=================================================================
# 学习进度更新
## 2008年9月19日:13-14
=================================================================
# Chapter 2: The combustion process
what is combustion?
The combustion process can best be described by breaking it down into components:
-First, what is it? It is a chemical reaction, which is irreversible.
-Second, what’s involved? Fuel, heat and oxygen.
-Third, what are the results of this chemical reaction? Energy is produced in the form of heat and light.
>燃烧是一个化学反应，主要有可燃物，热量和氧气的参与，最终会产生光和热等产物
Therefore, perhaps a comprehensive definition of combustion could be:
‘An irreversible**不可逆的**, chemical reaction between fuel and oxygen that produces energy in the form of heat and light.，
Just for comparison, the Oxford English Dictionary defines combustion as:
'Rapid chemical combination of a substance with oxygen，involving the production of heat and light.'
**Complete combustion**is where all available fuel is used due to an unlimited source of ventilation or oxygen.
**Incomplete combustion**occurs when all available fuel cannot be burned due to insufficient oxygen. This is the environment that firefighters encounter most often.
The basic fire triangle that all boy scouts learn is shown in Figure 3 in a slightly different way.
## 13
Figure 3: The fire triangle
Note that the graphic is shown with the point at the bottom. We will cover this slightly later in the book.
The old adage**谚语**is that if you remove one of the elements of the triangle then the combustion process will cease. While that may be the foundation upon which we base our tactics, indeed,**water cools**and**removes heat**, there are exceptions to the rule and due to modem lifestyles these are becoming far more frequent.
Let’s look the elements of the triangle in a little more detail.
Oxygen
A likely scenario can be encountered where fuel and heat exist in sufficient quantities to support combustion, all that is required is for oxygen to be added to complete the chemical reaction and initiate flaming combustion.
The addition of oxygen can be caused by as simple an action as opening a door, allowing air (containing oxygen) to flow towards the heat and fuel source.
In the effort to locate and extinguish a fire，firefighters must remain alert to this possibility and consider the effects of opening and closing doors en route to the fire.**This is called ‘flow path management’.**
>为了找到火点和灭火，消防员必须，要考虑开门和关门对火灾发展的影响
If a fire has**insufficient**oxygen to support combustion its growth and movement are dictated by the amount of ventilation available and is known as**‘ventilation controlled’**.
If a fire has**sufficient**ventilation to continue to burn until all available fuel is used, it is dictated by that fuel and known as** ‘fuel controlled’**.
A ventilation controlled fire can affect the other two elements of the fire triangle. If combustion ceases due to a lack of available oxygen then temperatures and heat energy may reduce.
>如果因为缺氧导致燃烧中断，那么温度和热量也会慢慢降低
However, fuel can still be available and heat may remain sufficient to auto ignite this fuel once oxygen is introduced and the fuel mixes to
>这时候可燃物依然存在，热量也足以支持燃烧，如果氧气进入，燃烧就会继续，
## 14
within its ‘limits of flammability’. This phenomena is covered later in this chapter (see p17).
While oxygen is a requirement for combustion, it’s important to remember that there are fuels that produce or release oxygen when subjected to heat and the pyrolysis process. These are known as oxidizing agents. Peroxides are a common example of this.
Fuel
Fuel can be referred to as the ‘fuel package，and can be found in any of the three states of matter: solid, liquid or gas. When exposed to heat, a fUel goes through a process of chemical decomposition known as ‘pyrolysis’. In short, the heat causes the fuel to move through its matter states while breaking down into its component chemicals in vapour form.
These chemical components are released as gases, known as pyrolysates and volatiziles and may bond with other components contained within that fuel to form a number of gases. This is also sometimes referred to as 'off-gassing’. These gases are what we have referred to for years as smoke. Let’s now christen them 'fire gases' ，a.k.a. unbumed pyrolysates/volatiziles/pyrolysis products.
Sometimes fuel doesn’t pass through a liquid stage to release gases, these types of fuel are known as ‘subliminals’and this process is called ‘sublimination’.
