B - Combustion Fundamentals

Chapter 1: Introduction to Combustion Fundamentals

  • Types of Combustion:

    • Fire: Requires oxygen, a heat source, and fuel.

    • Flash Fire: Requires fire components plus suspension.

    • Explosion: Requires fuel, oxygen, an ignition source, and containment.

  • Ignition Sources:

    • Open flames (welding, cutting)

    • Hot surfaces (dryer bearings, heat exchangers)

    • Mechanical impacts (friction from belts)

    • Static discharges and smoldering dust

  • Ignition Source Statistics (2015 Study):

    • Mechanical spark: 30% of explosions.

  • Properties Affecting Dust Explosion:

    • Chemistry of dust must react exothermically with oxygen.

Chapter 2: Mechanics of an Explosion

  • Key Factors:

    • Moisture Level: Affects burn capability and explosion severity.

    • Concentration of Dust: Both insufficient and excessive concentrations prevent explosions.

    • Particle Size: Smaller particles burn faster due to increased surface area.

    • Turbulence: Enhances burn rate and pressure increase.

  • Explosion Dynamics:

    • Combines fuel, oxygen, and ignition source to generate heat and oxides.

    • Key Metrics:

      • Kinetics (dpdtmax): Rate of heat and pressure change.

      • Thermodynamics (p max): Maximum heat released.

Chapter 3: Understanding KST and Pressure Dynamics

  • KST (Explosibility Index):

    • Measures dust severity in a 1 cubic meter vessel.

    • Categories:

      • ST 1: KST ≤ 200, p max ≤ 10 bar.

      • ST 2: 201 < KST ≤ 300, p max ≤ 10 bar.

      • ST 3: KST > 300, p max ≤ 12 bar.

  • Example: Cornstarch has a KST of 202 bar m/s with varying DPDT in different enclosures.

Chapter 4: Primary vs. Secondary Explosions

  • Secondary Explosions: Initiated by a primary explosion that ignites dust suspended in the air, often more destructive.

  • Dust Concentration: Explosible ranges are between 50 - 3000 grams/m³.

  • Imperial Sugar Plant Case Study (2006): Dust accumulation led to catastrophic secondary explosions.

Chapter 5: Dynamics of Secondary Explosions

  • Simulation: Shows significant differences in pressure and flame duration between primary and secondary explosions.

  • Pressure Piling & Flame Jet Ignition: Enhance explosion risks due to pre-compression of gases.

  • Deflagration to Detonation Transition: Transition from subsonic to supersonic combustion leads to extreme pressure spikes.

Chapter 6: Primary Explosion Sequence

  • Explosion Sequence:

    • Ignition of dust cloud results in pressure rupture.

    • Flame and pressure spread through ductwork.

    • Dust buildup is disturbed, causing further ignition.

  • Ductwork Considerations:

    • Bends can increase flame speed; design limits to two bends.

Chapter 7: Conclusion

  • Risk Mitigation:

    • Understanding explosion dynamics is crucial for safety.

    • Future modules will present protective measures by FICA for plant safety.