Combustion Chemistry: Free Radicals and Chain Reactions

Free Radicals

• Free Radicals are the fourth component of the Fire Tetrahedron
• They form in the combustion of fuels like Octane, which combines with oxygen to produce carbon dioxide and steam
• The reaction involves multiple stages, not a single step, due to the limited number of molecules that can collide simultaneously

Chain Reaction

• The combustion of Octane involves a chain reaction, with the initial reaction producing highly unstable products with incomplete molecular structures
• These products react immediately in the next stage, completing the reaction in stages

Extinguishing Agents

• The critical rate of application of an extinguishing agent depends on its capacity to quench Free Radicals
• Faster quenching requires less powder to extinguish the fire, and smaller particle size helps cover a larger area

Limitations of Dry Chemical Agents

• Dry Chemical Agents do not produce a lasting inert atmosphere above a flammable liquid's surface
• They should not be used in areas with relays and delicate electrical contacts, as they can render equipment inoperative
• They can be slightly corrosive and should be removed from undamaged surfaces after fire extinguishment
• The use of water and dry chemical agents should be avoided

Halons as an Extinguishing Agent

• Halogenated extinguishing agents are hydrocarbons with one or more hydrogen atoms replaced by atoms from the halogen series
• They are non-flammable, non-toxic, and leave no residue
• Halons create an inert, survivable atmosphere in the compartment, and are effective on Class B, C, and E fires, but less effective on Class A fires
• They are not to be used on Class D fires

Halon Nomenclature

• The first digit of the number represents the number of carbon atoms in the compound molecule
• The second digit represents the number of fluorine atoms, the third digit represents the number of chlorine atoms, and the fourth digit represents the number of bromine atoms
• The fifth digit represents the number of iodine atoms (if any), and hydrogen is not numbered

Halon Substitutes and Alternatives

• The Montreal Protocol has led to the phase out of ozone-depleting substances, including Halons
• The international community has formulated alternatives to Halons, including:
  • FC-3-1-10 (PerFluoroButane, C4F10)
  • HFC-227ea (HeptaFluoroPropane, CF3CHFCF3)
  • R-595 Blend (a blend of HydroChloroFluoroCarbons)
  • HFC-23 (TriFluoroMethane, CHF3)
  • Water Mist (an environmentally acceptable solution)

Water Mist

• Water Mist is defined as water having a droplet size of 1000 microns
• It combats fires by cooling, oxygen displacement, and radiant heat attenuation
• Water Mist systems are effective, clean, and absorb smoke and irritant gases, making them suitable for various applications, including Turbine Hoods, Flammable Liquid Storage, and Machinery Rooms.