
experimental data, and these models will hopefully be a
forerunner of other like studies.
59
-------
8. RESULTS AND CONCLUSIONS
Table 12 is a complete summary of the emissions and
emission factors for wildfires for the various geographic
areas and regions. The emission data was obtained from
laboratory fires, burning tower experiments, and prescribed
burning but not from wildfires.
Fuel loadings (fuel consumed) were obtained from each
geographic area or region. The average fuel loading was
indicative of the fuel consumed in a wildfire.
The present state-of-the-art shows that no approved ex-
perimental data exists on the relation of fire behavior para-
meters such as type of terrain, density of vegetation cover-
age, type of vegetation, wind speed, and humidity to the
emission factors. Wind and terrain are important complex
variables, since local gradient winds differ from regional
prevailing winds and their interplay with local diverse
topography, especially in the West, makes it difficult to
evaluate them with respect to their direct effect on the
emissions. Wind provides oxygen but has a cooling effect.
Wind provides mixing but conducts radiation ahead of the
flame front and thus increases pyrolysis products, especially
in a head fire. (As stated previously, a backfire is reputedly
a cleaner fire than a head fire.)
Lastly, the information search revealed that wildfire
emissions have not been measured. Eventually a wildfire
should be sampled for comparison with laboratory data.
60
-------
Table 12
COMPLETE SUMMARY OF EMISSIONS AND EMISSION FACTORS
GeoRranhic Area
I) Rocky Mountain
Group
Northern,
Region 1
Rocky Mountain,
Region 2
Southwestern,
Region 3
Intermountain,
Region 4
2) Pacific Group
California,
Region 5
Alaska,
Region 10
Pacific N.W.,
Region 6
3) Southern Group
Southern ,
Region 8
(Group and
Region are the
same)
4) North Central
Croup
Eastern,
Region 9
(Both Groups
are in
Region 9)
5) Eastern Group
/»{ ..i.
^witn
Region 9)
6) Total United
States
Forest Vegetation of the U.S.
(Aopendix B)
** Acreage
Consumed
by
Wildfire
(acres)
Western Larch-Western White Pine; 774,405
'onderosa Pine-Douglas Fir;
.odgepole Pine; Pinyon-Juniper
'onderosa Pine-Douglas Fir;
Pacific Douglas Fire; Redwood;
'onderosa Pine-Sugar Pine;
Pinyon- Juniper-Chaparral
Oak-Hickory; Oak-Pine;
Longleaf-Loblolly-Slash Pine;
Cypress-Tupelo-Sweetgum;
Chestnut-Chestnut Oak-
Yellow Poplar; Mangrove
Spruce-Fir; Jack, Red, and
White Pine; Birch-Beech-Maple-
Hemlock; Oak-Hickory; Chestnut-
Chestnut Oak-Yellow Poplar
Chestnut-Chestnut Oak-Yellow
Poplar; Birch-Beech-Maple-
Hemlock; Spruce-Fir
351,563
162,795
206,983
Wildfire [
Fuel 5
Consumption
(tons /acre)
37
60
30
10
' 53,064 8
1,161,138
46,941
1,046,542
67,655
1,992,339
1,992,339
232,749
349,000
116,251
4,276,882
19
18
16
60
9
9
11
11
11
17
Pollutants
Emission Factors
^articulate
17 #/Ton*
(#/acre)
629
1,020
510
170
136
323
306
272
1,020
153
153
187
187
187
289
CO H-C ***T NOX
140 #/Ton*]24 #/Ton*|4 if/Ton*
(#/acre) ' (if/acre) (#/acre)
5,180 888
:
8,400 , 1,440
4,200 720
1,400 • 240
i
1,120 I 192
2,660 456
2,520
432
2,240 ' 384
8,400
1,260
1,260
1,540
1,540
1,440
216
216
264
264
1,540
264
2,380
408
148
i-.O
120
40
32
76
72
64
240
36
36
44
44
44
68
** Emissions
Particulate
(tons)
243,550
179,297
41,513
17,593
3,608
187,524
7,182
142,330
34,504
152,414
152,414
21,762
32,632
10,870
618,009
CO
(tons)
2,005,709
1,476,560
341,870
144,887
29,716
1,544,314
59,144
1,172,127
284,147
1,255,179
1,255,179
179,217
268,730
89,513
5.089.490
H-C ***
(tons)
343,836
253,125
58,606
24,843
5,094
264,739
10,139
200,936
48,712
215,173
215,173
30,723
46,068
15,345
872,484
HOX
(tons)
57,306
42,187
9,768
4,140
849
44,123
1,690
33,489
8,118
35,862
35,862
5,120
7,678
2,558
145,414
* Pollutant Yield, Ib pollutant/ton fuel consumed
** Acreage Consumed by Wildfire and Emissions are for 1971.
*** Hydrocarbon as methane
-------
9. RECOMMENDATIONS FOR FUTURE WORK
This section has been divided in two parts: recommenda-
tions to the Office of Air Quality Planning and Standards;
and an overall recommendation to the EPA.
9.1 Office of Air Quality Planning and Standards R&D
9.1.1 Smaller Inventory Units
The present study showed that data is presently being
developed on fuel loadings for various forested areas by the
U.S. Forest Service and the U.S. National Park Service. Also,
the EPA, various state air pollution control agencies, and
forestry commissions have developed rules and regulations
for the reporting of open burning, managed burning, and pre-
scribed burning either by a permit system of by an oral or
written consent to burn. It appears feasible that an inten-
sive search and retrieval of pertinent information relative
to forest fuel burning may extend the development of emission
factors and inventories to smaller political boundaries than
those of a regional size. A study of this nature would have
the following potential benefits:
a.) Knowledge of a smaller unit's contribution to
emissions will tend to minimize the extremes in
variations found in large areas of diverse forest
types.
b.) The areas for which data is either unavailable,
inadequate, or incomplete can be pinpointed for
further studies.
9.1.2 System of Map Overlays
To facilitate recognition and location of various in-
ventory units, a system of transparent colored maps of fuel,
fuel loadings, and the geographic area of interest (state
size) would be prepared for use in pinpointing sources or
areas of high hazard, risk, and pollution potential.
62
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9.1.3 Inventory Needs and Fire Report Forms
Since the field surveys of fuel, timber, fire control and
damage are the responsibility of the U.S. Forest Service, a
cooperative, planned effort should be initiated so that data
on the whole forest fire process from location, fuel type,
fuel consumption, as well as to the emission description,
would be reported. This will develop a forest fire report
that will give data useful to the EPA and the Forest Service.
9.1.4 Field Study
An area for future consideration is a field study of a
real wildfire. The logistics of such a study will be tremen-
dous and will require planning and cooperation. For such a
study, suppression and extinguishment should be a parallel
effort. There are certain forested locations, usually in-
accessible, that may be allowed to burn freely. Another pos-
sibility- would be to sample in shrub country where periodic
burns do occur. In a field study, the top of the convection
column, whether as a plume or a diffuse layer over the fire,
should be considered the sampling site. Some experts con-
tend that sampling near the combustion zone, in the smolder-
ing area, or too far downstream, yields results that are
very different from the emission products and quantities
that actually are entering the atmosphere. To prepare for
a real wildfire, pilot studies should be designed and
formulated to minimize chances of failure.
9.2 General Recommendations
9.2.1 Wood Smoke Composition
Studies have been conducted on bushfire smoke in Western
Australia. An intensive study of wood smoke, identification
of various forest fuel components, chemical composition, and
reactivity in the ambient air (nucleating, scavenging, etc.)
would be studied. Particle size measurement^ as it relates
to distance and temperature from the source would be determined,
63
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The scanning electron microscope and the automatic scanning
ability of the image analyzer would be utilized. The role of
combustion products, specifically particulates from forest fires
in the ecosystem, need to be studied,
9.2.2 Modeling Studies of Emissions from Fires
Two conceptual models have been initiated and presented
in this report. At this time, there is a noticeable lack of
experimental data that could be utilized in modeling. The
usual approach to modeling has been to compile experimental
data for use in preparing a model. Since there was no wild-
fire emission data nor any qualified laboratory emission data
relative to fire behavior parameters, a recommendation is
made to prepare an experimental study such that the minimum
inputs necessary for a model is designated and then obtained
so that only the essential number of experiments and data
are taken. This will optimize the research effort in study-
ing the parameters that relate to the wildfire combustion
process.
64
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67
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68
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Washington, 10 pp.
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duction. U.S. Forest Service Research Paper INT-117.
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and Air Quality. USDA Forest Service, Pacific Northwest
Forest and Range Experiment Station, Portland, Oregon,
44 pp.
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Emissions, 1968. E.P.A. Office of Air Programs Publi-
cation No. AP-73, Research Triangle Park, N.C;, 36 pp.
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Linsky, 1963. The Contribution of the Open Burning of
Land clearing Debris to Air Pollution, J. of Air
Pollution Control Association 13 (11) 542-545, 564.
76. Darley, E. F., F. R. Burleson, E. H. Mateer, J. T.
Middleton, and V. P. Osterli, 1966. Contribution of
Burning of Agricultural Wastes to Photochemical Air
Pollution. J. of Air Pollut. Contr. Assn. 16_, 685-690.
70
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77. Darley, E. F., H. H. Biswell, G. Miller, and J. Goss,
1972. Air Pollution from Forest and Agricultural Burning,
Presented at the Spring Meeting, Western States Section/
The Combustion Institute, Seattle, Washington, 10 pp.
78. Boubel, R. W., E. F. Darley, and E. A. Schuck, 1969.
Emissions from Burning Grass Stubble and Straw. J. of
Air Pollut. Contr. Assoc. 19 (7) 497-500.