For example, the chemical equation for wood is C6H10O5. This can be broken down as follows to represent the atoms contained within:
CCCCCC HHHHHHHHHH OOOOO
When heat exposure causes pyrolysis, the C6H10O5 releases these atoms which can bond with others released to form the following:
■ If 2x hydrogen atoms bond with 1x oxygen atom it creates H20 or water vapour.
■ If 1x carbon atom bonds with 2x oxygen atom it creates CO2 or carbon dioxide.
■ If 1x carbon atom bonds with 1x oxygen atom it creates CO or carbon monoxide.
■ If carbon atoms don’t bond but are released they form the substance we commonly know as 'soot，or unbumed carbon molecules.
These atoms are all present in wood, or C6H10O5，but if we are also burning in a normal atmosphere we must also consider the presence of the chemical elements that compose the air we breathe.
15
Air is 99% composed of nitrogen (N) and oxygen (O). It also contains very small quantities of the noble gases, which can have an effect on fire development at an extremely scientific level.
Thus when we consider the carbon, hydrogen and oxygen already existing in the wood and add nitrogen, we can form dangerous mixtures such as hydrogen cyanide.
Although in small percentages, air also contains these noble gases:
If we apply the same process of pyrolysis to all of these elements and mixtures, we can see that there is potential for creating a large number of different gases, with a variety of features such as toxicity and flammability.
And this is only by burning a piece of wood in air. Let’s consider that modem dwellings and workplaces contain furniture, fixtures and contents that are made up of far more complex chemical elements than a wooden bench. There is clearly a potential for firefighters to be operating in an environment composed of a number of chemicals in gaseous form, as smoke, before the fire is even located.
For firefighters, how does smoke (gaseous fuels) affect our approach?
If we consider that the majority of these gases are flammable, then we must consider tHat all smoke is unbumed fuel, harmful, flammable and ignitable. A firefighter would not walk through a pool of gasoline and without recognising it as fuel, so why do we continue to walk through unburned fuel in the form of smoke when carrying out our operations at fires?
This environment was termed the '3D environment' by Paul Grimwood, Ed Hartin, John McDonough and Shan Raffel (for more information, see http:// wwW.3dfirefighting.com/). The presence of fire gases is three dimensional (3D) and requires a 3D or'all around’ approach to successfully address this and reduce risks.
16
Flammability limits
A relationship between fuel and air
Flammability limits are the ratios of concentration between a fuel in vapour (gaseous) form and the air in the immediate surrounding atmosphere.
Flammability limits are widely categorised into the following:
■ Lower Explosive Limit
■ Upper Explosive Limit
■ Ideal Mixture (aka Stoichiometric Mixture).
Lower Explosive Limit
This is the lowest concentration of fuel to air that will sustain combustion. It is sometimes referred to as a ‘lean mixture ’. If the concentration dips below this it can be referred to as ‘too lean to burn’.
Upper Explosive Limit
This is the highest concentration of fuel to air that will sustain combustion. It is sometimes referred to as a ‘rich mixture ’. If the ratio of fuel to air is too high it can be known as ‘too rich to burn ’. This can be a very dangerous situation and if a fire has been or is present it can be referred to as ‘Ventilation controlled’ 一 it’s just waiting for introduction of air to ‘dilute’ it back down to within its flammable limits where it can bum freely.
Ideal Mixture/Stochiometric Mixture
This is the ratio of fuel to air where a substance can bum most efficiently and with the greatest force.
17
Figure 4: Examples of flammability limits
Courtesy of Merseyside Fire & Rescue Service, UK.
For firefighters, what do the flammability limits mean for us?
As we can see, all fuels need a supply of oxygen to burn. We can take steps to control the amount of air a fuel can mix with through 'flowpath management’ and either ventilating or shutting down compartments.
We also need to be aware that we may be proceeding into danger if we know that a fuel/air mix is above its Upper Explosive Limit and‘too rich', and that if we open doors we may allow airflow to dilute it down to within its flammability limits. It shows that fuels are in vapour form everywhere in the 3D environment and must be dealt with to reduce our exposure to risk.
