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 combustionis 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 过氧化物 are a common example of this.

有一些可燃物也会产生氧气

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.

热量会使可燃物热解
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’.

可燃物跳过液态，直接变为气态，生华

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.