79. Fritschen, L., H. Bovee, K. Buettner, R. Charlson,
L. Monteith, S. Pickford, J. Murphy, and E. F. Darley,
1970. Slash Fire Atmospheric Pollution. USDA Forest
Service Research Paper PNW-97, 42 pp.
80. Gerstle, R. W. and D. A. Kemnitz, 1967, Atmospheric
Emissions from Open Burning. J. of Air Pollution
Control Assn. 17, 324-327.
81. DeBell, D. S., and C. W. Ralston, 1970, Release of
Nitrogen by Burning Light Forest Fuels. Soil Sci. Soc.
Amer. Proc. 34, 936-938.
82. Wells, Carol G., 1971. Effects of Prescribed Burning on
Soil Chemical Properties and Nutrient Availability.
Prescribed Burning Symposium Proceedings, Charleston,
South Carolina, USDA Forest Service, Southeastern Forest
Experiment Station, Asheville, North Carolina, 86-97.
83. Murphy, J. L., L. J. Fritschen, and 0. P. Cramer, 1970.
Research Looks at Air Quality and Forest Burning.
J. of Forestry 68 (9), 530-535.
84. Komarek, E. V., Sr., 1970. Controlled Burning and Air
Pollution: An Ecological Review. Tall Timbers Fire
Ecology Conference No, 10, 141-173.
85. Ward, D. E. and R. C. Lamb, 1970. Prescribed Burning
and Air Quality-Current Research in the South. Tall
Timbers Fire Ecology Conference, No. 10, 129-140.
86. Burckle, J. 0. and J. A. Ddrsey, 1968. Air Pollution
and Open Burning in Forestry Operations. Joint Meeting
of the Southern Fire Chiefs and I&E Chiefs, Houston,
Texas, 14 pp.
87. Southwest Interagency Fire Council, 1968. Prescribed
Burning and Management of Air Quality - A Seminar,
Tucson, Arizona, 81 pp.
88. Robinson, E. and R. C. Robbins, 1972. Emissions, Con-
centrations, and Fate of Gaseous Atmospheric Pollutants.
Air Pollution Control, Part II, edited by W. Strauss,
Wiley-Interscience (1-93).
71
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89. Hobbs, P. V. and J. D. Locatelli, 1969. Ice Nuclei from
a Natural Forest Fire. Science 166, 107-108.
90. Schaeffer, V., 1973. Air Quality of the Global Ecosystem.
13th Tall Timbers Fire Ecology Conference, Tallahassee,
Florida.
91. Langer, G., R. F. Pueschel, B. G. Mendonca, and C. J,
Garcia, 1972. Inventory of Ice and Condensation Nuclei
on the Island of Hawaii. Submitted for Publication,
20 pp.
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"Aerodynamic Capture of Particles", edited by E. G.
Richardson, Pergamon Press, London, 89-96.
93. King, N. K., D. A. MacArthur, D. R. Packham, R. J.
Taylor, and R. G. Vines, 1972. Studies on Bushfire
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ment Study Group, North American Forestry Commission,
FAO.
94. Personal communication from Thomas Y. Palmer; Forest
Fire Laboratory, U.S. Forest Service, Riverside,
California, 92507.
95. Chandler, C. C., T. G. Storey, and C. D. Tangren, 1963.
Prediction of Fire Spread Following Nuclear Explosions.
U.S. Forest Service Research Paper PSW-5, 64 pp.
96. Rothermel, R. C. and H. E. Anderson, 1966. Fire Spread
Characteristics Determined in the Laboratory. U.S.
Forest Service Research Paper INT-30, 33 pp.
97. Brown, James K., 1972. Field Test of a Rate-of-Fire-
Spread Model in Slash Fuels. USDA Forest Service
Research Paper INT-116, 24 pp.
98. Countryman, C. M, and C. W. Philpot, 1970. Physical
Characteristics of Chamise as a Wildland Fuel. USDA
Forest Service Research Paper PSW-66, 16 pp.
99. Personal Communication from Lee Langan, Environmental
Measurements, Inc., San Francisco, California 94111.
100. McCaldin, R. 0. and L. W. Johnson, 1969. The Use of
Aircraft in Air Pollution Research. J. -.if the Air
Poll. Coiitr. Assn. 19. (6), 405-509.
101. Adams, D. F. and R. K. Koppe, 1969. Instrumenting Light
Aircraft for Air Pollution Research. J. of the Air Poll.
Contr. Assn. 19. (6), 410-415.
72
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102. Lauer, Donald T., 1970. Rapid Advances in Remote
Sensing Make it Useful Tool for Forester. Candian Pulp
and Paper Industry, January 1970 (46-50).
103. Latham, R. P. and T. M. McCarty, 1972. Recent Develop-
ments in Remote Sensing for Forestry. J. of Forestry,
July 1972 (398-402).
104. Randerson, Darryl, 1968. A Study of Air Pollution
Sources as Viewed by Earth Satellites, J. of Air Poll.
Contr. Assn. 18 (4) 249-253.
73
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Appendix A
LIST OF CONTRIBUTORS
74
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LIST OF CONTRIBUTORS
1) Adams, Donald F.; Washington State University; Pullman,
Washington - 99163
2) Beal, Clay G.; Region 6, U.S. Forest Service; Portland,
Oregon - 97208
3) Beaufait, William R.; Region 1, U.S. Forest Service;
Missoula, Montana - 59801
4) Broido, Abraham; PSW-F & R Expt. Station; Berkeley,
California - 94701
5) Brown, James; Northern Forest Fire Laboratory, U.S.
Forest Service; Missoula, Montana - 59801
6) Cohen, Jack; University of Montana; Missoula, Montana -
59801
7) Cooper, Robert W.; Southern Forest Fire Laboratory,
U.S. Forest Service; Macon, Georgia - 31208
8) Cramer, Owen P.; PNW-F & R Expt. Station; Portland,
Oregon - 97208
9) Darley, Ellis F.; Statewide Air Pollution Research
Center; Riverside, California - 92507
10) Davis, John R.; Rocky Mountain R & F Expt. Station,
U.S. Forest Service; Ft. Collins, Colorado - 80521
11) DeBruin, Henry W.; Division of Fire Management, U.S.
Forest Service; Washington, D.C. - 20250
12) Fosberg, Michael A.; Rocky Mountain F & R Expt. Station,
U.S. Forest Service; Ft. Collins, Colorado - 80521
13) Fritschen, Leo; University of Washington, Seattle,
Washington - 98195
14) Hardy, Charles; Northern Forest Fire Laboratory, U.S.
Forest Service; Missoula, Montana - 59801
15) Heilman, Edward G.; Region 9, U.S. Forest Service,
Milwaukee, Wisconsin - 53203
16) Hurst, William D.; Region 3, U,S. Forest Service;
Albuqueque, New Mexico - 87101
17) James, Howard A.; Bay Area Air Pollution Control District;
San Francisco, California - 94109
75
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18) Komarek, Edwin V.; Tall Timbers Research Station;
Tallahassee, Florida - 32301
19) Koppe, Robert; Washington State University; Pullman,
Washington - 99163
20) Langan, Lee; Environmental Measurements, Inc.;
San Francisco, California - 94111
21) Langer, Gerhard; National Center for Atmsopheric Research;
Boulder, Colorado - 80302
22) Loughlan, R. B.; Ontario Forest Industries Association;
Toronto, Ontario, Canada
23) Lundeen, Charles R.; Region 5, U.S. Forest Service;
San Francisco, California 94111
24) McBride, Robert S.; Region 4, U.S. Forest Service; Ogden,
Utah - 84401
25) Milodragovich, John R.; Region 1, U.S. Forest Service;
Missoula, Montana - 59801
26) Munson, Edward; Monterey-Santa Cruz County; Salinas,
California - 93901
27) Nagamoto, Clarence; National Center for Atmsopheric
Research; Boulder, Colorado - 80302
28) Norum, Rod; Northern Forest Fire Laboratory, U.S. Forest
Service; Missoula, Montana - 59801
29) Palmer, Thomas Y.; Forest Fire Laboratory, U.S. Forest
Service; Riverside, California - 92507
30) Phillips, David; Region 2, U.S. Forest Service; Denver,
Colorado - 80225
31) Pickford, Stewart; PNW F & R Expt. Station, U.S. Forest
Service; Seattle, Washington - 98105
32) Robinson, Gordon; Sierra Club; San Francisco, California -
94104
33) Ruziska, Wayne E.; Region 8, U.S. Forest Service;
Atlanta, Georgia - 30309
34) Sanderson, J.E.; Region 2, U.S. Forest Service; Denver,
Colorado - 80225
76
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35) Steele, Robert; University of Montana; Missoula,
Montana - 59801
36) Wilson, Kenneth 0.; Region 6, U.S. Forest Service;
Portland, Oregon - 97208
37) Yurich, Steve; Region 1, U.S. Forest Service; Missoula,
Montana - 59801
38) Zwolinski, Malcolm; University of Arizona; Tucson,
Arizona - 85721
77
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Appendix B
MATHEMATICAL DESCRIPTION OF FOREST FIRE EMISSIONS
by Dave Becker
78
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MATHEMATICAL DESCRIPTION OF FOREST FIRE EMISSIONS
1. INTRODUCTION
When a forest burns, a large diversity of materials are
consumed: various species of trees, goundcover, d^iff, etc.
By estimating the amount of each of the materials consumed
and its average chemical composition, the total amount of
each freed element can be estimated, especially total carbon.
The question is then, in what ratios do the various possible
combustion products occur: carbon monoxide, carbon dioxide,
saturated hydrocarbons, etc? This question is extemely dif-
ficult and its complete solution would require a detailed
specification of the physical and chemical stages of the fuel,
the meteorological conditions and the detailed history of
each burn. This task would be monumental. The state of the
fuel alone involves the variation of the composition of the
forest with species, geography, season, climate and other
factors. Moreover, the many chemicals released during the
burning process can interact in many ways to produce many
products via many alternate pathways. Additionally, the
description of the burning process itself in terms of physi-
cal parameters is a detailed and not altogether solved
problem - even in principle. With these considerations in
mind, IITRI seeks to establish a model of the emission pro-
cess, which, though crude, would relate some of the average
chemical and physical properties of the evolved emissions to
the available parameters.
