ABSTRACT
Bush fire is a common hazard in South Eastern Nigeria as in other parts
of the country during the harmattan. Every year, thousands of hectares
of forests as well as suburban lands are severely burnt. These forest
fires have been catastrophic, destroying large areas of tropical rain
forests. However, some trees in these forests have proven to be fire
resistant. These tree species were identified by local people.
Flammability studies of fifteen of these fire tolerant trees of South
Eastern region of the country were carried out with a view to
encouraging their use in afforestation. The flame characteristics, viz.,
ignition time, flame propagation rate, after glow time, flame duration,
char formation, and limiting oxygen indices of these tree species were
carried out. In addition, physical properties such as densities and
moisture contents were evaluated in order to determine their effects on
their burning parameters. The values for these parameters vary among
the selected tree species. Density in particular, related to the ignition
and flame spread behaviours, although, for the denser hard woods above 0.50g/cm3 and this dependence is less straight forward. Attempts
were made to explain these observations on the basis of thermal
conductivities, cellular structures, and presence of special flame resistant
pyrolysates at flaming temperature.
TABLE OF CONTENTS
Title Page – – – – – – – – – i
Approval Page – – – – – – – ii
Dedication – – – – – – – – – iii
Acknowledgements – – – – – – – iv
Abstract – – – – – – – – – v
Table of Contents – – – – – – – vi
CHAPTER 1
INTRODUCTION – – – – – – 1
1.1 Background – – – – – – – 1
1.2 Fires: A Historical Perspective- – – – – 2
CHAPTER 2
LITERATURE REVIEW – – – – – – 8
2.1 Pyrolysis- – – – – – – – 8
2.2 Combustion – – – – – – – 14
2.3 Pyrolysis and Combustion – – – – – 20
2.4 Mechanism of Flame Retardancy – – – – 21
2.5 Fire Extinguishment- – – – –
2.6 The Burning Cycle – – – – – – 22
2.7 Tree Characterization – – – – – – 24
2.8 Fire Tolerance – – – – – – – 40
2.9 Factors that contribute to fire tolerance in Timbers- – 41
2.10 Flammability Testing – – – – – – 43
2.11 Scope of Work – – – – – – – 48
2.12 Objectives – – – – – – – – 48
CHAPTER 3
8
EXPERIMENTAL – – – – – – 49
3.1 Materials and Methods – – – – – 49
3.2 Research Methodology – – – – – 54
3.3 Characteristics of the Samples – – – – 56
CHAPTER 4
RESULTS AND DISCUSSIONS – – – – – – 61
4.1 Oral Interview – – – – – – – 61
4.2 Effect of Density – – – – – – – 61
4.3 Effect of Moisture Content – – – – – 67
4.4 Burning Behaviours – – – – – – 67
4.5 Flame Propagation Rate – – – – – 68
4.6 After Glow Time – – – – – – – 70
4.7 Ignition Time – – – – – – – 73
4.8 Flame Duration – – – – – – – 75
4.9 Ash Formation and Glowing Combustion – – 76
4.10 Limiting Oxygen Index – – – – – – 79
4.11 Conclusions- – – – – – – – 80
REFERENCES – – – – – – – – 81
APPENDICES – – – – – – – – – 91
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND
The use of dry natural wood in building, construction and for furniture is
well established[1]. Wood is generally regarded as possessing a high
degree of combustibility when sufficient quantity of heat energy is
applied[1]. It can be ignited by a variety of fire sources and once ignited
the flame may spread rapidly across the surface with slower progress
through the bulk until the fire becomes general. This is the cause of
significant numbers of injuries and fatalities in fires reported yearly by
various countries[2-4]. Because of this, and within the African continent,
the phenomenon of combustion in terms of pyrolysis and flammability
has been the subject of extensive studies directed towards three primary
interests: building and contents, forest and grassland fires[1].
In its broadest sense, the performance of wood in fires can be described
in terms of three distinct burning phenomena namely ignition, flamming
and glowing, which present different potential hazards, and should be
approached in different ways. In the past, many studies of the thermal
decomposition of cellulose or lignocelluose have been reported [5-10].