We have shown that products of combustion such as carbon monoxide have wide flammable ranges and can burn across a wide ratio of concentrations with air. But modern compartments have more materials than wood releasing C6H10〇5 and fire gases are a combination of a number of fuels in vapour form.
A vigilant and well prepared firefighter knows this and treats all vapours in the 3D
environment as flammable fuels ready to burn.
18
Vapours, and unburned pyrolysis products are flammable!
Note the exiting gases have been ignited (Photograph, Walker IFRA 2015).
Passive agents
The hidden fourth side of the fire triangle
Passive agents are materials within any compartment that absorb energy in the form of heat. All elements within a compartment’s structure or furnishing act as passives for a short time, until temperature increases, pyrolysis begins and the thermal capacity of that material is reached.
At this point the material is no longer passive and actively contributes to the fire as fuel or by transferring heat.
Gases released by passives undergoing pyrolysis can also act as passive agents. Water vapour and carbon dioxide given off in this way can suppress fire development in the early stages and in localised areas.
19
So what do passives do?
Passive agents take away energy from the fire which can slow its growth for such a time until they can absorb no more energy and start actively contributing to the fire’s growth as fuel.
Heat
Heat is perhaps the most misunderstood of the traditional three elements of the fire triangle. To dispel a popular misconception we need to know the difference between temperature and heat.
Heat is a measure of the energy contained by a body, both its potential (stored) and kinetic (moving) energy. It is measured in joules (J).
Temperature is a measure of the kinetic (moving) energy of that body. As heat is introduced, the molecules move faster and collide more frequently, producing more heat. Temperature is a measure of how frequently these collisions occur.
However, these are not inextricably linked. For example，the heat created by raising 500 litres of water to 100°C is substantially greater than that created by raising one litre to 100°C although the temperature is the same.
Or，to use another example, a cup of tea at 80°C is a higher temperature than a swimming pool at 30°C, but because the swimming pool is clearly a much larger quantity of water, the total thermal energy it contains (its heat) is a great deal higher.
Measuring heat:
The formula to measure heat is as follows:
Q= CMT
Where Q is heat, C is specific heat capacity, M is mass of the body and T is its temperature.
20
Auto ignition temperatures
When a fuel reaches a certain temperature, there is enough kinetic energy in it to ignite it without the presence of a flame or the introduction of an external ignition source.
Some common examples of auto ignition temperatures are:
21
Flashpoints and firepoints
When a liquid substance reaches a certain temperature there can be fire gases (pyrolysates/volatiziles/vapours) released that form an ignitable mixture in air that, when an ignition source is introduced, causes combustion to occur.
If the substance does not continue to bum when the ignition source is removed this is known as the ‘flash point’. If the substance continues to burn once the ignition source is removed then this is known as the ‘fire point’.
Accepted definitions are:
Flash point: the lowest temperature at which a substance vapourises so that the introduction of an ignition source causes a flame to momentarily ‘flash'across the surface of the fuel.
Fire point: the lowest temperature at which a substance vapourises so that the introduction of an ignition source results in continued combustion once the ignition source is removed.
Heat release rate
Heat release rate (or HRR) is a measure of power or the heat energy that is released per measure of time by a fire. It is influenced by many factors including the shape of a fuel and its surface area, for example，a sheet of paper burns more quickly and releases its heat energy more quickly than a log - although the log can generate more heat and greater temperatures over a longer period it releases it more slowly over a greater period of time.
It should be noted that the heat release rate of a fire dictates its size and its speed of growth. If the HRR is higher, then the pyrolysis process is quicker, more gaseous fuel is released and more oxygen is used up, resulting in incomplete combustion and quantities of unbumed fuel in a ventilation controlled environment. This is the MOST IMPORTANT factor in the intensity, speed and development of fire.