In constructing such a model for practical application,
it is essential that the simplifying assumptions yield a set
of equations that only rely on directly obtainable measure-
ments and, perhaps, a small number of free parameters.
Alternatively, the model can contain parameterized experi-
mental data, unsupported by basic theory -- a more pragmatic
approach. Whenever possible, the model should incorporate
the measurements and indices already being taken and calculated,
79
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respectivley, by the Forest Service. Fortunately, the U.S.
Forest Service has been studying forest fires for a long
time, and has accumulated data in standardized format on
several aspects of the fires that are relevant to silvicul-
tural and safety concerns. Several investigators have studied
this data and have established quantified relations among
the various fire aspects (rate of speed, wind velocity, fuel
loading, etc.)* However, these relations contain only half
of the necessary information. They describe the physical
conditions under which the chemical reactions occur. To
formulate an emissions model, the chemical reactions must
be quantified in terms of the physical conditions. There
are many ways in which this can be done, depending mainly
on what aspects of. the emission the model is expected to pre-
dict, and with what accuracy, In this report, the basis for
several different kinds of models -- based on several dif-
ferent degrees of chemical characterization of the emission --
are discussed. It is concluded that owing to a scarcity of
emission data, a desire to frame equations that will be
solvable in the field, and the incomplete current under-
standing of many of the underlying processes; the initial
effort should be concentrated on the most pragmatic approach.
Later, when the understanding is more complete, it will be-
come appropriate to study the more detailed models and to
present the results as a reference table, rather than equations
The following section will describe some mathematical
relations that have been derived from the available fire
models. These relations will be used to parameterize the
emission conditions and the chemical reactions. Following
that section, additional sections will discuss, in detail,
several different chemical models.
Some of the relations are founded in models; others
are only "curve fits".
80
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2. FIRE - PROCESSES AND PHYSICAL PARAMETERS
2.1 Time Dependent Behavior of the Emissions
Before beginning the mathematical treatment, consider
the time history of a given fuel element that is consumed in
the fire. As the fire approaches it, its temperature grad-
ually rises from that of the ambient atmosphere to that re-
quired for thermal decomposition. At the thermal decomposi-
tion threshold, the fuel element gradually begins to evolve
gas. The distribution of chemicals within that gas can be
well predicted from pyrolysis data at that temperature, as
there is no excess of atmospheric oxygen within the fuel
element. At that preignition temperature, the gas is not
further oxidized and the pyrolytic chemical distribution is
the same as the emission chemical distribution to the atmos-
phere. As the fuel element and its surroundings reach the
ignition temperature, the chemical composition of the emitted
gases is changed from the pyrolytic distribution by oxidation
in the flame. Moreover, on leaving the flame, the reaction
products are subject to coalescence and agglomeration.
These processes are primarily restricted to the time period
during which the products remain within the plume. There,
inter-emission particle collisions are most probable. On
emerging from the plume, the gases and particulates disperse
throughout the atmosphere as before. Figure B-i is a flow
diagram that depicts the processes described above.
The major point to be recognized is that the emission
characteristics of a forest fire vary with the time history
of the burning of each fuel element. Moreover, the burn
history varies greatly from fire to fire and is difficult to
predict because it is dependent upon some factors (especially
the geometry of the fuel arrangement) that are difficult to
measure with sufficient accuracy, in practice. Thus it is
proposed that the time history of the burn be treated as an
observable itself.
81
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Fuel Element
1
Fraction
Emitted at Pre
Post Ignition
Temperature
(TO
Fraction Emitted
at Highest Achieved
Temperature (T2)
Fraction Emitted
at Stable Burning
Temperature (T3)
Pyrolysis at
Pyrolysis at
Pyrolysis at
Oxidation in the
Flame (T2)
Oxidation in the
Flame (T3)
Agglomeration in
the Plume
Agglomeration in
the Plume
Dispersion Throughout the
Atmosphere
Figure B-l
COMBUSTION CALCULATION STAGES FOR A SINGLE FUEL ELEMENT
82
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Figure B-2 depicts the time histories of the burning of
three hypothetical fuel elements (a,b,c,). In some cases,
such as b, conditions (such as high fuel water content) pre-
clude open flames. In such cases, the fire may smolder for
many hours, or even days, until the fuel loading is dissi-
pated. In other cases, such as a or c, a well defined period
of maximum temperatures is present and is characterized by
large flames. In these cases, though the maximal temperatures
are present only for relatively brief periods, much of the
fuel is consumed during those periods, owing to the increased
fuel consumption rate with increased temperature. However,
since higher temperatures are conducive to higher oxidation
efficiency, a relatively greater fraction of carbon dioxide
and water are produced. In comparing the amounts of emissions
produced during the pre-ignition, the full-ignition and the
post-ignition periods, it is clear that, in general, none of
the periods can be ignored. The emission characteristics of
the three periods are roughly compared in Table B-l.
Table B-l
RELATIVE EMISSION CHARACTERISTICS OF THE THREE PERIODS
OF THE BURNING. OF A FUEL ELEMENT
Rate of Fuel
Consumption
Time
Duration
Combustion
Efficiency
Resultant
Emissions
Pre-ignition
Low
Short to
Long
Low
Low to
High
Full-Ignition
High
Short
Medium to
High
Low to
High
Post-Ignition
Low to
Medium
Medium to
Long
Low to
Medium
Low to
High
83
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0)
*J
4->
-------
To quantify the emission dependence on burn history for
each fire, records should be kept of the time duration of the
fire within the three above named burn periods; pre-ignition,
full-ignition and post ignition. The horizontal bars in
Figure B-2 indicate how these durations would be defined in
actual cases. Though this approximation is crude, it is
consistent with the accuracy to be expected from field data,
and, on that basis, it does not compromise the validity of
the model.
In succeeding sections, methodology will be described for
the calculation of emissions from the fire. These sections
will treat a fire as a constant temperature event. That is,
the flame will be treated as being of spatially and tempor-
ally uniform temperature, within its boundary. It is to be
understood that the uniformity is to apply to each of the
three burn periods independently, and that total emissions
are always found by summing the emission contributions of all
three periods. The temperatures of the three periods will
always be denoted herein as T, , T«, T~ in the order of pre-,
full-, and post-ignition; or as T; in general. The durations
of the periods will similarly be t,, t«, to, respectivley,
or t; in general.
2.2 Relative Significance and Parameterization of
Various Fuel-Related Factors
The leftmost column of Table B-2, which appears on the
following page, details 11 of the most significant factors
that determine the chemical composition of the emissions.
The second column of the Table lists those (of three) fire
behavior parameters to which each factpr relates. The
third column of that Table contains estimates of the relative
significance of each of the factors for each of the fire
behavior parameters. A "1" signifies high significance and
"2" and "3" signify progressive lesser levels of significance.
It is recommended, at least initially, that effort be con-
centrated on only the most significant factors.
85
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Table B-2
FACTORS AFFECTING THE CHEMICAL PROPORTIONS OF EMISSIONS
Factor
Wind
Rain/Humidity
Season of Year
Time of Day
Topography
Time History of
Burn
Control Measures
Applied
Fuel Chemical
Composition
Fuel Diameter -
Ave. and Dist.
Fuel Density
Fuel Moisture
Content
Fire Behavior
Fire Temperature
Fuel Mixing
Fuel Residence Times
Fire Temperature
Fuel Residence Times
Fire Temperature
Fuel Residence Times
Fire Temperature
Fuel Residence Times
Fuel Mixing
Fuel Residence Times
Fire Temperature
Fuel Mixing
Fuel Residence Times
Fire Temperature
Fuel Mixing
Fuel Residence Times
Fire Temperature
Fuel Residence Times
Fire Temperature
Fuel Residence Times
Fire Temperature
Fuel Residence Times
Fire Temperature
Fuel Residence Times
Order
of
Effect
1
1
1
1
1
1
1
2
2
3
1
2
2
1
1
1
2
3
3
3
2
2
3
3
1
1
1
1
1
1
86
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The three fire parameters, their definitions, and the
motivation for choosing them are:
Fire Temperature - The average temperature in and imme-
diately around the flame, if any; otherwise the hottest
fuel temperature. This parameter relates directly to
both the chemical and the physical aspects of the prob-
lem. Chemically, reaction rates are selectively depen-
dent on it, and physically it is determined by the fuel
factors.
Fuel Mixing - The degree to which the vaporized fuel is
dispersed with oxygen in the flame. Though it is almost
always true that there is more than sufficient oxygen
present to completely oxidize the fuel, frequently the
fuel forms pockets or streamlines that exclude the
oxygen and preclude complete combustion in the flame.
Inadequate mixing is a major cause of incomplete combustion.
Fuel Residence Times - There are really two fuel resi-
dence times of interest, but one of them relates more
to the completeness of oxidation and the other to the
size distribution of particulates. The former is the
amount of time that a typical fuel vapor element spends
within the physical flame. That is, the time between
vaporization and emergence from the top of the flame.
The other residence time is the amount of time that
the fuel vapor element spends within the plume. Note,
this definition of Fuel Residence Times is not the same
as that of the Forest Service. The two parameters may
be related, but not on a one to one basis.
Thus, for example, beginning at the top of the Table,
the first factor, "wind", has an effect upon the temperature
of the fire, the degree of mixing of the vaporized fuel with
the ambient air, and the residence times of the vaporized fuel
within the flame and the plume. More specifically, wind af-
fects the fire by several different mechanisms, among which are:
• Wind carries heat away from the flame by mass trans-
port thus lowering Fire Temperature.
