Enclosure Fires - Chapter 2: How a fire starts
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People usually say that “all fires are small to begin with”, which is absolutely true. We will now discuss why some fires remain small and therefore, do not cause any major damage either, as well as what factors cause a fire to grow in size. Two key factors are ignition and flame spread. These will be discussed in more detail.
This is only the First Chapter -
The diagram shows the temperature in the fire room along the vertical axis and time along the horizontal axis. A fire can develop in many ways, depending on the conditions. The start of the fire, which is the first part of the fire growth curve, is shown in Figure 4.
2.1. Initial fire
When talking about how a fire starts we use the term fire trig- ger. Fire trigger is used to mean the object which has caused the fire. A fire can be triggered by such objects as a hob, radio, TV, candles or an iron. We also speak about the cause of fire. The three most common causes are chimneys, fires started de- liberately and hobs left unattended. Technical faults are also common.
In the case of fatal fires, bedclothes are the object most fre- quently involved. This is due to the fact that the most common fire trigger in the case of fatal fires is cigarettes. Other usual causes resulting in people dying include their clothes somehow catching fire.
We will now go through the various stages in a fire’s behaviour in chronological order. The starting point is when an object catches fire. A crucial factor in the fire’s development is
Figure 5. Fire trigger and causes of fire.
In a fuel-controlled fire the heat release rate is controlled by the access to fuel. Ventilation control means that the amount of oxygen controls the heat release rate.
Figure 6. Fire behaviour can develop in two different ways. It can grow or subside.
whether the object has sufficient fuel itself or there is some nearby. If the fire does not spread it remains fuel controlled and burns itself out.
When the fire is fuel controlled the heat release is controlled by the access to fuel. In this situation, there is therefore sufficient oxygen for all the fuel to be able to combust. On the other hand, when the fire is ventilation controlled, it is the amount of oxygen and indirectly, the opening size, which control heat release. The fuel arrangement is also crucial to the fire’s behaviour.
How does the fire then grow? When there is a chance for the fire to spread the heat release rate will increase.
The heat from the initial fire will then cause other objects to ignite. Ignition is a vitally important phenomenon, which will be dis- cussed later on in the chapter. The material’s flame spread is also very important in terms of how the fire spreads further.
In most cases, the heat release rate from one object is not sufficient for the fire to cause a flashover.
We usually talk about the initial fire as the object which the fire starts with. This may be, for instance, a sofa or a candle on the table
Let us start with the initial fire. There are, in theory, two paths which the fire can choose, once it has started. It will either grow or subside.
Scenario 1 (the fire subsides – see Figure 8) is very easy to handle from a tactical point of view. There are often somesmoke gases in the compartment, but the actual fire is very easy to extinguish. This situation is very common with house fires in Sweden.
In the case of scenario 2 (the fire grows – see Figure 9), we need to give a bit more thought. Also, as it is interesting to see what happens when the fire spreads further, in the next section we will look at how and in what way the fire will be able to spread during the initial phase.
The fuel’s arrangement in the compartment is crucial to the fire’s continuing behaviour. Porous and wood-based mate- rials in furnishings contribute to the fire’s rapid development. Plastics sometimes cause fires to spread very quickly due to the fact that they drip to form pools of fire on the floor.
We will now look at how the material ignites and flames spread with objects. It is important to understand these processes in order to be able to learn how a fire’s intensity in- creases.
Figure 8. The fire does not spread.
Figure 9. The initial fire in the sofa grows bigger. The fire’s area increases.
Ignition
+ O2
Energy
+ H2O +CO2 +CO
+ carbon particles, etc.
Exothermic process
Combustion is a chemical reaction. It involves, to be more precise, a whole series of chemical reactions when the fuel is oxidized. Fuel and oxidizing agents react with each other. This releases heat and light. As a result, the chemical process is accompanied by physical effects. Heat is the physical energy which is released during the chemical process. Light is the physical consequence of the fact that there is energy stored in the soot particles, for instance.
Ignition is the first visible sign of combustion. The combustible material may auto-ignite due to the high temperature or it can be ignited by an external source such as a match or spark. In the case of solid materials, there is a critical temperature at which ignition takes place.
