Combustion Chemistry: Free Radicals and Chain Reactions
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Questions and Answers

What is the critical rate of application of a fire extinguishing agent dependent on?

  • The capacity to produce an inert atmosphere
  • The capacity to quench Free Radicals (correct)
  • The particle size of the extinguishing agent
  • The capacity to cool the surrounding area

    • Why are Dry Chemical Agents not suitable for use in telephone exchanges and computer equipment rooms?

  • Because they are corrosive
  • Because they can render electrical equipment inoperative (correct)
  • Because they produce a lasting inert atmosphere
  • Because they are not effective against electrical fires

    • What is the disadvantage of using Dry Chemical Agents in conjunction with water?

  • It produces a toxic gas
  • It creates a corrosive substance
  • It is not environmentally friendly
  • It reduces the effectiveness of the Dry Chemical Agent (correct)

    • What property of Dry Chemical Agents prevents them from producing a lasting inert atmosphere?

    <p>Their insulating properties</p> Signup and view all the answers

    What is the result of using Dry Chemical Agents in the presence of re-ignition sources?

    <p>Temporary extinguishment of the fire</p> Signup and view all the answers

    What is the benefit of using Halon extinguishing agents?

    <p>They create an inert atmosphere</p> Signup and view all the answers

    What is unique about the flame-inhibiting property of Halon extinguishing agents?

    <p>It creates an inert atmosphere</p> Signup and view all the answers

    Which type of Halon is more effective than others?

    <p>Bromine-based Halon</p> Signup and view all the answers

    What is the fourth component of the Fire Tetrahedron?

    <p>Free Radicals</p> Signup and view all the answers

    What is the combustion reaction of Octane with oxygen?

    <p>C8H16 + 12O2 → 8CO2 + 8H2O</p> Signup and view all the answers

    According to the Collision Theory, how many molecules can collide simultaneously?

    <p>3-4 molecules</p> Signup and view all the answers

    What is the reason for the reaction of Octane with oxygen not taking place in a single step?

    <p>Not more than 3-4 molecules can collide simultaneously</p> Signup and view all the answers

    What is the characteristic of the products of the first stage of the reaction?

    <p>They have incomplete molecular structures</p> Signup and view all the answers

    Why do the products of the first stage of the reaction react immediately in the next stage?

    <p>Their activation energy levels are low</p> Signup and view all the answers

    What is the result of the reaction of Octane with oxygen in the first stage?

    <p>All of the above</p> Signup and view all the answers

    How does the reaction of Octane with oxygen take place?

    <p>In several stages</p> Signup and view all the answers

    What type of fires are Halons not effective against?

    <p>Class D fires</p> Signup and view all the answers

    What is the primary mechanism by which Halons extinguish fires?

    <p>Choking off oxygen</p> Signup and view all the answers

    What does the first digit in the Halon nomenclature represent?

    <p>Number of carbon atoms</p> Signup and view all the answers

    What is the deadline for phase out of Halons in developed countries according to the Montreal Protocol?

    <p>01 Jan 1995</p> Signup and view all the answers

    What is the purpose of creating a Halon bank in the Navy?

    <p>To procure and store Halon for use until 2010</p> Signup and view all the answers

    What is the trend in toxicity and extinguishing properties of Halons from F to I?

    <p>Increase in toxicity and extinguishing properties</p> Signup and view all the answers

    What type of atom is not numbered in the Halon nomenclature?

    <p>Hydrogen</p> Signup and view all the answers

    What international agreement aims to phase out ozone-depleting substances, including Halons?

    <p>Montreal Protocol</p> Signup and view all the answers

    What is a characteristic of electrically non-conducting fire extinguishing agents?

    <p>They do not leave a residue upon evaporation</p> Signup and view all the answers

    What is the chemical name of the trade name PFC-410?

    <p>PerFluoroButane (C4F10)</p> Signup and view all the answers

    What is the primary way HFC-227ea achieves effective fire extinguishment?

    <p>By physical cooling of the fire</p> Signup and view all the answers

    Why is HFC-227ea safe to use on energized electrical equipment?

    <p>Because it is non-conductive</p> Signup and view all the answers

    What is the trade name of the blend of HydroChloroFluoroCarbons?

    <p>NAF S-III</p> Signup and view all the answers

    Why is R-595 Blend classified as a transitional substance?

    <p>Because it is a HydroChloroFluoroCarbon (HCFC)</p> Signup and view all the answers

    What is the chemical name of the trade name FE-13?

    <p>TriFluoroMethane (CHF3)</p> Signup and view all the answers

    How does the volume required for the stowage of HFC-227ea compare to that of Halon 1301?

    <p>It is approximately 1.6 times more than that of Halon 1301</p> Signup and view all the answers

    Why are inert gases not suitable for rapidly escalating hydrocarbon fires or explosion inertion in occupied areas of a ship?

    <p>Due to their long discharge time and large agent volume</p> Signup and view all the answers

    What is the droplet size of water mist?

    <p>1000 microns</p> Signup and view all the answers

    How does water mist combat fires?

    <p>By cooling, oxygen displacement, and radiant heat attenuation</p> Signup and view all the answers

    What happens to the water mist when it absorbs heat from the fire?

    <p>It gets converted to steam expanding up to 1700 times by volume</p> Signup and view all the answers

    What is an additional benefit of water mist systems?

    <p>They clean, absorb smoke and irritant gases, and particulate</p> Signup and view all the answers

    Why are water mist fire protection systems complex?

    <p>Due to their high pressure requirements and nozzle design technology for providing fine atomization</p> Signup and view all the answers

    Where do water mist fire protection systems find application?

    <p>In areas such as Turbine Hoods, Flammable Liquid Storage, Machinery Rooms, Emergency Generator Rooms, Switch Gear Rooms, and Engine Rooms</p> Signup and view all the answers

    What is an advantage of water mist systems?

    <p>They are environmentally acceptable and do not contribute to ODP or Global Warming</p> Signup and view all the answers

    Study Notes

    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.

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    Understanding the role of free radicals and chain reactions in combustion processes, including the combustion of fuels like octane.

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