8.2 Soot Formation and Oxidation

Soot formation in combustion is a complex process involving fuel breakdown, particle growth, and aggregation. Understanding these mechanisms is crucial for developing strategies to reduce harmful particulate emissions in various combustion systems.

Soot oxidation competes with formation, influenced by temperature, fuel composition, and oxidizing species. Particle characteristics and distribution evolve throughout combustion, affecting light scattering, transport, and health impacts. This knowledge guides emission reduction efforts.

Soot Formation Mechanisms

Precursor Development and PAH Formation

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• Soot precursors originate from fuel pyrolysis and incomplete combustion processes
• Fuel molecules break down into smaller hydrocarbon fragments under high temperatures
• These fragments recombine to form larger, more complex molecules
• Polycyclic aromatic hydrocarbons (PAHs) emerge as key intermediates in soot formation
• PAHs consist of multiple fused aromatic rings (benzene, naphthalene, anthracene)
• PAH formation occurs through various pathways including:
  • Hydrogen abstraction carbon addition (HACA) mechanism
  • Resonantly stabilized free radical (RSR) routes
  • Ion-molecule reactions

Nucleation and Growth Processes

• Nucleation marks the initial stage of solid particle formation from gas-phase species
• PAHs and other large molecules collide and stick together, forming nascent soot particles
• These initial particles typically measure 1-2 nm in diameter
• Surface growth involves the addition of gas-phase species to existing particle surfaces
• Acetylene serves as a primary growth species, adding carbon to the particle surface
• Surface growth significantly increases particle mass while maintaining a relatively constant number of particles

Particle Interactions and Aggregation

• Coagulation occurs when small particles collide and combine to form larger spherical particles
• This process reduces the total number of particles while increasing their average size
• Agglomeration involves the formation of chain-like structures from primary particles
• These structures can form complex, fractal-like geometries
• Agglomeration primarily affects particle morphology rather than total mass
• The extent of agglomeration depends on factors such as:
  • Particle concentration
  • Residence time in high-temperature regions
  • Local flow conditions

Soot Oxidation and Properties

Oxidation Mechanisms and Influencing Factors

• Soot oxidation competes with formation processes throughout combustion
• Oxidation rates depend on particle size, structure, and available oxidizing species
• Primary oxidizing agents include O₂, OH radicals, and O atoms
• Oxidation occurs more readily at particle edges and defect sites
• Flame temperature significantly affects oxidation rates
• Higher temperatures generally lead to increased oxidation and reduced soot emissions
• Fuel composition influences soot formation and oxidation tendencies
• Aromatic fuels typically produce more soot than aliphatic counterparts
• Oxygenated fuels can reduce soot formation by promoting more complete combustion

Particle Characteristics and Distribution

• Soot particle size distribution evolves throughout the combustion process
• Initial nuclei form a narrow distribution of very small particles
• Surface growth and coagulation broaden the distribution and shift it towards larger sizes
• Typical soot particles in flames range from 10-50 nm in diameter
• Agglomerated structures can reach several hundred nanometers in size
• Particle size distribution affects:
  • Light scattering and absorption properties
  • Particle transport and deposition behavior
  • Health impacts and environmental effects
• Measurement techniques for size distribution include:
  • Scanning mobility particle sizers (SMPS)
  • Transmission electron microscopy (TEM)
  • Laser-induced incandescence (LII)

Temperature Effects and Fuel Considerations

• Flame temperature plays a crucial role in soot formation and oxidation balance
• Lower temperatures (1300-1600 K) favor soot formation processes
• Higher temperatures (>1800 K) promote more rapid oxidation
• Temperature effects interact with other factors such as:
  • Fuel-air mixing
  • Residence time
  • Pressure conditions
• Fuel composition significantly influences soot propensity
• Factors affecting soot formation in different fuels include:
  • Carbon-to-hydrogen ratio
  • Presence of aromatic structures
  • Oxygen content
  • Molecular weight and volatility
• Fuel additives and blending strategies can be employed to reduce soot emissions
• Examples include:
  • Adding oxygenates to diesel fuel
  • Blending low-sooting components with high-sooting base fuels