But this generally varies ac-cording to the material which is burning and can therefore not be used as a measure of inflammability. With solid materials, the surface must be heated up to 300–400°C for ignition to occur with a pilot flame. If there is no flame nearby the surface temperature must be higher. Wood needs to reach a surface temperature of 500–600°C before it auto-ignites.
Inflammability in solid materials is estimated using the time it takes for ignition to occur. Ignition takes place when sufficient combustible gases have built up on the solid material’s surface for them to be ignited by a small flame.
Materials such as wood or paper (organic polymers) need to emit 2 g/m2s (grams per square meter and second) of combustible gases to be able to ignite. As for plastics (synthetic polymers), which have a high energy content, they need about 1 g/m2s of combustible gases to be able to ignite.
Pyrolysis gases
Figure 11. Energy balance on a surface. The figure shows how heat exchange occurs from the object, as well as how thermal conduction takes place through the object.
Figure 11 shows what happens on the fuel surface when the material is subjected to external radiation (heat radiation). The radiation makes the temperature rise to the level required for the material to pyrolyse. Pyrolysis involves the fuel decomposing. This process requires the external radiation to be at a certain level. If the radiation level is too low the material will never be able to ignite.
Experiments have shown the amount of heat required for a particular material to be able to ignite in proximity to a small flame. This can be measured using a device called a cone calori- meter. The material is placed in it under a cone which emits a certain level of radiation. There is a spark generator on top of the sample, continually trying to ignite the material. This is how the time until the material ignites is measured.
Figure 12 shows the radiation intensity (kW/m2), along with the time it takes to ignite the wood when it is subjected to different processes. Later on in the book, we will explain why radiation levels of around 20 kW/m2 are so important.
Figure 12. Ignition time as a function of incident radiation.
Surface temperature in solid materials
The surface temperature of a solid material Ts can be calculated using equation 1, which originates from what is known as the general thermal conduction equation. 6 This equation has been simplified somewhat, but is still adequate for our purpose.
2q" t 0.5
Ts – Ti =
p0.5 (k3c) 0.5
Equation 1
q" – heat supplied W/m2 – Radiation energy (in this case, from the fire)
Ts – surface temperature (°C) for fuel
Ti – initial temperature (°C) of fuel surface (original temperature)
k – thermal conductivity W/m2 °C (a high coefficient means that the material is a good heat conductor)
3 – density in kg/m3
c – specific heat capacity in J/kg °C (this means the material’s ability to store heat)
t – time in seconds
Figure 12 shows that coated pine is ignited only after a very long time has elapsed if the radiation intensity is lower than 20 kW/m2. Compared with this, untreated pine ignites in just 7 minutes at the same radiation level. 20 kW/m2 is equivalent to the level of radiation emitted by a smoke gas layer at a temperature of around 500°C. Inflammability for solid materials can therefore be estimated using the time taken for a certain heat impact to cause ignition to happen.
The surface heats up quickly in a material with a low thermal inertia, k3c, whereas a material with a high k3c value heats up slowly. Table 1 shows the differences in k3c (pronounced “kay-row-see”) for various materials.
For instance, we can compare the time it takes for chip- board and wood fibre board to ignite. Both materials are subjected to the same constant level of radiation, 20 kW/m2.
Chipboard ignites after 180 seconds. But the wood fibre board, which has a much lower k3c value, ignites after a considerably shorter time of just 50 seconds. The experiment was carried out using a cone calorimeter. There is therefore a spark generator for igniting the gases in this case.
Ignition time
Ignition time can also be calculated using Equation 2, which is a reformulation of Equation 1. Note that the heat resistance from the surface has been omitted and that the ignition temperature most often lies in the range of 300 °C–400 °C. When the ignition temperature Tsa is known the ignition time ta can be calculated:
(Tsa – Ti )2
ta = 4(q") 2 k3c × p
Equation 2
Let us take as an example a fire room where a flashover has occurred. If the temperature in the room is around 600 °C all the surfaces will be affected by radiation in the order of 30 kW/m2. If we calculate the length of time it takes to ignite combustible chipboard, for instance, the calculations to be carried out are as follows, assuming that the ignition temperature Tsa = 400 °C. The k3c value is taken from Table 1.