The ignition properties of cellulose materials have been reviewed and
discussed in various publications [11-13]. Nonetheless, it is surprising
that little literature is available on the thermal characteristics of tropical
timbers. There is no doubt that in the near future, the wood resources of
the temperate zones cannot support an increasing wood demand. The
vast resources of the tropical rainforests should therefore become of
10
decisive interest for future timber and forest planning, renewal and
development.
1.2 FIRES: A HISTORICAL PERSPECTIVE
Fire or flame, simply put, is a region of hot gases raised to
incandescence [14]. This definition implies that the burning material,
which are in most cases polymers, (compounds having very high molar
mass) such as cellulose, plastics, rubbers, etc, must be able to supply
gases that burn.
Man began to emerge from the cave when he learnt to use fire. The
acquisition of the skill to use and control fire and its products is no
doubt, one of the most fundamental inventions. The eventual spread of
man across the Earth’s surface is directly linked with his ability to make
and control fire. This is so because, otherwise, man would have been
restricted to live only in areas with a hospitable warm climate. The
importance of fire in the human experience is attested by the fact that it
has been observed in every human culture of the recent past [14].
It is reasonable to state that the advent of the match in 1680 when
Robert Boyle used white phosphorus to ignite sulphur-tipped wood
splints was a huge success for fire making[13]. An improvement on this
came in 1827 when John Walker developed a match which was ignited
by drawing its head between layers of sand paper (the friction match)
[13]. Finally the safety match, these days a common household item,
was perfected by the Swedes in 1855[13].
Fire is a natural environmental phenomenon and has been an integral
part of our eco-system for millennium. The population and development
of North America has repeatedly brought humans into contacts with fire
11
in all manner of circumstances including wildland fires. Over the past
400 years [15], Americans as a society have grown to gear all forms of
fire and have sought ways to suppress it as completely as possible.
Wildland fires, however, pose unique challenges to the fire service and
require vastly different approaches to its prevention, mitigation, and
suppression. As more people choose to leave the cities and build their
homes in the “wildland/urban” interface, it is critical that these concerns
are addressed.
Natural wildland fires are generally caused by lightning, which strikes the
earth an average of 100 times each second or 3 billion times every
year[16] and has caused some of the most notable wildland fires in the
United States (e.g. Yellowstone in 1988). Other natural causes include
sparks from falling rocks and volcanic activity.
Human activity, however, is the primary cause of wildland fires. Some
of these fires are intentional, such as those that were used by Native
American as signals or to drive game, those set by forestry experts, or
those set by arsonists. Others, however, have been accidental, caused
by carelessness or inattention by campers, hikers, or others traveling
through wildlands [17]. Table 1 illustrates the 10-year average of fires
by their cause and average burned.
Table 1: 10-year average of Wildland fire causes (1788-97) [18]. Human cause Lightning cause Number of fires 102,694 13,879 Percent of fires 88 12 Acres burned 1,942,106 2,110,810 Percent of acreage 48 52
The three primary classes of wildland fires are surface, crown, and
ground. These classifications depend on the types of fuel involved and
intensity of a fire. Surface fires typically burn rapidly at a low intensity
12
and consume light fuels while presenting little danger to mature trees
and root systems. Crown fires generally result from ground fires and
occur in the upper section of trees, which can cause embers and
branches to fall and spread the fire. Ground fires are the most
infrequent type of fire and are very intense blazes that destroy all
vegetation and organic matter, leaving only bare earth [19]. Large fires
actually create their own winds and weather, increasing the flow of
oxygen and “feeding” the fire [18].
There is a dichotomy associated with wildland fires; they threaten
resources we value, yet they are an essential part of most ecosystems.
Several plant species even depend on it to reproduce. Some pinecones
require fire to melt away a resinous coating before the seeds inside are
released, while others produce seeds that lie dormant in the seedbed
and germinate only after exposure to the heat from a fire [16].
Recovery from a wild land fire begins even before the last of the flames
are extinguished. After a wild land fire, essential nutrients are released
back into earth through the burning of mature plants and organic litter
[16]. Additionally, wildland creatures have learned to adapt to fires.
