Fire definitions, radical chemistry and chemistry of phosphorous

Question 1

What are the physics and chemistry definitions of a flame?

Answer 1

Physics: The zone of burning gases and fine suspended matter associated with rapid combustion; a hot glowing mass of burning gas/vapor. The condition of active, blazing combustion.

Chemistry: A region in which chemical interaction between gases occurs, accompanied by the evolution of heat and light.

Question 2

What is the fire triangle?

Answer 2

Three necessary conditions for combustion:

1. Fuel
2. Oxygen
3. Heat (enough to initiate combustion)

This is often presented as a fire triangle. Fires are extinguished/prevented by removing one or more of the three components of the triangle.

Question 3

What is the variant of the fire triangle?

Answer 3

The fire triangle concept can be extended to sustaining the fire after initiation.

• If the original ignition source is removed, sufficient heat must be generated by chemical reactions to sustain the fire.
• We must also consider chemical mechanism of fire. Free radicals are necessary for a hydrocarbon fire to be sustained. Removing these will extinguish the fire.
• Adding the chemical reaction to the triangle gives a “fire pyramid”

Question 4

What are the three stages of a free-radical reaction?

Answer 4

Initation - generation of the reactive intermediate.

Propagation: the reactive intermediate attacks a stable chemical species to generate other reactive intermediates (the reaction continues)

Termination: Two radicals combine to quench the unpaired electrons. This halts the reaction, often in the form of a byproduct.

Question 5

What is a primary fire, and what is a secondary fire?

Answer 5

Primary fires include all fires in buildings, vehicles and outdoor structures; or any fires involving casualties or rescues; or fires attended by five or more appliances.

Secondary fires are the majority of outdoor fires including grassland and refuse fires, unless they involve casualties or rescues, property loss or fiver or more appliances attended. They also include fires in derelict buildings.

Question 6

What are deliberate fires and what are accidental fires?

Answer 6

Deliberate: Include those where deliberate ignition was merely suspected and recorded by the fire and rescue service as “doubtful”

Accidental fires are those started accidentally. Those where the cause was recorded as “unknown” are grouped in these statistics.