(400 – 20)2
ta =
4(30 000)2
120 000 × p u 15 seconds
This is a rough estimate and must not be regarded as a precise value. In actual fact, the material will heat up at the same time as the surface cools down as a certain amount of heat radiation leaves the surface. If you decide beforehand that the surface should not be heated up beyond a certain temperature, you can calculate the length of time the surface can be subjected to a certain amount of heat, i.e. a certain amount of incident radiation, until it reaches the preset temperature.
Material k (W/mK)
c (J/kgK)
3
(kg/m3)
k 3c (W2s/m4K2)
Chipboard
0.14
1,400
600
120,000
Wood fibre
board
0.05
2,090
300
32,000
Polyurethane
0.034
1,400
30
1,400
Steel
45
460
7,820
160,000,000
Pine tree
0.14
2,850
520
210,000
Table 1. Thermal inertia for different materials.
Figure 13. The heat is blocked at the surface
when the material is well insulated. Com-
FIG
pare, for instance, wood fibre board (on the left) with chipboard (on the right).
Flaming combustion and smouldering
A combustion process can actually be divided into flaming combustion and smouldering.
Flaming combustion (homogeneous oxidation) occurs when fuel and an oxidising agent are in the same state, e.g. two gases.
Smouldering (heterogeneous oxidation) occurs on the surface when the fuel and oxidising agent are not in the same state, e.g. when the fuel is a solid and the oxidising agent a gas.
The surface of a material with a low thermal inertia, i.e. a low k3c, heats up quickly as less heat is conducted in the material. A low value means that more heat stays at the surface, which means that the surface reaches the temperature more quickly when there are sufficient combustible gases for ignition, usually between 300 °C
and 400 °C.
Combustion of gases and liquids comes under flaming combustion, whereas solid materials can burn with both types of combustion. We will look at flaming combustion in Chapter 3 and will therefore concentrate on looking at smouldering fires in this section.
Smouldering can occur on the surface or inside a porous material when it has access to oxygen, allowing oxidation to continue. The heat can even remain inside a porous material and support the pyrolysis process until auto-ignition possibly occurs.
The solid carbon layer on the charred residue is a porous material, which commonly smoulders. A smouldering fire most often produces a lot of pyrolysis products which do not oxidise all at once. In a compartment fire the pyrolysis products are emitted by the burning object and accumulate in the upper part of the room without having combusted. The compartment then gradually fills up with smoke gases, which mainly contain carbon monoxide (which is toxic if inhaled).
Smouldering fires can therefore result in people dying.
Smouldering or self-combustion is common in upholstered furniture. The fire starts with the cotton or viscose fabric beginning to smoulder on a layer of polyurethane stuffing, ignited by a cigarette, for instance (see Figure 14).
This type of stuffing material can withstand smouldering very well, without the covering. But in upholstered furniture the various materials combine in such a way that the fabric layer starts to smoulder and it progresses from there. While the fabric is smouldering, the foamed plastic starts to both smoulder and pyrolyse. Pyrolysis from the foamed plastic (the yellow smoke) combines and adds to the fabric’s smouldering. The fabric’s mass loss rate increases and an increased number of pyrolysis products are released. This results in the entire item of upholstered furniture becoming involved in the fire.
Smouldering fires can often occur inside structures, which then makes them very difficult to get at. In this oxygen-deficient environment you cannot get a flame, but the combustible gases can be transported away and ignite in other places. A smouldering fire burns very slowly, which means that it can go on for a long time.
There are only a few substances that can smoulder. But they are actually quite common. Charcoal is one example. Apart from charcoal, there are also substances which produce carbon on combustion, like wood. It even includes some metals, such as pulverized iron.
Surface flame spread
When the phrase flame spread is used in this book, it means the initial flame spread, i.e. when the fire starts. Obviously, flame spread occurs in the same way in a room which is close to flashover. Flame spread can also occur in a gas layer. The flames start far away from the location where the pyrolysis gases have accumulated.
Flame spread can also be viewed as a series of continuous ignition events. As ignition greatly depends on the thermal inertia of the material, which we mentioned earlier, flame spread will also depend on the material’s k3c value.
