Smoke Burns

Firefighters tend to focus on visible fire (like a moth to a candle). Smoke is toxic and makes it hard to see, but does not present much of a threat to a firefighter wearing protective clothing and self-contained breathing apparatus, or does it?

Smoke

Smoke is a visible product of incomplete combustion. But what exactly is it? Smoke is a complex aerosol. An aerosol is a mixture of a gas and/or vapor and liquid or solid particulates. Key constituents of smoke include gases such as carbon monoxide and hydrogen cyanide, formaldehyde and benzene vapor, carbon particulate, and unburned pyrolysis products. Most of these components of smoke are toxic and flammable.

How can flammable gases and vapors exist inside a fire compartment? Why doesn't the fire consume them? The answer to these questions goes back to the fire triangle. Combustion requires fuel, oxygen, and heat in the correct proportion. Fuel and oxygen must be within the explosive or flammable range in order for combustion to occur (see Figure 2)

The Lower Explosive or Flammable Limit (LEL/LFL) is the minimum concentration of fuel vapor in air that will support combustion. Below this level, there is insufficient fuel for combustion to occur. The Upper Explosive or Flammable Limit (UEL/UFL) is the highest concentration of fuel gas or vapor in air that will support combustion. Above this level there is insufficient oxygen to support combustion. The flammable range falls between the LEL/LFL and UEL/UFL.

During the incipient stage of a compartment fire, adequate oxygen is available and fire development is predominantly limited by fuel characteristics and configuration. The developing fire consumes most of the pyrolysis products given off by solid fuels. However as the oxygen concentration in the compartment drops, pyrolysis products that are not consumed by the fire (excess pyrolyzate) and flammable products of incomplete combustion (i.e. carbon monoxide) begin to accumulate within the compartment.

At some point in the combustion process within a closed compartment or compartment with limited ventilation the fire will be limited in its growth by the available oxygen supply. At this point the rate of heat release will stabilize and if ventilation is extremely limited will decline as illustrated in Figure 3.

It is important to point out that while the example in figure 3 illustrates a transition from fuel controlled to ventilation controlled prior to flashover, this is entirely dependent on the ventilation profile of the compartment. In other cases, the fire may reach flashover in the compartment of origin before becoming significantly ventilation controlled. Even though the rate of heat release slows when the fire becomes ventilation controlled, the temperature in the compartment can remain quite high. Radiant and convected heat from the fire can result in ongoing pyrolysis and further accumulation of pyrolysis products and flammable products of combustion within the compartment.

On a summer afternoon in 2005 Gresham Fire and Emergency Services responded to a fire in a doublewide mobile home. On arrival, smoke was pushing from windows on the Alpha and Bravo sides of the structure at moderate velocity. The windows were all darkly stained with condensed pyrolizate. After making entry through a doorway on Side Bravo, the crew on the first hoseline encountered little flaming combustion but substantial heat. Brief application of water to cool the hot gas layer and direct attack on the burning fuel quickly extinguished the fire. However, examination of the structure and its contents after the fire was extinguished illustrates the impact of radiant and convected heat in ventilation controlled fires (see Figure 4). Even though limited ventilation slowed fire development, the temperature in this compartment was high. Note the melted plastic on the microwave just to the left of center in the photo as well as pyrolysis and charring of the tops of the chair and couch. When firefighters arrived, this residence was full of hot, fuel rich smoke. The only thing missing was a source of air.