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.
CHAPTER 1
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 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 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 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].
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].
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 |