• Wind increases the burning rate by increasing the
Mixing within the flame and increasing the oxygen
supply.
© Wind changes the geometry of the flame and the fuel
transport rate which, in turn, changes the residence
time and the fractional oxidation.
87
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Similarly, all of the effects of all of the other listed
factors can be described in terms of these three basic para-
meters and the effects which the various factors have on
them. Hence, IITRI believes that the physical characteristics
of the fire -- insofar as they affect the chemical composition
of the emission -- can be adequately described by these three
parameters.
2.3 Calculation of the Three Fire Parameters
The calculation of explicit values for the fire parameters
for a given fire should be based, as much as possible, on the
standard measurements and indices of the Forest Service. This
can be readily achieved for the fire temperature parameter.
The residence time parameter can be determined via any of
several alternate methods. However, they will require some
new, but simple, measurements. The mixing parameter is ex-
tremely difficult to estimate, but, fortunately, it is be-
lieved to be the least variable of the three parameters.
2.3.1 The Fire Temperature Parameter
The fire temperature parameter can be adequantely deter-
mined in terms of the indices of the forest service. The
average pre-ignition temperature can be estimated by using
the ignitability index (1) and by assuming that the fuel is
close to the ignition temperature. The full-ignition tem-
perature can be expressed as a polynomial in the Energy
Release Component (ERG) (1) of the Burning Index. However,
these expressions require that the burning forests be clas-
sified according to the Forest Service Fuel Models (1).
2.3.2 Residence Time Parameter
To determine, on the average, how long an element of
gaseous fuel remains inside of the flame, two quantities must
be determined: the average height of the flame and the
average velocity of the gas through the flame. Fortunately,
both of these quantities can be expressed in terms of other,
88
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more readily measurable quantities, though direct observa-
tion of flame height and gas velocity is certainly feasible.
A recent Forest Service Publication (2) gives a deriva-
tion for the vertical gas velocity in terms of the plume
height. Starting from hydrodynamic equations, ignoring
coriolis forces, the vertical geometrical structure of the
volume, and smaller corrections, the following relation is
derived.
1/2
V = 2.6Z ' .» vertical gas velocity in meters/second
Z = height in meters
o
Another author gives a semi-empirical expression for
the height of the flame (3).
L = 18.6(Rpb'h) /J = flame height in meters
h = depth of the fuel bed in meters
p, ' = bulk density of the burned fuel
R = Rate of spread of the fire in meters/second.
Also, the same author gives an empirical expression for
the rate of fire spread under wind conditions, namely:
RPb' - k (1 + U)
*
where k = .07kg/m for all wildland fires
U = wind velocity
Thus, L « 18.6 Ck'Cl +U)h]2/3 - (18,6)(.07)2/3»
[(1 + U)h]2/3 - 3.16 [(1 + U)h]2/3
and the residence time in the flame is given by:
*- 1.22
tp -
89
-------
Similarly, the residence time of the emission pro-
ducts in the plume is given by the height of the
plume divided By the gas velocity
'
t = - -, ;A = "''• = *.385Z in seconds
p 2:. 6 Z 1/Z
o
i f'.,
2.3.3 Fuel Mixing Parameter
To date, IITRI has not found a suitable expression for
fuel/oxygen mixing, and, bne'may riot yet exist in a form
simple enough to be useful (4,5). However-, several authors
have discussed the problem under the heading "Microdiffusive
Combustion;", and more- work will be required to elucidate the
treatments. It is most likely feasible' to treat the effect
of incomplete mixing as. constant, or to .parameterize it in
terms of flame length and fuel temperature. Either way, it
seems that while the effect of mixing is of major importance,
its variation from case to case is minimal. -One check of
this idea can be found in observing smoke plumes. Since the
plumes usually have the same general" appearance, almost in-
dependent of the size of the fire, and since the visual
structure of the plume is almost always strongly suggestive
of streamline flowj the microscopic flow properties -- includ-
ing microdif fusion --are, also, probably 'nearly independent
of the size of the fire.
2 .4 Procedure in Calculating .the Emissions from a Fire
The following procedure proposes, steps to be followed
in calculating the emissions from the fire. The point
wherein chemical calculations enter the treatment is indi-
cated, but detailed discussion of those calculations does not
begin until Section 3. , __;- --.r>._- . v '
A. Measure the time duration (ti_, t?, to) of each of
the three major, -periad's'-of *the .fire (buildup,
peak, die out) . '.
90
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B. Characterize the flame temperatures (Tls To, T3)
during each of these three periods via either
1) the U.S. Forest Service Energy Index, 2) the
Ignitability Index, 3) direct evaluation, 4) after
the fact evaluation -- such as largest diameter
trunk burned, rock burned . . . natural scale.
C. Measure or estimate the Fuel Density (p), and the
depth of the fuel bed (h) and the wind speed U.
D. Measure the height of the plume (Z ) and estimate
the vertical gas velocity via °
V = 2.6 Z 1/2
o
E. Characterize the flame height during each of the
three burn periods via either 1) direct observation,
2) formula;
Flame height L = 3.16 [(1 + U)h]2/3
or 3) fire indices.
F. Calculate the residence time in the flame as
t- - 1.22 [(1 +_U)h]2/3 _ L
tf - 1/2 V
o
G. Either estimate the fractional burn of each of the
fuel types during each of the periods of the burn
or measure the total fractional burn for the entire
fire, ft, and use pyrolysis weight loss rate
graphs, as functions of T and t^ to estimate the
contribution of each period.
H. Either proceed to I or L for stepwise analysis or
overall parameterization, respectively.
I. Use the known Tj_, or To, 13 to evaluate the P
(Pyrolysis) matrix -*• depending on the particular
model chosen.
J. Use the known TI, T2,=tT3 and ti, t2, t3 to sequen-
tially evaluate the 0 matrix depending on the
particular chemical model chosen.
K. The fuel mixing coefficient will temporarily be
taken to be constant.
L. Solve for the emissions by multiplying.
91
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3. INTERFACING THE PHYSICAL MODEL WITH A CHEMICAL
MODEL-OPTIONS
Several kinds of information are required to predict
the degree to which oxidation and chemical reaction take
place in the flame:
o the concentration of each of the reactants,
o the distribution of the reactants
o the flame temperature, and
o the amount of time during which the reactants are
held in proximity of each other.
To specify the data requirements and procedures in
greater detail requires that the desired output be specified
in corresponding detail. The principal question is "How
should the emissions be characterized to yield a model that
is both field workable and yet yields useful numbers?"
Since there is a consensus among reporting authors that
nitrogen oxide formation is negligible (6) except in rare
extremely intense fires -- and since the sulfur content of
the forest is very small (7) -- the focus turns to the many
carbon compounds, and how they should be characterized.
Table B-3 details some of the possibilities that were ex-
plored, along with some of the associated advantages and
disadvantages of each.
The following subsections will discuss each of these
options in greater detail. However, before proceeding to
those discussions, there is one more consideration that must
first be covered -- the mathematical approach to the model.
Basically, there are two major types of approaches that may
be fruitful for quantifying the emissions: one that traces
the stepwise generation/reaction of the chemical species,
and another that does not look at the details but instead
quantifies the entire process in terms of a complete base
set of parameters. To be more explicit, the expression
92.
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Table B-3
OPTIONS IN CHARACTERIZATION OF CARBON COMPOUNDS
Option
a. Average molecular wt. of the
carbon chains
b. Fraction of carbon chains
above or below a given
molecular wt.
c. Distribution of the molecular
wt.
d. Fraction of the project that
has unsaturated bonds;
fraction that are saturated
e. Actual distribution of all
chemicals present
Advantage/Disadvantage
No structure information,
calculation ease
No structure information,
calculation ease
No structure information,
calculation ease
Some structure information,
calculation ease
Most complete structure
information, calculation
very difficult
would be:
where
C-O'P-F
E represents the emissions
C represents the effects of the plume
0 represents the effects of the flame
P represents the effect of pyrplysis
F characterizes the fuel
The variables E, C, 0, P and F could be scalars, vectors
or n-dimensional matrices depending on the precise formula-
tion of the problem (for instance, the options of Table B-l).
However, the order of the factors, in this approach, is al-
ways the same. The point is that this approach describes
the process sequentially and is based on the equations that
93
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describe each process within the sequence. When all the pro-
cesses are known and are describable, this approach is ideal.
When the practical difficulty, in using this kind of model
becomes large, it is still at least desirable to verify the
model by performing some calculations with it. However, it
is unlikely that this form of model could yield sufficiently
simple equations as to be useful in the field. More likely,
it would be solved on a computer to yield easy-to-use
reference tables.
The alternate approach is to quantify the entire process
in terms of a complete base set of parameters. If we assume,
temporarily, that the effect of microdiffusion (fuel mixing)
is roughly independent of the particular case, and that break
down of forest fuel by species is not essential, then the
emission process can be adequately characterized by three
parameters; the fire temperature and the two residence times.
Thus,
E. (Tf, t tf) - t.
where
E is the mass release rate per unit fuel element for
any emission product, and
E. is a function dependent only on:
tf = fuel residence time in the flame
t = emission residence time in the plume
T£ = temperature of the flame
t. = duration of fire period
Now, however, instead of trying to calculate E by exam-
ining the composite processes, proceed directly to emission
data and treat the data as a three dimensional (t^, t , T^)
curve fit for each chemical emission product that is to be
studied. This curve fit can be done by any of several methods:
regression analysis, linearization, Taylor Expansion, etc.
94
-------
If it turns out that it becomes necessary to treat individual
forest fuel species separately, then E. becomes a matrix, but
the methodology remains the same. In either case, the resul-
tant description will be sufficiently simple that, in prac-
tice, the emissions could be calculated in the field.
