How Chemical Flame Inhibition Works to Extinguish Fire

Fire is a rapid oxidation process that produces heat, light, and various reaction products. Fire requires three elements to sustain itself: fuel, oxygen, and heat. These three elements form the fire triangle, and removing any one of them can extinguish the fire.

However, some fires are more difficult to put out than others, especially those involving flammable liquids and gases. These fuels can produce vapors that ignite easily and burn rapidly, creating a self-sustaining chain reaction that feeds the fire. To extinguish these fires, simply cooling or smothering them may not be enough. A different method of fire extinguishment is needed: chemical flame inhibition.

Key Takeaways

What is chemical flame inhibition?How does it work?What types of fires can it extinguish?What are some common extinguishing agents?
Chemical flame inhibition is a method of fire extinguishment that interrupts the flame-producing chemical reaction and stops flaming.Some extinguishing agents, such as dry chemicals and halogenated hydrocarbons, interfere with the chain of events needed for combustion to occur and disrupt the fuel’s ability to react with oxygen.Dry chemicals, such as sodium bicarbonate and potassium bicarbonate, are widely used for Class B and C fires. Halogenated hydrocarbons, such as halons and halon alternatives, are also effective for Class B and C fires but have environmental concerns.Dry chemicals, such as sodium bicarbonate and potassium bicarbonate, are widely used for Class B and C fires. Halogenated hydrocarbons, such as halons and halon alternatives, are also effective for Class B and C fires, but have environmental concerns.

What is Chemical Flame Inhibition?

Chemical flame inhibition is a method of fire extinguishment that interrupts the flame-producing chemical reaction and stops flaming. Some extinguishing agents, such as dry chemicals and halogenated hydrocarbons (Halons), interfere with the chain of events needed for combustion to occur and disrupt the fuel’s ability to react with oxygen. This method of extinguishment is effective on gas and liquid fuels because they must flame to burn.

How Does Chemical Flame Inhibition Work?

To understand how chemical flame inhibition works, we need to understand how combustion works. Combustion is a complex process that involves many steps of chemical reactions between the fuel molecules and the oxygen molecules in the air. These reactions produce heat, light, and various products, such as carbon dioxide and water.

One way to simplify the combustion process is to use the Zeldovich mechanism⁴, which describes the main steps of combustion as follows:

  • Initiation: The fuel molecules are broken down into smaller fragments by heat or other sources of energy.
  • Propagation: The fuel fragments react with oxygen molecules to form intermediate radicals, such as hydroxyl (OH) and hydroperoxyl (HO2). These radicals are highly reactive and can initiate further reactions with other fuel or oxygen molecules.
  • Branching: The intermediate radicals can also react with each other or with other radicals to form more radicals, increasing the number of reactions and the rate of combustion.
  • Termination: The radicals eventually react with each other or with stable molecules to form final products, such as carbon dioxide and water.

The propagation and branching steps create a self-sustaining chain reaction that feeds the fire. The termination step limits the extent of combustion by consuming the radicals.

Chemical flame inhibitors work by breaking this chain reaction. They interfere with the chain of events needed for combustion to occur and disrupt the fuel’s ability to react with oxygen, which is necessary to keep the fire alive.

What Types of Fires Can Chemical Flame Inhibition Extinguish?

Fires are classified according to the type of fuel involved. The most common classification system is based on five classes: A, B, C, D, and K. Each class has different characteristics and requires different methods of extinguishment.

Class A fires involve ordinary combustible materials, such as wood, cloth, paper, rubber, and many plastics. These materials burn in the solid phase and produce glowing embers or char. Water is used in a cooling or quenching effect to reduce the temperature of the burning material below its ignition temperature.

Class B fires involve flammable liquids, greases, and gases. These materials burn in the vapor phase and produce flames that can spread quickly. Water is not effective for these fires, as it may spread the burning liquid or gas or cause a steam explosion. Chemical flame inhibition is one of the methods used to extinguish these fires, as it can interrupt the flame-producing chemical reaction and stop flaming.

Class C fires involve energized electrical equipment. These fires are essentially Class A or B fires but with the added hazard of electric shock. Water is not suitable for these fires, as it may conduct electricity and harm the firefighter or damage the equipment. Chemical flame inhibition is also one of the methods used to extinguish these fires, as it can interrupt the flame-producing chemical reaction and stop flaming, without conducting electricity.

Class D fires involve combustible metals, such as magnesium, sodium, potassium, and aluminum. These metals burn at very high temperatures and react violently with water, air, or other agents. Water is not only ineffective but also dangerous for these fires, as it may cause an explosion or intensify the fire. Chemical flame inhibition is not effective for these fires either, as some metals can oxidize by carbon dioxide or nitrogen, the two most common extinguishing agents. Special dry powders that absorb heat and form a crust over the burning metal are used to extinguish these fires.

Class K fires involve cooking oils and fats. These materials have high flash points and can ignite at very high temperatures. Water is not effective for these fires, as it may cause a violent boiling-over or splashing of the burning oil or fat. Chemical flame inhibition is also not effective for these fires, as it may not penetrate the surface of the oil or fat. Wet chemical agents that form a foam blanket over the burning oil or fat and prevent re-ignition are used to extinguish these fires.

What are Some Common Extinguishing Agents for Chemical Flame Inhibition?

There are two main types of extinguishing agents that work by chemical flame inhibition: dry chemicals and halogenated hydrocarbons.

Dry chemicals are powders that consist of fine particles of sodium bicarbonate, potassium bicarbonate, ammonium phosphate, or other salts⁸. They are widely used for Class B and C fires, as they can interrupt the chain reaction and stop flaming. They are also effective for Class A fires, as they can smother and cool the burning material. Dry chemicals are stored under pressure in portable fire extinguishers or fixed systems and are expelled by a propellant gas, such as nitrogen or carbon dioxide.

Halogenated hydrocarbons are liquids that contain atoms of fluorine, chlorine, bromine, or iodine. They are also known as halons or halon alternatives. They are effective for Class B and C fires, as they can interrupt the chain reaction and stop flaming. They are also electrically non-conductive and leave no residue. Halons were widely used in the past, but have been phased out due to their ozone-depleting potential. Halon alternatives are newer compounds that have less environmental impact but may still have some drawbacks, such as toxicity or global warming potential. Halogenated hydrocarbons are stored under pressure in portable fire extinguishers or fixed systems and are expelled by a propellant gas, such as nitrogen or carbon dioxide.

Conclusion

Chemical flame inhibition is a method of fire extinguishment that interrupts the flame-producing chemical reaction and stops flaming. It is effective on gas and liquid fuels, which are classified as Class B and C fires. Some common extinguishing agents that work by chemical flame inhibition are dry chemicals and halogenated hydrocarbons. These agents have different advantages and disadvantages and should be used with caution and proper training.

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