A smouldering fire in a foamed plastic mattress.
Most cellulose materials form a carbon layer which can smoulder. Even some plastics can smoulder.
Area not reached by combustion
Radiation from surface Flame radiation
image
Flame-surface convection
Radiation dominated area
Conduction
Radiation from surface
Flame radiation
Flame-surface convection Fuel vaporisation
Figure 15. Flame spread on a wall.
Convection dominated area ( 5 - 25 cm)
Conduction
Radiation from surface
Flame radiation
Flame-surface convection Fuel vaporisation
As we said earlier, rapid flame spread can contribute to the fire’s area increasing, and consequently, to an increase in the heat release rate too. This can gradually lead to a very dangerous situation. It is therefore very important to clarify what factors have an impact on flame spread.
Figure 15 shows what happens on the surface when a wall is on fire. The wall can be split into three sections. The bottom section is dominated by heat transfer to the surface via convection. In the middle section, flame radiation is the main factor, which is due to the flame’s width increasing with the height.
The wider the flame, the more heat transfer occurs via radiation. In the top section, the wall has not ignited yet. In the figure, the length of the arrows corresponds to the size of the various components.
The rate at which the flames spread over the material’s surface is mainly dependent on the following:
the material’s thermal inertia, k3c
the surface’s direction
the surface’s geometry
the surrounding environment.
Thermal inertia kρc
The flame spread rate depends, to a large extent, on the ignition time, which in turn is heavily dependent on the material’s thermal inertia (k3c), which is a material property. The larger the thermal inertia a material has, the slower the flame spread on its surface.
In the case of solid materials, the thermal conduction coefficient (k value) increases most often as the density increases. In most cases, the density determines how quickly flames spread across the surface. This means that the flame spread rate across the surface of a heavy material is usually slower than that across a light material. For instance, in the case of foamed plastics, the flames can spread extremely quickly.
Surface direction
The flame spread rate is mainly upwards. Flame spread rate downwards is much slower, which is due to the fact that the surface does not heat up in the same way. In between, the rate changes according to the surface’s gradient.
Figure 16. Flame spread on a light material (on the left) and on a heavy material (on the right).
image
8 H 8 H for T = 4
7 H
6 H
5 H
4 H 4 H for T = 3
3 H
2 H 2 H for T = 2
Figure 17. Diagram illustrating fire spread upwards.
H
2 3 4
H for T =
Time
image
2
Figure 18. Flame spread 1
in different directions.
Vertical flame spread 3
upwards and horizontal
spread along the ceiling 4
have the fastest rate.
image
In the case of vertical flame spread upwards, the height of the flames for many materials, such as wood fibre and chip- board, is roughly twice as large over the same period of time. This means that if it takes 30 seconds for a 25 cm flame togrow to 50 cm, then a 1 m high flame will grow to 2 m in about the same time, if the wall material is the same. (This value must be regarded as only an approximation.)
The same situation applies to flame spread along the under side of a horizontal surface as in the case of vertical flame spread upwards. In contrast to this, flame spread on the upper part of a horizontal surface or downwards on a vertical sur- face can be described as “creeping”, as it is slower than flame spread upwards.
Surface geometry
In a corner there is interaction between both burning surfaces, which increases the spread rate. The smaller the angle, the faster the flame spread. This is due to the heat getting trapped in the corner, which then heats up the material. The smoke gases which are forming heat up so that a smaller amount of air is sucked into the plume.
Surrounding environment
When the ambient temperature rises, the flame spread rate increases too. The surface is heated up and the ignition tem- perature is reached more quickly. The higher the temperature from the start, the faster the flame spread rate will be as well. Another consequence of this is that the higher the tempera- ture a material has from the start, the faster the surface pro- duces sufficient combustible gases.
Figure 19. Interaction in the corner makes the flame spread rate faster, compared with when the flame occurs in the middle of the wall.
image
Figure 20. Flame spread in every direction.
Let us take as an example of the scenario where a smoke gas layer heats up the ceiling material over a long period of time. By the time the flames have reached up along the wall, the ceil- ing material is already heated up and the flame spread will be very rapid.