(Probably simple polynomials in the three parameters.)
Thus, in conclusion, the stepwise calculation approach
yields the theoretical functional dependences and allows com-
parison of each step with auxiliary data, but yields equa-
tions that are time consuming to solve. The overall para-
meterization does not treat the steps independently but does
yield simple, consistent equations that summarize the data
in terms of the three parameters.
3.1 Complete Chemical Description of the Fire and
Emissions
Within this subsection will be discussed all the calcu-
lations and procedures that will be necessary to completely
model the chemical emissions from the fire. While IITRI does
not recommend that this option now be carried to completion,
it will be discussed in great detail because all of the
other options to be presented are restricted versions of
this one. Both approaches (stepwise analysis and overall
parameterization) will be discussed, and the former will
require more discussion than the latter.
3.1.1 The Overall Parameterization Approach
In this most complete of the IITRI models, the fuel is
characterized by specie in a vector*, each position of which
indicates the number of pounds of one specie consumed during
one of the three time periods (i = 1, 2, 3) of the fire,
per acre. The calculation of these consumption figures is
done in accordance with Procedure G. in Section 2.4. Thus,
* Actually an N-Tuple.
95
-------
F. = (f1s f9, £....£,) where k = number of species
1 x z J • to be considered
f. » pounds of specie j
^ consumed per acre
during time period i
For each of the k species, a separate regression relation
will exist for each of the emission products. Thus, for
instance,
EC02 " *C02/elm (tf V 'f> '
by the consumption
of elm trees during
one of the time
periods
Thus, the total emission factor for COo per acre for
the entire fire is the sum of the matrix products:
3 ,
F m T V (t" f- t- ^ • F
C02 i-1 fi> pi' fl i
The determination of the regression relations for the
overall parameterization method in this form would thus re-
quire measuring, for instance, the CO^ output of each of
the forest species separately over a varied range of condi-
tions. Since this is impractical -- an, most likely, unneces-
sarily detailed for the needs of the current program -- less
exact descriptions of this form will be considered in the
following subsections.
3.1.2 The Stepwise Analysis Approach
The stepwise analysis approach follows the flow diagram
presented as Figure B-l in Section 2.1. Beginning with the
fuel, the characterization proceeds as in Section 3.1.1. The
vector F is calculated, and, as before, it represents the
number of pounds of each specie consumed during each of the
three time periods (i = 1, 2, 3).
The first step in the combustion process is the volati-
lization and partial decomposition of the fuel molecules by
96
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pyrolysis. The dependence of the chemical composition of the
pyrolysis breakdown products on the species of the fuel and
the temperature will be retained in this most complete level
of description. Similarly, the chemical identity of each of
the significant chemicals produced will be retained. Since
pyrolysis data has been accumulated for many species of trees
at many temperatures (8), there should be little problem in
at least approximating its specie and temperature dependence.
Thus,
— » = — <
C = P • F
— *
where: C = (C-^, Z^* ••• cs ••• CM) and each C is one of the
N significant chemicals; P = a K X N matrix each of whose
elements describes the temperature dependence of the produc-
tion of one of the chemicals from one of the K species of
fuel.
The next step is the combustion itself, and is the most
complex step in the sequence. To discuss this step will re-
quire a brief digression into chemical kinetics (9) . The
rate of a first order, oxidation reaction can be expressed
in the form:
• exp [-Ers/RTJ • [0] • [Rs] for the
reaction 0 + R_ -» 0 • R0 Decomposed and
s s
oxidized products.
where :
= molar concentrations of oxygen and the
s ' th reactant, respectively
E ' = activation energy for the s'th reactant
going to the r'th product
R = molar gas constant
Z = a relatively insensitive variable containing
information on the geometry of the reacting
volume and the geometry of the reactants.
The microdif fusion information should be
placed within this variable.
97
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If Z and [0] are constant, the equation can readily be
s
solved
[R]s L"J CAF L rs'
S
,
-Zo [0] exp -E /RT dt'
Rs(t)
[0] exp [-Ers/RTj
Thus, the fraction of R that reacts in a short time
interval can be readily calculated, provided that the time
interval is sufficiently small that only a small fraction of
the total population changes and that the oxidation products
do not themselves interact, but only oxidize. Under these
circumstances, which IITRI believes to hold well for the
entire flame, the change in the distribution of chemical
products over a short time interval t, can be expressed as:
C (t + At) = 6 • C(t)
where:
C is the chemical distribution vector defined as before
0 is a matrix that describes, in terms of the activa-
tion energies and temperature, the decomposition
fractions and accumulated decomposition products
At < the mean reaction time for the shortest
lived reactant.
To find the chemical distribution of the emissions that
emerge from the flame, the 0 matrix must be applied m
times, where m is defined by:
m = "At
That is, the 0 matrix must be used a sufficient number of
times to fully account for the gases' entire residence time
in the flame. Thus, the total chemical emissions (CT) can
be described by: t
(T^) • P • F
98
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The size distribution of the particules would then be
calculated by applying the standard particle growth equations*
to the emergent chemical distribution, and by assuming that
the agglomeration processes are restricted to the residence
time of the emission products within the plume, t **.
The obvious advantages of this type of treatment of the
problem are that it characterizes the emission products in
great detail, and that it identifies the processes that are
responsible for each of the emission characteristics. However,
in view of:
1. the lack of many experiments that determine the
factors Zg, and the virtual impossibility of
calculating them;
2. the imprecision in the data ultimately to be col-
lected and evaluated for forest fire air quality
impact calculations; and
3. the involved calculational procedures required to
use the model, as presented above;
IITRI recommends that, at least initially, the model be sim-
plified to a level more appropriate.
The first approximation to the model that IITRI recom-
mends involves the characterization of the fuel. Since many
different species have cellulose and lignin that is virtually
identical, and since these are the major components of the
wood, it would be advantageous to group the forest growth
into a small number of categories, specifically:
• Hardwoods
• Softwoods
• Resinous shrubs
• Non-resinous shrubs
* See Notes A, B and C.
** See Section 2.3.2.
99
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The water content differences within these breakdowns
would be treated explicitly, but the chemical composition
would be assumed uniform.
The next approximation involves the activation energies
E . Since the emissions are almost exclusively carbon com-
ITS
pounds, excluding water, and since the bonds that are to be
broken are virtually all single or double carbon-carbon bonds,
assume that E has only two values, one for all single bonds
i o
and another for all double bonds. This approximation will
not compromise the accuracy of the model significantly.
Next, assume all of the factors Z to be constant. This
s
approximation will introduce some error, but since there are
only limited suitable data available for analysis, there is
little choice. IITRI expects that while this approximation
is serious, it will still be consistent with the accuracy of
the data.
The final set of approximations involves the character-
ization of the emissions. For the purposes of the EPA, it
may not be necessary to completely specify the chemical com-
position of the emission. For instance, in terms of air
quality it makes relatively little difference whether straight
chain saturated hydrocarbons are present as 3, 4, or 5 carbon
chains. Even for the organic molecules with ring systems,
it is usually more important to know the concentration of
ring molecules than to keep a separate record of each type.
Several of the possible chemical characterization schemes
have already been presented in Table B-3. Now each will be
discussed in greater detail.
3.2 Molecular Weight Characterization of Emissions
3.2.1 Stepwise Analysis
One way that the emissions can be characterized is by
their molecular weight (MW), or, since they are all composed
mostly of carbon and hydrogen, by the number of carbon atoms,
100
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C . The emissions can then be conveniently described in
terms of a distribution function. Suppose that, within the
accuracy of typical forest measurements, the C numbers
•XL
follow a gaussian distribution
-(C - C )2
N(C ) = N exp a 2a a— = fraction of the number
a ° of molecules in the
emission that have
exactly Ca number of
carbon atoms
where:
C" = average number of carbon atoms in a chain
a = parameter descriptive of the width of the
distribution
N = normalizing factor
Then, using this distributional form, the chemical characteri-
zation information is concentrated into three parameters:
a, N , and C . To describe how these three parameters evolve
o oc
in time is thus to describe the distribution.
Fortunately, these parameters have properties that make
their calculation straightforward. The initial values of
C (t=o), N (t«o), and C"a (t=o) are calculated from the
initial distribution produced by pyrolysis. The calculation
then centers on how the flame changes these parameters.
There are several ways to proceed at this point, but the
following is one of the simplest, and it can be used to
describe the others. Temporarily, suppose that all of the
carbon-carbon bonds in the emissions have the same bond
strength*. Then, except for geometrical factors, the prob-
ability of reacting at any one bond is roughly equivalent
to the probability of reacting at any other. Moreover, the
probability of the reaction occurring at any one site, during
* Either picture all bonds as being single bonds or assume
that the single and double bonds have roughly equal
strength.
101
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the short time interval t, can be explicitly calculated.
Using the result of Section 3.1.2,
= Z exp (-E/RT) [0] At
o
setting R/y = 1/2 yields the median time of reaction, for
L. ^ ' J -iTiof- Q-rtr* c* •
instance:
(At) = Jin [1/2] exg
where
E = common activation energy for all bonds
Z » [0], are defined as in Section 3.1.2
R(t) is taken to represent the number of carbon-carbon
bonds .
But, the concentration of oxygen is roughly the same for all
fires, so
(At) - In [1/2] exp (-E/RT)
where
Z [0] and is roughly a constant
When half of the bonds have reacted, on the average, Ca will
have been halved. So, in general:
(tf) « Ca (o) exp -tf Z' e-E/RT
The dependence of o (t) can also be approximated, o (o)
is known from the initial distribution of pyrolysis products.
However, it is also known that if the average number of car-
bons per chain becomes equal to 1, then o must equal o.
That is, since carbon atoms are discrete, an average of one
carbon per molecule means exactly one carbon per molecule.