Small animals generally hide in burrows, birds fly away, larger mammals
run away and fish are protected by the water in which they live. These
animals are capable of adjusting to radical changes in their habitat,
which endure until the next fire in that area [17].
Erosion is a critical concern as heavy rains in the wake of wildland fire
can cause landslides or debris flows, and run off can have damaging
effects on water sources. In some areas, if the fire was intense enough,
the soil actually becomes hydrophobic and cannot absorb water,
exacerbating the situation [19].
13
The first wildland fire control program was established in 1885 in the
Adirondacks Reserve in New York. By the following year, a program
was established in Yellowstone Park. Both were modeled on practices in
use in Germany, considered the model for forest management, which
were to extinguish all fires regardless of severity. In 1910, these policies
were reexamined after catastrophic blazes burned 5 million acres and
killed 79 fire-fighters [15]. As a result, the US Forest Service (USFS)
“declared war” on fires and launched an aggressive campaign on fire
prevention and control.
In 1926, after questions arose regarding the merits of light burning, the
USFS adopted a policy that would allow areas of 10 acres or less to
burn, but required the suppression of all fires over 10 acres. The
Tillamook burn in 1933 destroyed 3 million acres of virgin timberland in
the Northwest [15]. In its wake, the USFS reverted to an even more
stringent “ no burn” policy and mandated that all fires were to be
extinguished during the first duty shift after its discovery or by 10 am the
following day. This policy remained in effect and was reexamined in
1971 when the USFS changed its policies to allow some lightning fires to
burn as natural prescribed fires.
In 1978, the USFS again revised the policy, this time excluding the 10
am objective. The emphasis was shifted to managing fire suppression
costs so that they are consistent with land and resources management
strategies. By 1988, changes in Policy by the USFS and National Park
Services allowed many natural fires to burn on federal wildland [20].
14
Despite policy and myriad suppression efforts, wildland fire has been a
continued problem in America’s forests. Table 2 illustrates some
historically significant wildland fires.
Table 2: Selected Historically Significant Wildland Fires [18]
Date Name Location Acres Significance Oct 1871 Peshitog Wisconsin/Michagan 3,780,000 1,500 fatalities in Wisconsin Sep 1894 Hinckley Minnesota Undetermined 418 lives lost Sep 1894 Wisconsin Wisconsin Several million Undertermined; some lives lost Aug 1910 Great Idaho Idaho/Montana 3,000,000 85 fatalities 1949 Mann Gulch Montana 4,339 13 smokejumpers killed Sep 1970 Laguna California 175,435 382 structures destroyed 1987 Siege of ‘87 California 640,000 Valuable timber lost on the Klamath and Stanislaus National Forests 1988 Yellowstone Montana/Idaho 1,585,000 Large acreage Oct 1991 Oakland Hills California 1,500 25 lives lost and 2,900 structures destroyed Jul 1994 South Canyon Colorado 1,856 14 firefighters fatalities 1998 Volusia complex Florida 111,130 Thousands of people evacuated from several countries 1998 Flagler/St. John Florida 94,656 Forced the evacuation of thousands of residents May 2000 Cerro grande New Mexico 47,650 Originally a prescribed fire; 235 structures destroyed; damaged Los Alamos National Laboratory
Today, fire suppression agencies throughout the country are increasingly
challenged by wildland fires that affect structures located in areas that
are essentially wildland. The question what to do with the urban/wildland
interface has become one of the most controversial in the fire service.
Some have argued that it is not appropriate for publicly funded fire
suppression personnel to be dedicated to protecting homes built in this
dangerous area when they can be better utilized elsewhere. Others,
15
however, claim that one has the right to build home anywhere he or she
wants, in spite of the possible ramifications of such an action. Another
controversy is over how to thin the forests and lighten their fuel load.
Some argue the emphasis should be on prescribed burning while others
are proponents of mechanical thinning (cutting trees strategically) [15].
Given the political, ecological and economic implications of any decision
affecting residents and homes in the interface, these debates are likely
to continue for years to come.
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