Summary
Combustion is a chemical reaction process, w here fuel oxida- tion takes place. The first visible sign of combustion is ignition. When solid materials ignite, flame spread occurs almost at the same time, which can be regarded as a series of ignition events. In the case of solid materials, there is a critical tem- perature at which ignition takes place. But this is generally ir- respective of whatever material is burning and the surface temperature can therefore notbe used as a measure of inflamm- ability. With solid materials, the surface must be heated up to between 300 and 400°C for ignition to occur with a pilot flame. If there is no flame close by the surface must reach atemperature of between 500 and 600°C (wood).
Flammability in solid materials is estimated using the time it takes for ignition to occur. The combination of properties represented by k3c refers to the material’s thermal inertia and determines how quickly the material’s surface heats up. The surface of a material with a low thermal inertia heats up quickly, while the surface of one with a high k3c heats up slowly.
The lower the k3c value a material has, the shorter the ignition time. This means that a porous wood fibre board ignites more quickly than chipboard.
A combustion process can actually be divided into a flaming fire and a smouldering fire.
A smouldering fire can occur on the surface or inside porous materials where there is access to oxygen.
In the case of many fires which occur, rapid flame spread has been the cause of the serious consequences involved. Flame spread rate is dependent on a number of factors, especially the material’s thermal inertia, the surface geometry, the surrounding environment and the surface direction.
The flame spread rate is fairly slow on a surface made of a material with a high thermal inertia (which has, more often than not, a high density). This means that the flame spread across the surface of a heavy material is usually slower than that across a light material.
If the material has been heated up by, for example, a warm surrounding gaseous mass or by radiation from a smoke gas layer, the material can reach its ignition temperature fairly quickly. This means that surfaces which are heated up also lead to faster flame spread than unaffected surfaces.
The direction of the surface and flames are also key factors in the flame spread rate when a fire is spreading. It is mainly vertical flame spread upwards and flame spread along the surface of a ceiling in a compartment, which causes the fire to develop quickly.
In the case of flame spread upwards, where the difference in density and air flow push the flames upwards, the flames from the burning material heat up the part of the material which has not yet started to become pyrolysed.
Flame spread along the ceiling in a compartment can alsocause the fire to develop quickly. There are two reasons for this: firstly, the air flow forces the flames forward and secondly, the ceiling surface has been warmed up considerably by the hot smoke gases which have accumulated in the area of the ceiling.
Horizontal flame spread downwards along the lower section of walls in a compartment occurs at a much slower rate. But in certain cases, when the fire is close to a flashover, flames can spread very quickly downwards due to the surface being heated up via radiation.
We would just like to end by reminding you that this section deals with flame spread involving solid materials. Flame spread with both solid materials and in a smoke gas layer is crucial from the point of view of a fire spreading. Flame spread along the under side of a smoke gas layer is a very common sign indicating that something is changing in the fire room. This flame spread is an important sign for firefighters with breathing apparatus who need to fight the fire. Later on in this book, we will look specifically at flame spread in smoke gas layers.
Test your knowledge!
Let us assume that the surface of a material is heated up by a heat source. How hot does the surface need to be for the gases which form to be able to ignite?
It is a well-known fact that the flame spread rate varies according to the material. Let us compare two materials, such as chipboard and wood fibre board. Which material has the quickest flame spread rate? What does it depend on?
What is the abbreviation for thermal inertia? What do the different letters in the abbreviation stand for? Name some materials with a large thermal inertia.
What are the different ways in which heat can be transferred? Give some examples from everyday life of each type of heat transfer.
Flame spread is a key factor in the acceleration of a fire’s development. Name a couple of factors which have an impact on flame spread rate.
The flame spread rate varies with the direction in which the flames are moving. In which direction(s) do the flames spread fastest? Why is this the case?
A room is on fire and the temperature in the room is close to 500–600°C. Estimate how long it will take for chipboard to ignite if there is an ignition source. Chipboard is affected directly by this radiation. Hint: use the equation.
Flame spread is discussed in detail in this book. Why is knowledge about this so important for BA firefighters, for instance?
Let us assume that a surface is heated up by an external heat source. There is no ignition source. What temperature must the surface reach for the gases to be able to auto- ignite?
Name some materials whose surfaces have a very rapid flame spread rate.