Then, £• (t) _ 1
CT = a (o) • ~ -
Ca (o) - 1
102
-------
Since the total area under the N(C ) curve must be
conserved, as no carbon atoms are created or destroyed, the
dependence of N on t^ follows by normalization.
Thus, using this version of the model one can readily
predict the average MW, the fraction of emission above or
below a given MW, and the total distribution of MWs. More-
over, the concepts herein developed are not limited to
gaussian distributions. They can be applied readily to any
distribution that is parameterized in terms of mean values.
One interesting variation would be to define either two
distribution functions or two groups of chemicals. One func-
tion, or group would contain all molecules with at least one
unsaturated carbon bond and the other would contain the satu-
rated ones. Then, by following the time evolution of each,
the final emissions could be characterized according to the
degree to which it is saturated. Naturally, each would
evolve differently as the saturated bond activation energy
differs by about 20% from that of the unsaturated. This
option is especially interesting because the unsaturated
bonds are associated with photochemical smog. Thus, a
minimal amount of structural characterization could provide
sufficient information for some impact analysis.
3.2.2 Overall Parameterization
The approximation ideas of Section 3.1 can also be ap-
plied to the overall parameterization method. Thus, since
T£ and T. specify the emissions, along with the fuel charac-
terization — consider for instance:
-a (tf T> = S • F
where:
F is as defined before, except that it now has only
4 elements.
103
-------
S is also a 4 element vector, each element of which
depends on tf and T; and each is fit via an
independent regression relation.
Similarly, separate functions could be defined for
saturated and unsaturated bonded molecules. These are much
more tractable equations than those of Section 3.1.1.
104
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4. CONCLUSIONS AND RECOMMENDATIONS
The mathematical description of the emissions of a burn-
ing forest is a complicated problem. However, by making ex-
tensive use of Forest Service methodology, restricting ques-
tions to the larger aspects of emissions, and neglecting some
secondary effects, the problem can be managed.
As a next step in the development, IITRI recommends
testing the parametrical relations on actual fire data. The
various alternative methods of treatment of the problem vary
greatly in the amounts of effort that they require for solu-
tion. Also, the predictions of any of these models can be
expected to be no better than the data that is fed into them.
Thus, before investing effort in a complete version of the
model, it is probably best to try the overall parameteriza-
tion method, followed by a statistical study of those results,
Those results can be checked against the general predictions
of one of the more detailed models and either verify its es-
sential correctness or indicate where changes will be needed.
Moreover, the statistical analysis will also indicate the
situations under which additional data will be most urgently
needed. Since the amount of data available is small, the
analysis should not be lengthy -- but the results will be
correspondingly uncertain.
105
-------
NOTES
A. Decrease of Particulate Number Density with Time (10)
i . i=Kc and ^-. -Kn2
where:
n = number density at t' =0
n = number density at t' = t
K = experimental constant for a given geometry,
temperature and pressure
B. Increase of Average Particle Volume with Time (10)
a = aa + Kt
where:
a = average particle volume at t' =0
o « average particle volume at t' ** t
C. Particle Formation and Growth Rate (10)
-u TT r 1 )
v 3 KT /
Z(r) - C(2P)
where:
2P = interparticle collision probability
M B interparticle collision probability
P - particle mass density
j =» surface tension
K = Boltzman Constant
T o Absolute Temperature
r - particle radius
Z = the number of particles growing to the radius r',
per second
106
-------
REFERENCES
1. National Fire Danger Rating System; Deeming, John E.;
Lancaster, James W.; Fosberg, Michael A.; Furman,
R. William; and Schroeder, Mark J.; USDA Forest Service
Research Paper RM-84, February 1972, 165 p.
2. Slash Fire Atmospheric Pollution; Fritschen, L.; Bovee,
H.; et al.; USDA PNW-97, 1970.
3. Rates of Spread of Some Wind-Driven Fires; Thomas, P.M.;
Forestry 44 (2) p 155-175, 1971.
4. Diffusion and Heat Transfer in Chemical Kinetics;
Kamenetskii, D.A.F.; Plenum Press, London, 1969, p. 434.
5. Combustion Engineering; Fryling, Glenn R.; Combustion
Engineering Inc., 196b.
6. Release of Nitrogen by Burning Light Forest Fuels;
Debell, D.S.; Ralston, C.W.; Soil Science Society of
America Proceedings, Vol. 34, p. 936-938.
7. Sources. Abundance and Fate of Gaseous Atmospheric
Pollutants; Robinson. E.; Robbins. R.C.; Final Report.
SRI Project PR-6755, February 1968.
8. The Chemistry of Cellulose and Wood; Israel Program for
Scientific Translations, Jerusalem, 1966.
9. Diffusion and Heat Transfer in Chemical Kinetics;
Frank-Kamenetskii, David A.; Plenum Press, New York,
1969, 574 p.
10. Particulate Clouds; Dusts. Smokes and Mists; Their
Physics and Physical Chemistry and Industrial and
Environmental Aspects; Green. H.L. and Lane, W.R.;
Van Nostrand, 1964, 471 p.
107
-------
Appendix C
CONCEPTUAL MODEL OF PYROLYSIS-COMBUSTION OF FOREST FUELS
by Arthur Takata
108
-------
CONCEPTUAL MODEL OF PYROLYSIS-COMBUSTION OF FOREST FUELS
1. PYROLYSIS AND FIRE CONDITIONS
In this section we are concerned with assessing the
state of knowledge in regard to the generation of pyrolysis
products both during and following passage of the flame front.
Of primary concern here is the determination of the amounts
of volatile fuels distilled from wildland fuel during the
passage of the front. Following discussions of means for
predicting these emissions, the section will culminate with
a discussion of the convection columns to which these
emissions will be exposed.
To expedite these endeavors, considerable liberties have
been taken in simplifying analyses to afford practical pre-
dictive schemes. In this regard, semi-empirical approaches
were formulated based on the best sources of information --
t
whether they be experimental or analytical. Only by such
techniques, can one achieve the results desired.
109
-------
2. ESTIMATES OF DEGREE OF PYRQLYSIS
In order to accurately predict pollutant emissions, one
must first appreciate the quantities of pyrolysis produced
which depend on:
• amounts, sizes,and types of fuels
• rate of fire spread
o rates and degree to which fuels are consumed
In the remainder of this section, we shall consider each of
the above factors in the order presented.
2.1 Fuel Array Models
Since it is not practical to accurately measure the
sizes and quantities of wildland fuels, one must resort to
some approximate approach. In view of the similarities of
fuels, the most direct way is to categorize different types
of fuels into several model arrays with the differences
selected on the basis of fire behavior and on comprehensive
coverage of fuel arrays. Such categories have and are being
developed as part of the National Dire Danger Rating System
and presently include eleven fuel models. These models are
described in Reference 1 and are distinguished from each
other according to type, loading and sizes of fuels. Such
models represent the most practical means for rapid descrip-
tion of fuel arrays and are particularly useful for assessing
pollutant emissions produced by past wildland fires.
2.2 Prediction of Rates of Fire Spread
Rate of fire spread represent one of the most inade-
quately analyzed parameters required. For example,
Rothermel (1), considers that the rate of fire spread equals
the ratio of the "propagating heat flux" divided by the
energy required to ignite a unit volume of the fuel. Similar
approaches have also been used by Thomas (2) and by
Woolliscroft (3). Perhaps the most disturbing feature of
110
-------
these analyses is the neglect of the fact that the flux
various with distance beyond the fire front.
A more detailed spread criterion is afforded by the
model of Albini (4) which assumes that the total outgassing
(moisture and fuel vapor) is directly proportional to the
incident flux. Unfortunately, this approach does not con-
sider the sensible heat absorbed both prior and during
pyrolysis. In addition, the analysis does not provide for
the heat reradiated or convected away from heated fuels as
the outer layer of wood heats. This heat loss could be
quite large. Finally, none of the above analyses deals
with the very difficult problem of predicting flame propaga-
tion through variable mixtures of fuel vapors and air.
These observations and the fact that the experimental
and theoretical spread rates usually differ by a factor of 2
or more (5,6) emphasizes the need for more careful work.
Unfortunately, as indicated by the paper of Hottel (7), the
problem of predicting the rate of fire spread has not been
resolved for the case of radiant/connective heating, let
alone for the case of firebrands.
Until better predictive means are available for pre-
dicting rates of fire spread, it is recommended that one
predicts the rate of fire spread by using estimates of the
burn times and of the area burnt. If the estimates prove
too uncertain, it is recommended that one use the above
estimates or spread rates for similar fuels and conditions,
to assess whether the spread rate will be typically low,
average or high as defined below:
9 Typical low speed (12) =6.2 ft/min.
9 Typical average speed (see below) = 21.0 ft/min.
9 Typical high speed (12) =71.0 ft/min.
111
-------
The average speed here was calculated so that it was equidistant
percentage wise between the extreme speeds. High-spread rates
are associated with fine-grass and fine-grass mixtures and
high winds, while low speeds are associated with slash-type
fuels with low winds. To treat cases for which burn data are
not available, it is recommended that one use the predictive
scheme of Rothermel (1).
2.3 Factors Affecting Pyrolysis Ignition and Burning
of Wildland Fuels
On the basis of the work of Kilzer and Broido (8), it
has been found that cellulose (which constitutes the bulk of
dry wood) breaks down upon heating in the following five
steps:
1) Generation of noncombustible gases (^0, traces of
CO?, formic and acetic acids, and glyoxal) up to
200<5C.
2) Dehydration of cellulose to "dehydrocellulose"
between 200 and 280°C. Reactions are endothermic
and products are almost entirely nonflammable.
3) Depolymerization of cellulose between 280 and
340°C, resulting in the formation of volatiles.
4) Decomposition of dehydrocellulose into gases and
char residue via an exothermic reaction that
becomes dominant at about 320° C.