My old colleague, a fellow.adventurer, Station Officer Jim Dave of the States of Jersey Fire & Rescue Service, emphasises the importance of heat release rate in the following extract from his masters’ degree dissertation.
22
23
Heat release rate versus temperature
Underwriters’ Laboratories and the National Institute for Science & Technology
(UL/NIST) draw a distinction between HRR and temperature using candles as
an example. One candle and ten candles (of the same type,size and composition)
burn at the same temperature but the ten candles release ten times the energy,
hence ten times the heat release rate.
The graphic below shows a single candle that will burn at a temperature of 500-
1,400°C and give off a HRR of 80 watts.
Also shown are ten candles of the same type and sizs. These also burn at a
temperature of 500-1400°C,yet produce a HRR of 800 watts.
Figure 5: Heat release rate of candle
Heat transfer can be measured in terms of the heat transferred over a measured area in a measured amount of time. This is known as‘heat flux’
Let’s briefly examine each of these in turn.
24
Conduction
Conduction is the transfer of heat energy through direct contact of materials.
For example, Greg Gorbett and Jim Pharr in Fire Dynamics cite the example of a metal rod being held in a flame. The part of the metal rod inside the flame is increasing in temperature, so there is more molecular movement (collisions). These begin to strike and affect the neighbouring molecules and therefore create a chain reaction, with a temperature increase moving along the rod until it reaches the portion where it’s held.
Figure 6: Conduction
Firefighters should be aware that certain materials conduct heat extremely well and heat transfer and subsequent ignitions can occur in this way. Other materials do not conduct well and are known as`insulators’.
25
Convection
Convection is the transfer of heat energy through a ‘fluid medium’. Beware here that the term ‘fluid’does not necessarily mean a liquid. It is far more likely that the fluid medium encountered by firefighters is air.
Figure 7: Convection
Note the heat rises from the heater while cool air is drawn in towards the heat source at the lower levels as it cools and falls.
In a compartment, heated air/fire gas flows past a solid object and there is a temperature difference. The motion of the fluid and the conduction through air of molecules in the fluid, causes increased molecule activity (collisions) in the solid and therefore a rise in temperature (kinetic heat energy) in the solid.
Convection can transfer heat either ‘naturally’ or ‘forced’； consider how a hot pie cools down when you blow onto the surface (forced). This type of forced heat transfer should also be remembered when managing flowpaths as the actions of the firefighters may change the direction and speed of how the ‘fluid medium’ flows, affecting heat transfer and fire spread.
For firefighters, how does convection affect us?
In addition to managing flow paths, our nozzle techniques can rapidly increase a convection speed and fire spread if we misapply them.
26
Radiation
Radiation is the transfer of heat energy through electromagnetic waves. This can happen whether a material is in a solid, liquid or gaseous form. Radiation does not require any material such as a conductor or a fluid medium to transfer its energy.
The transfer of heat energy to earth by the sun is an example of radiation.
Radiant heat transfer is generally responsible for fire spread to other materials in a compartment fire. The radiant heat from the flame can ignite other fuels, and the radiant heat from the smoke layer moving downward can increase the spread of fire and progress it towards becoming a fully developed fire - the flashover stage.
Figure 8: Heat radiation
Firefighters should note that the biggest heat transfer process (thus the biggest cause of fire spread) in fire is radiation via soot particles in the smoke/ gas. Convection and conduction only form part of heat transfer, in lesser ratios than radiation.
27
Chapter 2: revision questions
■ Define combustion.
■ Explain the difference between complete and incomplete combustion.
■ What are fuels known as that release oxygen when subjected to heat?
■ Define pyrolysis.
■ Explain the difference between heat and temperature.
■ Define the terms ‘flash point’，‘fire point’ and ‘auto-ignition temperature’.
■ What is heat release rate (HRR) and give two examples that affect it?
■ What is the term that refers to measuring heat transfer in known timescales and over known measured areas?
■ List the three types of heat transfers with examples.
■ What is the name for materials that do not conduct heat energy well?
■ Which of the heat transfer processes is the biggest contributor to fire development?