Chemistry of Combustion

Question 1

What is combustion?

Answer 1

A chemical reaction between fuel and
oxygen that produces energy in the form of heat and light

Question 2

What are the components of the fire tetrahedron?

Answer 2

Oxygen
Heat
Fuel
Chemical chain reaction

Question 3

What are the types of fire spread?

Answer 3

Radiation - straight
Conduction - metals
Convection - air

Question 4

In which physical state do things burn?

Answer 4

Solid
Liquid
Gas

Question 5

What is pyrolysis?

Answer 5

The chemical decomposition of a
substance due to the application of heat which causes the production of flammable vapours.

When mixed with oxygen, if ignited, can combust, will
produce more heat.

Question 6

Why is the density of vapour and rate of production important?

Answer 6

Mixture needs to be sufficient to support combustion.

Question 7

What are the classifications of fire?

Answer 7

A - Solid materials
B - Liquid and liquifiable solids
C - Gases
D - Certain metals
F - Combustible cooking fuels

Question 8

What are passives?

Answer 8

Take no part in combustion

Absorb/steal heat

Question 9

What are actives?

Answer 9

Contribute to combustion

Give off energy (heat)

Question 10

What is an example of a passive?

Answer 10

Water

Question 11

What is the expansion rate of steam at 100 degrees?

Answer 11

1L water : 1700L steam

Question 12

What is the expansion rate of steam at 450 degrees?

Answer 12

1L water : 3500L steam

Question 13

Define smoke

Answer 13

A visible product of incomplete combustion which includes carbon monoxide, hydrogen cyanide and unburnt pyrolysis products

Question 14

What are the different types of flame?

Answer 14

Diffused
Premixed

Question 15

What are the features of a diffused flame?

Answer 15

Not mixed prior to ignition
Unclean burn
Cooler flame
Orange/red colour
Less noisy
Less efficient

Question 16

What are the features of a premixed flame?

Answer 16

Gases premixed prior to ignition
Cleaner burn
Hotter flame
Blue flame
High velocity
Noisy
Efficient

Question 17

What is the LEL?

Answer 17

Lower Explosive limit (too lean)

Question 18

What is the UEL?

Answer 18

Upper Explosive Limit (too rich)

Question 19

What is the IM?

Answer 19

Ideal mixture

Question 20

What is the flammable range?

Answer 20

The difference between the LEL and the UEL as a percentage of gas to air that will allow flame or combustion

Question 21

What is a Flash Point?

Answer 21

The lowest temperature at which a substance will produce sufficient vapour that when an ignition source
is introduced the vapour will momentarily ignite but NOT continue to burn

Question 22

What is the Fire Point?

Answer 22

The lowest temperature at which a substance will produce sufficient vapour that when an ignition source
is introduced the vapour will ignite and CONTINUE burning

Question 23

What is the Automatic Ignition Temperature?

Answer 23

The lowest temperature at which a substance will produce sufficient vapor which is already hot enough to ignite without the introduction of an ignition source

Question 24

What are the indicators of an impeding flashover?

Answer 24

• Supply of fresh air/ventilated fire
• Lowering neutral plane
• Remote surfaces pyrolysing
• Flames visible in the gas layer
• A rapid increase in temperature

Question 25

What is a flashover?

Answer 25

The stage of thermal radiation where the total thermal radiation from the fire plume, hot gases and hot compartment boundaries causes all exposed combustible surfaces to pyrolyse (release flammable gases) and ignite when there is adequate ventilation. This sudden and sustained transition of a growing fire to a fully developed fire is known as a flashover.

Question 26

What are the stages of fire development?

Answer 26

Incipient Growth Flashover Fully developed Decay

Question 27

What are the indicators of an imminent backdraught?

Answer 27

History of the fire Low neutral plane Blackened windows Hot doors and windows Pulsating smoke Whistling Inrush of air when opening is made Ghosting tongues of flame seen in the compartment Underventilated fire

Question 28

What is a backdraught?

Answer 28

‘Limited ventilation can lead to a fire in a compartment producing fire gases containing significant proportions of partial combustion products and unburnt pyrolysis products. If these accumulate, the admission of air when an opening is made to the compartment can lead to a sudden deflagration. This deflagration moving through the compartment and out of the opening is a backdraught

Question 29

What is ventilation?

Answer 29

Ventilation is the removal of heated air, smoke and other airborne contaminants from a structure. These are replaced with a supply of fresher air.

Question 30

What is fire gas ignition?

Answer 30

Fire gas ignitions occur when gases from a compartment fire have leaked into an adjacent compartment and mixed with the air within this additional area. This mixture may then fall within the appropriate flammable limits that, if ignited, will create an increase in pressure either with or without explosive force