5) Vigorous oxidation of charcoal above 500°C.
Most of the volatiles are produced between 300 and 400°C (9).
The fact that the activation energies associated with the
pyrolysis of cellulose are appreciably higher in air than in
nitrogen (10) indicates that the rate of pyrolysis will dif- '
fer somewhat depending on the amount of excess air. However,
the effect is not large enough to receive further consideration.
Basically, there are three ranges of wood temperatures
involved in spreading fire and in sustaining burning. These
are (11):
112
-------
1) Flame point: wherein the decomposition gases will
burn if an external ignition source is present --
225 to 260°C.
2) Burning point: wherein the decomposition gases
will burn spontaneously without an external
ignition source -- 260 to 290°C.
3) Flash point: wherein wood will ignite spontaneously --
330 to 470°C.
To assess the conditions for sustained burning, items (1) and
(2) are of critical importance -- item (2) because it indi-
cates the temperatures that must be achieved by at least a
portion of the fuel or other media if burning is to be sus-
tained; and item (1) because it indicates the minimum tem-
peratures that far out fuels must achieve to participate in
the burning.
Involvement of fuels in fire occurs in two steps: the
first being the initial heating of fuels forward of the
flame front; and the second being the subsequent involvement
of these fuels in flames. Most of the emissions produced by
heated fuels forward of the flame front will eventually enter
the convection column.
2.4 Estimation of Degree of Pyrolysis
Recognizing that fires can be treated in two stages, one
of flaming and one of smodering, we will first discuss means
for assessing the quantities of fuel vapor driven from wood
during the passage of the flame front. In this regard one
can approach the problem either from an experimental or
analytical basis. Here, we shall discuss both approaches.
t
2.4.1 Approach Based on Experimental Results
One method for appraising the amounts of pyrolysis pro-
ducts is to first assess the heat release rates and then
develop means for relating the two. In this regard Rothermel (1)
summarizes data needed to predict the heat release rates per
unit area of fuel bed. These results are presented in
113
-------
Figure 13 of Reference 1 for various packing ratios of
excelsior, and for 1/4" and for 1/2" wood cribs. Determina-
tions of the heat release rates for intermediate size fuels
requires interpolation. Extrapolation to larger size fuels
may be had by using the analytical approach discussed in
Section 2.4.2. Data needed to account for the presence of
moisture and mineral content are documented in Figures 7 and
8 of Reference 1.
The only drawback with this method is that it does not
provide any thermal description of the wood after flame
passage. This, of course, makes application to mixed fuels
difficult and is important to predict smoldering reactions
after flame passage. Overall, this approach appears to be
the most satisfactory of the approaches presently available
even though it requires data or additional analysis relating
the heat release rates and amounts of pyrolysis products.
The primary advantage of this approach is that one can take
advantage of existing heat release data and thereby circum-
vent the very complex problem involved with fuel arrays.
2.4.2 Preliminary Analysis of Degree of Pvrolysis
Another method for predicting pyrolysis is to analyze
the effect of thermal fluxes on the fuels. Here we shall
briefly suggest the kind of analysis we have in mind. Since
the evaporation of water largely controls the temperature
profile through wood, let us assume that a quasi-steady-state
condition exists in which heat absorbed at the surface of
the fuel is conducted in a steady-state fashion into the
depth at wh,ich water is being evaporated. This assumption
is considered reasonable in that only shallow depths are
involved and is satisfactory for interim determinations of
the surface temperatures. Also we shall neglect variations
of the flux over the surface of the fuel. As a result:
q - ?-c-T4 = K(T-Tb)/x (C-l)
114
-------
where:
q = flux absorbed by fuel surface
? = emittance of surface of fuel
a = Stefan-Boltzmann Constant
T = absolute temperature of surface of fuel
K = thermal conductivity of fuel
T, = absolute temperature of fuel at depth of boiling
given by x
x = depth beneath surface of fuel at which water is
being evaporated
If § is taken as 1, and K is taken as 0.1 Btu/ft-hr-°F, then
it is possible to evaluate the surface temperature T as a
function of the depth x for various values of q. To deter-
mine the depth of pyrolysis for various fluxes, exposure
times and moisture contents of wood, one additional equation
is necessary describing the effect of the incoming flux
(q - 5-cT'T ) on the depth of dehydration. To assess the
effect of the incoming fluxes, we shall neglect the rela-
tively small quantities of heat absorbed beneath the depth
at which water is being distilled as well as the heat
expended in vaporizing fuel vapors. Subject to this sim-
plifying condition, the heat AQ required to drive the water
from the depth x to x + Ax is approximately:
FT+T.
(C-2)
AQ - p-Ax
+ (T -
where:
o
p = density of dry wood, Ib/ft
x = depth of dehydration, ft
T = surface temperature, °R
115
-------
T, = temperature of boiling water, °R
T = initial temperature of wood, °R
Cf = specific heat of dry wood, Btu/lb-°R
M^ = ratio of weight of water of hydration to weight
of dry wood, dimensionless
Qv = latent heat of vaporization of water, Btu/lb
GW = specific heat of water, Btu/lb-°R
C = specific heat of steam (constant pressure),
s Btu/lb-°R
Substituting:
p = 30 lb/ft3
Cf = 0.46 Btu/lb-°R
Qv - 970 Btu/lb
T, - 212 + 460 - 672°R
D
C -1.0 Btu/lb-0R
w
C =0.5 Btu/lb-0R
S
into Equation C-2 and letting TQ = 70 + 460 = 530°R results
in:
AQ - 30 Ax [(0.23 + 0.5 Mf) T + 776 Mf - 89.2] (C-3)
Here AQ varies with time and can 'be found from Equation C-l
as follows:
AQ = (q - S-CT-T4) At (C-4)
By integrating Equations C-2 through C-4 over time, it is
possible to estimate the depth of dehydration as a function
of q, exposure time, and moisture content M1^, The results
of this endeavor are displayed by Figures C-l and C-2 for
fluxes of 10,000 and 20,000 Btu/ft2-hr, respectively.
In each of the two figures, estimates a-je given for the
depths of char (defined as the depth within which temperatures
116
-------
0.10
0.08
u
eg
o
0 0.06
a
a
•o
CD
" 0.04
M
w
0.02
Note: I I
Results apply^to fires with fluxes of
10,000 Btu/ft -hr and are intended only
for purposes of guidance. Char assuir.ed
to occur at temperatures > 260°C.
Properties for wood taken as:
P
C,
K
= 30 lb/ft3 (dry)
- 0.46 Btu/".b-°F
=0.1 Btu/ft-hr-°F
M
M
10 20 30 40 50 60 70
Exposure Time of Wood to Fire, Sec
Figure C-l
ESTIMATED DEPTHS OF CHAR AS FUNCTION OF MOISTURE9CONTENT AND EXPOSURE TIMES
FOR FLUX OF 10,000 BTU/FT -HR
80
-------
0.10
oo
0.08
0.06
OJ
O.
S 0.04
1
4J
w
0.02
Results apply to fires with fluxes of
20,000 Btu/ft^-hr and are intended only
for purposes of guidance. Char assumed
to occur at temperatures > 260°C.
Properties for wood taken as: —
p = lb/ft3 (dry)
K
30
0.46 Btu/lb-°F
0.1 Btu/ft-hr-°F
0 10 20 30 40 50 60 70 80
Exposure Time of Wood to Fire, Sec
Figure C-2
ESTIMATED DEPTHS OF CHAR AS FUNCTION OF MOISTURE-CONTENT AND EXPOSURE TIMES
FOR FLUX OF 20,000 BTU/FT -HR
-------
exceed 260°C) as a function of exposure time to the stated
fluxes for various moisture contents of the wood. Here Mf
refers to the ratio of the weight of free water to the weight
of dry wood. Exposure times (commonly termed reaction time)
associated with rapidly spreading fires are of the order of
several seconds (12) while exposure times associated with
slowly spreading figures are of the order of several tens of
seconds (12). Additional information pertaining to exposure
times are available in Reference 1.
Fires with the flux of 10,000 Btu/ft -hr used in
Figure C-l are typical of most wildland fires while fluxes
of 20,000 Btu/ft -hr are typical of large fires. Considering
that the exposures times are usually less than 100 sec, it
may be seen that volatile fuels will be evolved from layers
of the order of 0.1 in. or less during flame exposure.
To appreciate the consequence of the above results, it
is necessary to consider how much of the volatile fuel will
be driven out of char. Such data are illustrated in Table C-l.
Since we have used the arbitrary temperatures of 260°C in
defining the lower boundary of the char, the data in Table C-l
indicate that 60 percent or more of the wood within the
layers of char illustrated by Figures C-l and C-2 would be
pyrolyized. Activation energies and frequency constants for
the first-order decomposition of wood indicate the above
reactions are culminated in a matter of a few seconds (17)
after the wood achieves the stated temperatures. The above
analysis represents a preliminary examination of the problem
of predicting the amount of pyrolysis produced during passage
of the flame frant. Thin fuels such as grass would be
totally consumed. In order to achieve accurate results,
more detailed analysis along with experimental checks are
requires.
Next, it is important to consider emissions produced
after the passage of the flame front, namely emissions released
119
-------
by residual burning of incompletely pyrolyi?ed fuels and by
oxidation of char. While analysis can he used to predict
such emissions, it is highly complex in that it requires pre-
diction of the degree of oxidation or reaction of volatiles
during passage through burning char. This problem can best
be resolved by chemical analyzing the gases and particulates
produced experimentally by a variety of glowing fuel arrays
under various realistic combustion conditions. Such experi-
ments would also be valuable in indicating the extent to
which the pyrolysis and oxidation continues for each of the
fuel situations.
