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Products of combustion

In the matter of firefighting, wood is probably the most important fuel. The ignition and extinguishment of most normal fires depends to a very large extent on the properties of wood or charcoal. Charcoal is the product obtained when wood is carbonized or decomposed by heat in the absence of air. Wood is widely used as a fuel in countries rich in timber.

Burning wood and wood-based materials produce water vapour, heat, carbon dioxide and carbon monoxide. All burning materials produce hot gases called firegases, flame, heat and smoke resulting in decreased oxygen levels. The reduced oxygen levels and the carbon monoxide present the primary hazard.

Above-mentioned carbon dioxide, carbon monoxide and water vapour are predominant fire gases. Burning vegetable fibers such as cotton, jute, flax, hemp and sisal give off large amounts of dense smoke. Jute smoke is particularly acrid.

Burning wool gives off dense, grayish-brown smoke. Another product of wool combustion is hydrogen cyanide, a highly toxic gas. Charring wool form a sticky, black, tarlike substance.

Burning silk emits quantities of thin gray smoke, somewhat acrid in character. Silk may produce hydrogen cyanide gas under certain burning conditions.

The oldest well-known form of plastic, celluloid, is the most dangerous of plastics. Celluloid decomposes at temperatures of 121⁰ C (250⁰ F) and above with great rapidity, and without the addition of oxygen from the air. Flammable vapour is produced by the decomposition. If this vapour is accumulated and is then ignited, it can explode violently. It will burn vigorously and is difficult to extinguish.

The caloric value of rubber is twice that of other common combustible materials. Natural rubber decomposes slowly when first heated. At about 232⁰ C (450⁰C) or above, it begins to decompose rapidly, giving off gaseous products that may result in an explosion. The ignition temperature of these gases is approximately 250⁰C (500⁰F).

Synthetic rubbers behave similarly, though the temperature, at which decomposition becomes rapid, may be somewhat higher.

Liquid paint burns fiercely and gives off much heavy black smoke. Because of the dense smoke and toxic fumes, firemen fighting a paint fire in enclosed area should always use breathing apparatus.

Explosions are another hazard of liquid paint fires. Since paint is normally stored in tightly sealed cans or drums (of up to 150-190 litres capacity) fire may easily heat the drums and burst them.

The insulation on electrical conductors is usually made of rubber or plastic. Polyvinyl chloride known as PVC is widely used as electrical insulation. While burning this plastic hydrogen chloride releases which attacks the lungs with serious consequences. It is also believed that PVC contributes to the severity and hazards of fires.

Burns can be caused by direct contact with flames, or by heat radiated from the fire. Flames are rarely separated from the burning material. However, in certain types of smouldering fires, heat, smoke and gas can develop without visible flames. Air currents can carry them far in advance of the fire.

There exists one more principal difficulty-hazardous effect of extinguishing agents. It concerns primarily halogenated hydrocarbons. The vapours of carbon tetrachloride, CC1₄, are themselves quite dangerous. They may render a person unconscious and prevent escape from the fire. More serious still are the gases that are pro­duced from CC1₄ by reactions in the fire. In addition to other noxious fumes, small percentages of phosgene, COC1₂, may be generated, and this gas is most deadly. Certain of the freons are much less toxic than CC1₄, and less apt to give dangerous products.

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