FIRE MYTH BUSTING

Every fire sends out indicators that can help us to determine the seat of fire, the stage of fire development and the changes that are likely with or with out our intervention. Interpretation of these indicators in the modern context is a skill that is still developing. There are however a few myths that become law if repeated enough. Some of the most dangerous myths revolve around smoke colour and in particular, white smoke.

Myth 1. Pyrolysis smoke is white because of the water content.

It is true that pyrolysis smoke has a moisture content in the early stages, this is not the only reason it is white. Lighter coloured smoke (sometime almost white) is produced when the fuel is heated to the pyrolysation temperature and the volatile components of the fuel are released while the soot is left behind to form char. Caution: Don’t forget that smoke can  turn white after water has been applied to the fire as a result of the condensation of water vapour.

Myth 2. White smoke is too lean to burn

Nothing could be further from the truth. Pyrolysis is the chemical decomposition of a solid by the application of heat. As the external heat source breaks down the solid fuel, hydrocarbons are part of the volatiles released. If the process was to continue with out the fuel reaching the temperature at which the solid moves to self sustaining combustion, all of the potential energy in the fuel is now in gaseous form!!

In fact white smoke from pyrolysis is often fuel rich. The flammability range is relatively narrow because the temperature is much lower than smoke produced from flaming combustion. 

This photo was taken during a small scale demonstration called the Dolls House. The box is constructed of particle board and a small quantity of particle board is broken up, placed in the corner and ignited. It is possible to develop the fire to flashover and then to experiment with the ventilation available to the fire. In this photo the fire was allowed to flashover. The air inlet/outlet was then restricted and the fire quickly turns form flaming combustion to pyrolysis. Even though the fire is starved of oxygen, there is still sufficient residual heat to allow for chemical decomposition (pyrolysis). This white smoke is FUEL LADEN and is actually fuel rich. Because pyrolysis occurs at temperature much lower that flaming combustion, the smoke is cooler and the flammability range is narrower.  

Later in the same demonstration the white smoke that leaves the under ventilated compartment is hot enough to auto ignite! This photo has capture the moment when the outer edge of the smoke has diluted down into the flammabe range. In this case the dilution of the fuel rich smoke did not drop the temperature of the smoke below the auto ignition temperature. Thus the white smoke igntied. 

The next 2 photos show the 4 compartment Dolls House demonstration. Black smoke is uissuing from the top right compartment. The main fire is in the bottom left compartment and there is sufficient air to support flaming combustion. The smoke emerging on the top right is dark due to the presence of soot (nothing to to with the amount of unburnt hydrocarbons in the smoke). 

When the air supply to the bottom left fire compartment is severely restricted, flaming combustion ceases, but pyrolysis continues. Smoke emerging on the top right is now white and VERY fuel rich! At this stage of the demonstration we generally introduce a flame to the discharging white smoke to ignite it and thus prove that white smoke is fuel rich and potentially very dangerous. 

Myth 3. Black smoke that is pushed through narrow gaps will turn white

While I cannot say categorically that this if false, I have yet to see any scientific evidence to support this statement. While the soot component readily adheres to surfaces, the smoke is not transformed into a white colour. The thickness of the smoke is reduced, but is will still be black or grey. 

Myth 4. The blacker the smoke, the more unburnt hydrocarbon fuel present.

People assume that the hydrocarbons in aerosol or vapour form are black. This is NOT the case. The black colour is a result of the "free carbon" (soot) that is produced from incomplete combustion. If the air supply is good, the flame temperature is high and some of the soot is burnt in the flame zone which produces a luminous yellow flame. If the air supply is hindered, the flame temperature drops and less of the soot is burnt in the reaction zone (flame) and more of it is drawn up into the smoke plume. 

Consider gasoline, diesel or paraffin in a vapour or aerosol form. Are any of these black?

In the video below the fuel used is OSB and pine pallets. The fire is allowed to build up and then oxygen is excluded by closing down the container.

Do you still beleive that white smoke is too lean to burn?