Table C-l
DECREASE IN WEIGHT OF WOOD ON HEATING*
Heating
Temperature,
°C
160
170
180
190
200
210
Weight Loss Heating
(Volatile Products Temperature,
of Distillation), % ' °C
2.0
5.4
11.4
18.0
22.9
26.9
220
230
240
250
260
270
Weight Loss
(Volatile Products
of Distillation),!
32.5
44.6
49.2
51.3
58.8
62.9
Results presented in Chapter XXV in book by N. I. Nikitin.
120
-------
3. DESCRIPTION OF CONVECTION COLUMNS
In order to predict the consequence of combustion on the
products of pyrolysis, one must appreciate the types of en-
vironment through which the products will be exposed. Of
concern here are the temperatures, residence times (or flame
length and speed), mixing, and oxygen content associated with
the convection column. To this end, we shall first discuss
the applicability of fire modeling.
Modeling of fires involves the attempt to scale para-
meters such as gas density, heat input and distance so that
one can use measurements from small well-defined fires to
predict the characteristics of large-scale fires. Much work
has been conducted in this regard (13,14,15). Most models
assume a given rate of heat release over a given area of
ground and use dimensional analysis to determine how the
characteristics of the convection column vary with the
variables. The result is scaling laws to translate data
from one fire to another according to fire intensities and
shape of burn area. Because of the complex phenomena in-
volved in the formation of convection columns, care must
be exercised in using scaling laws. Whenever possible,
scaling laws should be checked against full-scale test data.
Lengths of flame L, of course, have a direct bearing
on residence time and are frequently predicted using the
following expression:
L . C-Q2/3 (C-5)
where:
C = constant of proportionality
Q = rate of fuel or heat release per unit length of
fire front
In order to better appreciate the accuracy of this relation-
ship, field test data were taken from References 12 and 19
121
-------
and plotted as shown in Figure C-3. Here we have fitted the
data with curves assuming the flame length is proportional to
2/7 1/3
Q /J and to Q ' . Two points should be observed. The first
is that the data are not consistent. This is not at all
unusual based on IITRI's experiences with large liquid-fuel
9
fires (400 to 2000 ft ) which are much more reproducible
than wildland fires. Secondly, it should be observed that
somewhat better agreement is had by relating flame length to
I/O 9/0
Q ' than to Q . This result also agrees with the results
of fires from circular pools of liquid fuels (18,19).
These liquid fuel baths ranged in size from tenths of
centimeters to a few thousand centimeters and covered all
flow regimes from laminar to turbulent. Over these regimes
1/3
the ratio L/Q ' fluctuated by +50% without an abvious trend,
2/3 —
while the ratio L/Q ' constantly decreased with fire size
by almost two orders of magnitude.
The effects of wind on flame length may be seen by the
five data points of Figure C-3 for which wind data are avail-
able. For these data wind does not appear to have a pro-
nounced effect on flame length. Therefore, at least for the
present, it is recommended that the flame length be predicted
without consideration of wind as follows:
L = 0.13'Q1/3 (C-6)
where:
L = flame length in meters
Q = rate of heat generation per unit length of fire
front, cal/cm-sec
From IITRI's experiences with liquid-fuel fires, flame
temperatures will peak within a few feet of the fuel and then
gradually decrease with height. However, because such tem-
peratures fluctuate widely with time, it is difficult to
estimate the residence times associated with the flame tem-
peratures. In view of this fact, it is recommended the flame
122
-------
5.3
5
to
14
V
u
JC
4J
00
c
0)
CO
2/3
0.0079 Q meters
Wind Velocities given in
cm/sec .
0 4000 8000 12000 16000
Rate of Heat Generation per Unit Length of Fire Front, Q, cal/cm-sec
Figure C-3
FLAME LENGTH VERSUS RATE OF HEAT GENERATION
-------
temperature be considered constant at least until more de-
tailed information becomes available. In this regard,
References 12 and 19 present the mean temperatures for nine
tests. These are 1055, 1050, 1080, 940, 1050, 1070, 1010,
1080, and 1070°C for an average of 1045°C (1913°F). While
this mean temperature seems somewhat high, its use is re-
commended until more detailed data are available.
124
-------
REFERENCES
1. Rothermel, R. C., "A Mathematical Model for Predicting
Fire Spread in Wildland Fuels, Intertnountain Forest and
Range Experiment Station Research Paper INT-115, 1972.
2. Thomas. P. H., "Rates of Spread of Some Wind-Driven
Fires/' Forestry 44(2), 1971.
3. Woolliscroft, M. J., "A Report on Forest Fire Fieldwork,"
Joint Fire Research Organization Fire Research Note
No. 744, 1969.
4. Albini, F. A., "A Physical Model for Firespread in Brush,"
llth Symposium (International) on Combustion, 1965.
5. Woolliscroft, M. J., "A Report on Forest Fire Fieldwork,"
Joint Fire Research Organization Fire Research Note
No. 693, 1968.
6. Brown, J. K., "Field Test of a Rate of Fire Spread Model
in Slash Fuels," Intermountain Forest and Range Experiment
Station, Forest Service U.S. Department of Agriculture,
Ogden, Utah, January 1972.
7. Hottel, H. C., C. Williams and F. R. Steward, "The
Modeling of Fire Spread Through a Fuel Bed," Tenth
Symposium (International) on Combustion, 1965.
8. Kilzer, F. J., and Broido, A., "Speculations on the Nature
of Cellulose Pyrolysis," Pyrodynamics 2, 151-163, 1965.
9. Lipska, A. E., "The Pyrolysis of Cellulosic and Synthetic
Materials in a Fire Environment," U.S. Clearinghouse Fed.
Sci. Tech. Inform. AD-645858, 1966.
10. Kato and Takahashi, N., "Pyrolysis of Cellulose. II.
Thermogravimetric Analyses and Determination of Carbonyl
and Carboxyl Groups in Pyrocellulose," Agr. and Bio. Chem.
31(5):519-524, (Tokyo), 1967.
11. Kollmann, F., "Occurrence of Exothermic Reactions in Wood,1
Holz als Rohund Werkstoff 18, 1960.
12. Woolliscroft, M. J., "A Statistical Analysis of Some Rates
of Spread of Forest Fires," Fire Research Note No. 732,
October 1968.
13. Lee, B. T., "Mass Fire Scaling with Small Electrically
Heated Models," U.S. Naval Radiological Defense Laboratory
Report for OCD on Work Unit 2536F, March 1969.
125
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14. Scesa, S. and Sauer, F. M., "Possible Effects of Free
Convection on Fire Behavior -- Laminar and Turbulent
Line and Point Sources of Heat, U.S. Dept. of Agriculture
Report 417 for Armed Forces Special Weapons Project,
September 1954.
15. Parker, W. J., Corlett, R. C. and Lee, B. T., "An
Experimental Test of Mass Fire Scaling Principles,"
U.S. Naval Radiological Defense Laboratory, Report
WSCI 68-14, San Francisco, California, April 1968.
16. Blinov, V. I. and G. N. Khudiakov, "Certain Laws Governing
Diffuse Burning of Liquids," Acd. Navik, SSR Dohlady, 113,
1094 - 1098, 1957.
17. Browne, F. L., "Theories of the Combustion of Wood and
its Control," Forest Products Laboratory Report No. 2136,
Forest Service, U.S. Department of Agriculture,
December 1958.
18. Hottel, H. C., "Review of Certain Laws Governing Diffuse
Burning of Liquids," Fire Research Abstracts and Reviews,
Vol 1, 2, January 1959.
19. Woolliscroft, M. J., "Notes on Forest Fire Fieldwork
(New Forest, 1967), Fire Research Note No. 740, February
1969.
126
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BIBLIOGRAPHIC DATA
SHEET
1. Report No.
EPA-450/3-73-009
3. Recipient's Accession No.
4. Title and Subtitle
Development Of Emission Factors For Estimating
Atmospheric Emissions From Forest Fires
5. Report Date
October 1973
6.
7. Author(s) George Yamate
IIT Research Institute
&• Performing Organization Kept.
No.
9. Performing Organization Name ajjd Address
TIT Research institute
10 West 35th Street
Chicago, Illinois 60616
10. Project/Task/Work Unit No.
11. Contract/Grant No.
68-02-0641
12. Sponsoring Organization Name and Address
U. S. Environmental Protection Agency
OAQPS, MDAD, NADB
Research Triangle Park, N. C. 27711
11 Type of Report & Period
Covered c on 70
14.
15. Supplementary Notes
16. Abstracts j^s ^p^ C0ntains emission factors (weight of pollutant per acre burned)
for estimating atmospheric emissions from forest fires (especially wildfires) for each
cf the ten U. S. Forest Service regions in the U. S. The pollutants considered are:
total particulates, hydrocarbons, carbon monoxide, nitrogen oxides, and sulfur oxides.
Data on acreage consumed By wildfires are used with the factors to estimate mass emissio
for each reoion. The effects of such variables as terrain, density of vegetation covera
type of vegetation, wind speed, and humidity are also discussed. Finally, proposed
approaches to mathematically correlate these variables' (via empirical and theoretical
models) with both emission factors and mass emissions are presented.
is
17. Key Words and Document Analysis. 17a. Descriptors
Air Pollution, Forest Fires
17b. Identifiers/Open-Ended Terms
17c. COSATI Field/Group
13B
18. Availability Statement
Release.,Unlimited. .Copies,May Be Obtained From:
Air Pollution Technical Information Center
Room 253, Chemstrand Building, Research Trianale Jart
19.. Security Class (This
Report)
AINCLASSIEIED^
20. Security Class (Tnis
Page
UNCLASSIFIED
21. No. of Pages
129
22. Price
FORM NTIS-35 (REV. 3-72)
£.11 \
U SCOMM- D C,,14 »S2-57
127
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FORM NTIS-9S (REV. 3-72) USCOMM-DC I49S2-P72
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