Smoke ignition

Sketch of a room with flue gas ignition

A smoke (gas) by igniting ( engl. Rollover or flame over ) is the sudden By igniting and burning of pyrolysis gas . This gas mixture is created by the action of heat or combustion. When flue gas is ignited, primarily in dedicated rooms, there is enough oxygen for ignition. This distinguishes the smoke ignition from the smoke gas explosion ( backdraft ), for the ignition of which further oxygen must first be added to the smoke gas. The main components of the flue gas are pyrolysis gases .

Process in an enclosed space

Development phase

A fire develops and, if there is sufficient oxygen, heats the room and its components, so that pyrolysis gases are formed ( wood and plastics emit flammable substances). These are thermally processed.

Full fire phase

Here, when the ignition point is reached, the flue gas can be ignited and then the full fire phase, etc. a. come with flames from the windows.

Acceptance phase

Then after the first full fire there is often a heat build-up from the accumulating smoke. This fills the space to the floor and displaces the oxygen. The result is a decrease in fire intensity. Due to the incomplete combustion, the flammable carbon monoxide is generated with the smoke ; The necessary prerequisites for energy and fuel are now available again for combustion. If oxygen is now supplied again, for example by opening a door, a flue gas explosion can occur.

origin

Pyrolysis gases arise when a substance is heated to the point where it emits flammable gases . Examples of pyrolysis gases are the acrid yellow-white smoke of a campfire , similar smoke from straw fires or the smoke after blowing out a candle . This smoke (gas and particle mixture) is flammable, which is easy to observe with candles, for example, if you hold a burning match a few centimeters above the still smoking wick. Pyrolysis gases are generated very easily when furnishing items burn, since a lot of plastic has been used in their production .

Pyrolysis gases are very hot and collect in the upper part of the room, heat other objects due to their high temperature and cause them to also release pyrolysis gases. If the room is now opened, fresh air with oxygen flows in, which mixes with the combustible pyrolysis gases, and an ignitable mixture is created. The mixture is ignited by the already burning fire in the room and the surroundings suddenly heat up. This enormous heat and the sudden rise in pressure even shatter windows. This allows the fire to expand further.

Therefore, closed rooms in which a fire is burning should not be opened by unprotected and untrained people. But even for members of the fire brigade , smoke ignition represents a danger that should not be underestimated, as temperatures of well over 1000 ° C can arise.

Rollover and flashover

The rollover is the ignition of the pyrolysis gases in a layer of smoke. As soon as enough flammable pyrolysis gases have accumulated in the smoke layer and there is enough air in the room to ignite, the smoke layer will ignite.
The flashover is the transition from an incipient fire to a full fire through the ignition of all combustible surfaces in the room by the radiant heat of the smoke gas layer.

The phenomena of flue gas ignition and flashover occur with every major fire in closed rooms that has exceeded the initial phase.

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